Thermochemical Instabilities at High Temperature Ceramic Surfaces

The surface of silicon-based high temperature ceramics exhibits thermochemical instabilities when exposed to oxygen-rich high enthalpy flows. These instabilities manifest as sudden temperature jumps of several hundred degrees and rapid material failure, when temperatures exceed 2000 K. Understanding and predicting these phenomena is critical to the design of thermal protection systems for sustained high speed flight vehicles. In this talk we review a series of test cases where surface temperature jumps were observed during plasmatron wind tunnel testing of ceramic materials, including ZrB2-SiC ultra-high temperature ceramics, C/SiC ceramic matrix composites, and silicone-based coatings for low-density carbon phenolic ablators. The underlying physical processes occurring when Si-containing ceramics are exposed to high enthalpy air flow include formation of passivating scales at low temperatures, passive to active oxidation transition, melting of oxide scales, changes in surface radiative properties, formation of porosity and changes in effective conductivity, surface catalytic recombination and transitions in catalytic properties, as well as high-temperature phase changes. The role of these processes in promoting thermochemical instabilities for the different material systems is discussed.