Microstructure Control of In-Situ-Toughened α-SiAlON Ceramics

Single-phase α-SiAlON with elongated grains is obtained from α-Si₃N₄ powder for a broad range of compositions of practical interest. Following the concept of nucleation and growth, two-step firing is used for microstructure control. This method takes advantage of the slow transformation reaction from α-Si₃N₄ to α-SiAlON at low temperature when the composition is near the α-SiAlON phase boundary and, hence, is marginally stable. For more-stable compositions, the seeding of α-SiAlON crystals is more effective, because it allows elongated grains to grow onto the seed crystals. The fracture toughness is strongly correlated with the microstructure and is enhanced greatly in the optimized materials.