INFLUENCE OF THE SURFACE-ROUGHNESS ON THE OXIDE SCALE FORMATION ON MCRALY COATINGS STUDIED IN-SITU BY HIGH-TEMPERATURE X-RAY-DIFFRACTION

In modem stationary gas turbines for electric power generation the lif e time of the blades is mainly controlled by the oxidation resistance of the coatings. In the case of protective overlay coatings the surfac es are generally shot peened, whereas bond coats for EBPVD thermal bar rier coatings need a smooth surface. For a better understanding of the influence of the surface roughness on the oxide scale formation, in s itu studies using high temperature X-ray diffraction were performed on an MCrAlYRe coating with 12% Al and 3% Re for the use as overlay and as bond coat. The measurements were performed in air at 950 and 1000 d egrees C. The results show differences in the oxide scale formation on the same coating with an as-sprayed and a polished surface. On the as -sprayed surface Cr2O3 and spinels were observed besides alpha-Al2O3 i n the initial state whereas on the polished surface a coherent alpha-A l2O3 scale with inclusions of YAlO3 was observed from the beginning. ( C) 1998 Elsevier Science S.A. All rights reserved.