Chemical vapor deposition of alpha aluminum oxide for high-temperature aerospace sensors

Thin-film thermocouples and strain gages are being developed for high-tempe rature application on aerospace propulsion hardware for both development te st purposes and as active control sensors. The critical technology necessar y in the fabrication of the sensor is an adherent, dense, and homogeneous d ielectric to provide electrical isolation at engine operating temperatures. Techniques are being developed to create a crystalline aluminum oxide diel ectric formed by a combination of a thermally grown oxide (TGO) from a NiCo CrAIY hardcoating, which is then enhanced with the addition of a chemical v apor deposited (CVD) crystalline aluminum oxide layer. This article will fo cus on the process development used to deposit the a alumina layer on the T GO using CVD in a coldwall reactor at 1100 degrees C, The chemistry employe d in this process is the pyrolitic decomposition of aluminum tri-isopropoxi de. The hexagonal (HCP) a phase is achieved at deposition temperatures of 1 000-1100 degrees C, as confirmed by x-ray diffraction analysis. By eliminat ing gas phase and hot wall decomposition, this approach minimizes precursor depletion effects, yielding a more dense and uniform him morphology. Confo rmal coatings up to 10 mu m thick with high resistivity and good adhesion a nd hardness have been observed on complex airfoil geometries. Growth rates up to 10 mu m/h are possible, although low growth rates lead to more desira ble film properties. The kinetics of the deposition indicate that the react ion proceeds by a mass transport limited mechanism. Uniform temperature con trol over highly complex geometry is desirable, but not essential for unifo rm film growth. Results indicate that the gas flow uniformity and the precu rsor transport rate are the critical variables. (C) 2000 American Vacuum So ciety. [S0734-2101(00)16904-0].