Spark Plasma Sintering of ultrafine-grained WC – Al2O3 ceramics

The sintering mechanisms of WC – Al2O3 nanopowder compositions with different contents of aluminum oxide particles (1, 3, 5 wt.%) were investigated. Samples of WC – Al2O3 ceramics were produced by Spark Plasma Sintering method (SPS) in vacuum, by heating to a temperature of 1450 °C at a rate of 50 °C/min under uniaxial stress 70 MPa. Plasma-chemical nanopowders of tungsten monocarbide and submicron powders of aluminum oxide were used to make the ceramics. The density, microstructure, phase composition, microhardness (Hv) and fracture toughness (KIC) of the ceramics were investigated. It was shown that the use of the SPS method makes it possible to obtain WC-Al2O3 ceramics with good relative density (95.4-98.1%) and a homogeneous microstructure with ultrafine grain size (0.1 – 0.2 μm). By the method of the X-ray phase analysis, it was established that in the process of SPS of WC-Al2O3 ceramics the formation of an undesirable W2C phase takes place, leading to decrease in the fracture toughness KIC. To reduce the intensity of W2C particle formation, colloidal graphite (0.1, 0.2, 0.3 wt.%) was added to WC – Al2O3 ceramics. Using the Young-Cutler model and the model of diffusion resorption of pores it was shown that the main mechanism of SPS of WC – Al2O3 ceramics is grain boundary diffusion. It is shown that the introduction of graphite leads to a decrease in the activation energy of SPS of WC – Al2O3 ceramics, which is probably due to a decrease in the W2C particle content to 0.5 wt.%.