Effect of grain size on oxidation resistance of WC-6wt%Co cemented carbide sintered by spark plasma sintering

In this study, two WC-6wt%Co cemented carbides of different fine grades were prepared via spark plasma sintering, and the average WC grain sizes in prepared WC/Co cemented carbides were 320.9 nm (ultrafine-grain) and 847.2 nm (fine-grain), respectively. The effect of grain size on the oxidation resistance of cemented carbide in the air at 600–800 °C was investigated. The results show that when the oxidation temperature is relatively low (600, 700 °C), grain refinement leads to an increase in the number of grain boundaries in the sample, which induce the formation of more fine cracks and pores in the oxide layer and adversely affect the oxidation resistance of ultrafine-grained (320.9 nm) cemented carbide. However, as the oxidation temperature increases to 800 °C, the antioxidant performance of the fine-grained (847.2 nm) cemented carbide decreases and is lower than that of the ultrafine-grained (320.9 nm) cemented carbide, with larger intergranular fracture and disintegration behavior of the oxide grains within the oxide layer being the main reasons for the deterioration of the antioxidant performance of the sample. In addition, the oxidation thermodynamics of WC/Co cemented carbide was discussed, and the macroscopic color inconsistency of the two samples was observed and explained. The apparent activation energies of WC-6wt%Co cemented carbides at 600–800 °C are calculated to be 162.78 kJ/mol and 229.69 kJ/mol, respectively for ultrafine- and fine-grained samples.