Influence of SiO<sub>2</sub> Impurity on Transformability of Tetragonal Phase of Zirconia Toughened by Al<sub>2</sub>O<sub>3</sub> Particles
The purpose of this work was to study the influence of silica additive on transformability (at concentrated loads) of the tetragonal phase of zirconia stabilized with calcium oxide, toughened by aluminum oxide. Methods. To study of phase transformations in the area of local deformation, a five-cycle indentation method with an increasing load was used. The effectiveness of tetragonal-monoclinic transformations was judged by the quantitative evaluation of the ratios of hysteresis loss energy to plastic deformation energy δ calculated from P-h diagrams. Results. It has been found that increase of SiO<sub>2</sub> impurity concentration from 0 to 5 mol.% in zirconia (stabilized by CaO and containing 5 wt.% Al<sub>2</sub>O<sub>3</sub>) causes an increase in the values of δ, that is, an increase in hysteresis losses recorded when the indenter is repenetration into the formed imprint. The reason of observed hysteresis losses are tetragonal-monoclinic transformations t-ZrO<sub>2</sub> → m-ZrO<sub>2</sub>. Therefore, the introduction of SiO<sub>2</sub> impurity contributes to improving the transformability of zirconia tetragonal phase. The reason for this may be the interaction of SiO<sub>2</sub> and CaO with the formation of calcium silicates and, as a result, imbalance in the concentrations of ZrO<sub>2</sub> and its stabilizer CaO. Conclusion. Increasing the transformability of the t-ZrO<sub>2</sub> tetragonal phase with the introduction of SiO<sub>2</sub> impurity enhances the role of transformation toughening in zirconia reinforced with Al<sub>2</sub>O<sub>3</sub> particles, which provides a significant increase (by more than 70 %) of its fracture toughness. However, exceeding the critical value of SiO<sub>2</sub> impurity concentration causes spontaneous (in the absence of mechanical loads) t-ZrO<sub>2</sub> → m-ZrO<sub>2</sub> transformation, which abruptly reduce the role of the transformation toughening and worsens the strength properties of the studied ceramics.