"Polycrystalline diamond (PCD) is a new superhard material that has been studied and used internationally since 1970s. It is formed by sintering at high temperature 1400 ℃ and high pressure 6gpa.
Polycrystalline diamond is not only an engineering material, but also a new functional material; It is not only a high-tech product, but also a high-efficiency product. With the development of modern industry, science and technology, polycrystalline diamond has been widely used in modern industry, national defense and high-tech fields because of its excellent mechanical, thermal, chemical, acoustic, optical and electrical properties.
Polycrystalline diamond cutting tool has become an indispensable and necessary means in modern machining. This is mainly reflected in the following aspects:
(1) High speed cutting, high stability machining
(2) Ultra precision mirror machining
(3) Dry cutting and clean machining
Performance introduction
(1) High hardness and wear resistance
The hardness of polycrystalline diamond is up to about 10000hv. It is the hardest material in the world at present. It is much higher than that of cemented carbide and engineering ceramics. Due to its high hardness and isotropy, polycrystalline diamond has excellent wear resistance.
(2) Low friction coefficient
The friction coefficient between polycrystalline diamond and some non-ferrous metals is lower than that of other materials, about 1 / 2 of that of cemented carbide. The low friction coefficient not only reduces the deformation and cutting force, but also does not produce chip nodules during cutting, thus reducing the machined surface roughness.
(3) High thermal conductivity
Polycrystalline diamond has high thermal conductivity, which is better than silver and copper, and much higher than ordinary cemented carbide. Therefore, the cutting heat is easy to dissipate in the cutting process, so the cutting temperature is low.
(4) High machining precision
Because the polycrystalline diamond tool has low coefficient of thermal expansion and high elastic modulus, it is not easy to deform in the cutting process. Under the action of cutting force, the tool can maintain its original parameters, keep sharp for a long time and have high cutting precision. Therefore, when using PCD tool for machining, it can reduce cutting force and cutting temperature, improve tool durability and cutting rate, and obtain good machining surface.
Main problems: the high hardness and wear resistance of PCD material make it very difficult to form and finish the surface, which seriously hinders its popularization and application.
Common machining methods: EDM, laser machining, chemical machining and ultrasonic machining.
Ideal machining method: grinding or grinding.
Introduction to processing technology
Grinding
In view of the high hardness and wear resistance of polycrystalline diamond, its grinding mainly includes diamond wheel grinding, electric discharge grinding and electrolytic grinding, among which the simplest and effective grinding method is diamond wheel grinding. In the actual machining process, different grinding methods can also be combined according to the needs.
(1) Diamond grinding wheel grinding
When Grinding Polycrystalline Diamond with diamond grinding wheel, the contact pressure and grinding force between grinding wheel and workpiece are large, which requires the grinding system to have sufficient stiffness and high precision. In recent years, many countries are developing special grinding machines and wheels for Grinding Polycrystalline Diamond (including polycrystalline cubic boron nitride). For example, CPG series high stiffness diamond tool precision grinder and CP series high efficiency special diamond grinding wheel developed by Osaka diamond industry company of Japan
(2) ELID mirror grinding of polycrystalline diamond
The on-line electrolytic dressing grinding wheel (ELID) precision mirror grinding technology is a new ultra precision machining technology developed by Dr. yoshiji Ohmori of Japan Institute of physics and chemistry in the late 1980s. The basic principle is that in the grinding process, the grinding wheel is continuously sharpened and trimmed by using the dynamic balance between the nonlinear electrolytic dressing effect and the electrolytic inhibition effect of the oxide insulating layer on the surface of the metal bonded superhard abrasive grinding wheel, so that the grinding particles of the grinding wheel can obtain a constant protrusion, so that the grinding wheel can always be processed continuously in the best grinding state, It is suitable for precision mirror grinding of hard and brittle materials.
Good results can be obtained by machining polycrystalline diamond with metal bonded diamond grinding wheel and ELID Precision Mirror Grinding Technology. It is a new way of precision machining of polycrystalline diamond and has great popularization value and application prospect.
(3) Electric spark grinding
The characteristics of pulse generator are very important in EDM, especially in machining PCD materials. PCD has certain conductivity, good thermal conductivity and high melting point, so it is very difficult to process polycrystalline diamond by traditional EDM.
The erosion mechanism of PCD in EDM grinding includes: gasification of diamond, oxidation of diamond, conversion of diamond to graphite and amorphous carbon, ejection force generated by EDM, microcracks on diamond surface caused by thermal stress, fracture and fracture of diamond grains, etc. The graphitization of diamond plays a key role in the process of polishing diamond film by EDM. It not only plays a conductive role, maintains the existence of discharge channel, and vaporizes the diamond film protruding from the top of the peak, but also the graphitization oxidation process of diamond continues, so as to etch the diamond.
EDM and EDM grinding are efficient and low-cost machining methods, but they can not effectively process large-area polycrystalline diamond.
Grinding
There are three common grinding methods for polycrystalline diamond: one is to use resin bonded fine-grained diamond grinding wheel for precision grinding; The other is to use quenched high hardness steel plate (or agate plate) and grind directly without any abrasive; Another is to use high-speed rotating cast iron disc, supplemented by diamond powder for grinding. Grinding can be used as a fine polishing process of polycrystalline diamond, and it is an important part of manufacturing polycrystalline diamond tools.
(1) Grinding of polycrystalline diamond with diamond wheel
Grinding Polycrystalline Diamond with diamond grinding wheel is similar to grinding wheel, but the model of grinding wheel is different. Generally, resin bonded grinding wheel with concentration of 50% - 70% and particle size of W40 - W7 is selected for grinding. When Grinding Polycrystalline Diamond with diamond grinding wheel, because the grinding of PCD material by diamond abrasive particles in the grinding wheel is essentially an interaction process between two objects with similar hardness and properties, which is essentially different from the traditional grinding process, PCD grinding mechanism and grinding process have their own characteristics.
(2) High speed steel disc grinding of polycrystalline diamond
The basic principle of Grinding Polycrystalline Diamond with high-speed steel disc is that PCD workpiece rubs with high-speed rotating high-speed steel disc under certain pressure to produce friction heat, which increases the temperature of sliding interface. The high temperature generated on sliding interface (grinding surface) is conducive to oxidation, graphitization, diffusion, bonding and thermal stress crushing of diamond grains at sliding interface in PCD, So as to achieve the purpose of grinding.
According to the chemical properties of diamond, some metal elements can react with it to promote its disintegration. If the material of the grinding disc itself contains the above metal elements, it will promote the diffusion, bonding and thermal corrosion of diamond at the grinding sliding interface, which will help to improve the efficiency of grinding. However, the temperature of grinding sliding interface should not be too high, otherwise it will affect the surface quality of grinding.
(3) Abrasive grinding of polycrystalline diamond
Abrasive grinding of polycrystalline diamond is one of the traditional machining methods. In the process of abrasive grinding, the cleavage brittle fracture of grains is one of the main removal methods in the process of grinding PCD; Sharp abrasive particles pass through PCD at a high speed and appropriate pressure, resulting in a large number of damaging scratches on the surface of diamond grains. These scratches can be divided into plastic scratches and brittle scratches. This method is also an important mechanical removal method of PCD materials. In addition, there are many thermochemical removal methods, such as graphitization removal, thermal etching removal, diffusion removal and oxidation removal, but thermochemical removal methods do not play a major role in the removal of abrasive PCD materials. Abrasive grinding has high precision, but very low efficiency. However, when ultrasonic grinding is assisted, the efficiency will be greatly improved.
Other processing methods
(1) WEDM
WEDM machine
WEDM realizes the cutting of workpiece materials by pulse discharge in the local area between electrode wire and workpiece. It is one of the common methods of PCD cutting. WEDM polycrystalline diamond is mostly used to make machining tools, geological drills and wire drawing abrasives. The shape of these workpiece molds is complex and changeable. Forming with WEDM is an economic and feasible way at present.
(2) Laser processing
The mechanism of laser processing polycrystalline diamond is: a laser beam with high energy density irradiates the polycrystalline diamond surface, part of the light energy is absorbed by the surface and converted into heat energy, and the temperature in the local area of the irradiation spot rises rapidly to tens of thousands of degrees, which makes the polycrystalline diamond material melt or even vaporize locally and form pits. At the same time, thermal diffusion begins, and the material around the spot melts. With the continuous absorption of laser energy, the steam in the pit expands and the pressure increases, and the melt is sprayed out at high speed in the form of explosion. The recoil pressure generated by the injection forms a strong shock wave in one direction inside the workpiece. In this way, polycrystalline diamond will erode some substances and form laser pits under the action of high temperature melting and vaporization and shock wave.
Laser processing belongs to non-contact processing. It has excellent characteristics in PCD material processing. It can process high-purity PCD materials (such as polycrystalline diamond film). The processing deformation and thermal deformation are very small, and the laser processing speed and efficiency are high. It is a more effective processing method. Especially in microporous processing and non-conductive superhard material processing, it shows its advantages. However, there are still some problems, such as difficult to control the dimensional accuracy and repetition accuracy, and micro graphitization on the machined surface, which need to be refined. At present, it can only be used for rough machining or semi finish machining. The laser parameters that play a decisive role in laser processing materials are pulse width, maximum pulse power and average pulse power.
(3) Chemical processing
The chemical processing of polycrystalline diamond is based on the characteristic that diamond can be dissolved in molten potassium nitrate, sodium nitrate and sodium phosphate. Molten nitrate is used to dissolve polycrystalline diamond. In view of the distribution state of surface free energy and the principle of reducing free enthalpy, the surface of polycrystalline diamond can gradually change from rough to smooth. At the same time, due to the oxidation reaction, an oxide film can be formed on the diamond surface, which can be removed by grinding and then continue to dissolve.
(4) Ultrasonic machining
The ultrasonic machining of polycrystalline diamond uses diamond micro powder (1-50um) as free abrasive particles. The main factors affecting the ultrasonic machining of PCD materials are vibration frequency, amplitude, diamond micro powder size, PCD material, unit pressure, machining area, etc. At present, polycrystalline diamond ultrasonic machining includes ultrasonic grinding and ultrasonic grinding, which are mostly used for the machining of PCD inner hole, PCD wire drawing die and sand blasting die. The core component of ultrasonic grinding method is an ultrasonic grinding device with axial ultrasonic vibration and high-speed rotation.
Application Introduction
PCD tools have very high durability (more than tens of times longer than cemented carbide tools), stable dimensional machining accuracy and good workpiece surface roughness. The main processing objects are non-ferrous metals, non-metallic materials, woodworking materials containing Al2O3, alloys, ceramics, various fiber and particle reinforced composites, plastics, rubber, graphite, glass, wood, etc., and can also do ultra precision processing on the above materials. At present, the main varieties include PCD woodworking saw blades and trimming knives, PCD indexable blades, PCD piston series knives, PCD wheel Yi knives, PCD commutator knives, PCD watch cases, jewelry knives, as well as various PCD welding turning knives and wok knives.
PCD cutting tools account for 60% of the application in automobile leading cities. PCD tools are mainly used to process skirt, pin hole, cylinder block, gearbox, carburetor, etc. of engine piston in the automotive field. Because the silicon content of these parts is high (more than 10%) and most of them are produced in large quantities by assembly line, they have high requirements for the service life of tools, and cemented carbide tools are incompetent. The durability of diamond tools is 10-50 times that of cemented carbide tools, which can ensure the dimensional stability of parts and greatly improve the cutting speed, machining efficiency and surface quality of workpieces.
Wood processing industry
At present, the widely used man-made wood plates have been obviously different from traditional wood. Synthetic plates are, in a sense, the concept of synthetic resins, simultaneous interpreting their simultaneous interpreting process. In particular, the development of man-made boards such as medium density fiberboard, plywood, particleboard and composite floor has accelerated their demand for superhard tools, so that diamond tools gradually replace the position of traditional woodworking tools in the market. Al2O3, the outermost layer of laminate flooring, has a great impact on the wear of cemented carbide tools. PCD tools can effectively solve this problem.
Electronic industry
The general cutting tool is easy to turn over when cutting the edge and durian of the printed board, which affects the installation and positioning of various electrical components, and the whole printed board is not beautiful. Cemented carbide cutting tools have the advantages of short service life, slow processing speed, low processing efficiency and poor processing accuracy. The multi edge electric printing board knife made of PCD material is not easy to wear, and the processing linear speed is as high as 1500-2000m / s, so it has high processing efficiency and low processing cost, and has been widely used.
Petroleum geological drilling industry
The polycrystalline diamond bit uses a sharp, high wear-resistant and self sharpening polycrystalline diamond cutting block as the cutting element, so that it can achieve high footage (4-6 times of the cone bit) and high ROP (more than 2 times higher than the cone bit) under low WOB (about 40kN). It has higher safety than cone bit, which can greatly improve drilling efficiency and reduce drilling cost. It has outstanding advantages in large section of soft to medium hardness formation, especially in high-cost offshore drilling, ultra deep well and slim hole drilling, PCD bit has incomparable advantages over cone bit.
Wire rod die
Wire drawing die is a very important die for wire drawing by various metal wire manufacturers (such as wire and cable factory, steel wire factory, welding rod and welding wire factory, etc.). The wire drawing die has a wide range of applications. It is mainly used for drawing linear difficult to process objects such as bar, wire, wire and pipe. It is suitable for the drawing of steel, copper, tungsten, equivalent metals and alloys.
When drawing copper wire with the same diameter, the service life of polycrystalline diamond die is 300-500 times that of cemented carbide die, 80-100 times that of nickel wire, 50-80 times that of cemented carbide die when drawing molybdenum wire, and 20-60 times that of cemented carbide die when drawing carbon steel. Polycrystalline diamond wire rod die in addition to the commonly used wire and cable drawing die, there are enamelled wire dies such as motors, electrical appliances and transformers, precision inner lead dies such as integrated circuits, etc.
Glass cutting processing
As a glass cutting tool with excellent performance, the cutter wheel made of polycrystalline diamond is mainly used for high-precision and high-quality cutting of liquid crystal glass in the electronic industry. At the same time, it can cut architectural and automotive glass. It is an ideal substitute for ordinary glass cutter and cemented carbide glass cutter wheel. Both mandrel and cutter wheel are polycrystalline diamond materials, which have the advantages of high machining precision, wear resistance and good consistency, and have extraordinary service life.
Gem processing
The application of polycrystalline diamond in this field is similar to that of traditional electroplated diamond sand
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