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

α-SiC/α-SiAlON composites with 80wt% α-SiC (6H phase) were fabricated by spark plasma sintering at 1800-2000°C in a 0.6atm 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 30s. The electric current was suspected of activating this polytype transformation.