Tool surface coating technology is a surface modification technology developed in response to market demand. Since its appearance in the 1960s, this technology has been widely used in the metal cutting tool manufacturing industry. 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 for high-speed cutting tool manufacturing. This technology forms a certain film on the surface of the tool through chemical or physical methods, so that the cutting tool can obtain excellent comprehensive cutting performance, thereby meeting the requirements of high-speed cutting.
In summary, cutting tool surface coating technology has the following characteristics:
1. The use of coating technology can greatly increase the surface hardness of the tool without reducing the strength of the tool. The hardness that can be achieved at present is 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, thus making high-speed cutting possible;
3. The lubricating film has good solid-phase lubrication performance, which can effectively improve the processing quality and is also suitable for dry cutting;
4. Coating technology, as the final process of tool manufacturing, has almost no impact on tool precision, and can be used for repeated coating processes.
The benefits brought by the coated cutting tool: can greatly improve the life of the cutting tool; effectively improve the cutting efficiency; significantly improve the surface quality of the processed workpiece; effectively reduce the consumption of tool materials and reduce processing costs; reduce the consumption of coolant Use, reduce costs, and help environmental protection.
Proper surface preparation on small round tools can increase tool life, reduce machining cycle times, and improve machined surface quality. But choosing the right tool coating for your machining needs can be confusing and labor-intensive. Each coating has both advantages and disadvantages in machining. If an inappropriate coating is selected, it may result in a lower tool life than an uncoated tool, and sometimes even cause more problems than before the coating .
There are many kinds of tool coatings to choose from, including PVD coatings, CVD coatings, and composite coatings that alternately coat PVD and CVD, etc. These coatings can be easily obtained from tool manufacturers or coating suppliers. layer. This article will introduce some common properties of tool coatings and some commonly used PVD and CVD coating options. Each property of the coating plays an important role in determining which coating is most beneficial for cutting operations.
Commonly used coatings
1. Titanium nitride coating (TiN)
TiN is a general-purpose PVD coating that increases tool hardness and has a higher oxidation temperature. The coating can be used for high-speed steel cutting tools or forming tools to obtain very good processing results.
2. Chromium nitride coating (CrN)
The good anti-caking properties of CrN coating make it the coating of choice in the processing prone to build-up edge. The machining performance of high-speed steel or carbide cutting tools and forming tools will be greatly improved after applying this almost invisible coating.
3. Diamond coating (Diamond)
CVD diamond coating can provide the best performance for non-ferrous metal materials processing tools, and is an 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, because machining steel generates a lot of cutting heat and causes a chemical reaction that destroys the adhesion layer between the coating and the tool).
4. Coating equipment
There are different coatings for hard milling, tapping and drilling, each with its own specific application. In addition, multi-layer coatings are also available, which have other coatings embedded between the surface layer and the tool substrate, which can further increase the service life of the tool.
5. Titanium carbide nitride coating (TiCN)
The carbon element added in the TiCN coating can increase the hardness of the tool and obtain better surface lubricity, which 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 the TiAlN/AlTiN coating can effectively improve the high-temperature machining life of the tool. This coating can be selected for cemented carbide tools mainly used in dry or semi-dry cutting. Depending on the ratio of aluminum and titanium contained in the coating, AlTiN coatings can provide higher surface hardness than TiAlN coatings, so it is another viable coating option for high-speed machining.
Coating Properties
1. Hardness
The high surface hardness brought about by the coating is one of the best ways to increase tool life. In general, the harder the material or surface, the longer the tool will last. Titanium carbide nitride (TiCN) coatings have higher hardness than titanium nitride (TiN) coatings. Due to the increase of carbon content, the hardness of TiCN coating is 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 as high as Hv9000 on cutting tools has been relatively mature. Compared with PVD coating cutting tools, the life of CVD diamond coating cutting tools has increased by 10 to 20 times. The high hardness of diamond coating and the ability to increase cutting speed by 2 to 3 times compared with uncoated tools make it a good choice for cutting non-ferrous materials.
2. Oxidation temperature
Oxidation temperature refers to the temperature value at which the coating begins to decompose. The higher the oxidation temperature value, 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 proved to be much more effective than TiCN in high temperature processing. The reason why the 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 chips, and 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 cemented carbide tools is usually higher, which makes TiAlN the coating of choice for carbide tools. Carbide drills and end mills often use this PVDTiAlN coating
3. Wear resistance
Abrasion resistance refers to the ability of a coating to resist abrasion. While some workpiece materials may not be too hard on their own, elements added and processes used during production may cause the cutting edge of the tool to chip or dull.
4. Surface lubricity
A high coefficient of friction increases cutting heat, leading to shortened coating life and even failure. Reducing the coefficient of friction can greatly extend tool life. A fine smooth or regularly textured coating surface helps reduce cutting heat because the smooth surface allows chips to slide off the rake face quickly, reducing heat generation. Coated tools with better surface lubricity can also be machined at higher cutting speeds than uncoated tools, further avoiding hot welding with the workpiece material.
5. Anti-adhesion
The anti-adhesion property of the coating prevents or mitigates the chemical reaction between the tool and the material being processed, avoiding the deposition of workpiece material on the tool. When machining non-ferrous metals (such as aluminum, brass, etc.), built-up edge (BUE) often occurs on the tool, which may cause chipping of the tool or out-of-tolerance workpiece dimensions. Once the material being machined begins to adhere to the tool, the adhesion continues to expand. For example, when processing an aluminum workpiece with a forming tap, the aluminum adhered to the tap will increase after each hole is processed, so that the diameter of the tap becomes too large in the end, causing the workpiece to be out of tolerance and scrapped. Coatings with good anti-bonding properties work well even in machining situations where the coolant properties are poor or insufficiently concentrated.
Coating Application
Achieving a cost-effective application of a coating can depend on many factors, but there are usually only one or a few viable coating options for each specific processing application. The correct choice of coating and its properties can mean the difference between a noticeable improvement in processability and little or no improvement. Depth of cut, cutting speed and coolant can all have an effect on how well a tool coating is applied.
Because of the many variables that exist in the machining 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 good to work with coating (tool) manufacturers to validate the latest and greatest tool coatings for machining applications.
