Molecular Dynamics And Experimental Research On Sliding Friction And Cavitation Behavior Of Polymer Materials In Water Environment

Polymer materials have been widely used in water environments as friction pair parts to replace metal materials because of their high elasticity,low density,high strength,high toughness,corrosion resistance,excellent self-lubrication,and mechanical processing properties.However,the friction pair parts of polymer materials that have been used in the water environments for a long time will change the physical and chemical state of the material surface due to water absorption and cavitation,which will affect the service life of polymer materials.Correspondently,there are few studies and reports about polymer friction pair materials in these aspects.In this paper,typical polymer materials were taken as the research object,and molecular dynamics simulation+ experimental methods were adopted to carry out the following:(1)The relationship between the tribological properties and the microcosmic properties of the friction interface(including the molecular structure,physical and chemical changes,etc.)of polymer materials in the process of sliding friction;(2)The dynamic characteristics of microscopic cavitation collapse in cavitation phenomena and the mechanism of cavitation damage of polymer materials;(3)The microscopic water absorption mechanism of polymer materials and the influence mechanism of water absorption on physical and chemical properties,mechanical properties,and tribological properties of materials are explored and studied.This work can lay a theoretical foundation and technical guidance for the design and application of polymer materials in wading engineering equipment.The research contents and main conclusions of this thesis are as follows:1.The molecular dynamics models of sliding friction in solid-solid and solid-liquid-solid contact forms were established using joint atomic and all-atomic methods,respectively,in the case of ultra-high molecular weight polyethylene(UHMWPE)with a single-chain structure.From the mesoscopic to the micro-atomic scale,the molecular chain energy,conformation,and physical and chemical changes of the friction interface during the friction process of polyethylene materials under different load and temperature conditions are studied.The results of the study found that the state of polyethylene(glassy state or rubbery state)has a significant effect on the normal force and friction force.The friction force is mainly composed of the deformation friction force in the cohesive zone and the adhesive friction force in the interface zone.The deformation friction force occupied the leading position,and the adhesive friction takes the secondary position;During the friction process,the surface material will undergo elastic and plastic deformation,and then wear.The elastic deformation energy is mainly related to the Van der Waals interaction energy between molecular chains,and the plastic deformation energy is mainly related to the bond angle energy and dihedral energy of molecular chains;During the sliding friction process of polyethylene,the molecular chains are stretched and oriented along the sliding direction,and aggregated and oriented along the loading direction;The flexibility of the molecular chain increases with the increase of temperature;During the friction process in the water environemt,the molecular chain structure of polyethylene has better chemical stability than polyoxymethylene.In the case of also meeting the requirements of tribological use,the ultra-high molecular weight polyethylene structure is more suitable for the friction pair engineering application than the polyoxymethylene in the water environment.2.The cavitation dynamics characteristics,such as the collapse time,the collapse energy,the strength of the collapse jet,and the mechanical and chemical effects of the collapse,were studied by molecular dynamics simulation.The research results show that:the higher the impact velocity,the shorter the collapse time of the cavitation bubble,and the higher the jet velocity generated;The jet generated by the collapse of the cavitation bubble will form a "fist"-like secondary water hammer impact;The impact pressure is proportional to the impact velocity and inversely proportional to the diameter of the cavity;When the impact velocity of water molecules is 1.0 km/s,the ice-VII structure phase change occurs in the water molecular structure.In addition,the cavitation erosion performance of five typical polymer materials was experimentally studied using ultrasonic vibration cavitation erosion equipment.The results show that the cavitation erosion resistance of UHMWPE and polyamide 6(PA6)materials is better than that of polyether ether ketone(PEEK),polytetrafluoroethylene(PTFE),and polyformaldehyde(POM)materials.The "water cushion" effect formed by surface cavitation pits can slow down the cavitation erosion of materials;When metal and polymer materials are mutually cavitation,the degree of cavitation corrosion of metal materials is directly related to the hardness of polymer materials.Polymer materials with lower hardness are easier to absorb cavitation impact energy and reduce the degree of cavitation damage to the mating parts.3.Using molecular dynamics and experimental methods to study the change mechanism of water molecules in the microscopic molecular structure of polymer materials and the influence mechanism of water absorption on materials' physical,chemical,mechanical and tribological properties.The research results show that: due to the hydrogen bonding force,water molecules are distributed in clusters in UHMWPE and PTFE,while they are evenly distributed in PA6,POM,and PEEK;The role of hydrogen bonding force makes it difficult for water molecules to diffuse inside PA6,POM,and PEEK,resulting in higher water absorption.UHMWPE and PTFE are not easy to form hydrogen bonds with water molecules,and the water absorption mass fraction is low;The water absorption mass fraction of polymer material has a power function relationship with the immersion time;The higher the water absorption,the more the surface hardness decreases,the more serious the degree of plasticization,and the decrease in mechanical properties and wear resistance.