Tribological Characteristics of Perf luoropolyether Liquid Lubricants under Sliding Conditions in High Vacuum

Masabumi Masuko Tokyo Institute of Technology, Tokyo, Japan, and Wil- liam R. Jones Jr and Larry S. Helmick NASA Lewis Research Center, Cleveland, Ohio, USA

Tribological characteristics of three PFPEs (Fomblin Z, Demnum, and Kry- tox) were studied under high vacuum using a four-ball apparatus with 440C steel specimens. Fomblin Z and Demnum exhibited initial scufJing-like high friction whereas Krytox did not. Steady state friction with Fomblin Z was the lowest among the three oils and that with Demnum and Krytox was almost the same. The lowest wear rates in air were provided by Krytox regardless of load and low wear rates in vacuum at high load were achieved with Krytox and Demnum. Results are explained by reactivity and pressure-viscosity charac- teristics of the oils.

KEYWORDS: perfluoropolyether, PFPE, sliding, air, high vacuum, friction, wear, load, pressure-viscosity

INTRODUCTION

Perfluoropolyethers (PFPEs) have been used as lubricants for space applica- tions for many years because of their wide liquid range, low volatility, high viscosity index, and chemical inertness.' Space mechanisms where liquid lu- brication is currently required include bearings for actuators, gyroscopes, mo- mentum wheels, scanning mirrors, and filter wheels.2 In addition, other mechanisms such as gears or harmonic drives, where contact conditions are more severe than in rolling element bearings, also require liquid or grease lu- brication.

Many PFPE stability studies, such as therma1,g oxidative: and catalyW6 have been reported concerning PFPE degradation. Studies concerning the tri- bological behaviour of PFPEs are less common,7.* particularly sliding experi- ments under high load in a vacuum environment.9 The objective of this study, therefore, was to compare the ability of three commercially available PFPE lu- bricants to reduce weadfriction in the boundary regime using a newly de-

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Figure 1 Vacuum tribometer

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Perjluoropolyether Lubricants under Sliding Conditions in High Vacuum 113

signed vacuum four-ball tribometer. Conditions included: dry air and vacuum (lo4 to Pa) environments, loads of 200 and 600 N, a rotational speed of 100 r/min and room temperature (-23°C).

EXPERIMENTAL

Vacuum four-ball tribometer

The apparatus has been previously described'" and is shown in Figure 1. The specimen configuration is the same as the standard four-ball apparatus. In this tribometer, 9.5 mm (3/8 in.) diameter bearing balls made of AISI 44OC stain- less steel were used as test specimens. The specimen stage was mounted in a vacuum chamber.

Tests were carried out under a constant sliding speed of 2.88 x 10" m / s (100 r/min) and two axial loads (200 N and 600 N). All tests were conducted at room temperature (-23°C). Vacuum tests were started after the pressure reached 10-5 Pa or less.

Measurement of friction and wear

Frictional force was recorded continuously. Wear scar diameters for the three stationary balls were measured at various sliding distances with an optical mi- croscope and then averaged.

The microscope stage allowed for wear scar measurements without disas- sembling the balls from the cup. Therefore, the experiment could be continued using the same set of balls. Wear volume was calculated from the wear scar diameter. Wear on the rotating (upper) ball was not measured.

Test lubricants

Three commercially available PFPE lubricants were studied (Demnum S 100, Fomblin 225, and Krytox 143AB). All have similar viscosities at room tem- perature. Properties are presented in Table 1, overleaf.

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114 Masuko, Jones, & Helmick

Table 1 Properties of sample oils

Oil name Fomblin 225 Demnum Kwox SlOO 143AB

Molecular weight 15000 5600 3700

Viscosity, mm2/s Q 20'C 255 250 230

0 40'C 135 96.0 75

Q 100°C 30.0 18.2 9.7

Viscosity index 360 209 113

Density, gkm3 1.851 1.878 1.894

Pour point, "C -66 -60 -40

Pressure-viscosity coefficient (a) 1.W 2.62 4.2l at 38'C, Pa-' x

' Reciprocal Asymptotic lsoviscous Pressure, a* (ref. 12). scant calculated for Demnum S-65 from ref. 13.

RESULTS

Comparison of wear rate

Wear volume as a function of sliding distance appears in Figure 2. Wear vol- ume increased almost linearly with sliding distance. This relationship was ob- served for all experiments. Therefore, wear rate, defined as the wear volume per unit sliding distance, mm3/mm, was calculated from the slope of this rela- tionship.

Wear rates are compared in Figure 3. From the comparison of the wear rate at 200 N, it can be seen that Krytox allows less wear than the other two PFPE oils, regardless of environment. In general, wear rates are higher in vacuum than in air. In particular, Fomblin Z exhibited a very high wear rate in vacuum at 600 N. Repeatability of the Fomblin Z results was poorer than for the other two lubricants.

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Perfluoropolyether Lubricants under Sliding Conditions in High Vacuum 115

Figure 2 Wear volume as a function of sliding distance. Demnum SlOO (600 N) in air

lo Regression line with a r slope of 6.94 x 10-9 mm3/ E

0 40 80 120 160 200 240 Sliding distance, rn

Comparison of friction coefficient

Friction coefficients were normally steady throughout the experiments, except for the initial region. Steady state friction coefficients obtained in vacuum are lower than those in air (5-20%), regardless of lubricant or load. Fomblin 2 yielded the lowest friction coefficient both in air (0.1 1) and in vacuum (0.10)

Figure 3 Comparison of wear rates

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116 Masuko, Jones, & Helmick

under both loads, but variation of the friction coefficient in the steady state region was greater than that obtained with the other two lubricants.

High initial friction coefficient

At 200 N, none of the lubricants exhibited high transient initial friction coef- ficients. At 600 N, initial high friction was not observed for Krytox in either environment, whereas Demnum and Fomblin Z exhibited transient initial high friction regions. However, their magnitudes were different (i.e., higher with Demnum (0.16) than with Fomblin Z (0.13)), whereas the durations were sim- ilar (<3 min).

DISCUSSION

Characteristic features of the results in vacuum are as follows: Fomblin Z yielded the lowest friction but the highest wear rate, among the three lubri- cants, particularly at 600 N. Both Fomblin Z and Demnum showed scuffing- like behaviour (transient high friction) during the initial stage of the experi- ments, but the friction coefficient in this stage was greater with Demnum than with Fomblin Z. Krytox did not exhibit any initial high friction although steady state friction was almost equal to that of Demnum.

It has been suggested that Fomblin Z easily decomposes thermally or cata- lytically forming reactive species. The stability of Fomblin Z in the presence of a catalytic surface was the lowest among the three PFPEs of this study.5 This instability occurs because of the presence of formal groups -(OCF,O)-. Fomblin Z is much more reactive than either Krytox or Demnum. In addition, it has been also shown that PFPE oils form metallic fluorides, both thermally and trib~logically.~ For a discussion of the effect of fluoride formation on boundary lubricating performance, see ref. 1.

The pressure-viscosity coefficient (a) of Krytox, which is indicative of the capability of elastohydrodynamic (EHL) film formation, is the greatest among the three test fluids (about 4.5 x 1 0 - 8 Pa-' at 38°C) whereas that of Fomblin 2 is about 1.5 x 1 0 - 8 Pa-'. Values for Demnum are intermediate (-2.6 x 10-8 Pa-'). Values are from references 12 and 13.

Taking account of both reactivity and pressure-viscosity behaviour, the tribological characteristics of the fluids can be explained as follows: a low steady state friction coefficient with an accompanying high wear rate which was obtained with Fomblin Z is similar to the behaviour of highly reactive

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Perjluoropolyether Lubricants under Sliding Conditions in High Vacuum 117

additives (such as EP agents). Since Fomblin Z is very reactive and forms met- al fluorides, as discussed earlier, it is reasonable to conclude that the tribolog- ical characteristics of Fomblin Z are similar to that of EP additives (i.e. chemical or corrosive wear).

This can also explain the initial high friction obtained with Demnum and Fomblin Z. Since the initial contact pressure is very high (mean Hertzian stress is 2.2 GPa (200 N) and 3.17 GPa (600 N)), local scuffing can occur. If the lubricant can react with the surface to minimise scuffing by forming EP layer-like surface films, the initial scuffing (high friction) can be effectively suppressed. Therefore, it is plausible that the less reactive Demnum would show higher initial transient friction than Fomblin Z.

Lubricant reactivity cannot explain the lack of initial high friction with Krytox, but EHL effects may come into play. EHL film thickness can be cal- culated with the following equation:'O

EHL minimum film thickness ( h ~ " ) can be calculated for the various lu- bricants based on initial contact conditions (i.e. before any wear or contact heating occurs). Fomblin Z yields the following results: 0.025 pm (200 N) and 0.023 pm (600 N). Krytox, having a greater a value, yields higher h,,,: 0.045 pm (200 N) and 0.041 (600 N). Demnum values are intermediate.

Based on a ball surface roughness of about 0.025 pm, initial h ratios (film thickness to composite surface roughness) are approximately one for Fomblin Z and two for Krytox. Therefore, all tests begin in the mixed film regime (b1) but as soon as any contact heating occurs, they are driven into the boundary regime (h ratios el),

However, the higher h ratio for Krytox will initially (first few metres of sliding) affect contact friction. Even though a certain amount of metal contact still exists with Krytox, the contact load supported at the actual solid contacts can be reduced because the oil film can support more load than Fomblin Z. This effective partial EHL film formation is reflected by the smoother friction traces with Krytox and a lack of initial high friction.

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118 Masuko, Jones, & Helmick

CONCLUSIONS

The following conclusions are drawn from the experiments:

(1) Wear rates are higher in vacuum than in air. (2) Krytox exhibits the lowest wear rate in air regardless of load. (3) Krytox and Demnum both exhibit low wear rates in vacuum at high load. (4) Demnum and Fomblin Z exhibit initial scuffing-like high transient fric- tion at high load but Krytox does not. ( 5 ) Fomblin Z exhibits the lowest steady state friction coefficient but the highest wear rate, particularly in vacuum at high load. (6) The tribological characteristics of the three PFTE lubricants can be rea- sonably explained by considering both reactivity and pressure-viscosity char- acteristics of oils. (7) Improved lubrication performance of unformulated PFPEs will require the development of a new series of boundary additives.

References

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Jones, Jr., W.R., ‘The properties of peffluoropolyethers used for space applications’, NASA TM 106275, July 1993. Fleischauer, P.D., and Hilton, M.R., ‘Assessment of the tribological requirements of ad- vanced spacecraft mechanisms’, Aerospace Report No. TOR-0090(5064)- 1 (1991). Helmick, L.S., and Jones, Jr., W.R., ‘Determination of the thermal stability of perfluoro- alkylethers’, NASA TM 102493 (1990). Jones, Jr., W.R., Paciorek, K.J.L., Ito, T.I., and Kratzer, R.H., ‘Thermal oxidative degra- dation reactions of linear perfluoroalkyl ethers’, Ind. Eng. Chem. Prod. Res. Dev., 22, 2,

Kasai, P.H., and Wheeler, D., ‘Degradation of perfluoropolyethers catalyzed by aluminum oxide’, Applied Sugace Science, 51,201-11 (1991). C a d , D.J., ‘The reaction of perfluoropolyalkylether oil with FeF,, AIF,, and AICI, at elevated temperatures’, ASLE Trans., 28, 1,40-6 (1985). Mori, S., and Morales, W., ‘Tribological reactions of perfluroalkyl polyether oils with stainless steel under ultrahigh vacuum conditions at room temperature’, Wear, 132, 11 1- 12 (1989). Carrt, D.J., ‘The performance of peffluoropolyalkylethers under boundary conditions’, Aerospace Report No. TR-0091(6945-03)-4 (1991). Masuko, M., Fujinami, I., and Okabe, H., ‘Lubrication performance of perfluoropoly- alkylethers under high vacuum’, Wear, 159,249-56 (1992). Masuko, M., Jones, Jr., W.R., Jansen, R., Ebihara, B., and Pepper, S.V., ‘A vacuum four- ball tribometer to evaluate liquid lubricants for space applications’, NASA TM 106264 (1993). Hamrock, B.T., and Dowson, D., Ball bearing lubrication: the elastohydrodynamics of el- liptical contacts, J. Wiley and Sons, New York (1981).

166-70 (1983).

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12. Jones, Jr., W.R., Johnson, R.L., Winer, W.O., and Sanborn, D.M., ‘Pressure-viscosity measurements for several lubricants to 5.5 XIOs Newtons per square meter (8 X104 psi) and 140°C (300”C)’, ASLE Trans., 18,4,249-62 (1975).

13. Tanaka, Y., Nojiri, N., Ohta, K., Kubota, H., and Makita, T., ‘Density and viscosity of lin- ear perfluoropolyethers under high pressure’, Int. J. of Thermophysics, 10, 4, 857-70 (1989).

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