Spectroscopic nondestructive evaluation of environmental barrier coating failure on silicon-based ceramics

Environmental barrier coatings (EBCs) are external ceramic coatings on silicon-based ceramic components to protect them from water vapor attack in combustion environments. A spectroscopic nondestructive technique to monitor the degradation of environmental barrier coatings was investigated. Lithium oxide was selected as a spectroscopic marker material after an extensive screening of various materials for their emissivity in simulated combustion environments. Spectral response of excited lithium atoms from mullite doped with 0.025-1 wt% lithium oxide was examined under an oxyacetylene flame. A three-layer EBC (Si/Li2O-doped mullite/BSAS) on a silicon carbide substrate, which had fine simulated cracks, showed significant lithium emission on exposure to the oxy-acetylene flame, which demonstrated the potential of emission spectroscopy as a nondestructive evaluation tool to monitor EBC degradation.