High temperature oxidation resistance of modified MCrAlY coatings for thermal barrier systems

MCrAlY coatings obtained by thermal spray techniques are widely used as bond coats in thermal barrier systems to protect the substrate from corrosion and high temperature oxidation and to guarantee the coupling between ceramic top coat and substrate. In this work, the high temperature oxidation resistance of MCrAlY coatings with modified compositions was evaluated; in particular the effect of reactive and refractory elements (Ta, Re, Hf) addition was studied. The investigated MCrAlY coatings were obtained by HVOF and plasma spray (VPS) techniques; the samples were exposed in air at 1150°C up to 100 hours and the oxidation kinetics were evaluated by measuring the thickness of TGO (Thermally Grown Oxide) scale at the several exposure times. Microstructural evolution induced by the heat treatments was studied by means of SEM, EDS and XRD analyses: kinetics data verified that the oxidation resistance of MCrAlY coatings is strictly related to the amount of the reactive and refractory elements in the starting powders and that a thorough understanding of microstructure modifications is essential in controlling the TGO growing. Moreover the effects of an innovative surface treatment were evaluated: selected MCrAlY coatings were protected with a sputtered ceramic film deposited by PVD technique; the system oxidation resistance was assessed and compared to that of unprotected coatings.