The need for ta-C: Drilling Performance

In the early 1970s, the first report about diamond-like carbon (DLC) coatings was published. Industrial use of these films started with automotive components, such as high-pressure diesel injection systems and powertrain components. Today, a family of DLC coatings exists with specific advantages in certain applications.

The coatings are usually classified by the ratio of sp³ to sp² bonds and the hydrogen content. When carbon is sp³-bonded, it will form diamond; sp² bonds will lead to graphite. With an increase in the ratio of sp³ to sp² bonds, the hardness of the coating will typically increase.

DLC coatings can be doped with metals like tungsten (W-C:H), where C is carbon and H is hydrogen, and other elements like silicon (Si-DLC) to change the coefficient of friction or temperature resistance of the layers. A known application for cutting tools is the combination of a hard nitride coating, such as TiAlN, with a softer, lubricating top coating, such as W-C:H. This combination shows excellent results in tapping and drilling applications because it enhances chip evacuation. The focus of this article will be on a specific DLC coating called tetrahedral amorphous carbon (ta-C).

Endmills coated with a ta-C coating via circular arc CARC+ technology display the coating’s typical rainbow color pattern. All images courtesy of IHI Hauzer Techno Coating

Ta-C is a hydrogen-free carbon coating with a high sp³/sp² ratio. Compared to other DLC coatings, ta-C films show higher hardness and temperature resistance and significantly lower coefficients of friction. The first application of the coating was in the automotive industry: It was deposited on tappets (valve lifters), where it is still used today.

One of the purposes of the coating in tribological, automotive applications such as tappets is to reduce friction, which leads to improved engine efficiency, less fuel consumption and lower CO₂ emission. The push for better fuel economy and less pollution has actually resulted in the use of ta-C coatings on cutting and forming tools. One way to improve the mileage of a car is to reduce its weight. Besides the introduction of ultrahigh-strength steels, the use of aluminum in car bodies and engines has risen over the past several decades. The use of plastics, including carbon fiber-reinforced plastics (CFRP), is also increasing in the automotive and aerospace industries.

When cutting these materials, the tool wear mechanisms are different compared with cutting steel. One of the main challenges is to keep the cutting edge sharp and reduce built-up edge. Because the edge radius of cutting tools for this application is quite small, it helps to keep the coating thickness as thin as possible. These factors contribute to the advantage of depositing ta-C coatings on cutting tools specifically for machining nonferrous metals and plastics.

The HiPIMS process on a carbon target for deposition of a ta-C coating.

Ta-C coatings show little adhesion of aluminum to the cutting edge. Given the high hardness of the coating, a coating thickness well below 1μm is typically sufficient on cutting tools such as drills and endmills. The first tests in the aerospace industry—drilling a titanium and CFRP sandwich material—showed that ta-C extended tool life and significantly improved hole quality. As ta-C has a maximum operating temperature of around 500° C, coolant applications are required in these cases.

DLC coatings, such as ta-C, have a certain degree of transparency. When the coating thickness is less than 1μm, tools will have rainbow colors due to an interference effect. As thickness increases, the color becomes gray-black.

Deposition Technologies

Various technologies are used for the deposition of DLC coatings. Historically, plasma-assisted chemical vapor deposition (PACVD) has been the most common technology.