Surface coating technology for cutting tools (coated carbide and coated high-speed steel tools) is a material surface modification technology that has developed in recent decades to meet market demands. Coating technology can effectively extend the service life of cutting tools, endow them with excellent comprehensive mechanical properties, and thus significantly improve machining efficiency. For this reason, coating technology, along with cutting materials and cutting processes, is considered one of the three key technologies in the field of cutting tool manufacturing. Cutting tool coating refers to applying a layer of material with high hardness and wear resistance to the surface of a mechanical cutting tool. To meet the requirements of modern machining for high efficiency, high precision, and high reliability, manufacturing industries worldwide are increasingly emphasizing the development of coating technology and its application in tool manufacturing. In factories of industrialized countries, coated tools account for nearly 60% of the total. Currently, the main coating technology methods include vapor deposition, sol-gel method, and thermal spraying. Among these, vapor deposition is more widely used and produces coatings of higher quality. Vapor deposition technology can generally be divided into physical vapor deposition (PVD, equipment shown in Figure 1) and chemical vapor deposition (CVD, equipment shown in Figure 2). Figure 1: Physical vapor deposition equipment. Figure 2: Chemical vapor deposition equipment. Methods for preparing cutting tool surface coatings using vapor deposition mainly include the following: magnetron sputtering deposition coating, arc ion plating deposition coating, high-temperature chemical vapor deposition coating, medium-temperature chemical vapor deposition coating, and plasma-enhanced chemical vapor deposition coating. Among these, high-temperature chemical vapor deposition, magnetron sputtering deposition, and arc ion plating are the most commonly used. The following will discuss the advantages and disadvantages of each coating technology in conjunction with their different mechanisms. Magnetron Sputtering Deposition Technology Magnetron sputtering deposition coating technology belongs to the glow discharge category and utilizes the principle of cathode sputtering for film deposition. The film particles originate from the cathode sputtering effect of argon ions on the cathode target during glow discharge. After the argon ions sputter the target atoms, they are deposited onto the workpiece to form the desired film layer. Because a magnetic field is introduced into the target part of the sputtering device, the magnetic field lines confine the electrons to the vicinity of the target surface, extending their trajectory in the plasma, thereby increasing their participation in the collision and ionization process of gas molecules. Magnetron sputtering deposition has the following advantages: (1) high deposition rate and low target voltage required to maintain discharge; (2) low bombardment energy of electrons on the substrate; (3) fine film structure, since the magnetron sputtering deposition coating is obtained by atomic state particles obtained by cathode sputtering, carrying the high energy obtained from the target surface to the workpiece, which is conducive to the formation of small cores and the growth of very fine film structure; (4) magnetron sputtering deposition coating can obtain large area thin film and can be widely used. However, this method also has the following problems: (1) uneven etching of the target material. Due to the uneven distribution of magnetic field strength, the utilization rate of the target material is low. This can be achieved by reasonably designing the target structure and adding an electromagnetic field to promote the change of the magnetic field strength of the target surface, realize discharge scanning, and thus effectively improve the utilization rate of the target material. (2) low metal ionization rate. To address this, the volume of the magnet at the target center can be increased (or decreased) as required, causing some magnetic field lines to diverge to the substrate, achieving unbalanced magnetron sputtering. It's worth noting that magnetron sputtering can also be used to prepare multilayer and nanofilms. With the rapid development of high technology and emerging processing industries, the demand for depositing multilayer and nanofilms with higher performance is increasing. Therefore, magnetron sputtering technology deserves further in-depth research and development, and its application prospects are excellent. Arc Ion Plating Deposition Technology Ion plating (IP) is a new technology developed based on vacuum evaporation deposition. It introduces various gas discharge methods into the field of vapor phase deposition, allowing the entire vapor phase deposition process to take place in plasma. Arc ion plating (AIP) belongs to the category of cold field arc discharge and is a solid-state evaporation source without a fixed molten pool, often using a circular cathode arc source. The advantages of AIP are: (1) high metal ionization rate, up to 60% to 90%; (2) high deposition rate of arc ion plating; (3) good film-substrate adhesion of the deposited coating; (4) easy to obtain compound coatings such as titanium nitride, and mass production can be carried out below 200℃. The disadvantages of AIP are: (1) the presence of coarse molten droplets in the film increases the surface roughness and increases the diffuse reflection of light, thus reducing the surface brightness of the ornament; (2) coarse molten droplets are easy to peel off during cutting, causing defects on the coating surface. There are many types of coatings prepared by arc ion plating, and the fields involved are extremely wide. It can be used for the deposition of hard protective coatings, and the coatings cover various metal oxides, carbides, nitrides and some metal and alloy materials. It can also be used for the preparation of multilayer structure coatings and nano multilayer structure coatings. It has the characteristics of simple operation of arc ion plating, large utilization of coating chamber space and high production efficiency. In recent years, it has developed into an important technology for depositing hard coatings and has been rapidly developed at home and abroad. In recent years, a new coating preparation system has adopted a composite coating technology that combines arc ion plating and magnetron sputtering deposition. The system is equipped with several arcs and magnetron sputtering cathodes. The arc layer serves as a transition layer or provides the necessary wear resistance for the entire coating, while the magnetron sputtering layer provides high-temperature and chemical stability. The advantages of this composite coating technology are that the deposition process is easy to control, has good stability and repeatability, and its deposition rate (≥0.5 μm/h) is sufficient to meet the practical requirements of saving processing time in industrial production. High-Temperature Chemical Vapor Deposition Technology High-temperature chemical vapor deposition (HTCVD, generally abbreviated as CVD) technology refers to the interaction of a mixture of coating materials and gases on the surface of a cemented carbide under certain temperature conditions. This causes some components in the mixture to decompose and form a hard coating of metal or compound on the tool surface. The key to the successful implementation of this method lies in: (1) the interaction between the mixed gas as the coating material and the surface of the cemented carbide, that is, the reaction between the mixed gas of the coating material on the surface of the cemented carbide to produce deposition, or the reaction between one component of the mixed gas of the coating material and the surface of the cemented carbide to produce deposition; (2) the deposition reaction must be carried out under certain energy activation conditions. High temperature chemical vapor deposition coating has the following advantages: (1) the preparation of the required coating source is relatively easy; (2) it can deposit single-layer and multi-layer composite coatings such as metal carbides, nitrides, and oxides; (3) the bonding strength between the coating and the substrate is high; (4) the coating has good wear resistance. It is undeniable that this method has inherent defects. The main ones are as follows: (1) the coating temperature is high. That is, the coating deposition temperature is higher than 900℃, which makes it easy to generate a brittle decarburized layer (η phase) between the coating and the substrate, thus causing brittle fracture of the cemented carbide material and a decrease in bending strength; (2) the coating is in a tensile stress state, which easily leads to the generation of microcracks during use; (3) the waste gas and waste liquid emitted during the coating process will cause industrial pollution and have a significant impact on the environment. For this reason, the development of this method was somewhat restricted in the mid-to-late 1990s. Medium temperature chemical vapor deposition technology The reaction mechanism of medium temperature chemical vapor deposition (MTCVD) coating technology is that organic compounds containing CN atoms, such as trimethylamine and methylimine, are used as the main reaction raw materials. They react with TiCl4, N2, H2 and other gases at a temperature of 700℃～900℃ to produce TiCN and other coatings through decomposition and combination reactions. The advantages of MTCVD are: (1) fast deposition rate and low deposition temperature; (2) thicker coating; (3) uniform coating for workpieces with complex shapes; (4) high coating adhesion; (5) low residual stress inside the coating. In view of this, this method is easy to industrialize and is a coating method that is superior to high temperature chemical vapor deposition coating. MTCVD also has disadvantages: (1) the coating is under tensile stress, which can easily lead to the generation of microcracks during use; (2) the waste gas and waste liquid emitted during the coating process will cause industrial pollution and are not environmentally friendly. The above reasons have also restricted the development of this technology to some extent. MTCVD technology can obtain a dense fibrous crystalline coating with a thickness of 8 to 10 μm. This coating structure has extremely high wear resistance, thermal shock resistance and toughness, and is suitable for use under high speed, high temperature, high load and dry cutting conditions. In terms of tool life, it can be about twice as long as ordinary coated tool life. Plasma-enhanced chemical vapor deposition (PECVD) technology refers to a method of depositing coatings on the surface of hard alloys by generating high-energy electrons through electrode discharge to ionize gas into plasma, or by introducing high-frequency microwaves into carbon-containing compound gas to generate high-frequency, high-energy plasma, and depositing coatings from the active carbon atoms or carbon-containing groups therein. Advantages of PECVD: (1) It uses plasma to promote chemical reactions, which can reduce the coating temperature to below 600℃; (2) Due to the low coating temperature, no diffusion, phase change, or exchange reaction occurs between the hard alloy substrate and the coating material, so the substrate can maintain its original strength and toughness. Disadvantages of PECVD: (1) Large equipment investment and high cost, and high requirements for gas purity; (2) The intense noise, strong light radiation, harmful gases, metal vapor dust, etc. generated during the coating process are harmful to the human body; (3) It is difficult to coat the inner surface of small pores. The processing temperature of PECVD has been reduced to 450-650℃, which effectively suppresses the η phase and can be used for coatings such as TiN, TiCN, and TiC on thread cutters, milling cutters, and molds. However, so far, the application of this process in the field of tool coating is not widespread. Figure 3 Overview of tool coating in factories of industrialized countries Conclusion (1) The coating technology for cutting tools is still dominated by vapor deposition technology. Sol-gel method and thermal spraying method need further research and development. (2) Physical vapor deposition has the advantages of low temperature and less environmental pollution, so it has developed rapidly in recent years. Among them, magnetron sputtering deposition and arc ion plating deposition technology are the most significant. (3) Physical vapor deposition and chemical vapor deposition technology will still coexist and complement each other in the coating of cutting tools, and each will occupy its own share in the coating ratio due to its own advantages. Generally speaking, PVD coating is ideal for steel tools such as high-speed steel, sharp carbide precision cutting inserts, and carbide solid multi-blade tools (such as end mills and twist drills); most other carbide inserts can be coated using CVD coating. (4) Physical vapor deposition and chemical vapor deposition still have their own shortcomings, so improving deposition process conditions and developing new deposition methods are still the focus of further research.