**4. Conclusions**

The concept of the combination of hard and soft coatings on elastomers has been investigated. In this research, DLC was taken as an example of a hard coating and MoS2 as a soft coating. It was proven that this concept can be used to improve the tribological properties of elastomers, especially under starved lubrication condition. There is not one coating that is optimal for all substrates. For different rubber substrates, the coating should be chosen individually, based on the substrate, coating properties, and their interaction. For a rubber substrate with low rigidity like FKM, soft coatings like MoS2 present better tribological properties than hard coatings like DLC. This is attributed

to the good shear characteristics and good deformation properties of MoS2. Meanwhile, for a substrate with a higher rigidity like HNBR, a hard coating like DLC is a better option. For NBR, whose rigidity is between FKM and HNBR, a hybrid coating is the best choice. It possesses both advantages of hard and soft coatings. For TPU, due to its totally different microstructures, a different wear mechanism was discussed. For a hard substrate with a smooth surface, MoS2 presented a better performance than a hard coating because the small particles of the hard coating can bring disadvantages during sliding motions.

Through the observation of microstructures on uncoated and coated surfaces the influence of the surface roughness and surface energy on tribological properties was investigated. The low surface energy of substrate leads to a porous and loose coating. As a consequence, the tribological properties could be adversely influenced.

The concept of the combination of hard and soft coatings will open new fields for the use of coatings in tribological applications on elastomers. Our data rule out the possibility that the application of DLC/MoS2 as a coating can improve the tribological properties of elastomeric seals, especially under dry or insufficiently lubricated conditions. This finding is promising and should be explored with different combinations of even more than two coatings.

**Author Contributions:** Conceptualization, A.H., J.M.L., and T.S.; Methodology, A.H., J.M.L., and C.W.; Validation, C.W.; Formal Analysis, C.W., A.H., P.N., T.S.; Investigation, C.W.; Data Curation, C.W.; Writing–Original Draft Preparation, C.W., P.N.; Writing–Review & Editing, A.H., J.M.L., T.S.; Visualization, C.W.; Data Supervisor, A.H., T.S.; Resources, T.S., J.M.L.; Project Administration, A.H., J.M.L., T.S.; Funding Acquisition, T.S.

**Funding:** This research was funded by the project of "Bionics4Efficiency", which is one project of the "Bridge Program" (84037) of the Austrian Research Promotion Agency (FFG).

**Acknowledgments:** The authors gratefully thank M. Mitterhuber, W. Waldhauser and H. Parizek for their technical and scientific support and useful discussions.

**Conflicts of Interest:** The authors declare no conflict of interest.
