The influence of post-sintering HIP on the microstructure, hardness, and indentation fracture toughness of polymer-derived Al2O3–SiC nanocomposites

Al2O3–SiC nanocomposites containing 3–8 vol.% SiC were prepared from fine α-alumina powder and a poly(allyl)carbosilane precursor of SiC by polymer infiltration of porous alumina matrix (composites IP), or by warm pressing of polymer-coated alumina powder (composites CW). The polymer was converted to SiC by careful heating of green specimens in inert atmosphere (Ar). The residual porosity was eliminated to less than 10% by pressureless sintering (PS) at temperatures between 1700 and 1850 °C. The post-sintering hot isostatic pressing (HIP) at 1700 °C eliminated the residual porosity to less than 1%, but also resulted in coarsening of the alumina matrix grains, and the inter- and intragranular SiC inclusions. The Vickers hardness of IP specimens sintered at T < 1850 °C increased by 1–10%, which is attributed to elimination of residual porosity. The hardness and indentation fracture toughness of specimens IP sintered at 1850 °C decreased after HIP by 6 and 15%, respectively. The HIP of CW composites increased their hardness and fracture toughness by approximately 10%. The maximum fracture toughness of 5.2 ± 0.2 MPa m1/2 was measured for the materials containing 8 vol.% of SiC. A correlation was found between the fracture toughness, and the mean size and volume fraction of intergranular SiC inclusions in composites CW.