Showing papers in "Wear in 1985"
TL;DR: Friction and wear of Si3N4 sliding on itself were measured at room temperature in different gaseous and liquid environments as discussed by the authors, showing that wear occurs by two fracture mechanisms: within 1 μm of the surface, asperity contact produces very large local stresses and cracking on a very fine scale; 3-5 μm deeper the fracture follows weaknesses of the material and is intergranular fracture with some transgranular cleavage.
Abstract: Friction and wear of Si3N4 sliding on itself were measured at room temperature in different gaseous and liquid environments. At low sliding speed the friction coefficient ƒ is 0.85 in dry argon and nitrogen and 0.8 in laboratory air and oxygen. In dry gases, wear occurs by two fracture mechanisms: within 1 μm of the surface, asperity contact produces very large local stresses and cracking on a very fine scale; 3–5 μm deeper the fracture follows weaknesses of the material and is intergranular fracture with some transgranular cleavage. No evidence of plastic deformation was obtained. In water- and humidity-saturated gases wear is predominantly by a tribochemical reaction which produces an amorphous protective layer in humid gas and dissolution in liquid water. In intermediate humidity, wear is a combination of fracture and tribochemistry; the latter increases adhesion between wear particles to form a layer of compacted wear particles on the wear track. The fact that humidity decreases wear in Si3N4 and increases it in A12O3 is explained by the differences in chemical reactivity and susceptibility to stress corrosion cracking between the two materials.
403 citations
TL;DR: It appears that most of the penetration of femoral heads into acetabular cups can be attributed to wear rather than to creep of the polyethylene, and the agreement between this laboratory wear factor and the deduced clinical wear factor is much better than has been recognized previously.
Abstract: A casting technique for assessing the extent of the penetration of femoral heads into polyethylene acetabular cups in explanted Charnley prostheses is described and the observed penetration rates are related to laboratory wear studies carried out in the Institute of Tribology at Leeds over some 16 years. Some 32 explanted high molecular weight polyethylene (RCH 1000) acetabular cups provided by Wrightington Hospital were examined, and 25 were found to be suitable for penetration measurements. Relevant patient details for the 25 explanted acetabular cups were provided and 15 of the subjects were male whilst 10 were female, with 14 of the cups coming from the right hip and 11 from the left. The ages of the subjects at the time of the removal of the prostheses ranged from 18 to 68 years, the average being 54.4 years. The average period of residence of the prosthesis in the body was 8 years 11 months. The correlation between the measured volumes of polyethylene removed and the penetration was found to be linear and therefore entirely in accord with the view developed in part I that the femoral head tunnels into the acetabular cup with a circle of penetration having a diameter comparable with that of the femoral head. This observation enabled the expression derived in part I for a clinical wear factor to be adopted and results are recorded for all 25 prostheses on the assumption that creep played little part in the total penetration process. The average clinical wear factor was found to be 2.9 × 10−6mm3N−1m−1, with the results ranging from 0.09 × 10−6 to 7.2 × 10−6mm3N−1m−1. It was observed that the femoral heads from the explanted prostheses exhibited a number of fine scratches and attention was thus drawn to the influence of counterface roughness on the wear of polyethylene. Measurements of surface roughness indicated that for a number of the femoral heads the roughness of the metal surface was greater than that normally associated with the highly polished finish of new prostheses. The average value of the surface roughness Ra was found to be 0.054 μm. For this roughness laboratory tests revealed a wear factor of 1.2 × 10−6mm3N−1m−1. The agreement between this laboratory wear factor and the deduced clinical wear factor is much better than has been recognized previously. The clinical observations indicate an average penetration range of about 0.19 mm year−1, whereas X-ray studies have previously suggested a mean penetration rate of only 0.07 mm year−1. The powerful role of counterface roughness on the wear rate of polyethylene has been noted elsewhere by Dowson et al. The present study indicates that when full account is taken of counterface roughness the agreement between laboratory and clinical observations is in much closer accord than had been recognized previously. Particles of acrylic cement were detected in the explanted acetabular cups and the role of such particles in determining the effective roughness of the metallic femoral heads and hence the wear rate of the polyethylene acetabular cups is discussed. It is concluded that every effort should be made to minimize the presence of acrylic cement particles between the articulating surfaces in total replacement joints. The major factors thought to be responsible for the remaining discrepancies between the average clinical and laboratory wear factors and the wide spread of results are considered. The results presented in this paper demonstrate the close accord now achieved between laboratory and clinical wear studies. They indicate that although there is undoubtedly a wide range of wear factors and effective penetration rates in both the laboratory and the clinical situations, the mean values are sufficiently close to promote confidence in the understanding of the wear processes involved in both situations. It thus appears that most of the penetration of femoral heads into acetabular cups can be attributed to wear rather than to creep of the polyethylene.
154 citations
TL;DR: In this article, it was shown that preferential removal of the cobalt binder is an important mechanism in the abrasive wear of cemented carbides in the WC-Co family.
Abstract: It has previously been proposed that preferential removal of the cobalt binder is an important mechanism in the abrasive wear of cemented carbides in the WC-Co family. It is here demonstrated that binder extrusion occurs also in metal-to-metal sliding wear contacts. The wear scar generated by sliding a hardened steel ball repeatedly over a polished WC-Co surface was studied by scanning electron microscopy. The extruded cobalt fragments accumulate at surface defects, such as cracks caused by the sliding loaded ball, and gradual microfragmentation of the carbide grains follows. The energy required to extrude the cobalt and to cause the gradual change in surface layer microstructure is provided by the frictional forces.
137 citations
TL;DR: In this article, the effect of additive chain length on friction coefficient and transition temperature was investigated under different test conditions, and it was concluded that, at very low speeds, boundary lubrication is controlled by adsorbed monolayers, whereas at higher speeds, where partial elastohydrodynamic conditions prevail, ordered multimolecular layers may influence lubrication.
Abstract: The effect of additive chain length on friction coefficient and transition temperature is investigated under different test conditions. Carboxylic acids and normal alcohols with 12 – 18 carbon atoms in the chain were used as additives in n-hexadecane base lubricant. It was found that the magnitude of the friction coefficient decreases as the additive chain length is increased. Matching of the additive chain length with the base lubricant had no effect on the friction coefficient. Similarly, the transition temperature at low sliding speeds increased with increasing chain length and was not affected by chain matching. At higher speeds, the acids containing 16 and 18 carbon atoms did not exhibit a transitional behavior within the studied temperature range. These results are discussed in terms of the separation distance between the contacting surfaces, the intermolecular dispersion forces and the structure of adsorbed layers. It is concluded that, at very low speeds, boundary lubrication is controlled by adsorbed monolayers, whereas at higher speeds, where partial elastohydrodynamic conditions prevail, ordered multimolecular layers may influence lubrication.
137 citations
AT&T1
TL;DR: In this paper, the authors reviewed the fretting of electrical contacts on the basis of work carried out by the present author and by other investigators, including a discussion of mechanisms of transfer, wear, oxidation and frictional polymerization which apply to the fretbing of contacts and of the influence of operational parameters such as cycle rate and force on contact resistance.
Abstract: The fretting of electrical contacts can lead to unacceptable increases in contact resistance. Although this phenomenon was recognized for many years, considerable new work has been published recently on its occurrence and possible effect on the reliability of separable electronic connectors. The reason for these studies has been economic. Escalation in the cost of gold, the preferred contact material because of its nobility, stimulated interest in potential replacement metals such as Sn-Pb solder plate and palladium. Unfortunately, connectors with these replacement metals are prone to fretting failure. In this paper the fretting of electrical contacts is reviewed on the basis of work carried out by the present author and by other investigators. Included are a discussion of mechanisms of transfer, wear, oxidation and frictional polymerization which apply to the fretting of contacts and of the influence of operational parameters such as cycle rate and force on contact resistance, a survey of materials whose fretting behaviors have been determined and an analysis of the role of lubricants in controlling fretting degradation.
125 citations
TL;DR: In this article, an experimental Cr-Mo alloy rail steel was heat treated to pearlite, bainite and tempered martensite microstructure at hardness levels of 38, 42 and 45 HRC.
Abstract: In order to find out whether it is worthwhile to produce premium rail steels harder than 36–39 HRC and to discover the best microstructure for wear-resistant rail an experimental Cr-Mo alloy rail steel was heat treated to pearlite, bainite and tempered martensite. Each microstructure was prepared at hardness levels of 38, 42 and 45 HRC. These were tested in a dual disk-on-disk machine that closely simulates wheel-rail contact in curves at 1/10 scale. Wear rates were established for dry, grease-sand and pure grease environments. Dry wear rates decreased significantly with increasing hardness in high rails with tempered martensite or bainite microstructures but were almost independent of hardness for pearlite in the range tested. Lubrication reduced the wear rates by up to two orders of magnitude for all microstructures. It is concluded that pearlite gives the best dry wear performance, and there is no advantage in increasing the hardness beyond 38–40 HRC. The superior performance of pearlite in dry wear appears to be due to a very pronounced work hardening near the wearing surface. Electron microscopy reveals a marked refinement in the pearlite microstructure near the wearing surface, and this may explain the high work hardening observed.
117 citations
TL;DR: In this article, a method of choosing parameters for the multiparameter estimation of roughness is proposed, on the basis of a comparative analysis of the roughness features given by individual parameters, a selection procedure was carried out and a set of basic parameters proposed.
Abstract: The parameters and functions used for the evaluation, analysis and modelling of roughness are reviewed in the present work. Since the estimation of the roughness performed with one parameter is ambiguous, a method of choosing parameters for the multiparameter estimation of roughness is proposed. On the basis of a comparative analysis of roughness features given by individual parameters, a selection procedure was carried out and a set of basic parameters proposed. Next, using the results of correlation analysis, a subset of four uncorrelated parameters was established. It is proposed that two-parameter characterizations should be based on parameters of this subset. This approach would considerably facilitate the exchange of licences, and of technical and scientific information.
117 citations
TL;DR: It was concluded that cement in the explanted polyethylene cups of total replacement hip joints contributed to the appearance of the worn regions and, no doubt, to the wear process itself.
Abstract: In this paper attention is focused on the findings of a recent microscopical study of explanted polyethylene hip joints from Wrightington Hospital. The project was initiated some years ago in collaboration with the late Professor Sir John Charnley, but the present study is based on an extended study of acetabular cups which have been sectioned for direct examination in the electron microscope for the first time. The account of the appearance of worn polyethylene acetabular cups reported in this paper provides a useful basis for the quantitative assessment of wear in the current form of Charnley low friction arthroplasties reported in part III of the present trio of papers. It has been confirmed that worn polyethylene acetabular sockets exhibit three distinct regions: a relatively smooth high wear region in the superior half of the cup, a rougher low wear area in which the original machining marks are still visible and a low ridge separating the two. Rim wear associated with impingement of the femoral stem is also evident in some of the heavily worn sockets. Some five cups representing service lives ranging from 1 to 14 years were selected for detailed visual examination. Descriptions of the high and low wear regions are offered and it will be evident from the geometry of the intermediate ridge region that creep cannot have contributed significantly to the overall penetration of the femoral head into the acetabular cup. This point is discussed further in part III. Perhaps the most significant finding is that cement was found in all the cups examined. The adhesive wear process evident in the high wear region was supplemented by scratching or abrasive wear associated with cement ingress. It was concluded that cement in the explanted polyethylene cups of total replacement hip joints contributed to the appearance of the worn regions and, no doubt, to the wear process itself. It is expected that new designs of acetabular cups and improved surgical techniques will be of considerable benefit in this context.
97 citations
TL;DR: In this article, the authors proposed that the process involved in the initial transfer events is a consequence of local shear instabilities which develop at large plastic strains. And they found that for a pin-and-disk system the disk should be made of the material having the higher cohesive strength.
Abstract: Results of pin-on-disk sliding tests in vacuum, with copper, CuNi, nickel and molybdenum sliding against iron, indicate that initial transfer events involve discrete fragments. We propose that the process involved in the initial transfer events is a consequence of local shear instabilities which develop at large plastic strains. In copper samples the initial transfer elements are lamellar, with layer thickness equal to the cell thickness in the highly deformed base material adjacent to the sliding interface. Prolonged sliding gives rise to the formation of transfer layers or patches on the specimen surface. These layers or patches are composed of finely mixed material derived from the two sliding counterparts as well as scattered pieces of more recently transferred fragments. Typical wear debris particles are generated from the transfer layers or patches. Transfer tendencies for different materials combinations can be predicted from an adhesion point of view if geometrical effects are properly considered. Combining theoretical predictions of transfer tendencies and experimental observations of transfer and geometrical effects, one finds that for a pin-and-disk system the disk should be made of the material having the higher cohesive strength.
96 citations
TL;DR: In this article, a hot-pressed silicon-nitride-based ceramic has been used for cutting cast irons at high speeds (10−16.75 m s−1 (600−1000 m min−1) and rates (about 1.0 mm rev−1), where conventional tools cannot be used.
Abstract: Silicon nitride, a nitrogen compound of highly covalent bond character, has low density, high hardness, low thermal expansion and high thermal conductivity and excellent oxidation and corrosion resistance. Therefore it is considered to be a promising material for high temperature engineering components. However, it has very poor sinterability compared with the conventional oxide ceramics owing to its low self-diffusivity. By investigating starting materials, sintering additives and processing, a hot-pressed silicon nitride ceramic having grains enveloped by crystalline boundary phases can be produced. The silicon-nitride-based ceramic has high strength, high Weibull modulus, fracture toughness and thermal conductivity, and low thermal expansion compared with existing cutting tool ceramics such as Al2O3 + TiC, and it is expected to have superior thermal shock resistance. This new ceramic cutting tool shows superior performance in cutting cast irons at high speeds (10–16.75 m s−1 (600–1000 m min−1)) and rates (about 1.0 mm rev−1), where conventional tools cannot be used.
90 citations
TL;DR: In this paper, the effect of oxide films on hot spot temperatures is deduced, and the division of frictional heat is also discussed, using an iteration process involving N, a, ξ and Tc.
Abstract: In this paper the early research on the thermal aspects of wear, where most of the attention had been on deducing the “hot spot” temperatures which occur between sliding surfaces, is briefly reviewed. More recent work in which the effect of oxide films on hot spot temperatures is deduced is then described. The division of frictional heat is also discussed. A surface model is used in which N individual contact areas (of radius a) occur at any given time, each with an oxide film of thickness ξ formed at the contact temperature Tc which occurs during the time of contact. By using an iteration process involving N, a, ξ and Tc and experimental measurements of heat flow down the pin of a pin-on-disc wear test rig, it is shown that the resulting ξ values are consistent with the measurements of oxide film thickness. Recent experiments with photography and IR video recording are then described, which give some measure of validity to the calculated values of N, a and Tc.
TL;DR: In this paper, a model of the unsteady heat conduction process in a multidisc wet clutch embodying alternately assembled discs of two types: discs made entirely of steel and discs with a steel core lined with layers of friction material.
Abstract: The subject of the considerations in this paper is a multidisc wet clutch embodying alternately assembled discs of two types: discs made entirely of steel and discs with a steel core lined with layers of friction material. A model of the unsteady heat conduction process in such discs has been developed with account taken of the non-homogeneity of the thermal properties of these discs. Heat conduction equations have been derived and initial and boundary conditions given. To solve this problem, the finite difference method was used. Next, the thermal stresses in the steel discs, regarded as quasi-static, were considered. The stresses were calculated by the finite element method. Calculations of the temperature fields and thermal stresses were performed for a specific typical clutch engagement. Discussion of the results obtained is presented. In the analysis of the temperature distributions and stresses, attention has been focused on the determination of factors permitting a reduction in stresses.
TL;DR: In this paper, the authors showed that the formation of α'-martensite in 304 steel gives a lower average value of friction coefficient with large fluctuations compared to 304 steel during early stages of sliding, whereas the friction continues to rise smoothly to a steady state value for 310 steel.
Abstract: Blocks of 304 (metastable) and 310 (stable) austenitic stainless steels were tested in argon against M2 tool steel rings. Scanning, transmission and scanning transmission electron microscopy analysis, together with microhardness measurements, showed that the strain-induced martensite transformation in 304 steel affects the form and composition of the wear debris and the nature of the transfer layer. The hardness of the transfer layer relative to the hardness of the adjacent deformed base material is also important, as shown in earlier work on Cu-Be. During early stages of sliding, the friction values for both alloys are similar, but whereas the friction continues to rise smoothly to a steady state value for 310 steel, the formation of α'-martensite in 304 steel gives a lower average value of friction coefficient with large fluctuations.
TL;DR: In this article, a polytetrafluoroethylene-impregnated Al 2 O 3 -TiO 2 coating on Ti-6Al-4V was evaluated against various coatings and surface treatments applied to the alloy.
Abstract: A Ti-6%Al-4%V alloy (Ti-6Al-4V), in the solution-treated overaged condition, was subjected to fretting exposures against uncoated Ti-6Al-4V as a baseline and against various coatings and surface treatments applied to Ti-6Al-4V. Fretting experiments were conducted in dry air and in saturated air by using a hemisphere of radius 4.7 mm on a flat surface. Typical conditions included a 1.47 N normal load, an 80 Hz frequency and a 70 μm amplitude at room temperature. The coatings evaluated included plasmasprayed tungsten carbide with 12% Co, aluminum oxide (Al 2 O 3 ) with 13% titanium oxide (TiO 2 ), chromium oxide, aluminum bronze with 10% aromatic polyester, polymer-bonded polyimide, polyimide with graphite fluoride, polyimide with molybdenum disulfide (MoS 2 ) and methylphenylsiliconebonded MoS 2 and the surface treatments included a pre-oxidation surface treatment, a nitride surface treatment and a sputtered MoS 2 coating. Results of wear measurements on both the coated and the uncoated surfaces after 3 × 10 5 cycles indicated that the polyimide coating was the most wear resistant and caused the least wear to the uncoated surfaces. Increased exposure, however, showed a sharp increase in fretting wear rate after 10 6 cycles. Surface-nitrided Ti-6Al-4V fretted against a polytetrafluoroethylene-impregnated Al 2 O 3 -TiO 2 coating on Ti-6Al-4V resulted in immeasurable wear to both surfaces.
TL;DR: In this paper, the machinability of an Al-40vol.%SiC composite alloy was evaluated using a large collection of experimental and commercial cutting tools, and it was observed that the wear rate of tools harder than silicon carbide was inversely proportional to their hardness.
Abstract: The results of a machinability study of an Al-40vol.%SiC composite alloy, using a large collection of experimental and commercial cutting tools, are described in this report. It was observed that the wear rate of tools harder than silicon carbide was inversely proportional to their hardness. All cutting tools evaluated, however, wore rather rapidly and only polycrystalline diamond tools provided somewhat useful tool life. The expected production cost of a precision part from this alloy would be high because of the short tool life and relatively high price of the only usable cutting tool for this application.
TL;DR: In this article, a functional relationship was developed between the single-pass wear rate under severe abrasive conditions (dominated by microcracking events) and a term considering the hardness and macrofracture energy of the composites as well as a probability factor for micro-cracking.
Abstract: Single-pass two-body abrasion tests were run on unfilled and glass-fibre-and glass-sphere-filled polyethylene terephthalate sliding dry against different abrasive papers. The abrasive grains varied in hardness and size. Wear mechanisms were studied by using scanning electron microscopy. A functional relationship was developed between the single-pass wear rate under severe abrasive conditions (dominated by microcracking events) and a term considering the hardness and macrofracture energy of the composites as well as a probability factor for microcracking. The probability factor includes parameters of the abrasive counterbody and some geometrical and frictional details of the composite investigated.
TL;DR: Further laboratory wear studies of PTFE on stainless steel in disc machines are reported and 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.
Abstract: 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.
TL;DR: In this paper, approximate solutions for the theoretical flash temperatures at circular and elliptical contact spots between two homogeneous materials when heat is evolved at the (assumed to be mathematically flat and uncontaminated) interface at a uniform rate, either independent of velocity as in the case of Joule heat or in proportion with the relative speed, i.e. as is the case for friction heat.
Abstract: Approximate solutions, reliable within a few per cent, have been obtained for the theoretical flash temperatures at circular and elliptical contact spots between two homogeneous materials when heat is evolved at the (assumed to be mathematically flat and uncontaminated) interface at a uniform rate, either independent of velocity as in the case of Joule heat or in proportion with the relative speed, i.e. as is the case for friction heat. The parameters appearing in the corresponding equations have been expressed in terms of known materials properties plus the number of contact spots and their axis ratio. The accuracy with which the flash temperatures can be known in practice is limited by uncertainty regarding those parameters and by the idealizations embodied in the model. Those theoretical uncertainties are probably no larger than the actual momentary fluctuations in the conditions at the contact spots.
TL;DR: In this article, the effects of turbulent diffusion on particle dispersion, and hence on erosion, are demonstrated numerically, and the relative rate of erosion decreases and the location of maximum wear is displaced toward the stagnation point as the turbulence intensity increases.
Abstract: Very few investigations on wear by particle impact have accounted for the influence of turbulence on erosion. In this study, the effects of turbulent diffusion on particle dispersion, and hence on erosion, are demonstrated numerically. Variations imposed on the level of turbulence intensity in a particle-laden jet impinging normally on a flat wall show a strong dependence of wear parameters on this quantity. Impacting particle velocities, trajectories and surface densities are predicted from lagrangian equations of motion. Eulerian equations are used to describe fluid motion, with the turbulence viscosity evaluated from a k-ϵ model of turbulence. Non-essential complexities are avoided by assuming one-way coupling between phases and using an adjusted Stokes' drag relation. Numerical calculations reveal that the relative rate of erosion decreases and that the location of maximum wear is displaced toward the stagnation point as the turbulence intensity increases. Efforts to remove present model limitations have been hindered by the unavailability of suitable experimental data.
TL;DR: In this paper, the effect of microstructure on the erosion of AISI-SAE 1078 and 10105 steels by 240 grit A1 2 O 3 particles was investigated at particle velocities V of 40-100 m s −1 and angles of impingement a of 10°-90° relative to the target surface.
Abstract: The effect of microstructure on the erosion of AISI-SAE 1078 and 10105 steels by 240 grit A1 2 O 3 particles was investigated at particle velocities V of 40–100 m s −1 and angles of impingement a of 10°–90° relative to the target surface. The microstructures investigated included spheroidite, pearlite, martensite and tempered martensite. Spheroidite and pearlite microstructures eroded by the ductile mode at all velocities, exhibiting a maximum erosion rate at an impingement angle of 40°. The effect of the angle of impingement on the erosion rate of martensite and tempered martensite varied with particle velocity, the erosion mode tending towards a brittle mode with increasing velocity. At all angles of impingement the erosion rate tended to increase with the volume fraction of Fe 3 C. Examination of the eroded surfaces by scanning electron microscopy showed the occurrence of localized plastic flow of appreciable magnitude. No subsurface cracking or void formation was evident. The erosion rate E r could be considered to vary with particle velocity according to the power law E r = kV n where n has a value of about 2 independent of the microstructure and the angle of impact.
TL;DR: In this paper, the authors compare the performance of two brake dynamometers, the inertial dynamometer and the Chase dynamometer, on several friction pairs under various temperature, sliding speed and load conditions.
Abstract: For effective and safe automotive brake system applications, friction materials must meet certain minimum requirements for performance, noise and durability. To ensure this, friction materials are subjected to a series of vehicle tests on a test track or on the road before they are released as commercial products. While vehicle tests are expensive, time consuming and subject to road conditions and weather variability, brake dynamometer testing in the laboratory is faster and less costly to screen or verify friction material characteristics. Furthermore, because of the capability to control test conditions precisely, a brake dynamometer serves as an excellent tool for the research and development of friction materials. In the U.S.A. two major types of brake dynamometers are commonly used: the inertial dynamometer which evaluates a full size brake or a brake system and simulates vehicle braking well, but is time consuming and expensive, or a smaller Chase dynamometer that features low capital expenditure and shorter test time using a small friction material sample against a large drum. The Chase dynamometer does not simulate brake conditions as well as the inertial dynamometer, and therefore is used primarily for rapid screening and/or for quality control only. The correlation, or the lack of it, between these two brake dynamometers is discussed in this paper. Friction and wear data from both test systems on several friction pairs under various temperature, sliding speed and load conditions are compared and discussed. The materials used were (a) non-asbestos organic disc pads against cast iron rotors, (b) semimetallic disc pads against cast iron rotors and (c) non-asbestos organic disc pads against copper rotors.
TL;DR: In this article, an investigation of the friction and wear behavior of two commercial Fe-12%Cr-base alloys, Rex 535 and Jethete M.152, during like-on-like reciprocating sliding in air at ambient temperatures of 300-600°C has been undertaken.
Abstract: An investigation of the friction and wear behaviour of two commercial Fe-12%Cr-base alloys, Rex 535 and Jethete M.152, during like-on-like reciprocating sliding in air at ambient temperatures of 300–600°C has been undertaken. In all cases, the coefficients of friction and wear rates were considerably less than during sliding at lower temperatures. Coefficient of friction and contact resistance measurements indicated rapid breakdown of any thermally formed oxide present on the surfaces at the commencement of sliding. However, a smooth adherent wear-protective oxide developed on the load-bearing regions after a very short period of sliding. This was formed from oxide debris produced by removal of oxide from the metal surfaces and by oxidation of metal wear debris. This debris was fractured, comminuted, compacted and plastically deformed by the sliding action. The deformation was facilitated by the very small size of the oxide particles and asperities and the influence of the hydrostatic pressure component of the stress field during sliding. Although wear-protective oxides were able to develop at temperatures below 300 °C, they were not stable under the sliding conditions used and broke down, enabling metal-metal contact to occur.
TL;DR: In this paper, two-dimensional finite element calculations are used to describe the interaction between a hard rigid asperity and an as perity with an elastic-plastic material behaviour.
Abstract: Two-dimensional finite element calculations are used to describe the interaction between a hard rigid asperity and an asperity with an elastic-plastic material behaviour. In the calculations the hard asperity is moved through the soft asperity while the separation of the surfaces is kept constant. The normal and shear forces and the friction coefficient are determined for different separations. The influence of adhesive friction in the contact zone and the influence of the radii of the asperities are also investigated. The finite element model is compared with an analytical model.
TL;DR: The friction and wear behavior of the graphite-fiber-reinforced glass matrix composite system is described in this paper, where both the friction coefficient and wear results are described as a function of fiber elastic modulus, fiber orientation and test configuration for unlubricated sliding.
Abstract: The friction and wear behavior of the graphite-fiber-reinforced glass matrix composite system is described. By reinforcing borosilicate glass with graphite fibers it has been possible to develop a composite whose friction coefficient and mechanical properties can be compared favorably with those of resin matrix composites and whose wear resistance and environmental stability can be comparable with those of glasses and ceramics. Both friction and wear results are described as a function of fiber elastic modulus, fiber orientation and test configuration for unlubricated sliding.
TL;DR: In this article, the authors used thermal strain analysis to predict the lifetime of railroad wheels experiencing hot spotting, a phenomenon known as hot spotting on the tread surface as braking occurs, and calculated the thermal strains of these hot spots.
Abstract: As a railroad wheel is heated during a brake application, a compressive stress is developed in the tread surface. Free thermal expansion is partially or completely constrained by the cooler underlying material. If yielding of the surface material occurs, a residual tensile stress could be developed. Constraint of thermal expansion can cause stresses which may eventually initiate and propagate fatigue cracks. Constraint of free thermal expansion is a necessary ingredient in the thermal fatigue process. The number of cycles that produces cracks would classify this process under the more general heading of “low cycle fatigue”. It has been observed that a phenomenon known as “hot spotting” develops on the tread surface as braking occurs. These areas of contact, roughly 20–40 mm in diameter, experience high localized stresses as a result of the severe temperature gradients associated with them. Thermal strains of these hot spots were calculated and in the case of total constraint all this strain is converted to mechanical strain. Thermomechanical fatigue testing provided the necessary data for the fatigue analysis. The result of this work provides a life prediction of railroad wheels experiencing hot spotting.
TL;DR: In this paper, an insulated, rigid, flat-ended punch moving steadily over the surface of a halfplane is considered and the effect of thermal distortion on the contact pressure distribution is found and the temperature profile under the punch is given.
Abstract: An insulated, rigid, flat-ended punch moving steadily over the surface of a half-plane is considered. It is assumed that the motion has been sustained for a sufficiently long period for a steady thermal state to have been achieved, the source of heat being the energy dissipated at the interface due to Coulomb friction. The effect of thermal distortion on the contact pressure distribution is found and the temperature profile under the punch is given. A closed-form solution is found for the case of large Peclet number.
TL;DR: In this article, a model was described to account for the wear rates, coefficients of friction and surface features observed when fretting a 0.64% C high tensile roping steel in seawater.
Abstract: A model is described to account for the wear rates, coefficients of friction and surface features observed when fretting a 0.64% C high tensile roping steel in seawater. The work includes a comparison of the fretting behaviour, both in air and in selected aqueous environments, with that of an austenitic stainless steel. When fretting in air, mechanical and oxidational components are suggested, although in aqueous solutions an additional electrochemical corrosion component becomes important. Although a liquid can help to reduce friction by acting as a lubricant, the effects of which are most noticeable in the early stages of fretting, under most circumstances the liquid will give rise to a corrosion process which accounts for high wear rates and ultimately to increasing coefficients of friction through the accumulation of trapped corrosion debris. Only in solutions of low ionic conductivity and/or very low dissolved oxygen concentration can the corrosion rate be reduced, but only by the application of a suitable level of cathodic protection can it be eliminated altogether to achieve desirable friction and wear behaviour. A number of experiments were carried out in artificial seawater to give some indication of the fretting corrosion behaviour of roping steels used in offshore environments.
TL;DR: In this article, the effect of different modes of aeration and of pyrrhotite addition on the ball wear using mild steel, high carbon low alloy steel and austenitic stainless steel balls was evaluated.
Abstract: The relative significance of corrosive and abrasive wear in ore grinding is discussed. Laboratory marked ball wear tests were carried out with magnetic taconite and quartzite under different conditions, namely dry, wet and in the presence of an organic liquid. The effect of different modes of aeration and of pyrrhotite addition on the ball wear using mild steel, high carbon low alloy steel and austenitic stainless steel balls was evaluated. Results indicate that abrasive wear plays a significant role in ore grinding in the absence of sulfides, and rheological properties of the ore slurry influenced such wear. The effect of oxygen on corrosive wear becomes increasingly felt in the presence of a sulfide mineral such as pyrrhotite. Wear characteristics of the three types of ball materials under different grinding conditions are illustrated.
TL;DR: In this paper, a general three-dimensional theory for thermomechanical cracking in an unflawed semi-infinite elastic medium from high speed asperity excitations is presented.
Abstract: In this paper the problem of thermomechanical cracking in an unflawed semi-infinite elastic medium from high speed asperity excitations is expounded. The paper is in three parts in which a general three-dimensional theory is established, the asperity characteristics with a single moving asperity are examined and the cumulative effect of repeated asperity excitations is analyzed. The need for a more realistic but more complicated three-dimensional formulation than the two-dimensional formulation is also presented. The general mathematical solutions are left in the expressions of double Fourier transforms, from which special solutions can be obtained by numerical inverse transforms. The general solutions are applied to the problem of a single moving asperity principally to study the various effects of the contact area configuration and of the uniformity of load distribution over the contact area. It is found that contact areas of the same asperity width (in the direction of motion) and of the same aspect ratio result in nearly the same stress state regardless of their shape. However, contact areas of higher aspect ratio, while having the same surface temperature, result in a lower temperature gradient at the subsurface and thus a lower stress there. In the limit, with the same uniformly distributed load, a two-dimensional model (aspect ratio approaching infinity) requires a load almost seven times that of a circular asperity contact area to initiate fracture. Finally, for the same total load, a nonuniformly distributed load causes higher stress than a uniformly distributed load. In the problem of repeated asperity excitations the cumulative effects in terms of the period and the number of repetitions are studied. At a periodic spacing of 600 asperity widths, neither the mechanical effect nor the thermal effect is cumulative. At a periodic spacing of six asperity widths, while the mechanical effect is non-cumulative, the thermal effect of preceding excitations does influence the temperature field, resulting in higher stress with increasing repetitions, approaching an upper bound. A numerical example shows that, for a single moving asperity to initiate cracking at a friction load of 98 MPa (14200 lbf in−2), a repetition of three asperities at a spacing of six asperity widths is sufficient to initiate cracking at a friction load of only 44 MPa (6350 lbf in−2).
TL;DR: In this article, Japanese valve train wear engine tests were carried out using oils containing a common zinc dialkyldithiophosphate (ZnDTP) and the results showed that the zinc decomposition results in the formation of zinccontaining compounds and phosphorus-containing intermediate products such as tetraalkylthioperoxidiphosphate.
Abstract: Japanese valve train wear engine tests were carried out using oils containing a common zinc dialkyldithiophosphate (ZnDTP). The ZnDTP decomposition products formed in the oils during the engine tests were analysed. It was found that the ZnDTP decomposition results in the formation of zinccontaining compounds and phosphorus-containing intermediate products such as tetraalkylthioperoxidiphosphate. During an engine test, the ZnDTP decomposes rather quickly, whereas the phosphorus-containing intermediate products were detected over a long period. The adsorption test results indicate that diesel soot adsorbs the zinc compounds but that the soot adsorbs little of the phosphorus compounds. From the valve train wear test results and four-ball wear test results it was found that the oils tested in the engine keep their antiwear performance after the ZnDTP has decomposed and that this performance is mainly attributable to the phosphorus-containing compounds.