Fracture toughness of nanocrystalline tetragonal zirconia with low yttria content

Nanocrystalline tetragonal zirconia samples are shown to reach toughnesses between 16 and 17 MPa(.)m(1/2), provided the conventional levels of 3 mol% yttria additive are lowered to 1 or 1.5 mol%. For 1 and 1.5 mol% compositions, maximum toughness occurs just below the critical grain sizes of 90 and 110 nm, respectively-i.e. just before spontaneous transformation to the monoclinic phase occurs. Moving away from the critical grain size, towards smaller grains, toughness decreases monotonically up to a factor of 5. These results indicate a critical parameter for maximizing toughness in nanocrystalline zirconia ceramics is proximity to the phase transformation boundary (critical grain size). Combining the present results with data from the literature, a linear relationship between yttria content and inverse critical grain size is found. A power law relationship between grain size and toughness increment is also obeyed by the present data. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.