A New Concept..
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Halogenated Gases for High Temperature Lubrication of Metals DONALD H. BUCKLEY and
ROBERT L. JOHNSON Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio
Nickel- and cobalt-base alloys have been effectively lubricated by halogenated gases to temperatures of 1200" F. without a corrosion problem
trend in lubrication systems for aircraft and particularly missiles is toward operation at extreme temperatures. For example, lubrication is required under -200' as well as at 1000' F., where organic liquids and greases are no longer useful. Gases such as the halogen-substituted methanes are thermally stable at the high temperatures and therefore are under consideration as lubricants. hfethane substituted with fluorine and chlorine or bromine satisfies the important requirement of chemical stability in contact with metal surfaces a t ambient temperatures to 1000' F. In sliding contact of lubricated metals, "flash" temperatures of surface asperities that are GOO0 C. above ambient have been reported ( 3 ) . These temperatures are sufficient to rupture chemical bonds of gases adsorbed on the surface. Active atoms thus released from the molecule, such as chlorine and bromine. then react at the hot spots to form metal halides; these reaction products function as solid lubricants. This mechanism has been postulated for extreme pressure lubrication of gears by additives in oils (5, 6, 9). Excessive corrosion was experienced
T H E
Prospect for the future parts
. . . . . . gaseous lubrication of metal aircraft and missile
at temperatures above GOOo F. in lubrication of ferrous materials by halogenated gases ( 7 ) . Other types of alloys, including nickel-base materials, should have adequate high temperature corrosion resistance in these gases. The selection of the proper combination of metal and gas is critical, in that the reaction film produced must have lubricating properties. Corrosion can also be reduced by employing the minimum number of active halogen atoms in the molecule. Previous experience (7, 7) showed, however, that two chlorine atoms per molecule were necessary for effective lubrication of steel. A bromine atom can be removed more readily from a mclecule than a chlorine atom, because of difference in chemical bond strength, and thus react to give good lubrication. Further, the catalytic effect of sulfur hexafluoride (SFs) as an additive might aid lubrication without excessive corrosion. The object of the research reported herein was to select materials that have resistance to corrosion, study their lubrication with various gases and gas combinations, and investigate their performance over a broad temperature range. The gases used were dichlorodifluoromethane (CFzClZ), dibromodifluoromethane (CF2Brz), monobromotrifluoromethane (CFsBr), and sulfur hexafluoride (2, 4). Experiments were run with iron-, cobalt-, and nickel-base materials sliding in an atmosphere of reactive gas from 75' to 1200' F. A hemisphere of 3/~6-inchradius under 1200-gram load contacted the flat surface of a rotating disk 2l/2 inches in diameter ( 7 ) . The sliding velocity was 120 feet per minute.
Results and Discussion Corrosion Study. The poor corrosion resistance of M-1 tool steel a t temperatures above GOOo F. ( 7 ) prompted a corrosion study of various materials. The experiments were conducted in an atmosphere of CF?ClQ with 170 SFG a t 7 j 0 , GOO", and 1200" F. Of the gases studied, this mixture of CIJ2Cln and SF,3 provided the most corrosive atmosphere. The materials investigated that showed the least corrosion were the nickel-base alloys. particularly 7.57c silicon-nickel, Inconel-X, and Hastelloy C. Alloy and Gas Combinations. Various corrosion-resistant alloys were explored as slider materials for lubrication by chlorine- and bromine-containing gases. Previous NACA solid lubricant research studies (8) have shown that certain AX2 type compounds (layer lattice crystal structure) have good lubrication properties. The results of solid lubricant studies were used as a guide in material selection for minimum friction and wear. For example, cobalt chloride was a good solid lubricant; hence, would be a desirable reaction product. Runs were made with corrosionresistant materials at 600" F. (threshold of severe corrosion with M-1 tool steel) to establish alloy-gas combinations for detailed study. Two disk materials were used. nickel-base Hastelloy C and cobalt-base Rexalloy 33. The rider materials were hl-1 tool steel, K102B Kentanium, Stellite 21, 98h12 Stellite, Inconel-X, and 7.570 silicon-nickel. In CF&r plus 1% SF6, two metal combinations showed significant reduction in friction and wear over results obtained in air: M-1 tool steel in sliding contact VOL. 51, NO. 5
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Figure 1. alloy 33
Friction and wear of 7.5% silicon-nickel on Rex-
Friction was high at room temperature but low at 800' F. Wear decreases rapidly from 75' to 800' F. CFzClz with 1 7 0 SFe, 120 feet per minute, 1200 grams, 1 hour
with Hastelloy C and 98M2 Stellite with Hastelloy C. With CF2C12 plus 1% SF6 as the lubricant, the best metal combination was a 7.5y0 silicon-nickel rider specimen in contact with a cobaltbase Rexalloy 33 disk.
Dichlorodifluoromethane Plus Sulfur Hexafluoride. The results obtained with some of the more effective alloygas combinations at 600" F. indicated that these combinations should be studied over a broad temperature range. For example, 7.570 silicon-nickel rider in combination with Rexalloy 33 disk specimen was a good material combination in CF2C12 with 1% SF6. Therefore, the data shown in Figure 1 were obtained a t temperatures from 75' to 1200' F. to indicate effectiveness of CFzClz with 1% SFe. Friction was somewhat high at room temperature but low (0.05) a t 800' F. This extremely low friction coefficient indicated that molten surface films of reaction products may have been present, as discussed for M-1 tool steel specimens (7). The wear decreased rather rapidly from 75' to 800' F. for 7.5y0 silicon-nickel on Rexalloy 33; above 800' F. the wear was relatively constant. This trend in wear was the reverse of that observed with M-1 tool steel. Agglomerates of reaction products appeared to have formed a t the grain boundaries of the silicon nickel as well as on the slider interface. The reaction mechanism involved has not been resolved but may include preferential reaction with silicon compounds. Dibromodifluoromethane. Data obtained a t 600' F. with various metals indicated that a good metal combination for lubrication with a brominecontaining gas would be 98M2 Stellite rider sliding against a Hastelloy C disk. This combination was run in CFzBrg at temperatures from 75' to 1200' F. Data presented in Figure 2 show that the friction was within the range of what is considered effective boundary lubrication (0.20 or less). The wear begins to increase above GOO0 F. The increase in
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Figure 2.
Friction and wear of 98M2 Stellite on Hastelloy C
Friction was within the range for effective boundary lubricotion. With CF2Br2 wear increased above 600' F. CF&z and CF& with 1 % SFs, 120 feet per minute, 1200 grams, 1 hour
wear may be attributed to the limited corrosion resistance of 98M2 Stellite at elevated temperatures. The susceptibility of the rider to corrosion can have a pronounced effect on the wear. One method of reducing corrosion would be to reduce the bromine content in the lubricating gas from two atoms to one atom per molecule. Monobromotrifluoromethane Plus Sulfur Hexafluoride. Previous data (2) involving similar experimental conditions have shown one chlorine atom in a gas molecule inadequate for effective boundary lubrication. The removal of a bromine atom from an analogous bromine-substituted gas molecule is, however, easier and may provide effective lubrication. The benefits obtained with 1% SFs as a catalyst added to CFzClz (7) suggested that the addition of SF6 to CF3Br might improve its lubrication characteristics. A series of runs was made with 98M2 Stellite rider specimens sliding against Hastelloy C disks in an atmosphere of CFaBr with 1% SF6 at various temperatures to 1200' F. (Figure 2 ) . The friction values obtained a t 1000' and 1200' F. are higher than with either CF3Br or SF6 alone. This effect of the mixture giving increased friction bears further consideration. The wear decreased as the temperature was increased to 800' F. Above 800' F. the wear was relatively constant. Two distinct wear trends in the two gaseous atmospheres are shown in Figure 2. The bromine content in CFzBr2 is such that the wear was constant to a temperature of 800' F., where corrosion began to influence wear. The bromine made available in the decomposition of CF3Br was sufficient for adequate lubrication at elevated temperatures without excessive corrosion, but wear was higher below 800' F. Mixtures of CFzBrz and CF3Br
AND ENGINEERING CHEMISTRY
may have merit for effective lubrication without objectionable corrosion over a greater temperature range. Summary of Results
The boundary lubrication of various materials with chlorine- and brominecontaining gases was studied from 75' to 1200' F. Corrosion-resistant alloys were lubricated effectively by reactive gases at temperatures from 75' to 1200' F. The best of materials and gas combinations with regard to friction, wear, and corrosion resistance varied with the temperature level. Thus, the optimum conditions for gas lubrication are highly selective. literature Cited (1) Allen, G. A , Buckley, D. H., Johnson, R. L., Natl. -4dvisory Comm. Aeronaut., NACA T N 4316 (1958). ( 2 ) Allied Chemical and Dye Corp.
General Chemical Division, New York 6, N. Y., Sulfur Hexafluoride Tech. Bull. TB-85602 (1955). (3) Bowden, F. P., Tabor, D., "Friction and Lubrication of Solids," pp. 288-38, Clarendon Press, Oxford, 1950. (4) Buffington, R. M., Fellows, H. M., Kinetic Chemicals, Inc., Tech. Paper 5. ( 5 ) Davey, W., J . Znst. Petrol. 31, 73, 154 (September 1954). (6) Gregory, J. N., Zbid., 34, 297, 670-6 (September 1949). (7) Murray, S.F., Johnson, R. L., Swikert, M. .4., Mech. Eng. 78, 233-6 (March 1956). (8) Peterson, M. B., Johnson, R. L., Natl. Advisory Comm. Aeronaut., NACA T N 3334 (October 1954). (9) Prutton, C. F., Turnbull, David, Dlouhy, George, J. Znst. Petrol. 32, 226, 90-1 18 (February 1946). RECEIVED for review December 1, 1958 ACCEPTEDMarch 2, 1959
Division of Petroleum Chemistry, 134th Meeting, ACS , Chicago, Ill., September 1958.
