The Effect of Cold Isostatic Pressing of VK8 Hardmetal Powder Blanks on Hardmetal Hardness and Phase Composition after Sintering 

Abstract

The paper examines how cold isostatic pressing (CIP) of powder samples preformed by uniaxial pressing at up to 0.4 GPa influence the density before and after sintering, coercive force, structure, hardness, and phase composition of the WC–8 wt.% Co hardmetal after sintering in vacuum. Cold isostatic pressing was conducted in a multiplication installation. The pressure transmission medium was transformer oil. The density of the CIP samples increased linearly with pressure prior to sintering and a maximum was found at 0.2 GPa after sintering of the CIP samples. The coercive force increased nonmonotonically with pressure. The WC grain size decreased when the CIP pressure increased to 0.2 GPa but increased slightly after CIP at 0.3 and 0.4 GPa. The Co layer became thinner with pressure increased to 0.3 GPa and slightly thicker at a pressure of 0.4 GPa. The intermetallic Co_0.8W_0.2 phase appeared in the samples that passed preliminary CIP at 0.4 GPa. Hardness measurements at different loads up to 300 N showed that hardness increased at the CIP pressure up to 0.3 GPa and slightly decreased in CIP at 0.4 GPa. Scanning electron microscopy photographs of the indenter imprints showed massive cracking of the WC grains. The results were analyzed using previously published experimental data on the effect of high isostatic pressure on the multiplication of dislocations and the fracture of single crystals. Those papers found that the deformation and fracture of single crystals under high isostatic pressure were accompanied by a sharp increase in the dislocation density. Each WC particle was compressed from all sides by the neighboring WC and Co particles, resulting in quasiisostatic compression. However, the compressive forces are not equal in quasiisostatic compression conditions and one force is obviously greater than the others. This force causes deformation or fracture leading to the mechanical activation of the powder as the imperfection of its particles increases and in turn influences the properties acquired by the hardmetal.