Peel ply treatment, sanding, grit-blasting, and plasma etching were used to prepare the surface of graphite/epoxy composites in order to improve surface adhesion performance. We measured the contact angles on the treated surfaces and subsequent wetting anisotropy was found on the sanded and grit blasted surfaces. Linear Kaelble method was employed to determine the surface energy. It was found that polar component of surface energy of sanded and girt blasted composites was between 0 mJ/m2 to 1.4 mJ/m2 while dispersive component of surface energy was between 41.0 mJ/m2 to 56.6 mJ/m2. The surface energy of composites after plasma etching with gap being 1.90 cm was 67.6 mJ/m2 with polar component of surface energy being 36.7 mJ/m2 and dispersive component of surface energy being 30.9 mJ/m2 while the surface energy of composites after plasma etching with gap being 2.54 cm was 41.4 mJ/m2 with polar component of surface energy being 3.0 mJ/m2 and dispersive component of surface energy being 38.4 mJ/m2. The surface energy results indicated the increase of the surface energy of sanded and grit-blasted composites was mainly due to the increase of the dispersive component of the surface energy while the increase of the surface energy after plasma etching treatment was attributed to the increase of the polar component of the surface energy. Moreover, the effect of plasma etching on the surface energy of composites was greatly influenced by plasma etching gap distance.
XPS results indicated that sanding and grit blasting did not cause significant surface chemical composition changes compared with surface chemical composition of peel ply treated surfaces. After plasma etching with gap being 2.54 cm, there was a slight increase of oxygen concentration and decrease of carbon concentration while plasma etching with gap being 1.90 cm greatly changed the surface composition by increasing oxygen concentration and decreasing carbon concentration. Therefore, low content of polar functional groups from oxygen and nitrogen on the sanded and grit blasted surface resulted in the low polar component of surface energy. The big increase of polar groups such as C-O and C=O and a decrease in non-polar groups such as C-H and C-C made the composite surface after air plasma etching more polar, and resulted in the big increase of polar component of surface energy.
SEM micrographs showed that sanding and grit blasting greatly changed the composites surface morphology and roughness while plasma etching did not cause significant surface morphology and roughness change compared with surfaces after peel ply treatments. Sanded surfaces were covered with oriented sanding scratches with sharp edges, random distributed debris and exposed and broken fibers while the main surface features of grit-blasted surfaces were micro-cracks and wear debris with different sizes distributed on them. The unique geometric morphology of sanded and grit-blasted surfaces controlled the liquid spreading on the surface and hence determined the contact angles and caused wetting anisotropy. Moreover, it could affect the surface energy by determining contact angles of probe liquids from which the surface energy was calculated.
It was evident that the plasma etching treatment offered a more satisfactory method of improving surface adhesion performance for graphite/epoxy composites without causing surface damages and graphite fiber exposures and breakages.