Nondestructive evaluation of the oxidation stresses through thermal barrier coatings using Cr3+ piezospectroscopy

The stresses in the aluminum oxide formed during high-temperature oxidation of a bond-coated superalloy are shown to be measurable through zirconia thermal barrier coatings. The basis for the measurements is the piezospectroscopic shift in the R-line fluorescence (photoluminescence) from Cr3+ impurities incorporated into the growing aluminum oxide scale. Measurements through the thermal barrier coating are feasible because (partially stabilized) zirconia coatings have some transparency at both the excitation and at fluorescence frequencies.