Progress in fretting maps

In recent years, fretting maps on fretting wear and fretting fatigue conditions proposed by various researchers have been frequently cited in literature. In this paper, four kinds of main fretting maps have been reviewed in detail. Some ambiguity, contradiction, deficiency and common ground have been noted on these maps. The application and further research of the fretting maps has also been noted.     

Elevated temperature fretting fatigue of Ti-17 with surface treatments

Fretting fatigue of Ti-17/Ti-8-1-1 contacts at 316 °C is examined experimentally. Different surface treatments are analyzed, including coatings, lubrication, and levels of shot peening. The evolution of friction is examined for a range of surface treatment. Fretting fatigue life for baseline specimens are obtained for a range of load parameters to determine loads that yield fretting fatigue lives of approximately 100,000 cycles. This applied load level was maintained constant for the different combinations of surface treatments to investigate the influence of surface treatment on fretting fatigue life. The Cu-Ni-In and Al-Br coatings and MoS₂ and Everlube lubricants are removed early in the fretting fatigue experiment; hence these surface treatments had little effect on fretting fatigue life. Shot peening increases fretting fatigue lives by about 60%. Block loading experiments show that minor cycles reduce fretting fatigue life.     

Influence of surface-coatings on titanium-alloy resistance to fretting fatigue in cryogenic environment

Fretting damage, also known as small amplitude oscillatory sliding motion, can lead to catastrophic failure in many industrial applications. The understanding of fretting fatigue and its reproduction in laboratory tests have enable us to evaluate the fretting resistance of homogeneous substrate. To reduce the damage caused by fretting fatigue increasing use has been made of coatings or heat treatments which result in non homogeneous solids. From a theoretical point of view, ascertaining the mechanical behaviour of materials so modified is quite complex due to insufficient definition of the contact parameters. This present study seeks to analyse a layered medium undergoing fretting fatigue and the improvement of its fatigue criterion.     

Effect of contact load on fretting fatigue behaviour of steel wires

Abstract

The tension–tension fretting fatigue tests of steel wires were performed on a self-made fretting fatigue test equipment under contact loads ranging from 40 to 70 N and a strain ratio of 0·8. The results showed that when the contact load increased, the fretting regime of steel wires transformed from gross slip regime to mixed fretting regime. The fretting fatigue life in the mixed fretting regime was significantly lower than that in the gross slip regime. The main fretting wear mechanisms in the gross slip regime, where there were serious fretting damage and a lot of wear debris, were abrasive wear and fatigue wear. Microcracks were observed in the fretting scar of the mixed fretting regime, and the main fretting wear mechanisms were adhesive and fatigue wears. The fretting wear scar was the fatigue source region, and the fatigue fracture surface could be divided into three regions.     

Improvement of the fretting damage resistance of Ti-811 alloy by Cu/Ni multilayer films

The Cu/Ni multilayer films were prepared on the titanium alloy surface by ion-assisted magnetron sputtering deposition (IAD) technique. The Cu/Ni multilayer films could significantly improve the resistance of fretting wear and fretting fatigue (FF) of Ti-811 alloy at room temperature. The FF resistance of the titanium alloy substrate did not increase monotonically with increase in the modulation period thickness of the Cu/Ni multilayer films. The Cu/Ni multilayer films with modulation period thickness of 200 nm had the highest FF resistance among the prepared Cu/Ni multilayer films for its comprehensive properties with high toughness, high strength and good lubricating action.     

Quantitative analysis of fretting fatigue degradation in 7075-T6 aluminum alloy

Fretting fatigue failures are commonly observed in the aviation industry. The objective of this study was to understand the fretting fatigue mechanism by characterization of fretting fatigue degradation to gain insight into the process of crack formation from pits in 7075-T6 aluminum alloy. This paper focuses on the quantitative analysis of fretting fatigue degradation in terms of pit depths and dominant crack formation. For 60 percent of the specimens, the dominant crack nucleated from a pit other than the maximum-depth pit observed on the fracture surface.     

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.     

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.     

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.     

Progress in standardization of fretting fatigue terminology and testing

This paper reviews the current ASTM, ISO, and other standards that pertain in part to fretting fatigue and fretting wear testing. A historical perspective gives some background on why there still are relatively few standards for fretting fatigue and fretting wear testing. Current standards on the books tend to be application specific. In the past few years, there have been some new activities in standardization. These developments along with future needs in standardization are discussed.     

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.     

On the mechanisms of various fretting wear modes

According to relative motion directions for a ball-on-flat contact, there are four fundamental fretting wear modes, e.g., tangential, radial, torsional and rotational modes. In this paper, the mechanisms of these four fundamental fretting wear modes, particularly for the later three modes, have been reviewed from results obtained by the authors in combination with results from literature. Some general features have been reported. Differences both in running and degradation behavior have been discussed in detail. Results showed that some similar laws for three fretting regimes (partial slip regime, mixed regime and slip regime), fretting maps (running condition fretting map and material response fretting map), wear and cracking mechanisms obtained from the classic mode (i.e. tangential fretting) were also identified and useful to characterize the other modes. Nevertheless, the occurrence of relative slip for the radial fretting, the formation of mixed regime for the torsional fretting, the evolution of surface morphology for the rotational fretting were quite different compared to that of the classical fretting mode.     

Observations on fretting damage transition to cracking; state of the art and preliminary observations

Interrupted fretting fatigue experiments were performed on 7075-T6 aluminum alloy and fretting damage characterized by confocal and scanning electron microscopy. Strain, induced by specimen fatigue, produces a small amplitude oscillatory motion between the fatigue specimen and the fretting pad. A fretting fatigue damage threshold exists in this material. Hundred percent fretting fatigue was defined as the average total cycles to fracture based on specimens 1–5 (both axial and normal forces were applied). Specimens had fretting damage induced at 100%, 80%, 60%, 40%, and 20% of the maximum fretting fatigue cycles to fracture. A positive correlation was not found between the depth of fretting damage and crack formation, but there appeared to be a stronger relationship between the fretting damaged surface areas, proximity of pits and crack nucleation sites.     

The tribological performance of DLC coatings under oil-lubricated fretting conditions

Whether or not the process of fretting occurs is to a large extent dependent on the coefficient of friction, because the coefficient of friction directly affects the amount of shear stress. As a result, the key factor when it comes to reducing the amount of fretting damage is to reduce the coefficient of friction. Various surface coatings, and especially hard, diamond-like carbon (DLC) coatings, are known to be able to produce surfaces with a low level of friction. Despite some such attempts in the past, which did not result in major improvements, the developments and improvements in DLC coatings in recent years suggest the need for a re-evaluation of these coatings for fretting applications. Another way to reduce the amount of friction in mechanical components is to apply lubricants, and recent studies on the lubrication of DLC coatings suggest that this combination could be very successful in preventing failures under boundary-lubrication conditions. Therefore, in this work we present the results of friction and wear measurements from three types of fretting contacts: steel/steel, steel/DLC and DLC/DLC. Boundary oil-lubrication conditions were investigated and a wide range of displacement amplitudes, i.e., from 25 to 500 μm, were selected to assess the fretting and sliding behaviours. The results show a significant difference between the fretting and sliding regimes. In the fretting regime, the DLC-containing contacts, and especially the self-mated DLC/DLC contacts, performed much better than the steel/steel contacts, and significantly reduced both the wear (a 3–10 times reduction with steel/DLC and DLC/DLC) and the friction (a more-than-two-times reduction with DLC/DLC). In the sliding regime, the lubrication effects governed the tribological performance, making the results for all three material combinations very similar.     

High temperature fretting wear behavior of WC-25Co coatings prepared by D-gun spraying on Ti-Al-Zr titanium alloy

Detonation gun (D-gun) spraying is one of the most promising spraying techniques for producing wear-resistance coatings. A thick layer (about 0.3 mm thickness) of WC-25Co with high hardness was covered on Ti-Al-Zr titanium alloy by D-gun spraying and the fretting wear behavior of WC-25Co coatings was studied experimentally on a high precision hydraulic fretting wear test rig. An experimental layout was designed to perform fretting wear tests at elevated temperatures from room temperature (25 °C) to 400 °C in ambient air. In the tests, a sphere (Si₃N₄ ceramic ball) was designed to rub against a plane (Ti-Al-Zr titanium alloy with or without WC-25Co coatings). It was found that the fretting running regimes of WC-25Co coatings were obviously different from those of Ti-Al-Zr titanium alloy. The mixed fretting regime disappeared in WC-25Co coatings, and the boundaries in the running condition fretting map (RCFM) showed hardly any change as temperature increased. The worn scars were examined using a laser confocal scanning microscope (LCSM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the coefficients of friction (COF) of WC-25Co coatings at elevated temperatures were nearly constant in the partial slip regime and very low in the steady state. The fretting damage of the coatings was very slight. In the slip regime, the WC-25Co coatings exhibited a good wear resistance, and the wear volume of the coatings obviously decreased with increasing tested temperature. The fretting wear mechanisms of WC-25Co coatings were delamination, abrasive wear and oxidation wear at elevated temperature. The oxide debris layer formed at higher temperature was denser and thicker on top of WC-25Co coatings, thus providing more surface protection against fretting wear, which played an important role in the low fretting wear of the coatings.     

Friction behavior of quenched and tempered steel in partial and gross slip conditions in fretting point contact

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.     

An experimental study on bending fretting fatigue characteristics of 316L austenitic stainless steel

Bending fretting fatigue tests of 316L austenitic stainless steel plates against 52100 steel cylinders have been carried out under same normal load and varied bending loads. Tests of plain bending fatigue were carried out as a control group. The S-N curves of the bending fatigue were made. The results indicated that there was an obvious drop of life under the condition of bending fretting fatigue due to higher local contact stress. A dislocation model of micro-crack nucleation mechanism, as a manner of zig-zag mode, was created to explain the nucleation of fretting fatigue cracks.     

Modeling of geometry effects in fretting fatigue

The stress field that results from two bodies in contact is an important aspect that governs the fretting fatigue behavior of materials. Applied loads as well as contact geometries influence the contact stresses. The profile of an indenter and the boundary conditions provide sufficient information from which the surface tractions and the corresponding subsurface stresses have been calculated in a semi-infinite halfspace using singular integral equations. In this investigation, a numerical subroutine was developed to calculate the surface tractions and the corresponding surface and subsurface stresses of an arbitrary finite thickness infinite plate subjected to loading through a random indenter. The results from the detailed stress analysis of the contact region are required by both an initiation and fracture mechanics approach. While initiation criteria involving stress gradient fields, such as sharp notches and edges of contact in fretting fatigue, are not well established or agreed upon, stress intensity factor calculations using tools such as weight functions are more reliable. The stress intensity analysis, which is used to determine whether an initiated crack will continue to grow if it is above the threshold, depends on many variables in the stress analysis such as pad and specimen geometry, loading configuration and friction coefficient. The contact stress analysis has been used to determine equivalent stress parameters that are related to the initiation of a crack. Similarly the numerical subroutine for the contact stresses is used in conjunction with the stress intensity analysis to determine the influence of the geometry, loading configuration and friction coefficient on the stress intensity factor. Results from high-cycle fretting fatigue experiments are used to determine the threshold stress intensity factor for a given configuration. The combination of the numerical and experimental analysis is then used to develop a tool for high-cycle fretting fatigue based on a threshold approach involving a go–no go criterion.     

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.     

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.