Investigation of tribological properties of two protic ionic liquids as additives in water for steel–steel and alumina–steel contacts

Highlights

• Investigated PILs possess good water solubility and low copper corrosiveness.

• t-oclylamine oleate and diethanolamine oleate greatly improved lubricity of water.

• Friction pair materials did not significantly affect PIL additive performance.

Abstract

Water-based lubricants are attractive owing to their low cost and eco-friendliness. An appropriate additive package must be considered to use water as a lubricant. Recently, protic ionic liquids (PILs) were emphasized for improving the lubricity of water. Most PILs are nontoxic and biodegradable. In this study, tert-octylamine oleate and diethanolamine oleate PILs were investigated by using them as additives (1% wt.) in water. Their lubrication properties were tested on a ball-on-plate reciprocating tribometer using steel-steel and alumina-steel friction pairs. The investigated PILs improved water's friction and wear reduction ability up to 80%. Both friction pairs performed similarly. However, the alumina-steel friction pair exhibited a more stable friction. The adsorption layer was assumed the most likely friction and wear reduction mechanism.

Introduction

The increasing demand for green, environmentally friendly lubricants has resulted in the intensive investigation of newly formulated lubricants and effective friction-reducing, anti-wear additives. Water is considered a low-cost lubricant of high cooling capacity and low compressibility. Moreover, it is readily available, nonflammable, and environmentally friendly. However, it exhibits poor lubricity, low viscosity, low boiling point, high melting point, and metal corrosiveness, thereby rendering it unacceptable for most tribological applications [1]. With the appropriate additive package, water-based lubricants are widely used in many industrial applications such as metalworking fluids (metal processing, machining, and cutting operations) and cooling processes. However, to be considered as a green lubricant, both the base fluid and additives must be environmentally friendly. Therefore, new eco-friendly additives are desired. Recent studies focus on nanoparticles [[2], [3], [4], [5]] and ionic liquids [[6], [7], [8], [9]] as water additives. Some of them are considered environmentally friendly.

Recently, protic ionic liquids (PILs) have been emphasized for lubrication purposes owing to their excellent tribological properties and environmental friendliness. Most PILs are composed of C, H, N, and O elements; they are halogen-free and do not contain unwanted species such as S, P, or Zn. PILs are formed by combining Brønsted acid and Brønsted base during a relatively simple neutralization reaction, as discovered by Walden in 1914 [10], who investigated ethyl ammonium nitrate PILs in aqueous solutions. However, PILs as lubricity-improving additives for water have been studied only recently. Espinosa et al. [11] investigated triprotic bis(2-hydroxyethylammonium) succinate PILs as a neat lubricant and additive in water. They discovered that bis(2-hydroxyethylammonium) succinate corresponded to ultralow friction and reduction in running-in period owing to boundary film formation. Aviles et al. [9] reported the study of di- and triprotic ammonium ionic liquids derived from stearic and palmitic fatty acids as lubricity-improving additives in water. It was discovered that the application of the investigated PILs in water could improve the friction coefficient up to 80%. A recent study by Carrion et al. [8] exhibited a synergistic effect between diprotic bis(2-hydroxyethyl)ammonium palmitate ionic liquid and nanodiamonds by using them as additives in water. In their study, the investigated additives reduced the wear rates by more than two orders of magnitude compared with water.

PILs do not contain heteroatoms; therefore, their reactivity with the surface is limited. Most studies reported the adsorption layer and wear surface oxidation as primary friction and wear reduction mechanisms. Excellent tribological properties of PIL were reported by Shi and Larsson [12]; however, no changes in wear surface composition was observed using EDX and XPS analysis. Song et al. [13] reported the adsorption layer as the primary wear and friction reduction mechanism of tetrabutylammonium-based amino acid PILs. Our previous study demonstrated no substantial change in the PILs lubricated surface composition [14]. In one of the studies, it was discovered that using PILs as an additive in water, wear surfaces become rich in oxygen, carbon, and nitrogen, which corresponded to the formation of metal oxides, hydroxides, and metal-ammonium complexes, respectively [11]. In a later study by Aviles et al. [9], it was shown that wear, friction, and wear surface roughness were related to wear trace composition. Water-lubricated surfaces, which suffers from high wear and friction, are rich in oxygen. With improved lubricity, the wear surface undergoes less composition and morphology changes.

In our latest study [14], we demonstrated the excellent lubrication ability of ammonium-based oleate PILs when used as neat lubricants. In our current study, we present the results of using two ammonium-based oleate PILs as additives in water. They were used to lubricate steel–steel and alumina–steel friction pairs. The surface interaction and friction mechanisms are discussed in detail.