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### TECHNICAL PAPERS

J. Tribol. 2007;129(3):453-460. doi:10.1115/1.2736426.

We numerically investigated the characteristics of contact force, adhesion force, and contact stiffness between a smooth contact pad and a small rough surface, such as a current magnetic disk surface. The computer-generated asperity had an isotropic Gaussian distribution with a small asperity height and high asperity density. We took asperity contact, bulk deformation, and meniscus force of a lubricant layer at contacting asperity into consideration in the calculations. We evaluated the effects of asperity density, contact pad area, asperity radius, root mean square (RMS) asperity height, and lubricant thickness on external and internal contact forces, adhesion force, and contact stiffness as a function of the separation between the contact pad and disk in both approaching and separating processes. We found that contact and adhesion force tend to change suddenly at the start and end of contact and exhibits hysteresis in the approaching and separating processes when asperity density becomes large and RMS asperity height becomes small comparable with current head sliders and magnetic disks. We also found that contact stiffness is governed by bulk deformation and that the contact stiffness and adhesion force can be regarded as constant during contact when the asperity density increases, the RMS asperity height decreases, and the contact area increases.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):461-466. doi:10.1115/1.2736427.

The objective of this study is to construct a continuous mathematical model that describes the frictionless contact between a nominally flat (rough) viscoelastic punch and a perfectly rigid foundation. The material’s behavior is modeled by assuming a complex viscoelastic constitutive law, the standard linear solid (SLS) law. The model aims at studying the normal compliance (approach) of the punch surface, which will be assumed to be quasistatic, as a function of the applied creep load. The roughness of the punch surface is assumed to be fractal in nature. The Cantor set theory is utilized to model the roughness of the punch surface. An asymptotic power law is obtained, which associates the creep force applied and the approach of the fractal punch surface. This law is only valid if the approach is of the size of the surface roughness. The proposed model admits an analytical solution for the case when the deformation is linear viscoelastic. The modified analytical model shows a good agreement with experimental results available in the literature.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):467-480. doi:10.1115/1.2736430.

In view of the difficulty in measurement of flash temperature rise at the contact between rough sliding bodies a good deal of work has been carried out in the last few decades to predict flash temperatures theoretically. However, as surfaces become smoother and loading decreases in applications such as MEMS, NEMS and magnetic storage devices measurement of flash temperature becomes increasingly more difficult due to the nanometer scale asperity interactions. Consequently measurement of flash temperature at the nanoscale asperity contact has not yet been possible. The analysis of flash temperature rise under these circumstances is no less challenging since it must consider not only the small-scale asperity height distributions but also the surface forces those may operate at very small surface separations. The paper attempts to predict the flash temperature rise analytically using a fractal approach to describe the nanoscale asperity interactions at low loads and also taking into account the influence of relevant parameters including the surface forces. The important observation here is that in addition to the dependence on load, speed, and material parameters the flash temperature steadily rises with surface adhesion but falls with the fractal dimension $D$ until a critical value of around 1.5, and then rises again. The flash temperature also falls with Fourier number. Under certain combinations of load, speed, and material parameters, extremely high flash temperature is predicted while under certain other parametric combinations extremely low flash temperature may occur. The later parametric combination is certainly of much practical importance.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):481-494. doi:10.1115/1.2736431.

Based on a hybrid superposition of an indentation contact and a rolling contact an analytical procedure is developed to evaluate the effects of surface adhesion during steady-state rolling contact, whereby two analytic solutions have been obtained. The first solution is a Hertz-type rolling contact between a rigid cylinder and a plane strain semi-infinite elastic substrate with finite adhesion, which is a JKR-type rolling contact but without singular adhesive traction at the edges of the contact zone. The second solution is of a rolling contact with JKR singular adhesive traction. The theoretical solution indicates that, when surface adhesion exists, the friction resistance can be significant provided the external normal force is small. In addition to the conventional friction coefficient, the ratio between friction resistance force and normal force, this paper suggests an “adhesion friction coefficient” which is defined as the ratio between friction resistance force and the sum of the normal force and a function of maximum adhesive traction per unit area, elastic constant of the substrate, and contact area that is characterized by the curvature of the roller surface.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):495-501. doi:10.1115/1.2736432.

This paper presents a numerical approach to simulate sliding friction between engineering surfaces with 3D roughness in point contacts. The numerical approach is developed on the basis of the deterministic solutions of mixed lubrication, which is able to predict the locations where the asperity contacts occur, and the pressure distribution over both lubrication and contact areas. If the friction coefficients over the contacting asperities have been determined, total friction force between the surfaces can be calculated by summing up the two components, i.e., the boundary friction contributed by contacting asperities and the shear stress in hydrodynamic regions. The frictions from asperity contact were determined in terms of a limiting shear stress or shear strength of boundary films while the fluid shear stress in the lubrication areas was calculated using different rheology models for the lubricant, in order to find which one would be more reliable in predicting fluid tractions. The simulations covered the entire lubrication, regime, including full-film Elastohydrodynamic Lubrication (EHL), mixed lubrication, and boundary lubrication. The results, when being plotted as a function of sliding velocity, give a Stribeck-type friction curve. This provides an opportunity to study friction change during the transition of lubrication conditions and to compare friction performance on different rough surfaces, which is of great value in engineering practice. Experiments were conducted on a commercial test device—universal material tester (UMT) to measure friction at a fixed load but different sliding velocities in reciprocal or rotary motions. The results also give rise to the Stribeck friction curves for different rough surfaces, which are to be compared with the results from simulations. The samples were prepared with typical machined surfaces in different roughness heights and textures, and in point contacts with steel ball. Results show that there is a general agreement between the experiments and simulations. It is found that surface features, such as roughness amplitude and patterns, may have a significant effect on the critical speed of transition from hydrodynamic to mixed lubrication. In the regime of mixed lubrication, rougher samples would give rise to a higher friction if the operation conditions are the same.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):502-508. doi:10.1115/1.2738477.

The effect of microdents within thin elastohydrodynamics (EHD) contacts has been studied by two measurement techniques. Phase-shifting interferometry was used to obtain topography of microtextured surface and thin-film colorimetric interferometry provided detailed information about film thickness changes within a lubricated contact. The behavior of microdents has been observed for positive slide-to-roll ratios when the disk is moving faster than the microtextured ball. The depth of microdents has been found to play significant role as to the lubrication films efficiency. The presence of deep microdents within lubricated contact results in film thickness reduction downstream that can even cause lubrication film breakdown. As the depth of microdents is reduced, this effect diminishes and beneficial effect of microdents on film thickness formation has been observed. No such an effect of microdent depth on lubricant film shape has been observed in case of negative slide-to-roll conditions when microdents do not cause film thickness reduction regardless of their depths. Obtained results suggest that surface texturing using microdents of an appropriate depth could help to increase lubrication films capabilities.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):509-516. doi:10.1115/1.2736433.

An elastohydrodynamic lubrication (EHL) model for coated surfaces in point contacts has been developed by combining the elastic deformation formulation for the coated surfaces with an EHL model. Inverse fast Fourier transform (IFFT) is employed first to obtain the influence coefficients (ICs) from the frequency response function (FRF). The subsequent calculation of elastic deformation is performed using the efficient algorithm of discrete convolution and fast Fourier transform (DC-FFT). The coating EHL model is verified by the comparison to available numerical results. The effects of coating on lubrication under various loads, speeds, rheological models, and pressure-viscosity behaviors are numerically investigated. Similar to the observations from dry contact, stiffer coatings in EHL tend to reduce the nominal contact radius but increase the maximum contact pressure, and vice versa for more compliant coatings. However, as coating thickness increases, the influence of coatings on film thickness, including the central and the minimum film thicknesses, does not follow a monotonic variation, and therefore, cannot be predicted by any simple film thickness equation. The reason for that is the pressure viscosity effect which tends to counterbalance the effect of coating. The average friction coefficient in lubricant film increases in stiff coating cases but decreases for compliant coating cases. Furthermore, two possible approaches to improving the minimum film thickness thus reducing friction and wear in mixed lubrication are indicated: a thin stiff coating for conventional EHL and a thick compliant coating for soft EHL.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):517-527. doi:10.1115/1.2736435.

An analytical approach for treating problems involving oscillatory heat source is presented. The transient temperature profile involving circular, rectangular, and parabolic heat sources undergoing oscillatory motion on a semi-infinite body is determined by integrating the instantaneous solution for a point heat source throughout the area where the heat source acts with an assumption that the body takes all the heat. An efficient algorithm for solving the governing equations is developed. The results of a series simulations are presented, covering a wide range of operating parameters including a new dimensionless frequency $ω¯=ωl2∕4α$ and the dimensionless oscillation amplitude $A¯=A∕l$, whose product can be interpreted as the Peclet number involving oscillatory heat source, $Pe=ω¯A¯$. Application of the present method to fretting contact is presented. The predicted temperature is in good agreement with published literature. Furthermore, analytical expressions for predicting the maximum surface temperature for different heat sources are provided by a surface-fitting method based on an extensive number of simulations.

Topics: Heat , Temperature
Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):528-535. doi:10.1115/1.2736436.

The paper presents a numerical model to investigate fretting wear either under partial or gross slip conditions. An efficient three-dimensional elastic–static contact model to solve both the normal contact problem and the tangential contact problem is presented. The contact model is validated with analytical solutions for a sphere on flat geometry. A wear law issued from the literature and based on the friction energy is used to simulate surface wear. Numerical friction logs are obtained and the wear rate evolution is found to be highly dependent on the tangential displacement.

Commentary by Dr. Valentin Fuster
J. Tribol. 2006;129(3):536-543. doi:10.1115/1.2736437.

This paper develops a three-dimensional (3D) thermal-structure coupling model, implements transient stress analysis of thermoelastic contact of disk brakes with a frictional heat variation and identifies the source of the thermal fatigue. This thermostructure model allows the analysis of the effects of the moving heat source (the pad) with a variable speed and integrates the heat flux coupling between the sliding surfaces. To obtain the transient stress/temperature fields of the brake under an emergency braking, the thermoelastic problem under this 3D model is solved by the finite element method. The numerical results from the analysis and simulation show the temperature/stress of the disk presenting periodic sharp fluctuation due to the continuous cyclic loading; its varying frequency corresponds to the rotated cycle times of the braking disk. The results demonstrate that the maximum surface equivalent stress may exceed the material yield strength during an emergency braking, which may cause a plastic damage accumulation in a brake disk, while a residual tensile hoop stress is incurred on cooling. These results are validated by experimental observation results available in the literature. Based on these numerical results, some suggestions for avoiding fatigue fracture propagation are further presented.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):544-552. doi:10.1115/1.2736439.

Sliding wear is a significant surface failure mode in many mechanical components. The magnitude of changes in surface topography due to wear may be comparable to or larger than the original surface roughness and elastic deformation. However, wear has rarely been incorporated into the numerical models used as predictive tools in engineering practice. This paper presents a numerical approach to simulate the wear process based on the deterministic mixed elastohydrodynamic lubrication (EHL) model developed and modified by Zhu and Hu (2001, Tribol. Trans., 44, pp. 383–398). It is assumed that wear takes place at locations where the surfaces are in direct contact, and the wear rate at those local contact spots is proportional to the relative sliding speed, the local contact pressure, and inversely proportional to the hardness of the surface. At each simulation cycle, the distributions of lubricant film thickness and contact pressure are calculated by using the mixed EHL model. The material removal at each contact location is evaluated and the surface topography modified correspondingly. The renewed surface topography is then used for the next cycle. The model is formulated such that any mathematically expressed wear law can be implemented, and therefore, the simulation can be applied to a wide variety of engineering applications.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):553-561. doi:10.1115/1.2736440.

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

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):562-569. doi:10.1115/1.2736442.

There are disk-drive data storage applications best served by single-sided recording configurations. These include situations where (i) storage requirements can be achieved on a single side of a disk and (ii) dimensional constraints on the disk drive prohibit the presence of a recording head and its associated mounting device on each side of the disk. Even if dimensional requirements are not a concern, the most cost-effective and operationally efficient slider-disk air-bearing interface for single-sided recording is one that does not include an air-bearing slider, pressure pad, or other air-bearing structure on the nondata side of the disk. A metal foil disk offers some of the best characteristics of both the hard disk and floppy disk for digital data storage. It offers hard disk recording densities, increased shock resistance, reduced manufacturing cost, and requires less operational energy than a hard disk. However, use of a conventional recording head slider assembly without opposing air-bearing support for single-sided recording on a high-speed metal foil disk presents a fundamental problem because the air-bearing surface of the slider produces a net transverse force to the disk. This force causes the disk to deflect and can result in flying height and stability problems at the slider-disk interface. The current work describes an air-bearing interface for low flying height single-sided recording on a high-speed metal foil disk that minimizes disk deflection and instability without the presence of air-bearing components on opposing sides of the disk. The new interface utilizes a vacuum cavity-type air-bearing with little or no preload. Examples will be presented and discussed for the new interface that illustrate the flying characteristics of a picosized slider on a $1.8in.$ stainless steel disk with thickness of $25.4μm$.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):570-578. doi:10.1115/1.2736456.

Flying height (FH) control sliders with thermal actuation have been introduced recently in commercial products for compensating the static FH loss and reducing the risk of head-disk contacts. In the research reported here, we investigated the effects of air-bearing surface (ABS) designs on the thermal actuation. We created a three-dimensional finite element model of an entire slider with a detailed read/write transducer structure and conducted thermal-structural coupled-field analysis using velocity slip and temperature jump boundary conditions to formulate the heat transfer across the head-disk interface when a slider flies over a spinning disk. An iteration procedure was used to obtain the equilibrium solutions. Four ABS designs with distinct features were simulated. We defined five measures of merit, including protrusion rate, actuation efficiency, power consumption, pressure peak, and temperature rise of the sensor to evaluate the performance of thermal actuation. It is found that the effect of the pressure is more significant than that of the FH on the heat conduction from the slider to the disk. The efficiencies of three conventional designs decrease as the FHs are continuously reduced. A new ABS design, called “Scorpion III,” is presented and demonstrates an overall enhancement, including virtually 100% efficiency with significantly less power consumption. Transient thermal analysis showed that it requires $∼1–2ms$ for the temperature to reach the steady-state values, and there is a trade-off between increasing the actuation bandwidth and decreasing the power consumption.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):579-585. doi:10.1115/1.2736448.

In order to achieve a magnetic recording density of $1Tb∕in2$, the spacing is expected to be less than $2–3nm$. However, a critical issue in achieving such an ultralow spacing is the dynamic instability of the head disk interface (HDI). That is, the experimentally observed hysteresis of fly sliders. The phenomenon of slider hysteresis has two features: slider touchdown and slider takeoff. The goal of this research is to experimentally clarify the effects of the lubricant bonded ratio as well as the lubricant film thickness on slider hysteresis behavior in detail. It also aims to determine the contributing factors. In this study, the difference in the touchdown and takeoff velocities was monitored by varying the lubricant bonded ratio and lubricant film thickness of the disks. Furthermore, the correlation between the observed phenomenon and the variation in the experimental parameters was investigated. The results showed that the touchdown velocities were almost independent of the lubricant bonded ratio, while the takeoff velocities were greater for a lubricant with a higher bonded ratio. These results were obtained for a constant lubricant film thickness of around one monolayer. Therefore, the slider hysteresis was greater for a lubricant with a higher bonded ratio. With regard to the effect of lubricant film thickness, it was observed that the touchdown and takeoff velocities were greater for thinner lubricants. These results for the effect of lubricant film thickness are very similar to those obtained by Ambekar, Gupta, and Bogy (2005, ASME J. Tribol., 127(3), pp. 530–536). However, the slider hysteresis was greater for thicker lubricants. Considering these experimental results as well as the experimental data for the effect of the surface roughness of a disk on the slider hysteresis obtained by (Tani (2006, J. Appl. Phys. , 99(8), pp. 08N104-1–08N104-3), it was suggested that the variation in the touchdown velocity is due to a variation in the intermolecular forces. Furthermore, it was suggested that the variation in the takeoff velocity is caused by a variation in the friction forces between the slider and disk surface. This occurs because the takeoff velocity was greater for a lubricant with a higher bonded ratio or a thinner lubricant, which only has a small fraction of free mobile lubricant. The results predicted by the simulations are consistent with those observed experimentally. In addition, a design guideline for next-generation HDI, with small touchdown and takeoff velocities, resulting in small slider hysteresis, is discussed in detail in this paper.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):586-594. doi:10.1115/1.2736450.

This paper aims to compare the tribo-mechanical properties and structure–property relationships of a wear resistant cobalt-based alloy produced via two different manufacturing routes, namely sand casting and powder consolidation by hot isostatic pressing (HIPing). The alloy had a nominal wt % composition of Co–33Cr–17.5W–2.5C, which is similar to the composition of commercially available Stellite 20 alloy. The high tungsten and carbon contents provide resistance to severe abrasive and sliding wear. However, the coarse carbide structure of the cast alloy also gives rise to brittleness. Hence this research was conducted to comprehend if the carbide refinement and corresponding changes in the microstructure, caused by changing the processing route to HIPing, could provide additional merits in the tribo-mechanical performance of this alloy. The HIPed alloy possessed a much finer microstructure than the cast alloy. Both alloys had similar hardness, but the impact resistance of the HIPed alloy was an order of magnitude higher than the cast counterpart. Despite similar abrasive and sliding wear resistance of both alloys, their main wear mechanisms were different due to their different carbide morphologies. Brittle fracture of the carbides and ploughing of the matrix were the main wear mechanisms for the cast alloy, whereas ploughing and carbide pullout were the dominant wear mechanisms for the HIPed alloy. The HIPed alloy showed significant improvement in contact fatigue performance, indicating its superior impact and fatigue resistance without compromising the hardness and sliding∕abrasive wear resistance, which makes it suitable for relatively higher stress applications.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):595-602. doi:10.1115/1.2736451.

A general solution scheme to account the surface roughness and the cross-film viscosity variation of lubricant due to its non-Newtonian behavior and rise in fluid-film temperature for the analysis of fluid-film bearings is presented. The combined influence of surface roughness, non-Newtonian behavior of lubricant, and thermal effects on the performance of a hole-entry hybrid journal bearing system has been investigated. The surface roughness, especially stationary roughness (i.e., rough bearing and smooth journal) with a transverse pattern was found to partially compensate for the loss in load-carrying capacity due to the thermal and/or non-Newtonian behavior of lubricant effects. It limits 18.86% loss in load-carrying capacity due to the thermal effect to only 1.6% and 33.99% loss due to the combined influence of non-Newtonian lubricant and thermal effect to 16.76%.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):603-610. doi:10.1115/1.2736452.

The effect of surface texture and roughness on shear and pressure forces in tribological applications in the lubrication regime is analyzed by means of lattice-Boltzmann simulations that take the geometry of real surface elements into account. Topographic data on representative surface structures are obtained with high spatial resolution with the application of an optical interference technique. The three-dimensional velocity field past these surfaces is computed for laminar flow of Newtonian fluids in the continuum regime. Subsequently, pressure and shear flow factors are obtained by evaluating the velocity field in accordance with the extended Reynolds equation of Patir and Cheng (1978, ASME J. Tribol., 100, pp. 12–17). The approach allows an efficient determination of the hydrodynamic characteristics of microstructured surfaces in lubrication. Especially, the influence of anisotropy of surface texture on the hydrodynamic load capacity and friction is determined. The numerical method used in the present work is verified for a simplified model configuration, the flow past a channel with sinusoidal walls. The results obtained indicate that full numerical simulations should be used to accurately and efficiently compute the characteristic properties of film flows past rough surfaces and may therefore contribute to a better understanding and prediction of tribological problems.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):611-620. doi:10.1115/1.2736704.

In recent years it has been shown experimentally by a number of workers that simple, Newtonian liquids can slip against solid surfaces when the latter are both very smooth and lyophobic. It has also been shown theoretically how, based on a half-wetted bearing principle, this phenomenon may be used to significantly reduce friction in lubricated sliding contacts and thus make possible the hydrodynamic lubrication of very low load contacts. This paper describes the experimental validation of this concept. A low load bearing is constructed and the influence of surface roughness and the wetting properties of the surfaces on friction are investigated over a wide range of sliding speeds. It is shown that liquid slip can be used to considerably reduce friction in full film, hydrodynamic conditions.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):621-627. doi:10.1115/1.2736454.

In order to improve the durability of metal-film magnetic tapes, novel lubricants, A20H, X-1P, and X, a modified phosphazene, were deposited on the tapes. The adhesion, friction, and wear of the unlubricated/lubricated tapes were investigated using an atomic force microscope (AFM). The degradation of the lubricants was studied using a mass spectrometer in high vacuum. The durability of various unlubricated/lubricated tapes was compared in ambient and in humid air. The AFM test results show that the A20H lubricated tape exhibited lower adhesion and friction than X-1P and X lubricated tapes. The lubricants were believed to be mainly degraded by tribochemical reaction and mechanical shear in high vacuum. In high humidity air, the various lubricated tapes exhibit higher friction than in ambient air. By comparing the tribological performances of the various lubricated tapes to metal particle (MP) tape, it was found that the lubricated metal-film tapes exhibit lower adhesion, friction, and wear than the MP tape.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):628-639. doi:10.1115/1.2736455.

A new foil gas bearing with spring bumps was constructed, analyzed, and tested. The new foil gas bearing uses a series of compression springs as compliant underlying structures instead of corrugated bump foils. Experiments on the stiffness of the spring bumps show an excellent agreement with an analytical model developed for the spring bumps. Load capacity, structural stiffness, and equivalent viscous damping (and structural loss factor) were measured to demonstrate the feasibility of the new foil bearing. Orbit and coast-down simulations using the calculated stiffness and measured structural loss factor indicate that the damping of underlying structure can suppress the maximum peak at the critical speed very effectively but not the onset of hydrodynamic rotor-bearing instability. However, the damping plays an important role in suppressing the subsynchronous vibrations under limit cycles. The observation is believed to be true with any air foil bearings with different types of elastic foundations.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):640-646. doi:10.1115/1.2736705.

A series of experiments was performed to study the behavior of grease-lubricated journal bearings. The results reveal that an oscillatory bearing undergoes a transition from boundary, to mixed, and to hydrodynamic regime. Another distinct feature is friction hysteresis that occurs as a result of oscillation. In this paper, we examine the effect of load, oscillating frequency, and lubricant on the friction hysteresis loop.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):647-654. doi:10.1115/1.2736706.

The paper presents a new experimental device made to analyze the thermoelastohydrodynamic (TEHD) behavior of connecting-rod bearings functioning in severe conditions. First, it focuses on the test bench description. The general principle of the test bench and then the main original technological solutions used with respect to the functional specifications are detailed. Two numerical models are described. They were developed in order to design and to validate two central components of the experimental device. Finally, the paper comments on the test results used to understand and validate the traction∕compression loading system, which is one of the key points in the test bench behavior.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):655-659. doi:10.1115/1.2736707.

A microbearing tester driven by an air turbine of $10mm$ diameter has been developed, and successfully used to test hydroinertia gas bearings with a shaft of $4mm$ diameter. The effects of bearing gas pressure conditions and bearing length to diameter ratio $(L∕D)$ on the maximum achievable rotation speed were investigated. The maximum rotation speed of $890,000rpm$, which corresponds to the $DN$ number (the product of a shaft diameter in millimeter and a rotation speed in rpm) of 3,560,000, was achieved. At $890,000rpm$, the tip speed of the turbine reaches approximately $470m∕s$. Using the bearing system developed, the turbo components of a $100W$ class gas turbine and an air pump for $1kW$ class fuel cells can be tested.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):660-668. doi:10.1115/1.2736708.

The damping capability of squeeze film dampers (SFDs) relies on adequate end sealing to prevent air ingestion and entrapment. The paper presents the parameter identification procedure and force coefficients of a test SFD featuring a mechanical seal that effectively eliminates lubricant side leakage. The test damper reproduces an aircraft application intended to contain the lubricant in the film lands for extended periods of time. The test damper journal is $2.54cm$ in length and $12.7cm$ in diameter, with a nominal clearance of $0.127mm$. The SFD feed end is flooded with oil, while the discharge end contains a recirculation groove and four orifice ports. In a companion paper (San Andrés and Delgado, 2006, ASME J. Eng. Gas Turbines Power, 119, to be published) single frequency–unidirectional load excitation tests were conducted, without and with lubricant in the squeeze film lands, to determine the seal dry-friction force and viscous damping force coefficients. Presently, tests with single frequency excitation loads rendering circular centered orbits excitations are conducted to identify the SFD force coefficients. The identified parameters include the overall system damping and the individual contributions from the squeeze film, dry friction and structural damping. The identified system damping coefficients are frequency and motion amplitude dependent due to the dry friction interaction at the mechanical seal interface. Identified squeeze film force coefficients, damping, and added mass, are in good agreement with predictions based on the full film, short length damper model.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):669-678. doi:10.1115/1.2738072.

The present work presents a theoretical approach for the analysis of textured annular “damper” seals. The data for the seal were extracted from the work of Childs and Fayolle (ASME J. Tribol.121(1), pp. 42–49). The texture of the stator consists of equally spaced cylindrical holes of an order of magnitude larger than the seal clearance. The main idea of the present work is that the static and dynamic characteristics of the textured annular seal can be predicted by using a slightly modified bulk-flow model. The modifications are introduced by considering the textured seal as being geometrically similar to a straight seal with the same clearance. The presence of the texture is taken into account by considering modified friction laws for the rotor and for the stator, separately. An additional inertia effect due to the texture is also added as a source term to the momentum equations. The modified friction laws and the inertia effect are deduced from a three-dimensional Navier-Stokes analysis of the flow in the textured seal. This computational analysis is carried on for a single texture element extracted from the round-hole pattern of stator by using periodicity boundary conditions. The stiffness and the damping of the annular seal were calculated by using the modified bulk-flow model and results were compared with the experimental data from Childs and Fayolle. The use of the present model shows a net improvement of the predictions for the direct and cross-coupling stiffness and for the cross-coupling damping. The results obtained for the direct damping are still under discussion.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):679-683. doi:10.1115/1.2736709.

The authors have carried out extensive measurements of electron triboemission from the scratching of ceramics and semiconductors that are briefly summarized in this paper. Analysis of the frequency-domain distribution of typical triboemission count-pulse outputs suggested that their occurrence is not Poisson’s (e.g., it is not random). This paper presents a study on the hypothesis of deterministic-chaos origin for triboemission data. Electron triboemission outputs from the ceramics alumina, sapphire, silicon nitride, and the semiconductors Si and Ge are analyzed by means of extracting deterministic-chaos metrics. The results suggest that the low-level electron-emission components may be described by multiplicative processes of random initiation, while the superimposed large burst-type components may be of deterministic origin.

Commentary by Dr. Valentin Fuster

### TECHNICAL BRIEFS

J. Tribol. 2007;129(3):684-688. doi:10.1115/1.2736730.

Approximate closed-form equations are derived for normal and tangential contact forces of rough surfaces in dry friction. Using an extension of the Greenwood and Tripp (1970, Proc, Inst. Mech. Eng., 185, pp. 625–633) model, in which the derivations permit asperity shoulder-to-shoulder contact and viscoelastic asperity behavior, mathematical formulae are derived for normal and tangential components of the contact force that depend not only on the proximity of the two surfaces but also the rate of approach and relative sliding. A statistical approach is forwarded in which dependence of the asperity tangential contact force on relative tangential velocity of two asperities can be cast as corrective factors in the mathematical description of tangential force. In this regard two corrective coefficients are derived: force directionality corrective coefficient and force–velocity directionality corrective coefficient. The results show that for a moderate to high load ranges the contact force can be analytically described to within 20% accuracy of that from a numerical integration of the contact equations, well below the uncertainties due to surface profile measurement.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):689-694. doi:10.1115/1.2736731.

This paper addresses the effects of pitch static attitude (PSA) and roll static attitude (RSA) on air bearing slider steady performance, especially for ultralow flying height sliders. We performed simulations for three different low flying sliders with flying heights (FHs) of $7nm$, $5nm$, and $3.5nm$ using the static simulator code of the Computer Mechanics Laboratory. We found that PSA and RSA have quite significant effects on the steady performance of these air bearing slider designs, and the effect is more important the smaller the size and the lower the FH of the slider. We also investigated the effects of suspension stiffness on the air bearing sliders’ flying attitude (pitch and roll) and found that these effects are similar to those of PSA and RSA.

Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):695-699. doi:10.1115/1.2736732.

A three-dimensional thermohydrodynamic model is developed to predict non-Newtonian lubricant behavior in slider bearings and channel flow. The generalized Reynolds equation is established using the concept of generalized Newtonian fluids (GNF) and the temperature field is determined with the energy equation. The chosen rheological models are the power-law, Bingham, and Hershel–Bulkley models. The last two models hold uniformly in yielded and unyielded regions using the approach proposed by Papanastasiou. The results present the evolution of the velocity, pressure, and thermal fields. The power loss, load capacity, and friction coefficient are analyzed. Comparisons are made with Newtonian lubricants and other recent non-Newtonian computational analyses.

Commentary by Dr. Valentin Fuster

### DISCUSSION

J. Tribol. 2007;129(3):700. doi:10.1115/1.2749229.
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Jackson, R. L., Chusoipin, I., and Green, I., “ A Finite Element Study of the Residual Stress and Strain Formation in Spherical Contacts,” ASME J. Tribol.JOTRE9 0742-4787[[XSLOpenURL/10.1115/1.1843166]], 2005, 127(3), pp. 484–493.Etsion, I., Kligerman, Y., and Kadin, Y., “ Unloading of an Elastic-Plastic Loaded Spherical Contact,” Int. J. Solids Struct.IJSOAD 0020-7683[[XSLOpenURL/10.1016/j.ijsolstr.2004.12.006]], 2005, 42(13), pp. 3716–3729.Kogut, L., and Komvoppoulos, K., “ Analysis of Spherical Indentation Cycle of Elastic-Perfectly Plastic Solids,” J. Mater. Res.JMREEE 0884-2914[[XSLOpenURL/10.1557/JMR.2004.0468]], 2004, 19, pp. 3641–3653.
Commentary by Dr. Valentin Fuster
J. Tribol. 2007;129(3):701. doi:10.1115/1.2749446.
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Jamari, J., and Schipper, D. J., 2007, “ Deformation Due to Contact Between a Rough Surface and a Smooth Ball,” WearWEARCJ 0043-1648, 262(1-2), pp. 138–145.Jamari, J., and Schipper, D. J., 2006, “ Experimental Investigation of Fully Plastic Contact of a Sphere Against a Hard Flat,” ASME J. Tribol.JOTRE9 0742-4787[[XSLOpenURL/10.1115/1.2164470]], 128, pp. 230–235.
Commentary by Dr. Valentin Fuster

### ANNOUNCEMENT

J. Tribol. 2007;129(3):702. doi:10.1115/1.2750333.
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Commentary by Dr. Valentin Fuster