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The development of high-performance and environmentally-compatible lubricants is crucial for minimizing energy losses in mechanical systems and increasing the lifetime of moving mechanical components, thus preserving our environment. While ionic liquids (ILs) have emerged as promising next-generation materials for lubrication purposes owing to their attractive physico-chemical properties, several challenges currently limit their use in engineering applications, including their high cost and corrosivity. Recently, eco-friendly, protic ILs (PILs) have been synthesized and showed great advantages compared to tradition (aprotic) ILs, such as low cost, ease of preparation, and good lubricating properties. Despite these advancements, remarkably little is known about the interrelationship between PIL molecular structure and lubrication mechanisms. In this work, the physico-chemical and lubricating properties of a family of PILs synthesized by using only renewable, biodegradable, and biocompatible products and constituted by the same choline cation and amino-acid anions with different side chains, were investigated. The molecular structures of the choline amino acid-based ionic liquids (AAILs) were confirmed through magnetic resonance and Fourier transform infrared spectroscopy, while their thermal behavior was evaluated by differential scanning calorimetry and thermogravimetric analysis. The antiwear and friction-reducing performance of the choline AAILs when used as neat lubricants was studied as a function of normal load by reciprocating ball-on-flat tribometry using steel-steel contact. Surface analytical measurements (Raman and XPS) performed on the worn steel surfaces confirmed that the excellent lubricating performance of choline AAILs originates from the formation of oxygen- and carbon-rich tribolayers. The formation of these protective layers are influenced by the applied normal load and the molecular structure of the amino acid. The results of this work open the path for the rational design of environmentally-friendly PILs for tribological applications. 

Abstract Surface performance is critically influenced by topography in virtually all real-world applications. The current standard practice is to describe topography using one of a few industry-standard parameters. The most commonly reported number is$$R$$ $R$a, the average absolute deviation of the height from the mean line (at some, not necessarily known or specified, lateral length scale). However, other parameters, particularly those that are scale-dependent, influence surface and interfacial properties; for example the local surface slope is critical for visual appearance, friction, and wear. The present Surface-Topography Challenge was launched to raise awareness for the need of a multi-scale description, but also to assess the reliability of different metrology techniques. In the resulting international collaborative effort, 153 scientists and engineers from 64 research groups and companies across 20 countries characterized statistically equivalent samples from two different surfaces: a “rough” and a “smooth” surface. The results of the 2088 measurements constitute the most comprehensive surface description ever compiled. We find wide disagreement across measurements and techniques when the lateral scale of the measurement is ignored. Consensus is established through scale-dependent parameters while removing data that violates an established resolution criterion and deviates from the majority measurements at each length scale. Our findings suggest best practices for characterizing and specifying topography. The public release of the accumulated data and presented analyses enables global reuse for further scientific investigation and benchmarking.