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.