| Interphase tailoring is investigated in carbon-fiber composites, through electrodeposited interphases of butadiene-co-maleic anhydride (BMA), ethylene-co-acrylic acid (EAA), and methyl vinyl ether-co-maleic anhydride (MVEMA). Optimized fiber coating conditions are formulated from a study of the effect of various coating parameters on the polymer deposit and SEM examinations of the coated fibers. Interlaminar shear (ILSS) and impact strengths (IS) of the epoxy composites made from untreated (UT), commercially treated (CT) and electrocoated fibers, are compared. Instrumented impact tests, SEM studies of fracture surfaces, and electron microprobe analysis, are used to discern failure modes and loci. Interfacial shear strength (IFSS), determined by fiber fragmentation in single-fiber composite specimens, is also used to estimate fiber-matrix bond strength. In a novel approch, lognormal distributions of fiber aspect ratios and fiber fracture stress are combined, to extract a distribution of IFSS.; In the BMA-coated fiber composites unsaturation in the butadiene segments leads to crosslinking of the interphase copolymer, and inadequate penetration of this layer by the matrix molecules. The result is a weak interphase/matrix interface and low ILSS, but improved IS. On the other hand, the saturated copolymer interphases (EAA and MVEMA) simultaneously improve impact properties and fiber-matrix bonding. Fiber strength is slightly reduced with electrocoating, the extent depending on coating conditions and copolymer deposited. The dramatic improvement in IFSS is attributed to the strong fiber/interphase/matrix bonds developed. ILSS results with these interphases are vitiated by the occurrence of failure by a mixture of modes, instead of pure shear. Improvement in IS results from the ability of the interphase to deform under impact, absorbing energy and blunting the growing crack tip. The improvements vary with the chemical structure and molecular weight of the interphase. Electrodeposition is thus demonstrated to be a viable technique for precisely tailoring the interphase to suit composite requirements, and gain an understanding of the influence of the chemical structure and physical characteristics of the interphase on ultimate composite properties. |
