Studies on the transient stage of oxidation of VPS and HVOF sprayed MCrAlY coatings

Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion. They consist of a MCrAlY bond coating and a ceramic top layer. The oxidation resistance of MCrAlY coating is based on the formation of alpha-Al2O3 in the steady-state stage of oxidation. The alpha-Al2O3 grows very slowly between the metallic bond coating and the ceramic top coating and is thtermodynamically stable. High-temperature oxidation experiments with vacuum-plasma-sprayed (VPS) and high-velocity-oxygen-fuel (HVOF)-sprayed MCrAlY coatings showed that the transient stage of oxidation for HVOF coatings is very short. Moreover, in contrast to the VPS coatings, where also Cr2O3, NiO or CoO are identified, on HVOF coatings, only metastable modifications of alumina were observed. The transformation of metastable alumina modifications to alpha-Al2O3 is very fast. Oxidation experiments in a high-temperature chamber attached to a X-ray diffractometer were carried out. The morphology and the structure of the formed oxide scale were characterized by X-ray diffraction and scanning electron microscopy (SEM). (C) 1999 Published by Elsevier Science S.A. All rights reserved.