Aircraft composites repair: Plasma’s potential for better bonds

Plasma treatment might be a key enabling technology in the pursuit of certification-worthy, fastener-free adhesively bonded repair patches applied to aircraft primary structure. CW discussed the promise of plasma in its 2015 article on the Defense Advanced Research Projects Agency’s (DARPA, Washington, DC, US) Transition Reliable Unitized STructure (TRUST) progra), which seeks to document adhesive bond quality in order to achieve certified bonded composite primary structures without mechanical fasteners during original equipment manufacture.

A sidebar to that article, “Plasma treatment as surface preparation for adhesive bonding,” reiterated that, although it is well known that surface preparation is critical to long-term bond performance, the focus, historically, has been on optimizing mechanical processes, such as hand sanding and grit blasting. However, TRUST lead investigator Lockheed Martin Aeronautics Co. (LM Aero, Palmdale, CA, US), cited more recent research that shows the key to long-term bond durability is achieving a chemical bond between substrate and adhesive. LM Aero chose to pursue development of atmospheric plasma because of its ability to activate the inert surface of cured epoxy composites and its compatibility with repeatable, scalable inline processing.

LM Aero’s testing of various carbon fiber/epoxy laminates and epoxy adhesives showed that atmospheric plasma treatment elevates the composite’s surface energy, the elevated level remains high even after 96 hours of out-time, and that the resulting bond is characterized by greater fracture toughness, lower interfacial failure and decreased laminate failure during subsequent bonding tests. In fact, even after 192 hours of out-time, plasma-treated epoxies exhibited a higher activation state than the baseline preparations possess after 0 hours out-time. It was also shown that 3 seconds of plasma treatment is the optimum balance between functionalization and processing speed for each of the three epoxies studied.

In a recent CW webinar (see Learn More), composite repair training firm Abaris Training (Reno, NV, US) and plasma treatment systems supplier Enercon Industries (Menomonee Falls, WI, US) reviewed how plasma treatment increases surface energy for composite bonding substrates and how it can be incorporated into inline processes. “We have proven that our … plasma surface treating systems can clean, etch and functionalize surfaces to improve adhesion and bond strength,” says Enercon VP marketing Mark Plantier. “Bond strength is also improved by plasma’s ability to modify the polar[ity] and disperse components of the surface.” He adds that Enercon’s plasma systems can be integrated with robotic automation, indexing and in some cases handheld treatment devices, all of which makes it possible to use them for localized treatment in a repair application. 

The task now is for the industry to continue working with plasma treatment system suppliers to define the necessary process parameters and standards that will enable this technology’s incorporation into the aerocomposites repair process chain.

This short article is a Side Story to s feature artile titled, "Aircraft composites repair moves toward maturity." To read this feature article, click on its title under "Editor's Picks" at top right.