CBN cubic strain sphalerite structure, F43m space group; h-BN hexagonal line graphite structure, P6/mmc space group; w-BN hexagonal line wurtz structure, P63mc space group. The properties of the three isomers vary widely, and h-BN has a structure very similar to graphite and has a very soft texture. In w-BN and CBN, B and N atoms are all formed into a tetracoordinate structure, which are all superhard materials. The CBN obtained by the high temperature and high pressure method is granular crystal, the highest microhardness is up to 84.3 GPa, and the highest microhardness of the CBN film is 61.8 GPa, which is no less than the diamond film. CBN is second only to diamond in terms of hardness and thermal conductivity, and has excellent thermal stability. It does not oxidize when heated to 1000 ° C in the atmosphere. CBN has extremely stable chemical properties for iron group metals. Unlike diamond, which is not suitable for processing steel, it can be widely used in the finishing and grinding of steel products. In addition to excellent wear resistance, CBN coating can process heat-resistant steel, titanium alloy and hardened steel at a relatively high cutting speed. It can cut high-hardness chill rolls, carbon-doped materials and tool wear. Very severe Si-Al alloys, etc. The methods for synthesizing CBN thin films by low pressure gas phase mainly include CVD and PVD methods. CVD includes chemical transport PCVD, hot wire assisted heating PCVD, ECR-CVD, etc.; PVD has reactive ion beam plating, active reactive evaporation, laser evaporation ion beam assisted deposition, and the like. CBN synthesis technology has a lot of work to do in basic research and application technology, including reaction mechanism and film formation process, plasma diagnosis and mass spectrometry, determination of optimal process conditions, and development of high-efficiency equipment. 2.4 Carbon nitride that may exceed the hardness of diamond In the late 1980s, American scientists I'IU and Co-henE4' designed a new compound p-C3N4 similar to p-Si3N4, using solid-state physics and quantum chemistry theory to calculate its phantom modulus, energy band and character constant. It was found that the modulus of carbon nitride reached the numerical range of diamond. Since the hardness of the substance is proportional to the moduli of the body, the hardness of the C3N4 is likely to reach the hardness of the diamond, which has attracted the attention of scientists all over the world. In 1994, I'IU published his new research achievement E53. He used the ab initio calculation of variable character model molecular dynamics (VCS-MD) to extend the theoretical study of low-energy C3N4 solids, pointing out that C3N4 may have three kinds. Structure: p-phase of hexagonal system, sphalerite structure of cubic system and graphite-like structure of trigonal line. In 1996, Jeter and Hemley in the United States still used the first-principle ab initio calculation, but changed the calculation process. When the initial conditions are used, the enthalpy gradient method is used to minimize the degree of electron freedom; when the boundary condition is used, the periodic function is used to spread the wave function of the electron as a plane wave; the extended standard conservation and the conservation of intensity (ENHC) are used. Five structures of C3N4 were obtained, which were n-phase, p-phase, cubic phase-deficient zinc blende structure, cubic-phase willemite E structure, and graphite-like phase. Except for the graphite-like phase, the other four are superhard materials. The cubic modulus of the Cu-zinc ore E structure c-C3N4 exceeds that of diamond. Therefore, it is possible for carbon nitride to have a hardness that meets or exceeds the diamond. The success of synthesizing carbon nitride is an outstanding example of molecular engineering. Carbon nitride, a superhard material, is expected to have many other valuable physicochemical properties, and research on carbon chloride has become a hot topic in the world of materials science. The main methods for synthesizing carbon nitride include DC and RF reactive sputtering, laser evaporation and ion beam assisted deposition, ECR-CVD, and dual ion beam deposition. The carbon steel film obtained by electron beam evaporation ion beam assisted deposition method at Okayama University in Japan has reached the highest microhardness of carbon nitride: 63.8 GPa. China's Tsinghua University also obtained high hardness carbon nitride of 60.8GPa. The hardness of the carbon nitride synthesized by Wuhan University reached 50.OGPa and deposited on the high-speed steel twist drill to obtain very good drilling performance. The key technique for preparing a carbon nitride superhard coating is to avoid precipitation of the graphite phase. 3 How to improve the use of coated tools The choice of tool material depends on the cutting conditions and on which face will be reground. For example, if the rake face of the tool is reground, it is more advantageous to use high speed steel with drills because the steel is more resistant to crater wear after the tool has no coating on the rake face. The advancement of tool materials has led to the use of high-speed steel, hard alloy, various toughened ceramics, milled-base cermets, poly-diamonds and c-BN, which greatly improved the machining efficiency of metal cutting. Knives of each material have their own advantages and disadvantages and are therefore of particular use. Coated tools place new demands on tool geometry. It is generally believed that improvements in tool geometry, such as the rake angle, chip evacuation space, etc., should focus on the chip removal capability to accommodate increased cuts at higher feed rates and higher speeds. The coated knife has a high processing efficiency, which allows for a higher feed rate and cutting speed (which can be increased to 2-3 times the original cutting speed). For difficult-to-machine materials, the coating improves tool performance. The reason why the tool with super hard coating has a small amount of wear is due to the high hardness, high melting point and excellent thermochemical stability of the superhard compound of the film layer. Superhard compounds are mostly composed of transition metal nitrides, carbides and borides. They are combined with strong covalent bonds and have a low standard free energy of formation, which constitutes a very stable system and does not significantly reduce hardness at high temperatures. These layers exhibit higher resistance to mechanical wear and thermal wear than tool materials such as cemented carbide and high speed steel. Coating conditions, process parameters, pre-plating substrate pretreatment, etc. are very important for the preparation of high quality coatings. The state of the tool surface is critical to the adhesion of the coating. The surface of the workpiece to be plated must be free of other layers, burns, rust, oil or other contaminants. The workpiece is subjected to strict sand blasting and degreasing cleaning, and ion bombardment cleaning is performed before the hard film is grown in a vacuum. The tool used for different coating materials has different effects. Low-speed cutting, TiC coating has an advantage; high-speed cutting, TiN is more suitable; HfN has higher thermochemical stability than TiN, suitable for working at higher cutting speeds. Compared to TiN and A1203 coatings, A1203 coatings have a significant advantage in high-speed cutting, while TiN-coated tools have a longer service life at low-speed cutting. Tool life and film thickness also have a certain relationship. If the blade surface wear is used as the reference, the tool life will increase with the increase of the film thickness, but the saturation will be achieved when the film thickness is 5μm, that is, the life is no longer significantly increased; but if the previous guilloche depth is the reference of the tool life, the tool life In proportion to the film thickness, no saturation phenomenon was observed. When the film is too thick, it is easy to cause peeling. Now the coating thickness of the turning tool is 5~m-10?m. For hard coatings of milling cutters, the effect of film thickness is different. When milling a steel workpiece, the tool life is the longest when the film thickness is about 2 μm, and the life is decreased when the film thickness is increased. However, when machining cast iron or the like having a small impact, the optimum film thickness changes in a thicker direction. TiC coatings have the best results in milling, while Al2O3 coatings do not show the advantages of turning. Carbide tools are usually coated by CVD, but PVD coating treatment hardly causes the edge strength to drop. PVD coated carbide milling cutters are more durable than CVD coatings. For the wear resistance of general high speed steel tools, CVD coatings are superior to PVD coatings, but precision, complex shapes, expensive, and non-reground high speed steel tools are mostly PVD coatings. It is a rather complicated technique to improve the use of coated tools and to make full use of the hard coating. In order to achieve an optimized combination, a database of coated tools is created, and different workpieces are selected by computer to select tool coating materials and processing parameters. The situation becomes simple and effective, thus achieving high-quality, high-efficiency, low-cost processing targets. Previous page