In:
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, American Vacuum Society, Vol. 18, No. 4 ( 2000-07-01), p. 1653-1658
Abstract:
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 article will focus on the process development used to deposit the α alumina layer on the TGO using CVD in a coldwall reactor at 1100 °C. The chemistry employed in this process is the pyrolitic decomposition of aluminum tri-isopropoxide. The hexagonal (HCP) α phase is achieved at deposition temperatures of 1000–1100 °C, 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 μm thick with high resistivity and good adhesion and hardness have been observed on complex airfoil geometries. Growth rates up to 10 μm/h 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 desirable, but not essential for uniform film growth. Results indicate that the gas flow uniformity and the precursor transport rate are the critical variables.
Type of Medium:
Online Resource
ISSN:
0734-2101
,
1520-8559
Language:
English
Publisher:
American Vacuum Society
Publication Date:
2000
detail.hit.zdb_id:
1475424-1
detail.hit.zdb_id:
797704-9
