ZrO2 Crystalline Phases Research Articles

Microstructural Changes and Mechanical Properties of Mullite-ZrO<sub>2</sub> and Mullite-ZrO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub> Composites Prepared Using In-Situ Reaction between Synthetic Zircon

Mullite-ZrO2 and mullite-ZrO2-Al2O3 composite ceramics were prepared by in-situ reaction between alumina and synthetic zircon powders. The mechanical properties of these composites were investigated as a function of Al2O3/SiO2 weight ratio in the matrix. Three-point bending strength at room temperature and fracture toughness increased with increasing content of Al2O3 over the entire range of the Al2O3/SiO2 ratio from 70/30 to 90/10. In particular, the mean bending strength was 650MPa in the vicinity of the composition ratio of 90/10. As judged by the crystalline phases and observation of the fine structure, a strengthening mechanism which acts significantly on the mechanical properties of the composite sintered bodies was indicated by the following: (1) grain size reduction of the mullite matrix by dispersion of the Al2O3 phase, (2) nano-structure in which Al2O3 and ZrO2 particles were finely dispersed in the mullite grains and (3) dispersion-effect of the Al2O3 and ZrO2 crystalline phase with a larger thermal expansion coefficient than that of the matrix.

Metastable Phase Selection and Partitioning in ZrO2—MgO Processed from Liquid Precursors

Aqueous mixtures of either zirconium acetate or zirconium nitrate and magnesium nitrate were dried and subsequently pyrolyzed at fast heating rates (upquenching) to form metastable crystalline phases of ZrO2 with various degrees of MgO supersaturation. The crystallization temperature was determined to be 380°C for the zirconium acetate, and 270°C for the zirconium nitrate at a heating rate of 5°C/min. The crystalline structures were characterized as a function of MgO content and thermal history for specimens containing 0 to 30 mol% MgO. Upquenching to 900°C, where monoclinic (m) ZrO2 and MgO are the equilibrium phases, yielded single‐phase tetragonal (t) ZrO2 (<8 mol% MgO), single‐phase cubic (c) ZrO2 (9 to 17 mol% MgO), and two‐phase c‐ZrO2+ MgO structures (>17 mol% MgO). The composition for which T0(t/c) = 900°C was estimated as 9 ± 1 mol% MgO. Compositions crystallizing as metastable t‐ZrO2 (<8 mol% MgO) partitioned at higher temperatures and/or longer times into two‐phase mixtures, following the general sequence t→t+m→m+ MgO. Similarly, compositions forming metastable c‐ZrO2 (10 to 30 mol% MgO) partitioned in the following sequence: c→c+t+ MgO →t+ MgO →t+m+ Mgo →m+ Mgo. The initial phase selection and subsequent partitioning sequence are discussed in light of phase hierarchies predicted from thermodynamic concepts and kinetic constraints which are introduced by the solute partitioning required to achieve equilibrium.