Tool coating is an important key to improving a tool’s life time. Choosing the right tool coating to match the different cutting conditions and methods can help extend tool life, and optimize production process.
What is Tool Coating?
Tool coating is a method that can improve the wear resistance of the tool. A thin layer of exceptionally hard, abrasive resistant metal or non-metallic compound is coated on the surface of a cemented carbide or high-speed steel (HSS) substrate by a vapor deposition method.
Carbide tool coating method:
The coating provides the tool with strong protection, acid resistance, oxidation resistance, and wear resistance. The coating can improve the surface hardness and thermal stability of the tool, and reduce friction during cutting, allowing for increased cutting speeds, thereby increasing the and tool’s lifetime and processing efficiency.
- Chemical vapor deposition (CVD):
CVD is a chemical process that is widely used in the surface treatment of cemented carbide cutting tools. In the CVD process a source material is introduced into a reaction chamber in the form of gas, and a chemical reaction is carried out that deposits the source material onto the surface of the substrate by internal diffusion. In the reaction process, different by-products may also be produced, but most of them will be carried away with the gas flow and will not remain in the reaction chamber.
CVD technology is mainly used for putting coatings on turning tools. These tools are then more suitable for high-speed machining during medium and heavy cutting. CVD equipment is relatively simple, the process is mature, the process is easy to control, and there is a high degree of penetration of deposited material into the base material. Multi-layer coatings can be made with uniform thicknesses. Its production process cost is low, and it is suitable for mass production.
- Physical vapor deposition (PVD):
PVD deposits a thin film on a material by a physical reaction under a vacuum. The vacuum aids in the evaporation of the added material during coating. The PVD process is mostly used for the surface treatment of cutting tools and various molds, as well as in the production of semiconductor devices. The difference between PVD and CVD is that the adsorption and desorption of PVD are physical, while CVD is chemical. PVD also has a wider range of applications. PVD can be used to apply films using a variety of materials, but production conditions that may affect the uniformity of film thickness and will need to be overcome.
PVD technology is highly suitable for solid carbide tools and high-speed steel tools and is widely used in the coating treatment of carbide drills, milling cutters, taps, and welding tools. The coating temperature of PVD is lower than the tempering temperature of high-speed tool steel, so it will not hurt the hardness and dimensional accuracy of the tool. After coating, there is no need for heat treatment. The thickness of the coating is only a few microns so will not affect the accuracy of the original workpiece.
What Types of Tool Coatings are there?
Often a single coating alone cannot meet the mechanical property requirements of the tool, and multiple coatings may be applied. There are various forms of composite coatings, and the thicknesses of component coatings are getting thinner and thinner, even to the depth of nanometers.
The following are some common coatings:
- Titanium Nitride coating (TiN):
TTiN is a general-purpose PVD coating that increases tool hardness and has a high oxidation temperature. Good machining results can be obtained for high-speed steel cutting tools or forming tools.
- Chromium Nitride coating (CrN):
TCrN coating is widely used for cutting tools as its high resistance to adhesion reduces friction during cutting. The almost invisible coating will improve the machining performance of high-speed steel or carbide cutting tools and forming tools.
- Diamond coating (Diamond):
CVD diamond coatings provide optimum performance for non-ferrous metal machining tools and are suitable coatings for machining graphite, metal matrix composites (MMC), high silicon aluminum alloys, and many other highly abrasive materials.
- Titanium Nitride Carbide coating (TiCN):
The carbon element of the TiCN coating can improve the hardness of the tool and reduce adhesion to give better surface lubricity. TiCN coating is highly suitable for high-speed steel tools.
- 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. Mainly used in dry or semi-dry machining, it is suitable for coating carbide tools. Depending on the ratio of aluminum and titanium contained in the coating, AlTiN coating can provide higher surface hardness than TiAlN coating, and is suitable for high-speed machining.
- Coating equipment:
Coatings for hard milling, tapping, and drilling are different and each is suitable for its specific application. In addition, multi-layer coatings can be used, which embed other coatings between the surface layer and the tool base, thereby increasing the service life of the tool.
What are the Advantages of the Tool Coating Process?
The coated tool has a higher surface hardness, better wear resistance, more stable chemical properties, higher heat resistance, and better oxidation resistance than uncoated tools. The life time of a coated tool can be three to five times longer than that of the uncoated tool. They can be used at higher cutting speeds and give better precision, resulting in more efficient production and lower costs.
Characteristics of Coated Tools:
- Hardness:
Due to the added carbon content of TiCN coatings, they have a higher hardness than TiN coatings. The TiCN coating has a 33% higher hardness than TiN. Its hardness varies from about Hv3000 to 4000. Tools coated with CVD diamond can be 2 to 3 times higher than that of uncoated tools and have a service life of about 10 to 20 times that of uncoated tools. The high hardness allows for higher cutting speeds and they are a suitable choice for cutting non-ferrous materials.
- Oxidation temperature:
Oxidation temperature refers to the temperature at which the coating begins to decompose. The higher the oxidation temperature value, the more favorable it is for machining under high-temperature conditions. The reason why the TiAlN coating can maintain its hardness at high temperature is that a layer of aluminum oxide can be formed between the tool and the chip, and the aluminum oxide layer can transfer heat from the tool to the workpiece or chip.
- Wear resistance:
Abrasion resistance refers to the ability of a coating to resist abrasion. Because some cutting tool materials may not have the optimal hardness desired for cutting, coatings can be added during production reduce wear of cutting edges.
- Surface lubricity:
A high coefficient of friction increases cutting heat, which can lead to reduced coating life or even failure. Reducing the coefficient of friction can extend tool life. A smooth or regular-textured coated surface helps reduce the heat of cutting, as the smooth surface reduces heat generation by allowing chips to slide quickly off the rake face. Coated tools with better surface lubricity can also be machined at higher cutting speeds than uncoated tools, further avoiding high-temperature fusion welding with the workpiece material.
- Anti-sticking:
The anti-stick properties of the coating prevent or mitigate chemical reactions between the tool and the material being machined, preventing workpiece material from depositing on the tool. When machining non-ferrous metals (such as aluminum, brass, etc.) a built-up edge often occurs on the tool, resulting in tool chipping or the workpiece size getting out of tolerance. Once the material being machined begins to adhere to the tool, the adhesion will continue to expand. Coatings with good anti-adhesion properties work well even in applications with poor coolant properties.
Application of Tool Coating:
The application of a good coating depends on many factors, so it can be a matter of trial and error to choose the right coating for different specific processing applications. The correct choice of coating and its properties will be critical in enhancing or improving processability. Depth of cut, cutting speed, and coolant can all have an impact on the performance of tool coating applied. Since there are many variables in the machining of workpiece material, one of the best ways to determine which coating to use is through trial cuts. Coating suppliers are constantly developing new coatings to further improve the high temperature, friction, and wear resistance of coatings.
The coating of a tool is one of the key factors affecting the life of the tool. The service life of the tool can be extended and the cost minimized. Different but suitable tool coatings can be chosen according to the cutting method and workpiece material to improve the tooling process and optimize production efficiency.
