Research On The Formation Mechanism,lubrication Mechanism And Measurement Technique Of Potassium Stearate Films On Diamond-like Carbon Surface

At present,the aerospace inertial navigation system places a higher demand on the performance of gyroscope.The service life and reliability of inertial navigation system is mainly determined by the operational lifetime of gyroscope,which is thus the important index for evaluating the performance of gyroscope.The aerodynamic gyro motor is the most important component in the gyroscope.Also,the operating lifetime of gyroscope is closely related to the starting performance of aerodynamic gyro motor.During the starting stage of aerodynamic gyro motor,sliding contacts between moving surfaces would occur because of the insufficiency in air bearing force.If the friction pairs within the aerodynamic gyro motor operates under poor lubricating conditions,the sliding contact would accelerate the wear of aerodynamic gyro motor parts.Furthermore,when the resistance between moving surfaces is too high,the starting torque of aerodynamic gyro motor even cannot overcome the starting resistance,and thus results in the failure of entire aerospace system.To solve this problem,the boundary lubrication technique has been widely employed to improve the starting performance of aerodynamic gyro motor in numerous countries.Up to present,researches on the formation mechanism and boundary lubrication mechanism of surface monolayers have just started in our country,which cannot provide helpful guidelines for the corresponding technology.And the preparation of surface monolayers on aerodynamic gyro motor parts is still completed with the manual method.Measurement and characterization techniques are not yet available,and automation level is still quite low.For these reasons,the controllability of motor starting performance is quite poor,which has limited the further development of aerospace inertial navigation techniques in our country.Therefore,this thesis would provide in-depth investigations about the formation mechanism,boundary lubrication mechanism,measurement techniques and preparation process of surface monolayers.Firstly,the diamond-like carbon(DLC)film over aerodynamic gyro motor parts,which serves as the adsorptive substrate for surface monolayers,was modeled.The lubricant molecule,potassium stearate,and multi-molecular clusters were also modeled.Based on this,the molecular dynamic(MD)model for studying the formation mechanism of potassium stearate monolayers on DLC substrate was obtained.Based on the existing research,the formation method of potassium stearate films on DLC substrate surface was preliminary analyzed.The interatomic interactions during the formation process of potassium stearate monolayers were investigated by analyzing the potential functions in MD model.Then,the substrate-molecule interactions were compared with molecule-molecule interactions.The free energy change during the adsorption/desorption process of single molecule and multiple molecules was studied through potential of mean force(PMF)calculations.Furthermore,the formation mechanism of potassium stearate films on DLC surfaces was investigated and discussed in detail.With the fundamental understanding about the formation mechanism of potassium stearate films on DLC surface,the influence of substrate surface state on surface monolayers was studied.For the convenience of analysis,the substrate surface morphologies were divided into the square groove,triangle groove and semi-circle groove.And thus the influence of geometric shape and geometric size of surface morphologies on potassium stearate monolayers was studied.Based on this,the MD model for investigating the boundary lubrication mechanism of potassium stearate monolayers on DLC surface was obtained.Through the MD simulation,the effect of molecular number and normal load on the structural details and frictional behavior of boundary lubrication films was investigated,and the kinematic behavior of surface monolayers during the boundary lubrication process was studied.Through the above analysis,detailed discussions about the boundary lubrication mechanism of potassium stearate monolayers on DLC surface were provided.Afterwards,measurement techniques for characterizing the molecular adsorption state of potassium stearate monolayers were studied.The scanning electron microscope(SEM)and atomic force microscope(AFM)are appropriate method for surface characterization under different scale.Therefore,the SEM combined with AFM was employed to characterize the surface monolayers during the different stage of preparation process,and the influence of preparation process on the surface morphology of potassium stearate films was investigated.Then,the contact-angle meter was used to measure the contact angle over different sample surfaces,and thus the corresponding surface free energy can be evaluated.For the purpose of providing effective methods for studying the frictional behavior of surface monolayers,the nanoscale tribological behavior of surface monolayers was first investigated through the AFM.Comparisons were made between the surface nanofriction over different surface regions,and thus the microscopic lubrication mechanism of surface monolayers was investigated experimentally.Through the pin-on-disk tribotests,the macroscale tribological behavior of surface monolayers was studied,and the influence of normal loads on the friction coefficient of surface monolayers and DLC surfaces was investigated.The above investigations can provide effective experimental methods for the following research about the monolayer preparation process.Finally,based on the research about the formation mechanism,lubrication mechanism and measurement techniques of potassium stearate films,the effect of smearing process on potassium stearate monolayers was further investigated through the MD simulation method,and then a specially designed monolayer preparation process for aerodynamic gyro motor parts was proposed.In order to improve the controllability of monolayer quality,a specially designed device was used for the monolayer preparation.Based on orthogonal tests,influences of preparation factors on the lubrication ability and process stability of monolayers were investigated in detail.Thus,the optimal parameter combinations for monolayer preparation were obtained.Then,the surface state,surface nanofriction and thickness information of surface monolayers prepared with the optimized preparation parameters were characterized with the AFM.Also,the lubrication performance of surface monolayers prepared with the optimal parameter combinations was analyzed and verified through the tribotests.