Abstract : Modern metal cutting requires high cutting speed, high feed rate, high reliability, long life, high precision and good cutting control. The emergence of coated tools has made a major breakthrough in cutting performance. It combines the tool base with the hard film surface. Due to the good toughness and high strength of the substrate, the hard film surface has high wear resistance. And a low coefficient of friction, which greatly improves the performance of the tool.
1 Modern metal cutting requirements for tools
Modern metal cutting, the requirements of the tool are high cutting speed, high feed rate, high reliability, long life, high precision and good cutting control. The emergence of coated tools has made a major breakthrough in cutting performance. It combines the tool base with the hard film surface. Due to the good toughness and high strength of the substrate, the hard film surface has high wear resistance. And a low coefficient of friction, which greatly improves the performance of the tool.
Since the advent of hard-coated tools in the early 1970s, chemical vapor deposition (CVD) and physical vapor deposition (PVD) technologies have been developed, opening a new chapter in the history of tool performance. Compared with uncoated tools, coated tools have significant advantages: they can improve machining efficiency, improve machining accuracy, and extend tool life, thus ensuring the quality of workpieces and reducing processing costs.
2 tool hard coating new material
2.1 Development of multi-component, composite hard coating materials
The hard film on the surface of the tool has the following requirements on the material: 1 high hardness and good wear resistance; 2 stable chemical properties, no chemical reaction with the workpiece material; 8 heat and oxidation resistance, low friction coefficient, and strong adhesion to the substrate. It is difficult for a single coating material to fully meet the above technical requirements. The development of coating materials has been developed from the initial single TiN coating, TiC coating, TiC-Al2O3-TiN composite coating and multi-composite coatings such as TiCN and TiAlN. Now the latest development of TiN/NbN, TiN/CN, and other multi-component composite film materials have greatly improved the performance of tool coatings.
Among the hard coating materials, TiN is the most mature and widely used process. At present, the use rate of TiN coated high speed steel tools in industrialized countries has accounted for 50%-70% of high speed steel tools, and the use rate of some non-reground complex tools has exceeded 90%. Due to the high technical requirements of modern metal cutting tools, TiN coatings are increasingly unsuitable. The TiN coating has poor oxidation resistance. When the temperature is up to 500 ° C, the film is obviously oxidized and ablated, and its hardness can not meet the needs.
TiC has a high microhardness, so the wear resistance of the material is good. At the same time, it adheres firmly to the substrate. When preparing a multi-layer wear-resistant coating, TiC is often used as the underlying film in contact with the substrate, which is a very common coating material in coating tools.
The development of TiCN and TiAlN has brought the performance of coated tools to a higher level. TiCN can reduce the internal stress of the coating, improve the toughness of the coating, increase the thickness of the coating, prevent the crack from spreading, and reduce the chipping of the tool. Setting TiCN as the primary wear layer of the coated tool significantly increases tool life. TiAlN has good chemical stability and anti-oxidation wear. When processing high-alloy steel, stainless steel, alloy, and nickel alloy, it has a life expectancy of 3-4 times that of TiN coated tools. If there is a high Al concentration in the TiAlN coating, a very thin non-state Al2O3 is formed on the surface of the coating during cutting to form a hard inert protective film, which can be used more effectively. High-speed machining. The oxygen-doped titanium-titanium carbide TiCNO has high microhardness and chemical stability and can produce a coating equivalent to TiC+Al2O3 composite coating. Some transition metal nitrides, carbides, borides and their multi-component compounds, some of which have a relatively high hardness, can be developed for coating tools, which will make a new breakthrough in the performance of coated tools. .
2.2 Application of low pressure gas phase synthetic diamond film
Among the above hard film materials, there are three types in which the microhardness HV can exceed 50 GPa: a diamond film, a cubic boron nitride CBN, and a carbon nitride β-C3N4. The emergence of these few ultra-high hardness film materials has opened up a very rare and expensive natural diamond for the development of hard films for coated tools, which is far from meeting the needs of modern industry. In the mid-1950s, General Motors of the United States artificially synthesized diamond to obtain granular and powdered diamond. Due to the difficulty in processing granular diamond, it is difficult to apply it to the surface of the tool. The polycrystalline diamond inserts (PCD) commonly used in the mechanical industry also limit their performance due to their single geometry, no chipbreaks and reasonable geometric parameters. In the early 1970s, diamond film was synthesized by low-pressure chemical vapor deposition. After more than 20 years of technical research, the technology of low-pressure gas phase synthesis of diamond finally made a major breakthrough. Research on diamond has become a hot topic in the world.
Diamond and graphite are allotropes, diamond bodies are cubic, belonging to the Fd3m space group; and graphite is a hexagonal line, belonging to the R3m space group. Due to the different bonding modes between atoms, the performance difference is very large. From the theory of thermodynamics, graphite is more stable than diamond. Low-pressure vapor-grown diamond, in the phase diagram of carbon, is carried out in a region where graphite is steady state and diamond is metastable. However, since the chemical potentials of the two phases are very close, both phases can be formed. The key technology for low pressure gas phase synthesis of diamond is to inhibit the graphite phase and promote the growth of the diamond phase. Commonly used synthetic methods include hot wire method, plasma enhanced chemical vapor deposition (PECVD), including microwave PCVD, electron cyclotron resonance ECR-PCVD, DC and RF PCVD, DC and high frequency arc discharge thermal plasma. The energy input during the reaction (such as RF power, microwave power, etc.), the activation state and optimal ratio of the reaction gas, and the nucleation mode of the deposition process are decisive for the formation of the diamond film. The crystal form and lattice constant of the substrate material have a great influence on the nucleation growth of the diamond film. When the diamond phase and the graphite phase simultaneously nucleate on the substrate, the graphite phase will grow rapidly. If a high concentration of hydrogen is present, it will corrode the grown graphite phase and remove the graphite phase. Although it can also corrode the diamond phase, it is much slower, thus inhibiting the growth of the graphite phase. purpose. Many deposited diamond films require temperatures from 600 ° C to 900 ° C, so this technique is commonly used to deposit diamond films on the surface of cemented carbide tools.
The commercialization of diamond carbide tools is a major achievement in coating technology in recent years.
2.3 cubic boron nitride CBN thin film technology yet to be broken
Compared with synthetic diamond film, the research work of synthetic CBN film is carried out later. BN has 3 isomers:
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