The Influences Of External Conditions On The Friction Properties For The Transition Metal Disulfides

With the rapid development of micro-nano manufacturing technology and nanomaterials science,the micro electro-mechanical systems(MEMS)also present new opportunities and challenges.Many two-dimensional(2D)materials,such as graphene and transition metal disulfides(TMDs)have been widely used in friction components and moving parts for MEMS.The molybdenum disulfide(MoS2)and tungsten disulfide(WS2)have also been used as solid lubricant and potential additive to base oils in aerospace and defense industry due to the excellent frictional properties.Further,friction is known to show the dependences on thickness of thin film,the relative sliding speed and relative humidity of the environment for various materials.However the influences of thickness,scanning velocity and relative humidity on the frictional properties are not yet clear,which seriously affects their stability in application devices.Hence,we focus on the influences of thickness,scanning velocity and humidity on the frictional properties.We propose the normal spring constant method to calibrate the lateral calibration factor for obtaining conveniently the friction force.In order to study the influences of mentioned factors on friction,the scanning velocity and relative humidity were set at the different scanning velocities and relative humidities,and the friction measurements were performed on the MoS2 and WS2 nanosheets.The main contents are given as follows:1.Distinguishing with the traditional cantilever mechanics method,we propose the normal spring constant method to calibrate the lateral calibration factor by using the normal spring constant obtained from atomic force microscopy(AFM)but not the Young’s modulus and the width of the cantilever.The special calibration grating is not indispensable any more for the wedge method,and the inaccurate cantilever’s Young’s modulus could be reasonably circumvented for the cantilever mechanics method.2.The MoS2 and WS2 nanosheets were successfully prepared by mechanical method and hydrothermal intercalation and exfoliation route.The AFM and Raman microscope were used to investigate the morphologies,thickness and crystalline structure.The adhesion force decrease monotonically with the increase of the nanosheet’s thickness for MoS2 nanosheets,and keep stable at the thickness larger than 6 layers.Under the different relative humidity,the adhesion force increase with the increasing of relative humidity.3.The measurements were performed for the influences of thickness,scanning velocity and humidity on the frictional properties.The friction forces and coefficients for MoS2 nanosheets and WS2 nanosheets decrease monotonically with the increase of the nanosheets’ thickness from 3 nm to 5 nm and keep stabilization at the thickness larger than 5 nm.The friction coefficients are of the constant values 0.007 ±0.0013 for MoS2 nanosheets and 0.013 ± 0.0020 for WS2 nanosheets at the thickness larger than 5 nm.The friction force is a constant value 1.7 nN at the scanning speed larger than the critical value 3.10 μm/s,while it logarithmically increases for the scanning speed less than the critical value.The friction force and friction coeficient increase with the relative humidity at the range of 30%～60%,and the friction coefficient is at the range of 0.010～0.013.