The characteristics of alumina scales formed on HVOF-sprayed MCrAlY coatings

Abstract

Turbine blades are protected against high-temperature oxidation by thermal barrier coating (TBC) systems, which consist of a ceramic top coating (ZrO2/Y2O3) and a metal bond coating (MCrAlY, M=Ni, Co). At high temperatures and under oxidative conditions between the bond coating and the ceramic top coating an oxide scale is formed, which protects the 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 isothermal 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 synthetic air between 850 and 1050°C for different periods. Oxidation experiments show that the oxidation rate of HVOF coatings is two times slower than the oxidation rate of corresponding VPS MCrAlY coatings. The oxidation mechanism is mainly changed in the transient stage (no metastable modification of Al2O3 was formed). It is supposed that the oxide dispersion favors the formation of α-Al2O3 scales. The presence of the fine oxide dispersion also influences the adherence of the oxide scale. The microstructure of the transient oxide scales were examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.