Ultra-short pulsed laser ablation of stainless steel is accompanied by the evolution of di erent mi- crostructures. Depending on the uence, accumulated energy and number of laser passes cones from impuri- ties, laser induced periodic surface structures and cone- like protrusion (CLP) evolve at the surface. These often unwanted morphologies can be inhibited by carefully choosing the strategy and laser parameters. Here, the identi ed region shows a small processing window for de ned 515 nm sub 1 ps ablation leading to low surface roughness. CLP are still not well understood and here a pre-cursor structure is reported. Subsequently, the CLP growth is grain orientation and polarization dependent and studied in more depth. Preferentially, CLP start to evolve at the (101) grain orientations with linear polarized laser radiation. Moreover, a nanoindentation study reveals robust mechanical properties, which could be bene cial for tribology applications in the dynamic regime. Keywords Laser machining Self-assembled struc- tures Cone-like protrusions Laser induced periodic surface structures Ultra-short laser pulses Orthogo- nal processing Nanoindentation Tribology 1 Introduction In recent years ultra-short pulsed laser machining is be- coming a viable technology for industrial application. The equipment costs have been constantly decreasing and several bene ts are leading to a strong increase in N. Ackerl G. Fisch K. Wegener Institute of Machine Tools and Manufacturing (IWF), ETH Zurich, Leonhardstrasse 21, 8092 Zurich, Switzerland J. Auerswald V-ZUG AG, Industriestrasse 66, 6302 Zug, Switzerland.usage. Having the option to precisely distribute pulses of energy into the material gives a vast number of appli- cation. Additionally, the high power density allows to ablate and modify otherwise hard-to-machine materials like ceramics and diamond as well as synthetic or natu- ral composite materials such as carbon ber re-inforced polymers or mother-of-pearl. Moreover, laser machining can bridge the gap between conventional manufacturing techniques like drilling, milling, electric-discharge ma- chining and semi-conductor technology with the power to span from sub micrometer precision over several 100s of millimeter. Laser micromaching of stainless steel is of high in- terest for such applications as e.g. watch dials, hands, appliques, housings, wrist bands or mechanical parts in watch industry, functionalized surface structures for large scale automotive applications, and corrosion re- sistant markings in the medical industry. However, dif- ferent microstructures like cones, laser induced peri- odic surface structures (LIPSS) and cone-like protru- sion (CLP) arise depending on the laser parameters [1]. In literature the origins of cones have been identi ed as slag inclusions from the steel production process, which are primarily Al 2O3and MgO [2]. Depending on the steel quality the number of cones near the threshold uence strongly increases. In parallel LIPSS of low spa- tial frequency are generated, if linear polarized light is used [3]. Increasing the uence leads subsequently to the growth of thermal LIPSS, which are perpendicu- lar to the former ones, originating from heat accumula- tion [4]. Hitherto, there are di erent regimes of ablation where heat accumulation and local power density is the driving force limiting a high-speed ablation process [5]. However, the CLP structure is not observed after single- pass laser ablation and multiple layers of ablation are necessary [6]. A dependence of local uence and surface 2 N. Ackerl et al. speed of the laser leading to precursor structures on the specimen is reported within this contribution. De ned ablation conditions are a prerequisite for low roughness and ecient laser ablation [7]. In prin- ciple the uence, laser repetition rate, and scan speed are interdependent [8] and no global optimum can be reached. However, depending on the desired process ei- ther high material removal rate or surface quality can be reached for 2.5 Dorthogonal strategies. This leads to limiting constraints, where high average power must be distributed on the workpiece by high scan-speeds and repetition rates [9]. To date there is plenty of laser power with high repetition rates available, but depend- ing on the necessary focal spot size the beam steer- ing [10] or controllers [11] are constraining the maxi- mal laser machining speed. Using the laser in a burst mode with small temporal pulse spacing can enhance the maximal applicable average power [12] and abla- tion rate [13{15]. However, the use of sub-nanosecond spaced ultra-short pulses is still a matter of controversy, where the enhanced eciency could not be observed [16]. Additionally, the laser energy can be distributed on rotational workpieces by a spindle axis, where one more degree of freedom in terms of processing strat- egy can be utilized [17,18]. Quasi-tangential laser ma- chining has a huge potential and could utilize the high power nowadays available for ablation , but with in- creased complexity in the laser path and hatch calcula- tion [19]. Microstructures are not necessarily an unwanted side e ect, but can be used to alter the physical interplay at the surface of di erent materials. Well-known exam- ples are the change in wettability [20,21], optical [22] and tribological properties [23{25]. The idea proposed in this manuscript is to utilize CLP structures for tri- bology application with high relative surface speed to reach the hydrodynamic lubrication regime. A nanoin- dentation study on cross-sections of CLP reveals rea- sonable mechanical properties, with 1:1 aspect ratio in the regime of several micrometers, potentially applica- ble to lower the coecient of friction. 2 Experimental techniques 2.1 Material All studies were carried out on austenitic stainless steel samples of di erent quality listed in table 1. The com- position is allowed to vary in the given range, where every batch can be slightly di erent. Steel 1.4310 ex- hibits a higher carbon content than the widest used austenitic stainless steel 1.4301, resulting in higher me- chanical strength desired in certain applications butalso in slightly lower corrosion resistance. 1.4404 con- tains more molybdenum and nickel making it more re- sistant against acids and saline chloride solutions, e.g. human sweat. For the parameter variation a standard Cr/Ni stainless steel 1.4301 with 50 mm2and 2 mm thickness served as substrate. The ablation characteris- tic was studied on the as received state without further sample preparation. Table 1 Di erent types of stainless steel inspected, where the percentage in weight of the adjacent species is given and iron lls the gap to 100 %. classi cation 1.4310 1.4301 1.4404 AISI301 AISI304 AISI316L carbon 0.05 - 0.15 0:07 0:03 chromium 16.0 - 19.0 17.0 - 19.5 16.5 - 18.5 nickel 6.0 - 9.5 8.0 - 10.5 10.0 - 13.0 molybdenum 0.0 - 0.8 0 2.0 - 3.0 silicon 2 1 1 manganese 2 2 2 grain size 2:5 µm 8 µm 35 µm The small grain size of these materials makes the study of grain dependencies on laser ablation challeng- ing. Therefore, an additive manufactured 1.4404 austenitic steel cube produced by selective laser melting (SLM) has been used [26]. Due to the powder bed fusion tech- nique with the well controlled energy and temperature gradient larger or similar oriented small grains larger than 30 µm are formed. These grains are elongated in build-up direction, which allows studying the evolution of CLP on the top and side ank of the cube. Here, the specimen was grinded to grit 4000 with SiC sandpa- per and following polished with Al 2O3suspension with 50 nm to a mirror-like surface. 2.2 Laser machine testbed Figure 1a shows the table-top experimental setup con- stituted by a free path beam delivery and a combination of optical and mechanical axes. An Amphos 200 laser with MOPA con guration, YB:YAG Innoslab ampli - cation, and a second harmonic unit serves as source. The fundamental 1030 nm have a pulse duration of about 1 ps, which leads after frequency doubling and the non- linearity to pulses of 0 :8 ps at 515 nm. The used Am- phos laser enables a wide parameter variation in terms of average power up to 120 W with green light, repe- tition rate from single shot up to 40 MHz, however in this study we kept the repetition rate at800 kHz . More- over, the compressor grating after the Innoslab allows to vary the pulse duration up to 10 ps. The laser ra- diation is coupled into a galvo scanner, where the raw Evolution of Microstructures on Stainless Steel induced by Ultra-Short Laser Ablation 3 Fig. 1 Laser test bed with mechanical (XYZ) and Galvo (UV) axes for de ned positioning (a). Pockets with altered polarization, feed rate and power (b) enables the assessment ablation depth, taper angle, and evolved structures. beam size is controlled by a Galilean telescope and po- larization set by zero-order wave plates. A suction unit with a directional air ow of 250 m3h1removes part of the the ablation debris. 2.3 Laser ablation strategy For the extended parameter study pockets are ablated with a 2.5 Dstrategy and rotating 15 adjacent layers by 23 deg per pocket. The geometry is 2 mm2with a cen- tered unablated region of 1 mm height 100 µm width to analyze the total ablation, taper ank angle, and edge precision of the laser process. Process parameters varied are the average power from 0 :2 W to 2 :8 W, feed rate between 100 mm s1to 1:6 m s1, and the hatch line distance between 1 µm and 5 µm. This leads to a total of 224 parameter elds per hatching distance, where the parameters are chosen to cover the evolution of cones, LIPSS, thermal ripples, and partial to full coverage of the pocket with CLP structures. Moreover, the depen- dence on the microstructural evolution on polarization state is attained by using half- and quarter-wave plate to adjust and switch the polarization from linear to cir- cular polarization in the focal plane. 2.4 Analysis methods The pockets are analyzed per set of parameters with op- tical microscopy using two Keyence devices. By the fo- cus shifting technique of the Keyence VH-X6000 the mi- crostructure is analyzed, depicted in gure 1b. Smaller structures are qualitatively assessed by scanning elec- tron microscopy (SEM) using a Hitachi SU-70. The electron back scattered di raction (EBSD) study was carried out on a FEI Quanta device. Confocal microscopy using a Keyence VK-X1000 device with blue laser al- lows to determine the ablation depth in an automated manner. To assess the modi ed region by the laser process, cross-sections are created by cutting, grinding and pol-ishing. Beside an optical inspection of the interface re- gion, nanoindentation reveals the mechanical proper- ties. A Nanomechanics iNano with the continuous dy- namic oscillation (CSM) method allows to determine the surface near mechanical properties. This technique oscillates the indenter with a sinusoidal driving force, here 110 Hz, and measures force plus displacement with high resolution [27]. The force and indentation depth was calibrated to measure bulk properties and not just the surface near oxide layer [28]. This allowed a indenta- tion distance of 2 µm, which is a distance of three times the indentation depth to avoid impact of neighboring indents. The CSM measurement allows to calculate the materials hardness and elastic modulus for each indent, revealing the spatial distribution of mechanical proper- ties. 3 Results and discussion 3.1 Microstructural evolution Upon the ablation of steel distinct structures are ob- served and gure 2 gives an overview of three kinds. Cones, shown in gure 2a, appear after ablating sev- eral layers near the threshold uence and origin from MgO and Al 2O3inclusions from the steel production. Due to the higher threshold uence compared to met- als the oxides are ablated less forming a cone structure. In parallel LIPSS with low and high spatial frequency appear in case of linear polarized laser light. Increas- ing the energy density leads to the formation of ther- mal ripples, which could be superimposed to the LIPSS depicted in gure 2b. These structural entities can be even formed in a single pass laser process with slow scanning speed and small hatching distance. However, at even higher uence cone-like protrusions emerge af- ter several layers of ablation, growing with each layer on precursor ripples observable in gure 2c. The mi- Fig. 2 Inclusions with higher threshold uence lead to cones (a). Near the threshold LIPSS are observed (b) and at higher uence and after multi-layer ablation CLP structure evolve, preferentially at precursor structures (c). crograph in 2c shows the CLP structure in black and grayish ripples on distinct grains of the SLM additively 4 N. Ackerl et al. manufactured stainless steel sample. Here, the sample was etched for 2 min with V2A to reveal the grains and observe preferential growth on certain orientations. The graph is after ablating 10 layers at 2 J cm2with a scan speed of 500 mm s1. Interestingly, this ripple structure serves as precursor to the CLP growth and it is assumed that the re ectivity and, therefore, en- ergy coupling to the material is altered. This leads to higher absorbance, which melts a small surface near re- gion even by using ultra-short pulses of less than 1 ps. With each subsequent pass of the laser both structures grow and at after a certain amount of layers the sur- face is covered with CLP. Figure 3 reveals the depen- dence of the microstructural evolution on uence and scan speed after ablating 15 layers with the introduced 2.5Dstrategy. The contour plot points to a saturation /s76/s70/s83/s76 /s84/s104/s101/s114/s109/s97/s108 /s67/s76/s80/s32/s112/s97/s114 /s67/s76/s80/s32/s102/s117/s108/s108 /s66/s117/s109/s112/s115 Fig. 3 Layer depth and microstructures observed in depen- dence of uence and feed rate after removing 15 layers with linear polarization. A transition from bumps, cones, LIPSS, thermal ripples, and CLP on laser process parameters is re- vealed. in terms of material removal rate and layer depth Zlayer in good agreement with proposed models in literature [7,9]. At low feed rates starting from moderate uence of 0:3 J cm2a bumpy structure due to heat impact is formed, where even semi-spherical structures can be ob- served. The structures evolve in certain regimes of feed rate, hatch distance and uence, where gure 3 shows the mapping over a wide eld. If the hatching distance is varied the whole dependency is shifted with compa- rable tendency. If a certain microstructure is desired, a set of parameter can be chosen from this plot or a cer- tain structure prevented. The LIPSS structure can be inhibited by the use of circular polarized light, which additionally has a strong impact on the precursor ripple formation and following CLP coverage of the surface.3.2 Impact of grain orientation and polarization To further assess the growth dependencies of CLP struc- tures EBSD mapping was carried out on the SLM sam- ple prior laser ablation. Subsequent stop-motion optical imaging after each ablated layer with 1 m s1and 2 W reveals the temporal evolution of the microstructures. Figure 4a shows the surface after ablating 17 layers. Clearly, on the (101) orientated grains shown in g- ure 4b the surface morphology di ers observed in a changed re ectivity in the micrograph. These are the ripple precursor structures shown in gure 2c and with each subsequent layer CLP start to grow randomly on exactly these regions. In total 80 layers have been subse- quently removed and the nal surface is superimposed with the EBSD map in gure 4c. The CLP structure Fig. 4 Optical micro-graph after 17 layer ablated with cir- cular polarization (a) and EBSD from the pristine polished marked surface (b). In (c) an overlay of EBSD measurement after 80 layers show the preferential evolution of CLP at (101) grains, e.g. marked by the red circle. evolves preferential on the (101) directions, however some structures are observed at di erent orientation. This is dedicated to underlying grains with the pre- ferred orientation after removing 80 layers and approx- imately 150 µm of material. The preferred growth at the (101) grains presumably is connected to the packing of the crystal and changes the coupling with the incident Evolution of Microstructures on Stainless Steel induced by Ultra-Short Laser Ablation 5 electromagnetic wave. A slight change in the thermal conductivity in di erent orientation of the grains could lead to an accumulation of heat and following thermal ripple formation. However, if this is true the growth and appearance should depend on the grain size, which was not observed within this study. A strong dependence on polarization direction is observed after ablating 160 layer at keeping the parameters unchanged, except the polarization state. Figure 5 clearly reveals a strong de- pendence, where the surface is almost completely cov- ered for linear polarization in contrast to circular one. This strong dependence on polarization direction ten- Fig. 5 Polarization dependency using circular (a) and linear (b) polarization on CLP evolution after 160 layer at 2 :5 m s1 at 3 J cm2. tatively is correlated with the LIPSS formation, which enhances the energy coupling and following thermal rip- ple creation. A preferred generation of LIPSS starting at defects and discontinuities a ecting the surface plas- mon polariton, e.g. grain boundaries, could explain the faster growth rate of precursor and CLP. Moreover, even at circular polarization a grain boundary could lead to LIPSS generation matching the defect and po- larization direction at a certain point. This is more un- likely and therefore the evolution of LIPSS, ripples and subsequent CLP lessened. However, this still has to be proven by stop-motion imaging in a SEM to resolve the evolution of the smaller structures. 3.3 Structure and mechanical properties of CLP A cross-sectional view on the CLP covered surface in gure 6a shows the evolved protrusions. The structure peaks and valleys are higher and deeper than the sam- ples surface, tentatively formed by surface-near melting and fast resolidi cation forming di erent sized CLP de- pending on the exact dynamics, compare gure 6b. A dependence on scanning speed, local uence and to- tal accumulated energy from literature [1] corroborates this interpretation. The grain size of the CLP is still under investigations, where edge charging inhibited a Fig. 6 Cross-section of the CLP structure revealing the size and shape (a). A magni ed micrograph shows the CLP width and height in more detail (b). straight forward measurement. However, the pseudo- periodic structures appear homogeneously over the whole surface. This could be taken as an approach for gen- erating microstructures for application in e.g. changing the wettability and/or tribology. For exploitation of the potential the mechanical stability plays a role in en- durance of the functionality on the surface. Therefore, a nanoindentation study was carried out with an indenta- tion depth of about 100 nm and a distance of 2 µm over a 10 000 µm2. Using the CSM method the sti ness of the sample is measured, which can be used to calculate hardness and E-modulus. Nanoindentation revealed a small a ected zone at the interface with reduced sti - ness, see gure 7, where as well free standing cone- like structures could interfere with the measurement. However, the calculation of the bulk regime points to a Fig. 7 Nanoindentation with the CSM method allows to measure the mechanical properties, indicating a small change in the surface near region to the embedding material to the bulk. 6 N. Ackerl et al. Young modulus of 160 GPa in good agreement with lit- erature and at the interface to the embedding material to 120 GPa, which corresponds to the green region in gure 7. The overall mechanical properties of the sur- face near region enable a potential use for tribological application. Dimples and microstructures are known to reduce the coecient of friction for wet lubricant appli- cations with higher surface speeds. Therefore, the CLP could be used in the hydrodynamic regime with fast surface speed, where the protrusions act as reservoirs to attain a well spread lm. However, this has to be studied in more detail and a preliminary test by a pin on disc measurements gathering the Stribeck curves up to several m/s surface speed pointed int the right di- rection. Nevertheless, a pin diameter with 4 mm and a normal force of 1 N was used, which lead to fast wear on the structures. A fresh study with higher pin diameter and less force will reveal the potential for tribological applications. 4 Conclusion and Outlook Ultra-short pulsed laser ablation on stainless steel re- veals self-organized microstructures, which are depen- dent on the laser and process parameter. A regime for de ned ablation without cones and CLP has been iden- ti ed, where LIPSS can be mitigated by circular po- larization. The CLP start to grow succeeding at least one ablated layer on ripple precursor structures. With each layer the CLP and ripple structure cover more surface area ending in full coverage. These two struc- tures preferentially assemble at the (101) direction of the face-centered cubic austenitic steel grains under in- vestigation. Moreover, a strong dependence on the laser polarization direction is reported. The CLP geometry can be changed by laser parameters, where the resolid- i cation of the surface molten layer plays the crucial role. Nanoindentation pointed to a slight change in sti - ness compared to the bulk properties with an E-modulus of about 120 GPa at the interface. Potentially, CLP structures are a viable candidates for tribological ap- plication to reduce friction and wear via laser surface structuring. A preliminary study pointed to a reduced coecient of friction in the hydrodynamic regime of lu- brication. Acknowledgments The authors want to thank the Swiss National Fund (FuSSiT/169654) for funding. Thanks to Keyence Switzer- land and, especially, Martin Banzer for support andthe possibility to implement a Keyence VH-X6000 tem- porarily in our laboratories. Moreover, N.A. wants to acknowledge Reto V ollmin from Keyence Switzerland for support with the confocal Keyence VK-X1000 and setting up the automated measurement routines. The help of Marianna Diamantopoulou with the EBSD stud- ies is highly appreciated. Special thanks from N.A. to Peter Hosemann at UC Berkeley for hosting during the conception and writing of this manuscript.