Zirconia ceramics have unique physical and chemical properties, such as high hardness, low thermal conductivity, high melting point, resistance to high temperature and corrosion, chemical inertness and amphoteric properties, and their applications in electronic ceramics, functional ceramics and structural ceramics have developed rapidly. As a special technical ceramic material, it has broad application prospects in high-tech fields such as electronics, aerospace, aviation and nuclear industry. However, the fatal shortcomings of zirconia ceramic materials are brittleness, low reliability and low repeatability. These shortcomings have seriously affected the scope of its application. Only by improving the fracture toughness of zirconia ceramics, realizing the strengthening and toughening of materials, and improving its reliability and service life, can zirconia ceramics truly become a widely used new material. Therefore, the toughening technology of zirconia ceramics has always been ceramic research hotspots. At present, ceramic toughening methods mainly include: phase change toughening, particle toughening, fiber toughening, self-toughening, dispersion toughening, synergistic toughening, nano-toughening, etc.
1. Phase Change Toughening
Phase transformation toughening means that the meta-stable tetragonal phase t-ZrO2 undergoes a phase transformation under the action of the stress field at the crack tip, forming a monoclinic phase, resulting in volume expansion, thereby forming a compressive stress on the crack, preventing crack propagation, and strengthening the crack. In addition, external conditions (such as laser shock, fatigue fracture toughness, low temperature, grain size and content, critical transformation energy, etc.) have a great influence on the phase transformation toughening of zirconia ceramics. If the phase transformation produces large stress and volume changes, the product is easy to break. Therefore, during the production process, the influence of external factors on the phase transformation and toughening of zirconia ceramics should be avoided.
2. Particle Toughening
Particle toughening refers to the use of particles as toughening agents and added to ZrO2 ceramic powder. Although the effect is not as good as whiskers and fibers, if the particle type, particle size, content and matrix material are properly selected, there is still a certain toughness effect. Its advantage is that it is simple and easy to implement, and the toughening will bring the improvement of high temperature strength and high temperature creep performance. The toughening mechanism of particle toughening mainly includes refining the matrix grains and crack turning and bifurcation.
3. Fiber Toughening
The principle of fiber and whisker toughening is that the crystal close to the crack tip is subjected to a closing stress due to deformation, which offsets the external stress at the crack tip, blunts the crack propagation, and plays a toughening role. In addition, when the crack columnar crystal is expanded, the friction force must be overcome when the columnar crystal is pulled out, and it will also play a role in toughening.
4. Self-toughening
Due to the existence of columnar crystals in zirconia ceramics, cracks will be deflected during the fracture process of zirconia ceramics, which will change and increase the path of crack propagation, so that the passivation of cracks increases the resistance to crack propagation and achieves the purpose of toughening.
5. Diffusion Toughening
Dispersion toughening mainly refers to the toughening of the ceramic matrix by the tetragonal ZrO2 particles. In addition to the phase transformation toughening mechanism, there is also the dispersion toughening mechanism of the second phase particles. Before the crack propagates, the internal residual strain energy of the ceramic itself must be overcome first, so as to achieve the purpose of toughening.
6. Micro-crack Toughening
Micro-crack toughening refers to the addition of ductile materials at the crack stress tip to produce micro-cracks to achieve the purpose of dispersing the stress, reducing the driving force of the cracks, and increasing the toughness of the material. When the material undergoes a phase transition, it often results in residual strain energy effects and microcracks. Therefore, the effect of phase transition toughening is significant.
7. Compound Toughening
Composite toughening refers to the simultaneous use of several toughening mechanisms in the actual toughening process of ZrO2 ceramics, thereby improving the toughening effect of ZrO2 ceramics. In the actual application process, the specific toughening mechanism is selected according to the different properties of the zirconia ceramic material to be prepared.
