Synthesis of silicon nitride-barium aluminosilicate self-reinforced ceramic composite by a two-step pressureless sintering

Dense 40wt%BAS/Si₃N₄ self-reinforced composite was synthesized by a two-step pressureless sintering process. The results showed that the BAS glass-ceramic not only served as an effective liquid phase sintering aid for attaining full densification and completing the α- to β-Si₃N₄ phase transformation, but also remained as a structural matrix. A bimodal microstructure could be obtained via this two-step process without the addition of special β-Si₃N₄ seeds. It is due that the first step could suppress the α- to β-Si₃N₄ phase transformation while allowing densification, and then at the higher temperature second step, the continued transformation could be used to facilitate the growth of the β-nuclei formed in the first step. The obtained composite exhibits excellent mechanical properties compared to unreinforced BAS matrix. The flexural strength and fracture toughness could reach 565 MPa and 7.4 MPa m^1/2, respectively. The crack deflection, grain bridging and pullout are considered as major toughening mechanisms in this composite.