Improving the Bond Strength of a Composite Repair with an Atmospheric Plasma Treatment.

OSBORNE, JOHN CHRISTOPHER. Improving the Bond Strength of a Composite Repair with an Atmospheric Plasma Treatment. (Under the direction of Dr. Jerome J. Cuomo.) The use of carbon fiber composite materials in aircraft structures is continually growing. As a result, greater importance is placed on maintenance and repair of these structures. This research focused on improving the adhesive bond in a carbon fiber composite repair. Two common repair joints: the scarf joint and the lap joint were chosen as typical repair models and the effects of various atmospheric plasma treatment parameters on the functional groups and eventual strength of repair were investigated. Mechanical tension testing, surface wettability, surface roughness, and X-ray Photoelectron Spectroscopy (XPS) were utilized to analyze the plasma treated composites surface. The initial studies evaluated differences in shear strength of the non-sanded lap, sanded lap and scarf joints that resulted from varying the number of plasma scans, the plasma scan rate, and the plasma working distance. A single plasma scan increased shear strength significantly for the non-sanded and scarf joints. Wettability was determined by contact angle measurements. Statistical significance was found between the contact angle measurements and the plasma processing parameters. The scarf joint and sanded lap joint samples gave similar results for identical plasma treatment conditions. The non-sanded lap joint gave significantly higher angle of contact measurements for identical plasma treatment conditions. The surface analysis results showed the sanded lap and scarfed samples to have increased polar functional groups on their surface as compared to the non-lap sanded specimen. The increased polar surface of the sanded lap and scarf joint specimens lead to a decreased contact angle measurement and therefore enhanced wettability. A profilometer was employed to measure the surface roughness. The model showed no statistical significance between the average surface roughness measurements and the plasma processing parameters. Topographical changes caused by plasma treatments were only evident in the 3D interferometry images of the non-sanded lap joint specimens. Increasing the plasma treatment by increasing the number of plasma scans, decreasing the plasma scan rate, or decreasing the plasma working distance resulted in increased etching of the epoxy matrix. Changes in the chemical composition of the plasma treated and control surfaces were examined using XPS. Evaluation of the C 1s spectra reveled two functional groups, carbonyl and carboxylic to exhibit substantial changes due to increasing plasma treatments. The carbonyl and carboxyl ratios increased with increasing plasma scans and trended similarly to the wettability data. Therefore, wettability was attributed to an increase in these functional groups. In the final analysis, an examination was made of the change in the C 1s ratios coupled with changes in shear strength as a function of number of plasma scans. An increase in the carbonyl and carboxyl functional group ratios on the scarf joint surface only, correlated to an improvement in shear strength. Improving the Bond Strength of a Composite Repair with an Atmospheric Plasma Treatment by John Christopher Osborne A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Master of Science Materials Science & Engineering Raleigh, North Carolina 2011