*2.1. Multi-Step Processes*

Generally speaking, almost every two texturing techniques can be combined to create multi-scale surface textures. Thereby, upon combining different texturing techniques, three factors have to be considered in particular: (i) the complexity of the individual processes and, hence, the overall effort for the texturing approach, (ii) the sequence of the texturing steps [54], and (iii) the respective size limitations of each method. Regarding tribologically effective surface textures, suitable texturing techniques are those which generate surface textures on significantly different scales. According to Hsu et al., wide and shallow textures improve the tribological behavior under hydrodynamic lubrication, whereas narrow and deep textures can reduce friction under mixed or boundary lubrication [55]. In their multi-scale approach, they used a triboindenter to fabricate the textures individually by nano-scratching. As shown in Figure 1, they manufactured a polished reference sample, single-scale textures, a mixture of different dimples as well as different overlapping dimples. Such mechanical texturing methods for which a specific pattern can be programmed and then machined (CNC machining) offer the advantage of very flexible texture arrangements. Nevertheless, such processes are rather slow, since every texture feature has to be created individually, which results in strong limitations regarding their industrial applicability [22].

Similarly, laser surface texturing can be used to create multi-scale textures by ablating predefined surface areas, whereby each feature is created individually. Therefore, the laser beam is directed to the desired surface areas either by moving the sample via a translation stage or by scanning the beam over the sample surface with a galvanometric scanner [42,56–58]. In between the spots where the textures are to be created, the laser beam can be blocked by a shutter system. The general texture shape and their morphology can be influenced by the laser parameters like laser fluence, wavelength, and pulse duration as well as the focusing system [59–63]. The technique for which the laser beam is merely focused and scanned over the sample surface is called direct laser writing (DLW). Using laser surface texturing and specifically DLW, Segu et al. created multi-scale textures on steel samples for tribological

purposes [64–66]. Examples of these textures are depicted in Figure 2. In this regard, it needs to be highlighted that again the advantage of this technique is the flexibility in terms of geometrical shapes, but it lacks speed, especially when creating very small texture features.

**Figure 1.** Multi-scale surface textures fabricated by nano-scratching using a triboindenter. In this work, single-scale, mixed, and overlapping textures have been produced. Adapted from [55].

**Figure 2.** Multi-scale surface textures created by laser surface texturing using a pulsed Q-switched Nd:YAG laser with a power of 24 W and a pulse duration of 200 ns. The smaller textures (squares, triangles) have a side length of 250 μm while the bigger circles have a diameter of 500 μm. Adapted from [66].

Additionally, chemical methods like etching processes can be used to create multi-scale textures [67,68]. Wang et al. used a combination of lithography and reactive ion etching to create multi-scale textures consisting of bigger circular dimples with a diameter of 350 μm and smaller square dimples having a side length of 40 μm on silicon carbide to improve its tribological properties. The advantage of these chemical methods is that the surface topography is modified while the chemical and mechanical properties of the surface remain fairly constant [42]. Furthermore, material removal can be efficiently controlled, the design of the surface textures is flexible, and irregular shapes and complex geometries can be textured. However, the method is constituted by rather complex procedures

like lithographic methods and deep textures are rather difficult to obtain due to electrolyte diffusion and ohmic polarization. Moreover, it is expensive to create high-resolution textures, and less versatile regarding possible materials than laser surface texturing [22,42].

Furthermore, different texturing methods can be combined to create multi-scale textures [47–49,54,69–74]. Resendiz et al. used end milling with a single crystal diamond cutter and micro shot-blasting to produce multi-scale surface textures on aluminum [47]. The bigger circular dimples fabricated by machining had a diameter of 150 μm and a depth of 30 μm. The smaller textures created by shot blasting with 10 μm aluminum oxide particles are completely covering the underlying primary machined textures and can be found inside the dimples and on the non-textured portions between the dimples. Their depth is not given but the roughness increases by 88% to 0.33 μm compared to the flat surface. With a similar method using laser surface texturing and micro shot-blasting with 25 μm alumina particles, Kim et al. manufactured multi-scale surfaces on sapphire wafers [73]. Thereby, the micro-patterns created by the laser process had a depth of 100–300 μm and a diameter of 40–160 μm. In contrast, the shot blasted textures are with a surface roughness (Rq) of 450 nm much smaller, even though their exact dimensions are not given. Gachot et al. and later Grützmacher et al. combined micro-coining with laser surface texturing, specifically direct laser interference patterning (DLIP), to create multi-scale surfaces [48,49,54,70,72]. DLIP is a technique, which uses overlapping laser beams to modulate the laser intensity spatially by interference. By applying this technique, multiple texture features can be created on the surface in a single laser shot [62]. Micro-coining is a fast process, which allows for the generation of high-quality textures in the range of 20–200 μm at low cost, while DLIP is a fast and versatile technique especially suited to create smaller textures with feature sizes between 200 nm and 30 μm [54]. It is shown that the process sequence is important for this combination. Thereby, micro-coining should precede the laser process because the inverted process sequence may lead to the partial destruction of the laser textures especially in highly deformed areas, such as the flanks of the micro-coined textures [54]. If performed correctly, however, this approach can lead to pronounced multi-scale textures with a homogenous distribution of both texture types over the surface, as can be seen in Figure 3.

**Figure 3.** Multi-scale surface textures created by a combination of micro-coining and direct laser interference patterning. The bigger micro-coined textures have a diameter of 180 μm and a depth of 43 μm, whereas the cross-like laser pattern has a width of 6 μm and a depth of 0.6 μm. Adapted from [70].

Additionally, multi-scale textures can be manufactured by replicating natural surfaces inhibiting a multi-scale surface topography like the lotus leaf [44–46,75]. Shafiei and Alpas mimicked the natural surface texture of lotus leaf and boa's skin using a cellulose acetate film. In a subsequent step, a nanocrystalline nickel layer is deposited onto this film to obtain the positive surface texture of the natural sample [45,46]. To add surface textures on another scale to the lotus leaf replicas, an additional electrodeposition step was used, which led to the deposition of spherical nickel droplets on the tips of the micro-textures as shown in Figure 4 [46].

**Figure 4.** Multi-scale surface textures on nickel created by replicating the natural surface of the lotus leaf and a subsequent electrodeposition step. Adapted from [46].

Finally, an innovative method to create multi-scale surface textures is to combine two different laser processes, namely DLW and DLIP [76,77]. Thereby, the bigger surface textures are created by DLW and the smaller textures by DLIP. This eliminates the disadvantage of low productivity for DLW when creating small textures and offers the advantage of creating both texture types with the same tool even though a two-step fabrication process is used. Thinking about the industrial application of such textures, this means that only one tool needs to be integrated into the production process leading to smaller costs and higher production speeds. Cardoso et al. used this technique to create multi-scale surface textures. They used a solid state laser having a pulse duration of 30 ns for the DLW process fabricating cross-like textures having a periodicity (distance from channel to channel) of 15–35 μm, a depth of 3–5 μm and a width of 15 μm. Subsequently, the smaller textures were fabricated with a DLIP system on top of the bigger DLW textures using an Nd:YVO4 laser with a pulse duration of 10 ps. The smaller textures showed a periodicity of 2.6 μm and a depth of roughly 1 μm [76]. Another laser process, which has been proposed to create multi-scale textures is the combination of DLW with the controlled generation of laser induced periodic surface structures (LIPSSs) [78]. Zhang et al. used this approach to fabricate multi-scale textures. The primary textures fabricated by DLW have a width of 40 μm and a depth of 50 μm, whereas the LIPSSs on top of these textures have a depth of merely 150 nm and a period of 550 nm [78].

It is worth mentioning that several other techniques like self-assembly of nanostructures by thermal evaporation [17,79], nano-imprinting [80] or special etching techniques [67] can be utilized. However, these techniques are quite complicated and not well suited for industrial applications.
