Engineering ceramic materials

alumina ceramic 

Alumina ceramics are a type of high-temperature structural ceramics with the widest range of applications, the most abundant raw materials, and the lowest price. They are the most mature high melting point oxide ceramics and have a wide range of applications in mechanical, chemical, electronic and other fields. Alumina raw materials come from a variety of sources and are inexpensive. According to their composition, they can be divided into two categories: alumina ceramics and high alumina ceramics.

Generally, the content of alumina in alumina ceramics is more than 99%, and the sintering temperature is high. When the particle size of raw materials is coarse, the sintering temperature can reach 1700 ℃. In order to improve the sintering performance and reduce the sintering temperature, a small amount of MgO, Cr203, TiO2, etc. are often added as sintering additives, which generate Solid solution or grain boundary phase, activate the lattice, inhibit grain growth, and thus promote sintering. The sintering of ceramic materials belongs to the solid phase sintering mechanism. The main crystalline phase of the sintered material is corundum phase, with good performance. Alumina ceramics have the following characteristics: ① high mechanical properties; ② High resistivity; ③ High hardness; ④ High melting point and corrosion resistance; ⑤ Excellent optical properties; ⑥ Ionic conductivity.

High alumina ceramics generally refer to ceramics with different alumina contents, such as 95 ceramics, 90 ceramics, 85 ceramics, 75 ceramics, etc., which are added with different amounts of silicate liquid phase sintering additives or other substances, resulting in lower sintering temperatures. At the same time, the material properties also decrease compared to alumina ceramics. According to the content of the main crystal phase generated, they are divided into corundum ceramics, corundum mullite ceramics, and mullite ceramics.

Zirconia ceramic

1) Crystal structure and Martensite transformation of zirconia ceramics

Zirconia has three crystal forms: cubic, tetragonal, and monoclinic. The monoclinic phase is a low-temperature stable phase. If it is heated above 1000 ℃ and transformed into a tetragonal phase, and continues to be heated to 2370 ℃, it will transform into a cubic phase.

2) Mechanism of zirconia phase transformation toughening

The toughening mechanism of zirconia phase transformation is mainly divided into stress induced phase transformation toughening and microcrack toughening.

(1) Stress induced phase transformation toughening. Under stress, the tetragonal zirconia particles undergo a transition to a monoclinic phase. As the phase transition progresses, it is accompanied by volume expansion and shear strain, which absorb energy and increase crack propagation resistance, thereby enhancing toughness

Function.

(2) Micro crack toughening. At the service temperature, if the ZrO2 grain is larger than the critical grain size, the tetragonal grain spontaneously transforms into monoclinic phase, and many microcracks or crack cores are generated around it due to volume expansion. When they are in the action zone before the main crack, because they extend and release part of the Strain energy of the main crack, the energy required for the main crack growth is increased, so that the crack growth is effectively made, and the Fracture toughness of the material is improved, Most of the elastic Strain energy of the material is converted into the surface energy of microcracks.

3) Types and characteristics of zirconia ceramics

According to the stability of phase structure, zirconia ceramics are divided into stable ZrO2 ceramics and partially stable ZrO2 ceramics.

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