The reasons for the low fracture toughness of ceramic materials

The low fracture toughness of ceramic materials is mainly due to the instability of their microstructure. Under the action of stress, ceramic materials develop tiny cracks, and these cracks will gradually expand as the stress increases, eventually leading to the rupture of the entire material.

Although ceramic materials are renowned for their high hardness, high strength, and high temperature resistance, behind these characteristics lies their inherent brittleness. Although ionic bonds and covalent bonds in ceramic materials give them excellent physical properties, they also limit the possibility of plastic deformation. When ceramic materials are subjected to external forces, due to the directional nature of covalent bonds and the repulsive characteristics of ionic bonds, it is difficult to have significant dislocation movement to relieve stress, thus resulting in the generation of brittleness. Moreover, the crack problem existing in ceramics is the root cause of its brittleness. Once a crack forms, it will rapidly expand within the ceramic material, like a domino effect, until the entire object breaks.

In practical applications, the fracture strength of ceramic materials is often much lower than their theoretical strength. This is mainly because ceramic materials are not completely dense, and there are various forms of pores inside, which are similar to cracks and weaken the strength of the material. At the same time, the complex organizational structure and uneven distribution within the ceramic also further affect its actual fracture strength.

Ceramic materials themselves do not possess particularly high fracture toughness. However, through specific manufacturing processes and toughening techniques, the fracture toughness of ceramics can be significantly enhanced.

Methods for improving ceramic fracture toughness

Although the fracture toughness of ceramic materials is not high by nature, through specific manufacturing processes and toughening techniques, we can effectively increase its fracture toughness. For example, by adding toughening agents, using special sintering processes, and conducting surface treatments, we can significantly enhance the impact resistance and impact toughness of ceramics.

Technological approaches to enhancing ceramic toughness

1. Phase transformation toughening (using zirconia as an example)

By adding yttrium oxide (Y₂O₃) stabilizer, zirconia undergoes a transformation from tetragonal phase to monoclinic phase under stress, resulting in a 3-5% volume expansion that can close cracks. The fracture toughness of such ceramics can reach 15 MPa·m¹/² (in accordance with JIS R1607 standard).

2. Fiber/whisker reinforcement

Introducing silicon carbide fibers (diameter 5-20 μm) into the ceramic matrix can cause the crack propagation path to be deflected, increasing the toughness by 3-8 times.