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Development of Carbon/Carbon multilayer with extremely low wear and low friction under dry and mixed-lubrication condition

The developed Carbon/Carbon multilayer (C/C-ML) is a coating for protecting metallic surfaces from wear and to reduce the friction force under tribological loading. It is a nanostructured amorphous coating consisting of 100% carbon. The layered nanostructure is obtained by periodic variation of chemical bonding state (sp-hybridization) between carbon atoms.

The film is composed of nanometre-layers with dominating sp3-content (diamond bonding) and dominating sp2-content (graphite bonding). Irrespective of partial graphite bonding the overall coating hardness is high (>40 GPa) resulting in extraordinary good wear resistance.

The nanolayering has two major benefits in comparison to not structured homogeneous coatings:
- Due to the nanolayering the intrinsic compressive film stress is reduced from 5-8 GPa to 1-3 GPa. As the compressive stress is a driving force for coating delamination, the nanostructured coatings show improved adhesion strength.

- Coating with an intrinsic nanostructure posses a higher resistance against crack propagation (=fracture toughness) due to energy dissipation on the tip of propagating cracks. For these two reasons the nanostructured C/C-ML is predestined for tribological application under demanding mechanical loading conditions.

The coating can be deposited by vacuum-arc discharge in dedicated PVD coating machines. There was developed a pulsed arc technique allowing for high-rate deposition of good-quality C/C-ML films in the thickness range 100 nm - 10 µm. The films can be deposited on complex three-dimensional metal substrates as steel components and tools.

The periodicity is obtained by periodical change of deposition parameters influencing the energy of depositing carbon species. The films have high potential for low-wear and low-friction application e.g. on components in automotive industry. The special benefit of the coatings is the extremely low friction and wear under dry and mixed lubrication conditions. First application tests in automotive industry revealed the high potential of these coating to obtain even super-low friction behaviour in the presence of adjusted lubrication oils.

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