Quantitative characterization of nanoindentation properties of CVI gradient SiC ceramic into CNT arrays

Vertically aligned carbon nanotube array/SiC composites (VACNT/SiCs), fabricated through chemical vapor infiltration (CVI) of SiC ceramics into the CNT arrays, have unique porous microstructures and microscopic properties along with internal CVI SiC gradient from surface to inside. The internal nanoindentation properties including modulus and hardness were characterized to be related to the CVI depths in horizontal and vertical directions, as compared to the external surface nanoindentation properties in the longitudinal and transverse directions. Results showed that the modulus and hardness of the VACNT/SiCs from surface to inside decreased linearly with the CVI depths. Besides, the fitting slopes of modulus with different CVI depths in the vertical and horizontal directions were nearly equal, but the fitting slope of hardness in the vertical direction was almost twice that in horizontal direction. Therefore, it can be indicated that the CVI deposited SiC gradient gives rise to reduction in both modulus and hardness, and has a larger effect on the latter which is mainly upon the deposited SiC quantity and density. Comparatively, the modulus along the external surface in the longitudinal was twice that in the transverse directions, 31.2 and 16.8 GPa, respectively. And the hardness in the longitudinal and transverse directions was nearly the same, 0.627 and 0.597 GPa. It can be inferred that the modulus of the VACNT/SiCs along the external surfaces shows anisotropy due to the CNT modulus anisotropy. Nevertheless, the hardness along the external surfaces was the nearly same, resulting from equally deposited SiC coating on all over the surfaces of composites.