Modelling of plowing and shear friction coefficients during high-temperature ball-on-disc tests

During high-temperature ball-on-disc tests of aluminium against steel, hot aluminium deforms and wear track evolves. The individual contributions of plowing and shearing to the apparent friction and the contact pressure are unknown. The aim of the research was to develop a model capable of determining these parameters. It was found that during high-temperature ball-on-disc tests, the plowing friction accounted for only about 1% of the apparent friction, although the plowing friction coefficient increased with increasing wear lap, while the shear friction played a dominant role in determining the apparent friction measured. The mean contact pressure decreased significantly as the test proceeded.     

Experimental discrimination of plowing friction and shear friction

The friction of a diamond spherical indenter sliding on CrN coated nitrided steel was investigated. A friction model was proposed that takes into account plowing and shear friction. With the model the separate contributions of substrate properties and surface condition to the friction were successfully extracted: the shear friction coefficient μ _sh was found to depend exclusively on the surface condition, i.e., not on load on the indenter, hardness of the substrate, and thickness of the coating. On the other hand, the plowing component of friction was independent on surface condition.     

The Relationship Between Contact Mechanics and Adhesion in Nanoscale Contacts Between Non-Polar Molecular Monolayers

Atomic and friction force microscopy were employed to examine adhesion and friction between dodecanethiol self-assembled monolayers in pure media as well as in two-component heptane/acetone mixtures. In media that did not contain hydrogen bond donors, the pull-off forces were found to be in very good agreement with theoretic predictions based on the Lifshitz theory. As the hydrogen bond donor ability of the medium increased, the adhesion energy was found to be increasingly underestimated by the model, illustrating the importance of the medium–medium interactions outside the contact area in determining the adhesive properties of the contact at the nanoscale. Exceptionally, in n-octanol, the pull-off forces were considerably lower than predicted and a dual slope linear friction–load relation was observed. These observations were rationalized by the formation of physisorbed layers of octanol on the surfaces. The friction–load relationship in the other media was found to be dependent on the magnitude of adhesion. For weakly adhering systems, the friction–load relationship was linear, but as adhesion increased, a sublinear relationship was observed. The data were rationalized by treating the friction as the sum of an adhesion-dependent shear term characterized by a surface shear strength τ and a molecular plowing term characterized by a coefficient of friction μ. Thus, Amontons’ law appears to describe the limiting case of very weak adhesion where viscoelastic plowing is primarily responsible for energy dissipation, while a sublinear friction–load relationship emerges in other situations due to the dissipation of energy in shearing adhesive contacts.     

石墨种类对铜基粉末冶金摩擦材料性能的影响

采用柔性石墨、造粒石墨和鳞片石墨分别制备粉末冶金烧结摩擦材料，研究不同种类片状石墨对摩擦材料摩擦磨损性能的影响。结果表明：不同种类石墨制备的摩擦材料的密度和力学强度差异，将影响材料基体在制动过程中的组织形态，使摩擦界面呈现不同的磨损形式，其中柔性石墨摩擦材料的主要磨损机制为氧化磨损，造粒石墨摩擦材料的主要磨损机制为犁削磨损和磨粒磨损，鳞片石墨摩擦材料的主要磨损机制为犁削磨损和黏着磨损；造粒石墨制备的摩擦材料在不同速度下制动和重复单次制动时的摩擦因数波动值较小，摩擦因数稳定性好，且具有适中的磨耗量，综合摩擦磨损性能最佳。     

Impact of Wire-EDM on Tribological Characteristics of ZrO<Subscript>2</Subscript>-based Composites in Dry Sliding Contact with WC–Co-Cemented Carbide

In the present investigation, experiments were conducted by unidirectional sliding of pins made of FCC metals (Pb, Al, and Cu) with significantly different hardness values against the steel plates of various surface textures and roughness using an inclined pin-on-plate sliding apparatus in ambient conditions under both the dry and lubricated conditions. For a given material pair, it was observed that transfer layer formation and the coefficient of friction along with its two components, namely adhesion and plowing, are controlled by the surface texture of the harder mating surfaces and are less dependent of surface roughness (R _a) of the harder mating surfaces. The effect of surface texture on the friction was attributed to the variation of the plowing component of friction for different surfaces. It was also observed that the variation of plowing friction as a function of hardness depends on surface textures. More specifically, the plowing friction varies with hardness of the soft materials for a given type of surface texture and it is independent of hardness of soft materials for other type of surface texture. These variations could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding. It was also observed that among the surface roughness parameters, the mean slope of the profile, Δ _a, correlated best with the friction. Furthermore, dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing taking place at the asperity level.     

Study of Friction and Transfer Layer Formation in Copper-Steel Tribo-System: Role of Surface Texture and Roughness Parameters

In the present investigation, experiments were conducted on a tribological couple—copper pin against steel plate—using an inclined pin-on-plate sliding tester to understand the role of surface texture and roughness parameters of the plate on the coefficient friction and transfer layer formation. Two surface characteristics of the steel plates—roughness and texture–were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture of the plate. The plowing component of friction was highest for the surface texture that promotes plane strain conditions while it was lowest for the texture that favors plane stress conditions at the interface. Dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing and hence the plane strain/stress type deformations taking place at the asperity level.     

双粗糙面滑动过程摩擦因数的变化分析

采用W-M函数建立具有分形特征的三维双粗糙面接触模型,考虑了接触界面间的黏着效应,在滑动速度、法向载荷及界面剪切强度等参数变化下,运用有限元方法探讨了粗糙体在滑动过程中摩擦因数的变化情况。结果显示,滑动速度、法向载荷及界面剪切强度等参数对摩擦因数的变化有一定的影响,边界润滑工况下平均摩擦因数为0.28,无润滑工况下平均摩擦因数为0.713,最大界面剪切强度时的平均摩擦因数为0.73;随着界面剪切强度的减小、法向载荷的增大、滑动速度的增加,滑动摩擦因数有所减小。与相关文献结论或实验结果进行比较,证明了上述结果的正确性。分析结果可为摩擦学设计和摩擦材料的制备提供理论参考。     

Analytical Model for Plowing Friction at Nanoscale

Existing analytical models for elastic–plastic friction, which have been developed for microscale single-asperity contacts, do not apply when the contact dimensions are reduced to the nanometer regime. We demonstrate that although the microscale models correctly describe elastic recovery behind a sliding nanoscale tip, they fail to predict the corresponding coefficient of friction. We show that the breakdown of microscale models can be attributed to the large contribution to friction from pileup. We propose an analytical model for plowing friction for single-asperity contacts, which includes the effects of both elastic recovery and pileup. Functional dependence of the average pileup height on elastic properties of the worn material and on the depth of cut is also proposed. Applicability of the new model is demonstrated in the examples of SiC and Cu.     

Friction of Undulated Surfaces Coated with MoS2 by Pulsed Laser Deposition

Solid lubricants such as molybdenum disulfide can provide very low friction, but their effectiveness especially in the geometrically constrained sliding pairs is limited by plowing of coated surfaces by wear particles. Even in the presence of solid lubricants wear particles cause higher friction by plowing the interface. To minimize plowing, undulated surfaces with microgrooves perpendicular to the sliding direction can be used to trap wear particles. Smooth and undulated stainless steel surfaces were coated with molybdenum disulfide by pulsed laser deposition (PLD) and friction tested. Under identical test conditions, the friction coefficient of coated undulated surfaces is between 20–40 percent lower than that of coated smooth surfaces. The friction coefficient of undulated uncoated surfaces is about 100–350 percent less than that of smooth uncoated surfaces. Moreover, the entrapment of wear particles at the interfaces of geometrically constrained bearings may lead to seizure even when bearing surfaces are coated with solid lubricants. The use of undulated surf aces on these sliding systems has shown improved operating time and a reduced maintenance cycle.     

Thermomechanical modeling and transient analysis of sliding contacts between an elastic–plastic asperity and a rigid isothermal flat

To investigate thermomechanical contacts between an elastic–plastic sphere and a rigid flat, simulations with slip rates ranging from 0.1 m/s to 10 m/s were performed. As interfaces with strong interfacial bonding but weak substrate were specifically targeted, slip initiation was treated as shear failure of the softer material in numerical simulations. The simulations show that both sliding friction coefficient and friction stress are significantly dependent on slip rate while the maximum static friction coefficient is independent of that. Moreover, the energy release during the transition from full stick to full slip is comparable to the shear fracture energy of the material.     

Two-dimensional two-fluid model for air-oil wavy flow in horizontal tube

This study concerns the development of a two-dimensional two-fluid model for wavy flows in horizontal tubes. To deal with the curved walls of the liquid and gas phases and the gas-liquid interface simultaneously, the bipolar coordinate system was used. Experiments on air-oil mixture flow in horizontal tubes with diameters of 20 and 40 mm were conducted to observe wavy flow patterns accompanying the two-dimensional (2D) and Kelvin-Helmholtz (KH) waves and to measure the pressure gradient under different flow conditions. Two different previous correlations for the interfacial friction factor were employed in the model for predicting the wavy flows with 2D and KH waves. Predictions of the model of the liquid film height, the average values of wall shear stresses of each phase, and the average interfacial shear stress were compared for different diameters and different superficial gas and liquid Reynolds numbers. Also presented are detailed predictions of the model for four different flow conditions, including the local values of interfacial shear stress, wall shear stress of the liquid phase, interfacial friction factor, liquid film height, and two-dimensional velocity distribution in the liquid phase at the cross-section of the tube.

     

In Situ Analysis of Third Body Contributions to Sliding Friction of a Pb–Mo–S Coating in Dry and Humid Air

Reciprocating sliding tests of ion-beam deposited (IBD) Pb–Mo–S coatings were performed with an in situ tribometer that allows real-time visualization and Raman analysis of the sliding contact through a transparent hemisphere. Experiments were performed in dry air, ambient air (∼50% RH) and mixtures of dry and humid air cycled between low and high humidity. Third bodies formed in the sliding contact were monitored through an optical microscope and analyzed by Raman Spectroscopy. Third body velocity accommodation modes were identified and correlated with friction behavior in dry and ambient air. The dominant velocity accommodation mode in both dry and humid air was interfacial sliding between the outer surface of the transfer film and the wear track; this interface, based on present and earlier studies, is crystalline MoS₂. Therefore, the friction coefficient was controlled by the interfacial shear strength of MoS₂ sliding against MoS₂. Humid air sliding was accompanied by a rise in the friction coefficient and a small but observable second velocity accommodation mode: shear/extrusion of the transfer film. It is concluded that the friction rise in humid air was due to an increase in the interfacial shear strength, and that the rise in friction caused the third body to deform rather than the deformation causing the friction to rise.     

Medical textiles with low friction for decubitus prevention

Decubitus or pressure ulcers in immobile patients are associated with prolonged pressure, shear and friction forces acting on the skin and the soft tissue underneath. We investigated the friction behaviour of hospital bed sheets in comparison with prototypes of low friction textiles in order to assess their potential for decubitus prevention. A specific textile composed of synthetic fibres showed a factor of three lower friction than normal hospital bed sheets under both dry and wet conditions. By absorbing and distributing interfacial water within the textile structure, the prototype additionally exhibited beneficial water transport properties.     

Scaling effects between micro- and macro-tribology for a Ti–MoS2 coating

The tribological properties of a Ti–MoS₂ coating (9 at% Ti) were studied at macroscopic length scales with an in situ tribometer and at microscopic length scales with a nanoindentation instrument equipped for microsliding experiments. Measurements were conducted in controlled environments at both low and high humidity (i.e. ～4%RH and ～35%RH). Reciprocating micro- and macro-sliding tests were performed with spherical diamond tip with a 50 μm radius and a sapphire tip with a radius of 3.175 mm, respectively. For both scales, the range of Hertzian contact pressures was between 0.41 GPa and 1.2 GPa. In situ video microscopy observations identified that the dominant velocity accommodation mode at macro-scale was interfacial sliding. However, an additional velocity accommodation mode, transfer film shearing, was also observed with higher humidity. Overall higher friction was observed with microtribology compared to macrotribology. The higher coefficient of friction was attributed to three different stages during the sliding process, which were identified with respect to different contact pressures, contact areas, tip shapes, and environmental conditions. The first two stages exhibited a solid lubrication behavior with some combination of interfacial sliding, transfer film shearing and microplowing. The transfer film thicknesses for these stages, normalized to the initial Hertzian contact radius, fell in a range of 0.001–0.1. For the third stage, the dominant VAM was plowing and the normalized transfer film thickness fell below this range. Comparisons between the two scales demonstrated that for dry sliding, microscopic contacts on Ti–MoS₂ deviate slightly from macroscopic behavior, showing higher limiting friction and microplowing. For humid sliding, microscopic contacts deviate significantly from macroscopic behavior, showing plowing behavior and absence of transfer films.     

Friction and Shear Strength of Polymers

The authors have measured the shear strength and friction of a number of polymers. The shear strength measurements were made on thin films of the polymers deposited on hard, smooth substrates. The values of the coefficients of friction obtained from these shear strength measurements have been compared with the values obtained when bulk polymer slides either on itself or on a hard, smooth surface. The results support the view that in the sliding of bulk polymer energy is dissipated within a thin region close to the sliding interface and it is the pressure in the contact which largely governs the magnitude of the interfacial shear strength.     

Effects of Normal Load,Sliding Speed,and Surface Roughness on Tribological Properties of Niobium under Dry and Wet Conditions

The effects of normal load, sliding speed, and surface roughness on the friction and wear of high-purity niobium (Nb) during sliding without and with an introduction of water were systematically investigated. Increasing the normal load or sliding speed decreased the friction of the Nb under the both dry and wet conditions because the increased wear of the Nb decreased the interfacial shear strength between the steel ball and Nb by promoting the surface roughening and the production of wear debris. However, the Nb tested at the lowest sliding speed under the lowest normal load with water exhibited the lowest friction and wear due to the formation of oxide layer on the wear track. The friction and wear of the Nb tested under the dry condition decreased with increased surface roughness because the higher interfacial shear strength between the steel ball and smoother Nb resulted in the earlier breakdown of the native oxide layer and direct contact between the steel ball and Nb. However, increasing the surface roughness of the Nb increased its friction and wear under wet conditions, probably due to the easier breakdown of the oxide layer that formed on the rougher surface during sliding. The tribological results clearly showed that the introduction of water during sliding had a significant influence on the tribological properties of the Nb.     

The effects of surface roughness on nanotribology of confined two-dimensional films

A molecular dynamics simulation is used to investigate the effect of surface roughness on the nano-rheology of ultra-thin films confined between two solid walls. The results show that friction is sensitive to the confining surface morphology and the fluid molecules near the two confining surfaces are in a layering solid-like structure. At a high shear rate, the interfacial layers of the film stick to the walls, resulting in partial slip inside the film and the development of shear stress in the viscous molecular fluid. The frictional resisting force is due to the shear in the viscous molecular fluid but not dependent on the critical yield stress of the solid-like structure as in the case of smooth boundaries.     

Simultaneous measurement of normal and friction forces using a cantilever-based optical interfacial force microscope

We measured normal and friction forces simultaneously using a recently developed cantilever-based optical interfacial force microscope technique for studies of interfacial structures and mechanical properties of nanoscale materials. We derived how the forces can be incorporated into the detection signal using the classical Euler equation for beams. A lateral modulation with the amplitude of nanometers was applied to create the friction forces between tip and sample. We demonstrated its capability by measuring normal and friction forces of interfacial water at the molecular scale over all distance ranges.     

Thermal activation in boundary lubricated friction

The friction coefficients for copper pairs lubricated with fatty acids and fluorinated fatty acids have been measured over a wide range of sliding speeds and temperatures. Sliding speeds in the range 10⁻⁷−10⁻² m s⁻¹ and temperatures in the range 4.2–300 K were used. The friction coefficients near 300 K are generally low and increase with sliding speed, while the friction coefficients at low temperatures are markedly higher and relatively independent of velocity. Each lubricant's friction vs. velocity behavior over the temperature range 150–300 K can be described by a friction-velocity master curve derived from a thermal activation model for the lubricant's shear strength. The activation energies deduced from this friction model are identical to those obtained in the same temperature range for a vibrational mode associated with low temperature mechanical relaxations in similarly structured polymers. These results suggest that thermally activated interfacial shear is responsible for the fatty acids' positive-sloped friction vs. velocity characteristics at low sliding speeds near room temperature.     

Tribological Properties of Alkoxyl Monolayers on Oxide Terminated Silicon

The tribological properties of alkoxyl monolayers on oxide terminated silicon surfaces have been investigated using interfacial force microscopy. For a C18 alkoxyl monolayer, both adhesion and frictional properties are similar to those of a self-assembled monolayer of octadecanethiol on gold. Friction is shown to increase as the alkyl chain length of the molecules decreases. Analysis using contact mechanics models has been carried out to estimate reduced modulus, adhesion energy, and friction shear strength. These interfacial mechanical properties are correlated to molecular structures at the interface.