Enhancing Surface Hardness of Titanium Alloy Ti-6Al-4V by Combined Nitriding and CVD Coating

Abstract

Titanium alloys show a relatively high strength to density ratio and good corrosion resistance. Their main disadvantage in many applications is the low hardness and hence low resistance to abrasive wear. The wear resistance of titanium alloys can be enhanced by nitriding, but the compound layer thickness is limited. The requirement of thicker compound layers could be fulfilled by a combination process. The duplex process of nitriding plus moderate temperature chemical vapour deposition (MTCVD) TiCN coating of titanium alloys appears promising. Either pressure nitriding or gas nitriding can be applied to Ti-6Al-4V. With pressure nitriding, homogeneous compound layers can be produced on complex shaped components; however, the combined process must be carried out discontinuously in two reactors, due to the different process parameters. When using gas nitriding, the combined process can be performed continuously in the CVD equipment. With the combined nitriding +MTCVD coating route, thick compound layers could be produced in relatively short process times. The surface microstructures consist of a nitrogen diffusion layer, an intermediate layer, a compound layer, and a TiCN coating. The Ti-6Al-4V surface was characterised by high hardness and good layer adhesion.