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Structural Ceramics Performance Difference

Hangzhou JWG Technology Co.Ltd | Updated: Jul 12, 2017

As we all know, zirconia is a heterogeneous system, Structural Ceramics affected by the temperature after three phase: monoclinic, tetragonal and cubic, Structural Ceramics but it is reversible phase transition process, at room temperature is only monoclinic zirconia.

The expansion properties of ZrO2 with different phase structure are very different. When the monoclinic ZrO2 is transformed into tetragonal ZrO2, the anisotropy expansion occurs, Structural Ceramics and the expansion coefficient along the three axes (a, b, c) is inconsistent, and the expansion is not obvious along the b - axis direction, and the a - At the time of transformation, the lattice parameters also change, and ZrO2 is transformed from a single oblique tetragonal at elevated temperature. Due to the absorption of heat, Structural Ceramics there is a significant volume shrinkage (5%), and volume expansion occurs when cooling (four-way monoclinic transformation) %), Which is the cause of cracking of ZrO2 ceramics. ZrO2 is converted from tetragonal to tetragonal phase, and the conversion temperature is usually between 1100 and 1200 ° C (1163 ° C). However, when the t-ZrO2 is converted to m-ZrO2 during cooling, the transition temperature is between 850 ℃ and 1000 ℃ (930 ℃) due to the difficulty in the formation of m-ZrO2 new crystal nuclei. Indicating that ZrO2 in the 930-1170 ℃ between the crystal phase transition occurs when the temperature hysteresis.

The results show that some oxides are added as stabilizers (Y2O3, CaO, Al2O3, CeO2, MgO, Sc2O3, etc.) in ZrO2. These oxides can form solid solution or complex in ZrO2, Structural Ceramics which can change the internal structure of the crystal. The lattice defects in ZrO2 can be filled to suppress the ZrO2 torsion and play a stable role. At room temperature, the metastable tetragonal phase or cubic phase is formed, Structural Ceramics which changes from a single monoclinic phase to the tetragonal and cubic phase of the twin crystal structure.

The addition of cationic radius is similar to that of Zr4 + (the difference is within 12%), and their properties are similar in ZrO2. They can form a monotonic, tetragonal and cubic crystal structures with ZrO2.

The degree of zirconia stabilization is directly related to the type, Structural Ceramics number, ion radius and valence of the addition of the cation. Some oxides and zirconia can completely form solid solution (such as ZrO2-Y2O3,). However, some cationic oxides and zirconia do not completely form solid solution, Structural Ceramics but part of the formation of solid solution, part of the formation of compounds (such as ZrO2-Al2O3).

When ZrO2 is stabilized with MgO, the ZrO2 and MgO are decomposed into tetragonal ZrO2 and MgO when cooled to below 1400 ℃. When the temperature is reduced to 900 ℃, Structural Ceramics the ZrO2 is decomposed to monoclinic ZrO2. So MgO stable ZrO2 can not be between 900 ~ 1400 ℃ for a long time heating, otherwise it will lose stability. Stabilizers can be used alone or in combination (binary or ternary). Structural Ceramics The bulk of the stabilized zirconia can greatly improve the performance of the structural ceramics.

According to the study, tetragonal zirconia has toughening properties, which is the theoretical basis of zirconia as structural ceramics. The basic principle of toughening is that the tetragonal zirconia is affected by external force (temperature and stress), Structural Ceramics Monoclinic structure change effect, Structural Ceramics absorb the damage energy, inhibit the change and extension of the crack. This change is called martensitic transformation, plays the role of toughening. Zirconia phase change can be divided into phase change during the firing process and phase change during use. The former is temperature induced, Structural Ceramics the latter is stress induced.

ZrO2 toughening mechanism is very complex. It is generally believed that the transformation of m-ZrO2 <==> t-ZrO2 is achieved by a single ZrO2 crystal, resulting in a phase transition in an unconstrained state. In fact, Structural Ceramics the transition of the zirconia complex containing the matrix to t-ZrO2 to m-ZrO2 is accompanied by a volume expansion, and the transition will be inhibited to varying degrees by the matrix. Structural Ceramics In addition to the outermost crystal, Structural Ceramics the remaining crystals are surrounded by a substrate , Then the transition to a constrained state occurs.

When the matrix has a great resistance to t-ZrO2 transition to m-ZrO2, monoclinic has higher free energy than tetragonal, and transformation can not occur. The tetragonal can be preserved. When there is external force, Structural Ceramics part of the lifting of the matrix of the binding force, Structural Ceramics the transition can be carried out. At this point the transition is called stress induced phase transition.

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