The military's fleet of aircraft, ships, and land-based vehicles require constant upkeep to ensure they are working properly to safeguard our service members. And to sustain the availability of the fleet, preventive maintenance and repairs must be performed with minimum downtime. Each year, more than $4 billion in fleet—hundreds of aircraft, thousands of engines, and several hundred thousand components—are overhauled or repaired.But before they can be repaired, these components must be cleaned.
Currently, pre-repair cleaning is done using liquid solvents such as methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK), and hexane. However, toxicity issues and cleaning effectiveness make these solvents less than ideal for use in critical processing of high-value parts. Moreover, environmental regulations prohibit the use of some of these solvents at different sites across the U.S., making it difficult to develop a standard procedure to implement across the armed services.
One example of a critical clean requiring new technology is the landing gear doors on the Navy's fighter aircraft. Over time, these doors become saturated with hydraulic fluid from the landing gear, and the fluid must be removed prior to repairing the doors' composite material. At most depots, the method in place calls for wiping the door with MIBK or hexane to remove the oil. The part then undergoes a solvent wipe and a drying cycle. But a major drawback is that this solvent wiping process produces a large quantity of hazardous waste that requires special handling, even if the area of repair is a small fraction of the total area of the door.
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The Surfx AtomFlo 400 desktop unit with 2-inch linear plasma source is shown with optional XYZ robot |
Due to this problem and the desire to develop a method that could be easily implemented across all service locations, Ed Harris, senior materials and processing engineer for composites at NAVAIR Materials Engineering Laboratory, contacted Surfx Technologies to determine whether the company's Atomflo atmospheric plasma process could be used instead of solvent treatment to remove hydraulic fluid from the doors.
Surfx sent a portable system and an applications engineer to NAVAIR's San Diego facilities for initial testing, where single lap shear tests showed failures at the first ply of the laminate as opposed to the failure in the bondline, demonstrating effective surface activation by the plasma. They also found that the atmospheric-pressure helium and oxygen plasma was as effective for cleaning the doors as the solvents NAVAIR was using. But the Atomflo offered some extra benefits—it does not generate any waste, is a localized treatment, and, as Surfx reports, the Atomflo product is completely safe around flammable liquids such as jet fuel.
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| This is the Surfx AtomFlo 400L2P portable unit used at NAVAIR’s materials research laboratory. |
The next test is to determine whether using the atmospheric plasma treatment is detrimental to the carbon-fiber matrix in any way. Initial data from the Defence Science and Technology Organisation of Australia indicate that the Atomflo does not damage their carbon-fiber matrix. However, more thorough in-house testing is underway at NAVAIR. Harris is optimistic. "I think it's a viable process for production," he says. "Maybe not as a point-of-use inline tool on aircraft carriers (due to the gas bottles), but certainly in a depot environment where we're taking aircraft offline for repairs."
Due to the great results obtained on the landing gear doors, the Materials Research Laboratory is investigating other applications for Atomflo within the Navy. The current process flow for metal bonding includes a solvent wipe, followed by a mechanical deoxidation of the surface, dry wipe to remove debris, chemical activation by sol-gel treatment, and primer application before applying the adhesive. For this application, Surfx's atmospheric plasma treatment could enhance the sol-gel surface preparation. Initial results for metal bonding are promising. Only cohesive failure was observed for the wedge crack extension testing on stainless steel and titanium without grit blasting. "The results are exciting," says Harris, who already has tests planned on other NAVAIR repair needs.
"We selected Surfx based on the availability of their fully ruggedized, commercial system as well as their plasma performance. We looked at a few other vendors that offered compressed air plasmas, but we found them to be too aggressive in removing the epoxy matrix of the composite parts," explains Harris. "The system price is also less than the replacement cost for a single landing gear door, so it was an easy sell internally as far as costs go," he continues. So far, the unit has performed up to the demanding standards required by the military.
