Microstructure, hardness and toughness of nanostructured and conventional WC-Co composites

The microstructure and mechanical properties of nanograin-sized WC-Co composites were investigated and compared with those of conventional cermets. The dislocation density in the nanometer-sized WC crystals is lower than in the conventional ones, and no inclusions are observed in them. Nanostructured composites have higher tungsten content in the binder phase and a higher FCCHCP ratio of the cobalt. An amorphous phase is observed in the binder phase of the nanostructured samples. Hardness and surface toughness were investigated by performing Palmqvist indentations at loads from 0.025 to 40 Kg. The hardness increases with decreasing binder mean free path of dislocation in the binder phase. The high hardness of nanostructured cemented carbides results not only from the ultrafine microstructure, but also from alloy strengthening of the binder phase itself. The variations of hardness with load suggest that the finer grade conventional carbides have higher microfracture strength, and the nanostructured WC-Co composites are superior to the conventional ones in this respect. Bulkfracture toughness is related to cracks developing through the phases of the material. Palmqvist indentation toughness measurements show that the toughness decreases with increasing hardness in conventional composites, whereas the increase of hardness in nano-structured composites does not further reduce their bulk fracture toughness. This implies that different dominant toughening mechanisms exist in the conventional and nanostructured composites.