Today, we share a long article on tool coatings. This article answers 4 questions about what tool surface coating technology is, what are the commonly used coatings, what are the characteristics of the coatings, and the application areas of the coatings.
Ⅰ. Overview of tool coating
Tool surface coating technology is a surface modification technology developed in response to market demand. Since its emergence in the 1960s, this technology has been widely used in metal cutting tool manufacturing.
Especially after the emergence of high-speed cutting processing technology, coating technology has been rapidly developed and applied, and has become one of the key technologies of high-speed cutting tool manufacturing.
This technology forms a thin 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.
To sum up, the cutting tool surface coating technology has the following characteristics:
- The coating technology can greatly increase the hardness of the tool surface without reducing the strength of the tool. The hardness that can be achieved at present is close to 100GPa;
- 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;
- The lubricating film has good solid phase lubrication performance, which can effectively improve the processing quality, and is also suitable for dry cutting processing;
- As the final process of tool manufacturing, coating technology has almost no effect on tool accuracy, and can repeat the coating process.
Benefits of coated cutting tools:
- Can greatly improve the service life of cutting tools.
- Effectively improve cutting efficiency.
- Significantly improve the surface quality of the processed work
- Effectively reduce the consumption of tool materials and reduce processing costs.
- Reducing the use of cooling fluid, reducing costs, and conducive to environmental protection.
Proper surface treatment of small circular tools can improve tool life, reduce processing cycle time, and improve the quality of processed surfaces.
However, the correct choice of tool coating according to processing needs can be a confusing and laborious task.
Each coating has both advantages and disadvantages in cutting.
If an inappropriate coating is used, it may cause the tool life to be lower than that of uncoated tools, and sometimes even cause more problems than before.
There are currently many types of tool coatings available, including PVD coatings, CVD coatings, and composite coatings that alternately apply PVD and CVD.
These coatings can be easily obtained from tool manufacturers or coating suppliers.
This article will introduce some common properties of tool coatings and some common PVD and CVD coating options.
Each characteristic of the coating plays a very important role in determining which coating is most beneficial for cutting.
Ⅱ. Commonly used coatings
- Titanium Nitride Coating (TiN)
TiN is a general-purpose PVD coating that can increase tool hardness and have a high oxidation temperature.
This kind of coating is used for high-speed steel cutting tools or forming tools to obtain very good processing results.
- Chromium nitride coating (CrN)
The good anti-adhesion properties of CrN coating make it the first choice coating in the process that easily generates built-up edge.
After applying this almost invisible coating, the processing performance of high-speed steel tools or carbide tools and forming tools will be greatly improved.
- Diamond coating (Diamond)
CVD diamond coating can provide the best performance for cutting 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-coated tools cannot be used to process steel parts, because a large amount of cutting heat is generated when processing steel parts, and a chemical reaction occurs, which causes the adhesion layer between the coating and the tool to be damaged).
- Coating equipment
The coatings suitable for hard milling, tapping and drilling are different, and each has its own specific application.
In addition, multi-layer coatings can also be used. Such coatings also embed other coatings between the surface layer and the tool base, which can further increase the tool life.
- Nitrogen titanium carbide coating (TiCN)
The carbon element added in the TiCN coating can improve the tool hardness and obtain better surface lubricity, and is an ideal coating for high-speed steel tools.
- Aluminum nitride titanium or aluminum nitride coating (TiAlN/AlTiN)
The alumina layer formed in TiAlN / AlTiN coating can effectively improve the tool’s high-temperature machining life.
This coating is available for carbide tools mainly used for dry or semi-dry cutting.
Depending on the ratio of aluminum to titanium contained in the coating, AlTiN coatings provide a higher surface hardness than TiAlN coatings.
Therefore, it is another feasible coating choice in the field of high-speed machining.
Ⅲ. Characteristics of the coating
The high surface hardness that comes with the coating is one of the best ways to improve tool life.
In general, the harder the material or surface, the longer the tool will last. Titanium carbide nitride (TiCN) coatings have a 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 range is about hv3000-4000 (depending on the manufacturer).
The application of CVD diamond coating with surface hardness up to Hv9000 on the tool has been more mature, compared with PVD coated tool, the life of CVD diamond coated tool has been increased by 10-20 times.
The high hardness and cutting speed of diamond coating can be increased by 2 to 3 times over uncoated tools, making it a good choice for cutting non-ferrous materials.
- Oxidation temperature
Oxidation temperature is the value of the temperature at which the coating begins to break down.
The higher the oxidation temperature value, the more favorable it is for cutting at high temperatures.
Although the TiAlN coating may be less hard at room temperature than the TiCN coating, it has proven to be much more effective than TiCN in high-temperature processing.
The reason why TiAlN coatings retain their hardness at high temperatures is that a layer of alumina can be formed between the tool and the chip, and the alumina layer transfers heat from the tool to the workpiece or chip.
Carbide tools typically cut faster than HSS tools, which makes TiAlN the coating of choice for carbide tools, and carbide drills and end mills typically use this PVD-TiAlN coating.
- Abrasion resistance
Abrasion resistance refers to the ability of a coating to resist wear.
While some workpiece materials may not be inherently hard, the elements added during the manufacturing process and the process used may cause the cutting edge of the tool to chip or dull.
- Surface lubricity
High coefficients of friction increase cutting heat, resulting in shortened or even failed coating life, while lower coefficients of friction can greatly extend tool life.
A fine smooth or textured regular coated surface helps reduce cutting heat because the smooth surface allows the chip to slide quickly away from the front face of the cutter, reducing heat generation.
Coated tools with better surface lubrication can also be machined at higher cutting speeds than uncoated tools, further avoiding high-temperature welding to the workpiece material.
- Adhesion resistance
The anti-sticking property of the coating can prevent or reduce the chemical reaction between the tool and the material to be processed, and prevent the workpiece material from being deposited on the tool.
When machining non-ferrous metals (e.g., aluminum, brass, etc.), BUEs are often generated on the tool, resulting in tool chipping or oversized workpieces, and once the material being machined begins to adhere to the tool, the adhesion will continue to expand.
For example, when processing aluminum workpieces with forming taps, the aluminum that adheres to the taps after processing each hole will increase, and eventually the tap diameter becomes too large, causing the workpiece size to be out of tolerance and scrapped.
The coating with good anti-sticking property can play a good role even in the processing occasions where the performance of the coolant is poor or the concentration is insufficient.
IV. Application of coatings
Achieving cost-effective coating applications may depend on many factors, but for each specific processing application, there is usually only one or several viable coating options.
The correct choice of coating and its properties may mean the difference between a significant improvement in processability and almost no improvement.
Cutting depth, cutting speed, and coolant can all have an effect on how well the tool coating is applied.
Because there are so many variables in the processing of workpiece material, one of the best ways to determine which coating to choose is through test cutting.
Coating suppliers are continually developing newer coatings to further improve the coating’s resistance to heat, friction and wear.
It’s always good to work with coating (tool) manufacturers to validate the latest and greatest tool coatings for use in machining.