| The application of thin film sensors on hot section components of gas turbine engines requires the formation a specific and complex layered structure to be 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 this work, test specimens consisting of an MCrAlY-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 test specimen structure, particularly in terms of superficial thermally-grown oxide (TGO) formation. Three different MCrAlY materials were applied to the Hastelloy-X substrate, with each coat material deposited by a different process. FeCrAlY, NiCrAlY, and CoNiCrAlY were applied by magnetron sputtering physical vapour deposition (MS-PVD), air plasma spray (APS) deposition, and high-velocity oxy-fuel spray (HVOF) deposition, respectively. The surfaces and cross-sectional features of the test specimens were examined using SEM and EDS analysis, together with the support of XRD analysis. Of the heat-treated bond coat-substrate combinations tested, the HVOF CoNiCrAlY-coated test specimens subjected to an air furnace heat treatment of 1100°C for 1 hour yielded the most suitable surface condition that could be processed further into a thin film sensor assembly prototype. The surface of this specimen consisted of a 1 mum-thick aluminum oxide TGO layer featuring the fewest distributed transition metal oxide clusters. |
