| With the deepening research and application of the micro-electromechanical systems(MEMS)in various fields,the tribological problems represented by adhesion,friction and wear have become the bottleneck of the MEMS development.The surface modification and lubrication of MEMS micro components are effective methods to reduce adhesion,friction and wear,thus improve the tribological properties of MEMS,which are valuable both for academic and application.According to the structure and reaction characteristics of different assembled molecules,the silane based bilayer self-assembled monolayers(SAMs)with basal-connecting layer of N-(2-aminoethyl)-3-aminopropyl trimethoxysilane(DA)and intermediate-strengthened layer of lauroyl chloride(LA),DA-LA,which are successively assembled on the monocrystalline silicon(Si)surface,were designed.Based on the analysis and calculation of the assembled reaction process of DA-LA from the view of chemical reaction and thermodynamics,the spontaneity mechanism of assembled reaction process was revealed.The variations of temperature and energy in the assembled process of silane-based SAMs were given by molecular dynamics simulation,which provided a new way to explore the mechanism of the films formation from the molecular level.Base on the mechanism study,the silane-based SAMs systems were prepared on Si substrates by the molecular self-assembly technique.The effects of solution concentration and assembly time on the films quality were investigated by atomic force microscopy(AFM).The optimum process parameters for the silane-based SAMs preparation were obtained.The wettability and chemical components of the silane-based SAMs under the optimum preparation conditions were characterized by the contact angle measuring instrument and the total reflection fourier transform infrared spectroscopy(ATR-FTIR),respectively.The results showed that the surface hydrophobic properties and internal bonding stability gradually increased with assembling of two molecular layers.The silane-based self-assembled monolayers/ ionic liquids composite films systems were prepared by dip-coating and spray painting,respectively.In the systems,the 1-dodecyl-3-methylimidazolium hexafluorophosphate ionic liquids(ILs)film was introduced on the silane-based SAMs as a surface-lubricating layer.The microscopic morphologies,wettability and chemical components of the composite films were characterized by the digital microscope,contact angle measuring instrument and ATR-FTIR,respectively.The experimental results showed that the prepared methods of ILs film and the underlying surface characteristics had great influence on the composite films.Particularly,DA-LA-ILs composite film prepared by the spray painting showed a uniform distribution state and good hydrophobicity,which provided experimental basis for studying the composite film preparation.The adhesion and micro frictional force of the silane-based SAMs were measured by AFM.The macroscopic antifriction and wear-resistant of the composite films system were investigated by the micro friction and wear testing machine and the super-depth microscope,respectively.Both in the microscopic and macroscopic scales,the tribological properties of the composite films were studied comparatively.The synergistic mechanism of the basal-connecting layer,the intermediate-strengthened layer and the surface-lubricating layer to reduce the surface friction and prolong the wear resistance were analyzed comprehensively.The results showed that the DA-LA-ILs composite film prepared by spraying method had the lowest friction coefficient.In the experiment period of 1350 seconds,wear phenomenon was not observed,which displayed the most excellent friction-reduction and wear-resistance properties.Furthermore,the DA-LA-ILs composite film can be used as an effective lubrication film.The theoretical foundation and experimental basis on the application of the self-assembled monolayers/ ionic liquid composite films in MEMS lubrication were provided. |
PDF Full Text Request