MXenes are a 2D materials (2DM) class with high industrialization potential, owing to their superb properties and compositional variety. However, ensuring high etching efficiency in the synthesis process without involving toxic, hazardous or non‐sustainable chemicals are challenging. In this work, an upscalable electrochemical MXene synthesis is presented. This novel protocol uses a non‐toxic and sustainable sodium tetrafluoroborate/hydrochloric acid (NaBF ₄ /HCl) electrolyte and increases etching efficiency by applying cathodic pulsing via pulse voltammetry. Hydrogen bubble formation restores electrochemical activity, and effectively supports 2D‐sheet removal, allowing continuous etching at higher yields in situ. In detail, yields of up to 60% electrochemical MXene (EC‐MXene) with no byproducts from a single exfoliation cycle are achieved. EC‐MXene had an excellent quality with high purity as assessed using chemical mapping by scanning electron microscopy with energy dispersive electron spectroscopy (SEM/EDX) and surface termination analysis performed with X‐ray photoelectron spectroscopy (XPS) and, for the first time, with low energy ion scattering (LEIS). Further properties of EC‐MXenes such as dimensions and adhesion energy of single flakes, vibrational peaks, and interlayer spacing are provided by atomic force microscopy (AFM), X‐ray diffraction (XRD), Raman spectroscopy (Raman), and transmission electron microscopy (TEM) respectively. Pulsed electrochemical synthesis is key to surface reactivation at the electrodes' interface, which results in improved exfoliation and quality of EC‐MXenes. This paves the way for scaling up and green industrialization of MXenes.

Purpose: The paper aims to investigate the possibilities to control friction in lubricated systems by surface patterning, making use of a multi-scale approach. Surface patterns inside the tribological contact zone tend to directly reduce friction, whereas surface patterns located in the close proximity of the contact area can improve the tribological performance by avoiding lubricant starvation and migration. Finally, optimized surface patterns were identified by preliminary laboratory tests and transferred to a journal bearing, thus testing them under more realistic conditions. Design/methodology/approach: Surface patterns on a large scale (depth > 10 µm) were fabricated by micro- and roller-coining, whereas surface patterns on a small scale (depth < 2 µm) were produced by direct laser interference patterning. The combination of both techniques resulted in multi-scale surface patterns. Tribologically beneficial surface patterns (verified in ball-on-disk laboratory tests) were transferred onto a journal bearing’s shaft and tested on a special test-rig. To characterize the lubricant spreading behavior, a new test-rig was designed, which allowed for the study of the lubricant’s motion on patterned surfaces under the influence of a precisely controlled temperature gradient. Findings: All tested patterns accounted for a pronounced friction reduction and/or an increase in oil film lifetime. The results from the preliminary laboratory tests matched well, with results from the journal bearing test-rig, both tests showing a maximum friction reduction by a factor of 3-4. Numerical investigations, as well as experiments, have shown the possibility to actively guide lubricant over patterned surfaces. Smaller periodicities, as well as greater structural depths and widths, led to a more pronounced anisotropic spreading and/or greater spreading velocities. Multi-scale surfaces demonstrated the strongest effects regarding the lubricant’s spreading behavior. Originality/value: Friction, as well as lubricant migration, can be successfully controlled by using micro-coined, laser-patterned and/or multi-scale surfaces. To the best of the authors’ knowledge, the study demonstrates for the first time the unique possibility to transfer results obtained in laboratory tests to a real machine component.

Laser surface texturing is an interesting possibility to tailor materials’ surfaces and thus to improve the friction and wear properties if proper texture feature sizes are selected. In this research work, stainless steel surfaces were laser textured by two different laser techniques, i.e., the direct laser interference patterning by using a nanosecond pulsed Nd:YAG laser and additionally by an ultrashort pulsed femtosecond Ti:Sa. The as-textured surfaces were then studied regarding their frictional response in a specially designed linear reciprocating test rig under lubricated conditions with a fully formulated 15W40 oil. Results show that dimples with smaller diameter lead to a significant reduction in the coefficient of friction compared to the dimples with a larger diameter and surfaces with a grid-like surface pattern produced by direct laser interference patterning.

Thermocapillary migration in tribological systems such as gearboxes, piston ring-liner assemblies, or bearings can lead to lubricant starvation and, consequently, to severe damages. Due to surface tension gradients induced by local temperature gradients on the frictionally heated surface, lubricant may flow out of the hotter contact zone to colder adjacent regions. In order to manipulate lubricant migration, several approaches exist to create migration barriers or to guide lubricant in a well-defined way back into the contact. These approaches are either based upon surface topography (designed surface textures) and/or surface chemistry (designed chemical patterns). In this review article, the state-of-the-art regarding thermocapillary migration is summarized from a theoretical and experimental point of view. After a brief introduction, theoretical models are presented to introduce the underlying theory of droplet spreading and thermocapillary migration. Subsequently, the influence of surface textures and chemical patterns on thermocapillary migration and the possibilities to use these approaches to manipulate lubricant migration are discussed. After addressing applied research in this field and potential applications, current challenges and future research directions are presented before closing with general conclusions.

Laser surface texturing has been successfully applied in the past to various machine elements in order to improve their tribological performance. In this study, direct laser interference patterning was used to produce periodic cross-like surface patterns on commercial cylinder roller bearings. The wear behavior of as-patterned bearings was studied by a modified FE8 test rig under boundary lubrication. A mineral oil (ISO VG 100) as a base oil additivated with 0.02 wt % zinc dialkyldithiophosphate (ZDDP) was used for the tribological tests which were performed under a Hertzian pressure of 1.92 GPa and maintained at 80 °C and 20 rpm for 2 h. The laser-patterned bearings showed a significantly reduced mass loss by two orders of magnitude compared to the unpatterned reference bearings. A closer look at the samples proved the formation of the characteristic blue-colored ZDDP tribofilm on top of the laser-induced topography maximum positions. Due to the higher contact pressure at the laser-induced peaks, the tribofilm formation was preferable at those positions thus protecting against wear. The laser patterns nearly remained unworn compared to the reference samples. A subsequent Raman analysis of the laser-patterned bearings clearly revealed the formation of zinc and iron sulfides as well as phosphates at the peak points.

Preventing the accretion of droplets on surfaces is vital and slippery liquid‐infused porous surfaces (SLIPS) have promising application prospects, such as surface self‐cleaning and droplet transportation. In this work, controllable self‐transport of bouncing droplets on ultraslippery surfaces with wedge‐shaped grooves is reported. The impact behaviors of droplets on SLIPS under various impact velocities and diameters are explored, which can be classified as hover, total bounce, partial bounce, Worthington jet, and crush. SLIPS with wedge‐shaped grooves were designed to transport accreted droplets. An energy and transport model is established to explain the impact and self‐transport mechanism, where the Laplace pressure and moving resistance between droplets play a key role. Finally, SLIPS with branched wedge‐shaped grooves were designed for droplet self‐transport and demonstrated advantages. This work provides a general reference for spontaneous motion control of sessile droplets, droplets with initial impacting velocity, or even liquid films.