Microstructure-toughening relation in alumina based multiwall carbon nanotube ceramic composites

Composites incorporating 0.0, 1.1, 6.4 and 10.4 vol.% of multiwall carbon nanotubes (MWCNTs) in alumina were processed by the spark plasma sintering. The effects of MWCNT contents on the fracture toughness, bending strength, hardness, elastic modulus were evaluated and potential toughening mechanisms were scrutinized. The fractured surfaces exhibit a new microstructure in which MWCNTs are positioned not only at the grain boundaries but also within the alumina grains. The observed longer and shorter pulled out lengths are generally commensurate with the corresponding thermal residual stresses residing at inter- and transgranular positions of alumina. The nanotubes protruding from inside alumina grains indicate the activation of classical fiber pullout induced toughening corroborated by the pullout induced residual holes observed first time in the composites. Meanwhile, nanotubes at the grain boundaries suggest the existence of stretching/disentangling toughening. The significant improvement in the toughness implies that there is a synergy between the two mechanisms.