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^1,2. This trade-off between strength and fracture toughness also extends into two-dimensional materials space^3,4,5. 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)^3,6. Hexagonal boron nitride (h-BN) is a dielectric two-dimensional material⁷ with high strength (about 100 gigapascals) and elastic modulus (approximately 0.8 terapascals), which are similar to those of graphene⁸. Its fracture behaviour has long been assumed to be similarly brittle, subject to Griffith’s law^{9,10,11,12,13,14}. 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.

如果散装材料可以承受高负荷而没有任何不可逆的损坏（如塑性变形），它通常很脆，可能会发生灾难性的故障^1,2。这种强度和断裂韧性之间的权衡也延伸到二维材料空间^3,4,5。例如，石墨烯具有超高的内在强度（约 130 吉帕）和弹性模量（约 1.0 太帕），但很脆，断裂韧性低（约 4 兆帕/平方根米）^3,6。六方氮化硼（h-BN）是一种介电二维材料⁷，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^8相似. 根据格里菲斯定律^{9、10、11、12、13、14 ，它的断裂行为长期以来一直被认为同样易碎}. 与预期相反，我们在此报告了单晶单层 h-BN 的高断裂韧性，其有效能量释放率比其 Griffith 能量释放率和石墨烯报道的能量释放率高出一个数量级。我们观察到单层h-BN中的稳定裂纹扩展，并获得了相应的抗裂曲线。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，裂纹偏转和分支反复发生，这从本质上使材料增韧并实现稳定的裂纹扩展。我们的原位实验观察得到理论分析的支持，表明单层 h-BN 具有更多的实际优势和潜在的新技术机会，例如为二维器件增加机械保护。