Cutting Tool Coating Chemistry 101

Manufacturers continually push the envelope in terms of materials and processes, which means that their tooling suppliers must do the same. As a result, inserts and solid tools alike now appear with a wide range of coatings in a muted rainbow of golds, blacks, silvers and blues. And each has its own complex grade designation and chemical formula. No matter how complex these coatings appear, however, one doesn’t need a degree in chemical engineering to understand how these materials work.

It helps that coatings are relatively new in metal cutting, with the first coated cemented carbide tools appearing in the 1970s. On the other hand, the main processes used to produce these coatings – physical vapor deposition (PVD) and chemical vapor deposition (CVD) – have been known for centuries. PVD is a low-temperature process that creates a thin coating that protects and strengthens sharp edges, whereas CVD requires higher temperatures for a chemical reaction that leaves a thick, wear-resistant coating.

However, the only key difference between PVD and CVD involves how the coating material is initially vaporized in a process called sublimation, in which a material passes from the solid state to the gas state with no intermediate liquid stage. Typically, this material is titanium, which reacts with nitrogen and/or carbon during the coating process to produce titanium nitride (TiN), titanium carbide (TiC), or titanium carbonitrides (TiCN). These innovations served as the foundation for most modern coatings.

Today, many tools and inserts feature titanium aluminum nitride (TiAlN), which reacts with high temperatures to form a layer of aluminum oxide (Al2O3), which adds further hardness to the coating. Other coatings have introduced ceramics like silicon nitride (SiN), which particularly excel in high-temperature applications. And rather than use heat to force the formation of Al2O3, manufacturers have coated tools in it directly for the highest level of crater and chemical resistance at high speed and high temperatures.

While understanding the principles behind cutting tool coatings is simple, however, the finer details get increasingly complicated. For example, the Duratomic® coating developed by Seco involves the molecular manipulation of coating components to produce a layer of 100% α-Al2O3, a stable crystalline structure similar to that found in sapphires and rubies. Duratomic is also one of many coatings that use multiple processes and materials to form layers of Al2O3 and various titanium compounds.

Luckily, tool manufacturers like Seco make it easy to use printed catalogs or online tools to find the right coating for a given material and application. To learn more, visit secotools.com or speak with your local Seco distributor.