Andrew W. Batchelor

Some considerations on the mitigation of fretting damage by the application of surface-modification technologies

Fretting is a surface-degradation process due to mechanical and chemical attack by small-amplitude oscillatory movement between two contacting surfaces and it is intimately related to wear, corrosion and fatigue. The introduction of surface treatments or coatings is expected to be an effective strategy against fretting damage. This paper discusses the application of several types of advanced surface-modification methods for the mitigation of fretting damage, such as physical and chemical vapour deposition (PVD and CVD), ion implantation, laser treatment and plasma nitriding, etc. Some coatings are effective in the mitigation of the fretting wear, whereas others are more effective under fretting fatigue conditions. The effects of surface-modification methods on fretting resistance are explained using fretting maps. There are at least five different mechanisms in using surface-modification methods to increase fretting resistance: (1) inducing a residual compressive stress; (2) decreasing the coefficient of friction; (3) increasing the surface hardness; (4) altering the surface chemistry; (5) increasing the surface roughness. Apart from this, the intrinsic properties of the coatings, such as their density and mechanical and chemical properties as well as the adhesion condition with the substrate, also significantly affect the performance of the coatings under fretting conditions. Based on this rationale, a coating-selection method was proposed to select the most appropriate surface treatments or coatings to minimise the probability of fretting damage. Selection of a process is guided primarily by identification of the fretting failure modes, and the ability to adjust and obtain the required surface properties, with a balance between the precise control of the surface properties and the process cost.

Improvement in fretting wear and fatigue resistance of Ti–6Al–4V by application of several surface treatments and coatings

Application of surface modification methods is expected to be an ideal solution to mitigate fretting damage. In this study, our aim was to improve the fretting wear and fretting fatigue resistance of titanium alloys by using several types of surface treatments and thin films, including shot-peening, ion-beam-enhanced deposition (IBED) CrN films, shot-peening+IBED CrN films as well as IBED CuNiIn films. Results showed that with the application of all the above surface coatings and treatments, the fretting wear and fretting fatigue resistance of Ti–6Al–4V were improved. However, the mechanisms and effects of several surface modification methods to mitigate the fretting damage were quite different. IBED CrN film exhibited the best fretting fatigue performance while the duplex treatment by shot-peening/IBED CrN film exhibited the highest fretting wear resistance. There are four mechanisms which can be used to explain the different fretting performance of these surface treatments and coatings: (1) to induce a compressive residual stress; (2) to decrease the coefficient of friction; (3) to increase the hardness; (4) to increase the surface roughness.

Fretting wear behaviors of thermal sprayed hydroxyapatite (HA) coating under unlubricated conditions

Abstract Fretting damage (fretting wear and fretting fatigue) is one of the modern plagues for the orthopedic implants because the oscillatory micromovement often often occurs at the interface between the implant and bone. In this investigation, the fretting wear behaviors of plasma sprayed hydroxyapatite (HA) bioceramic coatings on titanium alloy substrate were investigated under unlubricated conditions as a function of number of oscillatory cycles, normal load and amplitude. Coefficient of friction, wear volume and wear mechanisms were studied. For the different test conditions in this study, the fretting regime is gross-slip. Results showed that the fretting wear mechanisms of plasma sprayed bioceramic coating were mainly delamination and abrasive wear, whereas the wear mechanisms of the substrate Ti alloys were oxidation, delamination as well as abrassive wear. Under unlubricated condition, the fretting wear resistance of bioceramic coatings was not as good as that of Ti6Al4V substrate due to the porous, lamellar and loose structure of HA coating.

Wear behaviour of laser-treated plasma-sprayed ZrO2 coatings

Abstract The effect of laser remelting on the wear behaviour of plasma-sprayed ZrO 2 ceramic coatings was studied. The results showed that the porosity and roughness of the coatings were reduced significantly after laser treatment, and the bonding strength was apparently increased by the remelting process. However, there were extensive network cracks, as well as a few large bubbles, in the laser-treated coatings. Unlubricated pin-on-disc wear tests revealed that, when compared with as-sprayed ceramic coatings, the wear resistance was improved significantly after laser treatment. The wear resistance of laser-treated specimens increased with increasing laser power, and the minimum weight loss of the coating specimen occurred at a specific travel speed in laser processing. The main wear mechanism of the as-sprayed coatings was spallation of the coating, whereas the wear of laser-remelted specimens was dominated by ploughing and gouging (scratching).

Friction and wear changes in synovial joints

Abstract Synovial joints have complex and poorly understood friction and wear properties. The relative importance of synovial fluid and cartilage in terras of lubrication is still unclear. The friction and wear characteristics of cartilage surface was investigated by sliding the orthopaedic cartilage of an adult rat femur against a stainless steel plate. Tests were performed dry and with irrigation by synovial fluid or saline solution. Friction and wear of the cartilage were initially low but increased in severity as a superficial lubricating layer was progressively removed by wear. Irrigation of the cartilage by synovial fluid reduced friction to very low levels, but saline solution had no lubricating effect. Microscopic examination of worn cartilage surface showed that low friction coincided with limited damage to the cartilage surface. It has been concluded that the outer surface of orthopaedic cartilage is covered by a substance capable of providing lubrication for limited periods when synovial fluid is unable to prevent contact between opposing cartilage surfaces.

THE EFFECT OF METAL TYPE AND MULTI-LAYERING ON FRICTION SURFACING

Abstract Friction surfacing was attempted with a stainless steel, aluminium and brass consumables on a mild-steel substrate in the open air and in flowing nitrogen. Stainless steel formed a strongly-bonded thick layer but friction surfacing with both aluminium and brass was not possible. Both brass and aluminium consumables failed to form a heated layer in contact with the mild steel, high thermal conductivity of either metal being the probable cause of failure to perform friction surfacing. Nitrogen ventilation caused cooling of the heated layer between the stainless steel and the mild-steel substrate and lowered the quality of the coating. Multi-layer surfacing by stainless steel consumables was successful, with at least three strongly-bonded layers possible, provided that the surfaces are free of oily contaminants.

Friction surfacing of metal coatings on steel and aluminum substrate

Friction surfacing of (i) tool steel, inconel, aluminum and titanium rods onto mild steel substrates and (ii) stainless steel, mild steel and inconel onto aluminum substrates was investigated. It was found that tool steel and inconel were efficiently deposited onto steel to form a dense strong coating while aluminum was only deposited at high contact pressures. Titanium could not be deposited under the tested conditions. Stainless steel, mild steel and inconel could be deposited onto aluminum substrates to form dense coatings, however only SS displayed a lack of intermetallic compound formation with Al. Melting of Al by frictional heat was observed and this led to the formation of brittle and therefore undesirable intermetallic compounds at the interface of the coating. X-ray microscope investigations of the coatings confirmed that the structure of the coating is a continuous series of discrete layers inclined at a shallow angle to the phase of the substrate. Fissures were found between these layers close to the outer surface of the coating, which implies that the uppermost layers should be removed to obtain the best quality of coating. Microscopic observations revealed that there was little mixing between the coating and the steel substrate but some surface roughening and sub-surface deformation of the steel substrate by friction surfacing. The Al-alloy substrate showed similar types of surface alteration but to a far greater extent. Careful control of process parameters such as substrate, speed, load and consumable was necessary for coating quality.

Laser nitriding of pure titanium with Ni, Cr for improved wear performance

Titanium and its alloy suffer from galling, seizing, ploughing and adhesion during sliding contacts. A laser nitriding method to improve the tribological properties of pure titanium was investigated in this study. Before laser nitriding, a layer of nickel and chromium was plasma sprayed onto the surface of pure titanium in order to prevent the formation of cracks and pores during laser treatment. A scanning electron microscope (SEM) and image analyser were used to optimize the laser processing parameters and analyse the coating microstructure. X-ray diffraction (XRD), a Talysurf 5 tester and microhardness tester were used to analyse the phase crystallinity, surface roughness and micro-hardness of the laser treated coating. A ball-on-flat reciprocating wear machine was used to evaluate the sliding and fretting wear behaviours of both untreated and laser treated specimens. Results showed a substantial improvement in wear resistance after laser treatment which can be attributed to a significant improvement in hardness and corrosion resistance as well as a decrease in friction properties of nitrided layers.

Structure and tribological properties of plasma nitrided surface films on Inconel 718

Abstract Inconel 718 specimen was plasma nitrided for microscopic examination and tribological testing. Film structure and composition, depth of film hardness and surface roughness were measured as a function of coating parameters. The film growth rate appears to be diffusion controlled, (i.e. case depth ∝ √treatment time). Nitrogen was detected in the surface of the nitrided layer by Auger electron spectroscopy. The nitrogen was found to react with Cr to form CrN which was detected in the nitrided layer by X-ray diffraction analysis. Friction and wear coefficients as a function of atmosphere, load, sliding speed, and surface temperature were performed. Exterior film structure varied from columnar or pyramidal with high roughness to a smoother spherical structure after nitriding at a lower temperature. The distribution of nitrogen within the film appeared uniform and there was a sharp boundary between the film and substrate. A maximum microhardness of 1290 HK was reached after prolonged nitriding. Nitrided surfaces showed 4 times lower coefficient of friction than a plain Inconel surface until the nitrided layer was worn away. Coarse lamellar wear particles showing signs of extreme plastic deformation were formed during wear of nitrided inconel.

Materials degradation and its control by surface engineering

Mechanisms of Materials Degradation: Mechanical Causes of Materials Degradation Chemical Causes of Materials Degradation Materials Degradation Induced by Heat and Other Forms of Energy Duplex Causes of Materials Degradation Surface Engineering: Discrete Coatings Integral Coatings and Modified Surface Layers Characterization of Surface Coatings Application of Control Techniques: Control of Materials Degradation Financial and Industrial Aspects of Materials Degradation and Its Control.