A multi-step process of oxygen diffusion to improve the wear performance of a gamma-based titanium aluminide

Abstract

A gamma-based titanium aluminide with the composition of Ti–45Al–2Nb–2Mn–1B (at%) was surface treated through a process in which thermal oxidation in air and diffusion were carried out in several steps to investigate the effect of oxide forming elements and diffusion of oxygen on the wear performance of surface layers. The microstructure and phases on the surface were characterized using optical and electron microscopy, X-ray diffraction (XRD) and glow discharge spectroscopy (GDS). The constituents of the top surface layers were several layers of oxide compounds of TiO 2 and Al 2O 3, beneath which a small amount of nitride compounds of TiN and Ti 2AlN were appeared. After diffusion of oxygen a layer of Ti 3Al with diffused oxygen appeared as a hard layer under the oxide compound layer. The ball on disk wear tests showed the dominant adhesive wear on the untreated surfaces. The treated surfaces were in most cases hard enough to be worn by the steel ball counterface and in fact the material of steel was transferred to the surface. The wear of steel counterface was also due to the ploughing action of the projected angular crystals of TiO 2 and Al 2O 3 on the surface. The Ti 3Al layer should also have played a major role in resistance to wear after the removal of the compound layer as well as supporting the top layer during the wear. The wear resistance and load bearing capacity of the treated samples improved up to two orders of magnitudes higher than those of untreated samples.