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Magnetic Storage Tribology

2006;():819-824. doi:10.1115/IJTC2006-12040.

A new method is introduced for predicting particle and liquid droplet contamination on an air bearing surface. The method primarily relies on the analysis of flow patterns nearest the air bearing surface, restricted to two dimensions. In addition, a mathematical model for the 3-dimensional flow adds clarity to the contamination mechanisms. The predictions compare well with contamination patterns observed in prototype disk drives.

Commentary by Dr. Valentin Fuster
2006;():825-831. doi:10.1115/IJTC2006-12050.

This paper focuses on slider air bearing design in order to reduce the lift-off force during the unloading process while satisfying the desired static flying performances. Since it takes a huge amount of computational time to solve time-dependent dynamic L/UL equations, a simplified lift-off force model with respect to air bearing suction force and flying attitudes is created by the kriging method. The EMDIOS is employed as a design framework to wrap effectively and connect the analyzers to the optimizer. In this study, an optimization problem is formulated to minimize the amplitude of lift-off force during unloading process while keeping the flying height, pitch and roll angles within suitable ranges over the entire recording band as well as reducing the possibility of slider-disk contact in steady state. From a conventional negative pressure slider, a modified slider model is optimally designed for 1-inch disk drive with L/UL system. The simulation results show that the lift-off force can be reduced by about 60% in comparison with that of the initial design. It is demonstrated by the dynamic L/UL simulation that the optimum slider incorporated with the suspension is not only smoothly loaded onto the rotating disk, but also properly unloaded onto the ramp. It is proven that the proposed design approach, which uses a static analysis instead of a time-dependent dynamic L/UL analysis during iterative optimization process, works efficiently in designing a slider air bearing for L/UL applications.

Topics: Pressure , Stress , Design
Commentary by Dr. Valentin Fuster
2006;():833-841. doi:10.1115/IJTC2006-12062.

In order to achieve a magnetic recording density of 1 Tb/in2 , the spacing is expected to be less than 2–3nm. However, a critical issue in achieving such an ultra-low spacing is the dynamic instability of the head disk interface (HDI). That is, the experimentally observed hysteresis of fly sliders. The phenomenon of slider hysteresis has two features: slider touchdown and slider takeoff. The goal of this research is to experimentally clarify the effects of the lubricant bonded ratio as well as the lubricant film thickness on slider hysteresis behavior in detail; it also aims to determine the contributing factors. In this study, the difference in the touchdown and takeoff velocities was monitored by varying the lubricant bonded ratio and lubricant film thickness of the disks; further, the correlation between the observed phenomenon and the variation in the experimental parameters was investigated. The results showed that the touchdown velocities were almost independent of the lubricant bonded ratio, while the takeoff velocities were greater for a lubricant with a higher bonded ratio; these results were obtained for a constant lubricant film thickness of around one monolayer. Therefore, the slider hysteresis was greater for a lubricant with a higher bonded ratio. With regard to the effect of lubricant film thickness, it was observed that the touchdown and takeoff velocities were greater for thinner lubricants. These results for the effect of lubricant film thickness are very similar to those obtained by Ambekar et al. However, the slider hysteresis was greater for thicker lubricants. Considering these experimental results as well as the experimental data for the effect of the surface roughness of a disk on the slider hysteresis obtained by Tani et al, it was suggested that the variation in the touchdown velocity is due to a variation in the intermolecular forces. Further, it was suggested that the variation in the takeoff velocity is caused by a variation in the friction forces between the slider and disk surface; this occurs because the takeoff velocity was greater for a lubricant with a higher bonded ratio or a thinner lubricant, which only has a small fraction of free mobile lubricant. The results predicted by the simulations are consistent with those observed experimentally. In addition, a design guideline for next-generation HDI, with small touchdown and takeoff velocities, resulting in small slider hysteresis, is discussed in detail in this paper.

Topics: Lubricants , Disks
Commentary by Dr. Valentin Fuster
2006;():843-854. doi:10.1115/IJTC2006-12065.

As the areal density of magnetic disk storage continues to increase and head-disk spacing decrease, contact between the recording slider and the rotating media becomes imminent. In order to predict contact forces, fly-height modulations, and off-track motions, dynamic models are typically used. A critical element of these models is the contact stiffness and damping arising from the interfacial interaction between the slider and the disk. In this paper, we review different models for predicting contact stiffness based on roughness and layered media and then we report experimental data of both contact stiffness and contact damping of typical head-disk interfaces. It is found that the contact stiffness models (based on roughness alone), over-predict the contact stiffness of actual head-disk interfaces by as much as an order of magnitude. Also, it is found that the contact damping ratio is typically few percent and its behavior is substrate dependent. In addition, the effects of a molecularly thin lubricant and humidity on contact stiffness and damping were experimentally investigated and no significant effects were found.

Topics: Damping , Disks , Stiffness , Storage
Commentary by Dr. Valentin Fuster
2006;():855-861. doi:10.1115/IJTC2006-12086.

The replenishment characteristics of PFPE polar lubricant films after sliding-induced depletion arising on lubricant-coated magnetic disks have been clarified by measuring the profile changes of the lubricant film incurring in-contact sliding against a glass pin. Lightly loaded and slowly rotated test conditions were selected so that the molecular layering structure would not be strongly disturbed or destroyed. An unrecoverable phenomenon was first found to occur in which the replenishment function of lubricant films failed. This phenomenon was observed to occur when depletion depth reaches the monolayer surface or when the residual film thickness is only slightly thicker than the monolayer; when depletion depth invades the monolayer, unrecoverable features changed into recoverable features. Compared with the same tests performed using a nonpolar lubricant, the unrecoverable phenomenon was found to be peculiar to polar lubricants. In addition, replenishment characteristics were compared with diffusion characteristics currently obtained from step-boundary flow, which spreads outward from a step-shaped lubricant boundary, and the similarities and differences between the two were clarified.

Commentary by Dr. Valentin Fuster
2006;():863-868. doi:10.1115/IJTC2006-12088.

A simulation method, in which grooves are virtually distributed on the slider air-bearing surface instead of on grooved medium surface, is developed to investigate slider flying ability fundamentally. Its feasibility is verified. The characteristics of a slider flying over a discrete track medium (DTM) surface are simulated, including the flying attitude, groove parameters effect on flying profile, 3 σ flying height, and flying height sensitivity on altitude of the slider. Simulation results indicate that when a traditional slider is flying over a DTM surface, it will have a higher 3 sigma of flying height, a more sensitivity on altitude, and a bigger flying height loss.

Commentary by Dr. Valentin Fuster
2006;():869-873. doi:10.1115/IJTC2006-12097.

A drive level measurement of flying height modulation and a demonstration of slider-disk contact control were conducted. The results of the flying height modulation strongly agree with those obtained from a LDV (Laser Doppler Vibrometer) measurement. The modulation was mainly caused by curvature caused by disk clamping. Furthermore, feedback control of a slider-disk contact was successfully demonstrated. Friction force was controlled at a small value to maintain the slider so that it flew over the disk in the light contact regime.

Topics: Disks
Commentary by Dr. Valentin Fuster
2006;():875-884. doi:10.1115/IJTC2006-12106.

Using a coarse-grained molecular dynamics simulation based on the bead-spring polymer model, we reproduced the film distribution of molecularly thin lubricant films with polar end groups coated on the disk surface and quantified the film surface morphology using a molecular-probe scanning method. We found that the film surface morphology changed periodically with increasing the film thickness. The monolayer of a polar lubricant that entirely covers the solid surface provides a flat lubricant surface by exposing its nonpolar backbone outside of the monolayer. By increasing film thickness, the end beads aggregate to make clusters, and bulges form on the lubricant surface accompanying an increase in surface roughness. The bulges continue to grow up even though the averaged film thickness reaches or exceeds the bilayer thickness. With further increases in film thickness, the clusters start to be uniformly distributed in the lateral direction to clearly form a third layer. As for the formation of fourth-fifth layers, the process is basically the same as that for the second-third layers. Through our calculations of the intermolecular potential field and the intermolecular force field, these values are found to change periodically and are synchronized with the formation of molecule aggregations, which explains the mechanism of forming the layered structure that is inherent to a polar lubricant.

Commentary by Dr. Valentin Fuster
2006;():885-890. doi:10.1115/IJTC2006-12131.

In this report, in order to understand particle induced friction (PIF) in sliding friction, previous results and theory regarding PIF were reviewed and introduced into nano technology application “head-disk interface (HDI)” in hard disk drive (HDD) as example. The theoretical analysis of PIF in HDI provided the results, i.e. (1) interfering nano particles can generate high contact pressure more than a few GPa levels, (2) at the contact area generated by interfering particles, the PIF theory assumes generating extremely high temperature as well as the energy in a power generator, and (3) from the point of (2), in order to check the number of interfering nano-particle, TA (thermal asperity) measurement will be effective more than small vibrating signal by AE (acoustic emission) or friction force by piezo sensor, moreover, (4) the PIF theory can point the main cause in HDI issues, which are (i) load force and (ii) sliding velocity in HDI could not be down-scaled like particle size, contact area, flying height called “nano technology”.

Commentary by Dr. Valentin Fuster
2006;():891-898. doi:10.1115/IJTC2006-12193.

Contact induced vibration of air bearing-slider-suspension system is a crucial issue for slider flying stability and head positioning precision of 1 Tb/in2 hard disk drives. In this paper, the contact induced off-track vibrations of air bearing-slider-suspension system are investigated by simulation. A dynamic simulator is developed to calculate the interactions between the air bearing dynamics and vibrations of slider-suspension assembly. The simulation model consists of a finite element (FE) model of suspension assembly, an air bearing model based on the generalized lubrication equation, and a slider-disk contact model based on the probability distributions of surface roughness. A sequential method is used to couple all these models and analyses. The time history of the slider and suspension motions, together with the time-varying forces including air bearing force, air shear forces, contact force and friction force can be obtained. The effects of different contact conditions, such as the contact intensity, friction coefficient, and disk surface waviness on off-track vibrations are investigated numerically in details. The results reveal some mechanisms on how these factors contribute to the off-track vibrations of suspension assembly.

Topics: Bearings , Vibration , Disks
Commentary by Dr. Valentin Fuster
2006;():899-908. doi:10.1115/IJTC2006-12234.

Preferential surface texturing is expected to significantly improve tribological performance of ultra-low flying magnetic storage head-disk interfaces (HDIs) by modifying the roughness and reducing the contact area preferentially, thereby reducing the relevant interfacial forces such as friction, contact and adhesive forces. Due to the different etch rates in the titanium carbide (top surface) and alumina (bottom surface) portions of the slider air-bearing surface (ABS), during reactive ion etching the surface heights possesses a distinct bimodal distribution. In order to accurately and realistically capture the interfacial phenomena of the ultra-low flying HDI with a preferentially textured slider ABS, a probability density function was proposed by linking two different Gaussian asperity distributions. The proposed bimodal asperity distribution was then directly incorporated into a previously developed rough surface contact model to calculate the corresponding interfacial forces. The results were then directly compared to a single Gaussian approximation (ignoring the bimodality) as well as a high order polynomial curve-fit approximation (encompassing the bimodality). Comparative study revealed that the proposed bimodal distribution method has a main advantage of being able to resolve the top and bottom asperity contributions separately, thereby providing the interfacial force estimates that are more physically accurate. Other simpler methods, by assuming a single continuous distribution over the entire surface, are not able to isolate the top and bottom asperity distributions and thus are more likely to overestimate the interfacial forces in sub-5 nm flying HDIs.

Topics: Force , Modeling , Disks
Commentary by Dr. Valentin Fuster
2006;():909-910. doi:10.1115/IJTC2006-12330.

The effect of the loading velocity on the transition of the computer hard disk drive air slider from the high flying height state to the nominal flying height state is studied numerically. It is found that increasing the loading velocity decreases the air bearing force corresponding to the beginning of the transition period.

Topics: Stress , Bearings , Computers , Disks , Force
Commentary by Dr. Valentin Fuster
2006;():911-915. doi:10.1115/IJTC2006-12368.

A combination of a lubricant layer and carbon layer is generally used to provide wear and corrosion protection to the magnetic layer in a magnetic medium. Typically, the carbon layer has a diamond-like structure doped with hydrogen and/or nitrogen. After the vacuum coating is applied, the coated substrate is removed from the vacuum system and immersed in a dilute lubricant solution. The substrate is slowly withdrawn from the lubricant solution or placed in a tank from which the solution is drained. Lubricant thicknesses are controlled by varying the solution concentration and/or the drain rate. In this report, we will present results of evaporated X-1P as a single component lubricant instead of as an additive as has been used in the disk industry. The evaporation process has been chosen to substitute for the well known lubricant dipping technique as the deposition technique to coat the lubricant due to the following reasons: (i) eliminate solvent, (ii) improved control of the lubricant /carbon interface by reducing exposure to water and oxygen prior to lubricant deposition, (iii) combination of lubricant and carbon coating in one vacuum system, (iv) improve yield using shorter process time and (v) phosphazene molecules have low vapor pressure, high thermal stability and narrow molecular weight distribution.

Topics: Lubricants , Carbon
Commentary by Dr. Valentin Fuster

Manufacturing/Metalworking Tribology

2006;():917-922. doi:10.1115/IJTC2006-12039.

High temperature aluminum sheet forming places high demands on tooling materials and lubricants, and considerable effort is being exerted to determine tribological characteristics during the surface interaction of aluminum sheet and tool steel at high temperatures (up to 500°C). A new flat-on-flat test method of tribotesting was developed which provides an advantage over existing techniques by allowing measurement of operative friction coefficients and determination of the onset of sheet/tool adhesion.

Commentary by Dr. Valentin Fuster
2006;():923-928. doi:10.1115/IJTC2006-12077.

Lubricants bases for metalworking applications make extensive use of water-soluble additives to reduce friction and wear. In order to do such task, these additives must form a lubricating film that separates the contact surfaces, thus imparting good surface finishes to the worked parts. This paper presents a study on the tribological performance of aqueous solutions of tetraalkylammonium thiomolybdates and thiotungstates. Tests were carried out on a pin-on-disc tribometer for a steel-aluminum contact while keeping load, entrainment speed, sliding distance, temperature and concentration of the additive constant to study the lubrication effect of these two salts. Chemical analysis of the wear track indicates the presence of an in-contact formed solid film enriched with MoS2 and WS2 that reduces friction markedly. This reduction in friction, observed for both solutions, is highly dependent on the alkyl group.

Topics: Friction , Water
Commentary by Dr. Valentin Fuster
2006;():929-937. doi:10.1115/IJTC2006-12141.

This research investigates the tribological interactions in removal of nano-sized particles during post chemical-mechanical polishing (CMP) cleaning. Possible surface interactive forces are discussed in order to understand the particle adhesion and subsequent removal through physical and mechanical interactions. The investigation was carried out using theoretical analysis combined with experimental study. The theoretical analysis was based on the particle adhesion and removal. Surface interaction consideration includes brush load, frictional force, and hydrodynamic drag. Finite element modeling using ABAQUS was used. The polishing experiments were done on silicon wafers with SiO2 slurry. Cleaning tests were done using DI water to brush clean the adhered particles from hydrophilic silicon surface. An Atomic Force Microscope (AFM) was used to evaluate the effective cleaning topography. It was found that particles are removed through sliding and rolling. The fluid film behavior was successfully illustrated using a modified Stribeck behavior with considering of changing material properties. New particle removal mechanisms and lubrication model were proposed.

Topics: Force , Lubrication
Commentary by Dr. Valentin Fuster
2006;():939-948. doi:10.1115/IJTC2006-12210.

Membrane microfiltration is a promising technology that has been shown to extend metalworking fluid (MWF) life by eliminating contaminants while allowing the fluid to stay in use. However, the efficacy of this technology is compromised by the clogging of the filter pores in a process known as membrane fouling. In this paper the fouling issue is addressed by the development of a semi-synthetic MWF specifically designed to reduce fouling of microfiltration membranes. The composition of the designed MWF is discussed and compared with a commercial MWF. Cross-flow microfiltration fouling tests were carried out in low pressure, high velocity conditions on ceramic α-alumina membranes. Several common MWF components are shown not to be factors of membrane fouling on these membranes. The flux of the designed fluid was found to reach an immediate steady-state at about twice the value of the steady-state flux of the tested commercial fluid. SEM imaging was used to further evaluate membrane fouling by each fluid. The machining capabilities of the designed fluid were examined in terms of cutting forces and machining temperature.

Commentary by Dr. Valentin Fuster
2006;():949-957. doi:10.1115/IJTC2006-12221.

Precision machining may induce very different residual stress profiles. However, the effects of machining induced residual stresses on component performance in sliding contact have received little attention. This study develops a two dimensional finite element simulation model of sliding contact to study the effects of the distinct residual stress profiles induced by hard turning and grinding on sliding contact. A parametric simulation study based on the design-of-experiment method was also performed to assess the influence the sliding parameters of the applied load, speed, and friction of coefficient on sliding contact mechanics. The results have shown that the initial residual stress profiles have great influence on the normal stress in sliding direction, but have little effects on other stress components. Sliding relaxes initial residual stress and shifts the maximum subsurface stress to the top surface. The normal strains are slightly influenced by the residual stress, while the shear strain is hardly affected. Compared with friction of coefficient and sliding speed, the applied load has the greatest influence on contact stresses and strains. The increasing load shifts peak normal stress in sliding direction and von Mises stress from the subsurface to surface. The peak Hertz pressure, shear stress and von Mises stress increase nonlinearly with the applied load.

Commentary by Dr. Valentin Fuster
2006;():959-969. doi:10.1115/IJTC2006-12229.

Chemical mechanical polishing (CMP) is a manufacturing process in which a wafer surface is polished by pressing it against a rotating pad that is flooded with slurry. The slurry itself is a fluid containing abrasive particles. Past experimentation has shown that the distribution of suspended particles in the slurry is significantly related to the distribution of material removal on the wafer during CMP. Therefore, this study involves the development and simulation of a model that predicts the kinematics and trajectory of the abrasive particles. The simulation results compare well with data from shear cell experiments data conducted by other researchers.

Topics: Slurries
Commentary by Dr. Valentin Fuster
2006;():971-980. doi:10.1115/IJTC2006-12232.

This paper addresses the tribological challenges in the machining of compacted graphite iron (CGI) through an investigation of the effects of tool material, local tool-surface topography and Minimal Quantity Lubrication (MQL) on machining performance. CGI is a material that is being considered as an alternative to aluminum and gray cast iron (CI) for diesel engine applications — a market with large potential. Solutions for continuous cutting operations like turning and boring of CGI are constantly being sought. Prior limited work has attributed the tool wear problems encountered during CGI machining to the absence of a lubricating layer of manganese sulphide (MnS) on the cutting tool during continuous cutting at high speeds. With the aim of understanding the tribology of the tool-chip interface, experiments were undertaken using four different tool-inserts (flat-coated carbide, grooved-coated carbide, grooved-coated cermet and chamfered ceramic tool inserts) under dry and minimal quantity lubrication (MQL) conditions. Two cutting speeds (250 m/min. and 400 m/min.) with constant feed and depth of cut were tested. Post-machining analysis was conducted to determine the chip flow angle and the resultant force in each case. Results show that at low speed, the cermet tool produces a significant reduction in cutting forces in comparison to coated carbide. At high speed, cermet shows a minor increase in forces. MQL causes a decrease in forces only at low speed in all tools except the flat-coated carbide, in which case it shows a minor increase. Resultant forces show an increase with MQL usage at the high speed, suggesting a negative influence on the CGI machining performance. The work reveals that there are several complex effects observed due to the interaction between tool coating, tool topography, and lubricant action.

Commentary by Dr. Valentin Fuster
2006;():981-986. doi:10.1115/IJTC2006-12235.

Next generation integrated circuits (IC’s) will require the use of porous dielectric materials with low shear strengths and at the same time still require processing with chemical mechanical polishing (CMP). CMP polishes the substrate material (e.g. SiO2 ) by rubbing the surface with nanometer scale SiO2 particles. The particles are suspended in an aqueous slurry and are rubbed on the substrate by a porous polyurethane pad. The high friction of CMP can damage porous dielectric materials. This research is defining the source of this friction to enable development of CMP which will not damage porous (low dielectric constant) materials. Experiments were done to determine the contributions of the SiO2 particles and bare pad asperities to the total friction with an SiO2 substrate. Further experiments were done to determine the change in friction for various SiO2 particle sizes. This system level friction data was obtained using a bench-scale tribometer at contact stresses near that of commercial CMP. Very low speeds (∼ 10−3 m/s) were used to eliminate confounding hydrodynamic effects. AFM (Atomic Force Microscope) and instrumented indentation experiments will be used to determine the friction contribution of a single SiO2 particle/substrate contact. The bench-scale experiments showed that the coefficient of friction increased by as much as 10% with the increasing weight percent of SiO2 particles in the slurry. This data suggests that a constant real area of contact is distributed between bare pad asperity contacts and higher friction SiO2 particle contacts. As more SiO2 particles are added, their contact area increases as the pad asperity contacts decrease thereby increasing the coefficient of friction of the system. Further experiments revealed a trend of increasing coefficient of friction with smaller particle diameter. This friction increase is consistent with theories which suggest that particle real contact area is a function of the particle cross-sectional area as opposed to a purely Hertzian model. The results demonstrate a possible route to controlling CMP friction by varying the relative combinations of the pad friction, the percentage of abrasives in the slurry, and the abrasive particle size.

Commentary by Dr. Valentin Fuster
2006;():987-996. doi:10.1115/IJTC2006-12307.

The effect of various process parameters like pressure, velocity and process consumable characteristics like pad properties and slurry chemistries on copper during Chemical Mechanical Planarization (CMP) process have been extensively researched. However, the thermal aspects of CMP have not been understood in detail. In this research, we present the effect of slurry temperature on the coefficient of friction and the resulting micro-structural changes in the copper material due to the polishing process. We also investigated the work hardening effect of polishing on copper thin film. From the results, it was found that temperature has an effect on the coefficient of friction but the effect of process parameters influences the effect of temperature. The effect of temperature on coefficient of friction is also dependent on the material of polishing pad. From XRD measurements more crystallinity was observed for the sample polished at 29°C as compared to other samples. The mechanical properties of copper investigated before and after CMP show improved mechanical properties with polishing. Significant change was observed among the mechanical properties of samples polished at different temperatures.

Commentary by Dr. Valentin Fuster
2006;():997-998. doi:10.1115/IJTC2006-12337.

Recently, it has been demostrated that large plastic strains imposed on a chip, during machining, results in significant microstructure refinement [1]. Titanium chip obtained by machining is found to have a nanocrystalline structure with ∼100 nm size grains. The hardness and strength of the nanostructures are seen to be substantially higher than those of microcrystalline titanium. In this work, the tribological behavior of this nanocrystalline titanium created by large strain machining is compared with that of conventional pure titanium. Tribological tests on titanium samples have been carried out in a pin-on-disk tribometer, sliding against AISI 52100 steel pins. The wear performance of the nanocrystalline titanium, is shown to be superior than the microcrystalline material. Wear mechanisms are discused from SEM observation of wear tracks, wear debris morphology and transfer tribolayers.

Topics: Wear , Machining , Titanium
Commentary by Dr. Valentin Fuster

Nanotribology

2006;():999-1006. doi:10.1115/IJTC2006-12007.

In view of the difficulty in measurement of temperature rise at the contact between sliding bodies with engineering scale roughness a good deal of theoretical work has been carried out in the last few decades. However, as surfaces become smoother and loading decreases in applications such as MEMS, NEMS and magnetic storage devices measurement of flash temperature becomes increasingly more difficult due to the nanometer scale asperity interactions. Consequently measurement of flash temperature at the nano-scale asperity contact has not yet been possible. The analysis of flash temperature rise under these circumstances is no less challenging since it must consider not only the small-scale asperity height distributions but also the surface forces those may operate at very small surface separations. The paper attempts to predict the flash temperature rise analytically using a fractal approach to describe the nano-scale asperity interactions at low loads and also taking into account the influence of relevant parameters including the surface forces. The results show in general that the contact surface temperature steadily increases with load, nano-scale roughness and surface forces. Interestingly, the fractal analysis presents a wide spectrum of solutions. While under certain combinations of fractal and material parameters extremely high contact temperature rise is predicted, under certain other parametric combinations extremely low temperature rise can be seen. The later parametric combination is certainly of much practical use.

Commentary by Dr. Valentin Fuster
2006;():1007-1013. doi:10.1115/IJTC2006-12143.

Diamond-like carbon (DLC) coatings doped with different nitrogen contents were prepared by an ECWR-CVD deposition technique and vacuum-annealed at different temperatures. The structures, nano-mechanical properties and nano-tribological behaviors of the annealed coatings were characterized using Laser Raman Spectroscopic technique and Hysitron Triboindenter, respectively. The annealing temperature promotes the increase in the sp2 ratio and decreases evidently the nano-mechanical properties, including nano-hardness and reduced modulus. For the as-deposited DLC coatings with different nitrogen contents, the frictional coefficients (LF /NF ) deduced from nano-scratch testing at same loads are similar, and, therefore, seem to be independent of the added nitrogen content. However, Annealing treatment has a significant effect on the frictional coefficient for the N-doped DLC coatings. In addition, the increasing extent of the frictional coefficient with the annealing temperature for the annealed N-doped DLC coatings varies depending on the nitrogen content in the coatings.

Commentary by Dr. Valentin Fuster
2006;():1015-1020. doi:10.1115/IJTC2006-12154.

This research studies the effects of microstructures of poly(vinylidene fluoride) (PVDF) on its adhesion force with the probe of an atomic force microscope (AFM). The adhesion problem is currently a bottle neck for the development of micro-electro-mechanical systems (MEMS). Understanding the surface adhesion mechanisms is an important step to advance MEMS technology. In the present work, we fabricated PVDF thinfilms using spin casting and in situ corona poling methods. The microstructure was thus changed from α to the mixture of β and γ phases. Surface forces were then evaluated using an AFM. It was found that the adhesion forces between the AFM probe and the polymer surfaces were affected by microstructures. This paper discusses details of molecular structures. We propose a surface molecular structural model in order to achieve the level of adhesion forces desired.

Topics: Force , Adhesives
Commentary by Dr. Valentin Fuster
2006;():1021-1041. doi:10.1115/IJTC2006-12200.

Adhesion influences pure interfacial friction on ideal surfaces when the scale approaches micro- and nano-scales. For non-adhesive surfaces, the adhesion forces are composed of Van der Waals surface forces and the forces resulted from the contact. The van der Waals forces are basically conservative and by themselves do not provide energy dissipation process. The contact forces depend on surface atomic configuration, surface energy distribution, electron interactions, and the real area of contact, and are not conservative in nature, resulting in the fact that pull-off forces are always greater than the snap-on forces. This is referred to as the adhesion hysteresis. In this paper we propose a nanofriction model based on the energy dissipation mechanism of the adhesion hysteresis. We consider the contact of a rigid cylinder on an elastic plane, using the Dugdale approximation. The plane is modeled by continuously distributed elastic springs. For both sliding and rolling contacts, adhesive force at the rear of the cylinder is greater than at the front, which results in frictional resistance and energy dissipation due to excitation of the elastic oscillations. The relationship among adhesion, adhesion hysteresis, and friction is discussed.

Commentary by Dr. Valentin Fuster
2006;():1043-1052. doi:10.1115/IJTC2006-12214.

Using the reactive force field in molecular dynamics (MD) simulations, the three-dimensional structure of amorphous carbon at densities ranging from 2.4–3.4 g/cm3 has been predicted. The structures for these solid amorphous systems were generated by melting a cell with 512 carbon atoms, followed by rapid quenching from the liquid phase. At the density of 3.24g/cm3 , we find that 70% of the atoms have sp3 character, in good agreement with our experiment. Simulation results show that all of the sp3 atoms connect to form a percolating tetrahedral network, to which are attached isolated sp2 atoms or short chains of sp2 atoms. Hydrogen-free DLC surfaces were constructed by determining the lowest energy surface for cutting the bulk amorphous carbon cell. It is found that the surface C atoms react readily with glycerol to form a carbon surface containing OH-terminated groups, which is enhanced by sliding. Using MD simulations, we examined the friction properties for various DLC surfaces: bare, H-terminated, OH-terminated, and the passivated surfaces after reaction with glycerol and H2 O2 . Simulation shows that the bare DLC surface has friction coefficient of about 0.8, whereas the DLC surface passivated with OH/H by reacting with H2 O2 leads to friction coefficients down to 0.01. These results suggest that the origin of the superlubricity observed in the DLC system arises from the passivation of carbon surface by OH groups, which is consistent with experiment. The relationship between the friction and interfacial adhesion has also been investigated. The MD simulations suggest that friction is determined by variations in the adhesion during sliding, rather than the absolute value of the adhesion between interfaces. Larger variations (energy barriers) induce larger deformations of the sliding objects, leading to higher friction.

Commentary by Dr. Valentin Fuster
2006;():1053-1062. doi:10.1115/IJTC2006-12259.

The effect of surface textures on the friction of Poly(dimethylsiloxane) (PDMS) elastomer was investigated at both macro and micro scales using a nanoindenter. Friction tests were conducted by means of a stainless-steel bearing ball with a diameter of 1.6mm (macro scale tests) and a Rockwell diamond tip with a radius of curvature of 25 μm (micro scale tests), under normal loads of 5, 10, and 25 mN and with a sliding speed of 1 μm/sec. The coefficient of friction (COF) obtained using the textured surface is found to be much lower than that using the flat surface of the same material, and it was reduced by about 59% in the macro scale tests and 38% in the micro scale tests. COFs in different sliding directions on the groove-textured surfaces were compared, and the friction behaviors were analyzed. It is also found that surface texture (roughness) can tune surface wettability from medium hydrophobic to superhydrophobic. The experimental results (contact angles) were compared with theoretical predictions, and possible explanations were given.

Commentary by Dr. Valentin Fuster
2006;():1063-1064. doi:10.1115/IJTC2006-12390.

EHD oil films possess a remnant thickness of 10–20 nm at very low speeds. This behavior was justified numerically using an empirical equation for wall viscosity. This paper uses molecule-wall interaction potential and finds that the viscosity decreases exponentially with the distance to the wall. A thin attached layer, responsible for remnant oil film, is generated by wall attraction. At high speeds, EHD film occurs between attached layers. The effect of pressure and temperature is evidenced and thickness of nano-EHD oil film is found.

Topics: Thickness
Commentary by Dr. Valentin Fuster

Engineered Surfaces

2006;():1065-1080. doi:10.1115/IJTC2006-12032.

Latest development in high-power lasers made possible a wide variety of laser surface modifications. Such surface modifications include: glazing, shock peening, alloying, cladding and texturing. The main reason behind applying these treatments is to improve the tribological performance of the modified surfaces. In addition to reducing friction and wear, it is favorable to improve the scuffing resistance. Scuffing can be defined as a sudden catastrophic failure of a lubricated sliding surface characterized by a sudden rise in friction; resulting in severe surface damage through localized plastic flow. This paper investigates friction and scuffing performance of laser glazed 1080 steel and laser textured H13 stainless steel. Results showed that laser glazed surfaces reduced sliding friction under dry conditions by approximately 35% and improved wear resistance. In addition, laser glazed surfaces showed high resistance to scuffing compared to unglazed surfaces. Also, Laser surface texturing technique reduced sliding friction under lubricated conditions and improved scuffing resistance.

Topics: Lasers
Commentary by Dr. Valentin Fuster
2006;():1081-1107. doi:10.1115/IJTC2006-12095.

Railroad industry is very important to the U.S. Nearly 60% of the U.S. freight is transported by railroads. Fuel represents about 9.5 percent of the operating cost of a railroad; the industry is aggressively trying to find ways to improve fuel efficiency. One major contributor to the railroad energy losses is the friction between the wheel and the rail. Many technologies have been implemented to reduce the high friction between the wheel and the rail. Such technologies include the application of liquid lubricants as oil and grease to the gage side of the rail. Such lubricants, wash away in the rain, introduce environmental problems, and may cause a train to lose traction if they migrate to the top of the rail. On the other hand, a new technology has recently been developed at Argonne National Laboratory (ANL) showed desirable results. This technology involves the use of laser to treat (glaze) the gage side of the rail. Treating steel rails by “laser glazing” reduces friction between the rails and train wheels by approximately 60%, which may reduce rail cracking by up to 75%. Reducing friction can save an estimated $60 million in fuel costs and $16 million in rail replacements, as well as lower the risk of derailment. This paper focuses on the optimization of the laser glazing process for the railroad applications.

Commentary by Dr. Valentin Fuster
2006;():1109-1144. doi:10.1115/IJTC2006-12096.

Friction forces at wheel/rail interfaces are a significant parasitic energy loss that affects the efficiency with which goods are transported via rail. This paper reviews the development of a laser glazing process that is designed to improve fuel efficiency by treating the gauge face of rails to minimize wheel/rail forces. This research involved activities to develop the laser glazing process, characterize the microstructure of glazed rail steel, assess friction forces with benchtop rigs, and perform full-scale friction force measurements with sets of full-scale instrumented railroad wheels. The full-scale tests performed at the Canadian National Research Council Centre for Surface Transportation Technology in Ottawa, Ontario were performed with two objectives: first to confirm friction reduction observed in earlier (Association of American Railroads - AAR) rolling/sliding tests (and in subsequent lab-scale tests), and second to confirm the adhesion of a glazed layer to the underlying rail under typical loads (up to 38,000 lb). Demonstration of adhesion is critical not only for commercial acceptance of the process, but also for planning the next phase of research, which involves field tests of glazed rail segments. The full scale tests confirmed the reduction in friction observed in prior rolling-sliding, tests performed at the AAR/Pueblo facility, and in benchtop tests, with friction reductions up to 40%. Evidence of cracking was observed, thus raising concerns about durability; however, the extent and nature of the cracks were such that while they warrant further investigation, they should not preclude the field tests of glazed rail segments.

Commentary by Dr. Valentin Fuster
2006;():1145-1152. doi:10.1115/IJTC2006-12138.

Polyvinylidene Fluoride (PVDF) is a commercially available, piezoelectric polymer. It is widely utilized due to its advantageous mechanical, chemical, and electromechanical properties. In this paper, we discuss a PVDF microgripper and then characterize its gripping (frictional) force. This mechanical characterization of the PVDF will be helpful to design the microgripper. The microgripper has many applications like surgeries, microassembly and micromanipulation. The friction force is an important criterion that greatly affects the gripping. The actuation of the gripper is done by applying voltage. It is fundamentally interesting to understand the effects of the applied voltage on the coefficient of friction. The friction force was measured as a function of the applied voltage. It was found that the increasing voltage applied on the sample lead to an increase in the coefficient of friction measured on the surface of the sample. This work serves as an investigation on the effect of applied voltage on the friction coefficient of the sample. The possible mechanisms behind this phenomenon are also discussed here.

Topics: Friction , Grippers
Commentary by Dr. Valentin Fuster
2006;():1153-1162. doi:10.1115/IJTC2006-12157.

The impact and fatigue resistance of overlay coatings is significantly influenced by the residual strain (or stress) field induced during coating deposition, post-treatment and in-service loading. Optimization of residual stress field is therefore critical to the life and performance of components. Non-destructive measurement of these stress fields in relatively thinner (300 to 400 micron) thermal spray coatings however poses a challenge because conventional techniques such as deep hole drilling, x-ray diffraction, synchrotron diffraction, and changes in beam curvature either makes these technique destructive, and/or provides only a very near-surface strain measurement. This particularly complicates the strain analysis in cermet coatings, e.g. WC-Co deposited by the thermal spraying process, where the low penetration depth of x- and synchrotron- diffraction rays can only provide a through thickness measurement of stress profile via the destructive layer removal technique. Recent investigations have therefore concentrated on the use of neutron diffraction technique for such analysis, and this paper reports some of the early findings of the comparison of through thickness strain measurements in relatively thin (400 μm) as-sprayed and post-treated WC-12%Co coatings via neutron diffraction technique. Since neutrons are not charged, they do not interact with the electron cloud surrounding the atom (unlike x-ray), hence diffraction results from the interaction with the atomic nucleus. Neutrons therefore have greater penetration depth in most engineering materials and therefore provide a non-destructive through thickness strain measurement. Results of strain measurement are discussed with the structure property relationship and contact fatigue performance, and indicate that post-treatment of these coatings results in harmonization of the strain field within the coating, and at the coating substrate interface. This significantly influences the contact fatigue performance by improving both the cohesive and adhesive strength of these coatings.

Commentary by Dr. Valentin Fuster
2006;():1163-1167. doi:10.1115/IJTC2006-12159.

A surface processing method that combines electrostatic deposition of microparticles and dry etching is utilized to modify the surface topography of silicon surfaces to reduce adhesion and friction force. Microscale adhesion and friction tests were conducted on flat (smooth) and processed silicon surfaces with a low elastic modulus thermoplastic rubber (Santoprene) probe that allowed a large enough contact area to observe the feature size effect. Both adhesion and friction force of the processed surfaces were reduced comparing to that of the flat surfaces.

Commentary by Dr. Valentin Fuster
2006;():1169-1174. doi:10.1115/IJTC2006-12167.

In this study we attempt to find optimum geometrical parameters of square-shape micro-dimples imposed on parallel flat bearing surfaces which give the best tribological performance, including load capacity and friction coefficient. An analytical solution of Reynolds equation for the surfaces involving numerous dimples is presented, then considering the variations of number of dimples as well as dimple length and height ratios for a constant dimpled area, it is tended to get the optimum value of parameters. It is shown that despite the variations of different studied geometrical parameters, it seems the optimum value of these parameters remain nearly constant.

Commentary by Dr. Valentin Fuster
2006;():1175-1181. doi:10.1115/IJTC2006-12207.

The fabrication of nanocomposite coatings consisting of a hard matrix and solid lubricant islands holds promise developing highly wear resistant coatings with low friction coefficients. We have conducted research on methods for creating hard coating layers with a pattern of holes that can act as reservoirs for solid lubricant phases. The requirements for this method include low cost and the ability to create holes with nearly 1:1 aspect ratios. We first investigated methods to disperse polymer or ceramic microspheres on the substrate, followed by deposition of hard coatings (typically 1–3 microns thick) and then removal of the microspheres. This method resulted in a random distribution of 3 micron holes in the coating, and also permitted deposition of the hard coatings at elevated temperatures. The second part of this study investigated the behavior of a solid lubricant (molybdenum disulphide) when deposited on the surface in thin-film form and a powder (graphite) in liquid suspension form. The effectiveness of the patterned hole structure will be discussed in terms of the ability to reduce friction and wear.

Commentary by Dr. Valentin Fuster
2006;():1183-1184. doi:10.1115/IJTC2006-12233.

This paper describes the application of new scale-sensitive texture analysis methods in tribology experiments that are based on applying F-tests and regression analyses to the results of length-scale, area-scale [1], and filling-scale analyses of measured tribilogical surfaces, combined with physical testing. These methods can improve understanding of the interaction of surface roughness, or textures, and tribological phenomena. The challenges in using surface metrology to elucidate the texture-related mechanisms that control tribological interactions are to capture the relevant information in the surface texture in its measurement, extract it in the analysis, then connect it convincingly with the tribilogical interactions.

Commentary by Dr. Valentin Fuster
2006;():1185-1186. doi:10.1115/IJTC2006-12327.

Micropitting, a significant failure mode in gears, is a form of local fatigue cracking driven by cyclic plastic deformation arising from surface roughness. Oil additives have a marked effect on micropitting and hence on the useful life of lubricated components. In the present paper we investigate the proposition that the effect of oil additives on micropitting results from the way they influence the development of the surface roughness.

Commentary by Dr. Valentin Fuster
2006;():1187-1189. doi:10.1115/IJTC2006-12361.

This presentation discusses a study of low surface energy (LSE) diamondlike carbon (DLC) films. Plasma immersion ion deposition (PIID) technique was used to prepare various DLC films on silicon and 316 stainless steel (SS) substrates. To study the film properties and search for optimal coatings with lower surface energy, low friction and high wear resistance, various precursors were used to prepare the DLC film. The coating microstructures were studied using scanning electron microscopy (SEM), Raman spectroscopy, and atomic force microscopy (AFM); the mechanical properties were characterized using nano-indentation; the tribological properties were studied using a pin-on-disc tribometer; and the surface energy (contact angle) was measured.

Topics: Tribology , Coatings
Commentary by Dr. Valentin Fuster
2006;():1191-1192. doi:10.1115/IJTC2006-12362.

This work is concerned with the dynamic and pressure response of a hydrodynamic tilting pad bearing for a shaft with and without DLC coating. Here it is shown that the maximum pressure point it is moved towards the center of symmetry when the DLC coating is applied. This was attributed to the decrement of the shear stresses at the surface between the shaft and the fluid. It is also shown that the amplitude of the vibration signal when analyzed through wavelet theory is lower for the case with coating compared with the no coating.

Commentary by Dr. Valentin Fuster

Biotribology

2006;():1193-1202. doi:10.1115/IJTC2006-12133.

This paper describes the influence of lubrication on wear during testing of materials for artificial knee joints in a rolling/sliding tribotester built to simulate contact conditions in a total knee replacement. The test configuration consists of parallel cylinders (pucks) of ultrahigh molecular weight polyethylene (UHMWPE) and polished cobalt-chrome alloy in oscillatory rolling/sliding contact in a bath of dilute (25%) bovine serum. Wear tests of three different UHMWPE materials were run under constant load at 40% sliding for 1.5 million oscillation cycles at 1.5 cycles/second. Wear of the UHMWPE was determined by measuring the profile of the cylindrical contact surface of the puck before and after each test. Profile measurements were repeated after at least fourteen days to account for creep. Differences between initial and final profiles were attributed to wear of the UHMWPE. It was found that the largest wear depth occurred near the ends of the oscillatory contact area, with the most heavily irradiated material showing the least wear. Analysis of the elastohydrodynamic lubrication in the rolling/sliding contact was carried out assuming a line contact situation. The time dependent modified Reynolds equation and the elasticity equation with initial conditions were solved numerically using a multigrid technique with full approximation scheme, and using a Newton Raphson method to solve the highly nonlinear system of equations. The thickness of the lubricating film of bovine serum was determined for points along the length of the wear track. It was found that the smallest film thickness (hmin ) occurs very close to the location in the oscillating contact where the greatest wear occurs, owing to the very low entraining velocity near the ends of the oscillation cycle. The coefficient for wear of the UHMWPE (K) was found to be relatively constant over the central section of the oscillatory motion, but increased to a higher value where hmin decreased to near zero. Thus, the important influence of lubrication on wear of artificial knee bearings was demonstrated.

Commentary by Dr. Valentin Fuster
2006;():1203-1209. doi:10.1115/IJTC2006-12164.

Ultra-high molecular weight polyethylene (UHMWPE) is a popular choice for the liner material of the acetabular cup and forms one of the articulating surfaces in total joint replacements (TJRs). Evaluating the tribological characteristics of UHMWPE on immediate contact with the physiological fluid is essential to understand pathways and mechanisms of eventual failure. In this study, the friction response and interfacial shear strength of a UHMWPE-ceramic interface was quantified using atomic force microscopy (AFM) before and after exposure to bovine serum albumin (BSA) solution. A 10% protein solution concentration was used to closely mimic protein levels in human physiological fluid. Medical grade UHMWPE samples with two different surface finishing treatments, milling and melting/reforming were used in the experiments. Friction response as a function of normal load was monitored on a particular area on each sample. Fluorescence microscopy was used to assess the protein adsorption on the test area. The interfacial shear strength of the interface was calculated from the friction data using contact mechanics. Contact angle measurements were also performed on the surfaces to evaluate the surface energies before and after protein adsorption. Correlations between the friction behavior and surface energy of the surfaces are discussed.

Commentary by Dr. Valentin Fuster
2006;():1211-1212. doi:10.1115/IJTC2006-12359.

Pedestrian slips, trips and falls account for around one in three major workplace accidents. Many of these result from poor traction caused by liquid and particulate contamination. The mechanisms behind lubrication for liquid contaminants within the shoe-floor contact are well understood, the same cannot be said for particulate contaminants. This paper considers the key parameters controlling friction in a shoe-floor contact contaminated with particles of different size. Experiments were conducted using a Stanley Pendulum Tester. Results suggest the adhesive friction is significantly affected by particulate contaminants whilst the hysteretic component is not. Three lubrication mechanisms, sliding, shearing and rolling have been observed depending on floor roughness, particle size and shape factor. A simple map showing the regimes where these can occur is presented.

Commentary by Dr. Valentin Fuster

Emerging Technologies/Other

2006;():1213-1220. doi:10.1115/IJTC2006-12001.

Turbochargers (TCs) improve performance in internal combustion engines. Due to low production costs, TC assemblies are supported on floating ring bearings (FRBs) and showing subsynchronous motions of significant amplitudes over a wide speed range. However, the subsynchronous whirl motions generally reach a limit cycle enabling continuous operation. The paper advances progress on the validation against measurements of linear and nonlinear rotordynamic models for predicting shaft motions of automotive TCs. A comprehensive thermohydrodynamic model predicts the floating ring speeds, inner and outer film temperatures and lubricant viscosity changes, clearances thermal growth, operating eccentricities for the floating ring and journal, and linearized force coefficients. A nonlinear rotordynamics program integrates the FRB lubrication model for prediction of system time responses under actual operating conditions. Measurements of shaft motion in a TC unit driven by pressurized air demonstrate typical oil-whirl induced instabilities and, due to poor lubricant conditions, locking of the floating rings at high shaft speeds. Nonlinear predictions are in good agreement with the measured total amplitude and subsynchronous frequencies when implementing the measured ring speeds into the computational model. The computational tools aid to accelerate TC prototype development and product troubleshooting.

Commentary by Dr. Valentin Fuster
2006;():1221-1230. doi:10.1115/IJTC2006-12026.

Gas film bearings offer unique advantages enabling successful deployment of high-speed micro-turbomachinery. Current applications encompass micro power generators, air cycle machines and turbo expanders. Mechanically complex gas foil bearings are in use; however, their excessive cost and lack of calibrated predictive tools deter their application to mass-produced oil-free turbochargers, for example. The present investigation advances the analysis and experimental validation of hybrid gas bearings with static and dynamic force characteristics desirable in high-speed turbomachinery. These characteristics are adequate load support, good stiffness and damping coefficients, low friction and wear during rotor startup and shutdown, and most importantly, enhanced rotordynamic stability at the operating speed. Hybrid (hydrostatic/hydrodynamic) flexure pivot-tilting pad bearings (FPTPBs) demonstrate superior static and dynamic forced performance than other geometries as evidenced in a high speed rotor-bearing test rig. A computational model including the effects of external pressurization predicts the rotordynamic coefficients of the test bearings and shows good correlation with measured force coefficients, thus lending credence to the predictive model. In general, direct stiffnesses increase with operating speed and external pressurization; while damping coefficients show an opposite behavior. Predicted mass flow rates validate the inherent restrictor type orifice flow model for external pressurization. Measured coast down rotor speeds demonstrate very low-friction operation with large system time constants. Estimated drag torques from the gas bearings validate indirectly the recorded system time constant.

Commentary by Dr. Valentin Fuster
2006;():1231-1270. doi:10.1115/IJTC2006-12047.

The performance of Gas Foil Bearings (GFBs) relies on a coupling between a thin gas film and an elastic structure with dissipative characteristics. Due to the mechanical complexity of the structure, the evaluation of its stiffness and damping is still largely inaccurate if not arbitrary. The goal of this paper is to improve the understanding of the behavior of the bump type FB structure under static and dynamic loads. The structure was modeled with finite elements by using a commercial code. The code employed the large displacements theory and took into account the friction between the bumps and the support and between the bumps and the deformable top foil. Static simulations enabled the estimation of the static stiffness of each bump of a strip. These simulations evidence a lack of reliable analytical models that can be easily implemented in a FB prediction code. The models found in the literature tend to over-estimate the foil flexibility because most of them do not consider the interactions between bumps that seem to be highly important. The transient simulations allowed the estimation of the dynamic stiffness and the damping of a single bump of the FB structure. The presence of stick-slip in the structure is evidenced and hysteretic plots are obtained. The energy dissipation due to Coulomb friction is quantified in function of materials, excitation amplitude and frequency. Some energetic considerations allow the calculation of the equivalent viscous damping coefficient and the results are related to experimental data found in literature. The influence of the number of bumps is also briefly addressed.

Topics: Bearings
Commentary by Dr. Valentin Fuster
2006;():1271-1279. doi:10.1115/IJTC2006-12105.

Five-Degrees-of-Freedom (5-DOF) characterization of the stability of a gas lubricated conical bearing of the spiral groove design is determined for bearing numbers up to 500. Perturbation analysis is performed separately for axial, cylindrical and conical modes; the results are compiled into frequency-dependent modal impedance data sets that can be graphed as impedance contours to identify critical points in stability analysis. For a symmetrical rigid rotor, the stability-threshold with respect to each mode can be separately determined. For axial and cylindrical modes, the threshold parameter is the rotor mass. For the conical mode, both transverse and polar radii of gyration enter into the picture but the threshold parameter can still be interpreted in terms of rotor mass.

Topics: Stability , Bearings , Rotors
Commentary by Dr. Valentin Fuster
2006;():1281-1289. doi:10.1115/IJTC2006-12158.

Presented are stability analyses of a flexure pivot tilting pad gas bearing with radial compliance using linearized perturbation method. The radial compliance was intended to accommodate large rotor centrifugal growth at high speeds. The rotor centrifugal growth was considered in the perturbation analyses as a physical mechanism that reduces the gas film thickness. The rotor growth reduces effective bearing clearance and could be a stabilizing mechanism at intermediate speed ranges as well as a limiting factor to the maximum achievable rotor speed for given bearing clearance and pad radial stiffness. The bearing stability was very sensitive to the bearing clearance. The smaller clearance is desirable to achieve higher stability margin at low speed ranges below 100 krpm. However, at higher speeds above 100 krpm, the larger clearances provide more stability margins. The cross-coupled stiffnesses and damping coefficients become near zero at resonance frequency of pad tilting motion, and direct stiffnesses become near zero as excitation frequency approaches the resonance frequency of pad radial motion.

Commentary by Dr. Valentin Fuster
2006;():1291-1302. doi:10.1115/IJTC2006-12160.

A new foil gas bearing with spring bumps was constructed, analyzed, and tested. The new foil gas bearing uses a series of compression springs as compliant underlying structures instead of corrugated bump foils. Experiments on the stiffness of spring bumps show an excellent agreement with an analytical model developed for the spring bumps. Load capacity was measured to demonstrate the feasibility of the new foil bearing. Constructed bearing showed a load capacity of 96N at 20,000rpm under no cooling. Measured temperature rise for cooled bearing (with cooling air flow rate of 1350 cm3/sec) was less than 15°C up to 96N, indicating very effective cooling performance of the designed cooling jacket and a possibility of large margin of load capacity beyond the 96N. Initial selection of spring geometry rendered rather soft supports compared to other bump foil bearings, and thus allowed only limited loads during the test. Measured structural stiffness and damping evidence the existence of necessary damping for stable bearing operations. Structural stiffness was highly nonlinear and showed different behaviors between the static loading and sinusoidal dynamic loading. Under the small sinusoidal loadings, presumable stick slip at the interface between spring bumps and bearing sleeve rendered rather high bump stiffness close to the clamped-end case. The measured equivalent viscous damping coefficients increased with applied load amplitudes. The orbits and coast down simulations using the calculated stiffness and measured structural loss factor indicate that the damping of underlying structure can suppress the maximum peak at the critical speed very effectively but not the hydrodynamic rotor-bearing instability. The orbit simulations also show the foil gas bearings can accommodate large rotor excursion at speeds beyond the onset speed of instability.

Commentary by Dr. Valentin Fuster
2006;():1303-1314. doi:10.1115/IJTC2006-12173.

Silicon based power MEMS applications require the high-speed micro-rotating machinery to operate stably over a large range of operating conditions. The technical barriers to achieve stable high-speed operation using micro-gas-bearings are governed by: (1) stringent fabrication tolerance requirements and manufacturing repeatability, (2) structural integrity of the silicon rotors, (3) rotordynamic coupling effects due to leakage flows, (4) bearing losses and power requirements, and (5) transcritical operation and whirl instability issues. Over the past few years, a large body of research was conducted at MIT to address these technical challenges; many lessons were learned and new theories were developed related to the dynamic behavior of micro-gas journal and thrust bearings. Based on the above mentioned experience, a gas-bearing supported micro-air turbine was developed with the objectives of demonstrating repeatable, stable high-speed gas-bearing operation and verifying the new micro-gas-bearing analytical models. The key challenge in this endeavor involved the synthesis and integration of the newly-developed gas-bearing theories and insight gained from extensive experimental work. The focus of this paper is on the process and the outcomes of this synthesis, rather than the details and results of the underlying theoretical models which have been previously published. The characteristics of the new micro-air turbine include a four-chamber journal bearing feed system to introduce stiffness anisotropy, labyrinth seals to avoid rotordynamic coupling effects of leakage flows, a reinforced thrust bearing structural design, a redesigned turbine rotor to increase power, a symmetric feed system to avoid flow and force non-uniformity, and a new rotor micro-fabrication methodology. A large number of test devices were successfully manufactured demonstrating repeatable bearing geometry. More specifically, three sets of devices with different journal bearing clearances were produced to investigate the dynamic behavior as a function of bearing geometry. Experiments were conducted to characterize the “as fabricated” bearing geometry, the damping ratio, and the natural frequencies. Repeatable high-speed bearing operation was demonstrated using isotropic and anisotropic bearing settings reaching whirl ratios between 20 and 40. A rotor speed of 1.7 million rpm (equivalent to 370 m/s blade tip speed or a bearing DN of 7 million mm-rpm) was achieved demonstrating the feasibility of MEMS based micro-scale rotating machinery and validating key aspects of the micro-gas-bearing theory.

Commentary by Dr. Valentin Fuster
2006;():1315-1326. doi:10.1115/IJTC2006-12174.

Ultra-short micro-scale high-speed gas bearings exhibit a whirl instability limit and dynamic behavior much different from conventional hydrostatic gas bearings. In particular, the design space for stable high-speed operation is confined to a narrow region and involves singular behavior. The previously developed ultra-short gas bearing theory (Liu et al. [1]) assumed fully-developed flow in the journal bearing gap. There is experimental evidence that this assumption might not be fully applicable for the relatively short flow-through times in such bearings. This has an impact on the estimation of whirl instability onset, bearing operability and power requirements. In this paper, unsteady flow effects in the bearing gap are investigated with the goal to quantify their impact on bearing dynamic behavior. It is shown that although three-dimensional flow calculations in the ultra-short journal bearing are necessary to quantify the onset of whirl instability, the underlying mechanisms can be qualitatively described by the impulsive starting of a Couette flow. Using this description, two time scales are identified that govern the journal bearing dynamic behavior: the viscous diffusion time and the axial flow-through time. Based on this, a reduced frequency parameter is introduced that determines the development of the flow field in the journal bearing and, together with bearing force models, yields a criterion for whirl instability onset. Detailed three-dimensional CFD calculations of the journal bearing flow have been conducted to assess the criterion. Singular behavior in whirl-ratio as a function of the reduced frequency parameter is observed, verifying the refined stability criterion. Using high-fidelity flow calculations, the effects of unsteady journal bearing flow on whirl instability limit and bearing power loss are quantified, and design guidelines and implications on gas bearing modeling are discussed. The stability criterion is experimentally validated demonstrating repeatable, stable high-speed operation of a novel micro-bearing test device at whirl-ratios of 35.

Commentary by Dr. Valentin Fuster
2006;():1327-1335. doi:10.1115/IJTC2006-12189.

In spite of many inherent advantages, foil bearing’s design tools considered temperature effects are in the existence not much due to the non-linear property of bump foils. In this present, when the external force was applied to the foil bearing under the high temperature environment, the dynamic characteristics of the bump foils were investigated by plane strain-stress method of FE analysis. Test results are presented for the standard specimen which is thickness, 0.12mm, half length, 1.8mm and the bump height, 0.45mm, using by an high temperature exciter system up to 680°C. The bump foil is fixed at one end of the lower plate. Each pad is 50mm by 50mm and made of INC X-750 material. As the upper plate moves or vibrates to the vertical direction, bumps are deformed in both the vertical and horizontal directions. The measured data includes stiffness and damping of bump foil (l = 1.8∼2.3mm, h = 0.45mm). These results agree with that of FE results. When the bump foil are heated up, the stiffness characteristics of bump foil bearings decreases linearly and the damping effect of bump suddenly decreases due to the Coulomb friction between the top foil and the bump foil, and the bump foil and the plate, also, thermal effect of bump foils.

Commentary by Dr. Valentin Fuster
2006;():1337-1343. doi:10.1115/IJTC2006-12220.

Flows of granular particles in sliding contacts have been proposed as a “dry” lubrication mechanism. Granular flows are complex flows that exhibit fluid and solid behavior. A transparent granular shear cell (GSC) with an annular geometry was constructed to study their behavior. The GSC operates at variable speeds and its wheel roughness was fabricated to physically match the well-known theoretical boundary conditions of Jenkins and Richman. Effects of the physical variables on the local granular flow parameters are presented. A continuum modeling approach, known as granular kinetic lubrication (GKL), was employed to predict the experimental data. Good qualitative and modest quantitative agreements between the experiments and model were obtained.

Commentary by Dr. Valentin Fuster
2006;():1345-1360. doi:10.1115/IJTC2006-12244.

Research efforts related to dry particulates in sliding contacts are reviewed. In the tribology community, there are primarily two types of dry particulate lubricants that are studied—granular and powder. Granular lubricants usually refer to dry, cohesionless, hard particles which transfer momentum and accommodate surface velocity differences through shearing and rolling at low shear rates, and collisions at high shear rates. Powder lubricants refer to dry, cohesive, soft particles which accommodate surface velocity differences mostly by adhering to surfaces and shearing in the bulk medium, in a manner similar to hydrodynamic fluids. Spanning the past five decades, this review proposes a classification system for the scientific works in the dry particulate tribology literature in terms of theory, experiments, and numerical simulations. It also suggests that these works can be further categorized based on their tribosystem geometry—annular, parallel, and converging.

Commentary by Dr. Valentin Fuster
2006;():1361-1362. doi:10.1115/IJTC2006-12328.

High operating speeds and temperatures required for advanced turbomachinery necessitate the development of bearings capable of continuous operation between 3 and 4 million DN at temperatures up to 820°C. Non-contact oil-free bearings such as compliant foil bearings, active magnetic bearings and hybrid foil/magnetic bearings are alternate solutions to the current liquid-lubricated hydrodynamic and rolling element bearings, with limited life under these extreme conditions. A critical component in these oil-free bearings is the tribological coating system that must be used on the journal and the foil pads to ensure reliable operation during transient periods and start-stop cycles. The purpose of the present investigation was to assess the reliability of tribological coatings for a large (150 mm diameter) hybrid foil/magnetic bearing.

Commentary by Dr. Valentin Fuster
2006;():1363-1364. doi:10.1115/IJTC2006-12329.

One of the fields of tribology is to predict wear to ensure reliability for mechanisms that mechanical engineers design. Since the middle of the XXth century, lots of wear investigations have been done. Nowadays we can find a huge list of wear mechanisms (abrasion, adhesion, corrosion, etc) and more than one hundred wear laws, involving more than one hundred independent parameters. For Meng & al. [1], “the available equations are so confusing that few designers can use any of them to predict product life with confidence”. For Godet [2] “adhesion, abrasion, fatigue etc. are not wear but [only] particle detachment mechanisms”. And finally, from [1], we shall “develop full descriptions of the evolution of macroscopic events on sliding surfaces. This must include a description of the formation and movement of fragmented particles in the interface region”. The medium at the interface, constituted by detached particles from the materials or even by the oil in the case of lubrication, is then called “third body”.

Topics: Wear , Simulation
Commentary by Dr. Valentin Fuster
2006;():1365-1366. doi:10.1115/IJTC2006-12334.

Polymers for tribological applications in machines offer the advantages of simple designs, reduced manufacturing complexity and lower maintenance costs. However, polymers are far more sensitive to friction-generated temperature than are metals or ceramics. Some typical tribological polymers are being evaluated in dry sliding by a thermo tribometer that uses a thrust washer configuration [1,2]. The polymer is pressed against and slides on a ring-shaped, thermocouple-instrumented metal anvil mounted on a torque cell thus providing for simultaneous measurement of friction and temperature. Previously reported finite element estimates of temperature distribution in the polymer-metal couple have been used to guide the design of and testing with the thermo tribometer [3]. Reported here are experimental results of friction and temperature rise for three polymers (acetal, nylon 6/6, UHMWPE) sliding against metal anvils (stainless steel, aluminum) with different thermal diffusivities. Results were obtained with normal pressures of 93–465kPa and sliding speeds of 0.81–4.06 m/s during the first 300 seconds of sliding.

Commentary by Dr. Valentin Fuster
2006;():1367-1368. doi:10.1115/IJTC2006-12341.

Foil gas thrust bearings are a critical component enabling oil-free turbomachinery, but because of the strong effect bearing compliance has on the hydrodynamic gas film, current numerical simulation efforts require experimental measurement of gas film quantities for verification. In this study, experimentally measured bearing torque over a range of speeds and loads is used to draw conclusions about the gas film characteristics. Assuming an isothermal gas film of constant thickness, the torque measurements give typical reference values for bearing compressibility number and film thickness under normal operation. These data are presented for use in the development of more accurate foil thrust bearing numerical models.

Commentary by Dr. Valentin Fuster
2006;():1369-1373. doi:10.1115/IJTC2006-12364.

Recent breakthrough improvements in foil gas bearing load capacity, high temperature tribological coatings and computer based modeling has enabled the development of increasingly larger and more advanced Oil-Free turbomachinery systems. Successful integration of foil gas bearings into turbomachinery requires a step wise approach that includes conceptual design and feasibility studies, bearing testing, and rotor testing prior to full scale system level demonstrations. Unfortunately, the current level of understanding of foil gas bearings and especially their tribological behavior is often insufficient to avoid developmental problems hampering commercialization of new applications. A new approach loosely based upon accepted hydrodynamic theory, has been suggested by DellaCorte, et al (1). The proposed “Foil Gas Bearing Performance Map” is intended to guide the integration process. This map, which resembles a Stribeck curve for bearing friction, is useful in describing bearing operating regimes, performance safety margins, the effects of load on performance and limiting factors for foil gas bearings. The previously proposed “Foil Gas Bearing Performance Map” has been based, in large part, on qualitative experience in the authors’ test facilities as well as product development case studies. A limited amount of test data specific to the understanding of the performance map has been experimentally obtained. A project is underway to obtain a larger, more rigorous data set. The objective of the current work is to take an analytic approach toward understanding the behavior of the various regions of this map. These analytic approaches can then be useful in both guiding further experimental investigations as well as providing useful non-dimensionalization schemes which may allow the performance map to be generalized, akin to the Stribeck curve. The foil journal bearing possesses several unique features that distinguish its analysis from those of the traditional hydrodynamic journal bearing. These features include the use of a gas as a lubricant, compliant structures / hydrodynamic surfaces, and interference fits (statically preloaded bearings). The combination of these features is considered in the proposed approach towards explaining the features of the foil gas bearing performance map.

Topics: Gas bearings
Commentary by Dr. Valentin Fuster

Special Symposia on Contact Mechanics

2006;():1375-1376. doi:10.1115/IJTC2006-12213.

We have studied the lubricating properties of fatty acids with varying degree of unsaturation (0–2 double bonds), adsorbed onto steel surfaces. A correlation is seen between the molecular packing and the reduction of friction. A linear dependence of the friction force on load is found in systems with low adhesion, and a non-linear dependence when the adhesion is higher. A contact mechanics model by Sridhar, Johnson and Fleck (SJF) is used to evaluate the data obtained in systems with higher adhesion.

Commentary by Dr. Valentin Fuster
2006;():1377-1378. doi:10.1115/IJTC2006-12268.

This study presents a basic step towards the selection methodology of electric contact materials for microelectromechanical systems (MEMS) metal contact switches. This involves the interrelationship between the two important parameters, resistivity and hardness, since they provide the guidelines and assessment of the contact resistance, wear, deformation, and adhesion characteristics of MEMS switches. For this purpose, thin film alloys of three noble metals; platinum (Pt), rhodium (Rh) and ruthenium (Ru) with gold (Au) were investigated. The interrelationship between resistivity and hardness was established for three amounts of alloying of these metals with gold. Thin films of gold (Au), platinum (Pt), ruthenium (Rh), and rhodium (Ru) were also characterized to obtain their baseline data for comparison. All films were deposited on silicon substrates. When Ru, Rh, and Pt are alloyed with Au, their hardness generally decreases but resistivity increases. This decrease or increase was, in general, dependent upon the amount of alloying.

Commentary by Dr. Valentin Fuster
2006;():1379-1383. doi:10.1115/IJTC2006-12270.

An analytical model has been developed to investigate the rotational dynamics of a rigid microplate positioned at a liquid-liquid interface in a closed microfluidic channel completely filled with two immiscible liquids. The model considers two-dimensional microwaves that develop at the interface between two isotropic liquids with different densities, viscosities and surface tensions. Interfacial tension occuring at the liquid-liquid-solid perimeter line around the microplate can magnify the microplate’s response amplitude by orders of magnitude depending on microwave amplitudes and frequencies, as well as on microplate and microbeam principal dimensions. Such effect can be used to control the sensitivity of microsensor instrumentation as well as to enhance a microsensor’s output signal.

Commentary by Dr. Valentin Fuster
2006;():1385-1389. doi:10.1115/IJTC2006-12272.

The measurements of nanomechanics have been established to study the effects of strain burst and pile-up on the hardness and elastic modulus of single crystal copper samples. For nanoindentation of the single crystal copper subjected to various loadings, the residual depths have been analyzed and compared with the cross sectional depth profile obtained by the atomic force microscope (AFM). A penetration depth around 5 to 8 nm is observed before the strain burst. It is found that the critical load to cause the first burst slightly decreases with increasing loading rate. A new graphical method based on the 3-D image of indent was proposed to eliminate the hardness and elastic modulus errors induced by the pile-up effect. The hardness and elastic modulus are not influenced by the value of maximum load and reduced to around 95% of their direct measurement values based on the Oliver-Pharr method. In comparison, the hardness decreases with increasing maximum load and shows strong dependence with the maximum load applied when the work of indentation method was employed.

Commentary by Dr. Valentin Fuster
2006;():1391-1397. doi:10.1115/IJTC2006-12274.

The first part of the paper proposed a semi-analytical method for the tridimensional thermal-elastic-plastic contact between two hemispherical asperities. The algorithm has been described for both the load-driven (ld) and the displacement-driven (dd) formulations, and validated through a nano-indentation test simulation. The way to consider rolling and sliding motion of the contacting bodies consists of solving the elastic-plastic contact at each time step while upgrading the geometries as well as the hardening state along the moving directions. The derivations concerning the interference calculation at each step of the sliding process are then shown, and an application to the tugging between two spherical asperities in simple sliding (dd formulation) is made. The way to project the forces in the global reference is outlined, considering the macro-projection due to the angle between the plane of contact and the sliding direction, and the micro-projection due to the pile-up induced by the permanent deformation of the bodies due to their relative motion. Finally a load ratio is introduced and results are qualitatively compared to a two-dimensional FEM analysis presented elsewhere.

Commentary by Dr. Valentin Fuster
2006;():1399-1405. doi:10.1115/IJTC2006-12276.

The friction coefficient between a guide and a tape sample is determined experimentally as a function of nominal tension, speed and guide radius. A comparative study between metal particulate and metal evaporated tape is performed. The tape/guide contact pressure correlated to the experimental results. Tape temperature measurements are compared with the experimentally obtained friction coefficients.

Topics: Friction , Design
Commentary by Dr. Valentin Fuster
2006;():1407-1414. doi:10.1115/IJTC2006-12279.

One potential method to accomplish high-rate nanomanufacturing is to develop processes which allow for rapid transfer of nano-scaled devices from a template to a device wafer [1]. In order to accomplish this transfer, the device wafer must make intimate contact with the template. A similar situation exists in wafer bonding, except that in that case the two wafers remain in bonded contact due to surface energy even after the applied pressure is removed. We need to maintain intimate contact during transfer, while allowing for easy separation and limiting the contact pressure. Wafers typically have waviness and bow which cause a deviation of many micrometers from flatness over the 15-mm length scale of a typical chip. This non-flatness can be a serious problem in the transfer of nanometer-scale elements. We have developed a model that allows us to examine the effects of applied pressure, bow radius, and surface energy on the flattening of a spherically/cylindrically bowed chip. This model uses elastic plate theory and surface energy. An operating window is found which provides intimate contact while allowing for separation after the pressure is removed. It is also shown that the effect of adhesion is to produce a discontinuity in the internal bending moments at the separation boundary, which is proportional to the square-root of the adhesion energy.

Commentary by Dr. Valentin Fuster
2006;():1415-1416. doi:10.1115/IJTC2006-12281.

The behavior of an elastic-plastic contact of a deformable sphere and a rigid flat under combined normal and tangential loading with full stick contact condition is investigated theoretically. This allows static friction modeling under highly adhesive conditions. Combined loading begins with a normal preload that produces an initial contact area and causes elastic or elastic-plastic deformations in the contact zone. The full stick contact condition leads to a junction between the contacting bodies which can bear tangential loading. On the second stage of the loading process, the tangential load is being applied gradually, while the normal load remains constant. The maximum tangential load that can be supported by the junction prior to sliding inception is determined as the static friction. The effect of normal load on this static friction is investigated. The evolution of the contact area during the tangential loading revealed an essential “junction growth” mainly just before sliding inception.

Commentary by Dr. Valentin Fuster
2006;():1417-1418. doi:10.1115/IJTC2006-12282.

An experimental test rig was developed in order to investigate elastic-plastic single micro-spherical contact under combined normal and tangential loading. This novel apparatus allows in situ and real time direct optical measurement of the real contact area (RCA) evolution in pre-sliding. It also allows relative displacement measurements under very low rates of tangential loading (down to 0.01N/s) to capture accurately the fine details at sliding inception. The RCA measurement is realized by direct optical observation technique, whereas two different image processing algorithms were implemented for the elastic and the elastic-plastic contact regimes.

Commentary by Dr. Valentin Fuster
2006;():1419-1421. doi:10.1115/IJTC2006-12284.

The influence of both full stick and perfect slip contact conditions on the unloading and multiple loading of a deformable sphere by a rigid flat is investigated using the finite element method for various values of the Poisson’s ratio and the modulus of elasticity over the yield strength ratio. The residual deformations and stress field within the sphere tip, as well as changes in the size of the contact area following multiple loading cycles are analyzed.

Commentary by Dr. Valentin Fuster
2006;():1423-1424. doi:10.1115/IJTC2006-12285.

A recent paper on wear modeling has been completed [1]. A previous fast and robust three-dimensional contact computation tool taking into account the effect of cyclic wear induced from fretting solicitations under gross slip conditions is extended to partial slip conditions.

Topics: Wear , Modeling , Stick-slip
Commentary by Dr. Valentin Fuster
2006;():1425-1427. doi:10.1115/IJTC2006-12286.

The modeling of the heat source in TIG welding may take different forms, ranging from surface heat flux with uniform or Gaussian distribution, to volumetric heat distribution with complex shapes such as a double ellipsoid. Once the mathematical model for the heat source is chosen for a given steady state calculation, an efficiency parameter η is fitted to adjust the simulated temperatures considering the measured ones, on some points of the surface plate. It is shown here that the way in which the heat flux density is spread (at the surface or in volume) has limited impact on the results of the macroscopic thermo-mechanical simulation, if the total input energy ηUI is kept constant.

Commentary by Dr. Valentin Fuster
2006;():1429-1433. doi:10.1115/IJTC2006-12287.

Typically, conformal contacts are treated in the case of centric loading, excepting circular or elliptical cross-sections of equivalent punches having a flat front surface. This paper extends the investigation of eccentric load applied to equivalent rigid punches with smooth cross-sections and flat front surface, pressed against an elastic half-space. Then, an approximate solution is proposed for equivalent rigid punches of circular cross-section having a rounded front edge. Formulae for pressure distribution and contact elements are derived.

Topics: Pressure
Commentary by Dr. Valentin Fuster
2006;():1435-1440. doi:10.1115/IJTC2006-12288.

Recently, the punch profile in circular contact was modified to yield an advantageous pressure distribution at maximum contact load. This distribution consists of a nearly flat central plateau, surrounded by a peripheral zone of monotonous pressure decrease. This paper investigates the behavior of this improved contact at lower loads than the nominal. A pressure distribution at lower loads is proposed, which is a solution of the problem. In agreement with the principle of uniqueness of solution in elastostatics, this is the only solution. Equations for central pressure, contact radius and normal approach are derived. The main feature of pressure distribution for a nominal load is preserved at all load levels.

Topics: Stress
Commentary by Dr. Valentin Fuster
2006;():1441-1443. doi:10.1115/IJTC2006-12289.

Fundamental understanding of surface forces at the nanoscale is a paramount requirement in the design, fabrication and manipulation of nanosystems. Force-displacement curve measurement in the atomic force microscope (AFM) is widely used as a measure of adhesion to quantify surface forces. However, the role of separate constituent forces in nanoscale contact formation and interfacial adhesion is not obvious. In this work, a sharp Si3 N4 tip was used in adhesion measurements to study the effect of capillary condensation by comparing pull-off forces measured using AFM in different environments. To distinguish the contribution of the capillary meniscus on adhesion, and to understand its role in contact modification in ambient conditions, an equivalent stress field between two surfaces with a meniscus is proposed, based on the water-screened van der Waals interaction and capillary condensation. Maugis-Dugdale mechanics are employed to evaluate the contact modification in ambient conditions. Calculations supporting the experimental observation suggest that although the meniscus height is less than 1 nm it is enough to modify the nanocontact significantly.

Commentary by Dr. Valentin Fuster
2006;():1445-1446. doi:10.1115/IJTC2006-12290.

The electrical contact resistance of electrostatically actuated ohmic contact type MEMS relays has been investigated. Multi-contact MEMS relays using electrostatic comb-drive actuators have been used to obtain experimental data in this study. The electrical contact resistances versus various applied voltages and thus loads have been studied. For an applied DC bias voltage of 172 V, the movable fingers make contact with the fixed fingers. The resistance versus applied voltage characteristics has been measured for an applied DC bias voltage in the range of 172 V to 220 V. This results in a predicted load on the surfaces between 1.32 μN and 2.95 μN. A new multi-scale rough surface contact model was used to estimate the real area of contact and electrical contact resistance as functions of the applied force for these devices. Neglecting mechanisms such as adhesion, the multiscale model appears to over predict the electrical contact resistance. Thus the roles of dry adhesion, liquid meniscus adhesion, and scale dependant material properties are considered. The results suggest that liquid meniscus adhesion and scale-dependant properties play a more significant role than dry adhesion in governing the electrical contact resistance. When these effects are considered, the predicted electrical resistance from the theoretical model appeared to match the measured resistance values fairly well, and without the use of any fitting parameters.

Commentary by Dr. Valentin Fuster
2006;():1447-1448. doi:10.1115/IJTC2006-12291.

A special microdevice was designed and tested to characterize electrical effects on interfacial adhesion in microelectromechanical systems (MEMS). The adhesion force was measured as a function of the voltage applied across the contact interface. An increase in the adhesion force was attributed to electrostatic attraction resulting from trapped charges. A thermal effect at contacting asperities enhanced the adhesion force significantly. At a critical voltage, microwelding at asperity contacts caused the surfaces to adhere strongly to each other. The adhered device could not be released within its operation limits, indicating the development of a very high adhesion force.

Commentary by Dr. Valentin Fuster
2006;():1449-1450. doi:10.1115/IJTC2006-12292.

The current work presents a new methodology for modeling the impact between elasto-plastic spheres. Recent finite element results modeling the static deformation of an elasto-plastic sphere are used in conjunction with equations for the variation of kinetic energy to obtain predictions for the coefficient of restitution. A model is also needed to predict the residual deformation of the sphere during rebound, or unloading, of which, several are available and compared in this work. The model predicts that a significant amount of energy will be dissipated in the form of plastic deformation such that as the speed at initial impact increases, the coefficient of restitution decreases. This is due to the maximum contact force also increasing as the initial speed at impact increases. This work also derives the initial critical speed which causes plastic deformation in the sphere and decreases the coefficient of restitution below a value of one for the completely elastic case.

Commentary by Dr. Valentin Fuster
2006;():1451-1457. doi:10.1115/IJTC2006-12293.

Micromechanical switches for RF communications often use gold as a contact material. While significant data exists for such contacts at contact forces larger than 200 mN and smaller 1 mN, little data exists at medium forces between these extremes. Moreover, previous studies have been unclear about the use of contact heating, particularly since voltages less than 100 mV can cause significant heating. This paper presents contact resistance data for both heated and unheated contacts with force from 0.01 to 1000 mN. Without heating, 1–10 mN of force is required to produce low, predictable resistance. However, with heating, forces as small as 0.05 mN produce low-resistance contacts, suggesting that contact heating is a valuable tool in removing insulating films and creating low-resistance, predictable contacts.

Commentary by Dr. Valentin Fuster
2006;():1459-1460. doi:10.1115/IJTC2006-12294.

An experimental study is performed on micro textured surfaces in order to better understand how to predict the changes in friction due to surface micro texturing. Surface testing shows that the effect of each texturing parameter on the Stribeck curve is dependant on the particular lubrication regime. By systematically changing the surface texturing parameters, the effects of surface texturing on the Stribeck curve are shown to be highly predictable. Scaling arguments must be developed to better understand and quantify the mechanisms of reduced friction.

Topics: Friction , Mechanisms
Commentary by Dr. Valentin Fuster
2006;():1461-1468. doi:10.1115/IJTC2006-12295.

A finite element model was built to study the contact of a micro hemisphere in elastic-plastic contact with a rigid flat. The effect of adhesion on the deformation and stress fields is included, using the Lennard-Jones potential, which makes this model applicable to a wide range of the Tabor parameter. Both loading and unloading are considered. In the case of Ruthenium (Ru), the results demonstrate reasonable agreement with a previous model applicable to the DMT region. However for a Gold (Au) contact, which is in the JKR region, the results differ significantly.

Commentary by Dr. Valentin Fuster
2006;():1469-1473. doi:10.1115/IJTC2006-12296.

A three-dimensional elastic-perfectly-plastic finite element model of asperity interaction has been developed. This model consists of two neighboring connecting hemispheres which contact a rigid and flat surface. For two asperities of the same radius and same height, as the approach of the rigid surface is increased, the contact area changes from isolated nearly circular contacts, to a wasp-waist area, and finally to a single nearly elliptical contact region. For asperities of different heights the results can be qualitatively similar to those for equal heights, but also exhibit a shielding effect. Graphs are given of the contact area boundary as a function of the approach, and also of force vs. interference, and contact area vs. interference for various values of asperity tip interferences, and for different asperity height differences.

Commentary by Dr. Valentin Fuster
2006;():1475-1479. doi:10.1115/IJTC2006-12297.

For many applications, radio frequency microelectromechanical systems (RF MEMS) switches require very fast switching times so that they are capable of switching thousands of times each second. Contact adhesion tends to slow switches by increasing the time required to break the contact and open the switch. Previous work showed that typical switches have a contact opening time exceeding 1 ms, which is far too slow for many applications. This paper presents a dynamic, transient model to predict the time required to break the contact. The model considers both the motion of the switch body and the adhesion at the contact. The model has been validated by comparing it to experimental data. The model shows that vibrations in the switch body can be instrumental in dramatically reducing contact opening time by a factor of nearly 1000, leading to acceptable switch speeds.

Commentary by Dr. Valentin Fuster
2006;():1481-1485. doi:10.1115/IJTC2006-12299.

The use of asymptotic solutions for the characterisation of the states of stress relevant to several contact problems is introduced. The relevance of the proposed formulations to engineering applications as a correlation device for contact fatigue is discussed, and their application to the ‘sharpening up’ of numerically obtained contact stress solutions presented.

Commentary by Dr. Valentin Fuster
2006;():1487-1492. doi:10.1115/IJTC2006-12300.

Two different elastic contact stress modeling approaches were compared to results from a flat punch compression experiment. The two approaches, which employed the Greenwood and Williamson contact stress model and the Volume Pixel (“Voxel”) asperity modeling simulation, were used to analyze the force-displacement response of a sample surface when pressed down by a smooth, rigid, flat plane to a prescribed load. The results of the model and simulation are compared to experimental measurements from the flat punch compression performed using a nanoindenter.

Commentary by Dr. Valentin Fuster
2006;():1493-1494. doi:10.1115/IJTC2006-12302.

This study models the electrical contact resistance (ECR) between two surfaces separated by an anisotropic conductive film. The film is made up of an epoxy with conductive spherical particles (metallic) dispersed within. In practical situations the particles are often heavily loaded and will undergo severe plastic deformation and may essentially be flattened out. In between the particles and the surfaces there may also be an ultra-thin insulating film (consisting of epoxy) which causes considerable electrical resistance between the surfaces. In the past this effect has been neglected and the predicted ECR was much lower than that measured experimentally. This added resistance is considered using electron tunneling theory. The severe plastic deformation of the spherical particles is modeled using a new expanded elasto-plastic spherical contact model. This work also investigates the effect of compression of the separating epoxy film on the electrical contact resistance. The model finds that the high experimental ECR measurements can be accounted for by including the existence of a thin insulating film through the electron tunneling model.

Commentary by Dr. Valentin Fuster
2006;():1495-1498. doi:10.1115/IJTC2006-12303.

Adhesion becomes increasingly important in small-scale devices owing to typically large surface area to volume ratios and the need to maintain small gaps between various components, particularly in the presence of liquid films. Understanding the coupling between elasticity and capillarity is critical to the successful modeling of liquid-mediated adhesion between elastic bodies. A previous study established a generalized model for the contact of elastic spheres in both wet and dry conditions, with and without solid-solid contact. In this paper, the model is applied to investigate the approach and detachment process of two elastic spheres. The pull-off forces conditions are extracted from the force vs. separation curves. The variation of contact radius and pull-off force with liquid volume at the interface is studied for a fixed relative strength of liquid capillarity and solid-solid adhesion. Results of the calculations for vanishing liquid volume are compared to the classic JKR model.

Commentary by Dr. Valentin Fuster
2006;():1499-1502. doi:10.1115/IJTC2006-12304.

A critical factor in the successful operation of an electromagnetic launcher (EML) is the maintenance of good electrical contact between the armature and rail. While there have been numerous experimental and theoretical investigations of armature-rail interfaces, a well-accepted model of the contact conditions has not yet been established. In the current study, we consider the interface between a stationary metallic flat and a translating pin with a hemispherical tip. Operating conditions of the experiments are designed to yield the contact pressures (∼500 MPa) and electrical current densities (∼5 GA/m2 ) existing in typical EMLs, albeit with much lower sliding speeds (∼3 m/s). Electrical current to the interface is supplied by a programmable 8V/350A power supply. The test apparatus is configured so that the pin can translate continuously along a 1.5 m rail, its carriage being powered by a direct drive linear motor. Loading of the pin against the flat is provided by means of a pneumatic actuator. Current, voltage, load, friction, surface temperature and position are monitored throughout the sliding event. Changes in surface topography and morphology are assessed via optical and stylus profilometry as well as optical microscopy. Arcing and phase change events are observed.

Commentary by Dr. Valentin Fuster
2006;():1503-1504. doi:10.1115/IJTC2006-12335.

A recently developed thermo-mechanical model was used to study the temperature distribution in a sliding contact of a cylinder and a coated real rough surface. The model conducts a full thermo-mechanical analysis of the contact including the interactions between the thermal and elastic displacements and full heat division. Following a brief description of the numerical model, results are presented to illustrate the thermo-mechanical effects of various contact parameters, coating properties and surface roughness structure.

Commentary by Dr. Valentin Fuster
2006;():1505-1506. doi:10.1115/IJTC2006-12339.

Most of the statistical proprieties of a rough surface can be derived from knowledge of two statistical functions: the frequency density function and the autocorrelation function, Bakolas [2], Bushan [3].

Commentary by Dr. Valentin Fuster
2006;():1507-1513. doi:10.1115/IJTC2006-12346.

An axisymmetrical hemispherical asperity in contact with a rigid flat is modeled for an elastic-perfectly plastic material. The single asperity finite element contact model is used for determining the contact parameters for the normal contact between two rough surfaces. The objective of this paper is to show the difference in the statistical model developed by Kogut-Etsion Model (KE Model) and Jackson-Green Model (JG Model) for the materials having higher yield strength, in calculating the contact parameters. The present analysis extends the work of KE Model and JG Model. KE model predicts that when the interference ratio (ω/ωc ) reaches 110; the mean contact pressure to yield strength ratio (p/Y) reaches 2.8, after that plastic deformation starts. JG Model extends KE model for materials of different yield strengths and concludes that hardness does not depends upon the strength of the material alone. Based on their results, they developed two statistical models to calculate the contact parameters for the entire surfaces. But JG model results violate Abbott-Firestone (AF Model) work based contact area ratio limitation. The present analysis covers higher interference ratios. The elastic perfectly plastic assumption made in the present analysis satisfies AF model results for higher yield strengths and the contact area ratio (A*/ω*) never crosses 2. The predicted results shows that the statistical model developed by KE and JG models are still not enough to calculate accurately the real contact area and contact load for the entire surfaces based on FEM single asperity model for all the yield strengths.

Commentary by Dr. Valentin Fuster
2006;():1515-1516. doi:10.1115/IJTC2006-12348.

By applying the line integral of Barnett-Lothe tensors on oblique planes, the three-dimensional rough surface contact problem for a semi-infinite anisotropic elastic half-plane in contact with a rough rigid sphere is formulated. The steady-state heat transfer condition is assumed and the technique of analytical continuation is employed. The general solutions due to varying degrees of anisotropy and mechanical boundary conditions are obtained.

Commentary by Dr. Valentin Fuster
2006;():1517-1518. doi:10.1115/IJTC2006-12352.

This work seeks to characterize the component of friction which arises from energy being dissipated via elasto-plastic deformation during sliding contact (as apposed to adhesive mechanisms). The sliding interaction between spheres is analyzed using two approaches (a semi-analytical and finite element simulation). These analyses are used to formulate empirical equations which describe the average tangential and normal forces resulting from the sliding interaction. A parametric study of the properties of typical metals is then used to help verify the effectiveness of the empirical equations. The study shows that the effective friction coefficient between spherical asperities increases with the elastic modulus, decreases with yield strength, and increases with the interference between the contacts (dependant on the normal load).

Commentary by Dr. Valentin Fuster

Special Symposium on Nanotribology

2006;():1519-1526. doi:10.1115/IJTC2006-12393.

As the head-disk spacing reduces in order to achieve the areal density goal of 1 Tb/in.2 , dynamic stability of the slider is compromised due to a variety of proximity interactions. Lubricant pickup by the slider from the disk is one of the major reasons for decrease in the stability as it contributes to meniscus forces and contamination. Disk-to-head lubricant transfer leads to lubricant pickup on the slider and also causes depletion of lubricant on the disk. In this paper we experimentally and numerically investigate the process of disk-to-head lubricant transfer using a half-delubed disk, and we propose a parametric model based on the experimental results. We also investigate the dependence of disk-to-head lubricant transfer on the disk lubricant thickness, lubricant type and the slider ABS design. It is concluded that disk-to-head lubricant transfer occurs without slider-disk contact and there can be more than one timescale associated with the transfer. Further, the transfer increases non-linearly with increasing disk lubricant thickness. Also, it is seen that the transfer depends on the type of lubricant used, and is less for Ztetraol than for Zdol. The slider ABS design also plays an important role, and a few suggestions are made to improve the ABS design for better lubricant performance.

Topics: Lubricants , Disks
Commentary by Dr. Valentin Fuster

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