Effect of inclusion size on mechanical properties of alumina toughened cubic zirconia

The relationship between alumina inclusion size and mechanical properties of particulate cubic zirconia-alumina composites was studied. The composites of the diverse size and content of alumina inclusions and of the nearly constant size of zirconia grains were used. Physical mixtures of the 8 mol% Y₂O₃-ZrO₂ nano-powder and the γ-Al₂O₃ or α -Al₂O₃ micro-powder were cold isostatically pressed and then pressurelessly sintered for 2 h at 1300^∘C in air. The γ -Al₂O₃ and α -Al₂O₃ powder was composed of the particles of 0.17 and 0.36 μ m in size, respectively. Crystallites of the zirconia powder had the size of 6 nm. Microstructural features of the composites have been characterised quantitatively. Hardness, critical stress intensity factor and bending strength of the composites was measured and correlated with the microstructural features. Depending on the size and content, the alumina inclusions influenced strength of the composites by influencing their fracture toughness and the presence of flaws of critical size. An increase in size of the alumina inclusions was accompanied by the increase of fracture toughness due to the additional contribution of large alumina inclusions to the crack deflection mechanism. It was found that decreasing the alumina inclusion size significantly below the cubic zirconia matrix grain size (more than 3 times) did not lead to the increased values of fracture toughness of the composites. The highest increase in fracture toughness (up to 3.9 MPa⋅ m^0.5) has been found when the inclusion size was comparable to the matrix grain size.