Copper-chromium (CuCr) and nickel-chromium-aluminum-yttrium (NiCrAlY) coatings were applied by kinetic metallization to GrCop-84 substrates (an advanced copper-based alloy developed at the NASA Glenn Research Center specifically for rocket thrust chambers, ref. 1). The coatings showed excellent cyclic oxidation behavior at 600 °C. In addition, the CuCr coatings showed excellent compatibility with the GrCop-84 substrates during thermal exposures. Similarly, when a CuCrAl bond coat was used with the NiCrAlY coating, the two-layer coating also showed excellent compatibility with the GrCop-84 substrate. Both coatings show outstanding potential to provide oxidation protection for copper-based rocket thrust chambers.
Advanced rocket thrust chambers, such as those on the Space Shuttle Main Engines, are made of copper-based alloys possessing both high strength and high thermal conductivity. Although the high thermal conductivity allows the cryogenic fuel to cool the copper alloy, a thin oxide scale forms on the surface exposed to the hot gas. Because the gases flowing in the combustion chamber are extremely turbulent, this thin oxide scale can be repeatedly reduced to metallic copper and reoxidized in a process known as "blanching," which can weaken the chamber lining. Consequently, coatings were explored to provide oxidation protection for higher temperature use and to eliminate this blanching effect.
CuCr coatings form a protective oxide scale based on chromium oxide (Cr2O3) that is extremely stable and able to protect thrust chambers from blanching attack (ref. 2). NiCrAlY coatings, used routinely to provide oxidation protection in aeroengines, form protective alumina scales with even greater stability than Cr2O3 and, consequently, are not susceptible to blanching. Various coating techniques have been explored to deposit NiCrAlY and CuCr coatings on copper-based thrust chambers. One technique is kinetic metallization--an impact consolidation process in which solid-state metallic powders are deposited without melting to produce a coating (ref. 3). An inert carrier gas, commonly helium, is used to accelerate the powder particles through a specially designed sonic nozzle to velocities just below the sonic speed of the gas. Since the powder particles are deposited at low temperatures and in an inert gas, there is almost no oxidation of the powder or substrate during deposition, an important consideration for coating copper-based components.
For any coating, a prime concern is adherence of the coating to the substrate. The best adherence results when strong metallurgical bonds form. Short diffusion anneals can allow interdiffusion between the coating and substrate strengthening this bond. However, for certain materials, interdiffusion can result in the formation of porosity, known as Kirkendall porosity, at or near the interface, which can degrade coating adherence. In addition to thermal exposure during operation, thrust chambers often undergo a higher temperature thermal exposure during fabrication associated with the brazing operation that attaches the liner to an outer jacket. This brazing operation, although short, is typically several hundred degrees higher than the normal chamber operating temperature and results in the greatest amount of interdiffusion.
Weight change during thermal cycling at 600 °C for NiCrAlY and three blended elemental Cu-Cr coatings compared with that of uncoated GrCop-84.
Kinetic metallization coatings were deposited on coupons of GrCop-84 by Innovative Technologies, Inc. (Inovati), of Goleta, California. NiCrAlY coatings, with and without a bond coat, as well as three CuCr coatings were deposited and evaluated. Both NiCrAlY and the CuCr coatings showed excellent oxidation behavior during thermal cycling at 600 °C (see the graph). In agreement with earlier results for similar CuCr coatings (ref. 4), none of the coated samples showed any weight loss, in contrast to results for the uncoated copper alloy. After a simulated braze anneal (950 °C for 30 min), significant porosity developed when the NiCrAlY was deposited directly on the substrate (see the left image below). However, when a CuCrAl bond coat was deposited prior to the NiCrAlY coating, no porosity developed after annealing (see the center image). Similarly, the CuCr coatings showed excellent compatibility with the GrCop-84 substrates during thermal exposures, with no porosity observed after annealing (see the right image). Consequently, both coating systems, NiCrAlY with a CuCrAl bond coat, and CuCr coatings demonstrate excellent potential for protecting copper-based rocket thrust chambers from oxidation.
Microstructure of the coated GrCop-84 substrate after a simulated braze anneal of 950 °C for 30 min. Left: NiCrAlY applied directly on GrCop-84. Porosity indicated by arrows. Center: NiCrAlY with CuCrAl bond coat. Right: CuCr applied directly on GrCop-84.
Find out more about the research of Glennís Durability and Protective Coatings Branch: http://www.grc.nasa.gov/WWW/EDB/
Dr. James A. Nesbitt, 216-433-3275, James.A.Nesbitt@nasa.gov
Author: Dr. James A. Nesbitt
Headquarters program office: Space Exploration
Programs/Projects: Constellation Systems
Last updated: October 16, 2006
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