The effects of fiber surface treatments on the mechanical properties of graphite fiber/bismaleimide composites.

摘要

The surfaces of carbon fibers (Hercules AU-4) were treated by cold plasmas of oxygen, argon and ammonia. A two-component bismaleimide system (Matrimid{dollar}sp{lcub}rm TM{rcub}{dollar} 5292 CIBA-GEIGY) was used as the matrix for composites containing either the untreated or the treated fibers. Research efforts were made to demonstrate the capability of various plasmas in improving the interfacial adhesion between carbon fibers and BMI resin. Emphasis was placed on the determination of predominant adhesion promotion mechanisms as influenced by various plasma treatments. The changes in surface composition of various treated carbon fibers were characterized by X-ray photoelectron spectroscopy (XPS). The magnitudes of fiber surface free energies and their polar and dispersive components were determined using a micro-Wilhelmy technique. The transverse tensile test was utilized to study the variations of interfacial strength induced by plasma treatments. Scanning electron microscopy (SEM) was used to assess the effect of plasmas on the fiber surface topology and the fracture mechanisms of composites. A correlation was established between the surface characteristics of fibers and the mechanical properties of composites.; Oxygen and argon plasmas were found to promote mechanical keying by increasing fiber surface roughness and porosity. The wettability of carbon fiber surface by the matrix resin was also enhanced by plasmas through increasing the fiber surface energies. This facilitates better fiber-resin contact during composite processing. A possible chemical bonding also exists between the oxidized fibers and the resin system. For the cases of ammonia and ammonia/argon plasma treatments, possible chemical bonding between bismaleimide and the plasma-deposited amine groups is one important promoter of interfacial bonding. In these cases increased wettability was also observed. Ammonia and ammonia/argon plasmas appear to be the more appropriate treatments considering that they only induce a slight reduction in the bulk strength of the fiber. In contrast, excessively prolonged exposure of carbon fibers to oxygen or argon plasma could lead to an appreciable reduction in fiber strength.