Tool surface coating technology is a surface modification technology developed in response to market demand. Since its appearance in the 1960s, this technology has been widely used in the metal cutting tool manufacturing industry. Especially after the emergence of high-speed cutting technology, coating technology has been rapidly developed and applied, and become one of the key technologies of high-speed cutting tool manufacturing. This technology can form a film on the surface of cutting tool by chemical or physical methods, so that the cutting tool can obtain excellent comprehensive cutting performance, so as to meet the requirements of high-speed cutting.
In summary, the surface coating technology of cutting tools has the following characteristics:
1. Coating technology can greatly improve the surface hardness of the tool without reducing the strength of the tool. At present, the hardness can reach nearly 100 GPa.
2. With the rapid development of coating technology, the chemical stability and high temperature oxidation resistance of the films become more prominent, which makes high-speed cutting possible.
3. Lubrication film has good solid lubrication performance, which can effectively improve the processing quality and is also suitable for dry cutting.
4. Coating technology, as the final process of tool manufacturing, has little effect on tool accuracy and can be repeated.
Benefits of coated cutting tools: can greatly improve the life of cutting tools; effectively improve cutting efficiency; significantly improve the surface quality of the workpiece being processed; effectively reduce the consumption of tool materials, reduce processing costs; reduce the use of coolant, reduce costs, and benefit environmental protection.
Correct surface treatment of small circular cutters can improve tool life, reduce processing cycle time and improve surface quality. However, it may be a confusing and laborious job to choose the correct tool coating according to the processing requirements. Each coating has both advantages and disadvantages in cutting. If improper coating is selected, the tool life may be lower than that of uncoated tools, and sometimes even more problems than before.
At present, there are many kinds of tool coatings available, including PVD coatings, CVD coatings and composite coatings alternately coated with PVD and CVD. These coatings can be easily obtained from tool manufacturers or coating suppliers. This paper will introduce some common properties of tool coatings and some commonly used PVD and CVD coatings selection schemes. Each characteristic of the coating plays an important role in determining which coating is most beneficial for cutting.
TiN is a general-purpose PVD coating, which can improve tool hardness and oxidation temperature. The coating can be used for cutting tools or forming tools of high speed steel to obtain good processing effect.
Chromium Nitride Coating (CrN)
CrN coating is the preferred coating in the processing of chip tumors because of its good adhesion resistance. With this almost invisible coating, the machinability of HSS or cemented carbide tools and forming tools will be greatly improved.
CVD diamond coating can provide the best performance for cutting tools of non-ferrous metal materials. It is an ideal coating for processing graphite, metal matrix composites (MMC), high silicon aluminium alloy and many other high abrasive materials. A chemical reaction occurs, which destroys the adhesion layer between the coating and the cutting tool.
Coatings suitable for hard milling, tapping and drilling are different and have their own specific application occasions. In addition, multi-layer coatings can also be used. Other coatings are embedded between the surface layer and the tool matrix, which can further improve the service life of the tool.
TiC coating (TiCN)
Carbon element added in TiCN coating can improve tool hardness and obtain better surface lubricity. It is an ideal coating for high-speed steel tools.
Nitrogen-Aluminum-Titanium or Nitrogen-Titanium-Aluminum Coatings (TiAlN/AlTiN)
The alumina layer formed in TiAlN/AlTiN coating can effectively improve the high-temperature working life of cutting tools. The coating can be used for cemented carbide cutting tools mainly used for dry or semi-dry cutting. According to the different proportion of Al and Ti in the coating, AlTiN coating can provide higher surface hardness than TiAlN coating, so it is another feasible coating choice in the field of high speed machining.
Characteristics of Coatings
High surface hardness caused by coating is one of the best ways to improve tool life. Generally speaking, the higher the hardness of the material or surface, the longer the tool life. TiCN coatings have higher hardness than TiN coatings. Due to the increase of carbon content, the hardness of TiCN coating is increased by 33%, and the hardness range is about Hv3000-4000 (depending on the manufacturer). The application of CVD diamond coatings with surface hardness up to Hv9000 on cutting tools has been mature. Compared with PVD coated cutting tools, the life of CVD diamond coated cutting tools has increased by 10-20 times. The high hardness and cutting speed of diamond coated tools can be 2-3 times higher than that of uncoated tools, making them a good choice for cutting non-ferrous materials.
The oxidation temperature is the temperature at which the coating begins to decompose. The higher the oxidation temperature is, the more advantageous it is for cutting at high temperature. Although the room temperature hardness of TiAlN coatings may be lower than that of TiCN coatings, it has been proved that TiAlN coatings are much more effective than TiCN coatings in high temperature processing. The reason why TiAlN coating can maintain its hardness at high temperature is that a layer of alumina can be formed between the tool and the chip, and the alumina layer can transfer heat from the tool to the workpiece or chip. The cutting speed of cemented carbide tools is usually higher than that of high speed steel tools, which makes TiAlN the preferred coating for cemented carbide tools. The PVDTiAlN coating is usually used for cemented carbide drills and end milling cutters.
Wear resistance refers to the ability of the coating to resist wear. Although the hardness of some workpiece materials may not be very high, the elements added in the production process and the process adopted may cause tool cutting edge cracking or bluntness.
High friction coefficient will increase the cutting heat, which will shorten the coating life and even cause failure. Reducing friction coefficient can greatly prolong tool life. Fine, smooth or regularly textured coated surfaces help to reduce cutting heat, because smooth surfaces allow chips to slip rapidly off the rake face and reduce heat generation. Compared with uncoated cutting tools, coated cutting tools with better surface lubricity can also be processed at higher cutting speed, thus further avoiding high temperature welding with workpiece materials.
Anti caking property
The bonding resistance of the coating can prevent or reduce the chemical reaction between the tool and the material being processed, and avoid the deposition of the workpiece material on the tool. In the processing of non-ferrous metals (such as aluminium, brass, etc.), cutters often produce debris tumors (BUE), resulting in tool breakdown or workpiece size oversize. Once the processed material begins to adhere to the tool, the adherence will continue to expand. For example, when aluminium workpiece is processed with formed tap, the adherent aluminium on the tap will increase after each hole is processed, and eventually the diameter of the tap will become too large, resulting in the scrap of the workpiece size. Coatings with good adhesion resistance can play a very good role even in processing occasions where coolant performance is poor or concentration is insufficient.
Application of Coatings
Implementing high performance-to-price applications of coatings may depend on many factors, but for each specific processing application, there is usually only one or several feasible coating options. Correct selection of coatings and their properties may mean that there is a difference between marked improvement in processing performance and almost no improvement. Cutting depth, cutting speed and coolant may affect the application effect of tool coating.
Because there are many variables in the processing of a workpiece material, one of the best ways to determine which coating to choose is through trial cutting. Coating suppliers are continually developing more new coatings to further improve the high temperature, friction and wear resistance of coatings.