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Formation of Boundary Film from Ionic Liquids Enhanced by Additives
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0002-0851-8475
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.ORCID iD: 0000-0003-4582-0902
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.ORCID iD: 0000-0002-4755-5754
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0002-8972-2944
2017 (English)In: Applied Sciences, E-ISSN 2076-3417, Vol. 7, no 5, article id 433Article in journal (Refereed) Published
Abstract [en]

Room temperature ionic liquids (RTILs) have several properties that make them interesting candidates as base fluids for extreme conditions. However, a lack of compatibility with tribo-improving additives combined with an often overly aggressive nature is limiting their use as base fluids. To overcome these drawbacks, hydrocarbon-imitating RTIL base fluids have recently been developed. In this study, the effects of several common additives in the novel RTIL (P-SiSO) were examined by laboratory tribotesting. A reciprocating steel-steel ball-on-flat setup in an air atmosphere was used, where the lubricant performance was evaluated over a range of loads and temperatures. Surface analyses after testing were carried out using optical profilometry, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Neat P-SiSO displayed high performance in the tribotests. At an elevated load and temperature, a shift in lubrication mode was observed with an accompanying increase in friction and wear. Surface analysis revealed a boundary film rich in Si and O in the primary lubrication mode, while P was detected after a shift to the secondary lubrication mode. An amine additive was effective in reducing wear and friction under harsh conditions. The amine was determined to increase formation of the protective Si–O film, presumably by enhancing the anion activity.

Place, publisher, year, edition, pages
Basel: MDPI, 2017. Vol. 7, no 5, article id 433
Keywords [sv]
gränsskiktssmörjning, jonvätska, friktion, nötning, tribologi
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear) Chemical Process Engineering
Research subject
Machine Elements; Chemical Technology
Identifiers
URN: urn:nbn:se:ltu:diva-63211DOI: 10.3390/app7050433ISI: 000404449000002Scopus ID: 2-s2.0-85018920929OAI: oai:DiVA.org:ltu-63211DiVA, id: diva2:1092368
Note

Validerad; 2017; Nivå 2; 2017-05-10 (rokbeg)

Available from: 2017-05-02 Created: 2017-05-02 Last updated: 2023-09-05Bibliographically approved
In thesis
1. Lubrication mechanism of hydrocarbon-mimicking ionic liquids
Open this publication in new window or tab >>Lubrication mechanism of hydrocarbon-mimicking ionic liquids
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Lubrication is critical in order to achieve high efficiency and reliability of machine elements such as gears, bearings, and other moving mechanical assemblies (MMA). In space applications, tribological properties of lubricants are quickly growing more important. Traditional space systems such as satellites imply MMA such as gyroscopes, antenna pointing mechanisms, and solar array drives. These MMA operate in high vacuum (<10-5 Pa) under lightly loaded conditions. Modern space missions on the other hand, such as remotely operated vehicles used for in-situ Mars exploration relies on different types of MMA. In these robotic systems, electromechanical actuators are being used extensively to provide controlled motion. Gears and bearings in these actuators operate in an atmosphere mainly consisting of CO2 at ~10+3 Pa under heavily loaded contact conditions. In these conditions, the tribosystem is likely to operate in the boundary lubricated regime, with consequent risk of high friction and wear.

High molecular weight fluids have significant heritage in space because of their low vapor pressure. They are currently employed as lubricants in a wide range of space applications, as they meet high demands on resistance to vacuum outgassing. Unfortunately, the large molecules are susceptible to degradation under heavy load.

Ionic liquids (ILs) on the other hand, are synthetic fluids that consist entirely of ion pairs with opposing charge. The resulting ion bonds enable inherently low vapor pressure of the fluid without the need for a high molecular weight. For this reason ILs have been advocated as potential lubricants for space applications, but so far compatibility issues have hampered their use as lubricants. Countless IL variations are possible, and solutions are thus likely to exist. Constituent ions can be designed individually and combined in various configurations. However, the fundamental understanding of the lubricating mechanism of ionic liquids is still incomplete, and consequently the optimum molecular structure for IL lubricants remain unknown.

In this thesis, a stepwise approach to molecular design of IL lubricants is described, and the resulting hydrocarbon-mimicking ionic liquids are evaluated in tribological experiments. In this thesis, the experiments focus on tribological performance, using steel-steel tribopairs in air environment under boundary lubrication (Paper I). Boundary film formation under a range of contact pressures and temperatures, is analyzed after tribotesting by optical profilometry, scanning electron microscopy (SEM), and energy dispersive X- iii ray spectroscopy (EDS) in Paper II. The analysis reveal formation of a highly effective boundary film based on silicate, that can be further enhanced by amine additives. This thesis demonstrates the feasibility of improving tribological performance of ionic liquids by molecular design.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
P-SiSO, boundary film, silicate, friction, wear, anti-wear, friction modifier, base fluid
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-65505 (URN)978-91-7583-953-0 (ISBN)978-91-7583-954-7 (ISBN)
Presentation
2017-11-02, E231, Luleå, 09:00 (English)
Supervisors
Projects
Projekt: Rymdforskarskolan 2015
Available from: 2017-09-06 Created: 2017-09-05 Last updated: 2018-01-13Bibliographically approved
2. Ionic Liquid Lubricants for Space Applications
Open this publication in new window or tab >>Ionic Liquid Lubricants for Space Applications
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lubrication is critical to the efficient and reliable operation of machine elements such as gears, bearings, or any other moving mechanical assembly (MMA). On Earth, machine designers are accustomed to the access of a wide range of liquid lubricants that enable predictable and reliable long-term operations of high performance MMA. In space applications on the other hand, engineers are constrained to a comparatively limited choice of lubricant candidates that can meet the stringent demands of tribosystems operating in a space environment. At the same time, repair or maintenance are seldom options that are possible in space, and consequently lubricant failures are potentially critical. As international space agencies are converging on the goal of establishing a permanently crewed lunar Gateway for human presence on the Moon and eventually onwards to Mars, there is a need for radical improvements in many aspects of space exploration technology, including space tribology and space grade lubricants.  

Liquid lubricants are enablers of high performance. A thin fluid film – even in the submicron scale – is often sufficient to separate opposing surface boundaries from direct contact, and thereby prevent excessive friction and wear. Liquid lubricants are therefore attractive for use in space mechanisms. Unfortunately, liquid lubricants must overcome several issues in order to be effective in the space environment. Vacuum, microgravity, and low temperatures are all factors that oppose the effective supply of liquid lubricants into the tribological contact of MMA. If the tribological contact becomes starved of oil, the surfaces enter the boundary lubrication regime where seizure is an ever-present threat. 

There are very few types of fluids available that meet the stringent space grade lubricant requirements. Perfluoropolyalkylethers (PFPE), or multiply alkylated cyclopentanes (MAC) are two fluids with significant heritage in space applications. These fluids are currently employed as lubricants in a wide range of space applications, as they are rare examples of fluids that meet the high demands on resistance to vacuum outgassing. Unfortunately, these compounds are susceptible to degradation under boundary lubrication conditions, and unlike conventional lubricants employed on Earth, these fluids have poor compatibility with the boundary lubrication additives that are commonly employed in conventional oils. 

Ionic liquids (ILs) have emerged as potential liquid lubricant candidates in space. These synthetic fluids are composed of anions and cations. The resulting ionic interaction enables the substance to have low vapor pressure with relatively low molecular weight. For this reason, ILs have been advocated as one of the candidate lubricants for space applications. When employing ILs as lubricants, the ionic charge provides Coulombic interaction with surfaces to enable the formation of a boundary lubricating film. This is an important part of the IL lubricating mechanism, but successful lubricant performance requires integrating the lubricant candidate into the tribosystem, taking into account operating conditions and environment. Therefore, the boundary film formation should be tunable to the application at hand. Ionic liquids are designable fluids, with properties dependent on the combination of anion and cation as well as incorporated functional groups. 

Based on this background, this work focused on evaluating the feasibility of employing ionic liquid lubricants for space applications. In this thesis, the molecular design of an IL lubricant was described Paper [1], and the resulting hydrocarbon-mimicking ionic liquid (P-SiSO) was evaluated in tribological experiments in boundary lubricated conditions. Boundary film formation by neat P-SiSO was studied  in Paper [2], and in Paper [3] we describe the use of P-SiSO as a multipurpose performance ingredient in MAC. A test methodology was devised in Paper [4] in order to evaluate the lubrication performance under component scale experiments in space relevant conditions. The designed ionic liquid lubricant was evaluated in Paper [5] by the specific methodology. Advanced surface analysis was employed to understand the tribo-mechanism of P-SiSO in both the model scale experiments as well as the component scale. The lubricated surfaces were analyzed in terms of surface topography- and chemistry, and mechanisms of lubrication are discussed. A highly effective boundary film based on ionic adsorption and formation of silicate was observed by these ionic liquids. This thesis demonstrates the feasibility of employing ionic liquids for lubrication of moving mechanical assemblies in space applications. 

Place, publisher, year, edition, pages
Luleå, SE: Luleå University of Technology, 2021. p. 60
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-82223 (URN)978-91-7790-747-3 (ISBN)978-91-7790-748-0 (ISBN)
Public defence
2021-02-23, A-109, Luleå University of Technology, Lulea, 09:00 (English)
Opponent
Supervisors
Projects
Rymdforskarskolan
Available from: 2021-01-11 Created: 2021-01-08 Last updated: 2021-03-26Bibliographically approved

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Nyberg, ErikMouzon, JohanneGrahn, MattiasMinami, Ichiro

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