Zirconia toughened alumina (ZTA) ceramics can be prepared by introducing zirconia into alumina ceramics. Zirconia can play the role of phase transformation toughening and microcrack toughening in alumina ceramics, toughening and strengthening alumina ceramics, so as to improve the toughness of alumina ceramics, so ZTA ceramics is a very promising ceramic material.
Phase transformation toughening
Phase transformation toughening is the toughening effect caused by the transformation from metastable tetragonal zirconia to stable monoclinic zirconia. Phase transformation is induced by the tensile stress at the crack tip. The transformation process is accompanied by 4% volume expansion and 6% shear strain, resulting in compressive stress which ultimately prevents the crack growth. It needs to increase the external force to further expand the crack, thus increasing the toughness of ZTA ceramics.
In ZTA, microcracks are mainly toughened in two ways. One is that the grain size of zirconia is larger than the critical size in the sintering process. Therefore, in the process of ceramic cooling, the volume expansion caused by phase transformation results in a large number of microcracks. The other is due to the thermal expansion mismatch between zirconia particles and alumina phase, resulting in microcracks. When the stress acts on the microcrack area, the microcrack can absorb part of the energy, reduce the stress concentration at the end of the main crack, so as to slow down or control the continuous spread of the crack, so as to increase the toughness of the material.
Strengthening and toughening of intragranular structure
The formation of nano effect is due to the internal crystal structure, and the final result will make the internal crystal toughened. However, it is difficult to control the grain size of alumina and zirconia below 100nm in the process of preparing ZTA nanocomposite ceramics. Therefore, generally, only zirconia particles can be controlled at nanometer level. Therefore, ZTA nano composite ceramics refers to the introduction of zirconia nano particles into alumina matrix in the process of ceramic sintering. The final sintering result is that not only zirconia grains exist between alumina grains, but also a considerable part of zirconia particles are surrounded by alumina grains, which leads to the toughening of inner crystal when subjected to external force.
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