Alumina ceramic materials are corundum-type structure, which itself has the characteristics of ionic bonds, making the sliding system is far from the metal so much, which leads to the lack of certain toughness, plasticity. So the fracture toughness of the lower, which greatly limits the wide application of alumina ceramic. What are the commonly used toughening methods of alumina ceramics?
1, lamellar structure toughening
Natural materials such as bamboo, shells and so on, the overall performance is very good, because its structure was layered distribution. People from these natural structures inspiration, the use of bionic structure to improve the brittleness of ceramic materials, improve its toughness.
Laminated composite ceramic material is composed of multi-layer material. The elastic modulus and linear expansion coefficient of each layer are different, which results in macroscopic stress between layers and compressive stress on the surface. By external force, can maximize the absorption of strain energy, and the crack along the interface have repeatedly deflected, turning. In order to achieve the purpose of improving the surface properties and overall toughness.
For example: Al2O3 / Ni layered ceramic, using the linear expansion coefficient of nickel is about alumina) times in the Al2O3 layer should produce pressure, crack deflection ability, so the material has good toughness. Laminated ceramics is a new type of material with broad prospects. However, its weakness is mainly that the weak intercalation will decrease the strength of the material, and the properties of the parallel and perpendicular to the interlayer direction are very different and anisotropic.
Therefore, the industry experts put forward the idea of strong sandwich, prepared a strong sandwich ZTA / Al2O3, impact toughness of 10 Mpa.m1 / 2 or more, 2.8 times that of ZTA, Al2O3 ceramic 5.6 times. Some scientists through the computer simulation of layered composite ceramics, found that if the soft layer material strength is too high, too low will reduce the overall toughness, and improve the hard and soft layer thickness and elastic modulus ratio, the hard layer uniformity are Can improve the ceramic toughness. This provides some research ideas and optimization ways for the toughening ceramic.
2, fiber composite toughening
The results show that the toughening efficiency of continuous fiber is higher than other toughening methods, which is the highest toughness of ceramic series up to 20Mpa.m1 / 2, so it is a very effective way to improve the brittleness of ceramic materials.
This method disperses fibers with higher strength and modulus of elasticity in the ceramic matrix. Composite material in the external force, part of the load borne by the fiber, in order to reduce the load itself. Moreover, the fiber in the matrix in the bear greater than the strength of its fracture, the fiber extraction mechanism. In addition, these fibers also have cracks in the matrix bridge, deflection to prevent the expansion of the crack. These three toughening mechanisms work together to improve the toughness of ceramic materials.
At present, the fiber used for Al2O3 ceramic mainly carbon fiber, silicon carbide fiber, aluminum silicate fiber and so on. It was found that the toughening effect can be improved by increasing the aspect ratio of the fiber. In the use of fiber form, the use of fiber, three-dimensional braided toughening effect is better. And fiber similar to the current use of whisker toughened Al2O3 porcelain are also more effective. Because the whisker is a single crystal structure growth, very small diameter (usually less than 3 um) short fibers. The crystal defects are few, the atomic arrangement of highly ordered, close to the strength of adjacent atoms between the theoretical value of the bonding force. It has been proved by theory and practice that it is applied to the toughening of ceramics and has a certain effect on improving toughness. Such as the silicon carbide whisker (volume fraction of up to 20% ~ 30%) into the Al2O3-based ceramics, the segment toughness of up to 8 ~ 8.5 Mpa.m1 / 2.
In addition to the mechanism of whisker toughening, such as pull out, crack deflection, crack bridging, pinning and other mechanisms, its high strength is also a reason. Therefore, in theory, improve the whisker strength, reduce its elastic modulus, improve the aspect ratio can improve the toughening effect. Fiber, whisker toughening Al2O3 ceramic shortcomings is difficult to guarantee uniformity of mixing.
3, since toughening
The so-called self-toughening, is in a certain process conditions, the growth of toughening, enhanced phase. It can eliminate the physical or chemical incompatibility between the matrix phase and the toughening phase to some extent, and ensure the thermodynamic stability of the matrix phase and the toughening phase.
For Al 2 O 3 ceramics, the toughening of Al 2 O 3 into Al 2 O 3 ceramics is a hot spot for overcoming the brittleness of alumina ceramics. The main mechanism is to control the growth direction of Al2O3 grain by process measures, and make it grow into rod shape and long column shape along some crystal face, and play the role of toughening similar to whisker. When the alien loads are applied, the cracks are produced in the tail of the crack. Moreover, the Al 2 O 3 ceramics also have the toughening mechanisms such as pulling out and crack deflection, and the toughness of the alumina ceramics is improved.
4, phase transformation toughening
This is a relatively early and common toughening method. It is artificially created in the material in a large number of very fine cracks to absorb energy, prevent crack propagation. Which mainly concentrated in the ZrO2 martensitic transformation research, more successful ZTA, ZTM and other ceramic materials. ZrO2 dispersed in the Al2O3 matrix, due to the different coefficient of linear expansion, cooling, ZrO2 particles subject to compressive stress, phase change is blocked. Then, when the material is subjected to external force, the pressure on the ZrO2 particles is relaxed, and the tetragonal phase changes into monoclinic phase. After the volume expansion, microcracks are generated in the matrix, and the energy of the main crack is absorbed to achieve toughening effect. This is the stress-induced phase transition toughening mechanism.
In the toughening mechanism, in addition to ZrO2 induced phase transition mechanism, the phase transition produces volume expansion, in the crack region does not occur to the phase transition zone extrusion phenomenon, the crack was closed trend, expansion difficulties, can also improve toughness. Some researchers use ZrO2 volume fraction of 10% to 30% prepared ZTA ceramics found that the amount of ZrO2 volume fraction of 20% toughening effect is the best.
Ceramic toughening technology in the future for a long time will be the material industry's hot technology. Ceramic materials inherent high strength, high temperature, low expansion coefficient and other characteristics if combined with high toughness, it will be the material industry dream of high-performance materials, the application of a very wide range
