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Cold Spray: Metallic Coatings at Room Temperature
12-26-02
Thermal spray methods are often used to apply metallic coatings to substrate materials, but the resulting deposits can contain flaws and create deleterious residual stresses due to the deposition of molten or semi-molten metal droplets. A technique called cold gas-dynamic spray, or "cold spray" for short, claims to solve these problems by depositing the metal particles at or near room temperature. Efforts are accelerating in areas of scientific understanding of the process, availability of production-scale equipment, and investigation of potential commercial applications.
The invention of the cold spray process is credited to Anatolii Papyrin, a Russian researcher who developed it in the mid-1980's at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Sciences. Patents were obtained in the US and Europe in the mid-1990's, and subsequently there has been both academic and commercial interest in the process.
Conventional thermal spray processes utilize molten or semi-molten particles that have been heated and accelerated with a plasma, arc, detonation, or combustion process. It was believed that melting of the metal powders was required in order to make the deposit "stick". The breakthrough that led to the cold spray process was the recognition that, above a certain critical particle velocity, the deposition efficiency of solid metal particles could be increased significantly. While the specifics of the bonding of the metal particles to the substrate material are the subject of ongoing studies, it is thought that the high velocity impact of the particles disrupts the oxide films on the metal particle and the substrate, allowing a kind of "explosive welding" of the materials. Key parameters for producing successful cold spray deposits vary somewhat by alloy, but typically involve metal particles in the 10-50 micron range accelerated to 300-1200 m/s at temperatures lower than the melting temperature of the particle, often at or near room temperature. So far, metals deposited include aluminum, copper, steels, nickel-base superalloys, and discontinuously reinforced aluminum composites. Research topics being explored relative to the cold spray process include jet gas dynamics, physics of high-speed particle impact, and evaluation of a range of powder materials.
Advantages cited for the cold spray technology vs. conventional thermal spray are:
- Improved integrity of the coating, due to reduced metal powder oxidation, retention of the properties of the metal powders, and elimination of unfavorable residual stresses.
- An increased range of metal powders can be deposited, including thermally sensitive alloys and small powders, with the potential for deposition of dissimilar materials.
- High deposition rates, with the ability to collect and reuse the overspray powders, fewer requirements for surface preparation, and minimal substrate heating.
Efforts have been underway to transition the cold spray technology from R&D to commercialization. An initial group of companies under the leadership of the National Center for Manufacturing Sciences (NCMS) developed and published the first property information in the mid-1990's. More recently, a consortium of eight companies and Sandia National Laboratory has been formed. The companies are Alcoa, Daimler Chrysler, Ford, Pratt & Whitney, Praxair, Siemens/Westinghouse, Jacobs Chuck Mfg., and Ktech Corp. This group will seek to investigate the application of cold spray technology to a range of possibilities in the automotive, aircraft engine, electronics, and other markets. Also, there is interest in the process for net shape fabrication of powder parts layer-by-layer as well as for specialized joining for dissimilar meals and for joining at low temperatures. The stated goal is for the broad application of the technology within three years.
For those interested in learning more about the cold spray technology, the web sites of the Sandia National Laboratory at www.sandia.gov and of the commercial company holding the patent license, Ktech Corp. at www.ktech.com would be useful.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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