l2O3-based ceramic materials have broad application prospects due to their excellent wear resistance, high temperature resistance, corrosion resistance, high hardness, and strong chemical stability. However, its disadvantage is that its toughness is low and it is prone to brittle fracture. Therefore, improving the toughness of alumina ceramic materials is the prerequisite for its further application in various fields.

1. Particle dispersion toughening

Particle dispersion toughening is mainly to add non-metal or ductile metal with high elastic modulus to the Al2O3 ceramic matrix as the second phase particles. The high elastic modulus particles prevent the shrinkage of the transverse section when the matrix material is stretched. To achieve the same lateral shrinkage as the matrix, the longitudinal tensile stress must be increased to strengthen the material. Increasing the external tensile stress makes the material consume more energy, so it has a toughening effect. Particle dispersion toughened Al2O3-based ceramic materials are mainly divided into metal particle toughened and non-metallic particle toughened.

1 metal particle toughening

Al2O3-based ceramic materials can improve their mechanical properties by adding metal particle phase, and the particle disperse phase can be introduced into ductile metal phase. At present, the ductile metal phase has also been proved to be a promising toughening method, and the added metal particles mainly include Al, Ni, Ag, Cu, Fe, etc. The toughening of metal particles mainly uses the unbroken particles at the crack tip to act as bridges on the upper and lower surfaces of the crack. On the one hand, it prevents the opening of the crack and reduces the stress intensity factor at the crack tip, and on the other hand, the particles undergo plastic deformation due to crack propagation. , to consume the energy at the tip of the crack to achieve the purpose of toughening.

1. Particle dispersion toughening

Particle dispersion toughening is mainly to add non-metal or ductile metal with high elastic modulus to the Al2O3 ceramic matrix as the second phase particles. The high elastic modulus particles prevent the shrinkage of the transverse section when the matrix material is stretched. To achieve the same lateral shrinkage as the matrix, the longitudinal tensile stress must be increased to strengthen the material. Increasing the external tensile stress makes the material consume more energy, so it has a toughening effect. Particle dispersion toughened Al2O3-based ceramic materials are mainly divided into metal particle toughened and non-metallic particle toughened.

1 metal particle toughening

Al2O3-based ceramic materials can improve their mechanical properties by adding metal particle phase, and the particle disperse phase can be introduced into ductile metal phase. At present, the ductile metal phase has also been proved to be a promising toughening method, and the added metal particles mainly include Al, Ni, Ag, Cu, Fe, etc. The toughening of metal particles mainly uses the unbroken particles at the crack tip to act as bridges on the upper and lower surfaces of the crack. On the one hand, it prevents the opening of the crack and reduces the stress intensity factor at the crack tip, and on the other hand, the particles undergo plastic deformation due to crack propagation. , to consume the energy at the tip of the crack to achieve the purpose of toughening.

Researchers at Osaka University in Japan have prepared a composite material composed of alumina (Al2O3) ceramics and titanium (Ti), that is, Al2O3/Ti ceramic matrix composites, which have excellent mechanical properties and can also be used for EDM. They will also be used in industrial products and biomaterials as a new multifunctional composite material with an active surface layer with antimicrobial properties and the ability to photocatalytically degrade pollutants.

2 Non-metallic particle toughening

The toughening of non-metallic particles is to add SiC, TiC, WC, TiB2, ZrB2 and other non-metallic nanoparticles to Al2O3-based ceramic materials to achieve the purpose of toughening alumina-based ceramic materials. For example, researchers add SiC particles (about 280nm) to Al2O3-based ceramics, and prepare Al2O3-based ceramic composites by hot pressing and sintering. The hardness is 23.4Gpa, and the fracture toughness is 4.8MPa·m1/2. Ceramics (3MPa·m1/2), the fracture toughness has been improved to a certain extent. The main reason for the improvement of mechanical properties is that the fracture form changes from intergranular fracture to transgranular fracture, which enhances the ability to resist crack propagation and improves fracture toughness.

2. Phase transformation toughening

Phase transformation toughening is to use the toughening characteristics of ZrO2 to realize the toughening of alumina ceramic materials. The toughening mechanism is that when the metastable tetragonal phase ZrO2 ceramics are subjected to external stress, the tetragonal phase ZrO2 particles will transform into the allotropic monoclinic ZrO2 phase, and at the same time produce a volume expansion of 3% to 5%. Strain energy and bridge cracks, thereby increasing the fracture toughness of the material. At the same time, the volume expansion caused by the phase transition produces compressive stress on the crack and hinders the expansion of the crack, which is manifested in the decrease of the stress intensity factor at the crack tip, thereby improving the ability to resist crack growth. Moreover, the martensitic transformation of zirconia causes microcracks and residual stress in the Al2O3 matrix, which can improve the toughness of alumina ceramics. The toughening effect mainly comes from the refinement of grains, microcracks and crack deflection. .

It is worth noting that the effect of ZrO2 particles toughening Al2O3-based ceramics is closely related to the composition of the material, sintering temperature, grain size and other factors. Although the effect of phase change toughening is obvious, it is greatly affected by temperature. By adding a small amount of stabilizer, the high temperature adaptability of ceramics can be greatly improved, so the method of phase change toughening can be widely promoted and applied.

3. Toughening by whiskers, fibers and carbon nanotubes

1 whisker toughened

Whisker toughening is mainly to significantly improve the toughness of Al2O3-based ceramics through the bridging, pinning or deflection of cracks in ceramics by whiskers and the pulling out of whiskers. The whisker toughening mechanism mainly includes pull-out bridging mechanism and crack deflection mechanism.

(1) Pull-out bridging mechanism: there is a critical value lp for the pull-out length of whiskers. When the distance from one end of the whisker to the main crack is less than this critical value, the whisker will be pulled out from this end; breaks in the process. In addition, the interface bonding strength between whiskers and matrix directly affects the toughening mechanism and toughening effect of composites.

(2) Crack deflection mechanism: When the crack expands to the vicinity of the whisker, the crack is not easy to deflect through the whisker due to the high elastic modulus of the whisker, which greatly increases the crack propagation distance and absorbs more energy. , to achieve the effect of toughening. At present, whiskers have good mechanical properties, which makes whisker-toughened ceramic matrix composites develop rapidly and become one of the most promising high-temperature structural materials. Commonly used whiskers are SiC, Al2O3, ZrO2, Si3N4, mullite and carbon fiber.

Whisker-toughened Al2O3-based ceramics is a kind of ceramic-based composite material that has been studied relatively early. The introduction of microscopic whiskers into Al2O3 can increase the fracture toughness by 3 to 4 times. The SiC crystal with a content of 20% to 30% is prepared by hot pressing. The flexural strength and fracture toughness of Al2O3-based ceramics that must be toughened reach 650MPa and 8-8.5MPa·m1/2, respectively.

2 fiber toughening

Two conditions need to be met between the fiber and the Al2O3-based ceramic material to achieve the toughening effect:

One is that the elastic coefficient of the reinforcing fiber must be higher than that of the alumina ceramic matrix;

The second is that the fiber must be compatible with the Al2O3-based ceramic material.

Commonly used reinforcing fibers are: carbon fiber, SiC fiber, etc. The combination between Al2O3 matrix and various fibers is not a simple mixture, but an organic complex, which is organically combined through a very thin interface.

3 carbon nanotube toughening

Carbon nanotubes have extremely high strength, toughness and modulus of elasticity. Its modulus can reach 1TMpa, which is almost the same as that of diamond. Using carbon nanotubes as a reinforcement of Al2O3-based ceramic materials can exhibit good strength, elasticity, fatigue resistance and isotropy.

The mechanism of carbon nanotubes toughening Al2O3-based ceramic materials is: using short fibers to toughen, carbon nanotubes have excellent mechanical properties. When pulling out and breaking, more energy is consumed, which is beneficial to prevent the expansion of ceramic cracks. In addition, the bridging and pinning effects of carbon nanotubes on ceramic grains can achieve the purpose of transmitting and sharing loads, transforming the ceramic crack propagation mode from intergranular fracture to transgranular fracture, which can significantly improve the toughness of Al2O3 ceramics. Carbon nanotubes can also form a unique network structure with ceramics to deflect cracks along grain boundaries, which also helps to improve the fracture toughness of Al2O3 ceramics.

4. In situ growth whisker toughening In situ growth whisker toughening is to prepare whiskers by in situ growth. The inside of the matrix grows and arranges according to a specific orientation, forming whiskers in the matrix.

The advantage of this toughening method is that the obtained whiskers are evenly distributed in the matrix, which effectively increases the content of whiskers in the matrix, improves the uniformity of whiskers in the matrix, and makes the mechanical properties of ceramics reach isotropy. Effect.

5. Composite toughening At present, improving the toughness of Al2O3-based ceramic materials by using a variety of toughening mechanisms has attracted widespread attention. The main methods are: multi-phase particle composite toughening, whisker-particle composite toughening, whisker- Phase change composite toughening, etc.