*2.2. Single-Step Processes*

Single-step processes to create multi-scale patterns are scarcely spread. However, some can be found in the literature [59,81–83]. However, often these multi-scale patterns occur rather randomly. Qiu et al. used an electrochemical growth approach to fabricate multi-scale cobalt textures [82]. In their work, they demonstrated cobalt crystals with several levels of hierarchy having a flower-like morphology.

However, a single-step method to fabricate multi-scale samples in a controlled fashion is again the laser process. By making use of the formation of LIPSSs during laser processing with suitable laser parameters, bigger laser textures can be created by DLW, while simultaneously smaller LIPPS are formed [81]. Ahmmed and Kietzig used a femtosecond Ti:sapphire laser system with a wavelength of 800 nm and a pulse duration <85 fs to fabricate multi-scale textures on copper [81]. The laser inscribed primary pattern demonstrated a width of 14–80 μm and a depth of 60–130 μm whereas the LIPSSs were on a scale of several hundred nanometers. Thereby, the primary textures can be controlled by the laser parameters (i.e., laser power, pulse duration, scanning speed, etc.), whereas the secondary LIPSSs textures can be controlled by the laser wavelength, the incidence angle, and the polarization of the laser beam [57].

### **3. E**ff**ect of Multi-Scale Textures on Friction and Wear—Experimental Studies**

A significant amount of research work has been conducted to replicate the multi-scale surface topographies of the Lotus leaf, the Rice leaf, shark skin, snakeskin, among others and test these surface with regard to their potential to optimize friction and wear under dry and lubricated conditions. Shafiei and Alpas fabricated bio-inspired multi-scale surface textures by mimicking biological surfaces such as the Lotus leaf and the snakeskin using replica film followed by the electrodeposition nanocrystalline nickel. For the replicated, multi-scale Lotus leaf sample, a 30% reduction of the maximum COF under dry sliding was observed compared to a flat surface. The authors attributed the observed effect to a reduction of the real contact area [45]. Following the same approach, Shafiei and Alpas fabricated multi-scale surfaces by replicating the Lotus leaf and combining it with a chemical surface treatment (PFPE) to achieve superhydrophobicity and low friction. Using this combined treatment, a maximum friction reduction of 60% under dry friction was shown, which was explained by the reduction of the real area of contact [46]. Similarly, Wang et al. fabricated bio-inspired superhydrophobic surface textures in nickel (Lotus- and Rice-leaf) by combining a replicating technique with nickel electroplating. Though this approach, multi-scale surface textures with protruding and depressing textures were realized. The final fabrication step was a chemical modification of the resulting surface textures, applying PFPE as a lubricant. Afterwards, they studied the tribological behavior of the fabricated surface textures with and without final PFPE surface treatment. The best performance with significantly reduced COFs over the entire sliding time was found for the multi-scale surface textures with subsequent PFPE treatment irrespective of the type of textures (protruding or depressing). Additionally, they verified that the wear resistance was improved when using a final chemical PFPE treatment. The observed results were traced back to the good lubrication abilities of PFPE films as well as the possibility to reduce the real area of contact and to trap wear particles in the surface textures [44]. Using a combination of nanocasting, electroplating, and physical vapor deposition, Wang et al. fabricated diamond-like carbon films with Lotus leaf-like textures thus aiming at generating hard and flexible coatings with superhydrophobicity and good tribological properties. The main drawback of this innovative idea is the number of steps necessary to fabricate these surfaces. The fabricated textures showed a pronounced reduction in the COF over time with PFPE lubrication (without any further oil), which was traced back to the possibility to store wear particles [84]. However, it remains questionable if this is really the only contributing aspect leading to the improved friction and wear performance. The research group of El Mansori has put considerable attention to the analysis and imitation of python skin [43,85,86]. In their tribological investigations under dry conditions, they impressively demonstrated that pythons naturally have tribologically optimized surface features, which would be worth to mimic by laser surface texturing. The main conclusion of these works is that reptile skin typically follows an aperiodic and asymmetric pattern, which is in contrast to the deterministic idea of surface texturing such as arranging dimples or other shapes in a regular matrix. It can be figured that by copying more ideas from nature, improved texture designs with superior friction and wear performance can be achieved [43,87]. The research group led by Greiner conducted research towards a similar direction thus copying the surface texture of snakes and lizards to optimize

the friction response under dry and lubricated conditions. They verified that these bio-inspired surface textures reduce the COF by about 40% under dry conditions, while a 3-fold friction reduction was observed in lubricated systems [88,89]. Moreover, Greiner et al. observed a pronounced size effect when fabricating and testing multi-scale surface textures with variable diameter. Surface textures with the biggest diameter showed the lowest frictional results. The explanation of the obtained results is not straight-forward and requests more in-depth analysis of the underlying phenomena, which will be subject of ongoing research work [89].

Wang et al. studied the tribological performance of textured SiC contacts under water lubrication. Initially, they verified a 2.5-fold increase in the critical load (for the transition from full-film to mixed lubrication) for single-scale textures with the lowest area density, a low depth, and an intermediate diameter [90]. In a follow-up paper, they used reactive ion etching to fabricate multi-scale textures having small and large dimples to optimize the texture effect of SiC–SiC pairings by increasing both the hydrodynamic pressure (big dimples) and the lubricant supply (small dimples). The multi-scale texture showed the best tribological performance under water lubrication with a 3.3-fold increase in the resulting load carrying capacity (Figure 5). The authors attributed the positive effects induced by surface textures to an additional hydrodynamic pressure and lubricant reservoir effect. Additionally, surface textures helped to improve the running-in process, thus leading to a smoother surface with lower roughness values. In the case of the multi-scale surface, it was speculated that the finer textures improved the water supply, thus offering more water in the tribological contact zone, which is beneficial to induce tribochemical reactions between water and SiC [91]. A summary of the research conducted by Wang et al. on textured SiC surfaces can be found in [92].

**Figure 5.** Coefficient of kinetic friction (COF) versus load curves in order to determine the critical load for an untextured reference as well as single-scale (large and small dimples) and multi-scale surface textures (mixed). Adapted from [91].

Segu et al. studied the effect of multi-shape textures combining circular, elliptical and triangular textures using a pin-on-disk set-up. The texture combination was fabricated by laser surface texturing using a nanosecond ND-YAG laser with a pulse duration of 200 ns. For all combinations, the structural depth was kept constant at 6.5 microns. Moreover, two area densities, namely 12 and 20%, were selected. The authors recorded Stribeck-like curves and compared the frictional behavior of the multi-shape surfaces with two reference surfaces (grinded and polished). All texture combinations showed beneficial frictional properties with a faster transition (i.e., at lower sliding speed) to low frictional values indicating hydrodynamic lubrication. In addition, they also proved the possibility to positively affect the friction and wear performance under dry sliding conditions using multi-shape textures. The textures showed a reduced averaged COF, which was traced back to the possibility to store

wear debris in the textures thus removing it from the tribological contact. Furthermore, they also demonstrated beneficial frictional properties under lubricated conditions for longer sliding times. The beneficial effects of these multi-shape surfaces under lubricated conditions were explained by the increased possibility to build-up additional hydrodynamic pressure [66]. The contribution presented by Segu et al. gives interesting insights into the frictional behavior of multi-shape surfaces under dry and lubricated conditions but lacks on the presentation of carefully selected reference measurements. The data of the grinded and polished references are not sufficient to really justify the frictional efficiency of the multi-shape surfaces since the tribological results of single-shape textures (purely circular, elliptical, and triangular) have not been presented for comparison. Following the promising results of the combination of circular and elliptical multi-shape textures, Segu et al. fabricated multi-shape textures consisting of circular and elliptical textures having a depth between 3.5 and 7.5 microns as well as a density between 5 and 20%. With regard to the structural depth, they verified the best frictional behavior with the lowest COF for an intermediate depth of 5.5 microns. They explained this observation with the interplay between the oil film thickness, the structural depth of the textures and the potential pressure build-up. Although demonstrating interesting results, this study again lacks the presentation of suitable reference data as well as a deep elucidation of the obtained results [65].

The research group of Hsu conducted also important research related to the effects and mechanisms of surface textures in tribological contacts. An initial study aimed at investigating different texture geometries under low load and high-speed conditions as well as under high load and low speed conditions. Related to the first conditions, elliptical textures oriented perpendicular to the sliding direction led to the best results with the greatest friction reduction under boundary and mixed lubrication. Different contributions such as reserve lubricant flow, cavitation, the storage of lubricant inside the textures as well as a squeeze effect must be taken into consideration and may act simultaneously. Under high load and low speed conditions, all surface textures induced detrimental effects with increased friction, which has been mainly traced back to undesired edge effects [93]. Following this systematic study on single-scale surface textures with the main conclusions that large/shallow textures reduce friction under full-film lubrication and small/deep textures are effective under mixed and boundary lubrication, Hsu et al. extended their texture design to multi-scale surface textures. The general idea was to combine small but deep textures with large but shallow textures to be efficient under different lubricated conditions. Hence, Hsu et al. created a mixture of textures on different scales but also followed design rules found in nature, which favor an overlapping of textures on different scales, as shown in Figure 1 in Section 2. For experiments performed under a low contact pressure, all textured samples irrespective of single- or multi-scale were efficient to reduce friction. Particularly, the multi-scale textures with overlapping features showed a maximum friction reduction of up to 80% (Figure 6). Even under higher contact pressure, this multi-scale texture reduced friction by 70%, which is a significant advance in the design of surface textures for high contact pressure applications. Moreover, the multi-scale samples showed negligible wear features, which was traced to the transition from mixed to full-film lubrication even under higher contact pressures. Generally speaking, this study impressively demonstrated that by combining surface textures on different scales (each one optimized for a different lubrication regime) can bring an overall improvement of the frictional performance with a significant friction reduction across all lubrication regimes [55].

**Figure 6.** COF versus sliding speed for different reference cases (polished and single-scale textures) and different designs for multi-scale surface textures (mixture and overlapping textures) depending on the acting contact pressure (**a**) 157 MPa, (**b**) 314 MPa and (**c**) 470 MPa. The black baseline represents the polished reference surface, whereas baseline 2 and 3 stand for the single-scale surface textures. Adapted from [55].

Inspired by a significant friction reduction induced by micro-coined dimples as verified by [94], Grützmacher et al. overlapped micro-coined surfaces with finer textures fabricated by direct laser interference patterning (DLIP). For their study, they selected hemispherical micro-coined surfaces having a depth of 50 and 95 microns. Single-scale micro-coined samples with a depth of 50 microns showed beneficial frictional results, while deeper micro-coined samples demonstrated detrimental results regarding friction and wear. With this selection of the micro-coined geometries, Grützmacher et al. aimed at addressing the effect of additional finer cross-like laser textures thus answering the question whether this kind of texture pattern may improve the frictional behavior of single-scale samples and either compensate the negative effects of deep dimples or further improve beneficial samples. Interestingly, the overlapped laser textures downgraded the frictional behavior of the initially beneficial micro-coined sample with a depth of 50 microns. In contrast, the additional laser textures helped to significantly improve the frictional behavior of the sample with a depth of 95 microns. Another interesting aspect has been realized during the analysis of the resulting wear scars. For both multi-scale samples, the wear scars show a rather irregular behavior with deflections from its original, circular trajectory. For this to happen, the tribological counter-body (ball) needs to interact with the underlying surface textures, which reflects the potential pressure build-up induced by the textures. Grützmacher et al. interpreted the obtained results in the following way. The improved friction behavior of the multi-scale texture was traced back to potentially reduced cavitation. For deep structures, it is well known that cavitation is more likely to occur. Combining deeper, coarser textures with fine cross-like textures may, therefore, help to reduce cavitation thus improving the distribution of lubricant in the contact area. This may induce a larger oil film thickness, thus increasing the resulting load-bearing capacity and reducing friction [48]. Following this approach, Grützmacher et al. investigated the effect of single- and multi-scale surface textures applied on the shaft of journal bearings by recording Stribeck-like curves. In order to manufacture these textures, DLIP and roller-coining have been utilized. Though DLIP, finer cross-like textures with a periodicity of 6 microns and a depth of about 1 micron have been realized, while roller-coining aimed at fabricating coarser textures with depths of up to 45 microns. Compared to the polished reference shaft, all textured single- and multi-scale surfaces led to a significant improvement of the frictional performance. As can be seen in Figure 7, under mixed lubrication, a reduction of friction by a factor of about 2–3 was observed, whereas, under hydrodynamic lubrication, a 4.6 fold decrease of the resulting COF was observed for the multi-scale texture combing the finer cross-like laser textures with the deeper micro-coined textures. The improved friction behavior of the aforementioned multi-scale texture was traced back to potentially reduced cavitation. Combining deeper, coarser textures with fine cross-like textures may help to reduce cavitation thus increasing the load bearing capacity as well as reducing the COF under hydrodynamic lubrication [72]. A comprehensive overview of the research efforts of Grützmacher et al. related to the effect of multi-scale surface textures in tribological contacts can be found in [71].

**Figure 7.** COF versus rotational speed for an unpolished reference, a purely laser-textured sample (laser), a solely micro-coined sample (A2) and the respective multi-scale samples combining direct laser interference patterning (DLIP) and micro-coining. Adapted from [72].

Inspired by the positive effects of multi-scale textures related to the running-in behavior and the transition between mixed and full-film hydrodynamic lubrication, Rosenkranz et al. studied the friction and wear performance of these textures under mixed lubrication. Following the beneficial effects observed for single-scale textures fabricated by DLIP [34] and micro-coining [94], they combined cross-like DLIP textures (depth about 0.6 microns and periodicity about 6 microns) with hemispherical micro-coined textures inhibiting two different depths and periodicities. Using an additive-free PAO oil, Rosenkranz et al. investigated the temporal evolution of the COF over time with a special emphasis on the time when the COF suddenly increases, which has been defined as the maximum oil film lifetime. The largest effect in terms of extending the oil film lifetime has been found for the sample with the deepest and widest micro-coined textures combined with the cross-like laser texture (Figure 8). The obtained results were attributed to an improvement of the lubricant's distribution and the additional pressure build-up due to the finer laser texture, while the coarser micro-coined texture tended to store produced wear particles thus removing them from the tribological contact zone [70].

**Figure 8.** COF versus laps in order to determine the maximum oil film lifetime of the polished reference, the cross-like laser textures (LAS), the micro-coined surfaces (A series) and both corresponding multi-scale textures (A1L and A2L). Adapted from [70].

As already outlined in Section 2, Resendiz et al. have combined inclined end milling and micro shot blasting to create multi-scale surface textures in aluminum samples. Using end milling, circular-shaped dimples with a diameter of about 75 microns and a depth of 30 microns were created, while shot blasting with aluminum oxide particles (diameter of 10 microns) superimposed a finer roughness of the coarser milled textures. Using experimental and numerical approaches, the authors tried to evaluate the respective effect of each texturing method as well as the combination of both under lubricated conditions. In Stribeck-like curves, surface textures fabricated by the combination of end milling and shot blasting showed the best frictional behavior with a significant friction reduction compared to the untreated reference surface (Figure 9). The observed experimental findings were addressed by simulations thus verifying a cavitation effect inducing an additional pressure build-up around the textured surfaces. For the textures fabricated by a combination of both techniques, the greatest film thickness was found. Additionally, they proved that the tribological performance was notably improved by the storage of wear debris, which underlines that two effects are responsible for the superior friction and wear behavior of the multi-scale surface textures [47].

**Figure 9.** COF versus lubrication parameter for a flat surface, a shot-blasted sample, a dimpled sample, and a shot-blasted dimpled (i.e., multi-scale) surface. Adapted from [47].

Zhang et al. used pulsed nano-second and femto-second lasers to fabricate micro- and nano-scale surface textures as well as a combination of both ending up in a multi-scale texture. After having fabricated the respective textures, all surfaces were covered by a TiAlN coating. While the general idea is promising, the tribological characterization of the fabricated samples is not sufficient to draw any significant conclusion. Additionally, the combination of texturing and coatings is further complicated by the use of MoS2 as a solid lubricant. As already outlined in Section 2, the combination of lasers having a different pulse duration is promising in the context of creating multi-scale textures, but simpler tribological experiments need to be conducted in order to explore the full potential of this approach [78].
