6.2 Cubic Boron Nitride (CBN) Coating
CBN is another superhard material that emerges after synthetic diamond. It has many excellent physical and chemical properties similar to diamonds (such as ultra high hardness, second only to diamond, high wear resistance, low friction coefficient, low In addition to the coefficient of thermal expansion, etc., it also has some properties superior to diamond. CBN is chemically inert to iron, steel and oxidizing environments and forms a thin layer of boron oxide during oxidation. This oxide provides chemical stability to the coating, so it is heat resistant when processing hard iron and grey cast iron. It is extremely excellent, and it can cut heat-resistant steel, hardened steel, titanium alloy, etc. at a relatively high cutting temperature, and can cut high-hardness chill rolls, carburized and quenched materials, and silicon-aluminum alloys with very serious tool wear. Processing materials. Since the successful preparation of pure CBN coatings by Inagawa et al in 1987, the research boom of CBN hard coatings has been launched internationally. The methods for low pressure gas phase synthesis of CBN coatings are mainly 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. The research results show that the progress has been made in the synthesis of CBN phase, good adhesion to cemented carbide matrix and suitable hardness. At present, the maximum amount of cubic boron nitride deposited on cemented carbide is only 0.2-0.5μm. In order to achieve commercialization, reliable technology must be used to deposit high-purity and economical CBN coatings with a thickness of 3 to 5 μm, which is confirmed in actual metal cutting.
6.3 CNx coating
In the 1980s, American scientists Liu and Cohen designed a new β-C3N4 compound like β-Si3N4. Using solid physics and quantum chemistry theory, it was calculated that its hardness might reach diamond, which attracted the attention of scientists all over the world. Synthetic carbon nitride has become a hot topic in the world of materials science. F Fujimoto of Okayama University in Japan used electron beam evaporation ion beam assisted deposition to obtain a carbon nitride coating of 63.7 Gpa. The hardness of carbon nitride synthesized by Wuhan University reached 50GPa, and it was deposited on high-speed steel twist drill to obtain very good drilling performance. The main methods for synthesizing carbon nitride include true flow and RF reactive sputtering, laser evaporation and ion beam assisted deposition ECR-CVD, and dual ion beam deposition.
7 soft coating
The research and development of soft-coated tools has opened up new ways to improve tool performance, providing new ideas and new research fields for the design of new coated tools. Under special conditions, the soft coating film of the soft coated tool will be transferred to the surface of the workpiece material to form a transfer film, so that the friction during the cutting process occurs between the transfer film and the lubricating film, thus having excellent tribological properties. Such as: low friction coefficient, high load-bearing limit, good chemical stability at high temperature, can adapt to working temperature above 1000 °C, suitable for use under special environmental conditions such as high temperature and large load, and can be applied to dry cutting, which can reduce processing. Cost, etc., have great theoretical and practical significance and have broad application prospects. Soft coating materials mainly include MoS2-based soft coating materials and WC/C medium-hard coating materials. The former can greatly improve the cutting performance of the cutter and prevent the formation of built-up edge; the latter has a slightly higher friction factor than the MoS2 coating, but its wear resistance is better. The soft coating can be used alone, or it can be hard coated on the surface of the tool and then coated with a soft coating of MoS2, which is good for cutting steel or processing high silicon aluminum alloy.
Soft-coated tool technology has just started in foreign countries, and there are few reports on this aspect in China. Recently, a new process called “MOVIC†soft coating was developed in Switzerland. A layer of MoS2 was applied to the surface of the tool and tested under industrial production conditions, and preliminary results were obtained. Germany's Guhring Tool Company's new MolyGlide soft coating has a hardness of only HV20~50. It is coated with a 0.2-0.5μm MoS2 anti-friction coating on the basis of TiN hard coating. The friction coefficient is only 0.05~. 0.10, which is 1/4 of the friction coefficient between TiN coating and steel. It has the function of non-adhesion and self-lubrication. It avoids the phenomenon of sticking knife and built-up edge when processing high-plasticity and high-toughness working materials. It is very suitable for dry type. Light metal materials such as aluminum alloys and titanium alloys are drilled under micro-lubrication conditions. The soft and hard coatings exhibited at CIMT 2005 are TiAlN/MoS2, TiAlN/WC/C and TiN/TiCN/MoS2. Other sulfides such as WS2, TaS2 and other soft coatings have also made some progress.
8 Conclusion
The tool coating technology better solves the contradiction between tool strength and toughness. By optimizing the lattice structure of the coating, adding alloying elements and multi-layer coating combination to improve the toughness of the coating to adapt to different cutting conditions, Greatly improved tool durability and cutting speed. At present, the research hotspot is to develop a new tool coating material with nano-multilayer structure and nano-composite coating. The soft coating of tool with special properties will also become a new research field of tool coating material.
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