An investigation of the fretting behaviour of low friction coatings on steel

Three different low friction coatings on steel (electrolytically deposited cadmium, PTFE solid lubricant in epoxy resin and PTFE solid lubricant in polyimide resin) were studied in order to relate their fretting behaviour with mechanical properties. Particular importance was given to adhesion which was measured using a scratch test. Fretting tests were carried out on the steel substrate and on the coatings under the same conditions. The major parameters of the tribological system were identified and then quantified. The values of the parameters obtained for each coating were compared with the corresponding values for uncoated steel. The mechanical characteristics of the coatings and their fretting parameters were plotted using a polar diagram in order to give an overview of the fretting behaviour of each coating. Differences were noted and the corresponding parameters were identified. The influence of the adherence of the coating and of the fretting test load on the lifetime of the coating was determined.     

Fretting damage of TiN coated zircaloy-4 tube

Fretting has been reported and investigated for over 50 years. However, it is still one of the modern plagues for several industrial machineries. Especially, fretting of fuel rod cladding material, zircaloy-4 tube, in pressurized water reactor (PWR) must be reduced and avoided. Thin hard coatings are employed to improve the tribological properties such as friction and wear of conventional engineering materials. Among these coatings, physical vapor deposition (PVD) TiN coating is probably one of the most frequently and successfully used PVD coatings for the mitigation of fretting wear. Therefore, in this study a fretting wear experiment was performed using TiN coated zircaloy-4 tube as the fuel rod cladding material and uncoated zircaloy-4 tube as one of the grids. The fretting tester was designed and manufactured for this experiment. The number of cycles, slip amplitude and normal load were selected as main factors of fretting. The type of contact was cylinder-to-cylinder contact. The worn surface was observed by optical microscope, 3-D surface measuring instrument and scanning electron microscope (SEM). The results of this research showed that the wear volume of TiN coated zircaloy-4 tube decreased about 1.2–3 times more than uncoated tube and wear mechanisms were brittle fracture, fatigue fracture, adhesion, abrasion and oxidation.     

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.     

Radial fretting fatigue damage of surface coatings

Radial fretting tests with a 52100 steel ball-on-flat contact have been carried out under different normal loads. TiN, MoS₂ and TiN+MoS₂ coatings on a 1045 steel flat were examined. The normal loads amplitude used were 200, 400 and 800 N at speeds of 12 and 1.2 mm/min. Dynamic analysis in combination with microscopic examinations by SEM and EDX have been performed. It was observed that the vertical stiffness increased with the increase of loading speed and number of cycles. The metallographic examinations showed that little damage was observed for the MoS₂ coating, which exhibited excellent radial fretting fatigue resistance. For the TiN coating, micro-cracks appeared at the lower load while delamination occurred at the higher load. For the TiN+MoS₂ composite coating, the vertical stiffness increased but accompanied by some micro-cracks. As a result of the study, the radial fretting test is proposed as one possible new method to evaluate coating life.     

The use of nickel graphite composite coatings for the mitigation of gross slip fretting wear on Ti6Al4V interfaces

In metallic contacts, surface oxides, adhesion, and material transfer play a primary role in the initial stages of fretting wear degradation. Given this behavior, the focus of this study was to mitigate fretting wear within Ti6Al4V contacts at room temperature and 450 °C with the use of thermally sprayed nickel graphite composite coatings with 5–20% graphite. The results show that the embedded graphite particles reduced the friction of the nickel thermal sprayed coatings during both low and high temperature fretting wear experiments. Friction and wear mechanisms are discussed with correlations of contact chemistry, morphology, and mechanical performance. Wear on the mated Ti6Al4V surfaces was reduced by the formation of uniform transfer films that were identified as graphitic based at room temperature and NiO based at 450 °C.     

Unlubricated gross slip fretting wear of metallic plasma-sprayed coatings for Ti6Al4V surfaces

Plasma-sprayed Al–bronze or CuNiIn coatings are often applied to protect against fretting wear and extend the operational life of Ti-alloy compressor blades in turbine engines. In order to develop a fundamental understanding of how these coating systems perform under gross slip fretting conditions, bench level fretting wear tests were conducted at room temperature to simulate cold engine startup. Alternative coatings such as plasma-sprayed molybdenum and nickel were also evaluated because of their potential for reducing fretting wear under certain simulated engine conditions. The combination of scanning electron microscopy (SEM), surface profilometry, surface chemistry (EDS), and friction analysis were used to study coating performance and evaluate the interfacial wear mechanisms. In this study, it was determined that all coatings caused significant damage to the mating Ti6Al4V surfaces and that the wear mechanisms were all similar to those of the uncoated baseline case.     

Coatings for fretting prevention

The purpose of the program to study coatings to prevent fretting fatigue was to determine the optimum fretting fatigue resistant coatings for titanium/steel mating surfaces found in helicopter rotor hub assemblies. Here several problems have been encountered during component accelerated fatigue tests of the hub assembly where fatigue failures of the titanium hub initiated at areas of severe fretting. In order to address this problem a program was developed to evaluate candidate coatings in terms of their ability to reduce or eliminate fretting of Ti-6Al-4V when in contact with 17-4 PH steel.This paper describes the new fretting test apparatus, the results obtained thus far on screening selected coatings which might have potential for fretting protection and an evaluation of bend bar test specimen configurations designed to achieve fretting fatigue under conditions comparable to those experienced in the actual rotor hub assemblies.     

Life estimation of Ti-6Al-4V specimens subjected to fretting fatigue and effect of surface treatments

Suitability of different multi-axial parameters in predicting fretting fatigue life of Ti-6Al-4V specimens has been investigated. Ameliorating effect of surface treatments on fretting fatigue has been studied. In simple uni-axial/multi-axial fatigue tests, nucleation as well as propagation of cracks occur under the influence of identical stresses. Hence nucleation accounts for most of the total life. Fretting fatigue crack nucleation occurs due to very large contact stresses, effect of which is felt only close to the surface (due to steep gradients). Propagation mostly occurs due to lower stresses in the bulk of the material (negligible influence of contact tractions) and forms a significant portion of total life. Total life has to be taken as sum of initiation life calculated from different multi-axial fatigue parameters and propagation life from conventional fracture mechanics approach. Steep stress gradients necessitate the adoption of a statistics based approach to predict the crack initiation life, based on an assumed distribution of flaws. The quality of comparison between predicted and experimentally observed failure lives provides confidence in the notion that conventional fatigue life prediction tools can be used to assess fretting fatigue failure. Effect of surface treatments like shot-peening with or without additional surface coatings on total life of the specimen and on friction coefficient has been studied.     

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.     

Evaluating and predicting durability of bonded solid lubricant coatings under fretting conditions

Fretting wear is a common failure at contact surfaces of tight assemblies in industries, such as those of transport, power transmission and nuclear power station. Using friction reduction coatings is one of the most effective methods to palliate fretting failure. However, in view of numerous available coatings, it is still a tough task to evaluate them and to select the optimum one for a given application. In this paper, based on the investigation of fretting behaviors of three bonded solid lubricant coatings, an initial maximal dissipated energy density (Ed_0 max ini) approach and a local Archard factor (K_A0) approach were suggested to evaluate and predict coating durability. The lifetime of each coating under different values of test parameters can be fitted by one master curve. The master curves of a coating may be used to predict the coating lifetime only by running a new test for few cycles under relevant test conditions. For a given test condition, the durability of coatings can be easily evaluated by comparing their K_A0 master curves. Ed_0 max ini master curves include comprehensive tribological performance, which is helpful for coating selection.     

Fretting of WC/a-C:H and Cr2N Coatings Under Grease-Lubricated and Unlubricated Conditions

The fretting phenomenon was investigated experimentally in contacts between coated and uncoated steel rod and ball specimens generating a circular Hertzian contact. A fretting wear test rig equipped with a video camera was used to observe the effects of fretting on coated steel surfaces in both grease-lubricated and unlubricated environments. Tungsten carbide reinforced amorphous hydrocarbon (WC/a-C:H) and chromium nitride (Cr₂N) coatings were tested and compared. Fretting wear volumes and surface profiles are presented for both grease-lubricated and unlubricated conditions. Videos of a coated ball fretting against a transparent sapphire flat were recorded and screen captures are presented. The role of normal load, lubrication, frequency, and amplitude of motion on the fretting wear of coatings is discussed. The lubricant released from the grease was observed to flow through channels in the stick zone of the fretting contacts. Both coatings were found to reduce fretting wear. WC/a-C:H was more effective at reducing wear under unlubricated conditions. WC/a-C:H decreased fretting wear more than Cr₂N when delamination was avoided in grease-lubricated contacts.     

Some Observations on Frictional Force During Fretting Fatigue

Frictional force behavior during fretting fatigue and its interdependence on other fretting variables are investigated. Both coefficient of static friction and the normalized frictional force (i.e., the ratio of frictional force and normal contact load) increase during the earlier part of a fretting fatigue test and then both reach to a stabilized value. The variation of temperature in the contact region and normalized frictional force with increasing cycle numbers and bulk stress show similar trend implying that normalized frictional force represents the average friction in the contact region during a fretting fatigue. An increase in bulk stress, relative slip, and hardness of pad material results in an increase of the normalized frictional force, while an increase in contact load, frequency and temperature decreases the normalized frictional force. The normalized frictional force is also affected by the contact geometry. On the other hand, coefficient of static friction increases with an increase in the hardness of mating material, temperature and roughness from shot-peening treatment, but is not affected by contact geometry and displacement rate. Further, the normalized frictional force is not affected by the contact geometry, roughness and applied bulk stress level when fretting fatigue test is conducted under slip controlled mode, however it increases with increasing applied relative slip and decreasing contact load in this case.     

Coatings tribology—contact mechanisms and surface design

The fundamentals of coating tribology are presented by using a generalised holistic approach to the friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, nanomechanical and tribochemical contact mechanisms, and material transfer. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design regarding both friction and wear can be achieved by new multi-layer techniques which can provide properties such as reduced stresses, improved adhesion to the substrate, more flexible coatings and harder and smoother surfaces. The differences between contact mechanisms in dry, water- and oil-lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel and an alumina ball. The mechanisms of the formation of dry transfer layers, tribolayers and lubricated boundary and reaction films are discussed.     

Fretting fatigue of zinc coated low carbon steel EN H320 M

The object of the present study was to investigate the influence of zinc coatings on steel sheets during fretting fatigue and fatigue tests. The influence of the fatigue stress range, normal pressure and amplitude of slip, on the fracture life was studied for both coated and uncoated EN H320 M steel. The wear produced by fretting was measured and compared with the fracture life evolution for different values of slip amplitude. The wear scars and the fracture surfaces were examined by scanning electron microscopy to identify the degradation mechanism. Although zinc films do not influence the fatigue life of the tested steel, when fretting is superposed on to a fatigue stress the coating markedly improves the fracture life.     

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.     

Palliatives in fretting: A dynamical approach

Fretting damages are connected to numerous aspects like friction, wear, contact mechanics, fatigue and material sciences. Its quantification also requests to consider the loading history as well as the sliding condition. Based on a “fretting sliding” approach, and considering fretting wear test conditions, various palliative solutions have been investigated. Shot peening treatment, introducing compressive residual stresses, appears pertinent against crack propagation but ineffective against crack nucleation due to the activation of surface relaxation phenomena. Hard thin coatings present stable residual stresses independently of the sliding conditions. However, they only delay the crack nucleation process, when the coating is worn through, cracking phenomena are activated. To quantify the coating endurance against wear, an energy density approach has been developed. The stability of this approach has been confirmed regarding the contact size effect and illustrated through the analysis of synergic interaction between soft thick coating and solid lubricant.     

Fretting wear characteristics of cold gas-dynamic sprayed aluminum alloys

Process selection for repair of mechanical components due to wear and corrosion, e.g. damage of aluminum casting housings of fuel injection systems, is based on cost and response time factors, provided that the mechanical performance is maintained within acceptable limits. One of the promising and emerging repair technologies is Cold Gas-Dynamic Spray (CGDS) coating, where a high-pressure gas propels fine powder particles to very high velocities to produce surface coating. It is essential to identify the optimum process conditions and powder composition to produce repaired surfaces with tribological properties close to those of the originally manufactured part (without coating). The objective of this work is to compare the dynamic friction and fretting wear properties of the repaired surfaces using various types of coating composition and spraying techniques. Eight types of CGDS coatings, applied to AMS 4260 aluminum specimens, were fretted against 440C stainless steel specimens at low and high nominal loads to assess their fretting wear resistance, dynamic friction properties and damping capacity. The optimum coating composition and process conditions were identified. In comparison to the uncoated specimen, this optimum coating offered tribological characteristics close to the uncoated material and even better dynamic friction properties.     

Comprehensive tribological characterization of thin TiN-based coatings

Innumerable papers have been published so far describing tribological investigations of thin hard coatings based on TiN. Analysis of the presented results demonstrates a large dispersion of measured friction and wear numbers, whereas TiN-coated pieces and tools have proved their benefits in a broad area of application. Therefore an attempt was made to clarify the influences on friction and wear test results by varying the coating process, the tribological stresses due to sliding, fretting and rolling motion and by changing the surrounding medium. The results reveal that machining of substrate surfaces and type of tribological stresses due to sliding, fretting and rolling have an important influence. The formation of reaction layers is dominating the tribological behaviour in most cases.     

Fretting Fatigue Behavior of Cavitation Shotless Peened Ti–6Al–4V

Fretting fatigue behavior of cavitation shotless peened (CSP) titanium alloy, Ti–6Al–4V was investigated. Constant amplitude fretting fatigue tests were conducted at several maximum stress levels, σ_max, ranging from 400 to 555 MPa with a stress ratio of 0.1. Test results showed that the fretting fatigue life was enhanced by CSP treatment as compared to the unpeened specimen, but the enhancement was not as large as that from the shot-peening treatment. Residual stress measurements by X-ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Before fretting, CSP specimen had higher compressive residual stress on the surface than the shot-peened specimen. However, greater residual stress relaxation occurred in CSP specimen such that the relaxed compressive residual stress profile near the contact surface of CSP specimen was lower than that of shot-peened specimen. This lower compressive residual stress from fretting fatigue was the reason for shorter fretting fatigue life of CSP specimen as compared to shot-peened specimen at the applied stress level.     

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.