Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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galling issues. The latter is a particular concern for electrical connectors; an aluminum-to-aluminum interface could result in excessive mating forces, or even unmateable connectors1 (the incorporation of dry film lubricants have been proposed to resolve this issue, but this would have an adverse effect on elec- trical connectivity). In summary, while IVD aluminum may be viable to replace cadmium in many applications, it is not anticipated to be a direct replacement for electrical connectors. In fact, an Air Force study has recognized that IVD aluminum will not easily replace more than about 50% of cadmium plating requirements.17 METAL-FILLED PAINTS AND CERAMICS Organic paint systems that are loaded with sacrificial metals (generally alu- minum and zinc metal powders) have demonstrated significant corrosion resis- tance 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-replace- ment 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 pri- marily 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 heat-treat 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 with- in the same chamber. Recent work conducted by Boeing 1, 5 found that sputtering provides a bet- ter 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 cad- mium 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, aluminum- tungsten, aluminum-manganese, aluminum-zinc, and aluminum-magne- sium-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 start-up and operational costs, and may not be cost-effective for smaller parts such as electrical connector shells.5, 22 317

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