Modeling And Analysis Of Fractal Nanoscale Surfaces

The micro-nano morphology and contact characteristics directly affect the tribological behavior between the frictional contact surfaces.With the advancement of nano-fabrication technology,the size of components has developed to the nanoscale,making the micro-morphology more prominent on the friction and wear characteristics between contact surfaces.The surface structure in engineering usually exhibits fractal characteristics.However,at the nanoscale,the model is made up of atoms and the surface structure of the model is no longer continuous.The method of directly using the fractal theory to establish a rough surface fails at this scale.Simplifying a nano rough surface to a smooth surface or a surface of a regular geometric object will result in the loss of much important information on the actual surface.Based on this,this thesis researches and experimentally verifies the digital characterization method of the nano fractal surface,shows the method of determining the modeling parameters,and uses the established model to perform the nano contact analysis of a rough surface and a rigid plane.In this thesis,an atom deletion method is proposed.Through the discrete Fourier transform and filtering of the fractal rough surface information,the fractal rough surface model under nanoscale is established,and the complete flow of modeling and analysis is listed,the effectiveness of this method is verified by experiments.The research shows that the non-filtered nano fractal surface model in the simulation process will appear suspended atom groups which will make the model unsuitable for simulation.But the filtering can improve this situation,and the filter's passband radius D0 has a great impact on the surface roughness.On the one hand,the smaller Do is,the more stable the value of Sa,Sq and the height distribution of asperities are after the generated surface's energy minimization and relaxation.On the other hand,as D0 decreases,the fractal dimension of the surface will gradually decreases,and the more rough details of the surface will be lost.In order to obtain a stable nano fractal model with more fractal features,a comprehensive consideration of changes in surface parameters is required.Based on the above modeling and analysis theory,in this thesis,a molecular dynamics model of a fractal rough substrate and a rigid plane is established,and the contact behavior between a nano fractal model and a rigid plane is also studied.The research shows that after the nano fractal model relaxes,a large number of crystal defects will appear in the asperities.The forms of defects mainly consist of line defects and stacking faults.In addition,some of the single asperities have a forest dislocation at the top.Since the height of the asperities is consistent with the Gaussian distribution,when the rigid plane is pressed into the asperities,the change of the contact area will show continuity,and there will be no abrupt changes,so the change of the contact force will show this continuity too.The contact force generally has an exponential relationship with the contact area.During the contact process,there are cases where small contact points are combined with each other to form a large contact point.This combination can relieve the stress at the center of the rough peak.An analysis of the stress near the surface layer of a rough substrate shows that stress concentration areas are mainly located in two locations:one is near the asperities with higher initial height,the other is located at the peaks and valleys where the original rough peaks are densely distributed.The research of this thesis will contribute to establishing a comprehensive nano contact model which is closer to the actual working condition and will also lay a certain foundation for revealing the friction and wear mechanism under nanoscale.