PREPARATION AND NANO-TRIBOLOGICAL PROPERTIES INVESTIGATION OF SELF-ASSEMBLED AMINOSILANE/RARE EARTH COMPOSITE FILMS

PREPARATION AND NANO-TRIBOLOGICAL PROPERTIES INVESTIGATION OF SELF-ASSEMBLED AMINOSILANE/RARE EARTH COMPOSITE FILMS
　　With the rapid development of microelectromechanical systems(MEMS) and nanoelectromechanical systems(NEMS)nowadays,nano-tribology has become one of the most focused areas in nano-tribology research.With the purpose of resolving the lubrication and zero-wear problems in nano-scale,it is imperious to initiate the research of surface films which possess low-friction and excellent anti-wear properties.Self-assembled monolayers technology(SAMs)is one of the Neodymium Magnets most effective approaches to solve friction and stiction of MEMS/NEMS for it can be used to investigate interfacial reciprocity and nano-friction/stiction mechanism.
　　Due to the small-size effect of micro-machine,the surface stiction, nano-friction and the adhesive force were extremely prominent compared with its volume,and thus become the key factors in determining its performance,stability and work life.Therefore,to fully reveal the tribology mechanism of the micro-machine,the nano-tribological properties which take the moleculars and atoms on the interface as the study objects must be carried out. Interfacial adhesion toughness is the critical factor to the properties of SAMs.Based on this principle,rare earths were used to prepare amino silane/rare earth composite films by a self-assembling process in this research.
　　The nano-tribology properties and nano-wearness performance of the SAMs under micro-scale were investigated with the assist of atomic force microscope(AFM).Through in-depth exploration,the mechanism of how rare earth improved the structure and the properties of the thin films and how the nano-tribological properties were enhanced was revealed and introduced. Some significant original results have been obtained.
　　Firstly,the self-assembling process of rare earth on the surface of amino silane film was investigated through the contact angle curve and the AFM images.The thickness of the composite film is 15nm which indicates the aminosilane/rare earth film was successfully assembled on the substrate.
　　Secondly,by thermodynamic calculation,free energy variation value of rare earth elements is negative in self-assembly process with the bond energy gap of-336 KJ/mol;rare earth can spontaneously adsorb through chemical reaction on the substrate.By the surface free energy formula,surface energy of the films is obtained.The results indicated that the phosphorylated aminosilane film has the lowest surface energy,which provides the theoretical evidence for the self-asemble process.
　　Thirdly,the assemble dynamics of aminosilane/rare earth composite film was investigated.And the influence of the PH and the concentration of the rare earth solution on the assemble process was analyzed.By studying the change of contact angle of water on the surface of the prepared film,the absorbing process of Lanthanum was investigated:the first stage was electrostatic adsorption;then the La ion was chemically bonded with the phosphate group.With the aid of Materials Studio,the adsoption process of the film was simulated using molecular dynamics simulation.The energy variation through the whole process was elaborated in a direct way.
　　Fourly,the nano-friction and nano-wear behaviors of silicon substrate and prepared films at the micro-scale were studied by AFM.The sensitivity of the friction force and adhesion force with respect to the variables such as applied load,sliding velocity,relative http://www.everbeenmagnet.com/en/products/110-sintered-neodymium-magnets humidity and number cycles was discussed.The results showed that friction forces increased with scan rate, applied normal load and relative humidity.Adhesion forces increased with the relative humidity,but remained unchanged when scan rate and applied normal load increased.It was also found that aminosilane/rare earth composite films exhibited superior nano-tribological properties at the same condition comprared with Si substrate,aminosilane-SAM and phosphorylated aminosilane-SAM.
　　Fifthly,the experiment procedures to characterize the nano-wear properties of the film under micro-scale were designed.And the nano-wear performances of the prepared films were investigated using AFM.The sensitivity of the micro-wear depth with respect to the variables such as applied load,lengthwise sliding velocity,widthwise step length and number cycles was discussed.The results showed that the wear depth increased linearly with the applied load and the number cycles,while the influence of the sliding velocity on the wear depth was relatively slight.With the increase of the widthwise step length,the wear depth decreased linearly.
　　Sixthly,in nano-scale,the applied load and the adhesive force were of the same magnitude.Therefore,the influence of the adhesive force on the friction force could not be neglected.According to the Laplace,Kelvin and Van der waals equations,the relation between relative humidity and adhesive force was deduced.It is found that in lower humidity,the Van der waals force plays the major role in the adhesive force;while in higher humidity,the capillary force is much more prominent.The model of spring double-oscillator under nano-friction conditions and the model of cobble under micro-wear conditions were raised to explain the nano-tribological behaviours of the prepared film.
　　Based on the preparation of aminosilane/rare earth composite films with superior friction reduction and wear resistance,the process and forming mechanisms of aminosilane/rare earth composite films were discussed.The nano-tribological properties and nano-wear mechanism of the films at the micro-scales were systematically investigated.The result of this study provided a new approach to investigate lubrication in MEMS/NEMS under nano-scale and promoted surface engineering application of rare earths.