Cutting with coatings: Knowing what’s best for your application
Coatings may seem simple on the surface, but when deciding what is best for your application, things become more complex. When looking at a cutting tool, there are three main components that make up the tool—the material you are making the cutting tool out of, the geometry and the coating you put on it. With all the innovations within the industry, there have been more developments in coating technology than in material or geometry technologies. Therefore, deciding which coating would be beneficial for a particular application requires learning more about the various options available, the newest innovations and what will best support or improve your application, but, most importantly, you must look at all aspects of your application when choosing the coating for your cutting tools.
Over the last 50 years, coatings have revolutionized cutting tools. Without them, there is much less wear resistance or heat resistance during a machining application, but the benefits of running tools coated instead of uncoated are numerous. Not only do you have better tool life with coated tools, but coatings also provide thermal barriers, which allow you to run faster. Coatings take the surface hardness of your tool and increase it while adding a level of lubricity to the tools. Ultimately, you have much less friction in the cut, and the lubricity gained from the coating also helps to eliminate built up edge among other issues that you could face with uncoated tools.
Now that it is clear that the benefits of coated tools outweigh those of uncoated tools, the question may become whether the benefits vary between the different types of coatings. The easy answer is yes as different coatings have different properties in terms of wear resistance and hardness, but the benefits depend highly on the application. There are coatings with higher lubricity, coatings that are harder than other coatings, coatings that have higher heat thresholds and coatings for specific material groups. Exemplifying this are P, K and N grade inserts where different material grades are put together with different coatings in order to excel within their materials groups. Steels might require more aluminum chromium-based coatings whereas cast iron might require silica nitride or aluminum titanium because of the abrasive nature of cast iron. Lastly, aluminums and other nonferrous materials require coatings that have higher lubricity because aluminum is a material that wants to build up on an edge more than other materials; it is soft and has a lower melting point than other materials. All in all, the benefits you get from coated tools depend on what you are trying to machine and the material that you are cutting because certain coatings work better in certain materials.
Whether looking at TiN, TiCN, TiAlN, or a material-specific coating for your application, knowing what coatings are available may require you to refer to the manufacturer of the tooling you are using, but many basic concepts for coatings remain consistent throughout the industry. TiN, the first physical vapor deposition or PVD coating developed, remains the most often used although it is not the most wear resistant. The second PVD coating to be created was TiCN, which was the first multilayer coating to be developed with layers of TiN and TiCN interlayered. Next came TiAlN. Ultimately, these three coatings are somewhat universal, so you can use them in about every different type of application; however, coatings more recently have become more material specific. Because a new universal type coating has not been developed, these material-specific coatings focus on getting to the next level of heat resistance, hardness and wearability through different combinations and configurations of chemical elements, causing innovators to continue to push the envelope with coatings.
Even knowing the different types of coatings does not guarantee a clear answer on what will work best in a specific application; you must examine the many different factors involved. First and foremost, know the material you are cutting and what geometry is needed for the application. Then you will be able to use your tooling manufacturer’s recommendations in terms of what coating to use in that material. Additionally, coating choice is dependent upon what type of cutting you are doing. There is a lot of variation between thread mills, drills and end mills for example. If knowing this information does not lead to a clear answer on which coating to use, there are other questions you can ask of your application and the environment that you are working in, including heat generated:
- How hard is the material you are cutting?
- What is the machine tool that is involved?
- What is the horsepower of your machine?
- What type of coolant is available?
- Do you have high pressure coolant?
- What kind of penetration rate are you looking to have?
Ultimately, you have to look at the application in its entirety in order to make a sound decision as far as what tool should be used and what coating should be on the tool. Because there are several variables in the cutting tool process, there is a chance the coating choice may be incorrect; however, you can reexamine the application and choose a coating that works to produce a successful application.
Simply stated, coatings are extremely beneficial for cutting tools for heat and wear resistance alone. Combining different elements like aluminum, titanium, chromium or silica nitride in a coating system with different layering techniques created the coatings that exist today, which is the same concept being used to further enhance these combinations and create new coating innovations. Ultimately, most of the 92 chemical elements available have been examined through the lens of whether they can be used for a wear resistant coating in a cutting tool, but the continual efforts to create new coatings means there are few elements left to work with. So while you are asking the questions needed to determine what coating is best for your application, the industry as a whole is asking another major question—can new element combinations be created for use in coatings for cutting tools? Nevertheless, while choosing the right coating involves a complex chemical understanding, having ISO-specific geometries within the industry has removed most of the guesswork and begun the transition of not needing to analyze what will be the best coating to cut with.