| The new demands for static friction and friction control have been appeared with the rapid development of microelectromechanical systems (MEMS) and nanoelectrome- chanical systems (NEMS). As a result, the lubrication in nano-scale and zero wear are imperatively to be solved. The technology of self-assembled monolayers (SAMs) is one of effective approaches to solve friction and stiction of MEMS/NEMS because it can be used to investigate interfacial reciprocity and friction/stiction mechanism in nano-scale. However, currently organic self-assembled monolayer is not really practical due to its poor structural stability, interfacial adhesion toughness, load-carried capacity, wear resitance as well as sensitivity to temperature and humidity which seriously restricts its application in MEMS/NEMS.In view of the importance of interfacial adhesion toughness to SAMs, rare earths (RE) were used to prepare 3-Aminopropyltriethoxysilane– rare earth (abbreviated form of APTES/RE) composite films by a self-assembling process in this dissertation which effectively improved interfacial adhesion toughness of APTES-SAM and reduced its surface energy, and then, enhanced the tribogical properties of APTES/RE composite films due to the special physical chemical characteristics of rare earths. Some significant original results were obtained.Firstly, Gibbs's free energy and enthalpy change of the phosphorylated APTES- SAM during the preparation of APTES/RE composite films were -27.12kJ/mol and 14.23 KJ/mol by thermodynamic calculation, respectively. It indicated that RE could be spontaneously absorbed on the substrate to form APTES/RE composite films connected with chemical bond. Secondly, APTES was prepared on Si/SiO2 substrates to form two-dimensional self-assembled monolayer (SAM). The terminal -NH2 groups in the film were in situ phosphorylated to -PO(OH)2 group to endow the film with good chemisorption ability. Then APTES/RE composite films were deposited on the phosphorylated APTES-SAM making good use of the chemisorption ability of the -PO(OH)2 groups. Surface morphology and contact angles were analyzed by means of atomic force microscope (AFM) and contact angle measurements, respectively. The results showed that the optimum APTES/RE composite films could be obtained when the prepared APTES-SAM with phosphorylating time 30min were immersed in rare earth solution with the concentration of 2mol/ml and PH value of 5～6 for coating time of 5h.Thirdly, X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical composition of typical elements in APTES/RE composite films. The results showed that rare earth reacted with P=O and P-OH of the surface of substrate by complexing bond and formed steady RE complexs,which accordingly improved interfacial adhesion toughness of APTES-SAM.Fourthly, the tribological properties at the macroscale of the prepared films were analyzed by Universal micro-tribometer (UMT). The results showed that APTES/RE composite films were effective in reducing friction and resisting wear compared with the APTES-SAM and the phosphorylated APTES-SAM under the same condition. The superior friction reduction and wear resistance of APTES/RE composite films were attributed to the enhanced load-carrying capacity of the inorganic lanthanum particles in the APTES/RE composite films as well as strong interfacial adhesion toughness.Fifthly, scanning electron microscopy (SEM) was used to investigate wear mechanism of the samples. Based on the SEM observation, the wear of APTES-SAM and phosphorylated APTES-SAM was characteristic of brittle fracture and severe abrasion; whereas the APTES/RE composite films were dominated by slight abrasion and micro-crack It was attributed to strong interfacial adhesion toughness between RE and -PO(OH)2 groups that resisted the impact caused by applied load. The large-scale transfer and rubbed-off of APTES-SAM were restrained, which could avoid sever brittle fracture happened on the surface of the film in the friction and wear test that led to the invalidation of the film.Sixthly, the friction and wear behaviors of silicon substrate and prepared films at the nanoscale were studied by AFM. The sensitivity of the friction force and adhesion force with respect to the variables was discussed, such as applied load, sliding velocity, relative humidity and so on. 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 APTES/RE composite films exhibited superior nano-tribological properties at the same condition comprared with Si, APTES-SAM and phosphorylated APTES-SAM. It may be attributed to low surface energy of non-polar terminal groups and the strong interfacial adhesion toughness of the APTES/RE composite films.Based on the preparation of APTES/RE composite films with superior friction reduction and wear resistance, the process and forming mechanisms of APTES/RE composite films were discussed. The tribological properties and wear mechanism of the films both at the macro- and nano-scales were systematically investigated. The result of this study provided a new approach to investigate lubrication in MEMS/NEMS and promoted surface engineering application of rare earths.
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