Crack propagation resistance behavior of Si₃N₄ composites reinforced by BN nanotubes

BNNT/Si₃N₄ composites were prepared with Si₃N₄ ceramics enhanced by BN nanotubes (BNNT). Bending strength and fracture toughness of BNNT/Si₃N₄ composites were tested by three point bending strength and SENB method. BNNT/Si₃N₄ composite microstructures were observed by SEM. The mathematical model of the crack propagation resistance of Si₃N₄ ceramics crack shielding region enhanced by BNNT was constructed, which was based on the crack propagation resistance formula of BNNT reinforcing Si₃N₄ ceramics. The calculated results used this model were compared with crack propagation resistance of Si₃N₄ ceramics. The results show that bending strength and fracture toughness of BNNT/Si₃N₄ composites are obviously higher than those of pure Si₃N₄ ceramics and BNNT increases the crack propagation resistance of Si₃N₄ ceramics. It can be seen that the friction resistance to pull out is the main reason of reducing the Si₃N₄ ceramics crack propagation capability and that Si₃N₄ ceramics added BNNT has clear appreciation of the resistance curve behavior. The finite element simulation results on the crack tip stress indicate that BNNT makes maximum stress of the crack tip to deflect nanotubes and reduce stress of the Si₃N₄ ceramics crack tip, which can improve the crack propagation resistance behavior of Si₃N₄ ceramics.