Dynamic Features And Experimental Investigations In Boundary Lubrication

The high-tech equipment is crucial to the national development.The components with high precision and high durability have been widely used in the industrial fields,such as the aviation industry,high-speed train system,and wind power system,and determine the performance and reliability of high-tech equipment.The key elements industry has large-scale production but low technical substance in China,which restraints the national industrial development In addition,modern engines are required to have small volume,light weight and long service life.Downsizing would maximise tribological effects since the load will be carried by smaller components.The key elements often operate under high speed,high temperature and highly dynamic loading conditions,resulting in being run in the boundary lubrication regime for a large part of their operational cycle.It is well established that the tribofilm can be formed due to tribochemical reactions between the surfaces and the lubricant additives under the boundary lubrication regime The tribofilm buildup has a direct impact on friction and wear evolutionIt is boundary lubrication,as a final safeguard,that protects friction pairs from scuffing failure.Frictional pairs tend to undergo the collapse of the boundary film before adhesive wear.Thus,it is necessary to develop the boundary lubrication theory,which plays an important role in the explanation to surface failure mechanisms and the prediction on the lubrication performance,more importantly,helps engineers for the development of optimized energy-efficient systems.However,there have been no sophisticated theories and mathematical tools which can predict the friction and wear with consideration of dynamic features in the boundary lubrication regime.Why does research continue at such a pace?The answer is fourfold.Firstly,it is very difficult to understand the mechanism of transient boundary lubrication and propagation of scuffing in the micro scale,based on the present experimental techniques and theoretical analyses.Secondly,the boundary film is composed of physisorbed film,chemisorbed film and chemically reacting film,and involves many factors and complicated interactions.Thirdly,there are dynamic processes in the regime of boundary lubrication which involves the tribofilm formation and removal.This is the most remarkable challenge and an enigma in the fundamental study of friction.Fourthly,there is lack of sophisticated framework to predict the wear and friction in the boundary lubrication,based on the dynamic process of tribofilm buildup during rubbingPart 1 introduced the current status and developments of boundary lubrication and its application in the industry The challenges and bottleneck problems in the experimental and theoretical investigations on the tribochemical reaction were outlined.Five major research contents,including the digital surface generation,the elastoplastic deformation of asperities and flash temperatures,experimental studies on the tribochemical reactions of the tribofilm,experimental studies on the tribofilm removal,as well as the numerical modelling of tribofilm buildup and its effect on the boundary friction,have been establishedIn Part 2,a new model of rough surface generation based on wavelets was proposed,which can simulate isotropic and anisotropic Gaussian rough surfaces.With combination of Johnson translator system,this model can generate non-Gaussian rough surfaces with given skewness and kurtosis.The results and associated analyses show that this method can adequately produce rough surfaces whose statistical characteristics match the prescribed values Furthermore,the three-dimensional bearing area curve was defined,and it is better than the two-dimensional one Then,the defined micro-asperity curve and micro-cavity curve can provide the basis for the future researches on the analyses of micro-lubrication,micro-friction and micro-wearIn Part 3,a deterministic contact model of rough surfaces was established which included the elastoplastic deformation of the asperities,the flash temperature and the micro scuffing.Some important parameters,e.g.real contact area,contact pressure and plastic deformation,were calculated.Junction growth and its variations with the sliding friction coefficient under different Tabor constants were also investigated.The rate of junction growth increases as the sliding friction coefficient increases.The maximum von Mises stress increases and its location moves upward to the surface when considering the interfacial shear stress.Then,the effect of junction growth on the flash temperature was particularly investigated,which has not been explored by previous numerical studies.The results indicate that the maximum flash temperature occurs exactly where the contacted asperities undergo massive plastic deformation,which numerically verifies that surface temperature spikes are associated with the agglomerations of neighboring asperities contactsIn Part 4,Raman microscopy and Atomic Force Microscopy(AFM)have been used with the aim of understanding the link between the friction behavior and the MoDTC/ZDDP tribofilm formation and removal Tribotests were coupled with a collection of ex-situ Raman intensity maps to analyse the molybdenum disulfide(MoS2)tribofilm build-up.Post-test AFM analyses were implemented on the ball wear scar to acquire the average MoDTC/ZDDP tribofilm thickness.A good correlation was achieved between the friction coefficient measurements and Raman maps when using a linear relationship between the microscopic friction and the local amount of MoS2 tribofilm.After a rapid increase,the average MoDTC/ZDDP tribofilm thickness levels out to a steady state as the friction drop ceases.This is the first study that sets out a framework to link MoS2 amount and coverage to the friction behavior,providing the basis for developing numerical models capable of predicting friction by taking into account tribochemistry processesIn Part 5,a series of tribotests were run to investigate the effects on the MoS2 tribofilm removal behavior and associated friction coefficient performance.The results show that the oil temperature,load,rubbing time as well as the lubricants can affect the MoS2 tribofilm removal and subsequently have influence on the friction increase.In-situ Raman spectra analyses were carried out to detect the MoS2 tribofilm removal.The removal rate of MoS2 from tribofilms,obtained at different temperatures,suggests that the MoS2 tribofilms are much easier to be removed from tribocontacts compared to anti-wear ZDDP tribofilmsIn Part 6.a semi-deterministic numerical friction model was proposed to take into account the MoS2 tribochemical reaction dynamics in the boundary lubrication regime.This model consists of the MoS2 tribofilm dynamics model and contact mechanics model.In order to obtain the formation and removal rates of MoS2 tribofilm,a specific calibration methodology was developed to couple the experimental data with the numerical models.Then,the full numerical procedure was implemented under rubbing of two real rough surfaces.The simulation results show how the MoS2 tribofilm buildup on the contact surfaces,in terms of the local MoS2 amount in the micro scale as well as the distribution of MoS2 tribofilm in the macro scale.The friction coefficients obtained from the simulated Raman intensity maps show a close agreement with the experimental data,confirming that the proposed model is capable of predicting friction at the set-up conditionsThe key conclusions and the novelties of this work are concluded in Part 7.The future research work in terms of boundary lubrication and associated tribochemical reactions are outlined.