Preparation and characterisation of aluminium nitride–silicon carbide composites

Aluminium nitride–silicon carbide (AlN–SiC) composites were prepared to increase the fracture toughness of AlN-based ceramics. The composites (0–30 vol.% SiC) were made by pressureless sintering at 1870 °C, using Al2O3–Y2O3 (2 wt.%) as sintering additive. The solubility of SiC in AlN was determined to be between 10 and 15 vol.% at 1870 °C. Particulate AlN composites were formed for >10 vol.% SiC and the SiC grains acted as grain boundary pinning centres during sintering. Vickers hardness and Young’s modulus increased monotonically when adding SiC, 65% for 30 vol.% SiC and 13% for 20 vol.% SiC, respectively. Both bending strength and fracture toughness increased ∼22% when 20 vol.% SiC was added. The toughening of the composites is mainly caused by residual strain due to thermal expansion or elastic modulus mismatch between SiC grains and the AlN–SiC matrix. Elongated α-SiC grains were observed for >15-vol.% SiC materials, which increased the toughening of the materials. The fracture toughness was also calculated using various models based on Vickers indentation, and most models underestimated the fracture toughness compared to the single edge notched beam (SENB) method.