Characterization of zirconium diboride for thermal protection systems

Zirconium diboride (ZrB2) composites containing either silicon carbide (SiC) or molybdenum disilicide (MoSi2) were investigated as potential thermal protection system (TPS) materials for future reusable launch vehicles (RLVs). The additions were investigated because of the ability of SiC and MoSi2 to form an oxidation resistant surface layer over ZrB2. Samples of pure zirconium diboride and billets containing 10, 20, and 30 volume percent of either SiC or MoSi2 were prepared by hot pressing. Microstructures were characterized by examining polished cross sections using scanning electron microscopy. The density, four point bend strength, Vickers' hardness, and elastic modulus were measured for all materials. Oxidation behavior was characterized using thermogravirmetric analysis. The addition of SiC and MoSi2 improved the strength of ZrB2, reaching a maximum of similar to1 GPa at 30 volume percent additives. SiC additives also improved the fracture toughness, with 30 volume percent SiC increasing toughness to 5.25 MPa-m (1/2). The addition of SiC or MoSi2 improved the oxidation resistance of the composites, with 30 volume percent MoSi2 having the lowest weight gain of 0.004 mg/mm(2) after heating in air to 1500degreesC.