On the microstructure of the initial oxide grown by controlled annealing and oxidation on a NiCoCrAlY bond coating

Different pre-annealing and pre-oxidation treatments were conducted on a dual phase gamma+beta Ni-21Co-18Cr-22Al-0.2Y (at.%) bond coating for 1 hr at 1373 K (i) with or without a native oxide upon heating, (ii) in two different atmospheres upon heating, and (iii) under various oxygen partial pressures (pO(2)) in the range of 0.1-10(5) Pa during oxidation. The chemical composition, structure, morphology and phase constitution of the resulting oxide layers were investigated using a range of analytical techniques. It is found that the exclusive formation of a continuous alpha-Al2O3 layer without the simultaneous formation of NiAl2O4 spinel was promoted for oxidation at low pO(2). The formation of metastable theta-Al2O3 was suppressed for a low fraction of the beta phase, coupled with a high fraction of segregated Y at the initial bond coat surface. Initial Y segregation and incorporation of Y2O3 and Y3Al5O12 within the developing oxide layer was promoted in the absence of a native oxide and for heating in an inert atmosphere. The development of protrusions (i.e. pegs) at the oxide/coating interface, as a result of the incorporation of internal Y2O3 precipitates by the inward growing oxide layer, was most pronounced upon heating in an inert atmosphere, followed by oxidation at an intermediate pO(2).