Nitriding of Titanium

Abstract

A kinetic study of the nitriding process of titanium in nitrogen stream was carried out in order to establish a method to improve the frictional properties of titanium. The diffusion layer formed by nitriding was covered by a very thin compound layer (TiN). The maximum hardness of the diffusion layer was above Vickers 1000, but the rate of its growth was comparatively low. The rate of reaction of titanium with nitrogen was found to follow the parabolic law at 800∼900°, and the rate-controlling process was considered to be the diffusion process in both the compound layer and the diffusion layer. As the rate of growth of the diffusion layer is considered to be controlled exclusively by the solubility and the diffusion coefficients of nitrogen in α-titanium, both of which are functions of temperature only, it is necessary to operate at higher temperature and for longer time in order to make the hardened layer deeper. However, the nitriding at a temperature above the transformation point of titanium is not practical because of the formation of surface reliefs produced in the course of β→α transformation and the deformation of material nitrided at high temperature. Therefore, it is concluded that the most practical nitriding temperature is about 850°C. The diffusion coefficients of nitrogen in α-titanium and the compound layer were also determined at 850°C as follows: D_α=1.3×10⁻¹¹ cm²/sec and D_γ=1.4×10⁻¹² cm²/sec.