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Langmuir 1990,6, 1522-1524
Improved Surface Adhesion and Coverage of Perfluoropolyether Lubricants Following Far-UV Irradiation David D. Saperstein’ IBM General Products Division, 5600 Cottle Rd., San Jose, California 95193
L. Judy Lin IBM Research Division, Almaden Research Center, 650 Harry Rd., San Jose, California 95120-6099 Received March 20, 1990. I n Final Form: July 30, 1990 Far-UV treatment of a surface with a thin film of a perfluoropolyether improves adhesion and lowers surface free energy substantially compared to the unirradiated, lubricated surface. With 185-nm radiation, approximately one monolayer can be “fixed”to surfaces such as amorphous carbon, silica, and gold. These lubricated surfaces become extremely hydrophobic after UV treatment (advancing water contact angle increases from about 65’ to 2 110’) and are not removed by fluorinated solvents. In the absence of farUV radiation, most perfluoropolyethers show poor adhesion to carbon and are removed easily by rinsing with fluorinated solvents.
Introduction Perfluoropolyethers are composed of repeat units of small perfluorinated aliphatic oxides such as perfluoroethylene oxide or perfluoropropylene oxide.lJ As a class of compounds, they exhibit several important features1p2 such as uniform viscosity over a wide range of temperatures, low surface tension, resistance to oxidation a t high temperature, low vapor pressure, high chemical stability, low toxicity, and moderately high solubility for They are the lubricant of choice for most magnetic media applications,6 and they have important uses in spacecraft’ and other demanding applications such as high-vacuum pump oil,899 where performance of the lubricant is critical to the application. Perfluoropolyethers and other lubricants work by protecting surfaces which come in contact during sliding or other motion. In some applications, the surfaces will r u b many times and so t h e lubricant is subject t o continuous stress. To extend the life of these lubricants, it may be desirable to attach the molecules to the rubbing surfaces by making part of t h e molecule chemically reactive.l+12 This paper demonstrates for the first time13 that far-UV irradiation (e.g., 185 nm) of a thin film of any perfluoropolyether on most inorganic substrates, e.g., amorphous carbon, silica, and metals, causes improved (1) Gumprecht, W. H. A S L E Trans. 1966,9,24-30. (2) Sianesi, D.; Zamboni, V.; Fontanelli, R.; Binaghi, M. Wear 1971, 18,85-100. (3) Wesseler, E. P.; Iltis, R.; Clark, L. C., Jr. J.Fluorine Chem. 1977, 9, 137-146. (4) Snyder, C. E., Jr.; Gschwender, L. J.; Tamborski, C. Lubrication Eng. 1981, 37, 344. (5) Jones, W. R., Jr.; Paciorek; K. J. L., Ito, T. I.; Kratzer, R. H. Ind. Eng. Chem. Prod. Res. Deu. 1983,22, 166-170. (6) Moulder, J. F.; Holland, L.; Smith, K. L. Appl. Surf. Sci. 1986,25, 446-454. (7) See, for example: Zehe, M. J.; Faut, 0. D. N A S A Technical Memorandum 101962;1989. (8) See: Laurensen, L.; Dennis, N. T. M.; Newton, J. Vacuum 1979, 29,433-437 and references therein. (9) Falcone, E. Chim. Oggi 1983, 27-30. Storage (10) Yanaaisawa, M. Trib. and Mech. of. Magnetic - Systems 1 9 8 5 , ~sp-ig, 16-20. (11) Scarati, A. M.:. Caporiccio. G. I E E E Trans. Map. - 1987.. Mae-23, . 106-108. (12) Miyamoto, T.; Sato, I.; Ando, Y. Trib. and Mech. Of Magnetic Storage Systems 1988, 5, SP-25, 55-61.
adhesion and coverage of the lubricant to the surface without need for chemical derivatization. Adhesion is observed for polymers with light absorbing end groups such as AM200114and more surprisingly for polymers with no apparent absorption peak in the wavelength range of the experiment such as the nonreactive Fomblin Z 1~bricanta.l~ The coverage appears more uniform after treatment of a perfluoropolyether on a substrate such as amorphous carbon or silica because water is no longer able to wet the lubricated surface. Since far-UV irradiation will fix fluorinated lubricants to most inorganic surfaces used in contact applications, it is expected that this treatment will improve the useful life of many lubricated surfaces.
Experimental Section For convenience, we define the expression “UV-fixed lubricant” (also referred to as ‘fixed lubricant”) to mean a perfluoropolyether thin film on a substrate such as amorphous carbon (sputtered hard carbon), silica, or gold that is not removed by rinsing with Freon 113 (1,1,2-trifluorotrichloroethane)after farUV treatment. We also define the expression ‘free lubricant” to mean a perfluoropolyether which can be removed from a surface with Freon 113 whether or not it has been irradiated with far-UV light. Several different polymers are available commercially with molecular weights from 2000 to 25 000 daltons, such as Fomblinls Z (random copolymer of CFzCFzO and CFzO units) and Y (random copolymer of CF(CF3)CFzOand CFzO),Krytox16 (a homopolymer of CF(CF&CFzO),and Demnum17(a homopolymer of CFzCF2CFzO). To prepare a UV-fixed perfluoropolyether on a surface, the lubricated substrate is exposed to the radiation from a lowpressure Hg lamp which emits primarily over the range 254185 nm. The flux from the lamp at 254 nm is approximately 5 mW/cm2 (6 X 1015photons/cmz per s); the flux at 185 nm is approximately 20% of this value. Oxygen, which forms ozone in ( 1 3 ) Improvement of the adherence of thin metal films t o semiconductors by UV radiation has been previously demonstrated. See: Kellock, A. J.; Nyberg, G.I. Williams, J. S. Vacuum 1985, 35, 625-628 and references therein. (14) Fomblin Z lubricant with piperonyl end groups. See: Caporiccio, G.;Strepparola, E.: Scarati, M. A. European Patent Appl. 0,165,650 and 0,165,649, 1985. (15) Sianesi, D.; Fontanelli, R. US Patent 3,665,041, 1972. (16) Gumprecht, W. H. et al. US Patent 3,242,218, 1966. (17) Ohsaka. Y.: Tohzuka, T.: Takaki, S. European Patent A w l . 0,148,482, 1985.
0743-7463/90/2406-1522$02.50/00 1990 American Chemical Society
Langmuir, Vol. 6,No. 9,1990 1523
Letters far-UV radiation, and other reactive gases are excluded during treatment by continually flushing the expossure chamber with nitrogen or another inert gas.'* The length of exposure is adjusted to the lamp emittance and to the UV absorbance of the lubricant and substrate. After the UV treatment, the free lubricant is rinsed off the substrate with room temperature Freon 113 in order to measure the amount of lubricant adhering to the surface. To verify that the perfluoropolyether was fixed to the substrate, several procedures were attempted to accelerate ita removal from the surface. Some samples of the UV-fixed lubricant were heated in refluxing fluorinated solvent for 24 h (50 "C),others were sonicated in Freon, and still others were rubbed aggressively under nonpolar and polar solvents to try and remove the lubricant. These procedures had negligible effect on the UV-fixed lubricant. The quartz in the Hg lamp used for UV treatment transmits 185-nm radiation. When an ozone-free germicidal lamp was used instead, which eliminates the 185-nm radiation while passing 254-nm light, no UV-fixed 2-15 on amorphous carbon was observed. The thickness of the lubricant layer has been measured with ellipsometry (Rudolph Research), FT-IR (Bruker/IBM), and XPS (SurfaceScience Labs). Water contact angle^^^,^ are measured manually by a goniometer manufactured by Rame-Hart with the samples open to the ambient air. The advancing angle measurement is made about 15 s after the 10-pL drop is placed on the surface to allow equilibration of the drop and the surface. Receding angles are measured by removing enough liquid to cause the perimeter to shrink without the drop moving laterally.
Results and Discussion The UV, as opposed to far-UV, photochemistry of perfluoropolyethen has been studied for many years.2l In fact, some synthetic methods depend on UV photochemistry to produce the desired purity of the perfluoropolyethers.22 In the presence of oxygen, long-term exposure to UV radiation may degrade some of the polymers, producing small radicals such as F, COF, etc.= It is quite unexpected, therefore, that exposure to far-UV radiation produces little degradation of the polymer and causes improved adhesion and coverage effects. The commercial perfluoropolyethers are named by their backbone (e.g., Z = (CF&F20),(CF20),), by their average molecular weight (e.g., 2-03 = 2000-3000 daltons and 2-25 12000-17000 daltons of the same random copolymer of perfluoroethylene oxide and perfluoromethylene oxide), and, if they have a nonfluorinated end group, by a n acronym (e.g., Z-DOL = Fomblin 2-03 modified with bifunctional CH2OH end groups).24 Table I shows that UV treatment is applicable to perfluoropolyethers of high and low molecular weight, to perfluoropolyethers with and without fluorinated end groups, to linear and branched polymers, and to perfluoropolyethers with and without perfluoromethylene oxide links. The results in Table I also (18) Ozone will remove ambient organics and perfluoropolyethersfrom a surface and thereby increase the surface hydrophilicity. When a surface is first irradiated with far-UV light and then lubricated in the laboratory with a perfluoropolyether, there is no measurable increase in adhesion or water contact angle over that measured for an unirradiated surface. (19) See: Troughton, E. B.; Bain, C. D.; Whiteaides,G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4,365 for a discussion of contact angles. (20) Also see: Dettre, R. H.; Johnson, R. E., Jr. J.Phys. Chem. 1965, 69, 1507 and references therein on the measurement of advancing and receding contact angles. (21) Sianesi,D.; Pasetti, A.; Bernardi, G. C.; Caporiccio, G . La Chimica e L'lndustria 1973,55, 208 and references therein. (22) Marchionni, C.; Viola, G. T.; European Patent App EP 193122, 1986. (23) Faucitano,A.;Buttafava, A.; Martinotti, F. F.; Caporiccio, G.; Corti, C.; Maini, S.; Viola, G. T. J. Fluorine Chem. 1980, 16, 649. (24) See: US Patent 4,268,556 and 4,267,238.
Table I. Adhesion of the Lubricants after UV Treatment initial exposure UV-fixed thickness, time, thickness,a lubricant surface A min A
2-DOL Krytox 143AD Demnum SlOO
carbon carbon carbon carbon carbon carbon carbon carbon
2-15 2-15 2-15
Si02
50 50 30 35 35 50 50 50
ZrO2 Au
50
2-03 2-15 2-25
YR AM2001
50
10 10 10 10
l8k 1 22 25 20
10
30
10
15 15 18 20 20
10 10 5 5 10
50
18
Without UV treatment,
