*2.1. Laser Setup and Material*

Figure 1a shows the experimental laser setup schematically. It consists of a pulsed Yb:YAG disk laser source (TruMicro 5050 of Trumpf GmbH, Ditzingen, Germany) emitting a laser beam with a wavelength of 1030 nm, maximum pulse frequency of 400 kHz, pulse energies up to 125 μJ and a fixed pulse duration of 6.7 ps. The fluence profile of the focal laser spot is nearly Gaussian (*M*<sup>2</sup> < 1.3). The polarization of the laser beam exiting the laser head is linear. Besides exposing the material to linear laser polarization, also a quarter wave plate was included in the setup to achieve circular polarization, which may lead to triangular shaped LIPSS textures. The beam was focused on the surface of the samples, using a telecentric F*θ* lens (Ronar of Linos GmbH, Göttingen, Germany) with a focal length of 80 mm, resulting in a focal spot with an *<sup>e</sup>*<sup>−</sup>2-diameter of *<sup>d</sup>* = 33.6 ± 1.6 <sup>μ</sup><sup>m</sup> (see Section 2.2.1).

The samples consists of polished CoCrMo discs with a diameter of 25 mm and a thickness of 3 mm. The surface roughness (*R*<sup>a</sup> = 0.003 ± 0.0003 μm, Rq = 0.004 ± 0.0004 μm) of the discs was measured with an atomic force microscope (NX10, Park Systems Corp., Suwon, South Korea). manufacturer's headquarters. The beam was scanned over the substrate using a galvoscanner (intelliSCAN14 of ScanLab GmbH, Puchheim, Germany) at normal incidence in air, see Figure 1b. Different shapes and sizes of LIPSS were produced by adjusting the type of polarization (linear or circular) and by adjusting the laser peak fluence (*F*0) and the number of overscans of the laser spot over the surface (*N*OS). The scan velocity of the laser spot (*v*), the laser pulse frequency (*f*F) and the spatial pitch between laser pulses on the surface (Δ*x*, Δ*y*) were kept constant in this study at *v* = 2 m/s, *f*<sup>F</sup> = 1000 Hz and Δ*x* = Δ*y* = 5 μm, respectively, see Figure 1b. This yields a geometrical pulse-to-pulse overlap (OL) in both, *x*- and *y*-direction, of *OL* = 1 − *v*/(*d* · *f*F) ≈ 0.85. All samples were cleaned in an ultrasonic bath with ethanol for 20 min and dried in ambient air after laser treatment. Table 1 shows the chemical composition of the samples.

**Table 1.** Cobalt–Chrome–Molybdenum (CoCrMo) alloy composition in weight percent, the composition is balanced (Bal.) with Cobalt.


**Figure 1.** Schematic representations of the laser setup and the scanning trajectory of the laser spot. (**a**) Schematic representation of the laser setup; *λ*/2: half-wave plate; BSC: beam splitter cube; BD: beam dump; *λ*/4: quarter-wave plate (optional). (**b**) Scanning trajectory of the laser spot; *f*F: pulse frequency; *v*: scan velocity; *d*: beam diameter; *OL*: geometrical pulse-to-pulse overlap; *N*OS: number of overscans; Δ*x*, Δ*y*: geometrical pitch between subsequent laser pulses in *x*- and *y*-direction.

#### *2.2. Analysis Tools*

#### 2.2.1. Laser Beam Characterization

The laser power was measured using an thermopile power sensor (PM30 of Coherent, Santa Clara, CA, USA) with a measurement uncertainty of ±1%, connected to a readout unit (FieldMaxII-TOP of Coherent, Santa Clara, CA, USA). The focal spot diameter 33.6 ± 1.6 <sup>μ</sup>m (*e*<sup>−</sup>2) was measured using a laser beam characterization device (MicroSpotMonitor of Primes GmbH, Pfungstadt, Germany).

## 2.2.2. Surface Morphology Dimensions

Laser-induced surface structures were analyzed using a scanning electron microscope (JSM-7200F, Jeol, Tokyo, Japan). From SEM micrographs, the periodicity of LIPSS areas were analyzed with the help of a 2D fast Fourier transform (FFT) algorithm using a MATLAB [19] script. Details of the script are reported in our earlier work [20].

The roughness of the surface textures was analyzed by means of an atomic force microscopy (NX10, Park Systems Corp., Suwon, South Korea) in true non-contact™ mode using a non-contact cantilever (PPP-NCHR, 125 × 30 × 4 μm, Tip < 10 nm, Park Systems Corp., Suwon, South Korea). The roughness parameters that were extracted from these measurements are used to characterize the surface by means of root mean square surface area roughness (*R*q), average surface area roughness (*R*a), maximum peak height (*R*p), maximum valley depth (*R*v), skewness (*R*sk), kurtosis (*R*ku) and the ratio between the real surface area and the projected area (*σ*).
