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TECHNICAL PAPERS

J. Tribol. 1999;123(3):441-443. doi:10.1115/1.1286152.

The application of nanoparticles in tribology has received considerable attention in recent years. In this paper, we described the preparation of Ni nanoparticles in microemulsions of sodium dodecyl sulphate (SDS)/isopentanol/cyclohexane/water and their tribological performance as additives in oils. The size of these nanoparticles is about 10 nm, and their shape is spherical. The four-ball test results indicate that Ni nanoparticles are potential additives for lubricating oils, and the tribological performance of lubricating oils can be improved significantly by dispersing Ni nanoparticles in oils. The maximum non-seizure load (PB ) has been increased by 67 percent as compared with background oil. The wear scar diameter has been reduced from 0.71 mm to 0.49 mm. The relative percentage in friction coefficient has decreased 26 percent. The tribological mechanism is that a deposit film in the contacting regions was formed, which prevented the direct contact of rubbing surfaces and greatly reduced the frictional force between the contacting surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2001;123(3):444-454. doi:10.1115/1.1353181.

A comprehensive method of thermo-elastohydrodynamic lubrication analysis for connecting rod bearings is proposed, which includes thermal distortion as well as elastic deformation of the bearing surface. Lubrication film temperature is treated as a time-dependent, two-dimensional variable which is averaged over the film thickness, while the bearing temperature is assumed to be time-independent and three-dimensional. It is assumed that a portion of the heat generated by viscous dissipation in the lubrication film is absorbed by the film itself, and the remainder flows into the bearing structure. Mass-conserving cavitation algorithm is applied, and the effect of variable viscosity is included in the Reynolds equation. Simulation results of the connecting rod bearing of an internal combustion engine are presented. It is shown that the predicted level of the thermal distortion is as large as that of the elastic deformation and the bearing clearance, and that the thermal distortion has remarkable effects on the bearing performance. Therefore, the thermo-elastohydrodynamic lubrication analysis is strongly recommended to predict the performance of connecting rod bearings in internal combustion engines.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):455-461. doi:10.1115/1.1310158.

A recently postulated concept of friction between solids states that the process exhibits elements of quasi-hydrodynamic lubrication in which the wear particles shed from one or both surfaces form an interface layer acting as a lubricant. Pellets were formed by compacting MoS2 powder. A series of pellet-on disk tests were conducted to intentionally deposit wear particles on to a disk. The purpose of the tests are to aid in characterizing the particle detachment process of the pellet. This process consists of analyzing the transfer film, wear and frictional behavior of the pellet. This work also seeks to show that a MoS2 pellet can transfer a film capable of exhibiting tribological characteristics similar to those of liquid lubricants. For example, some tests were correlated with a dimensionless grouping of variables known as the Sommerfeld Number, to suggest the presence of quasi-hydrodynamic behavior in powder films. The pellet parameters that were analyzed were the particle size, compaction pressure and the effects of external loading during testing. Some of the conclusions arrived at were that smaller compaction pressures yield smaller friction coefficients and greater normal loads placed on pellets help to ensure the proper performance of the lubricating powder films. Data from the wear rate of the pellet and friction coefficient, in addition to the appearance of the wear particles were studied. An interesting feature of the pellet friction is that the longer the rest period in between test runs, the greater the friction values are at the resumption of testing. The present work is part of a continuing series to demonstrate that the process of so-called “dry” friction is not confined to the morphology of the interacting surfaces but that it is also a function of the shear and flow occurring at the interface.

Commentary by Dr. Valentin Fuster
J. Tribol. 1999;123(3):462-468. doi:10.1115/1.1332396.

An elastohydrodynamic lubrication model is presented for the coupled problem of a hydrodynamic lubricating fluid in an elastic structure that includes distributed structural inertia. The problem is formulated and the governing equations solved with the finite element method for an illustrative journal bearing subject to dynamic loading. Inertia effects are demonstrated through comparisons with an existing quasi-static model. While it is true that structural inertia can be neglected without significant loss of accuracy for many journal bearing applications, the new model presented does capture effects of distributed structural inertia where such effects are important and exhibits improvements over existing methods with respect to numerical stability.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):469-473. doi:10.1115/1.1308009.

An important mechanism reducing the lifetime of the deepdrawing tools is material transfer from the workpiece to the tool, resulting in the buildup of lumps on the tool surface. In this study, a lump growth model, based on transfer mechanisms observed in experiments, is developed. The development of the summit height distribution of the tool surface is calculated from the growth behavior of the individual lumps. Important parameters influencing material transfer are determined from the calculated summit height distributions. It follows that lump growth can be decreased and, under certain conditions prevented, by a higher (surface) hardness of the workpiece, a lower roughness of the tool surface, a lower nominal contact pressure, and a lower shear strength of the interface. These results are in general agreement with results obtained from experiments. Experimental results are presented in an accompanying paper (de Rooij and Schipper, 2000).

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):474-478. doi:10.1115/1.1308023.

In this study, the lump growth model, described in an accompanying paper (de Rooij and Schipper, 2000) is validated by means of experiments performed on a deepdrawing simulator. In the experiments, the influence of material and roughness properties of both sheet and tool on the galling behavior is determined. For these experiments, a deepdrawing simulator and a selection of aluminum and zinc coated sheets with several (coated) deepdrawing tools are used. Good agreement is found between results of the lump growth model and the sheet metal forming experiments.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):479-485. doi:10.1115/1.1308010.

Frictional heating due to the relative motion of contacting surfaces causes temperature rise and thermal distortion, which in turn affects the contact geometry and pressure distribution. A fast and effective method is presented for the calculation of the normal surface displacement of an elastic halfspace due to arbitrary transient surface heating. The method uses Fourier-transformed Green’s functions (frequency response functions), found in the closed form by using the approach of Seo and Mura and the heat conduction analyses of Carslaw and Jaeger. The frequency response functions are shown analytically to be the frequency domain representations of the Green’s functions given by Barber. The formulation for the surface normal displacement is in the form of three-dimensional convolution integrals (over surface and time) of the arbitrary transient heat flux and the Green’s functions. Fourier transforms of these convolution integrals are taken, avoiding the Green’s-function singularities and giving a simple multiplication between the transformed heat flux and the (known) frequency response functions. The discrete convolution–fast Fourier transform (DC-FFT) algorithm is applied for accurate and efficient calculations of the normal surface displacement from the frequency response functions for an arbitrary transient heat input. The combination of the frequency-domain formulation and the DC-FFT algorithm makes the solution of transient thermoelastic deformation extremely fast and convenient.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):486-493. doi:10.1115/1.1308022.

A procedure is developed to evaluate and rank tribomaterials based on wear parameters using a vectorial approach. The method known as the Hasse Diagram approach allows one to graphically compare tribomaterials based on a large number of wear parameters or test results which might otherwise be very confusing. The inclusion of uncertainty and a provision to include weighting factors so that the ranking scheme does not change when additional tribomaterials are added have also been discussed. An example of eight tribomaterials has been presented for ranking based on five wear parameters.

Topics: Wear , Wear testing
Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):494-500. doi:10.1115/1.1308026.

Two possible, long standing speculated mechanisms are theoretically investigated in an attempt to understand previous experimental observations of pressure build up in the cavitation zone of a submerged journal bearing. These mechanisms are (1) the shear of the cavity gas bubble by a thin lubricant film dragged through the cavitation zone by the rotating shaft and (2) the mass transfer mechanism which dictates the rate of diffusion of dissolved gas out of and back into the lubricant. A comparison with available experimental results reveals that while the cavitation shape is fairly well predicted by the “shear” mechanism, this mechanism is incapable of generating the level of the experimentally measured pressures, particularly towards the end of the cavitation zone. The “mass transport” mechanism is found inadequate to explain the experimental observations. The effect of this mechanism on the pressure build up in the cavitation zone can be completely ignored.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):501-508. doi:10.1115/1.1308027.

Recently, graphite porous material has been used successfully in an aerostatic bearing. In actual bearing design, it is often necessary to reduce the thickness of porous material to make the bearing smaller. However, a reduction in thickness results in a reduction in the strength of the porous material. In particular, when the diameter of porous material is large, it is difficult to supply the air through the full pad area of porous material because it deforms. Therefore, in this paper, two types of air supply method (the annular groove supply and the hole supply) in a circular aerostatic porous thrust bearing are proposed to avoid the deflection of the bearing surface. The static and dynamic characteristics of aerostatic porous bearing with these air supply methods are investigated theoretically and experimentally. In addition, the effects of a surface restricted layer on the characteristics are clarified.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):509-516. doi:10.1115/1.1308033.

In this paper, two types of stationary guides are presented to improve the web spacing characteristics; hybrid hollow porous guide for maintaining the sufficient web spacing and grooved guide for removing the entrained air. In the hybrid hollow porous guide, the pressurized air is added to the lubrication air flow between web and guide from inside the guide. Then, the web spacing characteristics are improved by the hybrid effects of hydrodynamic pressure due to web movements and hydrostatic pressure due to pressurized added air through the guide. The relations between web spacing and web velocity are measured under various supply pressures. Moreover, the relations between web spacing and web tension are examined. The contactless optical sensor, which can measure the variation of the quantities of reflected light from the back surface of web according to the variation of web spacing, is used to obtain the web spacing under various conditions. In the grooved guide, the method for the estimation of web spacing by using the concept of equivalent spacing between web and guide is presented, and the web spacing is measured by the same experimental apparatus used in the case of hollow porous guide. From the calculated and measured results obtained, the effectiveness of two types of guides on the web spacing is clarified. Finally, based on the assumptions of Gaussian distributions of asperities, the simple relations to predict the friction coefficient between web and guide surfaces are formulated, and the variation of friction coefficient with velocity is shown for various values of web tension.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):517-524. doi:10.1115/1.1308028.

In all previous numerical simulations of the rotary lip seal, the sealing surface was modeled by regular periodic structures. In the present study, a more realistic quasirandom surface is used. A mixed elastohydrodynamic analysis is used to generate predictions of such seal operating characteristics as friction coefficient, reverse pumping rate, film thickness distribution, hydrodynamic and contact pressure distributions, contact area, and cavitation area. The results are in qualitative agreement with previous experimental observations. In the course of the simulations, a new physical mechanism of reverse pumping has been identified.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):525-532. doi:10.1115/1.1308029.

A thermohydrodynamic (THD) analysis of a fully circumferentially grooved hydrodynamic bearing is presented. The pressure distribution is obtained using the short bearing approximation taking into account the viscosity variation in the radial and circumferential coordinates. The axial temperature variation is also included by an axial averaging technique, which incorporates the supply pressure and film entry temperature in the energy equation. It is found that the determination of the lubricant temperature at the entry to the film plays an important role in the overall temperature distribution in the bearing. A simplified approach for determining this temperature is presented. An extensive set of experimental results performed by Maki and Ezzat (1980, ASME J. Lubr. Technol., 102 , pp. 8–14) is used for validation purposes. The results show that mixing in the inlet groove may cause the film entry temperature to be significantly different from the nominal supply temperature and hence have a significant influence on the bearing temperature.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):533-540. doi:10.1115/1.1308030.

The lubrication mechanism when the magnetic head is sliding on the magnetic disk in the drag test with cyclic interval rest periods, is discussed. A long rest period decreased the wear of disks and friction force. This result indicated that the reflow of lubricant into the contact areas decreased the wear of texture asperities. Next, the molecular spreading behavior of a lubricant film was calculated by Monte Carlo method to estimate the effect of lubricant recovering velocities. The lubricant replenishment into a thinner lubricant area made by an etching technique was measured. The measured profiles of lubricant replenishment were approximately agreed with the calculated profiles using a spreading model. It was clearly estimated that the reflow velocities changed in various cases of the environmental temperature, the radius of removal lubricant area, and the thickness of the lubricant films. Experimentally, the dependencies of temperature, the radius of removal lubricant area, and the lubricant thickness were measured in the drag test. This lubricant-spreading model adequately explained these results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):541-547. doi:10.1115/1.1308031.

Flying attitudes of the slider, which are flying height, pitch, and roll, are affected by air-flow velocity, skew angle, and manufacturing tolerances. In the traditional design process of air-bearing surfaces, we have considered only the steady state flying attitude over the recording band. To reduce the flying height variation during track seek as well as in steady state, we design a new shape for air-bearing surfaces. An optimization technique is used to improve the dynamic characteristics and operating performance of the new air-bearing surface shapes. The quasistatic approach is used in the numerical simulation of the track seek operation because the skew angle effect dominates the inertial effect even at high seek velocities. The perturbation method is applied to the lubrication equation to obtain the air-bearing stiffness. We employ the method of modified feasible directions and use the weighting method to solve the multicriteria optimization problem. The optimally designed sliders show enhanced flying and dynamic characteristics. The steady state flying heights are closer to the target values and the flying height variations during track seek operation are smaller than those for the original ones. The pitch and roll angles are kept within suitable ranges over the recording band during track seek operation as well as in steady state. The air-bearing stiffnesses of the optimally designed sliders are larger than those of the original ones.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):548-554. doi:10.1115/1.1308035.

Time-frequency analysis and wavelet transforms are employed to investigate transient contact dynamics at the head/disk interface of computer hard drives. Wavelet transforms are used to resolve multiple short consecutive contacts at high time resolution at high frequencies. The reassignment method is applied to the time-frequency distribution to enhance the time-frequency resolution, thereby allowing to resolve air-bearing and slider body frequencies simultaneously. The results indicate that strong impacts between slider and disk can lead to excitations of slider body and suspension vibrations. Finite element modal analysis of nano and pico glide sliders is found to be in good agreement with experimentally measured frequencies.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):555-560. doi:10.1115/1.1308036.

A lubrication model for the head-media interface is presented which includes the effect of porosity in the media coating. Experimental data is shown which illustrates the reduction in head-media spacing as porosity is increased. A modified Reynolds equation is derived to account for the effects of coating porosity. Other authors have considered a very thin porous layer to simulate a liquid lubricant or surface microstructure on a nonporous substrate. This study considers a porous layer that can be much larger than the bearing clearance. Darcy’s law is used in the porous layer. Velocity-slip effects, resulting both from rarefaction and the porous boundary, are considered. The modified Reynolds equation is applied to a simple capillary model of a porous layer as an illustrative example. The modified Reynolds equation was incorporated into a finite-element model for the head-media interface. Computations show reduced head-media clearance as porosity and permeability are increased in agreement with experimental data.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):561-565. doi:10.1115/1.1308037.

To achieve an areal density approaching 50 Gb/in.2 for the magnetic storage of data in hard disk drives requires reduced mechanical and magnetic spacing. Off-track jitter caused by airflow or contact can cause track misregistration on the order of 20–70 nm which may be excessive for adequate servo performance. The magnetic signal can be used to identify both the vertical spacing modulation due to the air bearing modes and off-track jitter due to suspension modes with nanometer resolution. We find that the off-track jitter in the flying regime is driven by airflow and is a strong function of the disk velocity and the suspension type. In the contact regime, the vertical spacing modulation and off-track jitter increase due to contact. Using a laser Doppler vibrometer we identified the leading contribution to the off-track jitter to be primarily the first torsional mode (T1) and to a lesser extent the first bending mode (B1) of the suspension.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):566-571. doi:10.1115/1.1308038.

The paper presents an analytical and experimental investigation aimed at eliciting the thermal characteristics of air lubricated compliant foil bearings. A Couette Approximation to the energy equation is used in conjunction with the compressible Reynolds equation to obtain a theoretical temperature distribution in the air used as a lubricant. The effect of temperature on the thermal properties of the working fluid is included. In parallel, an experimental program was run on a 100 mm diameter foil bearing operating at speeds up to 30,000 rpm employing cooling air across the bearing. The temperature rise of the cooling air provided an indication of the amount of heat energy conducted across the top foil of the bearing from the hydrodynamic film. The temperatures resulted from some tests are compared with the temperatures predicted by the analysis, and maximum over-prediction of about 19 percent was obtained. This simplified approach provides us with reasonably predicted temperatures. By comparing the theoretical heat dissipation obtained from the analytical predicted temperatures with the amount of heat carried away by the cooling air it was possible to arrive at the relative quantities of heat transferred from the bearing by convection via side leakage and by conduction via the top foil. From these comparisons it was deduced that about an average of 80 percent of the heat energy is carried away by conduction. The transient temperatures of the foil bearing in conducted tests for various speeds and loads are also presented.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):572-581. doi:10.1115/1.1308039.

The in situ vapor-phase lubrication of M50 steel, in combined rolling and sliding contacts at 540°C using nitrogen atmospheres containing acetylene, is achieved. Acetylene partial pressures of 0.05 atmospheres are capable of providing continuous lubrication to combined rolling and sliding contacts through pyrolytic carbon deposition. In these tests, friction coefficients as low as μ=0.01 are found for contacts at 2.0 m/s rolling speed, 10 cm/s sliding speed, 100 N load (1.3 GPa Hertzian contact pressure), and ambient temperature of 540°C, with even lower values observed at more modest sliding speeds. One example of a model for vapor phase lubrication of combined rolling and sliding contacts is developed which predicts the lubricant steady-state fractional coverage of the contact surfaces, and from this makes friction coefficient predictions using a linear rule-of-mixture. Friction coefficient responses to step changes in acetylene partial pressure, sliding speed, and disk wear-track diameter are measured. Increased partial pressure of acetylene and increased area available for deposition are observed to be beneficial, while increased sliding speed is detrimental to lubrication performance. Shapes and trends of steady-state friction coefficient versus acetylene partial pressure, sliding speed, and disk wear-track diameter are described and curve-fit by the model. In combined rolling and sliding this example model predicts large regions of operating conditions over which friction coefficient is independent of rolling speed, as well as regions of independence of vapor partial pressure. In the special case of pure sliding, a region of friction coefficient independence of a ratio of partial pressure to sliding speed and another region of independence of a ratio of partial pressure to the product of sliding speed and normal load are predicted.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):582-588. doi:10.1115/1.1308043.

Results given in this paper are threefold. In the case of Hertzian line contact, a new load/deformation relationship is derived analytically and use is made of the thickness of the outer race section. A minor effect of the section thickness is shown. The exponent on the deformation is 1.074 (instead of 1.1 usually accepted). Results calculated with the new relationship are successfully compared to results calculated with other published relationships and also are compared successfully to some available experimental results. For the case of point contact, useful relationships, obtained by curve-fitting, are given to calculate easily the load versus deformation, maximum Hertzian pressure and ellipse contact dimension as a function of a dimensionless load parameter and ratio k of equivalent radii (instead of sum of curvatures and elliptical integrals before). A large range of k is covered, from 0.05 (found at roller rib contact) to 13,000 to cover all bearing cases, from ball bearings to spherical and tapered roller bearings. Finally, an important analytical relationship, based on curve-fitting, also is suggested to describe a smooth transition from point contact to line contact as the load increases. It is recommended to define bearing setting and bearing preload with the suggested relationship.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):589-594. doi:10.1115/1.1308042.

Roller/race misalignment and deformation are used for calculating analytically the pressure distribution along the roller/race contact and the final roller/race load and moment. Use is made of the surface crowns and race undercuts for calculating contact dimensions with their possible truncations at large misalignment or loads. The pressure distribution is not symmetrical when misalignment occurs. This analytical development was possible by using a slicing technique in which the local roller/race geometrical interference was calculated in each slice of the contact. A mix of point and line contact Hertzian solutions developed in a companion paper “Part I” is used for obtaining the final load per slice. The final analytical solutions (load, moment and pressure) are successfully compared to two numerical solutions described briefly. The analytical model has been slightly fine-tuned using correction factors obtained by curve-fitting for matching the results to the numerical ones. In the curve-fitting, the single radius profile and multi-radius profile are distinguished.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):595-602. doi:10.1115/1.1308044.

The rough surface contact in a tribological process involves frictional heating and thermoelastic deformations. A three-dimensional thermal-mechanical asperity contact model has been developed, which takes into account steady-state heat transfer, asperity distortion due to thermal and elastic deformations, and material yield. The finite-element method (FEM), fast Fourier transform (FFT), and conjugate gradient method (CGM) are employed as the solution methods. The model is used to analyze the thermal-mechanical contact of typical rough surfaces and investigate the importance of thermal effects on the contact performance of surface asperities.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):603-607. doi:10.1115/1.1308040.

Three-dimensional stream functions for flows in elastohydrodynamically lubricated elliptical conjunctions are formulated. Closed-form and numerical solutions for the stream functions on special planes are obtained. Streamlines on these special planes are plotted to reveal the trajectories of the lubricant particles that pass by, pass through, or flow back from the Hertzian contact zone. Furthermore, a conceptual column stream function and column streamlines are introduced to present the three-dimensional flow in a two-dimensional manner. Thereby, the column streamlines can be plotted to visualize and quantify the flow rates of the lubricant that passes by or passes through the Hertzian zone.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):608-615. doi:10.1115/1.1308041.

The paper reports results of the experimental investigation into the steady state performance characteristics of a tilting pad thrust bearing typical of design in general use. Simultaneous measurements are taken of the pad and collar temperatures, the pressure distributions, oil film thickness, and power loss as a function of shaft speed, bearing load, and supplied oil temperature. The effect of operating conditions on bearing performance is discussed. A small radial temperature variation is observed in the collar. A reduction in minimum oil film thickness with load is approximately proportional to p−0.6, where p is an average bearing pressure. It has also been found that the oil film pressure profiles change not only due to the average bearing load but also with an increase in shaft speed and temperature of the supplied oil.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):616-623. doi:10.1115/1.1308045.

The dynamic behavior of pico sliders is investigated during slider-disk contacts as a function of velocity, pitch angle, crown height, and lubricant thickness using laser Doppler vibrometry and acoustic emission sensors. Analog and digital filtering methods are applied to distinguish air bearing and slider body resonances from frequencies related to disk runout, load beam design, and gimbal structure. Sliders with high pitch angle and small crown were found to exhibit smaller vibration amplitudes after slider-disk contacts than sliders with low pitch angle or large crown. The lubricant thickness of the disk was found to affect both the glide avalanche height of the disk and the dynamic behavior of the slider. The results show that roll and pitch frequencies of the air bearing increase with decreasing disk velocity.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):624-631. doi:10.1115/1.1330736.

The nanomechanical properties of various polymers were examined in light of nanoindentation experiments performed with a diamond tip of nominal radius of curvature of about 20 μm under conditions of maximum contact load in the range of 150–600 μN and loading/unloading rates between 7.5 and 600 μN/s. The elastic modulus of each polymer was determined from the unloading material response using the compliance method, whereas the hardness was calculated as the maximum contact load divided by the corresponding projected area, obtained from the known tip shape function. It is shown that while the elastic modulus decreases with increasing indentation depth, the polymer hardness tends to increase, especially for the polymers possessing amorphous microstructures or less crystallinity. Differences in the material properties, surface adhesion, and time-dependent deformation behavior are interpreted in terms of the microstructure, crystallinity, and surface chemical state of the polymers. Results obtained at different maximum loads and loading rates demonstrate that the nanoindentation technique is an effective method of differentiating the mechanical behavior of polymeric materials with different microstructures.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):632-640. doi:10.1115/1.1327583.

Three-dimensional rough surfaces were generated using a modified two-variable Weierstrass-Mandelbrot function with fractal parameters determined from real surface images. The number and size of truncated asperities were assumed to follow power-law relations. A finite element model of a rigid sphere in normal contact with a semi-infinite elastic-plastic homogeneous medium was used to obtain a constitutive relation between the mean contact pressure, real contact area, and corresponding representative strain. The contact model was extended to layered media by modifying the constitutive equation of the homogeneous medium to include the effects of the mechanical properties of the layer and substrate materials and the layer thickness. Finite element simulations of an elastic-plastic layered medium indented by a rigid sphere validated the correctness of the modified contact model. Numerical results for the contact load and real contact area are presented for real surface topographies resembling those of magnetic recording heads and smooth rigid disks. The model yields insight into the evolution of elastic, elastic-plastic, and fully plastic deformation at the contact interface in terms of the maximum local surface interference. The dependence of the contact load and real contact area on the fractal parameters and the carbon overcoat thickness is interpreted in light of simulation results obtained for a tri-pad picoslider in contact with a smooth thin-film hard disk.

Commentary by Dr. Valentin Fuster
J. Tribol. 2000;123(3):641-650. doi:10.1115/1.1339977.

The dependence of the nanotribological properties of ultrathin amorphous carbon (a-C) films, deposited on Si(100) substrates by radio frequency sputtering, on their nanomechanical properties was investigated using surface force microscopy. The thickness and nanohardness of the a-C films were found to be in the range of 7–95 nm and 9–44 GPa, respectively. Sharp conical diamond tips with a 90 deg included angle and radius of curvature of about 20 μm and 100 nm were used to perform friction and wear experiments, respectively. The effect of the substrate compliance on the nanomechanical and nanotribological properties of the a-C films is interpreted in terms of the indentation depth and the film thickness. The coefficient of friction and wear rate of the a-C films are related to their nanomechanical properties, thickness, and surface roughness. The dependence of the coefficient of friction on contact load and the dominant friction mechanisms of elastically and plastically deformed films are discussed in light of friction force and surface imaging results. High effective hardness-to-elastic modulus ratio and low surface roughness characterize high wear resistance a-C films. Below a critical load, the steady-state removal rate of the film material is insignificantly small, revealing a predominantly elastic behavior.

Commentary by Dr. Valentin Fuster

TECHNICAL NOTE

J. Tribol. 2000;123(3):651-654. doi:10.1115/1.1340632.

The purpose of this paper is to study the effect of non-Newtonian lubricant on the stability of oil film journal bearings mounted on flexible support using linear perturbation technique. The model of non-Newtonian lubricant developed by Dien and Elrod is taken into consideration. The dynamic co-coefficients are calculated for different values of power law index and length to diameter ratio. These are then used to find stability margin for different support parameters to study the effect of the non-Newtonian lubricant.

Commentary by Dr. Valentin Fuster

DISCUSSION

J. Tribol. 2001;123(3):655. doi:10.1115/1.1352743.
FREE TO VIEW

Suk,  M., and Gillis,  D. R., 1998, “ Effect of Slider Burnish on Disk Damage During Dynamic Load/Unload,” ASME J. Tribol., JOTRE9120, pp. 332–338.jtiJOTRE90742-4787Johnson, K. L. 1985, Contact Mechanics, Cambridge University Press, Cambridge.

Topics: Tribology , Stress , Disks
Commentary by Dr. Valentin Fuster

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