Characterization of the kinetic and mechanistic differences between free-surface and bulk grain growth in WC-Co materials

The rate of grain growth at different high temperatures in the bulk and at the free surface of a WC-Co substrate was measured. The microstructure, phase evolution, and elemental composition at the free surface were characterized at various stages of the grain-growth process and compared to equivalent characteristics of the bulk. A dramatic difference in the rate of grain growth between the free surface and the bulk of the material was observed. Grains in the free surface grew at a much faster rate than those in the bulk. Since this fast rate of growth was found to coincide with the vaporization of the binder phase from the free surface, it is suggested that this increase in the rate of growth is related to a change in the growth mechanism from an interfacial reaction limited growth in the bulk to a surface diffusion rate limited growth at the free surface. The contact points between,grains provide bridges for atomic transport from high free-energy regions (small grains) to low free-energy regions (large grains); hence, the contiguity of the material has a strong influence on the rate of growth.