Zirconia generally has three crystal forms: monoclinic ZrO2 (m-ZrO2), tetragonal zirconia (t-ZrO2) and cubic ZrO2 (c-ZrO2). Below 1170°C is the stable temperature of m-ZrO2, and its density is 5.68g cm-3; 1170°C to 2370°C is the stable range of t-ZrO2, and its density is 6.10g cm-3; 2370°C to 2680°C is The stable range of c-ZrO2 has a density of 6.27g·cm-3. Due to changes in external conditions, the crystal forms of zirconia can be transformed into each other. At 1100~1200°C, m-ZrO2 will be transformed into t-ZrO2; t-ZrO2 will be transformed into c-ZrO2 at about 2370°C; The formation of nuclei is difficult, resulting in a lag in the transformation temperature, and it is generally transformed into m-ZrO2 at 850~1000°C. The relationship between ZrO2 crystal transformation is expressed as: m-ZrO2t-ZrO2c-ZrO2 solution.
Toughening of Zirconia in Refractories
Adding ZrO2 to improve the performance of the original refractory material, especially to improve its thermal shock stability, is inseparable from the toughening effect of ZrO2. There are many theories about the toughening mechanism of ZrO2, and the following are currently recognized.
1. Stress Induced Phase Transformation Toughening
ZrO2 in the refractory matrix will exist in the form of t-ZrO2 at the firing temperature; when cooled, it will transform into m-ZrO2, accompanied by a volume expansion of 7%. But constrained by the surrounding matrix, the transition temperature from t-ZrO2 to m-ZrO2 drops. By making this change in the properties of the matrix, t-ZrO2 can be maintained to room temperature. The transition from t-ZrO2 to m-ZrO2 is only triggered when the matrix around ZrO2 decreases its confinement effect due to external force. The external energy is consumed due to the phase transformation, so as to achieve the toughening of the material.
2. Microcrack toughening
In the composite material containing ZrO2, if the particle size of t-ZrO2 is larger than the critical diameter, the volume expansion generated when t-ZrO2 transforms into m-ZrO2 will cause more microcracks near m-ZrO2. When the main crack is subjected to thermal stress or other external forces, some of the energy will be consumed when encountering these microcracks, which will increase the energy required for the main crack to expand to a certain extent, thereby achieving toughening of the material.
3. Crack deflection and bending toughening
In multiphase materials, due to the mismatch between the various phases, the main crack will be inclined and deflected to a certain extent when passing around the second phase particles, prolonging the distance of crack propagation, which will consume more driving force required for crack propagation , so as to achieve the toughening effect on the material. The toughening mechanism of zirconia is very complicated, but it is certain that the zirconia toughened material is at least the result of the simultaneous action of the above two different toughening mechanisms.
