Preparatiion And Tribological Investigation Of Rare Earth Nanocomposite Thin Films

Micro-electromechanical-systems (MEMS) technology has been received much attention for its characteristics such as small size, light weight,compact integration and advanced aptitude. Friction, wear and stiction are the important factors which affect the performance and reliability of MEMS. Surface modification of the microstructure materials has been an effective means to improve friction, reduce wear, restraining stiction and enhancing stability of the MEMS. Molecular thin films technique which has been developed in past 30 years gives an effective approach to solve the problem.Self-assembly technique is a new technique developed in the recent years preparing super thin film. Using self-assembly technique, moleculars can be engrafted by chemical adsorption on surface to form molecular brushes which can reduce wear. Many researchers are interested in the excellent lubricant performance of self-assembled monolayers. The research on preparation and tribological behavior of self- assembled monolayer is a hot spot.In this study, self- assemble technique has been used to prepare thin films in view of the defects of currently used surface modification methods in MEMS. The mechanical and tribological properties have been improved due to the special physical chemical characteristics of rare earths. Some significant original results have been obtained.Firstly, the variation value of the rare earth, free energy is negative in self-assembly processes by thermodynamic calculation, therefore RE can chemically adsorb on the substrate. By the relation of Legendre differential equation and Maxwell relations, the expression of films'the surface energy was obtained. The relation between the chemically adsorbing and the films'surface energy was investigated, which provides the theoretical evidence of the self-assembly of RE thin films.Secondly, silane coupling regent (3-mercaptopropyl trimethoxysilane (MPTS)) was prepared on a single-crystal silicon substrate to form a two-dimensional self-assembled monolayer (SAM). The terminal ? SH group in the film was oxidized in situ to - SO₃Hgroup to endow the film with good chemisorption trend. Lanthanum-based thin films were deposited on the oxidized MPTS-SAM by the chemisorption of the ? SO3H group. The effects of kinetics condition on preparing the thin films were discussed. The optimum self-assembled condition of rare earth film and silane film was also determined. The results showed that the optimum thin films were obtained when the content of rare earth elements was 0.5wt% and the pH value of the rare-earth solution was 4-5.Thirdly, the surface energy, chemical composition, phase transformation and surface morphology of the films were analyzed by contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The experimental results strongly indicated that the lanthanum-based thin films were absorbed on the oxidized MPTS-SAM and the rare-earth film could react with the substrate by chemical bond which improved the bonding strength of the coating-substrate.Fourthly, the friction and wear behaviors of the RE thin films under macro tribology condition were investigated systematically. The results showed that the RE thin films had superior friction reduction and longer wear life. It is attributed to the special flexible characteristic of the rare earth and the rigid molecular chains of 3-Mercapto-propyl trimethoxysilane which can bear the load and improve lubricant capability.Fifthly, the friction and wear behaviors of the RE thin films under micro tribology condition were investigated. The sensitivity of the frictional behavior with respect to the variables such as applied load, sliding velocity, humidity and temperature was discussed.Sixthly, atomic force microscopy was used to analyze the micro wear of RE thin films surface. The results showed that, due to very low surface adhesion and strong bonding strength with sulfonate group, RE thin films have excellent wear resistances.This study is the first effort to prepare RE thin films using self-assembly technique, and also the first time to use atomic force microscopy, X-ray photoelectron spectroscopy, contact angle measurement were used to characterize the films. The mechanism and characteristic of RE thin films were clarified. The friction and wear behaviors of the RE thin films under both macro and micro conditions were investigated systematically. The interrelationship between the friction and wear behaviors and experimental conditions was obtained. The wear mechanism was analyzed. This study is promising to be applied in MEMS devices surface modification and the application of the RE thin films self- assembling technique in MEMS.