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RESEARCH PAPERS

J. Tribol. 1998;120(2):145-151. doi:10.1115/1.2834401.

Dynamic stability is investigated for a mechanical seal configuration in which both seal elements are flexibly mounted to independently rotating shafts. The analysis is applicable to systems with both counterrotating and corotating shafts. The fluid film effects are modeled using rotor dynamic coefficients, and the equations of motion are presented including the dynamic properties of the flexible support. A closed-form solution for the stability criteria is presented for the simplifled case in which the support damping is neglected. A method is presented for obtaining the stability threshold of the general case, including support damping. This method allows instant determination of the stability threshold for a fully-defined seal design. A parametric study of an example seal is presented to illustrate the method and to examine the effects of various parameters in the seal design upon the stability threshold. The fluid film properties in the example seal are shown to affect stability much more than the support properties. Rotors having the form of short disks are shown to benefit from gyroscopic effects which give them a larger range of stable operating speeds than long rotors. For seals with one long rotor, counterrotating operation is shown to be superior because the increased fluid stiffness transfers restoring moments from the short rotor to the long.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):152-158. doi:10.1115/1.2834402.

A simulation has been developed to model the transient wear of particle-filled polymer composites as a function of sliding distance. All inputs are parameters of physical significance, including filler bulk volume fraction, specific wear rate (relative to that of the matrix), and contact pressure. Run-in wear behavior is simulated by consideration of the accumulation of wear-resistant filler particles and the formation of a volume fraction profile near the composite sliding surface, facilitated by matrix cold flow. Simulation outputs include time-dependent volume fraction profile, and composite wear rate and wear volume. The simulation may be used for evaluation of candidate materials for applications in which nonsteady-state run-in wear effects are important, as well as a guide for the engineering of composite surfaces with graded volume fraction profiles that may provide resistance to initial transient wear contributions.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):159-164. doi:10.1115/1.2834403.

A means of evaluating the surface roughness effect on contact fatigue life has been proposed. To account for stress variations caused by random surface roughness, an effective stress concept based on damage accumulation theory was employed. A point EHL analysis along with a comprehensive interior stress analysis has been performed to obtain the effective stress field under lubricated conditions. Numerical simulations were performed for surfaces produced by different finishing processes. Results show that surface roughness can cause significant stress variations in the near-surface. As a result, the effective stress at the near-surface is increased. The increased effective stress is responsible for the life reduction of the contact. Life reduction factors for contact surfaces with different finishing processes were compared.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):165-172. doi:10.1115/1.2834404.

Static and dynamic characteristics are experimentally investigated for annular plain seals with porous materials applied to the seal surface by insertion into the middle of the seal. Experimental results show that annular plain seals with porous materials have a higher leakage flow rate, larger main stiffness coefficients, and smaller cross-coupled stiffness coefficients and main damping coefficients than conventional annular plain seals with solid surfaces. In the porous seals, an increase of approximately 30 percent in the leakage flow rate and reduction of the same amount in the main damping coefficients are obtained, whereas the main stiffness coefficients for the porous seals are four to six times as much as those for the solid seals due to the increase in the hydrostatic force induced by a function of the hydrostatic porous bearing. This suggests that the quantitative effects of the porous materials on the main stiffness coefficients are much more significant than the effects on the leakage flow rate and the other dynamic coefficients. The larger main stiffness coefficients for the porous seals yield larger radial reaction force for a small concentric whirling motion, which would contribute to rotor stability from the viewpoint of increasing speed limits due to a stiffer rotor support.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):173-178. doi:10.1115/1.2834405.

Recent advances in high pressure rheometry have elucidated the shear response of liquid lubricants at the high shear stress characteristic of the traction generating region of lubricated concentrated contacts. These new measurement techniques are used to characterize the shear response of shear thinning liquids at low (<10 MPa) shear stress. A recently developed numerical scheme for calculating film thickness is extended to accommodate sliding. Film thickness predictions are compared with measurements using shear thinning liquids including a polymer/mineral oil blend, a highly elastic liquid, and synthetic base oils. Useful insights are provided concerning the effects of pressure-viscosity behavior for Newtonian liquids, sliding, and starvation for non-Newtonian liquids and the relevant shear stress for film forming.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):179-183. doi:10.1115/1.2834406.

Single- and dual-cathode DC magnetron sputtering was used to produce TiB2 coatings and CNx /ZrN multilayers, respectively, with hardness exceeding 40 GPa. The composition, structure, topography, and mechanical properties were determined by various techniques, including Auger electron spectroscopy, X-ray diffraction, high-resolution electron microscopy, atomic force microscopy, and nanoindentation. An optimum combination of the sputtering pressure and substrate bias results in the production of ultrasmooth TiB2 coatings with hardness up to 50 GPa and excellent wetting properties. The rationale for studying the CNx /ZrN system is that ZrN(111) provides excellent lattice match to the hypothetical β-C3 N4 (0001) face (β-C3 N4 was predicted to have mechanical properties comparable to diamond). Using a dual-cathode sputtering system, we produced crystalline multilayers of CNx /ZrN with bilayer thickness of 1–2 nm. Using various combinations of nitrogen partial pressure, target powers, and substrate bias, we found that the hardness of these coatings correlates very strongly with the occurrence of (111) texture of ZrN, consistent with the lattice-match strategy. Even with a ZrN volume fraction of 70 percent, such multilayer coatings have been synthesized with hardness in the 50 GPa regime.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):184-190. doi:10.1115/1.2834407.

The purpose of this investigation is to clarify the role of roughness on rolling contact fatigue. Tests have been carried out on a two-disk machine, for two rolling bearing steels (52100 and M50), two surface roughnesses corresponding to EHL and micro-EHL conditions (two different surface finishing), three normal loadings (1.5, 2.5 and 3.5 GPa), and under pure rolling or rolling plus sliding conditions. No surface damage has been observed up to 50 106 cycles for tests with smooth specimens. Tests with rough specimens have produced a typical surface damage, called here surface distress, made of a large population of asperity-scale micro-cracks and micro-spalls. The paper describes the surface distress observed, such as micro-cracks and micro-spalls. Surface damages obtained are different for tests under pure rolling conditions and tests under rolling plus sliding conditions. Therefore, the role of the friction direction is underlined. A link is made between our experimental observations and calculations that have been carried out using a transient EHL model. The influence of an indent in a line contact, simulating a micro-spall, is studied. Surface pressure and associated sub-surface stress field are analyzed versus the sliding direction.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):191-197. doi:10.1115/1.2834409.

A mathematical model that predicts the transient behavior of gas or liquid lubricated hydrostatic mechanical seals has been developed. The analysis includes an evaluation of the fluid, contact, and deformation mechanics of a mechanical seal subject to constant or varying rotational speed and sealed pressure. Squeeze film effects are included. For gas seals, slip at the walls is also taken into account. Results include predictions of film thickness distributions, contact forces, leakage rates, pressure distributions, heat generation rates, thermal deformation, and mechanical deformation.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):198-205. doi:10.1115/1.2834410.

A mixed-TEHD (thermal elastohydrodynamic) model was developed for journal bearings working at large eccentricity ratios in order to facilitate a better understanding of mixed-lubrication phenomena for conformal-contact elements. The model consists of a mixed-lubrication process that considers the roughness effect and asperity contact, a thermal process for temperature analyses, and a thermal-elastic process for deformation calculations. In this model, the interactive journal, lubricant, and bearing were treated as an integrated system. Finite-element, finite-difference, and influence-function methods were utilized in the numerical process. The overall solution was achieved by the iteration method. Analyses of a simulated bearing-lubricant-journal system working under mixed-lubrication conditions were conducted, and the influence of the changes of lubricant flows as a result of the asperity contact on the system heat transfer and temperature distributions was numerically investigated.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):206-213. doi:10.1115/1.2834411.

Investigation of the mixed lubrication of journal-bearing conformal contacts is very important for failure prevention and design improvement. This paper studies the asperity contact in heavily loaded journal bearings with Lee and Ren’s asperity contact theory in a newly developed mixed-TEHD (Thermal Elasto-Hydro-Dynamic) model and analyzes the performance of simulated journal bearings under high eccentricity ratios. The effects of operating conditions, bearing structures, and thermal conditions on the contact severity were numerically investigated. The results indicate that the asperity contact pressure and the performance of journal bearings in the mixed lubrication are strongly affected by the geometric design and the thermal-elastic deformations. The heat transfer of the bearing-lubricant-journal system was also shown to play a role.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):214-220. doi:10.1115/1.2834412.

A newly developed large two-pad journal bearing with cooling ditches and viscous pump for turbines of electric utilities was analyzed theoretically. The turbulent thermohydrodynamic lubrication analysis was done in connection with the flow analysis of oil circulating system through the cooling ditches and the viscous pump. The complicated simultaneous equations concerned with the oil flow in the bearing were solved and used to design a bearing of 535 mm diameter which will make its appearance in a near future. Full size bearing tests were carried out to check the bearing performance and viscous pump performance and to confirm the accuracy of the calculation method. Good agreement was obtained between the theoretical and experimental results.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):221-227. doi:10.1115/1.2834413.

Cryogenic fluid damper seals operating close to the liquid-vapor region (near the critical point or slightly sub-cooled) are likely to develop a two-phase flow region which affects the seal performance and reliability. An all-liquid, liquid-vapor, and all-vapor, i.e., a “continuous vaporization” bulk flow model is presented for prediction of the seal dynamic forced response. Continuity, momentum, and energy (enthalpy) transport equations govern the two-phase flow of a homogeneous saturated mixture in thermodynamic equilibrium. Static and dynamic force performance characteristics for the seal are obtained from a perturbation analysis of the governing equations. Theoretical predictions and comparisons to experimental measurements in a liquid and gaseous nitrogen seal are presented in Part II. The effects of two-phase flow regimes on the dynamic force coefficients and stability of an oxygen damper seal are also discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):228-233. doi:10.1115/1.2834414.

Cryogenic fluid damper seals operating close to the liquid-vapor region (near the critical point or slightly sub-cooled) are likely to develop a two-phase flow region which affects the seal performance and reliability. An all-liquid, liquid-vapor, and all-vapor, i.e., a “continuous vaporization” bulk flow model for prediction of the seal dynamic forced response is given in Part I. The numerical method of solution of the flow equations is detailed here. Computed predictions for static seal characteristics, leakage and axial pressure drop, correlate well with existing measurements for a gaseous nitrogen seal and a liquid nitrogen seal with two-phase at the seal exit plane. The effects of two-phase flow regimes on the dynamic force coefficients and stability of an oxygen damper seal are discussed. Fluid compressibility effects, particularly for mixtures with low mass content of vapor, are of utmost importance. Under these conditions, an increase on seal direct stiffness and reduction of whirl frequency ratio are shown to occur.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):234-240. doi:10.1115/1.2834415.

Currently, the herringbone groove journal bearing (HGJB) has important applications in miniature rotating machines such as those found in the computer information storage industry. Grooves scribed on either the rotating or stationary member of the bearing pump the lubricating fluid inward thus generating support stiffness and improving its dynamic stability when operating concentrically. The narrow groove theory (NGT), traditionally adopted to model the concentric operation of these bearings, is limited to bearings with a large number of grooves. A finite element analysis is introduced for prediction of the static and rotordynamic forced response in HGJBs with finite numbers of grooves. Results from this analysis are then compared to available experimental data as well as to estimates from the NGT. A bearing geometry parametric study is then conducted to determine optimum rotordynamic force coefficients. A discussion on the temporal variation of the bearing reaction forces and force coefficients for a rotating journal with a small number of grooves is also presented. These changes can be significant at high operating eccentricities, possibly inducing a parametric excitation in rotating systems employing this type of bearing.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):241-248. doi:10.1115/1.2834416.

Articular cartilage is the remarkable bearing material of diarthrodial joints. Experimental measurements of its friction coefficient under various configurations have demonstrated that it is load-dependent, velocity-dependent, and time-dependent, and it can vary from values as low as 0.002 to as high as 0.3 or greater. Yet, many studies have suggested that these frictional properties are not dependent upon the viscosity of synovial fluid. In this paper, a theoretical formulation of a boundary friction model for articular cartilage is described and verified directly against experimental results in the configuration of confined compression stress-relaxation. The mathematical formulation of the friction model can potentially explain many of the experimentally observed frictional responses in relation to the pressurization of the interstitial fluid inside cartilage during joint loading, and the equilibrium friction coefficient which prevails in the absence of such pressurization. In this proposed model, it is also hypothesized that surface porosities play a role in the regulation of the frictional response of cartilage. The good agreement between theoretical predictions and experimental results of this study provide support for the proposed boundary friction formulation.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):252-258. doi:10.1115/1.2834419.

This paper describes a theoretical model for piston-ring lubrication considering the combined effects of surface roughness and oil film temperature variation for refrigeration compressors. In the model, the piston-ring is treated as a one-dimensional dynamically loaded bearing with combined sliding and squeezing motion. The one-dimensional modified Reynolds equation, based on the average flow model by Patir and Cheng, is used to determine the pressure distribution, and the one-dimensional energy equation, considering the heat generated due to contact of asperities, is applied to calculate the oil film temperature distribution. In the analysis of the modified Reynolds equation, the flooded condition and Reynolds condition are employed at the leading edge and trailing edge of piston-ring, respectively. On the other hand, in the analysis of the modified energy equation, a constant temperature equivalent to the cylinder wall temperature is assumed at the leading edge. From numerical results of the minimum film thickness, pressure and temperature distributions and friction force, the combined effects of surface roughness and oil film temperature variation on these lubrication characteristics are clarified.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):259-265. doi:10.1115/1.2834420.

A new on-line methodology has been developed for both evaluation and analysis of head/tape interface phenomena. It uses the direct relationships between the power in the acoustic emission (AE) signal and wear, and between the power in the AE signal and the normal force acting on the magnetic head. The AE transducer is directly mounted onto the head assembly of a VCR mechanism in order to measure the head/tape contact behavior. This technique has been used to: (a) monitor the effect of relative humidity on head wear; (b) investigate the wear characteristics of Mn-Zn ferrite as a function of crystallographic orientation without direct measurement of the wear volume; and (c) evaluate tape abrasiveness. Explicit correlations were also obtained for the normal force acting on the magnetic head, the tape wrap angle around the head, and the reproduced signal flatness in terms of the tape transverse stress distribution. Moreover, the normal force as a function of the tape stress distribution is closely related to the longitudinal tape vibration at the head exit region.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):266-271. doi:10.1115/1.2834421.

Recently, laser texturing has captured the attention of head/media interface engineers in the hard disk drive industry because it provides precision in the landing zone placement while eliminating the transition zone of a mechanically textured landing zone. It also offers excellent tribological performance in terms of low CSS stiction and good durability. These advantages make it the solution of choice for high-end magnetic hard disk drives. This paper models the effects of laser bumps and laser textured disk surfaces on the Headway AAB slider’s flying characteristics. Two commonly used laser bump profiles (“Sombrero” and “Volcano” types) and various texture patterns are numerically generated in the simulator. The slider’s dynamic responses to these moving laser bumps and textures under two outer rail flying conditions are simulated, and the effects of various bump/texture parameters on the slider’s fly height, pitch, roll and their modulations are discussed. The laser texture mechanism is explained by examining the air bearing pressure profiles induced by the moving laser textures.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):272-279. doi:10.1115/1.2834422.

Proximity recording introduces some new head/disk interface characteristics as compared to the conventional noncontact recording. The different design philosophy has led to several distinct partial contact tripad air bearing designs, including the shaped rail negative pressure and straight rail positive pressure tripad sliders. The impact of these new characteristics and design differences on the head/disk reliability needs to be thoroughly understood in order to ensure a successful proximity recording. This paper investigates the partial contact air bearing characteristics of three representative tripad slider designs. The contact stiffness is used as a measure of the air bearing compliance. Simulations of the fly height, contact force, altitude sensitivity, substrate waviness induced spacing and contact force modulations, contact take-off, track seeking and crash stop dynamics are performed for the three tripad slider designs. The results demonstrate that the negative pressure tripad sliders can be designed such that they can achieve both flat fly height and contact force profiles and, fast take-off, while still maintaining a small contact stiffness. However, the air bearing compliance and waviness following performance have to be compromised for the correct partial contact air bearing pitch stiffness.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):280-288. doi:10.1115/1.2834423.

The ever shortening product cycle for magnetic recording disk drives demands a fast and accurate numerical prediction of the slider’s flying characteristics during the design stage. A computationally efficient multigrid control volume method is developed in this paper for the solution of the very high bearing number and shaped rail air bearing problems. The control volume schemes for discretizing the Reynolds lubrication equation are based on various convection-diffusion formulations, while the solution of the resulting discretization equations is accelerated by an additive correction based multigrid method. A comparison study using the 50 percent tripad and Headway AAB sliders demonstrates a significant improvement in the solver’s performance as compared to the single-grid method.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):289-295. doi:10.1115/1.2834424.

Two flat layered elastic half-spaces, of different material properties, are pressed together and slide against each other with a constant coefficient of friction. Although a nominally steady-state solution exists, an analysis of the dynamic motion yields complex eigenvalues with positive real parts, i.e., a flutter instability. These results demonstrate that self-excited (unstable) motion occurs for a wide range of material combinations. The physical mechanism responsible for this instability is that of slip-wave destabilization. The influence of the properties of the layers on the destabilization of sliding motion is investigated. These dynamic instabilities lead either to regions of stick-slip or to areas of loss-of-contact. Finally the dynamic stresses at the interfaces between the layers and the semi-infinite bodies are determined and compared to the nominally steady-state stresses. These dynamic stresses are expected to play an important role in delamination.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):296-303. doi:10.1115/1.2834425.

A model for calculating the static friction coefficient of contacting real (rough) surfaces in the presence of very thin liquid films (sub-boundary lubrication) is developed. The liquid has a very high affinity for the surfaces and its thickness is of the order of the surface roughness average. An extension of the Greenwood and Williamson (GW) asperity model and an improved Derjaguin, Muller and Toporov (DMT) adhesion model are utilized for calculating the contact and adhesion forces, respectively. The effects of the liquid film thickness and the surface topography on the static friction coefficient are investigated. A critical film thickness is found above which the friction coefficient increases sharply. The critical thickness depends on the surface roughness and the external normal load. This phenomenon is more profound for very smooth surfaces and small normal loads, in agreement with published experimental work on magnetic hard disk interfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):304-309. doi:10.1115/1.2834426.

Rail dark spot defect, also termed squat failure or shelling, which is a kind of rolling contact fatigue failure and occurs frequently on running surfaces of railway rails carrying high speed traffic, is one of the most dangerous rail failures. The dark spot crack is characterized by a principal crack propagating in the direction of traffic and a second crack growing in the direction opposite to traffic. By using a newly developed two-disk machine, the authors have succeeded in reproducing very similar dark spot cracks to those which appear in actual rails. It is found that the dark spot defects are caused by frequent repetitions of dry and wet runnings, and that the traction force plays an important role for the occurrence of the cracks. The principal crack may occur from a tiny pit formed a posteriori on the contacting surface and after that, the second crack is formed by cracks branched from the extended principal crack. It has also been proved experimentally that water is capable of entering the tip of the crack. Furthermore, a possible mechanism for the dark spot cracking has been proposed on the basis of the fracture mechanics approach.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):310-318. doi:10.1115/1.2834427.

A numerical simulation of the temperature rise for a three-dimensional rough surface sliding against a smooth surface in mixed lubricated contact has been developed. The effects of lubricant film friction and solid asperity friction are considered in the simulation. The moving grid method, which greatly reduces the required computer memory size and computing time, is used to solve the coefficient matrix of temperature equations. The time-dependent surface temperature rise at very small subregions is obtained. Different friction coefficients for lubricant shearing, surface film shearing and dry solid asperity contact are used to simulate the change of frictional heat in mixed lubricated contact. A critical temperature criterion is used to determine whether the friction coefficient is controlled by lubricant film, surface film, or dry solid asperity contact. Solutions for different contact conditions are presented for verification of the present simulation

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):319-324. doi:10.1115/1.2834429.

Nowadays, tilting-pad journal bearings are used under more and more severely demanding operating conditions. Three limits of safe operation were defined (Leopard, 1976): the hydrodynamic limit, the mechanical limit and the thermal limit. The purpose of this study is to determine the hydrodynamic limit of safe operation during start-up for a tilting-pad journal bearing. During start-up, the rapid increase of the temperature in the bearing solids leads to the thermal expansion of both the pads and the shaft. The operating bearing clearance decreases and, when it tends toward zero, seizure occurs. The evolution of the main characteristics (temperature, pressure, film thickness and displacements) versus time is analyzed in the case when a seizure occurs.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):325-331. doi:10.1115/1.2834430.

A test apparatus was designed and constructed to investigate the friction and heat generation characteristics of sliding polymeric U-cup seals used in high-pressure nitrogen gas springs. Tests were run at three speeds; and friction and temperature data were obtained. Seal specimens were examined under a scanning electron microscope before and after each test. Evidence of degradation and thermal softening of the seal material was found for the highest relative speed tests where elevated temperatures were measured. In addition, a semi-empirical transient finite-difference thermal analysis of the rod and the high-pressure chamber in the test apparatus was also developed. Numerical simulations of the three tests were performed using measured friction force data, and temperature predictions from the model were compared with those measured at different locations in the test apparatus. Predicted values exhibited reasonable agreement with the measured values, particularly under steady-state conditions. The variations observed for certain transient conditions appeared to be due to overestimated convective heat transfer coefficients at lower speeds, and due to the transient nature of thermal resistances between the seal cartridge and the high-pressure chamber, which may vary with a change in temperature.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):332-338. doi:10.1115/1.2834431.

Two of the most difficult issues to resolve in current design of head/disk interface in magnetic recording devices are stiction and durability problems. One method of overcoming these problems is by implementing a technology known as load/unload, where the system is designed so that the slider never touches the disk surface. One potential problem with this type of system is slider/disk contact induced disk defects. The objective of this paper is to show that the likelihood of disk scratches caused by head/disk contacts during the load/unload process can be significantly decreased by rounding the edges of the air-bearing surface. Using the resistance method, we observe that head/disk contacts burnish the corners of the slider and thereby decrease exponentially with load/unload cycles. A well burnished slider rarely causes any disk damage thus resulting in an interface with significantly higher reliability. A simple Hertzian contact stress analysis indicates that the contact stress at the head/disk interface can be greatly decreased by increasing the radius of curvature of the air-bearing surface edges.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):339-344. doi:10.1115/1.2834432.

This paper provides the results of an extensive sliding wear testing program to evaluate wear resistance of several material couples currently used for high temperature applications such as ground based gas turbines and aircraft engines. Nickel and cobalt base superalloys and iron base stainless steels were tested in different combinations, and their wear rates compared to determine optimal wear resistance. The results show that an alloy’s wear resistance is highly dependent on operating temperature and its coupling with another material. The influences of friction, hardness, and oxide formation on the alloy’s wear resistance are also presented and discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):345-352. doi:10.1115/1.2834433.

A new method for characterizing the dynamic behavior of gas films in triboelements is developed. The new method is based on an expansion of the step jump method. In this method, the dynamic character of the gas film is preserved in the form of its force response to a step jump stimulus. Transforming this step response into the frequency domain yields the frequency dependent stiffness and damping properties of the gas film. By approximating the step response with analytic constitutive models and using the elastic-gas film correspondence principle, it is possible to determine the system characteristic equation in analytic form and to find closed-form solutions for stability, transient and forced responses. Requirements are given for choosing constitutive models that comply with the second law of thermodynamics. The new analytical solution method offers a significant time savings compared to direct numerical methods, and it is much more conducive to parametric studies.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):353-357. doi:10.1115/1.2834434.

In this paper a novel and economical method of generating three-dimensional micro-patterns on single crystal silicon without the need for a mask is presented. The technique is based on the fundamental understanding of frictional interaction at light loads. Micro-patterning is done through a two-step process that comprises mechanical scribing and chemical etching. The basic idea is to induce micro-plastic deformation along a prescribed track through frictional interaction between the tool and the workpiece. Then, by exposing the surface to a chemical under controlled conditions, preferential chemical reaction is induced along the track to form hillocks about 5 μm wide and 1 μm high. This method of micro-machining may be used for making patterns in micro-electro-mechanical systems (MEMS) at low cost. Furthermore, this process demonstrates how microtribological processes can be utilized in the fabrication of micro-structures.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):358-368. doi:10.1115/1.2834435.

A theory for obtaining meniscus forces and profiles for any given liquid-mediated interface is presented that includes the effects of surface interactions, adsorption and evaporation of liquid films. The meniscus force is obtained from the derivative of the total free energy of liquid-mediated interface, which requires the meniscus profile to be known. The meniscus profile is the solution of a second-order differential equation, as derived from Pascal’s law for static incompressible liquids with inclusion of surface interactions. For nonvolatile liquid films, the total liquid amount at the interface is a conserved quantity, whereas for volatile liquids, the liquid films are in thermodynamic equilibrium with their respective vapor phase. Two typical types of initial liquid conditions are considered. Type I represents the case in which one surface is wet and the other is initially dry, having a finite contact angle with the liquid. Type II represents the situation in which both surfaces are wet by either a liquid or by two different liquids before making contact. If two or more types of liquids are involved at the interface, miscibility of the liquids and interactions due to other liquid(s) have to be also considered. For contacts with azimuthal geometry, which is merely a mathematical convenience, such as ellipsoidal/spherical, conical or crater, the theory generates several analytical formulae for calculating meniscus forces without involving meniscus profiles. These formulae can be handily applied to various surface probes techniques such as Scanning Probe Microscopy or Surface Force Apparatus. The proposed theory is also applicable to “meniscus rings” formed around crater geometry, such as encountered in laser-textured magnetic disks. In this case, the outer meniscus ring can be asymmetric to the inner meniscus ring if no liquid passage exists between the inner and outer meniscus ring. Even for the case of spherical contact geometry, the calculated meniscus profile is very nonspherical with a much larger volume than that of the widely assumed spherical meniscus profile for Type I conditions, leading to an under-estimation of the meniscus force in the previous models. It is found that for a spherical or a crater contact geometry, the surface interactions have little effect on the meniscus force provided the lateral meniscus dimension is much smaller than the radius of the sphere or of the crater. However the surface interactions have a large effect on the meniscus force for other contact geometries, such as conical contact geometry. The calculated meniscus forces are compared with the normal component of the stiction force measured at the laser textured surfaces and good agreement is found. The calculated meniscus profiles are also found in good agreement with that measured using light interferometer technique between two cross cylinders. One very interesting finding of our theory is that the meniscus volume grows first and may then shrink, as observed experimentally by others, because the initially dry surface become wetted and the boundary conditions change over from Type I to Type II.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):369-378. doi:10.1115/1.2834436.

The objective of this paper is to demonstrate, from experiments and modeling, how and why molecular orientation of functional end groups of perfluoro-polyether (PFPE) lubricants play an important role in the tribological performance of thin film magnetic disks. These disks typically have an amorphous carbon overcoat upon which a thin lubricant layer is deposited using dip-coating technique. Glancing-angle FTIR (Fourier Transform Infra-Red Spectrometry) is used for measuring molecular orientation of planer functional end groups. A molecular orientation index (MOI) was defined as 1 for randomly oriented functional end groups. The MOI is mathematically derived as 3 (maximum) for lubricant molecules oriented with their functional end groups perpendicular to the surface, and as 0 (minimum) if lubricant molecules oriented with their functional end groups parallel to the surface. The MOI is shown to depend on processing conditions and lubricant film thickness. The tribological performance of the lubricant films was evaluated using drag-mode contact start-stop testing. It was found that wear durability of the lubricant films (~2 nm) with MOI ~ 1.5 is a few times better than those with MOI ~ 0.5 to 1.0. No significant difference in the amount of bonded lubricant film was detected over the range of MOI studied. Nor was there a detectable relationship with hydrophobicity. It was inferred from decreased MOI values due to thermal effects and storage time that a smaller MOI value corresponds to a lower free energy state of the lubricant film. Interestingly, MOI values for bonded lubricant films for Process A are found to be close to 3.0, suggesting that almost all functional end groups in the bonded films are oriented perpendicular to the carbon surface, close to 2.0 for process B, and close to 0 for process C, meaning that almost all functional end groups in the bonded films from process C are oriented parallel to the carbon surface. Relationship between physical/chemical bonding configurations and MOI values are graphically presented in detail. Based on this relation, a simple model on lubricant film structures for the three processes studied is presented. The model MOI values agree very well with measured MOI values as a function of lubricant thickness for all three processes, and the model also appears to account for the observed tribology performance for the MOI values studied (0.5 ~ 1.5).

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):379-384. doi:10.1115/1.2834437.

Regardless of environment (ultrahigh vacuum, humid air, dry nitrogen, or water), ion-beam-deposited diamondlike carbon (DLC) and nitrogen-ion-implanted, chemical-vapor-deposited (CVD) diamond films had low steady-state coefficients of friction (<0.1) and low wear rates (≤ 10−6 mm3 /N·m). These films can be used as effective wear-resistant, self-lubricating coatings regardless of environment. On the other hand, as-deposited, fine-grain CVD diamond films; polished, coarse-grain CVD diamond films; and polished and then fluorinated, coarse-grain CVD diamond films can be used as effective wear-resistant, self-lubricating coatings in humid air, in dry nitrogen, and in water, but they had a high coefficient of friction and a high wear rate in ultrahigh vacuum. The polished, coarse-grain CVD diamond film revealed an extremely low wear rate, far less than 10−10 mm3 /N·m, in water.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):385-392. doi:10.1115/1.2834438.

The complete atomic-scale indentation cycle is analyzed using molecular dynamics simulations. A hysteresis is observed in the instantaneous normal force versus surface separation distance curve obtained with an atom or a rigid tip indenting and, subsequently, retracting from a dynamic face-centered-cubic substrate consisting of argon or copper. The generation of irreversible deformation in a Lennard-Jones solid is revealed in light of simulation results for indentation by a single atom. The direction of irreversible deformation is shown to coincide with that of macroscopic plastic flow. The compressive yield strength decreases with increasing substrate temperature and decreasing indentation speed. The phenomena of tip wetting by substrate atoms and connective neck formation, elongation, and rupture at the tip/substrate interface are elucidated by simulation results for the unloading process. It is shown that energy dissipation decreases as the substrate temperature increases and the energy consumed by irreversible deformation is always greater than that due to heating.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):393-398. doi:10.1115/1.2834439.

The friction characteristic of paper-based friction materials is one of the most important factors that influence the shifting performance of oil immersed clutches in automobile automatic transmissions. Visco-elasticity and porosity of the material are considered to be important properties that may control the friction characteristics of the material. In this study, the relationship between the friction characteristics and the visco-elasticity of a paper-based friction material is discussed. In order to vary the visco-elasticity while retaining the ingredients almost constant in a paper-based friction material, the formulation of the sample materials required changing the compression rate during the clutch plate bonding process and changing the volume percentage of resin. The properties of the sample materials such as porosity (pore diameter, pore diameter distribution, porosity percentage), and visco-elastic deformation in the compressive and the shearing directions, etc. were measured. In order to clarify the relationship between the friction and visco-elasticity of friction material, friction characteristics were measured. Friction characteristics of the sample materials were evaluated by a breaking-away test and continuous slip tests with a newly designed friction testing machine. From the experimental results, which showed that the lower the longitudinal modulus of the friction material, the higher the friction coefficient, it was found that friction characteristics of oil immersed paper-based friction materials are influenced by the visco-elasticity.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(2):399-405. doi:10.1115/1.2834440.

Thermohydrodynamic lubrication is complicated by the multiplicity of heat transfer paths from the site of dissipation to the surroundings. This has impeded a general understanding of thermohydrodynamic phenomena. The method of lumped variables is applied to a simple slider bearing (as an example) to explore thermohydrodynamic lubrication. This approach leads to a better understanding of the interplay between the different modes of heat transfer and the influence of the associated temperature rises on performance, and how this can be captured by the use of appropriate non-dimensional measures. Such measures also allow the mapping of the problem into a set of regimes characterized by the relative importance of different heat transfer modes.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

TECHNICAL BRIEFS

J. Tribol. 1998;120(2):406-408. doi:10.1115/1.2834441.
Abstract
Commentary by Dr. Valentin Fuster

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