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New alkylether-thiazolium room-temperature ionic liquid lubricants. Surface interactions and tribological performance Tulia Espinosa, José Sanes, and María-Dolores Bermúdez ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05888 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016
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ACS Applied Materials & Interfaces
New alkylether-thiazolium room-temperature ionic liquid lubricants. Surface interactions and tribological performance Tulia Espinosa, José Sanes, María Dolores Bermúdez* Departamento de Ingeniería de Materiales y Fabricación, Universidad Politécnica de Cartagena, 30202-Cartagena, Spain.
[email protected] Abstract The use of newly synthesized alkylether-thiazolium ionic liquids as lubricants is described for the first time. Two ionic liquids composed of a thiazolium cation and a bis(trifluoromethanesulfonyl)amide ([Th][Tf2N]) or dicyanamide ([Th][(NC)2N]) anion have been studied and their tribological behaviour has been compared with that of 1butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Im][Tf2N]) in pin-ondisk tests using sapphire balls against AISI 52100 or AISI 316L steels. All ionic liquids show higher contact angles on AISI 316L steel than on AISI 52100, the lowest values found for ([Im][Tf2N]) on both steel surfaces. AISI 52100 shows similar friction coefficients for all lubricants, and negligible wear rates for the ionic liquids containing the bis(trifluoromethanesulfonyl)amide anion. Immersion tests show no corrosion of AISI 52100 in imidazolium or thiazolium bis(trifluoromethanesulfonyl)amide ionic liquids.
AISI
316L
shows
similar
friction
coefficients
for
both
bis(trifluoromethanesulfonyl)amide ionic liquids, but the lowest wear rate is obtained for [Th][Tf2N]. An increase in friction coefficient and wear rate is observed for thiazolium dicyanamide. This increase is related to a tribocorrosion process due to decomposition of the thiazolium cation. XPS show the formation of iron sulfide on the wear track on AISI 316L after lubrication with thiazolium dicyanamide. No tribocorrosion processes take place for the [Tf2N] ionic liquids.
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Keywords: Thiazolium and imidazolium ionic liquids, contact angles, friction, wear, tribocorrosion, surface analysis, SEM, XPS.
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1. INTRODUCTION The present interest in room temperature ionic liquids (ILs), stable ionic fluids in a wide range of temperatures, is due to a variety of properties from their high thermal stability, wide electrochemical window, high conductivity and negligible volatility, to nonflammability, which have open up an increasing number of applications, from conventional solvents and catalysts to lubricants and thermal fluids. In the field of surface science and engineering, ionic liquids have shown outstanding potential as lubricants and lubricant additives1-16, even in the friction and wear reduction of highly reactive light alloys17-20, polymers21-26, and also as nanophase modifiers23,24, electrolytes27-29, corrosion inhibitors27-36 or promoters of surface coatings37-40. Imidazolium bis(trifluoromethanesulfonyl)amide ILs41-46 have been studied as lubricants or lubricant additives in the search for less corrosive substitutes of tetrafluoroborate and hexafluorophosphate derivatives, which are more hydrophilic and susceptible to hydrolysis in the presence of moisture. Thiazolium ILs46,47, where an imidazole nitrogen is replaced by sulphur, have been used as catalysts in various reactions. Thiazolium-type poly(ionic liquids) bearing a bis(trifluoromethanesulfonyl)amide anion have been used as high performance binder for lithium-ion battery electrodes47. Imidazolium and pyrrolidinium ILs with the dicyanamide anion have also been described48-50. The use of alkylthiazolium, in particular, octylthiazolium bis(trifluoromethanesulfonyl)amide in steel-steel lubrication was reported by Schneider et al.52. Thiazolium-based ionic liquids having different oxyethylene side chains have only very recently been synthesized51. The ionic liquids used in the present study belong to this new family, and this is the first report of their use in tribology. Preliminary results were recently presented at a conference53.
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The thiazolium cation in the ILs which are the subject of this study contain a moiety which is also present in biochemical molecules, in particular in thiamine (vitamin B1). Thiazolium cations have a slightly larger size, a lower polarity, and are relatively less moisture sensitive than the corresponding imidazolium ones. Regarding the anions, ILs containing the bis(trifluoromethanesulfonyl)amide anion are hydrophobic and present high thermal, chemical and electrochemical stabilities. ILs with the smaller size dicyanamide anion54 which decreases viscosity, can coordinate to transition metal atoms, is a hydrogen-bond acceptor55 and has a stronger basicity than the bis(trifluoromethanesulfonyl)amide anion56. The main purposes of the present work are to determine the tribological performance and tribochemical interactions of the new thiazolium-based ionic liquids as a function of the
anion,
comparing
the
behaviour
of
the
fluorine-containing
bis(trifluoromethylsulfonyl)amide with the halide-free dicyanamide, and of the cation, comparing thiazolium and imidazolium ILs. The sliding materials were selected to study wear mechanisms and tribocorrosion processes for steels with different wear resistance and corrosion susceptibility.
2. EXPERIMENTAL SECTION Newly synthesized thiazolium ILs [Th][Tf2N] and [Th][(NC)2N] were kindly provided by I. de Miguel et al. (Instituto de Química Orgánica General,CSIC, Madrid)51, and used as received.1-butyl-3-methylmidazolium IL [Im][Tf2N] (table 1) (purity >98%) was purchased from Solvionic (France) and used as received. Viscosities (table 1) were determined with an AR-G2 rheometer (TA Instruments). Degradation temperatures were determined with a TGA 1HT (Mettler-Toledo). Contact angles (table 2) were measured using the Drop Shape Analyzer DSA30B (Krüss,
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Germany). The materials used in tribological tests were sapphire balls (Al2O3 99.9%; hardness: 2750 HV) with 1.5 mm sphere radius, AISI 52100 steel (nominal composition in weight percentage: 0.95-1.05C; 1.30-1.65Cr; 0.25-0.45Mn; 0.15-0.35Si;
