1 Introduction Diamond is a crystal of a single carbon atom, and its crystal structure is an equiaxed face-centered cubic system (a system with the highest atom density). Since the bond between carbon atoms in diamond is sp3 hybrid covalent bond, it has strong binding force, stability and directionality. The unique crystal structure gives diamond the highest hardness, rigidity, refractive index, thermal conductivity and excellent resistance to wear, corrosion and chemical stability in natural materials. Natural diamonds are ideal for precision cutting tool materials because they meet the most demanding requirements for precision and ultra-precision cutting tools. The uniform crystal structure of natural diamond without internal grain boundary makes the cutting edge of the tool theoretically reach the atomic level flatness and sharpness. The cutting ability is strong, the precision is high, the cutting force is small; the hardness and wear resistance of natural diamond are Corrosion resistance and chemical stability ensure long tool life, ensure long-term normal cutting, and reduce the impact of tool wear on part accuracy; its high thermal conductivity reduces cutting temperature and thermal deformation of parts . Therefore, natural diamonds have an important position in the field of machining as superhard tool materials and have been widely used, especially in the field of ultra-precision machining (such as processing various mirrors for nuclear reactors and other high-tech fields, for missiles or The ultra-precision parts of the navigation gyro, computer hard disk substrate, accelerator electron gun, etc. in the rocket, using natural diamond tools have obvious advantages in terms of price and precision than traditional processing methods. Table 1 shows the accuracy and price comparison results of processing various mirrors using natural diamond tools and traditional grinding and polishing methods.
Table 1 Comparison of processing precision and price between natural diamond tools and grinding and polishing methods
In addition to high-tech fields, the application of natural diamond tools in the processing of general industrial and civil products has also increased year by year, from the development of traditional watch parts to the processing of aluminum pistons, jewelry, pens, high-cursors, non-ferrous metal decorative parts, etc. It can be said that natural diamond tools have been deeply involved in various fields of machining and play an increasingly important role.
On the other hand, due to the unique properties of natural diamonds, processing of itself is very difficult. Due to the extremely high hardness of natural diamonds, special methods must be used for grinding. The technology of the operators is also very high. Because natural diamonds have good chemical stability, they cannot be welded for a long time. They can only be clamped by mechanical means. Large-grain diamonds result in wasted diamond material and high tool prices, while also affecting the accuracy of tools and parts.
In the past 100 years, scientists and technicians have carried out long-term research and development on the important role, application prospects and implementation methods of diamond tools in mechanical processing, and have achieved fruitful results. The following is a review of the development of natural diamond tool technology in stages, introduces the latest cutting edge technology of diamond tools, and explores future development trends.
2 Natural diamond tool traditional processing technology Natural diamond tool was developed after the Second World War to meet the processing needs of watch precision parts, lighting parts and jewelry carving. Its manufacturing process originated from the grinding and polishing technology of diamond jewelry, and its development brought A major revolution in watches and related manufacturing technologies.
The diamond jewellery is polished and polished by applying a diamond paste mixed with olive oil to the upper surface of the cast iron grinding disc, and the diamond micropowder is embedded in the tiny pores of the cast iron surface, and the diamond is fixed on the clamp skateboard. The jewellery is ground on a high-speed rotating grinding disc by the weight of the jig.
When grinding natural diamond tools with diamond jewelry processing methods, the following special processes are required:
1. Blade processing and inspection: The quality of the blade will directly affect the quality of the parts being machined.
2. Orientation: Place the most severe wear surface of the tool on the hardest crystal surface of the diamond, so that the tool has the longest service life. Conventional orientation methods are generally oriented with the naked eye.
3. Loading: During the cutting process, the natural diamond tool must withstand the cutting force from all directions. To ensure continuous and stable cutting, the diamond tool must be firmly mounted on the tool holder. Since diamond brazing technology was not invented at the time, mechanical clamping was only possible.
Because the traditional diamond tool processing method is simple in process and low in equipment cost, it is still used for rough machining of diamond tools until today. In order to further improve the diamond tool processing technology, the scientific and technical personnel have carried out extensive research on the physical and chemical properties of diamond crystals, the grinding mechanism of diamond tools, the formation mechanism of cutting edges, cutting theory, brazing technology, precision sharpening equipment, etc. for half a century. It has laid a solid foundation for the development of ultra-precision machining technology for natural diamond tools, and many research topics are still going on today.
3 Ultra-precision machining technology of natural diamond tools In the late 1970s, in the research of laser nuclear fusion technology, a large number of high-precision soft metal mirrors were required, which required the surface roughness and shape accuracy of soft metal to reach ultra-precision level. . If the traditional grinding and polishing methods are used, not only the processing time is long, the cost is high, the operation is difficult, and the required precision is not easily achieved. Therefore, there is an urgent need to develop new processing methods. Under the impetus of real demand, natural diamond ultra-precision mirror cutting technology has been rapidly developed. Based on the existing diamond turning technology, ultra-precision natural diamond is developed by improving the precision and rigidity of the machine tool, strictly controlling the vibration and temperature drift during processing. Tools, etc., form a mirror cutting process and develop into a specialized technology. As one of the key technologies of ultra-precision mirror cutting, natural diamond tool technology has achieved remarkable innovation and development in theory and practice, mainly reflected in the following aspects:
1. Development of ultra-precision diamond tools (1) Introducing a wiper between the main cutting edge and the minor cutting edge, so that the theoretical roughness of the machined surface is close to zero and the wiper is tested under a microscope of 500× or higher. No defects.
(2) The machining accuracy of the critical angle of the diamond tool reaches 2â€.
(3) The arc precision of the circular arc tool used for turning the inner curved surface reaches the order of micrometers.
2. Development of precision sharpening equipment Since the traditional grinding equipment can not meet the processing requirements of ultra-precision diamond tools, an arc and blade grinding machine using air bearings has been developed, and the sharpening precision can reach 0.1 μm.
3. Precision Orientation Technology and Equipment The purpose of orientation of the diamond tool is not only to maximize the tool life, but also to minimize the friction between the tool flank and the machined surface and the cleavage surface near the blade. To do this, a more sophisticated X-ray diffractometer is required for orientation.
4. Invention of vacuum brazing technology Vacuum brazing is one of the most important breakthroughs in diamond tool manufacturing technology. On the one hand, the traditional mechanical clamping method may cause the diamond tool to produce small displacement and vibration defects during cutting, which affect the processing quality; on the other hand, due to the high chemical stability of the diamond itself, it is difficult to interact with other metals under normal conditions. The reaction takes place to achieve welding, ie the diamond is non-weldable. In order to solve this contradiction, after long-term research and exploration, the specific conditions of brazing diamond (high vacuum environment) and brazing alloy (silver-based alloy with titanium as active element) were finally found.
5. Establishment of tool wear mechanism Through research, it is found that during the cutting process, the wear of diamond tools is mainly chemical bond wear, and there are other forms of wear such as mechanical wear. The establishment of the tool wear mechanism determines the principle of tool orientation: the crystal face with the best chemical stability is placed on the flank of the tool.
6. Systematic research on mechanical grinding method 7. Progress in diamond theory research Through the in-depth study of diamond theory, the macroscopic plastic deformation of diamond was found (in the past, diamond was only slightly elastically deformed without plastic deformation); The trace impurities are classified into four types according to the different impurities, so that different types of diamonds can be selected according to different uses; in addition, systematic theoretical research on the fracture characteristics, cleavage characteristics and surface formation mechanism of diamond is obtained. A large amount of data has formed various theories and has a more scientific and deeper understanding of diamonds.
The development of diamond tool technology at this stage is characterized by the fact that diamond tools have been used in national defense, high-tech and other fields, so they have received a lot of capital investment. They have used cutting-edge equipment, instruments and the latest scientific methods in research, and have made great progress.
4 Latest processing technology for natural diamond tools In the late 1980s, micromachines developed rapidly as a new field of research. When machining the micro components used in the manufacture of micro-robots (such as Ø0.1mm micro-precision gears, 0.3mm micro-motors, etc.) by mechanical means, the tool nose radius is required to be 3 to 5 μm, and the arc precision can be controlled. And can achieve a very long tool life.
From tool material analysis, only single crystal natural diamond can meet the above requirements. At the same time, after nearly a decade of rapid development, the theory and technology of diamond tools have been accumulated, and basically have the ability to develop the above-mentioned high-precision tools. However, there is still a distance between the diamond tool and the micromachining tool processed by the above-mentioned diamond tool ultra-precision machining technology, and further development of a more advanced processing method is required. In recent years, methods have been developed to grind diamond tools using various chemical mechanisms. The three processing methods described below can achieve atomic level processing.
1. Thermochemical method The mechanism of the thermochemical method is: at a temperature of 800 ° C, if the diamond surface is in contact with iron, the carbon atoms in the diamond crystal can be freed from the lattice of the crystal and diffused into the lattice of the iron crystal.
The grinding process using the thermochemical method is: heating the iron grinding disc to 800 ° C in a hydrogen atmosphere, bringing the diamond into contact with the grinding disc and sliding relative thereto, and the carbon atoms in the diamond crystal lattice diffuse into the crystal lattice of the iron crystal to reach The purpose of grinding diamonds; the carbon atoms entering the iron lattice react with hydrogen to form methane and dissipate into the air with the air stream. The grinding rate using thermochemical methods is 40 to 2000 atomic layers per second.
2. Vacuum plasma chemical polishing First, a layer of fine-grained silicon oxide is plated on the surface of the grinding disc by vacuum plasma physical vapor deposition, and then the diamond surface is activated in a high vacuum while being in contact with the rotating grinding disc. The carbon atoms whose surface is in an activated state react with the silicon oxide on the grinding disk to form a diamond for grinding; the carbon monoxide or carbon dioxide gas generated by the reaction is pumped out of the reaction chamber by a vacuum pump. The reaction rate of this method is from 1 to 3000 μm 3 /s (about 0.25 to 750 atomic layers per second).
3. Non-destructive mechanochemical polishing method Adding a certain amount of fine diamond powder and finer (nano-scale) silicon powder to the NaOH solution, the silicon powder with strong negative static electricity will be adsorbed on a single diamond particle much larger than it. A diamond abrasive powder having a silicon adsorption layer was formed and then coated on a porous cast iron grinding disk. When grinding diamond, the silicon powder adsorbed on the diamond abrasive powder prevents the direct impact of the diamond powder on the surface of the diamond to be processed, protects the surface of the processed diamond from being deeply damaged, and on the other hand reacts with the surface of the diamond to be processed and passes through the weak The grinding action removes the reaction layer. The grinding speed of this method is very low, only 1 atomic layer per minute.
The above three methods have the following common features: 1 avoiding the surface impact groove marks inherent in the machining method, the surface of the processed diamond has no impact, the surface is abnormally smooth, and the surface roughness can reach Ra1nm (the machining method can only reach the order of 100 nm) ). 2 The contact force between the diamond and the grinding disc is extremely small, and the high quality cutting edge can be easily ground. 3 The damage of the diamond surface and the metamorphic layer are very shallow, which improves the tool life. 4The grinding speed is very low and can only be used for ultra-precision polishing after fine grinding.
The above method not only satisfies the processing requirements of processing diamond tools for micromachines, but also facilitates the manufacturing process of small wedge angle diamond tools such as ultra-thin bio-dissection knives and ophthalmic scalpels.
5 Development direction of diamond tool technology Throughout the development process of diamond tool technology, its technical development route is: traditional process and mass production → theoretical research → ultra-precision machining technology and mass production → theoretical breakthrough → the latest cutting edge grinding technology. It can be predicted that the future development direction of diamond tool technology will be the mass production of the latest cutting-edge processing technology and theoretical research based on these cutting-edge technologies.
A new processing technology, from the successful development to the realization of mass production, the main problems to be solved are to reduce production costs, improve process efficiency and simplify the operation process. The realization of mass production of diamond brazing technology is a typical example: the diamond vacuum brazing technology invented in the late 1970s required a set of expensive vacuum equipment and long vacuuming time, high processing cost, low efficiency and complicated operation. . The protective atmosphere brazing technology that emerged ten years later effectively solves these problems and basically meets the requirements of mass production. The equipment cost is reduced to 1/5 of vacuum brazing, and the welding time is shortened from 2 hours of vacuum brazing to 20 minutes. The semi-open welding chamber used greatly simplifies the operation.
The latest chemical mechanical polishing methods also have the problems of high cost, low efficiency, complicated operation process, etc. It is necessary to systematically study the influence of grinding mechanism and various processing parameters on processing efficiency and quality, and propose a new realization scheme based on this. After trials and improvements, the requirements for mass production will eventually be reached.
In addition to the study of chemical mechanical polishing methods, the grinding mechanism of mechanical grinding methods is also an ancient and challenging research topic. From the carbonization and cleavage of the 1950s to the cleavage and plastic deformation theory of the 1990s, although they can explain the anisotropy of diamond grinding hardness, they lack a rigorous and complete theoretical system and persuasive The experiment proves that the reason is that the diamond surface is abnormally smooth, and it needs to be analyzed by atomic level surface state, micro stress and micro crack observation means. With the continuous development of science and technology, these observation methods are gradually becoming possible.
In addition, most of the mechanical, physical, and chemical properties of diamond lack accurate data. How to accurately determine the hardness of diamond (currently only 0.4 to 1.3 GPa range) is also a subject that needs further research in the future.
Development of natural diamond tool technology
Abstract: This paper reviews the development history of natural diamond tools and their processing technology, introduces the latest processing technology of natural diamond tools, and discusses the development direction of diamond tool technology.