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IN THIS ISSUE


TECHNICAL PAPERS

J. Tribol. 2005;127(1):1-9. doi:10.1115/1.1828065.

A model is developed to predict the behavior of two sliding bodies undergoing oscillatory motion. A set of four dimensionless groups is introduced to characterize the transient dimensionless surface temperature rise. They are: the Peclet number Pe, the Biot number Bi, the amplitude of oscillation A, and the Hertzian semi-contact width α. Also considered in the analysis is the effect of the ratio β=A/α of the amplitude to the semi-contact width. The results of a series of simulations, covering a range of these independent parameters, are presented, and examples are provided to illuminate the utility of the model.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):10-23. doi:10.1115/1.1828073.

Recent developments in the solution of models of rough contact have demonstrated the advantages of fast matrix multiplication methods in increasing the size of problem that can usefully be solved. However, to date, no attempt has been made to compare these methods with existing approaches. In this paper a survey of solution methods reveals that three main classes of solution method have been developed: active set methods with direct matrix solution; active set methods with iterative matrix solvers; iterative methods based on fast matrix multiplications. The best features of each class are assembled from the literature and combined to formulate a “best-possible” algorithm in each class. These algorithms are implemented and compared, with emphasis on their solution speed, accuracy and memory requirements. For smaller problems with fewer than 2000 grid points in contact, the direct matrix solution method proves to be most efficient, and is always the most accurate. However, the method is limited by memory requirements as the contact region increases. For large problems with more than 2000 grid points in contact, the multilevel multisummation approach requires less memory, and performs faster, but gives errors in predicted pressure and contact region of approximately 0.5%. The active set method with iterative matrix solver requires less memory than direct matrix solution, is more accurate than the multilevel multisummation approach, but is always slower than one of the other methods.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):24-29. doi:10.1115/1.1828077.

The Greenwood and Williamson (1966) model is an elegant and often-cited paradigm for predicting the load-displacement behavior of contacting rough surfaces given the height distribution of the contacting asperities. By use of an inverse technique and available load-displacement data, this work provides an alternative method to determine the asperity distribution directly from topographic measurement and subsequent data reduction. This method produces distributions that are consistent with the Greenwood and Williamson representation of the load-displacement data and demonstrates how the asperity height distribution can evolve with loading.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):30-36. doi:10.1115/1.1829717.

The effective radius is a very important factor in determining frictional torque under the bolt head and nut in bolted joints. Since the effective radius is hard to be precisely determined in practice, the mean radius, which is the mean value of the inner and outer radii of the contact area under the bolt head and nut, is used. In this paper, contact mechanics analysis is used to determine the real contact pressure distribution between the bolt head and nut and the joint surface. Based on the pressure distribution, the effective radius can be precisely calculated. The effect of the surface roughness, underhead load distribution, underhead load value, and the ratio of the maximum to minimum contact radii are investigated. The results are compared with the result from theoretical formulas and the value of mean radius. These data will be very useful in enhancing the reliability and safety of bolted joints.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):37-46. doi:10.1115/1.1829722.

Scale effect on dry friction during multiple-asperity contact is considered. The coefficient of friction is a sum of the adhesion component, two-body deformation component, three-body deformation (plowing) component, and a so-called ratchet component of friction. These components of the friction force depend on the relevant real area of contact (dependent on roughness and mechanical properties) and relevant shear strength during sliding. A model of rough surface topography based on an empirical power rule for scale dependence of roughness and contact parameters is proposed. The effect of load on contact parameters is also considered. Comparison of the model to experiment data on scale and load dependence of friction is presented.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):47-50. doi:10.1115/1.1843834.

An experimental investigation is presented to evaluate recently published models for the contact and sliding inception of a deformable sphere loaded against a smooth rigid flat. The effects of the normal load on the contact area, junction growth, and the static friction force in the elastic–plastic contact regime are presented. Very good correlation is found between the predicted and measured contact area. A dramatic decrease of the static friction coefficient with increasing normal loading is observed, similar to the trend predicted by the model. The quantitative agreement is, however, less satisfying. Some possible reasons for the poor agreement are pointed out.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):51-60. doi:10.1115/1.1828074.

A direct comparison between experimental and numerical results for the passage of an array of 3D flat-top, square shaped surface features through an EHL point contact is presented. Results for pure rolling conditions show that the features’ deformation in the high-pressure region is governed by their ability to entrap lubricant both underneath and in the grooves during their passage through the inlet zone. Film perturbations associated with each defect occur as locally enhanced regions of lubricant and film thickness micro-constrictions. Under sliding conditions the features sustain further deformations as they traverse the high-pressure conjunction and meet the highly viscous lubricant entrapped in the grooves, which moves at a different velocity. Lubricant is also seen to accumulate just in front or behind the features depending on the slide-to-roll ratio. Overall, the results highlight the importance of understanding the effects of the defects structure and the lubricant rheology on the film thickness to unravel the effects of real roughness patterns.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):61-69. doi:10.1115/1.1828452.

The paper presents results obtained using a transient analysis technique for point contact elastohydrodynamic lubrication (EHL) problems based on a formulation that couples the elastic and hydrodynamic equations. Results are presented for transverse ground surfaces in elliptical point contact that show severe film thinning and asperity contact at the transverse limits of the contact area. This thinning is caused by transverse leakage of the lubricant from the contact in the remaining deep valley features between the surfaces. A comparison is also made between the point contact results on the entrainment center line and the equivalent line contact analysis. The extent of asperity contact is shown to be dependent on the Hertzian contact aspect ratio. It is also shown that transverse waviness (superimposed on the roughness) of even relatively small amplitude can lead to large increases in asperity contact rates over all waviness peaks in the contact.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):70-81. doi:10.1115/1.1843161.

A non-Newtonian numerical solution system for the thermal elastohydrodynamic lubrication (EHL) problems in point contacts has been developed. The Eyring rheology model has been used to describe the non-Newtonian flow of the lubricant. An effective viscosity has been defined for the Eyring fluid. The Newtonian solver can be applied easily to the non-Newtonian problems when the viscosity of the Newtonian fluid is replaced by the effective viscosity. A novel technique for the determination of the effective viscosity is proposed. Numerical solutions for the conventional point contact and normally crossing cylinders contact problems are presented and the effects of the entraining velocity, the load, the slide-roll ratio, and the characteristic shear stress of the Eyring fluid on the lubricating performance are discussed. The results indicate that the non-Newtonian thermal EHL theory predicts more realistic film temperatures and traction coefficients.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):82-88. doi:10.1115/1.1829719.

A closed-form time- and position-dependent model for coverage, based on the adsorption of environmental contaminants and their removal through the pin contact, is developed for reciprocating contacts. The model employs an adsorption fraction and removal ratio to formulate a series expression for the entering coverage at any cycle and location on the wear track. A closed-form solution to the series expression is presented and compared to other coverage models developed for steady-state coverage for pin-on-disk contacts, reciprocating contacts, or the time-dependent center-point model for reciprocating contacts. The friction coefficient is based on the average coverage under the pin contact. The model is compared to position- and time-dependent data collected on near-frictionless carbon self-mated contacts on a reciprocating tribometer in a nitrogen atmosphere. There are many similarities between the model curves and the data, both in magnitude and trends. No new curve fitting was performed in this paper, with all needed parameters coming from previous models of average friction coefficient behavior.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):89-95. doi:10.1115/1.1757490.

The aim of this study is to evaluate the temperature and the heat distribution into the two components of a disc brake system by combining macroscopic and microscopic effects. The major difficulty of the thermal problem is to determine how the heat is generated and how it is distributed in the two components in contact during transient state. Contrary to classical approaches assuming equal temperature at the contact surfaces, a contact interface is introduced in the model as a thin layer of third body with uniform volumic heat generation. This micro-macro model gives original indications on the temperatures near the contact surfaces, on the thermal gradients between the two components and on the heat partition between the two bodies during the braking time. Comparison with classical thermal models is discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):96-102. doi:10.1115/1.1828067.

Results of a numerical study of the influence of micro-patterned surfaces in hydrodynamic lubrication of two parallel walls are reported. Two types of parameterized grooves with the same order of depth as the film thickness are used on one stationary wall. The other wall is smooth and is sliding with a specified tangential velocity. Isothermal incompressible two dimensional full film fluid flow mechanics is solved using a Computational Fluid Dynamics method. It is shown that, by introducing a micro-pattern on one of two parallel walls, a net pressure rise in the fluid domain is achieved. This produces a load carrying capacity on the walls which is mainly contributed by fluid inertia. The load carrying capacity increases with Reynolds number. The load carrying capacity is reported to increase with groove width and depth. However, at a certain depth a vortex appears in the groove and near this value the maximum load carrying capacity is achieved. It is shown that the friction force decreases with deeper and wider grooves. Among all geometries studied, optimum geometry shapes in terms of hydrodynamic performance are reported.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):103-111. doi:10.1115/1.1828075.

In this paper, the dynamic characteristics of an oil-lubricated, sealed squeeze film damper (SFD) with a central feeding groove are analytically derived based on an enhanced dynamic pressure field analysis, accounting for the effect of the seal and feeding grooves, and its validity is experimentally verified. A test rig using an active magnetic bearing system as an exciter is introduced to identify the dynamic characteristics of SFDs with high accuracy and efficiency. Experiments are conducted with the seal gap varied, in order to investigate its effect on the dynamic characteristics of the SFD. The estimated and analytical damping and inertia coefficients for the sealed SFD with a groove are compared and it is found that the pressure field analysis, when neglecting the effect of the seal groove, tends to far underestimate the SFD dynamic coefficients.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):112-119. doi:10.1115/1.1828451.

A modified governing equation is derived incorporating the effects of roughness and cavitation in a journal bearing. The available theories of Reynolds roughness and cavitation algorithm proposed by Elrod are utilized in this work to develop a numerical procedure for stability analysis of a liquid lubricated rough journal bearing. The Elrod generalized theory of Reynolds roughness provides a governing equation based on the surface configuration. The Elrod cavitation algorithm conserves mass throughout the bearing and automatically predicts the full film and cavitation regions using a switch function. The roughness is considered on either or both the bearing and journal surfaces. The instability threshold speed increases significantly for the roughness patterns on the grooved bearing surface only at higher eccentricity ratios. The threshold speed increases significantly with increase in the inclination of herringbone type striated roughness patterns on the journal surface from 110° to 150°.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):120-129. doi:10.1115/1.1828454.

A numerical model has been developed to analyze both static and dynamic characteristics of a coupled porous journal and thrust bearing system that is used to support a rotating shaft in a magnetic hard disk drive. The analyzed system is composed of a porous sleeve, a herringbone-grooved solid thrust plate and a flanged shaft, where the bottom end is closed to form a cantilever spindle. The inner surface and the bottom face of the porous sleeve operate as a herringbone-grooved journal and thrust bearing, respectively. The model is based on the narrow groove theory for the bearing oil film, and Darcy’s law for the internal flow in the porous sleeve. The pressure distribution, static equilibrium position of the shaft and dynamic coefficients are obtained under a given external axial load. There exists a window of permeability of the porous sleeve that presents significant advantage to prevent the creation of a sub-ambient condition and to maintain a large thrust bearing film thickness at the expense of some loss of dynamic performance.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):130-140. doi:10.1115/1.1829721.

A modification of the Elrod and Ng turbulence model is presented. The order of magnitude of the Reynolds number in thin lubricant films varies between 102 and 105. For Reynolds numbers higher than 103, the fluid flow becomes turbulent. It is well accepted in lubrication to use a zero-equation turbulence model of the type developed by Constantinescu (1962, ASME J. Basic Eng., 84 (1), pp. 139–151), Ng (1964, ASLE Trans., 7 , pp. 311–321), Ng and Pan (1965, ASME J. Basic Eng., 87 , pp. 675–688), Elrod and Ng (1967, ASME J. Lubr. Technol., 89 , pp. 346–362), or Hirs (1973, ASME J. Lubr. Technol., 95 , pp. 137–146). The Elrod and Ng approach is certainly the most efficient for combined pressure and shear flows where the Reynolds number is above 104. This paper proposes a modification of the Elrod and Ng model in order to ensure a good correlation with experimental data obtained with low Reynolds number turbulent flows. The present model, coupled with a scaling factor for taking into account the transition to turbulence, is therefore accurate for all of the typical Reynolds number values recorded in lubrication. The model is then applied to hydrostatic noncontacting face seals, which usually operate at Reynolds numbers varying from 103 to 104. The accuracy of the model is shown for this particular application of radial rotating flow. A special study is made of the transition to turbulence. The results are compared with those obtained using the initial Elrod and Ng model. The axial stiffness coefficient and the stability threshold are significantly affected by the turbulence model.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):141-148. doi:10.1115/1.1828076.

Squeeze-film lubrication of the human ankle joint during walking is numerically analyzed, the effect of surface sliding being neglected at this stage. Biphasic mixture models are considered for synovial fluid (an ideal and viscous fluid phases) and for articular cartilage (an ideal interstitial fluid and an elastic porous matrix). In the model, the ideal fluid phase passes through the articular surface and matrix pores. The cartilage matrix is considered both normal and pathological (with primary osteoarthrosis). Calculations show that water and small solutes of synovial fluid imbibe into the articular cartilage during the stance period, while the interstitial fluid of the cartilage exudes and enriches the lubricant during the swing period in a central part of the contact at each step. Soon after the onset of walking, repeatedly near the load culmination of each step, the synovial fluid should be turned into a synovial gel and, shortly after, changed back again into a fluid there. In the pathological case, the protective synovial gel layer is quickly depleted after several steps and the surfaces may come briefly into contact in each cycle. With normal cartilage, however, the protective intermittent gel film (formed briefly at each step) maintains its thickness for a longer time. Normal cartilage also behaves more favorably, when a long walk is broken and then resumed shortly afterwards. With normal articular cartilage, maintenance of a lubricating fluid film is much aided by the cyclic nature of the loading encountered in walking, compared with the steady loading in standing where the fluid film is quickly filtered out into a protective permanent gel film.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):149-154. doi:10.1115/1.1828453.

The focus in this paper is to automatically design the air-bearing surface (ABS) considering the randomness of its geometry as an uncertainty of design variables. Designs determined by the conventional optimization could only provide a low level of confidence in practical products due to the existence of uncertainties in either engineering simulations or manufacturing processes. This calls for a reliability-based approach to the design optimization, which increases product or process quality by addressing randomness or stochastic properties of design problems. In this study, a probabilistic design problem is formulated considering the reliability analysis which is employed to estimate how the fabrication tolerances of individual slider parameters affect the final flying attitude tolerances. The proposed approach first solves the deterministic optimization problem. Beginning with this solution, the reliability-based design optimization (RBDO) is continued with the probabilistic constraints affected by the random variables. Probabilistic constraints overriding the constraints of the deterministic optimization attempt to drive the design to a reliability solution with a minimum increase in the objective. The simulation results of the probabilistic design are directly compared with the values of the initial design and the results of the deterministic optimum design, respectively. In order to show the effectiveness of the proposed approach, the reliability analyses by the Monte Carlo simulation are carried out. And the results demonstrate how efficient the proposed approach is, considering the enormous computation time of the reliability analysis.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):155-163. doi:10.1115/1.1843156.

A major cause of magnetic spacing losses in data tape systems is pole tip recession (PTR). This study is an investigation of PTR in a linear data tape recording system and identification of the mechanisms responsible for these effects, but the results have implications for any head where the tape bearing surface is Al2O3/TiC, AlTiC. Tape cycling experiments were performed using the linear tape open system as the experimental platform with metal particle tape. All experiments were conducted within a matrix of pressure and humidity, which encompassed the system operating extremes. Atomic force microscopy was used to analyze the surface topography of the heads. Auger electron spectroscopy and x-ray photoelectron spectroscopy were employed to analyze the chemical changes on the surface of the heads and tapes. Environment was found to have a significant influence on the head/tape interface. Head wear and PTR was highest at high temperature and humidity. Water vapor was found to transform the surface layers on the TiC grains in the tape-bearing surface to TiO2. This process results in the production of TiO2 fragments that become trapped in the recessed pole tip region, acting as three-body abrasive particles. The TiO2 present on the TiC grains and on the surface of heads increases with the water content after cycling against tapes. The hypothesis is supported by the presence of Ti on the poles.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):164-170. doi:10.1115/1.1843828.

In this paper, we investigate the effect of lube textures from the viewpoint of lubricant spreading. Lube textures, designating a patterned distribution of bonding strength between lubricant molecules and disk surface, were formed by irradiating ultraviolet rays through a stripe-patterned mask onto a magnetic disk surface which was partially coated with one-monolayer film of perfluoropolyether. Surface characteristics of the lube textures were evaluated by surface energies ascertained from contact angle measurements. Spreading of the lubricant film was measured by scanning microellipsometry on the striped lube textures in the directions parallel and perpendicular to the stripes. The thickness-dependent diffusion coefficients extracted from the spreading profiles show that lubricant spreading in the regime of film thickness less than 0.2 nm is faster along the stripes, indicating the possibility of controlling the behavior of lubricant films with lube textures.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):171-179. doi:10.1115/1.1843832.

To achieve a higher storage density in a hard disk drive, the fly height of the air bearing slider, as part of the magnetic spacing, has to be minimized. At an ultralow fly height, the intermittent–continuous contact at the head–disk interface (HDI) is unavoidable and directly affects the mechanical and magnetic performance of the hard disk drive, and is of great interest. The HDI wear has a nonlinear and time-varying nature due to the change of contact force and roughness. To predict the HDI wear evolution, an iterative model of Coupled Head And Disk (CHAD) wear, is developed based on the contact mechanics. In this model, a composite transient wear coefficient is adopted and multiple phases of the wear evolution are established. A comprehensive contact stiffness is derived to characterize the contact at the HDI. The abrasive and adhesive wear is calculated based on the extended Archard’s wear law. The plastic and elastic contact areas are calculated with a three-dimensional (3D) sliding contact model. Based on the CHAD wear model, for the first time, the coupling between head and disk wear evolutions is thoroughly investigated. Accelerated wear tests have also been performed to verify the disk wear effect on the slider wear. A wear coefficient drop with time is observed during the tests and it is attributed to a wear mechanism shift from abrasive to adhesive wear. A shift in the type of contact from plastic to elastic accounts for the wear mechanism change.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):180-189. doi:10.1115/1.1828066.

We studied the traction developed between a thin, flexible web and a rotating circumferentially grooved cylindrical roller. We have developed a new two-dimensional analytic model that couples air film pressure, web deflection, and asperity contact to predict traction for circumferentially grooved rollers with arbitrary wrap angles. The entrance effects are incorporated into our new traction model by adapting the squeeze film concept using the distance from the entrance as a surrogate for time. We have verified this model experimentally on a series of 14 rollers and 19 webs. We tested both nongrooved and circumferentially grooved rollers. We showed experimentally that rough, ungrooved rollers that have their low areas unconnected produce significantly lower traction and do not fit the model introduced here. Such rollers should be avoided where traction is important. We introduce dimensionless groups that the roller designer can use to quantitatively assess the interactions of process variables (e.g., speed, tension, etc.) with design variables (e.g., groove depth, groove pitch, roughness, etc.) over the full range of practical wrap angles.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):190-197. doi:10.1115/1.1828068.

Chemical Mechanical Polishing (CMP) is a highly effective technique for planarizing wafer surfaces. Consequently, considerable research has been conducted into its associated material removal mechanisms. The present study proposes a CMP material removal rate model based upon a micro-contact model which considers the effects of the abrasive particles located between the polishing interfaces, thereby the down force applied on the wafer is carried both by the deformation of the polishing pad asperities and by the penetration of the abrasive particles. It is shown that the current theoretical results are in good agreement with the experimental data published previously. In addition to such operational parameters as the applied down force, the present study also considers consumable parameters rarely investigated by previous models based on the Preston equation, including wafer surface hardness, slurry particle size, and slurry concentration. This study also provides physical insights into the interfacial phenomena not discussed by previous models, which ignored the effects of abrasive particles between the polishing interfaces during force balancing.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):198-205. doi:10.1115/1.1829718.

Previous experimental work has shown that negative fluid pressure does develop at the disk/pad interface during chemical mechanical polishing. However, these studies dealt with one-dimensional measurement and modeling. To better understand the problem, two-dimensional pressure mapping is carried out. In addition, the orientation of the disk is measured with a capacitive sensing technique. Results reveal a large negative pressure region at the disk/pad interface that is skewed toward the leading edge of the disk. The disk is also found to be leaning down toward the leading edge and toward the center of the pad. A mixed-lubrication model based on the Reynolds equation and taking into account the disk orientation angles has been developed. Modeling and experimental results show similar trends, indicating the tilting of the disk as a dominant factor in causing the negative pressure phenomenon.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):206-212. doi:10.1115/1.1829723.

The stiction forces that exist in microelectromechanical systems (MEMS) are characterized by surface energy and surface roughness. To simulate this contact condition, a three-dimensional fractal surface geometry and an adhesive contact model for a single asperity are used together to create a numerical adhesive rough surface solution methodology. This novel method of solution determines the characteristic adhesive contact type for each individual asperity uniquely at the time of load and area integration. Such a simulation more accurately represents the physics of the asperity-based contact. Numerical results for the adherence force are presented as a function of surface topography, interface compliance, and the work of adhesion for a MEMS interface. The magnitude of the force required to separate an adhesive rough surface interface is given in relation to a polysilicon system.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):213-222. doi:10.1115/1.1828071.

Multistage centrifugal pumps and compressors are among the most widely used pieces of rotating machinery in industry. A typical application demands the arrangement of several impellers or wheels mounted on a shaft that spins within a stationary case. Annular seals are the most common sealing devices used in this type of machinery. The annular seal design affects both (i) machinery performance in terms of energy conversion efficiency, and (ii) stability due to the interaction within the rotor and the stator through the fluid flow within the seals. Traditionally, the “bulk-flow” theory due to Hirs (ASME J. Lubrication Technol., pp. 137–146) has been used to estimate annular seals leakage and dynamic coefficients. To predict the flow behavior through the seal, this theory relies on empirical friction factor correlations. While leakage is well predicted, the dynamic coefficients are not. The discrepancy is attributed to the friction factor model. Several experiments have produced seal leakage data indicating that friction factor increases as the seal clearance is increased, contradicting predictions based on Moody’s pipe-friction model. A Computational Fluid Dynamics (CFD) commercial code was used to simulate flat-plate-channel-flow experimental tests of water flowing with deliberately roughened surfaces, showing an increase of friction factor with clearance increase. The higher friction factor characteristics of these deliberately roughened surfaces are governed by their ability to develop a high static pressure in the trailing face of each roughness cavity, while the wall shear stresses on the smooth land play a secondary role. In a certain Reynolds number range, the maximum friction factor observed on a specific roughness pattern size is independent of the actual clearance, which we have referred to as the friction-factor-to-clearance indifference behavior. This phenomenon is found to be related to the roughness cavity size and its length-to-clearance ratio.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):223-229. doi:10.1115/1.1828069.

In previous work it was shown that some functionalized polymers used as viscosity index improvers are able to form thick boundary lubricating films. This behavior results from adsorption of the polymer on metal surfaces to form a layer of enhanced viscosity adjacent to the surface. In the current work the behavior of one such polymer in rough surface contact conditions is studied, using both model and real rough surfaces. It is found that the polymer is able to form a thick boundary film in rough surface contact, just as it does with smooth surfaces. It is also shown that the effect of this boundary film is to significantly reduce friction in rolling-sliding, rough surface, lubricated contact.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):230-237. doi:10.1115/1.1828437.

Numerical investigations are carried out to simulate wear and the analysis of these simulations leads to proposing an original new wear law that takes into account interfacial particles in the contact. A 3D Discrete Element Model is presented that simulates the detachment of particles, their flow in the contact and their ejection. It shows that a layer of detached particles can be formed at the interface, separating the solids in contact. The simulations show how influential the contact geometry and the properties of the interfacial particles are in studying wear. The processes of material degradation and particle ejection are then studied separately. Their physical behavior is analyzed and simple analytical expressions are proposed. Consideration of the mass balance of the contact provides an analytical law for wear, involving the fate of the detached particles. Classical wear laws (such as Archard’s law), assuming that no particle stays in the contact, appear to be a limit case of this model.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):238-244. doi:10.1115/1.1829720.

A study has been made of the effect of an externally imposed, low-frequency modulation (≤100 Hz) on the action of a fluid in machining. It is shown that in conventional machining, fluid action in terms of lubrication is essentially confined to the edges of the chip-tool contact along the tool rake face, with little or no change in the friction condition over much of this face. However, the effectiveness of the lubricating action is significantly enhanced when a controlled low-frequency modulation of sufficient amplitude, such as to break the chip-tool contact, is imposed in the direction of cutting. Measurements show that the friction coefficient between tool and chip is reduced by a factor of up to three in the presence of such a modulation. The extent of the secondary deformation zone in the chip material close to the rake face is also significantly reduced. Direct observations of the tool rake face show that when the modulation is applied, the fluid penetrates into much of the intimate contact region between chip and tool.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):245-247. doi:10.1115/1.1843157.

Bair,  S., Qureshi,  F., and Winer,  W. O., 1993, “ Observations of Shear Localization in Liquid Lubricants Under Pressure,” ASME J. Tribol., JOTRE9115(3), pp. 507–514.jtiJOTRE90742-4787Bowden, P. B., 1973, in The Physics of Glassy Polymers, R. N. Haward, ed., Wiley, New York, pp. 279–339.Bair, S., and Winer, W., 2000, “A Study of Mechanical Shear Bands in Liquids at High Pressure,” XIII International Congress on Rheology, pp. 3-191–3-193.Bair,  S., Qureshi,  F., and Khonsari,  M., 1994, “ Adiabatic Shear Localization in a Liquid Lubricant Under Pressure,” ASME J. Tribol., JOTRE9116(4), pp. 705–708.jtiJOTRE90742-4787Bai, and Dodd, 1992, Adiabatic Shear Localization, Pergamon Press, Oxford.Bair,  S., and Winer,  W., 1979, “ Shear Strength Measurements of Lubricants at High Pressure,” ASME J. Tribol., JOTRE9101(3), pp. 251–257.jtiJOTRE90742-4787Bair, S., Winer, W., and Distin, K., 1993, “Experimental Investigations Into Shear Localization in an Operating EHD Contact,” Proc. Leeds-Lyon Symposium, Thin Films in Tribology, D. Dowson et al., eds., Elsevier Sicence Publishers B. V., pp. 383–388.Plint,  M. A., 1967, “ Traction in Elastohydrodynamic Contacts,” Proc. Instn. Mech. Engrs,ZZZZZZ182(1,14), pp. 300–306.Willermet, P. A., McWatt, D. G., and Wedeven, L. D., 1999, “Traction Behavior Under Extreme Conditions,” SAE International Fall Fuels and Lubricants Meeting and Exposition, Toronto, Ontario, Canada, Oct. 25–28, SAE paper 1999-01-3612.Webster, M. N., Lee, G. H., and Chang, L., 2003, “Effect of EHL Conditions on the Behavior of Traction Fluids,” Tribol. Trans. (submitted).Chang, L., Qu, S., Webster, M. N., and Jackson, A., 2003, “On the Mechanisms of the Dramatic Reduction in the EHL Traction Under Low Temperature Conditions,” Tribol. Trans. (submitted).Kaneta,  M., Nishikawa,  H., Kameishi,  K., Sakai,  T., and Ohno,  N., 1992, “ Effects of Elastic Moduli of Contact Surfaces in Elastohydrodynamic Lubrication,” ASME J. Tribol., JOTRE9114(1), pp. 75–80.jtiJOTRE90742-4787Yang,  P., Qu,  S., Kaneta,  M., and Kaneta,  M., 2001, “ Formation of Steady Dimples in Point TEHL Contacts,” ASME J. Tribol., JOTRE9123(1), pp. 42–49.jtiJOTRE90742-4787

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;127(1):248-253. doi:10.1115/1.1828070.

Surface texturing has emerged in the last decade as a viable option of surface engineering resulting in significant improvement in load capacity, wear resistance, friction coefficient etc. of tribological mechanical components. Various techniques can be employed for surface texturing but Laser Surface Texturing (LST) is probably the most advanced so far. LST produces a very large number of micro-dimples on the surface and each of these micro-dimples can serve either as a micro-hydrodynamic bearing in cases of full or mixed lubrication, a micro-reservoir for lubricant in cases of starved lubrication conditions, or a micro-trap for wear debris in either lubricated or dry sliding. The present paper reviews the current effort being made world wide on surface texturing in general and on laser surface texturing in particular. It presents the state of the art in LST and the potential of this technology in various lubricated applications like mechanical seals, piston rings and thrust bearings. The paper also describes some fundamental on going research around the world with LST.

Commentary by Dr. Valentin Fuster

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