Analysis and control of physical and chemical bonding at the fiber/matrix interface of reinforced composite materials

Low power gas plasma treatments have been used to modify the surface composition and morphology of carbon fibers. Angle resolved X-ray photoelectron spectroscopy (ARXPS) showed that treatments restrictively oxidized only the outermost surfaces, introducing alcohol, carbonyl, and carboxyl species onto the fibers. O$\sb2$ plasma treatments introduced a weakly bound carbonyl species which was pumped off the surface over an 8 hour period under vacuum. The remaining surface oxygen was stable under UHV and showed a net increase in the $\rm C\sb{oxidized}/C\sb{graphite}$ intensity ratio $(\theta = 25\sp\circ)$ from 0.12 to 0.66 for fibers treated for 5 minutes in an O$\sb2$ plasma. A patchy overlayer model suggested the oxidized carbon layer was homogeneous with a composition of about 6% alcohol, 58% carbonyl, and 36% carboxyl species, was approximately 19 A thick, and covered 75% of the surface. Scanning electron micrographs of treated fiber surfaces revealed no significant change in the overall fiber diameter, and mechanical testing confirmed that no loss in fiber strength occurred even after 60 minute treatments. Atomic force microscopy (AFM) revealed that short treatments initially roughen surfaces, creating pits on the order of 100 A. Microbond pull-out experiments showed an enhancement in fiber/matrix adhesion for samples prepared with plasma treated fibers versus untreated fibers. Nitric oxide plasma treatments introduce nitrogen and oxygen functionality onto fiber surfaces, including amine, amide, and nitrate species. In situ thermal desorption analysis using XPS showed losses of surface oxygen and nitrogen on treated fibers, but N/C and O/C intensity ratios for treated surfaces heated to 150$\sp\circ$C remained considerably higher than analogous values for untreated fibers. Finally, the plasma polymerization of allylamine has been investigated using XPS and SEM. XPS results revealed that the coating contained both amine and amide functionality. The conversion of amines to amides occurred with increased treatment time. SEM revealed that the coatings filled in the fiber grooves. Finally, XPS intensity ratios indicated that some polymer remained on fiber surfaces even after a 15 minute post O$\sb2$ plasma treatment. Overall, gas plasma treatments show promise as a method to manipulate surface chemical composition and morphology in order to improve fiber/matrix adhesion and ultimately enhance composite mechanical properties