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Carboxylic acids are well known for their friction-reducing abilities driven by the formation of low-shearstrength films on the steel surface. However, understanding of the adsorption mechanisms especially in polar solvents is yet not well explored. In this work Atomic Force Microscopy (AFM) was used to visualise the adsorption behaviour of various carboxylic acids in both polar and less-polar solvents. The work was continued with a tribological study of the lubricants additivated with carboxylic acids in a laboratory scale ball-on-disc tribometer. During this study, the effect of concentration and carboxylic acid chain length was studied in polar media (water-based lubricants) and compared with commonly used synthetic non-polar lubricant (poly-α-olefin, PAO). It was observed that for both polar and less-polar lubricants, surface coverage of carboxylic acids increased with increasing length of hydrocarbon tail. In less-polar lubricants carboxylic acids adsorbed to the surface by spreading on it evenly, whereas in polar lubricants very dense multi-layered formation was promoted. Friction reduction achieved with the use of carboxylic acids in the non-polar lubricant was not as efficient as in the case of the polar lubricant. This was associated with the more pronounced multilayer formation of carboxylic acids in the polar lubricants, facilitating higher friction reduction as compared to the adsorption of carboxylic acids in a dense monolayer form seen in the lesspolar lubricant.

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Self-lubricated coatings have been a major topic of interest in thermal spray in the last decades. Self-lubricated coatings obtained by thermal spray are exclusively based on solid lubricants (PTFE, h-BN, graphite, MoS 2 , etc.) embedded in the matrix. Production of thermal spray coatings containing liquid lubricants has not yet been achieved because of the complexity of keeping a liquid in a solid matrix during the spraying process. In the present article, the first liquid-solid self-lubricating thermal spray coatings are presented. The coatings are produced by inserting lubricant-filled capsules inside a polymeric matrix. The goal of the coating is to release lubricant to the system when needed. The first produced coatings consisted solely of capsules for confirming the feasibility of the process. For obtaining such a coating, the liquid-filled capsules were injected in the thermal spray flame without any other feedstock material. Once the concept and the idea were proven, a polymer was co-sprayed together with the capsules to obtain a coating containing the lubricant-filled capsules distributed in the solid polymeric matrix. The coatings and the self-lubricated properties have been investigated by means of optical microscopy, Scanning Electron Microscopy, and tribological tests.

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