Effect of BN grain size on microstructure and mechanical properties of the ZrB₂-SiC-BN composites

ZrB₂-20vol.%SiC composites containing 10vol.% h-BN particles (ZSB) with average grain sizes ranging from 1μm to 10μm were hot-pressed. The fracture toughness of the ZSB composites was higher than reported results of monolithic ZrB₂ (2.3-3.5MPam^1/2) and SiC particle reinforced ZrB₂ composites (4.0-4.5MPam^1/2). The improvement in the fracture toughness of the ZSB composites was due to the high aspect ratio of h-BN and weaker interface bonding, which could enhance crack deflection and stress relaxation near the crack-tip. Compared with the flexural strength of the ZrB₂-SiC composites, the reduction in the flexural strength of the ZSB composites was attributed to the weaker interface bonding and the lower relative density. Furthermore, improvement in toughness and the reduction in the strength were valuable to improve the thermal shock resistance of the ZSB composites. The ΔT_c of ZSB5 material is 400°C which is higher than ZrB₂-20%SiC and ZrB₂-15%SiC-5%AlN.