Molecular Dynamics Simulation Of Friction Behavior On Rough Surfaces Of Polycrystalline Zirconium

Nuclear power is a low-carbon and efficient clean energy.The development of nuclear power is of great significance to replace traditional energy,reduce pollution,improve the environment and achieve sustainable development.Nuclear power equipment is a high-tech equipment with high safety operation requirements,and zirconium alloy is widely used in nuclear power system because of its good mechanical properties and corrosion resistance.The complex fretting wear failure of zirconium alloy cladding tube due to fluid-induced vibration,reducing the service life of equipment and endangering the safety of nuclear power system.A large number of scholars have carried out experimental macro studies on fretting wear of zirconium alloy.With the rapid development of modern computer technology,this thesis uses molecular dynamics method to simulate the friction behavior of zirconium and study the microscopic damage mechanism of zirconium.In this thesis,combined with fractal theory,a contact model between single rough peak and rough substrate was established based on polycrystalline zirconium substrate.The effects of the main factors such as velocity,temperature,surface roughness and random surface topography on the friction behavior of polycrystalline zirconium were studied by molecular dynamics simulation method.The law of friction force and wear loss during the friction was discussed.From the dynamic point of view,the plastic deformation evolution of zirconium substrate was systematically studied by dislocation extraction algorithm and atomic strain analysis method.From thermodynamic point of view,the plastic energy ratio R was used to measure the influence of adhesion effect and plowing effect during the friction.It was found that the larger the plastic energy ratio R was,the more obvious the influence of plowing effect was.This indicated that increased subsurface damage resulted in an increase in the average friction coefficient.The main conclusions are as follows:(1)In this thesis,a polycrystalline zirconium model with rough surface was established based on fractal theory,and an atom removal method based on polycrystalline substrate was proposed.The model used Weierstrass-Mandelbrot(W-M)fractal function as the boundary condition,and preliminarily obtained the rough surface by removing the atoms that didn’t meet the boundary conditions.Then combined filtering and relaxation to obtain the rough surface that can be used in molecular dynamics simulation.(2)The friction behavior between polycrystalline rough zirconium substrate and spherical-cap diamond tool was simulated at different speeds and temperatures.The results showed that friction coefficient and plastic energy ratio R increased with the increase of sliding speed in the range of 0.2～3?/ps,and there was no significant difference in wear loss.When the slip speed is 3?/ps,the plowing effect was significant and the subsurface damage of the substrate was serious.In the temperature range of 100 K to 900 K,the friction coefficient and the plastic energy ratio R decreased with increasing temperature,while the wear rate increased.At high temperatures,the adhesion effect predominated.(3)Rough substrates coupled with five root-mean-square roughness and three random phases were established,and the sliding process was simulated combined with the sphericalcap diamond tool.The results showed that the plastic energy ratio R and friction coefficient decreased with the increase of roughness,and the damage degree of subsurface layer was smaller when the surface roughness was larger.The random surface topography had no obvious effect on the trend of friction coefficient,but had a significant effect on the wear loss and the evolution behavior of grain boundaries during sliding.