The influence of surface state on the moisture sensitivity of carbon fiber was analyzed by applying a T800 grade carbon fiber with five different surface conditions,namely,with and without surface oxidation,in the presence or absence of sizing agent.The interfacial properties of their composites in the presence of two epoxy matrices(respectively EP07 and EP10) were also characterized by micro-droplet tests.The overall results show that both oxidized and sizing-coated fibers have higher moisture equilibrium content than that of the pristine unsurface-treated fiber,due to higher amount of activated carbon groups.After moisture absorption of the carbon fibers,almost all the fiber/epoxy systems show decrease in the interfacial shear strength and the unsurface-treated fiber system exhibits the largest decline.Moreover,both interfacial shear strength and interlaminar shear strength of carbon fiber/EP10 composite demonstrate better water resistance performance than that of the carbon fiber/EP07 composite,consistent with DSC results of the two resins.
Three sample preparation techniques, focused ion beam(FIB), ion beam(IB) etching,and ultramicrotomy(UM) were used in comparison to analyze the interphase of carbon fiber/epoxy composites using transmission electron microscopy. An intact interphase with a relatively uniform thickness was obtained by FIB, and detailed chemical analysis of the interphase was investigated by electron energy loss spectroscopy. It shows that the interphase region is 200 nm wide with an increasing oxygen-to-carbon ratio from 10% to 19% and an almost constant nitrogen-to-carbon ratio of about 3%. However, gallium implantation of FIB tends to hinder fine structure analysis of the interphase. For IB etching, the interphase region is observed with transition morphology from amorphous resin to nano-crystalline carbon fiber, but the uneven sample thickness brings difficulty for quantitative chemical analysis. Moreover, UM tends to cause damage and/or deformation on the interphase. These results are meaningful for in-depth understanding on the interphase characteristic of carbon fiber composites.
This paper aims to study the effect of processing temperature on interfacial behavior of HKT800 carbon fiber composites with epoxy and Bismaleimide(BMI) matrix.Referring to the processing conditions of the composite, various processing-heat treatments were conducted on HKT800 and the extracted sizing content declines with increasing temperature.Chemical analysis shows that the HKT800 sizing is epoxy-type and reactive at 200 C.The interfacial shear strength(IFSS) of HKT800/epoxy and HKT800/BMI was investigated by micro-droplet method, for which the composites were fabricated with modeling temperature schemes referring to different diffusion, cure and post-cure stages.It shows that diffusion temperature and conversion degree of the resin both enhance the interfacial adhesion of HKT800/epoxy composite.For the HKT800/BMI composite,the diffusion temperature shows a insignificant effect on the IFSS.FTIR analysis indicates that sufficient reactions are achieved between HKT800-sizing and epoxy resin, however only partial reactions are observed between the sizing and BMI.Moreover, the presence of the sizing can evidently improve the wettability of HKT800 with epoxy resin but is unfavorable for HKT800 with BMI.
