Effet Of Interphase Containing Nano Components On Interfacial Properties Of Carbon Fiber/Epoxy Composites

Carbon fiber reinforced polymer composites have been widely used in the aero and space fields because of their low weight, high strength and stiffness. Interphase, which affects the load transfer mode and the crack diffusion, is an important component in the composites. The strength, toughness and environmental stability depend highly on the properties of the interphase. However, smooth and inert fiber surface usually results in poor matrix compatibility and weak adhesion between fibers and matrix. So carbon fiber surfaces are needed to be activated in order to introduce reactive groups on the fiber surface, change fiber surface energy to increase the wettability or increase the fiber surface roughness to enhance mechanical interlocking. However, enhancement of the interfacial adhesion is usually accompanied by a simultaneous decrease in toughness. In this paper, based on the carbon fiber/epoxy composites, polyhedral oligomeric silsesquioxanes (POSS) and amine functionalized carbon nanotubes (CNTs) were introduced into the interphase of the composites through coating or chemical grafting methods, and the interfacial strength and impact toughness of the composites were enhanced simultaneously.Aminopropylphenyl POSS, glycidylphenyl POSS and octaglycidyldimethyl- silyl POSS were added into epoxy solution, and coatings with different concentrations were prepared. The rare carbon fibers and electrochemical oxidized carbon fibers were coated with the coatings, and the physical and chemical properties of carbon fiber surface were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and dynamic contact angle analysis. The experiment results show that the fiber surface roughness and surface energy were increased. When the POSS concentration was 0.5wt%, the quality of the coatings on the fibers was the best. The coatings with the octaglycidyldimethylsilyl POSS had the best effect because of its similar structure of the side chains compared with the matrix resin. The interfacial strength and impact toughness testing results showed that the coatings containing POSS molecules could improve the interfacial properties of the composites, which were due to the inducing cracks and changing the direction of the crack propagation by POSS molecules. When the POSS concentration was 0.5wt%, the interfacial properties were the highest. The octaglycidyldimethylsilyl POSS had the best modification effect compared with another two POSS due to its soft and reactive side chains. In addition, effect of the coating modification was lower than that of electrochemical method. Combining the two methods would further increased the interfacial properties of the composites. Amine functionalized CNTs were added into epoxy solution to make coatings with different CNT concentrations. The rare fibers and electrochemical oxidized fibers were coated with these coatings. The experiment results showed that when CNT concentration was 0.5wt%, the composites had the highest interfacial properties. The mechanical interlocking and restricting of the crack propagation caused by CNTs were why the interfacial properties of the composites were enhanced.In order to enhance the chemical bonding between nanocomponents and fiber surface, we prepared hierarchical reinforcement through grafting CNTs onto the carbon fiber surface using octaglycidyldimethylsilyl POSS as the linkage. The mechanical property testing results showed that POSS grafting and binary grafting, especially for the binary grafting, could increase the interfacial properties of the composites. The reinforcing and toughening mechanisms were also discussed by studying the fiber surface functional groups and interfacial chemical reactions, interfacial mechanical interlocking, the wettability between fibers and matrix, and the stiffness around the interphase region. Experimental results showed that the glycidyl groups on the octaglycidyldimethylsilyl POSS side chains could react with the hardener in the matrix, and formed a strong chemical bonding between fiber surface and the matrix. POSS and CNTs increased the fiber surface roughness. The mechanical interlocking between fibers and matrix restricted the mobility of the resin molecules in the interphase region. When the composites were fractured, the nanocomponents could efficiently restrict the crack propagation, disperse stress and augment fracture area. The increase of the polar groups and fiber surface roughness could increase the fiber surface energy and improve the wettability of fiber surface, which could maximize the degree of the molecular contact and enhance the interfacial physical and chemical bonding. The formation of the chemical bonding and the enhancement of the mechanical interlocking increased the crosslink density of the resin around the interphase, and formed an interphase with gradient mechanical properties, which could transfer loads from matrix to fibers uniformly.