Phase transformation toughening for Zirconia toughened alumina

Phase transformation toughening is an early and commonly studied toughening method. It artificially creates a large number of extremely fine cracks in the material to absorb energy and prevent crack propagation. Among them, the research mainly focuses on the martensitic transformation of ZrO2, with relatively successful ceramic materials such as Zirconia toughened alumina and ZTM. ZrO2 is dispersed in the Al2O3 matrix, and due to their different linear expansion coefficients, ZrO2 particles are subjected to compressive stress and phase transformation is hindered during cooling. Then, when the material is subjected to external forces, the pressure on the ZrO2 particles is relaxed, and the tetragonal phase transforms into a monoclinic phase. After volume expansion, microcracks are generated in the matrix, which absorbs the energy of the main crack and achieves toughening effect. This is the mechanism of stress induced phase transformation toughening.

Among them, the stability of metastable tetragonal and cubic ZrO2 is mainly ensured by the oxygen ion vacancies formed after adding additives, as well as the size, charge, and concentration of cations. Basu found that an increase in the concentration of oxygen vacancies formed after the addition of Y2O3 would increase the disorder of the ZrO2- Y2O3 system. In addition, the decrease in critical size of tetragonal ZrO2 and the increase in free energy for the transition from tetragonal to monoclinic phases will enhance the stability of tetragonal ZrO2 and maintain it at room temperature. The defect reaction generated by doping Y2O3 in ZrO2 is:
Y2O3- ZrO2->2Yzr ‘+3OO+VO
The Zirconia toughened alumina prepared in this way has a fracture toughness of over 7.66 MPa. m1/2, with the highest reaching 15 MPa. m1/2. The material obtained by mixing 20% YTZ and Al2O3 powder in a molar fraction ranges from 1650 to 1700. C has good superplasticity. When Chen Deyong et al. prepared ZTA ceramics using ZrO2 with a volume fraction of 10% to 30%, they found that the toughening effect was best when the volume fraction of ZrO2 was 20%.

In the toughening mechanism, in addition to the induced phase transformation mechanism of ZrO2, phase transformation generates volume expansion, which compresses the crack area towards the non phase transformation area, making the crack appear closed and difficult to expand, which can also improve toughness. In addition, ZrO2 particles are dispersed in the matrix and also play a role in refining the grain size of the second phase. Yu Qinghua and others introduced nanotechnology into the development of Zirconia toughened alumina. They used the huge surface area of nano powder to reduce the sintering temperature and inhibit the abnormal growth of grains to improve the toughness. Among them, the intragranular type refers to the formation of microcracks and secondary interfaces within the grain by nanoparticles, weakening the main grain boundary effect and reducing the tip elastic modulus, thereby slowing down crack propagation. The intergranular type is characterized by the dispersion of ZrO2 particles at the Al2O3 grain boundaries, resulting in pinning, which leads to transgranular fracture and improved toughness.

Therefore, the Zirconia toughened alumina is mainly a composite toughening with phase transformation toughening. According to the principle, materials with martensitic transformation similar to ZrO2 can be used as toughening phases, but currently, other phase transformation toughening phases are rare in the research of ceramic materials. Moreover, at high temperatures, these phase transitions will reverse and the toughening effect will fail. Considering this, the influence of phase change SiC whiskers and phase change SiC particles on the toughness of Al2O3 ceramics was studied. It was found that the two can interact with each other, and their synergistic effect can improve the toughness of ceramics, overcoming the disadvantage of poor PSZ toughening performance at high temperatures.