Effects of Oxygen Plasma Treatments on Surface Functional Groups and Shear Strength of Carbon Fiber Composites

The morphology and chemical function of surfaces strongly influence adhesion at interfaces within carbon fiber composites (i.e., carbon fiber-reinforced polymer) and the mechanical strength of the composite materials. Reactive plasma discharges modify these properties of composite surfaces and can improve adhesion. However, the outcome depends upon the specific composite–adhesive used, which is likely due to a subtle difference in the formulation of these components. Here, we select a model system widely used for aerospace applications (T300 composites formed by the combination of T300 fiber, Cycom 934 epoxy resin, and FM377 adhesives) to investigate the influence of oxygen (O2) plasma treatments on the topology and chemistry of composite surfaces and the differences in mechanical strengths correlated to these changes. Lap shear tests show that shear strengths increase by 10% because of low-pressure O2 plasma treatments. These plasma treatments increase surface roughness by 2-fold, which may improve mechanical modes of interlocking at composite–adhesive interfaces. Chemical analysis of composite surfaces by X-ray photoelectron spectroscopy (XPS) shows that treated composites expose a greater number of imide moieties (N 1s binding energies near 400.7 eV), and reactive titrations with fluorescent probe molecules corroborate these results. The differences in the surface coverage of imide groups strongly correlate with the shear strength of these composites, and both increase with the power and time of the O2 plasma treatment, which is presumably due to greater cumulative exposure to reactive oxygen species. The imide functions that promote adhesion likely form by the oxidation of aniline derivatives present in the T300 composite, and we propose that these functions form covalent bonds with epoxides within the adhesives to improve interfacial chemical bonding. These findings suggest that O2 plasmas increase both mechanical interlocking and interfacial chemical bonding and thus provide greater shear strengths for carbon fiber composites.