Densification Mechanisms in the Tungsten Carbide—Cobalt System

Abstract

An investigation has been made into the sintering of tungsten carbide in the presence of small amounts of various metal additives. It has been found that a very marked degree of densification occurs when a small amount (1 vol.-% or less) of iron-group metal is added to WC. This arises from a process of activated sintering due to enhanced surface and interfacial diffusion, which has been observed by other workers to take place when nickel or other Fe-group metals are added in small quantities to W. Tungsten carbide forms low-energy prismatic interfaces in the presence of cobalt and other Fe-group metals. Densification takes place by the close-packing of particles along irregular boundaries which comprise segments of these low-energy interfaces. This configuration is far removed from that required by the usual sintering criterion of minimum surface and interfacial area. The segments of low-energy interface set up activity/diffusion gradients which bring about rapid material transport to achieve this irregular close-packing of particles. On the basis of these results it has been possible to review previous data on the sintering of WC–Co alloys and to conclude that in this system densification takes place in three stages: (I) high-density aggregates of carbide are formed; (II) collapse and filling of voids at the eutectic temperature; and (III) a third stage in which further sintering occurs more slowly of the ‘larger effective particle size’ aggregates formed in Stage I.