Friction and wear of potentiometer contacts in vacuum

Friction and wear of potentiometer contacts in vacuum

Wear - Elsevier Sequoia S. A., Lausanne - FRICTION AND WEAR Printed in the Netherlands OF POTENTIOMETER 355 CONTACTS IN VACUUM J. R. JONES Hug...

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Elsevier Sequoia S. A., Lausanne -

FRICTION

AND WEAR

Printed in the Netherlands

OF POTENTIOMETER

355

CONTACTS IN VACUUM

J. R. JONES Hughes

Aircraft

(Received

Company, Culver City, Calif.

January

(U.S.A.)

16, 1968)

SUMMARY

Wire-wound potentiometers are frequently used to sense the position of remote devices such as antennae and TV cameras. Under earth-ambient conditions the sliding surfaces of such potentiometers are lubricated for life by thin films of oil or grease. These lubricants always exert some finite vapor pressure. In hard vacuum this small volatility can result in contamination of adjacent surfaces, even when labyrinth seals are used. Therefore, on space vehicles, oil or grease is not permissible on potentiometers in the vicinity of optical surfaces. Yet, if no lubrication is provided, the contacting metals will undergo microscopic welding, followed by gross seizure and destructive wear, with attendant electrical noise and inevitable failure. In an attempt to provide lubrication without contamination of TV camera lenses, two types of materials were investigated in a vacuum apparatus simulating the sliding contacts of a potentiometer : solid lubricant powders, and electrically conductive composites. Several materials in both categories gave satisfactory performance with respect to friction and wear. INTRODUCTION

Numerous investigators have demonstrated the tendency for absolutely clean metals to weld on contact under conditions of high vacuum where the formation rate of protective oxide films is extremely low. This phenomenon was manifested on the Surveyor space vehicle in a relatively inconspicuous but important area. A TV camera system was being operated in the laboratory under an atmosphere of approximately 1o-l1 torr. The potentiometers, used to sense position and focal length, had been degreased to prevent contamination of adjacent lens surfaces. These were wire-wound, three-turn potentiometers, driven by stepper motors of very limited power, approximately 0.18 in. oz. of torque. It was calculated that in order for these potentiometers to become inoperable, the coefficient of friction (fe) between the sliding contact and the resistance element would have to equal 1.41. Under high vacuum, mechanical stoppage was experienced on several of the potentiometers and subsequent component tests indicated that metallic seizure was taking place between the sliding contacts. Wear, II (1968)

1. Ii. JONES

356 DESCRIPTIOS

OF POTENTIOMETER

The resistive element of these wire-wound potentiometers is in the form of a spiral, three turns of an insulated 0.04 in. diameter copper mandrel, upon which is wound 0.001 in. diameter nickel-chromium wire. This wire has a hardness of Rockwell C30 (Knoop 311). A simplified cross section view of this potentiometer is shown in Fig. I. The individual windings on the mandrel are separated from each other by approximately two diameters, in order to improve linearity in selecting or measuring resistances. The sliding contact is a hard, precious metal alloy (Paliney No. 7) containing palladium, silver, gold, platinum, copper, and a little zinc. The hardness is approximately Rockwell C38 (Knoop 380).

DRIVE SHAFT

\

TERMINALS

Fig. I. Simplified cross-section view of wire-wound potentiometer.

Due to manufacturing difficulties, two dimensions vary significantly: the final outside diameter of the wound element, and the distance separating individual wires. The sliding contact is therefore touching a varying number of wires at a constantly changing distance from the center of the potentiometer. Thus, the friction pattern is commonly somewhat erratic. In addition to the friction between the sliding contact and the resistive element, drive motors for these potentiometers must overcome the friction of the slip ring, the support bearings, and the lip of the carriage, which guides the contact over the three-turn spiral. The support bearings are ball bearings having a self-lubricating retainer. Thus there is no oil or grease anywhere in the system. EXPERIMENTAL

EQUIPMENT

This paper is concerned only with the friction and wear occurring between the sliding electrical contact and the wire windings. The test apparatus designed to simulate this portion of the potentiometer is presented schematically in Figs. 2 and 3. Two contacts contained in the upper specimen oscillate in sliding motion on three small Wear,

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(1968)

FRICTION AND WEAR OF POTENTIOMETER CONTACTS

357

resistive elements which are epoxy-bonded in three parallel grooves on the lower specimen. The contacts are shaped and positioned so that the load is applied (on the three resistive elements) at three lines which define a plane parallel to the ground. The angles in the forward contact assist in maintaining the line of action of the drive rod. PLAN

RESISTIVE ELEMENTS

ELEVATION FORWARD

ELEVA-TION

UR’ER SPECIMEN LOAD

BY CONTACT REAR ELEMENT ‘LOWER

Fig. 2. Schematic

representation

SPECIMEN

of vacuum test apparatus.

Fig. 3. Enlarged view of test specimens.

The upper specimen is a very close, slip fit into the upper specimen holder, thus minimizing any possible rocking which might occur during stick-slip or reversal of direction. The load weights are a snug fit into the top of the upper specimen for the same reason. All tests were conducted with a stroke length of 0.19 in. The motion of the drive rod.derives from an eccentric crank on the external gear box, and is therefore sinusoidal; the maximum linear velocity occurs at the center of the stroke. As an example, the maximum at 60 c/min is 35.2 in./min. In all the tests, the total (normal) load on the resistive elements was approximately 14 g. An external view of the equipment is shown in Fig. 4. The extremely small friction forces were detected by a thin-walled (0.003 in.) beryllium-copperforce transducer (strain ring) to which were bonded four strain gages. The strain gage leads were connected via a flanged electrical feed-through to an amplifier-recorder which produced a pen-and-ink chart trace on paper. The force transducer was calibrated by applying known, constant forces with a dynamometer and recording pen deflections. Calibration tests indicated that the response to friction force was esw&W,fl (1968)

Fig.

4.

External view

sentially

of

test apparatus

linear in both tension

and compression

modes, and symmetrical

about the

zero point over the oscillation ranges of interest (ctbo c’min). The vacuum system consisted of a pair of liquid nitrogen-cooled sorption rough ing pumps, coupled to a sputter ion pump having a nominal rating of 75 1;‘sec. When this equipment was used with bake-out, it proved capable of drawing the atmosphere of the test chamber down into the 10-9 torr range within 24 h. Pressure in the test chamber was determined by a calibrated scale based on the pump current. After pumping for extended periods, the pressure was probably below 9.1~10 ment was not capable of sensing accurately in this range.

torr, but the equip-

TESTPROCEDURE

Just

prior to assembly,

the test specimens

were ultrasonically

degreased,

first

in mildly alkaline detergent solution, then in deionized water. They were next washed in A.C.S. grade acetone and dried by blowing with dry, filtered nitrogen gas. After being ultrasonically degreased once more, this time in clean Freon TF, all specimens were examined under a microscope (40 x ) in order to be certain that no dirt, wax, lubricant, resin, or other macro-size particles were present. After cleaning, and during subsequent assembly and test operations, parts were handled with clean, white, nylon gloves or with clean stainless steel tweezers. The force transducer was calibrated just prior to and immediately after each test. After assembly, the drive was turned on briefly at 2-3 c/min, under atmospheric conditions, in order to make any final mechanical adjustments. The apparatus was operated in air at 40 c/min for IOOO cycles, to simulate Wear, II (1968)

the

FRICTION AND WEAR OF POTENTIOMETER CONTACTS

359

run-in of a potentiometer during build-up and bench testing of space vehicle systems. Relative humidity in the laboratory varied from 45 to 55%. Cycling was continued during all the preliminary phases of vacuum pumping, but was stopped during the bake-out period in order to assure uniformity of test procedure. The inside of the chamber was found (by measurement with a thermocouple) to reach a maximum of 200’F during bake-out. All testing was at room ambient temperature (70-75°F). Operation under high vacuum was conducted at 40 c/min until a total of 10,000 cycles had been completed, then at 60 c/min for a total of IOO,OOO cycles, except in those cases where friction became so high that there was no practical value in continuing. DATA AND DISCUSSION

No lubrication

Excessive friction (fk=o.S) occurred under vacuum conditions in both of the tests in which no lubricant was used. These tests were therefore discontinued after 10,000 cycles. Inspection under a 40 x microscope revealed considerable galling of the relatively soft resistance wires, see Fig. 5. These preliminary results served to confirm the findings of the earlier tests on actual hardware, and established the usefulness of the present equipment. Evaluation

of solid lubricant

The simplest

powders

and cheapest

modification

of these wire-wound

potentiometers

Fig. 5. Appearance of resistive elements after tests with various solid lubricants. (a) Test No. 2, precious metal contact, no lubricant; (b) test No. *a, precious metal contact, NbSea (40~) lubricant; (c) test No. 18, precious metal contact, MoSez(* 5 p) lubricant: (d) test No. 21, precious metal contact, MO& (5-10 p average) lubricant. Wear,

II

(1968)

would be the application of some lubricative, electrically conductive, and non-outgassing substance to the sliding contact and the resistance windings. In the recent literature, considerable attention has been given to several members of the chemical family

known

as metal

dichalcogenides.

These

have the general

formula

MX2,

in

which M is a metal from Groups VB or VIB of the periodic table and X is a nonmetal from Group VIA. The naturally occurring member in this family is the familiar MO& (molybdenum disulfide) which is commonly referred to as a lubricative semiconductor. BOES~ described the frictional and electrical properties of synthetic disulfides, diselenides and ditellurides of molybdenum, tungsten, niobium (columbium) and tantalum. He reported volume resistivities of several low-friction materials, Table I. TABLE I

..--.. _____-.

Compound _

MO& (synthetic)

MoSez NbSez Graphite

Friction coefficient* (fk) at 35 ft./w& and 170 P.S.;.

Volume resistivity (ohm-cm)

0.18 0.22

851

18.6.10-3 0.555.10-3

O.II

2.64.10-z

(not tested) --

* Tested under normal atmospheric TABLE

conditions.

II

EVALUATION

OF

SOLID

LUBRICANTS

FOR

PRECIOUS

METAL

CONTACTS

IN

WIRE-WOUND

Total cycles

Remarks

POTENTIOMETERS

Lubricant

Range ffk)

None

0.15

0.3-0.8

0.7

10,000

None

0.15

0.3-0.8

0.7

10,000

NbSez (40 p)

o.zo-o.14

0.07

0.08

NbSez (40 EC) O.IQ-0.14

0.07-0.38

0.20

MoSez (-5

0.14-0.10

0.04-0.06

0.07-0.08

100,000

o.ro-O.II

0.04-0.05

0.08-0.09

100,000

p)

MO&* *

of coefficient

of friction

* After completing vacuum portion of test. ** Molykote 2. Particle size ranges from sub-micron Wear, II (1968)

100,000

~0.000

Galling; friction.

high

wear;

erratic

Galling; friction.

high

wear;

erratic

Considerable debris; friction “pattern” improved after return to atmospheric pressure. Friction much higher and more variable than test No. PA. Negligible indication of wear. Slight debris from application of powder. Excellent film formation. No indication of wear. Polished spot on contact. Slight debris from application of powder. Excellent film formation.

to approximately

70,~.

FRICTIONAND WEAROF POTENTIOMETER CONTACTS

361

In the present work, two of the synthetic, electrically conductive solid lubricants (NbSes and MoSez) were compared with the unlubricated system and with Molykote 2, a technical grade, natural Moss (molybdenum disulfide). MO& was inbecause earlier work by DEVINE~ cluded in spite of being a “semiconductor”, indicated that in very thin films, MO& does not prevent electron flow. In DEV~NE’S work, the noise level in rolling element slip rings was quite low, and the only apparent limitation on the use of Moss was wear life of the film. The solid lubricants were applied to the precious metal contacts and to the wire windings of the resistive elements by dipping a clean, cotton Q-tip in the powder and rubbing the Q-tip on the applicable surfaces to produce a very thin film. An excess of the lubricant was used in each case, and the non-adhering, powdery residue was removed (prior to test) by tapping the specimen on a hard surface. Results of the unlubricated tests and the tests with the solid lubricants are presented in Table II. No satisfactory explanation can be given at this time for the poor repeatability with NbSez (niobium diselenide) . Lack of homogeneity within the sample and deterioration in storage are possibilities. The other data indicate that the equipment and procedure yield quite repeatable results. (It should be noted that in actual potentiometers NbSez, applied by the same technique, performed very satisfactorily. At present, however, no data are available on the effect of storage time on this performance). Moses (molybdenum diselenide) and MO& were consistent in friction pattern, and both were considerably lower in coefficient of friction than NbSes. Figure 5 indicates that the amount of wire wear with all these solids was negligible after IOO,OOO cycles. Moses and MoSz offer one further advantage over NbSes under these conditions: film formation was achieved with great ease with the first two, while considerable rubbing was required to obtain metal discoloration with the latter. Reporting a friction experiment carried out in three different atmospheres, RITTENHOUSE et al.3 noted that fk for Moss is considerably lower in a laboratory-produced vacuum than in air, and still lower on an actual space vehicle. The respective values were 0.14, 0.10, and 0.04. Numerous investigator+,5 have observed that fk for Moss is considerably lower under conditions of high vacuum. This phenomenon has been attributedeFTto the removal of loosely adsorbed or adhering moisture. DEACON ANDGOODMAN* consider the mechanism (of increased friction) to be hydrogen bonding at the contacting edges of MO!% crystallites. In the present author’s work, all the solids tested exhibited lower friction in vacuum than at atmospheric pressure. In all cases the value was approximately half of the lowest value obtained in air prior to the vacuum portion of the test. The lowest and most uniform friction was observed with MOSS(0.04-0.05). Evaluation of self-lzcbricatilzg compacts Another potential solution to the problem of friction and wear of wire-wound potentiometers in vacuum is the use of self-lubricating metal composites, which can be soldered or suitably bonded to the moving electrical contact. The material need not be a perfect conductor, but it should have a reasonably constant surface resistance, and a uniform friction pattern. Stick-slip should be minimal, because discontinuities in the contact of the sliding surfaces will produce electrical “noise”, due to momentary open circuit events. Several investigators have evaluated self-lubricating, electrically conductive Wear,

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362

J.R.

JONES

compacts formed by ordinary powder metallurgy techniques. CLAUS AND KINGERYg studied slip ring brush compositions containing silver, copper, and MoS2. He concluded that the optimum mixture to use, for high and low values of both speed and current density, is approximately 82.5/2.5/15.0 of silver,lcopper/MoSz, although he did not rule out other proportions. MOBERLY AND JOHNSON~~ compared two similar compacts in slip ring applications: 85115 silver/MoSs and 85115 of silver/NbSez. The compact containing NbSez exhibited somewhat better performance with respect to contact resistance, although both materials were excellent in all respects, and the authors predicted both extremely long wear life and very low radio noise levels in a space environment. In the present work, the composition selected from this general family of compacts was &/a0 silver/NbSez. Several years ago CAMPBELLANDVANWYK~’ reported the development of high temperature bearing materials by special powder metallurgy techniques. Various metals were combined with MO!% (and/or other metal dichalcogenides) under very high temperatures and pressures, in graphite dies, using an inert atmosphere (argon gas). In the successful composites, metal dichalcogenides were combined in weight percentages ranging from 50 to 80%. The compacts were not only very close to theoretical density, but were found to be electrically conductive. Several recently developed materials of this type were therefore evaluated by the present author as potentiometer contacts. The data presented in Table III indicate that all of these high temperature composites exhibit much lower friction, both in air and in vacuum, than the

TABLE

III

EVALUATION

OF SELF-LUBRICATING

Material

COMPACTS

AS ELECTRICAL

Range of coefficient of friction

Total

(fd

Cj&S

Air

Vacuum

CONTACTS

IN POTENTIOMETERS

Remarks

Test 120.

Ai+ Heavy wear and debris formation.

6

100,000

Negligible contact wear. Very slight debris.

5

0.20

100,000

Negligible contact wear. Very slight debris.

I4

0.08-0.24

0.38

100,000

Moderate contact wear and debris formation. Erratic friction in vacuum.

13

0.17-0.28

0.18-0.56

0.34

100,000

Light wear. Erratic, high friction in vacuum.

No. 04r)-45

o.I~-o.og

0.04-0.19

0.12

100,000

Negligible wear and debris.

16

No. 04”~45

0.16

0.04-0.07

0.12

100,000

Negligible wear and debris.

I9

Silver/NbSea**

0.67

0.57-0.67

0.57

100,000

Heavy wear and debris.

0.21

0.28-0.37

0.28

No. R30

0.18

0.06-0.10

0.18

No. R30

0.1g-0.25

O.IO-0.11

No. R30-SI

0.17-0.29

No. 33-1-H

No.

114-S

9,100

* After completing vacuum portion of test. ** Commercial sintered compact containing 8076 silver, Wrar, 11 (1968)

20’$/,

NbSez

20

4

FRICTION AND WEAR OF POTENTIOMETER

CONTACTS

363

silver/NbSez compact. It should be noted that the latter composition may not be equivalent in physical properties to those tested by CLAUSS AND KINGERYg. The chemical elements and compounds used to make the high temperature compacts are listed in Table IV. One of the compacts, 114-5, wore away so rapidly TABLE

IV

CHEMICAL

CONSTITUENTS

Compact number

OF HIGH

TEMPERATURE

Lubricative compounds NbSez

MO.%

114-5 R3o R30S1 33-I-H 046-45

Other elements or compounds present PbO

Ta

X

X

X X

that the material cycles

without

lowest

wear

comparison

MO

Ag

Fe

debris of wear

X

X

X

X

X

X

X X

The

lowest

friction,

of all the tests on composites on the 046-45

of the quantity

sioz

X

X

was ruled out of further consideration. wear.

Pt

X

X X

heavy

B

X

X

X

indication

COMPACTS

specimens,

of debris formed

uniform

was observed

after

IOO,OOO

All the otherscompleted

most test,

(on resistance

friction with

was a thin, wires)

pattern,

and

046-45. The only polished

by several

line.

A

compacts

is shown in Fig. 6.

Fig. 6. Appearance of resistive elements after tests with various compacts. (a) Test No. 5. Boeing No. R-30 contact; (b) test No. 6, Boeing No. 114-s contact; (c) test No. 16, Boeing No. 046-45 contact; (d) test No. 20, Boeing No. 33-IH contact. Wear, II

(1968)

(‘orupacts o_+b-qj and 1C.30were the only ones which exhibited significant reduction in ,fk under conditions; of Iligh vacuum. In addition, ft for these t\vo compact-; returned to the original “air” values when air was bled back into the test chamber. ‘l‘hi~ apparently indicates that the transfer film of lubricant from the compact is predominantl>, RIoSz, and is strongly affected bv moisture. The cxccllent performance of hot11 these materials makes them potential -candidates for unlubricated tests in actual F”)tentiomcter~. Friction

putterns The general shapes of the oscillating friction patterns observed in this study are shown in Fig. 7(a). Aberrations were to be expected, because of the irregular nature of the contacting surfaces. These commonly took the form of simple variations in friction force Fig. 7(b), but in the unlubricated tests, and at very low speeds with solid lubricants, considerable stick-slip occured Fig. 7(c). In the friction traces, stick--slip is readily discernible as a slow rise in friction force followed by a very rapid drop. Examples of actual friction traces are shown in Fig. 8. Note that the amplification of the signal from the force transducer was varied for convenience and clarity. AVERAGE ‘FRICTION FORCE

TEN;m.

0

TIMEa

TYPICAL

FRICTION

TRACE

b

ENLARGED

PORTION

OF a

c

ENLARGED

PORTION

OF 0

ILLUSTRATING

Fig. 7. General appearance

ILLUSTRATING

SIMPLE

VARIATIONS

STICK-SLIP

of friction traces.

Some very interesting changes in the friction patterns of the powders occurred when air was bled into the vacuum chamber after testing. In practically every case in which friction had become erratic, the pattern became xqgular again, possibly indicating that bare, metallic areas which were exposed after long periods of wear, had rapidly adsorbed oxygen and/or water. An unexpected observation was made when comparing the low speed friction patterns of specimens lubricated with MO!& and either NbSe2 or MoSe2. At 2-3 c/min, U’PUY,

II

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FRICTION AND WEAR

OF POTENTIOMETER

CONTACTS

365

prior to evacuating the chamber, all of these solids exhibited stick-slip behavior. Although this could partially be attributed to incomplete film formation, the stick-slip disappeared almost entirely with MO& under vacuum conditions; nor was stick-slip behavior resumed (with MoSZ} when air was again bled into the chamber. With NbSez and MoSez, however, stick-slip was quite evident under all conditions, at the low speed. In spite of this, wear was low with both substances. The general shape of the NbSez friction pattern became considerably more uniform under atmospheric conditions after test. This was not the case with MoSea: in air, following vacuum, the pattern gradually became rougher, and the magnitude of stick-slip increased. Under vacuum conditions, the friction patterns observed with the high temperature compacts were somewhat more erratic than those of the powders, even though fk became quite low in several cases. Usually the pattern improved (became more uniform and “square-shaped”) after air was bled back into the test chamber. A striking similarity can be seen by comparing the lubricity (fk) of MO& and NbSe2 in the compacts with their respective performance as powders. In both cases, with MoS2

aTEST

NO. PUNLUBRICATED

PALINEY

NO.7

AT 10,000

CHART

b. TEST 9300

NO. 2o-NbSe2(40p) CYCLES

LUBRICATED

CYCLES

SPEED:

PALINEY

NO. 7 AT

C;;~,;;~ED: 60 CPM VAC = 9 I: IO*

c TEST NO. 2l-MoS2(10-20~) 52,000 CYCLES

LUBRICATED

PALINEY

TORI,

NO. 7 Al

Fig. 8. Friction patterns from actual tests.

the reduced value off* in vacuum was accompanied by elimination of stick-slip. With NbSez, stick-slip was present in vacuum, at low speeds, with both compact and powder. Although the complete chemical composition of these compacts has not yet been fully established, it is presumed that a considerable portion of the original solid lubricant powder is still present after sintering. Weav.

II

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J. I<. JOh’ES

366

It was very apparent that, under all conditions, MO& provided the lowest coefficient of friction and the smallest variations in friction of any material tested. The use of MoSz should definitely be evaluated in actual potentiometers, even though its resistivity (as a solid) is about five orders of magnitude higher than other solids. CONCLUSIONS

In order to function properly in high vacuum, wire-wound potentiometers must be provided with some form of lubrication for the sliding electrical contacts. This paper has described the evaluation of several lubricative powders and electrically conductive composites as substitutes for fluid or semifluid lubricants. With respect to reducing friction and wear in vacuum, it is concluded that the material combinations listed below are effective, and should be evaluated further. The combinations are listed in a tentative, descending order of merit. Contact

Lubricant

Precious metal (Paliney No. Precious metal (Paliney No. High temperature composite High temperature composite Precious metal (Paliney No.

7) 7) 046-45 Rio ‘_ 7)

MO& MoSe2 None None NbSes

It should be noted that these conclusions do not in any sense reflect the electrical behavior of these materials in potentiometers. ACKNOWLEDGMENTS

The author wishes to express his appreciation to M. N. GARDOS for his invaluable assistance in preparing test specimens, conducting tests, and reducing the data, to H. B. LYMAN for operating and maintaining the vacuum equipment, and to the Jet Propulsion Laboratory for sponsoring the original work. He also wishes to thank Hughes Aircraft Company for permission to publish this paper. This paper presents results of one phase of research carried out under Contract 950056 for the Jet Propulsion Laboratory, California Institute of Technology, under Contract NAS 7-110 sponsored by the National Aeronautics and Space Administration. REFERENCES I D. J. BOES, New solid lubricants space, 2

2 (2) (1964)

: preparation, properties and potentials,

457-466.

E. J. DEVINE, Rolling element slip rings for vacunm application,

NASA

IEEE Tech.

Trans. Note

Aero-

D-2261,

1964. 3 J. B. RITTENHOUSE, L. D. JAFFE, R. G. NAGLER, AND H. E. MARTENS, Results of Ranger I flight friction experiment, AIAA J. I (8) (1963) IgIp--IgI5. 4 V. R. JOHNSONAND G. W. VAUGHN, Investigations of the mechanism of MO& lubrication in vacuum, J. AppZ. Phys. 27 (IO) (1956) rI73--I17g. 5 A. J. HALTNER, An evaluation of the role of vapor lubrication mechanisms in MO&, Wear, 7

(1964) 102-117. 6 H. F. BARRY AND J. P. BINKELMAN, MO.% lubrication (4) (1966) rx-145. Wear, rr (1968)

of various metals, Lubrication

Eng.

22

FRICTION AND WEAR

OF POTENTIOMETER

CONTACTS

367

7 J. GANSHEIMER, Neue Erkentnisse iiber die Wirkungsweise von Molybdandisulfid als Schmierstoff, Schnaiertechnik, II (1964). 8 R. F. DEACON AND J. R. GOODMAN, Lubrication by lamellar solids, Proc. Roy. Sot. (London). Ser. A, 243 (1958) 464-482. 9 F. J. CLAUSS AND M. K. KINGERY, Sliding electrical contact materials for use in ultra-high vacuum, J. Spacecraft Rockets, 4 (4) (1967) 480-485. IO L. E. MOBERLY AND J. L. JOHNSON, Electrical sliding contacts for application in space environments, IEEE Trans. Aerospace, Supphnent (1965) 252-257. II M. E. CAMPBELL AND J. W. VAN WYK, Development and evaluation of lubricant composite materials, Lubrication Eng., 20 (12) (1964). Wear, II

(1968)