| The effect of surface pretreatment on the physical and chemical properties of carbon fiber-polyimide matrix composite surfaces was evaluated. Eight pretreatments were studied: methanol wash, gritblast, sulfuric acid soak, ammonia plasma, argon plasma, argon plasma followed by ammonia plasma, nitrogen plasma, and oxygen plasma. The pretreated surfaces were chemically characterized through the use of XPS (X-ray photoelectron spectroscopy), ISS (ion scattering spectroscopy), and PAS-FTIR (photoacoustic Fourier transform infrared spectroscopy). Surface fluorocarbon contamination was appreciably reduced with gritblasting, argon plasma and oxygen plasma pretreatments. Specific functional groups were incorporated into the composite surfaces through the choice of gases used in the pretreatment. Physical changes were determined through the use of HR-SEM (high resolution scanning electron microscopy). With the exception of the macroroughening produced by gritblasting and the pitting produced by long exposure times in the oxygen plasma, no significant differences in the topography of the pretreated composites were observed. The wettability of the pretreated composite surfaces increased, as demonstrated with contact angle measurements, due to a combination of a decrease in the fluorocarbon contamination and an increase in the surface functionality present.; LaRC-TPI was used for all priming and adhesive bonding of the pretreated composites. Both the single lap shear test and the wedge test were used to evaluate the effect of surface pretreatment on the strength and durability of the adhesively bonded composite joints. Both types of joints were placed in three different environments: (i) room temperature, desiccator, 1000 hours (ii) 204{dollar}spcirc{dollar}C, 1000 hours, and (iii) immersion in boiling water, 3 days for single lap shear test, until failure for wedge test. The oxygen plasma was found to be the best pretreatment. The improvement in adhesive bonding following exposure to an oxygen plasma in comparison to all of the other surface pretreatments was attributed to several factors--a reduction in fluorocarbon contamination and an increase in oxygen surface functionality. This increased surface functionality and decrease in fluorocarbon contamination cause better wetting of the composite surface and may also lead to covalent bonding at the composite-primer interface. |
