Tribology behavior of Zr-N alloying layer on Ti6Al4V alloy surface at elevated temperature

The dense Zr-N alloying layers were prepared on the surface of Ti6Al4V alloy by two kinds of surface alloying processes to improve the wear resistance of Ti alloy at elevated temperature. One process is Zr-diffusing with nitriding simultaneously (synchronous surface alloying), and the other is nitriding after Zr-diffusing (asynchronous surface alloying). The microstructure and tribological property at elevated temperature of the modified surface layers were investigated. Results show that both the Zr-N diffusion layers consist of ZrN phase, and the asynchronous surface alloying with thick Zr-Ti solid solution layer is beneath the surface. The surface microhardness of the Ti6Al4V alloy was significantly increased by both Zr-N alloying layers. With the same time and temperature, the thickness of the asynchronous surface alloying layer was about 6 times of the synchronous surface alloying layer, because the asynchronous surface alloying has excellent solution and low diffusion inhibition. The results of the ball-on-disc wear tests indicate that the wear resistance at 300°C of the synchronous surface alloying layer was not significantly improved because of the shallow diffusion depth. The asynchronous surface alloying reduced the wear rate obviously by two orders and the friction coefficient by 50% due to the high surface hardness, thick depth, good thermal oxidization resistance and big surface load-bearing ability.