Adam L. Chamberlain

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 thermogravimetric analysis. The addition of SiC and MoSi2 improved the strength of ZrB2, reaching a maximum of ~1 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 1500oC. Introduction Future designs for hypersonic reentry vehicles call for significant reductions in the radii of the leading edges. Studies have shown that this reduction will provide significant improvements in vehicle safety by allowing increased cross range during reentry and increased time during ascent when a safe abort to land can be performed [1]. However, the decrease in leading edge radii will subject these areas to temperatures that will exceed 2000oC. The current leading edges on the space shuttle orbiter consist of a reinforced carbon-carbon (RCC) composite with a SiC coating. Oxidation resistance is provided by a protective oxide layer that forms by the passive oxidation of the SiC coating. This layer hinders the oxidation of the carbon substrate, but is only able to protect up to 1600oC. Above 1600oC, the oxide layer is unable to protect because of the change from passive to active oxidation, which is accompanied by the formation of SiO [2,3]. The transition to active oxidation leads to loss of the SiC coating and eventual exposure and degradation of the underlying RCC substrate. The inability of current TPS materials to provide the necessary oxidation resistance in extreme environments has increased the need for new leading edge materials. One family of materials that has been investigated is ultrahigh temperature ceramics (UHTCs), which include Zrand Hf-based nonoxide ceramics. These materials have the potential to operate at temperatures greater than 2000oC due to melting temperatures in excess of 3000oC. Of the UHTCs, ZrB2 is of particular interest because of its ability to maintain 125 MPa strength at 1800oC [4]. ZrB2 also has the lowest density of the UTHCs at 6.09g/cm 3 ; therefore, having the potential for providing the best specific properties (e.g. strength to weight ratio). In this study the effects of SiC or MoSi2 additions on the microstructure and properties of ZrB2 were investigated. These additives were chosen due to their ability to form a passive oxide layer. MoSi2 was investigated due to its potential to provide a protective layer at higher temperatures than SiC. Thermodynamic calculations suggest that MoSi2 may provide passive oxidation up to 2000oC; Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 493-496 doi:10.4028/www.scientific.net/KEM.264-268.493