Carbon Coatings for Tools and Components

High mechanical and abrasive stresses significantly shorten the service life of tools and components. Coatings made of tetrahedral amorphous carbon (ta-C) reduce friction and effectively protect surfaces.

The Fraunhofer Institute for Materials and Beam Technology IWS recently achieved two breakthroughs that could significantly expand the industrial application of ta-C coatings. Researchers from Dresden developed a plasma filter technology for the laser-arc process to specifically reduce defects. Furthermore, for the first time, they succeeded in reliably producing ultra-thick ta-C coatings up to 100 micrometers in thickness, thereby paving the way for the protection of surfaces subjected to extreme mechanical stress.

Tetrahedral amorphous carbon is the hardest form of diamond-like carbon coatings. A high proportion of sp³-hybridized diamond bonds accounts for its high hardness and exceptional wear resistance. At the same time, ta-C significantly reduces friction in tribological systems. Industrial companies have been utilizing these properties for years to reduce friction losses and protect components under demanding operating conditions. The industrial production of ta-C coatings requires controlled vacuum arc processes. This process inevitably produces particle-like droplets. They detach from the cathode, enter the plasma beam, and deposit onto the growing layer. These defects increase surface roughness and often require time-consuming, downstream smoothing steps. At the same time, high compressive residual stresses limit the achievable film thickness, as they increase the risk of film failure. The widespread industrial adoption of ta-C films depends crucially on whether these challenges can be overcome.

The Fraunhofer IWS is consistently advancing the laser-arc process for industrial use. High deposition rates and stable processes are among the key advantages of this technology. Together with industry partners, the researchers have transferred the process to series production applications for tribological protective coatings on engine and machine components as well as tools.

In parallel with the industrialization of the laser-arc process, Fraunhofer IWS addressed two key challenges in ta-C production through targeted further development: the deposition of layers more than 20 micrometers thick with low internal stresses, and the production of smooth, defect-free ta-C layers.

To achieve this, researchers adapted the pulsed power supply technology and the layer architecture, reducing the compressive residual stresses to a subcritical level without compromising layer hardness. At the same time, the team optimized the long-term stability of the laser-arc technique at high deposition rates. Today, the systems operate stably for periods exceeding 24 hours and enable reproducible processes for thick films. In addition, Fraunhofer IWS developed specific adhesion layer systems tailored to the respective material. These enable the reliable deposition of ta-C coatings up to 100 micrometers thick on engineering substrates such as roller bearing steel, cemented carbide, cast materials, and selected plastics. In addition, the researchers integrated a plasma filter technology into the laser-arc process. A 90-degree deflection filter now selectively separates the ion current from particles. As a result, the film grows uniformly and exhibits a particularly low defect density, while the process maintains a high deposition rate.

Dr. Volker Weihnacht, head of the Thin Film Technology Department at Fraunhofer IWS, explains: “Our laser-arc technology combines high deposition rates with stable processes for high-quality ta-C films. The targeted reduction of internal stresses, high long-term stability, and optimized adhesion layer concepts now make it possible for the first time to deposit ultra-thick layers on an industrial scale—a true breakthrough for conventional thin-film systems, which often fail under high loads due to the ‘eggshell effect.’ Our “thick” ta-C films now provide the necessary structural support required for high loads.”

The application determines the design of the coating system. Thick coatings are used where severe abrasive or erosive stresses occur. The coating acts as a self-supporting structure and prevents the “eggshell effect”—that is, premature coating failure due to localized overloading. Coating thicknesses of up to 100 micrometers thus provide reliable and long-lasting protection when extreme operating conditions require it.

For applications involving hard base materials, coating thicknesses of one to five micrometers are sufficient. Plasma filtration technology enables the deposition of smooth, defect-free ta-C coatings. The filtration process removes droplets from the plasma jet, resulting in homogeneous coatings with a smooth surface. This enhances the surface quality of the components and improves the stability of tribological systems during operation. Typical applications include plain bearings, shafts, guides, and tools for machining and forming. The customized coating design allows the system to be specifically tailored to the respective load and function.

“The expansion of the laser-arc process to include ultra-thick and smooth ta-C coatings strengthens our contribution to optimizing the service life of industrial systems,” explains Prof. Christoph Leyens, director of the Fraunhofer IWS. “This new development further reduces wear, stabilizes production processes, and helps industry use resources sparingly and efficiently.”

In addition to wear protection, ta-C coatings offer another property: their nearly universally low friction. Under a wide variety of lubricating media—from motor oil and machine oil to greases and waxes, as well as water-based media, acids, and bases—even friction pairs coated with ta-C on only one side exhibit stable friction behavior with coefficients of friction ranging from 0.1 to 0.2.

Even under unfavorable operating conditions, the tribological system remains stable. Insufficient lubrication or a temporary loss of lubrication does not immediately lead to damage to the contact surfaces. This robustness expands the range of applications from food processing to applications in the drinking water sector and medical technology, all the way to implant coatings.

ta-C coatings also offer potential in superlubrication, a state in which friction coefficients are virtually no longer measurable. Researchers at Fraunhofer IWS, together with partners from academia and industry, have been investigating for years how this effect can be transferred to real machine components. The CHEPHREN research project, funded by the Federal Ministry for Economic Affairs and Climate Action, achieved a significant breakthrough: for the first time, smooth ta-C coatings produced using plasma filtration technology reached the state of superlubricity under conditions simulating real-world applications. The long-term stability of this friction reduction offers significant economic potential. It increases the energy efficiency of drive and machine components and reduces the energy consumption of motors, pumps, and systems by up to ten percent. Dr. Volker Weihnacht summarizes: “ta-C offers great potential for wear protection and energy savings in numerous industrial applications. The advanced laser-arc technology expands this range of applications by enabling ultra-thick and exceptionally smooth ta-C coatings. This makes it possible to implement applications that were previously technically unfeasible.” (OM-9/26)