Analysis on Toughening Technology of Alumina Ceramics

Alumina ceramic is the most stable material among the oxides, which has high mechanical strength, high electrical insulation, and low dielectric loss. It has a broad application prospect in aerospace, aviation, textile, construction, and other fields.

Alumina ceramic is the most stable material among the oxides, which has high mechanical strength, high electrical insulation, and low dielectric loss. It has a broad application prospect in aerospace, aviation, textile, construction, and other fields.

However, because of its high brittleness and poor homogeneity, it affects the safety of ceramic parts. Therefore, improving the toughness of alumina ceramics is an important problem to be solved urgently.

In order to reduce the brittleness of alumina ceramics, in addition to the advanced preparation technology, the toughening technology of alumina ceramics needs to be studied extensively and deeply. At present, the research is mainly focused on the following aspects:

Phase transformation toughening

When phase transformation is used as a ceramic toughening method, the research on improving the thermal shock resistance of partially stabilized zirconia begins, and remarkable results have been achieved.

Due to the characteristics of zirconia phase transformation, zirconia toughened alumina ceramics have a good toughening effect.
At present, ZTA based on phase transformation toughening has been used as structural materials for many components.

Pure zirconia has solid phase transformation around 1000 ℃: square phase (t) → monoclinic phase (m), which belongs to martensitic transformation and will produce a volume expansion of 3% to 5%.
When the crack propagates into the region containing t-ZrO2 grains, under the action of the stress field at the crack tip, a process zone is formed at the crack tip, that is, the t-ZrO2 in the process zone will undergo a t-ZrO2 phase transition, which absorbs energy not only because of the new fracture surface, but also because of the volume effect (expansion) of the phase transition. At the same time, the compressive stress on the crack is caused by the volume expansion of t-m phase change particles in the process zone, which hinders crack propagation.

Relatively speaking, the critical stress intensity factor-fracture toughness at the crack tip of the material is improved. The toughness of the alumina matrix can be significantly improved by introducing the t→m transformation toughening of ZrO2 and the microcrack toughening and residual stress toughening derived from t→m phase transformation into alumina matrix.

So far, the transformation toughening of partially stabilized zirconia is one of the most successful toughening methods.

However, because many brittle materials do not necessarily have this kind of phase transformation which is beneficial to toughen, and it is also greatly affected by temperature, this toughening method can not be widely used.

Toughening of whiskers, fibers, and carbon nanotubes

Compared with the transformation toughening of alumina ceramics, whisker and fiber toughening is a promising toughening technology. When the whisker is pulled out and broken, it consumes a certain amount of energy, which helps to prevent crack propagation. Increasing the strength of the whisker and decreasing the elastic modulus of the whisker is beneficial to improve the toughness of the material.

Increasing the whisker size (length, radius, and aspect ratio) can improve the toughening effect of the whisker. Ceramic composites with high strength and toughness can be obtained by adding directional or disordered fibers to the ceramic matrix, which has become one of the development directions in the field of alumina ceramics.

In order to achieve the purpose of fiber composite toughening, two conditions must be satisfied between the fiber and the matrix material:

(1) the elastic coefficient of the reinforced fiber must be higher than that of the alumina ceramic matrix;
(2) the fiber and the matrix must be compatible.

Particle dispersion toughening

The mechanical properties of ceramic materials can be improved by adding granular metal phase, and the introduction of ductile metal phase into brittle ceramics has been proved to be a promising toughening method. The introduction of metal particles into the ceramic matrix as a ductile second phase can not only improve the sintering properties of the ceramics, but also hinder the crack propagation in the ceramic materials in many ways, and improve the bending strength and fracture toughness of the composites.

When the shape is granular, the toughening mechanism is mainly crack deflection, while the plastic deformation of metal mainly occurs in composites in the shape of fiber, sheet, and so on. Ceramics and intermetallics are materials that can be used at high temperatures.

Through the refinement of matrix grains and the shielding effect of cracks, the driving force of crack progress is dissipated to achieve the goal of toughening. Although the effect is not as good as that of fiber and whisker, the process is simple and the cost is low. As long as the type, size, and content of particles are selected properly, the toughening effect is still very obvious.

Nanotechnology toughening

The study of nanomaterials and nanotechnology may lead to a revolutionary breakthrough in ceramic toughening technology. On the one hand, due to the grain refinement of nano-ceramics, the number of grain boundaries will be greatly increased. At the same time, if the pore size and defect size of nano-ceramics are reduced to a certain size, the macroscopic strength of the material will not be affected. As a result, the strength and toughness of the material will be greatly increased. On the other hand, the introduction of nano-dispersed phase into ceramic matrix and composite can not only greatly improve the strength and toughness of ceramics, but also significantly improve the high-temperature resistance of ceramics.

Therefore, nanocrystallization of alumina ceramics and nanocomposites has become one of the important ways to improve their fracture toughness.

The strengthening and toughening mechanism of nanocomposite ceramics is mainly reflected in the following aspects:

The main results are as follows:

(1) the introduction of dispersed phase can effectively restrain the grain growth of the matrix and reduce the abnormal grain growth.
(2) there is local stress around the dispersed phase or dispersed phase, which refines the grains and weakens the main grain boundary effect.
(3) the high temperature of nanoparticles suppresses the movement of dislocations, thus improving the mechanical properties at high temperatures, such as hardness, strength, and creep resistance.