Characterization of transient oxide formation on CoNiCrAlY after heat treatment in vacuum and air

The application of thin film sensors on hot section components of gas turbine engines requires the formation a specific and complex layered structure that is functional, reliable, and durable. In the case of conducting substrates, such as the nickel- or cobalt-based superalloys, an electrically-insulating layer (typically aluminum oxide) is required between the sensing layer elements and the substrate. In order to achieve suitable bonding of alumina to the substrate, a MCrAlY type bond coat is commonly applied to the metallic substrate and the subsequent formation of dense, adherent oxide(s) on the MCrAlY coating surface can ultimately determine the durability of the layered structure. In this work, test specimens consisting of a high-velocity oxy-fuel (HVOF) CoNiCrAlY coated nickel-base substrate were subjected to selected vacuum and air furnace heat treatments in order to observe the effects of the heat treatments on the transient oxide formation on the CoNiCrAlY surface. The surfaces and cross-sectional features of the test specimens were examined using scanning electron microscope (SEM) and energy dispersive spectrometry analysis (EDS), together with the X-ray diffraction (XRD) analysis. Of the heat treatment conditions examined, the vacuum heat treated CoNiCrAlY specimens exhibited the formation of a denser alumina layer while the air furnace heat treatments resulted in a more ridge-like alumina formation on the surface. Increasing heat treatment temperature and time reduced the amount of transition metal oxide and encouraged more uniform alumina formation.