Low temperature hot-pressed BAS matrix composites reinforced with in situ grown Si₃N₄ whiskers

A series of Si₃N₄/BAS compositions of 20, 40, 60, 80 and 100 wt.% barium aluminosilicate (BAS) were hot-pressed at relatively low sintering temperature (1450 °) for a 30-60 min soak at a pressure of 10-20 MPa under N₂ atmosphere. Elongated rod-like β-Si₃N₄ whiskers were formed for compositions with 20-80 wt.% BAS. No metal ions were dissolved in Si₃N₄ crystal lattices. The microstructure varied with composition. The relative content of rod-like β-Si₃N₄ whiskers increased with increasing BAS content. Densities of all the specimens with BAS contents more than 20 wt.% were > 99% of the theoretical density. With a 30 min soak, the room temperature (RT) flexural strength increased from 259.1 to 323.0 MPa from 20 to 40 wt.% BAS and then decreased to 70.6 MPa for 100 wt.% BAS. The fracture toughness increased from 2.40 to 3.24 MPa m^1/2 from 20 to 60 wt.% BAS and then decreased to 1.78 MPa m^1/2 for 100 wt.% BAS. With a 60 min soak, the RT flexural strength of 40 wt.% BAS without monoclinic BAS seeds was 379.6 MPa and retained 87.9, 93.8 and 94.2% of their RT values at 1000, 1100 and 1200 °C, respectively; in contrast, the RT flexural strength of 40 wt% BAS containing 8 wt% BAS seeds was 519.4 MPa and retained 87.3, 63.0 and 51.9% of their RT values at 1000, 1100 and 1200 °C, respectively. Monoclinic BAS seeds had little effect in accelerating the hexagonal→monoclinic phase transformation of BAS glass ceramics.