Abstract
The effect of initial particle size on the sintering behaviour, microstructure, and physical and mechanical properties of submicrometre grain sized hardmetals manufactured from WC powders of different particle sizes (0·3 – 0·8 μ m) has been studied. The composition of the powders was WC–6Co with additions of VC and Cr3C2 grain growth inhibitors. The sintering was carried out in vacuum followed by hipping at 1380°C for 1 h. Ultrafine hardmetals exhibit substantially improved mechanical properties with a good combination of hardness, fracture toughness, and transverse rupture strength when hipping is applied after sintering. The finer the initial WC powder, the higher is the degree of shrinkage during the initial solid stage of densification. The analysis of sintering kinetics using dilatometric techniques showed that initial densification occurs by an interfacial diffusion mechanism in ultrafine powders as well as in regular powders, while the activation energy of the fine powders is considerably lower. The microstructure of sintered hardmetals is characterised by very fine grain size. A high volume fraction of direct WC–WC low angle and high angle grain boundaries with no Co interlayer was observed by scanning transmission electron microscopy/energy dispersive spectroscopy techniques. PM/0733
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