Grain growth during sintering of tungsten carbide ceramics

Abstract

Grain growth and abnormal grain growth in tungsten carbide cobalt composites (cemented carbides, hardmetals) are usually discussed with respect to liquid phase sintering (Ostwald ripening). Densification and grain growth during solid state sintering are not as thoroughly studied but do play an important role in sintering hardmetals and, particularly tungsten carbide ceramics (binderless hardmetals). In this work the influences of sintering temperature, carbon content, additions of grain growth inhibitors, defects and dislocations (microstrain) introduced by milling on the densification and microstructure of WC ceramics were studied including density, micro structural, thermal and X-ray analysis. Microstrain promotes densification and results in lowering the sintering temperature, whereas free carbon seems to hinder densification at low temperatures and to promote it slightly at higher temperatures. Depending on sintering regime, free carbon and microstrain may drastically boost abnormal grain growth. By adding grain growth inhibitors, densification is shifted to higher temperatures. However, the addition prevents abnormal grain growth regardless of C-content and microstrain. Like in hardmetals grain growth inhibitors also inhibit normal grain growth. The findings are relevant for sintering of WC ceramics and hardmetals alike.