Kinetics of growth of superhard boride layers during solid state diffusion of boron into titanium

Abstract

Solid state boriding of titanium is a very effective and inexpensive way to create surface layers that can impart high hardness and wear resistance. In this work, the growth kinetics of hard titanium boride layers created by the solid-state boron (B) diffusion through titanium (Ti) surface at various temperatures has been determined. The hard boride layers comprised of a monolithic titanium diboride (TiB2) layer at the top and a titanium boride (TiB) sub-layer that mostly consisted of TiB whiskers with high aspect-ratio growing normal to the surface. The structure and the crystallography of the layers were also confirmed by X-ray diffraction. Experimental results show that the growth rates of the TiB2 as well as the composite (TiB2+TiB) coating layers follow the parabolic kinetics. It is shown that the thickness development as a function of time can be predicted reasonably accurately, using the error function solutions for the two simultaneously growing boride layers developed in our previous work. The maximum degree of growth as dictated by the diffusivity of B in the boride phases was achieved using a solid state B powder pack in an ambient furnace atmosphere.