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RESEARCH PAPERS

J. Tribol. 1996;118(3):449-456. doi:10.1115/1.2831557.

Experimental results from a gas thrust bearing test facility are reported in this paper. This fundamental investigation of the thermo-fluid dynamics of a gas lubricated bearing forms part of a wider programme aimed at their introduction into the aeroengine environment. The experiments provide the most detailed measurements to date of a hydrodynamic lubrication film which uses gas as a working fluid. The bearing tested was lubricated with air. The thrust pads had an outer radius of 0.125 m and during operation the clearance between the bearing surfaces was typically 10 μm. The design speed of the bearing was 10,000 rpm. Mean pad pressure and temperature distributions are presented for a range of bearing loads up to 3/4 kN, which were measured under steady operating conditions. Transient pressure and surface clearance measurements from fast response probes are also reported. These measurements show a complex relative motion of the bearing surfaces, the exact nature of which could only be determined from a more detailed set of surface clearance measurements than those in the present work. However, the transient pressure measurements show that these surface clearance fluctuations do not exert a strong influence on the lubrication film pressures, which can be considered to be essentially steady. The test data are compared with predictions obtained by solving Reynolds equation numerically, using a finite-volume based procedure. Thermal distortion of the bearing surface is not included in the model and this limits the agreement between measurements and calculations. The results suggest that this must be accounted for in the design of practical bearing systems, even though the lubrication film can be treated as isothermal. The measurements also show evidence of the inlet effects which raise the static pressure of the flow entering the bearing to above the ambient level.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):457-465. doi:10.1115/1.2831558.

Coating adherence, friction coefficient, surface temperature, and wear mass loss are determined for a set of scale reduced steel brake disks on which NiCr cement based coatings and low conductivity subcoatings are plasma sprayed. The related pads are made of either sintered iron copper or sintered aluminum titanate. It is mainly shown that NiCr-Cr3 C2 cermet coating on the disk, coupled with sintered aluminum titanate pad, leads to good damage resistance and stable friction coefficient in the simulation of high speed train braking. Low conductivity sublayers are also involved and it is found that temperatures in disc or pad may be also controlled.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):466-472. doi:10.1115/1.2831559.

A three-dimensional numerical method for the prediction of transient temperatures reached during realistic braking operations in a ventilated disk brake assembly is presented in this paper. This Layered FFT-FEM numerical method (Fast Fourier Transform-Finite Element Method) takes advantage of the high Peclet number “skin effect” to decouple the moving boundary contact problem from the spatially periodic but non-axisymmetric cooling associated with the vents. The first problem is modeled according to the FFT-FEM method developed previously by the authors. This technique succeeded in using high Peclet numbers for axisymmetric moving solids submitted to general loading conditions. No numerical convergence problems are encountered and short computer time is required. The second problem is modeled according to traditional three-dimensional finite element techniques. The Layered FFT-FEM results are compared with analytical results. A braking operation is simulated for an industrial ventilated disk brake.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):473-482. doi:10.1115/1.2831560.

Time dependent thermal EHL circular contact results with measured surface roughness were obtained to analyze the effects of roughness on pressure, film thickness, temperature, and coefficient of friction. Both contact surfaces are considered to be rough. Multilevel multigrid techniques (with multigrid integration) were used to solve the system of two dimensional Reynolds, elasticity and three dimensional energy equations simultaneously. The effects of surface roughness under various loads, speeds, and slip conditions have been studied. Surface roughness causes pressure and temperature spikes and increases the coefficient of friction, and surface roughness flattens due to the high pressure in EHL contact. The higher the load, speed and slide to roll ratio, the more significant the effect of the surface roughness. A comparison between rough EHL and smooth EHL results indicates that surface roughness cannot be ignored in EHL analysis.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):484-491. doi:10.1115/1.2831563.

Previous investigations have demonstrated the principle and feasibility of quasi-hydrodynamic lubrication using powders. With conventional lubricants, the hydrodynamic behavior of a journal bearing can be summarized in the form of a plot of friction coefficient against a nondimensional load parameter, usually referred to as a Stribeck curve. One of the main features of the curve is that, as the thickness of the lubricant film increases, the friction coefficient initially decreases until it reaches a minimum and then increases due to shearing of the lubricant film. Previous tests on a three pad journal bearing using molybdenum disulfide powder as the lubricant showed that its friction/load characteristics followed the Stribeck form. The experimental work reported herein describes further tests carried out on a three-pad journal bearing to evaluate its thermal stability and long term wear characteristics using tungsten disulfide powder as the lubricant. Dry tungsten disulfide powder lubricant has been shown to provide excellent wear resistance. Its friction properties are similar to molybdenum disulfide but it can be used at much higher temperatures. Bearing tests were carried out at speeds up to 30,000 rpm and loads up to 236 N. Over the load/speed range stable thermal operation was achieved. From friction torque measurements it was found that the coefficient of friction for the bearing was essentially constant over the range of loads and speeds. Unlike liquids, powders have a limiting shear strength property. When the limiting shear stress is reached in a powder flow, the powder essentially shears along a plane and does not flow in a viscous manner. The constant friction coefficient seen with the tungsten disulfide powder suggests that the powder had a low limiting shear strength and that this dominated the operation of the bearing in the tests. The total accumulated test time was more than eighteen hours. Periodically, the test journal and bearing pads were inspected but there was no measurable wear or visible damage.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):492-497. doi:10.1115/1.2831564.

Further increase in magnetic recording density requires a reduction in slider flying height. The current study employs the short-bearing approximation to determine analytically the static equilibrium configuration of a slider supported by a starved liquid bearing that operates between ideally smooth surfaces. The solution incorporates rheological behavior based on previously acquired data. The accuracy of the short-bearing approximation is assessed by determining how well the resulting solution satisfies the Reynolds equation. The analysis suggests a means of designing a slider to achieve head/medium spacings in the neighborhood of 20 nm.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):498-502. doi:10.1115/1.2831565.

The characteristics of two misaligned journal bearings were experimentally studied. A bearing test rig was developed. The experimental journal was freely alignable in two possibly misaligned journal bearings. In Part 1, the static characteristics, such as the distributions of oil film thickness and the pressure and temperature of two misaligned bearings, are measured; the relationships of eccentricity, attitude angle, and side flow to the Sommerfeld number are experimentally determined. In Part 2, the traditional harmonic excitation method is extended to estimate the sixteen dynamic force coefficients of two misaligned journal bearings. Sixteen force coefficients of two circular journal bearings, each with two axial grooves, are estimated and the threshold stable speeds are predicted. Various experimental errors and the uncertainty of the estimated coefficients are analyzed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):503-508. doi:10.1115/1.2831566.

The traditional harmonic excitation method was modified to estimate 16 dynamic force coefficients of two misaligned journal bearings. To avoid the effects of different running conditions on the estimated coefficients, two harmonic forces with different frequencies were excited on the rotor simultaneously and all data were acquired during one test run. The excitation forces and their displacement responses were transferred to the frequency domain in which eight linear (complex) equations are derived. All 16 bearing force coefficients can then be obtained by solving these simultaneous linear equations through one operation. Applicable experimental procedures and data processing techniques are presented. The force coefficients of two circular journal bearings with two axial grooves each are estimated and correlate well with theoretical values when misalignment is considered. Various experimental errors are discussed and the uncertainty in the estimated coefficients is analyzed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):509-519. doi:10.1115/1.2831567.

A theory for analyzing the effects of elastic deformations of the seal wall on the dynamic characteristics of high pressure cryogenic annular seals under concentric operation is presented. The bulk flow continuity, axial and circumferential momentum, and the energy transport equations are utilized to determine the pressure distribution in the seal. Thermophysical properties of the cryogenic fluid are assumed to be functions of the local pressure and temperature. The wall deformations are obtained using an iso-parametric, axi-symmetric Finite Element formulation of the seal wall. A perturbation analysis is employed to arrive at the first order solution which yields the rotordynamic coefficients. Results obtained for the case of Space Shuttle Main Engine Oxygen Turbopump (SSME-HPOTP) Preburner Seal show a significant impact of seal flexibility on the dynamic coefficients.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):520-526. doi:10.1115/1.2831568.

This paper presents a new theory on frictional vibrations of paper-based friction material considering the interaction between its deformation and the consequent lubricant flow inside it. A frictional vibration model is proposed, in which a poroelastic body saturated by a viscous liquid is introduced, and then a stability criterion is derived on the assumption that vibrations in two directions, tangential and normal, are coupled. Compared with the experiment, it is confirmed that the proposed criterion can predict the stability limit more accurately than the conventional one, which does not take the deformation of the friction material into account and depends only on the slope of friction coefficient versus sliding velocity. Based on the present criterion, influences of material properties on the stability to frictional vibrations are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):527-531. doi:10.1115/1.2831569.

Seizure phenomena in pin-on-disk tests have been studied for “soft” and “hard” steel specimens. Differences in competing and dominant wear mechanisms under steady state friction have been preserved for “soft” and “hard” specimens in the region of transition to seizure or galling. Severe wear was observed for “soft” specimens under all loads tested, while adhesion and splitting off of wear particle conglomerates (microseizure) were identified for “hard” specimens. The contact temperature, calculated in accordance with the temperature model of plastically deformed contact spots (Kuhlmann-Wilsdorf), has appeared to be low for “soft” specimens and not sufficient for adhesion interaction. The effect of oxide films on the friction of “hard” specimens has been estimated in accordance with the temperature model for a coated semi-infinite body (Tian and Kennedy). The insulated oxide films on the surface of “hard” specimens create the “skin effect” and lead, therefore, to raising the temperature up to the temperature of adhesion interaction. Temperature instability of hard surfaces has been demonstrated to result from the “skin effect” and from a disturbance in equilibrium of formation and failure of oxide films. It has been shown that for “soft” specimens the prime cause of transition to seizure was the mechanical interlocking between the wear particles and the soft disk surface combined with mechanical instability, while for “hard” specimens the cause was temperature instability. A more realistic temperature model of the contact has been considered, which takes into account some competing wear mechanisms (oxidational wear, ploughing, delamination) and the effect of wear particles.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):532-538. doi:10.1115/1.2831570.

In this work, a test machine, experimental results and the comparison between theoretical TEHD results and experimental data are presented. The tested bearings are located in two speed increasing and reduction gearboxes (back-to-back test bed). The shaft is driven by a 1 MW motor. The bearing diameter and the bearing length are equal to 160 mm. The rotational speed varies from 2700 rpm (22 m/s) to 11,880 rpm (100 m/s). The applied load is up to 88,000 N. Temperatures in the bearing (film/pad interface, oil, pad and housing), power losses and oil flow are measured. The influence of the bearing design and of the pivot position on the pad is analyzed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):539-542. doi:10.1115/1.2831571.

A refined technique using Acoustic Emission has been developed for use in head/disk interface contact detection. These refinements are focused on the design parameters of the Acoustic Emission electronics, amplification and filter settings, which permit accurate and repeatable contact detection measurements. A technique for determining the slider body natural frequencies is described in order that the proper filter design parameters can be determined. Experiments confirm the existence of these frequencies for a Al2 O3 TiC 50 percent slider are in excess of 600 KHz.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):543-548. doi:10.1115/1.2831572.

Amorphous carbon nitride coatings of thickness of 5 and 30 nm were deposited onto 65 and 95 mm magnetic thin-film rigid disks surfaces using single-cathode and dual-cathode magnetron sputtering systems containing nitrogen-argon plasmas. Under optimum deposition conditions, amorphous carbon nitride coatings can be synthesized on ultrasmooth thin-film disks with no significant pinholes at thickness down to 5 nm, with hardness 22–28 GPa (compared to 7–12 GPa for amorphous carbon), and r.m.s. roughness as low as 0.25 nm. These amorphous carbon nitride coatings were shown to have better contact-start-stop performance and three-to-four times better pin-on-disk contact durability compared with amorphous carbon overcoats under identical testing conditions. Amorphous carbon nitride appears to be a promising candidate overcoat material for replacing amorphous carbon in the next-generation magnetic thin-film rigid disk systems.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):549-554. doi:10.1115/1.2831573.

Some fundamental characteristics of the subambient pressure air bearing suction force were investigated analytically and numerically. The performance of air bearing suction force is strongly determined by the cavity bearing number, in which the cavity region recess depth is used as the characteristic film thickness. Although the optimal recess depth for maximum, suction force varies for different operation conditions, the optimal cavity bearing number can be found in a wide range of applications. The analytical model was confirmed by a finite element analysis. Examples of different disc velocities, slider dimensions, and ambient pressure effect were presented.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):555-563. doi:10.1115/1.2831574.

In this paper, an efficient and accurate numerical procedure to determine the thermo-hydrodynamic performance of cavitating bearings is described. This procedure is based on the earlier development of Elrod for lubricating films, in which the properties across the film thickness are determined at Lobatto points and their distributions are expressed by collocated polynomials. The cavitated regions and their boundaries are rigorously treated. Thermal boundary conditions at the surfaces, including heat dissipation through the metal to the ambient, are incorporated. Numerical examples are presented comparing the predictions using this procedure with earlier theoretical predictions and experimental data. With a few points across the film thickness and across the journal and the bearing in the radial direction, the temperature profile is very well predicted.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):564-570. doi:10.1115/1.2831575.

In-situ measurement of head flying height and attitude using image processing of fringe patterns formed by Michelson interferometry is studied. A wide laser beam is applied to illuminate the slider back surface and disk surface simultaneously to create interferometric fringe patterns. Employing the relationships arising between the two fringe patterns, the calculation procedure is formulated to yield the slider’s parallel, pitch and roll displacements. Experimental fringe patterns are captured in a single visual field by a high-speed CCD camera. Image processing for a higher signal-to-noise ratio, such as smoothing, filtering, amplification and ridge line extraction is then applied to the image data. Additionally, average processing with respect to multiple fringe lines to produce higher accuracy is successfully applied. Measured values of flying height and pitch and roll displacements are confirmed to be in good accordance with the calculation results, demonstrating excellent applicability of the present method down to the near-contact region.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):571-579. doi:10.1115/1.2831576.

The results of extensive amount of thermohydrodynamic simulations are presented in the form of design charts for journal bearings. These charts enable one to evaluate the maximum temperature and an effective hearing temperature. Numerous examples are presented that show the applicability of these charts in predicting bearing performance parameters.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):580-588. doi:10.1115/1.2831577.

Application of grain flow theories to powder lubrication of hydrodynamic bearings requires careful consideration of slip of the flow velocity at the boundaries together with appropriate formulation of the fluctuation (thermal) velocity conditions at the boundaries. Generalized boundary conditions that can be applied to a wide variety of surface and grain material characteristics are developed for a Couette flow configuration. The equations account for both the energy loss at the boundary due to inelastic grain-wall collisions and the energy supplied by the boundary due to collisional momentum supply. The predictions of the model are compared to a number of published results that are based on direct computer simulation of grain-grain collisions. The model’s flexibility is demonstrated by simulating the effects of unequal surface roughnesses.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):589-600. doi:10.1115/1.2831578.

Models for thermohydrodynamic lubrication in the turbulent regime are developed for a mechanical end face seal with various combinations of asperity height and roughness pattern. A surface wear model, based on deformation, is established for mixed lubrication such that the displacement is at most equal to the mean asperity height. Only normal load is involved in the solution of asperity deformation, and the mean film thickness is determined based on a total volume conservation hypothesis, in conjunction with an elastic-exponential hardening model. The singularity problem, present in the expected form of the Reynolds equation for a seal surface with circumferentially-oriented roughness grain sphere grooves, is avoided by viewing the seal roughness as porous material, thereby introducing roughness permeability. Flow permeability is thus obtained by combining Darcy’s law for porous material with the average flow model developed by Patir and Cheng for mixed lubrication. The hydrodynamic pressure and thereby the hydrodynamic load support are relatively higher from a seal with radially-oriented roughness. Both the mean film thickness and the hydrodynamic load support are substantially elevated by increasing the composite rms roughness, raising the inlet-flow pressure, and decreasing the rotational speed. Good agreement has been obtained from the comparison between the results herein and Lebeck’s experimental results.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):601-607. doi:10.1115/1.2831579.

This paper examines a class of experimental techniques used to develop constitutive models for lubricants, by simulating the shearing of a thin lubricant layer while accounting for transient phenomena. The complete transient thermal problem with fully nonlinear constitutive relations is solved, and heat conduction is accounted for both in the lubricant layer and into the walls. Numerical simulations are used to examine the shear stress history, the velocity profile, and the temperature profile as functions of time. As a particular example, the high-rate torsional Kolsky bar rheometer (Feng and Ramesh, 1993) is simulated. The computations indicate that the Kolsky bar experiments, which are able to examine the time-histories of the stresses and of the motion, can he used to obtain material properties for lubricants at high shear rates. A full numerical analysis may be required to properly interpret some of the data available from the Kolsky bar experiments, since at longer times (greater than that associated with the peak shear stress) the thermal softening may dominate the response and the velocity field may become strongly inhomogeneous. The numerical simulations are performed using both rate-dependent and limiting stress constitutive laws, and the effects of the layer thickness and the rise time of the relative velocities are examined. The simulations show that the film thickness and the rise time of the relative velocities can have strong effects on the character of the solution when the transient phenomena are included in the analysis. The computations also demonstrate that highly inhomogeneous and even localized flows may occur within rheometers as a result of transient effects. The development of these flows depends on the layer thickness, the rise-time of the boundary velocity, the thermal boundary conditions, and the constitutive behavior of the lubricant.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):608-616. doi:10.1115/1.2831580.

A centrally grooved short squeeze film damper (SFD), together with its lubricant supply mechanism (LSM), is analyzed, using an integrated theoretical model. It is shown that the traditional analysis for such a damper, where the effects of the central groove and the LSM are ignored, can lead to a seven-fold underestimation of the magnitude of the hydrodynamic force coefficients. The new theory gives predictions for the damping coefficients which are in good agreement with corresponding experimental results. Moreover, a five-fold improvement is obtained for both the temporal and convective inertia coefficients, at low values of eccentricity. The new model leads to the prediction of a nonzero fluid static force which, in conformity with experimental results, is linearly related to the supply pressure. The existence of this static force has not been explained by previous theoretical work on SFDs.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):617-622. doi:10.1115/1.2831581.

Analytical expressions for the hydrodynamic forces, and four related dynamic coefficients, are presented for finite length squeeze film dampers (SFDs) executing combined radial and tangential motion around the bearing center, with small amplitude. Previous analyses by Mulcahy (1980) and San Andres and Vance (1987) are shown to be particular cases of the present treatment. The influence of combined motion on the coefficients is found to differ, in several respects, from that which can be deduced from results for one dimensional radial motion and circular centred orbital motion. The effects of combined motion on the mean flow velocity and the wall stress are also studied. The study provides further insight into the validity of bulk flow assumptions, often used when dealing with lubrication problems where fluid inertia effects are significant.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):623-628. doi:10.1115/1.2831582.

The lubricating air film between two rotating rollers in close contact was studied numerically. The numerical model used in this study accounts for the effects of air compressibility, material deformation, and the slip flow which occurs when the air film thickness is not much larger than the mean-free-path of the air molecules. The air film profiles and the pressure profiles for the nip region between the rollers were calculated. It was found that the calculated air film thicknesses are lower than predicted by the liquid elastohydrodynamic calculation. From this study, equations for the minimum air film thickness, the air film thickness at the center of contact, and the amount of air that passes through the nip were obtained. This study has application to the prediction of the amount of air entrained in a winding roll.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):629-636. doi:10.1115/1.2831584.

Measurements of friction in rolling of aluminium strip on an experimental mill are described. Friction depended most strongly on the ratio Λ of the smooth film thickness to the combined roughness of the roll and strip, and on the reduction in strip thickness. Whether the greater roughness was on the roll or on the strip was found to be unimportant. Varying the oil temperature from 40 to 60°C was also found to be of secondary importance. Profilometry results suggested that friction was determined by the mean film thickness between the surfaces. At the slowest speeds and smallest films, friction was close to the value of 0.09 found in separate measurements in a disk machine of the boundary additive properties. At the highest speed the friction values, which were less than 0.01, could be explained by hydrodynamic lubrication. The transition between these two extremes occurred when the film thickness was of the order of the additive molecule length of 3 nm.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):637-643. doi:10.1115/1.2831585.

In this paper, by accounting for the effect of dynamic behavior in valve train mechanisms on the surface contact load of a cam/tappet pair, a full numerical solution of nonsteady EHL for a high-speed diesel engine cam/tappet pair is obtained by a fast solution scheme. The shape of the lubricant film and the characteristics of pressure distribution on cam/tappet pair surfaces are further studied. The lubrication process of a cam/tappet pair at the opening and closing sides and in the most dangerous zone in normal working conditions is also analyzed. This analysis provides the theoretical basis which best approximates practical working conditions for designing cam profiles and establishing reasonable running-in condition.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):644-650. doi:10.1115/1.2831586.

As the head/disk spacing continues to decrease, the demand for thin film disks with glide capability below 20 nm becomes more pressing. As a consequence, the design of such media requires an ever increasing control of the surface topography to a nanometer level. This paper is an attempt to analytically predict the intrinsic glide capability of a textured disk, given the knowledge of its peak height distribution, as measured by a surface profilometer. This model also takes into account the long wavelength component of the topography, or waviness, by treating it as an independent variable leading to a broadening of the peak height distribution. This analysis also predicts relationships between various roughness parameters. Experimental data obtained on a total of 27 media surfaces of various types compare favorably to the theoretical predictions.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):651-656. doi:10.1115/1.2831587.
Abstract
Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):657-662. doi:10.1115/1.2831588.

A theoretical approach to evaluating capacitance, resistance, capacitive reactance, and impedance of the lower and upper lobes of a two-lobe elliptical hydrodynamic journal bearing under various operating conditions is developed. It is established that the change in capacitance and resistance with the change in eccentricity ratios is nonlinear. The capacitance and resistance, thus determined, are correlated with the dynamic coefficients of bearings using an electrical analogy. It is found that the stiffness and damping coefficients are higher for two-lobe bearings as compared to those of cylindrical bearings having identical dimensional parameters and operating under similar conditions. The analysis may have the potential to diagnose the stability regime of a bearing through the bearing’s electrical parameters. The electrical analogy may be a useful alternative to conventional techniques.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):663-668. doi:10.1115/1.2831589.

The tribological behavior of ultrathin films of hydroxyl terminated perfluoropolyethers attached to smooth silicon surfaces was studied in pin-on-disk experiments. The frictional properties of the films, which consisted of either physically adsorbed or chemically bonded polymer or a combination of both, was measured in sliding contact and lubricant loss was studied simultaneously by scanning microellipsometry. Prior to these experiments the kinetics of attachment of the polymer to silicon surfaces and the behavior of the polymer at high temperatures was investigated in order to obtain reproducible conditions for the deposition of the lubricant films and to rule out the occurrence of thermal degradation during film deposition. The tribological investigations showed that the lifetime of the surfaces with the attached boundary lubricant was strongly dependent on the mode of attachment of the polymer. The best tribological behavior was observed in films which contained both chemisorbed and physically adsorbed material.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):669-675. doi:10.1115/1.2831590.

The surface scuffing occurring in line-contact lubrication is related to the roller’s roughness pattern and asperity height. For surfaces with same contact asperity height, the magnitudes of friction power (Pf = fWVs ) relevant to various roughness patterns are found to have the same sequence as the critical local temperatures. Instead of using the nominal contact area, the real contact area (At ) is used to obtain the true friction power intensity (Ptfi = Pf /At ). A new scuffing failure model (Ptfi · σ−0.317 = C, where σ denote rms roughness) shows that the scuffing resistance of surfaces with transverse roughness pattern is higher than that of surfaces with longitudinal and oblique patterns. For certain roughness patterns, a high root mean square roughness height a is always associated with the high Ptfi value just before scuffing.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):676-680. doi:10.1115/1.2831591.

The copper(II) complex with alkylphosphonic acid monoalkyl ester (CuAMP) was synthesized, and its structure shown to be laminar on the basis of fast atom bombardment mass spectroscopy (FABMS). The extreme-pressure, antiwear and antiseizure performance of CuAMP was assessed using a four-ball machine. The results showed that CuAMP exhibits better extreme-pressure and antiwear properties than tricresyl phosphate. Based on the results of surface analysis, the antiwear action mechanism of CuAMP is discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):681-686. doi:10.1115/1.2831592.

Lithium soap and calcium complex soap-based lubricating greases are studied when fresh and after use in roller and ball bearings. Examination of the rheological properties of these greases using rotating rheometers reveals changes that have occurred during use in the bearings. These modifications are not uniform throughout the hearing. Physical and chemical analyses enable these modifications in rheological properties to be linked to variations in composition or structure, depending on the type of grease and bearing. Several mechanisms involved in lubricating a bearing with a lubricating grease are confirmed or clarified.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):687-692. doi:10.1115/1.2831593.

With the aim of seeking a more reasonable solution for the squeeze film force for large amplitude motion with low viscosity fluids, an elliptical velocity profile and the corresponding squeeze film force equations are derived for a two dimensional rectangular plate model using three different approximation methods. Through comparisons with existing theoretical studies which use parabolic velocity profiles for moderate to large Reynolds numbers, it is shown that the squeeze film model using the elliptical profile may be closer to reality. The force level of the dominant temporal inertia component using the elliptical profile (with either the iterative or the energy methods) is between that obtained using the traditional parabolic profile with the momentum method and that obtained with either the iterative or the energy methods.

Commentary by Dr. Valentin Fuster
J. Tribol. 1996;118(3):693-697. doi:10.1115/1.2831594.

Based upon previous results for thin solid film lubrication, a contact model has been proposed which can describe the physical essence of the steady-state friction condition. By using this contact model, a theoretical calculation method has been established. Good agreement between theoretical results and experimental results obtained for tribo-coatings shows that this theory can be used for practical application to obtain and maintain good solid lubrication.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

TECHNICAL BRIEFS

Commentary by Dr. Valentin Fuster

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