Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints: I: Charnley prostheses with polytetrafluoroethylene acetabular cups

A quarter of a century has now elapsed since the late Professor Sir John Charnley introduced his low friction arthroplasty of the hip based on a stainless steel femoral component and an unfilled polytetrafluoroethylene (PTFE) acetabular cup. The advantages of the very low friction resulting from the use of PTFE and a femoral head of diameter 22 mm were unfortunately outweighed by a very rapid penetration of the femoral head into the acetabular cup. Charnley abandoned the use of PTFE after some 3 years and 300 operations and subsequent measurements revealed a penetration rate of 2.26 mm year^?1. In 1962 he adopted ultrahigh molecular weight polyethylene as the socket material and this material is now used almost exclusively in total replacement joints.In this paper we report further laboratory wear studies of PTFE on stainless steel in disc machines and compare the laboratory wear factors with those deduced from clinical observations. Charnley's clinical penetration rates have been analysed in terms of Paul's loading cycles for steady walking and measurements of the walking activity of elderly patients by Wallbridge and Dowson.It is shown that the wear factor for PTFE and stainless steel is about 3 × 10^?5mm³N^?1m^?1 and that there is excellent agreement between the mean value obtained from a variety of laboratory experiments involving environments of atmospheric air, distilled water, bovine synovial fluid and Ringers' solution with various counterface roughnesses and the wear factors derived from clinical observations.     

In Vacuo Tribological Behavior of Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites: The Importance of Water for Ultralow Wear

Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many practical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of magnitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 × 10^?6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1–10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 × 10^?7 mm³/(Nm).     

Low Wear Steel Counterface Texture Design: A Case Study Using Micro-pits Texture and Alumina–PTFE Nanocomposite

Laser-induced surface micro-pits pattern has been successfully used under fluid lubrication to reduce friction and wear through mechanisms of enhanced hydrodynamic lubrication and fluid retention. Limited successes of friction and wear reduction using solid lubricant and textured surfaces have been reported in the literature, and there still lacks an efficient way of finding textures that produce desired tribological performances. This study evaluates the effect of counterface micro-pits texture on wear of a notable alumina–PTFE nanocomposite and uses the Taguchi method and “Simplex Method” to find the micro-pits parameters producing the lowest wear of the composite material. The optimum texture found yields a composite wear rate of 1 × 10^?7 mm³/Nm, a value identical to the material’s wear rate against untextured counterface. However, when slid against a freshly replaced composite pin, the existing transfer film on the optimum texture reduces composite’s wear volume at low wear transition by 90% and yields a steady-state wear rate of 3.9 × 10^?7 mm³/Nm. On the contrary, preexisting low wear transfer film on untextured counterface increases wear of the newly replaced pin by 10× and yields a wear rate of 4.4 × 10^?6 mm³/Nm. Results in this study suggest larger, shallower and sparser counterface pits are more favorable for debris entrapment, transfer film formation and wear reduction when slid against polymeric solid lubricants. It also raises new possibilities of self-adapting low wear counterface texture design that could potentially support low wear without requiring large amounts of run-in wear volume of bulk solid lubricants.     

Transfer Solid Lubrication of Aluminum Sliding Contacts

The effects of transfer from solid lubricant sticks of unfilled, glass-filled, and bronze-filled PTFE on the room-temperature wear and friction of trailing primary contacts of aluminum (6061 T6) rods in repetitive intermittent contacts were investigated in a ring-on-rod configuration. The materials of the ring countersurfaces upon which the solid lubricants transferred and against which the trailing aluminum rods wore included steel, aluminum, copper, and an oxide dispersion-strengthened copper alloy. This sliding of the unlubricated copper ring countersurfaces against the aluminum led to the roughening of the copper as large (> 1 mm) aluminum particles embedded themselves upon the countersurface, with consequent transitions in the aluminum wear rate and the coefficient of friction to values exceeding 6 × 10^{? 3} mm³/Nm and 0.6, respectively, after an incubation period of several initial contacts of lower wear rate and friction. The other ring countersurface materials resulted in similarly high aluminum rod wear rate and coefficient of friction, more nearly from the onset of sliding. The application of unfilled PTFE solid lubricant transfer reduced the aluminum's gouging of the copper countersurfaces and correspondingly reduced the aluminum rod wear rate and the coefficient of friction against the copper, as well as against all other countersurface materials, towards 2 × 10^?3 mm³/Nm and 0.3 or less, respectively. Glass- and bronze-filled PTFE transfer lubricants provided reductions in the wear rate of the aluminum rod comparable to or in some cases better than the unfilled PTFE, though the unfilled PTFE transfer lubricant in several cases provided better friction reduction.     

The tribological characteristics of JS material under lubrication of oil

A material composed of a steel backing, a sintered porous bronze middle layer and a layer of reinforced PTFE, which is named JS material, was prepared. The friction, wear and limiting PV values of this material under dry friction as well as the lubrication of number 20 mechanical oil were studied using a MPV-1500 friction tester. The worn surface of JS material and the transfer film formed on the counterface of carbon steel were investigated using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results show that the friction, wear and limiting PV values of JS material can be greatly improved with the lubrication of oil. The results of SEM and EPMA analyses indicate that, under dry friction conditions, the solid lubricant PTFE and Pb easily transfer to the steel counterface and results in the reduction of friction and wear; while under the lubrication of oil, little transference of PTFE and Pb to the steel surface occurs and very small friction and wear are achieved. Analyses of frictional surfaces also suggest that the Pb filler gets enrichment on the rubbing surfaces, which is beneficial in increasing the adhesion of the transfer film with the steel surface.     

Epoxy, ZnO, and PTFE nanocomposite: friction and wear optimization

To lower the friction coefficient and increase the wear resistance of epoxy, nanoparticles of zinc oxide and polytetrafluoroethylene (PTFE) were added in small volume percents to an epoxy matrix. Tribological testing of the samples in this study was completed on a linear reciprocating tribometer with a 250 N normal load and a 50.8 mm/s sliding speed. Several samples were made and tested following a modified Simplex Method optimization procedure in order to find a volume percent for optimized wear resistance and friction coefficient. The sample with the optimum wear rate consisted of 1 volume percent of zinc oxide nanoparticles and 14.5 volume percent of PTFE nanoparticles. It had a wear rate of k = 1.79 × 10⁻⁷ mm³/Nm; 400× more wear resistant than neat epoxy. The sample with the optimum friction coefficient consisted of 3.5 volume percent of zinc oxide nanoparticles and 14.5 volume percent of PTFE nanoparticles and had a friction coefficient of μ = 0.113, which is almost a 7× decrease in friction coefficient from neat epoxy.     

Experimental Evaluation of Friction and Wear Properties of Solid Lubricant Coatings on SUS440C Steel in Liquid Nitrogen

The friction and wear properties of the prevailing different solid lubricant coatings (Ion-plated Au, Ion-plated Ag and RF-sputtered PTFE on SUS440C stainless steel) used in the bearings of high-speed cryogenic-turbo-pumps of liquid rocket engines were experimentally evaluated in liquid nitrogen immersed conditions. Also the above experiments were carried out with two newly proposed solid lubricant coatings of sputter-ion-plated MoSTi and a new ion-plated Pb on SUS440C stainless steel. The friction coefficient and wear rates of the coatings of ion-plated Au, ion-plated Ag, RF-sputtered PTFE, the new ion-plated Pb and MoS₂Ti-SIP (with coating thickness of 0.7±0.1 μm) on SUS440C steel against SUS440C stainless steel ball in liquid nitrogen were compared. Worn surfaces were examined microscopically with a microscope and a profilometer for understanding the mechanisms of friction and wear and transfer film lubrication in liquid nitrogen. It is found that the newly proposed solid lubricant coatings are showing promising results for their use in liquid nitrogen immersed conditions. The sputter-ion-plated MoSTi coating on SUS 440C steel shows a minimum value of friction coefficient (μ=0.015) and wear rate (w_c=0.56 × 10⁻⁶ mm³/N m ) in liquid nitrogen.     

Silicon segregation in sliding wear

Friction experiments were conducted on a couple consisting of an Fe-Ni pin sliding against a tool steel disk. The Fe-Ni pin contained a small amount of silicon (0.18%). In a mild vacuum environment (0.1 mmHg at 10% relative humidity) at loads below a critical value (13.0 N, at a sliding speed of 0.22 m s^?1), silicon segregation to the pin surface took place, resulting in the formation of a glassy film. Under these experimental conditions the friction coefficient and wear values were very low (0.28 and 5 × 10^?3mm³km^?1 respectively). At loads higher than the critical value (in the same mild vacuum environment) as well as under atmospheric conditions, no silicon segregation could be detected. The corresponding values of the friction coefficient and the wear rate were much higher (0.54 and about 0.4 mm³ km^?1 respectively). It is suggested that the beneficial glassy layer can build up only when the rate of diffusion of silicon to the surface is higher than the rate of material removal (wear).     

Effect of the Synergetic Action on Tribological Characteristics of Ni-Based Composites Containing Multiple-Lubricants

Ni-based self-lubricating composites with multiple-lubricants addition were prepared by a powder metallurgy technique, and the effect of multiple-lubricants on tribological properties was investigated from room temperature to 700?°C. The synergetic effects of graphite, MoS₂, and metallic silver lubricants on the tribological characteristics of composites were analyzed. XRD analysis showed that new Cr _x S _y and Mo₂C phase were formed in the composites containing graphite, MoS₂ and metallic Ag lubricants during the sintering process. The average friction coefficients (0.69?C0.22) and wear rates (11.90?C0.09?×?10^?5?mm³?N^?1?m^?1) were obtained when rubbing against Inconel 718 alloy from room temperature to 700?°C due to synergetic lubricating action of multiple-lubricants. A smooth lubricating was gradually generated on the worn surface, and the improving of tribological properties was attributed to the formation of lubricious glaze film on the worn surface and their partially transferred to the counterface. The graphite played the main role of lubrication at room temperature, while molybdate phase and graphite were responsible for low friction coefficients and wear rates at mid/high temperatures. The synergetic lubricating effect of molybdate (produced in the rubbing process at high temperatures) iron oxide (transfer from disk material to the pin) and remaining graphite multiple-lubricants play an important lubricating role during friction tests at a wide temperature range.     

Tribological Behavior and Tribochemistry of Self-mated Ti3SiC2 in Ethanol

The tribological behavior of self-mated Ti₃SiC₂ in low viscosity fluids, that is, water and ethanol, is investigated. The results show that both water and ethanol effectively reduce the friction and wear of self-mated Ti₃SiC₂. The friction coefficient and total wear rates of Ti₃SiC₂ tribo-pair are 0.82 and 2.17 × 10^?2 mm³/N m under dry condition, 0.41 and 6.50 × 10^?4 mm³/N m in water and 0.16 and 2.22 × 10^?4 mm³/N m in ethanol. As sliding distance is prolonged from 0.5 to 22.9 km, the total wear rates of Ti₃SiC₂ in ethanol can be further reduced. Mechanical wear of Ti₃SiC₂ is partly inhibited in water and greatly inhibited in ethanol. Surface polishing of Ti₃SiC₂ pin and tribochemistry are the two distinct characteristics of self-mated Ti₃SiC₂ in ethanol. The surface film in ethanol is composed of TiO₂ and SiO. Tribo-products in ethanol, that is, silica gel and titanium dioxide, are found. Polycondensation of ethanol by the catalysis of TiO₂ and SiO_2?x gives birth to paraffins and olefins.     

Tribological behavior of PEEK components with compositionally graded PEEK/PTFE surfaces

PEEK is a high strength engineering thermoplastic that suffers from a high friction coefficient and a friction induced wear mode. Past studies with 10 μm PEEK and PTFE powders resulted in composite solid lubricant that (at the optimal composition) had a wear rate of k = 2 × 10⁻⁹ mm³/Nm with a friction coefficient of μ = 0.12. A compositional grading of PEEK and PTFE is implemented in this study to create a bulk composite with the functional requirements of component strength, stiffness and wear resistance while providing solid lubrication at the sliding interface. The tribological performances of three functionally graded PEEK components were evaluated on linear reciprocating, rotating pin-on-disk and thrust washer tribometers. Wear rates comparable to samples of the bulk solid lubricant and comparable or improved frictional performance were achieved by compositionally grading the near surface region of PEEK components.     

Effect of Particle Size on the Wear Resistance of Alumina-Filled PTFE Micro- and Nanocomposites

It was long supposed that the ability of hard particle fillers to reduce the wear rate of unfilled PTFE (typically ～ 10^{? 3} mm ³ /Nm) by an order of magnitude or more was limited to fillers of microscale or greater, as nano-fillers would likely be encapsulated within the large microscale PTFE wear debris rather than disrupting the wear mechanism. Recent studies have demonstrated that nano-fillers can be more effective than microscale fillers in reducing wear rate while maintaining a low coefficient of friction. This study attempts to further elucidate the mechanisms leading to improved wear resistance via a thorough study of the effects of particle size. When filled to a 5% mass fraction, 40- and 80-nm alumina particles reduced the PTFE wear rate to a ～ 10^?7 mm ³ /Nm level, two orders of magnitude better than the ～ 10^?5 mm ³ /Nm level with alumina micro-fillers at sizes ranging from 0.5 to 20 μm. Composites with alumina filler in the form of nanoparticles were less abrasive to the mating steel (stainless 304) countersurfaces than those with microparticles, despite the filler being of the same material. In PTFE containing a mixture of both nano- and micro-fillers, the higher wear rate microcomposite behavior predominated, likely the result of the continued presence of micro-fillers and their abrasion of the countersurface as well as any overlying beneficial transfer films. Despite demonstrating such a large effect on the wear rate, the variation of alumina filler size did not demonstrate any significant effect on the friction coefficient, with values for all composites tested additionally falling near the μ = 0.18 measured for unfilled PTFE at this study's 0.01 m/s sliding speed.     

Transition of the lubricated wear of carbon steel

To study the transition of the lubricated wear of 0.53% C steel in the steady state, wear tests were carried out by rubbing the annular surfaces of two cylindrical test pieces in machine oil with no additives. The ratio of mating areas was varied to approach actual contact conditions. Three regions of variation in the coefficient of friction with contact load were determined. Fatigue wear, characterized by a friction coefficient μ ≈ 0.05 and a specific wear rate ω_s ≈ 0.005 × 10^?6 mm³ N^?1 m^?1, occurs in the first region.A transition from fatigue wear to adhesive wear, with μ ≈ 0.05?0.12 and ω_s ≈ (0.005?0.05) × 10^?6 mm³ N^?1 m^?1, takes place gradually within a specific load range. Finally, adhesive wear predominates above the load level that marks the end of the transition. The same behaviour was analysed through stepwise loading tests. The onset of transition and seizure occurs at constant mean surface temperatures. However, the end of transition is also affected by factors other than temperature. The results are compared with the transitions reported by the International Research Group on Wear of Engineering Materials of the Organization for Economic Cooperation and Development.     

The Role of Microstructure in Ultralow Wear Fluoropolymer Composites

Past studies have shown that the inclusion of fillers in a polytetrafluoroethylene (PTFE) matrix can improve wear resistance by nearly four orders of magnitude. These discoveries have prompted several tribological experiments over the past decade that have highlighted the importance of particle size, tribofilm formation, filler percentage, and environment. To evaluate the effect that microstructure plays on a composite’s tribological performance, PTFE-filled polyamide-imide (PAI) composites were made and tested. To investigate the role of microstructure on the tribological performance of fluoropolymer composites, 12 composite formulations of PTFE and PAI over a range of 0 to 100 vol% PAI were tested. PTFE–PAI composite samples were slid against a stainless steel countersample using a linear reciprocating tribometer under a nominal 6.35?MPa contact pressure at 50.8?mm/s sliding speed. Of the samples tested, the 25 vol% PAI showed a remarkable mean steady-state wear rate of k?=?3?×?10^?9 mm³/Nm over an extreme distance of 360?km. A serial imaging investigation revealed that a mechanical interlocking of the two polymers occurred during the sintering process, which possibly contributed to the ultralow wear rates observed in this polymer–polymer composite.     

Mechanical and unlubricated tribological properties of titanium-containing diamond-like carbon coatings

Titanium-containing diamond-like carbon (Ti-DLC) coatings were deposited on steel with a close-field unbalanced magnetron sputtering in a mixed argon/acetylene atmosphere. The morphology and structure of Ti-DLC coatings were investigated by scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy. Nanoindentation, nanoscratch and unlubricated wear tests were carried out to evaluate the hardness, adhesive and tribological properties of Ti-DLC coatings. Electron microscopic observations demonstrated the presence of titanium-rich nanoscale regions surrounded by amorphous carbon structures in Ti-DLC coating. The Ti-DLC coatings exhibit friction coefficients of 0.12–0.25 and wear rates of 1.82 × 10^?9 to 4.29 × 10^?8 mm³/Nm, depending on the counterfaces, sliding speed and temperature. The Ti-DLC/alumina tribo-pair shows a lower friction coefficient than the Ti-DLC/steel tribo-pair under the identical wear conditions. Increasing the test temperature from room temperature to 200 °C reduces the coefficient of friction and, however, clearly increases the wear rate of Ti-DLC coatings. Different wear mechanisms, such as surface polishing, delamination and tribo-chemical reactions, were found in the tribo-contact areas, depending on different wear conditions.     

Friction and Wear Behaviors of Ag/MoS2/G Composite in Different Atmospheres and at Different Temperatures

Silver-based composite with 15?vol% MoS₂ and with 5?vol% graphite was prepared by powder metallurgy method. The impacts of the counterface materials, atmosphere, and temperature on the tribological behavior of the composite were investigated. It was found that when sliding against brass less effective lubricating film formed, causing a higher friction and wear comparing with ASTM-1045 steel. With the increasing proportion of oxygen in the O₂/N₂ atmosphere, the wear rate and friction coefficient ascended slightly. At 200?°C, the combination lubrication of graphite, MoS₂, and Ag contributed to a low friction coefficient (0.07) and wear rate (6.56?×?10^?6?mm³/Nm). At 400?°C, graphite lost its lubricating role, while silver became excessively soft. Large amount of MoS₂ was oxidized into MoO_3, and the residual MoS₂ formed some island-like lubricating films. Severe adhesive wear occurred on the contact surface, which led to a high friction coefficient (0.25) and a great increase of the wear rate (23.2?×?10^?6?mm³/Nm). At 600?°C, a relatively low friction coefficient (0.1) was obtained because of the formation of high-temperature solid lubricants, (Ag₂Mo₄O₁₃ and Ag₂Mo₂O₇) and liquid Ag₂Mo₂O₇. However, the wear rate at 600?°C was the highest (32.6?×?10^?6?mm³/Nm) due to the thick transfer layer.     

Comportement sous ultra-vide de matériaux auto-lubrifiants chargés en fluorure de graphite (The ultra-high-vacuum behaviour of graphite fluoride filled self-lubricating materials)

Self-lubricating materials were obtained when graphite fluoride, (CF_x)_n, was introduced into a polyimide matrix. Tests have been undertaken to evaluate the wear rate and coefficient of friction of these materials using stainless steel as a counterface, with medium loads of 1–10 N and speeds of 0.4 and 0.8 m s^?1. In ambient air, the specific wear rate was found to be very low being about 0.7 × 10^?7 mm³ N^?1 m^?1, and in ultra-high vacuum (10^?8–10^?9 Torr) the wear rate still remained low. Thus materials containing (CF_x)_n appear to behave quite differently in vacuum in comparison to graphite powder or carbon fibre-filled polymers, for which specific wear rates 100 and 400 times greater, respectively, have been observed.     

Biopolymer wear screening rig validated to ASTM F732-00 and against clinical data

Abstract

The ASTM standard F732-00, 'standard test method for wear testing of polymeric materials used in total joint prostheses' offers several pieces of data against which a test rig for such biopolymers can be validated. These conditions include a linear wear in the range from 0˙07 to 0˙2 mm/million cycles for ultrahigh molecular weight polyethylene (UHMWPE), and a wear rate ranking of poly tetra fluoro ethylene (PTFE)>>polyacetal≥UHMWPE. Using a modified pin on plate wear test rig, these three biopolymers were tested under a 40 N load against polished stainless steel plates while using dilute bovine serum heated to 37°C as the lubricant. When subject to multidirectional motion, mean wear factors of 37×10^–6 mm³ Nm^–1 for PTFE, 3˙8×10^–6 mm³ Nm^–1 for polyacetal and 1˙1×10^–6 mm³ Nm^–1 for UHMWPE were obtained. Therefore the wear ranking recommended by the ASTM standard was achieved. When the corrected mean weight loss of the UHMWPE test pins was converted to a linear wear rate, a value of 0˙12 mm/million cycles was obtained. Therefore the ASTM recommended target for linear wear rate was met. When the wear factors from the test materials were compared with clinical wear factors of 37×10^–6 mm³ Nm^–1 for PTFE, 3˙5×10^–6 mm³ Nm^–1 for polyacetal and 0˙95– 1˙45×10^{– 6} mm³ Nm^–1 for UHMWPE, good correspondence between in vivo and in vitro wear factors was obtained, further validating the modified pin on plate wear test rig.     

Effect of MoO3 Tabular Crystals on TiAl Matrix Composites under Different Test Loads

The friction and wear behavior of TiAl matrix self-lubricating composites (TMSCs) with MoO₃ tabular crystals (MTCs) sliding against a GCr15 steel ball is tested using a constant speed of 0.2 m/s at room temperature under different loads from 6.65 to 16.65 N. The result reveals that TMSCs show a consistently lower friction coefficient in a certain range from 0.2 to 0.6 and less wear rate from 0.29 × 10^?4 mm³ N^?1 m^?1 to 0.49 × 10^?4 mm³ N^?1 m^?1 compared to TiAl-based alloy. Moreover, the friction coefficient and wear rate of TMSCs decrease with an increase in test load. MTCs in the deformed layer will be refined to produce interfacial shear slip and reduce the shear stress because of the weak binding force of MTCs in the sliding process, which can facilitate the formation of a deformed layer and protect the deformed layer from spalling failure. In addition, MTCs on the worn surface of TMSCs can reduce the shear stress directly. Hence, MTCs can promote antiwear of the deformed layer and reduce the friction on the worn surface of TMSCs. MTCs can play a better role in antiwear and antifriction when the test load is higher.     

Performance of graphic aluminium particulate composite materials

Friction characteristics of journal bearings made from cast graphic aluminum particulate composite alloy were determined under mixed lubrication and compared with those of the base alloy (without graphite) and leaded phosphor bronze. All three materials ran without seizure while the performance of the particulate composite and leaded phosphor bronze improved with running. Temperature rise in the journal bearing under mixed/boundary lubrication was also measured. It was found that with 0.3D/1000 to 1.5D/1000 clearance and a low lubrication rate (typical value for a bearing of diameter 35 mm × length 35 mm is 80 mm³/min) and at a PV value of 73 × 10⁶ Nm m^?2 min^?1 graphitic aluminium alloy journal bearings operate satisfactorily without seizure and excessive temperature rise. In comparison, the bronze bearings, with all the other parameters remaining the same, could not run without excessive temperature rise at clearances below D/1000 at lubrication rates lower than 200 mm³/min