Study of the wettability behavior of stainless steel surfaces after ultrafast laser texturing
Graphical abstract
Introduction
The wettability of rough surfaces is a complex problem which continues to attract interest, thanks to new technologies and materials that allow obtaining surfaces with controlled micro- and nano-roughness [1]. Starting from the work of Neinhuis and Barthlott [2] that explained the origin of the superhydrophobicity and self-cleaning properties of the lotus leaf (the so-called “lotus effect”), a lot of efforts have been devoted to reproduce the micro and nano features of such surface, characterized by a water contact angle higher than 150° and, in general, low surface energy. Several techniques already exists to produce large-area superhydrophobic surfaces: transparent coatings have been developed for different kinds of window, as automobile windows [3], and eyeglasses, or to increase the performance of solar cells [4], to control bio-adhesion [[5], [6], [7]] and bio-fouling [[8], [9], [10]]. Nevertheless, most of these techniques, as for instance chemical vapor deposition (CVD) [11,12], electro-chemical deposition [13], and sol-gel method [14], involve the use of chemical coatings and show several drawbacks, especially in applications in which the release of coating particles into the environment is extremely critical. As an example, in food industry it is strategic that the coating used for tools undergoing chemical attack, scratches or wear, do not contaminate the products, which they contribute to cut, convey or package.
In this work, the possibility to use chemical-free ultrafast laser treatment to obtain hydrophobic and superhydrophobic stainless steel surfaces was investigated on AISI 316L, generally employed as food contact material. This technique offers several advantages over the other previously mentioned. Laser texturing is a one-step contactless approach that can be exploited with a flexible experimental apparatus, with the chance of creating a large variety of textures by tuning laser parameters and the environmental conditions [15,16].
This is the reason why, in the last five years, several approaches have been devoted to the fabrication of superhydrophobic surfaces using femtosecond lasers. Ultrashort laser pulses bring about an interaction process with the target material with a nearly negligible thermal transfer to the workpiece, allowing for a reproducible surface texture with virtually no heat affected zone [17]. Furthermore, such a technique can be potentially employed for the functionalization of large scale surfaces, with remarkable implications in the technological implementation. A thorough review of obtainable surface features as a function of the ruling process parameters can be found in [15,18]. Among the multiple functionalities that femtosecond lasers may induce within a surface, several contributions in literature exist showing the possibility to generate lotus-leaf like surfaces by ultrashort pulse laser texturing to modify the surface wettability [19]. One of the earliest and most comprehensive studies on the matter is due to Kietzig et al. [20] who realized ultrashort laser textures on AISI 304L stainless steel with static water contact angles (CA) between 120° and 150° and contact angle hysteresis (CAH) of 3°. A main finding of Kietzig et al. was that the superhydrophobic properties of the samples were discovered to be time-dependent. While the laser treated surfaces showed superhydrophilic behavior immediately after production for steel, titanium and aluminum, over the course of several weeks the wetting properties changed from superhydrophilic to superhydrophobic. Since surface roughness does not change with time, the reason of this behavior was attributed to surface chemical changes. In [20], the authors observed, by XPS analysis, that carbon and oxygen proportions of the surfaces changed in time, to which they attributed slow decomposition of carbon dioxide on the laser treated material surface. In a successive work [21], Kietzig et al. assert that the laser process causes a surface reaction, which changes the inherent wettability of pure metallic interfaces. The surface morphology after the treatment plays a minor role in the resulting contact angle as compared to surface chemistry changes.
Confirmation of the aforementioned findings was presented in [22], stating that the evolution of contact angles from superhydrophilic to superhydrophobic in the post laser treatment period was correlated with the amount of carbon on the structured surface and was independent of the chemical composition of the material. Nevertheless in [23], the explanation relating the increased superhydrophobicity to the amount of carbon on the surface was confirmed for aluminum substrates but not for steel. Conversely, the authors found that no hydrophobic functional groups were created on steel after laser treatment. Cunha et al. [24] found that laser treatment, carried out in air, led to the complete oxidation of both titanium and aluminum surfaces, which were reported to be hydrophilic. The equilibrium contact angles of these surfaces were similar or lower than 21°, the value reported by Kietzig et al. [20] for surfaces immersed in water after the laser treatment. Vorobyev and Guo [19] gave more importance to surface topography since laser-induced surface nanostructures play an important role in enhancing chemical interactions due to nanochemical effects, the first being oxidation of the ablated material and the second oxidation of the hot liquid/solid nanostructured surface after termination of ablation.
From the above state-of-the-art it is possible to see that despite experimental results showing that metal surfaces with tuned wettability are feasible by means of femtosecond lasers, the process is far from being completely understood.
A further complexity relates to the hypotheses of Vorobyev and Guo [19], who showed that lasers may create very different surface morphologies depending on the used parameters. Low laser fluence generates typical laser-induced periodic surface structures (LIPSS) on a submicron level. The apparent contact angle (CA) on surfaces covered by such features is generally lower than 150° [25]. With increasing laser fluence, periodic ripples and periodic cone-shaped spikes on a micron scale can be fabricated, both covered with LIPSS. The stainless steel surfaces with micro- and submicron double-scale structures have higher apparent Cas [26]. Rukosuyev et al. [27] found that by adjusting the fluences and with specific use of the focal volume of the laser beam, a micron scale ridge-like structure with superimposed submicron convex features could be produced. It was shown that the hydrophobic behavior was mainly caused by the surface texturing obtained as a result of laser ablation and not due to the intrinsic properties of the base materials.
From a topographic point of view, a multi-scale surface morphology made of nano- and micro-scale periodic ripples seems the best configuration to shift the wetting behavior to a superhydrophobic regime [28]. Nanoscale structures tend to become predominantly microscale with an increasing number of pulses and overlap [29]. While surface complexity increases, double-roughness patterns comprising nano- and micro-scale periodic ripples exhibited static CA >150° and low CAH as well [30]. The high permanent superhydrophobicity of this pattern is due to the special micro/nano-structure of the surface that facilitates the Cassie–Baxter state, which led Fadeeva et al. [31] to create titanium lotus-like surface structures having self-cleaning properties than can even reduce the bacterial adhesion.
Based on this background, the aim of this work is to investigate the wetting properties of AISI 316L stainless steel surfaces textured by ultrashort pulse laser in order to establish the correlation between process parameters and physico-chemical behavior of the treated surfaces. A deeper understanding of the phenomena generated by the laser on metals is indeed fundamental for a reproducible process in view of industrial applications.
Moreover, it should be noted that texturing rates obtained to date do not allow treatment of large areas for practical industrial applications. As an example, the texturing rate of the pilot work of Kietzig et al. is about 7.5 · 10−3 mm2/s taking into account the scanning speed and laser spot diameter used in [20]. Such slow processing times would hinder any use of ultrafast laser technology over macro-scale mechanical components. Therefore, in view of increasing the overall process throughput, this work is focused on maximizing the scanning speed by making use of repetition rates up to 1 MHz, to guarantee sufficient pulse overlap to generate hierarchical nanofeatures at scanning speeds two orders of magnitude higher than those adopted so far.
The wetting behaviour, being dependent on both the surface roughness and the water–solid adhesion forces, were assessed by CA hysteresis, roll-off angle and surface energy measurements. In fact, it is now widely accepted that a superhydrophobic surface is adequately characterized when the advancing and receding CAs, which define the CAH, are measured together with the static CA in order to discriminate between the so-called “Lotus effect” (high CA and low CAH) and the “Rose petal effect” (high CA and high CAH). Finally, to define the effect of ageing on the surface properties, the wettability and the chemical properties were evaluated in time by static CA and X-ray photoelectron spectroscopy (XPS) measurements.
Section snippets
Femtosecond laser treatments
The irradiated samples consist of AISI 316L stainless steel square plates with lateral dimension of 50 mm and thickness of 2 mm. In order to disregard the influence of pre-existing surface roughness on the final results, the initial samples were mirror-polished with an as-received average roughness Sa < 0.05 μm. Before the laser treatment, the samples were washed for 5 min in acetone in an ultrasound bath, following which laser texturing was performed without shielding gas in line.
Textured
Evolution of surface wettability with laser-induced surface morphology
In Fig. 1 SEM-FEG images of surfaces obtained with different values of RR and for increasing values of energy doses, E, are reported. At low energy doses, periodic surfaces structures, also known as LIPSS (Laser Induced Periodic Surface Structures) [33], which are oriented perpendicularly to the laser polarization and with a period comparable to the laser wavelength, are visible. The presence of LIPSS on steel in similar conditions of irradiation was previously assessed [34]. By increasing the
Conclusion
The obtained results demonstrated that all the experimental conditions allowed to obtain AISI 316L stainless steel surfaces with very high static CA, about 30 days after laser treatment. Different nano and microstructures can be obtained by changing the laser parameters (RR, v and dose), leading to surfaces characterized by high CA and low hysteresis (lotus effect) or by high CA and high hysteresis (rose-petal effect) demonstrating the key role of laser parameter to obtain self-cleaning
Acknowledgements
This project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No 687613.
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