Phase Evolution and Microstructural Changes in Air-Sintered Alumina/SiC Composites

The use of monolithic alumina is limited by its intrinsic brittleness, which is commonly addressed through second-phase reinforcement. Silicon carbide (SiC) is an attractive reinforcement due to its high-temperature stability; however, its oxidation behavior strongly influences composite processing and properties. In this study, alumina/SiC composites containing 1, 5, and 10 wt.% SiC were prepared by conventional powder mixing, calcined at 800 °C for 1 h, and pressureless sintered at 1400 °C in air. Phase evolution, microstructure, densification, and mechanical properties were investigated using XRD, SEM/EDS, density–porosity measurements, and flexural testing. Air sintering led to SiC oxidation and the formation of silica-rich glassy phase and mullite, which significantly affected densification. The composite containing 1 wt.% SiC exhibited the best performance, with a flexural strength of 248.7 MPa, a Weibull modulus of 5.7, an average grain size of 1.86 µm, and a porosity of 11.08%. Higher SiC contents resulted in excessive porosity and severe degradation of mechanical properties.