DENSIFICATION PROCESSES IN THE TUNGSTEN CARBIDE-COBALT SYSTEM

Abstract

Densification of the tungsten carbide-cobalt system has been investigated by determining the effect of the principal sintering variables-composition, temperature and time of sintering, particle size, ball-milling-and by studying the processes that occur. Considerable shrinkage takes place during heating, before the eutectic temperature is attained. A 9% cobalt alloy sintered entirely in the solid state to give comparable density and mechanical properties to those attained by liquid-phase sintering, but the sintering time was increased by a factor of 10. Densification proceeds from nuclei created by ball-milling, which packs the porous cobalt agglomerates with tungsten-carbide particles; if the cobalt particles are only mixed with the tungsten carbide, then on sintering they flow out into the matrix leaving behind voids that do not fill. Densification is characterizedby two features: first, tungsten-carbide particles cement together with cobalt between grains to form clusters and filaments; secondly, the clusters and filaments contract. The solubility of the tungsten carbide in the cobalt is important, since densification occurs far less rapidly when copper is used as the binder phase. Shrinkage can virtually cease before the compact is fully dense, either because voids form as a result of unsatisfactory mixing, or, with a small amount of cobalt, because the periphery of the compact sinters to full density before the interior, preventing further overall densification.

Notes

* Manuscript received 25 October 1966.