Surface texturing has been frequently used for tribological purposes in the last three decades due to its great potential to reduce friction and wear. Although biological systems advocate the use of ...hierarchical, multi-scale surface textures, most of the published experimental and numerical works have mainly addressed effects induced by single-scale surface textures. Therefore, it can be assumed that the potential of multi-scale surface texturing to further optimize friction and wear is underexplored. The aim of this review article is to shed some light on the current knowledge in the field of multi-scale surface textures applied to tribological systems from an experimental and numerical point of view. Initially, fabrication techniques with their respective advantages and disadvantages regarding the ability to create multi-scale surface textures are summarized. Afterwards, the existing state-of-the-art regarding experimental work performed to explore the potential, as well as the underlying effects of multi-scale textures under dry and lubricated conditions, is presented. Subsequently, numerical approaches to predict the behavior of multi-scale surface texturing under lubricated conditions are elucidated. Finally, the existing knowledge and hypotheses about the underlying driven mechanisms responsible for the improved tribological performance of multi-scale textures are summarized, and future trends in this research direction are emphasized.
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.
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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.
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.
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.
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.
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.
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•Critical assessment of MXenes' structure, synthesis, coating deposition, and cross-scale tribology.•Discussion on MXenes' usage as solid lubricant coatings, additives, and fillers in composites.•Identification of MXene's cross-scale tribological potential and mechanisms to control friction and wear.•Existing bottlenecks of MXene tribology are identified, and future research is envisaged to guide tribologists.
The present study experimentally examines the effect of selected single-scale and multi-scale surface patterns fabricated by roller-coining and/or direct laser interference patterning on the ...frictional performance of journal bearings. For this purpose, surface patterns showing beneficial effects in preliminary laboratory tests were selected and fabricated onto the shaft of journal bearings made of stainless steel (AISI 304). The frictional performance of these patterns was evaluated on a special test rig by recording Stribeck-like curves. The results show greatly reduced coefficients of friction and a shift in the transition from mixed to hydrodynamic lubrication to smaller rotational speeds for all patterned samples compared to the reference sample. The observed friction reduction matches well with results observed in the previous laboratory tests.
•Successful fabrication of single- and multi-scale surface patterns on curved shafts.•The burnished reference sample demonstrates a Stribeck-like behavior.•All patterned samples show a reduced COF over the entire range of rotational speeds compared to the reference.•The transition from mixed to hydrodynamic lubrication is shifted to smaller rotational speeds for all patterned samples.•The results match very well with preliminary laboratory tests on a ball-on-disk test rig.
The present study aims at numerically predicting the frictional performance of journal bearings with single- and multi-scale surface patterns considering the real 3D surface topographies after ...wearing-in in mixed-elastohydrodynamic (mixed-EHL) simulations using a multi-body simulation (MBS) environment. For this purpose, the extended Reynolds equation with flow factors according to Patir and Cheng has been combined with a deterministic asperity contact model, which can be further utilized in the design process to optimize the tribological response of engineering systems. For all patterned surfaces, a shift to smaller rotational speeds in the transition from mixed to hydrodynamic lubrication with a notably reduced coefficient of friction has been demonstrated. The largest frictional improvement (- 80%) has been achieved with single-scale surface patterns fabricated by direct laser interference patterning.
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•Numerical mixed-EHL simulation of journal bearings considering wearing-in.•Effects of single- and multi-scale surface patterns on journal bearing friction.•Significantly improved friction and wear performance induced by surface patterning.•Maximum friction reduction of 80% has been observed.
To investigate the influence of centrifugal forces on the oil film stability and the lubrication regime, mirror polished steel surfaces (AISI 304) were tested using a ball-on-disc tribometer in ...rotational sliding mode with a steel counter body (AISI 52100). Different sliding radii (1.5 and 6 mm) were used to vary the angular velocity. The tests were conducted under fully-flooded conditions using Poly-alpha-olefin oils. The measured coefficients of friction were complemented by the measurement of solid-solid contact ratios and the analysis of the respective wear tracks. It could be shown that the centrifugal forces have a great influence on the oil film lifetime. Thereby, the oil film lifetime is longer for larger sliding radii and higher viscosities.
•Measurement of the influence of centrifugal forces on the oil film lifetime.•Combined measurements of coefficient of friction, solid-solid contact ratio and wear.•Longer oil film lifetime for greater sliding radii and therefore smaller centrifugal forces.•Longer oil film lifetime for higher viscosities.
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.
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