| Ultrasonic motors(USM)is a new type of actuator that utilizes the inverse piezoelectric effect of piezoelectric ceramics and the movement are obtained by means of frictional contact forces drived by vibration of the elastic stator.It was widely used in the aerospace,weaponry equipment,biomedical device and intelligent robot because of many advantages,such as no electromagnetic interference,fast response,power self-locking,light weight,flexible structure,and high precision.However,USM transmits the power via friction between the stator and rotor in the process of motion conversion of the friction interface,in which wear is inevitable.It essentially determines the output performance,such as torque,velocity stability,environmental suitability and service life of USM.The friction material should not only match the stator vibration characteristics,but also meet the requirements of friction and wear under different conditions.It is challengable to develop suitable friction materials with high performance.The systematical study on the friction and wear mechanisms from theoretical and experimental methods is significant to the develop of USM.In this study,the effect of nano-fillers,vacuum,load,velocity,temperature and humudity on the tribological properties of polyimide(PI)nanocomposites were systematically studied to explore the friction and wear mechanisms from a smaller scale.Firstly,the tribo-model of friction materials sliding with copper is built according to the actual contact of USM and study the effect of modifiers on the tribological properties of friction materials.Then,the friction and wear behavior of polyimide nanocomposites under different environments and conditions are evaluated to analyse their dynamic state from an atomic level during the process of wear.Finally,the wear mechanisms are discovered by monitoring the variations of friction coefficient,wear rate,relative concentration,velocity,temperature and energy of system.This study will provide a theoretical guidance for designing high performance friction materials for USM.And the main research contents are as follows:1.Firstly,the development of friction materials of USM is summarized and analyzed.Then,the novel design methods are considered based on the traditional modification on the friction materials.Based on the normal measurement on the mechanical and tribological properties of friction materials from macroscopic characterization,the molecular dynamics simulations are employed to evaluate and predict the performance of friction materials with advanced nano-fillers under different conditions.Finally,the wear mechanisms will be discovered from both experiments and simulations.2.The polyimide with excellent performance is selected as polymer matrix of friction material of USM and modified by filling cabon nanotube,graphene and zirconia nanaparticles.The geometric optimization and dynamic balance are carried out after building PI nanocomposites model with multidimensional modifications.The simulated results indicate that the Young’s modulus and shear modulus of polyimide are increased by 169.88% and 126.78% after multi-dimentional modification,respectively.Then,the electrostatic adsorption and Van der Waals(VDW)force between PI molecular and nano-fillers were calculated and analysized to reveal reinforcement mechanisms of multi-dimentional modification from an atomic level.3.According to the characteristics of the friction interface material of the USM,the model of the friction between the PI nanocomposite material and the copper was established.The simulated results show that the friction coefficient of PI nanocomposite decrease by 59% without obvious wear after multi-dimentional modification.By experimental measurement,the friction coefficient and wear rate also decrease by 33.3% and 62.3%,respectively.Importantly,the interaction between polyimide molecule,the nano-modifier and the copper atom is analyzed by obtaining the radius distribution function,bond energy,relative concentration,temperature rise on the frictional interface.Combined with the variations of the dynamic state of friction materials,the adhesive wear mechanisms are explored through analysizing the electrostatic adsorption and VDW force.4.The effect of vacuum,temperature,load,velocity and humidity on the tribological properties of friction materials are studied based on the application of USM.The MD simulated results show that the temperature on the frictional interface reach to 473.9 K under the high vacuum of 1×10-6 Pa,which is close to Tg of PI Matrix and contribute to the PI molecular soft and aggravate adhesive wear.The MD simulation also predicted the limitied service conditions of PI composite according to the degree of wear,such as 400 K,0.15 (?)/ps and 0.4 GPa.On the contrary,the friction coefficient will decease by 75.2% without obvious wear when the water layer forms on the frictional interface under the high humidity environment.The dynamic state and adhesive wear mechanisms under different conditions are analysized by MD simulations after considering the molecular interaction,energy variation,glass transition temperation of polyimide,dry friction and water lubrication.This study can provide a theoretical foundation for designing high performance friction materials with high load,speed,vacuum,temperature and wear resistance. |
PDF Full Text Request