Single-walled carbon nanotube-reinforced composites based on ZrO2 obtained by vacuum pressureless sintering

The results of a study of sintered composites based on yttria-stabilized zirconia (ZrO2) reinforced with single-walled carbon nanotubes (SWCNTs) are presented in this paper. Mixing of ZrO2 nanopowder with SWCNTs was carried out in ethanol using an ultrasonic bath and a magnetic stirrer. Composite powders with 0.1, 0.5 and 1 wt.% SWCNTs and ZrO2 nanopowder were pressed into compacts at a pressure of 100 MPa, and then they were sintered in a high temperature vacuum furnace for 2 h at a temperature of 1500°C with a heating rate of 300°C / h. Changes in the microstructure, phase composition, and mechanical properties were investigated depending on the SWCNT content in the samples. It was found that in the selected sintering mode, high density samples (99.2 – 97.5 %) were obtained. It was found by scanning electron microscopy that undivided, entangled SWCNT bundles / aggregates and individual nanotubes, which together formed a continuous reinforcing structure, were observed in the microstructure of the composites. Moreover, SWCNTs led to the refinement of the microstructure of composites; the average grain size of composites was 16 % lower than that of ZrO2 ceramics. It was found by X-Ray diffraction that only high temperature modifications of zirconia (cubic and tetragonal) were present in ZrO2 ceramics and composites, and SWCNTs led to a slight decrease in the size of coherent scattering domain. It was found by the Vickers indentation method that the composite based on ZrO2 with 0.5 wt.% SWCNT was optimal in terms of mechanical properties, since it had the highest microhardness (13.6 GPa, which was 6 % higher than that of ZrO2 ceramics) and had a 12 % higher fracture toughness.