01-05 cracked cup

[Image above] Credit: PxHere

Happy New Year and welcome to 2021!

I know many of us breathed a sigh of relief when the clock struck midnight on January 1. Though the flip of the calendar is more a symbolic rather than literal change, it gives us the opportunity to recenter our thoughts and prepare for the year ahead.

As we outline our goals for the coming months, reviewing what we’ve already accomplished is a good start—both for setting personal goals and for deciding what lab experiments to conduct next. That’s why we’re kicking off this year with an open-access review article on self-crack-healing in ceramics.

Ceramics are increasingly used in automotive and aerospace applications, including as turbine blades and coatings for high‐temperature components. However, these applications subject the ceramics to thermal and mechanical stresses, which can cause microcracks and component failure.

Ceramics with self-crack-healing ability would be beneficial in these applications, for example, by reducing maintenance and inspection costs while enhancing component reliability and lifespan. As a result, many researchers have investigated self-crack-healing in ceramics at least since 1958.

In the review paper covered today, Israa Hammood and Gary Barber at Oakland University along with their colleague Bingxu Wang at Zhejiang Sci‐Tech University look at some of the experimental and numerical studies on self-crack-healing in ceramics published to date. In addition to offering detailed summaries of specific studies, they also provide a solid overview of the self-crack-healing topic.

Classification of self-healing mechanisms

The researchers begin their review by describing two approaches to classify self-healing mechanisms.

The first approach—intrinsic and extrinsic classification—is inspired by the natural healing ability of biological systems.

Intrinsic healing is inherent to the material, and it occurs due to physicochemical reaction of healing agents embedded into the material matrix. In contrast, extrinsic healing involves dispersing encapsulated healing agents within the matrix and, upon cracking, the healing agent in the capsules flows into and fills the gaps.

The second approach—autonomic and nonautonomic classification—depends on what triggers the healing process.

Autonomic self-healing does not require a stimulus to initiate healing because it is inherent to the system. On the other hand, nonautonomic self-healing requires an external trigger such as heat or light to initiate the healing process, which occurs through chemical reaction.

Self-healing mechanisms in ceramics

In ceramics, healing typically is based on a chemical oxidative reaction that is triggered when specimens are sintered or heat-treated. The process can be either intrinsic or extrinsic, depending on the specific ceramic material used.

In the paper, Hammood, Barber, and Wang give some details of the healing process for various ceramics, including the temperature range in which the healing mechanism takes place, the volume expansion that accompanies oxidation, and the bonding energy between crack faces.

Ultimately, they conclude that “TiO2, ZrO2, Al2O3, SiO2, HfO2, Nb2O5, and Y2O3 are among the most promising/effective oxides to have an effective healing process.”

Requirements for self-crack healing

The researchers end their paper by summarizing requirements for a complete and efficient self-crack-healing process, including

  • The healing must be triggered by cracking,
  • Healing occurs at high temperatures,
  • The strength of the healed zone must be equal to or superior to the base material,
  • The healing particles located at the crack walls should react with oxygen in the air to produce healing products, and
  • The volume between the crack walls should be filled with the self‐healing reaction products.

They also include requirements for the healing agent, specifically melting point, thermal mismatch, and volume expansion upon oxidation.

The researchers note that in some cases, cracks may be too large to repair through self‐healing alone. “In such cases, additional healing particles could be added to the crack before heating as shown in Figure 5 to assist the healing process,” they write.

In the future, they urge that additional research be carried out on localized heat sources and optimum use of additional self-healing particles to expand the applications and use of self-healing ceramics.

Figure 5 of the article, which shows steps for the preparation of self-crack-healing ceramics that require additional healing particles before heating. Credit: Hammood, Barber, and Wang; International Journal of Ceramic Engineering & Science (CC BY 4.0)

The open-access paper, published in International Journal of Ceramic Engineering & Science, is “A review of some of experimental and numerical studies of self‐crack‐healing in ceramics” (DOI: 10.1002/ces2.10071).