James A. Zaykoski

Oxidation-based materials selection for 2000°C + hypersonic aerosurfaces: Theoretical considerations and historical experience

Hypersonic flight involves extremely high velocities and gas temperatures with the attendant necessity for thermal protection systems (TPS). New high temperature materials are needed for these TPS systems. A systematic investigation of the thermodynamics of potential materials revealed that low oxidation rate materials, which form pure scales of SiO2, Al2O3, Cr2O3, or BeO, cannot be utilized at temperatures of 1800°C (and above) due to disruptively high vapor pressures which arise at the interface of the base material and the scale. Vapor pressure considerations provide significant insight into the relatively good oxidation resistance of ZrB2- and HfB2-based materials at 2000°C and above. These materials form multi-oxide scales composed of a refractory crystalline oxide (skeleton) and a glass component, and this compositional approach is recommended for further development. The oxidation resistance of ZrB2-SiC and other non-oxide materials is improved, to at least 1600°C, by compositional modifications which promote immiscibility in the glass component of the scale. Other candidate materials forming high temperature oxides, such as rare earth compounds, are largely unexplored for high temperature applications and may be attractive candidates for hypersonic TPS materials.

Mechanical, Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

Abstract The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB2, HfC0·98, HfC0·67, and HfN0·92 ceramics. The oxidation resistance of ceramics in the ZrB2–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC0·98 to 1100°C for HfC0·67 ceramics. The transition temperature of HfB2 was 1100°C. The ZrB2/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB4, and C using displacement reactions. The ceramics with ZrB2 as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB2 and ZrC ceramics. The ceramics with ZrB2/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.