Synthesis of α-SiC/α-SiAlON composites by spark plasma sintering: Phase formation and microstructures development

α-SiC/α-SiAlON composites with 80 wt% α-SiC (6H phase) were fabricated by spark plasma sintering at 1800-2000 °C in a 0.6 atm nitrogen atmosphere. The effects of the temperatures on the phase development, microstructures and mechanical properties were investigated. The results showed the Si3N4, AlN, Al2O3, and Y2O3 particles were isolated by the 6H-SiC to prevent α-SiAlON formation at 1800 °C. The Si3N4 decomposed at 1900 °C and above, thus added Si in the phase compositions. The α-SiC grains grew anisotropic in the sintering liquids at 1800 °C and 1900 °C, forming the self-reinforcing microstructures, and accordingly increased the flexural strength and fracture toughness. In cooling down immediately after the temperature reached 2000 °C, a transitory hold at 1700 °C transformed the 6H-SiC into the 3C polytype in 30 s. The electric current was suspected of activating this polytype transformation.