The distinctions of the interfacial nanostructures and properties between carbon fiber reinforced fast and conventional curing epoxy composites subjected to hydrothermal treatments

Abstract

The vital role of the interphase in loading transfer in carbon fiber reinforced resin composites has received considerable critical attention. In this study, to quantitatively distinguish between the effects of hydrothermal aging on the interfacial nanostructures and properties of carbon fiber reinforced fast and conventional curing epoxy composites, peak force quantitative nano-mechanics (PF-QNM) technique by atomic force microscopy (AFM) was utilized to determine the interfacial modulus, adhesion and thickness. In addition, the shear behavior was studied by short-beam strength test, and the water uptake was investigated by water adsorption test. Based on the modulus contrast of the interphase in PF-QNM modulus image, the average interfacial thickness of the fast curing epoxy composites after 18 h of hydrothermal aging was 67 nm, increased 236 % when compared with the unaged composites. It was much higher than the rise of 70 % of the conventional curing epoxy composites after 18 h of hydrothermal aging while the interfacial thickness was 69 nm. Consistently, the average interfacial thickness of the fast and conventional curing epoxy composites determined by the adhesion were 68 nm (widened 247 %) and 68 nm (widened 67 %). In a word, the interfacial thickness of the fast curing epoxy composites was about 4 times than that of the conventional curing epoxy composites after 18 h of hydrothermal aging, because of the fast curing caused microcracks and flaws. Then, after 192 h of hydrothermal aging, the decreasing of short-beam strength of the fast curing epoxy composites were almost 1.6 times than that of the conventional curing epoxy composites, due to the faster growing of microcracks and delamination on the side surface of the fast curing epoxy composites short-beam samples during testing. Lastly, the maximum water absorption and diffusion coefficient of the fast curing epoxy composites were 1.2 % and 5.5 × 10−7 cm2/s respectively, which were both 1.3 times that of the conventional curing epoxy composites. Significantly, all the results proved that, attributing to the fast curing caused microcracks and flaws in the fast curing epoxy composites, through which water preferred to enter a composite along the fibers and interphase, the interphase in the fast curing epoxy composites were more sensitive to the hydrothermal aging than the conventional curing epoxy composites. Therefore, the water uptake, interfacial thickness and short-beam strength of the fast curing epoxy composites changed faster.