Metal Finishing Guide Book


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METAL-FILLED PAINTS AND CERAMICS Organic paint systems that are loaded with sacrificial metals (generally aluminum and zinc metal powders) have demonstrated significant corrosion resistance in several applications. However, they are generally not considered for cadmium replacement due to poor galvanic corrosion performance and poor adhesion (compared to electroplating).5 Metal-filled ceramic coatings are being considered for some cadmium-replacement efforts. One supplier offers a coating that incorporates aluminum flakes in a ceramic matrix. The coating can be applied via brush or spray. It is used primarily for larger components in aircraft such as landing gear (specifically the F-22), as well as for high-temperature applications. Drawbacks to this candidate include sole source (only one supplier provides the coating, and they only license to major users), high cost, limited available data, and the requirement to heattreat the coating before use.1,5 Also, coating conductivity has apparently not been determined. As such, this candidate is likely not feasible for electrical connectors. SPUTTERED ALUMINUM AND ALLOYS Sputtering, or magnetron sputtering, is another PVD process. In this process, a part is placed in a vacuum chamber, where it is glow discharge cleaned after the system is evacuated. The ionized gas (typically argon) is attracted to the biased aluminum target, and aluminum atoms are ejected from the target and condense on the substrate to form a coating. The "Plug and Coat" method of sputtering allows both inner diameters (IDs) and outer diameters (ODs) to be coated within the same chamber. Recent work conducted by Boeing 1, 5 found that sputtering provides a better quality aluminum coating than IVD, with lower porosity. Through the "Plug and Coat" process, parts can be 100% PVD aluminum-coated (IVD Al on OD, sputter Al on ID). In addition, the process is non-hazardous as compared to cadmium plating (no air emissions, water emissions, or solid waste). Sputtered aluminum alloys have also showed promise to replace cadmium. They include aluminum magnesium, aluminum-molybdenum, aluminumtungsten, aluminum-manganese, aluminum-zinc, and aluminum-magnesium-zinc.5, 6 While promising, magnetron sputtered aluminum is still under development for coating aircraft parts. Susceptibility to environmental embrittlement has yet to be determined, and more recent work has generated mixed results.22 Also, while technically acceptable, this process involves high startup and operational costs, and may not be cost-effective for smaller parts such as electrical connector shells.5, 22 OTHER DEPOSITION TECHNOLOGIES Aluminum and its alloys can be readily deposited with thermal spray processes, such as flame spray, but these coatings are usually very thick—typically 76 to 127 microns (0.003'' to 0.005'')—and exhibit high roughness and porosity in the as-deposited state. The process also imparts a high degree of heat to the substrate. The latter issue can be partly alleviated by utilizing "cold spray" processes; however, the former issues restrict the use of this technology for electrical connectors. As mentioned previously, the use of ionic liquids (salt mixtures that melt 359

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