How to choose the correct tool coating
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There are many types of tool coatings available today, including PVD coatings, CVD coatings, and composite coatings that alternately coat PVD and CVD. These coatings are readily available from tool makers or coating suppliers. Floor. This article will introduce some of the common properties of the tool coating and some common PVD, CVD coating options. When determining which coating is most beneficial for cutting, each characteristic of the coating plays an important role.
1. Characteristics of the coating
(1) Hardness
The high surface hardness of the coating is one of the best ways to increase tool life. In general, the higher the hardness of a material or surface, the longer the life of the tool. The titanium carbonitride (TiCN) coating has a higher hardness than the titanium nitride (TiN) coating. Due to the increased carbon content, the hardness of the TiCN coating is increased by 33%, and its hardness varies from approximately Hv 3000 to 4000 (depending on the manufacturer). The application of CVD diamond coating with surface hardness up to Hv9000 on cutting tools has been mature. Compared with PVD coated tools, the life of CVD diamond coated cutting tools has been improved by 10 to 20 times. The high hardness and cutting speed of diamond coatings can be 2 to 3 times better than uncoated tools making them a good choice for non-ferrous materials.
(2) Wear resistance
Abrasion resistance refers to the ability of a coating to resist abrasion. Although the hardness of some workpiece materials may not be high, the elements added during the production process and the process used may cause the tool cutting edge to crack or dull.
(3) Surface lubricity
High coefficient of friction increases cutting heat, leading to shortened or even failure of coating life. Lower friction coefficient can greatly extend tool life. Delicate smooth or regular texture of the coated surface helps to reduce the heat of cutting, because the smooth surface allows the chips to quickly slip off the rake face and reduce heat generation. Compared to uncoated tools, coated tools with better surface lubricity can be machined at higher cutting speeds, thus further avoiding high-temperature welding with the workpiece material.
(4) Oxidation temperature
The oxidation temperature is the temperature at which the coating begins to decompose. The higher the oxidation temperature, the more favorable the cutting process is under high temperature conditions. Although the TiAlN coating may have lower ambient hardness than the TiCN coating, it has proven 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 the chip. The aluminum oxide layer can transfer heat from the tool to the workpiece or the chip. Compared to high-speed steel tools, carbide cutting tools usually have higher cutting speeds, which makes TiAlN the preferred coating for cemented carbide tools. The carbide drills and end mills usually use this PVD TiAlN coating.
(5) Anti-adhesion
The anti-adhesion of the coating prevents or reduces the chemical reaction between the tool and the material being processed, and prevents the workpiece material from depositing on the tool. When processing non-ferrous metals (such as aluminum, brass, etc.), the built-up edge of the tool (BUE) is often generated on the tool, resulting in tool chipping or excessive workpiece size. Once the material being processed begins to adhere to the tool, the adhesion will continue to grow. For example, when machining aluminum workpieces with a forming tap, the amount of aluminum adhered to the tap increases after each hole is drilled, and eventually the diameter of the tap becomes too large, causing the workpiece to be oversized and scrapped. Coatings with good anti-adhesive properties can even work well in poor performance or insufficient concentration of cooling fluids.
2. Common coating
(1) TiN Coating
TiN is a general-purpose PVD coating that increases tool hardness and has a high oxidation temperature. The coating is used for high speed steel cutting tools or forming tools to get a very good processing results.
(2) TiN coating (TiCN)
The addition of carbon to the TiCN coating increases tool hardness and provides better surface lubrication, making it an ideal coating for HSS tools.
(3) TiAlN or Al-TiAl coating (TiAlN/AlTiN)
The aluminum oxide layer formed in the TiAlN/AlTiN coating can effectively increase the high temperature processing life of the tool. This coating can be used for carbide tools that are mainly used for dry or semi-dry cutting. Depending on the ratio of aluminum and titanium contained in the coating, the AlTiN coating provides a higher surface hardness than the TiAlN coating, making it a viable coating choice in high-speed machining.
(4) Chromium nitride coating (CrN)
The good anti-adhesion of the CrN coating makes it the preferred coating in the process of producing a build-up edge. After applying this almost invisible coating, the processing performance of HSS cutting tools or carbide cutting tools and forming tools will be greatly improved.
(5) Diamond coating
CVD diamond coating provides the best performance for non-ferrous metal machining tools. It is the ideal coating for machining graphite, metal matrix composites (MMC), high silicon aluminum alloys and many other highly abrasive materials (Note: Pure diamond coating The tool cannot be used to machine steel parts because the machining of steel parts generates a large amount of cutting heat and causes a chemical reaction that destroys the adhesion layer between the coating and the tool).
Coatings for hard milling, tapping, and drilling are all different and each has its own specific application. In addition, multi-layer coatings can also be used. Such coatings also have other coatings embedded between the skin and the tool body, which can further increase the service life of the tool.
3. Successful application of coating
Achieving cost-effective application of the coating may depend on many factors, but usually there is only one or several possible coating options for each particular processing application. The correct choice of coating and its properties may mean the difference between significantly improved processing performance and little improvement. Depth of cut, cutting speed, and coolant can all have an effect on the application of the tool coating.
Because there are many variables in the processing of a workpiece material, one of the best ways to determine which coating to use is through the test cut. Coating suppliers are constantly developing more new coatings to further improve the coating's high temperature, friction, and wear resistance. It is always a good thing to verify with the coating (tool) manufacturer that the latest and best tool coating application in processing.