Fabrication of functionally graded TiN coating by reactive plasma spraying

Abstract

The fundamental nature of the nitriding of solid state Ti alloy by rf plasma has been investigated. A linear relationship was found between the thickness of the nitride layer and the square root of nitriding time. It therefore seems that the plasma nitriding process is diffusion dominated. The diffusion rate of nitrogen in the Ti alloy (R_d) was estimated from the experimental results obtained, and the plasma spraying of Ti powder was conducted with a deposition rate less than R_d. By using this process, it was possible to fabricate a thick nitride coating at a high deposition rate; the degree of nitriding of the coating could be controlled by changing the deposition rate. On the basis of this result, a functionally graded coating, whose composition changed from the coating/substrate interface to the coating surface, was fabricated by changing the deposition rate. Wear tests demonstrated that the wear resistance of the nitride coating was equivalent to that of a nitrided layer on the Ti alloy.