Research On Gradient Interface Construction And Reinforcement Mechanism Of Carbon Fiber Composite

Carbon fibers(CFs)reinforced polymer composites are widely used in aerospace,transportation,electronic information and other fields as national long-term strategic development materials due to their excellent properties such as light weight,high strength and high specific stiffness.However,the high degree of graphitization and poor surface chemical activity of CFs results in weak chemical and physical interactions with resins.In addition,the large gap between the modulus of fiber and resin leads to the inability of the interface to transfer stress uniformly,resulting in stress concentration.Based on the construction of multi-scale gradient interfacial layer,and from three perspectives of optimizing the modification method,increasing the interfacial scale and bi-directional structure design,the interaction between fiber and resin was enhanced and the modulus of both was balanced to achieve significant enhancement of interfacial properties.In this paper,the relationship between the chemical element composition,microstructure,wettability,interfacial thickness and modulus of CFs and their interfacial properties was investigated,and the mechanism of interfacial enhancement was described.The main studies are as follows:1.The π-π interactions of pyrene derivatives(aminopyrene and pyrene butyric acid)with CF and carbon nanotubes(CNTs)were used to functionalize them in a simple and non-destructive way,and then CNTs and polyetheramine(PEA)were chemically grafted to the fiber surface by a condensation agent(HATU)to create a multi-level gradient modulus "rigid-flexible" interfacial layer.The multi-stage gradient modulus "rigid-flexible" interfacial layer was established.The chemical composition and structure of the fiber surface were analyzed by various characterization methods to determine the successful construction of the "rigid-flexible" structure,and the effect of the constructed structure on the interfacial properties of the composite was investigated by testing the mechanical properties of the composite.The experimental results showed that the interfacial shear strength(IFSS),interlaminar shear strength(ILSS),transverse fiber bundle tensile(TFBT)and bending strength of CF-CNTs-PEA/epoxy composites were71.1%,58.3%,89.7% and 45.7% higher than those of CF/epoxy composites,respectively.In addition,the interfacial strengthening mechanism of the composites was discussed by analyzing their failure sections,where the abundant reactive groups on CF-CNTs-PEA not only chemically bonded with the resin,but also the polymer chains were entangled with the epoxy resin chains and participated in the curing,which significantly enhanced the fiber-resin interaction.In addition,the formed "rigid-flexible" gradient modulus interface layer can effectively smooth the modulus transition from fiber to resin and effectively transfer stress.2.In order to make CF composites with better interfacial properties,the modulus of fiber and resin were further balanced and the interfacial phase was thickened by increasing the interfacial structure scale.A multi-scale "rigid-flexible-rigid" "Nunchaku-like" structure was constructed by introducing CNTs,PEAs and CNTs on the CF surface in turn through chemical grafting.The experimental results showed that the strength of CF-CNTs-PEA-CNTs/epoxy composites increased by 73.4%,65.4% and137.0% for IFSS,ILSS and TFBT,respectively,compared with the untreated CF/epoxy composites.The successful construction of thicker multi-scale gradient interfacial phases was demonstrated by analyzing the C-element changes obtained by SEM-EDS in the composite interface and the modulus changes measured by atomic force microscope(AFM)in mechanical mode.The wettability,roughness,and activity of CF-CNTs-PEA-CNTs enahnced,promoting mechanical interlocking and chemical bonding between CF and resin.More importantly,the construction of "rigid-flexible-rigid" structure enabled the composite to obtain a thicker gradient modulus interfacial layer,which more adequately smoothed the modulus transition from fiber to resin and achieves multi-level stress depletion.3.The above research proved that the construction of a multi-level gradient modulus structure on the CF surface can effectively slow the transition from fiber to resin modulus.Considering that the modulus difference between fiber and resin is two orders of magnitude,reinforcing the modulus of resin can effectively enhance the interfacial properties of composite materials.The bi-directional structure design of the composite was achieved by modifying the CF surface and resin using CNTs-PEA.The chemical composition and microstructure characterization of CF surface proved the construction of multi-scale gradient modulus,as well as the mechanical property test of CNTs-PEA modified resin proved the enhancement of resin modulus.Finally,the effectiveness of the bi-directional structure on the optimization of the interfacial properties of the composites was demonstrated by comparing the interfacial properties of CF,CF-CNTs-PEA/EP,CF/CNTs-PEA@EP,and CF-CNTs-PEA/CNTs-PEA@EP.The experimental results showed that the ILSS of CF composites designed with bi-directional structure improved by 41.2% compared with the original CF composites,and by 17.7% and 8.0% compared with CF surface modification only and resin modification only,respectively.The mechanism of interfacial enhancement was discussed.For CF-CNTs-PEA/CNTs-PEA@EP,the modulus difference between CF and resin was further reduced by enhancing the resin modulus,while the modulus from CF to resin was smoothed through the multi-level gradient interface generated by the "rigid flexible" structural design on the CF surface.The combination of the two above enhanced the matching of fiber and resin modulus to optimize the interface performance.