What are the toughening methods for alumina ceramics?

The structure of alumina ceramic materials is corundum type, which has its own ionic bonding characteristics, making the slip system much less than that of metals, which leads to a certain lack of toughness and plasticity. Therefore, it exhibits low fracture toughness, which greatly limits the widespread use of alumina ceramics.
Common toughening methods of alumina ceramics
1、Layer structure toughening
Natural materials such as bamboo, shells, etc., comprehensive performance is very good, because its structure is laminar distribution. People get inspiration from these natural structures, the use of bionic structure to improve the brittleness of ceramic materials, improve its toughness.
Laminated composite ceramic materials are composed of multiple layers of materials. The layers have different elastic modulus and linear expansion coefficients, which in turn lead to macroscopic stresses between the layers and compressive stresses at the surface. When subjected to external forces, it can absorb strain energy to the maximum extent and cause repeated deflection and inflection of cracks along the interface. In this way, the surface properties and overall toughness can be improved.
For example: Al2O3/Ni laminated ceramics, using the linear expansion coefficient of nickel is about (alumina) times, in the Al2O3 layer to produce stress, crack deflection ability, so the material has a good toughness.
Laminated ceramics is a new type of material with a promising future, but its drawback is mainly that the weak interlayer will reduce the strength of the material, and the nature of the parallel and perpendicular to the interlayer direction varies greatly and is anisotropic. Therefore, experts in the industry have proposed the idea of using strong interlayer and prepared ZTA/ Al2O3 strong interlayer with impact toughness of more than 10 Mpa.m1/2, which is 2.8 times of ZTA material and 5.6 times of Al2O3 ceramic. Some scientists have simulated the laminated composite ceramics by computer and found that if the strength of the soft layer material is too high or too low will reduce the overall toughness, while increasing the ratio of hard and soft layer thickness and elastic modulus, and the uniformity of hard layer can all improve the ceramic toughness. This provides a certain research idea and optimization path for layer toughened ceramics.
2, fiber composite toughening
Research shows that continuous fiber toughening efficiency of ceramics than other toughening methods, is by far the highest toughness that can be achieved by ceramic series, can reach 20Mpa.m1/2 or so, so it is a very effective way to improve the brittleness of ceramic materials.
The method disperses fibers with high strength and elastic modulus in the ceramic matrix. When the composite material is subjected to external forces, part of the load is borne by the fibers as a way to reduce the load on the matrix itself. Moreover, the fibers in the matrix produce a fiber pull-out mechanism when a force greater than their strength breaks. In addition, these fibers also have crack bridging and deflection in the matrix to prevent crack expansion. These 3 toughening mechanisms work together to make the ceramic material much tougher.
At present, there are many kinds of fibers used for Al2O3 ceramics, such as carbon fiber, silicon carbide fiber, and aluminosilicate fiber. It is found that increasing the length-to-diameter ratio of fibers can improve the toughening effect. In the form of fiber use, the use of fiber,s three-dimensional braid toughening effect is better. Similar to the fiber, the use of whisker toughening Al2O3 porcelain is also more, the effect is also very good. Because the whisker is a single crystal structure growth, very small diameter (usually less than 3 um) of the short fiber. Its crystal defects are few, the atomic arrangement is highly ordered, and the strength is close to the theoretical value of the bonding force between adjacent atoms. Theoretically and practically, it has been proven to be useful in improving the toughness of ceramics by applying it to the toughening of ceramics. For example, if silicon carbide whiskers (volume fraction up to 20%-30%) are introduced into Al2O3-based ceramics, the segment toughness can reach 8-8.5 Mpa.m1/2.
The mechanism of whisker toughening is not only the mechanism of pull-out, crack deflection, crack bridging and pegging, but also the high strength of itself is a reason. Therefore, in theory, increasing the whisker strength, decreasing its elastic modulus, and increasing the aspect ratio can improve the toughening effect. The disadvantage of fiber and whisker toughened Al2O3 porcelain is that the mixing uniformity is difficult to ensure.
3、Self-toughening
The so-called self-toughening, is in a certain process conditions, the growth of toughened, enhanced phase. It eliminates the physical or chemical incompatibility of the matrix phase and toughened phase to a certain extent, while ensuring the thermomechanical stability of the matrix phase and toughened phase.
For Al2O3 ceramics, anisotropic growth of grain-toughened Al2O3 has become a hot research topic to overcome the brittleness of alumina ceramics. The main mechanism is to control the growth direction of Al2O3 grains through process measures, so that they grow into rods and long columns along certain crystallographic planes to play a toughening effect similar to whiskers. When subjected to external loading, the crack tail produces a bridging way; and these anisotropic growth of Al2O3 will also produce pull-out, crack deflection and other toughening mechanisms, and the entire alumina ceramic toughness is improved.
4、Phase change toughening
This is a relatively early and common study of a toughening party. It is artificially caused by a large number of very fine cracks in the material, in order to absorb energy and prevent crack expansion. ZrO2 dispersion in the Al2O3 matrix, due to the difference in their linear expansion coefficients, cooling, ZrO2 particles are subject to compressive stress, the phase change is blocked. And then, when the material is subjected to external force, the pressure on the ZrO2 particles is relaxed, and the tetragonal phase is transformed into monoclinic phase, and the volume expands to produce microcracks in the matrix, while absorbing the energy of the main crack to achieve the toughening effect. This is the stress-induced phase change toughening mechanism.
In the toughening mechanism, in addition to the induced phase change mechanism of ZrO2, the phase change generates volume expansion and squeezes the phenomenon in the crack region toward the non-phase change region, which makes the crack tend to be closed and difficult to expand, and also improves the toughness. Some researchers preparing ZTA ceramics with a volume fraction of 10% to 30% of ZrO2 found that the best toughening effect was achieved when the ZrO2 dosage was 20% by volume.