Investigation of CVD Processes to Perform Dense α-Alumina Coating on Superalloys

Abstract

Coated nickel-based superalloys are widely used as superior material systems in gas turbines, where the overlay coatings help withstand hot corrosion and serve as bond coats for the thermal barrier layer. However, interdiffusion may limit the coatings' lifetime, a potential remedy being a diffusion barrier layer. In this context, the chemical vapor deposition ͑CVD͒ process was investigated to deposit ␣-Al 2 O 3 on superalloy substrates. Deposited films using an AlCl 3 /CO 2 /H 2 gas mixture at low and atmospheric pressure were characterized. Difficulties including whisker growth at low pressure and the growth of noncompact large crystals at atmospheric pressure are discussed jointly with strategies for suppressing them. In addition, the potential of alternative precursors to produce ␣-Al 2 O 3 films was investigated using aluminum tri-isopropoxide in an adequate reactor geometry.

Inhibition Effect of APCVD Titanium Nitride Coating on Coke Growth during n-Hexane Thermal Cracking under Supercritical Conditions

“…It is generally acknowledged that alumina coatings can be deposited in amorphous, metastable, and stable crystalline phases, but amorphous and metastable phases transforming to the thermodynamically stable corundum phase (α-Al 2 O 3 , the hardest phase and chemically very inert) often requires substrate temperatures above 1000 °C. The range of deposition temperatures reported in the literature reviews by Maruyama and Arai, Huntz et al, and Pranhan et al for amorphous films was between 250 and 550 °C. − The thermodynamically stable α phase was grown at temperatures between 900 and 1200 °C, as reported by Bahlawane et al and Müller et al , Moreover, it is worth pointing out that the alumina coatings will generate a lot of cracks and delamination by the MOCVD method as their thickness increases . They explained that there existed a critical thickness during deposition of the thin alumina coatings owing to internal stress at the outer side of the film, and it was elaborated in detail by Haanappel et al about 1 μm during deposition of the thin alumina films on AISI304 .…”

Integrated Approach for the Development of Advanced, Coated Gas Turbine Blades

“…[19][20][21][22] The Al 2 O 3 layer should be as thin as possible in order to minimize thermomechanical load from lattice mismatch, but if it is less than about 3 lm thick it may become unstable during high temperature exposure and not block interdiffusion completely. [19] Chemical Vapour Deposition (CVD) of Al 2 O 3 on metallic substrates may lead to whisker growth, [19][20][21]23,24] …”

Alumina coatings on silica powders by Fluidized Bed Chemical Vapor Deposition from aluminium acetylacetonate

“…Alumina coatings have attracted much interest this last decade, due to their interesting physical and chemical properties [7]. Alumina is a relatively hard material, chemically inert and stable at high temperature, which makes it interesting as diffusion barriers [8], or hard coatings [9]. CVD is still today the economically most favorable technique for producing high-quality alumina coatings [10].…”