Alumina composites reinforced with multi-walled carbon nanotube (MWCNT) content up to 3.7 vol.% were prepared by precursor method. The MWCNT used in this study were modified with the acid treatment for 0-4 hours. The rationale behind the acid treatment was to introduce nanoscale defects and adsorb negatively charged functional groups on the surface of the MWCNT. In order to evaluate the effects of the acid treatment of the MWCNT on the mechanical properties and nano/microstructures of the composites, we investigated the dispersibility of MWCNT and morphology of the fracture surface. We found that there was an optimal acid treatment time for improving the mechanical properties of the composites, i.e., the bending strength and fracture toughness of the composites were correlated highly with the dispersibility of the MWCNT in the matrix. The dispersibility of MWCNT and mechanical properties of the composites were simultaneously increased up to 2 hours, and then they were decreased gradually with increasing acid treatment time. TEM observations revealed that peel-off of a few layers in the MWCNT structure was observed for the MWCNT powders acid-treated for 3 and 4 hours. These results implied that the excessive acid treatment of the MWCNT resulted in degradation of the dispersibility of MWCNT and mechanical properties of the composites. This may be due to the decrease in the number of the negatively charged functional groups by the peel-off of a few layers in the MWCNT structure.
