To improve the strength of the adhesive-bonded carbon fiber reinforced polymer (CFRP) joints, atmospheric pressure plasma treatment (APPT) was used to treat a CFRP substrate surface. This study investigated the effects of nozzle distance (i.e., the distance between plasma nozzle and CFRP substrate) and nozzle speed (i.e., the moving speed of plasma nozzle relative to CFRP substrate) of APPT on the lap-shear strength of adhesive-bonded CFRP joints. Results show that the lap-shear strength of plasma-treated CFRP joints increased to a peak value and then decreased as the nozzle distance increased, and the nozzle distance associated with the peaked joint strength depends on the applied nozzle speed. The lap-shear strength of plasma-treated adhesive-bonded CFRP joints reaches up to 31.6 MPa, compared to 8.6 MPa of the as-received adhesive-bonded CFRP joints. The surface morphology of plasma-treated CFRP substrates was investigated by scanning electron microscope observation, and the mechanism associated with the improved joint strength after applying APPT was revealed through surface chemistry analysis. It is found that APPT not only effectively removed the content of Si element and –CH3
(i.e., the main compositions of release agent) from the as-received CFRP substrate surface, but also generated many polar groups (i.e., –NH2
, –OH, –COOH, etc.), which has a positive effect on increasing the wettability and interfacial bonding strength of CFRP substrates and consequently results in a significant improvement of lap-shear strength of plasma-treated CFRP joints. In addition, the result of differential scanning calorimetry (DSC) test shows that the surface temperature of CFRP substrate should not exceed 175.3 °C during APPT. In this study, an empirical model governing temperature, nozzle distance and nozzle speed was established to guide the selection of atmospheric pressure plasma treatment process parameters in industrial manufacture.
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