Development of a friction energy capacity approach to predict the surface coating endurance under complex oscillating sliding conditions

In the case of surface coatings application it is crucial to establish when the substrate is reached to prevent catastrophic consequences. In this study, a model based on local dissipated energy is developed and related to the friction process. Indeed, the friction dissipated energy is a unique parameter that takes into account the major loading variables which are the pressure, sliding distance and the friction coefficient. To illustrate the approach a sphere/plane (Alumina/TiC) contact is studied under gross slip fretting regime. Considering the contact area extension, the wear depth evolution can be predicted from the cumulated dissipated energy density. Nevertheless, some difference is observed between the predicted and detected surface coating endurance. This has been explained by a coating spalling phenomenon observed below a critical residual coating thickness. Introducing an effective wear coating parameter, the coating endurance is better quantified and finally an effective energy density threshold, associated to a friction energy capacity approach, is introduced to rationalize the coating endurance prediction. The surface treatment lifetime is then simply deduced from an energy ratio between this specific energy capacity and a mean energy density dissipated per fretting cycle. The stability of this approach has been validated under constant and variable sliding conditions and illustrated through an Energy Density–Coating Endurance chart.     

Contact fatigue and wear behaviour of bismaleimide polymer subjected to fretting loading under various temperature conditions

Cracking and wear induced by fretting is a critical problem for industrial composite structures. Thermosetting bismaleimide resin is a promising material due to its good mechanical and thermal properties. The effect of temperature regarding fretting cracking and fretting wear is presently investigated. The temperature effect on crack initiation and propagation is quantified combining experiments and modelling. The fretting wear is explained using a friction energy wear approach. A bilinear evolution of wear volume versus the dissipated energy is identified and related to a protective third body layer. These various damage evolutions are compared to the viscoelastic properties of the polymer.     

Low-Friction MoS x Coatings Resistant to Wear in Ambient Air of Low and High Relative Humidity

The investigation of the tribological performance of MoS₂-based coatings in air of high humidity is critical for the future use of such low-friction and high-wear-resistant coatings in ambient air. Sulfur-deficient MoS _x coatings with a basal plane (x = 1.3) and a random (x = 1.8) crystallographic orientation were produced by planar magnetron sputtering. The coefficient of friction and the wear loss of MoS_x coatings in comparison with TiN and amorphous TiB₂ coatings were investigated in bi-directional sliding fretting tests performed in ambient air of different relative humidity. The wear rate expressed as a volumetric loss per unit of dissipated energy was determined. From these results, the best friction and wear performance was achieved with basal-plane-oriented MoS _x coatings tested at a relative humidity in the range of 10-50%. A coefficent of friction of 0.06-0.08 and a wear rate of 4 × 10³ m³J^-1, at a normal load of 1 N and a fretting frequency of 10 Hz, were recorded for that type of MoS _x coatings.     

Fretting corrosion damage of total hip prosthesis: Friction coefficient and damage rate constant approach

This paper analyzes friction coefficient evolution between materials related to total hip prosthesis. Fretting corrosion tests were conducted with stainless steel and poly(methyl methacrylate) interacting surfaces. In the course of fretting corrosion tests, the Coulomb friction coefficient is determined as a function of the number of cycles. It was found that the friction coefficient growth rate can be expressed as a power-law function. The influences of ionic strength, applied potential, pH, and albumin content on fretting corrosion were then investigated on the basis of the evolution of the friction coefficient. Finally, we identify the damage rate constant as being relevant for linking the mechanical and chemical parameters in the evolution of damage.     

The effect of temperature on gross slip fretting wear of cold-sprayed nickel coatings on Ti6Al4V interfaces

Fretting wear is an accumulation of damage that occurs at component interfaces that are subjected to high contact stresses coupled with low-amplitude oscillation. In metallic contacts, surface oxides, adhesion, and material transfer play a primary role in the initial stages of fretting wear degradation. Given these behaviors, the focus of this study was to determine the effect of temperature on inter-metallic fretting wear between Ti6Al4V (titanium, 6% aluminum, 4% vanadium) and cold-sprayed, commercially pure nickel coatings. The results presented herein show that increased temperature decreases friction through the formation of a uniform NiO layer, and by a reduction of Ni₂O₃ in contacts. In addition, it was found that a localized minimum friction coefficient is achieved at approximately 300 °C, above which friction increases slightly due to annealing of the cold-sprayed coatings.     

Fretting corrosion of materials for orthopaedic implants: a study of a metal/polymer contact in an artificial physiological medium

The fretting corrosion behaviour of a 316L SS flat against a PMMA counterface has been investigated in an artificial physiological medium. A specific device has been used to visualize the in situ degradation at the contact interface. Simultaneous analysis of the coefficient of friction and free corrosion potential has shown four distinct stages during fretting experiments. An energy-oriented approach to the fretting process was conducted in tandem with measurement of wear. This method has shown a linear progression in the wear volume of the samples as a function of the interfacial energy dissipated during fretting. The presence of chlorides contributes to a considerable acceleration of the degradation of the stainless steel surface. This process was explained by a mechanism related to crevice corrosion activated by friction.     

Evolution of Tribological Properties of Reactor-Grade NiCr-B Coating

The tribological properties of reactor-grade NiCr-B hardfaced coating were studied at a constant load and sliding velocity. The objective of the present article is to investigate the changes in coefficient of friction that accompanies morphological and phase changes occurring in wear scars. Such changes result from the sliding with a spherical steel ball. The transition from the low to high coefficient of friction at higher sliding distance is attributed to severe cracking as well as fretting wear–induced deformation of surface oxide scales. Increased value of coefficient of friction arises from protracted sliding over longer distances continued to deform and detachment of weakly adhered oxides like Fe₂O₃ and Cr₂O₃. Such scales are tribochemically formed on the wear scars and contribute to alteration in the coefficient of friction. The evolution of oxide phases in wear scars is found to be one of the main mechanisms for dissipation of frictional force.     

Antiwear Properties of Bonded MoS2 Solid Lubricant Coating under Dual-Rotary Fretting Conditions

Bonded MoS₂ solid lubricant coatings are widely used in tribology for their friction-reducing and antiwear properties. However, such coatings have been rarely investigated in complex fretting conditions, such as dual-rotary fretting (DRF). DRF is a complex fretting wear mode that combines torsional fretting with rotational fretting. In this work, the antiwear properties of bonded MoS₂ solid lubricant coating under dual-rotary fretting conditions were studied. Results indicated that the MoS₂ coating had better friction-reducing and antiwear properties than the substrate for alleviating DRF wear. The coating can greatly influence the fretting regimes and reduce the coefficient of friction. Furthermore, the service life of the coating was strongly dependent on the competition of the two fretting components and was reduced as the rotational fretting component increased.     

Reorientation of Randomly Oriented MoSx Coatings During Fretting Wear Tests

The reorientation of as-deposited randomly oriented sulphur-deficient MoS_x coatings during fretting wear tests has been investigated by X-ray diffraction (XRD), Scanning Electron Microscope and Transmission Electron Microscope. The driving force for the reorientation seems to be the plastic deformation induced by the tangential friction force during fretting tests. The evolution from an as-deposited randomly orientation into a basal one occurs above a threshold normal load and fretting frequency. This reorientation results in a lowering of the coefficient of friction as well as in a high resistance to humidity, but unfortunately a high wear rate is noticed. The effect of normal load and fretting frequency on that reorientation process in MoS_x coatings is discussed.     

The influence of water vapour in air on the friction behaviour of pure metals during fretting

An investigation was conducted to determine the effect of water vapour content in air on the frictional behaviour during fretting of pure metals: iron, aluminium, copper, silver, chromium, titanium and nickel. The fretting experiments were carried out under various humidity levels, ranging from dry air to 50% relative humidity at 23°C. During the experiment the frictional force between fretting surfaces was measured. Pure metals, except iron, were found to have a maximum value of the coefficient of friction during the steady-fretting stage (μ_s) at a specific humidity (RH_max). Iron showed a rapid decrease in μ_s with increasing humidity at RH_max. Each pure metal also exhibited maximum fretting wear at RH_max. The value of μ_s at RH_max for each metal was strongly related to the heat of formation of the lower metal oxide, indicating that the adhesive contact area was larger at RH_max for the fretting of metals with less chemical activity. At high humidity levels water vapour generally reduced the coefficient of friction, μ_s.     

Effect of contact configuration on the durability and friction coefficient of pressure‐sprayed MoS2 coatings under fretting conditions

Fretting wear is often found at the contact surfaces of a tight assembly where small‐amplitude oscillatory movement occurs, which can be the concealed origin of some enormous accidents. Employment of solid lubrication coatings, as one of effective measurements to palliate the fretting damage, has been widely acknowledged. The present work studied the fretting behaviour of a molybdenum disulphide coating on SUS 316 stainless steel substrate by a relatively cheap and easy‐to‐use process: pressure spraying. Two contact configurations (cylinder‐on‐flat and ball‐on‐flat) were used in the tests with different displacement amplitudes (from 5 to 75 µm) and normal loads (from 100 to 400 N for ball‐on‐flat and from 400 to 1000 N for cylinder‐on‐flat). The results showed that large displacement amplitude is adverse to friction coefficient and coating lifetime and that under a critical contact pressure, coating endurance is improved contact pressure increases. Contact configuration influences friction coefficient by changing contact area and distribution of contact pressure. One master curve of average dissipated energy per cycle in initial stable stage was obtained for two contact configurations, which can be employed to approximately predict coating lifetime. Copyright © 2009 John Wiley & Sons, Ltd.     

Application of an energy wear approach to quantify fretting contact durability: Introduction of a wear energy capacity concept

A friction energy formalism is considered and adapted to formalize the fretting wear responses of adhesive wear and non-adhesive wear interfaces. It is shown that for non-adhesive wear tribocouples like hard coatings (TiN, TiC, etc.) the wear kinetics can be formalized using the accumulated friction dissipated energy. By contrast, adhesive wear contacts involving aluminium and titanium alloys display a critical dependance regarding the applied sliding amplitude. The wear kinetics of such systems is captured by considering a sliding reduced energy wear formulation. A combined composite energy wear formulation is then introduced to formalize the fretting wear response whatever the tribocouple behaviour. It is shown that a local approach, focusing on wear depth analysis, is required to predict interface durability. A FEM investigation demonstrates that the wear depth kinetics can be predicted by considering the accumulated energy density. It concludes that interface durability can be related to a single energy density capacity variable (χ) defined as the maximum accumulated energy density which can be dissipated in the interface before contact failure.     

Tribological behaviour of polymeric coatings. Part I. Aramid particle‐reinforced epoxy nanocomposite systems

An epoxy‐based nanocomposite containing a low concentration of nanometric TiO₂ (4 vol. %), graphite powder (7.21 vol. %), and 2–14 vol. % aramid particles was developed as a coating material. The mechanical and tribological performance of the composites was investigated. The epoxy filled only with TiO₂ possessed significantly improved impact strength and flexural properties, whereas the further incorporation of graphite and aramid particles had a deleterious effect on most of the mechanical properties, except the modulus. The tribological behaviour of the composites was tested in sliding and fretting modes. Under sliding conditions, the addition of nanometric TiO₂ alone significantly improved the wear resistance and decreased the coefficient of friction compared to the neat epoxy. The sliding wear and friction behaviour was further enhanced with the incorporation of graphite and aramid particles. Contrary to the sliding wear behaviour, the fretting wear and friction behaviour was worse for the epoxy filled only with TiO₂, but was significantly enhanced by the incorporation of graphite and aramid particles. The optimum aramid particle content for sliding and fretting wear of the epoxy‐based nanocomposite was determined as 10 vol. %.     

Effect of shot peening on the fretting wear of Ti–6Al–4V

In this paper, we report on the fretting wear behaviour of polished and shot peened Ti–6Al–4V specimens. For fretting experiments, due to micro-displacements at the interface between two contacting surfaces, two types of damage can be observed: crack initiation and debris formation. Shot peening, which is already well known for improving fatigue resistance of titanium alloys, is shown to have a beneficial effect on the crack initiation and propagation under fretting wear loading, as cracks observed on specimens after cylinder-on-flat fretting tests are shorter in shot peened specimens than in polished ones. It is also demonstrated that shot peening decreases the friction coefficient only at the beginning of the test, as long as the asperities induced by shot peening are not worn-off. The effects of displacement amplitude, normal force and test duration on the wear volume have been investigated: in all cases, shot peening has no significant impact on the wear process. The same amount of debris are formed and ejected for both polished and shot peened specimens. Moreover, it is found that, for both types of specimens, the linear relation, developed for steels and hard coatings, between wear volume and cumulated dissipated energy is not valid in the present case as different wear volumes are measured for the same cumulated dissipated energy, depending on the experimental conditions (normal force, displacement amplitude). Using the test duration as the variable parameter, energy wear coefficients are calculated for different experimental conditions.     

A proposal for a fretting wear criterion for coated systems with complete contact based on accumulated friction energy density

This paper presents the results of basic studies that investigate fretting behaviour in different steel-pairing coatings. The experiments enabled the identification of the variation in the temporal and tribological behaviour of each pairing as a result of the chosen coating. The experiments were performed on a test bench for the determination of friction coefficients with a standardised torsion test. Using flat annular contact surfaces that were pressed together under a pressure of 25 or 50 MPa, the tested samples were alternatingly loaded with twisting angle amplitudes of 0.23° and 0.5° (or 46 and 100 μm slip, respectively). This technique enabled the recording of optical damage (fretting, etc.) and strength-affecting mechanisms (damage of the substrate material). The results were evaluated using an appropriate damage criterion, thereby identifying the accumulated dissipated friction energy as a fail-related physical quantity.     

Corrosion wear behaviors of micro-arc oxidation coating of Al2O3 on 2024Al in different aqueous environments at fretting contact

The fretting wear behavior of micro-arc oxidation (MAO) coating of Al₂O₃ on an aluminum alloy 2024Al flat against a 440C stainless steel ball was investigated in artificial rainwater, artificial seawater and distilled water by using a ball-on-flat configuration with 300 μm amplitude at room temperature for 1 h. The morphology of the wear scars were observed and analyzed using scanning electron microscopy; the 3D-morphology and wear volume-loss were determined using a non-contact optical profilometer. Potentiodynamic anodic polarization was used to measure the corrosion behavior of the MAO coating before and after the corrosion wear test. The influences of the load, frequency and aqueous medium on the friction coefficient and wear volume-loss of the coatings were also analyzed. Results show that the friction coefficient decreases generally with an increase of the frequency in the three aqueous solutions; whereas it presents different variation trends as the load increased. In addition, aqueous environment does significantly influence the friction coefficient, the friction coefficient was the largest when fretting occurred in distilled water, smaller when fretting occurred in rainwater, and the smallest when fretting occurred in seawater. Particularly the remarkable antifriction effect of the seawater is of note. The wear-loss of the MAO coating in the distilled water is the largest at low frequency; however, it increases rapidly in rainwater and seawater at high frequency due to the corrosion effect of Cl^? ion as well as its accelerating effect to the wear process, and results in larger wear-loss than that in distilled water, which implies a positive synergism between corrosion and wear.     

Study on rotational fretting wear of bonded MoS2 solid lubricant coating prepared on medium carbon steel

The rotational fretting wear behaviors of the bonded MoS₂ solid lubricant coating and its substrate steel were comparatively studied under varied angular displacement amplitudes, constant normal load, and rotational speed. Dynamic analysis in combination with microscopic examinations was performed through SEM, EDX, XPS, optical microscope, and surface profilometer. The experimental results showed MoS₂ changed the fretting running regimes of substrate. The friction coefficients of MoS₂ were lower than those of the substrate. For MoS₂, the damage in partial slip regime was very slight. The damage mechanism of the coating in slip regime was main abrasive wear, delamination, and tribo-oxidation.     

Fretting wear behaviors of nanometer Al2O3 and SiO2 reinforced PEEK composites

The influence of diameter and content of Al₂O₃ particles on the tribological behaviors under fretting wear mode was investigated. The surface of PEEK composite and steel ball were examined by SEM and EDS, to identify the topography of wear scar and analyze the distribution of chemical elements in the friction counterparts, respectively. It can be found that the filling of Al₂O₃ powder improves the fretting wear resistance of PEEK composite. With the increase of Al₂O₃ diameter, the area of wear scar on specimen increases first and decreases afterward. However, the wear of composites increases monotonically with increasing Al₂O₃ content. Although the filling of 10 wt.% and 200 nm PTFE powder in PEEK makes the lowest wear of all specimens, no synergistic effect was found when Al₂O₃ and PTFE were filled into PEEK composite together. For the friction pair of PEEK composite and steel ball, abrasive wear and adhesive wear dominate the fretting wear mechanism during fretting. Thermal effect plays a very important role during fretting; thus the property of temperature resistance for polymer material would affect the wear degree on the surface of wear scar.     

Nanoscale fretting wear study by scanning probe microscopy

Nanoscale fretting wear was studied by using scanning probe microscopy (SPM) and a newly proposed unified approach of slip index. The production of SiO₂ colloidal probes and the SPM calibration are described. Partial and gross slip fretting with displacement amplitudes from 5 to 500 nm were used for the study. Friction coefficient and nanowear results are presented showing a substantial increase of the friction at the transition from partial to gross slip and a significant difference between damaged surfaces in the two fretting regimes.