Mechanism of tribofilm formation with P and S containing ionic liquids
Graphical abstract
Introduction
Ionic liquids offer greater flexibility for obtaining task specific properties by using a particular set of cation and anion, hence are available in many combinations for different applications. Ionic liquids have been used as electrolytes in batteries, supercritical fluids, heat transfer fluids, active pharmaceutical ingredients, solvents for chemical synthesis, engineering fluids etc. [1], [2], [3], [4], [5] and are still finding new applications, one of which is as lubricants in tribology [6], [7], [8]. With the growing interest for the use of environment friendly ashless lubricant additives in engine and motor oils, ionic liquids are a possible alternative to traditional additives. Ionic liquids are synthetic salts with a melting point below 100 °C. A commonly used term in tribology is room temperature ionic liquids (RT-IL׳s) that have a melting point at or below room temperatures. The Ionic liquid of interest for lubrication contains a cationic and anionic species of which one or both are organic. Either cation or anion or both has/have delocalized charge, which prevents the formation of a stable crystal, resulting in a poor co-ordination of these ions. Hence these IL compounds are liquid at room temperatures [9], [10].
Ionic liquids have negligible vapor pressure, high polarity, high thermal stability, non-flammability, non-volatility, miscibility with water and with organic solvents and electrochemical properties that are highly desirable for tribological applications. Minami [7] reviewed various ionic liquids for their tribological properties and proposed a relationship between the chemical structure of IL׳s and their lubricant properties.
Since the first article on the use of RT-IL׳s as a lubricant was published in 2001 [11] a number of research studies have focused on finding promising ionic liquid structures to meet the required tribological properties as well as to understand the underlying mechanism of tribofilms formation [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. In tribology, most commonly studied IL׳s include cation׳s such as imidazolium, ammonium/aromatic amine and phosphonium and anions those are, tetrafluoroborate, hexafluorophosphate, sulfonates and bis (fluoroalkylsulfonyl)-amides [7], [14], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33].
Jimenéz et al. [30] studied that the wear surfaces generated using 1-N-alkyl-3-methylimidazolium IL as lubricant as well as lubricant additive (1 wt%) using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) in steel–aluminum contacts and reported the formation of tribolayers on steel balls containing aluminum and phosphorous. X-ray diffraction was also used to detect the formation of boron oxide over the wear track generated using 1-octyl,-3-methylimidazolium tetrafluoroborate IL on steel–titanium contact [20]. XPS has been used most commonly to investigate the chemical states of critical elements of IL derived tribofilms. Liu et al. [19] investigated wear mechanism of phosphonium based IL׳s using XPS and identified formation AlF3, Al2O3, AlO(OH), Al(OH)3 as well as AlF3,B2O3 and AlPO4 under the tribo-chemical reaction of anions with the fresh surface. Kamimura et al. [34] used XPS and time of flight-secondary ion mass spectroscopy (TOF-SIMS) to study the chemistry of worn surfaces derived from imidazolium, pyridinium, ammonium (cation) and tetrafluoroborate and bis(trifluoromethanesulfonyl)imide (TFSI−) (anion) based IL׳s and detected organic fluoride, iron fluoride and iron sulfates over the worn surfaces. Their study revealed that elements derived from the anion moiety of IL׳s are present over worn surfaces in the form of organic fluoride, iron fluoride and iron sulfates suggesting that adsorption of the anionic moiety took place followed by the tribo-chemical reaction. In another study, Minami et al. [24] studied the tribo-chemistry of phosphonium derived IL׳s using Auger electron spectroscopy (AES) and XPS surface analytical techniques. Boundary film composed of phosphate and fluoride structure was identified. They also suggested that formation of phosphate film inhibited the reaction of the bis(trifluoromethanesulfonyl)imide anion that yielded metal fluoride on the rubbed surfaces. Their study revealed that phosphate boundary film exhibited better tribological properties than those of fluoride boundary film. Gabler et al. [27] analyzed the tribolayer chemistry using XPS depth profiling as well as XPS imaging techniques for bis(trifluoromethanelsulfonyl)imide ionic liquid with various cationic moieties. In their study, it was found that no measurable chemical modification occurred on the structure of cationic moiety during the tribotest, suggesting cation׳s were not involved in the formation of the tribolayer. However, it was observed that depending on the presence of cation, degradation of anionic moiety varied in the order of phosphonium>imidazolium>pyrrolidinium>sulfonium>ammonium. Similarly, in a recent study [35] influence of cationic structure on physiochemical and lubrication properties of IL׳s has been studied. It was reported that larger cation׳s are more soluble with six carbons per alkyl chain a critical minimum for oil miscibility (found in their study). Besides it was also reported that symmetric cation׳s show better tribological outcomes in comparison to asymmetric cation׳s with identical anion. Qu et al. [36] studied the nanostructure, film thickness and compositional change of boundary film on IL lubricated metallic surfaces using cross sectional TEM coupled with EDS and compositional depth profiling by XPS. The measured mean film thickness on cast iron, steel and aluminum worn surfaces was 300, 60 and 200 nm respectively. TEM analysis of boundary film on ferrous alloys revealed very fine nano crystal structure well dispersed in amorphous phase matrix while the film on aluminum comprises many larger size (tens of nm) metallic particles in less orderly manner. Recently, Qu et al. [37], [38] studied oil miscible phosphonium–phosphate ionic liquids (PP-IL) as lubricant additives in base oil as well as in fully formulated oil and showed improved wear performance in the presence of ionic liquids. Qu et al. [39] also showed comparable or even superior antiwear and anti-scuffing properties of phosphonium IL in comparison with the conventional additive i.e. ZDDP in PAO base oils at 0.1% Phosphorous.
In this study, two ionic liquids, choline bis(2-ethylhexyl) phosphate (IL-P) and choline dibutyl dithiophosphate ( IL-TP), are studied as lubricant additives in group1 mineral base oil (BO) and fully formulated oils with no zinc and no phosphorous (FFO). Tribological behavior of these IL׳s was evaluated using a cylinder on reciprocating flat Schwing–Reib–Verschleiss (SRV®) tribotester (Optimol Instruments Prüftechnik GmbH, Munich, Germany). Tribological behavior of IL׳s as lubricant additives is compared with zinc dialkyl dithiophosphate (ZDDP) at equal phosphorus level (0.1% P) in BO and FFO. Surface morphology of the worn surfaces was studied using scanning electron microscopy (SEM) and 3D profile of wear track was acquired using scanning probe microscopy (SPM). Chemical nature of the tribofilms was characterized using X-ray absorption near edge structure spectroscopy (XANES). A simple phenomenological model was developed to explain the formation of tribofilms from ionic liquids in comparison to how they form with ZDDP.
Section snippets
Chemistry of antiwear additives
Table 1 shows the details of the chemical structure of the antiwear additives used in this study. Neutral −100 base oil and zinc dialkyl dithiophosphate (ZDDP) were purchased from commercial vendors. The ZDDP used in this study is a secondary alcohol derived ZDDP with approximately 70% neutral and 30% basic characteristics. Ionic liquids are provided by AC2T Research GmbH.
Thermogravimetric analysis of additives
Thermogravimetric analysis of the ionic liquids was performed using a Shimadzu TGA-51 Thermogravimetric analyzer. TGA was
Thermogravimetric analysis of ionic liquids
Fig. 1 is a plot showing weight loss as a function of temperature for IL-P and IL-TP. The onset of decomposition for IL-P was approximated to be about 258 °C whereas onset decomposition temperature of IL-TP was recorded to be about 172 °C. The PO bond has a dissociation energy of 544 kJ/mol while PS has a dissociation energy of 335 kJ/mol, hence it is easier for the PS linkage to be the point of initial thermal decomposition in IL-TP leading to a lower decomposition temperature in comparison to
Discussion of XANES and development of phenomenological model of tribofilms
In depth characterization of IL and ZDDP lubricated rubbed surfaces using XANES at different edges for the critical elements such as P, S and O provides a comprehensive understanding of the chemical nature of the tribofilms. A possible phenomenological model of tribofilms thus can be drawn for all six formulations as shown in Fig. 9. Based on the information obtained from P L edge and P K edge, it can be deduced that both IL-P BO and IL-TP BO tribofilms consist of a layered structure of
Conclusions
The tribofilms formed using phosphorous and sulfur containing IL׳s are very similar to those formed when ZDDP antiwear additives are used. Zn polyphosphates formed when ZDDP is used are replaced with iron polyphosphates when IL׳s are used indicating that the underlying substrate reacts with the degradation products of the ionic liquids to form protective tribofilms in base oils. Pad like tribofilms are formed on the wear surface in fully formulated oils where as in the case of base oil
Acknowledgments
XANES studies described in this paper was performed at the Canadian Light Source, which is supported by the Canadian Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. TGA studies were conducted at the Shimadzu User facility. SPM imaging and SEM studies were conducted at
References (70)
- et al.
A review of ionic liquids towards supercritical fluid applications
J Supercrit Fluids
(2007) Properties of ionic liquid solvents for catalysis
J Mol Catal A: Chem
(2004)- et al.
AFM-based nanotribological and electrical characterization of ultrathin wear-resistant ionic liquid films
J Colloid Interface Sci
(2008) - et al.
Room temperature ionic liquid 1-ethyl-3-hexylimidazolium-bis(trifluoromethylsulfonyl)-imide as lubricant for steel–steel contact
Tribol Int
(2004) - et al.
Friction and wear behaviors of ionic liquid of alkylimidazolium hexafluorophosphates as lubricants for steel/steel contact
Wear
(2004) - et al.
Comparison of the tribological behavior of steel–steel and Si3N4–steel contacts in lubricants with ZDDP or ionic liquid
Wear
(2014) - et al.
Friction and anti-wear properties of two tris(pentafluoroethyl)trifluorophosphate ionic liquids as neat lubricants
Tribol Int
(2014) - et al.
Tribological performance of phosphonium based ionic liquids for an aluminum-on-steel system and opinions on lubrication mechanism
Wear
(2006) - et al.
Thermo-oxidative stability and corrosion properties of ammonium based ionic liquids
Tribol Int
(2012) - et al.
Influence of cationic moieties on the tribolayer constitution shown for bis(trifluoromethylsulfonyl)imide based ionic liquids studied by X-ray photoelectron spectroscopy
Tribol Int
(2014)