Since their introduction to industry in the 1970s, a Fabricating & Metalworking article this month begins, "coatings have steadily emerged to become a routine part of the cutting-tool equation."

Indeed, coatings have become an indispensable part of most cutting-tool grades. Cutting-tool coatings improve wear resistance, increase tool life, broaden a given grade's application range and enable higher-speed use. In improving performance, coatings help cutting-tool manufacturers respond to changing work-piece materials and process requirements.

Most coatings are applied using one of two methods: chemical vapor deposition (CVD) and physical vapor deposition (PVD).

CVD was used to produce the first cutting-tool coatings in the late 1960s and early '70s. In the CVD process, the tools are heated in a sealed reactor up to 1000ºC, thus providing enough thermal energy to activate reactions in a vacuum chamber to form the coating. Thickness of CVD coatings can range from ~5 to 20 µm. Because the coating materials have a higher coefficient of thermal expansion than carbide, CVD coatings on carbide substrates are in residual tension at room temperature. (Because such heat is detrimental to most tool steel, CVD so far has been limited to carbide tools.) The stresses may be relieved by transverse cracks that do not affect coating adhesion but may initiate tool fracture during interrupted cutting.

The high process temperature used in CVD ensures good bonding between the substrate and the coating material. However, it also can cause embrittlement and other microstructural changes in the substrate material, reducing tool life and increasing the potential for catastrophic failure.

In response to this, the medium-temperature CVD (MTCVD) process was developed in the 1980s to allow coating deposition at temperatures from 700 to 900ºC. The reduced processing temperature and faster deposition of MTCVD work to maintain toughness of the substrate material and to reduce thermally induced cracking in the coating. (The article noted that medium-temperature, or MT-CVD, lowers the reaction temperature to about 900ºC. Though an improvement, it remains too hot for most substrates.)

The latter method, PVD, emerged in the 1980s as a viable process for applying hard coatings to cemented carbide tools. This second major process used for producing cutting-tool coatings, unlike CVD, however, relies on electrical plasma to create the reactions. The plasma's charged particles lower the reaction temperature to about 500ºC and are known to offer more control over the process.

In PVD, the coating is deposited in a vacuum, noted a Manufacturing Engineering article in 2002. The metal species of the coating, obtained via evaporation or sputtering, reacts with a gaseous species (e.g., nitrogen or ammonia) in the chamber and is deposited onto the substrate. Because PVD is a low-pressure process, the coating atoms and molecules undergo relatively few collisions on their way to the substrate. Therefore, PVD is considered a "line-of-sight" process, one that requires moving fixtures to ensure uniform coating thickness.

The primary difference between CVD and PVD is the latter's relatively low processing temperature, 500ºC. This lower processing temperature results in multiple benefits for PVD coatings: a very fine grain structure of the coating, for example, the result of which is a very smooth, bright coating with a low coefficient of friction; and PVD coatings are essentially free of the thermal cracks so common in CVD coatings.

A further advantage of the PVD process is the ability to coat tools with sharp edges and complex chipbreaker geometries. (CVD-coated tools require a hone, as the high-temperature process results in formation of eta phase in the carbide substrate. Eta-phase formation is especially prevalent on sharp edges.) In PVD, processing temperatures are low enough that eta-phase formation is eliminated, thus allowing deposition of PVD coatings on sharp edges. Ability to coat sharp edges also is enhanced by PVD coatings' relative thinness versus CVD.

As well, PVD coatings have very high built-in compressive stresses that help them resist crack initiation and propagation. Minimizing crack formation and propagation can help prevent premature tool failure, improving tool edge security.

Coatings have come a long way since the first CVD offerings in the 1970s. Now they can be both engineered for specific properties and aimed at niche applications. However, it remains highly unlikely that the industry will see one coating for all applications, as coatings must be tailored for the application, cutting edge, substrate and the material to be cut.

There are many, many factors to consider when trying to decide on CVD or PVD coatings. Our posted article does not cover a significant number of these factors. We encourage you to check out the full Fabricating & Metalworking cover story.

References

The Physics Behind The Coating
by Tim Heston
Fabricating & Metalworking, March 1, 2006

Cutting Tools 101: Coatings
by Jim Destefani
Manufacturing Engineering (via the Society of Manufacturing Engineers), October 2002