Study of nanocrystalline plasma electrolytic carbonitriding for CP-Ti

Abstract

Commercially pure titanium is subjected to carbonitriding according to a new procedure called plasma electrolytic carbonitriding. The influence of voltage and time on the thickness of carbonitrided layers is investigated. The kinetics of growth of the layers is analyzed according to their thickness and microhardness. The coatings formed on the titanium substrate are multilayer. They are formed by a combination of TiN, TiO2, and the α-phase in the form of inclusions of high concentrations of nitrogen and carbon in a close-packed hexagonal crystal lattice of titanium. All layers are very hard with hardnesses varying from 1100 (Vickers-Knoop hardness) for the TiN layer and 900 for the α-phase to 250 for the nonmodified surface. There exist strong metallic bonds between the layers of the coating. The time dependence of the form t 1/2 is established for the diffusion of nitrogen and carbon in the outer layers. The results of wear tests reveal that the mass losses of carbonitrided samples become 4–17 times lower than for the intact samples. Thus, the procedure of plasma electrolytic carbonitriding enables one to get highly efficient stable coatings on titanium.