| Type II, PAN-based carbon fibers (unsized and commercially treated) have been treated using oxygen plasma and the surface compositions have been determined using angle-resolved x-ray photoelectron spectroscopy (ARXPS) and ion scattering spectroscopy (ISS). The adhesive properties have also been characterized by applying mechanical tests to the fiber/epoxy matrix composites via single fiber fragmentation and single fiber tensile tests. The fibers were exposed to oxygen plasma at various powers (20, 50 and 200 W) and times (0.5, 3, and 10 minutes). The surface treatments enhanced the concentration of oxygen at the fiber surface to a point of saturation, however the changes were not directly proportional to the treatment power. There was a decrease in the XPS O/C ratio from 20 to 50 W, and then an increase from 50 to 200 W. The trend is explained by proposing that the 20 W treatment only effected the outermost surface of the fibers (10-15 A) which is probed by XPS, but the decrease which occurred from 20 to 50 W is probably due to much deeper penetration of the oxidation ({dollar}>{dollar}100 A), and therefore undetectable by XPS. When the fibers were exposed for 3 minutes at 200 W, there was an increase, suggesting that the higher intensity of the plasma totally ablated the surface, which, in turn, exposed the underlying fiber layers allowing for more oxidation within the fiber microstructure. The resulting O/C atomic ratios, therefore, traversed a minimum. Finally, the results of mechanical testing revealed that as the exposure time and/or power increased, there was a concurrent decrease in the carbon fiber/epoxy IFSS relative to the as-received fibers. This suggests that any increase in fiber/matrix adhesion due to enhanced oxidation of the fiber surface was offset by the tensile strength losses. |
