Development and tribological behaviour of UHMWPE filled epoxy gradient composites

In this study, ultra high molecular weight polyethylene (UHMWPE) filled epoxy gradient composites have been developed. Samples were prepared for different centrifugation time periods. SEM and optical microstructures confirmed the graded dispersion of UHMWPE particles in the epoxy matrix. Quick estimation of gradient characteristics has been done by abrasive wear measurements. Sliding wear tests were conducted by using a pin-on-disc machine. The sliding wear rate of composites reduced on increasing centrifugation time. Reduction in sliding wear rate in UHMWPE filled epoxy gradient composites has been attributed to the reduction of tensile contact stresses as a result of the lubricating effect of UHMWPE's smooth surface and highly entangled chain structure of UHMWPE.     

Tribological Performance of UHMWPE/GNPs Nanocomposite Coatings for Solid Lubrication in Bearing Applications

Ultra-high molecular weight polyethylene (UHMWPE)/ graphene nanoplatelets (GNPs) nanocomposite coatings were developed to reduce friction and wear in the absence of liquid lubrication. UHMWPE nanocomposite powders with different loadings (0.25, 1, and 2 wt.%) of GNPs were prepared and electrostatic spraying technique was then used to deposit the nanocomposite powders on aluminum alloy to form a thin coating. Friction and wear tests were conducted on the coatings against a flat-end pin, made of hardened tool steel to determine the best loading of GNPs. That was further tested to investigate the effect of sliding speed and contact pressure on its tribological properties and to establish coating operating limits. Results showed that UHMWPE nanocomposite coating reinforced with 1 wt.% GNPs showed the best tribological performance. It reduced wear rate by about 51% as compared to the pristine UHMWPE coating. The coating sustained a maximum sliding speed of 1 m/s at a contact pressure of 4 MPa equivalent to a pressure and velocity (PV) value of 4 MPa.m/s.     

Vacuum tribological behaviour of self‐lubricating quasicrystalline composite coatings

High‐temperature‐resistant self‐lubricating coatings are needed in space vehicles for components that operate at high temperatures and/or under vacuum. Thick composite lubricant coatings containing quasicrystalline alloys as the hard phase for wear resistance can be deposited by a thermal spray technique. The coatings also contain lubricating materials (silver and BaF₂ CaF2 eutectic) and NiCr as the tough component. This paper describes the vacuum tribological properties of TH103, a coating of this type, with a very good microstructural quality. The coating was deposited by high‐velocity oxygen fuel spraying and tested under vacuum using a pin‐on‐disc tribometer. Different loads, linear speeds, and pin materials were studied. The pin scars and disc wear tracks were characterised using a combination of scanning electron microscopy and energy dispersive spectrometry. A minimum mean steady friction coefficient of 0.32 was obtained when employing an X750 Ni superalloy pin in vacuum conditions under 10 N load and 15 cm/s linear speed, showing moderate wear of the disc and low wear of the pin.     

Sliding wear response of a cast iron under varying test environments and traversal speed and pressure conditions

This study pertains to the examination of sliding wear behaviour of a gray cast iron over a range of sliding speeds and applied pressures in dry and (oil and oil plus graphite) lubricated conditions. Wear properties characterized were wear rate and frictional heating. The cast iron revealed various forms and sizes of graphite particles in a matrix of pearlite and limited quantity of free ferrite. Different solidification patterns, as controlled by the chemical composition and/or carbon equivalent of the alloy and rate of cooling, were thought to be responsible for the varying morphology of the graphite phase formed in the material matrix. Occasional decohesion of graphite at ferrite/graphite interfacial regions was also observed.The wear rate of the cast iron increased with the speed and pressure of sliding due to increasing severity of wear condition. The specimens tended to lose proper contact with the disc at larger pressures when slid dry. This was attributed to severe cracking tendency of the material. On the contrary, specimen seizure was noticed in the oil and oil plus graphite lubricated conditions; the seizure resistance (pressure) decreased with sliding speed in presence of the lubricants. The wear rate versus pressure plots attained different slopes, i.e. the rate of increase in wear rate with pressure, depending on the test environment. One slope and inappreciable effect of pressure on wear rate were noticed due to substantial cracking tendency of the cast iron when tested in dry condition. In the oil lubricated condition also, virtually one slope was observed but it was higher than that in dry condition indicating greater sensitivity of wear rate towards the applied pressure. Also, the samples attained lower wear rate in oil than in dry condition in view of suppressed cracking tendency causing more stable lubricating film formation in presence of the oil lubricant. Addition of graphite particles to the oil lubricant caused a further reduction in wear rate because of the enhanced possibility of a more stable lubricant film formation due to smearing of the graphite particles. In this case, the slope of the wear rate versus pressure plots was the least in the intermediate range of pressures irrespective of the sliding speed owing to more stable lubricating film formation.A higher rate of temperature increase with test duration (intermediate sliding distance) in the beginning was attributed to the abrasive action of the hard debris generated through the fragmentation of the initially contacting asperities. A subsequently observed lower rate of increase at longer durations could be owing to the occurrence of mild wear condition in view of less stressing of the contacting asperities and increased stability of the lubricant film formed. Increase in the rate of frictional heating at still longer durations resulted from destabilization of the lubricating film.Frictional heating increased with applied pressure and sliding speed in view of increasing severity of wear condition. The rate of increase in frictional heating was low initially up to a specific pressure followed by a higher rate of increase at still larger pressures when the tests were conducted in oil plus graphite at both the sliding speeds and in the oil lubricant at the lower speed. A constant (high) rate of increase in frictional heating with pressure was noticed in the dry condition at both the sliding speeds and in the oil lubricant at the higher speed. Low rate of frictional heating with pressure was attributed to the occurrence of mild wear condition while a higher rate of frictional heating with pressure resulted from the occurrence of severe wear condition. As far as the influence of test environment on frictional heating is concerned, least frictional heat was generated in the oil plus graphite lubricant mixture while the maximum was noticed in dry condition, intermediate response of the samples being observed in oil. Formation of more stable lubricating film was thought to be responsible for lower frictional heating in the lubricated conditions; the presence of graphite in the oil lubricant increased the extent of lubricating film formation and stability of the film so formed.The wear response of the samples has been explained in terms of cracking tendency and lubricating effects of graphite, predominance of the counteracting effects of the two parameters over each other, and lubricating film formation by the external oil (plus graphite) lubricant on the sliding surfaces in specific test conditions. Characterization of wear surfaces, subsurface regions and debris particles of the material enabled to further substantiate the observed wear performance of the samples.     

Role of contact frequency on the wear rate of steel in discontinuous sliding contact conditions

This work presents a novel approach of sliding ball-on-disk wear tests where the disc material is investigated. Each part of the wear track on the disc is in discontinuous contact with the counterbody. The contact frequency at each part of the wear track on the disc with the counterbody is defined by the rotation frequency of the disc. The sliding speed is however a function of both the rotation frequency and wear track diameter. In this work, the effect of the contact frequency on friction and wear was investigated on carbon steel in discontinuous sliding contact with corundum balls. Various sliding speeds were used while maintaining the contact frequency at a fixed value, and various contact frequencies were applied at constant sliding speeds.The wear rate of the disk material is shown to depend not only on the usual wear test parameters, namely sliding speed and contact load, but also on contact frequency. Moreover, contact frequency is shown to be a key factor determining the wear mode even at constant sliding speed and load. At contact frequencies above 9 Hz, the dominant wear mechanism is oxidational wear, while at frequencies below 4 Hz the dominant wear mechanism is adhesive wear. This transition from adhesive to oxidational wear takes place together with a change in the type of debris generated and in the value of the coefficient of friction.The validity of the Garcia-Ramil-Celis model proposed earlier for discontinous sliding contact conditions, is demonstrated for the case of carbon steel disks sliding against a chemically inert counterbody.     

Large-scale friction and wear tests on a hybrid UHMWPE-pad/primer coating combination used as bearing element in an extremely high-loaded ball-joint

The surfaces of a heavily loaded ball-joint were initially covered with a sliding spray and suffer wear. A solution is found by incorporating UHMWPE pads (Ultra high molecular weight polyethylene) with a carbon fibre/epoxy reinforced ring as sliding material into the chairs of the structure, while the steel ball-side is covered with a Zn-phosphate primer coating, protecting against corrosion. The local static and dynamic behaviour of the hybrid UHMWPE pads in contact with steel or Zn-coated counterfaces has been large-scale tested on loading capacity, low friction and wear resistance. For protection of the sliding counterface against wear, a polymer lip covering the carbon ring has been experimentally designed to flow over the carbon ring under high contact pressures, assuming the retained polymer disc under hydrostatic conditions. As such, the soft coating resists extremely high contact pressures (150 MPa) with good adhesion to the steel ball. However the application method should be carefully selected, sprayed coatings are the most favourable for low initial static friction. Calculated bulk and flashtemperatures revealed that the UHMWPE melting temperature is not exceeded, although softening of the coating under high contact pressures may be favourable for a ‘self-repairing’ ability. Pre-sliding creep and intermediate wear paths as manifesting in the ball-joint were simulated, indicating that the maximum design coefficient of friction is not exceeded. Test results are compared to FEM-calculations to verify the practical applicability of the modified sliding system.     

Experimental analysis of tribological behavior of UHMWPE against AISI420C and against TiAl6V4 alloy under dry and lubricated conditions

Friction and wear behavior of ultra-high molecular weight polyethylene (UHMWPE) sliding against AISI420C austenitic stainless steel and against TiAl6V4 alloy under dry and lubricated conditions were investigated with a reciprocating pin-on-flat tribometer for comparative purposes. The tests were conducted by varying frequency of the pin alternative motion and the applied normal load. For the tests in lubricated conditions a fluid containing a large amount of sodium hyaluronate has been chosen. By using an electronic precision balance the wear mass loss of the UHMWPE samples was evaluated accordingly. Friction is greatly reduced by the presence of UHMWPE and this is believed to be due to the formation of a lubricating film of UHMWPE in the contact zone. Furthermore, the experimental investigation shows that the AISI420C/UHMWPE gives, in dry conditions, better values in the wear rate and in the friction coefficient than the TiAl6V4/UHMWPE.     

Friction and Wear Behavior of Ultra-High Molecular Weight Polyethylene Sliding Against GCr15 Steel and Electroless Ni–P Alloy Coating Under the Lubrication of Seawater

The friction and wear behavior of ultra-high molecular weight polyethylene (UHMWPE) sliding against GCr15 steel and electroless Ni-P alloy coating under the lubrication of seawater was investigated and compared with that under dry sliding and lubrication of pure water and 3.5 wt.% NaCl solution, respectively. It was found that under the lubrication of aqueous medium, the friction and wear behavior of UHMWPE mainly depended on the corrosion of counterface and the lubricating effect of the medium. Because of serious corrosion of counterface by the medium, the wear rates of UHMWPE sliding against GCr15 under the lubrication of seawater and NaCl solution were much larger than that under other conditions, and such a kind of wear closely related to the corrosion of counterface can be reckoned as indirect corrosive wear. However, when sliding against corrosion-resistant Ni–P alloy under the lubrication of seawater, the lowest coefficient of friction and wear rate of UHMWPE were obtained, owing to superior lubricating effect of seawater. Moreover, periodic ripple patterns were observed on the worn surfaces of UHMWPE sliding against GCr15 under the lubrication of seawater and NaCl solution, which were ascribed to the intelligent reconstruction of surface microstructure of UHMWPE upon large plowing effect of the counterface asperities. Based on scanning electron microscopic (SEM) and three-dimensional (3D) profile analyses of the worn surfaces of UHMWPE, a stick–slip dynamic mechanism was proposed to illustrate the pattern abrasion of UHMWPE. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

湿式换挡离合器摩擦片磨损量计算方法的研究

介绍了综合传动湿式换挡离合器的工作原理,通过分析离合器结合分离过程的油压和滑摩转速变化规律,拟合出离合器工作油压与转速的特性方程,建立了离合器摩擦片磨损量计算模型。依据离合器摩擦副磨损试验结果,确定了油压、滑摩转速影响指数与结合次数的关系。利用组建的综合传动湿式换挡离合器磨损量测试试验系统,进行了18 000次离合器换挡试验。试验结果表明,湿式换挡离合器磨损量可以表示为工作油压与滑摩转速的函数。     

Friction and wear properties of UHMWPE/Al2O3 ceramic under different lubricating conditions

Friction and wear behavior of ultra-high molecular weight polyethylene (UHMWPE) sliding against Al₂O₃ ceramic under dry sliding, and lubrication of fresh plasma, distilled water and physiological saline were investigated with a self-made pin-on-disk apparatus at 37±1°C. The worn surfaces were examined with a scanning electron microscope (SEM). The results show that the friction behavior of UHMWPE is very sensitive to its water absorption state. The wear rate of UHMWPE under dry sliding is the highest and under plasma lubrication is the lowest. The wear mechanisms are different under dry friction and various lubricating conditions.     

不同润滑条件对纳米Al2O3改性UHMWPE摩擦磨损性能的影响

用MPV 2 0 0型摩擦磨损试验机对纳米Al2 O3改性超高分子量聚乙烯 (UHMWPE)塑料合金材料在不同润滑条件下 (干摩擦、水润滑 )进行了摩擦学性能测定 ,分别考察了载荷、速度以及运行时间等对材料摩擦学性能的影响。为纳米Al2 O3改性超高分子量聚乙烯塑料的实际应用提供理论指导     

Experimental study of the friction and wear behaviour of a polymer disc/primer coating combination used in ball‐joints by means of large‐scale testing

The surfaces of a heavily loaded ball‐joint were initially covered with a sliding spray, and suffer wear. A solution is found by incorporating ultra high molecular weight polyethylene (UHMWPE) discs with a carbon fibre/epoxy reinforced ring as sliding material into the chairs of the structure. The ball side is covered with a zinc phosphate primer coating. For design purposes the local static and dynamic behaviour of the hybrid UHMWPE discs in contact with steel or Zn‐coated counterfaces should be large‐scale tested in terms of their loading capacity, low friction and wear resistance. Also the influence of creep and wear on friction is examined. After the large‐scale verification tests in laboratory, a good correlation is found with a test in the field. Copyright © 2006 John Wiley & Sons, Ltd.     

Sliding wear of an unlubricated frictional combination of ductile metal and brittle abrasive

The concept of determining wear from hypotheses concerning shear effects in metal surface regions was applied to the sliding wear of a smooth elastoplastically deformable metal substance and a rough brittle abrasive. Interfacial layers or lubricating depositions on either surface were neglected. The form of the contact was modelled using random fields for the roughness profiles. The resulting intensities of wear may be compared with published experimental results. The wear intensity level was quantitatively determined. Dependence on the nominal pressure and properties of the rough abrasive is thus explained. Instead of hardness the compressive strength of abrasive peaks is the dominant characteristic parameter.     

The effect of kinematic conditions on the wear of ultra-high molecular weight polyethylene (UHMWPE) in orthopaedic bearing applications

It is known that wear mechanisms differ between the ultra-high molecular weight polyethylene (UHMWPE) components of total hip replacement (THR) and total knee replacement (TKR). The difference in relative contact position or 'kinematic conditions of contact' between the metal and polymer components is thought to contribute to the contrast in observed wear mechanisms. A reciprocating wear tester was used to evaluate three basic kinematic contact conditions: sliding, in which the relative contact position on the polymer remains stationary; gliding, where the contact position on the polymer reciprocates; and rolling, where the contact position on the polymer varies and the relative velocities of both components are equal. All static load tests used cast Co-Cr alloy and irradiated Chirulen UHMWPE in a 37 degrees C environment lubricated with bovine serum albumin. UHMWPE test sample wear was measured gravimetrically at intervals of 600,000 cycles. The results indicated a difference in wear factor (volume lost due to wear per unit load per unit sliding distance) between the three groups with varying relative motion. The study indicates that screening tests which evaluate wear properties of new materials for total joint replacement should reflect the different kinematic contact conditions.     

A new formulation for the prediction of polyethylene wear in artificial hip joints

Artificial joints employing ultra-high molecular weight polyethylene (UHMWPE) are widely used to treat joint diseases and trauma. Wear of the polymer bearing surface largely limits the use of these joints in younger and more active patients. Previous studies have shown the wear factor used in Archard's law for the conventional polyethylene to be highly dependent on contact pressure and this has produced variability in experimental data and has constrained the reliability and applicability of previous computational predictions. A new wear law is proposed, based on wear volume being dependent on, and proportional to, the product of the sliding distance and contact area. The dimensionless proportional constant, wear coefficient, which was independent of contact pressure, was determined from a multi-directional pin on plate study. This was used in computational predictions of the wear of the conventional UHMWPE hip joints. The wear of the polyethylene cup was independently experimentally determined in physiological full hip joint simulator studies. The predicted wear rate from the new computational model was generally increased, with an improved agreement with the experimental measurement compared with the previous computational model. It was shown that wear in the UHMWPE hip joints increased as head size and contact area increased. This resulted in a much larger increase in the wear rate as the head size increased, compared with the previous computational model, and is consistent with clinical observations. This new understanding of the wear mechanism in artificial joints using the UHMWPE bearing surfaces, and the improved ability to predict wear independently and to address previously described discrepancies offer new opportunities to optimize design parameters.     

极端工况下不同水润滑轴承材料摩擦磨损性能对比研究

在干摩擦工况下模拟水润滑膜严重破坏的极端情况,研究未经改性聚四氟乙烯(PTFE)、超高分子量聚乙烯(UHMWPE)和聚醚醚酮(PEEK)3种材料在不同转速和载荷下的摩擦磨损性能。结果表明:干摩擦工况下UHMWPE材料具有优异的耐磨性和良好的自润滑性能,压力对摩擦因数影响比转速大;PTFE材料具有稳定的摩擦因数,压力和转速对摩擦因数影响明显,耐磨性较差;PEEK材料摩擦因数较大,且相对容易受转速和压力变化的影响,但具有良好的耐磨性能。综合分析,在极端工况下UHMWPE的适应能力最好,PEEK次之, PTFE最差。     

Adhesive wear and frictional characteristics of UHMWPE and HDPE sliding against different counterfaces under dry contact condition

Abstract

The current work evaluates the wear and frictional performance of ultrahigh molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE) sliding against different metal counterfaces, stainless steel(SS), mild steel (MS) and aluminium (Al), under dry contact condition. The experiments were conducted using pin on disc machine at different sliding distances (0–40·32 km), 15 N applied load and 2·8 m s^–1 sliding velocity. Interface temperatures and frictional forces were measured simultaneously during the sliding, while specific wear rates were determined for every 1·68 km sliding distance. Based on the optical microscopy of the worn surface and wear track, frictional and wear results were analysed and discussed. The experimental results showed that the type of counterface material significantly influences both frictional and wear performances of the selected polymers. This was mainly due to the film transfer characteristics. Higher temperature and friction coefficient for UHMWPE and HDPE were evident when sliding took place against Al counterface. Sliding the polymers against stainless steel showed low friction coefficients compared to other counterfaces.     

Exploration on tribological behaviours of perrhenates as potential lubricating oil additive during a wide temperature range

The perrhenates of calcium, copper and lead used as oil additive were synthesised by using the aqueous solution method. The tribological properties of these perrhenates were examined by using a ball‐on‐disc tribotester during a wide temperature range and a four‐ball testing machine at room temperature. The four‐ball test results showed that all synthesised perrhenates presented good extreme pressure property to a certain degree and demonstrated satisfactory wear resistance at room temperature. The ball‐on‐disc experiment results showed that the base oil with perrhenate additives significantly decreased friction and wear compared with pure base oil at elevated temperatures. An antifriction and low‐shear layer was verified to form on sample surface after testing under high temperature. This layer effectively prevented the direct contact of sliding surfaces and decreased the friction coefficient. The antifriction behaviour of perrhenates implied that they could be developed as the candidate of oil additives for a hybrid lubricating mode. Copyright © 2015 John Wiley & Sons, Ltd.     

Tribological behaviours of PA/UHMWPE blend under dry and lubricating condition

Dry-sliding and lubricated friction and wear behaviours of polyamide (PA) and ultra-high molecular weight polyethylene (UHMWPE) blend were studied using a pin-on-disc method (polymer pin sliding against a stainless steel disc) at room environment. The tribological performance of PA and UHMWPE were also investigated for the purpose of comparison. The worn surfaces were examined using a scanning electron microscope (SEM) and optical microscope. It was observed that PA specimen demonstrated highest friction coefficient, UHMWPE the lowest in both dry-sliding and lubricated sliding test. The friction of PA could be sufficiently decreased by blending with UHMWPE. Statistical analysis suggested the relationship between the wear volume loss and the sliding distance could be expressed by a linear model for dry-sliding, while a logarithmic model was determined for lubricated sliding. The difference in wear modes between both sliding series suggested that there was change in the mode of material removal process. The lower wear rate in lubricated sliding was attributed to the elastohydrodynamic or partial elastohydrodynamic lubrication through the development of a continuous lubricant film between the polymer and the counterface, while the high wear rate of the specimens, in dry-sliding test, was mainly caused by fatigue process due to the repeated action of tearing and crack-propagation.     

Comparative study of micro- and nano-ZnO reinforced UHMWPE composites under dry sliding wear

This is a comparative study between ultra-high molecular weight polyethylene (UHMWPE) reinforced with micro-zinc oxide (ZnO) and nano-ZnO under different filler loads. These composites were subjected to dry sliding wear test under abrasive conditions. The micro- and nano-ZnO/UHMWPE composites were prepared by using a hot compression mould. The wear and friction behaviours were monitored using a pin-on-disc (POD) test rig. The pin-shaped samples were slid against 400 grit SiC abrasive papers, which were pasted, on the stainless steel disc under dry sliding conditions. The worn surfaces and transfer film formed were observed under the scanning electron microscope (SEM). Experimental results showed that UHMWPE reinforced with micro- and nano-ZnO would improve the wear behaviour. The average coefficient of friction (COF) for both micro- and nano-ZnO/UHMWPE composites were comparable to pure UHMWPE. The weight loss due to wear for nano-ZnO/UHMWPE composites are lower compared to micro-ZnO/UHMWPE and pure UHMWPE. The optimum filler loading of nano-ZnO/UHMWPE composites is found to be at 10 wt%. The worn surface of ZnO/UHMWPE composites shows the wear mechanisms of abrasive and adhesive wear. Upon reinforcement with micro- and nano-ZnO, the abrasive and adhesive wear of worn surfaces transited from rough to smooth.