AN EVALUATION OF THE SUBSTRATE EFFECT ON THE PERFORMANCE OF YSZ EB-PVD THERMAL BARRIER COATINGS

摘要

The substrate effect of eight different nickel base superalloys on the cyclic failure of thermal barrier coatings is studied. Substrates include six third generation superalloys, one second generation superalloy, and the alloy IN738. The thermal barrier coatings include yttria stabilized zirconia (YSZ) deposited by electron beam physical vapor deposition (EB-PVD), a (Ni,Pt) Al underlayer deposited by chemical vapor deposition (CVD) and grit blasted to provide a rough surface. The study conducted included the cyclic exposure of the specimens between 1100 ºC and room temperature. During the cyclic heat exposure, a thermally grown oxide (TGO) grew. After failure, the specimens were categorized according to the number of cycles obtained before failure. Later, specimens were examined by macroscopic and microscopic methods and chemical analysis was obtained by energy dispersive spectrometry (EDS). Specimens all failed by ratcheting along the YSZ/TGO interface showing little or no effect of the substrate compositions. Nevertheless, large differences in the cyclic lives existed among the third generation superalloys and between third and second generation superalloys, and single crystal vs. polycrystalline superalloys. Phase transformations were observed in the bond coat. Substrate elements such as Ta, Cr, and Co appear to play a role in the transformation of these phases. Bond coat deformation is attributed to the phase transformation, and the bond coat phase properties are attributed to chemical content. Carbon and tantalum play a role in the cyclic failure of the specimens.