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

Abstract

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.

Characterization of atmospheric plasma as a surface preparation process for the bonding of space composite materials

“…Details regarding optimization of the plasma conditions are discussed elsewhere. 5 The test parameters have been chosen to promote the optimum environment for the interaction of oxygen-free radicals in the afterglow with the composite surfaces while limiting damage to the composite materials. Figure 2(a) shows the lap shear strength (LSS) of our epoxy composite system as a function of plasma exposure in terms of number of passes.…”

“…The PTFE film thickness used was 0.0127 mm. Further details are described in a previous publication 5. …”

A comparison of chemical and atmospheric plasma assisted copper plating on carbon fiber reinforced epoxy polymer surfaces

“…Low pressure plasma treatments [5] of large objects would suffer by a small throughput resulting from the batch character of the treatment, long treatment times (typically tens of minutes) and a substantial investment into costly vacuum system equipment. Some of these shortcomings can be addressed by plasma treatment done at atmospheric pressure, especially by using dielectric barrier discharge (DBD) treatment [5] or plasma jet treatment [8]. However, despite offering an efficient treatment at considerably shorter treatment times (units of seconds) the problem of up-scaling the treatment to the large-size object remains.…”

Atmospheric pressure plasma effects on the adhesive bonding properties of stainless steel and epoxy composites

“…Abrasive techniques are labor and materials intensive, and can damage some materials, such as polymer composites. 14,16,17 Plasma activation of the stainless steel and polymer composite surfaces prior to bonding offers an alternative to chemical etching and mechanical abrasion. However, previous research into plasma activation of stainless steel has been limited to vacuum-based systems which have a number of drawbacks.…”