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Nanotribology

2009;():1-3. doi:10.1115/IJTC2009-15076.

This paper presents the techniques used to comprehensively characterize the effect of current density on tribological, mechanical and adhesion properties of nanocrystalline coatings.

Topics: Tribology , Coatings
Commentary by Dr. Valentin Fuster
2009;():5-7. doi:10.1115/IJTC2009-15083.

Wear reduction by Carbon Nanotube (CNT) addition in composites with Ultra-High Molecular Weight Polyethylene (UHMWPE), Polyimide (PI), Polytetrafluoroethylene (PTFE), and epoxy resins have been reported separately. We studied Polypropylene (PP) and Polyamide (PA) composites and showed that with the addition of Multi-wall Carbon Nanotube (VGCF: Vapor Grown Carbon Fiber), wear decreased for PA composites but increased for PP composites. Differences in tribological characteristics of CNT composites with different resins were not well understood. In this paper, we compared tribological and mechanical characteristics of VGCF composites with PE, PP, and Polyacetal (POM) resins. Ball-on-Disk wear tests and mechanical strength measurements were performed. It was found that with the increase in VGCF content, specific wear amount (SWA) of VGCF-PE composite decreased while SWA of VGCF-POM composite stayed almost constant and SWA of VGCF-PP composite increased. On the other hand, with the increase in VGCF content, the tensile strength of VGCF-PE composite was increased but those of VGCF-PP and VGCF-POM composite were decreased. Decrease in SWA of VGCF-PE composite corresponded to the increase in tensile strength with VGCF content. We considered that the intermolecular force between side wall of VGCF and PE was strong enough to make both the SWA small and the tensile strength large.

Commentary by Dr. Valentin Fuster
2009;():9-11. doi:10.1115/IJTC2009-15095.

Dispersed nanoparticles of solid lubricants in sheet metal forming fluids are studied for enhanced lubrication that can lead to improved product surface quality and reduced tool wear. Molybdenum disulfide (MoS2 ) and hexagonal boron nitride (hBN) nanoparticles of varying size and concentrations have shown marked reduction in wear of steel counterfaces representing the tool and in scoring of titanium sheet surfaces. The most effective particle concentration and size ranges were 0.25–4% and 70–100 nm. The counterface wear was reduced by 50–75% while the friction coefficient only marginally improved.

Commentary by Dr. Valentin Fuster
2009;():13-15. doi:10.1115/IJTC2009-15120.

Topographical changes within the contact area as a function of cycling could be a critical factor causing failure and reliability issues in RF MEMS switch operation. In this paper, gold-to-gold contact, cantilever-type RF MEMS switches were tested (cold-switching mode) for different number of cycles, namely, 10, 102 , 103 , 104 , 105 , and 106 . After the cycling tests, the contact area of each switch was scanned using optical microscopy, scanning electron microscopy and atomic force microscopy to quantify the exact gold-to-gold contact surface changes, leading to adhesion failures (at about 106 cycles). Detailed roughness analysis was carried out to better quantify topographical changes on the contact surface and relate them to failures. It was found that the material transfer from the top beam to the bottom substrate was dominant, and observed after only few cycles. Adhesion failure of gold-to-gold contact switches could be attributed to large protrusions formed on the bottom surface as the switch cycles over 105 times.

Commentary by Dr. Valentin Fuster
2009;():17-19. doi:10.1115/IJTC2009-15140.

Wear occurs at most solid surfaces that come in contact with other solid surfaces. While biological surfaces and tissues usually have the ability for self-healing, engineered self-healing materials only started to emerge recently. An example of a smart self-healing material is the materials with embedded microcapsules, which rupture during crack propagation and release a healing agent that repairs the crack. We discuss the conditions under which the self-healing occurs and provide a general theoretical framework and criteria for self-healing using the concept of multiscale organization of entropy and non-equilibrium thermodynamics.

Topics: Wear
Commentary by Dr. Valentin Fuster
2009;():21-23. doi:10.1115/IJTC2009-15184.

Carbon nanotubes (CNTs) are the subject of extensive research in nanoelectromechanical devices due to their excellent mechanical and electrical properties. This study focuses on modeling the actuation of a CNT-based nanoswitch. The CNT is simulated as a wire with negligible bending stiffness. The distributed force due to the applied voltage difference between it and the electrode causes the CNT deflect toward the electrode. The 2nd order nonlinear ordinary differential equation (ODE) is first solved by the finite difference method (FDM) with fixed point iteration. However, this result is only found in a portion of the stable domain. The second method algebraically reduces the governing equation to a 1st order ODE. After removing the singularity, a numerical integration method is applied without iteration. This method gives complete results in the stable and unstable domains that coincide with the results by FDM in its convergent range.

Commentary by Dr. Valentin Fuster
2009;():25. doi:10.1115/IJTC2009-15192.
FREE TO VIEW

Microswitches have the potential to be used in numerous applications [1]. Microswitches are slower and currently less reliable than semiconductor switches. However they have the advantage of reduced power consumption, less insertion loss, and better isolation. The operation of a microswitch involves many interesting phenomena which will be reviewed. In an electrostatically actuated switch a voltage difference is applied between the beam and the gate. The resulting electrostatic force pulls the beam toward the gate such that its tip contacts the drain and completes the circuit. However the momentum of the beam causes it to deflect further, even after the tip is in contact with the drain. This stored elastic energy can be sufficient to cause the tip to separate from the drain resulting in what is often referred to as “contact bounce” [2]. Contact bounce causes several problems. First the switch cannot be used until the bouncing has stopped, thus increasing the effective time-to-closure. Second the tip and drain are subjected to more mechanical open-close contacts than are intended. Third the impact force between the tip and drain can be several times greater than the force in the closed position. These last two phenomena can prematurely degrade the contact surfaces. Attempts have been made to tailor the actuation voltage in order to reduce both the impact force and contact bounce [3]. Switches can fail in either of two failure modes. In one mode the contact resistance decreases with cycling. This behavior, while seemingly not a problem, is symptomatic of the contact being cleaned due to repeated contacts (i.e. “contact scrub” as it is known in the industry). As the resistance decreases the force of adhesion increases between these clean metal-to-metal contacts, eventually causing the switch to stick shut when the restoring force in the beam is insufficient to pull the contacts apart [4]. Another failure mechanism is a steady increase in contact resistance until the point at which the switch is no longer useable. These failures are attributable to the growth of contamination in the form of a friction polymer on the contacting surfaces [5]. Another measure of switch performance is intermodulation distortion which quantifies the distortion of a pair of harmonic RF signals with nearly equal frequencies. The beating between two such components causes the power to vary relatively slowly, i.e. at the beat frequency. This beat frequency may be sufficiently close to the thermal response time of the switch leading to distortion and false signals in the frequency range of interest [6].

Commentary by Dr. Valentin Fuster
2009;():27-29. doi:10.1115/IJTC2009-15205.

The prediction of dishing and erosion caused by CMP is desired as they adversely affect the electrical properties of interconnects in integrated circuits. For a model to properly capture these phenomena, it must account for the time dependent surface evolution in CMP. This work employs the previously introduced Particle-Augmented Mixed Lubrication (PAML) model to predict dishing and erosion in CMP. By using PAML to model the polishing of a patterned wafer, it is possible to predict the dishing and erosion experienced during CMP.

Commentary by Dr. Valentin Fuster
2009;():31-33. doi:10.1115/IJTC2009-15210.

The development of self-powered electronic devices is essential for emerging technologies such as wireless sensor networks, wearable electronics, and microrobotics. Of particular interest is the rapidly growing field of piezoelectric energy harvesting (PEH), in which mechanical strains are converted to electricity. Recently, PEH has been demonstrated by brushing an array of piezoelectric nanowires against a nanostructured surface. The piezoelectric nanobrush generator can be limited to sub-micron dimensions and thus allows for a vast reduction in the size of self-powered devices. Moreover, energy harvesting is controlled through contact between the nanowire tips and nanostructured surface, which broadens the design space to a wealth of innovations in tribology. Here we propose design criteria based on principles of contact mechanics, elastic rod theory, and continuum piezoelasticity.

Commentary by Dr. Valentin Fuster
2009;():35-37. doi:10.1115/IJTC2009-15212.

We have previously shown that, for non-adhesive conditions, an atomic scale contact can be adequately represented by a continuum analysis despite the physical shortcomings at this scale. Here we have extended the approach to include effects of the adhesive forces that become significant at this level of contact. Adhesive forces are obtained directly from the surface separation across the contact rather than through a total surface energy approach; this allows a complete representation of local surface features. The pull-off characteristics and pressure profiles have been obtained for several different atomistic AFM tip profiles and compared to those obtained from molecular dynamics simulations presented in the literature [1].

Topics: Adhesives
Commentary by Dr. Valentin Fuster
2009;():39-41. doi:10.1115/IJTC2009-15241.

Conventional friction force microscopes (FFMs) had the disadvantage of low force sensitivity due to mechanical interference between torsion caused by friction force and deflection by normal force. In order to overcome disadvantage, we developed a dual-axis micro-mechanical probe, which measures the lateral force by the double cantilever and the vertical force by the torsion beam. However, the calibration method of the lateral force has not been established. In this study, we present a new calibration method using a step-structure.

Commentary by Dr. Valentin Fuster

Biotribology

2009;():43-45. doi:10.1115/IJTC2009-15075.

The authors investigated the viability of UHMWPE composites for use in orthopaedic bearing applications as an alternative to crosslinking. Two composite systems were used, both based on a UHMWPE matrix: PtZr quasicrystals and Zr particle fillers. Through wear testing and impact toughness measurement it was shown that these two fillers produced composites that had high wear resistance and more impact toughness than the crosslinked polymer. These results suggest that further investigation into the use of composites for implant bearing surfaces is warranted and that appropriate criteria for filler materials must be developed to spur development of longer-lasting devices.

Commentary by Dr. Valentin Fuster
2009;():47-49. doi:10.1115/IJTC2009-15134.

Thin films exhibiting nonfouling behavior toward protein and cell attachment were grafted onto different substrates by plasma polymerization and deposition. By combining nonfouling film grafting and partial film etching by Ar ion sputtering through the windows of a shadow mask (Si or PDMS), chemical patterns of different shapes and sizes were produced on polymer substrates. Results from cell culture studies illustrate the effectiveness of the present fabrication process to produce surface micropatterns for controlling the cell shape and morphology, with direct implications in vascular pathology.

Commentary by Dr. Valentin Fuster
2009;():51-53. doi:10.1115/IJTC2009-15169.

Improvements in surgical procedures, installation techniques and properties of materials have resulted in a remarkable reduction in the failure of artificial hip joints (AHJ) due to infection. However, the durability of these replacements is greatly limited by premature osteolysis and eventual joint loosening, caused by macrophage activity in response to the release of submicron particles of ultra-high molecular weight polyethylene (UHMWPE) cup material [1–4]. The wear debris is mainly due to wear between the bearing surfaces, and these wear rates are known to be accelerated by the third body action of polymethylmethacrylate (PMMA) bone cement particles and metallic fragments of the femoral head material scattered within the synovial fluid lubricant [5]. This study is focused on development of a model that simulates the motion of UHMWPE particles in the synovial fluid between the AHJ bearing surfaces during articulation.

Commentary by Dr. Valentin Fuster
2009;():55-57. doi:10.1115/IJTC2009-15174.

Diffusion treatments such as chromising and aluminising of high carbon steel results in the formation of Cr and Al rich layers on the surface. These layers will be responsible for formation of protective Cr2 O3 and Al2 O3 scale on exposure to corrosive environment. In view of extensive application of 316 Stainless Steel in biomedical engineering, present work is undertaken to examine the influence of diffusion treatment of high carbon steel on the reciprocating wear in Hank’s solution and to compare the same behavior with that of 316 SS. In order to achieve the above mentioned objectives, the wear rates of 316 SS, 304 SS, high carbon steel in aluminised and chromised condition have been evaluated. Though the general characteristics of the wear behavior, such as higher wear rate at higher load and at higher reciprocating speed, for all the materials are similar, aluminised and chromised high carbon steel exhibited best wear resistance especially at high and intermediate load. In simulated body fluid condition, 316 SS exhibited two different material removal mechanisms. In the first case, which is observed at higher load, material loss is by nucleation and propagation of crack. In the second mechanism which is prevalent at low and intermediate load, wear takes place by deterioration of composite layer formed on the surface by mixing the corrosion product and the substrate. Consequently it is noted that wear rate decreases with increase of hardness at higher load. At intermediate and at low load wear rate is less dependent of hardness.

Commentary by Dr. Valentin Fuster
2009;():59-61. doi:10.1115/IJTC2009-15251.

Obtaining lubricant film thickness values within a compliant contact is a challenging problem for several reasons [1]: • Lubricant film thickness covers a wide range of values. • The required measuring range is from fractions to hundreds of microns. • Contact area is considerably large when compared to “hard”, metallic contacts. • Many soft components have a high roughness compared to surfaces usually investigated with established techniques.

Topics: Film thickness
Commentary by Dr. Valentin Fuster

Engineered Surfaces (Tribomaterials)

2009;():63-65. doi:10.1115/IJTC2009-15005.

Si3 N4 reinforced Al6061 composite is fabricated by liquid metallurgy technique. Si3 N4 particles are nickel coated prior to addition to molten metal to improve its wettability and to ensure excellent bond between matrix and the reinforcement. Metallographic studies, friction and wear tests were carried out using pin on disc type machine. Coefficient of friction and wear rate were measured at loads varying from 20–100N and sliding velocities from 0.314–1.574m/s. It is observed that Al6061-4wt%Si3 N4 composites exhibited lower wear rate and lower coefficient of friction when compared with matrix alloy under all the test conditions studied.

Commentary by Dr. Valentin Fuster
2009;():67-69. doi:10.1115/IJTC2009-15044.

Employing friction reduction coatings is one of the most effective methods to palliate the fretting damage. However, facing numerous available coatings, how to compare them and select the optimum one for a specific application is still a challenging task. In this paper, based on the investigation of the fretting behaviors of several bonded solid lubricant coatings, an energy approach in terms of “initial maximal dissipated energy density” was suggested to compare the tribological response of coatings. According to test results, the lifetime of each coating under different test parameters can be fitted by one master curve. The definition of this master curve for a given coating may be used for the prediction of the coating lifetime only by knowing the initial energy dissipated in the contact. The comparison of different master curves for different coatings can be employed to help the coating selection.

Commentary by Dr. Valentin Fuster
2009;():71-73. doi:10.1115/IJTC2009-15061.

Tribological characteristics of Al-Si alloy impregnated graphite composite (ALGR-MMC) in contact with bearing steel were investigated under insufficiently lubricated reciprocating sliding conditions. Ball (JIS SUJ2 bearing steel) on disk (ALGR-MMC) type wear tests were conducted under unlubricated conditions, i.e., in air with high (70%) and low (≤ 1%) relative humidity (RH), under drop-feed lubrication, i.e., oil-drops were deposed on the disk surface before commencing the tests, and under immersion lubrication, i.e., the ball-on-disk contact was submerged in a oil (SAE30) bath. Reciprocating sliding movement was maintained, under a contact load of 40 N at a frequency of 3.3 Hz and amplitude of 3.5 mm, for 100,000 cycles. Tribological properties were continuously monitored. Variation of the kinetic friction coefficient (KFC) versus the number of reciprocating cycles (NRC) was obtained for various amounts of oil (lubrication conditions) and two values of the air RH. For drop-feed lubrication the KFC has at beginning the same value as that under immersed lubrication, but if breakdown of the oil film has occurred after a certain NRC, it increases towards the value corresponding to unlubricated conditions. Connected to the oil film breakdown, there is a certain transition range, expressible both in terms of the oil amount and NRC. Minimum quantity of oil to avoid the film breakdown was found. Due to its self-lubricating properties, ALGR-MMC was able to endure adverse lubrication conditions such as insufficiently lubricated reciprocating sliding.

Commentary by Dr. Valentin Fuster
2009;():75-77. doi:10.1115/IJTC2009-15090.

Increased requirements for improved performance and reliability of mechanical systems dictate the use of new advanced materials and surface engineering techniques, which would increase efficiency and reduce energy losses. Already with a small change in surface roughness or topography lubrication mechanism can be change, thus leading to substantial improvement in tribological behaviour of contact surfaces. One of the ways of altering surface topography, known as surface texturing is by introducing small dimples or channels at the contact surface. Surface texturing is already successfully used in different applications, however, they are mainly based on trial and error approach. The aim of this investigation was to identify surface roughness parameters which could be used as a design parameter when preparing textured surfaces and to determine the effect of texturing parameters on roughness and tribological properties of boundary lubricated surfaces. Results of this investigation show that textured surfaces can be treated as an ordered roughness, with skewness and kurtosis parameters showing good correlation with the change in tribological behaviour of contact surfaces. Negative skewness and high kurtosis values, which can be achieved through wedge-shaped dimples, reduced dimple size and increased dimple spacing, were found to result in lower friction.

Topics: Lasers , Design
Commentary by Dr. Valentin Fuster
2009;():79-81. doi:10.1115/IJTC2009-15098.

The friction characteristics of lubricated contacts are usually determined by the shearing of three interfacial structural elements; namely lubricant fluid film, tribochemical reaction films and the near-surface materials. The shear behavior of the fluid film components of lubrication can be adequately described by either the hydrodynamic, or elastohydrodynamic theories and calculations. The shear behavior of the near surface materials can also be approximated by contact mechanics. However, due to the complexity and lack of adequate information, description and analysis of the tribochemical film contributions to frictional behavior is more challenging. In this study we present evaluation of the friction behavior of a lubricated contact as a function of tribological test duration, as the tribochemical surface films were being formed. Tests were conducted with two different formulated lubricants, in addition to an unformulated PAO basestock lubricant. One of the formulated lubricants consists of PAO synthetic basestock with 2.5% each of ZDDP and MoDTC additives. The other formulated lubricant is a fully formulated commercial gear oil with similar viscosity. During the friction tests in a fully flooded reciprocating sliding roller-on-flat contact configuration, the friction coefficient decreased exponentially as the tribochemical films were formed; highlighting the impact of boundary films on friction. In the test with the basestock fluid, the friction remained constant for the duration of the test; although an oxide layer was formed. Nanoindentation hardness and modulus measurements were conducted on two of the tribochemical films to measure their mechanical properties and the consequent impact on measured friction during testing.

Topics: Thin films
Commentary by Dr. Valentin Fuster
2009;():83-85. doi:10.1115/IJTC2009-15141.

Granular flows have been proposed as an alternative lubrication mechanism to conventional liquid lubricants in sliding contacts due to their ability to carry loads and accommodate surface velocities. Their load carrying capacity has been demonstrated in the experiments of Yu and Tichy [1]. Alternate lubrication techniques are becoming necessary due to the failure of conventional liquid lubricants in extreme temperature environments, and their promotion of stiction in micro-/nanoscale environments. Yet, understanding granular behavior has been difficult due to its non-linear and multiphase behavior. Cellular Automata (CA) has been shown to be a viable first order approach to modeling some complex aspects of granular flow. Previous work by the authors successfully modeled granular shear with a CA model [2]. Additional work combined CA computational efficiency with particle dynamics to effectively model collision events. This work builds upon and modifies the prior CA modeling approaches by adding friction modeling and spin of particles. This modification maintains the computational efficiency of CA, while increasing accuracy of the predicted granular flow properties, such as speed, solid fraction, and granular temperature. The current work compares the CA model with friction and spin physics relations to the authors’ prior CA model which neglected friction. Both CA models are also evaluated against experimental data to quantify the benefits of including friction and spin in the CA modeling approach for granular flows.

Commentary by Dr. Valentin Fuster
2009;():87-89. doi:10.1115/IJTC2009-15143.

The flow of solid granular material has been proposed as an alternative lubricant to conventional liquid lubricants. Since granular flows are also in numerous industrial and natural processes, they have been the subject of numerous studies. However, it has been a challenge to understand them because of their non-linear and multiphase behavior. There have been several past experiments, which have gained insight into granular flows. For example, previous work by the authors sheared grains in a two-dimensional annular shear cell by varying the velocity and roughness [1]. The present experimental work attempts to further insights from the previous work by specifically studying the interaction between rough surfaces and granular flows when the global solid fraction and grain materials are varied. A two dimensional annular (granular) shear cell, with a stationary outer ring and inner driving wheel, was used for this work. Digital particle tracking velocimetry was used to obtain local granular flow data such as velocity, local solid fraction, and granular temperature. Slip between the driving wall and first layer of granules is also extracted. This slip can be interpreted as momentum transfer or traction performance in granular systems such as wheel-terrain interaction. Parametric studies of global solid fraction and the material of the rough driving surface, attempt to show how these parameters affect the local granular flow properties.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2009;():91-93. doi:10.1115/IJTC2009-15162.

The tribological behaviour of Stellite 6, Tribaloy 700 and Tribaloy 400C have been studied during self-mated unlubricated rotational sliding between 20 to 600°C, at 3ms−1 for 1 hour using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Testing was conducted using a specially designed wear rig in which bearings and shafts of the materials were machined and tested within a furnace. Tribaloy 700 exhibited high bearing wear rates for all temperatures. Tribaloy 400C and Stellite 6 bearing wear rates generally decreased with temperature until very low wear rates were recorded at 600°C. The bearing wear rates appeared to be related to the formation of wear resistant oxide films often known as ‘glazes’. Adherent ‘glazes’ formed on Tribaloy 400C and Stellite 6 at 450°C and 600°C. However, ‘glazes’ formed on Tribaloy 700 showed poor adherence to the substrate causing high wear rates.

Commentary by Dr. Valentin Fuster
2009;():95-97. doi:10.1115/IJTC2009-15177.

From comparative dry sliding pin-on-plate experiments on distinctive WC-Co and WC-Ni cemented carbides, machined by grinding or wire-EDM, correlations are derived between wear rate, wear volume and coefficient of friction and contact load, sliding distance, microstructure and surface finish condition. The EDM induced surface modification turns out to deteriorate wear resistance, especially during the wear-in stage of sliding. These findings are in agreement with X-ray diffraction measurements of the residual stress level in the WC phase.

Commentary by Dr. Valentin Fuster
2009;():99-101. doi:10.1115/IJTC2009-15182.

Nanocrystalline diamond (NCD) offers great potential for many micro-mechanical systems, particularly where low fiction, thermal mismatch and mechanical integrity are of great importance. The reduction of grain size in NCD films to a few nanometers is, however, accompanied by a significant change in morphological, chemical, mechanical and wear properties. The evaluation of these properties is therefore necessary before designing NCD-coated micro-mechanical components for which reliability, wear and long lifetime are critical issues.

Topics: Wear , Diamond films
Commentary by Dr. Valentin Fuster
2009;():103-105. doi:10.1115/IJTC2009-15185.

Laboratory abrasive wear tests have been reported on permanent moulded toughened austempered ductile iron. The influence of austempering temperature on the abrasive wear behavior have been studied and discussed. The results indicate that with increase in austempering temperature from 300°C to 350°C, the abrasive wear resistance increased, and as the austempering temperature increased to 400°C, there was reduction in the abrasive wear resistance. These results have been interpreted based on the structural features and graphite morphology.

Topics: Wear , Nodular iron
Commentary by Dr. Valentin Fuster
2009;():107-109. doi:10.1115/IJTC2009-15186.

Erosion due to solid particles has a detrimental effect on the performance of components used in many critical applications. The problem becomes more acute at high temperatures. Thermal sprayed ceramic coatings are widely used in such situations wherein the present scenario considers better coatings. The ceramic composite coatings of alumina-zirconia were developed and their response to particle erosion and high temperature sliding wear and are experimentally evaluated. Based on a brittle erosion model, parametric influence on the variation of erosion rate was mathematically modeled adopting regression analysis and its validity is verified through a set of confirmation tests. Alumina-zirconia exhibits better performance with higher wear resistance and lesser coefficient of friction, at higher temperature, compared to plain alumina or alumina-titania.

Commentary by Dr. Valentin Fuster
2009;():111-113. doi:10.1115/IJTC2009-15187.

The friction and wear behaviors of Si3 N4 /GCr15 were studied on the pin-on-disk machine under three different lubrication conditions. The results showed that both the friction and the wear were significantly lower in glycol than in deionized water and kerosene. The steady friction coefficient was 0.02 in glycol, 0.05 in deionized water and 0.20 in kerosene. Scanning electron microscopy revealed that formation and dissolution of the film may occur in deionized water and glycol. The main wear mechanisms were tribochemical and abrasive wear in deionized water, fretting wear for tribochemical film by esterification of oxidized surface of Si3 N4 in glycol, corrosive and adhesive wear in kerosene.

Commentary by Dr. Valentin Fuster
2009;():115-117. doi:10.1115/IJTC2009-15189.

Cr-N and Cu-Cr-N coatings with Cu content between 3–65 at.%, Cu/Cr ratios in the 0.04–4.5 range and 21–27 at.% N, synthesized by twin e-beam Physical Vapor Deposition (EBPVD) at 450°C, were investigated. Using X-ray photoelectron spectroscopy (XPS), glancing angle X-ray diffraction (GAXRD) and scanning electron microscopy (SEM), in combination with nanoindentation mechanical property measurements and laboratory controlled ball-on-disc sliding experiments, it is shown that Cu-Cr-N coatings with low Cu content (3 at.%) possess sufficient wear resistance for high temperature demanding tribological applications.

Commentary by Dr. Valentin Fuster
2009;():119-121. doi:10.1115/IJTC2009-15190.

Natural refrigerants have gained significant interest due to environmental reasons. Among different natural refrigerants such as water, air, and ammonia, Carbon dioxide (CO2 ) is the main candidate to replace commonly used hydrofluorocarbon (HFC) refrigerants in air-conditioning compressors due to its environmental benefits. One of the main advantages of implementing CO2 , from an environmental point of view, lies in its relative low global warming potential (GWP) which is three orders of magnitude lower than commonly used HFC refrigerants. Coupled with the new refrigerants is also the need to use some form of protective coatings on the tribopairs. In this study unlubricated (presence of CO2 refrigerant only) experiments were performed to evaluate the tribological performance of three different polymeric-based coatings deposited on Durabar G1 gray cast iron disks. Specifically, Fluorolon 325, Impreglon 1704 PEEK, and PEEK/Ceramic/ PTFE blends were tested against 52100 hardened steel pins. Results showed lower friction coefficient and wear in the Fluorolon 325 case compared to the Impreglon 1704 PEEK and PEEK/Ceramic/PTFE coatings. These results are comparable with earlier studies using PTFE-based coatings [1].

Commentary by Dr. Valentin Fuster
2009;():123-125. doi:10.1115/IJTC2009-15215.

Explicit finite element method modeling of granular flow behavior in an annular shear cell has been studied and presented in this paper. The explicit finite element method (FEM) simulations of granular flow in an annular shear cell with around 1633 particles were performed, where the inner wheel rotated at a very high speed and the outer disk remained stationary. The material properties of the particles and the outer wheel were defined as elastic steel whereas the inner wheel was elastic aluminum. In this investigation, the explicit FEM model mimicked granular flow in an experimental set up where the inner wheel was rotated at a speed of 240 rpm. The FEM results for shearing motion and solid fraction were compared with experimental results from a granular shear cell.

Commentary by Dr. Valentin Fuster

Lubricants and Additives

2009;():127-129. doi:10.1115/IJTC2009-15049.

The tribological properties of a new unique thermo-reversible Gel-Lubricant (TR Gel-Lube) were investigated. TR Gel-Lube which includes 10–40% of amide type gel agent in base fluid, is repeatedly able to change from gel-state to liquid-state at the melting point of its gel agent. High-pressure rheological tests were performed in order to characterize the behavior of base oil and TR Gel-Lube as a function of pressure and temperature. The effect of TR Gel-Lube on ball bearings fatigue life was carried out by systematic tests using thrust ball bearings. The results of L10 life tests of TR Gel-Lube showed a longer life than the conventional greases. This result was investigated from oil film formation. It was found that gel agent played a key role in the lubricating properties. Some mechanisms such as adsorbed film formation and solid like formation are proposed.

Topics: Tribology , Lubricants
Commentary by Dr. Valentin Fuster
2009;():131-133. doi:10.1115/IJTC2009-15093.

Differential scanning calorimetry (DSC) and thin film micro oxidation test (TMOT) were employed to evaluate the antioxidation properties of tin dialkyldithiocarbamate (SnDDC) with p,p′ dioctyldiphenylamine (DODPA) antioxidant in pentaerythritol ester (PE) or polyalphaolefin synthetic lubricant (PAO), and their chemical structure were identified by fourier transform infrared spectroscopy (FTIR) analysis. DSC test shows that incipient oxidation temperature and oxidation induction time of DODPA-containing PE were improved significantly by SnDDC addition, SnDDC shows a good oxidative synergism with DODPA antioxidant. TMOT results indicates that the combination of SnDDC and DODPA in polyalphaolefin can also effectively reduce the weight lost, carbonyl peak square index and diposits of oxidized oils, which confirm that the combination of SnDDC and DODPA exhibit good synergistic antioxidation properties and deposits inhibition.

Commentary by Dr. Valentin Fuster
2009;():135-137. doi:10.1115/IJTC2009-15130.

Scuffing wear is usually used as an important indication of the initiation of lubrication failure. Components with scuffing wear should be readily replaced in order to avoid severe wear. This paper observed the change of surface structure in the scuffed area of the worn surface of oil-lubricated vanes-on-ring specimens from a microscopic and nano-scale observation and material characterization. There existed a kind of Bénard cell-like special surface shape found from the observation of the TEM image of the scuffed surface. In order to see whether there was very hot surface temperature to produce Benard cell, structure of the surface ferrous material was used as an indirect evidence of high temperature. It showed that the temperature in the scuffed area suffered a very high temperature which was over 800°C from the TEM Result. It showed that very strong fluid convection or even evaporation may occur inside the scuffed surface. The material strength of the scuffed surface would be much lower due to this high temperature. The oil pressure of that high temperature area was also very high. These factors made it possible to form the Bénard cell structure in microscale.

Commentary by Dr. Valentin Fuster
2009;():139-141. doi:10.1115/IJTC2009-15157.

Tribological film formation from fully-formulated synthetic automotive lubricants on hydrogenated DLC/steel contacts was compared with that on steel/steel contacts. Four DLC coatings containing various metal dopants (W and Si) were evaluated. Lubricants formulated with organic friction modifiers performed better on DLC/steel than on steel/steel in both sliding and rolling/sliding contacts (lower friction and wear). In contrast, lubricants formulated with MoDTC friction modifiers had higher sliding contact friction on DLC/steel than on steel/steel. No evidence was observed for tribological film formation on the DLC surfaces.

Commentary by Dr. Valentin Fuster
2009;():143-145. doi:10.1115/IJTC2009-15253.

Oil-in-water emulsions are the most widely used lubricants in metal rolling, as they provide a combination of cooling and lubricating properties. These avoid severe metal-to-metal contact by forming a separating (elastohydrodynamic) film between the metal strip and the rolls so as to prevent scuffing, limit wear and control friction.

Topics: Emulsions , Water
Commentary by Dr. Valentin Fuster

Elastohydrodynamic Lubrication

2009;():147-149. doi:10.1115/IJTC2009-15091.

The paper focuses on the solution of a numerical model to explore the journal bearing performance under transient thermal elastohydrodynamic lubrication with non-Newtonian lubricants based on Carreau viscosity model. The newly derived time-dependent modified Reynolds equation and the adiabatic energy equation have been formulated using a non-Newtonian Carreau viscosity model. The simultaneous systems consisting of the modified Reynolds equation, elasticity equation and energy equation with initial conditions were solved numerically using the multi-grid multi-level method with full approximation technique. The analysis showed that the fluid characteristics as defined by the Carreau model, led to large differences in minimum film thickness and maximum temperature rise for bearing liners with low elastic modulus.

Commentary by Dr. Valentin Fuster
2009;():151-153. doi:10.1115/IJTC2009-15094.

The aim of this study is to develop an interferometric technique to measure micron level clearances occurring in small piston-cylinder arrangements; such as hermetically sealed compressors. A Cartesian model of the piston cylinder arrangement is manufactured and lateral and vertical motions are generated via a step motor, and micrometers, respectively, to simulate the piston motion and the change of clearance during the motion. Experiments are conducted in air and a lubricant. Measurements show the important effect of surface waviness of the mirror and the effect of the motion of the carriage at micron level measurements.

Commentary by Dr. Valentin Fuster
2009;():155-157. doi:10.1115/IJTC2009-15101.

Refined solutions of thermal lubrication problems generally require fine mesh and many iteration steps. To solve these difficulties, Elrod and Brewe [1–2] proposed an efficient algorithm based on the use of Lobatto point quadrature. Within this approach, the unknown temperature across the film is written in a series of Legendre polynomials. Over the years, the deterministic EHL approach has been widely applied, however, the probabilistic EHL methodology is still used when the main interest of the engineer is directed toward computations of bulk properties. This paper presents an application of the Lobatto point quadrature within a probabilistic EHL algorithm for the computation of the load supported by the fluid, the elastically deformed asperities and the plastically deformed asperities in a mixed EHL contact with either isotropic of non-isotropic roughness.

Commentary by Dr. Valentin Fuster
2009;():159-161. doi:10.1115/IJTC2009-15110.

EHL entrapments are known to be highly dependent on the lubricant viscous characteristics. One such property is fragility, whose significant effect on entrapments we investigate here for the first time. We employ both experimental measurements and a numerical transient EHL simulation. We find that fragility is an extremely important property and, at least for low viscosity lubricants, must be included in high pressure EHL calculations for accurate prediction of physical behavior.

Commentary by Dr. Valentin Fuster

Hydrodynamic Lubrication and Fluid Film Bearings

2009;():163-165. doi:10.1115/IJTC2009-15023.

Ingestion and entrainment of air into the Squeeze Film Dampers (SFDs) with open end result in a bubbly mixture that affects their damping capacity. The industrial applications demand dampers with low pressure supply and operating high speed, it means, it exists a necessity of propose a model to predict the air ingestion in this kind of bearing. Diaz and San Andrés reported results of an extend research to quantify the effect bubbly mixture in the performance of the SFDs. They also advanced an analytic model for short infinity bearings (L/D = 0) which estimate the air ingestion as function of non-dimensional number named by them feed squeeze flow number. Méndez et al. advanced the understanding of the effect air entrainment. They propose a new dimensionless map to estimate the air volume fraction in finite length bearings. Although, their results are immediate applicability, they do not have been validated. The present work looks for confirm the results presented by Méndez et al. To achieve this, it is solved a compressible Reynold equation with a different numerical method. The results indicate that is necessary another dimensionless number to estimate the entrapment air and the dimensionless map proposed by Méndez et al. is valid only to bearings with an eccentricity-clearing ratio of 0.5248.

Topics: Dampers
Commentary by Dr. Valentin Fuster
2009;():167-169. doi:10.1115/IJTC2009-15056.

A numerical study examined the combined effects of journal misalignment and hydrodynamic lubrication on the stress fields of bearing liner under steady state conditions. The oil pressure, obtained by solving Reynolds equation, is imposed on a finite element model of an elastic liner bearing to calculate its stress fields. It was found that large degree of misalignment increases remarkably the oil pressure, and consequently the stresses in the bearing liner become significantly higher.

Commentary by Dr. Valentin Fuster
2009;():171-173. doi:10.1115/IJTC2009-15057.

The effect of angular misalignment of plain journal bearings on the oil whirl and oil whip phenomena was investigated using a test rig comprising a flexible one inch diameter shaft supported on two cylindrical fluid film bearings. A matrix of experiments was designed where a precisely controlled angular misalignment between the shaft and the drive end, the non-drive end, or both bearings was induced. In each case we measure the onset of oil whirl and/or the onset of oil whip and the maximum speed that the test rig can reach before reaching a maximum vibration displacement of 1 mm at the location of the displacement sensor. The results show clearly that the angular misalignment of the bearings with respect to the shaft help delay the onset of oil whip from 3200 rpm to 4500 rpm in some cases. An interesting and new observation was the effect of the configuration of the misalignment on the overall stability characteristics. In particular, when the two bearings were misaligned in the same direction such that the two misaligned bearing planes were parallel to each other the onset of instability was more delayed and the overall operation was smoother.

Commentary by Dr. Valentin Fuster
2009;():175-178. doi:10.1115/IJTC2009-15066.

This article deals with the finite differential method of static performance of a foil journal gas bearing. A foil bearing is a self acting hydrodynamic device, which separates stationary and rotating components of high speed rotating machinery by a fluid film of air or other gaseous lubricant. The present work concentrates on common approach in foil bearing in calculating the carrying capacity for a given shaft position (figure-1). During this work the external load is fixed and related shaft position is investigated. For steady operating characteristics such as minimum film thickness and load capacity predicted for the foil bearing. The system of governing equation is solved numerically with FDM by a computer program written in the MATLAB computing environment. A generalized hydrodynamic analysis is conducted to systematically analyses the effect like bearing speed is discussed.

Commentary by Dr. Valentin Fuster
2009;():179-181. doi:10.1115/IJTC2009-15068.

Water lubricated bearings are often installed on new build and modernized ships. The main reasons are unit simplicity, no danger of pollution and low price.

Topics: Bearings , Water
Commentary by Dr. Valentin Fuster
2009;():183-185. doi:10.1115/IJTC2009-15070.

A powerful computational approach was developed for a complex rotor-bearing system to analyze the elasto-hydrodynamic lubrication (EHL) using computational fluid dynamics (CFD) and fluid solid interaction (FSI) techniques. To investigate the interaction of the dynamics and elastic deformation of the shaft and the lubrication of the journal bearing, a pure fluid bearing model and a rotor-bearing FSI model were created. The shaft was modelled as rigid and elastic respectively. Three different boundary conditions: Sommerfeld, Gümbel and cavitation, were employed and compared in this study. The cavitation boundary was implemented based on the phase change caused by the pressure change between a liquid phase and a vapor phase. The load applied on the model was in the vertical direction, with a rotational velocity, representative of real working conditions of an experiment of a marine journal bearing. The results for the hydrodynamic case were compared with an in-house lubrication code based on Reynolds equation and the Reynolds boundary condition, and showed that the phase change method was adequate to deal with the cavitation problem of a rotor-bearing system. FSI was shown to be a powerful tool for the investigation of the hydrodynamic and elasto-hydrodynamic lubrication of a rotor-bearing system.

Commentary by Dr. Valentin Fuster
2009;():187-188. doi:10.1115/IJTC2009-15082.

In this study, analytical evaluation of dynamic characteristics of the mechanical coupling and its effect on the coupled rotor-bearing system are presented. The results indicate that the oil film bearing supported rotor system may successfully be coupled by the flexible coupling.

Topics: Bearings , Rotors
Commentary by Dr. Valentin Fuster
2009;():189-191. doi:10.1115/IJTC2009-15084.

Air foil thrust bearings are the critical component available on high-efficiency turbomachinery which needs ability to endure the large axial force. Previous investigations about the static characteristics were obtained over the region of the thin air film using finite-difference method and the characteristics of the corrugated bump foil using finite-element method. Moreover, a recent study demonstrated that bearing performance is sensitive to tilting thrust pad condition. In this study, experimentally measured bearing static characteristics are compared with the numerical model of the foil thrust bearing considering tilting pad condition. Three geometrically different type foil bearings were tested to measure their load capacity under tilting conditions that have continuous angles from zero to 0.0002 rad. These data are presented for use i1 n the development of more accurate foil thrust bearing numerical models.

Topics: Thrust bearings
Commentary by Dr. Valentin Fuster
2009;():193-195. doi:10.1115/IJTC2009-15128.

The purpose of this paper is to present the experimental evaluation of the stiffness and damping coefficients of a pair of elastic rings used to support a fluid film bearing into a turbojet engine body. The experimental rig, the method of analysis and the experimental data are presented.

Topics: Bearings , Fluid films
Commentary by Dr. Valentin Fuster
2009;():197-199. doi:10.1115/IJTC2009-15132.

Squeeze film dampers (SFD) are devices utilized to control vibrations of the shafts of high-speed rotating machinery. A dual squeeze film damper (DSFD) consists of two squeeze film bearings that are separated by a sleeve, which is released when the rotor experiences abnormal operating conditions. In this part of our study of DSFD we analyze the case when both the inner and the outer oil films are active. Previous studies utilized closed form analytical expressions to describe the forces within the lubricant. In this paper the oil forces are modeled using pressure distributions obtained from numerical solutions of the Reynolds equation. Numerical results are compared with the experimental data.

Commentary by Dr. Valentin Fuster
2009;():201-203. doi:10.1115/IJTC2009-15144.

This study focuses on the thermohydrodynamic lubrication analysis of fluid film bearings with step on the surface such as a Rayleigh step bearing. In general, the Reynolds equation does not satisfy the continuity of the fluid velocity components at steps. This discontinuity results in the difficulty to solve the energy equation for the lubricants, because the energy equation needs the velocity components explicitly. The author has solved this problem by introducing the equivalent clearance height and the equivalent gradient of clearance height at steps. These parameters remove the discontinuity of velocity components and the energy equation as well, so that one can solve these equations on all of the bearing surfaces including the step region by finite differential method (FDM). The numerical results of pressure and temperature distributions by the proposed method for a Rayleigh step bearing were compared with the results obtained by a commercial CFD package. These results showed good agreement with each other. This method is extended to 2D unequal grid problems.

Commentary by Dr. Valentin Fuster
2009;():205-207. doi:10.1115/IJTC2009-15151.

Numerical analysis of the dynamic characteristics of a magnetic head with ultra-thin spacing is presented. The dynamic response of the air film is analyzed with the momentum equations and the time-dependent modified Reynolds equation based on the FK-Boltzmann modified model. The pressure distribution is numerically calculated by using an up-wind scheme for a very large bearing number with the finite difference method, and a harmonic average method is used for calculating the flux factor with abrupt change of film thickness. Then, the dynamic characteristics are analyzed in time domain. The numerical results show that under an external motivation the significant vibration is produced in the height direction and in the pitch direction, but the slider is capable of returning to the equilibrium position very quickly.

Commentary by Dr. Valentin Fuster
2009;():209-211. doi:10.1115/IJTC2009-15188.

Demonstrated gas foil bearing (GFB) operation at high temperature is of interest for gas turbine applications. The effects of (high) shaft temperature on the structural stiffness and damping parameters of a foil bearing must be assessed experimentally. Presently, a hollow shaft warmed by an electric heater holds a floating 2nd generation FB that is loaded dynamically by an electromagnetic shaker. In tests with the shaft temperature up to 184°C, the measurements of dynamic load and ensuing FB deflection render the bearing structural parameters, stiffness and damping, as a function of excitation frequency and amplitude of motion. The identified FB stiffness and viscous damping coefficients increase with shaft temperature due to a reduction in the FB clearance. The bearing material structural loss factor, best representing mechanical energy dissipation, decreases slightly with shaft temperature while increasing with excitation frequency.

Commentary by Dr. Valentin Fuster

Machine Components Tribology (Rolling Element Bearings; Engine Tribology)

2009;():213-215. doi:10.1115/IJTC2009-15027.

The elimination of sub-synchronous vibration is a major task of rotating machinery engineers. The industry has used applied imbalance to improve stability of vertical pumps which would otherwise be totally unstable. The current interest is for the application of a small imbalance to determine the influence on the level of instability frequency components for a small high speed turbocharger rotor. The initial experimental results for the application of a known imbalance on the compressor end of a high speed turbocharger, indicates a reduced level for the lower instability mode.

Topics: Vibration
Commentary by Dr. Valentin Fuster
2009;():217-219. doi:10.1115/IJTC2009-15032.

A logarithmic profile is an essentially optimal geometry for rolling machine elements such as bearing rollers and raceways. Under most conditions of loading, it yields less stresses to give longer endurance. Lundberg first suggested the basic profile, and some researchers followed him by modifying it to satisfy engineering requirements. In this paper, the authors propose a mathematical optimization method for various profiles including a logarithmic one in roller bearing applications. Moreover, rolling contact fatigue life tests are carried out to make a comparison among logarithmically-crowned, standard partially-crowned and modified partially-crowned rollers. Results show that the logarithmic and modified partially crowned rollers are comparable in fatigue life, although the logarithmic rollers require less working effort to process the crowning.

Commentary by Dr. Valentin Fuster
2009;():221-223. doi:10.1115/IJTC2009-15037.

Some types of mating parts, such as exhaust valves and seats in diesel engines, must perform at elevated temperatures and in oxidizing environments, while at the same time resisting the effects of repetitive impacts and interfacial slip. The wear that takes place under such situations is the net result of a complex process that involves plastic deformation, tangential shear, and oxidation. Tribolayers form, are removed, and reform. An apparatus was designed to simulate the key aspects of elevated temperature wear of exhaust valves. Three degrees of motion: impact, slip, and rotation were taken into account. Two different test geometries were developed: (a) inclined cylindrical pins striking rounded corners, and (b) actual exhaust valve sealing surfaces striking the edges of flat blocks. The features of the high-temperature repetitive impact device are described, and examples of the wear scars produced by the two test configurations are presented.

Commentary by Dr. Valentin Fuster
2009;():225-227. doi:10.1115/IJTC2009-15055.

Nickel-titanium (Ni-Ti) alloys are normally associated with applications that exploit their shape memory characteristics. When properly formulated and processed, however, Ni-Ti is dimensionally stable and offers a unique blend of attributes suitable for tribological applications. This paper demonstrates that Ni-Ti alloys are promising candidate structural materials for oil-lubricated rolling and sliding contact applications such as bearings and gears. Ni-Ti alloys offer a broad combination of physical properties that make it unique among bearing materials. Ni-Ti alloys are hard, electrically conductive, highly corrosion resistant, readily machined prior to final heat treatment, non-galling and non-magnetic. No other bearing alloy, metallic or ceramic encompasses all of these attributes. Spiral orbit tribometer (SOT) tests were conducted using 60NiTi balls loaded between rotating 440C disks, lubricated with synthetic oil and tested in a vacuum. Excellent tribological performance (life and friction) was observed and lubricant degradation normally associated with titanium alloys was absent. It is anticipated that the Ni-Ti family of alloys may form the basis of an entirely new class of bearing and component materials.

Commentary by Dr. Valentin Fuster
2009;():229-231. doi:10.1115/IJTC2009-15089.

Highly localized tribological contacts can modify the surface texture of rolling element bearing surfaces. The evolution of the surface texture generates modified subsurface stress fields within the contact and subsequently may alter the bearing rolling contact fatigue (RCF) life. In this present study, relationships between bearing surface topography and subsurface stress states are examined for their effects on bearing RCF life in the limiting case of boundary or dry contact. Results show that seemingly minor changes to surface topography may influence the stress distribution enough to affect RCF life under heavily loaded conditions.

Commentary by Dr. Valentin Fuster
2009;():233-235. doi:10.1115/IJTC2009-15096.

Machine components in the fuel cell vehicle and related hydrogen infrastructures are operating within high pressure hydrogen gas. Especially, polymer seals used in gas compressors and regulator valves should be articulating against their metal counter face in pressurized hydrogen gas. However, the effect of high pressure hydrogen gas on tribological behavior of sliding surfaces has not been identified yet. In this study, effects of the pressurized hydrogen gas environment on wear behavior of polymeric sealing materials were examined by exposing polymer specimens and their sliding counterface to the high pressure hydrogen gas prior to the wear test. Unfilled polytetrafluoroethylene (PTFE) and 15% graphite filled PTFE were tested as representative polymer sealing materials and 316L austenitic stainless steel was used as a sliding counterface. Results of X-ray photoelectron spectrometer (XPS) analysis of the exposed stainless surface indicated that metal oxides in the surface passive layer of 316L stainless steel could be reduced to some extent by high pressure hydrogen. Increased metal contents of the stainless surface enhanced the development of polymer transfer film and consequently lower the specific wear rate of PTFE and PTFE composites.

Commentary by Dr. Valentin Fuster
2009;():237-239. doi:10.1115/IJTC2009-15097.

It is well known that preloaded ball screws with double nut produced hysteretic motion between the rotational angle of the screw shaft and linear motion of the nut, which is called “lost motion”, causes positioning error of ball screws. Preloaded ball screws with a double nut have the characteristic that the loaded balls wedge into a right angle direction to a ball rolling direction because the directions of screw shaft and nut raceways differ from the ball rolling direction. Loaded balls of preloaded ball screws with a double nut have two contact points geometrically. When the ball screw is operated under the condition of screw shaft turning and nut stationary, the wedging forces of balls are produced by differences between the screw shaft and nut raceways, and then loaded balls wedge into nut grooves. When the ball wedging motion is significantly increased, loaded balls make contact with another flank of the nut groove, that is, three points contact occur. In the present study, the authors deal with the positioning errors of the X-table which is composed of a preloaded precision ball screw and a couple of linear air bearings, and the ball wedging behaviors. The ball wedging behavior is quantified by the variation of the distance from ball surface to gap sensors which are inserted into several holes. These hole passing through the top of Gothic arch groove are bored in the perpendicular to the outer surface of the cylindrical nut.

Topics: Motion , Screws , Mechanisms
Commentary by Dr. Valentin Fuster
2009;():241-243. doi:10.1115/IJTC2009-15107.

Fretting wear is a complex phenomenon that occurs at component interfaces that are subjected to low amplitude oscillation under high contact pressure. In turbomachinery fretting occurs also at the blade tip interfaces where shrouds, that have the aim to reduce the blade resonant vibration amplitude, are machined. To diminish the fretting damage coatings are applied to the blade tips. The aim of this study is to compare the fretting wear behaviour of single crystal CMSX-4 superalloy interfaces with and without plasma sprayed T-800 coating. Experiments have been conducted with hemispherical surface in contact with a flat surface of the same materials at temperature of 800 °C. The hysteresis cycles have been measured through the experiment. The comparison of the hysteresis cycles shown that the tangential contact stiffness of the coated surfaces is greater then that of the surfaces without coating. At the end of wear process, the mating surfaces have been characterized by three-dimensional optical interferometry and SEM analysis. After 10×106 wear cycles, the uncoated surfaces show a large change in the contact parameters and fretting cracks on the flat surface. On the other hand, the coated surfaces do not shows a measurable change in the contact parameters while the coating damage on the flat surface leads to predict an incipient catastrophic wear.

Commentary by Dr. Valentin Fuster
2009;():245-247. doi:10.1115/IJTC2009-15112.

A white layer on a machined surface is often produced at abusive machining conditions. However, the effect of white layer on frictional and wear performance has received little attention. This study has shown that the existence of a turned white layer slightly decreases the coefficient of friction (COF), while a ground white layer significantly increases COF at dry conditions. At lubricated conditions, the turned white layer only slightly increases COF while the ground white layer slightly reduces it. The third body wear debris may act as solid lubricants leading to reduced friction.

Topics: Wear , Machining
Commentary by Dr. Valentin Fuster
2009;():249-251. doi:10.1115/IJTC2009-15158.

A detailed bondgraph model of rolling element bearings with direct correspondence between parameters of the model and physical system components was developed as multibody systems, with vector bondgraphs to represent dynamics. The model incorporates gyroscopic and centrifugal effects, contact elastic deflections and forces, contact slip, and bearing faults. The model clearly defines how a healthy or faulty bearing behaves. Dents and pits are modeled through changes in surface profiles. Simulation results in form of time/frequency domain data, orbit plots and Poincare maps show the effectiveness of the multi-DOF bearing model in fault detection.

Commentary by Dr. Valentin Fuster
2009;():253-255. doi:10.1115/IJTC2009-15166.

The end effect attenuation in finite length line contacts is mainly approached for cylindrical bodies. Multi-radius crowning may remove end effects in tapered roller contacts. Another method for leveling maximum pressure in these contacts is the use of polynomial generatrix. This paper investigates the effect of this generatrix in tapered roller contacts. An improved pressure distribution is obtained. This has a nearly flat maximum value along most of contact length.

Topics: Rollers
Commentary by Dr. Valentin Fuster
2009;():257-259. doi:10.1115/IJTC2009-15193.

A physics-based model is proposed to predict load dependent (mechanical) power loss of spur gear pairs by using a specialized gear elastohydrodynamic lubrication (EHL) model. The EHL model includes time variations of all key contact parameters such as surface velocities, radii of curvature and normal load in their continuous forms such that a continuous analysis of a tooth contact from its root to tip can be performed. The EHL model has the capability to simulate any gear contacts represented by condition ranging from full EHL to mixed or boundary EHL conditions. Predicted transient pressure and film thickness distributions are used to determine the instantaneous as well as the overall mechanical power loss of the gear mesh. Correction factors are introduced to account for thermal effects. At the end, capabilities and accuracy of the proposed model are demonstrated by comparing its predictions to experimental data.

Topics: Gears , Spur gears
Commentary by Dr. Valentin Fuster
2009;():261-263. doi:10.1115/IJTC2009-15201.

The effect of inorganic fullerene-like (IF) WS2 nanoparticles added to mineral oil which has a potential to reduce engine friction was investigated using a reciprocating piston ring - cylinder bore bench test system. The test system simulates actual engine conditions controlling oil temperature, speed, and normal load. Frictional characteristics of various IF-WS2 nanoparticle concentrations in mineral oil were experimentally analyzed. After a certain period of running in mixed lubrication regime, a significant reduction in friction coefficient was observed when the IF-WS2 formulated oil was used. This reduction remains to some extent with the reference mineral oil even after solvent cleaning. The results show that a thin tribofilm gradually forms on the piston ring and the cylinder bore surfaces that reduce the coefficient of friction in mixed lubrication regime. The effects of lubrication regime, load, speed, temperature and surface roughness on tribofilm formation were also discussed.

Commentary by Dr. Valentin Fuster
2009;():265-267. doi:10.1115/IJTC2009-15209.

As conventional refrigerants phased out, Hydrocarbons such as Isobutane emerged as the main alternative in refrigeration industry. However, compatibility of these new ozone friendly refrigerants with compressor oils and bearing materials need to be evaluated. This study investigates wear performance of Titanium and Niobium added MoS2 coatings running against 100Cr6 steel under isobutane exposure. Wear performance is established under starved lubrication condition by comparing the maximum achieved sliding distance before the sudden increase and fluctuation of the friction curve. Sinter steel samples were coated with MoS2 , MoS2 -Ti, MoS2 -Nb thin films by magnetron sputtering. The concept of durability distance has been defined as the distance until a sudden fluctuation of the friction curve is observed. It was found that addition of Titanium and Niobium adversely affects wear performance of the MoS2 coatings under isobutane exposure. This can be attributed to the possible degradation of MoS2 lamellar structure that acts as solid lubricant under such exposure.

Topics: Wear , Coatings , Titanium
Commentary by Dr. Valentin Fuster
2009;():269-271. doi:10.1115/IJTC2009-15252.

In this paper we present a coupled method for modelling fluid-solid interaction within a crack generated in rolling contact fatigue (RCF) in the presence of lubrication. The technique describes the fluid flow in the contact area and within the crack and explores how this affects the elastic deformation of the solid while the moving load traverses the cracked region. It is argued that this approach sheds light on the instantaneous response of the system, therefore providing a more physically-accurate description of the phenomenon under investigation.

Commentary by Dr. Valentin Fuster

Contact Mechanics

2009;():273-275. doi:10.1115/IJTC2009-15009.

Contact problem of the frictionless indentation of elastic half-space by smooth rigid punch of curved profile is investigated. An exact expression of the contact pressure distribution for a curved profile punch in terms of integral involving the pressure distribution for sequence of flat punches is derived. The method is illustrated and validated by comparison with some well-known analytical solutions.

Commentary by Dr. Valentin Fuster
2009;():277-279. doi:10.1115/IJTC2009-15018.

High friction/stiction caused by adhesive force is among the main issues in devices (ranging from macro scale down to mirco/nano scale) having contacting interfaces with relative motion. Relevant parameters, i.e., surface roughness, meniscus and viscous forces due to the presence of liquid films, need to be studied to provide a fundamental understanding of the physics of the experienced problems. The simulations of separation of two surfaces from micro menisci are performed. The roles of the involved forces which contribute to the adhesive force are examined.

Commentary by Dr. Valentin Fuster
2009;():281-283. doi:10.1115/IJTC2009-15031.

The contact area, friction force and relative displacement evolution at the very early stage of fretting are investigated experimentally. Copper and steel spheres of various diameters are loaded against a hard sapphire flat by a range of normal loads deep into the elastic-plastic regime of deformation. A reciprocating tangential loading is then applied with a maximum loading below the static friction to avoid gross slip. Real-time and in situ direct measurements of the contact area, along with accurate measurements of the friction force and relative displacement, reveal substantial junction growth and energy dissipation mainly in the first loading cycle. The so called “slip amplitude” is found to be attributed to residual tangential plastic deformation rather than to interfacial slip. Elastic shake-down is observed for the 2.5% hardening steel spheres while plastic shake-down is observed in the case of the elastic perfectly plastic copper spheres.

Commentary by Dr. Valentin Fuster
2009;():285-287. doi:10.1115/IJTC2009-15043.

This paper presents numerical and experimental studies of nanoindentation of a silicon substrate covered by patterned Ni nanodot asperities. A multi-asperity contact model was developed in this study to simulate the contact between a spherical indenter and the Ni nanodot asperities. In this model, the silicon substrate is considered to be deformable and the nanodots are allowed to interact with each other through the deformation of the substrate. The load-deformation relationship predicted by the model was found to be in good agreement with the experimental results. This model can also be used to predict indentation load-deformation relationships of a deformable substrate covered by nanodots with known size and location distributions, but not necessarily following a repetitive pattern.

Topics: Nanoindentation
Commentary by Dr. Valentin Fuster
2009;():289-291. doi:10.1115/IJTC2009-15051.

Coupled systems with frictional interface subjected to periodic loading may lead to either shakedown or cyclic slip depending on the initial condition. In this paper, we explore the phenomena in the context of the simplest coupled system involving two contact nodes, and findings show the possibility that a reduced form of Melan’s theorem might apply to such systems under suitable restrictions on the loading factor.

Commentary by Dr. Valentin Fuster
2009;():293-295. doi:10.1115/IJTC2009-15052.

The coefficient of restitution for the impact of a compound pendulum with a flat surface is an important quantitative measure in contact analysis. The impact is analyzed for different lengths of the pendulum, different angles of impact, and different initial angular velocities of the pendulum. The impact with friction is studied using an elasto plastic force developed by Jackson and Green for the three phases of impact: elastic compression, elasto-plastic compression, and elastic restitution phase.

Topics: Force , Pendulums
Commentary by Dr. Valentin Fuster
2009;():297-299. doi:10.1115/IJTC2009-15054.

The Cattaneo-Mindlin concept of interfacial slip in tangentially loaded compliant bodies is revisited and its basic simplifying assumptions are critically examined. It is shown that these assumptions, which, in the absence of modern numerical techniques, were essential in 1949 to enable an elegant quantitative solution of the basic problem of presliding between contacting bodies, are actually non physical. An alternative approach to the same problem that is based on treating sliding inception as a failure mode involving material plastic yield is discussed. This alternative approach was suggested even before 1949 but for the same lack of modern numerical techniques could only be promoted qualitatively. Some recent theoretical models, that are based on this earlier alternative approach, and in which the simplifying assumptions of the Cattaneo-Mindlin concept were completely relaxed are described along with their experimental verification. It is shown that the pre-sliding problem between contacting bodies can be accurately solved by these models using realistic physical assumptions and failure criterion.

Topics: Failure
Commentary by Dr. Valentin Fuster
2009;():301-303. doi:10.1115/IJTC2009-15099.

This paper investigates some of the modeling techniques available for predicting the real area of contact between two surfaces. These models are then applied to an idealized case of a rubber block in contact with a rough surface representing a tire on the road. A description of how the models work is presented. The various contact models are compared and analyzed. Qualitatively the models compare very well. Several models also compare well quantitatively.

Topics: Roads , Tires
Commentary by Dr. Valentin Fuster
2009;():305-307. doi:10.1115/IJTC2009-15104.

This work presents a combination of two dimensional (2D) and three dimensional (3D) finite element analysis (FEA) of structural and electrical contact between two nonconforming hemispheres at various vertical interferences. Items of particular interest include contact forces, current densities, and magnetic forces. The results are normalized to be applicable to micro and macro-scaled contact models. To test the validity of the analysis, the results are compared to another work focusing on contact between a hemisphere and rigid flat.

Commentary by Dr. Valentin Fuster
2009;():309-311. doi:10.1115/IJTC2009-15106.

For practicing engineers in industry it is important to have closed-form, easy to use equations that can be used to predict the real contact area, and relate it to friction, wear, adhesion, and electrical and thermal contact resistance. There are quite a few such models in the literature, but their agreement or their effectiveness has not been determined. This work will use several measured surface profiles to make predictions of contact area and contact force from many elastic contact models and compare them to a deterministic FFT based rough surface contact model. The results show that several of the models show good quantitative and qualitative agreement despite having very different mathematical foundations.

Commentary by Dr. Valentin Fuster
2009;():313-315. doi:10.1115/IJTC2009-15109.

Surface asperities can range widely in size. Therefore it is important to characterize the effect of size and scale on the contact mechanics. This work presents a molecular model of asperity contact in order to characterize small scale asperity contact. The effects of lattice orientation and radius (number of particles) are examined. It appears that lattice orientation has a noticeable effect, but nominally may not be important in the contact of asperities and rough surfaces. The size appears to be important until a certain minimum size is achieved.

Commentary by Dr. Valentin Fuster
2009;():317-319. doi:10.1115/IJTC2009-15115.

An improved rough surface contact model is proposed accounting for bulk substrate deformation and asperity interaction. The asperity contact stiffness is based on Hertzian solution for spherical contact, and the bulk substrate stiffness on the solution of Hertzian pressure on a circular region of the elastic half-space. The contact behavior of a single asperity composed of hemi-spherical asperity deformation as well as bulk substrate deformation is calculated by introducing the concept of spring-in-series. Based on the single asperity model, the contact stiffness for the rough surface is calculated including the effect of asperity interaction. Analytical simulation results using the proposed rough surface contact model were compared with the CEB model and experimental measurements.

Topics: Deformation
Commentary by Dr. Valentin Fuster
2009;():321-323. doi:10.1115/IJTC2009-15121.

A generalized abrasive wear model of a three-dimensional rough (fractal) surface sliding against a relatively softer material is presented. The model is based on a slip-line field of a rigid spherical asperity (or spherical wear particle) that plows through a soft surface, resulting in material removal by a microcutting process. The analysis yields a relationship of the abrasive wear coefficient in terms of the interfacial adhesion characteristics of the interacting surfaces, topography (fractal) parameters, elastic-plastic material properties, and applied normal load. Numerical results illustrate the effects of surface roughness and interfacial adhesion (lubrication effect) on the abrasive wear coefficient of fractal surfaces.

Topics: Plasticity , Wear , Fractals
Commentary by Dr. Valentin Fuster
2009;():325-327. doi:10.1115/IJTC2009-15122.

A finite element analysis of the indentation of an elastic-perfectly plastic half-space by a rigid sphere was performed for a wide range of material properties. The post-yield deformation behavior was found to consist of four deformation regimes, namely linear elastic-plastic, non-linear elastic-plastic, transient fully-plastic, and steady-state fully-plastic deformation. The boundaries of these deformation regimes were determined numerically in terms of elastic-plastic material properties. The deformation behavior in different regimes was examined in the context of finite element results showing the evolution of subsurface plasticity for different material properties.

Commentary by Dr. Valentin Fuster
2009;():329-331. doi:10.1115/IJTC2009-15123.

In order to elucidate contact and friction characteristics of rubbers, numerical analysis of asperity contact mechanics of a rubber piece with a smooth glass plate was carried out on the basis of an asperity contact model that considers van der Waal’s (vdW) pressure. First, by ignoring vdW pressure and the elastic deformation of the mean height surface, asperity contact characteristics were analyzed using the measured Young’s modulus, and surface parameter values that could yield the measured contact area were estimated. Next, asperity contact characteristics were analyzed by considering the vdW pressure and elastic deformation of a rough sphere that is a model of a large-scale asperity having small-scale asperities. It was found that the actual contact area was similar to the measured contact area; this result could not be obtained without assuming an rms asperity height of ∼0.1 μm for the small-scale asperities. It was also found that the friction coefficient decreased with an increase in the applied pressure in the cases where the friction force is proportional to the real area of contact and to the real internal contact pressure.

Commentary by Dr. Valentin Fuster
2009;():333-335. doi:10.1115/IJTC2009-15126.

The elastic-plastic contact between a spherical shell and a rigid flat is analyzed using finite element analysis. The effect of spherical shell geometry and material properties on the onset of plastic deformation is determined by finding the critical normal load and the critical interference which correspond to the onset of plastic yielding. The location of initial plastic deformation is identified as a function of material properties and spherical shell geometry. This work provides the dimensionless critical load as a function of the shell parameter λ = (t/R)*(E/Y), where t, R, E and Y are the spherical shell thickness, radius, Young’s modulus and yield strength, respectively. The numerical results are compared with the solid sphere model based on Hertz’s contact theory and classical shell theory in the elastic regime. The behavior of a spherical shell contacting a rigid flat plate is a strong function of the shell parameter λ.

Commentary by Dr. Valentin Fuster
2009;():337-339. doi:10.1115/IJTC2009-15138.

This paper presents the study of a stringer of inhomogeneities in an isotropic half space under prescribed load. It shows that the presence of stiff inhomogeneities causes the increase in the stresses in the regions where they are formed. Stress concentration generally occurs within the inhomogeneities, particularly near their edges or corners, and increases as the location of the inhomogeneities approaches the material surface and as the friction force increases.

Commentary by Dr. Valentin Fuster
2009;():341-342. doi:10.1115/IJTC2009-15156.

The formulation and modelling of tribological contacts is useful for lifetime predictions, in particular in power transmission and aeronautical systems. For this reason, contact problems are largely studied by both analytical and finite element (FE) methods. Analytical methods have been recently used for hardening problems, but are still limited to homogeneous bodies. This paper presents an elastic-plastic approach for inhomogeneities and damage using a fully-coupled contact code.

Commentary by Dr. Valentin Fuster
2009;():343-345. doi:10.1115/IJTC2009-15165.

Half-space contact theory cannot be applied when either contacting bodies are thin or they possess small transversal dimensions. The former situation is often dealt with, but the latter seems to be neglected. This paper investigates the effect of outer radius of cylindrical bodies upon the contact stress field. The method consists in adding supplementary displacements and stresses to the half-space solution in order to fulfill the boundary conditions and the force balance equation on load direction. It is found that the half-space theory is applicable if transversal radius exceeds contact radius.

Commentary by Dr. Valentin Fuster
2009;():347-349. doi:10.1115/IJTC2009-15167.

Contact modeling of rubber bodies is based either on viscoelasticity, nonlinear elasticity or adhesion. This paper advances a simple approach to the analysis of the contacts involving incompressible, nonlinear elastic bodies and reports experimental results on rubber to justify this approach. A good agreement is found between theory and experiments.

Commentary by Dr. Valentin Fuster
2009;():351-353. doi:10.1115/IJTC2009-15168.

This paper presents the discrete counterpart of an existing continuous formulation for an elastic layer loaded symmetrically. The influence coefficients based numerical approach allows for computing contact stresses induced in the elastic layer by arbitrary shaped indenters. The newly developed code is validated against existing pressure distributions in layer contact for quadratic form punches.

Topics: Stress
Commentary by Dr. Valentin Fuster
2009;():355-357. doi:10.1115/IJTC2009-15179.

This work presents an analytical solution of a multi-scale contact model for nominally flat rough surfaces. Based on an extension of the multi-scale model of Jackson and Streator incorporating certain fractal properties, the model investigates the resolution-dependent contact area as a function of load. The progression of contact from full contact to clusters of contact regions, and from elastic to plastic asperity behavior is described in a concise, analytical formulation. Moreover, using an appropriately defined dimensionless scale number, this progression is found to be essentially independent of the roughness. The latter, however, determines how fast the progression is traversed. In a comparison with previous models, e.g. the Persson diffusion model, the present model is found to show good quantitative and qualitative agreement in spite of its more simplistic construction.

Commentary by Dr. Valentin Fuster
2009;():359-361. doi:10.1115/IJTC2009-15219.

In this paper, an analytical model for predicting the contact stress and wear distribution between a textured surface and a compliant flat is presented. The modeling formulation is based upon a two-dimensional stress analysis of the flat, and it allows the contact stress distribution to be found from the distribution of the sample deflection into the flat surface. The wear evolution was calculated from the contact stress.

Topics: Wear , Stress
Commentary by Dr. Valentin Fuster
2009;():363-365. doi:10.1115/IJTC2009-15221.

The differences in load and unload paths in an elastic-plastic contact of a rough surface and a flat leads to hysteretic energy loss. The nonlinear normal vibration response of the block is examined when subject to an external compressive load. The vibration response corresponds to the application of a constant compressive external It is shown that vibration decay rate is constant as opposed to the exponential decay rate for the linear vibrating systems. Closed form equations relating contact damping and contact natural frequency to the surface parameters are given.

Commentary by Dr. Valentin Fuster
2009;():367-369. doi:10.1115/IJTC2009-15225.

An indirect boundary element scheme is employed for the first time to solve two-dimensional, frictional contact problems in the presence of coupling between normal and tangential tractions due to material mismatch and/or geometrical characteristics of the problem under investigation. A fully incremental contact algorithm is used which accounts for changes in the contact regime and the deformed shape of the bodies. The developed algorithm is first validated against the analytical solution obtained for a rigid flat and rounded tilted punch indenting an elastic finite body. The extension of the formulation to treat surface and subsurface cracks, wear and large deformations is also discussed.

Commentary by Dr. Valentin Fuster
2009;():371-374. doi:10.1115/IJTC2009-15229.

Electrical contact resistance is important to the performance of electrical switches and other current-carrying interfaces. This study investigates the behavior of electrical contact resistance for an aluminum sphere-on-flat contact as s function of current through the interface. It is observed that the contact resistance may either increase or decrease with increasing current, depending on the current level as well as the current history. At low current levels the voltage drop across the interface increases initially with increasing current until it saturates, after which the voltage level remains constant. If the current is increased beyond the value corresponding to saturation, a subsequent decrease in current yields a corresponding decrease in voltage, so that the associated current cycle shows substantial hysteresis. However, subsequent cycles of current are reversible so long as the voltage remains below the saturation point. Such behavior suggests that irreversible morphological changes occur at the interface when the current exceeds the level associated with the attainment of voltage saturation.

Commentary by Dr. Valentin Fuster
2009;():375-377. doi:10.1115/IJTC2009-15231.

Knowledge of contact stresses generated when two surfaces are in contact play a significant role in understanding most mechanisms of friction and wear. Most of present contact models are based on the Greenwood-Williamson (GW) single asperity contact model and a statistical approach is adopted to calculate the real contact area for the entire surface based on the assumption that all the summits have uniform radius of curvatures and their heights vary randomly. But in real cases, the asperity radii vary. For a clear understanding about those aspects, a multiple asperity contact model, based on 3-D rough surface generated is analyzed using a commercial FEM package. Salient aspects of the present model are presented here and results are compared with a single asperity contact model.

Commentary by Dr. Valentin Fuster
2009;():379-381. doi:10.1115/IJTC2009-15234.

In recent years, porous rubber has been used as a tread matrix for studless tires. It is said that the pores in the tread rubber remove water between the tire and the wet road surface; however, the water removal is not sufficiently well understood. In this study, a rotating rubber specimen was rubbed against a mating prism to observe the contact surface. The friction force was also measured simultaneously with observation of contact surface. The water entering the pores was distinguished by the continuity method. As the result of these experiments, the coefficient of friction for rubber having pores on the surface was found to be larger than that of rubber without pores. Moreover, the difference in the coefficient of friction for rubber specimens with and without pores tended to be larger at lower sliding speeds. No water entered pores 3mm or less in diameter at any sliding speed in this experiment. An experiment to make the rubber specimen collide with the mating prism was conducted since actual tires seem to be deformed by the vehicle weight, such that the tire surface might contact the road collisionally. In the resulting collision experiment, the water did enter pores 3mm in diameter.

Topics: Friction , Rubber
Commentary by Dr. Valentin Fuster
2009;():383-385. doi:10.1115/IJTC2009-15237.

The wear particles generated from a tire model made of vulcanized natural rubber were analyzed using ELISA (Enzyme-Linked Immunosorbent Assay). Antibodies to 4 different allergens were used to analyze the extracted proteins from the wear particles. Latex allergens were detected when the slip ratio was large. However, no clear relationship between the latex allergen content extracted from the wear debris and either the average size or the number of wear particles was observed.

Commentary by Dr. Valentin Fuster
2009;():387-389. doi:10.1115/IJTC2009-15244.

This article presents an investigation of frictional shakedown for layered systems subjected to periodic contact loading. The term shakedown is here referred to as the possibility of developing interfacial residual stresses at the layer/substrate interface such that frictional slip, originally activated by the applied external contact load, ceases after a few loading cycles. The two-dimensional elastic problem of an elastic strip uniformly pressed against an elastic half-plane and subjected to a concentrated oscillating tangential force is studied. A quadratic programming technique which enables to resolve interfacial micro-slip adopting the distributed dislocation technique is proposed to analyse the behaviour at the layer/substrate interface. The solution to the problem is first validated against classical solutions and subsequently used to map the interfacial behaviour of layered systems undergoing different loading scenarios.

Commentary by Dr. Valentin Fuster

Magnetic Storage Tribology Track and Symposium

2009;():391-393. doi:10.1115/IJTC2009-15025.

A femto air bearing slider with thermal protrusion is studied based on three groups of surface roughness. It is observed that the air bearing force and the contact force are proportional to the value of average roughness in the minimum flying height region studied, while the intermolecular force or the electrostatic force is the smallest for the smoothest surfaces when the minimum flying height is above a certain value. As a result, the total force on the slider is the largest on the smoothest surfaces in a certain minimum flying height region. When the minimum flying height is designed in that region, the fly-ability of the slider is maximized.

Commentary by Dr. Valentin Fuster
2009;():395-397. doi:10.1115/IJTC2009-15034.

Lubricant on thin-film magnetic disks is divided into two parts. One is bonded lubricant that is fixed on disk surface and remains on disk surface after solvent rinsing. The other is mobile lubricant that can move on disk surface and is removed by solvent rinsing. It has been believed that mobile lubricant is imperative to design reliable hard disk drives. However, mobile lubricant would also be a cause of high friction between heads and disks. We started our studies on disks without mobile lubricant to find the possibility of disks with only bonded lubricant. In this paper, tribological characteristics of disks with and without mobile lubricant at the same lubricant thickness were compared using our transparent pin-on-disk wear tests. The results showed that pin wear was smaller on disks without mobile lubricant than those on disks with mobile lubricant in low load or slow speed conditions. The reason of this was considered to be the contact force increase by meniscus force due to lubricant bridge.

Commentary by Dr. Valentin Fuster
2009;():399-402. doi:10.1115/IJTC2009-15079.

Small perturbation and modal-analysis methods were employed to systematically study a damped slider’s dynamic characteristics. We found that a design with grooves distributed on a trailing pad effectively improved the slider’s damping at higher frequencies, and the damping ratio was dependent on the number of grooves, their depth, location, width, length, distribution, orientation, and types. A higher damping ratio could be obtained by optimizing these parameters. The femto slider with distributed damping grooves on a trailing pad had a higher damping ratio in the third mode, and hence, its responses to disk parallel and wavy motion were greatly reduced. This new design for the damped slider was an effective solution reducing the slider’s modulation.

Topics: Bearings , Design , Disks
Commentary by Dr. Valentin Fuster
2009;():403-405. doi:10.1115/IJTC2009-15118.

Thermal flying-height control (TFC) is now a key technology used in hard disk drives (HDD) to push the magnetic spacing to sub-5nm. The precise control of the flying height (FH) actuation is a major consideration in improving the read/write capability as well as increasing the reliability. In this paper, we investigate the response of TFC sliders to altitude change with a focus on the actuation efficiency variation with altitude. Numerical and experimental results both indicated an increase in the actuation efficiency at higher altitudes. Simulations are conducted which disclose that increased protrusion and less pushback near the transducer contribute to the efficiency increase at higher altitudes. This study is of practical importance for improving the heater and ABS designs to reduce HDD sliders’ sensitivities to altitude changes.

Commentary by Dr. Valentin Fuster
2009;():407-410. doi:10.1115/IJTC2009-15119.

Scratch-related magnetic signal degradation can occur during magnetic storage hard disk drive operation when the read-write heads contact the spinning multilayer disks. To investigate this phenomenon controlled nanoscratch experiments were performed on perpendicular magnetic recording media using various indenters of different radii of curvature. Various loading conditions were used to cause permanent scratches that were measured using atomic force microscopy. The nanoscratch experiments were simulated using finite element analysis (FEA) that included the detailed nanometer scale thin-film multilayer mechanical properties. The permanently deformed field in the sub-surface magnetic recording layer was extracted from the FEA results. The residual scratch widths measured on the surface of the magnetic storage disk were directly compared with the residual sub-surface widths of the region on the magnetic recording layer where extensive permanent lateral deformation was present. It was found that the sub-surface widths of the deformed regions were significantly larger than the surface scratch widths. Thus, sub-surface thin-film layers, such as the magnetic recording layer could be damaged without observable damage to the protective top surface carbon overcoat. The exact location and extent of damage to the magnetic recording layer depends on the scratch load, size of scratch tip, and the friction at the interface. Such permanent deformation in magnetic recording layer could lead to demagnetization, which has been reported in the literature.

Commentary by Dr. Valentin Fuster
2009;():411-413. doi:10.1115/IJTC2009-15124.

Transient thermo-mechanical contact between a sphere and a moving flat is investigated using finite element analysis. Both the sphere and the flat are treated as elastic plastic bodies with isotropic hardening. Friction is related to heat generation at the contact interface. Strain and temperature distributions of the contacting bodies are obtained. For conditions corresponding to contact between a slider and a disk in a hard disk drive, plastic deformation and high temperatures are found to occur at the interface.

Commentary by Dr. Valentin Fuster
2009;():415-417. doi:10.1115/IJTC2009-15127.

A new partial contact head disk interface (HDI) with thermal protrusion is proposed for magnetic recording with densities of 1 Tbit/in2 and above. This HDI has the advantage of maintaining light contact between the slider and the disk, so that both the bouncing vibration amplitude and the contact force are small compared with a traditional partial contact HDI. The slider’s dynamic simulations are carried out to analyze the effect of various factors within the HDI on the slider’s dynamic performance, including the friction and adhesion between the slider and the disk, the track profile morphology of the disk and the air bearing design. It is found that the bouncing vibration amplitude can be reduced to the level of the flying height modulation (FHM) of a non-contact air bearing slider without thermal protrusion.

Topics: Disks
Commentary by Dr. Valentin Fuster
2009;():419-421. doi:10.1115/IJTC2009-15137.

Slider with thermal fly height control (TFC) uses a thermal heater to produce localized thermal protrusion and adjust the vertical position of the read/write head. This paper reports authors’ efforts in exploring large protrusion stroke with minimal heater power input whilst preserving heater robustness in the TFC slider, with an optimized thermal nano-actuator design. Effects of both heater line width and line spacing on TFC slider performances are investigated. A novel ‘Stream-River’ heater design approach is proposed. Simulation results conclude that the “Stream-River” approach is of both high power-protrusion efficiency and high heater robustness.

Commentary by Dr. Valentin Fuster
2009;():423-425. doi:10.1115/IJTC2009-15172.

In this study, fundamental research on lubricant depletion due to laser heating in thermally assisted magnetic recording was conducted. In particular, the effect of lubricant film thickness on lubricant depletion was investigated. The conventional lubricant Zdol2000 was used. As a result, it was found that the lubricant depletion characteristics due to laser heating depend largely on the lubricant film thickness. In addition, it was suggested that the lubricant depletion mechanism involves the evaporation of the mobile lubricant molecules, when the maximum attained temperature is not very high. Another suggested lubricant depletion mechanism involves the thermocapillary stress effect induced by the disk surface temperature gradient resulting from the non-uniformity of the laser spot intensity distribution.

Commentary by Dr. Valentin Fuster
2009;():427-429. doi:10.1115/IJTC2009-15176.

A stochastic analysis was derived for the lapping process of magnetic recording heads. Numerical results of the final surface roughness, material removal rate, and wear coefficient are presented in terms of material properties, applied pressure, topographies of original ceramic surface and lapping plate surface, kinematics, diamond particles density and sizes of lapping plate. The estimated final roughness is in good agreement with experimental results.

Commentary by Dr. Valentin Fuster
2009;():431-433. doi:10.1115/IJTC2009-15207.

Particle contamination on a slider in a hard disk drive (HDD) affects the HDD’s reliability. With the introduction of the thermal flying-height control (TFC) slider, the temperature in the head-disk interface (HDI) becomes non-uniform, which induces a temperature-gradient dependent force on particles moving in the HDI. This paper investigates the effect of this force, the so called thermophoretic force, on a particle’s motion in the HDI as well as its effect on particle contamination on the TFC slider. By numerical simulation of the particle’s trajectory together with an analytical analysis, we show that the thermophoretic force is always negligible compared to the Saffman lift force, which points to a direction parallel to the thermophoretic force. We conclude that the current particle contamination simulator without any thermophoretic forces included would not be significantly altered by the inclusion of these forces.

Commentary by Dr. Valentin Fuster
2009;():435-437. doi:10.1115/IJTC2009-15208.

Wear particles in a hard disk drive may cause the head/disk interface to fail. We have experimentally investigated wear particle generation resulting from fretting wear between the dimple on the suspension and the gimbal spring. We have found that increasing the normal load as well as using a low friction coating reduces the formation of wear particles.

Topics: Wear
Commentary by Dr. Valentin Fuster

Manufacturing/Metalworking Tribology

2009;():439-441. doi:10.1115/IJTC2009-15071.

This study investigates the chip clogging phenomenon which occurs in dry drilling of aluminum alloys and composites, and yet, is seemingly critical to the drill life. This issue must be resolved in order to facilitate dry drilling. A highspeed dry drilling test using common twist drills was conducted with the workpiece temperature pre-heated to a wide range, room to 265 °C. The drills after testing were then examined in terms of aluminum residue that strongly adheres with the flute surfaces. A possible transition temperature range that may result in severe aluminum build-up and chip clogging was identified under the tested condition.

Commentary by Dr. Valentin Fuster
2009;():443-445. doi:10.1115/IJTC2009-15072.

In this study, nanostructured diamond (NSD) films were grown, by microwave plasma assisted chemical vapor deposition (MP-CVD) technology, on WC-Co drills for performance enhancement in dry drilling of A390 alloys. Surface cobalt was removed by a well-controlled precision etching process. H2 /CH4 gas mixture with a small amount of N2 was applied to produce NSD films. Moreover, slight hone was applied to the cutting edges prior to the NSD deposition to relieve residual stresses generated by thermal mismatch in depositions. The results show feasibility of dry drilling of A390 alloys by NSD coated drills, substantially outperforming carbide drills.

Commentary by Dr. Valentin Fuster
2009;():447-449. doi:10.1115/IJTC2009-15108.

Fully formulated metalworking fluids have been thoroughly described in literature, but the influence of individual additives on tribological performance of such compositions is still not fully clear. In this study we want to establish the mechanism of how different additives affect tribological properties of the system, and to select the best compounds and concentrations for this purpose. The influence of various individual additives on friction behaviour, anti-wear and anti-corrosion properties of metal working fluids was examined. Additionally, chemical characterisation of the surfaces before and after tribological tests was done. The results show that additives can enhance one property while adversely affecting another. The best results were obtained for triethylamine and 1, 4-buthylene glycol, however mixing both additives gives opposite behaviour.

Topics: Tribology , Lubricants , Water
Commentary by Dr. Valentin Fuster
2009;():451-453. doi:10.1115/IJTC2009-15113.

Surface micro dents may act as lubricant reservoirs to reduce friction and wear in sliding and rolling contact applications. Surface patterning has become a valuable technique for fabricating micro dents. Alternative methods such as micromachining present obvious limitations in comparison with laser shock peening (LSP). In this paper, the use of LSP along with an automatic X-Y table proves to be an attractive and reliable method for producing micro dent arrays with enhanced surface integrity and free of cracks. Surface topography, residual stress, and microhardness of the fabricated micro dent arrays on polished Ti-6Al-4V have been characterized. It was found that a 10% density of micro dent array reduces coefficient of friction compared with a smooth surface. However, a higher dent density not necessarily reduces coefficient of friction.

Commentary by Dr. Valentin Fuster
2009;():455-457. doi:10.1115/IJTC2009-15195.

Understanding the tribological aspects of machining processes are essential for increasing the dimensional accuracy and surface integrity of products, as well as gaining a better control of tool wear, chip handling and power consumption. The objective of this investigation is to develop numerical models that accurately predict the chip formation and stress profiles in the work-piece during orthogonal metal cutting using the explicit finite-element method (FEM). In our simulations, a damage material model was utilized to capture the work-piece chip separation behavior and the simultaneous breakage of the chip into multiple fragments. In the simulation, the rigid steel cutter of different rake angles was moved at different velocities against a stationary aluminum work-piece at constant friction for a cutting depth of 1 mm. Overall, the results indicate that the explicit FEM is a powerful tool for simulating metal cutting and discontinuous chip formation. The rake angle had a significant effect on the formation of chip during metal cutting. The formation of discontinuous chip along the contact interface was hypothesized to be due to the internal crack initiation and propagation in front of the tool and above the cutting edge, rather than from the free surface.

Commentary by Dr. Valentin Fuster
2009;():459-461. doi:10.1115/IJTC2009-15196.

In the present investigation, sliding experiments were conducted using pins made of pure Al, Al-4Mg alloy, Al-8Mg alloy, Mg-8Al alloy and pure Mg against steel plates of various surface textures using a pin-on-plate apparatus under both dry and lubricated conditions. The primary focus of the study was to investigate the influence of alloying elements on the coefficient of friction and transfer later formation in Al-Mg systems. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. It was observed for a given surface texture that the alloying element addition decreased the average coefficient of fiction to lower values under both dry and lubricated conditions. For a given material pair, the coefficient of friction and formation of transfer layer depend on the surface texture of the hard surfaces.

Commentary by Dr. Valentin Fuster
2009;():463-465. doi:10.1115/IJTC2009-15197.

In this paper, chip formation process during mechanical cutting of rock is simulated by using an explicit finite element code, LS-DYNA. In the simulation, the work-piece material properties have been modeled using the damage constitute material model. This model simulates the separation of the chip from the work-piece and the simultaneous breakage of the chip into multiple fragments. In the simulation, a rigid steel tool was moved at various sliding velocities, namely 1, 4, 10, 50 and 100 mm/s against a stationary rock material. For a given sliding velocity, the simulations were carried out for various cutting depths, namely 1, 2, 3 and 4 mm. The variation of stresses and the amount of chip formation at different depths of cut and velocities have been investigated. Overall, the results indicate that the explicit FEM is a powerful tool for simulating rock cutting and chip formation. More specifically, the separation of chip from the work-piece at different depths of cut was distinctly shown using this numerical model.

Commentary by Dr. Valentin Fuster
2009;():467-469. doi:10.1115/IJTC2009-15198.

In the present investigation, unidirectional grinding marks were attained on the steel plates. Experiments were then conducted using pins of Al-Mg alloy against the prepared steel plates using an inclined pin-on-plate sliding tester. The goal of the research is to understand the influence of grinding mark direction and inclination angle of hard material on the friction and transfer layer formation during sliding. The inclination angle of the plate was held at 0.2°, 0.6°, 1°, 1.4°, 1.8°, 2.2° and 2.6° in the tests. The pins were slid both perpendicular and parallel to the grinding marks direction. Experiments were conducted under both dry and lubricated conditions on each plate in ambient environment. Results showed that the coefficient of friction and formation of transfer layer depend on the grinding marks direction and inclination angle of the hard surfaces. For a given inclination angle, the coefficient of friction and transfer layer formation were found to be more for the pins slid perpendicular to the unidirectional grinding marks when compared to parallel to the unidirectional grinding marks under both dry and lubricated conditions. The stick-slip phenomenon was observed only under lubricated conditions at the highest tilt angle for the sliding perpendicular to the grinding marks direction. These variations could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding.

Commentary by Dr. Valentin Fuster

Herbert S. Cheng Symposium

2009;():471-473. doi:10.1115/IJTC2009-15038.

Performances of point contact EHL on multilayered or coated substrates have been analyzed in this paper via computer simulations, with emphasis on comparing the effects of Newtonian and non-Newtonian lubricants. The lubrication system consists of a rigid ball in contact with a smooth coated flat. The coating is perfectly bonded to an elastic substrate and it has a uniform thickness. The rigid ball has surface velocity U relative to the contact point. The hydrodynamic pressure p is governed by a generalized Reynolds equation in which the non-Newtonian effects of lubricants are characterized by two factors whose values are determined based on lubricant rheology. The Papkovich-Neuber potentials were employed to get the response functions in frequency domain for layered contact problems, and the influence coefficients relating pressure to surface displacements and stresses can be obtained via invert Fourier transform. The surface deformation was then calculated in terms of the pressure-displacement influence coefficients and the DC-FFT method was used to speed up the computation. The distributions of pressure, film thickness and subsurface stress have been analyzed for lubricants with different rheological behaviors, from which pressure and film thickness profiles along the rolling direction are calculated for Newtonian and Non-Newtonian lubricants. The central film thickness become thicker for stiffer coatings in the case of Newtonian lubricants, but the trend is reversed for Non-Newtonian lubricants. The surface stresses along the rolling direction show a spike corresponding to the pressure, which is more significant with stiffer coatings in the Newtonian case, but the spike is less visible for Non-Newtonian lubricants.

Commentary by Dr. Valentin Fuster
2009;():475-477. doi:10.1115/IJTC2009-15041.

Step bearings are frequently used in industry for better load capacity. Analytical solutions to the Rayleigh step bearing and a rectangular slider with a finite width are available in literature, but none for a fan-shaped thrust step bearing. This study starts with a known solution to the Laplace equation in a cylindrical coordinate system, which is in the form of infinite summation. An analytical solution to pressure is derived in this paper for hydrodynamic lubrication problems encountered in the fan-shaped step bearing. The presented solutions can be useful for designers to maximize bearing performance as well as for researchers to benchmark numerical lubrication models.

Topics: Pressure , Thrust , Bearings
Commentary by Dr. Valentin Fuster
2009;():479-480. doi:10.1115/IJTC2009-15042.

Recent numerical simulation results of hydraulic rod seals are reviewed. These include steady-state simulations of U-cup seals. Challenges for future research are discussed.

Commentary by Dr. Valentin Fuster
2009;():481-482. doi:10.1115/IJTC2009-15050.

Surface topography plays an important role in the efficiency of lubricated contacts formed between highly loaded machine parts. Gears, rolling bearings, cam and followers etc. subjected to high loads and/or slow speeds are operated under mixed lubrication when lubrication film is not able to completely separate rubbing surfaces. Such an effect becomes even more serious under transient conditions that bring the risk of the surface damage because of asperities interactions. This paper focuses on the effects of both artificially produced and real roughness features on mixed lubrication film formation during start up motion of non-conformal contacts operated under rolling/sliding conditions. The observation of the effects of surface dents artificially produced on the ball surface helped to understand better the behavior of real surface topography. It was found that the presence of shallow surface features can help to separate mixed lubricated rubbing surfaces more efficiently than it could be suggested from the results obtained with smooth surfaces.

Topics: Lubrication
Commentary by Dr. Valentin Fuster
2009;():483-485. doi:10.1115/IJTC2009-15173.

The average flow model proposed by Patir and Cheng offers a great convenience for the analysis of rough surfaces in lubrication. The contact factor introduced by Wu and Zheng helps to solve a difficulty in local film evaluation using the average flow model. This paper reports a simple method to calculate the contact factor. Method validation is demonstrated by the comparison of the contact factors for Gaussian surfaces obtained with the present method and the fitting formula of Wu and Zheng. The proposed method can not only easily compute the contact factor values for Gaussian surfaces, it can also be used for those of non-Gaussian and measured surfaces, especially those with unknown probability density distribution of the roughness height.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2009;():487-489. doi:10.1115/IJTC2009-15211.

Recently, we developed a closed-form solution to the stress field due to a point eigenstrain in an elastic full plane. This solution can be employed as a Green’s function to compute the stress field caused by an arbitrary-shaped Eshelby’s inclusion subjected to any distributed eigenstrain. In this study, analytical expressions are derived when uniform eigenstrain is distributed in a planar inclusion bounded by line elements. Here it is demonstrated that both the interior and exterior stress fields of a polygonal inclusion subjected to uniform eigenstrain can be represented in a unified expression, which consists of only elementary functions. Singular stress components are identified at all the vertices of the polygon. These distinctive properties contrast to the well-known Eshelby’s solution for an elliptical inclusion, where the interior stress field is uniform but the formulae for the exterior field are remarkably complicated. The elementary solution of a polygonal inclusion has valuable application in the numerical implementation of the equivalent inclusion method.

Topics: Stress , Shapes
Commentary by Dr. Valentin Fuster

Wind Turbine Tribology

2009;():491-493. doi:10.1115/IJTC2009-15206.

Ring-on-ring rolling/sliding contact tests were conducted to evaluate the impact of ground, black oxided, isotropically finished (ES20), and tungsten carbide-incorporated amorphous hydrocarbon coated surfaces (ES322) on friction and wear in high slip contacts relevant to wind turbine gearbox bearings. The testing was performed in three phases, including Stribeck testing, adhesive wear testing, and cyclic acceleration testing. Results indicated that neither black oxide treatment nor ES20 had long-term adhesive wear protection benefits. However, ES20 exhibited the lowest traction coefficients in boundary lubrication during run-in as compared to the other test surfaces. Only the contacts containing the ES322 hard-coated surface consistently survived the harshest slip and boundary lubrication conditions.

Commentary by Dr. Valentin Fuster
2009;():495-497. doi:10.1115/IJTC2009-15220.

The drive for an increase in the amount of sustainable energy sources has given rise to increasing interest in the development of wind turbine technology. Although wind turbines are being increasingly installed around the world, several of the components in a wind turbine have challenges related to tribology that can drastically reduce their expected lifetimes.

Commentary by Dr. Valentin Fuster
2009;():499-502. doi:10.1115/IJTC2009-15255.

Passage of current through moving conductive contacts results in electrical discharge and then melting of the material, which leads to wear. Such kind of bearing wear is common in electrical machines. There are however certain patterns which are unique to this kind wear. This wear pattern is called ‘fluting’, which are repetitive in nature. Electrical discharge can create higher surface roughness. Also the thermal and rheological properties of the lubricant play a big role in the film thickness formation. The passage of current through the lubricant also changes this and is determined by the electrical properties of the lubricant. In this work effect of bearing currents on a 7204 angular contact ball bearing is studied. This is tested with and without different cage materials with an axial load and no radial load, rotating at 2700 rpm. Four experiments were done at different-level of voltage, lube and cage material. Type of lubricant was seen to play a significant role in fluting.

Commentary by Dr. Valentin Fuster

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