Formation of wear resistant coatings on Ti–6Al–4V by calciothermic reduction

A nitriding method defined as, “calciothermic assisted immersion nitriding (CAIN)” has been developed to modify the surface of titanium alloys and improve their tribological properties. The CAIN process described here involves the heat treatment of titanium alloy specimens (Ti–6Al–4V) in an air environment within a sealed vessel containing a molten flux of calcium and calcium chloride, resulting in the preferential diffusion of nitrogen atoms into the samples. The nitriding process was found to produce a consistent TiCxNyOz surface layer, where the exact chemical composition was dependent on the processing conditions. The tribological properties of the surface layers were tested using a high frequency reciprocating rig under dry lubrication conditions and demonstrated that the CAIN process can be used to produce layers with good adhesion to the Ti–6Al–4V substrate and wear resistance comparable to industrially produced physical vapour deposition (PVD) coatings.