Influence of elongated zirconia particles on microstructure and mechanical properties of yttria stabilized zirconia polycrystals

Yttria stabilized tetragonal zirconia polycrystals were prepared using mixtures of two different powders. One is composed of nanometric particles of 3.5 mol% Y2O3 solid solution in ZrO2 and the other one of pure zirconia particles. The latter shows elongated particles of about 200 nm length and monoclinic symmetry and the former consists of isometric tetragonal particles of about 7 nm size. Both powders were synthesized under hydrothermal conditions at 240 °C in water (the 3.5 mol% Y2O3 – ZrO2 solid solution) or in 4 M NaOH solution (the pure ZrO2 powder). Two homogenous mixtures of these powders were prepared; the one with 5 wt.% and the other one with 10 wt.% of the elongated zirconia particles. Compacts of 10 mm diameter were isostatically pressed (250 MPa) and sintered in a dilatometer furnace at 1400 °C with no soaking time. A set of samples heated up to the selected temperatures allowed us to follow phase changes of the materials vs. temperature by X-ray diffraction. It was confirmed that all materials show tetragonal symmetry at the final temperature (1400 °C). Density and mean grain size decrease with the elongated zirconia particle additives. The dense samples were polished and their hardness and fracture toughness were determined by Vickers indentation. No hardness changes, due to the elongated zirconia particles additives, were observed, but essential increase of fracture toughness occurred. The observations of the crack runs suggest crack deflection as a potential mechanism of the fracture toughness increase