**2. Materials and Methods**

#### *2.1. Laser Surface Texturing*

To compare the different laser surface texturing methods, the structures were applied on a chrome steel (100Cr6) substrate with a thickness of 6.6 mm. Before the laser modification is carried out, the base plate was prepared by grinding and polishing with a multi directional polishing using several abrasive grits and afterwards polished with suspensions down to a grain size of 1 μm to achieve a reference surface with a roughness of *Ra* = 14 nm.

#### 2.1.1. LSFL

Generally, LIPSS are categorized into 3 groups, namely low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL), and cone-like protrusions (CLP) [8]. In this study, we use LSFL to alter tribological properties. These structures appear upon laser irradiation with linear polarized light. The origin of LSFL is commonly explained by an interference effect of the incident laser light and a surface electro-magnetic wave generated by a laser-induced surface plasmon polarition (SPP) [8]. Due to this origin, LSFL occur with a spatial periodicity in the range of the used laser wavelength and an orientation perpendicular to the polarization of the laser light. Several research groups have shown that LSFL properties like spatial period, orientation, and homogeneity can also be controlled by

the applied laser fluence [27,28], pulse to pulse overlap [29], and initial surface roughness of the solid material [30]. For laser surface processing, we used a micro-machining station (MM200 USP, Optec, Frameries, Belgium) equipped with an ultrashort pulsed laser (Pharos 10-600-PP, Light Conversion, Vilnius, Lithuania) having a pulse duration of 220 fs (FWHM) at a repetition rate of 300 kHz. For the generation of the micro- and nano-structures, the fundamental emission wavelength of 1030 nm was utilized. Figure 1a shows the experimental setup for the surface treatment. The energy of the laser was adjusted by an external attenuator based on a rotating wave plate and a polarizer. Using a half wave plate in front of the focusing unit, the linear polarization of the laser beam was rotated orthogonal to the scanning direction. A galvo scanner (RTA AR800, Newson, Dendermonde, Belgium) was used in combination with a telecentric lens (f = 100 mm) to focus the beam onto the sample with a spot diameter of 38 μm (1/e2).

**Figure 1.** Methods for laser surface structuring (**a**) low spatial frequency LIPSS (laser-induced periodic surface structures, LSFL) setup (**b**) laser beam interference ablation (LBIA) setup.
