Charge density fluctuation determined the interlayer friction in bilayer MN4 (M = Be, Mg, and Pt)

Abstract

MN₄ (M = Be, Mg, and Pt) represents a new class of van der Waals materials. These materials are characterized by exceptional electrical and thermal conductivities, remarkable intralayer mechanical strength, and weak interlayer interactions, making them prone to shearing and slipping. Therefore, MN₄ has significant potential applications as a solid lubricant. However, until now, there have been only limited comprehensive theoretical investigations focusing on the frictional properties of MN₄ systems. Here, the frictional performances of MN₄ are systematically analyzed by applying first-principles high-throughput calculations. The results reveal that interlayer friction of MN₄ decreases from MgN₄ to BeN₄ and then to PtN₄. The friction is directly determined by charge density variations during the sliding processes. The periodic formation and breaking of quasi-σ bonds in bilayer MgN₄ leads to substantial variations in charge density and large interlayer friction. In contrast, the weak charge density alternations in PtN₄ lead to rather low frictions in PtN₄. Moreover, surface functionalization effectively diminishes friction within bilayer MgN₄, but amplifies interlayer friction within bilayer PtN₄, and under surface functionalization interlayer friction can be efficiently modulated by out-of-plane polarizations. Interestingly, HBr–MgN₄ exhibits two orders of magnitude lower COF compared to intrinsic bilayer MgN₄, leading to a phenomenon resembling superlubricity. These results significantly contribute to our understanding of the friction properties, offering valuable guidance for the practical implementation of MN₄ in solid lubricants.