Researchers from Dresden have developed a plasma filtration technique for the laser-arc process to specifically reduce defects. For the first time, they also succeeded in reliably producing ultra-thick ta-C layers up to 100 µm. This has opened the door to protecting 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 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 produces particle-like droplets. They detach from the cathode, enter the plasma beam, and deposit onto the growing coating. 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 has further developed the laser-arc process for industrial use. High deposition rates and stable processes are among the key advantages of this technique. Together with industry partners, the researchers adapted the process for series production of tribologically protective coatings on engine and machine components as well as tools.
Achieving ultra-high coating thicknesses and smooth surfaces
In parallel with the industrialization of the laser-arc process, Fraunhofer IWS addressed two key challenges in ta-C production: the deposition of layers more than 20 µm 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 coating architecture, reducing the compressive residual stresses to a subcritical level without compromising coating 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 layers up to 100 µm on engineering substrates such as rolling bearing steel, cemented carbide, cast materials, and certain plastics. In addition, the researchers integrated a plasma filter technology into the laser-arc process. A 90° deflection filter now separates the ion current from particles. As a result, the film grows uniformly and exhibits a particularly low defect density, while maintaining 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 reduction in residual stresses, the high long-term stability, and the use of adhesive layer concepts make the industrial deposition of ultra-thick layers possible for the first time—a breakthrough for conventional thin-film systems, which often fail under high loads due to the ‘eggshell effect.’ Our thick ta-C coatings 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 µm thus provide reliable and long-lasting protection when operating conditions require it.
For applications with hard base materials, coating thicknesses of 1 to 5 µm are sufficient. Plasma filtration technology enables the deposition of smooth, low-defect ta-C coatings. Filtration removes droplets from the plasma jet and results 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 tailored to the specific 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 and stabilizes production processes.”
Reduced Friction Improves Energy Efficiency
In addition to wear protection, ta-C coatings offer another advantage: their nearly universally low friction. Under various lubricating conditions—ranging 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 damage the contact surfaces. This robustness expands the range of possible applications.
ta-C coatings also offer potential in the field of superlubrication, a state in which friction coefficients are virtually immeasurable. Researchers at Fraunhofer IWS, together with partners from academia and industry, have been investigating for years how this effect can be applied to 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 filter technology reached the state of superlubrication under conditions close to real-world applications.