Department of Materials Science and NanoEngineering, Rice University, Houston, TX, USA.
Department of Mechanical Engineering, University of Maine, Orono, ME, USA.
Nature. 2021 Jun;594(7861):57-61. doi: 10.1038/s41586-021-03488-1. Epub 2021 Jun 2.
If a bulk material can withstand a high load without any irreversible damage (such as plastic deformation), it is usually brittle and can fail catastrophically. This trade-off between strength and fracture toughness also extends into two-dimensional materials space. For example, graphene has ultrahigh intrinsic strength (about 130 gigapascals) and elastic modulus (approximately 1.0 terapascal) but is brittle, with low fracture toughness (about 4 megapascals per square-root metre). Hexagonal boron nitride (h-BN) is a dielectric two-dimensional material with high strength (about 100 gigapascals) and elastic modulus (approximately 0.8 terapascals), which are similar to those of graphene. Its fracture behaviour has long been assumed to be similarly brittle, subject to Griffith's law. Contrary to expectation, here we report high fracture toughness of single-crystal monolayer h-BN, with an effective energy release rate up to one order of magnitude higher than both its Griffith energy release rate and that reported for graphene. We observe stable crack propagation in monolayer h-BN, and obtain the corresponding crack resistance curve. Crack deflection and branching occur repeatedly owing to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which intrinsically toughens the material and enables stable crack propagation. Our in situ experimental observations, supported by theoretical analysis, suggest added practical benefits and potential new technological opportunities for monolayer h-BN, such as adding mechanical protection to two-dimensional devices.
如果一种块状材料能够承受高负荷而不造成任何不可逆的损坏（如塑性变形），那么它通常是脆性的，可能会灾难性地失效。这种强度和断裂韧性之间的权衡也延伸到二维材料空间。例如，石墨烯具有超高的本征强度（约 130 吉帕斯卡）和弹性模量（约 1.0 太帕斯卡），但脆性大，断裂韧性低（约 4 兆帕每平方根米）。六方氮化硼（h-BN）是一种介电二维材料，具有高强度（约 100 吉帕斯卡）和弹性模量（约 0.8 太帕斯卡），与石墨烯相似。其断裂行为长期以来一直被认为是脆性的，符合格里菲斯定律。与预期相反，我们在这里报告了单晶单层 h-BN 的高断裂韧性，有效能量释放率比其格里菲斯能量释放率和报道的石墨烯高一个数量级。我们观察到单层 h-BN 中稳定的裂纹扩展，并获得了相应的裂纹阻力曲线。由于裂纹尖端的不对称边缘弹性特性和裂纹扩展过程中的边缘交换，裂纹偏转和分叉反复发生，从而使材料本质上得到增强，并实现了稳定的裂纹扩展。我们的原位实验观察结果得到了理论分析的支持，这表明单层 h-BN 具有额外的实际效益和潜在的新技术机会，例如为二维器件提供机械保护。
