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Effect of low-content of carbon nanotubes on the fracture toughness and hardness of carbon nanotube reinforced alumina prepared by sinter, HIP and sinter  +  HIP routes

Alumina ceramic composites were prepared with additions of 0.1 wt% of different kinds of carbon nanotubes (CNTs) by pressureless sintering (PLS), hot isostatic pressing (HIP) and sintering  +  hot isostatic pressing (sinter  +  HIP) routes. The composites showed relative density higher than 94% with a wide range of grain sizes and fracture toughness values, while the hardness remained practically the same. For the same content of CNTs, some isolated mechanisms associated with toughening behaviour such as crack deflection, crack bridging, and CNT pull-out were exhibited in some composites. Since the same dispersion process was used for all composites, the differences found in the homogeneity of the CNTs within the matrix are due to their intrinsic characteristics combined with the fabrication routes. As a result, the best performance was obtained for the alumina composite with CNT identified as MWCNT-CIMAV, for which the fracture toughness increased 63% as compared to monolithic alumina, both prepared by the HIP route.