HfB2/SiC as a protective coating for 2D Cf/SiC composites: Effect of high temperature oxidation on mechanical properties

In the field of thermal shielding for aerospace applications C_f/SiC composites are raising great interest, provided that they are protected from oxidation by suitable coatings. Conversely, ultra high temperature ceramics, and in particular HfB₂, are among the best oxidation resistant materials known. A coating made of a HfB₂/SiC composite (20% weight SiC) was tested as an oxidation protection on a C_f/SiC composite. The composite was produced by Polymer Impregnation Pyrolysis (PIP), which is a simple and low cost method; the coating was applied by painting a slurry on the surface of the composite and by heat treating. The thermal behaviour was studied by thermo-gravimetric analysis, and mechanical tests were conducted before and after oxidation. The HfB₂/SiC composite seems to effectively protect the underlying C_f/SiC composite, with a mechanical strength reduction of only 20% after 30 min at 1600 °C, even if some weight loss due to partial carbon fibre damage is observed. A first analysis of thermal cycling in oxidizing environment suggested that the HfB₂/SiC coating reduces continual damage thanks to the sealing effect of the glassy surface layer.