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 diamond film composed of polycrystals on a cemented carbide substrate. It is used to process non-ferrous metals such as silicon aluminum alloy and copper alloy, engineering materials such as glass fiber and cemented carbide, and the tool life is 50 to 100 times that of ordinary cemented carbide tools. Diamond coating uses many diamond synthesis technologies, the most common of which are hot wire method, microwave plasma method and DC plasma spray method. By improving the coating method and the bonding of the coating, diamond-coated tools have been produced and applied in industry.
Diamond-like coating has obvious advantages in the mechanical processing of certain materials (Al, Ti and their composite materials). The microstructure of diamond-like coating deposited by low-pressure vapor phase is still quite different from that of natural diamond. In the 1990s, low-pressure vapor phase 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 internal stress. The hydrogen in DLC will slowly release at higher temperatures, causing the coating to work unstably. The hardness of hydrogen-free DLC is higher than that of hydrogen-containing DLC. 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 structural design of depositing superhard DLC coating is Ti-TiC-DLC gradient
transformation coating, so that the hardness gradually increases from the softer steel substrate to the superhard DLC coating on the surface. This type of composite coating not only maintains high hardness and low friction coefficient, but also reduces brittleness, improves bearing capacity, bonding force and wear resistance.
After years of research, it has been shown that due to the high internal stress, poor thermal stability and catalytic effect between diamond-like coating and ferrous metal, the SP3 structure is transformed into SP2, which determines that it can only be used to process non-ferrous metals, thus limiting its further application in machining. However, recent studies have shown that the hardness of diamond-like coatings (also called graphite-like coatings) based on SP2 structure can reach 20-40GPa, but there is no problem of catalytic effect with ferrous metals. Its friction coefficient is very low and it has good moisture resistance. It can be used with coolant or dry cutting during cutting. Its life is doubled compared with uncoated knives. It can process steel materials, which has aroused great interest from coating companies and tool manufacturers. 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 that appeared after artificial 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 thermal expansion coefficient, etc.), it also has some properties that are superior to diamond. CBN is chemically inert to iron, steel and oxidizing environment. When oxidized, it forms a thin layer of boron oxide, which provides chemical stability for the coating. Therefore, it has excellent heat resistance when machining hard iron and gray cast iron. It can also cut heat-resistant steel, hardened steel, titanium alloy, etc. at relatively high cutting temperatures, and can cut hardened rolls with high hardness, carburized and quenched materials, and silicon aluminum alloys that are very difficult to machine, which are very prone to tool wear.
Since the successful preparation of pure CBN coatings in 1987, a research boom in CBN hard coatings has been set off internationally. The methods for low-pressure gas phase synthesis of CBN coatings mainly include CVD and PVD. CVD includes chemical transport PCVD, hot wire assisted heating PCVD, ECR-CVD, etc.; PVD includes reactive ion beam plating, active reaction evaporation, laser evaporation ion beam assisted deposition, etc. Research
The results show that progress has been made in synthesizing CBN phases, good bonding to cemented carbide substrates and suitable hardness. Currently, the maximum cubic boron nitride deposited on cemented carbide is only 0.2~0.5μm. If commercialization is to be achieved, reliable technology must be used to deposit high-purity and economical CBN coatings with a thickness of 3~5μm, and its effect must be confirmed in actual metal cutting processing.
