Synthesis of β-Sialon from Si₃N₄ and Aluminum-Iso-Propoxide

Abstract

β-sialon with z=0.25, 0.5, 0.75, 1 and 2 in Si_6-zAl_zO_zN_8-z was fabricated from Al(Oi-Pr)₃/tetrahydrofuran solution containing α-Si₃N₄ particles. The solution was ball milled for 24h, hydrolyzed, dried and calcined at 900°C for 2h in air. The mixture consisted of α-Si₃N₄ and alumina was hot-pressed at 1850°C for 1h under a pressure of 300kg/cm². Three-point bending strength, phases and microstructure were investigated. β-sialon with z=0.25 was not fully densified (88% of theoretical). β-sialon with z=0.5, 0.75, 1 and 2 showed complete densification. β-sialon with z=0.25 consisted of β-sialon, a small amount of α-Si₃N₄ and O′ phase. β-sialon with z=0.5 consisted of β-sialon and a small amount of O′ phase. β-sialon with z=0.75 consisted of β-sialon, a small amount of O′ and X phases. β-sialon with z=1 and 2 consisted of β-sialon and X phase respectively. β-sialon with z=0.5, 0.75 and 1 consisted of 0.5-1μm equiaxed grains and boundary phase. Its microstructure was very homogeneous and defects such as pore, clusters of large grains which were found in β-sialon fabricated from the mixture of α-Si₃N₄ and α-Al₂O₃ powders were not observed. In most of all β-sialon, fracture originated from surface flaws caused by machining. The strength of β-sialon with z=0.5 was much improved (67kg/mm² in average, 72kg/mm² in maximum) comparing with β-sialon (z=0.5-1) fabricated from the mixture of α-Si₃N₄ and α-Al₂O₃ powders (50kg/mm² in average). The strength of β-sialon with z=2 showed a very low value of 14kg/mm² in average which was caused by the aggregation of Al(OH)₃ during hydrolysis of Al(Oi-Pr)₃.