Tool coating material: superhard material coating

Diamond coating is one of the new tool coating materials. It uses low-pressure chemical vapor deposition technology to grow a layer of diamond film composed of polycrystalline on the cemented carbide substrate, and uses it to process non-ferrous metals such as silicon aluminum alloy and copper alloy, engineering materials such as glass fiber, and cemented carbide. The tool life is 50 to 100 times that of ordinary carbide tools.

1. Diamond, diamond-like carbon (DLC) coating

Diamond coating is one of the new tool coating materials. It uses low-pressure chemical vapor deposition technology to grow a layer of diamond film composed of polycrystalline on the cemented carbide substrate, and uses it to process non-ferrous metals such as silicon aluminum alloy and copper alloy, engineering materials such as glass fiber, and cemented carbide. The tool life is 50 to 100 times that of ordinary carbide tools. Many diamond synthesis techniques are used for diamond coating, the most common being hot wire, microwave plasma and DC plasma jet. By improving the coating method and the bonding of the coatings, diamond-coated tools have been produced and used in industry.

In recent years, countries such as the United States, Japan and Sweden have successively launched diamond-coated taps, reamers, milling cutters, and diamond-coated carbide drills for processing small holes on printed circuit boards and various indexable Blades, such as CD1810 of Sweden Sandvik Company and KCD25 of American Kennametal Company. A new process of laser plasma deposition of diamond developed by Turchan Company in the United States. Using this method to deposit diamond, because the plasma field surrounds the entire tool, the coating on the tool is uniform, and the deposition speed is 1000 times faster than the conventional CVD method. The diamond coating formed by this method has a real metallurgical bond with the substrate, and the coating has high strength, which can prevent the coating from falling off, cracks and cracks and other defects. CemeCon has a unique CVD diamond coating technology. In 2000, a production line was established to make the diamond coating technology reach the level of industrial production. It has high technical content and can produce diamond coatings in batches.

Diamond-like coatings have distinct advantages in machining certain materials (Al, Ti and their composites). The microstructure of diamond-like carbon coatings deposited by low-pressure vapor deposition is still quite different from that of natural diamond. In the 1990s, low pressure vapor deposition of DLC in the presence of activated hydrogen was often used, and the coating contained a large amount of hydrogen. Excessive hydrogen content will reduce the bonding force and hardness of the coating and increase the internal stress. The hydrogen in the DLC is slowly released at higher temperatures, causing the coating to work erratically. The hardness of DLC without hydrogen is higher than that of DLC with hydrogen, and it has the advantages of uniform structure, large area deposition, low cost, and smooth surface. It has become a hot spot in DLC coating research in recent years. American scientist A.A.Voevodin proposed that the structure of the deposited superhard DLC coating is designed as a Ti-TiC-DLC gradient transformation coating, which gradually increases the hardness from the softer steel substrate to the superhard DLC coating on the surface. This type of composite coating maintains high hardness and low coefficient of friction, reduces brittleness, and improves bearing capacity, bonding force and wear resistance. Sumitomo Corporation of Japan has launched the DL1000 coating coated with diamond DLC on cemented carbide inserts, which is used for cutting aluminum alloys and non-ferrous metals, resists sticking, and can effectively reduce the roughness of the machined surface.

After years of research, it has been shown that due to the high internal stress, poor thermal stability of the diamond-like carbon coating, and the catalytic effect between the ferrous metal and the transformation of the SP3 structure to SP2, it is determined that it can only be used in the processing of non-ferrous metals. for its further application in machining. However, recent studies have shown that the hardness of diamond-like coatings (also known as graphite-like coatings) based on SP2 structure can also reach 20-40GPa, but there is no problem of catalytic effect with ferrous metals, and its friction coefficient is very high. It has low temperature and good moisture resistance. It can be used with coolant or dry cutting during cutting. Its life is doubled compared with uncoated knives. great interest. In time, this new type of diamond-like coating will be widely used in the cutting field.

2. Cubic Boron Nitride (CBN) Coating

CBN is another superhard material after synthetic diamond. In addition to having many excellent physical and chemical properties similar to diamond (such as ultra-high hardness, second only to diamond, high wear resistance, low friction coefficient, low In addition to thermal expansion coefficient, etc.), it also has some properties better than diamond. CBN is chemically inert to iron, steel and oxidizing environments, and forms a thin layer of boron oxide when oxidized. This oxide provides chemical stability to the coating, so it is also heat resistant when machining hard iron and gray cast iron. Extremely excellent, it can also cut heat-resistant steel, quenched steel, titanium alloy, etc. at a relatively high cutting temperature, and can cut high-hardness chilled rolls, carburizing and quenching materials, and silicon-aluminum alloys with very serious tool wear. processing material.

Since the successful preparation of pure CBN coating by Inagawa et al. in 1987, the research upsurge of CBN hard coating has been set off in the world. The methods of low-pressure gas phase synthesis of CBN coatings mainly include CVD and PVD methods. CVD includes chemical transport PCVD, hot wire assisted heating PCVD, ECR-CVD, etc.; PVD includes reactive ion beam plating, active reactive evaporation, laser evaporation ion beam assisted deposition, etc. The research results show that progress has been made in the synthesis of CBN phase, good adhesion to cemented carbide substrate and suitable hardness. To achieve commercialization, reliable technology must be used to deposit a high-purity and economical CBN coating with a thickness of 3 to 5 μm, and its effect has been confirmed in actual metal cutting.

3.CNx coating

In the 1980s, American scientists Liu and Cohen designed a new compound similar to β-Si3N4, β-C3N4. Using the theory of solid state physics and quantum chemistry, it was calculated that its hardness might reach that of diamond, which attracted the attention of scientists from all over the world. Synthesis of carbon nitride has become a hot topic in the field of material science in the world. The carbon nitride coating obtained by FFujimoto of Okayama University in Japan by electron beam evaporation and ion beam assisted deposition method reaches 63.7 Gpa. The hardness of carbon nitride synthesized by Wuhan University reaches 50GPa respectively, and it is deposited on the high-speed steel twist drill to obtain very good drilling performance. The main methods for synthesizing carbon nitride include true flow and radio frequency reactive sputtering, laser evaporation and ion beam assisted deposition, ECR-CVD, dual ion beam deposition, etc.