ABSTRACT
Since CVD diamond thin films have many unique characteristics such as high hardness, high elastic modulus, low frictional coefficient, good thermal conductivity and chemical stability, so CVD diamond thin films have been widely used in cutting tools and wear resistant parts. Especially for the newly appeared and hard-to- cut materials, such as high silicon-content aluminum alloys, and non-metal based composite materials,engineering ceramics and fiber reinforced plastics, diamond coated tools have potential wide application. At the same time, the diamond films not only can be used as the exterior surface, but also can be used as interior surface, such as the interior surface of drawing dies and nozzles. However, the deposition technologies are not perfect. When using CVD diamond film in cutting tool applications, the poor adhesion of the diamond film on the substrate is the key factor of abnormal tool failures and becomes the main technical barrier for the successful development and commercialization of diamond-coated tools. The surface roughness of the diamond film is the main cause to lead to the tool wear. Aiming at solving these problems, lots of research works on key technologies and investigations were carried out in this dissertation systematically. Some innovative accomplishments in the dissertation based on the above intention were listed as follows:
1. The non-equilibrium thermodynamic coupling model of the diamond thin film deposition was studied, the non-equilibrium equation was obtained. On the basis of these,the effect of deposition factors on the diamond thin film was investigated. The new pretreatment method for the cemented tungsten substrates with smooth surface was proposed for the first time was methanol pretreatment, and with the introduction of Ar,nanocrystalline diamond films with high quality were deposited on cemented tungsten substrates successfully using bias-enhanced hot filament CVD system for the first time,which solved the problems of high surface roughness of diamond films. The surface was extremely smooth. The grain size of the nanocrsytalline diamond films was about 50nm.The rough surface Ra was less than 100nm. The influence of the deposition parameters on the quality of the conventional and nanocrystalline diamond films were investigated deeply.
The optimized deposition technology of conventional and nanocrystalline diamond films was obtained. The comparative tribological tests of conventional and nanocrystalline diamond films were carried out.
2. The study on the new pretreatment methods for the cemented tungsten substrates without destroying the original surface was investigated, which solved the problem of the high rough surface of conventional diamond films. The introduction of rare earth can enhance the borionization effect; increase the depth of borionization layer. The borionization pretreatment not only could effectively suppress cobalt diffusion to the surface, but also could effectively avoid roughening to the surface of substrate and ensure the roughness of the diamond films. The lifetime of diamond coated tools were enhanced greatly compared with those of uncoated tools through the cutting tests. The effect of the distance between the substrates and hot filaments on the quality of diamond films was researched, the high quality diamond coated tools with large grain size were obtained and the rough cutting tests were carried out. The lifetime prolonged 10times.
3. On the basis of studying the two-step method, the pretreatment method for complex shaped cemented tungsten substrate was obtained for the first time, that is to say methanol and alkali two-step method, which overcame the difficulties to be abraded and the low adhesive strength between the substrate and diamond film. The new method for the deposition of diamond coated integral rotational tools was put forward, which solved the key problem of the uniformity of diamond coated integral rotational tools. Using the conventional cemented tungsten carbides YG6(Co 6%)drills and keyway milling cutters as the substrates, conventional and ultra-fine grained diamond coated drills and keyway milling cutters were obtained through the improved deposition device by adjusting the deposition parameters. During the deposition process, the tools were kept rotating under certain speed, which ensured the uniformity of diamond films. The diamond films had uniform thickness and better adhesive strength. The cutting performance of these cutters was verified by the experiments of cutting SiC particles reinforced composite materials.The wear mechanism and lifetime of diamond coated tools were studied. The lifetime was 5 and 10 times longer than those of uncoated ones. The diamond like carbon (DLC) coated micro-rotational drills was fabricated using the same system and the cutting performance was carried out. The wear and machined quality of workpiece were demonstrated, which showed the excellent characteristics of DLC.
4. The inner hole shape of the drawing dies was optimized and designed by finite element analysis, and the nanocrystalline diamond composite films were deposited on the inner hole surface by the straight hot filament passing through the die hole successfully.The stress distribution and maximum stress point in the inner hole surface were analyzed.The optimal stress distribution was obtained by adjusting the angle and the length of calibrating strap of inner hole surface of drawing die to enhance the lifetime of dies and be more suitable for deposition of diamond coating. The quality of nanocrystalline diamond composite films were characterized by different kinds of techniques and the practical drawing wire test was performed in the welding material factories. The results proved that the lifetime was extended 10times compared with that of uncoated one. The nanocrystalline diamond coated dies not only could increase the lifetime of dies and production efficiency, but also could assure the stability of diameter of drawn wires and improve the quality and surface finish of drawn wires save the raw materials. It is of very importance for the electric line and cable industries.
5. The new method for fabrication of long thin diamond tubes and new idea about their bonding and transferring were put forward, which expanded the application field of diamond films. The study on deposition of diamond thin films on tungsten wire substrate with the gas mixture of acetone and hydrogen by using bias-enhanced hot filament CVD with the tantalum wires being optimized arranged was investigated. The self-supported diamond tubes were obtained by etched away the tungsten substrates. The quality of diamond film before and after the removal of substrates was tested and analyzed. The compressive stress in diamond film formed during the deposition was released when the substrate was etched away. There was no remaining stress in diamond tube after substrate removal. The modified epoxy resins which could bond the diamond coating and steel or aluminum was discussed and the bonding mechanism was analyzed preliminarily.
KEY WORDS: Diamond-coated tool, Bias-enhanced hot wire CVD, Nanocrystalline diamond film, Pretreatment, Surface roughness, Cutting performance
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