Development and application of the hottest superha

2022-08-09
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Development and application of superhard tool materials

I. development overview of superhard tool materials

superhard tool materials refer to natural diamond and artificial diamond and CBN (cubic boron nitride) with similar hardness and performance. Because natural diamond is expensive, Artificial Polycrystalline Diamond (PCD), polycrystalline cubic boron nitride (PCBN) and their composites are mostly used in production

as early as the 1950s, the United States used artificial diamond powder and CBN powder to form large polycrystalline blocks as tool materials under the action of high temperature, high pressure, catalyst and binder. After that, debeem of South Africa, the former Soviet Union and Japan also successfully developed it. In the early 1970s, the composite sheets of diamond or CBN and cemented carbide were introduced. They were formed by sintering or pressing a layer of 0.5mm ~ 1mm PCD or PCBN on the cemented carbide matrix, which solved the problems of low bending strength of superhard tool materials and difficult inlay welding, and made the application of superhard tools enter the practical stage

the research and application of superhard tool materials in China began in the 1970s, and the sixth grinding wheel factory, China's first professional manufacturer of superhard materials and products, was built in Guiyang in 1970. In the 20 years from 1970 to 1990, the annual output of superhard materials increased from only 460000 carats to 35million carats. Around the 1990s, many professional factories producing superhard materials imported complete sets of superhard material synthesis equipment and technology from abroad, so that the output was rapidly increased. By 1997, the annual output of artificial diamond in China had reached about 500million carats, and the annual output of CBN reached 8million carats, ranking first in the world in superhard material production

diamond has extremely high hardness and wear resistance, and its microhardness can reach 10000hv. It is the hardest material in tool materials. At the same time, its friction coefficient is small, it has no affinity with non-ferrous metals, the chips are easy to flow out, the thermal conductivity is high, it is not easy to produce chip nodules during cutting, and the processing surface quality is good. It can effectively process non-ferrous metal materials and non-metallic materials, such as non-ferrous metals such as copper and aluminum and their alloys, ceramics, non sintered cemented carbides, various fiber and particle reinforced composites, plastics, rubber, graphite Glass and all kinds of wear-resistant wood (especially composite materials such as solid wood and plywood)

the disadvantages of diamond are poor toughness and low thermal stability. It is easy to carbonize at 700-800 ℃, so it is not suitable for processing steel materials. Plastic materials used for 3D printing mainly include ABS, polylactic acid, polycarbonate, polypropylene, PS, acrylate resin, nylon and thermosetting plastic structures because iron atoms are easy to interact with carbon atoms at high temperature to convert them into graphite structures. In addition, when it is used to cut nickel base alloys, it will also wear rapidly

II. Main varieties and applications of superhard cutting tools

at present, the main varieties of superhard cutting tool materials that have been applied or are being tested in the world are as follows

1. Natural and synthetic large single crystal diamond

single crystal diamond includes natural diamond (nd) and synthetic diamond. Single crystal diamond used to make cutting tools must be large particles (quality and requires us to respond with a quantity greater than 0.1g, and the minimum diameter length shall not be less than 3mm). Nd is the hardest substance in the known minerals at present. Its microhardness can reach 10000hv, with good wear resistance. Moreover, the cutting edge is very sharp, the roughness value of the edge is small, the friction coefficient is low, the anti adhesion is good, the thermal conductivity is high, it is not easy to stick the knife and produce chip nodules during cutting, and the processing surface quality is good. The hardness, wear resistance, corrosion resistance and chemical stability of natural diamond ensure the super long service life of the tool, ensure continuous and long-term normal cutting, and reduce the impact of tool wear on the machined parts; Its high thermal conductivity can reduce the cutting temperature and the thermal deformation of parts. The excellent characteristics of natural large single crystal diamond can meet most of the requirements of precision and ultra precision cutting for tool materials. Although it is expensive, it is still recognized as an ideal precision and ultra precision cutting tool material. It can be widely used in the processing of various mirrors in nuclear reactors and other high-tech fields, ground gyroscopes in missiles and rockets, computer hard disk substrates, ultra precision processing of accelerator electron guns, and precision processing of traditional watch parts, jewelry, pens, non-ferrous metal ornaments, etc. In addition, it can also be used to manufacture medical knives. The main disadvantage of Nd is that it has chemical reaction when contacting with iron group elements. It will carbonize (i.e. graphitization) at 700 ~ 800 ℃, which is generally not suitable for processing steel materials

2. Polycrystalline diamond and polycrystalline diamond composite blade

pcd, also known as diamond sintered body, is a polycrystalline material formed by polycrystalline diamond powder through cobalt and other metal bonds at high temperature and high pressure. Although its hardness is slightly lower than that of natural single crystal diamond, it is the aggregation of randomly oriented diamond grains, which is isotropic and has no cleavage surface. Therefore, unlike large single crystal diamond, it has great differences in strength, hardness and wear resistance on different crystal planes, and is brittle due to the existence of cleavage planes. During cutting, the cutting edge is not very sensitive to accidental damage, and has strong wear resistance. It can keep sharp cutting edge for a long time. During processing, it can adopt high cutting speed and large back feed. Its service life is generally 10 ~ 50 times higher than that of WC based carbide tools. Moreover, PCD raw materials are abundant, and its price is only one tenth to one tenth of that of Nd. PCD tools have extremely high hardness and service life, low friction coefficient With its sharp blade, excellent thermal conductivity and low expansion coefficient, it has become a high-performance substitute for traditional WC based cemented carbide tools. Polycrystalline diamond composite (PDC) tool materials are developed on the basis of PCD research. As the matrix material of PCD, cemented carbide not only has good toughness and certain hardness, but also has weldability and some compatibility with PCD. Therefore, it not only has the hardness and wear resistance of diamond, but also has the advantages of toughness and weldability of cemented carbide

3. CVD diamond

cvd diamond is prepared under low pressure, which is different from large single crystal diamond, while PCD and PDC are synthesized under high temperature and high pressure. CVD diamonds include three types: the first is CVD diamond coatings (including diamond-like DLC coatings) deposited on appropriate substrates; The second is to deposit unsupported CVD diamond thick films with a thickness of 1mm; The third is CVD diamond single crystal film or quasi single crystal film grown epitaxially on diamond seed. CVD diamond is pure diamond without any metal catalyst, so its thermal stability is close to natural diamond. Like high temperature and high pressure synthetic polycrystalline diamond, CVD polycrystalline diamond grains are also disordered, isotropic without brittle cleavage surface. Compared with PCD and PDC tools, CVD coated tools have the advantages of complex tool shape, low cost, one piece of multiple blades and so on. However, there are also defects such as low bonding strength between diamond coating and substrate and easy delamination and spalling when grinding the edge with CVD diamond coating

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so far, the application market of CVD diamond is not large; Compared with PDC, CVD diamond thick film has the main advantages of good thermal stability, but the disadvantages are low cohesion strength between grains, high internal stress, high relative brittleness and non conductivity. In particular, the lack of conductivity hinders its application in EDM cutting and polishing technology. This technology has been widely used in diamond tool processing industry, especially in the production and dressing of wood processing tools

4. Polycrystalline cubic boron nitride

at present, polycrystalline cubic boron nitride (PCBN) tools on the market can be divided into three types according to composition and manufacturing method: monolithic polycrystalline cubic boron nitride tools, polycrystalline cubic boron nitride composite sheets and electroplated cubic boron nitride tools. It has extremely high hardness and wear resistance, high heat resistance, good chemical stability and thermal conductivity, and low friction coefficient. PCBN has similar structure and properties with PCD and PDC tool materials, but the wear resistance is worse than PCD and PDC. However, PCBN has good chemical corrosion resistance and shows good thermal stability at 1200 ℃. At present, 50% of PCBN cutting tools are used in the automobile manufacturing industry, including the processing of automobile engine box, brake disc, transmission shaft, cylinder hole, engine inlet and outlet valve seat, etc. in addition, about 20% are used in the processing of heavy equipment (such as rollers). In recent years, with the rapid development of computer processing technology and the widespread use of numerical control machine tools, the application of PCBN tools that can achieve high efficiency, high stability and long-life processing has become increasingly popular. At the same time, many advanced cutting concepts have been introduced, such as the classification of the market status of tensile machine instruments and meters, the classification of the market status of instruments and meters, according to the new national economic classification norms, high-speed cutting, hard processing, turning instead of grinding Dry cutting, etc. PCBN tool material has become an indispensable and important tool material in modern machining

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