Brittleness is a common fatal weakness of inorganic non-metallic materials. The brittleness of ceramics is intuitively manifested by the fact that fracture is unpredictable and violent under applied load. Indirectly, it is manifested by:poor resistance to mechanical shock and rapid temperature change.

Brittleness, which is also one of the important characteristics to measure the performance of ceramic materials, is the dense weakness of ceramic materials.

The nature of ceramic brittleness is mainly determined by the nature of the chemical bond and crystal structure, the lack of independent slip system in ceramics, once the material is in a state of stress it is difficult to relax the stress through the plastic deformation caused by slip. From the microstructure, the root cause of brittleness is the existence of microcracks, which can easily cause a high concentration of stress, followed by microcrack expansion to fracture.

The brittle characteristics of ceramic materials.

1. Covalent bonding characteristics
There are many gaps between the atoms of the chemical bonds in ceramic materials, which makes it difficult to cause dislocation movement.
Covalent bonds are directional, which can make the crystal structure complex and have a high resistance to distortion and impede the only movement.

2、Microstructure characteristics

Ceramic materials are polycrystalline, multi-phase structure, its grain boundaries will impede displacement, the aggregated displacement will cause the formation of cracks, coupled with the existence of points, lines, surface defects in the actual crystal structure, and the existence of microscopic and sub-microscopic cracks inside, its structural inhomogeneity is inevitable.
In addition, grain boundaries, porosity, crystalline phase, two-phase inclusions and cracks and other microstructural factors, can lead to ceramic materials show brittle.

3, no plastic change characteristics

Most ceramic materials at room temperature under the action of external forces without or only a small plastic deformation, which leads to ceramic materials fracture are relatively sudden, that is, showing brittle.
Brittle fracture is when the material is stressed in the case of lower than its own bond strength for stress redistribution, and the rate of applied stress exceeds the rate of stress redistribution when there is no other energy absorption process, the stress can not relax, it is concentrated on the expansion of the crack, making the expansion rate is very rapid, and finally lead to sudden damage.
Brittle fracture is the end of crack expansion.
Decrease. The increase of pores decreases the density of the material, i.e. reduces the loading area. The presence of pores at grain boundaries causes stress concentrations, which tend to form cracks under external forces.
An increase in porosity reduces the contact area between grains and shortens the pore spacing, which facilitates the formation and expansion of cracks and increases the brittleness of the material.
Multi-phase junction of the pores, itself is equivalent to cracking.

3, grain boundaries and cracking

For ceramic materials composed of a single crystalline phase, the expansion of the crack under the action of external forces encounter grain boundaries will often be suspended, if the grain boundaries have pores and stress concentration, the crack will be elongated along the grain boundary. Impurities and second phase grains in the grain boundary phase of polycrystalline materials, sometimes due to its brittleness and anisotropy caused by stress that becomes the path of crack propagation, and sometimes because of the high energy barrier and play a role in preventing crack expansion.
Several common solutions to the brittleness of ceramic materials

1, increase the number of microcrystalline, improve the density and purity of porcelain body

①Improve the quality of raw material micronized powder. In the premise of meeting other process requirements, so that the powder as fine as possible, the size and shape of the particles are uniform, chemical purity and phase structure of a single good.
②Select the sintering temperature scientifically and choose the best process conditions to prevent grain growth.
③ Determine the appropriate additive and addition amount to inhibit the abnormal growth of grains and promote densification.

2、Phase change toughening

Overcome ceramic brittleness by controlling the phase change in the ceramic body. That is, the use of phase change occurs when the volume change, reduce the stress concentrated in the crack tip, so as to improve the purpose of ceramic brittleness.

3、Toughening of composite materials

Add another particle material or fiber material in the ceramic matrix to form a composite material to toughen.
The plastic deformation of the particles can absorb part of the energy, so that the highly concentrated stress in the crack tip area can be
The plastic deformation of the particles absorbs part of the energy, thus allowing the highly concentrated stresses in the crack tip area to be partially removed and increasing the resistance of the material to crack expansion.
High strength, high modulus fibers can share most of the applied stress for the matrix, but also can prevent the expansion of cracks in the matrix.

4、Pre-stressing

The artificial introduction of residual pressure on the surface of the material through appropriate heating and cooling process can improve the tensile strength of the material and improve the brittleness of the material.
This method not only causes compressive stress on the surface, but also makes the grain refinement.

5、Process optimization

Reasonable control of process conditions to make the ceramic structure as uniform and dense as possible, reduce porosity, reduce cracking and improve brittleness.

Article source: Advanced Ceramic Materials
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