SwRI: Development of Nanostructured Cu-Cr Coatings: Responsive specialists for any corrosion-related problems Responsive specialists for any corrosion-related problems

	Microbalance system for studying high-temperature oxidation and corrosion

Copper-based alloys and composites are candidate materials for high heat-flux structural applications in reusable launch vehicles, such as NASA's crew exploration vehicles.

New nanostructured copper-chromium (Cu-Cr) coatings are being developed at Southwest Research Institute (SwRI) to protect underlying copper-based materials from:

• High-temperature oxidation

• Hot gas erosion

• Cyclic oxidation and reduction environments

Properties of Nanostructured Cu-Cr Coatings

The effectiveness of nanostructured Cu-Cr coatings for protecting advanced copper alloys from high-temperature oxidation has been demonstrated in isothermal and cyclic oxidation. The coatings can protect the combustion chamber wall and nozzle ramps against oxidation / reduction degradation in hypersonic vehicles. SwRI has several large vacuum chambers that are essential to evaluating the coating processes as articles are scaled up to prototype size.

Oxidation of copper alloys at elevated temperature is controlled predominantly by outward diffusion of copper ions to form external copper oxides. Protective coatings are needed to shield these materials from high-temperature oxidation.
 

Two vacuum-based surface engineering techniques are being explored for depositing the nanostructured Cu-Cr coatings:

• Ion beam assisted deposition (IBAD)

• Magnetron sputtering deposition (MSD)

Procedures for Applying Nanostructured Cu-Cr Coatings

GRCop-84 (Cu-8 atomic percent Cr-4 atomic percent Nb), an advanced copper material developed by NASA Glenn Research Center, was used for coating deposition and oxidation studies. SwRI has produced multilayered Cu-Cr coating by ion beam deposition of alternating layers of copper and chromium. A dual-gun electron beam vapor deposition system was also constructed to produce a single layer of nanostructured Cu-Cr coating.

Transmission electron micrographs indicated that copper and chromium phases with sizes less than 10 nm are finely dispersed in the coating. High-temperature oxidation exposure tests indicated that a transition between porous surface Cu-oxides formation and a slow-growing Cr-rich oxide formation occurred between Cu-10 weight percent chromium and Cu-15 weight percent chromium.

The uncoated GRCop-84 forms porous, thick Cu oxides, which are not protective. In contrast, the IBAD Cu-15 chromium coating surfaces formed a fine-grained, protective Cr-rich oxide. The much finer grain size of the ion beam deposited coating promotes the selective oxidation of chromium to form protective chromia scale.

For more information about development of nanostructured Cu-Cr coatings capabilities at SwRI or how you can contract with SwRI, please contact Ken Chiang, Ph.D., at kchiang@swri.org or  (210) 522-2308.