Construction of a continuously layered structure of h-BN nanosheets in the liquid phase via sonication-induced gelation to achieve low friction and wear

Herein, to endow h-BN nanosheets with gelling ability, a diurea compound was decorated on the h-BN nanosheets via designed adsorption and in situ reaction processes. The prepared h-BN-based gelator, BTO, exhibited excellent dispersibility in non-polar liquid media, and the gelation of BTO dispersions could be readily triggered by ultrasonic treatments. The sol-gel transformation of the system was found to be highly reversible by stirring and sonication. Based on the investigation on the self-assembly behavior of BTO nanosheets in the liquid phase, it was proposed that a continuous and layered structure formed by BTO during sonication was the key factor for the gelling properties of these nanosheets. The viscoelasticity of the sonication-induced gel was studied using a rheometer. Tribological evaluations show that the prepared h-BN nanogel exhibits outstanding lubricating performances, and more importantly, it has been proved that the gel state of the h-BN nanosheets provides superior and more reliable lubricating performances than the corresponding dispersion state under certain conditions; this can be ascribed to the formation of a continuous and uniform structure of modified h-BN nanosheets during gelation. Thus, this study not only clarifies the key role of the assembly structure in the tribological performances of 2D nanomaterials, but also demonstrates the potential of gelation in improving the macroscopic friction reduction and wear resistance of 2D nanomaterials.

Robust superlubricity by strain engineering

Structural superlubricity, a nearly frictionless state between two contact solid surfaces, has attracted rapidly increasing attention during the past few years. Yet a key problem that limits its promising applications is the high anisotropy of friction which always leads to its failure. Here we study the friction of a graphene flake sliding on top of a graphene substrate using molecular dynamics simulation. The results show that by applying strain on the substrate, biaxial stretching is better than uniaxial stretching in terms of reducing interlayer friction. Importantly, we find that robust superlubricity can be achieved via both biaxial and uniaxial stretching, namely for stretching above a critical strain which has been achieved experimentally, the friction is no longer dependent on the relative orientation mainly due to the complete lattice mismatch. The underlying mechanism is revealed to be the Moiré pattern formed. These findings provide a viable approach for the realization of robust superlubricity through strain engineering.