Cadmium has been widely used to coat high-strength steels due to its excellent corrosion resistance, adhesion, and lubricity characteristics. However, cadmium is a carcinogen, a teratogen, and a toxic metal that can be easily leached causing potential contamination of the groundwater supply and food chain. Although several alternative coating technologies have emerged--use of electroplated zinc-nickel (Zn-Ni) and tin-zinc (Sn-Zn) from aqueous plating baths, metal-filled polymer composites, novel stainless steel alloys, electroplated aluminum from an organic plating bath, and ion vapor deposited (IVD) aluminum--none meet the overall processing simplicity, coating performance, production throughput, and relatively low cost offered by cadmium plating. Atmospheric pressure chemical vapor deposition (APCVD) is a relatively simple, environmentally benign coating technology that has been used in the electronics industry and has the potential to be used for discrete parts in the military and commercial aircraft industry. This project investigated the use of APCVD to produce high-quality aluminum coatings for corrosion protection of high-strength steels in aircraft components such as landing gear.
The New Jersey Institute of Technology (NJIT) effort explored the use of the environmentally benign chemical vapor deposition (CVD) precursor triethylaluminum (TEA) for producing high-quality aluminum coatings. TEA dissolved in octane is a non-toxic, non-pyrophoric, and non-explosive precursor. The most promising coatings then underwent performance testing based on joint service requirements. The results of these tests were used in a feedback mode to optimize the APCVD process and achieve the required data for technology insertion. Coatings were sent to Naval Air Systems Command and the Army Research Laboratory for initial performance testing. An investigation of scale-up procedures followed. The optimal processing conditions achieved on the bench-top reactor were transferred to a high throughput, commercial APCVD system capable of processing large numbers of parts with complex geometries. Select coatings produced were evaluated in terms of their properties and performance, with the results used in an iterative fashion to meet the required criteria. The effect of post-treatments on overall performance of coated parts was also investigated.
The results obtained with the coatings deposited at the lower temperatures (230° to 250°C) are promising, in so far as better performance was observed than in the range of 275° to 300°C, but it is obvious that, to meet all the specified test criteria, further optimization of the precursor and other deposition parameters is needed to determine the optimum deposition temperatures and post-treatments for each high-strength alloy of interest. For lower strength steels and other substrate materials, where loss in strength as a result of the time at temperature during deposition is not an issue, technology insertion is possible and one commercial production facility has been commissioned to apply aluminum coating to small discrete steel parts.
In addition to offering an environmentally benign alternative to cadmium plating, APVCD promises to provide high production throughput, low cost, conformal coverage (not limited by line-of-sight access to surfaces), and coatings with the desired properties and performance. The process will be amenable for use at Department of Defense depots, original equipment manufacture (OEM) and subcontractor facilities, and logistics centers.