[Image above] Credit: Ricardo Castro
Editor’s note: This report comes to us from Ricardo Castro, associate professor in the Department of Materials Science & Engineering at the University of California Davis. For more information about Castro and his UC Davis lab, visit the Nanoceramics Thermochemistry Lab website.
By Ricardo Castro
One of the major manufacturing and application challenges in ceramics to date is its characteristically low toughness. Although composites are known to improve toughness of ceramics, having single-phased ceramics with high toughness is desirable, but not easily attainable, given that many functional properties, such as optical, are based on monolithic products.
It was proposed that grain size reduction would improve toughness by allowing grains to slide against each other, much like woven polymers absorb impacts. However, it turns out that most nanoceramics do not show the predicted high toughness, because grain–grain interfaces show much higher friction than expected, and so defects cannot move easily along boundaries, leading to facile crack propagation.
A team of researchers from University of California, Davis, in collaboration with researchers from University of Illinois at Urbana-Champaign, have explored the weak interfaces in nanoceramics to demonstrate a new toughening mechanism for nanoceramics.
In fibrous composites, fibers create an alternative crack path that deflects cracks and improves toughness. In nanocrystalline ceramics, the U.C. Davis team proposed that the extensive grain boundary network is a dormant crack path waiting to be awakened. In other words, there is a great number of grain boundary possibilities for the cracks to propagate.
However, cracks typically go in one direction and are mostly linear, which lowers toughness. This is because grain boundaries are not all equal in crystalline materials. Some grain boundaries are weaker than others and hence, cracks mostly propagate through those that are easier to break, resulting in clean and linear cracks. It stands to reason, then, that eliminating disparity in grain boundary strength would remove a “path of least resistance” and lead to a more distributed or tortuous crack path, as shown in the image below.
Credit: Ricardo Castro
The team designed a way for all of the grain boundaries to be more alike. By using dopants that segregate to the grain boundaries, researchers showed they can reduce local energies and improve toughness of individual boundaries. However, more importantly, the researchers acknowledged the existence of weak as well as already strong boundaries. Therefore, they target the weakest grain boundaries primarily so they can be made as strong as the already strong boundaries. This causes all of them to be equally strong, and thus, cracks branch often as they propagate through grain boundaries and meet a triple joint (where three grain boundaries meet). A 20% improvement in toughness was observed. The researchers demonstrated the concept with zirconia as a model system, but there is no reason to believe the mechanism cannot be applied in other compositions.
Other than an increment to textbooks on toughening mechanisms, this method to improve toughness could impact design of more reliable monolithic ceramics. The mechanical stability and performance of many functional oxides, such as battery electrodes and capacitors, is limited due to the intrinsic brittleness of the material. This method could potentially enable impact resistance without compromising functional properties.
You can read more in Bokov et al. in the Journal of the European Ceramic Society, Volume 38, Issue 12, September 2018, Pages 4,260-4,267.
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