TGO Stress Growth for Pt-Modified Aluminide Bond Coats during High Temperature Oxidation

Microstructure and compressive stress in thermal growth oxide (TGO) of Pt-modified aluminide coating on nickel base superalloy CSMX4 for thermal barrier coating (TBC) application after discontinuous oxidation at high temperature in air have been studied. Using a micro Raman instrument technology, photo-stimulated-luminescence-spectra (PSLS) of theta- and alpha-Al2O3 formation are obtained for Pt-modified aluminide coating after 20 h oxidation exposure at 900 degrees C in air; however, a compact and protective alpha-Al2O3 layer is formed over the coating at 1100 degrees C in air. Thermal growth stress in alpha-Al2O3 was calculated according to the spectra shift compared with stress-free alumina spectra, a little higher than 3.0 GPa for TGO in TBC system after oxidation exposure. The stress within "ridges" of alumina surface over the coating is lower due to crack's formation during high temperature oxidation. Alumina rumpling is obvious for Pt-modified aluminide coating due to absence of ceramic layer. At the same time, Al element's quick diffusion along substrate grain boundary results in more gamma'-Ni3Al phase formed along boundary during oxidation. Coefficient of thermal expansion for the coating is changed with phase transformation from beta-NiAl to gamma'-Ni3Al, and finally affects the stress in TGO and coating lifetime during high temperature oxidation.