*3.2. Surface Structures Processed with Circular Polarization*

Triangular nanopillars (TNP), hexagonally packed, can be produced by exposing the surface to either circular polarized ultra-short laser pulses [13], or to double-pulsed (bursts of pulses), linear cross-polarized, ultra-short laser pulses [29,30]. These types of structures might be preferred over LSFL for the aimed application, since TNP are symmetric in three directions, whereas LSFL are symmetric in only one direction. Because hip joints rotate with respect to the *x*-, *y*- and *z*-axis, the tribological characteristics of the bearing should ideally be equal in any direction.

The physical phenomena behind the formation of triangular LIPSS are still under debate [13,29,30]. e.g., Fraggelakis et al. [30] proposes that the convection flow of the molten material layer as a cause for this type of LIPSS, whereas Liu et al. [29] claims the 2D nanotriangle structures develop due to the interference of surface plasmon polaritons (SPP's) with the incoming laser light. Since Liu et al. applied cross-polarized, time delayed double-pulses, these authors argue that the first pulse induces SPP's and the interference with the laser light leads to transient, spatially periodic meta-gratings of a modified refractive index on the surface with a wave vector parallel to the laser polarization. Further, they claim that the second cross-polarized pulse also induces SPP's at the surface due to surface roughness with a wave vector parallel to the laser polarization. The latter SPP then interferes

with the transient refractive index meta grating of the first pulse and could diffract into two SPP's with different wave vectors. The interference of the laser light with these three SPP's in different directions leads to ablation of a hexagonal pattern, resulting in triangular shaped nanostructures.

Figure 4 shows SEM micrographs of surface structures processed on CoCrMo with circular polarization with *N*OS = 1 and increasing peak fluence levels. It can be observed from Figure 4a, that LSFL with a periodicity of about <sup>Λ</sup>LSFL ≈ 860 nm form at a peak fluence level of *<sup>F</sup>*<sup>0</sup> = 2.87 J/cm2. This structure may be an indication that the polarization is not perfectly circular, but actually elliptically polarized with the main axis perpendicular to the processed LSFL. It can also be observed in Figure 4a, that "interruptions" of the LSFL features start to appear in the direction and the periodicity of the hexagonal shapes, see Figure 4a and the indicated frequencies on the 2D-FFT map of Figure 4a. Further it can be recognized when comparing Figure 4a,b, that these "interruptions" are indeed a surface morphology "proceeding" the formation of grooves in two different directions, which then form the triangular nanopillars if the fluence is increased. At a fluence level of *F*<sup>0</sup> = 5.23 J/cm2, regular TNP are formed, which become less regular and less pronounced for higher fluence levels, see Figure 4c. When comparing the laser processing conditions and groove periodicities between the hexagonal nanopillars with earlier studies (see Table 2), it becomes evident, that the hexagonal pattern processed either with single pulses of circular polarization or with cross polarized pulses with linear polarization origin from the same physical phenomena. Hence, the physical explanation of the origin for those patterns has to apply for each case of laser processing condition listed in Table 2. The physical explanation of hexagonal nanopillars exceeds the scope of this paper.

**Table 2.** Comparison of laser processing parameters and groove periodicity of hexagonal nanostructures with earlier studies.


High spatial frequency LIPSS (HSFL) were found between the formed LSFL in Figure 4a and the triangular nanopillars in Figure 4b,c, with a periodicity of ΛHSFL ≈ 80 nm. Liu et al. [29] processed triangular nanopillars with two consecutive, cross-polarized pulses with a pulse duration of 50 fs and with a time delay of 1.2 ps on tungsten in air and in vacuum. In the latter study, HSFL were not observed when processing tungsten in air, but have been observed when processing tungsten at low pressures of 10−<sup>3</sup> Pa. It was claimed, that the formation of HSFL is attributed to a slower cooling rate of the molten, liquid material layer at lower pressures. In the latter case, less air exists in the experimental environment, to transfer the heat from the molten layer to. Therefore, heat remains in the molten layer for a longer period of time and the cooling rate decreases. Thus, when the liquid material cools down, there is more time for shrinking and film fragmentation of the melt into HSFL then when processing in air. Compared to the latter study, the pulse duration of the laser used in this work, is in the order of two magnitudes larger. Hence, more heat is introduced into the lattice, which might explain the occurrence of HSFL between LSFL and triangular nanopillars when processed in air.

**Figure 4.** SEM micrographs of surface structures processed on CoCrMo using circular polarized laser radiation at *N*OS = 1 and various peak fluence levels at two different magnifications ((**a**–**c**) 3000×; and (**d**–**f**) 15,000×). (**g**–**i**) 2D-FFT maps if the micrographs of the processed areas (**a**–**c**). The periodicity of the LSFL in (**a**) is about ΛLSFL ≈ 860 nm. The periodicity of the nanopillars is constant in the same range ΛTNP ≈ 860 nm. The arrow in micrograph (**a**) indicates the direction of the E-field of the laser polarization.
