| Diamond is not only the hardest known material in the world, but also it is a kind of versatile material with superior physical and chemical properties. It has a very small friction coefficient, good thermal conductivity, excellent electrical insulation, quite wide transmission band, excellent semiconductor property and strong chemical inertness, so it is not only widely used in such as military defense, aerospace and other high-tech fields, but also it has a broad application prospect in ultra-precision machining field. With the successful preparation of the synthetic diamond, the use of diamond both in width and depth gets a significant development. Whether natural or synthetic diamond, they all need precision and ultra-precision machining to make it meet all the various application requirements. But the unique and excellent characteristics of diamond will cause great difficulties for precision and ultra-precision machining, becoming the technical bottleneck of machining diamond in practical application. So the high-efficiency precision and ultra-precision machining of diamond material becomes the important subject which needs to be solved urgently.Currently widely used polishing methods of diamond materials, whether physical removal or chemical removal, contact or non-contact, all exist their own merits and faults, and there are big differences between polishing mechanism, polishing efficiency, size limit and equipment requirements. Dynamic friction polishing (DFP) is a new method which uses a synergistic effect of mechanical and chemical to promote the graphitization of diamond and get a high-efficiency material removal. Compared with traditional thermal-chemical polishing, the method of friction heat production make DFP have no need for heating device and vacuum or special atmosphere, which greatly reduces the technical requirements of special polishing equipment, so it is a very promising polishing method of diamond. Due to high-efficiency removal and low damage of DFP technology, it has a good application prospect in rough machining field of diamond crystal materials. But the studies on the preparation of polishing plate and polishing mechanism of DFP are less, there are still many difficulties on the polishing efficiency and surface polishing quality, etc. Aiming at the above problems of DFP, This paper proposed a new formula of polishing plate and studied the key technology and related theories of DFP diamond crystal, and main works are included as follows:(1) In order to increase the removal rate of diamond and reduce the wear rate of polishing plate in DFP, we need to develop an alloy which has both strong catalytic ability of graphitization of diamond and good physical and chemical properties. Based on the material removal mechanism of DFP. unpaired d electrons catalysis principle of transition metal and requirements of processing conditions on the polishing plate material properties, seven polishing plate formulas of Fe, Ni, Mn and W based alloys are selected. According to preparation of polishing plate and polishing performance contrast test, WMoCr alloy shows the most excellent comprehensive performance. The removal rate of diamond film is up to 1.5 ?m/min, but the wear rate of polishing plate is only 0.35 mm3/min and gets the biggest grinding ratio 23.06. And on this basis the adding of rare earth element Y in the alloy can further improve the microstructure and polishing performance of the polishing plate.(2) In order to improve the comprehensive performance of polishing plate, in the preparation of the WMoCr alloy polishing plate, mechanical alloying (MA) method is used to synthetize pre-alloying powders, and vacuum hot-pressed sintering (HPS) method is used to prepare alloy polishing plate with high hardness and high density. By the process test of MA, the effect rules of the process parameters of ball milling on powder characteristics are studied, and the reasonable process parameters are below:300 rpm rotation speed,60 h milling time, 15:1 ball-to-powder weight ratio and 10% PC A content. Excellent organizational performance and fine grain of pre-alloying powders supply guarantees for improving the polishing performance of polishing plate. On that basis, according to the studies of sintering temperature, sintering pressure and holding time etc., the effect rules on alloy material properties such as density and hardness are analyzed. The results show that under the conditions of 1400? sintering temperature,30 MPa sintering pressure and 30 min holding time, the relative density of prepared alloy is 96.49% and Vickers hardness is 777.78 HV. Alloy polishing plate shows small and uniform grain, compact structure and small porosity.(3) Using chemical reaction thermodynamics Gibbs principle, this paper discusses the effects of pressure and temperature on the equilibrium constant of graphitization of diamond and measures to speed up the reaction. And on this basis, in the built DFP polishing plate performance test bench, the single factor experiments of polishing process for DFP single crystal are designed, and optimized polishing process parameters are seletced. The experimental results show, under 65 N polishing pressure and 8000 rpm polishing speed, it can get high removal rate and high surface quality of diamond and low wear rate and oxidation degree of the polishing plate.(4) By observing the experiment process and detecting reaction product composition, combined with the basic principles of chemical thermodynamics, the polishing mechanism of DFP is studied. The changes of surface chemical components and internal organization of diamond and polishing plate are analyzed before and after DFP, and the process of graphitization of diamond is studied. Unique material removal principle of diamond by WMoCr alloy is discovered:except for the graphitization and diffusion effects, diamond phase C reacts with intermediate WO3 to generate WC as another material removal method.(5) On the basis of above work, the prepared WMoCr alloy cup wheel is used to polish single-crystal diamond by DFP, and its machining effect is compared to the traditional diamond grinding wheel. The experiment results show that WMoCr alloy cup wheel can get a higher material removal rate and higher surface quality. Finally the modification method of diamond grinding wheel by WMoCr alloy polishing plate is explored, and the feasibility and the effect of processing are analyzed... |
PDF Full Text Request