Polycrystalline Diamond (PCD) is a new type of superhard material that has been researched and used internationally since the 1970s. It is sintered at a high temperature of 1400 ° C and a high pressure of 6 GPa. Polycrystalline diamond is both an engineering material and a new type of functional material; it is both a high-tech product and a highly efficient product. With the development of modern industry and science and technology, polycrystalline diamond has been widely used in modern industry, national defense and high-tech fields with its excellent strength, heat, chemistry, sound, light and electricity. Polycrystalline diamond tools have become an indispensable tool in modern machining. This is mainly reflected in the following aspects: (1) High-speed cutting, high-stability processing (2) Ultra-precision mirror processing (3) Dry cutting and cleaning processing Performance introduction (1) High hardness and wear resistance Polycrystalline diamond has a hardness of up to 10,000 HV. It is the hardest material in the world. It is much harder than hard alloys and engineering ceramics. Because of its extremely high hardness and isotropic properties, polycrystalline diamond has extreme Good wear resistance. (2) Low friction coefficient The coefficient of friction between polycrystalline diamond and some non-ferrous metals is lower than that of other materials, which is about 1/2 of that of cemented carbide. The low coefficient of friction not only reduces the deformation and cutting force, but also causes no built-up edge during cutting, thus reducing the surface roughness of the machined surface. (3) High thermal conductivity Polycrystalline diamond has a high thermal conductivity, which is better than silver and copper. It is much higher than that of ordinary hard alloys. Therefore, the cutting heat is easily dissipated during the cutting process, so the cutting temperature is low. (4) High processing accuracy Because the polycrystalline diamond tool has a low coefficient of thermal expansion and a high modulus of elasticity, the tool is not easily deformed during the cutting process. Under the action of the cutting force, the tool can maintain its original parameters, keep sharp for a long time, and have high cutting precision. Therefore, when machining with a PCD tool, the cutting force can be reduced and the cutting temperature can be lowered, the tool durability and the cutting rate can be improved, and a good machined surface can be obtained. The main problem: the high hardness and high wear resistance of PCD materials make molding and surface finishing very difficult, which seriously hinders its popularization and application. Common processing methods: EDM, laser processing, chemical processing, ultrasonic processing. Ideal processing method: grinding or grinding. Processing technology introduction Grinding In view of the high hardness and wear resistance of polycrystalline diamond, the grinding process mainly includes diamond grinding wheel grinding, electric discharge grinding and electrolytic grinding. The simplest and most effective grinding method is diamond grinding wheel grinding. In the actual machining process, different grinding methods can also be combined according to the requirements. (1) Diamond grinding wheel grinding When the diamond grinding wheel grinds the polycrystalline diamond, the contact pressure and the grinding force between the grinding wheel and the workpiece are large, and the grinding processing system is required to have sufficient rigidity and high precision. In recent years, many countries have been developing and developing special grinding machines and grinding wheels for grinding polycrystalline diamond (including polycrystalline cubic boron nitride). Such as CPG series high rigidity diamond tool precision grinding machine developed by Osaka Diamond Industry Co., Ltd. and CP series high efficiency special diamond grinding wheel (2) ELID mirror grinding of polycrystalline diamond Online Electrolytic Dressing Wheel (ELID) Precision Mirror Grinding Technology is a new ultra-precision machining technology developed by Dr. Daisuke, Institute of Physical Chemistry, Japan in the late 1980s. The basic principle is to use the non-linear electrolytic dressing action and the dynamic balance of the metal oxide super-abrasive grinding wheel surface oxide insulating layer to suppress the electrolysis during the grinding process, and continuously trim the grinding wheel to obtain the grinding wheel abrasive grain. Constant amount of protrusion, so that the grinding wheel is continuously processed in the best grinding state, it is suitable for precision mirror grinding of hard and brittle materials. The diamond grinding wheel with metal bond and ELID precision mirror grinding technology can obtain good effect. It is a new way of precision processing of polycrystalline diamond, which has great promotion value and application prospect. (3) Electric spark grinding The characteristics of the pulse generator during EDM are very important, especially for processing PCD materials. PCD has a certain conductivity, good thermal conductivity and high melting point, so the traditional EDM processing of polycrystalline diamond is very difficult. The erosion mechanism of EDM grinding PCD includes: gasification of diamond, oxidation of diamond, conversion of diamond to graphite and amorphous carbon, throwing force by electric spark discharge, thermal stress to microcrack on diamond surface, and diamond Breakage of crystal grains, etc. Among them, the graphitization of diamond plays a key role in the process of electric spark polishing of diamond film, which not only acts as a conductive force, but also maintains the existence of the discharge channel, so that the top of the diamond film protrudes from the top of the peak, and the graphitization-oxidation process of the diamond continuously Perform to remove the diamond. EDM and EDM are the most efficient and cost-effective methods of processing polycrystalline diamonds, but they do not efficiently process large areas of polycrystalline diamond. Grinding There are three main methods for grinding polycrystalline diamond: one is to use a resin bond fine-grained diamond wheel for precision grinding; the other is to use a hardened high-hardness steel plate (or agate plate) without any abrasive. Direct grinding; another is the use of high-speed rotating cast iron discs, supplemented by diamond micro-powder grinding. Grinding can be used as a fine polishing process for polycrystalline diamond, which is an important part of the production of polycrystalline diamond tools. (1) Grinding of diamond grinding wheel with polycrystalline diamond Grinding of diamond grinding wheel Polycrystalline diamond is similar to grinding of grinding wheel, except that the type of grinding wheel is different. Grinding wheel with resin concentration of 50%-70% and particle size of W40-W7 is generally used for grinding. When the diamond grinding wheel is used to grind polycrystalline diamond, the grinding of the PCD material by the diamond abrasive grains in the grinding wheel is essentially a process of interaction between two objects with similar hardness and properties, which is essentially different from the traditional grinding process. The PCD grinding mechanism and grinding process have their own characteristics. (2) High-speed steel disc grinding of polycrystalline diamond High-speed steel disc grinding polycrystalline diamond, the basic principle is that the PCD workpiece is rubbed under a certain pressure and high-speed rotating high-speed steel disc, generating frictional heat, increasing the temperature of the sliding interface, sliding interface (grinding surface) The high temperature generated on the surface is beneficial to the oxidation, graphitization, diffusion, bonding and thermal stress cracking of the diamond grains in the sliding interface of the PCD, thereby achieving the purpose of grinding. From the chemical properties of diamond, some metal elements can react with them to cause their disintegration. If the material of the grinding disc itself contains the above-mentioned metal element component, it will promote the diffusion, adhesion, thermal erosion and the like of the diamond at the sliding sliding interface, which helps to improve the efficiency of the grinding process. However, the temperature of the grinding sliding interface should not be too high, otherwise it will affect the quality of the grinding surface. (3) Abrasive grinding of polycrystalline diamond Abrasive grinding of polycrystalline diamond is one of the traditional processing methods. During the abrasive grinding process, the cleavage of the crystal grains is one of the main removal methods in the process of grinding PCD; the sharp abrasive grains are at a high speed. With proper pressure across the PCD, a large number of damaging scratches are produced on the surface of the diamond grains. These scratches can be divided into plastic scratches and brittle scratches, which is also an important mechanical removal method for PCD materials. In addition to this, there are many thermochemical removal methods such as graphitization removal, thermal erosion removal, diffusion removal, and oxidation removal. However, the thermal chemical removal method does not occupy a major position in the removal of the polished PCD material. Abrasive grinding has high precision, but the efficiency is extremely low. However, when assisted in ultrasonic grinding, the efficiency is greatly improved. Other processing methods (1) Wire EDM Wire EDM machine The electric discharge wire cutting process realizes the cutting processing of the workpiece material by the pulse discharge of the local region between the electrode wire and the workpiece, and it is one of the commonly used methods for PCD cutting processing. Most of the wire-cut polycrystalline diamonds are used to make machining tools, geological drills and wire-drawing abrasives. These workpieces are complex and variable in shape, and it is economically feasible to form them with EDM machines. (2) Laser processing The mechanism of laser processing polycrystalline diamond is: a laser beam with extremely high energy density is irradiated onto the surface of polycrystalline diamond, part of the light energy is absorbed by the surface and converted into heat energy, and the temperature of the local area of ​​the irradiated spot rapidly rises to tens of thousands of degrees. The polycrystalline diamond material is partially melted or even vaporized and forms a pit. At the same time, thermal diffusion started, and as a result, the material around the spots melted. As the laser energy continues to be absorbed, the steam in the crater expands, the pressure increases, and the melt is ejected at high speed in an explosive form. The recoil pressure generated by the injection forms a strong shock wave inside the workpiece. Thus, the polycrystalline diamond etches away some of the substances under the action of high temperature melt vaporization and shock waves to form a laser etch pit. Laser processing is non-contact processing. It has excellent characteristics in PCD material processing. It can process high-purity PCD materials (such as polycrystalline diamond film), processing distortion and thermal deformation are small, and laser processing speed is fast and efficient. It is a more effective processing method. Especially in microporous processing and non-conductive superhard material processing, it shows its superiority. However, there are problems such as difficulty in controlling dimensional accuracy and repeatability, and micro-graphitization on the machined surface, which requires further finishing. Currently only used for roughing or semi-finishing. The laser parameters that determine the function of the laser processing material are pulse width, maximum pulse power, and average pulse power. (3) Chemical processing The chemical processing of polycrystalline diamond is based on the characteristics that diamond is soluble in molten potassium nitrate, sodium nitrate and sodium phosphate, and the polycrystalline diamond is dissolved by molten nitrate. In view of the surface free energy distribution state and the principle of free enthalpy reduction, The surface of the polycrystalline diamond is gradually smoothed from rough to smooth. At the same time, an oxidation film can be formed on the surface of the diamond due to the oxidation reaction, which can be removed by grinding and then continue to dissolve. (4) Ultrasonic machining Ultrasonic processing of polycrystalline diamond is processed by using diamond micropowder (1-50um) as free abrasive. The factors affecting ultrasonic processing of PCD are vibration frequency, amplitude, diamond powder size, PCD material, unit pressure and processing area. At present, polycrystalline diamond ultrasonic processing includes ultrasonic grinding and ultrasonic grinding, etc., and is mostly used for processing PCD bores and PCD drawing dies and sand blasting dies. The core component of the ultrasonic grinding method is an ultrasonic grinding device that performs ultrasonic vibration on one side and high-speed rotation on one side. Application introduction Automotive and aerospace PCD tools have a very high durability (more than a few times higher than the life of cemented carbide tools), stable dimensional machining accuracy and good workpiece surface roughness. The main processing objects are non-ferrous metals, non-metallic materials and woodwork materials, alloys, ceramics containing Al2O3, various fiber and particle reinforced composite materials, plastics, rubber, graphite, glass, wood, etc., and can also be ultra-precise for the above materials. machining. At present, the main varieties are PCD woodworking saw blades and trimming knives, PCD indexing blades, PCD piston series tools, PCD wheel yoke tools, PCD commutator tools, PCD case, jewelry knives and various PCD welding tools, 镬Knife. PCD tools account for 60% of the automotive industry. PCD tools are mainly used in the automotive field to process skirts, pin holes, cylinder blocks, gearboxes, carburetor, etc. of engine pistons. Due to the high silicon content (10% or more) of these components, and most of them are produced in large quantities by pipeline, the service life of the tool is high, the carbide tool is difficult to handle, and the durability of the diamond tool is cemented carbide tool. 10-50 times, the dimensional stability of the parts can be guaranteed, and the cutting speed, processing efficiency and surface quality of the workpiece can be greatly improved. Wood processing industry The artificially-used panels that have been widely used have been significantly different from traditional woods. The synthetic panels are in a sense the concept of synthetic resins, and the processing process is different from that of traditional wood. In particular, the development of man-made panels such as MDF, plywood, particleboard and laminate flooring has accelerated the demand for superhard tools, which has gradually replaced the position of traditional woodworking tools in the market. Among them, Al2O3, which is the outermost layer of laminate flooring, has a great influence on the wear of cemented carbide tools. PCD tools can effectively solve this problem. it Generally, the cutter is easy to turn over when the printed board is trimmed and cut, which affects the installation and positioning of various electrical components, and the whole printed board is not beautiful. Carbide tools have a short life, low processing speed, low processing efficiency and poor processing accuracy when processing electrical printed boards. The multi-blade electrical printing plate cutter made of PCD material is not easy to wear, and the processing line speed is as high as 1500-2000 m/s, so the processing efficiency is high and the processing cost is low, which is widely used. Petroleum geological drilling industry Polycrystalline diamond drill bit with sharp, high wear resistance and self-sharp polycrystalline diamond cutting block as cutting element, which can achieve high footage (4-6 times for roller cone bit) under low drilling pressure (about 40kN) And high drilling speed (more than 2 times higher than the roller bit). It has higher safety than roller cone drills, which can greatly improve drilling efficiency and reduce drilling costs. The large-scale soft to medium-hardness formation has outstanding advantages, especially in high-cost offshore drilling and ultra-deep wells and small borehole drilling. PCD drill bits have the incomparable advantages of roller cone drills. Wire mould Drawing die is a very important mold for drawing various kinds of wire rod manufacturers (such as wire and cable factory, steel wire factory, welding wire and wire factory, etc.). The wire drawing die has a wide range of applications, and is mainly used for drawing straight, difficult-to-machine objects such as bars, wires, wires, pipes, etc., and is suitable for drawing processing of steel, copper, tungsten, equal metal and alloy materials. When drawing copper wire of the same diameter, the service life of polycrystalline diamond die is 300-500 times of the life of cemented carbide die and 80-100 times of drawing of nickel wire. When drawing molybdenum wire, its life is only hard. The life of the alloy mold is 50-80 times, and when the carbon steel is drawn, the life of the polycrystalline diamond mold is 20-60 times that of the cemented carbide mold. In addition to the commonly used wire and cable drawing die, polycrystalline diamond wire moulds also include enameled wire molds for motors, electrical appliances, transformers, etc., precision inner lead molds such as integrated circuits. Glass cutting As an excellent glass cutting tool, the cutter wheel made of polycrystalline diamond is mainly used for high-precision and high-quality cutting of liquid crystal glass in the electronics industry. It can also cut glass for building and automotive, and is a common glass knife and hard. An ideal replacement for alloy glass cutter wheels. Both the mandrel and the cutter wheel are polycrystalline diamond materials, which have the advantages of high processing precision, wear resistance, good consistency, etc., and have an extraordinary service life. Gem processing The application of polycrystalline diamond in this field is compared with the traditional electroplated diamond grinding wheel. The most outstanding advantage is that the tool has a long service life. In many cases, the electroplated diamond tool is a single layer of diamond. When grinding high hardness jade, It will soon be worn out and lose its ability to work, and polycrystalline diamond completely overcomes this weakness and greatly improves processing efficiency. Gem processing is a newly developed application field of polycrystalline diamond tools in recent years, and the market prospect is very promising, greatly expanding the market of polycrystalline diamond. The hardness of this new type of polycrystalline diamond is lower than that of polycrystalline diamond used in drilling and other fields, and it has made some special treatments for gem processing. Other uses Polycrystalline diamond has achieved good results in applications where high wear resistance, high dimensional accuracy, and good contact are required. The ball bearing of semi-automatic grinding wheel frame is made by replacing the natural diamond with polycrystalline diamond, and the service life is 2500h, which is far better than traditional materials. Polycrystalline diamond trim pens can be used to trim almost all grinding wheels, including cubic boron nitride grinding wheels. Development requirements and trends (1) Specifications are getting larger and larger. (2) Grain refinement, quality optimization, and uniform performance. Early polycrystalline diamond products generally use about 50um of diamond micropowder, and now it has been developed to use 2um or even 0.5um, so that polycrystalline diamond tools and drawing die are no longer inferior to single crystal diamond in processing precision. (3) The wear ratio is getting higher and higher, and the wear resistance of polycrystalline diamond is a city-oriented index to measure its quality level. As a new type of super-hard material product, after years of research and production, its quality level is continuously improved and worn. It is getting higher and higher. (4) Diversified shape and structure. In the past, polycrystalline diamond products were generally in the form of sheets and cylinders. Due to the increase in size and processing technology (such as EDM and laser cutting technology), the shape of the triangle, the chevron and the gable Spherical surfaces, curved surfaces and various other irregular materials have increased. In order to meet the needs of special cutting tools, there are also wrapped, sandwich and coiled polycrystalline diamond products.
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Abstract Polycrystalline Diamond (PCD) is a new type of superhard material that has been researched and used internationally since the 1970s. It is sintered at a high temperature of 1400 ° C and a high pressure of 6 GPa. Polycrystalline gold...