All photos courtesy of Oerlikon Balzers

Expensive and time consuming, diamond-like carbon (DLC) coatings have become popular for racing applications because they allow highly stressed components to handle higher loads, higher temperatures, and longer use cycles without adding mass to the part.

However, several problems occur at the limits of DLC technology.

For the past 25 years, race teams, supercar producers, and some traditional automakers have used amorphous carbon with hydrogen (a-C:H) DLC coatings on critical engine parts using a plasma-assisted chemical vapor deposition (PACVD) technique that creates extremely hard, low-friction, thin coatings that adhere well to substrates.

Marc Hervé, segment manager for motorsports for coatings manufacturer Oerlikon Balzers says a-C:H coatings have become more affordable throughout the past two decades, but performance improvements have peaked.

“When a customer seeks higher performance than an a-C:H coating can deliver, the only option to date has been a more expensive and time-consuming hydrogen-free DLC,” he adds.

Camshafts in racing engines must withstand intense pressures, strain, and rotational speeds. Coating them with a low-friction diamond-like carbon (DLC) material can improve system function.

Roughness issues

Companies produce hydrogen-free coatings using a physical vapor deposition (PVD) by arc method that produces tetrahedral amorphous carbon or ta-C. The resulting coatings are dense, hard, and adhere well to substrates, but small droplets created in the vapor deposition process create a rough surface finish that must be polished for the low-friction needs of race components.

“The high hardness we achieve with DLC starts to work against you with ta-C coatings,” Hervé explains. “Because the coatings are so hard and durable, it can be very difficult to polish them.”

Another issue is availability. Oerlikon coats parts for race teams from one facility in France, and Hervé says the heat chambers used for PVD are small, limiting throughput. Both limitations keep the technology limited to the smaller world of motorsports, he says.

Racing pistons and rings face extreme temperature and pressure loads, making them good candidates for engineered coatings.

Sputtered coatings

To increase availability of coatings and possibly lower costs, Oerlikon Balzers engineers studied various deposition techniques to achieve hydrogen-free DLC durability with the smoothness of hydrogen-containing coatings. The resulting scalable pulsed powera plasma (S3p) process they developed combines arc evaporation and sputtering. Arc evaporation produces dense coatings with high adhesion, and the conventional sputtering technique ejects atoms of a source material toward a substrate, creating smooth surfaces.

S3p technology generates a high level of diamond tetrahedral bonds with hardness up to 40 gigapascals (GPa). Typical DLC coatings range from 20GPa to 30GPa. The Baliq Carbos coatings the engineers developed are 3x more resistant to wear than the 20GPa DLC coatings. They also achieve 0.03µm Ra roughness levels without requiring polishing.

For now, Hervé says the company is targeting race components, but the S3p process could make DLC coatings viable for a wider range of components.

Racing powertrain parts coated with Oerlikon Balzers’ Baliq Carbos diamond-like carbon can withstand higher temperatures and forces than uncoated parts. Baliq Carbos is smoother than earlier attempts at hydrogen-free coatings, removing a polishing step.

Further development

S3p is cooler than other DLC processes, operating below 200°C, while PVD operates closer to 350°C. Thinner-walled steel and aluminum components that couldn’t withstand the higher-temperature coating processes without warping may be suitable for Baliq Carbos coatings, Hervé says.

Engineers continue to refine the process, exploring larger chamber sizes to scale S3p for larger components or higher volumes of small parts.

“We’re beyond the R&D stage at this point, and we’re running a lot of tests with customer s to understand the capabilities of Carbos,” Hervé says. “But we’re still in the early stage.”

Oerlikon Balzers

About the author: Robert Schoenberger is the editor of TMV. He can be reached at rschoenberger@gie.net or 216.393.0271.