Stabilization of Silica Bubbles in Ultra High Temperature Ceramic Nanocomposites

Critical components in extreme aerothermal environments, such as the leading edge of a hypersonic platform or re-entry vehicle, must remain shape stable while being subjected to temperatures in the excess of 2000oC. Ultra-high-temperature-ceramics (UHTCs, e.g., borides/carbides/nitrides of the Group IV&V elements) are an exciting class of structural materials known for possessing high strength, melting point, emissivity, and thermal conductivity – all of which are necessary to manage the thermal gradients and thermal shock that arise during atmospheric heating. However, despite these advantages, oxidation and evaporative mass-loss in these materials severely limit their long-term service. To address this shortcoming, we propose to study the synthesis and high-temperature oxidation behavior of nanoporous UHTCs infiltrated with SiO2. This dual-phase nanocomposite will possess high-temperature structural integrity and oxidation resistance unattainable in its monolithic counterparts: molten SiO2 will act as a high-temperature barrier to prevent UHTC oxidation, whereas the nanoporous UHTC scaffolding will prevent aeroshearing of molten SiO2 and increase the superheat required to nucleate the deleterious SiO vapor that plagues Si-based compounds. Although this program will entail the fabrication of a novel UHTC nanocomposite, our aim is to use this material as a platform to advance the fundamental understanding between material architecture (i.e., the local curvature, microstructural length scale, and composition) and the nucleation kinetics of vapor bubbles at the nanoscale. We postulate that, as the porosity length scale is decreased into the nanometer regime, the threshold for heterogeneous nucleation will increase (due to a decrease in active surface sites) and vapor will eventually only nucleate homogeneously at superheats well above its typical boiling point. In addition, the nucleation rate can be further tuned by altering the interfacial energy and ratio of positive-to-negative curvature. If successful, this program will form the foundation of a new class of nanoarchitectured UHTC materials with unprecedented high-temperature behavior.