The roles of contact conformity, temperature and displacement amplitude on the lubricated fretting wear of a steel-on-steel contact

This paper investigates the effect of contact geometry, temperature and displacement amplitude on the fretting behaviour of an aero-turbo oil lubricated cylinder-on-flat contact. To be effective, the lubricant needed both to penetrate the contact and then offer protection. Lubricant penetration into the fretting contact is found to be controlled by two physical parameters, namely (i) the width of the contact that remains covered throughout the fretting test and (ii) the lubricant viscosity. The protection offered by the lubricant (assuming that it has successfully penetrated the contact) is influenced by four physical parameters, namely (i) lubricant viscosity, (ii) traverse velocity, (iii) nominal contact pressure, and (iv) chemical effects. The relationship between the three experimental parameters which were varied in the programme of work (temperature, fretting displacement and cylinder radius) and physical parameters which influence the protection offered by the lubricant film can be competing, and therefore complex wear behaviour is observed. The roles of the various parameters in controlling the wear behaviour are presented in a coherent physical framework

Fretting wear mapping: the influence of contact geometry and frequency on debris formation and ejection for a steel-on-steel pair

This paper examines the influence of contact geometry and oscillation frequency in a steel cylinder-on-steel flat fretting contact, with contact geometry being varied via the cylinder radius. Fretting frequency did not significantly impact the wear behaviour for more-conforming contacts, but did so for less conforming contacts where at high frequency, the wear rate is 50 % of that observed for low frequency fretting. It is proposed that frequency and contact conformity fundamentally control wear behaviour through influence of both the debris type and the retention or ejection of that debris from the contact. The debris type (either oxide or metallic) is influenced by fretting frequency (which controls the interval between asperity contacts), and by contact conformity (which controls the distance that oxygen has to travel to fully penetrate the contact). Debris retention within the contact is promoted by higher fretting frequencies (the associated higher contact temperature promotes debris agglomeration and sintering in the contact) and by higher contact conformity (which acts as a physical barrier to debris egress). Maps are presented which categorize the observed behaviour and outline a phenomenological framework by which the basic physical processes which influence fretting behaviour can be understood

The dependence of wear rate on wear scar size in fretting: the role of debris (third body) expulsion from the contact

The paper ‘The third-body approach: a mechanical view of wear’ by Maurice Godet (Wear, 100 (1984), pp 437–452) was perhaps the first to articulate clearly the key role of the rate of debris expulsion from a fretting contact in controlling the overall rate of wear. Whilst subsequent research over the past four decades has acknowledged this, the issue is generally addressed qualitatively rather than quantitatively. There are many parameters which will affect the rate of debris expulsion from a fretting contact, and amongst them is the physical size of the fretting contact. In this paper, for the first time, a physically-based relationship is proposed between the debris-expulsion limited wear rate and the contact size. This relationship is able to account for differences in wear rates observed in tests conducted with different (and evolving) contact geometries (non-conforming contacts) over a range of durations, thus clearly demonstrating the validity of the approach

Investigation on the plasticity accumulation of Ti-6Al-4V fretting wear by decoupling the effects of wear and surface profile in finite element modelling

A finite-element-based wear modelling methodology and a computational device for decoupling wear effects is presented in this study. The decoupling of wear effects facilitates the capture of plasticity accumulation on a particular wear-scarring profile after a specific number of cycles. It was determined that significant plasticity accumulation due to plastic shakedown was predicted in a partial-slip case, while a saturation of plastic deformation was predicted in a gross-sliding case. It was also predicted that a significant amount of plasticity does not meaningfully contribute to the stress and strain range observed in the contact region. It was assumed that plasticity accumulation contributes towards wear of the material and feeds the stress changes, which indirectly affects fatigue life

Abrasive wear behaviour of conventional and large-particle tungsten carbide-based cermet coatings as a function of abrasive size and type

Abrasive wear behaviour of materials can be assessed using a wide variety of testing methods, and the relative performance of materials will tend to depend upon the testing procedure employed. In this work, two cermet type coatings have been examined, namely (i) a conventional tungsten carbide-cobalt thermally sprayed coating with a carbide size of between ∼0.3 – 5 μm and (ii) a tungsten carbide-nickel alloy weld overlay with large spherical carbides of the order of ∼50 – 140 μm in diameter (DuraStell). The wear behaviour of these two materials has been examined by the use of two abrasion tests, namely the micro-scale abrasion test using both silica and alumina abrasives (typically 2-10 μm in size), and the dry sand-rubber wheel test (ASTM G65), again with both silica and alumina abrasives (typically 180 – 300 μm in size). It was found that when the abrasive particles were of the same scale or larger than the mean free path between the hard phase particles, then the matrix phase was well protected by the hard phases. Testing (in both test types) with alumina abrasives resulted in wear of both the hard carbide phases and the matrix phases in both the thermally sprayed coating and the weld overlay, with the thermally sprayed coating exhibiting lower wear rates. The wear behaviour of the materials with the more industrially relevant silica abrasive was more complex; the thermally sprayed coating exhibited a lower wear rate than the weld overlay with the fine abrasive in the micro-scale abrasion test due to effective shielding of the matrix from abrasive action due to the fine reinforcement particle size. In contrast, with the coarser silica abrasive in the dry sand-rubber wheel test, the weld overlay with the large carbides was able to provide matrix protection with low rates of wear, whereas the thermally sprayed coating wore by fracture of the more brittle microstructure. These findings demonstrate the importance of selection of appropriate laboratory test procedures and abrasives to simulate behaviour of materials in service environments

An experimental study on the key fretting variables for flexible marine risers

This paper presents an experimental investigation into the effects of contact conformity, contact pressure and displacement amplitude on the gross-slip fretting behaviour grease-lubricated cylinder-on-flat contacts in the context of flexible marine riser pressure armour wire, and compares behaviour with that observed in unlubricated conditions. Characterisation of friction and wear is critical to fretting fatigue life prediction in flexible risers since friction directly controls trailing-edge fretting stresses and hence fatigue crack initiation, on the one hand, and on the other hand, directly affects wear via relative tangential slip (displacement). Wear can have a beneficial or detrimental effect on fatigue crack initiation and propagation, depending on relative slip and slip regime. It is shown that friction and wear are higher for dry conditions than for grease-lubricated conditions. For grease-lubricated conditions, behaviour is determined by whether grease can be retained in the contact (as opposed to being extruded out). Retention (or replenishment) of grease in the contact results in low rates of wear and low coefficients of friction; these conditions are favoured by fretting displacements above a critical value, by low contact conformity, and by low applied loads

Debris development in fretting contacts: debris particles and debris beds

In this study, the formation and destruction of compacted beds of oxidized debris particles are investigated. Fretting tests of steel specimens were conducted, employing a cylinder-on-flat geometry with displacement amplitude being varied. The debris was examined, both in the form of the debris beds and in the form of loose debris; together, these were characterized to better understand the mechanisms of debris bed formation and development throughout the fretting process. XRD was employed to determine the phase makeup of the debris, with SEM imaging and particle size analysis employed to understand the evolution of the debris from nanoparticles into agglomerates and then into sintered bed structures

Removal of heat-formed coating from a titanium alloy using high pressure waterjet: Influence of machining parameters on surface texture and residual stress

© 2015 Elsevier B.V. All rights reserved. Titanium alloys are widely used in the aerospace and medical industries owing to their high strength to weight ratio and outstanding corrosion resistance. A problem for titanium or titanium alloys is the existence of a hard, brittle and oxygen-enriched layer on the surface (so called alpha case). This is usually formed during hot forming processes or after long-term service at elevated temperatures in an open-air environment. With the development of waterjet systems, high pressure waterjet has shown its capability for the removal of such hard and difficult-to-machine coatings. Waterjet machining is usually associated with a surface roughening, which is unwanted for most of aerospace applications, but is beneficial for medical application where fixation is required (e.g. metal orthopedic implants). A potential benefit of waterjet material removal is that the process may introduce compressive residual stress to the machined surface and subsurface layers. In this study, Ti-6Al-4V with an alpha case layer was subjected to plain waterjet impact over a range of parametric conditions, to fully remove the alpha case layer. The resulting surfaces were then analyzed to demonstrate the influence of process parameters on both surface roughness and residual stress measured using X-ray diffraction (XRD)

Influence of KCl and HCl on a Laser Clad FeCrAl Alloy: In-Situ SEM and Controlled Environment High Temperature Corrosion

This study investigated the effects of HCl and KCl on a laser-clad FeCrAl coating at 450 °C in an in-situ ESEM followed by a furnace exposure. In all in-situ TEM cross-sections, three major phases were identified: an iron rich oxide, an iron-chromium mixed oxide and an aluminium enriched layer. HCl allowed chlorine based corrosion to occur which suggests interaction from its gas phase. EDX of the regions around KCl crystals showed a decrease in chromium which is an indication of chlorine selectively removing chromium. Moreover, the mass gain in HCl with KCl was significantly lower than that observed in air with KCl

Sliding wear of a self-mated thermally sprayed chromium oxide coating in a simulated PWR water environment

Bearing surfaces in the primary circuit of pressurized water reactors (PWR) are prone to damage due to aggressive chemical and tribological conditions under which they operate, and a wide range of materials have been examined in this regard. One of the most promising candidates is chromium oxide in the form of a thermally spayed coating, and in this work, the behaviour of a commercially available Cr2O3 coating in self-mated sliding was considered. Tests consisted of a number of start-stop cycles of sliding between a crowned pin and a rotating disc in a water environment in an autoclave in an attempt to simulate the most aggressive phase of bearing run-up and run-down. Wear and damage mechanisms were examined at temperatures from ambient up to 250 C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS). Across the temperature range, wear was mild, with no evidence of coating delamination. A five-fold increase in wear was observed between 80 C and 250 C (with wear depths of generally less than 8 µm being observed on the disc samples even at the higher temperature), despite there being only very small changes in hardness of the coating over the same temperature range. Debris was observed on the wear tracks following testing, with the evidence together suggesting that this debris was a very fine-grained mixture of Cr2O3 and amorphous -CrOOH, a corrosion product of Cr2O3