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Special Symposia on Contact Mechanics

2004;():1-8. doi:10.1115/TRIB2004-64331.

A three-dimensional numerical model based on a semi-analytical method in the framework of small strains and small displacements with respect of Hertz’s hypotheses is presented for solving an elastic-plastic dented contact with friction. The calculation of surface deformations and pressure distribution, which is the most time consuming step during the elastic-plastic algorithm, is obtained using a method based on a variational principle with a Fast Fourier Transform (FFT) and a Conjugate Gradient Method (CGM). The method is fast enough to allow investigating the effect of a small size surface defect, here a debris denting, on the subsurface elastic-plastic stress state, requiring a fine mesh with around 106 surface grid points. Further, the FFT approach is also involved in the calculation of internal stress state. The plasticity model is based on an incremental load and Von Mises yield criterion. The effects of the contact pressure distribution and residual strain on the geometry of the contacting surfaces yield from the Betti’s reciprocal theorem with initial strain. The code is used to compute a few smooth and dented contacts, with several types of contact interfaces conditions, including frictionless and Coulomb friction. The effects of surface dents and friction on the contact pressure and subsurface stress field are presented and discussed.

Commentary by Dr. Valentin Fuster
2004;():9-15. doi:10.1115/TRIB2004-64332.

The quality of electrical contact resistance is important to the performance and reliability of metal-contact microelectromechanical system (MEMS) relays and switches. The cyclic degradation of contact resistance was examined for a metal multilayer sphere-on-flat contact at low contact force. The relationships between the degradation and underlying mechanisms, particularly surface contamination effects, were investigated through experimentation and analysis. Results indicated that the degradation severity could be significantly decreased with environmental control and contact current reduction. Micro-arcs at very small surface gaps, which can decompose adsorbed contaminants and leave an insulating surface film, were proposed as a cause of the resistance degradation.

Commentary by Dr. Valentin Fuster
2004;():17-21. doi:10.1115/TRIB2004-64333.

Adhesion plays an important role in microelectromechanical systems (MEMS). It is a major concern in MEMS reliability and oftentimes excessive adhesion forces lead to permanent adherence of MEMS surfaces resulting in microdevice failure. The role of residual stresses in the adhesive contact between a pre-stressed membrane and a rigid flat-ended cylindrical punch is studied. Breaking the contact can be achieved under either fixed-load or fixed-grips configuration. The influence of the residual stress on the pull-off force, punch displacement, and contact area at pull-off is studied. It is shown that residual stresses have significant influence on interfacial contact behavior in MEMS and, hence, should be taken into consideration in formulating the adhesion contact mechanics.

Topics: Stress , Stiction
Commentary by Dr. Valentin Fuster
2004;():23-28. doi:10.1115/TRIB2004-64334.

Microball bearings can potentially provide robust and low friction support in micromachines such as micromotors and microgenerators. Their microtribological behavior needs to be investigated for design and control of such micromachines. In this paper a vision-based, non-intrusive measurement method is presented for characterization of friction in linear microball bearings. Infrared imaging is used to directly observe the dynamics of microballs and track the motion of bearing components. It is verified that microballs roll most of the time with occasional sliding or bumping resulting from fabrication nonuniformity. The friction-velocity curve demonstrates evident hysteresis. The dependence of frictional behavior on several factors is studied.

Topics: Bearings
Commentary by Dr. Valentin Fuster
2004;():29-32. doi:10.1115/TRIB2004-64335.

Many state-of-art microelectronic, photonic and MEMS devices are based upon or created using small-scale contacts. These include, for example, high frequency, microscale electromechanical switches and nanopatterning of organic optoelectronic materials by contact adhesion, cold welding, and lift-off. The initial stages of contact occur between asperities of micro- and/or nano-scopic dimensions. As a consequence, understanding the processes that occur at the atomic level when two rough surfaces are bought into contact is fundamentally important for a wide range of problems including adhesion, contact formation, contact resistance, materials hardness, friction, wear, and fracture. The centrality of single asperities in the fundamental micromechanical response of contact between two rough surfaces has long been recognized. A wide range of experiments has shown that the conductance of small contacts changes abruptly as a function of contact size. In some cases, the conductance through individual asperities increases in a stepwise manner as the two surfaces are pressed into contact. These jumps conductance appear to be correlated with jumps in the force. The observed force-displacement relation appears to be poorly described by JKR theory during loading, while JKR provides a reasonable description of the behavior in unloading. In this presentation (see Acta Materialia 52 , 3983 (2004) for more details), we report the results of molecular dynamics simulations of single asperity contact during multiple cycles of loading and unloading at room temperature. We focus on the mechanisms by which contact deformation occurs and the relationship between contact conductance (and contact area) and the deformation. These simulations account for adhesion, elastic deformation, dislocation generation and migration, the formation of other types of defects and morphology evolution. In order to study the elastic and plastic deformation of the asperities on a rough surface, we set up a model system, as shown in Fig. 1. For simplificity, we consider a single deformable asperity on a deformable substrate that interacts with a flat, rigid plate. We calculate the conductance of the contact during loading and unloading through the modified Sharvin model [12]. To our knowledge, this study represents the first dynamic, atomistic simulation of the elastic and plastic deformation behavior of a single asperity and the corresponding evolution of the contact area and contact conductance. The present simulation results reproduce a large body of existing nano-contact experimental results, including the stepwise variation of contact area and conductance with displacement and the hysteresis in the contact radius and contact resistance versus force curves.

Commentary by Dr. Valentin Fuster
2004;():33-35. doi:10.1115/TRIB2004-64336.

When two surfaces are brought together, contact occurs initially between asperities on the surface. If the mechanical loads are small, complete contact is never achieved and the behavior is dominated by asperity contact. The contact area and asperity morphology may evolve in time as a result of mechanical and capillarity (surface tension) effects, mediated by plastic deformation and/or diffusion. If a current passes through the contact, as in the case of micro-electro-mechanical switches, the evolution may be controlled by electromigration. This effect may be especially important if the voltage drop across the contact is fixed and the fractional contact area is small, such that the current is concentrated in a small number of contacts (see Fig. 1). Electromigration occurs as a result of the voltage driven electrons scattering off and imparting momentum to the atoms in the solid (see Fig. 2). Typically, the electromigration atom flux is opposite the direction of the electrical current (i.e., in the same direction as the electron flux). At small homologous temperatures (i.e., the temperature normalized by the melting temperature) and in small structures (such as asperities), atomic transport will be dominated by surface, rather than bulk, diffusion. In this presentation, we consider the evolution of an idealized asperity under the action of both capillarity and electromigration.

Commentary by Dr. Valentin Fuster
2004;():37-40. doi:10.1115/TRIB2004-64337.

A model for the electrical degradation of MEMS switch metal microcontacts under many-cycle operation is presented. This model utilizes an asperity-based description of contacts and a general model for the degradation evolution. Calculations of switch resistance using this model predict a resistance evolution that shows good qualitative agreement with measurements. Implications of this model are discussed.

Topics: Metals , Cycles
Commentary by Dr. Valentin Fuster
2004;():41-45. doi:10.1115/TRIB2004-64340.

An innovative RF-MEMS varactor has been developed for use in distributed RF-MEMS circuits such as phase shifters and tunable matching networks where the capacitance ratio between on- and off- state must be limited to 2–10. By fabricating standoffs on the bottom side of the beam, this device eliminates the intimate contact between the FR-MEMS and dielectric, that is found in typical RF-MEMS beam capacitive switches. These standoffs limit the range of motion, allowing the capacitance ratio to be set, and also greatly reduces the contact area thus preventing stiction from occurring. The RF-MEMS switch has been fabricated with initial measurements demonstrating a capacitance ratio of 2.5. Preliminary reliability testing results demonstrate that this RF-MEMS design is very robust.

Commentary by Dr. Valentin Fuster
2004;():47-50. doi:10.1115/TRIB2004-64344.

A polarization vector of the piezoceramic element and location of excitation electrodes on its surface determine the resonance modes of the high frequency vibration exciter. In its turn the modes of vibration play a key role in the functionality of ultrasonic motor. Two different regimes of operation are analyzed — when the contact zone of the resonator performs elliptic and unidirectional motions. Though the mechanical characteristics of the ultrasonic motor in both cases are comparable, detailed analysis of the contact surface shows very different wears. Computational simulation and experimental investigations are used to analyze the contact zone of ultrasonic motor. Laser holography is used to identify and control the regimes of motion. Atomic force microscopy is used for analysis of contact surfaces.

Commentary by Dr. Valentin Fuster
2004;():51-53. doi:10.1115/TRIB2004-64345.

A novel quantitative nano/micro-tribometer with integrated SPM and optical microscope imaging has been developed to characterize numerous physical and mechanical properties of liquid and solid thin films and coatings, with in-situ monitoring their changes during micro and nano indentation, scratching, reciprocating, rotating and other tribology tests. Both the materials properties and surface topography can be assessed periodically during the tests.

Commentary by Dr. Valentin Fuster
2004;():55-58. doi:10.1115/TRIB2004-64346.

For faster signal propagation in integrated circuits, new materials with lower dielectric constant (low-k) values are required with copper metal lines. Although integration of low-k materials (k<3.0) has been demonstrated, but ultra low-k materials possess many challenges due to their poor mechanical integrity and weak adhesion to other interconnects. During chemical mechanical planarization (CMP) generation of several defects including delamination of low-k materials is severe problem in the integration of these materials. Different slurries and pad introduces different levels of defect and also batch-to-batch variation in consumables is often makes process more difficult. In this study we have investigated the tribological properties of CMP pad and wafer interface while monitoring coefficient of friction and acoustic emission data. Signals are analyzed in order to online defect monitoring, batch-to-batch consumable variations and different consumables effects.

Topics: Tribology
Commentary by Dr. Valentin Fuster
2004;():59-64. doi:10.1115/TRIB2004-64347.

The behavior of a nano-scale cylindrical body (e.g. a fiber), lying on a substrate and acted upon by a combination of normal and tangential forces, is the subject of this investigation. As the scale decreases to the nano level, adhesion becomes an important issue in this contact problem. Thus this investigation treats the two-dimensional plane strain elastic deformation of both the cylinder and the substrate during a rolling/sliding motion, including the effect of adhesion using the Maugis model. For the initiation of sliding, the Mindlin approach is used, whereas for rolling, the Carter approach is utilized. Each case is modified for nano-scale effects by including the effect of adhesion on the contact area and by using the adhesion theory of friction for the friction stress. Analytical results are given for the normal and tangential loading problems, including the initiation of sliding and rolling in terms of dimensionless quantities representing adhesion, cylinder size, and applied forces.

Commentary by Dr. Valentin Fuster
2004;():65-68. doi:10.1115/TRIB2004-64348.

Cycling of a micromechanical switch with gold-on-gold contacts demonstrates that the contact resistance decreases and the adherence force increases. An experimental setup using a Scanning Probe Microscope (SPM) is allowing the fundamental physics of this behavior to be better understood. The setup includes two side-by-side cantilevers — one of high stiffness which applies the repeated loading and the other a standard profiling cantilever allowing in-situ measurements of topographical changes.

Commentary by Dr. Valentin Fuster
2004;():69-74. doi:10.1115/TRIB2004-64349.

The effect of a height-dependent asperity radius of curvature is accounted for in a recently developed scale-dependent model of contact and friction. The contact and friction model includes the effects of adhesion, using the Maugis model, and of scale-dependent friction, using the Hurtado and Kim single asperity friction model. This multi-asperity model has been modified to include the effect of non-contacting asperities. The results indicate the types of conditions under which the effects of a height-dependent asperity radius of curvature affects friction.

Topics: Friction
Commentary by Dr. Valentin Fuster
2004;():75-82. doi:10.1115/TRIB2004-64350.

Previous models and measurements of MEMs adhesion have focused on steady-state adhesion behavior. This approach is inadequate for micromachined switches (such as RF MEMS switches) because it gives no information about the time required for contact opening, an important specification. We propose a technique to measure the switch opening time and present substantial experimental data for switches with gold-gold contacts. The data demonstrate that contact opening time increases dramatically as contact dimple area increases or as pull-apart force or contact resistance decrease. A model of opening time is also presented with model parameters that fit the experimental data.

Commentary by Dr. Valentin Fuster
2004;():83-85. doi:10.1115/TRIB2004-64351.

Microelectromechanical systems (MEMS) radio frequency (RF) switches hold great promise in a myriad of commercial, aerospace, and military applications. MEMS switches offer important advantages over current electromechanical and solid state technologies including high linearity, low insertion loss, low power consumption, good isolation, and low cost [1–21]. However, there is little fundamental understanding of the factors determining the performance and reliability of these devices. Our previous work investigated fundamentals of hot-switched direct current (DC) gold (Au) contacts using a modified microadhesion apparatus as a switch simulator [1]. Those experiments were conducted under precisely controlled operating conditions in air at MEMS-scale forces with an emphasis on the role of surface forces and electric current on switch performance, reliability, and durability [1]. Electric current had a profound effect on deformation mechanisms, adhesion, contact resistance (R), and reliability/durability. At low current (1–10 μA), asperity creep and switching induced adhesion were the most important observations, whereas, at high current (1–10 mA), lack of adhesion and switch shorting by nanowire formation were prominent [1].

Commentary by Dr. Valentin Fuster
2004;():87-92. doi:10.1115/TRIB2004-64352.

The nanoindentation method is becoming increasingly popular for materials characterization, particularly for thin-films, coatings, and other materials that do not easily lend themselves to standard mechanical test methods. However, nanoindentation methods are limited when it comes to their description of a material behavior that deviates from simple elastic or visco-elastic responses. In this paper, we describe a four-parameter constitutive model that has been implemented in a PC-based analysis tool that combines non-linear finite element modeling with data mining gradient-based optimization methods to solve for the unknown parameters from an experimental nanoindentation creep curve. This “VNDM” (virtual nanoindentation and data mining) method, then, is used to complement experimental nanoindentation creep tests for more thorough material characterization. The results verify that the parametric model is capable of describing a wide-range of material behavior, including elastic, visco-elastic, and visco-plastic responses.

Commentary by Dr. Valentin Fuster
2004;():93-97. doi:10.1115/TRIB2004-64354.

We designed a laboratory model to investigate the spontaneous rupturing along a frictionally held interface. A frictional contact, which mimics a material interface or geological fault, is simulated using two photoelastic plastics plates (Homalite or Polycarbonate) held together by friction and compression. The pre-uniaxial static loading is exerted by a hydraulic press. A unique design used in our experiments, which starts the spontaneous rupturing, is the exploding wire technique. It triggers the rupture within a controlled environment while keeping the spontaneous nature of the rupturing. The fault is oblique to the compression axis to provide the shear driving force for continued rupturing. We performed the spontaneous repturing experiment along an interface between both similar and dissimilar materials. Under proper loading conditions, a shear rupture which initially propagates at Rayleigh wave speed, jumps to a supershear speed (close to the longitudinal wave speed) after propagating a finite distance L in a similar material system. For the dissimilar material system, directionality of rupture propagation is observed. A rupture propagating at the generalized Rayleigh wave speed is observed in the same direction as the sliding of the more compliant material and in the opposite direction, supershear rupture is possible under proper loading condition.

Commentary by Dr. Valentin Fuster
2004;():99-102. doi:10.1115/TRIB2004-64355.

Results from recent and ongoing investigations of frictional sliding under dynamic loading conditions are discussed. The configuration analyzed consists of two identical elastic plates with an interface characterized by a rate- and state-dependent frictional law. The calculations are carried out within a framework where two constitutive relations are used: a volumetric constitutive relation between stress and strain and a surface constitutive relation that characterizes the frictional behavior of the interface. The simulations discussed predict a variety of sliding modes including a crack-like mode and several pulse-like modes as well as circumstances where the sliding tip speed can exceed the longitudinal wave speed.

Commentary by Dr. Valentin Fuster
2004;():103-106. doi:10.1115/TRIB2004-64357.

A recent theorem due to Barber shows an analogy between conductance and incremental stiffness of a contact, implying bounds on conductance based on peak-to-peak roughness. This shows that even a fractal roughness, with bounded amplitude, has a finite conductance. The analogy also permits a simple interpretation of classical results of rough contact models based on independent asperities such as Greenwood-Williamson and developments. For example, in the GW model with exponential distribution of asperity heights, the conductance is found simply proportional to load, and inversely proportional to a roughness amplitude parameter which does not depend greatly on resolution, contrary to parameters of slopes and curvatures. However, for the Gaussian distribution or for more refined models also considering varying curvature of asperities (such as Bush Gibson and Thomas), there is dependence on sampling interval and the conductance grows unbounded. An alternative choice of asperity definition (Aramaki-Majumdar-Bhushan) is suggested, which builds on the geometrical intersection of the rough surface, with the consequence of a finite contact area, and converging load-separation and load-conductance relationship. A discussion follows, also based on numerical results.

Commentary by Dr. Valentin Fuster
2004;():107-111. doi:10.1115/TRIB2004-64359.

Microelectromechanical systems (MEMS) technology employed to generate switches and relays has the potential to replace traditional solid-state devices due to broader operating temperature range, higher breakdown voltage, and much higher off-state resistance. Interest in MEMS switches and relays has surged recently, principally due to the demonstrated performance in switching radio-frequency signals. The role of surface topography in the performance and lifetime of MEMS switches and relays is the objective of this review paper. Special emphasis is given to an approach that accounts for effect of roughness and to the role of surface topography in the pull-in voltage and contact bouncing. A critical assessment of emerging topics where experimental and theoretical studies are still required in order to obtain better insight into the role of surface topography in MEMS switches and relays is presented.

Commentary by Dr. Valentin Fuster
2004;():113-116. doi:10.1115/TRIB2004-64360.

We have measured pre-sliding tangential deflections (PSTD) between micromachined surfaces of up to 200 nanometers in length before the static friction event using a polysilicon nanotractor actuator [1,2]. The detailed PSTD structure is resolved by a one-nanometer-resolution in-plane optical metrology we have developed, and may be a manifestation of discrete asperity-asperity interactions leading to an effective spatial distribution of friction coefficients. Results indicate a dependence on surface treatment, with a perfluorinated eight-carbon chain monolayer coating showing substantially different PSTD than an eighteen-carbon chain hydrocrabon monolayer. This behavior may qualitatively be related to variations in dynamic versus static friction. We present a simple phenomenological model that captures some of the behavior of PSTD, and suggest some possible microscopic interpretations.

Commentary by Dr. Valentin Fuster
2004;():117-118. doi:10.1115/TRIB2004-64361.

In this extended abstract, we briefly describe a thermodynamic model to treat capillary adhesion energies in MEMS. We first determine the constitutive laws of a capillary pendular ring bridging an asperity to a substrate. We find that the work of adhesion, W, depends on the surface separation rate. For the constant volume case (rapid surface separation), W = 2γ (where γ is the surface energy per unit area of the liquid-air interface), but if thermodynamic equilibrium is maintained (slow surface separation), W = γ. Thermodynamic analysis indicates that heat from the system walls can lower the work of adhesion at slow separation rates. We extend these constitutive laws to a simple multi-asperity surface in which the asperities are all of constant height. At low vapor partial pressures (p/psat ), adhesion can be several orders of magnitude below γ because of incomplete wetting. As vapor partial pressure increases, condensing liquid fills in the geometric irregularities of the surface. As this filling takes place, W approaches 2γ for both the slow and rapid separation rates.

Commentary by Dr. Valentin Fuster
2004;():119-122. doi:10.1115/TRIB2004-64362.

A numerical procedure for the surface deformation and adhesion between elastic half-spaces interacting by the Lennard-Jones surface potential is presented. The problem is solved based on the superposition of Boussinesq solutions for a half-space, with an iteration procedure to incorporate the nonlinear surface force-depression relation. First the axisymmetric contact between a sphere and a plane surface is demonstrated to show the validity of the procedure for a wide range of the Tabor parameter. The method is then applied to the contact between a sphere and a wavy surface, and the influence of the surface waviness on the adhesive behavior is illustrated.

Commentary by Dr. Valentin Fuster
2004;():123-126. doi:10.1115/TRIB2004-64367.

Thermoelastic contact problems can posess non-unique and/or unstable steady-state solutions if there is frictional heating or if there is a pressure-dependent thermal contact resistance at the interface. These two effects have been extensively studied in isolation, but their possible interaction has not been investigated until recently. We shall discuss some idealized geometries in which the two effects are seen to form limiting cases of a more general stability and existence behavior. In most cases, frictional heating has a destabilizing effect relative to the static contact problem, but if the thermal contact resistance is very sensitive to pressure, cases of stabilization can be obtained. Also, the critical sliding speed depends on the contact pressure in contrast to results obtained in the absence of thermal contact resistance.

Commentary by Dr. Valentin Fuster
2004;():127-132. doi:10.1115/TRIB2004-64368.

It is well known that the conventional Coulomb friction condition can lead to non-uniqueness of solution in elastostatic solutions if the friction coefficient is sufficiently high. Interest in this field has centered on discrete formulations, particularly with reference to the finite element method. More recently Hild has demonstrated the existence of a multiplicity of non-unique solutions to a simple problem in two-dimensional continuum elasticity and showed how to determine the conditions for such states to exist by formulating an eigenvalue problem. Both the discrete and continuum examples of non-uniqueness seem to be related to the well known physical phenomenon whereby a frictional system can become locked or ‘wedged’ in a state of stress even when no external loads are applied (the homogeneous problem), but the equivalence is not complete because of the influence of unilateral inequalities in the physical problem. We shall discuss the relations between these concepts in the context of simple continuum and discrete problems in two-dimensional linear elasticity.

Commentary by Dr. Valentin Fuster
2004;():133-139. doi:10.1115/TRIB2004-64370.

Investigation of contact and friction at multiple length scales is necessary for the design of surfaces in sliding microelectromechanical system (MEMS). A method is developed to investigate the geometry of asperities at different length scales. Analysis of density, height, and curvature of asperities on atomic force microscopy (AFM) images of actual silicon MEMS surfaces show these properties have a power law relationship with the sampling size used to define an asperity. This behavior and its similarity to results for fractal Weierstrass-Mandelbrot (W-M) function approximations indicate that a multiscale model is required to properly describe the surfaces.

Commentary by Dr. Valentin Fuster
2004;():141-146. doi:10.1115/TRIB2004-64374.

An experimental investigation was conducted to study the nature of dynamic rupture caused by shear loading, through impact, of interfaces which are held together by friction under external pressure. The dynamic stress field developed during rupture propagation was recorded in real time by a high-speed camera in conjunction with a classical dynamic photoelasticity set-up. Visual evidences of different dynamic propagating rupture modes were recorded. Unlike classical shear cracks in coherent interfaces of finite strength, rupture in frictional interfaces seems to grow without noticeable acceleration phases and at various discreet speeds. At low impact velocities a crack-like rupture mode occurs which becomes super-shear with the increase of the impact speed and a shear Mach cone emanates from the rupture point. For higher impact speeds super-shear and super-sonic Mach lines are observed formatting an unstable slip pulse which subsequently vanishes whereas evidences for extended stick and slip regions are recorded. Increasing the external applied pressure the observed features are intensified.

Commentary by Dr. Valentin Fuster
2004;():147-152. doi:10.1115/TRIB2004-64375.

Important end effects occur in Hertz-like finite length line contacts. If the length of shorter contacting cylinder is bounded by flat surfaces, the pressure tends to infinity at both ends. Many design measures were advanced to reduce or attenuate these pressure riser effects. These imply modification of contact geometry and, in most cases, numerical investigations. Few experiments were performed to check the actual contact between modified surfaces. Applying a previous proposal, contact area between a modified steel roller and a sapphire window is measured by scanning the reflectivity of metallic surface. A typical “dog bone” shape for this area is found. Lateral extensions of contact area, measured experimentally for a roller with rounded edges, agree well with numerical results obtained by a new, refined numerical procedure.

Commentary by Dr. Valentin Fuster
2004;():153-160. doi:10.1115/TRIB2004-64377.

This paper investigates the dynamic contact between an axi-symmetric rigid paraboloid and a standard solid (Zener) viscoelastic half-space. The load consists of a pulse cosine applied over a step function. Following the theory developed by Ting, analytical expression of contact radius for an advancing contact is derived. For receding contact, contact radius is numerically obtained. Contact pressures for increasing and decreasing contacts are derived. The influence of load frequency is evidenced. At low frequencies, contact pressure has an initial Hertz shape and then, as the contact increases, a relaxed one. As the contact reaches its maximum, the pressure tends again to an elastic pattern. Experimental tests were carried out on a contact between a polymeric sphere and a glass plate. A good agreement is found with theoretical results by measuring contact radius.

Commentary by Dr. Valentin Fuster
2004;():161-168. doi:10.1115/TRIB2004-64381.

The micro-slip developed in contact area points is involved in rolling friction. It occurs in some points whilst in others the surfaces move together. Contact area is divided into micro-slip zones, subjected to opposite shear stresses. These are computed in each point and their values yield the power lost by interface friction and related micro-slip torque. Theoretical results are checked by comparison with published data. The specific power loss, defined as the power lost by friction on a unity of contact area, does not depend on direction of shear and its integral over contact area yields the lost power due to micro-slip. The micro-slip component of rolling friction coefficient is calculated by dividing power losses by the product between normal load and relative angular velocity. Global rolling friction coefficient includes as well hysteresis losses. Experimental evidences confirm well the advanced model.

Commentary by Dr. Valentin Fuster
2004;():169-176. doi:10.1115/TRIB2004-64383.

In laser display applications, the necessity to create images free of distortions imposes specific requirements on the motion of scanning devices. We present an approach of a scanning micromirror operation that is aimed to fulfill the requirements of motion linearity, high operational frequency and low actuation voltages. The operational mode incorporates a contact event between the mirror and an elastic constraint followed by a bouncing event and a subsequent inversion of motion. A stable limit cycle with voltage-dependent frequency and triangular response signal is obtained by the application of an actuation voltage which is piecewise constant in time. Approximate expressions relating the frequency and amplitude of the response with the actuating voltage are obtained by energy balance method. The influence of contact losses on the response as well as the stability of the limit cycle are studied numerically.

Topics: Lasers
Commentary by Dr. Valentin Fuster
2004;():177-181. doi:10.1115/TRIB2004-64385.

The transient modal analysis method (TMA) has been used to solve the inhomogeneous (loaded) transient thermoelastic contact problem (ITTEC). In the TMA method, the solution of the inhomogeneous transient problem is expressed in modal coordinates, corresponding to eigenfunctions of the homogeneous (unloaded) problem. However, for the large-scale ITTEC problem, this method is found to be extremely time-consuming, because of the computation-intensive of the eigen-solutions. This paper describes a new approach to solve the large-scale ITTEC problem with a dramatic reduction in computational complexity. The method is referred to as fast speed expansion method (FSE). With the FSE method, full eigen-solutions are performed only at a limited number of sparsely located speeds. For speeds between these speeds, eigenvectors are solved by linear interpolation, while the eigenvalues are computed from Taylor series. The method is illustrated with application to an automotive clutches.

Commentary by Dr. Valentin Fuster
2004;():183-190. doi:10.1115/TRIB2004-64387.

The process of unloading an elastic-plastic loaded sphere in contact with a rigid flat is studied by the Finite Element Method. The sphere material is assumed isotropic with elastic-linear hardening. The numerical simulations cover a wide range of loading interference deformation of various values of Young’s modulus and Poisson ratios of the sphere material. The contact loads, stresses, and deformations in the sphere during both loading and unloading, are calculated for the range of interferences. Empirical dimensionless expressions are presented for the unloading load-deformation relation, the residual axial displacement and the residual curvature of the sphere after complete unloading.

Commentary by Dr. Valentin Fuster
2004;():191-196. doi:10.1115/TRIB2004-64389.

In order to understand the mechanisms of micro-slip initiation and propagation at the contact interface, a visualization method of partial micro-slip at the contact interface was developed. The process of micro-slip initiation and propagation at the contact interface was successfully observed, and the relationship between tangential coefficient and partial slip distribution at the contact interface was analyzed by using the newly developed method. It was clarified that the partial slip distribution was affected by contact morphologies such as spherical ball, triangular stamp, rectangular stamp and concave stamp contact. The effect of uneven area of the contact surface on the micro-slip initiation and propagation was also clarified. It is confirmed from observed results that the visualization of the partial slip at the contact interface is useful for understanding the mechanism of the micro-slip initiation and the propagation at the contact interface.

Commentary by Dr. Valentin Fuster

Tribomaterials

2004;():197-206. doi:10.1115/TRIB2004-64011.

This study is focused on characterization of microstructural changes linked to deformation and crack formation mechanisms in duplex Ti-6Al-4V specimens used in displacement controlled fretting only experiments. In particular, the effect of slip displacement amplitude and number of fretting cycles on the evolution of grain morphology, grain orientation, misorientation distribution, composition, and microhardness is investigated using electron backscatter diffraction (EBSD), energy dispersive X-ray analysis (EDX) and nanoindentation. A strong basal microtexture and significant oxygen diffusion were observed in the Ti-6Al-4V specimen that exhibited the most significant cracking. A critical slip amplitude threshold may exist for which a combination of mechanisms such as plastic deformation, grain reorientation, and oxygen diffusion occur during fretting that make conditions ideal for crack formation. The results provide insights for development and validation of computational crystal plasticity models with application to fretting and sliding contact problems. New fretting damage assessment measures have also been identified and have application for components that suffer from fretting wear and/or fatigue related failures.

Commentary by Dr. Valentin Fuster
2004;():207-217. doi:10.1115/TRIB2004-64028.

The friction and wear properties of PTFE and POM were investigated using a ball-on-steel ring tester under dry conditions and conditions lubricated by paraffin and 10W-30 oils. SEM, EDAX, FT-IR and surface wettability techniques were used to characterize and assess the morphology and chemical composition of the original surfaces as well as wear track, transfer film and wear debris for different loads and speeds. Although the friction was high, similar behaviours to those reported were observed. The experimentally determined surface temperature of the plastic revealed optimum loading levels for each sliding speed at which the friction and wear rates become minimal and the thermal effect stabilizes. In boundary-like lubrication using both oils, friction and wear were significantly reduced with the exception of an increase in wear rate with load when POM was lubricated with 10W-30 oil. Surface analysis revealed that the formation of lubricious protective layers on the surfaces in contact is crucial to reducing friction and more importantly wear. FT-IR results confirmed that film transfer occurs in the case of POM.

Topics: Friction , Wear , Lubricants
Commentary by Dr. Valentin Fuster
2004;():249-257. doi:10.1115/TRIB2004-64156.

Micro sleeve bearings intended for micro rotational machinery were fabricated by X-ray lithography and Ni electroplating. Coated to the working surfaces of the bearings was a 900nm thick uniform tungsten hydrocarbon (W-C:H) coating using an inductively coupled plasma (ICP) assisted, hybrid chemical vapor deposition (CVD)/physical vapor deposition (PVD) tool. Tribological characteristics and mechanical properties of as-electrodeposited Ni micro bearings, annealed Ni micro bearings at 800°C, and W-C:H coated micro bearings were investigated. Potential applications of the micro bearings may involve very light contact pressure (5∼30MPa) and high sliding speed, such as micro motors and micro turbines. Conventional pin-on disc test methods on top flat surfaces, (001) planes, cannot effectively predict tribological characteristics because these micro bearings use the sidewall (110 plane) as a working surface. A special micro wear tester and friction tester were developed. Surface morphologies of new and worn bearing surfaces were studied using SEM. Raman Spectroscopy and X-ray Photoelectron Spectroscopy (XPS) characterized the W-C:H coated micro bearings. Test results of the W-C:H coated micro bearings (wear characteristics and friction) are also presented. W-C:H coated micro bearings had much lower wear rate than uncoated bearings. During the wear test, a transfer layer formed on the counter steel shaft even under very small contact pressure, leading to low steady state friction and high wear resistance.

Commentary by Dr. Valentin Fuster

Lubricants and Additives

2004;():373-377. doi:10.1115/TRIB2004-64115.

The evaporation loss of lubricating oils is an issue of particular importance in lubrication. It is known that the volatility of oil has a significant effect on oil consumption. Portions of oil can evaporate under high temperature conditions, potentially altering oil properties such as viscosity. In this paper, the volatility of the thin oil films was studied. Different kinds of oils were used for the test. The work was to investigate the effect of the different working conditions on the volatility of lubricating oil. Conditions such as the oil film thickness, contacting surface area and the surface material were studied. Encouraging results were obtained with respect to the effect of working conditions on the volatility of thin oil films. This emphasizes that the working conditions play a significant role in the evaporation rate and an appropriate set hand to be considered to minimize the loss.

Commentary by Dr. Valentin Fuster
2004;():387-394. doi:10.1115/TRIB2004-64136.

Different compositions of bronze materials are used for sliding bearings in various applications including those involving oscillatory motion and moderate temperatures. The compatibility of new environmentally adapted synthetic esters with different contacting surface materials is important to ensure good tribological performance. The present work deals with tribological studies on oscillatory journal bearings manufactured with three different compositions of bronze materials lubricated with EALs under boundary lubricated conditions. Under certain operating conditions, dissolution of alloying constituents from the bronze bearing material dissolute into the synthetic ester lubricant occur leading to the formation of a copper enriched bearing surface layer. Tin-bronze with synthetic ester lubricant showed more stable friction values and roughly ten times better wear results than the harder aluminium- and manganese-bronze and also outperformed the mineral oil under the same conditions.

Commentary by Dr. Valentin Fuster

Elastohydrodynamic Lubrication

2004;():439-445. doi:10.1115/TRIB2004-64017.

The Eyring sinh law is presently the most well accepted model for shear-thinning of EHD lubricants at high pressure. It was, however, not accepted for this purpose by Eyring who found it to be of only limited usefulness for thixotropy. Then, it is extremely important that these models receive a critical review using data obtained by modern methods to evaluate the actual Eyring models and show how they differ from the model in use today. Data from high-pressure viscometers were used to validate the actual Eyring models. The Ree-Eyring model for shear-thinning obeys time-temperature-pressure superposition. A film thickness calculation shows it to suffer from the same anomalous behavior in sliding contact as does the sinh law.

Commentary by Dr. Valentin Fuster
2004;():447-457. doi:10.1115/TRIB2004-64044.

The paper presents results obtained using a transient analysis technique for point contact elastohydrodynamic lubrication (EHL) problems based on a formulation that couples the elastic and hydrodynamic equations. Results are presented for transverse ground surfaces in elliptical point contact that show severe film thinning and asperity contact at the transverse limits of the contact area. This thinning is caused by transverse leakage of the lubricant from the contact in the remaining deep valley features between the surfaces. A comparison is also made between the point contact results on the entrainment centre line and the equivalent line contact analysis. The extent of asperity contact is shown to be dependent on the Hertzian contact aspect ratio. It is also shown that transverse waviness (superimposed on the roughness) of even relatively small amplitude can lead to large increases in asperity contact rates over all waviness peaks in the contact.

Commentary by Dr. Valentin Fuster
2004;():459-468. doi:10.1115/TRIB2004-64073.

This paper describes an experimental study on dimple formations under elastohydrodynamic lubrication (EHL) conditions. The oil film thickness between a ball surface and a sapphire disk was measured using optical interferometry and the temperatures of both the surfaces and of the oil film averaged across it were measured by an improved infrared emission technique. It was found that temperature profile across the oil film varies abruptly along the direction of the oil film thickness and the Couette flow decreases due to the viscosity wedge action as the oil flow is close to the dimple zone. The maximum temperature rise in the dimple zone sometimes reached above 400 K and thus the phase transition of the oil from liquid to glass may not occur.

Commentary by Dr. Valentin Fuster
2004;():479-493. doi:10.1115/TRIB2004-64250.

The paper presents the results of a thermal analysis of a set of disk experiments carried out by Patching et al. to investigate scuffing. The experiments used crowned steel disks at 76 mm centres with maximum Hertzian contact pressures of up to 1.7 GPa. Experimental measurements of contact friction were used as the basis for a thermal analysis of the disks and their associated support shafts. Temperatures measured by embedded thermocouples 3 mm below the running tracks of the disks were used to determine the heat partition between the faster and slower running disks in order to match the experimental with calculated temperatures. This partition was found to vary approximately as a function of the product of sliding speed and surface temperature difference. A transient (flash) temperature analysis of one of the experiments was also carried out. This shows large differences between the disk transient surface temperatures. These surface temperature distributions were compared with those obtained from corresponding elastohydrodynamic lubrication (EHL) analyses using two different non-Newtonian lubricant formulations. The EHL analyses show that the heat partition obtained depends on the form of non-Newtonian behaviour assumed, and that to achieve the same partition as is evident in the experiment a limiting shear stress formulation is necessary. It is suggested that the combination of heat transfer and EHL analysis presented in the paper could be used as a sensitive tool for distinguishing between different non-Newtonian lubricant models under realistic engineering loads and with high sliding speeds.

Topics: Heat , Interior walls
Commentary by Dr. Valentin Fuster
2004;():495-501. doi:10.1115/TRIB2004-64289.

Liquid lubricants in elastohydrodynamic lubrication (EHL) are subjected to high pressures of the magnitude of 109 N/m2 (GPa). Under these high pressures, the lubricants in room temperatures are usually in the glassy state behaving as amorphous solids. Similar to the polymers, the lubricants exhibit elastic or visco-elastic behavior under low-shear-stress loading and plastic or visco-plastic behavior with high shear stresses. Recent laboratory experiments have revealed two types of localized shear responses that can take place in the lubricant bulk. The localized shear may be active in the EHL film and may play a significant role in the responses exhibited by the contact conjunction. A typical example is the EHL traction. This paper summarizes the up-to-date research related to the shear localizations.

Commentary by Dr. Valentin Fuster

Hydrodynamic Lubrication and Fluid Film Bearings

2004;():503-515. doi:10.1115/TRIB2004-64026.

The order of magnitude of the Reynolds number in thin lubricant films varies between 102 and 105 . For Reynolds numbers higher than 103 , the fluid flow becomes turbulent. It is well accepted in lubrication to use a zero equation turbulence model of the type developed by Constantinescu [1] or Elrod, Ng and Pan [2–4] or Hirs [5]. The Elrod and Ng approach is certainly the most efficient for combined pressure and shear flows where the Reynolds number is above 104 . This paper proposes a modification of the Elrod and Ng model in order to ensure a good correlation with experimental data obtained with low Reynolds number turbulent flows. The present model, coupled with a scaling factor for taking into account the transition to turbulence, is therefore accurate for all the typical Reynolds number values recorded in lubrication. The model is then applied to hydrostatic noncontacting face seals, which usually operate at Reynolds numbers varying from 103 to 104 . The accuracy of the model is shown for this particular application of radial rotating flow. A special study is made of the transition to turbulence. The results are compared with those obtained using the initial Elrod and Ng model. The axial stiffness coefficient and the stability threshold are significantly affected by the turbulence model.

Commentary by Dr. Valentin Fuster
2004;():517-527. doi:10.1115/TRIB2004-64027.

The present work deals with the Navier-Stokes and bulk-flow analysis of hybrid bearings intended for use in aerospace applications. These bearings are expected to work at high rotational speeds and high feeding pressures. In such a case, the pressure in the shallow pockets of the bearing is no longer constant and is influenced by hydrostatic and hydrodynamic effects. It has been shown in the literature that the recess pressure pattern can have an important influence on the dynamic characteristics of the bearing. The present work investigates the pressure field in the recess of centered hybrid bearings with radial and angled injection by using a numerical Navier Stokes analysis. The recess pressure pattern is then subsequently characterized by combining these results with some parametric descriptions. For calculating the dynamic characteristics of the bearing, the parametric pressure pattern is then injected into a bulk-flow model that is an alternative analysis of the one introduced by San Andrés [3, 4]. In order to evaluate the validity of the bulk-flow code, the numerical predictions are compared with experimental data taken from the literature for radial and angled injection. The favorable effect of the counter-rotating angled injection is then explained by using the velocity field issued from the Navier Stokes analysis and the pressure field given by the bulk-flow model.

Commentary by Dr. Valentin Fuster
2004;():529-537. doi:10.1115/TRIB2004-64042.

Experimental dynamic-stiffness-coefficient results are presented for a high-speed, lightly loaded, load-on-pad, flexible-pivot tilting-pad bearing. Results show that the real part of the direct dynamic-stiffness coefficients are quadratic functions of the excitation frequency. This frequency dependency is modeled well by an added-mass coefficient, and the resultant [M], [K], and [C] matrix model is frequency-independent versus a conventional [K] and [C] model that is frequency dependent. The dynamics introduced by the additional pad degrees of freedom (including pad inertia and web moment stiffness) and the effects of fluid inertia in the lubricant film account for part of this frequency dependency. Experimental results are compared to numerical predictions from models based on: (i) the Reynolds equation, and (ii) a Navier-Stokes (NS) equations bulk-flow model that retains the temporal and convective fluid inertia terms. The NS bulk-flow model results correlate better with experimental dynamic stiffness results, including added-mass terms. Both models underestimate the measured added-mass coefficients for the full excitation range; however, they do an adequate job for excitation frequencies up to synchronous frequency. The frequency dependency predicted by using a [K] and [C] model can be removed by adding a mass matrix to the reaction-force model with either a Reynolds equation or a bulk-flow NS model, with a very considerable speed up in calculation of damped eigenvalues for rotor-bearing systems.

Commentary by Dr. Valentin Fuster
2004;():539-550. doi:10.1115/TRIB2004-64064.

Direct lubrication tilting pad journal bearings (DLTPJ bearings) have rarely been applied to large scale rotating machinery, such as turbines or generators whose journal diameters are more than 500mm. In this paper, static and dynamic characteristics of a 580mm (22.8in.) diameter DLTPJ bearing were studied experimentally using a full scale bearing test rig. In the static test, distribution of metal temperature, oil film pressure and bearing loss were measured in changing oil flow rate, with mean bearing pressure ranging up to 2.9MPa. Maximum metal temperature of DLTPJ bearing was compared to that of conventional flood lubrication bearing, and it was confirmed that the direct lubrication can increase load capacity. In the dynamic test, spring and damping coefficients of oil film were obtained by exciting the bearing casing that was floated by air bellows. These data will be used for analysis and design of steam turbine rotors and their bearing systems. Also, vibration of pads was investigated, because metal failure on upper pads due to vibration has been found in some actual machines. In order to generate oil film pressure on the surface of upper pads, Rayleigh-Step was machined there. And it was confirmed that vibration was reduced by Rayleigh-step.

Commentary by Dr. Valentin Fuster
2004;():551-557. doi:10.1115/TRIB2004-64123.

The time-dependent thermal compressible elastohydrodynamic (EHD) lubrication of sliding line contact has been developed to investigate the effect of a sudden load change. The time-dependent modified Reynolds equation with non-Newtonian fluids has been formulated using power law’s model. In this study, the non-Newtonian dilatant fluids for liquids-solid lubricants have been purposed experimentally using the common solid particles namely, Molybdenum disulfide (MoS2 ) and Polytetrafluoroethylene (PTFE). The simultaneous systems of modified Reynolds and elasticity and energy equations with initial conditions were solved numerically using multigrid multilevel technique. The performance characteristics of the thermoelastohydrodynamic under line contact were presented with varying time for the pressure distribution, temperature distribution and oil film thickness. The transient response of the line contact between two surfaces was simulated under a heavy step load function. The coefficients of friction were also presented in this work at steady state condition with varying particle concentration. This simulation showed a significant effect of liquid-solid on thermoelastohydrodynamic (TEHD) lubrication under heavy load conditions.

Commentary by Dr. Valentin Fuster
2004;():559-568. doi:10.1115/TRIB2004-64133.

The present work deals with the flow characteristics induced by a two-dimensional roughness pattern. The roughness pattern consists of identical and equally spaced rectangles with characteristic lengths at least one order of magnitude larger than the clearance of the thin film. Periodic boundary conditions enable the analysis of a single groove and the complete Navier-Stokes analysis is carried on for turbulent Reynolds numbers. The analysis is performed for shear driven flows (Couette), pressure driven flows (Poiseuille) and combined Couette-Poiseuille flows. First, the presence of inertial forces generated by the groove is underlined by the momentum balance performed for the computational cell. The peculiar effect of the groove is also depicted by the rotor and the stator shear stresses variations. Finally, it is shown that despite the presence of inertial forces, cell-averaged rotor and stator shear stresses obtained for pure Couette or Poiseuille flows can be added or subtracted for approximately obtaining the characteristics of combined shear and pressure driven flows.

Commentary by Dr. Valentin Fuster
2004;():569-579. doi:10.1115/TRIB2004-64140.

This paper presents a novel finite element elastohydrodynamic lubrication analysis appropriate for gas journal bearings under dynamic conditions. The method employs gas pressure as a state variable, and structural sleeve deformation is represented by a linear combination of pre-selected mode shapes obtained from a related eigenvalue problem. The method takes into account temporal variation of journal position and velocity, and second-order slip flow boundary effects are included at no additional computational cost. The formulation is subsequently applied to a particular example (flexible large-aspect ratio, high-speed, MEMS-scale journal microbearing), where it is shown that a judicious choice of structural sleeve elasticity can significantly improve bearing performance (as measured by pressure distribution and load capacity) when compared with results obtained using rigid bearing surfaces.

Commentary by Dr. Valentin Fuster
2004;():581-587. doi:10.1115/TRIB2004-64144.

This paper describes a numerical procedure for analyzing the dynamics of transient and steady state vibrations in a wavy thrust bearing. The major effects of the wavy geometry and the operating parameters on the dynamic characteristics of the bearing had been discussed in previous work. The present study thus concentrates on examining the relationships between the development of the transient and steady state vibrations when operating conditions including rotational speed and load magnitude are parametrically varied. Special attention is given to the development of steady state vibrations from initial transients with comparisons and consequences to the overall system stability. Numerical based vibration signature analysis procedures are then used to identify and quantify the transient vibrations. The conclusions provide general indicators for designing wavy thrust bearings that are less susceptible to transients induced by external perturbations.

Commentary by Dr. Valentin Fuster
2004;():603-613. doi:10.1115/TRIB2004-64178.

Results of a combined theoretical and experimental investigation into the operation of thrust bearings with PTFE-faced pads are reported. Bearing performance is analysed in terms of temperature, power loss, oil film thickness and pressure. These parameters are first calculated using a THD model. The effect of PTFE facing on bearing thermal performance is then presented and discussed. A TEHD model is subsequently employed. Obtained TEHD results show that oil film thickness and temperature are strongly affected by the PTFE layer. Theoretical results are compared with measured temperature, oil film thickness and pressure.

Topics: Thrust bearings
Commentary by Dr. Valentin Fuster
2004;():731-736. doi:10.1115/TRIB2004-64321.

Homogenization is a formal mathematical two-scale averaging process that can be applied to roughness problems and can replace previous heuristic averaging procedures which have sometimes led to ambiguous results. This procedure was previously mathematically developed and applied to compressible flow problems. The purpose of this paper is the development of a special form of Reynolds equation for such homogenized conditions applied to the incompressible Newtonian case. The equation allows the calculation of the operating characteristics of a contact by taking into account the local geometry of surfaces, while making a substantial improvement in computing time. The method allows for the study of rough surfaces, but requires considerably fewer calculated points than for traditional deterministic discretization methods.

Commentary by Dr. Valentin Fuster
2004;():737-747. doi:10.1115/TRIB2004-64388.

A commercial CFD code has been applied to model lubricant flow behaviour within 2D and 3D linear pad bearings having closed pockets or recesses. The study indicates that the presence of closed pockets can result in a significant reduction in bearing friction coefficient. At high convergence ratios, no cavitation is predicted within the pockets. This means that suitably-positioned pockets in the high-pressure region of the bearing result in a much greater reduction in local shear stress than in local pressure, so that there is an overall reduction in friction coefficient. At low convergence ratios, cavitation occurs in the inlet to the pockets so that each pocket acts an effectively-independent step bearing. This results in the overall bearing supporting a higher load and thus having lower friction coefficient than is the case without pockets.

Topics: Friction , Bearings
Commentary by Dr. Valentin Fuster

Rolling Element Bearings

2004;():749-755. doi:10.1115/TRIB2004-64012.

This paper deals with a design of crowning to reduce the ball passage vibrations of a linear guideway type recirculating linear ball bearing (linear bearing) with the crowning length LC (1.2sLC ≤ 1.7s, where s is the distance between the adjacent balls in the load zone). First, a fundamental condition to reduce the ball passage vibrations is provided. Second, a design method of the crowning by applying an approximate curve (an assumed functional formula) to the control points (satisfying the fundamental condition or being the intersection of straight and crowning areas of a raceway) is presented. Third, the design procedure is explained by giving an example. In the design example the power function crowning fits the control point best. In addition, the calculated amplitude for the power function crowning is lower than those for the circular arc crowning and the crowning of the commercial linear bearing. Finally, the validity of the presented design method is confirmed experimentally.

Commentary by Dr. Valentin Fuster
2004;():757-764. doi:10.1115/TRIB2004-64031.

The effect of raceway curvature on ball bearing contact stress and fatigue life is analyzed with both Hertzian theory and FEM. A numerical program and a 3-D FEM model are developed to calculate the contact stress and deformation at the bearing ball and raceway. The simulation results of the contact stress and deformation are discussed. The accuracy is evaluated by contrasting finite element results with analytical solutions from Hertzian theory. The effect of bearing race curvature on the contact maximum stress and area between ball and races is discussed. The results show that the race curvature is very sensitive factor to affect the bearing contact stress. The raceway curvature effect on the bearing thrust load capability and bearing running temperature is analyzed and discussed as well. For the validation, A. B. Jones’ program has been used to calculate bearing life with different raceway curvatures.

Topics: Design , Ball bearings
Commentary by Dr. Valentin Fuster
2004;():771-779. doi:10.1115/TRIB2004-64142.

More often then not, the rolling element bearings of rotating machinery are the mechanical components that are first prone to premature failure. Early warning of an impending bearing failure is vital to the safety and reliability of high-speed turbo-machinery. Presently, vibration monitoring is one of the most applied procedures in on-line damage and failure monitoring of rolling element bearings. This paper presents results from an experimental rotor-bearing test rig where quantified damage was induced in the supporting tapered ball bearings. Subsequently the vibration signature due to damage at the inner race of the bearing is examined. Four on-line vibration signature analyzing schemes are used concomitantly: (i) time averaging, (ii) frequency domain analysis, (iii) joint time-frequency analysis (Wigner-Ville and wavelet transforms) and (iv) chaotic vibration analysis (modified Poincare diagrams). The size/level of the damage is corroborated with the vibration amplitude and the resulting relationships are linearized to provide quantification criteria for bearing progressive failure prediction. The results from the above mentioned methodologies are compared for accuracy and redundancy, thus increasing the reliability for early detection of bearing damage and failure. It is shown that the use of the modified Poincare map can provide an effective way for identification and quantification of bearing damage in rolling element bearings.

Commentary by Dr. Valentin Fuster
2004;():781-793. doi:10.1115/TRIB2004-64246.

Three decades have passed since the introduction of silicon nitride rollers and balls into conventional rolling-element bearings. For a given applied load, the contact (Hertz) stress in a hybrid bearing will be higher than an all-steel rolling-element bearing. The silicon nitride rolling-element life as well as the lives of the steel races were used to determine the resultant bearing life of both hybrid and all-steel bearings. Life factors were determined and reported for hybrid bearings. Under nominal operating speeds, the resultant calculated lives for deep-groove, angular-contact, and cylindrical-roller hybrid bearings are respectively, 3.8, 3.3, and 5.5 times that using the Lundberg-Palmgren equations. An all-steel bearing under the same load and nominal operating speeds will always have higher life than the equivalent hybrid bearing operating under the same conditions. Under these conditions, hybrid bearings are predicted to have a lower fatigue life than the all-steel bearings by 58 percent for deep-groove bearings, 41 percent for angular-contact bearings and 28 percent for cylindrical roller bearings.

Commentary by Dr. Valentin Fuster
2004;():795-806. doi:10.1115/TRIB2004-64275.

This study is devoted to evaluate the performance of an automatic ball-type balance system (ABB) installed in optical disk drives (ODD) with consideration of the rolling friction between the balancing balls and the ball-containing race of the ABB. Researches have been conducted to study the performance of the ABB by investigating the nonlinear dynamics of the system; however, the rolling friction model adopted was a simple stick-slip type, which does not reflect the true contact dynamics between rolling balls and their races, leading to an inaccuracy in predicting ABB performance. In this study, a complete dynamic model of the ABB including a detailed rolling friction model based on contact mechanics is established. The method of multiple scales is then applied to formulate a scaled model to find all possible steady-state ball positions and analyze stabilities. It is found that possible steady-state residing positions of the ball inside the race are multiple and form continuous ranges. Numerical simulations and experiments are conducted to verify the validness of the theoretical findings. The obtained results are used to predict the level of residual vibration, with which the guidelines on dimension design and material choices of the ABB are distilled to achieve desired performance.

Commentary by Dr. Valentin Fuster

Engine Tribology

2004;():831-840. doi:10.1115/TRIB2004-64058.

A probabilistic analysis is presented for studying the variation effects on the main bearing performance of an I.C. engine system, under structural dynamic conditions. For computational efficiency, the probabilistic analysis is based on surrogate models (metamodels), which are developed using the kriging method. An Optimum Symmetric Latin Hypercube (OSLH) algorithm is used for efficient “space-filling” sampling of the design space. The metamodels provide an efficient and accurate substitute to the actual engine bearing simulation models. The bearing performance is based on a comprehensive engine system dynamic analysis which couples the flexible crankshaft and block dynamics with a detailed main bearing elastohydrodynamic analysis. The clearance of all main bearings and the oil viscosity comprise the random variables in the probabilistic analysis. The maximum oil pressure and the percentage of time within each cycle that a bearing operates with oil film thickness below a threshold value of 0.27 μm at each main bearing constitute the system performance measures. Probabilistic analyses are first performed to calculate the mean, standard deviation and probability density function of the bearing performance measures. Subsequently, a probabilistic sensitivity analysis is described for identifying the important random variables. Finally, a Reliability-Based Design Optimization (RBDO) study is conducted for optimizing the main bearing performance under uncertainty. Results from a V6 engine are presented.

Commentary by Dr. Valentin Fuster
2004;():849-857. doi:10.1115/TRIB2004-64100.

The wear between the plunger and plunger sleeve of rotary diesel fuel injection pump causes considerable decrease in injection pressure and the quantity of fuel to combustion chamber of an engine, which ultimately leads to failure of engine assembly. This research investigates the cause of failure particularly focusing on surface roughness effects to hydrodynamic lubrication and scuffing failure due to abrasive contaminant. The surface roughness of plunger and plunger sleeve were measured and incorporated in Reynolds equation to analyze roughness effects on hydrodynamic lubrication. The critical particle size of the dust normally present in the diesel fuel is evaluated to determine which test dust sample could cause systems to fail. Based on this information, scuffing failure of pumps due to an abrasive contaminant partially penetrated in the plunger sleeve is analyzed. The abrasive contaminant is modeled as a spherical shaped rigid particle. Excessive temperature rise between the particle-plunger interface is used as an indication of whether scuffing would take place. Experiments were conducted to determine parameters such as particle size of dust samples, surface roughness of plunger and plunger sleeve, specific heat of diesel fuel, diesel fuel density, quantity of fuel flow and radial clearance. These experimentally determined parameters are then used as input in our computer program to lend more confidence to our predicted results.

Commentary by Dr. Valentin Fuster
2004;():859-867. doi:10.1115/TRIB2004-64101.

The presence of Elasto-hydrodynamic Lubrication (EHL) film between opposing piston and liner surfaces prevents possible solid-to-solid contact and wear. This enhances engine life manifold as compared to when the EHL film is non-existent in the initial engine startup or breaks down during normal engine operation. Forced dry sliding of piston during engine cranking followed by partial lubrication in the initial engine startup leads to adhesive wear. This research investigates the possibility of an EHL film on such an occasion by considering elastic deformation of opposing piston skirt and liner surfaces due to Elasto-hydrodynamic (EHD) pressures. The geometry of piston skirts is defined and governing equations are applied to determine hydrodynamic pressures. The EHL film thickness profile generated by inverse solution technique and its expression is defined by incorporating contact geometry and EHD pressures in the piezoviscous regime. A computer code is developed and used to simulate the performance parameters and their behavior during initial engine startup. Due to critical factors such as engine speed, redial clearance between piston skirts and liner and lubricant viscosity, a time dependent 2-D EHL film profile is generated. The simulated results indicate that, despite piston eccentricities due to secondary oscillatory motion, EHL film established between the opposing piston skirts and liner surfaces prevented possible solid-to-solid contact in the entire duration of 720-degree crankshaft rotation, which corresponds to four piston strokes.

Commentary by Dr. Valentin Fuster
2004;():869-892. doi:10.1115/TRIB2004-64191.

Frictional losses in an IC engine are observed between 17–19% of total induced horsepower. 35–45% frictional losses observed due to piston ring assembly only from the above-referred total frictional loss. Lubrication system is for reducing the frictional losses and under the total hydrodynamic lubrication system, if made it feasible, above referred losses can be reduced considerably. Wear normally observed at TDC and BDC during the power stroke. Experimental set-up is prepared by using used piston-cylinder assembly of an engine. Experiment methodology is adopted based on certain assumption and simulated the entire system with an extra drive system by an electric motor with a provision of various speed availability. Various pockets on cylinder liner of 2mm diameter are located on the periphery of cylinder liner to offer lubrication to the system. Care was taken to control the rate of lubrication flow with the help of control knob. Seven different profiles on piston ring were generated and measured. Friction force is calculated by power consumption measurement under different dynamic condition with a variation of 5-speed, 3- lubricants and different 8- types of piston ring geometry are experimented under different combination and results are tabulated. Graphs are plotted for friction force v/s speed for different lubricants and piston ring profiles. Effect of lubricants (SAE30, 15W40& 2T) and ring geometry are compared.

Commentary by Dr. Valentin Fuster
2004;():893-906. doi:10.1115/TRIB2004-64198.

Piston assembly friction measurement has been carried out on a single cylinder gasoline engine using the IMEP (indicated mean effective pressure) method at realistic engine speeds and loads without any major engine modifications. Instantaneous and mean piston assembly friction were measured under motored and fired conditions at different lubricant temperatures. The forces acting on the piston assembly were carefully determinated by measuring the cylinder pressure, crankshaft angular velocity and strain in the connecting rod. The difference between the resulting gas pressure, inertia and connecting rod axial forces acting on the piston yields the piston assembly friction. To achieve this with confidence, an advanced instrumentation, telemetry and data acquisition system was designed and developed, giving special attention to the synchronisation and simultaneous sampling of analogue and digital channels. Experiments are reported for piston assembly friction at a range of engine operating conditions with different lubricant formulations, with and without a friction modifier.

Commentary by Dr. Valentin Fuster
2004;():907-921. doi:10.1115/TRIB2004-64199.

A piston assembly friction model has been developed to predict the individual performance of compression rings, the oil control ring and the piston skirt. Validation of this model has been undertaken by comparing the predicted results with the experimental measurements of piston assembly friction in a gasoline engine under fired conditions using the IMEP (indicated mean effective pressure) method. The experimental results for an SAE 0W20 without friction modifier were compared with the predictions. The predicted results correlate very well with the measurements, especially at higher lubricant inlet temperatures. Piston skirt friction was predicted using both a simple concentric / cylinder model and a more realistic but computationally intensive method incorporating piston secondary motion. The results clearly indicate than the latter more realistic method is required to achieve satisfactory correlation with the measured data.

Commentary by Dr. Valentin Fuster
2004;():941-949. doi:10.1115/TRIB2004-64319.

It is well known that the crankshaft bearing loading conditions are of complex nature. Also at high loads, elastic and thermal distortion of the bearing solids occurs, moreover, under these harsh conditions, misalignment can occur during operation. Due to the complexity of the problem, few researches have been done to study the dynamic performance characteristics of crankshaft bearing under such conditions. In the present work, it is intended to investigate the effect of bearing parameters on hydrodynamic pressure, film thickness and friction losses in the crankshaft main bearing considering shaft misalignment and bearing distortion. The main conclusion of the research is that, it is essential to consider misalignment as it is greatly deteriorating the film thickness leading to higher frictional losses. It is also found that the degree of misalignment is having little effect on the bearing mean load capacity.

Topics: Lubrication , Bearings
Commentary by Dr. Valentin Fuster

Machine Components Tribology

2004;():1017-1023. doi:10.1115/TRIB2004-64154.

An unsteady numerical model of a mechanical seal, with mixed lubrication, has been developed. The thermal analysis is performed using Duhamel’s method is combination with a numerical experiment to determine Duhamel’s auxiliary function. The results using this semi-empirical approach compare well with those from a finite element analysis. The model has been used to predict the performance of a mechanical seal during startup and shutdown.

Commentary by Dr. Valentin Fuster
2004;():1025-1030. doi:10.1115/TRIB2004-64196.

The vibration caused by friction force is a wet friction system was studied in the present paper. In these systems, unexpected vibration or noise occasionally occurs during the engagement process of the input shaft to the output one. Such a vibration has been explained as self-excited vibration caused by friction force. When friction force increases with decreasing relative sliding speed, this friction characteristic is generally expressed as negative damping. In this study, a testing machine was developed to evaluate the vibration characteristics of wet friction systems with paper-based materials. The acceleration of the input shaft and the separator plate were directly measured in the engagement process together with the variation of transmitted torque. The effects of the driving conditions and design factors of wet friction systems with paper-based friction materials on the vibration and their characteristics were discussed. It is shown that the in-plane and out-of-plane vibration of the separator plate were induced by the torsional vibration of the input shaft during the engagement process of the friction plate and separator plate. While the torsional vibration of the input shaft was observed, the Torque-V slope was estimated to be negative, and its magnitude grew faster as the number of friction plates increased even under the same applied normal load condition.

Topics: Friction , Design , Vibration
Commentary by Dr. Valentin Fuster
2004;():1031-1040. doi:10.1115/TRIB2004-64256.

The relative motion of the friction and separator plates in wet clutches during the disengaged mode causes viscous shear stresses in the fluid passing through the 100 microns gap. This results in a drag torque on both the disks that wastes energy and decreases fuel economy. The objective of the study is to develop an accurate mathematical model for the above problem with verification using FLUENT and experiments. Initially we start with two flat disks. The mathematical model enables us to calculate the drag torque on the disks and 2D axisymmetric solver verifies the solution. The surface pressure distribution on the plates is also verified. Then 3D models of one grooved and one flat disk are tested using CFD, experiments and an approximated 3D mathematical model and three parameters namely number of grooves, depth of groove and clearance between the disks are studied to understand the effect they have on the drag torque. Conclusions are finally drawn and design implementations are suggested to reduce drag.

Commentary by Dr. Valentin Fuster
2004;():1065-1096. doi:10.1115/TRIB2004-64281.

A new theoretical model for estimating the entrained air film thickness between a web and roller is presented for both impermeable and permeable webs. A simple closed-form formula for estimating the air film thickness, which considers the effects of air leakage from the web edges and air diffusion due to the permeability of web, was obtained based on a large number of simultaneous numerical solutions of the compressible Reynolds equation and the web equilibrium equation. The variation of air film thickness with roller velocity is measured for three typical webs: PET (polyethylene terephthalate), coated paper, and newsprint. The effects of web permeability, web width and web tension on the air film thickness are examined theoretically and experimentally for a wide range of roller velocity. Reasonable agreement is seen both quantatively and qualitatively between the predicted and measured results. The validity of the formula for the first order estimation of web-roller interface problems is verified experimentally.

Topics: Permeability , Rollers
Commentary by Dr. Valentin Fuster
2004;():1097-1114. doi:10.1115/TRIB2004-64285.

An experimental procedure is proposed for studying the underhead and thread friction in fasteners. The effective bearing friction radius, the underhead friction coefficient, and the thread friction coefficient are experimentally determined for fasteners with standard hexagonal heads and for flanged head fasteners. Hence, greater accuracy has been achieved in determining the value of the torque components that are consumed in overcoming friction in threaded fasteners. This would lead to a more reliable torque-tension correlation and would enhance the safety and quality of bolted assemblies. A Design of Experiment (DOE) procedure is presented in order to investigate the effect of fastener material class, the thread pitch, and the fastener size on thread friction coefficient. For the underhead bearing friction, an experimental model is presented in order to determine the effect of the contact area radii ratio on the bearing friction radius.

Commentary by Dr. Valentin Fuster
2004;():1115-1125. doi:10.1115/TRIB2004-64299.

The mathematical formulation relates the tribological events at micron-scale and the macroscopic scale vibration response of a two-disk brake system. This is accomplished by a visco-elastic account of interaction at the micron scale, its statistical quantification through the approximate analytical representation of the statistical expectation of contact force and the introduction of the contact force into the macro-scale dynamics of the two-disk system. Steady-state analysis of the system establishes the relation between friction torque and speed and supports observed behavior of many mechanical systems with friction. It is shown that, as a result of coupling of the macro-system’s dynamics and contact, there are combinations of parameters at the micro- and macro-scale that yield negative slope in friction torque/sliding speed relation, a well known source of dynamic instability. This results in an effective negative damping that tends to reduce with decrease in the normal load and/or increase in structural damping of the system.

Topics: Friction , Vibration
Commentary by Dr. Valentin Fuster

Contact Mechanics

2004;():1139-1146. doi:10.1115/TRIB2004-64014.

The effective mechanical properties of a layered surface vary as a function of indentation depth and the values of these properties range between the value of the layer itself and of the substrate. In this paper, a layered surface is modelled like a solid that has effective mechanical properties as a function of indentation depth by assuming that the layer is perfectly bounded to the substrate. The normal load as a function of indentation depth of sphere pressed against a flat layered surface is calculated using this model and is in agreement with the experimental results published by El-Sherbiney (1975), El-Shafei et al. (1983), Tang & Arnell (1999) and Michler & Blank (2001). A deterministic contact model of a rough surface against a flat layered surface is developed by representing a rough surface as an array of spherically shaped asperities with different radii and heights (not necessarily Gaussian distributed). Once the data of radius and height of every single asperity is obtained, one can calculate the number of asperities in contact, the real contact area and the load carried by the asperities as a function of the separation.

Commentary by Dr. Valentin Fuster
2004;():1147-1156. doi:10.1115/TRIB2004-64015.

The contact of rough spheres is of high interest in many tribological, thermal, and electrical fundamental analyses. Implementing the existing models is complex and requires iterative numerical solutions. In this paper a new model is presented and a general pressure distribution is proposed that encompasses the entire range of spherical rough contacts including the Hertzian limit. It is shown that the non-dimensional maximum contact pressure is the key parameter that controls the solution. Compact expressions are proposed for calculating the pressure distribution, radius of the contact area, elastic bulk deformation, and the compliance as functions of the governing non-dimensional parameters. The present model shows the same trends as those of the Greenwood and Tripp model. Correlations proposed for the contact radius and the compliance are compared with experimental data collected by others and good agreement is observed.

Commentary by Dr. Valentin Fuster
2004;():1157-1166. doi:10.1115/TRIB2004-64023.

Numerical investigations are carried out to simulate wear and the analysis of these simulations leads to proposing an original new wear law that takes into account interfacial particles in the contact. A 3D Discrete Element Model is presented that simulates the detachment of particles, their flow in the contact and their ejection. It shows that a layer of detached particles can be formed at the interface, separating the solids in contact. The simulations show how influential the contact geometry and the properties of the interfacial particles are in studying wear. The processes of material degradation and particle ejection are then studied separately. Their physical behaviour is analysed and simple analytical expressions are proposed. Consideration of the mass balance of the contact provides an analytical law for wear, involving the fate of the detached particles. Classical wear laws (such as Archard’s law), assuming that no particle stays in the contact, appear to be a limit case of this model.

Topics: Wear , Simulation
Commentary by Dr. Valentin Fuster
2004;():1167-1177. doi:10.1115/TRIB2004-64040.

The thermal surface distortion of an anisotropic elastic half-plane is studied using the extended version of Stroh’s formalism. In general, the curvature of the surface depends both on the local heat flux into the half-plane and the local temperature variation along the surface. However, if the material is orthotropic, the curvature of the surface depends only on the local heat flux into the half-plane. As a direct application, the two-dimensional thermoelastic contact problem of an indenter sliding against an orthotropic half-plane is considered. Two cases, where the indenter has either a flat or a parabolic profile, are studied in detail. Comparisons with other available results in the literature show that the present method is correct and accurate.

Commentary by Dr. Valentin Fuster
2004;():1179-1208. doi:10.1115/TRIB2004-64092.

As the size of contacting and sliding tribosystems decrease, intermolecular or adhesive forces become significant partly due to nanometer size surface roughness. The presence of adhesion has a major influence on the interfacial contact and friction forces as well as the microtribosystem dynamics and thus influences the overall dynamic friction behavior. In this paper, a dynamic friction model that explicitly includes adhesion, interfacial damping and the system dynamics for realistic rough surfaces was developed. The results show that the amplitude and mean value of the time varying normal contact and friction forces increase in the presence of adhesion under continuous contact conditions. Also, due to the attractive nature of adhesion, its presence delays or eliminates the occurrence of loss of contact. Furthermore, in the presence of significant adhesion, dynamic friction behavior is significantly more complicated compared to the no adhesion case, and the dynamic friction coefficient predictions may be misleading. Thus, it is more appropriate to discuss dynamic friction force instead of dynamic friction coefficient under dynamic conditions.

Commentary by Dr. Valentin Fuster
2004;():1209-1215. doi:10.1115/TRIB2004-64095.

The random Gaussian surface model proposed by Nayak is important to many statistical summit-based micro contact models. A Gaussian distribution is usually assumed for the summit heights as many surfaces have a Gaussian distribution of surface heights. In this work, based on Nayak’s models the skewness and kurtosis of the summit heights distribution are derived as a function of the bandwidth parameter α. The correctness of these two equations verified using a numerical scheme that generates random Gaussian surfaces with various α values. Also, practical contact simulations are performed to demonstrate the significance of the proposed equations and also to show the error of using a Gaussian distribution versus a correct asymmetric distribution for the summit heights.

Commentary by Dr. Valentin Fuster
2004;():1217-1224. doi:10.1115/TRIB2004-64106.

An instability model is proposed to describe the process of separating a sphere from a flat in the presence of a lubricant. The model is based on consideration of the effects of sphere inertia in characterizing the critical point of separation — namely, that there is a sudden onset of significant sphere acceleration. Good agreement is found between the predictions of the instability model with those of a full numerical simulation. The instability model provides an improvement over an “impulse” model found in the literature, which is unable to predict the point of separation.

Commentary by Dr. Valentin Fuster
2004;():1225-1235. doi:10.1115/TRIB2004-64120.

The apparent contact area of curved rough surfaces can be larger than that predicted by the Hertz theory due to asperity interaction outside the Hertzian region. In the present study, simple theoretical formulas for the contact semi-width and radius were derived, and a numerical contact model was developed based on an iterative scheme for the elastic deformation of the macroscopic surface profile and the asperity deformation. Both the theoretical formulas and the numerical model are based on a general power-law relationship between the local apparent pressure and real-to-apparent contact ratio. Numerical results of the contact semi-width agree well with the prediction of the formula. The apparent contact region becomes increasingly larger than the Hertzian region as the dimensionless roughness parameter increases, or as the dimensionless load parameter decreases, while the effect of the load exponent is relatively small. The ratio of the contact semi-width to the Hertzian semi-width is mainly determined by a dimensionless contact parameter involving the root-mean-square roughness, the equivalent radius and the Hertzian semi-width or radius. When applied to fractal regular surfaces, the present theory indicates that the influence of the fractal dimension on the contact behavior is due to its effects on both the area-load coefficient and the load exponent.

Commentary by Dr. Valentin Fuster
2004;():1237-1243. doi:10.1115/TRIB2004-64132.

Many classical applications in machine design and recent ones in the field of electrical contacts or micro-contacts involve surface circular contacts which show important edge effects. To optimize these contacts, a uniform pressure distribution must be generated over an important part of contact area. This requires a specifically profiled front surface. Previously, these authors proposed a solution based on an optimum pressure distribution. This leads numerically to a punch profile which is approximated by a polynomial. The pressure generating this polynomial profile is found and compared to initial proposal. Recent investigations establish a correspondence between a polynomial punch surface and generated pressure. Starting from this correspondence, a new general approach is offered. The same optimum pressure as previously is accepted. Its profile is approximated by a function advanced in the paper. This function yields directly an even polynomial punch profile. Formulae for central pressure and normal approach are derived.

Commentary by Dr. Valentin Fuster
2004;():1245-1268. doi:10.1115/TRIB2004-64152.

The stiction forces which exists in micro-electro-mechanical systems are characterized by surface energy and surface roughness. To simulate this contact condition, a three dimensional fractal surface geometry and an adhesive contact model for a single asperity are used together to create a numerical adhesive rough surface solution methodology. This novel method of solution determines the characteristic adhesive contact type for each individual asperity uniquely at the time of load and area integration. Such a simulation more accurately represents the physics of the asperity based contact. Numerical results for the adherence force are presented as a function of surface topography, interface compliance, and the work of adhesion for a MEMS interface. The magnitude of the force required to separate an adhesive rough surface interface is given in relation to a polysilicon system.

Commentary by Dr. Valentin Fuster
2004;():1269-1281. doi:10.1115/TRIB2004-64153.

When adhesive forces are taken into consideration, contacting asperities can still interact after intimate contact is broken. Current theories that predict the contact behavior of adhesive cylindrical asperities fail to capture the forces in this regime. In the present investigation, prior solutions for adhesive cylindrical asperities will be extended to include the condition where the asperities are not in physical contact but are still interacting through adhesive forces. In the extended results, relationships between the adhesive contact radius and the applied normal load will be developed and discussed with respect to the design of micro-scale components.

Topics: Adhesives , Cylinders
Commentary by Dr. Valentin Fuster
2004;():1307-1318. doi:10.1115/TRIB2004-64215.

A thermomechanical analysis is presented for semi-infinite elastic solid sliding against a rigid rough surface characterized by fractal geometry. A piecewise-linear distribution of the contact pressure was obtained by superposition of overlapping triangular pressure elements. The normal surface displacements due to the effects of contact pressure, shear traction, and thermoelastic distortion caused by frictional heating are incorporated in the influence coefficients of the matrix inversion method. Results for a smooth cylindrical surface sliding over a semi-infinite elastic solid demonstrate the accuracy of the analysis and provide reference for comparison with results obtained with the rough (fractal) surface. The effects of surface topography and interaction between neighboring asperity microcontacts on the surface and subsurface temperature rise and stress field of the elastic semi-infinite solid are discussed in the context of numerical results. The significance of frictional heating on the contact pressure, temperature rise, and stresses in interpreted in terms of the Peclet number and topography (fractal) parameters. The results provide insight into the likelihood for cracking and plastic flow at the surface due to the combined effects of mechanical and thermal surface tractions.

Topics: Fractals
Commentary by Dr. Valentin Fuster
2004;():1319-1325. doi:10.1115/TRIB2004-64225.

When two surfaces touch each other, intimate contacts occur at the tips of the asperities and adhesional interaction between the solids arising out of the surface forces becomes significant. This effect need be considered in MEMs, micro-machines, magnetic storage systems other such situations where the surfaces are inherently smooth and loads are extremely low. Surfaces in these and many other tribological contacts may have sub-micron or even nanometric levels and the stochastic model for rough surfaces that typically applies to engineering surfaces is not suitable. The rough surface model is these circumstances must cover asperities ranging from nanometer to micrometer level and this essentially needs a fractal approach. The paper describes a theoretical study of adhesive wear at the contact between surfaces with nanometric level asperities at low loads using a fractal contact model and taking into account the surface forces. The model predicts wear between solids with wide range of material and surface properties. The results broadly confirm the experimental observation such as dependence of wear volume on normal load and also on adhesion due surface forces. Furthermore the fractal analysis gives a generalized solution and depending on the combinations of material and fractal parameters specific solutions, relevant to realistic situations may be arrived at, are obtained. Under certain parametric combinations high wear even under tensile load is predicted while near zero wear is expected for some another set of parameters. These predictions are certainly advantageous in the selection of surface and material properties in applications where loads are small and surfaces are ultra smooth.

Commentary by Dr. Valentin Fuster
2004;():1347-1352. doi:10.1115/TRIB2004-64255.

The effective radius is a very important factor in determining frictional torque under bolt head/nut is bolted joints. Since the effective radius is hard to be precisely determined in practice, the mean radius, which is the mean value of the inner and outer radii of the contact area under bolt head/nut, is used. In this paper, contact mechanics analysis is used to determine the real contact pressure distribution between bolt head/nut and the joint surface. Based on the pressure distribution, the effective radius can be precisely calculated. Effect of the surface roughness, underhead load distribution, underhead load value, and the ratio of the maximum to minimum contact radii are investigated. The results are compared with the result from theoretical formulas and the value of mean radius. These data will be very useful in enhancing the reliability and safety of bolted joints.

Commentary by Dr. Valentin Fuster
2004;():1353-1360. doi:10.1115/TRIB2004-64266.

Impact of a rigid sphere moving at constant velocity on elastic homogeneous half-space was analyzed by the finite element method. Frictionless dynamic contact was modeled with special contact elements at the half-space surface. A dimensionless parameter, β, was introduced to study the effect of wave propagation on the deformation behavior. For small surface interference (β), the front of the faster propagating dilatational waves extends up to the contact edge, the real contact area is equal to the truncated area, and the contact pressure distribution is uniform. However, for large surface interference (β ≤ 1), the dilatation wave front extends beyond the contact edge, the real contact area is less than the truncated area, and the contact pressure exhibits a Hertzian-like distribution. The mean contact pressure increases abruptly at the instant of initial contact, remains constant for β ≤ 1, and increases gradually for β > 1. Based on finite element results for the subsurface stress, strain, and velocity fields, and simple theoretical model that yields approximated closed-form relationships for the mean contact pressure and kinetic and strain energies of the half-space was derived for small surface interference (β ≤ 1), and its validity was confirmed by favor comparisons with finite element results.

Commentary by Dr. Valentin Fuster
2004;():1361-1372. doi:10.1115/TRIB2004-64271.

A contact mechanics theory of static friction is presented for isotropic rough surfaces exhibiting fractal behavior. The analysis is based on a piece-wise power-law size distribution and a normal slope distribution of the asperity contacts and elastic-fully plastic deformation models. Numerical integration yields solutions for the normal and friction forces in terms of fractal parameters, elastic-plastic material properties, and interfacial shear strength. The variation of the static coefficient of friction with normal load is related to the effect of the surface topography on the dominant deformation mode at the asperity contacts. Plastic deformation of the smaller asperity contacts dominates at low loads and elastic deformation of the larger asperity contacts at high loads. The critical load signifying the transition from predominantly plastic to elastic deformation depends on the fractal parameters and material properties. In the low-load range, the static coefficient of friction decreases with the increase of the load, while in the high-load range it increases relatively faster with the load. Numerical results for copper fractal surfaces illustrate the effects of normal load, surface topography, and interfacial shear strength on the static coefficient of friction.

Topics: Fractals , Stiction
Commentary by Dr. Valentin Fuster
2004;():1373-1380. doi:10.1115/TRIB2004-64296.

Previous work by this group on an average lubrication equation for grain flow with roughness effects is extended to include grain-grain collision elasticity ranging from perfectly elastic to perfectly inelastic. The average lubrication equation is based on Haff’s grain flow theory, with flow factors from Patir and Cheng and Tripp’s use of perturbation. The derived flow factors are obtained as functions of rough surface characteristics, grain size and collision pattern. As collision energy loss approaches zero, the inelastic results approach those for perfectly elastic grain collision. The mathematical formulae for flow factors, grain/grain collision elasticity, grain size and roughness are presented, discussed. Predictions for the elastic and inelastic cases are graphically demonstrated and compared.

Commentary by Dr. Valentin Fuster
2004;():1391-1396. doi:10.1115/TRIB2004-64309.

Numerical integration has been widely used in commercial FEA software to solve transient problems. However, for the large-scale inhomogeneous thermoelastic contact problem (ITEC), this method is found to be extremely computation-intensive. This paper introduces a new approach to solve the ITEC transient problem with much lower computational complexity. The method is based on the transient modal analysis (TMA) method in conjunction with the fast speed expansion (FSE) method. The TMA method is used to obtain the inhomogeneous transient solution by expressing the solution in modal coordinates, corresponding to eigenfunctions of the homogeneous (unloaded) problem. If the sliding speed is constant, the eigenfunctions can be found by one run of the commercial software program ‘HotSpotter’. However, if the speed varies, the eigenfunctions change and numerous runs of HotSpotter are needed, making the method computationally inefficient. However, the FSE method employs an efficient algorithm to interpolate and expand the eigenfunctions and eigenvalues over a range of speeds. This reduces the number of eigenvalue solutions required and results in a significant reduction in computation time. The method is illustrated with application to an axisymmetric transmission clutch problem.

Commentary by Dr. Valentin Fuster
2004;():1397-1408. doi:10.1115/TRIB2004-64391.

A model is developed to predict the behavior of two sliding bodies undergoing oscillatory motion. A set of four dimensionless groups is introduced to characterize the transient dimensionless surface temperature rise. They are: the Peclet number Pe , the Biot number Bi , the amplitude of oscillation A, and the Hertzian semi-contact width α. Also considered in the analysis is the effect of the ratio β = A/α of the amplitude to the semi-contact width. The results of a series of simulations, covering a range of these independent parameters, are presented and examples are provided to illuminated the utility of the model.

Topics: Temperature
Commentary by Dr. Valentin Fuster

Magnetic Storage Tribology

2004;():1409-1416. doi:10.1115/TRIB2004-64050.

To achieve a higher storage density in a hard disk drive, the fly height of the air bearing slider, as part of the magnetic spacing, has to be minimized. At an ultra-low fly height, the intermittent / continuous contact at the head–disk interface (HDI) is unavoidable and directly affects the mechanical and magnetic performance of the hard disk drive, and is of great interest. The HDI wear has a non-linear and time-varying nature due to the change of contact force and roughness. To predict the HDI wear evolution, an iterative model of Coupled Head And Disk (CHAD) wear, is developed based on the contact mechanics. In this model, a composite transient wear coefficient is adopted and multiple phases of the wear evolution are established. A comprehensive contact stiffness is derived to characterize the contact at the HDI. The abrasive and adhesive wear is calculated based on the extended Archard’s wear law. The plastic and elastic contact areas are calculated with a 3-D sliding contact model. Based on the CHAD wear model, for the first time, the coupling between head and disk wear evolutions is thoroughly investigated. Accelerated wear tests have also been performed to verify the disk wear effect on the slider wear. A wear coefficient drop with time is observed during the tests and it is attributed to a wear mechanism shift from abrasive to adhesive wear. A shift in the type of contact from plastic to elastic accounts for the wear mechanism change.

Topics: Wear , Disks
Commentary by Dr. Valentin Fuster
2004;():1417-1423. doi:10.1115/TRIB2004-64062.

The focus in this paper is to automatically design the air-bearing surface (ABS) considering the randomness of its geometry as an uncertainty of design variables. Designs determined by the conventional optimization could only provide a low level of confidence in practical products due to the existence of uncertainties in either engineering simulations or manufacturing processes. This calls for a reliability-based approach to the design optimization, which increases product or process quality by addressing randomness or stochastic properties of design problems. In this study, a probabilistic design problem is formulated considering the reliability analysis which is employed to estimate how the fabrication tolerances of individual slider parameters affect the final flying attitude tolerances. The proposed approach first solves the deterministic optimization problem. Beginning with this solution, the reliability-based design optimization (RBDO) is continued with the probabilistic constraints affected by the random variables. Probabilistic constraints overriding the constraints of the deterministic optimization attempt to drive the design to a reliability solution with minimum increase in the objective. The simulation results of the probabilistic design are directly compared with the values of the initial design and the results of the deterministic optimum design, respectively. In order to show the effectiveness of the proposed approach, the reliability analyses by the Monte Carlo simulation are carried out. And the results demonstrate how efficient the proposed approach is, considering the enormous computation time of the reliability analysis.

Commentary by Dr. Valentin Fuster
2004;():1425-1432. doi:10.1115/TRIB2004-64067.

To reduce the risk of head crash when a slider contacts a smooth disk during flying height adjustment, a novel tapping method for reducing the friction and lowering the risk of head crash was developed. An active-head slider was designed and fabricated to demonstrate the usefulness of tapping approach. It is also found that micro-vibration is significantly effective in avoiding a high friction force and protecting a head from the crash due to head-disk contact during the flying height adjustment.

Topics: Friction
Commentary by Dr. Valentin Fuster
2004;():1433-1439. doi:10.1115/TRIB2004-64070.

We demonstrated the direct visualization of molecularly thin lubricant films on magnetic disks with a thickness resolution of 0.1 nm by using an ellipsometric microscope with a white light source. It was able to reduce the optical interference noise that arises in conventional laser-based ellipsometric microscopes, and to provide a large SNR by a factor of about 6 compared to a laser-based ellipsometric microscope. The wavelength width should be given the first priority in designing a white light source ellipsometric microscope, and the width should be determined after considering the required coherence length and thickness resolution. Theoretical calculations indicate that a wavelength width of less than 10 nm can provide a thickness resolution of 0.1 nm. A white light source ellipsometric microscope can provide real-time visualization of a molecularly thin lubricant film with a thickness resolution of 0.1 mm, which is useful in investigating the kinetic behavior of molecularly thin lubricant films on magnetic disks.

Commentary by Dr. Valentin Fuster
2004;():1441-1446. doi:10.1115/TRIB2004-64083.

Lubrication by an extremely thin film has become very important in micro machines, magnetic recording disks and so on. Molecularly thin perfluoropolyether (PFPE) films are considered a good lubricant for these micro devices. When the thickness of the PFPE film is thinned to several nanometers, it is possible to assume that the film consists of mobile and chemically bonded molecules. In this paper, we investigated the role of these molecules from the viewpoint of the vibrational stability of the sliding ball with the disk surface. From experiments by the ball on disk type tribotester, it is found that chemically bonded molecules prevent direct contact between the slider and the disk surface. Furthermore, when mobile molecules exist on bonded molecules, the stability is improved in wide load range.

Topics: Stability , Lubricants
Commentary by Dr. Valentin Fuster
2004;():1447-1453. doi:10.1115/TRIB2004-64088.

The unstructured triangular mesh is successfully applied to the steady simulations of the slider due to its flexibility, accuracy and mesh efficiency in capturing various complex ABS rails and recess wall regions. This paper introduces a new implicit algorithm with second order time accuracy for the time-dependent simulations of the slider dynamics based on the unstructured triangular mesh. The new algorithm is specially developed for the unstructured triangular mesh and the finite volume method. It is applied to simulate the load/unload details based on the 9-D model, such as the influences of the limiter position on the flying height during the unload process, and the development and the 3-D profiles of the air bearing pressure during the load process. Because of the mesh efficiency of the unstructured triangular mesh and the flexibility of applying greater time steps, the simulation times are significantly shorter than those of the structured rectangular mesh. The simulation results based on the algorithm are in good correlation with the experimental results.

Commentary by Dr. Valentin Fuster
2004;():1455-1482. doi:10.1115/TRIB2004-64104.

Recently, tribomicroplasma has been discovered around sliding contacts. This must be the unknown high energetic state that has been sought after for many years as the source of tribochemical reaction. On the other hand, the mechanism of tribochemical decomposition of the perfluoropolyether (PFPE) fluid, Z-dol, used as hard disk drive lubricant, is not fully explained by traditional reaction mechanisms. A mixture of volatile decomposition products evolved from lubricated sliding contacts is collected by a sampling tube packed with an adsorbent and separated by gas chromatography (GC) into individual components, the chemical structures of which are determined by a Quadra pole mass spectrometer (Q-MS). The volatile products at the sliding contact coincided completely with that of the gas-discharge plasma decomposition product. These results verify that PFPE fluid is decomposed by triboplasma. This verification opens a new mechanism of tribomicroplasma reaction in the field of tribochemistry.

Topics: Fluids
Commentary by Dr. Valentin Fuster
2004;():1483-1490. doi:10.1115/TRIB2004-64121.

As the flying height decreases to achieve greater areal density in hard disk drives, different proximity forces act on the air bearing slider, which results in fly height modulation and instability. Identifying and characterizing these forces has become important for achieving a stable fly height at proximity. One way to study these forces is by examining the fly height hysteresis, which is a result of many constituent phenomena. The difference in the touchdown and takeoff rpm (hysteresis) was monitored for different slider designs, varying the humidity and lubricant thickness of the disks, and the sliders were monitored for lubricant pickup while the disks were examined for lubricant depletion and modulation. Correlation was established between the observed hysteresis and different possible constituent phenomena. One such phenomenon was identified as the Intermolecular Force from the correlation between the lubricant thickness and the touchdown velocity. Simulations using 3D dynamic simulation software explain the experimental trends.

Topics: Disks
Commentary by Dr. Valentin Fuster
2004;():1491-1495. doi:10.1115/TRIB2004-64127.

Intermolecular and surface forces contribute significantly to the total forces acting on air bearing sliders for flying heights below 5nm. Their contributions to the total forces increase sharply with the reduction in flying height, and hence their existence can no longer be ignored in air bearing simulation for hard disk drives. Various experimentally observed dynamic instabilities can be explained by the inclusion of these forces in the model for low flying sliders. In this paper parametric studies are presented using a 3-DOF model to better understand the effect of the Hamaker constants, suspension pre load and pitch angle on the dynamic stability/instability of the sliders. A stiffness matrix is used to characterize the stability in the vertical, pitch and roll directions. The fly height diagrams are used to examine the multiple equilibriums that exist for low flying heights. It has been found that the system instability increases as the magnitude of the van der Waals force increases. It has also been found that higher suspension pre load and higher pitch angles tend to stabilize the system.

Commentary by Dr. Valentin Fuster
2004;():1497-1501. doi:10.1115/TRIB2004-64134.

In this paper, a mesh-less method, least square finite difference (LSFD) method, was applied to solve the slider air bearing problems in hard disk drives. The LSFD method derives from weighted least square approximation procedure and a Taylor series expansion of pressure distribution to directly discretize the Reynolds equation. The scheme is able to approximate the second-order derivatives to the second-order accuracy in this study. The mesh-free method does not require maintaining detailed structural information about the computational meshes in the computational domain. Furthermore, adaptive node distribution can prevents pressure oscillatory in the high-pressure gradient regions. In addition, Successive Over Relaxation iteration technique is applied to solve nonlinear system equations. Finally, the static flying attitude of the Taper Flat, Tripad and the Transverse Pressure Contour (TPC) sliders are calculated.

Topics: Bearings , Disks
Commentary by Dr. Valentin Fuster
2004;():1523-1529. doi:10.1115/TRIB2004-64193.

This paper describes the effect of end-group functionality and molecular weight of ultra-thin liquid lubricant films on contact slider dynamics in hard disk drives. In the experiments, the contact slider dynamics as well as ultra-thin liquid lubricants behavior are investigated using three kinds of lubricants which have different end-groups and molecular weight as a function of lubricant film thickness. The dynamics of a contact slider is mainly monitored using Acoustic Emission (AE). The disks are also examined with a scanning micro-ellipsometer before and after contact slider experiments. It is found that the lubricant film thickness instability due to de-wetting occurs as a result of slider-disk contacts, when the lubricant film thickness is thicker than one monolayer. Their unstable lubricant behavior depends on the chemical structure of functional end-groups and molecular weight. In addition, it is also found that the AE RMS values, which indicate the contact slider dynamics, are almost equivalent, independent of the end-groups and molecular weight for the lubricants, when the lubricant film thickness is around one monolayer. The molecular weight, however, affects the contact slider dynamics, when the lubricant film thickness is less than one monolayer. In other words, the AE RMS values increase remarkably as the molecular weight for the lubricant increases. When the lubricant film thickness is more than one monolayer, the AE RMS values decrease because of the effect of mobile lubricant layer, while the lubricant de-wetting instability affects the contact slider dynamics. Therefore, it would be concluded that the lubricant film thickness should be designed to be around one monolayer thickness region in order to achieve contact recording for future head-disk interface.

Commentary by Dr. Valentin Fuster
2004;():1531-1536. doi:10.1115/TRIB2004-64212.

System identification methods have been used to study the response of a magnetic recording slider during contact with a scratch on the disk surface. In addition, the slider response was studied taking into account the effect of disk micro-waviness at various disk rotational speeds. The simulated slider response was compared with the measured slider dynamic behavior. Very good agreement was found between simulated and measured data. The flying height modulation of the slider, due to disk micro-waviness, was found to depend on disk velocity.

Topics: Disks
Commentary by Dr. Valentin Fuster
2004;():1537-1547. doi:10.1115/TRIB2004-64224.

This paper deals with the experimental identification of elastic, damping and adhesion forces in the dynamic collision of a spherical slider with a stationary magnetic disk. We used rough Al2 O3 TiC and smooth glass spherical sliders with a radius of 1 mm, and magnetic disks with four different lubricant film thicknesses of 0, 1, 2, and 3 nm. We found that the Al2 O3 TiC slider shows ordinary approach and rebound processes, whereas the glass slider showed a velocity drop at the end of the rebound process when the lubricant thickness was 1, 2 and 3 nm. We identified the elastic force factors in the approaching and rebound processes, based on the Herztian contact theory, and the damping force factors based on a damping force model that is proportional to slider velocity and penetration depth (contact area). From the drop in velocity when the slider and disk separated, we found that the dynamic adhesion force is almost equal to the static pull-off force, except for with a 3nm lubricant thickness. The dynamic adhesion force with 3 nm lubricant thickness is significantly higher probably because of squeeze damping effect.

Commentary by Dr. Valentin Fuster
2004;():1549-1554. doi:10.1115/TRIB2004-64251.

In a magnetic recording disk file, pitch moment exerted by the flexure on the slider is usually treated as a product of flexure pitch-static-attitude (PSA) and pitch-stiffness (Kp ), both measured in the absence of preload (gram-load). However in the presence of preload, pitch moment due to dimple friction must be considered. We shall show by elementary beam theory that lever-arm of the dimple friction is related to the flexure bow-height, and that to minimize uncertainty in the pitch moment, hence improving the fly height distribution, the flexure must bow toward the disk. These results will be confirmed experimentally.

Topics: Friction
Commentary by Dr. Valentin Fuster
2004;():1571-1578. doi:10.1115/TRIB2004-64306.

The objective of the present work is to present the numerical analysis of the computer hard disk slider. The pressure between slider and disk surfaces is calculated using the finite element method for the generalized Reynolds’ equation solution, with application of tabulated data for the flow rate coefficient. The search for the steady state attitude of the slider is formulated in terms of optimization and solved by a combination of the conjugate gradient method, method of feasible directions and quadratic curve fitting. The advantages of the perturbation method in static analysis are considered. The steady state attitude of the femto slider is studied for various radial positions and skew angles. The parameters having effect on the objective function Hessian matrix are studied for the purpose of improvement of the search process convergence rates.

Commentary by Dr. Valentin Fuster

Manufacturing/Metalworking Tribology

2004;():1579-1586. doi:10.1115/TRIB2004-64020.

The effects of textured tubes on the tribological performance in Tube Hydroforming (THF) are discussed. Textured surfaces, namely sand blasted, knurled, and as rolled surfaces were tested under various interface pressure and sliding velocity conditions. Sand blasted textured tubes were found to have the best tribological performance. It was also found that the interface pressure has a great influence on the attainment of Micro-Plasto HydroDynamic Lubrication (MPHDL) and Micro-Plasto HydroStatic Lubrication (MPHSL) conditions at the tool-workpiece interface. Preliminary finite element simulations on the deformation behavior of tube surface shows that surface textures can be optimized to enhance tribological performance.

Topics: Tribology
Commentary by Dr. Valentin Fuster
2004;():1617-1626. doi:10.1115/TRIB2004-64310.

A study has been made of the effect of an externally imposed, low-frequency modulation (≤ 100 Hz) on the action of a fluid in machining. It is shown that in conventional machining, fluid action in terms of lubrication is essentially confined to the edges of chip-tool contact along the tool rake face, with little or no change in the friction condition over much of this face. However, the effectiveness of the lubricating action is significantly enhanced when a controlled low-frequency modulation of sufficient amplitude, such as to break the chip-tool contact, is imposed in the direction of cutting. Measurements show that the friction coefficient between tool and chip is reduced by a factor of up to three in the presence of such a modulation. The extent of the secondary deformation zone in the chip material close to the rake face is also significantly reduced. Direct observations of the tool rake face show that when the modulation is applied, the fluid penetrates into much of the intimate contact region between chip and tool.

Commentary by Dr. Valentin Fuster

Nanotribology

2004;():1627-1635. doi:10.1115/TRIB2004-64013.

This paper adopts an atomic-scale model based on the nonlinear finite element formulation to analyze the stress and strain induced in a very thin film during the nanoindentation process. The deformation evolution during the nanoindentation process is evaluated using the quasi-static method, thereby greatly reducing the required computation time. The finite element simulation results indicate that the microscopic plastic deformation in the thin film is caused by instability of its crystalline structure, and that the magnitude of the nanohardness varies with the maximum indentation depth and the geometry of the indenter.

Commentary by Dr. Valentin Fuster
2004;():1651-1656. doi:10.1115/TRIB2004-64284.

Nanoindentation and nanotribological experiments were performed on an antiwear tribofilm produced from a blend consisting of gear base oil and phosphorus-containing additive. Electrical contact resistance measurements were used to determine in situ the formation of the tribofilm on AISI steel surfaces at 100 °C under conditions favoring sliding in the boundary lubrication regime. Nanoindentation experiments were carried out with a surface force microscope on a small sector of a disk specimen that contained part of the wear track. A Berkovich diamond tip with a nominal radius of curvature equal to 100 mm was used to perform indentations on and off the wear track under a normal load between 100 and 600 μN. The phosphate tribofilm exhibited an average hardness of 6.0 GPa and reduced elastic modulus of 122.7 GPa compared to 12.5 GPa and 217.6 GPa of the steel substrate, respectively. In addition to the nanomechanical properties, the nanotribological properties of the tribofilm were evaluated in light of nanofriction tests performed with a 20 μm radius of curvature conical diamond tip under a normal load in the range of 100–200 μN. The original steel surfaces exhibited constant friction, whereas the wear track covered by the phosphate tribofilm, whereas the wear track covered by the phosphate tribofilm revealed a higher and more fluctuating coefficient of friction, which is attributed to the roughness of the wear track and the greater plasticity of the tribofilm than the substrate.

Commentary by Dr. Valentin Fuster

Engineered Surfaces

2004;():1657-1665. doi:10.1115/TRIB2004-64009.

Results of a numerical study of the influence of micro-patterned surfaces in hydrodynamic lubrication of two parallel walls are reported. Two types of parameterized grooves with the same order of depth as the film thickness are used on one stationary wall. The other wall is smooth and is sliding with a specified tangential velocity. Isothermal incompressible two dimensional full film fluid flow mechanics is solved using a Computational Fluid Dynamics method. It is shown that, by introducing a micro-pattern on one of two parallel walls, a net pressure rise in the fluid domain is achieved. This produces a load carrying capacity on the walls which is mainly contributed by fluid inertia. The load carrying capacity increases with Reynolds number. The load carrying capacity is reported to increase with groove width and depth. However, at a certain depth a vortex appears in the groove and near this value the maximum load carrying capacity is achieved. It is shown that the friction force decreases with deeper and wider grooves. Among all geometries studied, optimum geometry shapes in terms of hydrodynamic performance are reported.

Topics: Lubrication
Commentary by Dr. Valentin Fuster
2004;():1667-1674. doi:10.1115/TRIB2004-64047.

High-power laser surface treatments in the form of glazing, shock peening, cladding, and alloying can significantly affect material tribology. In this paper, effects of laser glazing, laser shock peening, and their combination on the tribological behavior of 1080 carbon steel were investigated. Laser glazing is a process in which a high-power laser beam melts the top layer of the surface, followed by rapid cooling and resolidification. This results in a new surface layer microstructure and properties. Laser shock peening, on the other hand, is a mechanical process in which a laser generates pressure pulses on the surface of the metal, similar to shot peening. Five conditions were evaluated: untreated (baseline), laser shock peened only (PO), laser-glazed only (GO), laser-glazed then shock peened last (GFPL), and laser shock peened then glazed last (PFGL). In pin-on-disc testing, all laser-treated surfaces reduced dry friction, with the GFPL surface having maximum friction reduction of 43%. Under lubricated conditions, all laser-treated surfaces except the PO sample lowered friction. Similarly, all glazed samples reduced wear by a factor of 2–3, while the PO sample did not change wear significantly. These tribological results are associated with changes in the near-surface microstructure and properties.

Commentary by Dr. Valentin Fuster

Biotribology

2004;():1689-1694. doi:10.1115/TRIB2004-64008.

In addition to confounding mass-based wear measurements in serum-lubricated hip simulator experiments, fluid absorption by the acetabular cups may simultaneously modify the wear resistance of the ultra-high molecular weight polyethylene (UHMWPE) from which they are composed. To decouple the fluid absorption and wear processes enabling clearer investigation of this effect, absorption was first imposed during an initial stage where UHMWPE was exposed to pressurized (10MPa) fluid. This was followed by a second stage, where resultant wear behavior was assessed by a multidirectional pin-on-flat technique that, though still providing a serum-lubricating environment, does not promote the simultaneous fluid absorption occurring in hip simulator testing. Both unirradiated and highly-crosslinked UHMWPE were investigated, each with both bovine calf serum and water soaking exposures of duration to 129 days. The pressurized soaking of a highly-crosslinked UHMWPE decreased its wear resistance, causing an increase in wear rate by approximately 50% during subsequent serum-lubricated multidirectional pin-on-flat sliding tests as compared to non-soaked material. The magnitude of this effect did not appear to depend on whether the soaking fluid was water or serum, nor did it appear to depend on soak time provided it was at least of a 14-day duration during which more rapid transient fluid absorption occurs. Such soaking did not produce as pronounced an effect on unirradiated UHMWPE, at its lack of wear resistance likely causes the absorption-affected surface region to the completed removed within the earliest stages of sliding contact.

Commentary by Dr. Valentin Fuster
2004;():1703-1713. doi:10.1115/TRIB2004-64150.

Background: Multi-directional sliding motions between total knee replacement materials is a suspected primary wear mechanism of ultra-high molecular weight poly(ethylene) (UHMWPE). Method of Approach: A wear testing machine was developed to quantify damage from crossing contact pathways on candidate biomaterials. A cyclic five-pointed star pattern was used to evaluate the tribological differences between linear and cross-motion surface tribology of stainless steel pins on flat UHMWPE. Results: Volumetric reconstruction of resultant damage showed that cross-shear volume loss was 2.94(± 0.88) times that of linear loss during testing. Conclusions: Basic multi-axis, cross-shear wear testing provides quantifiable measures of complex biomaterials wear phenomena.

Commentary by Dr. Valentin Fuster
2004;():1715-1725. doi:10.1115/TRIB2004-64166.

The effects of contact area and contact stress on friction and wear of polyethylene-metal articulation were evaluated using a bi-directional pin-on-disk apparatus. A doubling of the contact area under fixed loading conditions led to a 50% increase in the coefficient of friction and a doubling of the wear rate. There appeared to be a relationship between the increase in wear rate and the increase in the coefficient of friction. A model was developed to explain the mechanism by which engagement of asperities results in the increasing wear rate with increasing contact area despite the decreased stress.

Commentary by Dr. Valentin Fuster
2004;():1767-1773. doi:10.1115/TRIB2004-64272.

Modification of the microstructure of ultra-high molecular weight polyethylene (UHMWPE) is essential for improving the wear resistance of orthopaedic implants. A common approach is to crosslink the polymer by gamma irradiation. The objective of this study was to examine the tribological behaviors of untreated and gamma irradiated UHMWPE under physiologically relevant contact conditions. Emphasis was given on the identification of the dominant wear mechanisms in the early stage of polymer wear. The irradiation dose exhibited as strong effect on the tribological properties of UHMWPE sliding against Co-Cr alloy in a bath of bovine serum. Transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM) were used to examine the microstructure and morphology of the worn surfaces. Regularly spaced folds with average spacing depending on the irradiation dose (i.e., crosslink density) formed on the wear tracks. Surface folding was related to plastic flow and the degree of mobility of the crystalline lamellae. SEM and TEM results elucidated the roles of the crosslink density and crystalline lamellae in the wear process. Based on the experimental evidence, a deformation model was obtained that provides explanation for the dependence of surface folding on the crosslink density and lamellae reorientation during sliding.

Commentary by Dr. Valentin Fuster
2004;():1775-1783. doi:10.1115/TRIB2004-64328.

This paper describes an experimental method, Bio-Ferrography, to separate ultra high molecular weight polyethylene (UHMWPE) wear debris, generated in hip simulators, from bovine serum lubricating fluid. A total of 54 experiments were performed in which an enzyme digestion “cocktail” was developed and used to clean the bovine serum samples of extraneous sugars, proteins and lipids that interfere with the UHMWPE particle separation. Erbium chloride was used to marginally magnetize particles in the fluid prior to passing through the ferrographic device. The particles were captured and separated from the fluid by traversing the treated serum across a magnetic gap of a bio-ferrograph. Morphology of the captured and separated wear debris was compared with particles from samples of fluid filtered through a paper sieve arrangement with pores of 0.05 micrometers in diameter. The UHMWPE wear debris collected using the described experimental method, were found to be between 0.1 and 20 micrometers in diameter with spherical and pill-shaped particles. The filtered UHMWPE particles were also in the same size range as the debris separated using bio-ferrography, 0.1 to 20 micrometers. To show that the experimental method captured UHMWPE particles, the spectra of the chemical composition of UHMWPE from an acetabular cup insert of a hip implant and of UHMWPE particles separated using Bio-Ferrography were compared and found to be the same. To further demonstrate that polyethylene could be captured and separated through the experimental method, manufactured polyethylene microspheres in the diameter range of 3 to 45 micrometers, were captured and separated using the bio-ferrographic process.

Topics: Wear , Fluids
Commentary by Dr. Valentin Fuster

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