Modified MCrAlY coatings for high temperature applications in oxidative environment

MCrAlY (M = Ni, Co, Fe) system derives from some binary alloys (Ni-Al, Fe-Al, Co-Al): at high temperature and above a threshold of Al content forms an alumina (α-Al2O3) scale, slowly growing, retarding further oxidation. MCrAlY coatings are widely used as bond coats in thermal barrier systems to protect the substrate from 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 thermal spray techniques (HVOF and vacuum plasma spray); 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 and microstructure evolution of MCrAlY coatings are strictly related to the amount of the reactive and refractive elements in the starting powders. 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 oxidation resistance of the system was assessed and compared to those of unprotected coatings.