Investigation On Interphase Design Based On Sizing Process And Its Effect On Interfacial Properties Of CFRP

Carbon fiber reinforced epoxy (CF/EP) composites attracted a long-standing attention, since epoxy shows excellent mechanical properties and good processing manufaturability. However, the common drawbacks for CF/EP composites are that they are very brittle, hence have reduced damage tolerance, and consequently the damage tolerance of the composites. Another problem for CF/EP is that they cannot be recycled. On contrast, the carbon fiber reinforced thermoplastic polymer (CFRTP) composites show the advantages of good toughness and environmental friendliness. With the rapid development of high performance thermoplastic resin, application of CFRTP composites in aeronautics and astronautics fields was promoted which also makes the basic research particularly important. To a large extend, the overall performance of the composite materials is determined by the interphase which is critical for efficient load transfer. Generally, for CF/EP composites, the interface is brittle because there are chemical bonds at the interphase. In the case of CFRTP composites, the interface between carbon fiber and the thermoplastic matrix is weak since there could be no or little chemical bonding at the interface. To solve these problems, this paper aims at improving interphase toughness of CF/EP composite and interfacial adhesion of carbon fiber reinforced poly (phthalazinone ether ketone)(PPEK) composite through interphase designing.Study on the affecting factors about CF/PPEK composites shows that approximately80%of the apparent interfacial shear strength in CF/PPEK composite system was attributed to residual radial compressive stress at the fiber/matrix interface. IFSS of the composite was determined by physiochemical molecular interactions at the interface, coefficient of static friction and the compressive thermal residual stress. Interfacial design was performed by contra-posing the affecting factors. In this thesis, a new high temperature-resistance thermoplastic sizing agent was developed to size carbon fiber. Surface energy and wetting performance of carbon fiber were improved, resulting in a21.6%enhancement in IFSS of CF/PPEK composite.Introducing carbon nanotube (CNT) into the interface of CFRP can effectively improve the interfacial friction and mechanical interlocking between carbon fiber and PPEK resin. As a result, interfacial bonding strength was enhanced. In this thesis, preparation of carbon nanotube/carbon fiber (CNT/CF) multi-scale fiber in a facile way was achieved by combining sizing process and electrophoretic deposition by adjusting the form of electrophoretic electrode and the electric field. There are a large number of CNT on the carbon-fiber surface. Surface roughness was improved attributing to the deposition of CNT on the fiber surface which results in a35.6%improvement in interfacial shear strength of the CF/PPEK single fiber composite. Another fabricated method of CNT/CF multi-scale fiber was developed by sizing carbon fiber with CNT contained sizing agent. To get a good dispersion in the sizing agent, CNT was chemically modified. Compared to the commercial carbon fiber reinforced PPEK composite, CNT/CF multi-scale fiber composite showed18.5%higher interfacial shear strength.The thermoplastic sizing agent was mixed with an active monomer and amortized to size carbon fiber. Due to the re-sizing process, interfacial fracture toughness and interfacial adhesion of carbon fiber/epoxy (CF/EP) composite were both improved. Formula used to calculate interfacial fracture toughness was established based on the principle of function transformation and Scheer-Nairn shear lag model. The calculation indicated that interfacial fracture toughness of the CF/EP composite was improved56.1%by re-sizing carbon fiber with the modified sizing agent. Mechanical testing showed that the interfacial shear strength (IFSS), interlaminar shear strength (ILSS) and Mode II interlaminar fracture toughness were improved15.5%,12.0%and51.3%, respectively.Unsized carbon fiber tows were sized with the thermoplastic sizing agent and CNT contained sizing agent. Unidirectional CF/PPEK laminates were prepared to characterize macroscopic mechanical properties of composite laminate such as ILSS and impact fracture toughness. Composite reinforced by the thermoplastic sizing agent sized carbon fiber showed36.6%and22.2%improvement in ILSS and impact fracture toughness. In the case of CNT/CF multi-scale fiber, ILSS and impact fracture toughness were improved115.4%and113.9%, respectively, compared to the commercial carbon fiber composite. Mechanical testing results indicated that ILSS and impact toughness were enhanced by sizing carbon fiber with ether the thermoplastic sizing agent or CNT contained sizing agent.Experimental and theoretical analysis confirmed that interphase design based on the carbon-fiber sizing process can effectively improve interfacial/interlaminar bonding strength of CF/PPEK composites and interfacial/interlaminar fracture toughness of CF/EP composites. The mechanical performances of CFRP composites were improved by introducing CNT at the interface. The idea of interphase design based on the sizing process makes a good sense and can be promoted in industrial scale.