Castilho Cintia J, Li Dong, Xie Yiheng, Gao Huajian, Hurt Robert H
School of Engineering, Brown University, Providence, RI, USA.
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.
Carbon N Y. 2021 Mar;173:410-418. doi: 10.1016/j.carbon.2020.10.079. Epub 2020 Oct 27.
Liquid-phase deposition of exfoliated 2D nanosheets is the basis for emerging technologies that include writable electronic inks, molecular barriers, selective membranes, and protective coatings against fouling or corrosion. These nanosheet thin films have complex internal structures that are discontinuous assemblies of irregularly tiled micron-scale sheets held together by van der Waals (vdW) forces. On stiff substrates, nanosheet vdW films are stable to many common stresses, but can fail by internal delamination under shear stress associated with handling or abrasion. This "re-exfoliation" pathway is an intrinsic feature of stacked vdW films and can limit nanosheet-based technologies. Here we investigate the shear stability of graphene oxide and MoSe nanosheet vdW films through lap shear experiments on polymer-nanosheet-polymer laminates. These sandwich laminate structures fail in mixed cohesive and interfacial mode with critical shear forces from 40 - 140 kPa and fracture energies ranging from 0.2 - 6 J/m. Surprisingly these energies are higher than delamination energies reported for smooth peeling of ordered stacks of continuous 2D sheets, which we propose is due to energy dissipation and chaotic crack motion during nanosheet film disassembly at the crack tip. Experiment results also show that film thickness plays a key role in determining critical shear force (maximum load before failure) and dissipated energy for different nanosheet vdW films. Using a mechanical model with an edge crack in the thin nanosheet film, we propose a shear-to-tensile failure mode transition to explain a maximum in critical shear force for graphene oxide films but not MoSe films. This transition reflects a weakening of the substrate confinement effect and increasing rotational deformation near the film edge as the film thickness increases. For graphene oxide, the critical shear force can be increased by electrostatic cross-linking achieved through interlayer incorporation of metal cations. These results have important implications for the stability of functional devices that employ 2D nanosheet coatings.
剥离的二维纳米片的液相沉积是新兴技术的基础，这些技术包括可书写电子墨水、分子屏障、选择性膜以及防污或防腐蚀保护涂层。这些纳米片薄膜具有复杂的内部结构，它们是由范德华（vdW）力结合在一起的不规则平铺的微米级片材的不连续组装体。在刚性基板上，纳米片vdW膜对许多常见应力具有稳定性，但在与处理或磨损相关的剪切应力下可能会因内部分层而失效。这种“再剥离”途径是堆叠vdW膜的固有特征，可能会限制基于纳米片的技术。在这里，我们通过对聚合物-纳米片-聚合物层压板进行搭接剪切实验，研究了氧化石墨烯和MoSe纳米片vdW膜的剪切稳定性。这些三明治层压结构在混合内聚和界面模式下失效，临界剪切力为40 - 140 kPa，断裂能为0.2 - 6 J/m。令人惊讶的是，这些能量高于连续二维片材有序堆叠的平滑剥离所报道的分层能量，我们认为这是由于纳米片膜在裂纹尖端拆卸过程中的能量耗散和混沌裂纹运动所致。实验结果还表明，膜厚度在确定不同纳米片vdW膜的临界剪切力（失效前的最大载荷）和耗散能量方面起着关键作用。使用具有薄纳米片膜边缘裂纹的力学模型，我们提出了剪切到拉伸失效模式的转变，以解释氧化石墨烯膜而非MoSe膜的临界剪切力最大值。这种转变反映了随着膜厚度增加，基板约束效应减弱以及膜边缘附近旋转变形增加。对于氧化石墨烯，通过层间掺入金属阳离子实现的静电交联可以增加临界剪切力。这些结果对采用二维纳米片涂层的功能器件的稳定性具有重要意义。
