The Effect of Cold Isostatic Pressing of Powder Billets Produced from the VK8 Hardmetal on its Hardness and Phase Composition After Sintering.

The paper examines how cold isostatic pressing (CIP) of powder samples preformed by uniaxial pressing at up to 0.4 GPa influenced the density, coercive force, structure, hardness, and phase composition of the WC–8 wt.% Co (VK8) hardmetal after sintering in vacuum. A multiplication installation was employed for cold isostatic pressing. Transformer oil served as the pressure transmission medium. The density of the CIP samples increased linearly with pressure prior to sintering and reached its maximum at 0.2 GPa for the sintered CIP samples. The coercive force increased nonmonotonically with pressure. The WC grains refined when CIP pressure increased to 0.2 GPa but slightly coarsened after CIP at 0.3 and 0.4 GPa. The Co layer became thinner with pressure being increased to 0.3 GPa and slightly thicker at a pressure of 0.4 GPa. An intermetallic Co_0.8W_0.2 phase emerged in the samples subjected to preliminary CIP at 0.4 GPa. Hardness measurements at different loads up to 300 N showed that hardness increased at CIP pressures 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 experimental data published previously 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 pressures 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 quasi-isostatic compression. However, the compressive forces are not equal in quasi-isostatic compression conditions, and there is always one force that is greater than the others. This force causes deformation or fracture leading to the mechanical activation of the powder because the imperfection of its particles increases and in turn influences the properties acquired by the hardmetal. [ABSTRACT FROM AUTHOR]