Electroplated Hard Chromium Replacements for Landing Gear Applications

Electroplated hard chromium (EHC) involves hazardous and/or toxic chemicals. Therefore, cost effective alternatives have been sought by the United States Air Force (USAF) to provide comparable performance, reduce compliance burdens, and address worker safety concerns for nearly 20 years. As part of these investigations, the Air Force Research Laboratory working with Concurrent Technologies Corporation (CTC), has been evaluating chromium (Cr)-free coatings for complex geometry (i.e., non line-of-sight) applications encountered in maintenance, repair, and overhaul activities for the last twelve years. More recently, regulations have become more stringent such that Occupational Safety and Health Administration (OSHA) permissible exposure limits (PELs) for Cr(VI) have been reduced to 5.0 micrograms per cubic meter (μg/m3) of air as an 8-hour time weighted average (TWA) and the American Conference of Government Industrial Hygienists (ACGIH) for Cr(VI) threshold limit values (TLVs) have been reduced to 50 μg/m3 for water soluble compounds and 10 μg/m3 for insoluble compounds as an 8-hour TWA. As a result, it has become increasingly harder to maintain compliance with the environmental, health and safety regulations.  The implementation of EHC alternatives for complex geometries continues to be a need for the USAF,  ALCs, and original equipment manufacturers. The AFRL and CTC have continued work to fully optimize and test electroless deposited nickel-phosphorus (Ni-P) coatings with occluded silicon carbide (SiC) particles and electrodeposited cobalt-phosphorus (Co-P) coatings with occluded SiC particles. The current paper will discuss the results that were obtained from the testing performed to address EHC replacements for landing gear applications. The composite coating properties are being compared to those of EHC as well as a benchmark coating, nanostructured Co-P.