Surface modification of aircraft used composites for adhesive bonding

Due to the inherent low surface energy of the polymer matrix portion of a given composite material, poor adhesion properties are exhibited and must be overcome in order to achieve strong adhesive bonds. Mechanical methods to improve adhesion have typically included manual abrasion like sanding or grit blasting. Energetic techniques, such as laser and plasma, are garnering continued attention towards the same end. This work describes results of recent investigations of atmospheric pressure plasma treatment (APPT) of composite materials based on glass and carbon fiber reinforced toughened epoxy resin systems for adhesive bonding. Chemical, physical and APPT treatments were compared in terms of enhancing surface energy and interfacial fracture toughness. Surface treatments were followed by characterization of wetting properties using traditional contact angle techniques as well as ballistic liquid deposition. The effects of APPT on the substrates were characterized by taking into account both chemical and morphological changes. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) were used to confirm the elimination of fluorine and the introduction of oxygen and nitrogen. Etching effects of plasma were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The double cantilever beam (DCB) test configuration was used to investigate treatment effects on adhesive bond performance. Results exhibited the effectiveness of physical procedures in cleaning surfaces, while APPT generated a higher hydrophilic behavior. All the samples tested by DCB yielded cohesive failure mode within the laminates.