**1. Introduction**

Laser surface texturing enables a precise and reliable method to modify surface properties and provides new ways for the design and fabrication of novel surface functionality towards smart surfaces. In this wide field of surface engineering, the controllable modification of friction and wear is of high interest. A recent study reveals that about 23% of the world's energy consumption and 8120 Mt/year of CO2 emission are caused by these tribological effects, in turn posing the necessity for wear and friction control [1]. While laser-based surface functionalization has already been intensively studied during the last decade, femtosecond laser technology has recently expedited surface structuring on different length scales to advance tribological properties [2–7]. In terms of laser surface texturing, different methods are available to modify the friction behaviour. On the one hand, regular ablation processes using short (pulse duration *τ* > 10 ps) and ultrashort (pulse duration *τ* < 10 ps) laser pulses are capable of generating meso- to micro-structures with spatial feature size limitation of the applied laser spot size, typically several tens of micrometers. On the other hand, advanced processing techniques can fabricate surface texturing patterns with feature sizes magnitudes below the applied laser spot size. Out of this research field, we use Laser-Induced Periodic Surface Structures (LIPSS) [8] and Laser

Beam Interference Ablation (LBIA) [9] to fabricate periodic linear patterns on solid surfaces with feature sizes of approx. 1 μm. The general difference between LIPSS and LBIA is their origin and the respective profile depth. While LBIA structures are generated by a special interferometer-based optical setup, LIPSS occur in a self organizing process based on the interference of the laser irradiation and Surface Plasmon Polaritons (SPP) [10]. Ultrashort laser-based periodic surface structures can be generated on all kind of solid materials [11–13]. Both laser-induced surface morphologies, LIPSS and LBIA, can be generated in a single-step process and provide multiple possibilities to modify different surfaces properties. Possible applications of this laser surface texturing methods involve colorization due to diffraction at the periodic structures [14,15], modification of surface wetting properties [16,17], influence on surface cell growth [18,19], and friction management [20–22]. The friction properties of surfaces depend not only on the specific material and several properties of the sliding surface itself, but also on the properties of the counter body, the sliding speed, the environmental conditions, and the nature of any lubricant. Therefore, it is obvious that numerous experimental studies are needed to get fully acquainted with the effects of laser generated periodic surface structures on such tribological properties.

With growing possibilities of flexible methods for laser-based surface functionalization, the question arises, which method leads to the required effects in the field of surface engineering. A line-wise laser surface texturing can be achieved by both methods, LBIA and LIPSS, and they both have the potential to modify friction behaviour. In different tribological test scenarios, LIPSS fabricated surfaces with features in the range of 1 μm and below have shown that it is either possible to increase [23] or to decrease [24] the coefficient of friction (COF). While Gachot et al. [20] report a friction reduction after applying LBIA structures on 100Cr6 while performing a dry linear reciprocating ball-on-disc test, Kasem et al. [25] identify a friction increase on LBIA structures on SAE 1035 steel in a lubricated linear sliding test. Beside the structural modification, the laser-based surface temperature increase, caused by the laser beam absorption, can influence the chemical properties and thus lead to a modification of sliding friction. The important role of the surface chemistry is supported by the studies of Gachot et al. [26] for LBIA and Bonse et al. [7] for LIPSS.

To the best of our knowledge, a direct comparison of the tribological modification behaviour of different laser generated micro structures of approx. 1 μm is not investigated yet. Thus, we use both laser surface texturing methods and demonstrate the possibilities of friction modification and show which method results in which friction behaviour.
