Tribological behavior of ammonium-based protic ionic liquid as lubricant additive
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ISSN 2223-7690 CN 10-1237/TH
RESEARCH ARTICLE
Tribological behavior of ammonium-based protic ionic liquid as lubricant additive Hong GUO, Patricia IGLESIAS* Mechanical Engineering Department, Rochester Institute of Technology, New York 14623-5603, USA Received: 19 March 2019 / Revised: 11 June 2019 / Accepted: 28 February 2020
© The author(s) 2020. Abstract: In this study, the tribological behavior of an ammonium-based protic ionic liquid (PIL) as an additive in a base mineral oil (MO) is investigated on a steel–steel contact at room temperature and 100 °C. Tri-[bis(2-hydroxyethylammonium)] citrate (DCi) was synthesized in a simple and low-cost way, and the ionic structure of DCi was confirmed by proton nuclear magnetic resonance (1H NMR). The stability measurement of 1 wt% DCi to a MO was investigated, and the lubricating ability and anti-wear properties of DCi as an additive in MO were also examined using a custom-designed reciprocating ball-on-flat tribometer. Optical microscope and profilometry were used to obtain the worn morphology of the steel disks. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were carried out to investigate the wear mechanism and to analyze the surface interactions between the rubbing components. When 1 wt% DCi is added into the base MO, frictional performance is improved at both temperatures studied with a friction reduction of 29.0% and 35.5%, respectively. Moreover, the addition of 1 wt% DCi to MO reduced the wear volume 59.4% compared to the use of MO. An oxygen-richened tribolayer is confirmed by EDS on the disk surface when DCi was used as additive under 100 °C. Keywords: protic ionic liquid (PIL); tri-[bis(2-hydroxyethylammonium)] citrate (DCi); steel–steel; friction; wear
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Introduction
It is estimated that the energy consumption from tribological contacts accounts for 23% which is around 119 EJ of the world’s total energy consumption [1]. In order to reduce the energy losses and increase the energy efficiency of mechanical systems, one approach is to add additives, such as friction modifier additives or anti-wear additives, to the lubricant to obtain improved tribological performance [2]. Ionic liquids (ILs) have attracted attention of the research community as lubricants or lubricant additives since they been explored for lubrication in 2001 [3]. ILs are synthetic salts that consist of cations and anions with melting point below 100 °C or even lower. The most commonly used cations of ILs are ammonium [4, 5], imidazolium [6], pyridinium [7], or phosphonium [8, 9], where the * Corresponding author: Patricia IGLESIAS, E-mail: [email protected]
anions could be either organic or inorganic [10]. ILs have a number of unique properties which make them outperform conventional lubricants or lubricant additives, such as high temperature stability, low flammability, negligible volatility as well as high thermal conductivity [3, 11, 12]. Oil-miscible ILs are explored and reported with promising anti-wear properties in early 2012 [13, 14], and since the
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