Effects of short carbon fiber on mechanical properties of ZrB₂-SiC ultra-high temperature ceramics

Zirconium diboride-silicon carbide based ceramics are attractive for application in aircraft propulsion systems and thermal protection system for hypersonic vehicle because of their high melting point, high thermal and electrical conductivity and good chemical stability. However, the major problems associated with ZrB₂/SiC are lack of damage tolerance and insufficient strength as structure components. ZrB₂-20% (volume fraction) SiC ceramics with the additional short carbon fiber was developed to promote graceful failure using hot pressing technique. The results show that the introduction of short carbon fiber into the ZrB₂-20%SiC matrix results in an apparent increase in fracture toughness from 2.0 MPa·m^1/2 to 6.56 MPa·m^1/2 and not evidently change in flexural strength from 502 MPa to 445 MPa and elastic modulus from 414 GPa to 322 GPa. Meanwhile the influence of short carbon fiber on the strength, and modulus is analyzed and estimated using a modified role-of-mixture formulation and that on the toughness is also calculated based on the toughening mechanisms of crack bridging, fiber pullout and crack deflection. The calculated results of the strength, elastic modulus and fracture toughness of the composite agree well with the experimental results.