Influence of Water on Tribolayer Growth When Lubricating Steel with a Fluorinated Phosphonium Dicyanamide Ionic Liquid

Luigi Urtis,Daniel Rauber,Peng Zhang,Antonella Rossi,Nicholas Spencer,Andrea Arcifa,Junxiao Du,Tobias Kraus,Marzia Fantauzzi,Rolf Hempelmann

doi:10.3390/lubricants7030027

Abstract

This work aims to elucidate the role of environmental humidity on the tribological behavior of steel surfaces lubricated with an ionic liquid comprised of a fluorinated phosphonium cation—tributyl-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl-phosphonium—and a dicyanamide anion (i.e. N(CN)2−). Ball-on-disk tribotests were carried out at room temperature and at various levels of relative humidity (RH). Water was found to be required to promote the formation of a tribofilm over the contact area. The reaction layer exhibited a patchy morphology, which resembles that observed formed with conventional antiwear additives such as ZnDTP. A surface-chemical analysis of the tribofilm indicated that the tribofilm is composed of fluorides, oxides, and phosphates, pointing to a stress-induced degradation of the ions and corrosion of the sliding counterparts, which is enabled by the presence of water at the sliding interface.

Highlights

Ionic liquids (ILs) are salts with melting points lower than 100 ◦ C, generally comprised of large, asymmetric organic ions
We investigate the tribological behavior of self-mated 100Cr6 steel lubricated with a hydrophobic fluorinated IL—tributyl-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl-phosphonium dicyanamide (F6)
A patchy tribofilm was observed over the contact area of samples tribostressed in the presence of humid air (Figure 2a)

Summary

Introduction

Ionic liquids (ILs) are salts with melting points lower than 100 ◦ C, generally comprised of large, asymmetric organic ions. The promising tribological performance reported in the first publications on IL-mediated lubrication triggered a significant interest among various research groups, as demonstrated by the number of scientific papers available on the topic [2,3,4,5] These tribological investigations are often complemented by the surface-chemical analysis of the tribostressed counterparts, so as to characterize the formation of Lubricants 2019, 7, 27; doi:10.3390/lubricants7030027 www.mdpi.com/journal/lubricants. It is often not clear to what degree the presence of these films may affect friction or reduce wear In this respect, it should be noted that the potential impact of contaminants deriving from the exposure of the IL to air during sliding of IL-lubricated counterparts has been scarcely investigated. This is important in the case of steels, due to the corrosivity of several ILs toward these materials [9,10,11,12,13]

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