Effect of high temperature treatment on the microstructure and elastoplastic properties of polyacrylonitrile-based carbon fibers

Abstract

The effects of high temperature treatment (2000–2800 °C) on the microstructure evolution and elastoplastic properties of polyacrylonitrile-based carbon fibers were investigated. It is found that the uneven shrinkage of the carbon fiber surfaces leads to formation of bulges whose length-to-width ratios increase with increasing heat treatment temperature. Because of the shrinkage stress, a closely stacked and curved graphene layer can be found in the sub-surface region of the carbon fibers. In the longitudinal section, the continuity of the graphite-like ribbons in the sub-surface region is worse than that in the skin region and the cross-linking regions between the graphite-like ribbons still contain a lot of defects after heat treatments. As the indentation depth increases, the axial and transverse elastic modulus of the carbon fibers decrease, which are related to the needle-like pores and the ordered graphite-like crystallites. The improvement of the slip between the graphene layers and the breakage of the graphite-like crystallites and the cross-linking regions are responsible for the decrease in axial elastic modulus with increasing heat treatment temperature. By contrast, the transverse elastic modulus is low and the excellent elasticity in the transverse direction is due to the curved graphene layers and the needle-like pores.