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Structural Ceramics Expansion Performance Difference

Hangzhou JWG Technology Co.Ltd | Updated: Oct 11, 2017

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 uniaxial 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 addition of cationic radius is larger than Zr4 + or smaller than Zr4 +, Structural Ceramics filled and dispersed in large cations, forming a filled solid solution (or dispersed solid solution).

The degree of zirconia stabilization is directly related to the type, 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 cooling is continued 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). The bulk of the stabilized zirconia can greatly improve the performance of the structural ceramics.

According to the study, tetragonal zirconia has the 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), 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, 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. Structural Ceramics In fact, the transition of the zirconia complex containing the matrix to the m-ZrO2 is accompanied by a volumetric expansion, and the transition will be inhibited to varying degrees by the matrix. In addition to the crystal of the outermost surface, the remaining crystals are surrounded by the matrix , Then the transition to a constrained state occurs.

In the composites, if the particle size distribution is wide, Structural Ceramics the larger t-ZrO2 is first transformed into m-ZrO2, which will produce a large stress, so that some grain boundaries become weak and separation, in the main crack end The interaction between the crack and the main crack causes the main crack to branch and cause the closed grain boundary to open or extend, absorb the energy, ease the concentrated stress at the end of the main crack, and tilt and bend the crack path Also increased the surface area of the fracture, thus improving the toughness of the material.

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