AVS 46th International Symposium
    Thin Films Division Tuesday Sessions
       Session TF-TuA

Paper TF-TuA9
Chemical Vapor Deposition of Alpha Aluminum Oxide for High Temperature Aerospace Sensors

Tuesday, October 26, 1999, 4:40 pm, Room 615

Session: Fundamentals of Si and Dielectric PVD
Presenter: R.H. Niska, AlliedSignal Aerospace Co.
Authors: R.H. Niska, AlliedSignal Aerospace Co.
A.P. Constant, Iowa State University
T. Witt, Iowa State University
O.J. Gregory, University of Rhode Island
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Thin film thermocouples and strain gages are being developed for high temperature application on aerospace propulsion hardware for both development test purposes and as active control sensors. The critical technology necessary in the fabrication of the sensor is an adherent, dense, and homogeneous dielectric to provide electrical isolation at engine operating temperatures. Techniques are being developed to create a crystalline aluminum oxide dielectric formed by a combination of a thermally grown oxide [TGO] from a NiCoCrAlY hardcoating which is then enhanced with the addition of a chemical vapor deposited [CVD] crystalline aluminum oxide layer. This paper will focus on the process development used to deposit the alpha alumina layer on the TGO using CVD in a coldwall reactor at 1100C. The chemistry employed in this process is the pyrolitic decomposition of aluminum tri-isopropoxide. The hexagonal [HCP] alpha phase is achieved at deposition temperatures of 1000C-1100C, as confirmed by X-ray diffraction analysis. By eliminating gas phase and hot wall decomposition, this approach minimizes precursor depletion effects, yielding a more dense and uniform film morphology. Conformal coatings up to 10 microns thick with high resistivity and good adhesion and hardness have been observed on complex airfoil geometries. Growth rates up to 10 microns per hour are possible although low growth rates lead to more desirable film properties. The kinetics of the deposition indicate that the reaction proceeds by a mass transport limited mechanism. Uniform temperature control over highly complex geometry is desireable, but not essential for uniform film growth. Results indicate that the gas flow uniformity and the precursor transport rate are the critical variables.