A study of the reciprocating-sliding wear performance of plasma surface treated titanium alloy

Triode 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 ball-on-plate reciprocating-sliding is correlated to debris generation, changes in relative humidity - and the resultant wear mechanisms involved.