A comparative study of the influence of plasma treatments, PVD coatings and ion implantation on the tribological performance of Ti-6Al-4V

The mechanical and tribological properties of the titanium alloy Ti-6Al-4V have been characterized by low-frequency reciprocating wear, rotating-bending fatigue and Knoop microhardness tests and surface roughness (R(a)) measurements before and after various treatments. These treatments included ion-beam nitrogen implantation, plasma-assisted physical vapour deposition (PAPVD) of TiN and plasma diffusion of nitrogen and carbon. Using a commercial thermionic triode plasma-assisted PVD system, hot-filament-supported glow discharges were produced with different compositions of nitrogen, carbon and hydrogen. The plasma-diffusion treatments all increased the load support provided by the substrate but reduced the bulk fatigue stength considerably. The PAPVD TiN coating increased surface microhardness but provided limited load support. Nitrogen-ion implantation increased low-load surface microhardnesses and improved the fatigue resistance without improving the overall load support provided by the substrate. It was found that the elevated temperatures and increased process times in plasma-diffusion treatments increased the surface roughness significantly, although this roughening effect could be reduced by depositing a thin titanium layer by PAPVD, prior to the plasma treatment.