Highly Uniform Activation of Carbon Fiber ReinforcedThermoplastics by Low-Temperature Plasma

Nonequilibrium plasma treatments of carbon fiber reinforced thermoplastic (CFRTP) composites can activate the robust, chemically inert material into a nonequilibrium state that can be used to fabricate bonded structural assemblies with high toughness. The nonequilibrium surface is unstable; thus, the development of nondestructive characterization techniques is of paramount importance to assess the material before use. Herein, a discovery is reported that plasma-activated CFRTP surfaces are characterized by a sharp, well-defined surface potential distribution, the width of which is correlated to the fracture toughness of bonded assemblies. The activated surface has a maximum lifetime on the order of days to weeks and is hypothesized to be composed of metastable radical-ion complexes. The hypothesis is consistent with several independent pieces of evidence, including Kelvin probe force microscopy, contact angle measurements, magnetic force microscopy, and radical probe experiments.