Study On Preparation Mechanism And Process Of Large Size Spherical Diamond Thick Film

Diamond films synthesized by chemical vapor deposition (CVD) have some outstanding properties such as high thermal conductivity, high band gap, low electrical conductivity, high IR transparency, high hardness, and high chemical inertness, etc. It has been considered as a promising material for many applications in mechanics, electron, optics and biology engineering. In recent years, convex diamond film has been widely more regarded in national defence field. But study on large size spherical diamond thick film just now begins, and preparation mechanism and process of diamond film are rarely reported.Thus it is important to develop experiment and theory investigation on large size spherical diamond thick film. The main work and the results obtained in this dissertation are as follows:1. FEM simulation study was carried out on the growth of diamond film by FLUENT. The results showed that the velocity field of spherical molybdenum substrate was not uniform, and it was much lower in center spherical substrate than that of the edge of spherical substrate. Fluctuation of plasma velocity field is harmful to growth of spherical diamond film. The pressure is the biggest in center of spherical substrate and tends to decrease at the ambient. In center small area of spherical substrate, plasma velocity was so low, which results in very high pressure field in center area of spherical substrate. Fluctuant velocity and pressure distribution have negatively influenced on the growth of spherical diamond film. Plasma better flows in convex substrate than the concave substrate, which is help to prepare high quality diamond film. The temperature field of the substrate is uniform, which is one of the major facts to successfully synthesize large size spherical diamond thick film. The results also shows that, although velocity and pressure field simulated are non-uniform, high quality spherical diamond film can be prepared. Meantime, it was found that diamond film has more stress by concave spherical substrate.2. Preparation mechanism and the growth course of large size spherical diamond thick film by DC plasma jet CVD were widely studied, meantime, growth dynamics, nucleation and growth mechanism of diamond film were also investigated. It was found that, in non-equilibrium state, the existence of super atom H, activated radical as CH3, C2H2 was necessary factors for preparing large size spherical diamond thick film. It was also showed that substrate curvatures, adhesion power of between diamond film and substrate, and plasma arc behavior have important influences on diamond film quality.3. Preparation process of large size spherical diamond thick film was widely investigated and the results showed that it has an important influence on the quality of diamond film. Substrate temperature, CH4 concentration, chamber pressure, substrate curvature and distance between substrate and plasma are closely related with diamond film quality. To optimize these processes parameters, and high quality large size spherical diamond thick film can be prepared.4. The techniques based on X ray diffraction (XRD), scanning electron microscopy (SEM), atom force microscopy (AFM), X ray photoelectron spectroscopy (XPS), Raman spectroscopy etc. were used to investigate microstructure, nucleation density, surface roughness, orientation evolution and impurities of spherical diamond film respectively.5. Study on the mechanism and distribution of the stress of diamond film was carried out by XRD, and the stress has an important influence on performance of the diamond film. At the same time, the fracture mechanism and microhardness were also analyzed in detail. It is advantaged to synthesize high quality diamond in a low methane concentration and suitable substrate temperature.