Ⅰ. Tool Coating Overview
Tool surface coating technology is a surface modification method developed to meet market demands. Since its introduction in the 1960s, it has been widely used in the production of metal cutting tools. The advent of high-speed cutting processing technology has led to the rapid development and application of coating technology, making it a crucial aspect in the manufacture of high-speed cutting tools.
The technology involves forming a thin film on the tool surface through chemical or physical methods, resulting in excellent comprehensive cutting performance that meets high-speed cutting requirements.
In summary, cutting tool surface coating technology has the following characteristics:
- The coating increases the surface hardness of the tool without compromising its strength, with current achievable hardness approaching 100GPa.
- With the advancement of coating technology, the film's chemical stability and high-temperature oxidation resistance have become more pronounced, making high-speed cutting possible.
- The lubricating film has excellent solid phase lubrication properties, improving processing quality and is suitable for dry cutting.
- As the final step in tool production, coating technology has minimal impact on tool accuracy and is repeatable.
Using coated cutting tools offers several benefits, including:
- Significant improvement in the useful life of cutting tools.
- Greater cutting efficiency.
- Significant improvement in the surface quality of the processed work.
- Reduction in tool material consumption and processing costs.
- Less use of cooling fluid, resulting in cost savings and greater environmental protection.
Proper surface treatment of small circular tools can lead to an increase in tool life, shorter processing cycle time and an improvement in the quality of processed surfaces.
However, choosing the right tool coating to meet specific processing needs can be a complicated and time-consuming task. Each coating has its own advantages and disadvantages when cutting. Using an inappropriate coating can result in shorter tool life than uncoated tools and even create additional problems.
There are several types of tool coatings available on the market, including PVD coatings, CVD coatings, and composite coatings that alternate between PVD and CVD. These coatings can be easily obtained from tool manufacturers or coating suppliers.
This article will provide an overview of common tool coating properties and highlight some common PVD and CVD coating options. The characteristics of each coating play a crucial role in determining which coating is best suited for cutting.
Ⅱ. Commonly used coatings
- Titanium nitride (TiN) coating
TiN is a commonly used PVD coating that can increase the hardness of the tool and has resistance to oxidation at high temperatures. This coating is used on high speed steel cutting tools or forming tools to achieve optimal processing results.
- Chromium nitride (CrN) coating
CrN coating is highly sought after due to its excellent anti-adhesion properties, making it the preferred coating for processes that frequently result in built-up edges. Once applied, this nearly invisible coating significantly improves the processing performance of high speed steel tools, carbide tools and forming tools.
- Diamond Coating (Diamond)
CVD diamond coating is the best choice for cutting tools used in processing non-ferrous metal materials. It offers excellent performance when cutting graphite, metal matrix composites (MMC), high silicon aluminum alloys and other highly abrasive materials.
Please note that tools with pure diamond coating cannot be used to process steel parts because the high cutting heat generated during processing causes a chemical reaction that damages the adhesion layer between the coating and the tool.
Related Reading: Ferrous vs Non-Ferrous Metals
- Coating equipment
The coatings suitable for hard milling, threading and drilling are unique and have their respective specific applications. In addition, multilayer coatings can also be used, which consist of other coatings incorporated between the surface layer and the base of the tool, resulting in an extended tool life.
- Titanium Nitrogen Carbide (TiCN) Coating
The addition of carbon elements in the TiCN coating increases the hardness of the tool and provides better surface lubrication. This coating is ideal for high speed steel tools.
- Aluminum nitride, titanium or aluminum nitride coating (TiAlN/AlTiN)
The alumina layer formed on the TiAlN/AlTiN coating significantly improves tool life in high temperature machining. This coating is suitable for carbide tools mainly used for dry or semi-dry cutting.
The ratio of aluminum to titanium in the coating determines the surface hardness of the coating, with AlTiN coatings providing a higher surface hardness than TiAlN coatings. As a result, it is a viable option in the area of high-speed machining.
Ⅲ. Coating characteristics
- Toughness
High surface hardness is a reliable method for improving 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. The hardness of TiCN coatings increases by 33% due to increased carbon content, with a hardness range of approximately HV3000-4000 (varying depending on the manufacturer).
CVD diamond coatings with surface hardness up to HV9000 have become more prevalent in tooling applications, resulting in a 10- to 20-fold increase in tool life compared to PVD-coated tools. The high hardness and cutting speed of diamond coatings, which can be 2 to 3 times that of uncoated tools, make it an excellent 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, the better the cutting at high temperatures.
Although TiAlN coatings may have a lower hardness at room temperature compared to TiCN coatings, they are much more effective in high temperature processing. The reason for this is that an alumina layer can form between the tool and the chip, which transfers heat from the tool to the workpiece or chip, thus retaining the hardness of the TiAlN coating at high temperatures.
Carbide tools generally cut faster than HSS tools, making TiAlN the preferred coating for carbide tools. Carbide drills and end mills generally use PVD-TiAlN coatings.
- Abrasion resistance
Abrasion resistance refers to the ability of a coating to resist wear and tear. Although some part materials may not be naturally hard, elements added during manufacturing and the processing method can cause the cutting edge of the tool to chip or dull.
- Surface lubricity
High friction coefficients generate increased cutting heat, shortening or compromising coating life, while lower friction coefficients significantly extend tool life.
A thin, smooth or evenly textured coated surface reduces cutting heat by allowing chips to slide quickly away from the front face of the cutter, thus reducing heat generation. Coated tools with enhanced surface lubrication can also be machined at higher cutting speeds compared to uncoated tools, further avoiding high-temperature welding of the workpiece material.
- Adhesion resistance
The anti-adhesion property of the coating prevents or reduces the chemical reaction between the tool and the material being processed and prevents the deposition of workpiece material on the tool.
When machining non-ferrous metals (such as aluminum and brass), built-up edges (BUEs) often occur on the tool, causing tool chipping or oversized parts. Once the material begins to adhere to the tool, the adhesion will continue to expand. For example, when processing aluminum parts with forming cores, the aluminum that adheres to the cores after processing each hole will increase, eventually causing the diameter of the core to become too large and resulting in out-of-tolerance parts that must be discarded.
Coating with good anti-adhesion properties can be effective even in situations where the refrigerant performance is poor or the concentration is insufficient.
4. Application of coatings
The cost-effectiveness of coating applications can depend on a number of factors, but for each specific processing application, there are typically only a few viable coating options. Choosing the correct coating and its properties can make a significant difference to processability, while an incorrect choice can result in minimal improvements.
Cutting depth, speed and coolant used can affect the effectiveness of the tool coating. To determine the best coating for a specific application, test cutting is often the most effective method.
Coating suppliers are constantly developing new coatings to increase resistance to heat, friction and wear. It is beneficial to work with coating (tool) manufacturers to evaluate the latest and most advanced tool coatings for machining applications.
-End-
