Effect of thermally grown Al2O3 on electrical insulation properties of thin film sensors for high temperature environments

Abstract

NiCrAlY thin films are employed as buffer layers for high temperature thin film sensors, and thermally grown alumina (TG Al2O3) scales are formed by vacuum segregation and oxidation of Al processes at high temperature, which could synergistically enhance the insulation properties of the thin film sensors. In this work, nickel-22% chromium-10% aluminum-1% yttrium (NiCrAlY) thin films were deposited on GH3536 Ni-based superalloys by direct current (DC) magnetron sputtering. The samples were vacuum heat-treated at 1050 °C for 6 h and then subjected to isothermal oxidation at 1050 °C for different time. The morphological and electrical insulation properties of the TG Al2O3 scales were then investigated. The X-ray diffraction peaks of TG Al2O3 gradually increased with increasing oxidation time, while the surface became compact, as confirmed by scanning electron microscopy. The gradual structure from metal alloy to oxide was tailored. Several creaks were observed in the cross-section of TG Al2O3 after prolonged oxidation; thermal mismatch stress is mainly responsible for this phenomenon, which is detrimental to the adhesion of thin film sensors. Furthermore, the results of insulation resistance tests on the TG Al2O3 scales showed that the insulation resistance increased by an order of magnitude with increasing oxidation time, reaching a maximum of ~5 kΩ at 800 °C. Finally, a layer of Al2O3 was deposited by electron beam evaporation to form a multilayer insulating structure, in which the TG Al2O3 was oxidized for 6 h. A favorable insulation resistance (>2 MΩ) was obtained at 800 °C; the electrical insulation properties were assessed using a palladium chromium (PdCr) thin film strain gauge, which exhibited excellent repeatability over four thermal cycles from 25 °C to 800 °C.