Surface modification of boron fibres for improved strength in composite materials

Abstract

When boron fibres are combined with an organic matrix, such as an epoxy resin, a high-performance composite structure is created. This study investigates the surface chemistry of plasma- and organosilane-treated boron fibres with the key aim to improving the adhesion properties between the boron fibre and the epoxy matrix. Optimisation of this interfacial region plays a critical role in influencing the mechanical behaviour of composite materials and has considerable industrial applications in the aerospace and manufacturing industries. The surface chemistry of a model boron surface and boron fibres was monitored using a combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Initial investigation of the as-received fibres showed the presence of silicone contamination on the fibre surface, which would affect adhesion. Removal of this contaminant through solvent cleaning and plasma oxidation provided an ideal surface for attachment of the organosilane adhesion promoter. A model for the interaction of the organosilane with a boron surface is proposed. The pull-out strength of boron fibres, with different surface treatments, embedded in the epoxy resin was measured using a custom designed adhesiometer. Compared with as-received boron fibres, a 6-fold improvement in the apparent interfacial shear strength was achieved for the organosilane treated fibres. Optical microscopy was used to determine the failure mechanisms between the fibre and epoxy resin. Typically, as the surface treatment improved adhesion, the locus of failure changed from the boron–epoxy interface to failure within the epoxy and ultimately fibre breakage.