a b s t r a c tTriode plasma nitriding was used in conjunction with electron-beam plasma-assisted physical vapour deposition of TiN and CrAlN to enhance the wear resistance of Ti-6Al-4V titanium alloy. Linear reciprocating-sliding ball-on-plate wear tests were performed to assess the tribological performance of the treated alloy. Wear volumes were correlated to changes in coefficient of friction, which is often indicative of breakdown of the surface treatment. Debris generated during wear testing was characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and laser diffraction for particle size analysis. Surface micro-profilometry and SEM were used to characterise the wear scars. The results obtained indicate that, in order to assess the relative improvement in wear behaviour attained by using surface treatments it is insufficient to compare linear wear rates alone. A clear comparison is only possible if testing is carried out in steps of increasing sliding distance, until the treated/deposited layers have been completely removed. Also, it is shown that the number of repeated tests necessary to evaluate clearly the treatment and/or coating can vary substantially, depending on the observed test progression; typically this necessitates the greatest number of repeats around the point of wear at which the ball counterface contact area is in transition from the treatment layer(s) to the substrate bulk. The inherent variability in wear performance under linear ballon-plate reciprocating-sliding is correlated to debris generation, changes in relative humidity -and the resultant wear mechanisms involved.

a b s t r a c tThe effect of triode-plasma enhanced low-pressure oxygen and/or nitrogen diffusion treatments, either as a single process or in conjunction with plasma-assisted physical vapour deposition (PAPVD) on Ti6Al-4V has been studied under rotating-bending fatigue testing. Following the diffusion treatment, samples exhibit a hardened case, more than 30 lm deep. Semi-logarithmic S-N plots are used to demonstrate and compare the significant changes in fatigue resistance obtained from each process. Fractography and residual stress measurements show that, compared to annealed samples, the fatigue strength of the diffusion-treated samples was superior; although the result changed depending on the processing parameters and microstructure of the substrate material. Also, unsupported and mechanically uncompliant ceramic coatings, such as TiN, promote the initiation of multiple crack sites, which lead to premature failure of the Ti-alloy substrate and a consequent reduction in endurance limit.

In this paper dynamic ball-on-plate impact wear testing is utilised to evaluate the intrinsic fatigue strength of the surface of triode plasma diffusion treated, single-layered TiN-, CrAlN-, and WC/C-coated and duplex diffusion treated/PVD-coated Ti-6Al-4V. The test is used to assess the resistance of surfaces to dynamic, highcycle loading caused by the repeated impact of a cemented carbide ball. The subsequent observation and comparison of the wear craters produced (and their measured volumes) was used to identify which diffusion treatment (or treatment/coating combination) provided the most marked reduction in contact-induced deformation and overall improvement in wear behaviour. A combination of nanoindentation, Knoop hardness microindentation, scratch adhesion, stylus profilometry, optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and atomic force microscopy test and evaluation methods, was used to characterise the surfaces under investigation. Experimental results revealed that triode plasma diffusion treatments can provide exceptional improvements in the impact fatigue resistance, particularly when the diffusion process has been designed to maximise the resultant hardened case depth. Also, amongst the three coatings tested, PVD CrAlN was found to be the most suitable for applications involving such dynamic impact loading. Finally, the results presented show that an appropriate sequential triode plasma oxidation and nitriding diffusion pretreatment, in combination with a hard and tough PVD ceramic coating, can provide a significant reduction in surface impact wear when compared to either plasma diffusion treatments alone, or PVD ceramic coatings deposited on non-pretreated Ti-alloy substrates.

Sequential triode plasma oxidation and nitriding have been used to provide enhanced load support for physical vapour deposited (PVD) hard coatings. The diffusion process has been designed to maximise process efficiency and coating adhesion, thereby significantly improving the tribological properties of the Ti-6Al-4V alloy -particularly at high contact pressures. This has been demonstrated using unlubricated linear reciprocating-sliding ball-on-plate wear tests and micro-scratch adhesion testing. Also, surface microprofilometry, nano/micro-indentation hardness testing, scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and glow-discharge optical emission spectroscopy (GDOES) data are presented to corroborate the effect of the several plasma diffusion processes and duplex diffusion/coating combinations discussed here. The results presented show that the novel processing technique developed permits the use of oxygen diffusion in order to obtain relatively large case depths in shorter treatment times without compromising the adhesion strength of subsequently deposited PVD layers.

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