The effects of the multi-walled carbon nanotubes (MWCNTs) addition on mechanical properties and microstructures of spark-plasma-sintered MWCNT/alumina composites are investigated by using four types of MWCNTs that are annealed at different temperatures. The annealing temperatures are 1200, 1800, 2200 and 2600°C, and MWCNTs contents are 0.9, 1.9 and 3.7 vol.%. The bending strength of the composites increases up to 1800°C, and then decreases gradually with increasing annealing temperature regardless of the MWCNTs content. This strength behavior can be explained in terms of the matrix grain size effect. Although the addition of 0.9 vol.% MWCNTs have led to higher bending strength compared to the MWCNTs-free alumina, the bending strength is observed to decrease with increasing the MWCNTs content. On the other hand, the MWCNTs annealed at 1200°C is the most effective in improving the fracture toughness of the composites, which may be attributable to the uniform crack bridging effect by MWCNTs. In contrast, the annealing temperatures higher than 1200°C give almost the identical fracture toughness that is lower compared to that for 1200°C. The reduction in the fracture toughness may be due to the decrease in the dispersibility of MWCNTs.
