Assessing the radial microstructural heterogeneity of polyacrylonitrile (PAN)-based carbon fibers using nanoindentation

The radial heterogeneity of five commercial polyacrylonitrile (PAN)-based carbon fibers is qualitatively analyzed by observing the cross-sectional morphologies after oxidative ablation. Three types of radial structures are identified: skin-transition layer-core structure, skin-transition layer structure, and homogeneous structure. The radial distribution of the reduced modulus ( E r ) and hardness ( H ) in both the axial and transverse directions is systematically studied by single-point nanoindentation tests. The results indicate that the core exhibits inferior axial mechanical properties but superior transverse mechanical properties compared to the skin. Additionally, a mechanical specificity of the transition layer is demonstrated in T800H and T1000G. Furthermore, the Partial-Unload tests is conducted on T700S and T800S. A general decrease in E r and H is observed with the accumulation of cycle loading, except for the axial mechanical properties in the core of T700S. The radial distribution of the orientation of graphene sheets is evaluated by the ratio of axial E r to transverse E r . Moreover, the relationship between the average E r and the degree of orientation (D) determined by Wide-angle X-ray diffraction (WAXD) is established. Finally, a mechanical model incorporating five deformation modes is proposed to elucidate the dependence of nanoindentation performance on orientation.