Sintering and characterization of a hard-to-hard configured composite: Spark plasma sintered WC reinforced α−SiAlON

Abstract

In this study, a composite comprising WC particulate-reinforced α−SiAlON was fabricated by spark plasma sintering (1750 °C/40 MPa/25 min) in order to develop a hard-to-hard phase configuration and to toughen the hard matrix through particulate reinforcement. Irrespective of the composition, the sintered samples were almost theoretically dense. The nature of the overall sintering process for the composite appeared to be guided by the liquid phase sintering of the SiAlON phase. Microstructural analyses using scanning and transmission electron microscopy indicated the presence of both equiaxed and elongated α−SiAlON grains. The WC grains principally appeared equiaxed in nature. A reaction product was not observed at the WC/α−SiAlON interface. High-angle annular dark-field scanning transmission electron microscopy imaging indicated the reasonable distribution of the elements within the constituent grains and grain boundary. The presence of an intergranular glassy phase was confirmed, which was principally rich in oxygen and yttrium, with some occasional tungsten in the case of triple junctions. The composite exhibited an acceptable combination of flexural strength, hardness, and fracture toughness with values around 489 MPa, 20 GPa, and 6 MPa-m0.5, respectively. In contrast to expectations, a decline in hardness was observed up to ≤30 wt% WC. Presumably, the WC grains acted as defects/inclusions with similar dimensions, which eventually resulted in inadequate interfacial performance and reduced hardness. Improvements in the Vickers hardness and fracture toughness were obtained at a WC loading of 40 wt%. The indentation size effect and load dependence of the fracture toughness were also determined for some selected specimens. Higher damage rates for the β−Si3N4 counterbody against the 40 wt% WC/α−SiAlON composite were observed up to 30 N under unlubricated conditions compared with those obtained against the monolithic constituent phases, i.e., α−SiAlON and WC. The formation of an adherent tribolayer was observed.