Understanding coatings and how they lower machining costs

This article provides basic information about the coatings and processes used to coat cutting tools, dies, wear parts and other components. HSS, carbide, tool steels and stainless steels are among the most commonly coated materials.

Physical and Chemical Vapor Deposition

Physical vapor deposition describes a family of coating processes. The most common of these processes are evaporation (typically using cathodic arc or electron beam sources) and sputtering (using magnetic enhanced sources, aka magnetrons, or cylindrical or hollow cathode sources).

PVD processes occur in a vacuum, typically at a working pressure of 10-2 to 10-4 mbar. They generally involve bombarding the substrate to be coated with energetic, positively charged ions during the coating process. In addition, reactive gases, such as nitrogen, acetylene or oxygen, may be introduced into the vacuum chamber during metal deposition. These gases create various compound coating compositions. The result is a very strong bond between the coating and tool substrate.

Image by Alan Richter.

The applications for PVD coatings are constantly expanding. That said, they can be separated into two broad categories: functional and decorative coatings.

Functional PVD coatings are engineered to improve the life and overall performance of a tool or component, thereby reducing the per-part manufacturing cost. An example of a functional PVD coating is TiN on an HSS endmill.

Decorative PVD coatings are deposited to improve the appearance of a part and provide wear resistance. These coatings improve both form and function. An example of a decorative PVD coating is the deposition of a zirconium-based film onto a stainless steel door handle. The resultant brass-colored coating provides better resistance to wear and tarnishing than real brass.

Chemical vapor deposition is an atmosphere-controlled process conducted at about 1,925° F (1,052° C) in a CVD reactor. During this process, thin film coatings form as the result of reactions between various gaseous phases and the heated surface of substrates in the CVD reactor. As different gases are transported through the reactor, distinct coating layers form on the tooling substrate. For example, TiN forms from the following chemical reaction:

TiCl4 + N2 + H2 1,000° C (1,832° F) → TiN + 4 HCl + H2

TiC forms as the result of the following chemical reaction:

TiCl4 + CH4 + H2 1,030° C (1,886° F) → TiC + 4 HCl + H2

The final product of these reactions is a hard, wear-resistant coating that bonds in a chemical and metallurgical manner to the substrate. CVD coatings provide excellent resistance to wear and galling.

Rationale for Coatings

When using PVD or CVD coatings in a tooling application, the primary motivation is simple: to lower manufacturing costs. Users consistently experience longer tool life while also being able to operate at increased speeds and feeds. The savings calculation is easy:

Reduced downtime for preventive maintenance and to change tools

+ increased production rates

+ decreased tooling costs due to increased tool life