The correct surface treatment of small round cutting tools can improve tool life, reduce machining cycle time and improve machined surface quality. However, choosing the right tool coating for your machining needs can be a confusing task.
Each coating has both advantages and disadvantages in cutting. If the wrong coating is selected, it may result in a tool life that is lower than that of an uncoated tool, and sometimes even cause more problems.
There are many types of tool coatings available, including PVD coatings, CVD coatings, and composite coatings that alternate PVD and CVD coatings, which can be easily obtained from tool manufacturers or coating suppliers.
This article will briefly introduce some common properties of tool coatings and some commonly used PVD and CVD coating selection options. Each coating property plays an important role in determining which coating is most beneficial for cutting.
Coating properties
1. Hardness
High surface hardness brought by the coating is one of the best ways to improve tool life. Generally speaking, the harder the material or surface, the longer the tool life.
Titanium carbide (TiCN) coating has a higher hardness than titanium nitride (TiN) coating. Due to the increased carbon content, the hardness of the TiCN coating has increased by 33%, and its hardness ranges from about Hv3000 to 4000 (depending on the manufacturer).
The application of CVD diamond coating with a surface hardness of up to Hv9000 on cutting tools has been relatively mature. Compared with PVD coated tools, the life of CVD diamond coated tools has increased by 10 to 20 times. The high hardness and cutting speed of diamond coatings can increase the cutting speed by 2 to 3 times compared with uncoated tools, making it a good choice for cutting non-ferrous materials.
2. 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 during the production process and the processes used may cause the cutting edge of the tool to crack or blunt.
3. Surface lubricity
High friction coefficients increase cutting heat, resulting in a shortened coating life or even failure. Reducing the friction coefficient can greatly extend tool life. Fine and smooth or regularly textured coating surfaces help reduce cutting heat because the smooth surface allows chips to slide quickly off the front cutting edge and reduce heat generation. Compared with uncoated tools, coated tools with better surface lubricity can also be processed at higher cutting speeds, further avoiding high-temperature fusion welding with the workpiece material.
4. Oxidation temperature
The 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 under high temperature conditions. Although the room temperature hardness of TiAlN coating may be lower than that of TiCN coating, it has been proven to be much more effective than TiCN in high-temperature processing. The reason why TiAlN coating can still maintain its hardness at high temperatures is that a layer of aluminum oxide can be formed between the tool and the chip, which can transfer heat from the tool to the workpiece or chip. Compared with high-speed steel tools, carbide tools usually have higher cutting speeds, which makes TiAlN the preferred coating for carbide tools. Carbide drills and end mills usually use this PVD TiAlN coating.
5. Anti-adhesion
The anti-adhesion property 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.), built-up edge often occurs on the tool, causing tool chipping or workpiece size deviation. Once the material being machined begins to adhere to the tool, the adhesion will continue to expand.
When machining aluminum workpieces with forming taps, the aluminum adhered to the tap will increase after each hole is machined, and finally the tap diameter becomes too large, causing the workpiece size deviation to be scrapped. Coatings with good anti-adhesion properties can play a good role even in machining occasions with poor coolant performance or insufficient concentration.
Commonly used coatings
1. Titanium nitride coating (TiN)
TiN is a general-purpose PVD coating that can increase tool hardness and has a high oxidation temperature. This coating can achieve very good machining results when used for high-speed steel cutting tools or forming tools.
2. Titanium carbide nitride coating (TiCN)
The carbon element added to the TiCN coating can increase the hardness of the tool and obtain better surface lubricity, making it an ideal coating for high-speed steel tools.
3. Nitrogen Aluminum Titanium or Nitrogen Titanium Aluminum Coating (TiAlN/AlTiN)
The aluminum oxide layer formed in the TiAlN/AlTiN coating can effectively improve the high-temperature machining life of the tool. This coating can be used for carbide tools mainly used for dry or semi-dry cutting. Depending on the ratio of aluminum and titanium contained in the coating, AlTiN coating can provide higher surface hardness than TiAlN coating, so it is another viable coating option in the field of high-speed machining.
4. Nitrogen Chromium Aluminum Coating (AlCrN)
The good anti-adhesion property of AlCrN coating makes it the preferred coating in machining prone to built-up edge. After applying this almost invisible coating, the machining performance of high-speed steel tools or carbide tools and forming tools will be greatly improved.
5. Diamond
CVD diamond coatings provide optimal performance for non-ferrous material machining tools and are ideal for machining graphite, metal matrix composites (MMC), high silicon aluminum alloys and many other highly abrasive materials (note: pure diamond coated tools cannot be used for machining steel parts because machining steel parts generates a lot of cutting heat and causes a chemical reaction that destroys the adhesion layer between the coating and the tool).
Different coatings are suitable for hard milling, tapping and drilling, and each has its own specific application. In addition, multi-layer coatings can be used, which have other coatings embedded between the surface layer and the tool substrate to further improve the tool life.
Successful application of coatings
The cost-effective application of coatings may depend on many factors, but for each specific machining application, there are usually only one or a few viable coating options.
The correct choice of coating and its characteristics can mean the difference between a significant improvement in machining performance and almost no improvement. Cutting depth, cutting speed and coolant can all affect the application of tool coatings.
Because there are many variables in the machining of a workpiece material, one of the best ways to determine which coating to use is through test cutting. Coating suppliers are constantly developing more new coatings to further improve the coating's high temperature resistance, friction and wear resistance. It is always a good thing to verify the application of the latest and greatest tool coatings in machining with the coating (tool) manufacturer.
