High-Temperature Stability of Amorphous Al 2 O 3 Deposited on Si and CeO 2 -stabilized ZrO 2 by Metalorganic Chemical Va
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ABSTRACT Aluminum acetylacetonate and water vapor were used to deposit an amorphous A120 3 seal coating on Si and CeO 2-stabilized ZrO2 (CSZ) produced by air plasma spray. The A120 3 coating prepared at -500tC was -2.5pam thick, uniform, and non-porous. The crystallization and adhesion characteristics of the A120 3 coating were examined by thermally annealing the A12 0 3coated Si and CSZ substrates in air. The amorphous coating crystallized to metastable A120 3 phases within 20 hours at temperatures as low as 700'C. The coating on Si spalled upon annealing whereas the coating on CSZ did not spall, but microcracked extensively due to the significant volume shrinkage associated with crystallization. INTRODUCTION Thermal barrier coatings (TBCs) have been developed to improve the performance of aircraft engines, power generation gas turbines, and diesel engines [1-3]. For diesel engines, CeO 2-stabilized ZrO 2 (CSZ) is applied by air plasma spray (APS) over a graded NiCrAlY bond coating to insulate and protect cast iron components from high-temperature combustion environments [2].
The APS-CSZ coating is intentionally made porous for strain tolerance and
for enhanced adhesion at the metal-ceramic interface region. However, recent engine test results have shown that the porous CSZ coating may absorb fuel prior to ignition in the combustion chamber and thus may adversely affect overall engine efficiency [2,3]. A pragmatic solution to the fuel entrainment problem is to seal the CSZ surface with a thin coating. Such a seal coating must be non-porous, impermeable, conformal, adherent, and stable upon thermal cycling from 800-900°C. Another major constraint is that the seal coating must be prepared at temperatures below 500'C to avoid tempering of cast iron components. A120 3 is an ideal candidate coating material because: (1) it is one of the most thermochemically stable materials with respect to ZrO 2-based ceramics as well as in hightemperature corrosion environments and (2) it has a coefficient of thermal expansion (CTE) similar to that of CSZ (-8x10"6 versus -0Oxl106 K-, respectively). However, these attractive properties are mainly reserved for crystalline a-A120 3 which is the thermodynamically stable polymorph of A120 3. It is generally known that ca-A1 20 3 is a difficult material to prepare by chemical or physical vapor deposition processes unless deposition temperatures above 1000'C are used [4]. From this perspective, the main objective of this work was to explore the possibility of using an amorphous A12 0 3 prepared by metalorganic chemical vapor deposition (MOCVD) at -.500*C with emphasis on evaluating the coating's physical integrity upon exposure to high-temperature environments. In particular, the significant volume shrinkage expected from the crystallization of the amorphous A12 0 3 coating (- -9%) [5] was a great concern. 103 Mat. Res. Soc. Symp. Proc. Vol. 555 01999 Materials Research Society
EXPERIMENTAL Al(acac) 3 (aluminum acetylacetonate: AI(C 5H 70 2)3) was chosen as an Al precursor because it de
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