Influence of chemistry and topography on the wettability of copper.

To understand the complex interplay of topography and surface chemistry in wetting, fundamental studies investigating both parameters are needed. Due to the sensitivity of wetting to miniscule changes in one of the parameters it is imperative to precisely control the experimental approach. A profound understanding of their influence on wetting facilitates a tailored design of surfaces with unique functionality. We present a multi-step study: The influence of surface chemistry is analyzed by determining the adsorption of volatile carbonous species (A) and by sputter deposition of metallic copper and copper oxides on flat copper substrates (B). A precise surface topography is created by laser processing. Isotropic topography is created by ps laser processing (C), and hierarchical anisotropic line patterns are produced by direct laser interference patterning (DLIP) with different pulse durations (D). Our results reveal that the long-term wetting response of polished copper surfaces stabilizes with time despite ongoing accumulation of hydrocarbons and is dominated by this adsorption layer over the oxide state of the substrate (Cu, CuO, Cu2O). The surfaces' wetting response can be precisely tuned by tailoring the topography via laser processing. The sub-pattern morphology of primary line-like patterns showed great impact on the static contact angle, wetting anisotropy, and water adhesion. An increased roughness inside the pattern valleys combined with a minor roughness on pattern peaks favors air-inclusions, isotropic hydrophobicity, and low water adhesion. Increasing depth of the primary topography can also induce air-inclusions despite increasing peak roughness while time dependent wetting transitions were observed.

Unveiling the tribological potential of MXenes-current understanding and future perspectives.

Reducing energy consumption and CO2 emissions by improving the tribological performance of mechanical systems relies on the development of new lubrication concepts. Two-dimensional (2D) materials have been the subject of extensive tribological research due to their unique physical and chemical properties. 2D transition metal carbides, nitrides, and carbonitrides (MXenes), with their tuneable chemistry and structure, are a relatively new addition to the family of 2D materials. MXenes' good strength and stiffness, easy-to-shear ability, capability to form wear-resistant tribofilms, and the possibility to control their surface chemistry make them appealing candidates to be explored for tribological purposes. This review provides a comprehensive overview of MXenes' tribology, covering their structure-property relationship, synthesis approaches, deposition methods to generate MXene coatings for tribological purposes, and their fundamental tribological mechanisms. Furthermore, detailed insights into studies exploring MXenes' tribological performance from the nano- to the macro-scale are presented with special emphasis on their use as self-lubricating solid lubricants, lubricant additives, and reinforcement phases in composites.

Se Nanopowder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions.

Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS2 , even though selenides and tellurides have demonstrated superior tribological properties. Here, an innovative in operando conversion of Se nanopowders into lubricious 2D selenides, by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films, is described. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in the shear-induced synthesis of selenide monolayers from nanopowders. The use of Se nanopowder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nanopowder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long-lasting problem of TMD-lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in operando and exploit their friction- and wear-reducing impact.

Topography versus chemistry - How can we control surface wetting?

HYPOTHESIS: Wetting characterization and the production of engineered surfaces showing distinct contact angles or spreading behavior is of major importance for many industrial and scientific applications. As chemical composition plays a major role in the wetting behavior of flat samples, wettability, capillary forces and resulting droplet spreading on anisotropic surface patterns are expected to be highly dependent on surface chemistry as well. EXPERIMENTS: To gain understanding of the fundamental principles of the interplay between surface topography and surface chemistry regarding water wettability, anisotropic line patterns were produced on steel samples in a direct laser writing process. Homogeneous surface coatings allowed for a chemical masking of the laser patterns and therewith the identification of the influence of surface chemistry on static contact angles and wetting anisotropy. FINDINGS: While a carbon coating leads to pronounced wettability and spreading along the topographic anisotropy, an inert gold-palladium coating can fully suppress anisotropic droplet spreading. Model calculations show that an amorphous carbon coating leads to Wenzel wetting while the gold-palladium coating causes air inclusions between the water and the surface in the Cassie-Baxter wetting state. Only in combination with the right chemical composition of the surface, directional patterns show their potential of anisotropic wetting behavior.

Superior Wear-Resistance of Ti3C2Tx Multilayer Coatings.

Owing to MXenes' tunable mechanical properties induced by their structural and chemical diversity, MXenes are believed to compete with state-of-the-art 2D nanomaterials such as graphene regarding their tribological performance. Their nanolaminate structure offers weak interlayer interactions and an easy-to-shear ability to render them excellent candidates for solid lubrication. However, the acting friction and wear mechanisms are yet to be explored. To elucidate these mechanisms, 100-nm-thick homogeneous multilayer Ti3C2Tx coatings are deposited on technologically relevant stainless steel by electrospraying. Using ball-on-disk tribometry (Si3N4 counterbody) with acting contact pressures of about 300 MPa, their long-term friction and wear performance under dry conditions are studied. MXene-coated specimens demonstrate a 6-fold friction reduction and an ultralow wear rate (4 × 10-9 mm3 N-1 m-1) over 100 000 sliding cycles, outperforming state-of-the-art 2D nanomaterials by at least 200% regarding their wear life. High-resolution characterization verified the formation of a beneficial tribolayer consisting of thermally/mechanically degraded MXenes and amorphous/nanocrystalline iron oxides. The transfer of this tribolayer to the counterbody transforms the initial steel/Si3N4 contact to tribolayer/tribolayer contact with low shear resistance. MXene pileups at the wear track's reversal points continuously supply the tribological contact with fresh, lubricious nanosheets, thus enabling an ultra-wear-resistant and low-friction performance.

Effect of Temperature on the Deformation Behavior of Copper Nickel Alloys under Sliding.

The microstructural evolution in the near-surface regions of a dry sliding interface has considerable influence on its tribological behavior and is driven mainly by mechanical energy and heat. In this work, we use large-scale molecular dynamics simulations to study the effect of temperature on the deformation response of FCC CuNi alloys of several compositions under various normal pressures. The microstructural evolution below the surface, marked by mechanisms spanning grain refinement, grain coarsening, twinning, and shear layer formation, is discussed in depth. The observed results are complemented by a rigorous analysis of the dislocation activity near the sliding interface. Moreover, we define key quantities corresponding to deformation mechanisms and analyze the time-independent differences between 300 K and 600 K for all simulated compositions and normal pressures. Raising the Ni content or reducing the temperature increases the energy barrier to activate dislocation activity or promote plasticity overall, thus increasing the threshold stress required for the transition to the next deformation regime. Repeated distillation of our quantitative analysis and successive elimination of spatial and time dimensions from the data allows us to produce a 3D map of the dominating deformation mechanism regimes for CuNi alloys as a function of composition, normal pressure, and homologous temperature.

Applying Ultrashort Pulsed Direct Laser Interference Patterning for Functional Surfaces.

Surface structures in the micro- and nanometre length scale exert a major influence on performance and functionality for many specialized applications in surface engineering. However, they are often limited to certain pattern scales and materials, depending on which processing technique is used. Likewise, the morphology of the topography is in complex relation to the utilized processing methodology. In this study, the generation of hierarchical surface structures in the micro- as well as the sub-micrometre scale was achieved on ceramic, polymer and metallic materials by utilizing Ultrashort Pulsed Direct Laser Interference Patterning (USP-DLIP). The morphologies of the generated patterns where examined in relation to the unique physical interaction of each material with ultrashort pulsed laser irradiation. In this context, the pattern formation on copper, CuZn37 brass and AISI 304 stainless steel was investigated in detail by means of a combination of experiment and simulation to understand the individual thermal interactions involved in USP-DLIP processing. Thereby, the pattern's hierarchical topography could be tailored besides achieving higher process control in the production of patterns in the sub-µm range by USP-DLIP.

Tribochemical Behaviors of Onion-like Carbon Films as High-Performance Solid Lubricants with Variable Interfacial Nanostructures.

Onion-like carbon (OLC), spherical nanoparticles consisting of carbon shells, is capable of providing exceptional lubrication effects. Nevertheless, the underlying mechanism, especially the tribo-induced evolution of interfacial nanostructures and their correlation with the friction states, is not clear. In this work, OLC films with a thickness of ∼1 µm were synthesized by electrophoretic deposition on the mirror-polished stainless steel. The lubricity was evaluated by tailoring the sliding aspects including applied normal load, contact time, and counterface materials. It is found that the friction reduction level is highly dependent on the material transfer and transformation of the OLC surface and the physicochemical nature of the as-formed tribolayer in the contact areas. The subsurface of the OLC film always undergoes a deep amorphization transformation upon sliding. It is interesting to note that the tribolayer formed on the bare steel ball is mainly composed of highly ordered graphene-like nanoflakes derived from the sliding-induced degradation of OLC nanospheres. In comparison, the nanospherical carbon structure can be retained in the topmost subsurface of the tribolayer formed on the ceramic Si3N4 ball. Such a nanosphere-/amorphization-coupled interface is capable of providing a robust lubrication state under high contact stresses. The findings identify a new lubrication mechanism for the spherical carbon nanostructure, rendering them effective solid lubricants.

The influence of mannitol on morphology and disintegration of spray-dried nano-embedded microparticles.

Nano-embedded microparticles represent a promising approach to deliver nanoparticles to the lungs. Microparticles with an appropriate aerodynamic diameter enable an application by dry powder inhaler and the transport of nanoparticles into the airways. By disintegration after deposition, nanoparticles can be released to exhibit their advantages such as a sustained drug release and delivery of the drug across the mucus barrier. The use of an appropriate matrix excipient to embed the nanoparticles is essential for the necessary disintegration and release of nanoparticles. In this context we investigated the influence of mannitol on the morphology, aerodynamic properties and disintegration behavior of nano-embedded microparticles. PLGA nanoparticles and mannitol were spray dried each as sole component and in combination in three different ratios. An influence of the mannitol content on the morphology was observed. Pure mannitol microparticles were solid and spherical, while the addition of nanoparticles resulted in raisin-shaped hollow particles. The different morphologies can be explained by diffusion processes of the compounds described by the Péclet-number. All powders showed suitable aerodynamic properties. By dispersion of the powders in simulated lung fluid, initial nanoparticle sizes could be recovered for samples containing mannitol. The fraction of redispersed nanoparticles was increased with increasing mannitol content. To evaluate the disintegration under conditions with higher comparability to the in vivo situation, spray-dried powders were exposed to >90% relative humidity. The disintegration behavior was monitored by analyzing roughness values by white light interferometry and supporting SEM imaging. The exposure to high relative humidity was shown to be sufficient for disintegration of the microparticles containing mannitol, releasing morphologically unchanged nanoparticles. With increasing mannitol content, the disintegration occurred faster and to a higher degree. Under these conditions, microparticles only composed of nanoparticles did not disintegrate. By enabling the release of nanoparticles from nano-embedded microparticles, mannitol was shown to be an ideal excipient to convert nanoparticles by spray drying into an inhalable dry power formulation.