Study On The Influence Of Substrate Bias On The Friction Properties Of Hexagonal Boron Nitride

Hexagonal boron nitride has good friction characteristics as well as excellent insulation and hydrophobicity.The research on the friction characteristics of hexagonal boron nitride under electric field is of great significance for its application as a tunable insulating lubricant in micro/nano-electromechanical devices.The nanofriction,adhesion and stick-slip of hexagonal boron nitride was studied under external electric field using atomic force microscopy.The results show that:the nanofriction increased after applying a bias voltage to the substrate.The increase of nanofriction under the positive bias was relatively greater than under the negative bias.The nanofriction on h-BN can be tuned stably,dynamically and reversibly by regulating the direction and magnitude of the external electric field.The change of adhesive force affected by substrate voltage is consistent with friction.Under the substrate electric field,the amplitude of the stick-slip motion of the tip on the surface of the hexagonal boron nitride becomes larger and the period decreases,resulting in a transition of stick-slip behavior from single-slip to multi-slip.This paper further theoretically studies the mechanism of the substrate electric field affecting the friction of hexagonal boron nitride.The contact area increases with the increase of the substrate electric field by Derjaguin-Müller-Toporov fitting the data of the variable load experiment under the substrate electric field.By establishing the theoretical capacitance model of tip-substrate,it is concluded that the expression of the relationship between the bias and the electrostatic force is a quadratic function of the left shift.Electrostatic interaction plays the dominant role in the increase of nanofriction,because of the enhancement of the electrostatic interaction between the tip and the substrate,resulting in a larger actual load and contact area of the tip.Both the inherent potential difference and the ice-like water layer confined between the Si O₂/Si substrate and h-BN layers contribute to the asymmetric influence of the electric field.The polarization electric field produced by the ordered arrangement of water molecules on the surface of silica affects the overall electric field strength.The improved Prandtl-Tomlinson model is used to find the substrate bias increases the interface barrier between the tip and the hexagonal boron nitride,and the movement of the tip requires greater energy consumption.This paper also studied the factors affecting the friction of hexagonal boron nitride surface under substrate bias.The results are as follows:1)annealing can reduce the frictional increment caused by the substrate bias,because the lack of ice-like water layer between the hexagonal boron nitride and the substrate can weaken the enhancement of the polarization electric field to the external electric field.2)Increasing the thickness of hexagonal boron nitride can also reduce the effect of substrate bias on friction.A thicker dielectric layer can enlarge the distance between the tip and the substrate,as well as the distance between the tip and the ice-like water layer,and reduce the overall dielectric constant,resulting in the reduction of electrostatic force on the tip.3)The decrease of the thickness of the substrate oxide layer can increase the friction increment caused by the substrate bias.The drop of hexagonal boron nitride and the change of the dielectric constant between the interfaces lead to the increase of electrostatic interaction on the tip.