Nitridation process and mechanism of Ti–6Al–4V particles by dc plasma spraying

Abstract

Ti–6Al–4V spherical particles (22–45 μm) injected in a dc plasma jet using N₂ as plasma forming gas, and collected in a controlled atmosphere chamber filled with nitrogen at 10⁵ Pa are directly nitrided from their surface to their core without any apparent nitrogen gradient. Particles are composed of small grains of the δ-TiN phase, with a uniform composition of TiN_0.55, strongly linked together by a secondary phase (Al–Ti–V–O). This result is achieved in very short times (∼ms) and nitrogen reacts only with titanium. The other elements (Al, V) play the role of oxygen getters, particularly aluminum which evolves probably as the volatile suboxide AlO. The mean composition of the particles is TiN_0.43O_0.05V_0.05Al_0.03.

The nitridation occurs inside the plasma jet core by the convective movement induced within the droplets by the high velocity plasma jet. The conditions for this convective movement (Reynolds number of the particles relatively to the plasma flow higher than 20, and the ratio of kinematic viscosities of the plasma to that of the alloy over 50) are fulfilled in the nitrogen dc plasma jet core. This convective movement continuously renews fresh alloy at the surface and entrains δ-TiN grains formed at the surface inside droplets, as well as dissolved nitrogen. Nitrided grains finally grow thanks to the nitrogen dissolution and diffusion within the liquid.

The interest of this process, compared to classical furnace nitridation of the alloy, is its rapidity, the lack of nitrogen gradient, and the favourable role of the secondary phase for some mechanical properties.