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Secondary air systems of civil aircraft engines require labyrinth seals with a minimum gap clearance for optimal functioning. In the case of high speed contact during the engine running-in period, an abradable material is deposited on the stationary part of the seal to limit the damage of the rotating shaft, which is made of a titanium alloy. Such situations are potentially critical for the seal; hence, the present study aims to observe the material behaviour during these contact conditions and to establish the tribological circuit of a third body through the interface. A high-speed contact test rig was developed to recreate contact conditions occurring in an aircraft engine. Two contact configurations occurring in different locations of the engine, with different materials and surface areas were explored. Thermal and mechanical instrumentation were used in each test. The influence of the contact geometry and the test conditions show that material flows through the contact determine the life cycle of the contact (by establishing a balance between the source, internal and material flow) and allows for the control of the thermomechanical constraints in a high-speed contact.

Lubrication is often employed in fretting contacts to reduce wear and stresses associated with high friction. Owing to the very small displacements associated with fretting, penetration of lubricating oils into the contact may not be effective. The efficacy of the penetration of the lubricant into the contact is very difficult to observe experimentally, and accordingly, this paper presents a numerical simulation of a lubricated fretting contact using a Coupled–Eulerian–Lagrangian (CEL) finite element method. Meso-scale CEL finite element models are developed to simulate the cylinder-on-flat arrangement used experimentally at the University of Nottingham in which the roughness of contact surfaces is characterized as fractal geometry by the Weierstrass-Mandelbrot (W-M) function. The fluid–solid and solid–solid contact in the lubricated fretting contact are simulated, and from these, wear and friction coefficients are determined. The effects of contact geometry on lubricated fretting contacts and lubricant on fretting wear are modelled and compared with experimental observations. Results indicate that oil lubrication reduces fretting wear and friction effectively in the less-conforming contacts but has little effect in the more-conforming contacts.

A new framework which describes the role of three key processes in fretting wear of metals is proposed, with these three processes being: (i) oxygen transport into the contact; (ii) formation of oxide-based wear debris in the contact and (iii) ejection of the wear debris from the contact. Based upon a physical understanding, rate equations for the three key processes are proposed, which provide a basis for the influence of test parameters (such as contact geometry, applied load, slip amplitude, fretting frequency etc) on the three rates to be understood. To maintain system equilibrium in steady-state fretting, the three processes must operate at the same rate as each other (debris cannot be ejected from the contact faster than it is formed, and debris cannot be formed faster than it is ejected). Accordingly, the observed wear rate is the rate of the process with the lowest rate of the three processes, with this process being termed the rate-determining process. The effect of test parameters on the three key processes differs, and thus the effect of changes in any test parameter on the observed rate of wear will itself be dependent upon which of the three processes is rate-determining. A number of assumptions have been made in deriving the equations which describe the key processes and it is recognised that these equations themselves may be refined in light of future research; however, any such revised equations can simply replace those proposed as part of the rate-determining process framework. The framework can be applied to both conforming and non-conforming contact geometries. In tests with non-conforming contact geometries, the contact size increases with wear; since it is proposed that the rates of two of the three processes (oxygen transport and debris ejection) are dependent upon the size of the contact, the rate determining process can change during such a test and the rate of wear will continually fall when either of these processes are rate-determining. This complexity means that the evolution of the wear volume can only be evaluated numerically through the use of a time-marching analysis in such cases. In contrast, in tests with conforming contact geometries, the contact size does not change as wear occurs, and so there will be no changes in the rate-determining process as wear proceeds. It is recognised that this framework addresses wear under steady-state conditions and does not consider the initial period of exposure where steady-state conditions are being developed. In the case of conforming contacts where the contact size is large, the duration of the initial transient period may be substantial and may form a significant proportion of the test duration or component lifetime. This needs to be recognised both in the design of tests and in the application of this framework.

Different origination theories for squat-type rail defects are examined and confronted with experimental evidence. Based on the morphology of the three-dimensional defect crack pattern, with a leading crack directivity governed by the tangential stress history, it is shown that theories assuming dynamic wheel-rail interaction as a direct initiation mechanism of the double-lobed defect violate the causality principle. Rail surface anomalies of three categories increase the tangential stress exposure and thus the risk of defect development: pertaining to the material properties, contact geometry (including both the global scale, involving the dynamic stress level transmitted by the contact area, and the local scale, involving transient stress redistribution within this area), and contact stiffness. Both detection measures focusing on geometrical surface deviations giving rise to dynamic wheel-rail interaction (such as axle-box acceleration measurements) and the idea of a critical diameter for such imperfections are inadequate. Instead, and apart from the surface geometry, steel micro-cleanliness and chemical composition, phase transformation mechanisms of surface material (due to operational conditions or treatment by grinding or milling) and welding deserve attention. Fluid entrapment remains a potentially contributing factor in early growth, while crack face oxidation and corresponding volumetric expansion may contribute at any stage after initiation.

Water-lubricated ceramic bearings are currently being used in engineering applications like water pumps, air compressors and marine systems, due to their higher efficiency, reduced environmental impact and simpler constructive designs. In search of low-cost and high-performance tribosystems, bearings made of dissimilar ceramics are not frequently studied, but have potential to show good tribological compatibility. In this study, tribological behavior of two dissimilar ceramic pairs, ZrO2-Al2O3 and ZrO2-ZTA (10 vol% ZrO2), was investigated. Water-lubricated ball-on-disc tests under loads ranging from 6.0 to 24.6 N and sliding speeds from 0.1 to 2.0 m/s were performed. Worn surfaces were characterized by optical microscopy, SEM and contact-type profilometry. Thermal (Sc,t) and mechanical (Sc,m) severity of contact models explained the mild to severe wear transition as function of tribosystem's variables such as load, speed, distance, materials’ properties, contact geometry and environment. During mild wear regime, wear rates were smaller than 10⁻⁷ mm³/Nm, friction coefficients remained low (0.1

In a dynamic radial shaft sealing (RSS) system good wear resistance is crucial for proper functionality. This contribution is dedicated to the analysis of wear in radial shaft seal applications in a sealing system itself and on a tribologically equivalent system. To make elastomers more wear resistant, new and innovative material concepts and compounds have to be evaluated. To do so a simple tribologically equivalent system, the ring-cone-tribometer (RFT), consisting of an elastomer ring and a coned shaft, was used. Two different radial shaft seal types were compared under identical test conditions on a multi-shaft test bench, to the results generated on the RFT. This was done to evaluate and compare the tribological properties of the RFT towards RSS. To avoid ageing or batch-to-batch influences the slabs for the production of the ring samples were molded from the same elastomer material batch as the radial shaft seals. Initial differences in terms of wear, friction, contact width and hardness between RSS and RFT were reduced step by step using an optimized, stiffer test setup on the RFT and a FE-simulation model to optimize the contact geometry. Data scattering on the RFT was reduced to a minimum so that the higher scatter within the results of the RSS test population was in a similar dimension as the deviation in both systems. To determine the wear during a test run, an online-wear measurement system, based on eddy current sensors, was developed that allows for the online estimation of seal wear progress on the RFT at any time. This process is based on a geometrical approach using the initial contact width and the axial displacement during the operation. The online-wear measurement correlates very well with the post-mortem wear measurement. All results form a set of tools which constitutes a solid foundation for online evaluation of rubber and seals in the future.

A continuum mechanics approach is applied to model fretting wear in gross-slip regime with accounting for the third-body layer, which is formed by wear debris trapped within contact interface. The developed modeling framework employs the known concept of a third-body conversion factor and utilizes it in a self-similar form. A two-dimensional Hertzian type contact geometry (cylinder on a plate of relatively large thickness) is assumed, though a major part of the presented analysis is curried out for an arbitrary initial gap function. A Winkler foundation model is used to describe the contact deformation of the third-body layer. The effect of elasticity is neglected in describing the self-similar evolution of the contact pressure profile in the advanced stage of fretting wear. A comparison with the FEM-based simulations available in the literature is given. The fact that the elasticity effect plays a minor role has been verified a posteriori in the considered example. Simple formulas are derived to estimate the continuous variation of the third body layer profile, thickness, and volume.

This paper investigates the effect that contact geometry on fretting wear rates and mechanisms for non-conforming contacts. Tests have been conducted in the gross sliding regime under a constant normal load in a cylinder-on-flat configuration with a high strength steel for a range of cylinder radii. Two distinct damage mechanisms have been identified, with the operative mechanism depending upon contact geometry. Fretting of less conforming contacts results in higher wear rates compared to that associated with more conforming contacts where the wear rate is significantly lower (between 20% and 50% of that of the less conforming contacts) in addition, much higher levels of adhesive transfer are observed in more conforming contacts, with beds of metallic debris particles being evident. It is proposed that the dependence of wear rate and mechanism on the contact geometry is not associated with differences in contact pressure (although these do exist), but is associated with the influence of contact geometry on debris flow out of the fretting contact and on the effectiveness of oxygen being excluded from the centre of the fretting contactpatch.

This study considers the approaches of Blok and Jaeger for the estimation of maximum and average temperatures in sliding elliptical contacts with both uniform and semi-ellipsoidal (Hertzian) heat distributions. The accuracy of each of these methods, which are based on single-point temperature matching between contacting bodies, is assessed relative to a numerical solution of the heat partition problem developed in a previous work, which imposed temperature matching at all nodal points. Comparisons are made for a wide range of Peclet numbers, as well as for moderate ranges of thermal conductivity ratio and elliptical ratio. It is found that the application of Blok's hypothesis yields remarkably accurate predictions of the maximum interfacial temperature, with typical errors less than 3%, whereas the hypothesis of Jaeger leads to predictions of the average interfacial temperature that have typical errors of around 6%. The authors also assess the accuracy of approximate formula developed by Tian and Kennedy to predict the maximum temperature at the interface for the case of sliding circular contacts and find the error to be no more than 2.6% for the full range of Peclet number. Further, the authors of the current study use fundamental heat source solutions developed by Tian and Kennedy to arrive at formulae for average temperature rise for circular contacts that are analogous to the Tian and Kennedy maximum temperature rise formulae. It is found that the formulae for computing the average interfacial temperature rise are also quite accurate, but have slightly more error than the maximum temperature rise formulae. Finally, in the present work, extensions are suggested to the maximum and average temperature rise formulae of Tian and Kennedy to include the effects of elliptical contact geometry. It is found that these formulae are at least 91% accurate for elliptical ratios between 0.4 and 5.0.

The wear behaviour of thin steel wires has been analyzed under oscillating sliding conditions in crossed cylinders contact geometry. The focus of this analysis was the influence of the crossing angle between the wires on the wear. The wires used had 0.45 mm in diameter and the material was cold-drawn eutectoid carbon steel (0.8% C) with a tensile strength higher than 2800 MPa. Two different types of tests were carried out, the first one representing the influence of the crossing angle for a constant load and the second one representing the influence of the crossing angle with constant contact pressure. In the first type of tests it was seen that as the contact angle decreases the contact pressure decreases too and hence less energy specific wear resistance is observed. As a consequence less wear is produced, thus increasing the life of the wires. In the second type of tests it was seen that with constant contact pressure but different crossing angles, nearly the same energy specific wear resistance was observed. This points at an identical wear behaviour in both type of tests but with a running-in and a steady state period as two different wear periods. The tests showed that the running in period may play an important role in the overall wear particle generation and hence the wear occurring in the steady state period is rather mild.

Fretting wear is a surface degradation process induced by small-amplitude oscillatory movements between contacting bodies. It may result in critical wear, especially in the blade/disk contact of aero-engines. To focus on this industrial issue, an experimental layout was designed to perform fretting wear tests up to 500 °C. A representative punch (Ti17)/plane (CuNiIn plasma coating) interface was investigated under air conditions. Wear regimes were identified regarding variations of parameters such as pressure, sliding amplitude and temperature. Chemical analyses (EDS, XRD, XPS) performed on fretting scars enabled the characterization of degradation mechanisms. A quantitative description based on an energy wear approach is introduced to predict the wear kinetics as a function of contact loading parameters. It is shown that temperature plays a minor role. By contrast, the mean pressure appears as a key factor, modifying the interface structure and consequently the wear kinetics: the higher the mean pressure, the higher the wear rate. Surface investigations show that above a threshold mean pressure ( p _th), Ti transfer from the punch toward the CuNiIn plasma coating is activated. The punch interface is then characterized by a composite structure displaying a central nitrided TTS layer, surrounded by an oxidized rim. The activation of the nitrided TTS structure is shown to be function of the mean contact pressure and the contact geometry. Indeed, the “nitriding process” which is induced by an over plastic strain accumulation, depends on the local concentration of oxygen within the interface. When most of the oxygen of the air is consumed in the lateral rim of the interface (oxided third body), the “contact nitriding process” takes place. Hence the global wear kinetics is greatly modified depending on whether the pressure is below or above the pressure threshold. However it is shown that considering a local energy wear approach and taking into account the Ti transfer layer on the CuNiIn surface, the global wear rate obtained under high pressure conditions (i.e. p > p _th) can be predicted using the energy wear rates identified under low pressure conditions ( p p _th).

This paper presents the experimental results obtained from the scratching of semicrystalline poly(etheretherketone) (PEEK). Various surface mechanical properties of the PEEK such as the hardness, the friction coefficient, the scratch depth and the prevailing deformation mechanisms are presented in the form of scratching maps, generated as a function of the contact conditions. These scratch maps provide a first order evaluation of the relative scratch resistance of materials for comparison purposes. The penetration depth dependent scratches (in the range 20–200 μm) were produced on the PEEK surfaces using rigid cones of different cone included angles and under different normal loads. The prevailing deformation mechanism and geometry of damage on the scratched polymeric surfaces were assessed using scanning electron microscope (SEM) and an optical profilometer. The deformation modes were seen to be influenced by the imposed experimental parameters; such as the contact geometry, the load, the nominal sliding velocity, the contact temperature and the contact lubrication. In addition to the common deformation mechanisms observed in amorphous polymers; ranging from elastic, ironing, viscoelastic plastic ploughing and brittle deformations, a fibrillation of the semicrystalline PEEK was seen when the polymeric surface was scratched under the severe contact conditions (the higher contact strains and the normal loads).

Subsurface initiated rolling contact fatigue cracks initiate in the vicinity of material defects. As these exist randomly in the material, fatigue will appear randomly under otherwise constant conditions. By combining statistical methods with a contact mechanics and fatigue analysis, probabilities of fatigue initiation are evaluated. The developed model takes as input contact geometry, contact load and the statistical properties of the material defects. It is shown that under some well-defined presumptions the probability of rolling contact fatigue initiation in a rolling wheel can be expressed by a simple formula. Based on this, the parametric influence is assessed and it is found that the mean material defect size has a large influence. The benefit of material cleanliness testing procedures and the influence of the allowed defect size on the risk of subsurface initiated rolling contact fatigue in railway wheels are then quantified with the aid of numerical simulations.

The important published papers on railway wheel profile wear in the past are reviewed and have been highlighted. A wheel profile wear prediction methodology is developed to improve an extant model and used to predict the wear of the wheels of railway vehicle operating on sharp curved tracks. The methodology includes the coupling dynamics of railway vehicle and track, the three-dimensional contact geometry analysis of wheel/rail, Kalker's non-Hertzian rolling contact theory, and Archard wear model. The normal loads, creepages and lateral displacements of the wheel/rail are first obtained through the coupling dynamics analysis of the vehicle/track. Then the wheel/rail contact geometry calculation is carried out to get the normal gap between the undeformed wheel/rail. Based on the attained parameters, the wheel–rail rolling contact is calculated using the modified Kalker's non-Hertzian theory, and the normal stress, slip and contact areas are obtained. Afterwards, the Archard wear model is used to calculate the wear depth on the wheel tread, and the two means, which are called the smoothing spline and the Super-smother, are used to smooth the wear distribution curve and the updated wheel profile due to the accumulated wear after a number of passages. A numerical example is presented to verify the present effective methodology. The obtained numerical results are reasonable, and indicate that the numerically reproduced wear phenomena of the wheels of the vehicle are consistent with those occurred at railway sites.

The scratch behaviour of an amorphous polymer was investigated to determine how the characteristics of the material affect the scratch resistance. A thermoplastic polymer (polymethylmethacrylate) was studied using both an experimental device allowing in situ observation of the contact area during scratching with spherical tips and a numerical approach using finite element modelling (FEM). The rheological properties of the PMMA surface were approximated by a simplified bilinear law, while the friction at the interface between the indenter and the material was modelled with Coulomb's local friction coefficient μ ad varying between 0 and 1, for each computed ratio a / R from 0.1 to 0.6 (where a is the contact radius and R the radius of the tip). FEM results for elastic–plastic contacts indicate that the contact geometry is directly related to the plastic strain field in the deformed volume beneath the indenter during scratching. The dimensions of the plastically deformed volume and the gradient of plastic strain both are shown to depend on the ratio a / R and also on the local friction coefficient μ ad . An equivalent average plastic strain is calculated by FEM over a representative plastically deformed volume. The average plastic strain increases with a / R , as predicted by Tabor's empirical rule, and with the local friction coefficient for a given ratio a / R . Clear correlations are demonstrated between the average plastic strain and the geometrical parameters classically used to describe the shape of the contact area.

Tangential traction caused by friction in contacting surfaces is a major factor in fretting fatigue that increases stress levels and leads to a reduction in fatigue life. Friction in fretting contact was studied in partial, mixed and gross slip conditions on quenched and tempered steel. Measurements were made with sphere-on-plane contact geometry for polished and ground surfaces. Friction was evaluated from on-line energy ratio and, after the tests, from wear marks. A maximum friction coefficient of over 1.0 was measured at mixed slip zone with polished surfaces, whereas ground surfaces promote lower values in similar operating conditions. The friction coefficient dependence on load cycles and loading frequency is also presented and briefly discussed. The friction data and understanding thus gained is to be used for evaluation of crack initiation with the numerical fretting fatigue model.

Wear behaviour of titanium alloy, Ti–6Al–4V under fretting-reciprocating condition was characterized under different loading conditions, contact surface conditions and contact configurations. Six series of tests were analyzed to investigate the effects of relative slip, contact load, number of cycles, bulk cyclic stress, contact geometry, and a lubricant. Dry and lubricated surface conditions produced “W” (scar with multiple valleys) and “U” (scar with single valley) shaped scars on both specimen and pad, respectively. WV (wear volume) was linearly related to the cumulative product of contact load and relative slip and to the total dissipated energy under different loading and sliding conditions for the cylinder-on-flat as well as flat-on-flat contacts for dry and lubricated conditions. The microscopy of wear scars showed that the wear was caused by both the adhesion and abrasion processes.

Previous studies by the authors have developed finite element (FE) based approaches to simulate fretting wear by material removal without consideration of the effects of debris. It was assumed that, wear particles, once generated, were immediately removed from the fretting interfaces and disappear. This method calculates the local wear as a function of local contact pressure and local slip, and incrementally updates the contact geometry with material removal. However, in reality, wear particles can become entrapped in the fretted regions and undergo a series of processes before they are eventually removed. The effects of trapped debris are important in fretting wear as debris naturally provides a load-carrying plateau. A revised modeling approach for incorporating the salient different effects of debris on the fretting wear process including the formation, trapping and removal of debris is therefore proposed in this study. In this approach the debris accumulated on the fretting interface is modeled as a layer structure with the mechanical properties being described by an anisotropic elastic-plastic material model. Migration of wear particles within the fretting surfaces after their formation is also considered, which determines the evolution of the thickness of the debris layer and its movement. The simulation tool incorporating the debris effects permits the redistribution of contact pressure and slip over the contact region to be estimated to ultimately predict debris effects on wear damage. The predicted results are calibrated and compared with measured fretting wear results from Hertzian fretting tests.

The prediction of fretting wear is a significant issue for the design of high-performance aeroengine spline couplings, due to the potential for slip between the highly loaded spline teeth under cyclic loads. The work reported here aims to demonstrate the feasibility and difficulties of applying a finite element based fretting wear simulation tool, which incrementally updates the contact geometry with material removal, to this problem. The method calculates the local wear that is the wear at each contact node, as a function of local contact pressure and local slip. It is calibrated using wear coefficient data from round-on-flat tests under representative loading conditions. The method has been previously validated against round-on-flat fretting test results for a range of contact loads. The problems of applying the simulation tool to the complex geometry of a helical spline coupling are discussed. The predictions are compared with results from fatigue tests on a reduced scale aeroengine type spline coupling and a previous approach to wear prediction which did not incorporate the evolving contact geometry.

Fretting fatigue is a complex problem and therefore it has generally been necessary to determine fatigue lives experimentally. This paper describes a method that integrates wear modelling with fretting fatigue analysis to permit the prediction of the effects of material removal, due to fretting wear, on fretting fatigue life. A finite element based method is employed to predict the fretting wear-induced evolution of contact geometry, contact stresses and a multiaxial fatigue damage parameter with cumulative damage effects, as a function of slip amplitude, for a laboratory fretting fatigue test arrangement. The results show that the approach is capable of capturing the experimentally observed effect of slip amplitude on fatigue life. The conditions simulated span across a range of partial slip and gross sliding conditions, corresponding to fretting fatigue tests available from the literature, and giving rise to the well-known phenomenon of a critical range of slip amplitude for reduced fatigue life. The methodology demonstrates that this is primarily due to wear-induced pressure redistribution, the nature of which is highly dependent on the slip regime.

In a railway wheel subsurface cracks may form at macroscopic defects under the wheel tread, propagate and, if not managed, cause catastrophic wheel failures. The deformation of such subsurface cracks under rolling contact conditions is the main issue of the current paper. Parametric studies are carried out with a two-dimensional elasto-plastic finite element (FE) model of a part of a wheel containing a subsurface crack. The purpose of the simulation is to study crack face displacements under varying conditions. The influence of crack length, crack location (depth), crack face friction, as well as wheel/rail contact load magnitude and contact geometry is investigated. The numerical results show that mode I deformations are negligible. Further, load magnitudes and contact geometry play crucial roles for the relative tangential crack face displacements. The current study of deformation of subsurface cracks under rolling contact conditions will form the basis for future studies on crack growth prediction.

In this work, the interfacial friction is investigated during a material removal test. Based on specially programmed experiments conducted on 6061 Al at 24 different conditions, it was found that the friction coefficient significantly varies in different temperature ranges. Using optical and transition electron microscopy, these variations were found to be dependent on phase changes within the workpiece crystalline structure and the amount of oxidation on the surface of the workpiece. The interfacial friction was also found to change with the geometry of the cutting tool, where larger tool wedge angles increased the shear stress in the shear plane.

The oscillating wear failure of the materials is frequently observed in various tribo-applications such as bearings, gears, bushes, palliatives, bolted and riveted joints and seals, etc. where oscillations of small amplitude or vibrations are inevitable. In general, the oscillating wear performance of the materials depends on the amplitude of oscillation, its frequency and duration, load, contact geometry, environment, etc. A vast literature is available reporting on the influence of fillers and fibres on the adhesive (unidirectional) and abrasive wear behaviour of polyamides (PAs) but not much on their oscillating wear behaviour. For the present study, PA 11 and its three composites containing short glass fibres (GF) and metallic powdery fillers such as bronze and copper were selected for the investigations. Experiments were carried out on the SRV Optimol instrument in pin on disc configuration, under various loads at room temperature. It was observed that the GF reinforcement greatly improved the friction and wear performance of PA 11. Incorporation of bronze and copper powders in GF reinforced composite further improved the friction and wear behaviour of PA 11. Copper proved to be beneficial filler than the bronze in this context. Worn surface studies were also done to investigate the wear mechanisms. The SEM and EDAX studies on the copper-containing composite indicated that the film transfer on the disc was very thin, uniform and coherent. It showed evidence of Cu transfer on the disc, which was thought responsible for strong adherence of the film to the mild steel disc resulting in the best friction and wear behaviour of the composite.

An understanding of abrasion resistance and the associated surfaces deformation mechanisms is of primary importance in materials engineering and design [Wear (1998)]. Instrumented scratch testing has proven to be a useful tool for the characterisation of the abrasion resistance of material. Although most studies on scratch resistance have been conducted for metals, assuming a purely plastic behaviour, recent studies focus on the effects of elastic and visco–elastic–plastic behaviour on the scratch mechanisms [Tribol. Int. (1998)]. In this study, normal and tangential scratch forces are measured during the scratch experiments using various indenter geometry and abrasive paper, under various temperature conditions (263–293 K). To complete the characterisation of the scratch process, three-dimensional topography of the scratched surface is measured. The comparison between single point scratch and multi-asperity abrasion testing is made possible thanks to morphological analysis of the abrasive paper topography. The single point scratch tests provide a good understanding of the effect of the contact geometry and temperature on the friction coefficient. These results are then used to explain the evolution of the friction coefficient obtained with two abrasive papers under various temperature and load conditions.

The selection of low friction coatings is of great interest to industrial applications. Nevertheless, regarding the lifetime of the coatings, the selection criteria often depend on the experimental apparatus and contact configuration and then cannot be applied to real cases. In this study, we use a model based on the local dissipated energy due to friction under gross slip conditions in fretting wear. Indeed, the maximum value of the local dissipated energy is a unique parameter that takes into account the two major variables in fretting wear experiments: the normal force and the sliding amplitude. Two kinds of contact geometry are tested to assess the stability of the proposed approach: a cylinder on plane contact and a plane on plane contact (which consists of a circular contact surface with rounded edges to remove the discontinuity). This approach is applied to a commercially available polymer bonded MoS2 solid lubricant film used in aeronautical applications to protect titanium surfaces in contact. The effects of normal force and displacement amplitude are investigated. The results show that, even if there is a great difference in contact size and pressure distribution between the two types of contact, the lifetime is the same if the maximum local dissipated energy is the same. Thus, the local variable used is representative of the wear kinetics and the “lifetime versus local dissipated energy” curve is independent of the contact configuration: the proposed approach can then be used to select coatings whatever the contact size.

This paper attempts to reevaluate the tool–chip interfacial boundary conditions in machining, at a fundamental level. Possible applications of fractals to model the tool–chip interface are suggested. Mathematical models are proposed, based on this conjecture, for describing the contact geometry and the proportion of sliding/sticking interfaces. The model assumptions are validated with data obtained from preliminary experiments.

Surface fatigue of rails is a growing concern for modern railways. Increased speed and higher axle loads provoke surface damage (known as headchecks) in shallow curves of conventional lines. A long-term experiment has been launched by the German Railways (DB AG) to specify a target profile with appropriate tolerances for grinding to limit or even prevent, headcheck development. Furthermore, the information is expected on determining an optimal rail maintenance policy by grinding that balances wear and surface fatigue. The objective is to maximize rail service life and consequently optimize the life cycle costs of the rails.

The mechanical behavior and microstructural changes in Ti–6Al–4V were determined in fretting tests, followed by axial fatigue tests. Prior to fatigue testing, specimens were subjected to fretting conditions over a range of contact stresses and fretting displacements. Fretting frequency was 100 Hz. High cycle fatigue (HCF) tests were run at 1000 Hz. The fretting test involved a flat-on-flat, bare Ti–6Al–4V/bare Ti–6Al–4V fretting system. The fretting process typically generated very shallow surface cracks at the ends of the wear scar. Subsequently, these shallow cracks were observed to propagate in axial fatigue tests, reducing the fatigue life significantly. Evidence of frictional heating during fretting was observed in the formation of scale-like oxide in the wear scar. Formation of oxides appeared to increase with increasing contact stress. Increased oxygen content was detected in the near surface regions of specimens. Large near surface deformation was typically observed within the wear scar. The contact geometry and slight tilting of the stationary fretting pad influenced the character of the fretting scar and the fretting-induced cracking. Fracture surfaces exhibited featureless, battered surfaces at the crack origins followed by (a) cleavage-type crack propagation, (b) formation of fatigue striations, and (c) final ductile tearing.

The fretting wear of a tube, which is in contact with a lateral support, is examined experimentally. A fretting wear tester is specifically designed. Elastic springs are used as the support, which can simulate the contact between a spacer grid and a fuel rod in pressurized water reactor fuel. The tubes and the springs are made of Zircaloy-4. The experiments are conducted in air at room temperature. The experimental conditions, i.e. the normal and shear forces on the contact, the slip range and the number of cycles, are set to be the same. To investigate the influence of the contact geometry on the wear, the spring supports have a concave, a flat or a convex contour. The influence on the axial and transverse slip directions is investigated to incorporate the actual tube motion caused by such a flow-induced vibration in the reactor. The wear on the tube is examined by the surface roughness tester, which measures the depth, and the contour of the worn surface of the tube. Since the shape and the distribution of wear are found arbitrary, a method for evaluating the wear volume is proposed using the signal processing technique. It is found that wear can be restrained when the slip direction is transverse, and if the support has a concave contour.

In recent years, synthetic hydrogels have been studied not only as possible replacements for articular cartilage but as platforms for growing cartilage in a damaged joint. In the present study, the tribological behavior of synthesized poly(2-hydroxyethyl)methacrylate (polyHEMA) hydrogels was investigated in a four-factor, two-level designed experiment using a device developed for biotribology research. The contact geometry consisted of a 6-mm diameter stainless steel ball on flat polyHEMA disks. The variables in the designed experiment were (a) applied load, (b) lubrication, (c) hydrogel crosslink density, and (d) degree of hydrogel hydration. Linear oscillating sliding contact tests were conducted for 30 min for each test. The results showed several significant main effects and first-order interactions. Increasing the applied load from 6 to 20 N increased average hydrogel wear by 125%. Increased crosslink density reduced wear by over 60%. And increased hydration resulted in an increase of 130% in wear. The coefficient of friction ranged from a low value of 0.02 to a high of 1.7 while linear wear varied by a factor of over 60. Interactions between (a) hydration and lubrication and (b) hydration and crosslinking on wear were highly significant. The single most important finding from this study was that, as expected, there was no correlation between friction — which is commonly reported in the literature on hydrogels — and wear. Most of the data fell into two groups, namely low wear/low friction and high wear/low friction. These results may be useful in the tribological design of hydrogels for both low wear and low friction.

The damage mechanisms of several widely used plasma-sprayed ceramic coatings were studied by single point scratch tests using diamond indenters. The ceramic coatings investigated were Al2O3, Al2O3/13% TiO2 and Cr2O3. Single scratching on polished virgin surface, repeated scratching over the same track and parallel interacting scratching were investigated. Microstructures of the coatings, their surface and subsurface damage patterns due to the scratching, and the morphology of wear debris were examined. It was found that the unique microstructure of the coatings determined their wear and related damage mechanisms. The predominant mechanism of material removal in the scratching of Al2O3 and Al2O3/13% TiO2 coatings was microfracture within the plastic region, which takes place preferentially at pre-existing microcracks and micropores and is driven by plastic strain. The mechanism of material removal in the scratching of the Cr2O3 coating depended on contact load and contact geometry. When the contact load was less than a critical value that was related to contact geometry, the predominant mechanism of material removal was the same as that of Al2O3 and Al2O3/13% TiO2 coatings. When the contact load exceeded the critical value, the predominant mechanism of material removal was macro-fracture controlled by lateral cracking.

The suitability and limitations of carbon fiber reinforced polyether ether ketone (PEEK) composites as bearing surfaces for total joint replacements are investigated. The wear behaviour of the composites is studied under both high-stress non-conforming contact and low-stress conforming contact conditions. The former mimics the total knee contact conditions while the latter simulates the total hip contact conditions. For each contact geometry, the study focuses on the effects of carbon fiber type, carbon fiber loading and counterface material on the wear behaviour of the composite.

The running-in behaviour of a diamond-like carbon (DLC) coating deposited on M42 tool steel substrate has been investigated on a ball-on-disk wear rig in dry air. During sliding, a classical running-in wear curve with a transition in wear rate from a high initial rate to a low rate with sliding was observed. Variations in friction coefficient with sliding distance were not related with this transition in wear rate. A transfer film was rapidly developed on the uncoated ball surface and wear of the ball was negligible. According to Talysurf measurements and optical observations, the width of the wear track on the disk specimen kept almost invariant with increase in sliding distance after the very initial sliding stages. Major change to the wear track cross-section profiles with sliding was the deepening of the wear track. A simulation model for the variation in wear volume and wear track profiles as a function of sliding distance is presented based on the assumption that there are two different dominant wear regimes with respectively a high wear rate and a low wear rate at contact pressures above and below some critical contact pressure, p c . The model has been assessed using the Hertzian elastic contact analysis and the elastic foundation contact model respectively to analyse contact pressures between the rubbing surfaces. It is shown that the running-in feature of wear results mainly from the presence of this transition in wear rate at different contact pressures. Linear increase in wear volume is expected if the Archard's specific wear rate is assumed to be the same at any contact pressure. Reasonably good agreement between the simulated results and experimental observations has been obtained. The elastic foundation model can be applied to simulating wear processes under any contact conditions.

The tribological behavior of MoS2-bonded-films was evaluated by the international standard testing methods, such as ASTM D 2625 and ASTM D 2714. Test results showed that friction and wear performance differed significantly with the contact configuration of tribo-tester. In order to investigate the effect of the test contact configurations on wear life, surface temperatures were measured and evaluated analytically in terms of a frictional heating analysis according to the contact geometry of two test standard methods. Ring-on-block tests in modified test conditions of ASTM D 2625 were also performed. The test results showed that the wear life of MoS2-bonded-films was affected significantly not only by the frictional heating and contact pressure, but also by the contact conformity of the test specimens.

In this work the friction and the wear resistance behavior of surface treated spheroidal and lamellar cast irons was studied. Thermochemical surface treatments included nitriding and nitrocarburizing. After characterization of the surface layers, dry sliding wear tests were performed on a prototype pin-on-disk apparatus in which cast iron disks were mated to hardened steel pins. The influence of the sliding velocity on the response of the various combinations of material and surface treatment was determined. Also, the influence of the contact geometry was investigated using hemispherical and flat pins.

An equation has been proposed to describe sliding wear caused by microfracture in polycrystalline alumina ceramics. The wear equation has been simplified, and grain size dependence for dry sliding wear data by Wu et al. has been shown to agree with this equation. For lubricated sliding in a ball-on-flat configuration, modification of the equation is necessary. A concept of equivalent “real” contact area for lubricated contacts is introduced to account for the different stresses at the interface as the contact conditions change before and after a wear transition. Microfracture occurs when superimposed stress exceeds a critical threshold. The decrease in surface conformity due to surface roughening and trapping of pulled-out grains can significantly increase the magnitude of the contact stresses, which leads to high wear rates. We incorporate the contact geometry effects into the microfracture wear equation. This new wear equation provides a self-consistent data fitting of load dependent wear transition in lubricated sliding of alumina observed by Cho et al. The implications of this model are discussed.

The study is based on unlubricated sliding self-mated tests with high-grade alumina in three different contact geometries. In each contact geometry, both mild and severe wear were observed; at the normal force of 30 N that was applied on each test, the transition into severe wear occurred at a velocity specific to the geometry. The wear transition involved surface fracture caused by mechanical and thermal stresses. Part of the wear debris produced under severe wear was compacted under friction and formed smooth tribofilms on the mating surfaces. Larger contact areas allowed slightly higher sliding velocities under a given normal force. The bearing capability of alumina, however, was quite low. Alumina can be recommended only for dry sliding applications in which the load and speed safely remain below the limit for the transition from mild to severe wear.

The underlying mechanisms of the adherence and of the sliding and rolling friction of rubber-like materials are studied on a microscale level, by scrutinizing phenomena occurring inside both static and rolling or sliding contact areas between a single hard transparent spherical or cylindrical asperity and the smooth and flat surface of a soft elastomer sample. Various mechanisms have been studied such as equilibrium conditions and kinetics of adherence of different rigid punches, contact geometry and rolling frictional force of a rigid cylinder, static friction, deformation and interfacial phenomena in sliding friction of a spherical asperity, influence of the radius of curvature of a blunt rigid asperity in sliding contact, stages of formation of reattachment folds, conditions for the appearance of detachment waves, friction of silica-filled compounds and the onset of wear.

To investigate the tribological behavior of a chemical vapor deposition (CVD) TiC—TIN multilayer composite coating, a systematic experiment was carried out. The CVD coating was made on the advanced computer-controlled equipment of the Bernex Corporation, Switzerland. An X-ray diffraction phase analysis was carried out for this coating and it was found that there are several phases in it. Friction and wear tests were carried out on an MT-1 model vacuum friction and wear testing machine with a ball-on-disk specimen contact geometry. A coated ball, a steel ball, a coated disk and a steel disk were used. The friction and wear behavior of these rubbing pairs was compared [1]. The results show that the multilayer coating performed better than the single layer coating, and both were much better than the uncoated steel. Scanning electron microscopy and energy dispersive spectroscopy analyses proved that there was no adhesive wear at all on the coated surface. The multilayer structure of this multilayer composite coating gives it excellent tribological properties. To explain this phenomenon, we proposed a wearresistant model of the multilayer structure, i.e. the multilayer wear mechanism. It is concluded that after further investigations this new CVD TiC—TiN multilayer composite coating could be used in a range of tribological applications.

Rolling contact fatigue defects are encountered in railway wheels used under high axle load conditions, in which substantial reductions in rail and wheel wear rates have been obtained through the previous introduction of higher strength materials and modified wheel and rail profiles. The metallurgical characteristics associated with defect initiation and growth are described. A fatigue initiation criterion based on exceedence of shakedown limits for the two wheel materials concerned has been assumed. An analysis of wheel-rail contact conditions in curved and tangent track was undertaken to identify which combinations of wheel load, wheel and rail profiles and vehicle dynamics result in stress levels which exceed shakedown limits. This analysis is supported by full-scale tests involving continuous monitoring of wheel-rail contact. Extreme dynamic loads, leading to the initiation of isolated defects, are more likely to occur on tangent track, while curved track conditions will favour fatigue crack growth, when contact geometry and vehicle tracking combine to concentrate higher stress cycles in the initiation zone. Additional field tests, using an instrumented vehicle, will be carried out to confirm these findings and allow appropriate remedial action to be identified.

This study considers the contact occurring in rail-wheel systems. Here the edge effect of the gauge face of the rail on the contact stress and contact geometry is investigated. The rail head is modeled as a three-dimensional elastic wedge which may have an arbitrary wedge angle. The solution procedure consists of deriving a Green's function for the normal displacement on the surface of an incompressible wedge loaded by a point normal force. The point force is then distributed to obtain an integral equation for frictionless contact. Numerical results are given for the rigid spherical indentation of an incompressible quarter-space.

In many rolling applications, especially where liquid lubrication is not allowed, wear is abetted by misalignments in the contact geometry. A rolling wear tester has been built where a measure of sliding is introduced by slight skewing of the rolling axes in the tangent plane of two contacting cylinders. Various contacting materials (plated and unplated) have been investigated under different conditions of speed, contact pressure and misalignment. An engineering wear analysis has been performed which is based upon the action of contact tractions during partial slip developed in the instantaneous contact area. The concept of “critical” misalignment angle θcr is introduced; for angles θ larger than θcr rigid body slip takes place without adherence. At misalignment angles θ less than critical the wear rapidly increases with θ.

A theoretical analysis of the contact geometry of an involute tooth mesh is presented. It is shown that the contact geometry between a pair of teeth in mesh changes with time. The distance between meshing teeth (the thickness of the pitch slot) has been determined by numerical computations and a new model is proposed. It is argued that the rate of change of the gap shape affects the lubricating film parameters.