Sintering of carbon nanotube-reinforced zirconia-toughened alumina composites prepared by uniaxial pressing and cold isostatic pressing

The effect of carbon nanotubes (CNTs) on hardness and fracture toughness in CNT-reinforced zirconia-toughened alumina (ZTA) composites prepared by uniaxial pressing (UP), cold isostatic pressing (CIP), and pressureless sintering (PLS) at 1520 °C in air during 1 h was investigated. Both pressing methods yielded nearly identical relative densities higher than 93%. The Vickers microhardness was increased in the composites treated by CIP, whereas the fracture toughness was decreased for the CIPed composites. This could be associated with the fracture mode and the toughening mechanisms present in the microstructure. CIPed ZTA composites with additions of 0.01 wt% MWCNTs obtained an increase in hardness of 26% and a decrease of 8% in fracture toughness compared to uniaxially pressed ones. From all studied composites, those with additions of MWCNTs yielded the better mechanical properties in comparison to composites with additions of SWCNTs, suggesting that MWCNTs are more easily dispersed into the ceramic matrix, thereby producing improvements as reinforcement agents. (paper)