| Chemical vapor deposited (CVD) diamond film has wide application prospects in many high-tech fields because of its extremely high hardness, low friction coefficient, excellent wear resistance and high thermal conductivity and other excellent performance. In order to explore the effect of nanocrystalline on the mechanical properties of CVD diamond film, study is carried out on the nanocrystalline diamond film (NCD) in this dissertation. The research contents include the CVD deposition system, growth process and mechanism and mechanical properties of NCD. All the results of this research provide the basis for the NCD application.The main work and the results obtained in this dissertation are as follows:1. The shortcomings of microcrystalline diamond film (MCD) and superiorities of NCD are analyzed firstly. The characterization technique, deposition process and mechanism and application foreground of NCD are reviewed.2. The study was carried out on the double bias hot filament CVD (HFCVD) system. The heat exchange of the HFCVD system was analyzed at first, then the substrate and atmosphere temperature field models are set up by the FEA method to investigate the large area HFCVD system. Multi-point substrate temperature measuring system and continuous deposition system are introduced to the HFCVD system to improve the measuring precision of the substrate temperature and the efficiency of sample preparation.3. By using the new method of double bias nucleation and grid bias growth, high purity and extremely smooth NCD was obtained on the different substrates such as single crystalline silicon, polycrystalline molybdenum and YG8 cemented carbide. Raman, SEM, XRD and AFM results show that the diamond films obtained have grain sizes less than 20nm, nucleation density higher than 1011cm-2 and the surface roughness less than 10nm. The thick diamond film with nanocrystalline surface was successfully obtained by the alternation between nucleation and growth process in the double bias HFCVD system.4. Effect of the grid bias and substrate bias on the NCD nucleation and growth process was studied in detail. It is shown from the experiment and theoretical analysis that the positive grid bias increases the activation, decomposition and ionization of hydrogen and methane molecules, while negative substrate bias helps positive carbon-containing ions bombard the substrate which leads to the high nucleation density of the diamond. It is shown that negative substrate bias will induce the increasing of non-diamond content in the film, while grid bias will generate a lot of hydrogen atoms that will maintain the purity of diamond film.5. The systematic characterization and invenstagation are carried out on the mechanical properties of NCD. The microhardness, Young's modulus, fracture toughness, adhesion strength and friction coefficient of NCD were measured and analyzed respectively. The decrease of diamond grain size results in the increase of grain boundary density and non-diamond content in the grain boundary in the film. The microhardness and Young's modulus will decrease with the decrease of diamond grain size, while fracture toughness will increase. The negative substrate bias current will induce the great decrease of microhardness and Young's modulus. The NCD friction coefficient is only 1/3 of MCD because of its smooth surface and lubrication of graphite content in the diamond boundary. The NCD has good adhesion performance because of its high nucleation density. The mechanisms of the effect of nano-meter size effect on the NCD mechanical properties were studied in detail.6. Study on the thermal stress of the diamond film by FEA model was carried out. By the FEA model, the magnitude and distribution of thermal stress were revealed. The intrinsic stress was measured and analyzed by XRD method and the mechanism of the nano-meter size effect on the NCD intrinsic stress was studied in detail. The non-diamond content in the grain boundary will cause the compressive residual stresses in the NCD.
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