**1. Introduction**

Achieving greater productivity at lower costs represents a significant challenge for the modern manufacturing industry. In this endeavor, new advanced materials and surface technologies are needed in order to enhance the efficiency of mechanical systems and reduce their energy consumption, which may be achieved through a reduction of the friction and wear of tribological contacts. This reduction of the tribological properties may be obtained through several different approaches, such as modification of the component's geometry, application of hard protective coatings, optimizing surface roughness and/or topography [1–5] or by the introduction of specific surface textures, which play a major role in lubrication, level of friction and the wear rates of tribological systems. Surface microtexturing has established itself over the last few years as a very promising approach to reduce friction in different materials in various lubricated applications [6–18].

Over recent years, a diverse array of manufacturing processes has been developed for the production of surface microtextures, such as mechanical micromachining, chemical etching or laser ablation, to name a few. All these industrial surface microtexturing processes show advantages or disadvantages. The latter process (laser ablation) was selected and used to produce the desired specific 3D microtexture to be investigated in the present study. The investigations consisted of producing, characterizing and tribologically testing the specific 3D microtexture.

Contour shaping, surface patterning and surface functionalization by femtosecond laser ablation (FLA) are becoming fundamental technologies in fields like tribology, microfluidic transport, fuel cells and medicine. Self-ordered hierarchical micro- and nano- structures can be generated on the majority of metals, semiconductors and dielectrics. Laser-induced periodic surface structures (LIPSSs) are significantly involved in the formation of such self-ordered microstructures and pinholes. LIPSSs have been attracting research interest for decades, but their formation is still not fully understood. The latest on the LIPSS formation mechanism and its potential deployment in tribological applications is outlined in [19]. However, the main interests of the authors are focused on a usable technology for contour shaping and surface patterning for specific tribological applications. In order to obtain the desired surface structure or surface quality, the understanding and handling of any feedback mechanisms occurring during the production of LIPSS, self-ordered microstructures or contour shaping is essential. The present authors recently focused themselves on the fabrication of membranes (based on AlGaN/GaN hetero-structure layers grown on 4H–SiC) for sensor fabrication. Feedback effects, which occur between small surface distortions and the slot waveguide function of LIPSS in SiC, were found to promote the formation of micropores in the membranes. To cope with this unwanted and challenging effect, the present authors developed a polarization steering procedure which interrupted the feedback loop and terminated the growth cycle of pinholes [20,21]. In the present study on steel samples, a similar pinhole formation phenomenon at locations where LIPSSs were interrupted by distortions was also observed. Such distortions can be triggered by the scanning laser beam itself, side wall irregularities at the laser generated cube structures, inhomogeneities inside the metal grain, laser generated debris particles or self-ordered bumps. In the current experiments, it is demonstrated that, among all the laser parameters contributing to the formation of such distortions, the most effective parameter is the orientation of the laser polarization with respect to the scan direction, which may suppress or promote the formation of self-ordered bumps and pinholes.

Tribological tests under lubricated conditions and also with an antifriction coating using the aforementioned 3D microtexture along with untextured (benchmark) samples were undertaken to evaluate the effectiveness of reducing the overall friction by decreasing the nominal contact areas through surface microtexturing.
