THE CHARACTERISTICS OF ALUMINA SCALES FORMED ON HVOF-SPRAYED MCRALY COATINGS

Turbine blades are protected against high-temperature oxidation by the rmal barrier coating (TBC) systems, which consist of a ceramic top coa ting (ZrO2/Y2O3) and a metal bond coating (MCrAlY, M=Ni, Co). At high temperatures and under oxidative conditions between the bond coating a nd the ceramic top coating an oxide scale is formed, which protects th e metal against further oxidation. The oxidation behavior of thermally sprayed MCrAlY coatings is influenced by the coating process and the composition of the metal alloy. This work is concerned with the isothe rmal oxidation behavior of high-velocity oxygen fuel (HVOF) MCrAlY (M = Ni, Co) coatings. During thermal spraying two elements, yttrium and aluminum, oxidize. As a consequence the HVOF MCrAlY coatings exhibit a microstructure with fine dispersed Al2O3 and aluminum yttrium oxides. Free-standing bodies of HVOF coatings were isothermally oxidized in s ynthetic air between 850 and 1050 degrees C for different periods. Oxi dation experiments show that the oxidation rate of HVOF coatings is tw o times slower than the oxidation rate of corresponding VPS MCrAlY coa tings. The oxidation mechanism is mainly changed in the transient stag e (no metastable modification of Al2O3 was formed). It is supposed tha t the oxide dispersion favors the formation of alpha-Al2O3 scales. The presence of the fine oxide dispersion also influences the adherence o f the oxide scale. The microstructure of the transient oxide scales we re examined by X-ray diffraction, scanning electron microscopy and tra nsmission electron microscopy. (C) 1998 Elsevier Science S.A. All righ ts reserved.