【Technology Knowledge】Talking about 5 kinds of commonly used tool coatings

Keywords tool, coating|2020-09-09 11:01:01

Correct surface treatment of small round tools can increase tool life, reduce machining cycle time, and improve the quality of the machined surface. However, the correct choice of tool coating according to processing needs may be a confusing and laborious task.

Each coating has both advantages and disadvantages in cutting processing. If an inappropriate coating is selected, it may lead to a lower tool life than uncoated tools, and sometimes even lead to more problems than before the coating .

There are many types of tool coatings to choose from, including PVD coating, CVD coating and composite coating of alternating PVD and CVD, etc. These coatings can be easily obtained from tool manufacturers or coating suppliers. Floor.

This article will briefly introduce some common attributes of tool coatings and some commonly used PVD and CVD coating options. Each characteristic of the coating plays a very important role in determining which coating is most beneficial to the cutting process.

1 Characteristics of the coating

hardness

The high surface hardness brought by the coating is one of the best ways to increase tool life. Generally speaking, the higher the hardness of the material or surface, the longer the tool life.

Titanium carbide (TiCN) coatings have higher hardness than titanium nitride (TiN) coatings. Due to the increased carbon content, the hardness of the TiCN coating is increased by 33%, and the hardness range is about Hv3000～4000 (depending on the manufacturer).

The application of CVD diamond coating with a surface hardness of up to Hv9000 on cutting tools is relatively mature. Compared with PVD coated cutting tools, the life of CVD diamond coated cutting tools is increased by 10 to 20 times. The high hardness and cutting speed of diamond coating can be 2 to 3 times higher than that of uncoated tools, making it a good choice for cutting non-ferrous materials.

Abrasion resistance

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

Surface lubricity

High friction coefficient will increase cutting heat, resulting in shortened coating life or even failure. And reducing the friction coefficient can greatly extend the tool life. A fine, smooth or regular-textured coating surface helps reduce cutting heat, because a smooth surface can quickly slide chips away from the rake face and reduce heat generation. Compared with uncoated tools, coated tools with better surface lubricity can be processed at a higher cutting speed, thereby further avoiding high temperature welding with the workpiece material.

Oxidation temperature

The oxidation temperature is the temperature at which the coating begins to decompose. The higher the oxidation temperature, the more advantageous it is for cutting under high temperature conditions. Although the normal temperature hardness of TiAlN coating may be lower than that of TiCN coating, it is proved that it is much more effective than TiCN in high temperature processing. Add WeChat: Yuki7557 send a macro program tutorial. The reason why TiAlN coating can maintain its hardness at high temperature is that a layer of aluminum oxide can be formed between the tool and the chips. The aluminum oxide layer can transfer heat from the tool to the workpiece or Chips. Compared with high-speed steel tools, the cutting speed of carbide tools is usually higher, which makes TiAlN the preferred coating for carbide tools. Carbide drills and end mills usually use this PVD TiAlN coating.

Anti-adhesion

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.), built-up edge (BUE) is often generated on the tool, which causes the tool to collapse or the workpiece size is out of tolerance. Once the processed material begins to adhere to the tool, the adhesion will continue to expand.

When machining aluminum workpieces with forming taps, the aluminum adhering to the taps will increase after each hole is processed, and eventually the diameter of the taps will become too large, causing the workpiece size to be out of tolerance. Coatings with good anti-blocking properties can play a good role even in processing situations where the coolant performance is poor or the concentration is insufficient.

2 Commonly used coatings

1 Titanium nitride coating (TiN)

TiN is a general-purpose PVD coating that can improve tool hardness and has a higher oxidation temperature. The coating is used for high-speed steel cutting tools or forming tools to obtain very good processing results.

2 Titanium Carbide Nitrogen Coating (TiCN)

The carbon element added to the TiCN coating can increase the hardness of the tool and obtain better surface lubricity. It is 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 increase the high temperature machining life of the tool. Cemented carbide tools mainly used for dry or semi-dry cutting can use this coating. According to the ratio of aluminum to 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 processing.

4 Nitrogen chromium aluminum coating (AlCrN)

The good anti-adhesion of AlCrN coating makes it the first choice in the processing that is prone to buildup. After coating this almost invisible coating, the processing performance of high-speed steel cutting tools or cemented carbide cutting tools and forming tools will be greatly improved.

5 Diamond coating (Diamond)

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

The 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. This type of coating also embeds other coatings between the surface layer and the tool base, which can further increase the service life of the tool.

3 Successful application of coating

Achieving cost-effective coating applications may depend on many factors, but for each specific processing application, there are usually only one or a few viable coating options.

The correct choice of coating and its properties may mean the difference between a significant improvement in processing performance and almost no improvement. Cutting depth, cutting speed and coolant may all have an impact on the application effect of tool coatings.

Since 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 coating's high temperature resistance, friction resistance and wear resistance. It is always a good thing to verify the application of the latest and best tool coating in machining with the coating (tool) manufacturer.

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