According to foreign media reports, hexagonal boron nitride (h-BN) is the "Iron Man" in 2D materials, and its crack resistance is so strong that it can violate the theoretical description that engineers still use it to measure toughness for a century. "What we observe in this material is amazing," said Lou, correspondent for the study at Rice University. "No one would have thought that this would happen to 2D materials. This is why it is so exciting. "

Lou explained the significance of this discovery by comparing the fracture toughness of h-BN and its cousin graphene. Graphene and h-BN are almost identical in structure. In each structure, atoms are arranged in an interconnected hexagonal plane lattice. In graphene, all atoms are carbon atoms, while in h-BN, each hexagon contains three nitrogen atoms and three boron atoms.

The carbon-carbon bond in graphene is the strongest in nature, which should make graphene the hardest material around. But there is a trap here. Even if only a few atoms are abnormal, the performance of graphene will change from extraordinary to mediocre. Lou pointed out that in the real world, no material is without defects, which is why fracture toughness or crack propagation resistance is so important in engineering.

"We measured the fracture toughness of graphene seven years ago, and it is actually not very resistant to fracture," Lou said. "If there are cracks in the lattice, a small load will destroy the material."

In short, graphene is very brittle. A.A.Griffith, a British engineer, published a groundbreaking theoretical study on fracture mechanics in 192 1, describing the failure of brittle materials. Griffith's work describes the relationship between the size of cracks in materials and the force required to make cracks grow.

The study of Lou 20 14 shows that the fracture toughness of graphene can be explained by Griffith's time test standard. Considering that the structure of hydrogen boron nitride is similar to graphene, it is expected to be brittle.

But this is not the case. The fracture resistance of hexagonal boron nitride is about 10 times that of graphene. Because the performance of this material in fracture test is so unexpected that it can't be described by Griffith formula.

"This work is so exciting because it reveals the inherent toughening mechanism of perfect brittle materials," said Gao Huajian, co-author of the study at Nanyang Technological University in Singapore. "Obviously, even Griffith could not foresee that the fracture behavior of two brittle materials with similar atomic structures would be so completely different."

Lou, Gao and their colleagues tracked the behavior of different materials and found a slight asymmetry because h-BN contains two elements instead of one. "Boron and nitrogen are different, so even if you have this hexagon, it is not exactly like a carbon hexagon because of this asymmetric arrangement," Lou said.

In addition, he also pointed out that the details of theoretical description are complicated, but the result is that cracks in h-BN tend to branch and turn. In graphene, the crack tip directly passes through the material. However, the lattice asymmetry of h-BN produces a "bifurcation" that can form branches.

"If the crack splits, it means it is turning. If you have this kind of steering crack, you basically need to consume extra energy to further drive the crack. Therefore, by making cracks more difficult to propagate, materials are effectively toughened, "Lou said.

Gao pointed out: "The inherent lattice asymmetry makes h-BN have a permanent tendency, that is, the moving crack will deviate from its path, just like a skier loses the ability to maintain a balanced posture and move in a straight line."

Because of its heat resistance, chemical stability and dielectric properties, hexagonal boron nitride has become an extremely important material in 2D electronics and other applications, which makes it not only a supporting foundation, but also an insulating layer between electronic components. Lou pointed out that the amazing toughness of h-BN also makes it an ideal choice for tear resistance of flexible electronics products made of 2D materials, which are usually very brittle.

"The niche area of 2D-based electronic products is flexible devices," Lou said. He said that in addition to applications such as electronic textiles, 2D electronic devices are thin enough to be used in more exotic applications, such as electronic tattoos and implants that can be directly connected to the brain.

"For this type of configuration, you need to ensure the mechanical strength of the material itself when bending," Lou pointed out. "The fracture resistance of h-BN is good news for the field of 2D electronics because it can be used as a very effective protective layer."

Gao said that these findings may also point to a new method of manufacturing tough mechanical metamaterials through asymmetric engineering structures.

"Under extreme loads, fracture may be inevitable, but its disastrous effects can be mitigated by structural design," Lou said.