Oxidation resistance of HfB2HfB2–SiC composites for protection of carbon-based materials

The oxidation behaviour of HfB2HfB2–SiC monoliths and coatings has been studied in the 20–1700∘C temperature range in flowing O2O2/He mixtures (PtotPtot=1 bar, PO2=1PO2=1–200 mbar). Two overlapping domains of oxidation were identified corresponding, respectively, to the reaction of HfB2HfB2 and an intermediate phase containing Hf/Si/B/C in the 600–800∘C temperature range and SiC nanoparticles in the 800–1000∘C interval. A protective borosilicate glass is quickly formed at low temperature. The good oxidation resistance up to 1700∘C and under low oxygen partial pressure (10 mbar) is discussed according to the degradation mechanisms involving compositional and microstructural changes and compared to previous results obtained on hot pressed HfB2HfB2–SiC composites. The specific microstructure of the materials studied in this work is of great importance for the composition of the protective glassy phase and leads to the formation of a refractory glass containing hafnium.