FORMATION OF BRIDGES BETWEEN DIAMOND PARTICLES DURING SINTERING IN MOLTEN COBALT MATRIX

Abstract

Large diamond particles of about 700-mu-m size mixed with 70 wt % fine cobalt powder have been sintered at 5.25 GPa and 1500-degrees-C after wrapping in zirconium foil. During the sintering treatment faceted inter-particle bridges form, and their pale yellow colour in contrast to the dark yellow colour of the original diamond particles indicates that the bridge is a new phase reduced in nitrogen content because of the gettering effect of the zirconium foil. The morphological characteristics of the bridge and the adjacent particles indicate that the bridge is formed by migration of the inter-particle grain boundary and solution-reprecipitation through the liquid cobalt matrix. It is possible that these processes are induced by the coherency strain energy produced by nitrogen diffusion from the region ahead of the retreating boundaries, as has been observed in metal systems.