This paper is a long article on tool coating, which answers four questions: what is the tool surface coating technology, what are the commonly used coatings, what are the characteristics of the coatings and the application fields of the coatings. Those who specialize in machining continue to look at it in the past.

Overview of tool coating

Tool surface coating technology is a surface modification technology developed according to the market demand. Since its emergence in the 1960s, this technology has been widely used in the manufacturing industry of metal cutting tools. Especially after the emergence of high-speed cutting technology, coating technology has been developed and applied rapidly, and has become one of the key technologies of high-speed cutting tool manufacturing. This technology forms a certain film on the tool surface 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.

To sum up, the surface coating technology of cutting tools has the following characteristics:

1. The coating technology can greatly improve the tool surface hardness without reducing the tool strength. At present, the hardness can be close to 100GPa;

2. With the rapid development of coating technology, the chemical stability and high-temperature oxidation resistance of the film are more prominent, which makes high-speed machining possible;

3. The lubricating film has good solid lubrication performance, which can effectively improve the machining quality, and is also suitable for dry cutting;

4. As the final process of tool manufacturing, coating technology has little impact on tool accuracy, and can carry out repeated coating process.

Benefits of coated cutting tools: it can greatly improve the service life of cutting tools; Effectively improve cutting efficiency; Obviously improve the surface quality of the machined workpiece; Effectively reduce the consumption of tool materials and reduce the processing cost; Reduce the use of coolant, reduce the cost and be conducive to environmental protection.

Correct surface treatment of small circular tools can improve tool life, reduce machining cycle time and improve machining surface quality. However, choosing the correct tool coating according to the machining needs may be a confusing and laborious work. Each coating has both advantages and disadvantages in cutting. If an inappropriate coating is selected, the tool life may be lower than that of uncoated tools, and sometimes even lead to more problems than before coating.

At present, there are many kinds of tool coatings to choose from, including PVD coating, CVD coating and composite coating alternately coated with PVD and CVD, which can be easily obtained from tool manufacturers or coating suppliers. This paper will introduce some common properties of tool coatings and some common PVD and CVD coating options. Each characteristic of the coating plays a very important role in determining which coating is most beneficial for machining.

Common coatings

1. Titanium nitride coating (TIN)

Tin is a universal PVD coating, which can improve tool hardness and have high oxidation temperature. The coating can be used for high speed steel cutting tools or forming tools to obtain good machining results.

2. Chromium nitride coating (CRN)

CrN coating has good adhesion resistance, which makes it the first choice in the processing of chip buildup. After coating this almost invisible coating, the machinability of high-speed steel tools or cemented carbide tools and forming tools will be greatly improved.

3. Diamond coating

CVD diamond coating can provide the best performance for non-ferrous metal material processing tools. It is an ideal coating for processing graphite, metal matrix composites (MMC), high silicon aluminum alloy and many other high abrasive materials (Note: pure diamond coated tools cannot be used for processing steel parts, because a large amount of cutting heat will be generated when processing steel parts, resulting in chemical reactions, Damage the adhesive layer between the coating and the tool).

4. Coating equipment

The coatings suitable for hard milling, tapping and drilling are different, and have their specific application occasions. In addition, multi-layer coatings can be used, and other coatings are embedded between the surface layer and the tool matrix, which can further improve the service life of the tool.

5. Titanium nitride carbide coating (TiCN)

The carbon element added in TiCN coating can improve the tool hardness and obtain better surface lubricity. It is an ideal coating for high speed steel tools.

6. Nitrogen aluminum titanium or nitrogen titanium aluminum coating (TiAlN / AlTiN)

The alumina layer formed in TiAlN / AlTiN coating can effectively improve the high temperature machining life of cutting tools. The coating can be used for cemented carbide tools mainly used for dry or semi dry cutting. According to the different proportion of aluminum and titanium 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.

Coating properties

1. Hardness

The high surface hardness brought 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 service life of the tool. Titanium nitride carbide (TiCN) coating has higher hardness than titanium nitride (TIN) coating. Due to the increase of carbon content, the hardness of TiCN coating is increased by 33%, and its hardness variation range is about hv3000 ~ 4000 (depending on the manufacturer). The application of CVD diamond coating with surface hardness up to hv9000 in cutting tools has been relatively mature. Compared with PVD coated tools, the service life of CVD diamond coated tools is increased by 10 ~ 20 times. The high hardness and cutting speed of diamond coating can be 2 ~ 3 times higher than that of uncoated tools, making it a good choice for cutting non-ferrous materials.

2. Oxidation temperature

Oxidation temperature refers to the temperature at which the coating begins to decompose. The higher the oxidation temperature, the more favorable it is for cutting at high temperature. Although the room temperature hardness of TiAlN coating may be lower than that of TiCN coating, it has been proved to be much more effective than TiCN in high temperature processing. The reason why TiAlN coating can maintain its hardness at high temperature is that it can form a layer of aluminum oxide between the tool and chip, which can transfer heat from the tool to the workpiece or chip. Compared with high-speed steel tools, the cutting speed of cemented carbide tools is usually higher, which makes TiAlN the preferred coating for cemented carbide tools. This pvdtialn coating is usually used for cemented carbide drill bits and end mills

3. Wear resistance

Wear resistance refers to the ability of the coating to resist wear. Although the hardness of some workpiece materials may not be too high, the elements added and the process adopted in the production process may cause the cutting edge of the tool to crack or blunt.

4. Surface lubricity

High friction coefficient will increase the cutting heat, resulting in the shortening of coating life and even failure. Reducing the friction coefficient can greatly prolong the tool life. The fine and smooth coating surface or regular texture helps to reduce the cutting heat, because the smooth surface can quickly slide the chips away from the rake face and reduce the generation of heat. Compared with uncoated tools, coated tools with better surface lubrication can also be processed at higher cutting speed, so as to further avoid high-temperature fusion welding with workpiece materials.

5. Adhesion resistance

The anti adhesion of the coating can prevent or reduce the chemical reaction between the tool and the processed material and avoid the deposition of workpiece material on the tool. When machining non-ferrous metals (such as aluminum, brass, etc.), chip buildup (bue) often occurs on the tool, resulting in tool edge collapse or workpiece size out of tolerance. Once the processed material begins to adhere to the tool, the adhesion will continue to expand. For example, when processing aluminum workpiece with forming tap, the aluminum adhered to the tap will increase after processing each hole, and finally the tap diameter will become too large, resulting in over tolerance and scrapping of the workpiece size. Coatings with good adhesion resistance can play a good role even in processing occasions with poor coolant performance or insufficient concentration.

Application of coatings

Achieving cost-effective application of coatings may depend on many factors, but there are usually only one or several feasible coating options for each specific processing application. Whether the coating and its characteristics are selected correctly may mean the difference between significantly improved and almost no improvement in processability. 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 constantly developing more new coatings to further improve the high temperature resistance, friction resistance and wear resistance of the coatings. It is always a good thing to work with coating (tool) manufacturers to verify the application of the latest and best tool coatings in machining.