Study On The Preparation And Tribological Properties Of Nanocomposite Films

The development of micro/nano-electromechanical systems(MEMS/NEMS) come up with many scientific problems, among which the reduction of friction and sticking forces during the fabrication and operation of micro-machines are urgent to be solved. Surface modification and lubrication are two key methods to reduce friction and wear, restrain sticking and enhance the stability of microsystems. In recent years, the deposition methods and molecular thin film assembly techniques give an effective approach for solving these problems. To explore new effective methods on enhancing the tribological properties of microstructures, some surface modification methods are discussed here and the main research contents and results in this dissertation are summarized as following:Firstly, a novel sandwich-structure nanocomposite film together with two other comparison films successfully prepared by deposition and self-assembly techniques has been proposed. Two kinds of coupling agents were reacted with the single crystal silicon surface by chemical bonds, and then an elastic poly (styrene-b-ethylene/butylenes-b-styrene) (SEBS) was connected to the coupling agent. Different chemical groups, reaction times and temperatures were taken into consideration for affecting the surface quality and the structure of the SAMs films. Results showed, good surface quality could be obtained when the self-assembly procedure was carried under a temperature of 220℃in vacuum. Ultrathin diamond-like carbon (DLC) films were deposited by using the electron cyclotron resonance chemical vapor deposition (ECR-CVD) and the filtered cathodic vacuum arc (FCVA) techniques, respectively.Secondly, the surface quality, the microstructure and the surface energy of the prepared samples were analyzed by the atomic force microscopy (AFM), X-ray photoelectron spectrometer (XPS), the Raman spectrometry and a contact angle goniometer. Results show that the DLC film prepared by the FCVA method contained more sp3 bonds. The surface energy of the coupling agentγ-(2,3-epoxypropoxy) propyltrimethoxysilane (A187) was higher than that of the coupling agent y-aminopropyl-triethoxysilane (APS), enhancing the anchor of elastomers (styrene-block-ethylene/butylene-block-styrene) (SEBS) on the coupling agent of A187.Thirdly, the tribological performance of the hard DLC films, the elastic SAMs(APSSEBS, A187SEBS) and the nanocomposite films(APSDLC and A187DLC) were investigated by a microtribolotester (UMT). Results showed that the frictional coefficient of the single silicon surface decreased from 0.6 to about 0.1 after the modification by the self-assembly and the deposition techniques. At low velocities, the naocomposite A187SEBS SAMs film could endure larger loads than that of APSSEBS SAMs film. While at high velocities with low loads, the tribological properties of the two SAMs films were similar and could keep at relatively low values. As to single DLC film, under both experimental conditions, the tribological performances of the DLC(F) film were better than those of the DLC(C) film. However, at the low load condition, the frictional coefficient of the naocomposite A187DLC(F) changed the least among all the tested samples, moreover, the DLC(F) still shows the excellent tribological performance at high loads.Fourthly, the results of nanoindentation test showed that the dimensional effect influenced the mechanical properties evidently with the load increasing since the films tested were ultrathin, and the mechanical properties of the three kinds of samples changed differently. With the increase of the indent depth, the hardness and the elastic modulus increased for the elastic SAMs(A187SEBS, APSSEBS) but decreased for the DLC films. As for nanocomposite of A187DLC and APSDLC films, the hardness and the elastic modulus decreased initially and subsequently increased with the indent depth.Fifthly, the linear relationship between the maximum indentation depth and the contact stiffness has been deduced and experimentally verified. The effects of the tip radius on the nanoindentation results were investigated theoretically. A semi-empirical relation depicting the relation of the hardness with the tip radius and the indentation depth was derived. The simulation results indicated that the tip size effect in the nanoindentaion tests was mainly caused by the underestimation of the contact area due to the real shape of the indenter tip. This size effect due to the tip radius becomes increasingly more obvious with the tip radius increasing.Sixthly, nanoscratch tests were carried out and the critical load of each sample was obtained. Results showed that the critical load of the DLC(F) film was the highest among all the samples reacheding 8000μN. The critical load of the A187DLC film was higher than that of the APSDLC film. The loading velocity had little effect on the test results, but once the film became failure, the measured lateral force increased with the decrease of the loading velocitySeventhly, the mechanical properties of plastic and elastic-plastic materials under different numbers of indenting cycles with various forces were investigated. Results showed that the hardness of both samples increased with the indenting cycle at low loads. At high loads, the hardness changed to the contrary and the changing rate of plastic materials was a little higher than that of elastic-plastic materials. With the increase of indenting cycles, the contact stiffness of both samples and the elastic modulus of the plastic material changed hardly, while the elastic modulus of the elastic-plastic material decreased distinctly.By comparing various properties of the three films, the nanocomposite film proposed in this dissertation shows excellent tribological and mechanical properties at low loads and can be potentially used in micromachines. Furthermore, it provides a novel method for solving the tribological problems between microstructures.This dissertation is supported by National Natural Science Fund of China (No.50475124).