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carbon/nitrogen (X) with bonded terminations (T x : -O 2 , -F 2 , -(OH) 2 , -Cl 2 , or their combinations) on the exterior transition metal surfaces. [6][7][8] MXenes' crystal structures and chemical formula are derived from their 3D crystalline precursors, which are usually MAX phases. [9] MAX phases are chemically denoted by M n+1 AX n (n = 1 to 4), for which the M-X layers found in MXenes are sandwiched by layers of A-group elements (mostly group 13-16 of the periodic table). [10] MXenes are synthesized by selectively removing/ etching these A-group element layers. Upon the removal of these A layers, surface terminations (T x ) occupy the bonding sites on M layers previously bonded to the A-group elements. [11] MXenes are excellent candidates for a range of applications (Figure 1a), including energy storage, [7] transparent electronics, [12,13] sensors, [14] electromagnetic interference (EMI) shielding, [15,16] and catalysis. [17] These applications involve MXenes as flexible and thin films, [13] MXenes as anchors in hybrid materials, [18] embedded in matrix materials, [19,20] or even as wearable fabrics. [21] In almost all these applications, fundamental understanding of the mechanical and tribological properties of MXenes is necessary (Figure 1b). The mechanical properties are vital to MXenes' applications in energy storage and catalysis, as MXenes in electrodes can act as active materials, conductive additives, strong binders, and current collectors, which can control the structure of the electrode and withstand expansion and contraction during charge and discharge cycles. [7,22] Additionally, the rise of flexible and wearable devices [13,21,23] necessitates mechanical characterization of MXene to withstand the required mechanical forces while maintaining the flexibility for these applications. Similarly, the potential applications of MXenes in triboelectric nanogenerators for energy storage and wearable electronics requires the characterization of MXenes' tribological properties. [24] More recently, MXenes have been used as reinforcement phases in composite materials. [25] The relevance of the mechanical and tribological properties in various applications is illustrated in Figure 1b. The rationale and support for this figure is summarized in Figures S1 and S2, Supporting Information, and discussed further in the Supporting Information.Despite well over 3000 publications to date on MXenes since their discovery, [26] only 4.9% of publications have studied their mechanical and tribological properties and their applications as reinforcement materials in composites, as shown in Figure 1a. Only two experimental studies on the mechanical stiffness 2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, were discovered in 2011 and have grown to prominence in energy storage, catalysis, electromagnetic interference shielding, wireless communications, electronic, sensors, and environmental and biomedical applications. In addition to their high electrical conductivity and electrochemically activ...

Surface texturing has gained great attention in the tribological community since precisely defined surface features can help to reduce friction and/or wear irrespective of the acting lubrication regime. The ability to positively influence tribological performance under different lubrication conditions makes surface texturing particularly interesting for machine elements since they may experience different conditions over the lifetime or sometimes even over one cycle/stroke. However, despite the great effort by both researchers and industry to introduce surface texturing in machine elements, many questions remain unclear regarding the optimal design of surface textures, as well as the positive and negative effects on the component's performance. The aim of this review article is to critically summarize the state of the art of surface texturing applied to machine elements, with a special emphasis on piston rings, seals, roller bearings, and gears. After a brief introduction, the first section focuses on surface texturing in sliding components (piston rings and seals), whereas the second section deals with surface texturing in rolling components (roller bearings and gears). Based upon the main evidence from the literature, the final section provides more general design guidelines for surface texturing in machine elements.

Surface texturing and solid lubricants have demonstrated the ability to substantially reduce friction and wear under dry conditions. In recent decades, these two technologies have been combined to leverage the advantages of both for superior tribological performance. This review article first summarizes the state-of-the-art regarding surface texturing and solid lubricants, including soft metals, polytetrafluoroethylene, diamond-like carbon and 2D layered materials. Then, the synergy between surface textures and solid lubricants is discussed, with particular emphasis on the underlying mechanisms. Finally, gaps in the existing understanding of these synergies are identified and opportunities for future research are suggested.

The frictional behaviour of plane converging bearings was experimentally and numerically studied for four texture geometries fabricated by ultra-short pulse laser texturing (single pocket, line-, cross-and dot-like texture) and convergence ratios under full-film lubrication in the presence of thick oil films (up to 100 µm). Regarding the experiments, small variations in the spread of results between different textures and a general improvement over the untextured reference can be observed. Numerical simulations help to clarify the expected variations and conditions under which these occur. For high convergences, the simulations