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Lubrication mechanism of hydrocarbon-mimicking ionic liquids
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. (Machine Elements)ORCID iD: 0000-0002-0851-8475
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 [en]
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: urn:nbn:se:ltu:diva-65505ISBN: 978-91-7583-953-0 (print)ISBN: 978-91-7583-954-7 (electronic)OAI: oai:DiVA.org:ltu-65505DiVA, id: diva2:1138567
Presentation
2017-11-02, E231, Luleå, 09:00 (English)
Supervisors
Projects
Projekt: Rymdforskarskolan 2015Available from: 2017-09-06 Created: 2017-09-05 Last updated: 2018-01-13Bibliographically approved
List of papers
1. Molecular design of advanced lubricant base fluids: hydrocarbon-mimicking ionic liquids
Open this publication in new window or tab >>Molecular design of advanced lubricant base fluids: hydrocarbon-mimicking ionic liquids
2017 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 7, no 11, p. 6364-6373Article in journal (Refereed) Published
Abstract [en]

This paper describes the molecular design and tribological evaluation of novel room-temperature ionic liquid (RTIL) lubricants{,} abbreviated as P-SiSOs. The RTILs are designed to mimic hydrocarbons{,} in order to ensure their compatibility with existing tribosystems as well as enable use of conventional additives. Steel-on-steel ball-on-flat reciprocating tribotests performed under atmospheric conditions show that the neat P-SiSOs exhibit favorable performances{,} resulting in friction and wear significantly lower than those in the case of the perfluoropolyether lubricants used as references. Tribotests performed at elevated loads and temperatures indicate the formation of friction-reducing boundary films of the neat P-SiSOs. The tribological performance of the P-SiSO is improved further by the incorporation of additives conventionally used in hydrocarbon oils. When used in a concentration of 5 wt%{,} the additives glycerol monooleate{,} dibenzyl disulfide{,} and oleylamine improve the tribological characteristics of P-SiSO. These results indicate that molecular-designed hydrocarbon-mimicking RTIL lubricants can exhibit suitable performances in the neat form and that their performances can be improved further by using conventional additives{,} as in the case of hydrocarbon base oil-additive systems.

Place, publisher, year, edition, pages
RSC Publishing, 2017
Keywords
additive compatibility, synthetic lubricant, friction modification, wear prevention, tetraalkylphosphonium, RTIL
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-61571 (URN)10.1039/C6RA27065D (DOI)000393755100024 ()2-s2.0-85010341504 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-02-07 (andbra)

Available from: 2017-01-21 Created: 2017-01-21 Last updated: 2018-07-10Bibliographically approved
2. Formation of Boundary Film from Ionic Liquids Enhanced by Additives
Open this publication in new window or tab >>Formation of Boundary Film from Ionic Liquids Enhanced by Additives
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
MDPI AG, 2017
Keywords
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:nbn:se:ltu:diva-63211 (URN)10.3390/app7050433 (DOI)000404449000002 ()2-s2.0-85018920929 (Scopus ID)
Note

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

Available from: 2017-05-02 Created: 2017-05-02 Last updated: 2018-07-10Bibliographically approved

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Nyberg, Erik

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