Structural Optimization of Amorphous Carbon Films for Renewable Energy Technologies

Abstract

DLC films are promising coatings for renewable energy technologies due to their ability to reduce frictional losses and offer superior corrosion protection. Their use, however, is limited by poor electrical conductivity and high internal stresses. Recently, it has become possible to produce novel undoped DLC coatings with low internal stresses, high thicknesses and excellent electrical conductivity. This was achieved using special deposition conditions, combining high temperatures with direct current plasma assisted chemical vapour deposition (DC PACVD). However, despite the beneficial properties of these films, they exhibit poor tribological performance and relatively low hardness. As it is well-known from literature that the properties of DLC films can be significantly improved by adding different foreign elements, i.e. doping, this work aims to resolve the drawbacks of the special high-temperature DLC coatings by adding two different doping elements: silicon and nitrogen. The novelty of this work lies in the fact that it is completely unknown how the two elements behave when they are added at these unusually high temperatures. The experimental analysis showed that doping with silicon improved hardness remarkably by a factor of 3-4 and internal stress increased compared to the undoped films: two findings which stand in complete contrast to previous literature reports. In addition, friction coefficient was reduced by up to 65 % and the adhesion to the substrate was significantly enhanced upon Si incorporation, paving the way for successful industrial application of the Si doped films to improve energy efficiency. Nitrogen incorporation into the DLC films also greatly improved film performance. Hardness increased by up to 48 %, while electrical resistivity decreased remarkably by up to several orders of magnitude, even surpassing the conductivity of graphite electrodes. Tribological testing showed that friction can also be effectively reduced by nitrogen doping at high deposition temperatures, contrary to previous literature. In addition, corrosion resistance of the films with the highest electrical conductivity was beneficial, enabling the production of novel DLC coatings with excellent conductivity for electrochemical industrial applications such as electrodes or bipolar plates.

Date of Award	2025
Original language	English
Supervisor	Daniel Heim (Supervisor)