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Objective:
Electrolytic hard chrome (EHC) plating is used extensively
by helicopter original equipment manufacturers (OEM) to impart
wear and corrosion resistance to many components, including
pistons, bearing joints, and support pins. Hard chrome plating
utilizes chromium in the hexavalent state (hex-Cr), which
is a known carcinogen. As a result, the Environmental Protection
Agency has issued air and water emission standards for chromium,
and recent epidemiological studies support lowering the permissible
exposure limit (PEL) for hex-Cr, established by the Occupational Safety
and Health Administration, by as much as two orders
of magnitude, which would greatly increase the cost of chrome
plating to OEMs and Department of Defense (DoD) facilities.
A tri-service/OEM/private-sector group, designated the Hard
Chrome Alternatives Team (HCAT), was formed to validate thermal
spray coatings as an environmentally-acceptable, superior-performance
alternative to EHC on many different types of aircraft components.
Technology
Description:
HVOF is a standard commercial thermal spray process in which
a powder of cobalt (Co)-cemented tungsten carbide (WC) or
a Tribaloy is injected into a supersonic flame of a fuel,
usually hydrogen, propylene or kerosene. The particles of
powder soften in the flame and form a dense, well-adhered
coating on the substrate. A detailed technology assessment
concluded that the optimum coatings for replacing EHC plating
on helicopter dynamic components were high-velocity oxygen-fuel
(HVOF) Tribaloy 400 (a Co-based alloy), WC (83%) / Co (17%),
and WC (86%) / Co (10%) Cr (4%). The technology can be used
to deposit 0.003 inch coatings on new OEM parts or to rebuild
0.015 inch layers on worn components.
Results:
Various
steel and other alloy landing gear components were coated
with WC/17Co and WC/10Co4Cr at the Ogden Air Logistics Center
(OO-ALC) at Hill Air Force Base in Utah, where aircraft landing
gear are overhauled, and also at the Naval Aviation Depot
Jacksonville in Florida, where fixed-wing aircraft are overhauled.
Materials and component testing, including axial fatigue,
salt-fog and cyclic corrosion, sliding wear, impact, and hydrogen
embrittlement, showed that the performance of HVOF WC/17Co
and WC/10Co4Cr was equal to, or superior to, EHC coatings.
Visual inspection of an F/A-18 E/F model nose landing gear
after rig tests and a P3 aircraft main landing gear after
flight tests showed no evidence of coating delamination or
wear.
Benefits
and Implications:
This technology will result in the elimination of hex-Cr emissions,
leading to reduced toxic waste disposal and a safer working
environment. HVOF guns can coat 900 square inches per hour,
which is fifty times faster than chrome plating. The superior
performance of the HVOF coatings will lead to decreased component
repair frequency, reduced process turnaround time, and increased
readiness. A detailed cost/benefit analysis estimated that
the annual cost avoidance for a landing gear overhaul facility
that processes more than 1,000 components per year would be
approximately $200,000, resulting in a 3.5 year payback period
on the capital investment. HVOF coating is being implemented
at OO-ALC, where the application of WC/17Co coatings up to
a thickness of 0.010 inches has been approved on 12 different
landing gear components. Commercial airlines also have successfully
completed tests on HVOF coatings under flight conditions. (Project Completed - 2004)
References:
1.
ESTCP Cost and Performance Report. May 2004. (PDF File
Format)
Principal
Investigator:
Mr. Bruce Sartwell
Naval Research Laboratory
Code 6176
4555 Overlook Avenue SW
Washington, DC 20375
Phone: (202) 767-0722
Fax: (202) 767-3321
E-mail: sartwell@nrl.navy.mil |
