Abstract
Fatty acids are natural products which have been studied as green lubricants. Ionic liquids are considered efficient friction reducing and wear preventing lubricants and lubricant additives. Fatty acid-derived ionic liquids have shown potential as neat lubricant and additives. Protic ionic liquid crystals (PILCs) are protic ionic liquids (PILs) where cations and anions form ordered mesophases that show liquid crystalline behavior. The adsorption of carboxylate units on sliding surfaces can enhance the lubricant performance. Ionic liquid crystal lubricants with longer alkyl chains can separate sliding surfaces more efficiently. However, they are usually solid at room temperature and, when used as additives in water, transitions to high friction coefficients and wear rates, with tribocorrosion processes occur when water evaporation takes place at the interface. In order to avoid these inconveniences, in the present work, a protic ammonium palmitate (DPA) ionic liquid crystal has been added in 1 wt.% proportion to a short chain citrate ionic liquid (DCi) with the same protic ammonium cation. A spin coated layer of (DCi + DPA) was deposited on AISI316L steel surface before the sliding test against sapphire ball. Synergy between DCi PIL and DPA PILC additive reduces friction coefficient and wear rate, without tribocorrosion processes, as shown by scanning electron microscopy (SEM)/energy dispersive X-ray microanalysis (EDX) and X-ray photoelectron spectroscopy (XPS) results.
Highlights
Fatty acids are an abundant natural resource, readily available from vegetable oils. Both fatty acids and some compounds derived from them could belong to the category of green lubricants
Gusain et al [2] proposed that the tribochemical reaction between the carboxylic groups present in fatty acid molecules form a boundary low shear layer on metal surfaces
We describe the use of a protic ammonium palmitic acid-derived protic ionic liquid crystals (PILCs)
Summary
Fatty acids are an abundant natural resource, readily available from vegetable oils. In this respect, both fatty acids and some compounds derived from them could belong to the category of green lubricants. Brønsted acids to ethanolamines [2,25,26] has made readily available the use of protic ionic liquid crystals (PILCs) with a variety of carboxylate anions, with promising tribological properties, both as neat lubricants and as additives [27,28,29] Both PILs and PILCs containing ammonium cations and carboxylate anions, have been used as additives in water, [28,29]. We describe the use of a protic ammonium palmitic acid-derived PILC (DPA) as additive in a short chain protic ammonium carboxylate PIL lubricant, with the same protic ammonium cation and a citrate anion (DCi). The main purpose of the present work, where DPA has been used as lubricant additive of DCi, is to study the feasibility of obtaining surface lubricant layers, without the need to use water as base lubricant, in order to increase stability, improve the tribological performance and avoid the corrosion processes associated to the use of water
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