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We report here the impact of different alloying elements in steels on friction and wear behavior by performing ball-on-flat lubricated reciprocating tribotesting experiments on 52100 ball on steel flats with different compositions (52100, 1045, A2, D2, M2, and a specialty Cu-alloyed steel) heat-treated to give similar hardness and microstructure, with polyalphaolefin (PAO-4) as the lubricant. There are small variations of coefficient of friction among these alloys. The major observation is that steels containing high concentrations (≥ 10 wt.%) of Cr, Mo, and V gave rise to markedly reduced wear compared with 52100 or plain carbon steels. D2 steel, which contains 11.5 wt.% Cr as the major alloying element was the most wear-resistant. The wear resistance is strongly correlated with the efficiency of formation of carbon-containing oligomeric films at specimen surfaces as determined by Raman spectroscopy. This correlation holds for steels heat-treated to have higher hardness and with n-dodecane, a much less viscous lubricant compared with PAO-4. Given the strong affinity of chromium to oxygen, chromium should exist as Cr2O3 at the steel surfaces during testing. We have performed molecular dynamics simulation on Cr2O3 and demonstrated its ability to catalyze the formation of carbon-containing oligomeric films from hydrocarbon molecules, consistent with its known catalytic activity in other hydrocarbon reactions. We believe that chromium-containing alloys, such as D2, and coatings, such as CrN, derive their wear resistance in part from the efficient in-situ formation of wear-protective carbon tribofilms at contacting asperities.

The ring-liner interface of an internal combustion engine is a very important interface because its behavior directly affects fuel efficiency of an automobile and the engine life. The liner surface...

Pour point, or the temperature below which a fluid ceases to flow, is an important lubricant property. A molecular-dynamics-based approach is developed to explore and identify the pour points of a ...

Under severe conditions, solid contacts take place even when parts are lubricated. Precise mathematical conditions are needed to describe the interior interface between fluid lubrication and solid-contact zones. In order to distinguish the conditions for this interface from conventional lubrication boundary conditions, they are named lubrication–contact interface conditions (LCICs). In this work, mathematical LCICs are derived with local flow continuity from the continuum mechanics point of view and pressure inequality across the interface. Numerical implementations are developed and tested with problems having simple geometries and configurations, and they are integrated into a new mixed/boundary elastohydrodynamic lubrication solver that uses a new method to determine solid-contact pressures. This solver is capable of capturing film thickness and pressure behaviors involving solid contacts.

The lubricants for traction-continuously variable transmissions (t-CVT) could generate high traction under a high pressure, which requires the design of a special class of base stock molecules. This paper reports the development of a model, based on non-equilibrium molecular dynamics (NEMD) simulations, for estimating traction coefficients to facilitate the design of base stock molecules prior to their synthesis. The pressure-viscosity coefficients (α) of a number of base stocks in traction lubricants are calculated and the results are correlated with the corresponding traction coefficient τ. A linear α - τ correlation is obtained with a coefficient of determination of as high as 0.85.

Mechanical power loss in hydraulic piston pumps comes from the friction between parts in relative motion, and wear is among the top pump failure mechanisms. Reducing friction and preventing wear require lubrication enhancement, which may be accomplished by proper surface texture design. Cylinder length is an important factor affecting the lubrication performance of the piston-cylinder system in a swash-plate pump. Our previous analyses revealed that under the same load, increasing the cylinder length could decrease the maximum pressure and increase the minimum film thickness, favorable for the pump operation with reduced wear; however, friction would become higher. The work reported in this paper intends to lower friction by textures in the piston surface while increasing, or at least maintaining, the load capacity of the piston system. Several textures are designed and their lubrication performances investigated, aiming at reducing the piston friction and improving the load-carrying ability of the system in a wide range of operation conditions. The effects of texture type, application location, and texture shape are studied, and their relationships with the friction and film thickness characteristics of the piston-cylinder interface are explored. Two optimal surface-texture designs are suggested, which are named the Step-Multiple-Grooves design and the Combine design.

We recently reported the use of cyclopropanecarboxylic acid (CPCa) as a model additive that can readily react under the combined effect of flash heating and stress in steel tribocontacts to form tribopolymers, along with marked improvement in tribological performance. In this paper, we present results of how chemical structural modification of CPCa may impact on the formation of tribopolymers and hence friction and wear properties, both by experiments and molecular dynamics simulation. Four lubricant additives, viz., CPCa, cyclobutanecarboxylic acid (CBCa), cyclopropane-1,1-dicarboxylic acid (CPDCa), and cyclobutane-1,1-dicarboxylic acid (CBDCa) consisting of a metastable ring structure and one or two carboxyl groups dissolved in an ester base oil were studied. Friction and wear rate using these additives rank in the order of CPDCa

Stress distributions and plastic deformation zones are factors directly influencing the fatigue life of components under cyclic contact. An effective approach to improving the resistance of a steel to contact fatigue failure is surface hardening, which builds gradient yield strength from the surface of the steel to the bulk. When using the distortion energy theory as the criterion to identify failure initiation for a case-hardened steel, contact yield starts in the subsurface wherever the von Mises stress reaches the local material strength, rather than at the point of the maximum von Mises stress in the subsurface. If the yield strength changes from the surface to the bulk following a straight line, the location of yield initiation should occur at the tangency of the strength line and the von Mises stress curve. Analyses on circular, rectangular, and elliptical contacts are presented to reveal the locations of contact yield initiation for such case-hardened steels subjected to rolling contact stresses, for which the influence of friction can be ignored. A group of formulas relating contact yield initiation, in terms of the critical pressure, location of the first yield, and plasticity index (transition to plasticity) to case-hardening parameters, such as the case slope, the minimum case depth, and surface and bulk strengths, are derived to facilitate contact element designs using case-hardened materials. The results are applied to examine the rolling contact behaviors of several case-hardened steels, and the data correlation suggests that their rolling contact fatigue lives are related to a nondimensional case-hardening slope besides external loading.

A transient mixed lubrication model is used to study the performance of a journal bearing subjected to impulse loading, considering mass conservation and the effects of asperities on flow and contact, to explore a novel journal bearing surface design methodology. The bearing surface features include an outlet pocket and axial lip for topographical design consideration. A data-driven approach for a steadily loaded bearing was first presented to illustrate the behavior of an indented pocket design at low and high loads, and Pareto optimization and sensitivity analysis methods were employed to analyze the data and provide insight to the design. The results show that the pocket location was the most influential parameter for the optimal bearing surface design for energy-efficient lubrication performance. For transient operation, a comprehensive parametric study was conducted, and the results reveal that, when compared to the results of the unmodified bearing, the bearing designed with the optimal outlet pocket can run at 9% lower mean friction while maintaining the baseline film thickness. The bearing with the lip feature shows a 20% minimum film thickness increase and 3% mean friction reduction. The design with outlet pocket and lip features combined can lead to 20% film thickness increase and 10% mean friction reduction.

Hydraulic machinery transfers energy between a fluid and a mechanical system. The swash plate pump is one of the most widely used pumps because of its simple and compact structure. The piston/cylinder system is the core of the swash plate pump, and its lubrication characteristics greatly affect the overall pumping performance. This study is aimed at the development of a transient hydrodynamic lubrication model for the pumps with varying length of the piston-cylinder interface and the investigation of the influences of cylinder length, clearance, as well as other design parameters, on the pump piston forces and friction. The changing domain and moving boundaries of the varying piston-cylinder interface impose a challenge to the modeling, and a novel equal-displacement-step method is developed to tackle this issue. The pressure, film thickness, and friction performances of varying and constant interface-length systems are studied, and the former is further analyzed in detail. The results indicate that increasing the cylinder length reduces the misalignment angle and raises the minimum film thickness, but it increases the maximum friction force at and slightly off the location for the maximum velocity because friction is related to velocity and the interfacial area. A longer piston is preferred, and the optimal length for stability should be L 0 / L min = 1.71 for the system analyzed in this study.

Mechanochemical reactions induced by external stress provide a unique approach for in situ synthesis of carbon tribofilms that can improve friction and wear performance. In this work, we studied how tribofilm formation and tribological performance might be related to the adsorption strength of three additives in polyalphaolefin (PAO4) as base oil, viz., cyclopropanecarboxylic acid (CPCa), cyclopropanemethanol (CPMA), and 1-cyclopropylethanol (CPEA) as characterized by two different surface-active groups –COOH and –OH. Tribo-testing results reveal that addition of 2.5 wt% CPCa to PAO4 gave the lowest friction coefficient and wear volume. FTIR and Raman analysis demonstrate substantial tribofilm formation only in the case when CPCa was used as the oil additive, not CPMA or CPEA, in spite of the fact that all three additives contain the same metastable cyclopropane ring. Thermogravimetric analysis and molecular dynamics simulations indicate the stronger adsorption of CPCa on the iron oxide surface compared with CPMA and CPEA. Weak adsorption of the latter molecules results in their desorption from the surface due to flash heating during tribotesting before they have the chance to participate in mechanochemical reactions required for tribofilm formation. The stronger binding of CPCa to the steel surface is a necessary condition for this type of surface mechanochemistry and appears critical to the efficient formation of carbon-containing tribofilms under our tribo-testing conditions.

Misalignment between the shaft and the bearing of a journal bearing set may be inevitable and can negatively impact journal bearing performance metrics in many industrial applications. This work proposes a convex profile design of the journal surface to help counteract the negative effects caused by such a misalignment. A transient mass-conserving hydrodynamic Reynolds equation model with the Patir–Cheng flow factors and the Greenwood–Tripp pressure–gap relationship is developed to conduct the design and analysis. The results reveal that under transient impulse loading, a properly designed journal profile can help enhance the minimum film thickness, reduce mean and peak bearing frictions, and increase bearing durability by reducing the asperity-related wear load. The mechanism for the minimum film thickness improvement due to the profile design is traced to the more even distribution of the hydrodynamic pressure toward the axial center of the bearing. The reason for the reductions of the friction and wear load is identified to be the decreased asperity contact by changing the lubrication regime from mixed lubrication to nearly hydrodynamic lubrication. Parametric studies and a case study are reported to highlight the key points of the profile design and recommendations for profile height selection are made according to misalignment parameters.

This research aims to evaluate the tribological performance of chromium molybdenum (CrMo) coatings under point and line-contact mixed elastohydrodynamic lubrication. This article studies the coatings made from two different methods and treated in an electrifying process of different durations, which produced microchannels and micropockets in the surfaces. The resulting surface topographies had varying impacts on lubricant film thickness, friction, and wear. Root-mean-square roughness (Sq) and porosity are used to characterize the surfaces and their performances in terms of film thickness, friction, and wear. The results suggest that the coated surfaces with a lower Sq and porosity density tended to yield higher film thickness. However, their influence on friction is complicated; lower roughness and porosity are preferred for lower wear, but certain levels of small roughness and surface pores may help to reduce boundary lubrication friction when compared with the frictional behaviors of porosity-free surfaces and those with higher roughness and higher porosity.

This work aims at a time-dependent contact model for the diffusion-affected interactions at the contacting interface between a rigid cylinder and a half-plane. The closed-form frequency response functions (FRFs) of species concentration, displacements, and stresses, are derived by using the Fourier-Laplace integral transforms. The mathematical solutions are so formulated that the fast numerical techniques can be utilized, including the combined Laplace-Talbot and discrete convolution-fast Fourier transform (LT-FFT) algorithms, and the conjugate gradient method (CGM). Numerical results reveal the mechanisms of diffusion-affected contact and diffusion-induced contact transition. A method to determine the critical diffusion flux density to contact instability is suggested and the limiting diffusion flux density without contact transition is identified.

This study examines the empirical relationship between frictional shear stress and pressure in macro- and micro-scale contact and sliding. Two types of friction tests are reported; the macro-scale tests deal with kinetic friction between stainless steel surfaces in a vapor phase lubrication condition, and the micro-scale tests measure kinetic friction at interfaces formed between MoS2 basal planes and surfaces of alumina or stainless steel specimens, using a quartz crystal microbalance microtribometer. A numerical model is used to calculate the contact areas in the macro-scale tests. The results from both friction tests confirmed that the interfacial shear stress in the contact area due to kinetic friction is proportional to the average contact pressure, and the constant of proportionality is close to the coefficient of friction (COF). These observations add to the validation of the Amontons’ law.

This paper reports a novel thermal-visco-elasohydrodynamic lubrication (TVEHL) model for analyzing the lubrication behavior of the interface formed by an elastic sphere and a polymer half-space. The temperature-dependent viscoelastic displacement of the polymer surface is calculated through the elastic-viscoelastic correspondence theory and frequency-temperature superposition. The discrete convolution and fast Fourier transform (DC-FFT) algorithm is used for efficient solution computation. The model is verified by comparing results from its degenerated forms with those from thermal-viscoelastic (TVE) contact and thermal-elastohydrodynamic lubrication (TEHL) theories. The results from the current model with and without considering temperature effect are also compared. The new TVEHL model is explored to study the viscoelastic material property, entraining speed, sliding-to-rolling ratio, and the coupled thermal-viscoelasticity effects.

Diamond-like carbon (DLC) is a class of amorphous carbon materials used as wear-resistant coatings in tribo-components. The chemical inertness of DLC surfaces, while important in many applications, makes them incompatible with additives in commercial lubricants. Specifically, DLC surfaces do not permit strong adsorption of friction modifier molecules. This study aims at improving the adsorption of these friction modifier molecules on DLC surfaces through an acid treatment. Water contact angle measurements show that such a treatment results in improved hydrophilicity of the DLC. XPS analysis demonstrates 50% increase in the uptake of ArmeenT, an organic friction modifier, on DLC after the acid treatment. This increased ArmeenT adsorption is accompanied by marked decrease in friction in micro-scale friction experiments.

The demand for increased energy efficiency continuously drives the development of new lubricants. Here we report the design and synthesis of hexahydrotriazine, triazine, and cyclen derivatives as friction modifiers (FMs) for enhanced fuel economy. This series of sulfur- and phosphorus-free oil-soluble heterocyclic ring-based molecules exhibits differing thermal and chemical stability depending on the degree of aromatization and number of linking spacers within the central heterocyclic ring. Thermally stable triazine and cyclen FMs significantly increase friction performance in the boundary lubrication regime. Cyclens in particular reduce friction by up to 70% over a wide temperature range. Detailed experimental investigations of the newly synthesized FMs at elevated temperatures demonstrate their favorable tribological performance under four operating conditions: variable-temperature sliding, linear speed ramping, reciprocating sliding, and rolling–sliding contact. These latest experimental findings suggest the potential of the application of “designer” heterocyclic FMs for reducing frictional loss in motor vehicles.

A mixed elastohydrodrynamic (EHL) model for journal bearings considering an axial flow due to shaft axial motion and misalignment is developed for lubrication performance evaluation. A new, faster mixed EHL computing technology utilizing the odd–even successive overrelaxation (OESOR) parallel numerical iterative method is proposed based on the red–black successive overrelaxation (RBSOR) method to minimize the execution time prolonged by the complexity caused by the axial flow and misalignments. The multithreaded computing scheme conducted by the OpenMP directive using different meshes and threads suggests that the OESOR method exhibits better efficiency. A series of transient analyses was conducted to solve the mixed EHL model with the parallel OESOR method. The results show that the axial flow and misalignments significantly affect the average pressure, hydrodynamic and asperity contact pressure, elastic deformation, and other characteristics of the journal bearings.

Proper friction control during a sheet metal forming process can positively influences the quality of the final product. The effects of textured tooling surface and the rolling direction of the strip surface with respect to the sliding direction, the sliding velocity, and the contact pressure on friction coefficient were investigated in this study with Aluminum 6111-T4 sheet and D2 tool steel. A flat-on-cylindrical setup was used to measure the friction force by pulling a strip sample across a tooling sample. The tool steel has laser-textured micro wedge-shaped dimples. The results showed a reduction in friction coefficient by using a textured tool. In addition, in the tests, the relative motion direction parallel to the sheet rolling direction leads a lowest friction coefficient compared to other orientations.

Extreme pressure (EP) lubricant additives form protective tribofilms at the site of contact using the heat and pressure of contact and relative motion. Common EP additives contain undesirable elements such as phosphorus and sulfur. A novel EP lubricant additive, which contains no phosphorus and sulfur, is presented for generating lubricious carbon films. The additive consists of a surface-active molecule with a metastable cycloalkane ring, which dissociates readily during tribological contact to form lubricious carbon films. Friction and wear performance of PAO4 with this additive under a range of loads and speeds were shown to be superior to that without the additive. Optical and scanning electron microscopy and Raman spectroscopy were used to analyze the tribofilms formed on post-test contact surfaces, providing direct evidence for the formation of carbon films. Quantitative kinetics for the carbon tribofilm formation was analyzed as a function of temperature and stress, from which the activation energy for carbon tribofilm formation was obtained.

Solving elastohydrodynamic lubrication (EHL) problems is a complex and time-consuming process due to the interactive solutions of the Reynolds equation and contact elasticity. Analyzing journal bearing EHL problems is even more difficult due to the scale difference in the structural and surface features, which may span four orders of magnitude. This article presents a fast EHL computing technology utilizing a parallel numerical iterative method (the red–black successive overrelaxation method) and multithreaded computing scheme conducted by OpenMP directives. The fast computational approaches allow the construction of high-density EHL meshes for effective descriptions of important texture features of journal bearing surfaces.

The work reported in this paper aims at understanding sliding friction anisotropy at the nano-, micro-, and macroscales with respect to surface asperity orientation and exploring the mechanisms behind this phenomenon. Experiments were conducted by probing surfaces with grooves parallel or perpendicular to the direction of relative motion. Continuum mechanics analyses with the FEM and a semi-analytical static friction model and the atomic molecular dynamics simulation were performed for the mechanism exploration. Friction anisotropy was understood from the differences in contact area, surface stiffness, stiction length, and energy barrier from the continuum mechanics prospective and from that in the stick–slip phenomena at the atomic level.

Material wear in the contact area may lead to fatigue failure of a structural component under torsional fretting. It is necessary to investigate the wear mechanisms accompanying torsional fretting, especially during the unloading process. This paper intends to analyze the torsional stress field during loading and unloading processes and to explore the link between fretting wear and the location of material first yield, for which a parametric study on factors influencing this location, such as coefficient of friction and stick zone ratio, is committed. Because the surface shear traction is complex during the unloading process, the subsurface stress field is calculated by means of an efficient semi-analytical method. The relationship between the first yield location and wear mechanism is examined through observations of worn surfaces from a set of torsional fretting experiments.

Modern lubricants face the task of providing lubrication over a wide range of temperatures, and extreme engine temperatures can exceed the thermal degradation limits of many engine oils. Soft metal additives can extend the life of engine oils at very high temperatures by providing solid lubrication to contacting surfaces. We report a new silver–organic complex which contains a high metal content and minimal supporting organic ligands. This silver pyrazole–pyridine complex is evaluated as a friction-reducing and anti-wear additive in engine oil at testing temperatures which thermally degrade the base oil. Two sets of ball-on-disk tests are performed: the first at a constant temperature of 200 °C and the second while increasing the chamber temperature from 180 to 330 °C. At 200 °C, the wear is considerably reduced compared with the base oil when the silver-organic additive is present at 2.5–5.0 wt%. Furthermore, the silver-based additive at 20 wt% in oil induces a remarkable friction reduction during the temperature ramp test, so much, so that the tribological transition from the oil as the primary lubricant to its degradation, and to the silver additive as the primary lubricant, is imperceptible.

Properties of the interface between the filler and the matrix of a composite material draw research attention due to their contributions to the overall properties of the material, especially when the filler and the matrix differ significantly from each other. The work reported in this paper investigates the interface between amorphous carbon and a cross-linked synthetic natural rubber. The interface was experimentally simulated with the surfaces of a sputtering-coated carbon on a spherical Al2O3 tip and a flat synthetic natural rubber sample. Step-loading and pull-out tests with a micro-/nano-indentation instrument were conducted. Fully relaxation of the samples occurred during both test procedures. The penetration depth, applied load, and experimental time were recorded during each test. The Johnson–Kendall–Roberts theory was used to analyze the data at the initial point (step-loading) and the final surface separation point (pull-out) to obtain the initial equivalent modulus, infinite equivalent modulus, work of adhesion, and the average normal interfacial strength at separation. It is found that the pull-out force and the work of adhesion depend on the unloading rate, but the infinite equivalent modulus and the average interfacial strength in the normal direction of the carbon–rubber interfaces are independent of the unloading rate in current experimental domain.

Properly designed micro-scale surface textures can have positive impact on adhesion reduction and lubrication enhancement, which can lead to lower friction and improved performance of a contact interface. The present study aims to utilize this function of textures to reduce the adhesion between a drill and a workpiece. In this study, rectangular surface textures were generated on the margins of drill bits using a diode-pumped Nd:YVO4 picosecond laser with a wavelength of 532 nm. Two designs were created in which the textures covered approximately 10 and 20 % of the margin surface area. Textured drills were tested by drilling a series of holes in a titanium plate while recording cutting forces, and the results were compared with the performance of baseline samples. Thermographic heat profiles and visual inspections of the drills were taken at increments of 5 and 10–15 holes, respectively. The comparison demonstrated an encouraging improvement in drill bit life as judged by the number of holes drilled before failure. Textured drills were found to reduce adhesion of titanium chips on the drill margins. This work has demonstrated the potential of texturing to significantly improve the lifetime of drill bits and similar cutting tools.

This work presents a numerical study of a spinning rigid sphere pressed against an elasto-plastic half space under combined normal and torque loading. The von Mises stresses and equivalent plastic strain under different torques are investigated. Results show that the torque shifts the maximum von Mises stress and plastic region in the half space closer to the surface at larger friction coefficient. An empirical formula to predict the contact area is suggested. The evolution of the plastic region in the half space is further examined. The region shows more complex shapes than those only under a normal load.

The Stribeck curve is an important means to demonstrate the frictional behavior of a lubricated interface during the entire transition from boundary and mixed to full-film lubrication. In the present study, a new test apparatus has been built that can operate under rolling–sliding conditions at a continuously variable speed in an extremely wide range, approximately from 0.00006 to 60 m/s, covering six orders of magnitude. Hence, a complete Stribeck curve can be measured to reveal its basic characteristics for lubricated counterformal contacts. The measured curves are compared with numerical simulation results obtained from an available unified mixed elastohydrodynamic lubrication (EHL) model that is also capable of handling cases during the entire transition. A modified empirical model for the limiting shear stress of lubricant is obtained, and a good agreement between the measured and calculated Stribeck curves is achieved for the tested base oils in all the three lubrication regimes, which thus well validates the simulation methods employed. Both the experimental and numerical results indicate that the Stribeck curves for counterformal contact interfaces behave differently from those for conformal contacts. When the rolling speed increases at a fixed slide-to-roll ratio, the friction continuously decreases even in the full-film lubrication regime due to the reduction of the lubricant limiting shear stress caused mainly by the rise of the surface flash temperature. In addition, the test results indicate that the boundary additives in a commodity lubricant may have considerable influence on the boundary lubrication friction but that on the friction in the mixed and full-film lubrication appears to be limited.

Trilayer materials consisting of a functional outer layer on a substrate containing one intermediate layer are widely used in data-processing devices, biomedical components, and mechanical elements. The recent analytical frequency response functions (FRFs) derived by the authors' group for the contact of multilayer materials lead to the novel deterministic modeling of frictionless and frictional contact involving a trilayer material system designed with various thickness and elastic property combinations. Displacements and stresses for point contacts are calculated effectively by employing the discrete-convolution and fast Fourier transform (FFT) method based on the influence coefficients obtained from the analytical FRFs. The maximum von Mises stress and its location, which are critical information for understanding the material contact status, are thoroughly investigated for a wide range of trilayer materials. The results provide an informative guideline for the design of bilayer coatings without contact failure.

Mixed lubrication is a mode of fluid lubrication in which both hydrodynamic lubricant film and rough surface asperity contact coexist. Mixed lubrication problems are usually associated with significant surface roughness effect. A common belief is that full-film lubrication occurs when the λ ratio, defined as average film thickness divided by composite root mean square roughness, is greater than 3.0, while boundary lubrication corresponds to λ

Time-dependent indentation creep behaviors of polydimethylsiloxane (PDMS) samples of different crosslink densities were studied through contact creep tests loaded with silica tips. Step loads from 0.1 to 10 mN were applied and held for 600 s. The data of penetration depth versus time were recorded during the holding period. A Hertz-type viscoelastic model was used to compute the creep compliance of the samples and the Johnson–Kendall–Roberts (JKR) theory was used to obtain the initial equivalent modulus, infinite equivalent modulus, and work of adhesion between the tested each pair of the PDMS and fused silica tip surfaces. The comparison between initial and infinite equivalent moduli obtained from the Hertz viscoelastic theory and the JKR theory shows that the adhesion between the tip and the sample surface plays an importance role in affecting the analysis results when the indentation strain is small. The influences of crosslink density, applied load, and tip size on the localized PDMS properties are discussed.

Hard coatings under cyclic contact loading are often subject to failure caused by chipping or spalling wear. A fracture-based model is developed to predict such wear damage and shows that particles are detached from the coating surface due to cracking, as observed in experiments. Particle detachment is found to depend on the depths of pre-existing cracks, the positions of the cracks relative to major surface asperities, and loading conditions. The model successfully predicts the size scale of the detached particles.

The influence of rectangle dimples with flat bottom on the friction of parallel surfaces at different sliding conditions is investigated based on lubrication equations. The elastic deformation of rough surfaces is evaluated using continuous convolution fast Fourier transform (CC-FFT). The friction coefficients for dimpled and non-dimpled parallel surfaces by simulation are compared with experimental results. Results show that this kind of dimples can reduce the friction coefficient for cases with the smaller ratio of film thickness to roughness (h/Rq), small roughness or large applied load. The friction force for the parallel surfaces can decrease due to the dimple effect over the range of the larger sliding speed, larger load or smoother surfaces.

Surface finish may significantly affect the lubrication performance of a tribological interface through the influence of topography on micro/nanoscale fluid flows around localized contacts at surface asperities. This paper aims to study the mixed lubrication performance of a group of engineered surfaces, including turned, isotropically finished, ground, and dimpled surfaces, under different operation conditions by means of a deterministic mixed elastohydrodynamic lubrication (EHL) model. The honed surface was used to mate with other surfaces. The results indicate that a longitudinal contact ellipse favors longitudinally oriented mating surface roughness and that a transverse contact ellipse, as well as a line contact, prefers a transversely orientated mating surface roughness for lubrication enhancement.

The geometry of micro-scale textures and the relative motion of surfaces in contact may affect the performance of an elastohydrodynamic lubrication interface. Reported in this paper are the investigations of the effects of texture bottom shape and surface relative motion on lubrication enhancement using numerically generated textures by means of model-based virtual texturing and numerical simulation. These textures are on one of the interacting surfaces in a triangular distribution and have the same density. The results suggest that the bottom shapes involving a micro-wedge and/or a micro-step bearing tend to yield thicker films. The lubrication of selected textured surfaces was also studied under three different relative motions: texture surface moving, un-textured surface moving, and both moving. The results indicate that textures on the faster moving surface offer stronger film thickness enhancement.

The fast Fourier transform (FFT) technique is becoming an important tool in tribological modeling because an FFT-based algorithm can be extremely accurate and efficient. However, certain subtle issues should be properly dealt with so that efficient and accurate FFT implementation can be achieved. This paper discusses the use of three typical convolutions, two convolution theorems, influence coefficients, and shape functions, as well as the influence of domain size. This paper also explores the applications of FFT to inverse problems and periodic contacts and summarizes available FFT-based algorithms for solving various contact problems.

Interaction of nominally flat engineering surfaces that leads to a large contact area exists in many mechanical systems. Considering periodic similarity of surface geometry, a numerical three-dimensional elasto-plastic contact model can be used to simulate the contact behaviors of two nominally flat surfaces with the assistance of the continuous convolution and Fourier transform (CC-FT) algorithm. This model utilizes the analytical frequency response functions (FRF) of elastic/plastic responses of materials and provides contact performance results, including the average surface gap, the contact area ratio, and the volume of plastically deformed material, which may be defined as performance variables. Following the digital filtration technology, rough surfaces can be numerically generated with specified autocorrelation length and the first four orders of statistical moments. A group of contact simulations are conducted with various working conditions. The effects of topographic and material properties on the contact behaviors are discussed. With a multi-variables regression method, empirical formulas are developed for the performance variables as functions of surface statistical characteristics, material properties, a hardening parameter, and the applied load in terms of pressure.

Designing thin-film coatings to meet engineering needs requires the knowledge of accurate mechanical properties of the coatings. Young's modulus and Poisson's ratio are two basic mechanical properties of materials, which should be conveniently measured. This paper reports a direct and non-destructive method for the measurement of the Young's modulus and Poisson's ratio of a thin-film coating and its substrate based on the extended Hertz theory for the contact of coated bodies. The theory is used to analyze load–displacement data from a spherical indentation in the elastic range, in which the substrate effect is intrinsically modeled. The Young's modulus and Poisson's ratio are determined at the same time through minimizing the difference between the measured and specially defined modified Young's moduli. Two sets of validation experiments are also reported. This new method does not require any assumptions on pressure distribution and Poisson's ratio and can be easily incorporated into current indentation analysis systems.

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.

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. DOI: 10.1115/1.2736439

The pressure–viscosity coefficient (the α value), which represents the variation of viscosity as a function of pressure, is an important parameter for elastohydrodynamic lubrication analyses. The properties of hydrocarbons in the C20–C40 mass range are of fundamental importance as they are basic constituents of synthetic- and mineral-based lubricant stocks. The conventional acquisition of the α value requires preparation of lubricant samples and experimental testing by means of a high-pressure viscometer. In this paper, we present a method to obtain the α value of a typical base oil (1-Decene trimer) based solely on the molecular dynamics simulations. Non-equilibrium molecular dynamics (NEMD) simulations were performed to calculate the shear viscosity of the lubricant at various temperatures and pressures up to 1 GPa. Elevated temperatures and time–temperature superposition (TTS)-based extrapolations were applied to further extend the ability of the NEMD simulations, and the rotational relaxation time was calculated and used to determine the validity of the NEMD calculations. The α value at 100 °C was calculated and compared with experimental results. Effectiveness of the extrapolation was evaluated with a 95 % confidence interval.

Steady-state mixed hydrodynamic lubrication of rigid journal bearing is investigated by using a finite difference form of the Patir–Cheng average Reynolds equation under the Reynolds boundary condition. Two sets of discretization meshes, i.e., the rectangular and nonorthogonal herringbone meshes, are considered. A virtual-mesh approach is suggested to resolve the problem due to the singularities of pressure derivatives at the turning point of the herringbone mesh. The effectiveness of the new approach is examined by comparing the predicted load with that found in the literature for a smooth-surface case solved in the conventional rectangular mesh. The effects of the skewness angles of symmetric and asymmetric herringbone meshes on the predicted parameters, such as load, friction coefficient, attitude angle, and maximum pressure, are investigated for smooth, rough, and herringbone-grooved bearing surfaces. It is found that the new approach helps to improve the computational accuracy significantly, as demonstrated by comparing the results with and without the treatment of the pressure derivative discontinuity although the latter costs slightly less computational time.

The “Stribeck curve” is a well-known concept, describing the frictional behavior of a lubricated interface during the transition from boundary and mixed lubrication up to full-film hydrodynamic/elastohydrodynamic lubrication. It can be found in nearly every tribology textbook/handbook and many articles and technical papers. However, the majority of the published Stribeck curves are only conceptual without real data from either experiments or numerical solutions. The limited number of published ones with real data is often incomplete, covering only a portion of the entire transition. This is because generating a complete Stribeck curve requires experimental or numerical results in an extremely wide range of operating conditions, which has been a great challenge. Also, numerically calculating a Stribeck curve requires a unified model with robust algorithms that is capable of handling the entire spectrum of lubrication status. In the present study, numerical solutions in counterformal contacts of rough surfaces are obtained by using the unified deterministic mixed elastohydrodynamic lubrication (EHL) model recently developed. Stribeck curves are plotted in a wide range of speed and lubricant film thickness based on the simulation results with various types of contact geometry using machined rough surfaces of different orientations. Surface flash temperature is also analyzed during the friction calculation considering the mutual dependence between friction and interfacial temperature. Obtained results show that in lubricated concentrated contacts, friction continuously decreases as speed and film thickness increase even in the full-film regime until extremely high speeds are reached. This is mainly due to the reduction of lubricant limiting shear stress caused by flash temperature rise. The results also reveal that contact ellipticity and roughness orientation have limited influence on frictional behaviors, especially in the full-film and boundary lubrication regimes.

When a conformal interface is under low velocity and heavy load conditions, solid contact (or dry contact) may occur even in a system with smooth surfaces. This paper presents two approaches for solving steady-state and transient mixed elastohydrodynamic lubrication problems of journal bearings with smooth surfaces under low rotating speed. The first approach uses the reduced Reynolds equation with a combined finite element–backward finite difference scheme and the second applies a zero film thickness equation to describe the mechanical behavior of mating surfaces at solid contact points. The major advantages of these two approaches are (1) no division of the solution domain into a lubricated area and a solid contact area is necessary and (2) the solid contact pressure, lubricant pressure, and eccentricity ratio can be solved simultaneously. Numerical examples are presented for the application of these approaches. For the steady-state cases under low velocity studied in this work, pressure distributions a...

The Stribeck curve is an overall view of friction variation in the entire range of lubrication, including the hydrodynamic, mixed, and boundary lubrication. This paper presents the results of research on the friction of a journal-bearing conformal contact system in the form of Stribeck curves through numerical simulation. The surface roughness of the journal and the bearing and material elasticity and thermoelasticity are included in the modeling, and their influences on the results of Stribeck curves are discussed. A three-dimensional Stribeck surface is constructed to illustrate the load effect on friction for the modeled journal-bearing structure. Review led by Gary Barber

When contact problems are solved by numerical approaches, a surface profile is usually described by a series of discrete nodes with the same intervals along a coordinate axis. Contact computation based on roughness datum mesh may be time consuming. An adaptive-surface elasto-plastic asperity contact model is presented in this paper. Such a model is developed in order to reduce the computing time by removing the surface nodes that have little influence on the contact behavior of rough surfaces. The nodes to be removed are determined by a prescribed threshold. The adaptive-surface asperity contact model is solved by means of the element-free Galerkin-finite element coupling method because of its flexibility in domain discretization and versatility in node arrangements. The effects of different thresholds on contact pressure distribution, real contact area, and elasto-plastic stress fields in contacting bodies are investigated and discussed. The results show that this model can help reduce about 48% computational time when the relative errors are about 5%.