Ionic Liquids IIIA - American Chemical Society

Chapter 19

Surface Chemistry and Tribological Behavior of Ionic Liquid Boundary Lubrication Additives in Water Downloaded by UNIV MASSACHUSETTS AMHERST on September 7, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch019

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Benjamin S. Phillips , Robert A. Mantz , Paul C. Trulove , and Jeffrey S. Zabinski * 2,

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Universal Technology Corporation, 1270 North Fairfield Road, Dayton, O H 45432-2600 U.S. Air Force Research Laboratories, Materials and Manufacturing Directorate, Nonmetallic Materials Division, Nonstructural Materials Branch, Wright-Patterson Air Force Base, O H 45433-7750 AFOSR/NL, 4015 Wilson Boulevard, Arlington, VA 22203

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Ionic liquids have been intensely studied regarding their properties as an environmentally friendly solvent. Conversely, little work in the field of tribology has been accomplished to date. Ionic liquids posses many favorable properties that would suggest that they have significant potential for use in the tribology field. One area that is discussed here is the use of ionic liquids as a boundary lubricant additive for water. The chemical and tribochemical reactions that govern their behavior were evaluated for two ionic liquids. Under water lubricated conditions, silicon nitride ceramics exhibit a characteristic running in period of high friction, through which surface smoothing and tribochemical reactions lead to low friction coefficients. The running-in period of high friction is the period where the majority of the wear occurs. The use of a suitable boundary lubricant to reduce the running-in period could be of significant importance. A promising candidate for this application is ionic liquids. We present the affects that ionic liquids have on the friction and wear properties of Si N , in particular the affects on the running-in period. Tribological properties were evaluated using a pin-on-disk and reciprocating tribometers. Solutions containing 2 wt % ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with x-ray 3

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© 2005 American Chemical Society

In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

245 photoelectron spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR). The test specimens were 1" diameter S i N disks sliding against ¼" Si N tells. The use of ionic liquids as a boundary lubricant additive for water resulted in dramatically reduced running-in periods for silicon nitridefromthousands to the hundreds of cycles. Proposed mechanisms controlling the friction and wear include the formation of a transfer film and the inception of an electric double layer. Downloaded by UNIV MASSACHUSETTS AMHERST on September 7, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch019

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Introduction The push to introduce cleaner technologies has led to the search for replacements for dangerous solvents. One promising solvent replacement is ionic liquids, which have received much attention in literature on this topic. They have many unique properties such as negligible volatility, nonfiammability, high thermal stability, and a low melting point, which qualifies them as excellent choices for solvents in many synthesis reactions (/-J). While interest in ionic liquids as environmentallyfriendlysolvents has increased, little work has been accomplished in the field of tribology. Ionic liquid are candidates for use as lubricants due to the same properties that make them excellent solvents. Recent studies have shown potential for imidazolium based ionic liquids as lubricants (4,5). Ye et al used a ball-on-disk configuration with reciprocating motion for study of two pure imidazolium based ionic liquids, l-methyl-3-hexylimidazolium tetraflouroborate and l-ethyl-3-hexylimidazolium tetraflouroborate. The friction coefficient did not rise above 0.065 for either ionic liquid tested on steel substrates. The authors suggest that the mechanism that provides lubrication is the breakdown of the tetraflouroborate ion to form fluoride, B 0 , and BN on the substrate surface (4). Related work using imidazoline borates as a lubricant was done be Gao et al (6)· Although, these materials are not classified as ionic liquids, their chemical structures have similarities. Using these chemicals as water additives on steel substrates, friction coefficients of near 0.1 were obtained in a four-ball test machine. Reports by Phillips et al show decreased friction coefficients and running in periods for Si N utilizing ionic liquids as water additives (7). These encouraging results should lead to more extensive work performed with ionic liquids in the tribology field. 2

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246 Water Lubrication of Ceramics Since the initial report of ultra-low friction coefficients of Si N sliding in water by Tomizawa and Fischer, significant research has been undertaken in die study of water lubrication of ceramics (8-19). Si N is die most commonly studied ceramic, although other ceramics such at alumina and silicon carbide have also been evaluated. The work done by Tomizawa and Fischer proposed that S13N4 forms a hydrated silicon oxide layer in the presence of water and that the layer is soluble, and upon dissolution the surface becomesfinelypolished. The mechanism controlling the lowfrictioncoefficient was ascribed to hydrodynamic lubrication (8). Under water lubricated conditions S i N initially exhibits high friction during a running-in period, where die surface is smoothed by dissolution and wear at asperity contacts (9). Certain sliding speeds and contact pressure enhance the tribochemical reaction that forms silicon oxide on the surface (10). Although hydrodynamic lubrication was die first mechanism proposed, other mechanisms have not been ruled out (13). For example, Xu et al suggests the behavior of silicon nitride is water is controlled by both hydrodynamic lubrication and boundary lubrication by a colloidal silica layer (/ /). Work done under solutions of varying pH by Mizuhara strengthens this argument (12). He reports that the dissociation rate of the silica layer is the key factor in the behavior of S i N in water. The behavior of S i N in water exhibits a highfrictionrun-in period, where the majority of the wear occurs. A significant advance would be to reduce the running-in period. This could be accomplished by the use of a boundary lubricant additive to water. Reducing the running-in process would decrease the wear of S i N and other similar materials. Typical boundary lubrication additives have several characteristics including: high load carrying capacity, low volatility, low shear strength, high stability, and environmentally friendly. As stated above, ionic liquids provide many of these characteristics and have shown promise in recent tribology testing (4). 3

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Experimental Ionic Liquids Two wt.% solutions of the ionic liquids in water were evaluated in the


247 tribology tests. The two solutions were comprised of l-methly-3-n-butylimidazolium hexaflourophosphate (PF ) and l-ethyl-3-methyl imidazolium tetraflouroborate (BF ). 6

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Tribo-Testing Friction and wear testing was performed using a pin-on-disk tribometer. The tribometer utilizes a stationary pin on a rotating disk and is setup to accommodate a Γ in diameter disc and a !4" ball. Multiple runs of self-mated samples were performed on each disc, with the test radius varying, keeping the linear speed constant at 0.120 m/s. The test apparatus is described in more detail by Phillips et al (7). Tribological testing was also carried out using a reciprocating tribometer. The tests were run with a 5mm stroke length and a period of 6 Hz. A normal load of200 Ν was applied, which correlates to 700 MPa maximum Heritian stress at the beginning of each test. Similar to the pin on disk testing the sample and pin were self-mated pairs. All tests were carried out in air in ambient conditions. The test apparatus is described in more detail by Cavdar et al (20).

Analytical Techniques 10

Compositional analysis was made with an XPS system operated at 8 χ 10" torr. Binding energy positions were calibrated using the Au 4fm peak at 83.93 eV, and the Cu 3s and Cu 2p / peaks at 122.39 and 932.47. A nominal spot size of300um was used for analysis. Charge neutralization was accomplished by flooding the sample surface with low energy electrons. 3 2

Results Chemical and Surface Analysis Surface roughness and wear track areas were obtained from a contact profilometer with a resolution of 0.5 Angstroms. The results were similar to water alone where smoothing of the asperity tips in the contact area occurred for both samples. For the water only test, the maximum depth of the wear scar was approximately 6000 Angstroms with a corresponding specific wear rate of 1.5 χ 10*5 mm /N*m. The maximum depth of the water plus ionic liquids test was approximately 2000 Angstroms with a specific wear rate of 4.58 χ 10*6 3



248 mm /N*m. The wear rate for the water plus ionic liquid test was an order of magnitude smaller than those for pure water. The wear rates were calculated over the first 10,000 cycles, which includes the high wear running-in period. Subsequent sliding would produce little wear and therefore increased sliding distance would reduce the specific wear rates. All composition data was taken from samples run in the oscillating friction tester. This was due to the very narrow wear scar created during die pin-on-disk test, which made it difficult to determine compositional changes with XPS (300 um spot). The wear scar generated from the oscillating friction tests was 5 x 5 mm wear scar, which enabled a more detailed study of the chemical changes within the system. Chemical analysis on die surface was completed after the water was allowed to evaporate from the samples. XPS and other chemical analyses were performed inside and outside of the wear area as well as before and after a methanol rinse. Little difference in the composition between the inside and outside of the wear scar during XPS analysis. The differences remained small after rinsing the sample with methanol. PF tests resulted in a significant amount of fluorine, near 20% F, and small amounts of phosphorus,

Ionic Liquids IIIA - American Chemical Society

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