Effect of Processing Parameter Changes on the Adhesion of Plasma-treated Carbon Fiber Reinforced Epoxy Composites

Atmospheric plasma treatment for the surface preparation of adhesively bonded composite joints appears promising as a replacement to current surface preparation techniques. However, questions remain regarding the sensitivity and optimization of various plasma processing parameters on final composite bond properties. In this study, we continue to investigate how plasma surface treatment processing variables ultimately affect the surface chemistry and bonding behavior of a graphite-epoxy composite. The plasma power level, the working distance of the plasma head, the carrier gas (helium) flow rate, the duration of plasma exposure, and the active gas (oxygen) concentration within the plasma were varied and correlated to surface chemistry variations using X-ray photoelectron spectroscopy (XPS). The carboxyl concentration on the surface was then measured as a function of these changes and correlated to lap shear strengths. In addition, samples were monitored using XPS to evaluate the decay behavior of the surface treatment as a function of time. Treated specimens in both inert and air environments exhibited similar decay profiles. Large changes were not observed until after 24 days of out-time. The effects of plasma treatment, duration of plasma exposure, and out-time on the crack delamination resistance (GIC) of bonded parts were assessed. G IC measurement indicated that solvent wiped bonded specimens exhibited a purely adhesive failure with unstable crack growth. Specimens with abrasion treatment exhibited reduced performance with cracks initiated in the adhesive traveling through both the adhesive-composite interface as well as the outer surface plies of the composite substrate. We believe damage to the composite substrate due to surface preparation caused this failure mode. On the other hand, plasma-treated specimens exhibited consistent failure modes for all treatments above 12 passes. The failures were entirely cohesive with the very high bond strength promoting crack propagation only within the adhesive. The GIC values indicated that the plasma-treated composites were two times as resistant to fracture as conventionally prepared specimens.