Structural evolution and high-temperature oxidation behavior of Hf-modified SiOC nanocomposite ceramics

Hafnium (Hf)-modified silicon oxycarbide (SiOC) ceramics were prepared by preceramic polymer pyrolysis using dimethyldimethoxysilane and methyltrimethoxysilane as precursors and hafnium acetylacetonate as a modifier, followed by spark plasma sintering (SPS). The structural evolution of SiHfOC ceramics with various Hf/Si molar ratios (0–0.08) and the oxidation behavior of SPS-densified ceramic bulks in air at 1500 °C were systematically investigated. From C0Hf to C0.05Hf, the oxide layer evolved from a cracked SiO 2 -rich layer into a thinner and more continuous Hf-Si-O composite layer containing more uniformly distributed HfSiO 4 particles. After oxidation for 32 h, C0.05Hf exhibited the lowest mass loss of 0.195 mg/cm 2 and the smallest oxide-layer thickness of 8.1 μm. Nanoindentation results further indicated improved resistance of the oxide layer to local deformation. The increased formation and more uniform distribution of HfSiO 4 phase reduced cristobalite-related phase transformation strain and thermal-expansion mismatch while enhancing the barrier effect against inward oxygen transport. By contrast, excessive Hf content (Hf/Si molar ratio = 0.08) led to the rapid development of an HfSiO 4 -rich outer region, accompanied by bubble growth, rupture pits, and local spallation, which damaged the integrity of the oxide layer. These results indicate that the Hf-content-dependent phase constitution and oxide-layer architecture govern the oxidation behavior of SiHfOC ceramics at 1500 °C.