Abnormal grain growth of WC with small amount of cobalt

Abstract

Abnormal grain growth (AGG) often appears in the synthesis of cemented tungsten carbide (WC), especially where ultra-fine powder is used. In this study, it was observed that AGG was strongly affected by the amount of Co. The obvious AGG was found to occur when the amount of Co was between 0.2 and 0.9 wt%. Careful examination of the etched samples found crack-like defects in abnormal grains, which were indications of grain boundaries in these large grains. Therefore, it is suggested that the considerable AGG of WC at lower Co concentrations be attributed to a grain boundary re-entrant edge (GBRE)-assisted two-dimensional nucleation process. Grain coalescence during sintering was the source of the grain boundaries. Due to the selective wetting of (0001) WC planes with the Co liquid, those grain boundaries were parallel to the (0001) planes resulting in platelet-like large grains with high aspect ratio. The amount of Co would affect the number of the wetted (0001) planes and the probability of grain boundary formation, and hence affect the AGG. The correspondence of the calculated weight percentage of Co required surrounding all the planes of WC grains and the experimental upper limit of the range supported this mechanism.