The magical self-healing ability makes ceramics immortal!

Not long ago, Texas A&M University and other institutions jointly discovered an unknown self-repair mechanism in certain types of ceramics. It is understood that in this new study, researchers found that a special type of ceramic-chromium aluminum carbide, can slow the propagation of cracks and even cure them.

As early as 2018, a team of researchers from the National Institute of Materials and Yokohama National University in Japan (NIMS-YNU team) added a healing activator at the grain boundary to allow ceramics to completely heal cracks in just one minute at operating temperatures up to 1000 °C in aircraft engines.

In fact, in the scientific research community, there has been research on the self-healing of ceramic materials a long time ago. In the 1970s, the self-healing properties of some materials were discovered one after another, such as ZnO, Al2O3, UO2 and MgO. Since then, SiC, Si3N4, Si3N4/SiC, Al2O3/SiC and other materials have been found to have crack self-healing ability.

1. Al2O3-based ceramics

Ando et al. studied the changes in the flexural strength of Al2O3-SiC ceramic samples with different crack sizes under the condition of healing at 1300 °C for 1 hour. The study found that samples with a crack size less than 200?m can achieve complete recovery of flexural strength. Al2O3-SiC nanocomposite ceramics can heal cracks with a length not exceeding 250nm and a depth not exceeding 100?m. At the temperature of 1273-1773K, when the oxygen partial pressure of Al2O3-SiC composite ceramics is greater than 50Pa, the strength of the ceramic cracked samples is completely restored.

2. Si3N4-based ceramics

Koji Takahashi et al. studied the crack self-healing phenomenon of Si3N4/SiC ceramic materials under different healing temperature conditions and found that Si3N4/SiC ceramic materials have higher flexural strength and static fatigue strength after healing.

3. Metallic ceramics and other ceramic materials

Jia Lin et al found that ZrO2 fibers and SiC whisker-toughened ZrB2-based ceramics showed the best crack healing behavior at 1200 °C. The surface cracks of the samples were filled with ZrO2 and SiO2, and the strength of the samples after healing was 95 MPa higher than that of the polished samples under the optimal healing conditions. Researchers have found that the crack self-healing mechanism of TiB2/SiC porous ceramics is that the oxidation reaction generates B2O3 and SiO2 glass phase to fill the crack, and the bending strength of porous ceramic materials is recovered after healing.

In recent years, many researchers have found that MAX phase ceramics can achieve self-healing of cracks at relatively low heat treatment temperatures, and Mn+1AXn phase ceramics are a ceramic material with ternary layered structure and good crack healing ability.

It is understood that this Texas A&M University and other institutions involved MAX ceramics, which are composed of alternating material layers. This structure gives them the ability to heal and the mechanisms behind it also play a role in geology. When a crack starts, a defect called a "kink band" forms between different layers. Under pressure, the crystals in the "kink band" rotate, which helps prevent further propagation of the crack. Even better, these rotating crystals can also actively heal cracks, so once the force on the material is relieved, they are almost undetectable.

Even more amazing, unlike previous studies, the ceramic self-healing process in this study was completed at room temperature, and this self-healing mechanism could close newly formed cracks over and over again, thus delaying the failure of the material.

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