Preparation And Tribological Properties Of Binary-doped Ionic Liquid Lubricating Films On Alkyl Silane Modified Silicon Surfaces

Single crystal silicon（Si）and its compounds are the main device materials of micro-electromechanical systems（MEMS）.With the widely applications of MEMS in different fields,the size of the Si devices is shrinking to the micro/nano scale.The scale effects make the adhesion and friction of the Si surface increased,which seriously restricts its reliable operation in MEMS.Therefore,it is urgent to deal with the surface of Si-based materials for adhesion reduction,friction reduction and anti-wear performances.The lubricating technology of nanoscale films has become an important method to solve this problem.Based on the structure and composition characteristics of nano-lubricating films,a silane-based binary-doped ionic liquid（ILs）lubricating film was designed.The films were prepared on Si substrates,and contain a connection layer of N-[3-（trimethoxyalkyl）propyl]et-hylenediamine（DA）molecules,an intermediate strengthening layer of 1-carboxyl-3-methyli-midazolium chloride（[CMIM]Cl）and a lubricating layer of 1-dodecyl-3-methylimidazolium hexafluorophosphate（[DMIM]PF6）.The molecular dynamics simulation method was used to study the molecular structure and wetting characteristics of the lubricating film.Based on the simulation results,self-assembly and spray coating technology were applied to prepare the lubricating film and the structure of the film was analyzed and verified by means of Fourier transform infrared spectroscopy contact angle measuring instrument and X-ray diffractometer.Then,a ball-on-disctribometer and atomic force microscopy were used to investigate the micro/nano tribological properties of the lubricating films.The results show that when the surface coverage of[CMIM]Cl molecules on Si-DA is50%,the adjacent molecules are arranged in 2a×2 2a,and the ratio of[CMIM]Cl and[DMIM]PF₆ molecules is 1:4.The simulation model structure of the silane-based binary-doped ILs lubricating film is the most stable.The wettability simulation results of the film show an optimal hydrophobic performance compared to the single-component ILs films,indicating that there was only a weak vander Waals force between the surface of the nano-droplet and the film.In the micro/nanotribology experiment,the silane-based binary-doped ILs lubricating film exhibits excellent tribological properties at the micro/nanoscale.In details,the adhesion of the film surface is reduced 63%relative to the bare Si surface,and the average friction value under different loads decreased by 60%at the nanoscale.Moreover,at the microscale,the wear life of the film was obviously enhanced at100 mN load,and the anti-wear time was up to 4500 s.The life expectancy is 70 times of that on the Si and Si-DA surfaces.When the load is increased to 500 mN,the film still exhibits the longest wear time,and the friction coefficients.This is mainly due to the formation of a denser and ordered layer structure of the bonded phase[CMIM]Cl,which enhances the stiffness and strength of the film.And the mobile phase[DMIM]PF₆ are easy to flow back to the wear track,achieving self-replenishing lubrication characteristics.In summary,the synergistic effect of the bonded and mobile phases are excellently improve the self-lubricating properties of the film.The results provide an effective method for solving the tribological problems of MEMS system device failure.