If you’ve ever eaten from a plate before, then you’re definitely familiar with ceramics. These materials are known for their high heat resistance and excellent compressive strength (I mean, try pinching your last dinner plate if you really want to find out). As any regular dishwasher in the family would also know, these materials are also exceptionally prone to catastrophic failure. With their great strength comes brittleness; one fall—or even just a chip—can cause cracks to form and propagate, shattering the whole piece. This limits the applications where engineers can insert ceramics into to take advantage of their properties. A recent study, published in the journal Science Advances, is set to challenge that notion, though.
The team of researchers, headed by doctoral student Hemant Rathod from Texas A&M University, created ceramics that are called MAX phases, labeled for their usual chemical formula of Mn+1AXn: M is a transition metal, like titanium (Ti) and chromium (Cr); A is a IIIA or IVA element in the periodic table, like aluminum (Al) and silicon (Si); X is either carbon (C) or nitrogen (N). These MAX phases exist in atomically alternating layers; the X and M elements form one layer in an octahedral structure, while A forms the second, single planar layer. This particular MAX phase ceramic in the study is chromium aluminum carbide (Cr2AlC).
When a MAX phase ceramic starts cracking, its deformation propagates through the layers of material, forming what are called kink-bands. Ceramic crystals in the kink-bands rotate once exposed to stresses, distorting its internal structure and “deflecting” any stresses from the current crack propagation that may induce more cracks to form. These rotated crystals also “heal” the material, according to the authors, lending the Cr2AlC self-healing properties. As a plus, this particular MAX phase ceramic can perform its self-healing process at room temperature; regular self-healing ceramics often need elevated temperatures to do their function.
Said Dr. Ankit Srivastava, co-author and assistant professor in the Department of Materials Science and Engineering at Texas A&M: “What’s really exciting about MAX phases is that they readily form kink-bands under loading which can self-heal cracks even at room temperature, making them suitable for a variety of advanced structural applications. So far, self-healing of cracks in ceramics has only been achieved at very high-temperatures by oxidation and that is why self-healing of cracks at room-temperature by kink-band formation is remarkable.” Rathod added: “What’s really exciting is that this kinking or self-healing mechanism can occur over and over closing the newly formed cracks, thus delaying the failure of the material.”
The authors believe these MAX phase ceramics hold the key to the next generation of engineering applications that may require self-healing ceramics at relatively low temperatures, like jet engines and nuclear reactors. They also believe that the layered structure of MAX phases may also mean that unexplored self-healing properties may also be present in similarly layered atomic structures, even if they’re not ceramics.
According to Siddiq Qidwai, program director in the National Science Foundation’s Directorate for Engineering: “This study demonstrates the serendipity of the scientific process. […] We have had self-healing soft materials and polymer composites, and now, remarkably, ceramics.”(This very study was funded by the National Science Foundation.)
References
- Irving, M. (2021, September 10). Kinky class of ceramics self-heals cracks at room temperature. New Atlas. Retrieved September 27, 2021, from https://newatlas.com/materials/self-healing-ceramics-max-phase-room-temperature/
- Patel, D. (2021, August 31). Under loading ceramics self-heal cracks by forming kink-bands. Texas A&M University Engineering. Retrieved September 27, 2021, from https://engineering.tamu.edu/news/2021/08/msen-under-loading-ceramics-self-heal-cracks-by-forming-kink-bands.html
- Rathod, H. J., Ouisse, T., Radovic, M., & Srivastava, A. (n.d.). Room temperature crack-healing in an atomically layered ternary carbide. Science Advances, 7(33), eabg2549. https://doi.org/10.1126/sciadv.abg2549
