Layer structure and mechanical properties of low pressure r.f. plasma nitrided Ti-6Al-4V alloy

Abstract

Ti-6Al-4V alloy has been nitrided in low pressure inductively coupled r.f. plasmas of nitrogen, hydrogen and argon. The structure and composition of the nitrided layers were investigated by a variety of methods including X-ray diffractometry, transmission and scanning electron microscopy and Auger electron spectroscopy. Mechanical properties, i.e. surface microhardness, erosion resistance and fatigue of nitrided samples, were determined as a function of preparation conditions. It was found that the nitrided layer is composed of an external compound layer, followed by a diffusion layer. The compound layer consists of the cubic δ-TiN and the tetragonal ε-Ti2N phases. The diffusion layer is composed of the ε-Ti2N and α'-(Ti,N) (solid solution of nitrogen in titanium) phases. The relative concentration of the different phases and their crystallographic orientation have been found to be strongly dependent on the plasma parameters, such as composition of gas feed and position in reactor. Surface microhardness as high as 29.4 GPa (3000 kfg mm-2) was obtained for the nitrided layers, with significant improvements in the fatigue life and erosion resistance of the nitrided alloy. Erosion resistance and fatigue improvements were obtained for different nitriding conditions: the erosion resistance was directly correlated with the surface microhardness and concentration of the δ-TiN phase, while the resistance to fatigue was correlated with the concentration of α' + ε phases. In this paper we present the structure of the nitrided layers obtained for different plasma parameters anf discuss the mechanism of formation of the nitrided layers and the correlation between their structure and the mechanical properties. © 1990.