Superhard tools play an important role in the automotive, aerospace, energy, military and mechanical fields. Super-hard tools refer to tools made of super-hard materials such as diamond and CBN and their composite materials. At present, PCD (Polycrystalline Diamond) tools and PcBN (polycrystalline cubic boron nitride) tools are mainly used. Polycrystalline diamond is a polycrystalline material obtained by sintering diamond fine powder with a metal binding agent such as cobalt under high temperature and high pressure. Although the hardness is slightly lower than that of single crystal diamond, its grains are arranged disorderly, isotropic, and have no cleavage plane. Polycrystalline diamond cutters have a low coefficient of friction, excellent thermal conductivity and low coefficient of expansion. Its hardness is 2 to 4 times that of cemented carbide, and the tool life is more than 10 times that of cemented carbide. Moreover, the price of raw materials used for polycrystalline diamond is dozens of times lower than that of natural diamond. Polycrystalline diamond composite chip cutters use cemented carbide as the base material, have weldability and good compatibility with polycrystalline diamond, and have good toughness and hardness. Therefore, polycrystalline diamond has both the hardness of diamond and the toughness and weldability of cemented carbide.
As a super-hard tool material, diamond has been used in processing for hundreds of years. During the development of the tool, the tool material for the knife was from about 1890 to 1950, and the typical material used was high-speed steel. Germany pioneered the development of cemented carbide materials in 1927, and subsequently, cemented carbide materials were widely used in the field of tool manufacturing. From 1950 to 1959, with Sweden and the United States successfully synthesizing synthetic diamonds, superhard materials—diamonds began to gradually replace cemented carbide materials as the main knife tool materials. From 1970 to 1979, with the successful birth of polycrystalline diamond prepared using high-pressure synthesis technology, synthetic polycrystalline diamond became an effective substitute for natural diamond. The material problems of diamond tools have been effectively solved, making the use of diamond tools from stone, electronics and automobiles to the aerospace industry.
Diamond tools have the advantages of high hardness, high compressive strength, good thermal conductivity and good wear resistance. These characteristics (advantages) are closely related to the crystal state of diamond. The reason why diamond is extremely hard is that in diamond crystals, four valence electrons of carbon atoms are bonded in a tetrahedral structure, and each carbon atom forms a covalent bond with four adjacent atoms. This structure has strong directivity and adhesion. Polycrystalline diamond is sintered with fine-grained diamond and binder with different orientations. Because of this, polycrystalline diamond has the characteristics of polyisotropy. Although its abrasion resistance and hardness are not as good as single crystal diamond, it is more difficult to crack along a single cleavage plane than single crystal diamond, and it also has hardness and abrasion resistance that are second only to single crystal diamond.
The surface Vickers hardness of polycrystalline diamond is greater than or equal to 1800 HV. The surface Vickers hardness of cemented carbides is far less than that of polycrystalline diamond. Polycrystalline diamond tools do not suffer from heat collection and workpiece burns due to slow heat extraction during the production process, because their thermal conductivity is higher than Cu, and is up to 700W / mK. The cutting force of polycrystalline diamond tools can be significantly reduced because the friction coefficient of polycrystalline diamond tools is generally only 0.1 to 0.3, which is only about one-third of the friction coefficient of cemented carbide. The coefficient of thermal expansion of polycrystalline diamond is only 0.0000009 to 0.00000118, which is about one fifth of that of cemented carbide. In addition, polycrystalline diamond cutters are not easy to produce sticky knives during production and use, and chips are not easily adhered to the tip to form chips, because the affinity between polycrystalline diamond and non-metallic and non-ferrous metal materials is extremely low.
Polycrystalline diamond tools are mainly used in the following two aspects:
(1) He is mainly used for the processing of non-ferrous metal materials. If ordinary tools are used, defects such as tool wear and low processing efficiency are easy to occur. Polycrystalline diamond tools can show excellent processing performance.
(2) For the processing of non-metallic materials, polycrystalline diamond cutters are suitable for processing non-metallic materials that are difficult to process, such as stone, hard carbon, CFRP and wood-based panels.