Our Technical Programs
Evaluation of Replacements for Hard Chrome Plating on Military AircraftSpeaker - Dr. Phillip Bretz
Metcut Research Inc.
The U. S. Department of Defense is faced with increasingly stringent requirements regarding the use of hard chrome plating in aircraft manufacturing and maintenance activities, due to the known carcinogenic properties of hexavalent chromium in plating baths. Various DOD-led efforts have been under way for many years, and were consolidated in 1996 under the Hard Chrome Alternatives Team (HCAT), whose principal sponsor is the Environmental Security Technology Certification Program. This effort is a multi-service/industry program that includes a cooperative effort with the Canadian Department of National Defense, and it is led by the U. S. Naval Research Laboratory.
The technical effort focuses on the evaluation of high velocity oxygen fuel (HVOF) thermal spray coatings as replacement candidates for Cr. This presentation will concentrate on the mechanical behavior evaluations that have been conducted or are planned to determine the applicability of these coatings for landing gear, propeller hub, gas turbine engine, helicopter, and hydraulic actuator components. The principal performance characteristics that have been studied to date include fatigue, corrosion, and wear, although many additional aspects of coating behavior have been addressed. In addition to laboratory evaluations, a number of flight service evaluations have been conducted on various aircraft.
At this time the evaluations for landing gear are well along. In this program 4340, 300M and Aeromet 100 alloys were evaluated with various coating of WC-17Co, WC-12Co-4Cr, and coating of the Triballoy family. The fatigue portion of the testing was performed by Metcut. It has found that under all conditions fatigue life with the WC coatings is better than with chrome. Nearly equal under "full reversing" load, the HVOF coating become significantly better in fatigue as the load ratio (tension/compresion) becomes lower. One problem is that the HVOF coatings can spall off, but despite this they still outperform chrome. The landing gear program also included wear tests both for fretting and a long stroke "rod and bushing" test. HVOF coating were equal to of better than hard chrome in these tests as well. Several landing gear parts are already in production using HVOF technology.
The Propellar Hup program has completed the stage of determining testing protocol. Testing is currently underway with initial results similar to the Landing Gear program. A surprising result was that the coating is actually carrying a significant amount of load during the fatigue tests (something that chrome coating never does).
Gas Turbine engine parts protocol is just now being formulated. The programs for Helicopter and hydraulic actuator evaluations have not yet gotten underway.
Much detailed information about the HCAT program is available at the HCAT web site: www.hcat.org.