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The effect of surface roughness, contact pressure and lubrication on the onset of galling of aluminum 6082-T6 is studied with a pin-on-plate tribometer. The design of experiments involves pins with mean arithmetic roughness Ra equaled to 0.2, 0.35 and 0.6 μm, normal loads ranging from 5 to 600 N, and 4 different lubrications: without lubricant, with MoS2 dry lubricant, with pure mineral oil and with a highly chlorinated paraffin oil. Tests are performed at room temperature and are repeated three times. Experimental results show the great sensitivity of the onset of galling to the roughness and the beneficial effect of dry and liquid lubricants. The occurrence of galling is study by optical micrographs, by the analyses of the coefficient of friction, and by the use of finite element simulations. It comes that the variation of the coefficient of friction is not self-sufficient to detect the onset of galling. The finite element computations involves Wilson's lubrication model and Xue's damage model. Galling is assumed when finite elements near the contact area reach the critical damage Dc. This numerical approach leads to a good prediction of the onset of galling for the tests performed under constant normal load. For tests performed with increasing normal loads and various lubricants, the numerical prediction overestimate the sliding length before galling occurs. • Galling of the aluminium alloy 6082 is predicted by means of finite element computations. • Pin-on-plate tests are performed to study the effect of roughness, contact pressure and lubricant on galling. • Wilson's lubrication model is used to compute the friction stresses, and a damage criteria to detect the onset of galling. • Good agreements are obtained between the numerical predictions of galling and experiments. [ABSTRACT FROM AUTHOR]

Lubrication is often employed in fretting contacts to reduce wear and stresses associated with high friction. Owing to the very small displacements associated with fretting, penetration of lubricating oils into the contact may not be effective. The efficacy of the penetration of the lubricant into the contact is very difficult to observe experimentally, and accordingly, this paper presents a numerical simulation of a lubricated fretting contact using a Coupled–Eulerian–Lagrangian (CEL) finite element method. Meso-scale CEL finite element models are developed to simulate the cylinder-on-flat arrangement used experimentally at the University of Nottingham in which the roughness of contact surfaces is characterized as fractal geometry by the Weierstrass-Mandelbrot (W-M) function. The fluid–solid and solid–solid contact in the lubricated fretting contact are simulated, and from these, wear and friction coefficients are determined. The effects of contact geometry on lubricated fretting contacts and lubricant on fretting wear are modelled and compared with experimental observations. Results indicate that oil lubrication reduces fretting wear and friction effectively in the less-conforming contacts but has little effect in the more-conforming contacts. • A novel approach of modeling lubricated contacts using a CEL finite element method. • The efficacy of the penetration of lubricant into the fretting contacts is simulated. • The wear and friction coefficients in a lubricated fretting contact are determined. • The effects of contact geometry on lubricated fretting contacts are simulated. [ABSTRACT FROM AUTHOR]

Adhesive wear in mixed elastohydrodynamic lubrication (EHL) has been one of the most prominent problems for heavy-loaded helical gears. However, this issue was rarely investigated in previous researches. In this work, an adhesive wear model for helical gears is established in line-contact mixed EHL. The contact parameters of the pinion and gear are derived according to the equivalent tapered roller contact model, and the load is attained in consideration of the varying contact line ratio. Then the asperity contact pressure is calculated according to Hertz's elastic contact theory and load sharing. Moreover, the sliding distances of the points on tooth surface of driving pinion and driven gear are achieved by a single point observation method, and Archard theory in dry contact is extended to the mixed lubrication to estimate the wear rate in mixed EHL by using fractional film defect. The modified Archard's wear model is then employed in formulating and accounting for the gear tooth wear. Effects of surface roughness, geometrical parameters and working parameters on wear depth of the driving pinion are furtherly investigated. The results show that the wear depth in mixed EHL is lower than that under dry contact, which indicates that tooth wear can be reduced with the reasonable lubricants. As the surface roughness becomes large, the asperity contact pressure, the tooth surface temperature and wear depth are increased. Additionally, the wear depths decrease with the increase in module, helix angle, pressure angle, tooth width or rotation speed but increase with input torque. • An adhesive wear prediction method is developed for helical gears in mixed EHL. • Effects of tooth surface roughness on the wear of helical gear are investigated. • Efficient lubrication can greatly reduce the wear depth of tooth surface. • Geometrical and working parameters optimization is beneficial for wear resistance. [ABSTRACT FROM AUTHOR]

Surface roughness is significant to friction and wear. However, the influence remains elusive, especially at the nanoscale. This study aimed to explore the effect of the nanoscale surface roughness on sliding friction and wear. For bearing steel tribo-pairs, the surface roughness can affect the lubrication state transition. The lower the surface roughness is, the easier it is to enter mixed lubrication and full film lubrication. Moreover, reducing the surface roughness can decrease friction and wear in mixed lubrication. At 94 mm/s sliding speed, as the surface roughness S q decreases from 769.7 nm to 21.1 nm, the average friction coefficient first remains unchanged and then decreases by 20%, while the wear rate decreases by 85%. However, further reducing the surface roughness may result in friction failure, raising friction and wear. At 10 mm sliding radius, about 20∼40 nm S q can lead to the minimal friction and wear. For the mechanism, as the surface roughness decreases, the asperity contact diminishes. Therefore, friction and wear decrease. However, when the surface roughness passes the critical point, asperities cannot fully accommodate the interfacial wear debris. Consequently, friction failure occurs. The findings may provide an enhanced understanding of the role of surface roughness in tribology. [Display omitted] • The surface roughness can affect the lubrication state transition. • Reducing the surface roughness can decrease friction and wear in mixed lubrication. • Too low surface roughness may result in friction failure, raising friction and wear. [ABSTRACT FROM AUTHOR]

The fretting behaviour of AISI 52100 steel and low alloy steel under grease lubricated conditions is investigated. The friction behaviour is characterized by a high friction regime for the smallest displacement amplitudes below and next to the partial-slip transition whereas a low friction regime is stabilized above a threshold sliding amplitude when a lubricating tribofilm is generated in the interface. In order to quantify the influence of different contact parameters like surface roughness, contact size and contact geometry, a new fast simplified fretting methodology is introduced. This method allows scanning the entire fretting regime through a single experiment. An experimental comparison of smooth and rough surfaces shows that the effect of surface roughness influences the stability and the performance of the lubricating tribofilm. Experimental results obtained through changing the contact size of the cylinder-on-flat configuration permit identifying two contact size regimes. For small interfaces, the effective fretting sliding amplitude triggering the tribofilm activation is proportional to the contact size, whereas for large contacts this amplitude appears to be constant. The comparison with a punch-on-flat interface suggests that the lubrication activation is better captured by considering the peak pressure width rather than the nominal contact size. • Experimental study of surface roughness, contact size and geometry effects in a grease-lubricated fretting contact. • Introducing simplified variable displacement method to assess grease effects. • Lubrication activation is better described considering peak pressure width rather than nominal contact size. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model of wheel/rail in rolling contact is established to study the adhesion characteristics under interfacial contamination considering surface roughness at high speed. The thermal partial elastohydrodynamic lubrication (EHL) theory in elliptical contact is used in the model. The numerical model is successfully solved by applying the iterative algorithm between the pressure field and temperature field. Multilevel method is used to solve modified Reynolds equation. Sweeping column method is used to solve energy and heat conduction equations. The effects of train speed, surface roughness amplitudes, roughness orientation and axle load on the adhesion coefficient under interfacial contamination are numerically investigated. A typical creepage-traction curve under oil contamination is obtained at high speed by the present model. In addition, the effects of temperature and the elastic-plastic deformation behaviour of asperities on the adhesion coefficient are discussed in the paper. Furthermore, the effect of different interfacial contaminations on wheel/rail adhesion using the present model on the adhesion coefficient is investigated. In order to validate the present model, the numerical results are compared with the experimental results that were obtained by JD-2 high-speed wheel/rail rolling contact machine under water/oil contaminations. [ABSTRACT FROM AUTHOR]

Friction modifiers (FMs) are widely used in wheel-rail applications. This paper presents a method to analyze the starved elastohydrodynamic lubrication (EHL) of the wheel-rail surface coated with FMs. Combined with rough surface simulation techniques, starved EHL analysis and film thickness analysis, the pressure distribution and adhesion coefficient under different film thickness states are obtained. The effect of film thickness of the FM supply layers and surface roughness on the adhesion coefficient is studied. The adhesion coefficient increases with the decrease in the effective FM film thickness and increase in the surface roughness. Based on the subsurface stress model, the subsurface stress distribution is studied. Then the influence of FMs on the fatigue of wheel and rail is analyzed on the basis of the shakedown map. Finally, the fatigue index of rail is explored in starved EHL state. • A friction modifier (FM) starved EHL numerical model considering the effective film thickness was established. • Roughness and the effective film thickness could remarkably affect the lubrication state of contact zone. • The adhesion coefficient gradually increases as decreasing the effective film thickness. • With the increasing roughness, the lubrication of wheel/rail surface gradually fails. • The fatigue index will be increased as increasing the roughness and decreasing the effective film thickness. [ABSTRACT FROM AUTHOR]

Determination of wear rate in spur gears requires treatment of the mixed elastohydrodynamic lubrication problem along the gear's line of action (LoA). For this purpose, a procedure is developed to predict the film thickness, traction coefficient and the wear rate with provision for the interactions between the surface asperities of the gear's teeth at each point on LoA. The results reveal that the surface finish has a more pronounced influence on the wear rate compared to the traction coefficient. The wear results are also compared to those obtained by an engineering approach that utilizes an estimate of the contact flash temperature with reasonable accuracy. Also investigated is the effect of starvation on the wear rate for applications where the inlet is not fully flooded. The results show that wear rate can be enormously large depending on the degree of starvation. [ABSTRACT FROM AUTHOR]

Abstract: A preliminary two dimensional numerical model of wheel/rail in rolling contact is established to study the adhesion characteristics of wheel/rail with considering surface roughness under oil and water mixed contamination. A partial elastohydrodynamic lubrication (EHL) theory in line contact is applied in the model. The present numerical model is used to investigate the effects of contact pressures of wheel/rail, the volume fraction of oil and their locations in the contact zone, train speed on the adhesion coefficient under oil and water mixed contamination condition. In addition, the effects of oil contamination, water contamination and water/oil mixed contamination on the adhesion coefficient are compared. The numerical results show that the adhesion coefficient is greatly influenced by the train speed, the volume fraction of oil and their locations in the contact zone. Furthermore, in order to verify the accuracy of the numerical model, the numerical results are compared with the experimental results that were obtained by JD-1 wheel/rail simulation facility. [Copyright &y& Elsevier]

Abstract: The continuum damage mechanics (CDM) approach in conjunction with the load sharing concept is applied to predict steady state lubricated line-contact wear. Empirical formula for the maximum contact pressure in dry rough contact is employed together with the lubricant film thickness equation to estimate the portions of the load carried by the asperities and the lubricant. Effect of contact temperature on wear is also included using a simplified thermo-elastohydrodynamic analysis in conjunction with fractional film defect concept. While most wear models require experimental calibration, current study attempts to predict wear purely based on numerical simulation. The proposed methodology is quite fast and easy to implement for practical engineering applications. Comparison of numerical calculations with available experimental wear data show very good agreement. [Copyright &y& Elsevier]

Abstract: This paper describes a method of utilizing oil-based nanolubricants containing abrasive nanoparticles for in-situ nanopolishing (ISN) of surfaces of rolling elements. Nanopolishing reduces the composite surface roughness of bearing elements and results in an increased film thickness ratio. Using a four-ball tester, the test and material conditions such as normal load, rotational speed, and initial surface roughness of balls were selected so that the initial film thickness ratio λ varied in the 1–3 range, i.e. within the partial (mixed) elastohydrodynamic (EHL) regime. The lubricant was either engine oil or engine oil-based nanolubricants containing diamond nanoparticles. The rolling ring track on the top ball was examined by a non-contact profilometer. The root mean square (RMS) roughness of the wear track using the nanolubricant was decreased. In-situ nanopolishing by the nanolubricant resulted in a significant increase in the film thickness ratio and a lower frictional torque. [Copyright &y& Elsevier]

Double helical gears are widely applied in the aviation, marine and energy equipment, and generally required to be modified as the crowned since the machining error and assembling error of the gear drive are inevitable. The crowned gears usually operate in point contact mixed elastohydrodynamic lubrication (EHL), and adhesive wear has been one of the most prominent problems, but few previous researches were focused on this problem. In this work, a wear life prediction model of crowned double helical gears in point contact mixed EHL is proposed. The tooth profiles of the crowned gears are obtained according to the generating principle, and the load is attained in consideration of multi-tooth pairs contact. The contact pressure is derived from Hertzian elastic contact theory and load sharing concept, and the sliding distance is determined by a generalized model. Then, the wear rate in point contact mixed EHL is evaluated by extending the Archard theory to lubricated case, and the extended Archard's wear model is used to formulate the gear tooth wear. Moreover, the wear life prediction model which takes account of rotation speed and input torque is established, and the influences of surface roughness and crowned amount on the wear life of the crowned gears are further studied. Results show that surface roughness has a great influence on the wear life at different rotational speeds, while input torque has a great influence on the wear life at different crowned amount. It indicates that the wear life of the crowned double helical gears can be improved by reasonable matching lubrication, modification and working parameters. • A wear life model is developed for crowned double helical gears in point contact mixed EHL. • Surface roughness has a great influence on the wear life at different rotational speeds. • Input torque has a great influence on the wear life at different crowned amount. • Lubrication, modification and working parameters optimization is beneficial for wear life. [ABSTRACT FROM AUTHOR]

The transient non-Newtonian thermal micro-elastohydrodynamic lubrication point contact model and the adhesive wear model in mixed lubrication point contact are applied to a heavy-loaded modified herringbone gear drive respectively in steady running phase and start-stop phase. The fundamental parameters of both elastohydrodynamic point contact and wear model are derived from the meshing characteristic and kinematics analysis of herringbone gears. The effects of pinion torque, rotational speed and surface roughness on the maximum pressure and temperature rise and the minimum film thickness are discussed in gear steady running condition for full film lubrication contact. Meantime, the asperity load ratio, contact flash temperature and tooth wear depth are investigated in gear start-stop phase for mixed lubrication point contact. The results show that the lubrication performance of herringbone gear is deteriorated by substantially increased pressure and temperature rise resulting from a larger tooth surface roughness and rotational speed. The tooth wear depth of herringbone gear in start-stop phase is likely greatly reduced with a larger start-stop acceleration. Moreover, the lubrication and wear performances for the complete running cycle of herringbone gear drive from start-up to stop indicate that large tooth surface roughness rather than high load are more prone to herringbone gear thermo-mechanical and wear failures. • A Non-Newtonian thermal transient point contact EHL model is applied to modified herringbone gears. • A mixed EHL point contact wear model is developed for herringbone gears. • Gear thermo-mechanical failure can occur at pitch point due to large tooth roughness. • Tooth wear in start-stop phase can be greatly reduced with large start-stop acceleration. [ABSTRACT FROM AUTHOR]

Experimental and numerical analyses were conducted to explore the influence of DLC/TiAlN-coated die surfaces in sheet-metal forming under dry and oil-lubricated conditions. In this study, ironing and deep-drawing experiments were performed to determine the potential of the DLC/TiAlN coating in the sheet forming of stainless steels under different tribological conditions. The performance and physical properties of the DLC/TiAlN-coated die surface were obtained through load, surface roughness, and wear measurements as well as hardness and microstructure examination. The experimental results indicated that the DLC/TiAlN coating strongly resists galling under dry friction and thin film lubrication conditions that reduces the friction and forming load. The presence of a thin oil film reduces the sliding-originated surface tensile stresses of the DLC/TiAlN coating, improving the wear resistance of the die surface even at high temperatures and high contact pressures. Thermomechanical numerical analysis supported the experimental results, which confirmed that the lubricant discharged the heat generated in the die–workpiece contact region to reduce the friction and forming load. With the DLC/TiAlN coating, the plain mineral oil with no extreme pressure additives can function as effective as chlorinated paraffin oil for protecting the die surface, thus extending the die service life. Image 1 • Ironing and deep drawing tests performed under different tribological conditions. • DLC/TiAlN coating resists galling under dry friction and thin film lubrication. • DLC/TiAlN utilized more environment-friendly oils than chlorinated paraffin oil. • TiAlN interlayer coating enhanced DLC performance. [ABSTRACT FROM AUTHOR]

The shear strength at the interface contributes to the overall friction force experienced by the contacting bodies sliding against each other. In this article, an experimental technique to characterize the shear strength at the interface of metallic bodies in sliding contact has been developed. The boundary layers formed at interface in a lubricating contact have been varied by using two different types of lubricants in combination with both zinc coated and uncoated steel sheets. The empirical relations between the experimental parameters such as contact pressure and sliding velocity and the interfacial shear strength have been expressed by fitting the experimental results. These expressions have been incorporated in the Material Point Method (MPM) based ploughing model. The coefficient of friction and ploughing depth obtained from the numerical simulations have been validated relative to the experimental results with a good agreement for both lubricated and unlubricated substrates, different loads and spherical indenter sizes. Furthermore, the interfacial shear strength has been varied in the MPM-based ploughing model and ploughing experiments to study the contribution of interfacial shear strength to overall friction, deformation and wear. • An experimental characterization technique of interfacial shear strength from the measured friction force and the contact area in line contact loading on coated sheet metals is developed. • The interface is varied using zinc coated and uncoated steel sheets, with different surface roughness and conditions, with or without using one of the two given lubricants. • The interfacial shear strength is measured and plotted for different interfaces over a range of contact pressures and sliding velocities and curve-fitted to mathematical relations. • The shear strength relations are included in the MPM-ploughing model whose results are compared with ploughing experiments on steel sheets for varying interfaces and applied loads. • The effects of varying interfacial shear strength on ploughing friction, substrate deformation and abrasive wear are studied using the MPM-ploughing model and ploughing experiments. [ABSTRACT FROM AUTHOR]