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

Abstract

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 charac- terised 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 rela- tive 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 sub- stantially, 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.