MOCVD of Aluminum Oxide Barrier Coating
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MOCVD of Aluminum Oxide Barrier Coating Jun C. Nable1, Malgorzata A. Gulbinska1, Steven L.Suib 1,2*, Francis S.Galasso1 1 Department of Chemistry, University of Connecticut, Storrs, CT 06269 2 Institute of Material Sciences, University of Connecticut, Storrs, CT * corresponding author Abstract High temperature materials such as nickel superalloys require the development of corrosion resistant coatings. These coatings must be thermally stable and chemically inert in order to protect the substrate material. Aluminum oxide is one such candidate. Aluminum acetylacetonate was used as the precursor for metal-organic chemical vapor deposition at atmospheric pressure. Uniform, amorphous, and multi-colored protective coatings were deposited onto nickel substrates at 400 to 500°C and dark gray, and powdery coatings were obtained at higher temperatures of 550 to 600°C. The coating was free from cracks when higher carrier gas flow rate were used. Introduction High temperature materials such as nickel superalloys are subjected to extremely high temperatures and corrosive environments. Nickel superalloys are used as major components for turbine parts [1]. Under normal operation these alloys are exposed to detrimental conditions. The process gases as well as the surrounding atmospheric environment at high temperatures result in rapid corrosion of the alloy. The corrosion eventually oxidizes the material and leads to failure of the engine component. Oxygen diffusion is one of the major issues, because this leads to oxidation of the bulk material. Aluminum oxide is a good candidate for a coating, which acts as a diffusion barrier against oxygen [2-5]. Chemical vapor deposition (CVD) is a widely employed coating method for high temperature materials. Conventional CVD of aluminum oxide utilizes chloride precursor, which results in dense, crystalline coatings [4,6,7]. However, during the coating process, significant property degradation can occur. The chloride precursor itself can contribute to corrosion of the substrate by reacting with metal impurities in the reactor. This process also is done at relatively high temperatures usually around 1000°C, which can be detrimental to the overall strength of the material [8]. An alternative would be to utilize metal-organic precursor for aluminum oxide deposition. Metal-organic chemical vapor deposition (MOCVD) has the advantage of being done at lower temperatures, usually from 200 to 600°C [7,9]. With proper conditioning and parameter control, different phases and microstructure can be obtained. Utilizing metal-organic compounds also avoid the undesired growth of whiskers and needles associated with using metal halide precursors [10]. Aluminum acetylacetonate was the chosen precursor for this MOCVD process. Aluminum acetylacetonate is a solid with an appreciable vapor pressure, unlike aluminum alkoxides. Hence this metal organic compound has a higher deposition rate. Aluminum
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acetylacetonate has been used to deposit dense, amorphous aluminum oxide thin films used for optoe
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