Enclosed Vacuum Evaporation Coatings

Graphic showing multiple layers on the inner surface of a 25 mmdiameter tube.

One of the unique capabilities within the Energy and Engineering Division of the Energy Sciences and Technology Directorate is the Enclosed Vacuum Evaporation (EVE) coating technology. The EVE technique was developed at Pacific Northwest National Laboratory (PNNL) starting in 1997, and some of its applications are nearing implementation.

The EVE coating technology is applicable to a variety of coating and substrate materials with a unique capability of producing smooth and uniform coatings on the inner surface of small-diameter, high-aspect-ratio cylindrical components or other confined geometries. The technique has been used to deposit reproducible coatings on the inner surface of tubes as small as 10 mm in diameter in lengths up to 3.8 m. For larger-diameter tubes or pipes in which radiant heating of the substrate from the source filament is impractical, separate resistive or inductive heating of the substrate to the desired temperature is used. The image above demonstrates one possible coating microstructure, showing multiple layers on the inner surface of a 25-mm-diameter tube.

Many applications in which wear, corrosion, or erosion cause design challenges involve geometries that are not amenable to traditional coating techniques. To address these challenges, exotic materials or sub-optimal design solutions are typically employed. The EVE coating technique offers a potential alternative to these practices, enabling performance improvement and/or manufacturing cost reduction.

In the automotive industry, the technique could be used to deposit a hard-facing iron- nickel- or chromium-aluminide coating on aluminum-alloy engine block cylinder walls to improve fuel economy and engine performance and decrease manufacturing costs. Another possible automotive application is corrosion-resistant coatings on the inner surface of exhaust gas recirculator components to increase their lifetime while maintaining low harmful emissions. In the defense industry, the EVE technique may offer an environmentally friendly replacement for hexavalent-chromium electroplating in military gun barrels for wear- and corrosion-resistance.

Other market opportunities may exist in the nuclear, chemical processing, and well-drilling industries. Most of the potential commercial applications for the EVE technique involve geometries that are less demanding than the application for which it was originally developed. Also, many potential commercial applications involve materials that may be evaporated from the solid state, further simplifying the process. The technique lends itself well to automation, and it is extremely flexible in terms of coating materials and geometry. Process times are short, allowing high throughput, no hazardous feed materials are required, and no hazardous effluents are produced. Each specific application requires fine-tuning of the filament design and process parameters, but the fundamental science behind the process is well understood and the path forward is essentially a matter of engineering.

Two EVE systems used for coating development are currently housed in the Material Sciences Laboratory (Building 326) in the Hanford 300 Area north of Richland, Washington. One of the systems (shown in photo at right) is used for process development on small-diameter tubes in lengths up to 30 cm. For shorter lengths, the other system can accommodate samples with larger diameters. The shorter system also is suitable for producing EVE coatings on exterior surfaces of moderate-sized samples. Two other EVE systems have been constructed: one suitable for development purposes that is capable of handling tubes in lengths up to 4 m, and another production system built for a specific application that coats four 4-m-long tubes simultaneously in each of three vacuum chambers. System control ranges from manual on the developmental systems to fully automated on the production system. Experience has been gained with a variety of coating and substrate material systems, including aluminum on stainless steels, aluminum and chromium on alloy steels, aluminum and iron on nickel-based alloys, nickel on zirconium-based alloys, and nickel and iron on aluminum-based alloys.

References

SD Knowles, DJ Senor, SV Forbes, RN Johnson, and GW Hollenberg. 2005. "Method of Coating the Interior Surface of Hollow Objects," US Patent Number 6,866,886. Washington, DC: United States Patent and Trademark Office.

Project Contact: D. Senor