Fracture toughness of nanocrystalline tetragonal zirconia with low yttria content

Abstract 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.