Microstructure and mechanical properties of Si₃N_4f/BN/Si₃N₄ composites by joint processes of PIP and CVI

The three-dimensional silicon nitride (Si₃N₄) fiber-reinforced silicon nitride matrix (3D Si₃N_4f/BN/Si₃N₄) composites with boron nitride (BN) interphase were fabricated by joint processes of precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI). The fracture morphologies and mechanical properties of Si₃N_4f/BN/Si₃N₄ composites with different PIP and CVI Si₃N₄ matrix content were studied. The composites with 50 vol% PIP matrix and 50 vol.% CVI matrix show the most optimal mechanical properties, especially the highest fracture toughness reported so far, whose flexural strength and fracture toughness are 198 ± 15 MPa and 7.5 ± 0.9 MPa·m^1/2, respectively. Meanwhile, the fracture characteristics were analyzed, and the strengthening and toughening mechanisms dominated by the fibers and high/low modulus matrices, corresponding to CVI/PIP matrices respectively, were also discussed. It is concluded that the multi-step fiber pull-out is conducive to improving both the strength and toughness, where the strengthening and toughening mechanisms include: (Ⅰ) relieving stress and resisting crack propagation by high/low modulus matrices; (Ⅱ) strengthening by short debonded fibers and toughening by long debonded fibers.