**4. Conclusions**

The present study has shown that the production of specific 3D-microtextures on steel substrates using an ultra-short pulsed laser was feasible and the reproducibility of the texture dimensions over the entire textured regions was extremely good.

LIPSSs are the initial features produced during the formation of self-ordered hierarchical microand nano-textures. By nature, self-ordered structures are not predictable in any minute aspect of their shape. It was demonstrated that knowledge of optical and structural feedback mechanisms can become a tool to steer the metamorphosis from LIPSS towards desired surface structures for specific applications, or to suppress their formation (for example pinholes) when a smooth ablation area is required. A very simple approach for controlling pinhole formation was found: to suppress pinhole formation, the rule is to orientate the laser polarization perpendicular to the scan direction; to promote their formation, the rule is to orientate the laser polarization parallel to the scan direction. Additional polarization flipping by 90◦ after some consecutive laser ablation scans leads to further improvements in terms of surface texture quality (smoothness). Pinholes are the intermediate state towards the formation of self-ordered microbumps. Applying these simple ablation rules provides a tool to generate tailored self-ordered, hierarchical micro- and nano-textures for implementation in tribological applications.

Unfortunately, the application of an antifriction coating onto the 3D-textures did, however, greatly modify their topography: 3D-textures became more rounded and the height differences between each plateau were drastically reduced.

Tribological investigations under oil-lubricated conditions have shown that the 3D-microtexture does not show any significant COF improvements when compared to the benchmark over the entire load range studied: COF for the oil-lubricated benchmark was between 0.10 and 0.12 and for the oil-lubricated 3D-textured samples, between 0.13 and 0.15.

However, tribological investigations with an antifriction coating showed that the 3D-microtexture does show significant COF improvements when compared to the unlubricated coated benchmark over the entire load range studied: COF for the coated benchmark was between 0.25 and 0.30, and for the coated 3D-textured samples, between 0.20 and 0.22. However, the friction measured for the coated 3D-texture was still slightly higher than for the oil-lubricated benchmark; therefore, from the point of view of comparing only the friction coefficients, the 3D-texture under study may not be easily seen as an interesting alternative to oil-lubricated blank steel.

Wear-based tribological tests (125 N; 2 h) have shown that the antifriction coating on the benchmark samples was completely depleted, which greatly influenced their friction and wear behavior. Wear-based tribological tests with 3D-textured samples have shown that linear grooves were produced on the cylinder surface by the 3D-texture, but only for the samples aligned with an angle of 90◦ to the direction of motion. This phenomenon is due to the geometrical aspects caused by the orientation of the 3D-texture with respect to the direction of motion during the wear tests.

A comparison of wear rate results showed that 3D microtextures with an orientation of 90◦ in combination with an antifriction coating may drastically reduce wear of both the upper and lower samples. For 3D microtextures with a 45◦-angle, disc wear remained similar to the benchmark, while cylinder wear slightly increased. The latter behavior for the 45◦-angled 3D-texture was explained through a different nominal contact area and resulting real contact pressures.

**Author Contributions:** Conceptualization, J.V., J.Z., S.K.; methodology, J.V., J.Z., S.K.; formal analysis, J.V., J.Z., S.K., F.A., I.V.; investigation, J.V. and J.Z.; writing-original draft preparation, J.V. and J.Z.; writing-review and editing, J.V., J.Z., S.K., F.A., I.V.; project administration, J.V. and J.Z.

**Funding:** The work presented herein was funded by the Austrian COMET Programme (Project XTribology, no. 849109) and carried out at the "Excellence Centre of Tribology" (AC2T research GmbH) in cooperation with V-Research GmbH.

**Acknowledgments:** Carl Bechem GmbH (Hagen, Germany) is gratefully acknowledged for the suggestion and the free of charge application of an appropriate antifriction coating onto the disc specimens for the present tribological investigations. The authors would also like to thank Stephan Kasemann for tireless support in proofreading the manuscript, Heinz Duelli for valuable technical advises to obtain proper REM pictures, Thomas Auer for test sample preparation, Giovanni Piredda for many inspiring scientific discussions and Johannes Edlinger (head of Research Centre for Microtechnology) to cultivate a scope for interdisciplinary research. Open Access Funding by V-Research GmbH.

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