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 Cf/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 Cf/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 Cf/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.