Study On Thermo-oxidative Aging Mechanisms And Damage Monitoring Of Carbon Fiber Reinforced Bismaleimide Composites

Since carbon fiber reinforced bismaleimide(CF/BMI)composites was the key materials for structure components with light weight and high temperature resistance in aerospace and military fields,the safety and durability of their applications under high temperature were the desired goals,which have become a focus in academia and industry.CF/BMI composites were generally endured long-term high temperature thermo-oxidative aging over 250 ℃,however,the low heat resistance of the interphase generated from commercial epoxy sizing and resin matrix limited their applications in high temperature composites.Therefore,the delay of thermo-oxidative aging failure and interfacial enhancement of CF/BMI composites were depended on the design of heat-resistance interphase.On the other hand,CF/BMI composites were highly susceptible to barely visible impact damage caused by mechanical force and long-term application under high temperature,the damage monitoring was important to avoid the development of the overall failure of composites.In this paper,waterborne polyamic acid was designed and prepared to establish high temperature resistance interphase,and the correlations of the thermal stability of the interphase with thermo-oxidative aging and interface performance of CF/BMI composites were studied.Molecular assembly interphase was constructed by using perylenediimide polycyclic molecular,and the correlation mechanisms with the high temperature interfacial properties and damage monitoring of composites were established.Perylenediimide-γCyclodextrins fluorescent probe was designed and used to construct novel mechanochromic interphase,and the interfacial enhancement the self-reporting damage of composites were achieved.(1)Waterborne polyamic acid(PAA)sizing agent with twisted non-coplanar structure was designed and prepared,and the high temperature resistance interphase was constructed on the modified carbon fiber(m-CF)surface.The effects of thermo-oxidative aging for 0,3,7 and10 days at 280 ℃ on the thermal degradation kinetic and microstructure of the interphase were qualitatively and quantitatively studied,and the correlations of thermal stability of the interphase with interfacial and thermo-oxidative aging properties of CF/BMI composites were also investigated.Compared with pristine CF(p-CF)and commercial CF(cCF),the surface activity and heat resistance of m-CF were significantly improved,and the surface morphology remained stable after thermooxidative aging for 10 days at 280 ℃.Thermal degradation activation energy(Ea)of PAA/BMI simulated interphase and m-CF composites obtained by Madhusudanan model was the highest after thermo-oxidative aging for 0,3,7 and 10 days at 280 ℃,resulting in enhanced TFBT strength and ILSS and retention rate of m-CF composites at high temperature.Before and after aging,interphase modulus and thickness of m-CF composites obtained by Peak Force Quantitative Nano-Mechanics(PFQNM)was hardly varied,and interphase height line profiles in atomic force microscope topographies was transferred from “U” to “V” shape after thermo-oxidative aging for 10 days,which was attributed to improved thermal stability of the interphase from PAA imidization and chemical bonding between PAA and BMI as well as suppressing oxidative degradation of interphase,resulting in the synchronous enhancement of interfacial and thermo-oxidative aging properties of m-CF composites.(2)Amino terminated PDI was designed and constructed a high temperature resistance molecular self-assembly interphase on modified carbon fiber(CF-PDI)surface,and the molecular assembly mechanism of PDI on CF surface was studied.The changes of physicochemical characteristics on CF-PDI surface at 25 ℃,100 ℃,200 ℃ and 250 ℃were analyzed,and the effects of high temperature interfacial properties and damage monitoring of composites at different temperatures were investigated.Compared with pristine CF(CF-desized),the surface roughness and active functional group content of CF-PDI were significantly increased,and PDI nanowires on CF surface were assembled by π-π stacking as the dominant force as well as hydrogen bonding and electrostatic adsorption as the driving force.The surface physicochemical properties of CF-PDI remained stable at 25 ℃ and 100 ℃,and the partial detachment and decreased surface activity of the nanowire assembly were observed at 200 ℃ and 250 ℃,which were attributed to the weakening ofπ-π stacking and the fracture of intermolecular hydrogen bond at high temperature.The TFBT strength and IFSS were increased by 76.6% and52.2% at 250 ℃ in comparison with those of CF-desized composites,which was ascribes to the enhanced interfacial bonding strength from high temperature resistance of PDI in molecular assembly interphase and the covalent bonding with resin matrix,resulting in the improvement of interfacial properties of the composite at high temperature.The strong green fluorescence was observed from CF-PDI and its composites at 25 ℃,and the fluorescence intensity was decreased with the increase of temperature,but still remained fluorescent characteristics at 250 ℃.The fluorescence disappearance at damage region was observed after high temperature impact damage,which contributed to the high temperature damage monitoring of composites.(3)Perylenediimides-γCyclodextrins(PDI-γCD)fluorescent probe was designed and prepared by supramolecular host-guest selfassembly method,and the novel mechanochromic interphase was constructed on modified carbon fiber(CF@PDI-γCD)surface.The surface physicochemical and fluorescence properties of pristine CF(CF-pristine),commercial CF(CF-commercial)and CF@PDI-γCD were investigated,and the effects of mechanochromic interphase on the interface property and reinforcing mechanism as well as interfacial self-reporting damage of composites were studied.Compared with CF-pristine and CF-commercial,the chemical activity and roughness of CF@PDI-γCD surface were significantly increased.The CF@PDI-γCD surface exhibited slight fluorescence instead of non-fluorescence on CF-pristine and CFcommercial surface.The IFSS and TFBT strength of CF@PDI-γCD composites were much higher than those of CF-pristine and CFcommercial composites,which was attributed to synchronous enhancement of mechanical interlocking and chemical bonding between PDI-γCD and resin matrix.The fluorescence switch was turned on by the PDI out of the cavity of γCD from the breakage of hydrogen bond under shearing force in CF@PDI-γCD monofilament composites,resulting in the appearance of green fluorescence on the interface damage region,realizing the damage self-reporting of composite.