INVESTIGATION OF SURFACE DAMAGE AND FRICTION CHARACTERISTICS OF 2D MATERIALS

Abstract

Two-dimensional (2D) materials such as single-layer hexagonal boron nitride (h-BN), molybdenum disulfide (MoS2), and graphene have great potential for use as protective and solid lubricant coating layers for nanoscale devices. These coating layers are used to primarily reduce surface damage and friction generated at contacting interfaces between mechanical moving parts, which is the main source of energy dissipation and severe damage in the systems. Therefore, comprehensive understanding of surface damage characteristics and frictional behaviors of single layer h-BN, MoS2, and graphene is essential. In this work, surface damage characteristics of these single-layer materials were systematically investigated using atomic force microscopy (AFM)-based progressive force and constant force scratch tests. The adhesion strengths to substrate of these atomically thin materials were carefully evaluated based on their critical forces determined using progressive force scratch test. The evolutions of surface damages with respect to normal force were further investigated using constant force scratch test. The results show that single-layer h-BN, MoS2, and graphene strongly adhered to the substrate, which may in turn significantly improve their tribological performances. Furthermore, the defect formation induced by scratch test was found to differently affect the topography and friction force of these atomically thin materials, prior to the failure, which indicates their distinctive surface damage characteristics. Interestingly, the residual strains commonly observed at scratched areas may suggest the scratch tests-induced in-plane compressive strains were dominant over tensile strains, thereby leading to the buckling formation of these atomically thin materials mostly in front of the scratching tip and eventually failure with sufficient strains. These behaviors could be considered as the general failure mechanism of these atomically thin materials due to scratch test. As the number of layer increased, the tribological performance of atomically thin h-BN, MoS2, and graphene were found to significantly improve because of the increase in adhesion strength to the substrate and decrease in surface damage and friction force. Friction characteristics of these single-layer materials were further investigated under various conditions using friction force microscopy (FFM). The low friction characteristics of these single-layer materials could be clearly observed from their normal force-dependent friction results, and by fitting to Hertz-plus-offset model the interfacial shear strengths could be further estimated. We also found the logarithmic increase in friction with increasing sliding speed, and based on the thermally activated Prandtl-Tomlinson model, some fundamental parameters which governed nanoscale friction such as the effective energy barrier and the effective shape of the interaction potential could be determined. In addition, friction was found to generally increase with increasing relative humidity, which was attributed to the water diffusion-induced increase in puckering effect on frictional behaviors of these single-layer materials as relative humidity increased. Furthermore, friction of the thermally annealed single-layer materials was found to be significantly lower than those of the as-exfoliated ones. Overall, these findings in this work, including distinctive surface damage characteristics or general failure mechanism of atomically thin h-BN, MoS2, and graphene would be useful for the design of effective and reliable nanoscale protective and solid lubricant coating layers based on these materials. In addition, the results also suggest that these single-layer materials used as coating layers should be operated under dry environment and reasonably low speed to achieve low friction and prolong the lifetime of devices. The simple thermal annealing could be useful for many practical applications such as stabilization, packaging, and storage of the nanoscale devices-based on these single-layer materials.

Key works: Atomic force microscopy, graphene, h-BN, MoS2, nanoscale tribology.