Elsevier

Tribology International

Volume 92, December 2015, Pages 353-364
Tribology International

Mechanism of tribofilm formation with P and S containing ionic liquids

https://doi.org/10.1016/j.triboint.2015.07.009Get rights and content

Highlights

  • XANES analysis of tribofilms from ionic liquids.

  • Phenemenological model of tribofilms from P and S containing ionic liquids.

  • SPM and SEM study of morphology and properties of tribofilm.

Abstract

Mechanism of tribofilm formation with two ionic liquids (IL), choline bis(2-ethylhexyl)phosphate and choline dibutyldithiophosphate was studied. XANES analysis of tribofilms indicates that the underlying mechanism of tribofilm formation with ionic liquids is similar to that formed when ZDDP is used. The chain length of glassy polyphosphates with IL in base oil is longer in length in comparison to that formed with ZDDP under identical conditions indicating a higher level of networking. In fully formulated oils, Ca replaces Zn and Fe (in the case of ZDDP) or Fe (when IL׳s are used) as the primary cationic species present in the polyphosphate network. The sulfur is present in the form of sulfates of different cationic species including Fe and Ca.

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 (BF4) 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 Pdouble bondO bond has a dissociation energy of 544 kJ/mol while Pdouble bondS has a dissociation energy of 335 kJ/mol, hence it is easier for the Pdouble bondS 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)

  • L Pisarova et al.

    Insight into degradation of ammonium-based ionic liquids and comparison of tribological performance between selected intact and altered ionic liquid

    Tribol Int

    (2013)
  • AE Jiménez et al.

    1-N-alkyl -3-methylimidazolium ionic liquids as neat lubricants and lubricant additives in steel–aluminium contacts

    Wear

    (2006)
  • M Yao et al.

    Imidazolium hexafluorophosphate ionic liquids as high temperature lubricants for steel–steel contacts

    Wear

    (2010)
  • AH Battez et al.

    Tribological behaviour of two imidazolium ionic liquids as lubricant additives for steel/steel contacts

    Wear

    (2009)
  • H Kamimura et al.

    Effect and mechanism of additives for ionic liquids as new lubricants

    Tribol Int

    (2007)
  • B Yu et al.

    Oil-miscible and non-corrosive phosphonium-based ionic liquids as candidate lubricant additives

    Wear

    (2012)
  • J Qu et al.

    Comparison of an oil-miscible ionic liquid and ZDDP as a lubricant anti-wear additive

    Tribol Int

    (2014)
  • M Kasrai et al.

    Sampling depth of total electron and fluorescence measurements in Si L-and K-edge absorption spectroscopy

    Appl Surf Sci

    (1996)
  • P Behrens

    X-ray absorption spectroscopy in chemistry: II. X-ray absorption near edge structure

    TrAC Trends Anal Chem

    (1992)
  • P Behrens

    X-ray absorption spectroscopy in chemistry: I. Extended X-ray absorption fine structure

    TrAC Trends Anal Chem

    (1992)
  • V Sharma et al.

    An analytical study of tribofilms generated by the interaction of ashless antiwear additives with ZDDP using XANES and nano-indentation

    Tribol Int

    (2015)
  • X Chen et al.

    Synthesis and tribological behavior of ashless alkylphosphorofluoridothioates

    Tribol Int

    (2013)
  • B Kim et al.

    Properties of tribofilms formed with ashless dithiophosphate and zinc dialkyl dithiophosphate under extreme pressure conditions

    Wear

    (2010)
  • G Pereira et al.

    A variable temperature mechanical analysis of ZDDP-derived antiwear films formed on 52100 steel

    Wear

    (2007)
  • M Najman et al.

    Combination of ashless antiwear additives with metallic detergents: interactions with neutral and overbased calcium sulfonates

    Tribol Int

    (2006)
  • M Kasrai et al.

    Sampling depth of total electron and fluorescence measurements in Si L- and K-edge absorption spectroscopy

    Appl Surf Sci

    (1996)
  • R Mourhatch et al.

    Tribological behavior and nature of tribofilms generated from fluorinated ZDDP in comparison to ZDDP under extreme pressure conditions—Part 1: structure and chemistry of tribofilms

    Tribol Int

    (2011)
  • M Patel et al.

    Structure and chemistry of crankcase and cylinder soot and tribofilms on piston rings from a Mack T-12 dynamometer engine test

    Tribol Int

    (2014)
  • M Fuller et al.

    Chemical characterization of tribochemical and thermal films generated from neutral and basic ZDDPs using X-ray absorption spectroscopy

    Tribol Int

    (1997)
  • H Zhao

    Innovative applications of ionic liquids as green engineering liquids

    Chem Eng Commun

    (2006)
  • WL Hough et al.

    Ionic liquids then and now: from solvents to materials to active pharmaceutical ingredients

    Bull Chem Soc Jpn

    (2007)
  • T Torimoto et al.

    New frontiers in materials science opened by ionic liquids

    Adv Mater

    (2010)
  • M Bermúdez et al.

    Ionic liquids as advanced lubricant fluids

    Molecules

    (2009)
  • I Minami

    Ionic liquids in tribology

    Molecules

    (2009)
  • F Zhou et al.

    Ionic liquid lubricants: designed chemistry for engineering applications

    Chem Soc Rev

    (2009)
  • Cited by (0)

    View full text