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Research status and development of TiCN coating on the surface of precision mold parts and cutting tools

As manufacturing technology develops in the direction of high speed, high efficiency, high precision, green and intelligent, intelligent processing has put forward higher requirements for the performance of precision mold manufacturing. For example, uncoated cutting tools have been developed due to defects such as low surface hardness and poor wear resistance. It is difficult to meet the cutting requirements of dry, high-speed conditions and difficult-to-machine materials such as titanium alloys. Coated tools have become the main choice for modern cutting processing due to their high surface hardness, good wear resistance and high temperature stability. TiC and TiN coatings are the earliest hard protective coatings applied to tool surfaces.The hardness of the TiN coating is about 2200HV, the friction coefficient with steel is 0.55, and the wear resistance is good. The maximum operating temperature is about 550°C. It is an ideal coating material for low-speed cutting tools and has been widely used. The hardness of the TiC coating is higher than that of the TiN coating, which is about 3300HV. During the friction process, due to the lubricating effect of the C element, the friction coefficient with steel is as low as 0.15. However, the TiC coating is brittle and has low toughness, which makes it extremely difficult to use during use. Easy to collapse and rarely used. Both TiC and TiN coatings belong to the cubic crystal system. TiC has a body-centered cubic structure and TiN has a face-centered cubic structure. TiC and TiN can dissolve in each other to form a TiCN solid solution.

TiCN coating combines the advantages of TiN and TiC coatings. Compared with TiN coating, the microhardness is significantly improved, and its toughness is better than TiC coating. The friction coefficient of TiCN coating is about 0.2, which has good friction reduction. Performance, widely used in taps, drills and milling cutters, especially suitable for processing aluminum alloy and other non-ferrous metals and alloys. The preparation methods of TiCN coatings mainly include surface coating technologies such as chemical vapor deposition technology, physical vapor deposition technology, plasma-assisted chemical vapor deposition technology, and plasma spraying technology. The disadvantage of TiCN coating is that it has poor thermal stability. It begins to oxidize at around 300°C. At temperatures above 400°C, it will be severely oxidized and peel off. Therefore, the modification of TiCN coatings has attracted much attention and has become one of the research hotspots of tool surface coating materials in recent years.The following table is the TICN related table of our factory

This article summarizes the structure, properties, preparation methods and processes of TiCN coatings, as well as the development direction of TiCN coatings at home and abroad in recent years.

1. Microstructure of TiCN coating

TiCN coating is a ternary hard coating formed by alloying based on TiN binary coating. C element is an implanted alloying element. The crystal structure of the TiN coating is a NaCl type face-centered cubic structure. After adding the C element, the N atoms in the TiN lattice are partially replaced by C atoms, forming a Ti(C,N) solid solution. The crystal structure of the TiCN coating is still NaCl. Face-centered cubic structure. The atomic radius of C is 0.091nm, and the atomic radius of N is 0.075nm. Since the C atomic radius is larger than the N atomic radius, when C atoms replace N atoms, the lattice expands and the lattice constant becomes larger, which is reflected in the XRD diffraction pattern. This is because the TiCN diffraction peak peak position shifts to a low angle. The microstructure of the TiCN coating is related to the C content in the coating. When the C content is low, the TiCN coating still maintains the structural orientation of the TiN coating, with the (111) crystal plane being the dominant one.As the N content in the coating increases, the peak intensity of the (111) crystal plane gradually decreases and the peak shape broadens, and finally the TiCN coating forms an amorphous state. It can also be considered that the TiCN coating is formed by introducing N element into the TiC coating. The N element is regarded as an alloying element. At this time, the diffraction peak of the TiCN coating is shifted to a higher angle than the TiC coating. For TiCN coatings with very low N content, the crystallinity of the coating is very low and is mainly amorphous. As the N content in the coating increases, the intensity of the diffraction peak of the TiCN coating phase increases, the peak shape becomes sharper, and the crystallinity increases. At the same time, the orientation of the diffraction peak of the coating gradually changes from (200) to (111) crystal. noodle.

The TiN binary coating forms a typical columnar structure during the deposition and growth process, and the columns are thick. Adding C element to the coating is beneficial to inhibit the formation of columnar grains. With the addition of C element, the columnar structure of TiCN coating is significantly weakened. Studies have pointed out that the columnar width of the TiN coating is about 30nm. The columnar width of the TiCN coating formed after adding the C element is reduced to 5nm, and the structure is obviously refined. As the C content in the coating increases, in addition to solid solution in the TiN lattice, C atoms will also be bonded to form an amorphous structure containing graphite-like (sp2) and diamond-like (sp3) structures. Phase, TiCN grains are embedded in the skeleton of the amorphous carbon structure, which inhibits the growth of TiCN grains, thus achieving the effect of refining the grains. The columnar or island-like structures originally formed by stacking growth are also eliminated, and the coating forms a non-directional dense structure. This nanocrystalline/amorphous composite structure has better thermal stability. The C element in the coating not only refines the grains, but also inhibits grain growth at high temperatures, stabilizing the coating structure.

2.Performance of TiCN coating

2.1 Hardness

Hardness is a very important performance indicator of tool surface coating, which determines the wear resistance of the coating. The hardness of TiCN coating is relatively high, usually 2300~3800HV, which is closely related to the C content in the coating. When the carbon content in the coating is low, C atoms exist in the form of solid solution. As the C content increases, the hardness of the coating increases linearly or approximately linearly. When the C atoms in the coating reach saturation or are close to saturation, the C atoms begin to form a C-C or CNx amorphous phase. At this time, the hardness of the coating reaches the maximum value. If the C content in the coating continues to increase, the coating’s The hardness gradually decreases. The hardening effects of C atoms mainly include C-Ti bond strengthening, solid solution strengthening and fine grain strengthening. The Ti—C bond energy is higher than the Ti—N bond, and the bond is stronger. When a small amount of Ti—C bonds are formed in the coating, the hardness of the coating can be increased. The hardness of the TiC coating is also higher than that of the TiN coating. However, the hardness of the TiC coating is still lower than that of the TiCN coating. This is because in addition to the role of Ti-C bonds in the coating, C atoms are solidly dissolved in the TiN lattice. Due to the different radii of C and N atoms, TiN The crystal lattice is distorted, making the coating harder. As mentioned before, C atoms can refine the grains. According to the Hall-Petch effect, the hardness of the TiCN coating increases with the addition of C atoms.

In addition to being related to the C atom content, the hardness of the TiCN coating is also related to the orientation of the coating crystal structure, organizational stress, coating thickness, preparation method and other factors. In face-centered cubic crystals, the (111) crystal plane is a close-packed atomic plane, and the hardness of TiCN coatings with (111) preferred orientation is often higher than that with (200) preferred orientation. The tensile stress in the coating is detrimental to the hardness, and a certain residual compressive stress is conducive to improving the hardness of the coating. However, if the stress is too large, the coating and the substrate will not be well bonded, and the coating will easily peel off. Usually, the greater the thickness of the coating, the higher the hardness of the coating. This is because the measurement of coating hardness will be affected by the matrix, making the measured hardness value lower. In order to reduce the interference of the substrate on the coating hardness measurement, the indentation depth is often controlled within one-tenth of the coating thickness. In addition, the thicker the coating, the greater the difference between the surface layer and the deep structure of the coating. The unevenness of the structure will also affect the hardness of the coating. There are differences in the hardness of TiCN coatings prepared by different preparation methods and processes. The hardness of TiCN coatings prepared by PVD is significantly greater than that of coatings prepared by CVD. By adjusting the coating deposition process within the same method, the hardness also increases. changes happened.

2.2 Wear resistance

Whether the coating is wear-resistant or not is directly related to the service life of the coated tool. Generally, the higher the hardness of the coating, the better the wear resistance of the coating. Due to various strengthening effects, the TiCN coating has higher hardness and therefore has better wear resistance. Research shows that under high-speed milling conditions, the flank wear rate of TiCN-coated milling cutters is only about one-third that of TiN-coated milling cutters. When drilling, the wear amount of TiCN-coated drill bits is still lower than that of TiN-coated drill bits when the number of holes drilled is twice that of TiN-coated drill bits. The higher the hardness of the TiCN coating, the greater the hardness difference with the base material, and the lower the bonding strength of the interface between the coating/base. The addition of C atoms distorts the TiN lattice, forming stress at the interface of the coating/base. Reduce the bonding strength. The scratch test shows that the bonding force between the TiCN coating and the steel matrix is ​​42N, which is lower than the bonding force between the TiN coating and the matrix.

In the past, coating research focused on solving the problems of hardening and toughening and enhancing the combination of coating and substrate. Under the new normal development of cutting processing technology and tool manufacturing technology, improving the thermal stability and red hardness of tool surface coatings has become a key issue. It is an equally important research topic as hardening, toughening and enhanced bonding. Multilayering, multi-component alloying and nanotechnology can delay the starting temperature of thermal decomposition of TiCN coating materials and improve the high-temperature hardness and high-temperature wear resistance of the coating. They are the development direction of tool surface coating materials in the future.

Chris

Greetings! I’m Chris, deeply fascinated by mechanical parts and engineering. Let’s embark on a journey through design, functionality, and innovation in the hardware world.

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