Investigation of the Passive-To-Active Oxidation Transition of SiC via Laser-Induced Fluorescence in an Atmospheric Pressure Inductively-Coupled Plasma Torch

Understanding of the passive-to-active oxidation transition of silicon carbide (SiC) is critical to limiting the extent of material erosion. This transition is investigated at 1 atm through the measurement of atomic silicon and silicon monoxide (SiO), which are associated with the consumption of both the passivating SiO2 layer and the SiC substrate. In this work, we evaluate two different SiC materials---monolithic SiC pucks and SiC coated graphite iso-Qs---and their response to exposure to high enthalpy air plasma. For cold wall heat fluxes of 220 W/cm2, the material surface temperatures range from 2025-2225 K, and both SiC specimen types move into a transitional regime, where silica-consuming and SiC-consuming reactivity is relevant. Planar laser-induced fluorescence (PLIF) measurements of atomic silicon reveal its presence away from the SiC surface, whereas SiO is localized near the surface. The onset of transition, which is defined here as the temperature at which atomic silicon and SiO fluorescence is first detected in the PLIF measurements, agrees well with correlations for the SiC transitional regime at 1 atm. Estimates of the recession rate of the SiC coating of the SiC iso-Q specimens are made based on the PLIF measurements, resulting in a mean recession rate of 2.9 microns/sec.