Ceramics take the heat

Sandia researchers are developing ultrahigh temperature ceramics (UHTCs) that withstand up to 2,000°C for use on hypersonic vehicles such as the space shuttle. The ceramics go in thermal-insulation on hypersonic vehicles. They also must resist evaporation, erosion, and oxidation, as well as exhibit low thermal diffusivity to limit heat transferring to support structures.

The ceramics are made of zirconium diboride, hafnium diboride, and silicon carbide and take the form of protective, oxidation-resistant coatings. "However, in their present state of development, UHTCs are weak and don't handle thermal shock well, deficiencies that keep them from being fully dense ceramics with good microstructures," says Ron Loehman, a senior scientist in Sandia's Ceramic Materials group. The poor properties come from agglomerates, inhomogeneities, and grain boundary impurities, all of which are errors in ceramic processing. Recently, researchers developed UHTCs that are 100% dense or nearly so. Also, researchers have hot pressed UHTCs with a wider range of SiC contents than used before. A range of compositions and microstructures give system engineers more flexibility with their designs.