Effect of dissimilar mating materials and contact force on fretting fatigue behavior of Ti–6Al–4V

Fretting fatigue behavior of a titanium alloy, Ti–6Al–4V, in contact with two pad materials having quite differing values of hardness and elastic modulus (aluminum alloy 2024 and Inconel 718) using “cylinder-on-flat” configuration was investigated at different applied stress levels and contact forces. Applied contact forces for both pad materials were selected to provide two Hertzian peak pressures of 292 and 441 MPa. Finite element analyses of all tests were also conducted which showed that an increase in contact force resulted in a smaller relative slip amplitude and a larger width of stick zone. These two factors, along with the lower coefficient of friction during fretting, resulted in less fretting damage on the contact surface of specimen subjected to higher contact force relative to that at lower contact force regardless of the hardness difference of mating materials. Also, an increase in hardness resulted in greater fretting damage on the contact surface of specimens only at higher contact force. Further, the fretting fatigue life decreased with an increase of applied contact force at higher applied effective stress, while it increased at lower applied effective stress with both pad materials. These observations suggest that there is complex interaction among hardness difference between mating surfaces, relative slip amplitude, and stress state in the contact region during fretting fatigue of dissimilar materials.     

The influence of current load on fretting of electrical contacts

The fretting corrosion behavior of tin coated brass contacts is studied at various current loads (1, 2 and 3 A). The typical characteristics of the change in contact resistance with fretting cycles are explained. The fretted surface is examined using scanning electron microscope, laser scanning electron microscope and energy dispersive analysis of X-rays to assess the surface morphology, extent of fretting damage, extent of oxidation, surface profile and elemental distribution across the contact zone. The degradation of contacts at high and low values of current is explained with reference to the thermal and electrical phenomena occurring at the contact interface. The results showed that irrespective of the current loads under study, the contact resistance is maintained at 1.0±0.02 Ω where the oxide debris formation and the electrical breakdown of oxide particles competed with each other maintaining the equilibrium. The number of cycles to failure of the contacts is delayed at lower current. The fretting corrosion degradation of tin coated contacts occurs much faster at higher currents as it generates more accumulation of oxide wear debris at the contact zone. The observed surface morphology and the tin profile of the fretted surface are in agreement with the experimental results.     

Fretting wear of Ti-48Al-2Cr-2Nb

An investigation was conducted to examine the wear behavior of gamma titanium aluminide (Ti-48Al-2Cr-2Nb in atomic percent) in contact with a typical nickel-base superalloy under repeated microscopic vibratory motion in air at temperatures from 296–823 K. The surface damage observed on the interacting surfaces of both Ti-48Al-2Cr-2Nb and superalloy consisted of fracture pits, oxides, metallic debris, scratches, craters, plastic deformation, and cracks. The Ti-48Al-2Cr-2Nb transferred to the superalloy at all fretting conditions and caused scuffing or galling. The increasing rate of oxidation at elevated temperatures led to a drop in Ti-48Al-2Cr-2Nb wear at 473 K. Mild oxidative wear was observed at 473 K. However, fretting wear increased as the temperature was increased from 473–823 K. At 723 and 823 K, oxide disruption generated cracks, loose wear debris, and pits on the Ti-48Al-2Cr-2Nb wear surface. Ti-48Al-2Cr-2Nb wear generally decreased with increasing fretting frequency. Both increasing slip amplitude and increasing load tended to produce more metallic wear debris, causing severe abrasive wear in the contacting metals.     

The use of notch and short crack approaches to fretting fatigue threshold prediction: Theory and experimental validation

In this paper the similarities between fretted and notched components in terms of stress gradient and the consequent “size effect” are discussed. Critical distance and short crack arrest approaches for the prediction of fretting fatigue thresholds are then presented and the predictions are compared with experimental results. Two geometry and alloy combinations are considered in order to validate the prediction by the means of experimental results. In particular, both Hertzian fretting tests performed on Al4%Cu alloys and experiments carried out employing ‘flat and rounded’ contact pads, made of Ti6Al4V alloy, are used for the comparison. It is shown that both criteria provide good predictive capabilities. However, the short crack arrest method is less empirical and can be adapted to a wider range of applications (e.g. surface treated components).     

Contact and thermal analysis of transfer film covered real composite-steel surfaces in sliding contact

For composite-steel surfaces in sliding contact an anisotropic numerical contact algorithm has been developed to study the ‘layer type’ problems. An FE contact analysis was applied to evaluate the contact parameters (real contact area, contact pressure distribution and normal approach). The contact temperature rise was determined by using both a numerical thermal algorithm for stationary and a FE transient thermal technique for ‘fast sliding’ problems.The effect of a continuous transfer film layer (TFL), that had built up during wear of the PEEK matrix material on the steel counterpart, was considered. Its thickness was assumed to be t=1 μm, and its material properties were that of PEEK at room temperature or, in the case of frictional heating, at a temperature of 150°C (i.e. above the glass transition temperature of the polymer matrix).Results are presented for a spherical steel asperity, with/without TFL, sliding over composite surfaces of different fibre orientation, and in addition, for real composite-steel surfaces (based on measured surface roughness data) in sliding contact. The TFL has an effect on the contact parameters especially at higher operating temperatures (i.e. 150°C); it results in the production of a larger contact area and a lower contact pressure distribution. The contact temperature rise is clearly higher if a TFL is present. Due to the low thermal conductivity of PEEK, the TFL is close to the melting state or it even gets molten within a small vicinity of the contact area.     

On the fretting wear mechanism of Zr-alloys

Zirconium alloys are highly desirable in nuclear applications due to their transparency to thermal energy neutrons and for their high corrosion resistance. The main objective of this study is to investigate the fretting wear mechanism of Zr–2.5%Nb alloy. The experimental work was carried out in air at 265 °C, using a specially designed fretting wear tribometer. The transfer of material, the change in the wear volume and the maximum wear depth with the number of cycles were measured through 3D mapping of the topography of the fretted surface. SEM and Fourier Transform Infrared Interferometry methods were used to examine the microspall pits and to measure the distribution of the thickness of oxide layer in the fretting region. For relatively small slip amplitude, the results showed that the fretting wear mechanism is initially dominated by adhesion and abrasion actions and then by delamination and surface fatigue. The time variation of the wear losses was shown to be cyclic until a steady state value is reached. At high slip amplitudes, however, abrasion and delamination are the only dominant wear mechanisms. The volumetric wear losses were found to decrease monotonically with the number of cycles. A novel approach was introduced, whereby the thermal and electrical contact resistances of the fretting interface are simultaneously measured. The results demonstrated the potential use of this non-intrusive approach for real-time monitoring of the fretting wear mechanism.     

Measurements of friction-induced surface strains in a steel/polymer contact

The paper describes an experimental study combined with analyses and numerical simulations of the surface strains developed in a metal–polymer contact under a variety of loading configurations. Specifically, a steel ball is caused to slide over a poly(methylmethacrylate) flat counterface under a fixed normal load where the imposed motions are small and consist of sliding and rotation and the combination of both. The surface strains have been measured directly using conventional strain gauges in two types of configurations specifically designed to monitor the strains for sliding and rotation. Calculations of frictional forces provide friction coefficients which are self-consistent and the computed ‘friction displacement loops’ correspond closely to those measured. In addition, the surface strain measurements provide a convenient and accurate insight into the stick–slip transitions in fretting contacts.     

Investigation into effects and interaction of various fretting fatigue variables under slip-controlled mode

Fretting fatigue behavior of unpeened and shot-peened Ti–6Al–4 V was investigated using a dual-actuator test setup which was capable of applying an independent pad displacement while maintaining a constant cyclic load on the specimen. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus relative slip range and the evolution of normalized tangential force. The fretting regime changed from stick to partial slip and then to gross slip with increasing relative slip range, and the transition from partial to gross slip occurred at a relative slip range of 50–60 μm regardless of the applied cyclic load, surface treatment, contact load and contact geometry. The fretting fatigue life initially decreased as the relative slip range increased and reached a minimum value, and then increased with increase of the relative slip range due to the transition in fretting regime from partial slip to gross slip. Shot-peened specimens had longer fatigue life than unpeened specimens at a given relative slip range, but the minimum fatigue life was found to be at the same value of relative slip range for both shot-peened and unpeened specimens. Tangential force was directly related to relative slip and this relationship was independent of other fretting variables.     

The fretting wear of an austenitic stainless steel in air and in carbon dioxide at elevated temperatures

The development of an experimental test facility designed to investigate the fretting wear of metals in high temperature gaseous atmospheres is described briefly. Small amplitude torsional fretting was produced between normally loaded annular contact surfaces at temperatures up to 650 °C. The information presented concerns the effects produced on fretting a type 321 stainless steel in air and CO₂ environments. Use of electron optical and surface profiling techniques has allowed a qualitative asessment of the degree and characteristic features of the damage to specimens fretted at room temperature and 650 °C under selected oscillatory wear conditions. During simultaneous oxidation and fretting at 650 °C the initially formed protective oxide is disrupted exposing a chromium-depleted surface which subsequently oxidizes to a less protective Fe-Cr spinel. This combination of effects could have unfortunate consequences on the long term wear behaviour.     

Effect of surface treatments on fretting fatigue damage of biomedical titanium alloys

Fretting fatigue is an adhesive wear damage caused by tangential micromotion under normal force at contact areas. It is observed along the contact points of hip implants and bone plates. Surface-modified biomedical titanium alloys offer better resistance against fretting damage. PVD TiN coatings and plasma nitriding have proved effective in minimizing friction and delaying the failure of materials. In the present study, attempt has been made to explain the fretting fatigue failure mechanism sequence of PVD TiN-coated and plasma-nitrided Ti–6Al–4V and Ti–6Al–4V couple through friction measurement and microscopic examination.     

An investigation of fretting behaviour of several synthetic base oils

The chemical and physical properties are quite different for mineral oil and synthetic oil. Compared to the investigation of mineral oil, less work on fretting behaviour of synthetic oils was reported. In this paper, a study of typical synthetic base oils such as polyalkylene glycol (PAG), polyalphaolefin (PAO) and silicone oil has been conducted. The contact consisted of a fixed flat specimen (GCr15 steel and 45 steel) opposite to a moving ball specimen (GCr15 steel) with a diameter of 12.3 mm. Other main parameters were as follows: the slip amplitude was ranged from 5 to 80 μm, the frequency was varied from 2 to 5 Hz; the normal load, temperature and relative humidity were respectively 100 N, 23 °C and 60%. Variations in the tangential force versus the displacement as a function of the fretting cycles were recorded. For comparison, fretting tests under dry condition have also been performed. The fretting scars were examined after tests. The evolution of coefficient of friction and wear volume were analyzed and compared at different fretting regimes for different synthetic base oils. The competitions between oil penetration into the interface and self-cleaning by fretting in different fretting regimes, the effect of physical properties such as surface tension, pressure–viscosity coefficient and compressibility on fretting behaviour have been particularly discussed.     

An Examination of Thermionic Emission Due to Frictionally Generated Temperatures

The emission of electrons from a surface due to heating-referred to as thermionic emission-is examined theoretically for sliding contact. High local temperatures generated by friction at the contacts between rubbing surfaces can activate the emission of electrons and influence tribochemical reactions. A thermal model previously developed by Vick and Furey [1,2] for sliding contact is used to predict the temperature rise over the surface. This predicted temperature rise, along with the bulk temperature of the material, is used in the Richardson--Dushman equation for thermionic emission to predict the current density from the surface. The total current discharged from the surface is obtained from an integration of the current density. Results demonstrate that high local temperatures generated by friction at the contacts between rubbing surfaces can activate the emission of electrons, with local spikes in the current density occurring in the vicinity of the peak temperature. In addition, relatively large changes in both current density and total current result from relatively small changes in either material properties or sliding conditions, such as velocity or applied load. Since the true area of contact is likely to evolve in a highly dynamic fashion, a study using time varying, multiple contacts was also conducted. Results suggest that the locations of high local temperatures and thermionic activity are likely to be short lived and random.     

Fretting fatigue behaviour of Ti–6Al–4V alloy under plane bending stress and contact stress

Fretting fatigue tests for Ti–6Al–4 V alloy were conducted by use of the plate fatigue specimen with bolt-tightened shoe on both sides of the plate. It was clarified that the repeated bending stress at the contact area where fretting fatigue failure starts linearly decreased as stress over the contact area increased. Fretting fatigue crack starts from the pit where stress concentrate. The pit initiates when fretting debris were removed from the surface striation formed due to the contact slip movement. The fretting fatigue crack initiation mode was transgranular, while the fretting fatigue crack propagation mode was striation.     

Effect of temperature on the fretting corrosion of tin plated copper alloy contacts

The effect of temperature on the fretting corrosion behaviour of tin plated copper alloy contacts in the temperature range of 25–185 °C, is addressed in this paper. The change in contact resistance with fretting cycles at various temperatures was determined. The contact zone after fretting corrosion test was analyzed using laser scanning microscope, X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray spectrometry (EDX), to assess the surface profile, phase content, morphology and compositional changes across the interface. The study reveals that temperature has a greater influence on the extent of fretting corrosion of tin plated copper alloy contacts. The softening of tin is responsible for the extended region of low contact resistance observed at 85 °C. The increase in thickness and the resistance of Cu–Sn intermetallic compounds (IMCs) is responsible for the decrease in surface roughness and the drastic increase in the contact resistance at higher temperatures. The study suggests that the tin plated copper alloy contact system should be considered as copper alloy/IMC/Sn/SnO₂ instead tin plated copper alloy. During fretting corrosion test at elevated temperatures, once the top surface layers are worn out, the contact interface is transformed from tin versus tin-to-tin-intermetallic versus tin-intermetallic. The study concludes that tin plated copper alloy contacts are not suitable for high temperature applications.     

Fretting corrosion of lubricated tin plated copper alloy contacts: Effect of temperature

The fretting corrosion behaviour of lubricated tin plated copper alloy contacts at ambient and elevated temperatures is addressed in this paper. At 27 °C, lubrication is very effective and the contact resistance remains stable for several thousand fretting cycles whereas at elevated temperatures (155 °C) the performance of lubricated contact is not appreciable. Surface profile and surface roughness confirm that the lubricated contacts have a smoother profile and experience a lesser damage at the contact zone at ambient as well as at elevated temperatures. The mechanism of fretting corrosion of tin plated contacts appears to be similar with and without lubrication at all the temperatures studied. The difference in performance of the lubricated contacts at ambient and elevated temperatures is due to the faster wear rate of tin coating at elevated temperatures. Oxidation of the contact zone of the lubricated contacts is prevented at all temperatures studied. The study concludes that lubrication is effective in improving the life of the tin plated copper alloy contacts under fretting conditions at ambient temperatures whereas at elevated temperatures lubrication provides only a marginal improvement in performance. The decrease in performance of lubricated tin plated contacts at elevated temperatures is due to the higher wear rate of tin coating and not due to evaporation of the lubricant.     

A tentative explanation for the tribochemical effects in fretting wear

In many fretting investigations, tribochemical reactions have been reported to critically determine the wear and friction behavior, however, different and contradictory assessments of the importance of mechanical and thermal effects on these reactions have been suggested. Since fretting is characterized by relatively slow sliding speeds, high temperatures are not generated over the entire nominal contact area. However, evidence for phase transformations, which are typical of high temperatures, have been observed many times in fretting experiments. In other words, there exists a discrepancy between the macro- and micro-scale observations. In our previous experimental and theoretical work, the tribochemical transformations of steel and ceramics were extensively investigated and the presence of very high flash contact temperatures under gross slip fretting was confirmed. In this paper we present a tentative explanation of the mechanism for the observed tribochemical changes under selected fretting conditions, which can also explain the discrepancy in the results from macro- and micro-scale studies. The proposed wear mechanism considers the tribochemical transformations at the asperity spot-to-spot contacts due to high flash temperatures, while the heat generation and dissipation at apparent contact area remain significantly lower. The observed overall wear transition occurs due to gradual accumulation of the transformed material, which in “closed” fretting contacts remains in great part within the contact.     

A study of tribopolymerisation under fretting contact conditions

The present study is aimed at determining whether or not tribopolymerisation can occur under conditions of fretting contact. Using a high contact stress system consisting of oscillating metal balls loaded against flat steel discs, effects of various monomers on friction, wear, and surface film formation were determined. Monomers were used at 1% concentration in hexadecane. Under the conditions used (90N load, 40 Hz frequency, 300 μm amplitude, for 1 hour), the monomers tested reduced friction or wear or both. Fourier Transform Infrared (FTIR) analysis of the test specimens showed that organic material is presented in the wear scars and depends on the metal system used, the monomer structure, location within the track, and the method of cleaning the surface after a test. With Al-on-steel, the addition of 1% styrene to hexadecane reduced volumetric wear of the disc by 65%; furthermore, positive FTIR evidence of polystyrene in the wear track was obtained. But adimer acid/glycol monomer formed metal soaps, no polymer, and had little effect on wear under these conditions. These results support the hypotheses that addition-type tribopolymerisation can be initiated by exoelectron emission. Additionally, it was found that not only does methyl methacrylate polymerise under the fretting conditions, but the polymer film formed also reacts with the friction contact surface. Taken as a whole, the results of this study of possible tribopolymerisation under fretting conditions support both major hypotheses, namely that: (i) for condensation-type monomers, the most important factor is the temperature of the rubbing surfaces. (ii) For addition-type monomers, it would appear that the effect of exoelectron emission can initiate surface polymerisation even at relatively low surface temperatures, e.g., 10–40°C above ambient. This is in agreement with the negative-ion-radical action mechanism (NIRAM) of boundary lubricant component. Finally, the results obtained in this study demonstrate that the principle of tribopolymerisation developed by Furey and Kajdas can be used as a novel and effective approach to designing specific molecular structures for boundary lubrication under various rubbing conditions.     

Modeling of fretting wear evolution in rough circular contacts in partial slip

This paper presents a numerical model that maps the evolution of contact pressure and surface profile of Hertzian rough contacting bodies in fretting wear under partial slip conditions. The model was used to determine the sliding distance of the contacting surface asperities for one cycle of tangential load. The contact pressure and sliding distance were used with Archard's wear law to determine local wear at each surface asperity. Subsequently, the contact surface profile was updated due to wear. The approach developed in this study allows for implementation of simulated and/or measured real rough surfaces and study the effects of various statistical surface properties on fretting wear. The results from this investigation indicate that an elastic–perfectly plastic material model is superior to a completely elastic material model. Surface roughness of even small magnitudes is a major factor in wear calculations and cannot be neglected.     

Characterization of fretting fatigue crack initiation processes in CR Ti–6Al–4V

A study was conducted to quantify fretting fatigue damage and to evaluate the residual fatigue strength of specimens subjected to a range of fretting fatigue test conditions. Flat Ti–6Al–4V specimens were tested against flat Ti–6Al–4V fretting pads with blending radii at the edges of contact. Fretting fatigue damage for two combinations of static average clamping stress and applied axial stress was investigated for two percentages of total life. Accumulated damage was characterized using full field surface roughness evaluation and scanning electron microscopy (SEM). The effect of fretting fatigue on uniaxial fatigue strength was quantified by interrupting fretting fatigue tests, and conducting uniaxial residual fatigue strength tests at R=0.5 at 300 Hz. Results from the residual fatigue strength tests were correlated with characterization results.While surface roughness measurements, evaluated in terms of asperity height and asperity spacing, reflected changes in the specimen surfaces as a result of fretting fatigue cycling, those changes did not correspond to decreases in residual fatigue strength. Neither means of evaluating surface roughness was able to identify cracks observed during SEM characterization. Residual fatigue strength decreased only in the presence of fretting fatigue cracks with surface lengths of 150 μm or greater, regardless of contact condition or number of applied fretting fatigue cycles. No cracks were observed on specimens tested at the lower stress condition. Threshold stress intensity factors were calculated for cracks identified during SEM characterization. The resulting values were consistent with the threshold identified for naturally initiated cracks that were stress relieved to remove load history effects.     

Fretting fatigue strength estimation considering the fretting wear process

In fretting fatigue process the wear of contact surfaces near contact edges occur in accordance with the reciprocal micro-slippages on these contact surfaces. These fretting wear change the contact pressure near the contact edges. To estimate the fretting fatigue strength and life it is indispensable to analyze the accurate contact pressure distributions near the contact edges in each fretting fatigue process.So, in this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage as follows:

W=K×P×S,