Effects of load and sliding speed on the tribological behaviour of Ti-6Al-4V plasma nitrided at different temperatures

Dry sliding tests were carried out under different load and sliding speed conditions on Ti-6Al-4V alloy plasma nitrided at three temperatures: 973, 1073 and 1173 K. The results were interpreted on the basis of the evolution of the friction coefficient and by characterizing the wear debris and worn surfaces in order to understand the acting wear mechanisms. A comparison was also made with the results obtained with untreated specimens, tested under the same conditions. Plasma nitriding can noticeably improve the dry sliding resistance of the Ti-6Al-4V alloy. The nitriding temperature must be chosen according to the main wear mechanism observed under specific load and sliding speed conditions. When wear is determined by the resistance of the compound layer (low loads and low sliding speeds), the nitriding treatment has to be carried out at 1073 K. In this case, the compound layer has optimal properties with respect to resistance to adhesion and fragmentation. When the material is exposed to delamination (high loads and high sliding speeds), the strength of the diffusion layer has to be maximized. In this case, the nitriding temperature should be as high as possible (1173 K in the present investigation) in order to enhance the hardening of the diffusion layer. (C) 1997 Elsevier Science S.A.