We usually have the impression that ceramics are a material that cannot be repaired. For example, a bowl will crack when it is cracked, unlike our human skin, which will heal on its own after a few days after being cut.
However, in recent years, with the development of materials science, a new type of material has become more and more well-known, this material is self-healing material, also known as self-healing material, its biggest feature is that after the occurrence of damage (generally refers to the production of cracks), without the intervention of external force, the self-repair of the structure and the self-recovery of performance can be completed. There are many kinds of self-healing materials, including self-healing polymer materials, self-healing metals, self-healing concrete, etc. Self-healing ceramics are also one of them.
The study of self-repairing ceramics first began in the 50s of the last century, but its development has been relatively slow. However, due to the great success of self-healing polymer materials in recent years, self-healing ceramics have also attracted more and more attention and have become a popular research direction.
So, how do self-healing ceramics work?
In principle, there are three self-healing mechanisms of ceramics: one is adsorption. When the crack sections come into contact together, the sections are rebonded due to the interaction of the forces between the atoms. This is the main way to heal cracks at low to medium temperatures. For example, the partial thermal shock crack healing phenomenon of alumina ceramics at room temperature is the effect of this mechanism. The second is diffusion. At a certain temperature, the particles on both sides of the section diffuse with each other, so that the gaps between the adjacent particles of the crack disappear. This mechanism occurs mainly at higher temperatures, such as thermal shock crack healing that occurs at 1400-1700 years for pure alumina. The third is chemical reaction. Under certain temperature and atmosphere conditions, the self-healing material inside the ceramic can undergo a chemical reaction to make the crack be bridged. This is the most important and currently the most studied repair mechanism. Those substances that have the ability to heal themselves, are called "healing agents".
According to current reports, "healing agents" mainly repair cracks through two mechanisms. One is to generate a less viscous liquid phase, which flows into the crack to fill it; For example, boride is added to ceramics as a "healing agent", and boron oxidizes at high temperatures to form molten boron oxide, which flows into the crack wall to heal the crack. The other is the formation of oxides, which allow the crack to bridge due to volume expansion. For example, silicon carbide "healing agent" is added to alumina ceramics, and silicon carbide can react with oxygen to form silica with a density of 329 cm3 reduced to 239 cm3, with a volume expansion of about 40%. This creates compressive stresses in the green body, allowing the crack to heal. Of course, silica can also promote the sintering of alumina, which can further promote the healing of cracks. Or sometimes, both of these mechanisms exist. For example, yttrium oxide has been introduced into silicon carbide-alumina composites. Not only does silicon carbide oxidize to silicon oxide when treated at high temperatures, but it can also form a low-viscosity liquid-phase eutectic compound that fills cracks and heals surfaces more effectively.
From the above introduction, we can find that it seems that when ceramics contain components that can undergo oxidation reactions, they have a certain self-healing ability. In fact, it is true that many non-oxides or metals can be used as "healing agents". If the ceramic itself is non-oxide, it is possible to heal the cracks under the right conditions. For example, it has recently been found that some ternary carbide and nitride ceramics have shown good crack healing ability. What's even more amazing is that some self-healing ceramics can also repair the damaged area repeatedly. For example, some researchers have achieved at least 7 cracking-healing cycles at specific locations on carbon-aluminum-titanium ceramics.
Interestingly, many studies have shown that the mechanical properties of ceramics are often improved after crack healing. For example, the self-healing behavior of silicon carbide-silicon nitride composite ceramics has been studied, and it has been found that its flexural strength is higher than that of intact specimens.24 However, in order to obtain a satisfactory self-healing effect, in addition to the performance of the "healing agent" itself, there are also strict requirements for the conditions of heat treatment, too high or too low temperature, too short or too long holding time, and unsuitable atmosphere will have an adverse effect on the self-healing process of ceramics, or even no self-healing at all. We'll have a chance to talk about this later.