Sliding wear of polymeric composites

The wear mechanisms of two different polymeric composites sliding against metal were investigated experimentally. The sliding distance, normal force and sliding speed were the test variables.Microscope observations showed that the mode of wear of the two materials is similar. Subsurface deformation, crack nucleation at the matrixharder particle or matrix-glass fibre interface, crack propagation parallel to the surface at a depth corresponding to the friction coefficient and crack shearing to the surface were found in both materials. The wear particles often have the shape of thin sheets. Comparison of the metal subsurfaces and wear particles with those of the polymers indicates that the mechanism of wear of semicrystalline composites is similar to delamination wear. Since film transfer greatly influences friction and wear it should be considered together with crack growth in fatigue in the prediction of wear of polymeric composites.     

Stages in the Wear of a Prow-Forming Metal

The stages in wear of a typical prow-forming metal, gold, were determined. They are: 1) prow formation at the initiation of sliding, characterized by a high wear rate, limited to the member having the larger surface involved in sliding (i.e., the flat in a rider-flat apparatus); 2) an intermediate stage of roughening and work hardening of the flat with reduction in its wear rate; and 3) a transition stage to the rider wear process. Soon after the transition, the flat gains mass, and the rider loses metal by transfer to the flat or as loose debris. The combined wear of both members for a given total sliding distance lessens with decreasing track length for unidirectional and reciprocating sliding—-both before and after the transition. Also, the number of passes to the transition is proportional to the length of track divided by the load. These phenomena originate in an increasing ratio of transfer to wear with decreasing track length, due to a diminished ability of wear debris to leave the zone of sliding. This increases the rate at which the track achieves the critical roughness and worked condition necessary for the transition.     

Friction and prow formation in a scratch process of copper by a diamond cone

A deformation process of flat copper surfaces scratched by diamond conical riders was observed by a metallographic technique and scanning electron microscopy. The geometric shape and the size of prows formed by material pile-up in front of the rider were examined and their influence on friction was investigated. It was found that, taking into account the actual shape of the prow, the friction force can be obtained theoretically for any cone angle.     

Sliding wear and friction of electroplated and clad connector contact materials: effect of surface roughness

A modeling study with bench apparatus was conducted with connector contact materials consisting of electroplated hard gold and gold flashed palladium on nickel underplatings mated to a clad noble metal. The clad metal contact was the ‘rider’, i.e., had the smaller surface involved in sliding compared to the plated ‘flat’ surface. This configuration is consistent with good engineering practice in commercial products. It was found that although a lubricant could reduce friction significantly and virtually eliminate adhesive transfer of metal, a requirement for negligible wear, i.e., the absence of loose particles, was that the plated surface be very smooth. If this contact was rough, such as might occur due to it having a nodular nickel underplate, tool marks, or burrsm abrasive wear of the cladding occurred in proportion to the magnitude of roughness. However, when both contacts were plated, lubricated wear was low and surface roughness was not a significant factor. Unlubricated plated contacts were severely with high friction by the prow formation adhesive wear mechanism regardless of their surface roughness.     

Investigation of the Transition State in the Wear of Polyphenylene Sulfide Sliding Against Steel

The effect of sliding variables, including counterface roughness, sliding speed, and contact pressure, on the run-in state of wear and friction was studied. Sliding was performed in the pin-on-disk configuration with a polyphenylene sulfide (PPS) pin resting on the flat steel counterface. Some experiments were also run to study the effect of air cooling and heating. Optical microscopy and scanning electron microscopy were used to study the shape and size of the wear debris, worn pin surface, and the transfer film formed on steel counterfaces. It was found that friction and wear in the run-in state were significantly affected by the sliding variables studied and their influence was closely related to the development of a transfer film during the run-in state. If the transfer film developed during initial sliding, the coefficient of friction increased and wear rate decreased. The wear rate in the run-in state increased with the increase in initial counterface roughness and there was an optimal counterface roughness of 0.06 m Ra for minimum steady state wear rate. A higher applied load led to a higher wear rate in the run-in state but that was not the case with steady state wear rate.     

Effect of load,area of contact and contact stress on the wear mechanisms of a bonded solid lubricant film

A pin-on-disk type of friction and wear apparatus was used to study the effect of load, contact stress and rider area of contact on the friction and wear properties of polyimide-bonded graphite fluoride films. Different rider area contacts were obtained by initially generating flats (with areas of 0.0035,0.0071, 0.0145 and 0.0240 cm²) on 0.476 cm radius hemispherically tipped riders. Different projected contact stresses were obtained by applying loads of 2.5–58.8 N to the flats. Two film wear mechanisms were observed. The first was found to be a linear function of contact stress and was independent of rider area of contact. The second was found to increase exponentially as the stress increased. The second also appeared to be a function of rider contact area. Wear equations for each mechanism were empirically derived from the experimental data. In general, friction coefficients increased with increasing rider contact area and with sliding duration. This was related to the build-up of thick rider transfer films.     

Vibration Reduces Metal to Metal Contact and Causes an Apparent Reduction in Friction

Experiments were conducted to determine the effect of vibration on friction. A rider, consisting of three fixed 1/3-inch diameter balls, slid along a flat steel plate under a dead weight load of 1 kg and a speed of 0.02 cm/second. The plate was vibrated by a speaker at frequencies of 20, 100, 500, and 1000 cycles per second. Acceleration, coefficient of friction, and electrical resistance of a contact were measured. For both lubricated (white oil plus oleic acid) and unlubricated conditions, apparent kinetic friction decreased rapidly after the acceleration of vibration approached and exceeded the acceleration due to gravity. Electrical resistance increased periodically with vibration. The data indicated that the vibration periodically reduced metal-to-metal contact due to reduced load. Thus, an apparent reduction of the coefficient of friction was observed. Microscopic observation of the unlubricated wear tracks on the plate showed absence of galling, brown film formation, and marked plastic deformation during sliding with vibration. The possibility of fretting-type oxide film was considered.     

Atomic-scale study of friction and energy dissipation

This paper presents an analysis of the interaction energy and various forces between two surfaces, and the microscopic study of friction. Atomic-scale simulations of dry sliding friction and boundary lubrication are based on the classical molecular dynamics (CMD) calculations using realistic empirical potentials. The dry sliding of a single metal asperity on an incommensurate substrate surface exhibits a quasi-periodic variation of the lateral force with two different stick-slip stage involving two structural transformation followed by a wear. The contact area of the asperity increases discontinuously with increasing normal force. Xe atoms placed between two atomically flat Ni surfaces screen the Ni-Ni interaction, decrease the corrugation of the potential energy as well as the friction force at submonolayer coverage. We present a phononic model of energy dissipation from an asperity to the substrates.     

Effects of Friction on the Contact and Deformation Behavior in Sliding Asperity Contacts

Finite-element analyses are carried out to study the effects of friction on the contact and deformation behavior of sliding asperity contacts. In the analysis, on elastic-perfectly-plastic asperity is brought in contact with a rigid flat at a given normal approach. Two critical values of the normal approach are used to describe the asperity deformation. One is the approach corresponding to the point of initial plastic yielding, and the other at the point of full plastic flow. Additional variables used to characterize the deformation behavior include the shape and size of the plastic zone and the asperity contact size, pressure, and load capacity. Results from the finite-element analysis show that the two values of critical normal approach decrease significantly as the friction in the contact increases, particularly the approach that causes plastic flow of the asperity. The size of the plastically deformed zone is reduced by the friction when the contact becomes fully plastic. The reduction is very considerable with a high friction coefficient, and the plastic deformation is largely confined to a small thin surface layer. For a low friction coefficient, the contact size, pressure and load capacity of the asperity are not very sensitive to the friction coefficient. For a moderate friction coefficient, the contact pressure is reduced and the junction size increased; the load capacity of the asperity is not significantly affected due to the compensating effects of the pressure reduction and the junction growth. For a high friction coefficient, the pressure-junction compensation is not longer sufficient and the asperity load capacity is reduced. The degree of the friction effects on these contact variables depends on the applied force or the normal approach. Although the analyses are conducted using a line-contact model, the authors believe that the effects of friction in sliding asperity contacts of three-dimensional geometry are essentially the same and the same conclusions would have been reached. These results may provide some guidance to the modeling of rough surfaces in boundary lubrication, in which the asperity friction coefficient can be high and vary significantly both in time and from one micro-contact to another.     

Influence of Cu Grain Size on Running-in Related Phenomena

We used a single-asperity microscopic tribosystem diamond sphere/Cu sheet to investigate the relevant phenomena affecting the dynamics of friction and wear in a macroscopic system. The influence of the average grain size of the softer of two tribopartners on friction and wear was investigated in particular. The observed tribosystem experienced a natural transition during the running time, from severe plastic flow to predominating boundary lubrication. This fact was used to study the influence of poly-α-olefine base oil and fully formulated engine oil Fuchs Titan SAE 5W45 on friction and wear during severe deformation and the boundary lubrication regime. It is shown that the initial grain size has a crucial influence on wear and friction only during first sliding interactions. During the initial sliding, the grain size rapidly decreases due to plastic deformation. The grains then become uniformly equal in size in every initial situation after approximately 30 cycles. Initially larger grains result in increased friction and wear as well as higher sensitivity to the kind of lubrication.     

The Wear of Electrodeposited Gold

The wear and friction of pure gold and gold alloy electrodeposits were determined. These platings are widely used in sliding electric contacts and include golds hardened with silver, cadmium, nickel, cobalt, or cobalt and indium. In preliminary work, the microstructures, hardnesses, crystallographic orientations of the surface, and topographies of the golds were examined; these properties vary widely, and relationships among them are discussed. Sliding was with thick deposits to eliminate effects of substrate. The experimental method involved repeat-pass unidirectional sliding at 100g in a rider-flat apparatus with plated flats against wrought gold or gold-plated riders. Wear and friction are described for both as-plated samples, and to eliminate distinguishing surface textures, with flats abraded to a common roughness.

Wear was found to be related to hardness, topography, and crystallographic orientation. A new result is that prow formation does not occur above a load-dependent critical roughness. For abraded pure gold, the critical value is 25–50 μin. center line average at 100g.

The prow formation wear mechanism dominated sliding in runs of 500 revolutions on a 1-in. diameter track. Exceptional were the very hard golds (about 200 KHN) against soft wrought gold riders (about 50 KHN), in which metal from the rider smeared onto the flat prior to the onset of prow formation. A 1% silver-gold alloy showed unusually high wear, and a 1% cadmium-gold alloy wore relatively, little in the as-plated form. The lowest wear rates were with nickel-or cobalt-hardened golds.

Coefficients of friction of electrodeposits change during running-in. This is related to changes produced by wear in the relative contributions of the adhesion, plowing and roughness components. The friction of very rough or very smooth as-plated gold decreases in repeat-pass sliding as a middle level of roughness is attained.     

The effect of contact stress on cartilage friction, deformation and wear

Following hip hemiarthroplasty, a metal femoral head articulates against natural acetabular cartilage. Cartilage friction and wear may be influenced by variables including loading time, contact stress, contact area, sliding distance, and sliding speed. The aim of this study was to investigate the effect of these variables on cartilage friction, deformation and wear in a simulation using idealized geometry model. Bovine cartilage pins were reciprocated against metal plates to mimic a hemiarthroplasty articulation under static loading. The effective coefficient of friction (micro elf) under contact stresses (0.5 to 16 MPa), contact areas (12 and 64 mm2), stroke lengths (4 and 8 mm), sliding velocities (4 and 8 mm/s), and loading time (1 and 24 hours) were studied. The permanent deformation of cartilage (after 24 hours of recovery) with and without motion was recorded to assess cartilage linear wear. The micro eff was found to remain < 0.35 with contact stresses < or =4 MPa. Severe damage to the cartilage occurred at contact stresses > 8 MPa and significantly increased micro eff after 12 hours of reciprocation. In long-term, contact area had no significant effect on micro eff, and sliding distance and velocity only affected micro eff under low contact stresses. The cartilage linear wear increased with contact stress, sliding distance and velocity.     

Tribological behavior of microwave processed copper–nanographite composites

Excellent properties offered by nanographite particles are exploited as a reinforcement to the copper matrix. The effects of graphite particle size, spatial distribution, normal load and sliding speed on the friction and wear performance of microwave sintered copper metal matrix composites were studied using a pin-on-disc tribometer. Copper–nanographite composites show higher wear resistance and low coefficient of friction compared to copper–graphite composites. High surface area of nanographite particles embedded in copper matrix exhibited high adherent graphite tribo-layer at the contact surface. Formed graphite layer reduces the sub-surface deformation of the composite by way of reduced frictional force.     

Wear,Friction, and Electrical Noise Phenomena in Severe Sliding Systems

The sliding mechanisms of unlubricated gold and palladium are described. Transfer, roughening, wear, friction, and contact resistance phenomena involve the same discrete events.

With rider-flat geometry, severely Worked transfer particles form a prow-shaped wedge which adheres to the smaller member. Even when rider and flat are different, prow material comes from the larger part. Loss of prow occurs by adhesive weld-back transfer and, to form loose debris, by shearing or fatigue fracture.

Friction rises in the early stages of sliding as prows grow. Soon, back transfer increases hardness of the larger member and friction falls to an equilibrium level. Friction also falls when prows are kneaded into rollers and become loose debris. During these stages, wear rate diminishes.

Contact resistance noise originates in stick-slip, roller formation, surface hardening, and changing composition at the sliding interface when dissimilar contact metals are involved.     

Wear and Friction of Filled Polytetrafluoroethylene Compositions in Liquid Nitrogen

Wear and friction behavior of slider materials at cryogenic temperature is important to the development of seals and bearings for missile powerplants. Data were obtained in liquid nitrogen (?320°F) with a series of molded and extruded polytetrafluoroethylene (PTFE) compositions containing various filler materials. A 3/16-in. radius rider specimen (PTFE materials) was caused to slide in a circumferential path on the flat surface of a rotating 2½-in. diameter disk specimen (usually type 304 stainless steel). The sliding velocity was usually 2300 ft per min and the load was 1000 grams.

As compared with reference steels and carbons used in conventional seals and bearings, the filled PTFE compositions gave low wear and friction (friction coefficients from 0.06 to 0.13) in liquid nitrogen. Several extruded compositions have particular promise for seal and bearing materials. An extruded glass-filled material gave wear and friction that was essentially unaffected by sliding velocities to 6000 ft per min.     

载流条件下铬青铜/纯铜摩擦副摩擦磨损性能研究

在自制的销盘摩擦磨损试验机上,对铬青铜/纯铜摩擦副进行载流条件下的干滑动模拟试验,研究了电流、速度、载荷对铬青铜/纯铜摩擦磨损性能的影响规律。试验结果表明:电流是影响摩擦副摩擦磨损性能最显著的因素,摩擦因数和磨损率都随着电流的增大而增大;速度和载荷对摩擦因数和磨损率也有显著影响;电流的存在,摩擦副间产生了比无电流时更严重的粘着磨损和塑性变形,同时增加了电化学腐蚀,使磨损更加严重。     

Sliding wear of graphite crystallized chromium white cast iron

The effects of sliding velocity, heat-treatment and graphite shape on sliding wear of graphite crystallized chromium white cast iron were studied. Two types of graphite crystallized chromium white cast irons having flaky or spheroidal and another type of 2.6C–15Cr white cast iron were prepared for this study. The effect of sliding velocity on wear resistance was studied by the Okoshi type and pin-on-disk type wear tests on materials which have experienced “as cast” and “heat-treated” conditions. The Okoshi type wear test results are divided into two relationships depending on sliding velocity or distance. Two regimes, initial wear and steady-state wear, existed for wear loss and sliding distance. A characteristic form of wear curve with a peak and a minimum was obtained when correlating wear loss and sliding velocity. The wear resistance of graphite crystallized chromium white cast irons were superior to that of 2.6C–15Cr white cast iron. In the results of pin-on-disk tests, there was no clear difference in the reported wear loss and friction coefficient among the alloys. However, an opposite tendency has appeared in the wear loss and friction coefficient: the wear loss value reached a peak in the wear curve at 0.52 m/s, while the friction coefficient reached a minimum at 0.52 m/s.     

表面粗糙度对滑动电接触磨损率的影响

在电气化铁路弓网系统中,磨损率是衡量列车运行状态与接触导线使用状态的重要指标。为了充分模拟弓网系统中磨损率情况,利用自行搭建的滑动电接触摩擦磨损试验机对滑板和接触导线进行摩擦磨损试验,分析滑板表面粗糙度、法向压力、接触电流与运行速度对磨损率的影响。得出结论:滑板磨损率随滑板初始表面粗糙度、接触电流、法向压力、运行速度的增加而增加,而高载荷下粗糙度对于磨损率的影响降低;滑板摩擦从磨合期进入稳定摩擦期存在一个临界表面粗糙度,当滑板初始表面粗糙度值等于临界粗糙度值时,其磨损率最低;不同初始表面粗糙度的滑板在跑合期内磨损过程不同,在稳定摩擦期内磨损过程趋于一致,且摩擦试验后滑板表面粗糙度也接近。     

Wear of water lubricated silicon nitride in ball on disc test

Abstract

The ball on disc test configuration is preferred to the flat ended pin on disc because the ball is self-aligned and measurement of wear on the ball is of higher accuracy, compared to the pin. Silicon nitride, sliding on itself in water, was tested with the ball on disc tribometer. Misalignment of the test ball from its proper position behind the disc axis of rotation leads to friction measurement errors, which were analysed. The disc wears non-uniformly, the wear track depth and width vary in longitudinal direction by a factor of 2–3. The uneven wear of the disc is explained by the combined effects of sliding surface anisotropy and disc material non-homogeneity on the one hand and by the friction force and the normal load periodic variation on the other hand. During the running-in process at particular sliding velocity amplitude modulated friction force was observed and an explanation by the mechanical vibration 'beating' phenomenon was suggested. Predictive model of the running-in process is presented, which describes the evolution of the ball wear scar area, the contact pressure and the wear rate. The model predictions are consistent with the experimental data.     

Frictional deformation and fracture in polycrystalline SiC and Si3N4

Electron optical microscopy was employed to study the friction and wear of commercial polycrystalline varieties of SiC and Si₃N₄ in air at ambient temperature. Friction and wear tests were conducted in a reciprocating configuration with conical riders (both diamond and ceramic) sliding on a flat ceramic substrate. Worn surfaces were examined by both scanning electron microscopy and transmission electron microscopy. In general, friction and wear in the diamond-ceramic couples were severe. Friction with ceramic-ceramic couples was low, with friction coefficients between 0.1 and 0.4, wear being absent in single-pass tests.With ceramic-ceramic couple multipass systems, wear of Si₃N₄ occurs by plastic deformation which increases in severity with sliding distance accompanied by a corresponding increase in friction coefficient. With SiC, wear occurs by a mixture of intergranular fracture due to grain boundary weakness and plastic deformation.