How can you choose the tool coating correctly?

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 task to select the correct tool coating according to the machining requirements. Each coating has both advantages and disadvantages in cutting process. If improper coating is selected, the tool life may be lower than that of uncoated tools, and sometimes even more problems than before coating.

There are many kinds of tool coatings available, including PVD coatings, CVD coatings and composite coatings of PVD and CVD, which can be easily obtained from tool manufacturers or coating suppliers. This article will introduce some common properties of tool coatings and some commonly used PVD and CVD coating options. Each characteristic of the coating plays an important role in determining which coating is most beneficial for cutting.

1 coating characteristics

The high surface hardness brought by hardness coating is one of the best ways to improve tool life. Generally speaking, the higher the hardness of materials or surfaces, the longer the life of knives. Nitrogen titanium carbide (TiCN) coating has higher hardness than titanium nitride (TiN) coating. The hardness of TiCN coating is increased by 33% due to the increase of carbon content. The hardness varies from Hv3000 to 4000 (depending on the manufacturer). The application of CVD diamond coating with surface hardness as high as Hv9000 on cutting tools is mature. Compared with PVD coated cutting tools, the service life of CVD diamond coated cutting tools is increased by 10-20 times. The high hardness and cutting speed of diamond coated tools are 2-3 times higher than that of uncoated tools, making them a good choice for cutting non-ferrous materials. Wear resistance and wear resistance is the ability of the coating to resist wear. Although the hardness of some workpiece materials may not be too high, the addition of elements in the production process and the process adopted may cause the cutting edge to crack or blunt. Surface lubricity and high friction coefficient will increase cutting heat, resulting in shortened service life and even failure. Reducing friction coefficient can greatly extend tool life. Fine, smooth or well-textured coated surfaces help to reduce cutting heat, because smooth surfaces allow the chip to slip away from the rake face quickly and reduce heat production. Compared with uncoated tools, coated tools with better surface lubricity can be processed at higher cutting speed, thus further avoiding high temperature welding with workpiece materials. Oxidation temperature, oxidation temperature refers to the temperature at which the coating begins to decompose. The higher the oxidation temperature is, the better the cutting process is 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 in high temperature processing than that of TiCN coating. The reason why TiAlN coating can maintain its hardness at high temperatures 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. Compared with high-speed steel tools, carbide tools usually have higher cutting speed, which makes TiAlN the preferred coating for carbide tools. This PVD TiAlN coating is commonly used for carbide drills and end milling cutters.

The adhesive resistance of the anti-adhesive coating can prevent or reduce the chemical reaction between the tool and the material being processed, and prevent the workpiece material from depositing on the tool. In the process of non-ferrous metals (such as aluminum, brass, etc.), chipping tumors (BUE) often occur on the tool, resulting in tool breakage or workpiece size excess. Once the processed material adheres to the cutting tool, the adhesion will continue to expand. For example, when machining aluminium workpiece with forming tap, the aluminium adhered to the tap after each hole is processed will increase, and eventually make the tap diameter become too large, resulting in excessive size of the workpiece scrapped. Coatings with good adhesion resistance can even play a good role in processing occasions with poor coolant performance or insufficient concentration.

2 commonly used coatings

TiN coating (TiN) TiN is a universal PVD coating, which can improve the hardness of cutting tools and has high oxidation temperature. The coating can be applied to high speed steel cutting tools or forming tools.

Carbon element added to TiCN coating can improve the hardness and surface lubricity of the tool, which is an ideal coating for high speed steel tools.

Alumina layer formed in TiAlN or TiAlN/AlTiN coatings can effectively improve the high temperature working life of cutting tools. The coating is suitable for hard or semi dry cutting of carbide tools. AlTiN coating can provide higher surface hardness than TiAlN coating according to the different proportion of aluminum and titanium in the coating, so it is another feasible coating choice in the field of high-speed machining.

Chromium nitride coatings (CrN) CrN coatings have good adhesion resistance, which makes it the preferred coating in the process of easily producing chip tumors. The machinability of high speed steel cutters or carbide tools and forming tools will be greatly improved by coating with this almost invisible coating.

Diamond coated CVD diamond coatings provide the best performance for non-ferrous metal cutting tools and are ideal coatings for graphite, metal matrix composites (MMC), high silicon aluminum alloys and many other high abrasive materials (Note: Pure diamond coated cutting tools cannot be used for steel parts because they are produced when machining steel parts. A large amount of cutting heat causes chemical reactions to damage the adhesive layer between the coating and the tool.

The coatings applied to hard milling, tapping and drilling are different, each having its specific application occasions. In addition, multi-layer coatings can be used, and other coatings are embedded between the surface layer and the tool substrate, which can further improve the tool life.

Successful application of 3 coating

The realization of high cost performance coatings may depend on many factors, but there is usually only one or more feasible coatings for each particular processing application. Whether the coating and its properties are chosen correctly or not may mean the difference between a marked improvement in workability 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 developing more and more new coatings to further improve the high temperature, friction and wear resistance of the coatings. It's always a good thing to work with the coating (tool) manufacturer to verify the latest and best tool coatings used in machining.