| Carbon fibers (CF) reinforced polymer (CFRP) composites are the typical representative of advanced composites, which have many advantages, such as low density, excellent mechanical properties, high and low temperature resistance, etc, so they have great potential in many cutting-edge industries including aerospace, military, automotive and sport fields, etc. However, the surfaces of CFs are smooth and inertness, and the interfacial adhesion between CFs and organic resin matrix is generally very weak, which largely limited their applications. In order to improve the interfacial adhesion between CF and resin matrix, polyacrylonitrile-based CF and diglycidyl ether of bisphenol A type epoxy (EP) resin were chosen as reinforced phase and resin matrix, respectively, to study the surface modification of CFs and CF/EP composites in this thesis.In this thesis, CFs were surface modified by using ammonia and nitric acid, and reinforced EP resin composites based on treated or original CFs were prepared. The changes in morphology, surface roughness, surface element composition, surface chemical structure, single fiber's tensile strength and surface wettability after surface treatment of CFs were characterized by scanning electronic microscopy (SEM), atomic force microscopy (AFM), element analysis (EA), X-photoelectron spectroscopy (XPS), tensile strength measurement and dynamic contact angle analysis (DCAA), respectively, and the interfacial adhesions of various composites were evaluated by interfacial shear strength (IFSS) tested using single fiber pull-out experiment.First, the effects of ammonia treatment on the surfaces of CFs and the interfacial properties of the composite were studied. Results show that after ammonia treatment, the surface roughness of CFs increases, but the surface chemical composition does not change. With the increase of the treating time, the surface wettability of CFs improves obviously; moreover the single fiber's tensile strength decreases, and decreased 18.8% when the treating time is 120h. Compared with the interfacial shear strength of the composite based on the untreated CFs, the interfacial shear strength of composite based on the treated CFs for 120h increases about 55.0% (from 25.1MPa to 38.9MPa). These attractive results prove that the surface roughness of CFs can effectively overcome the poor interfacial adhesions between CFs and organic matrix, and thus make it possible to fabricate advanced composites based on CFs.Second, the effects of nitric acid treatment on the surfaces of CFs and the interfacial properties of the composite were studied. Nitric acid treatment increases not only the surface roughness of CFs, but also the number of active functional groups on the surface. With the increase of the treating time, the surface wettability of CFs against denoized water improves obviously; moreover, the single fiber's tensile strength decreases, and decreases about 28.2% when the treating time is 10h. Besides, the surface free energy increases 8.1%, the IFSS of CF/EP composites enhances 77.2% when the treating time is 90min. By combining the surface characteristics of CFs and interfacial property of CF/EP composites, it can be confirmed that when both mechanical interaction and chemical bonding interaction play role at the same time, mechanical interaction has a dominant effect on interfacial adhesion of composites than chemical bonding interaction does.Last, in order to improve the interfacial adhesion of CF/EP composites, the mechanisms of above two surface-treating techniques for CFs were discussed, and the effect of the surface roughness on interfacial property of composites was studied. In addition, the relationship between the mechanical interaction and chemical bonding interaction, and which has the dominant influence on the interfacial adhesion of the composites were analyzed in this thesis. |
PDF Full Text Request