Microstructure and phase evolution of ceramic layers on Ta-2.5W alloy

A Ta-N ceramic layer on the surface of a Ta-2.5W alloy was synthesized by ion nitrocarburizing to improve its surface mechanical properties. Two processing parameters, temperature and holding time, were respectively investigated of the effects on the phase evolution of the ceramic layer. At 800 degrees C, only a single diffusion layer consisting of alpha-Ta (solution of nitrogen) was present. When temperature was raised to 900 degrees C and 1000 degrees C, a compound layer, mainly composed of theta-TaN phase, was formed to replace the upper part of the diffusion layer, resulting in the dramatic improvement of surface hardness. The thickness of compound layer increased with holding time and approached 20 mu m for the sample processed at 1000 degrees C for 3 h, corresponding to the optimal microhardness properties.