With their resistance to high temperatures and corrosion, ceramics are essential materials for bearings, nozzles, valves, and other industrial components. Yet their applications remain limited by some of their mechanical properties. Ceramics tend to be brittle, with low tolerance for strain. A ceramic with the strength and deformability of metals could be fabricated into, for instance, shock absorbers or buffers in high-temperature environments. Now researchers at Yanshan University in China report in Nature the synthesis of an unusually flexible ceramic material: twist-stacked boron nitride (TS-BN). It has a compressive strain that is an order of magnitude above that of conventional ceramics.
The key to the unique mechanical properties of TS-BN is its hierarchical nanostructure. The material consists of 2D layers of hexagonal boron nitride ceramics, a commonly used class of materials with low compressive strength and limited capacity for deformation. A surge of recent research has shown that stacking sheets of van der Waals materials, such as boron nitride, and introducing a twist between layers can lead to unique material properties (see the article by B. Andrei Bernevig and Dmitri Efetov, Physics Today, April 2024, page 38).
The researchers fabricate the TS-BN by sintering onion-like boron nitride nanoparticles into layers, which are stacked and twisted to form laminated plates measuring about 200 nm long and 40 nm thick. The final material is a 3D matrix of interconnected nanoplates that are oriented in all directions.
After investigating the structural properties of their new material, the researchers performed a series of mechanical tests. They found that at room temperature, TS-BN withstands axial compressive strains up to 14%, a nearly 10-fold increase over such ceramics as diamond, graphite, and silicon nitride, and it exhibits plastic behavior far exceeding other ceramics. Its compressive strength surpasses that of conventional hexagonal boron nitride ceramics by a factor of 6–10.
The strength and plasticity of the newly synthesized boron nitride ceramic results from the integration of twisted-layer 2D material and interlocked 3D structure. Because the intralayer atomic bonds in van der Waals materials are much stronger than the bonds between layers, the layers can easily slide, which can lead to increased elasticity and plasticity. Twisting between layers disrupts the crystalline symmetry, increases the layer spacing, and helps the material better flex and deform. Yet because of the random 3D orientation of the nanoplates, the deformations do not propagate from plate to plate, thus inhibiting the spread of cracks.
Although the new ceramic was assembled with standard sintering methods, its wide-scale application may be limited by its dependence on the precursor nanoparticles from which the TS-BN is constructed, says Gurpreet Singh, director of the Nanoscience and Engineering Lab at Kansas State University. The researchers created the onion-like precursor particles by using chemical vapor deposition, which is generally unsuited for production of larger or bulk ceramics samples.
With further advancements, TS-BN could replace weaker materials, such as rubber, for use in high-performance sealing parts or as lightweight shock absorbers, says study coauthor Zhisheng Zhao. The researchers are also exploring the possibility of creating similar hierarchical structures in other layered ceramics. Recently they have experimented with graphite-like carbon bulk materials. (Y. Wu et al., Nature 626, 779, 2024.)
