Effect of Carbon Nanotubes on Microstructure and Fracture Toughness of Nanostructured Oxide Ceramics

The structure and properties of the yttria-stabilized zirconia ceramics with an addition of high-modulus inclusions of carbon nanotubes are studied. The composite materials are produced by spark plasma sintering. An introduction of carbon nanotubes gives rise to an insignificant decrease in the density and grain size of the ceramics. A larger volume fraction of nanotubes results in an improvement of mechanical properties of the ceramic composites. The highest values of mechanical properties are demonstrated after adding 5 vol.% of carbon nanotubes and are found to be as follows: E = (246 ± 8) GPa, H = (12.7 ± 0.21) GPa, K ICI = (12.1 ± 0.35) MPa·m 1/2 , K ICN = (7.8 ± 0.29) MPa·m 1/2 . Two dissipative mechanisms contribute to the increase in fracture toughness upon introduction of nanotubes: tetragonal-to-monoclinic phase transformation of ZrO 2 and crack bridging by the nanotubes. As the amount of introduced inclusions increases, the contribution of martensitic transformation to fracture toughness decreases, which is due to a decrease in the grain size of the tetragonal zirconia phase and, accordingly, its transition to a stable state.