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Friction, Wear, and Evaporation Rates of VariousMaterials in Vacuum to 10-7 mm HgDonald H. Buckley a , Max Swikert a & Robert L. Johnson ba NASA—Lewis Research Center, Cleveland 35, Ohiob Lubrication Section, NASA—Lewis Research Center, Cleveland 35, OhioPublished online: 25 Mar 2008.

To cite this article: Donald H. Buckley , Max Swikert & Robert L. Johnson (1962): Friction, Wear, and EvaporationRates of Various Materials in Vacuum to 10-7 mm Hg, A S L E Transactions, 5:1, 8-23

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ASLE TRANSACTI ONS 5, 8-23 (1 962)

Friction, Wear, and Evaporation Rates of VariousMaterials in Vacuum to 10-7 mm Hg

By DONALD H. BUCKLEY,t MAX SWIKERT,2 and ROBERT L. JOHNSON3

Evaporat ion data on soft metals, lubricating inor ganic compounds, and varia li S reference ma terialsare repor ted for tem peratu res from 75 to 1000 Fin vacuum as low as 10 - 7 111m H g, Observa tionson 11I0des of vacuu m degradation (e .g., evaporation or dissociation ) and methods of experiment a­tion are related. Fricti on and wear data are present ed for several unl ubr icated m etal s (e .g., typ e440-C steel ) and m etals coated with inorganic (e .g., MoS2, CaF 2 ) as well as with soft metalfilm s in va cullm at ambient pressures between 10- 6 and 10- 7 111m H g,

Introduction

THE requirements for bearings and seals to operate inthe environment of space dictate a new area for lubri­cation research. The low ambient pressures encounteredin space can be expected to influence the behavior of oil,grease , and solid-film lubricants. The property of thesematerials most significantly affected by low ambientpressures is the evaporation rate. Various investigatorshave therefore measured the evaporation rates of oilsand greases in vacuum as one method of establishingtheir relative merit for space applic ations (1-3). Theresults of this work have given some indication as tothe oils and greases with the greatest stability at re­duced ambient pressures. Only limited experimentalwork, however , has been reported in the literature forinorganic solids and soft metals which have potentialuse as solid lubricant films or coatings for hard alloysubstrates [e.g. Reference (4)] . In general , the evapora­tion rates of these materials would be lower than thoseof oils and greases. These films might therefore be veryattractive as lubricants for high vacuum service.

The lack of oxygen in outer space creates anotherproblem for lubrication systems , namely, the absenceof the protective metal surface oxides normally encoun­tered in air. In the absence of these or other protectivesurface films, mass metal transfer, welding, and highcoefficients of friction may be experienced for metals insliding contact (5-7) . Some data are presented in theliterature for the fricti on and wear of metals and softalloys in vacuum [e.g. Reference (8)] . In general , thesedata were obtained with conventional oil diffusion pumps

Presented as an American Society of Lubrication Engineerspaper at the Lubrication Conference held in Chicago, Illinois,October 1961.

1 Aeronautical Research En gineer, NASA- Lewis Resear chCenter, Cleveland 35, Ohio.

2 Aeronautical Research En gineer, NASA-Lewis ResearchCenter, Cleveland 35, Ohio.

3 Head, Lu brication Section , NASA-Lewis Research Center,Cleveland 35, Ohio.

8

in the vacuum systems. One of the problems associatedWith such systems is that of back-migration of oil vaporsfrom the pump to the test chamber. This back -migrationeven with cold traps and baffles can serve as a sourceof specimen contamination and result in a pronouncedreduction in friction and wear.

There is a need for data on the evaporation rates oflubricants and also on the friction and wear character­istics of potential bearing and seal materials; these datamust be obtained in a vacuum environment to facilitatethe proper selection of lubricants and slider materialsintended for use in space. The friction and wear datashould be obtained in systems which are free fromsources of specimen contamination like that associatedwith the oil diffusion pumps.

The objectives of this investigation were to determinein vacuum (10- 6 to 10- 7 mm Hg) : (a) the evaporationrates for various organic and inorganic lubricants, (b)the evaporation rates of solid lubricant coatings, (c) thefriction and wear properties of unlubricated slider maoterials for reference , and (d) the friction and wear prop­erties of these slider materials coated with solid lubricantfilms. Evaporation-rate experiments were used to selectthe most promisin g solid lubricants for use as surfacefilms on slider materials in vacuum. Evaporation rateswere measured at temperatures from 55 to 1000 F . Fric­tion and wear experiments were conducted with a 3/16­in. radius rider sliding on a 20 in. diameter disk speci­men at a surface speed of 390 ft / min. The rider wasloaded against the disk with a 1000 g load. The dura­tion of the experiments was 1 hr .

Materials

The specimens used in the evaporation experimentsof this investigation were oils, greases, inorganic solids.metals, and solid lubricant coatings. The oils includedMIL-L-7808 (di-2-ethyl hexyl sebacate), two mineraloils (viscosities of 380 and 235 centistokes at 100 F )designed for high-temperature use, and a polyphenylether r1-(p-a-cumyl phenoxy) -4-phenoxy benzene1. Theether was a solid at room temperature. The greases

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Friction of Materials in Vac uum 9

T ABLE 1Alloy Com positions

Co m posit io n Hardn ess,Metal Fe :'\i Co Cr C M o w Ti Si l\In AI Ocher Rockwell

52100 Hal. 1.3 0.95 0.20 0.25 P , 0 .Q2 5 R~ 60-43of to to to to S. 0.025

cornp, 1.6 1.10 .35 .45

440-C Bal. 16.0 0.95 0.75 1.0 1.0 P . S R" 54oi to to M ax .

cornp . 18.0 1.20

Ni-Cr 0.250 Bal. 11.0 18.0 0.06 9.0 3.2 0.010 0.050 1.50 R c 43oi to to to to

co rnp . 12.0 20.0 .12 10.5

Ni-Cr-Fe 5.0 70.0 15.0 0.08 0.50 S,Cb R~ 29to AI,Mn

9.0

Cobalt alloy 3.0 2.5 43.0 .\ 1.0 2.4 17.0 1.0 R~ 52-63

Ni-bonded T iC 25.0 13.2 5.0 52.0 Cb , 4 .5 ; R .\ 89and CbC" Ta , 0.3

a Ti and Cb a re p resent as mi xed carbides.

F IG. I (a ) . Vacuu m appara tus for bulk evapora tion studies

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10 DONALD H. BUCKLEY, ]\ I AX SWIKERT, AND ROBERT L. JOHNSON

included were a radiation-resistant grease and twomineral-oil-base greases with phthalocyanine thickeners .The inorganics included chemically pure materials ob­tained from commercial supply houses: cadmium iodide(CdI2 ) , cobalt chloride (CoCI2 ) , nickel fluoride (NiF2) ,

lead oxide (PbO) , calcium fluoride (CaF2 ) , bariumfluoride (BaF2 ) , tungsten disulfide (WSJ , and molyb­denum disulfide (MoS2 ) . The nickel bromide (NiBr2)could not be readily obtained commercially and wastherefore prepared in the laboratory. The metals includedwere cadmium (Cd) , indium (In) , zinc (Zn) , magnesium(Mg) , tin (Sn), gallium (Ga) , lead (Pb), and silver(Ag). Two organics other than the oils and greases forwhich evaporation rates were measured were poly tetra­fluoroethylene (PTFE) and epoxy (diglycidyl ether ofbisphenol A with a diethylene triamine catalyst).

The alloys used in the friction and wear experimentwith their compositions are presented in Table 1.

I

The molybdenum disulfide coatings used were coatingsof the following types: silicon resin bonded, epoxy­phenolic resin bonded, ceramic bonded, and a metalmatrix coating. The coatings were applied to 0.003 in.thick metal tabs for evaporation experiments. Thethickness of the applied coating was 0.001 in. In thefriction and wear experiments, however , the coatingthickness on the disk specimens was 0.0002 to 0.0003 in.

Apparatus

The apparatus used in this investigation is presentedin Figs. 1(a) to 1(c). The evaporation apparatus con­sisted of a commercial bell-jar vacuum system with a4 in. oil diffusion pump system. Pressures in the 18 in.diameter chamber were measured with thermocoupleand hot cathode ionization gages . Inside the bell-jarchamber just above the throat of the diffusion pump,

ELECTRONIC BALANCE

LIQUID-NITROGENCOOLING COILS

CONDENSINGPLATE

FURNACE

FIG. l(b) . Vacuum evaporation apparatus with balance for continuous rate measurements

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Friction of Materials in Vacuum 11

a large pla te was used to support a heater and th etest specimen. The heater co ns isted o f a housing ofmachinable cera mic a nd a wire-wound tungst en heatingelement. T he specime n was p lac ed on th e heating ele­ment. A th ermocouple was position ed on the top edgeof the specimen for tempera tur e measurem ent and con­trol.

Approximately 4 in. above th e specime n was a con­densing shield . This shield was cooled with a liquid­nitrogen coil. a nd th e temperature measured at th e centerof the conden sin g shield when liquid passed throu gh th ecoil was - 20-l F . On th e side o f th e shield fac ing th especimen \ \'a 5 a copper ta b for making X -ray analysesof condensa te composition .

The bell-jar evapora t ion apparatus was modifi ed foruse with a continuous recording balance. The apparatuswith associat ed modi fications is present ed in Fi g. 1(b ) .The \H·jghin!! mech ani sm of a comme rc ia lly availableelectronic ba lan ce was mounted on a support in th eupper par! of th e bell ja r. Suspe nded from th e bala ncebeam \\"a ~ a thin wir e th at exte nded from th e bala ncebeam through th e co nde ns ing shield to a s t ir rup thatsupported the spe cimen. The specimen a nd st irrup weresuspended in th e mouth of a cylind rica l furnace . Thewalls of till" fu rn ace conta ined th e heating elements ofwound tunuste n wire. A d ummy specimen with thermo­couple placed beneath th e test specime n was used to

/ MOTOR

TO PUMP~

measure a nd cont ro l specime n temperatures . The d ummyspecimen techniqu e ha s bee n employed by others insimila r experimen ts (9) . W eight cha nges in th e specimenwere cont in uously recor ded on a s trip cha rt.

The vac uum frict ion a nd wea r apparatus is shownschema t ica lly in F ig . 1(c) . The ba sic elem en ts of theappa ra tus wer e a 2.0 in. dia meter d isk a nd a 3/ 16 in .radi us rid er specimen. The disk was ro ta te d by meansof a n indu cti on mot or with a ca nned ro tor a nd sta tor .The enclosur e of ro tor a nd sta tor pe rmitted a vacuumin th e mot or cha mb er. A vacuum valve connecte d th emot or cha mbe r with a mechanical pump sys tem. Thesystem included a liquid-nit rogen cold trap to inhibit oilmigrat ion from th e pump . The specimen chambe r con­taining th e disk and rider was a ttached to th e mot or.The motor drive shaft extende d into th e specimencha mber with th e d isk specimen bolt ed direct ly to th eend . The only sea l between cha mbers was that a ffordedby a ball bearing mounted on th e drive sha ft. Sinceth e presen ce of th e ball bea rin g did not provide a vacuumseal, th e ionization pump pumped both cha mbers . Whenth e pr essure in th e specimen cha mbe r was 10 - ; mm H g,it was approx ima te ly 10- 1 mm H g in the mot or cha m­ber.

The rider specimen was suppo rted in th e specimencha mb er by a reta ining a rm that was gimbal a nd be llowsmounted to th e cha mber. A lin kage a t th e end of th e

oLOAD

BAKEOUT HEATERS

MOTOR CHAMBER: 410 . mm Hg

SPECIMEN CHAMBER0-7I mm Hg

1' 1(; , 1(c) , Vacuu m irict ion a nd wea r a p pa ra t us

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12 D ONALD H. B UCKLEY, MAX SWIKERT, AND R OBERT L. J OH NSO N

retairung arm away from the rider specimen was con­nected to a strain-gage assembly . The assembly wasused to measure frictional force . Load was appli edthrough a dead-weight loading system.

Attached to the lower end of the specimen chamberwas a vacuum valve. The side of the valve away fromthe chamber had aT-section. On the lower portion ofthe T was connected a 400 liters/ second ionization pump.The remaining portion of the T connected with a vacuumvalv e and a mechanical pump using liquid-nitrogen coldtr aps.

The first high-vacuum pump to be used with thevacuum friction and wear app aratus was a conventionaloil diffusion pump. In initial experiments it was estab­lished , however , that specimen contamination by oilvapors had occurred (see Appendix A). Various liquid­nitrogen cold-trap designs were employed to elimin atethis problem. Subsequ ent specimen examinations, how­ever , indicated surface contamination. Combinations ofcold traps, optical baffies, and heating grids for decom­position of oil vapors were then tried with little or nosuccess. Since, even with the best trapping techniques,the possibility of specimen contamination did exist , thediffusion pump was replaced by an ionization pump. Thi spump employs no fluids or vapors in its opera tion.

Experimental procedure

The pro cedure used in the evapora tion studies con­sisted of first preparing the experimental specimens. Thiswas accomplished for the solids by oven-drying and de­hy drating the materials. When the powders were dry,small charges were placed in a mold and compressedinto tablet form und er a 10,000 psi compression force .When the specimens were removed, they measured % in .in diameter and were approximately ;Is in. thick . Thetablets were then placed into a small preweighed glazedceramic dish that had side walls of ;Is in . and insidediameter just lar ge enough to accommodate the tabletsp ecimen. The specimens and holders were then storedin a desiccator until ready for use. The oils and greaseswere placed directly in the dishes with the specimen fill­ing the dish .

Prior to an experiment the specimens were removedfrom the desiccator and weighed . The specimen andholder were then placed into the vacuum chamber, andthe chamber was evacuated. At a pressure between 10- 6

and 10- 7 mm Hg the experiment was started. In thoseexperiments conducted above room temperature, thespecimens were heated by a tungsten heater on whichthe specimen was placed.

The evaporation-rate experiments made with solidlubricant coatings were run using the electronic balancewith a continuous measure of weight change with time.During an experiment a tab was placed on the stirrupthat was suspended from the balance at the top end ofthe furnace , and the chamber was then evacuated. Ata pressure between 10-6 and 10-7 mm Hg the evapora­tion experiments were started.

The disk and rider specimens used in friction and wearexperiments were finish-ground at 4 to 8 f! in. Beforeeach experiment the disk and rider were given thesame preparatory treatment. This treatment consistedof (a ) a thorou gh rinsin g with acetone to remove oiland grease, (b ) polishing with moist levigated aluminaon a soft cloth , and (c) a thorough rinsing with tapwater followed by distilled wat er. For each experimen t(data point) a new set of specimens was used . In thoseexperiments in which disks with coatings were used , thecoatings were applied after the cleaning procedurePlated specimens were solvent-cleaned, and bonded coat­ings were carefully handled following coating in orderto avoid contamination.

The specimens were then placed in the apparatus.Mechanical pumps with liquid-nitrogen cold traps wereused to obtain a pressure of 10- 4 mm Hg in the speci­men chamber. In the startup of the ionization pump .ionization of chamber gases was produced by the pump .This ionization generally persi sted for 20 to 30 min.In the event that oil molecules had migrated throughthe liquid-nitrogen cold traps of the mechanical pumpsprior to being valved off from the specimen chamber.the ionization would decompose them.

When the specimen chamber pre ssure was between5.0 X 10-7 and 2.0 X 10- 6 mm Hg as measured bya hot cathode ionization gage and by the ionizationpump current , the friction experiment was started . A1000 g load was applied to the rider loading it againstthe disk. Friction force was continuously measur ed andrecorded on a strip chart. The wear was measured asvolume loss of the rider specimen upon completion of1 hr of running. The wear to the disk specimen sur facewas recorded with a surface profile measuring device .

Results and discussion

EVAPORATIO N DATA

The evaporation rates for some low-vapor-pressure oilsand greases were determined in vacuum at various tem­peratures. The results obtained in these evaporationexperiments are presented in Fig. 2. The sebacate MIL­L-7808 bubbled cont inuously while evaporating. Thehigh-temperature mineral oils (viscosities of 380 and235 centi stokes at 100 F) did not exhibit this character­istic , and the surfaces were rather quie scent duringevaporation at 55 and 200 F. The ether (l-a-cumylphenoxy-4-phenoxy-benzene was a solid at 55 F and ex­hibited no detectable weight change in 60.0 hr. At 200 Fthe ether was in a liquid state, and the evaporation ratewas quite high (2 .5 X 10- 6 g/ cm-/sec ).

The greases appeared to lose the base oil with in­crease in temperature, leavin g the bulk of the thickenerbehind. The evaporation experiments were terminatedbefore complete evaporation of the base oil had oc­curred. This was found desirable in order to avoid achange in evaporation rates once the base stock hadcompletely evaporated and the thickener with its owncharacteristic rate began to evaporate. The radia tion -

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Friction of Mater ials in Vacuum 13

A - EVAPORATION RATE LESS THAN I.OX IO- IO 9/CM2/SEC

GREASES

uwen<,

NLu<,c>

WI­<t0:::

Zo~ 10-50:::oa,<t>W

A

RADIATION­RESISTANTGREASE#159

\"0-"380­CENTISTOKEVISCOSITY

A

MINERAL -OILBASE WITHPHTHALOCYANINETHICKENER 0

r :::·] 55 0 F......

~ 2000 F

[-:,:-:-:,:1 3500 F......

OILS

FI G. 2. Evaporati on rat es for va rious oils and greases in vacuum. Ambient pressur e,8.0 X 10- 7 to 2.0 X 10- 6 mm H g.

A - EVAPORAT~N RATE LESS THAN

I.OXI0-10 g/ CM2 / SEC

8.5XI0 -5

r ,""JIlLNICKEL BROMIDE

G:7:1 3500 FCd

• 5000 F

o 550 F

~ 200 0 F

10 . 9 A ANICKEL FL UORIDE

uwCfl<,

"'L 10'7U<,o-

w· I0.9 L-_ --'I..U..;.,:.J.="-"-__""""'-'=-L:C:""-__~A--'" liJ%= "'---I- CADMIUM IODIDE~Q:

s 10.5I­~Q:

o~ 10.7>W

~A

COBALTCHLORIDE

FIG. 3. Evaporation rat es for possible coat ing constituents invacuum. Ambient pressure, 8.0 X 10- 7 to 2.0 X 10- 6 mm H g,

on th e specimen surface. T he cobal t chloride sampleexhibited a color chan ge from blu e to green at 500 F;however, th e evapora t ion rate was low. X -ray ana lys isof th e sample indi cated its composition to be cobaltch loride (CoCIJ . The cobalt chlor ide , cadmium iodide,and nickel bromide evaporated and condensed on th econde nsing shield as cobalt chloride, cadmium iodide,and nickel bromide, respecti vely. The epoxy composi­tion (diglycidy l ether of bisphenol A with a catalyst ofdiethy lene tr iam ine) showed evidence of deco mposit iona t 350 F .

resistant grease exhibi ted a very low rate of evaporationat 55 F. At 200 F , however, the rate increased and thespecimen became rubbery. T he greases using phthalo­cyanine as a thicke ner both showed evidence of thethickener evap ora ting wit h th e base stock, as determinedby examination of the condensate on the liquid-nit rogencondensing shield. T he evaporation rates of th e greaseswere tolerable at 200 F; however , a t 350 F the ra te wasquite high. T he total per cent of th e oil and greasesamples lost depended upon the evapora tio n rates ofthe various ma teria ls. For exa mp le a t 200 F only 3 pe rcent of the greas e with H thickener was lost in 20 hrwhile 86 per cent of the ~IIL-L-7808 had evaporatedin 14.0 hr. T he oil and greas e evapora t ion exper imentswere genera lly of 20 hr duration. In those cases wherethe evaporat ion ra tes were very high (l\H L-L-7808) th eexperiments were te rminated in shorter pe riod s. D upli­cate experiments indi cated a reproducibility of data with­in 5 per cent.

The evaporation ra tes for compressed disks of variouslubricant coating constit uents were determined , and theresults obtained in vac uum at va rious temperatures arepresented in F ig. 3. T he nickel fluoride showed no evi­dence of evapora tio n a t 55 or 200 F. At 350 F , however ,the nickel fluoride dissociated to metall ic nickel andfluorine. T he specimen surface changed from yellow­green to a black color. X-ray analysis of th e sur faceindicated th e black film to be metalli c nickel. T he leadoxide also dissocia ted at 350 F to metalli c lead andoxygen. X-ray analys is indica ted the presenc e of lead

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14 DONALD H. BUCKLEY, MAX SWIKERT, AND ROBERT L. JOHNSON

The evaporation rates for molybdenum disulfide , tun g­sten disulfide , calcium fluoride , and barium fluoride attemperatures to 1000 F are presented in Fig. 4. Theweight losses for these materials in vacuum at elevatedtemperatures were very low, and in general these ma­terials appeared to be the most stable of the inorganicstested. The rates for molybdenum disulfide , calciumfluoride , and barium fluoride were extremely close, anda single curve was drawn to represent the evaporationrate of these three materials. The molybdenum andtun gsten disulfide had to be heated in vacuum prior toan experiment to remove adsorbed gases.

The evaporation rates for polytetrafluorethylene (purepolymer without plasticizer) were determined at variousambient temperatures and pressures . The results ob­tained in these experi ments are prese nted in Fig . 5.While the polytetrafluorethylene exhibited some weightchange at various ambient press ures , this weight changewas extremely small . The evaporation rate of polytetra­fluorethylene at various temperatures was determined atan ambient pressure of 8.0 X 10- 7 to 2.0 X 10- 6 mmHg. The evaporation rate of polytetrafluoroethylene waslow to 350 F ; at 650 F the rate was high . Poly tetra­fluorethylene begins to decompose at temperatures above

500 F ; this may well account for the high evaporationobtained at 650 F .

The evaporation rates for various metals were deter­mined in vacuum over a range of temperatures, andthe results obtained in these experiments are presentedin Figs. 6 and 7. Cadmium and zinc exhibited relativelyhigh rates of evaporation. The rates were higher tha nobtained with polytetrafluorethylene. Lead, tin , andsilver had the lowest rates of evapo ration of the softmetals investigated.

When the evaporation rates determined experimental­ly in this investigation are compa red with data calculatedfrom vapor pressure presented in the literat ure (10), theresults appear to be in good agreement (Fig. 6) . Anydifferences observed could be due not only to experi­mental error but to error in the calculated val ues. Thecalculated values are based on thermodynamic relationswhere an error of 10 per cent is not unlikely . Since theevaporation rate can be determined from vapor pressure,the reverse must also be possib le. The use of evaporation

---- CALCULATED FROMVAPOR PRESSUREDATA (POINTS AREEXPERIMENTAL DATAl

o

950BOO650500TEMP. OF

350

A - EVAPORATlON RATELE SS THAN

LOX IO' IO 9/CM2/SEC

A

50

A - EVAPORATION RATE L ESS

THAN LOXIO' IO 9/ CM2/ SEC _-o J)- - -<r -SILVER-­_ 0- -

GALLIUMA A

IOIOL--'i?'-- --'i?'--- ....1....- - ....1....- - -'-- --'--- -'--

10 ' 6

FIG. 6. Evaporation rates for various metals in vacuum. Am­bient pr essure , 8.0 X 10- 7 to 2.0 X 10- 6 mm Hg,

A - EVAPORATION RATE LE SS

THAN LOXIO- IO 9/CM2/S

EC

u :;:~ ROOM TEMPERATUREUJU)<,

10 . 11

I I I'" 10.,2 I~u 10.2 10.3 10 .4 10.5 10. 6 10.7<,

'" AMBIENT PRESSURE, mmHgW 2.09xI0·4.... b<l0::

IZ -,La

10 .6;:::<l '"0:: -'"a0..<l 10.8>UJ

10" 0

zo~ti 10.8

o0..«>lLJ

Do TUNGSTEN DISULFIDEo MOLYBDENUM DISULF IDEo CAL CIUM FLUORIDEo BARIUM FLU ORIDE

500 650TEMP, OF

FIG. 4. Evaporation ra tes for various inor ganic compounds invacuum. Ambient pressure , 8.0 X 10- 7 to 2.0 X 10- 6 mm H g,

FIG. 5. Evaporation rate of PTFE at various temperaturesa nd pressures .

F IG. 7. Evaporation rates for various metals in vacu um . Am­bient pressure , 8.0 X 10- 7 to 2.0 X 10- 6 mm Hg.

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Friction of Materials in Vacuum 15

rates to determine vapor pressure ca n be accom pli shedfor pure metals by a simple equation :

~T

PIIlIll = 17.14 G -IJ1

where

Pmm vapor pressure in mrn H gG evaporat ion rate in g/ cm-/secT temperature in 0 KIn molecular weight

The vap or pressure of zin c IS det ermined from it sevaporation rate at 500 F:

( 0- 6) fllIi-33PIIIIII = 17.14 2.0 X 1 - _-6:>.38

PIIIIIl = 9.8 1 X 10 - :;

The vapor press ure of zinc at 500 F presented in th elit era tur e (10 ) is 1.0 X 10- 4 mrn H g.

Evapora t ion-ra te experiments wer e conducted withfour molybdenum disulfide coatings in vacuum at vari­ous temperatures . An elect ronic bal an ce was used to con­tinuously measure weight chan ges with t ime . The resultsob tai ned in these exp eriments a re presented in Fi g. 8.I n gene ral, th ere appears to be a break in evaporation

Em AIR~ VACUUM

~Ni -CrALLOY

'---.r--' ~440 -C COBALT-

BASE ALLOY

I-Z.4z o

~~uuU::: ~"-"-1J.J .20"-u o

0~

~2100

.6

IOOOXIO ·6

. 1

0:

~ ~:;: ;; 100: 'lJ.JZ0-52

F IG. 9. Fricti on a nd wea r of va ri ou s a lloy s in a ir a nd inva cuu m . Vacu um , 5.0 X 10- ' to 2.0 X 10- 6 mm H g ; slidingveloci ty , 390 ft / min j load , 1000 g ; duration of ru n , I h r.

95080 0650

::1 ~CERAMIC BONDED MoS2

: 0

~

:1METAL MATRIX MoS 2 COATING

~ 0 o I d"I

SILICO NE RESIN BONDED MoS 2

0 0 ::=:=:10 .9 I

zot=<lCl:oQ. PHENOLIC-EPOXY BONDED MoS 2

~ ',J~~I' L=:50 200 350

l1J....<lCl:

Ul1Jf/l<,

N

:ru<,0'

I500

TEMP. of

FIG. 8. Evap oration ra te o f various MoS~ coa tings in vacuum.Ambient pressure, 1.0- 2.0 X 10 - 6 m m H g ; O.OO I-in . MoS~ coa t ­ing on Ni- Cr a lloy .

• " 1. .. I

t \. ,:;l : . '' , I

I ~ :'. .. .. t .1

IAI DISK SPECIt:1EN. 5~100 TOOL, LSTEEL IN AIR· O.OOOS"

0.02S"

----~~-..,-----(Cl DISK SPECIMEN. 440-C STAINLESS

STEEL IN_ AIR

-~--(8) DISK 'SPECIMEN, 52100 TOOL

-STEEL IN VACUUM(D) DISK SPECIMEN, 440-C STA INLESS

STEEL iN VACUUM

F IG. IO(A ) - ( D ) . Ph ot omicro gra ph s a nd surface p rofile tracin gs o i wea r a reas on di sk spe cime ns , load , 1000 g j slid ingveloci t y , 390 ft /min ; duration o f r un , I h r j X 20. (A ) Di sk spe cime n, 52100 tool steel in a ir . (B ) Di sk spe cim en , 52100tool ste el in vacu um. (C ) Disk specime n, 440 -C sta inless st eel in air. (D ) Di sk spe cim en , 440- C sta inless steel in vacuum.

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16 DONALD H. BUCKLEY, MAX SWIKERT, AND ROBERT L. JOH NSON

. 1~~i I

;At ' :

~ l ~ , .p, . /. ,

(HI DISK SPECIMEN,Ni-Cr AllOY IN VACUUM '

l~Lp ISK S.~~CIMEN,

Ni-Cr · AllOY IN AIR0.0005" L

0.025"AIR

----~---~---------

(FI DISK SPECIMEN.COBALT-BASE ALLOY IN' VACUUM

-----~(EI DISK SPECIMEN.

CaBAL T-BASE l~Lf;lOY IN

I.,1~· .'

iEb

FIG. lO (E)- (H ) . Ph ot om icrographs a nd surface profile t ra cings of wear a reas on disk specimens, load, g; sliding veloci ty , 390fpm ; duration of run , 1 hr ; X 20 . ( E ) Disk specime n, coba lt-base a lloy in ai r . (F) Disk specimen, coba lt-base alloy in vacuum.(G ) Disk specime n, Ni -Cr -AI a lloy in air . (H ) Disk specimen, Ni -C r-Al all oy in vacuum.

(I) DISK SPECIMEN. Ni-:BONDtD TiCCERMET IN AIR f

AND CbC

0.0005"L

0.025 11

(J) DISK SPECIMEN, Ni-BONDED TiC AND esc I'CERMET-IN YAcuu".B:~:.......--.... ___...

F IG. 10 ( I ) , (J ) . Photomicro graphs a nd surface profile tr acings of wear areas on disk specimens,load , g ; sliding velocity , 390 fpm ; duration of run , 1 hr; X 20 . (I ) Specimen a nd rid er , Ni ­bonded Ti C a nd CbC cermet in air. (J ) Sp ecim en and rid er , Ni-bonded TiC and CbC cermetin vacuum.

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Friction of Xlaterial s in Vacuum 17

curves for all coatings ; thi s result indicates a change inevaporation rate. This change ma y be due to two dif­ferent evaporation rates, one for the binde r, and th eother for molybdenum disul fide itself.

F RI CTIO;-; Ar-;'O \VE AR D ATA

Unlubricatcd metals, Fri cti on and wear experimentswere conducted in a ir and vacuum with five a llov com­binations. The results obta ined in th ese experiments ar epresented in Fig. 9. The fricti on and wear values for521 00 steel sliding on itself were lower in vacu umthan in air. Examina tion of th e disk surfaces a fte r th eexperiments ind icated a chan ge in th e wear mechanism[Figs. 10(a) and (b) J. Free meta l wear particles a reoxidized in air, and th e resulting metal oxides on theparticles pr event adherence to th e base metal. At anambient pressure of lO- u mm H g, however , where oxygenavailability was ap preciably reduced, the wear surfaceof the disk showed evidence of considerable metal trans­fer as nascent wear particles readily adhered to theparent metal. The surface pr ofile tracings of Fig. 10 (b )indicate "buildup" on th e disk sur face. This was ap­parently due to th e transfer of met al from th e rid er tothe disk specimen.

The low fricti on coefficients experienced for 5210 0steel in vacuum ( lO- U mm Hg ) may be accounted forin that , even a t lO- t; mm Hg, oxyge n is pr esent. Bowdenand Tabor ( 11) hav e indicated that copper will oxidizeas rapidly a t 1O- :{ mm Hg pressur e as it will a t a tmos­pheric pressure ; also , it has been established elsewherethat , at 10- 6 mm Hg, a clean metal surface will ad sorba monolayer of gas in a second . Since some oxygen isavailable at 10- 6 mm Hg, it is pr obable th at as piecesof metal, which ar e quite hot , tr an sfer from one speci­men surface to th e other , localized oxida tion of th etransferred metal occurs . With the limited oxygen avai l­able the lower oxides of iron , FeO and F ea04 ' would

MASS WELDING OFSPECIM ENS

5 .0

~ 4 .0

I­U

~

u, 3.0u,oI-Zw 2.0~u,u,w8 1.0

form on ferrous metal and result in a fricti on coefficientlower than normally encountered in air at atmosphericpressure ( 12 ) where Fe:!O;j is one of th e oxides present.Results ob ta ined a t pr essures lower than lO- u mm Hgshould differ from th ose obta ined at lO- t; mrn Hg.

The coefficient of frict ion for 52 100 tool steel slidingon 52100 tool steel a t va rious ambient pressures is pr e­sented in Fig. 11(a) . The friction decreased from 0.45at 760 mm Hg pr essure to 0.2 between 10- 1 and 1O- :!mm Hg. The fricti on began to increase again at 10- .1

mm Hg, and a t 5.0 X 10- 7 mm Hg it was 0.375.In order to red uce th e oxygen concent ra tion availabl e

for surface reaction at pr essures of lO- u to 10- 7 mm Hga liquid helium condensing coil was added to th e fricti onand wear apparatus. Liqu id helium will cond ense allgases except helium . A friction experiment was made a t2.0 X 10- 7 mm Hg using th e liqu id helium cond enser.The result s obtained are pr esented in Fig. 11 (b ) . T heinit ial coefficient of fricti on was 0.2. This low valuebeing associated with th e pr esence of the lower ironoxides on th e specimen surfaces. As time pr ogressed,however , th e coefficient of fricti on began to increase andaf ter 30 min of opera tion when th e residual surfaceoxides had worn away, a fricti on of 5.0 was observed .The specimen then welded together and th e experimentwas termina ted by thi s complete seizure.

The fricti on and wear results ob tai ned with 440-Cstainless steel sliding on itself in vacuum were aboutthe same as in air (F ig. 9) . Examina tion of sur facephotomicrographs and pr ofile tr acin gs of th e disk weararea IFigs. 10 (c) and (d) 1 indi cat e that particles weretransferr ed to the disk specimen.

The friction and wear of a coba lt- base alloy weredet ermined in vacuum and air , and the resu lts obtainedar e pr esent ed in Fig. 9. The fricti on and wear were lowerin vacuum than in air, and examina tion of th e sur facetopography indica ted no evidence of mass metal transferin vac uum [Fig. IOU ) 1. Similar results have been ob­served for th is particular alloy in inert and reducin g

o

o RUN STARTED AT 10-7mmHg

o RUN STARTED AT 760mmHg

o DRY AIR

o

o.50

zQ .40t-~~

u, .30u,ot-Z!!:! .20~u,u,UJ

S .10

35

ol...:--l---l...:::--l.....-...l....,,--l...---lL....l..-.......L----l-~-'----l10 4 10 -1 10- 4 10-6 10 -B

PRESSURE , m m Hg

FIG. It (a ). Coefficient of fr iction for 52100 sliding on 52 100at var ious a mbient pressures. Slidin g velocit y , 390 fpm ; load ,1000 g; temperature , is F .

15TI ME , M I N

FI G. II (b ) . Coefficient of fr iction for 52100 sliding on 52100in vacuum with liquid helium cryogenic pumping; pr essure 2.0X 10 - 7 mm Hg., sliding velocity 390 fpm, load 1000 g.

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18 DONALD H. B UCKLEY, MAX SWIKERT , AND ROBERT L. J OH N SON

;-"

ITIS:

I I I'-§

- II'"'

I:i\IIIIIf

1.0X10-6

molybdenum disulfide coatings using various binderswere used as solid-film lubr icants for 440-C stainlessstee l in vacu um. The friction and wear resu lts obtainedwith 440-C sliding on 440-C coated with vario us molyb­denum disulfide films in vacuum are presented in Fig .12. From the results obtained, it appears the lubricantbinder plays an important role in the friction and wearresults obtained with molybdenum disulfide coatings.The best friction and wear were obtained with anepoxy-phenolic and a silicone resin-bonded molybdenumdisulfide coating. The metal matrix was less satisfactory,and the ceramic bonded gave the poorest results with acontinuous frictio n coefficient of about 0.3 over an entire1 hr of operating. These results, however, were obtainedat room temperature, and the epoxy-phenolic coatingmay not appear as promising at elevated temperatures.

Friction and wear experiments were made wit h othersolid film, and the resul ts obtained are presented inFig. 13. A lead oxide-silicon dioxide coati ng developedfor high-temperature use in air was applied to 440-Cstainless stee l and was run in vacu um. A fric tion co­efficient of about 0.17 was obt ained at 10- 6 to 10- 7 mmHg amb ient press ure. In air at room temperature, witha sliding velocity of 430 ft / min, th is particular coa tinghad a friction coefficient of 0.3 (14) . In vacu um th e sur­face became sufficiently hea ted witho ut an external hea taddi tion to give a friction coefficient similar to tha tobtained in air at 250 F in (14) .

A calcium fluoride solid lubricant coati ng developedfor high-temperature app lications was applied to aNi-Cr disk . A Ni-Cr-Fe rider was used again st the diskin vacu um. The friction and wear obtained are presentedin Fig. 13. The coating gave a friction coefficient of 0.18over a 1 hr period, and the wear to the rider specimenwas low. T he frictio n coefficient obtained with th is coat­ing in vacuum was lower than obtained in air at thesame temperature.

In the literature, instances are cited where thin, softmetal films were used as pro tect ive coatings for bearingsrun in vacuum (4, 15). Vario us soft metals were there-

COATINGS, 0 .0004 IN.IN THICKNESS

.000

Q::~wQ::

3= ~ .01Q:: ~ .

WZ0-0::

;Z .3W Zg lJ...Q . 2~ o tJW 0:: . 1o lJ...

RID~R__0':'-;"4 ~':"""~::'''''~~'''c~~~::---f7"~;:;--;'7'';~:---!''f'~';;----f':'~'='ON

DISK 440-C Ni-Cr- AI 440-CLUBRICANT-PbO · Si0 2 CoF 2 TINCOATING

FIG. 13. Friction and wear of alloys with various coatings invacuum . Ambient pressure, 8.0 X 10- 7 to 2.0 X 10-6 mm Hg ;sliding velocity , 390 fpm; load, 1000 g ; duration of run , 1 hr.

'---...,.-1

METALMATRIX

I.306,

CERAMICBONDED

.0 1

1.0XIO·6COATINGS, 0 .0002 TO 0 .0003 IN. IN THICKNESS

u, . 10oI- Z .0 8Z ol:jE.0 6

~ ~ .0 4wg .0 2

Q::

:g .000 1L--== "'--__~=L-_--"="""-_ _ __""=""____

0::

atmospheres (data in process of publication) . T his par­ticu lar alloy IS an air cast matenal and has about 1.0per cent siLcon in its composition. Th e influence ofsilicon on the friction and wear properties of materialsis discussed in Reference (11) . It was established in (11)tha t sma ll quantities of silicon rather markedly reducedthe friction and wear normally encountered for alloysin inert and reducing atmospheres. T his same mechan ismcould be expec ted to app ly in vacuum.

T he friction and wear properties of a vacuum-meltednickel-base alloy (Ni-Cr ) were next dete rmined. Theresults obtained in air and vac uum are presented inFig. 9. The coefficient of friction in vacuum was greaterthan in air. The rider wear , however, was about thesame in both environments. Examination of this weararea on the disk specimen indicated metal transfer invacu um [Fig. 10( h)] . From the photomicrograph itappears that pieces of metal were transferred from riderto disk and then smeared in the direction of sliding.Though the ride r wear was nearly the same in air andin vac uum, the wear mechanism on the disk surfaceswas quite different, as indicated by comparison of thephotomicrograph and surface profi le traces of F igs. 10(g)and 10( h) .

T he friction and wear values for a nickel-bondedtitanium and columbium carbide cerme t sliding on itselfis presented in Fig . 9. For this particular material bot hfrict ion and wear were higher in vac uum than in air.T he photomicrograph and surface profile tracing of Fig .10 (j) indicate evidence of metal transfer to the disksurface in vac uum.

Lubricated metals. The mechanism of wear for met­als in vacuum indicates that stable films should beemployed to lubricate these materials. Since molybdenumdisulfide has been shown to be a good dry film lubricantin air and has very low evaporation rates in vacuum ,

'---...,.-1 '-...-'SILICON PHENOLlC-RESIN EPOXY

BONDED BONDED

FIG. 12. Friction and wear of 440-C on 440-C with vario usMoS 2 films. Ambi ent pressure , 8.0 X 10- 7 to 2.0 X 10- 6 mmHg j sliding velocit y, 390 fpm j load , 1000 g; duration of run ,1 hr .

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Friction of Materials in Vacuum 19

fore appli ed to 44 0- C s ta inless stee l subs t ra tes to det er ­mine their inl1uen ce on th e fricti on and wear of th esesubstrates in vac uum . The results ob tai ned with a 440-Cstainless stee l rider slid ing on th e thin metal film (0 .000 4in. thickness ) coa te d 44 0-C disk s are p resented in Fi g.13. The fric t ion coeffi cient for lead , gold, and si lver wasless than 0.1 wh ile tin was abo ut 0.14. The wear for440-C sta inless stee l was low with all four coa tings . Thethin metal fi lms appear to have p roperties wh en appli edto hard subs t ra tes that make th em pot ential ca ndida tesas solid-film lubricants for use in vacuum .

Summary of results

Evapora tion ra tes , and fric tion and wear were ob­tained at a pressure o f 10 - 6 to 10-; mrn H g. Sincethe pressures enco unte red in sp ace may be man y orde rsof magnitude lower (a pp rox ima te ly IO -I~ mm H g ), th eresults obta ined in simila r expe rime nts a t lower p res ­sures may di ffer somewha t from th ose ob ta ine d herein .The results presented here, however , do give some in­dication of the effects of reduced a mbient pr essures onlubricant s, frict ion, and wear. The results obtai ned inthis study are s umma rized as follows :

1. The use of l\I oS~ coatings (e poxy-p he nolic resinand silicone resin bonded 0.000 2 to 0 .0003 in . thi ck ­ness) provided effec t ive solid lub ricant films for 440-Cstainless stee l a t pressures of 10 - 6 to 10- ; mm H g.Both fricti on and wear were ex t remely low with th esecoatings.

2. Some met al s (Ag, Sn, Au, and Pb ) when ap­plied to s ubs t ra tes such as 440 -C sta inless stee l (0 .0004in. thi ckn ess ) app rec ia bly redu ce th e fr ict ion a nd wearnormally enco unte red with thi s a lloy in vacuum ( 10 - 1

'

to 10-; mm H g ).3. The frict ion a nd wear results obta ined with

various met al s in vacuum ( 10-(; to 10 - ; mm H g )indicat e a wear mech ani sm unlike th at enco unte redin air. In gene ra l th e wear in va cuum was cha rac te r­ized by mass met al t ra ns fer . The friction coe fficientswere lower than obta ined in a ir for some alloys (e.g .,52100) and conside ra bly high er for othe rs (e .g., Ni­Cr alloy a nd Ni-bon ded TiC a nd CbC). Opera tio n atlower press ur es might be expec ted to further alterthe wear mech anism based on oxygen ava ilability.

4. The evapo ra tion data for some inorganics(CaF~ a nd ::\l oS~ ) and met al s (Ag a nd Sn) indica tethey have po te nt ia l for use as lubrican ts in vac uum(10- 6 mm H g ) to temperatures as high a t 1000 F .

5. T he evapora tio n data indicate that some oilsand greases may be used as lubricants at pressuresof 10- 6 mm H g for sho r t pe riods of t ime p rovidedthe ambient temperature is low (55 F ).

R E F E R E X C£ S

I. RIEHL. \V . A.. LOOXEY. W . C.. a nd CARUSO, S. D ., "Com­patibilit y of Engineering Materials wit h Space Enviro nme nt,"Army Ordnance Missile Co m ma nd . It em 1. P a rt II of ARPAOrder 92-59 (October , 1960) .

2. FREUXllL!CH . M . M ., an d H xx xox , C . H .. " P rob lems ofLubrica tion in Space," L ub ricat ion En g, 17 ( 2) , i2-77 (1961) .

3. J .\(;ODOWSKI, S. S.. and FREUIWLICH, M . M. , "Inve st iga tio nof P rop ert ies of Lub ricants in High Vac uum," Third P rogressRepor t-WA D D Report :\0 . i556- 1-3 Contract AF-33 (6 16)­6S-I5 (Octobe r, 1960) .

.i , ATI.EE. Z. J .. WII.SOX, J . T .. a nd FIDlER. J. C., " Lubricat ionin Vacu um by Vaporized T hi n Met alli c Fi lms," J . II pp l.Phys. 2 (9) . 611-6 15 (1940) .

5. Bowm.x , F . P .. an d H UGHES, T . P. , "The Fric tio n of Clea nM etals and th e Infl uen ce of Adsorb ed Ga ses. The T empera­t u re Coefficient of F ricti on," Proc. R oy . So c. ( L ondoni,A172, 263 ( 19.\9).

6. BOWDEX, F . P ., a nd YOUXG, J. E ., " Friction of Clea n M etal sa nd th e In fluence of Adsorb ed Films," Proc. R oy . So c. ( L on­doni , A208. 311-3 25 (195 1) .

7. BOWDE X, F. P ., a nd ROWE, G. \V., "The Adhesion of Clea nM etals." Proc, R oy . So c. (L ondon v; A233, 429-44 2 (1956) .

8. HAXSI·:X. Sn:(;FR IED, "Research P rogram on H igh VacuumF ric tio n." Litton Indust ries of Ca lifornia. Beve rly Hill s, AFCo ntract :\0 . 49 (6.\8)-343, (Ma rch , 1959) .

9. MO:--;K , G. W ., "A pparatus for Weighing in Vac uum ," J.Appl. Phys . 19 ( 5) . 485-486 ( 19-18 ) .

10. DUS Il ~I.\X. S., " Scientific Founda tions of Vac uum T echnique,"pp. 246-2-19, Wiley , Ne w York, 19-19.

11. BOWD!:»,', F . P ., and TABOR. D., "Frictio n a nd Lu bricat ion ,"p. 140 , Wil ey , Ne w York , 195i .

12. J OIlXSOX, R . L., GODFREY, D ., an d BISSOX, E . E., "Frictionof So lid Films on Stee l at Hi gh Slid ing Veloci ties ," NACAT :\ 1578 (1948) .

13 . BUCKLEY, D. H ., a nd J OH I'o"SOI'o" , R . L., "The I nflu en ce ofSilico n Additions on Fricti on a nd Wear of Nickel Alloysat T emp era tures to 1000 F ," Tr ans. liS L E 3 (1 ) , 93- 100(1960) .

1-/. J OHXSOX, R . L., and SLIXEY, H . E ., " H igh T empera tureF ricti on a nd Wear Propert ies of Bo nded Solid Lubrican tFi lms Co ntai ning Lead M on oxide," Lubrira tion Eng , 15 (1 2) ,

-I8i--I91:-196 ( 1959) .

15. e. B.S. Labora tories Technical Bullet in 463-6, " Vac uum Bear­ings a nd Dry Fi lm Lub rica nts."

BIBLIOGR APHY

1. BOWIH:X. F . P. , and H UGHES, T . P ., "T he Friction of Clea nMetals an d th e Influence of Adsorbed Gases. The Te mpera­tu re Coe fficient of Fricti on ." Proc. R oy . So c. ( L ondon v;A172, 263-2i9 (1939) .

2. HOWDEX. F . P ., a nd Yo uxc , J. E ., "Frict ion of Diam on d ,Graphite . and Ca rbo n a nd th e Infl uen ce of Surface F ilms,"Proc. R oy . Soc. ( L ondon'[ , A208, 444-455 (195 1) .

3. BOWIJ EX, F . P ., an d YOUX(;. J . E., " Frictio n of Clea n Met alsand th e Influence of Adsor bed Films." Proc , R oy . Soc . (Lon­don i , A208, 311-325 (195 1) .

.t , BOWlJEX, F. P ., a nd R OWE, G . W .• "T he Ad hesio n of Clea nM etals," Pro c. R oy . Soc. ( L ond onv . A233, 429 -4-12 (19 56) .

5. SI~lOl'I , 1., M cMAHox, H . 0 ., an d Bow cx , R. J ., "Dry M etal­lic F rict ion as a fu nct ion of T em perature betwee n 4.2 and600 K," J . II ppl. Ph ys. 22 ( 2) , 177-184 (19 51) .

6. S.\\'A(;E, ·R. L., "Gra phite Lu brica ti on," J. A pp l. Phy s. 19 (1 ) ,1-10 ( 1948) .

7. SA\"AGE, R . H ., and SCHA EFER, D . L. , "Vapo r Lubri cation ofGraphite Slid ing Co ntac ts," J . Appl . Ph ys. 27 ( 2) , 136-138(1956) .

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20 D ONALD H. BUCKLEY , MAX SWIKERT , AND ROBERT L. JOHNSON

SPECIMENS

EXPANSIONBELLOWS

PUMP

L1QUID- NITROGENCOLD TRAP

FIG. 14 (a ) . Va rio us baffle designs used in vac uum fric tio nand wear appara tus .

chambers. Numerous cold-trap and baffle designs havebeen reported in the literature and users of oil diffusionpumps have employed these and other modificat ions invacuum systems. There appears, in general, to be verylittl e if any stan dardizati on on cold-trap and baffle de­signs. Various cold traps and baffles were therefore triedin the vacuum friction and wear apparatus in an attemptto eliminate back-migrati on of oil vapors.

The first step taken was to place a liquid-nitrogenspiral cold trap in the apparatus between the experi­mental chambe r and the oil diffusion pump . A schematicof th is cold trap is shown in Fig. 14(a) . The gas being

Appendix

8. ] OHNSOX, V. R ., and VAUGHX, G. W ., " Invest iga tion of th eM echanism of MoS2 Lubrication of Vacuum," J . Appl.Ph ys. 27 (10 ) , 1173-11 79 ( 1956) .

9. BELL, M . E. , and FINDLAY, ]. H. , "Moly bde nite as a NewLubricant ," J . A m. Ph ysic. Soc. 59, 922 ( 1941) .

10. U.S. Patent No , 2,280,886- Ap plicat ion filed M arch , 1941by P ort er H. Bru ce, Assignor to Westinghouse Electric andM anufacturing Co .

11. L AVIK, M . T ., GROSS, G. E ., an d VAUGHX, G. W., " Investiga­tion of th e M echani sm of Tungsten Disulfide Lu brication inVacuum," Lubrication Eng. 15 (6) , 246- 249 :264 (1959) .

12. H ANSEN, SIEGFRIED, " Research Program on H igh Vac uumFriction ," Litton In du stries of Ca lifo rn ia, Beverly Hills, AFCo nt rac t No . 49(638)-343 (M arch, 1959) .

13. BELLER, WILLIAM, "Friction Resea rch Grinds to a Halt ,"M issiles and R ock ets 7 (9) , 23-2 4 ( 1960) .

14. RI EHL, W . A., LOONEY, \V. C. , and CARUSO, S. U., " Com­patibility of Engineering M at erial s with Space En vironment ,"Army Ordnan ce M issile Command, It em 1, Part II of ARPAOr der 92-59 (October, 1960) .

15. ATKINS,] . H. , BISPLINGHOFF, R . L ., H AMON, ]. L ., ] ACKSON,E . G., a nd SIMOXS, ] . C., ] R., "Effect of Space En vironmen ton M at erials," N RC Res. Report 40- 1-04, Ohio State Univ .R esear ch Foundat ion Proj ect No . 920 (August , 1960) .

16 . FREUNDLICH, M . M. , and H ANNOX, C. H ., "Prob lems ofLu br ication in Space," Lubrication Eng . 17 ( 2) , 72-77 (196 1) .

t r, FREUNDLICH, M . M ., and ] AGODOWSKI, S. S., "Lubr icants forHi gh -Vacuum Environments," WADD T echn ical Report 60­728, Part I , Co nt rac t No . AF3 3(6 16) -6845 Project No. 3044(November , 1960).

18. ] AGODOWSKI, S. S., and F REUNDLICH, M. M ., "Investi gat ionof P rop erties of Lubrican ts in Hi gh Vac uum," Third ProgressReport-WADD Report N o. 7556- 1-3 Co ntract AF33(616)­6845 (October, 1960) .

19. WILLIAMSON, ] . G., "Po la rity Ancho rs N ew Space Lu beTheory ," Chem , Eng . New s 39 (5) , 48-4 9 ( 1961) .

20. ] OHXSOX, ] . R. , VAUGH X, G. W ., and L AVIK, M . T ., "Ap­par atus for F ricti on Studies at Hi gh Vacuum ," R ev. Sci.In str. 27 (8) , 611-613 (1956) .

21. M ox x , G. W., " Apparatus fo r Weighin g in Vacuum," J.A pp'; Phys. 19 (5) , 485-486 (1948) .

22. C.B.s. Labor at ories T echnical Bull etin 463-6, " Vac uum Bear ­in gs a nd Dry Film Lubricant s."

23. ATLEE, Z. j ., WILSON, ]. T. , and FILMER, ]. C., "Lubrica­tion in Vac uum by Vaporized Thin M etalli c Films," J . Appl.Ph ys , 11 (9) , 611-6 15 (1940) .

A S TU DY OF THE EFFECTIV ENESS OF VA RIOU S TRAPPING

D EVI CES I N S TOP PING BA CK-MIGRATION OF DIFFUSIO N

P UMP OILS

The vacuum friction and wear apparatus used in thisinvesti gation originally employed an oil diffusion pumpto obtain high vacuum . The oil diffusion pump has as­sociated with it the undesirable characteristic of per­mitting oil vapors to migrate from the pump. In thevacuum literature it has been suggested that this prob­lem can be eliminated by the use of baffles and /or coldtraps between the oil diffusion pump and experimental

pumped had to travel a spiral path along liquid-nitrogen­cooled walls. A friction experiment was conducted witha disk and rider specimen in the experimental chamber.The friction coefficient obtained with clean 440-C stain­less steel was in a region (0.1) associated with effectiveboundary lubrication . The disk surface was checked withdistilled water after the experiment ; observations of highcontact angle and lack of wettability gave evidence ofan oil film.

An optical baffle plate was then added between thecold trap and specimens in an attempt to eliminate thecontamination. The baffle design used is shown in Fig.

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Friction of Ma terials in Vacuum 2 1

LHEATER

\ C_OPPER !I

L 'BAFFLE=

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II

FIG. 14 (b) , (c) , (d). Various baffl e designs used in vac uum fr ict ion a nd wear apparatus

'4 PTICALBAFFLE

=

TWNGSTENHEATER GRID

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.~-.4UNGSTEN El---IE3,GRID -HEATER

FIG. 14 (e) , ( f) , (g) . Various baffle design s used in vacuum frict ion a nd wear a ppa ra tus

~-

\ Ii·---'-=c:... lJllr'

~ --.- ~-~ f.-,

t~COOLINGCOILS"

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. I

TUNGSTENHEA_TERGRID

FIG. 14(h), (i), ( j) . Va rious baffle designs used in vac uum frict ion a nd wea r apparat us

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22 DONALD H. BUCKLEY, MAX SWIKERT, AND ROBERT 1. JOHNSON

14(b) . Friction experiments with 440-C stainless steelagain gave a friction coefficient indicative of effectiveboundary lubrication. Examination of the disk surfacewith distilled water showed evidence of a contaminatedsurface.

Th e use of a honeycomb copper structure as a baf­fle has been reported in the literature as a means oftrapping oil molecules. A baffle of this design was fittedto the optical baffle, and the unit was placed in thespecimen chamber above the liquid-nitrogen cold trap[Fi g. 14(c )] . This arrangement was not effective inpreven ting specimen contamination by oil vapors.

Another approach to eliminating the back-migrationproblem considered was that of using heat to decomposeoil vapors migrating to the specimen chamber. To achievethis , a heater was attached to the optical baffle previous­ly used. The optical baffle with heater placed betweenit and the liquid -nitrogen cold trap is shown in Fig. 14(d). This arrangement was also inadequate in eliminat­ing specimen contamination, as ascertained by frictionexperiments with clean 440-C stainless steel and wet­tabili ty checks . The heater plate was then replaced by aheater grid of tun gsten coils. These coils were heatedto an orange-yellow heat in an attempt to decompose oilvapors entering the specimen chamber. The heater gridand optical baffle used are shown in Fig . 14(e). This ar­rangement was also unsatisfactory.

A tun gsten-wound heater coil was then placed in thespecimen chamber directl y beneath the disk and riderin an attempt to keep vapors from the specimen surfaces[Fi g. 14(f)]. This arrangement was insufficient, and thespecimens again exhibited evidence of surface contami­nati on.

The design of the optical baffle previously used wasaltered, and the new optical baffle is shown in Fig. 14(g) ,where it was used in conjunction with the liquid-nitrogencold trap. In Fig. 14(h ) a tun gsten heater grid wasadded below the baffle. Neither of these systems accom­plished the elimination of oil contamination as deter-

mined by friction coefficients and specimen surface ex­amination.

The next app roach to the back-migration problemconsidered was to remove the expansion bellows of theappa ratus located between the specimen chamber andthe liquid-nitrogen cold trap and to replace it with aheater-baffle assembly. Thi s assembly can be seen inFig. 14(i ). It consisted of two water-cooled optical baf­fle plates with a tungsten-wound heater grid between theplates. The temperature of the space between the tun g­sten grids maintained during diffusion pump operationwas 1000 F. The outside walls of the chamber werewater-cooled. Thi s assembly in conjunction with theliquid-nitrogen cold trap minimized the specimen con­tamination. Examination of the disk surface with dis­tilled water indicated a contact angle less than previous­ly obtained ; however , the water still would not wet themetal surface, and the coefficient of friction for 440-Csta inless steel was about 0.2. An addition was thenmade to the system, namely, the optical baffle used inearlier trapping designs. Th e complete trapping systemcan be seen in Fig. 14(j). The results obtained wereabout the same as with the arrangement of Fig. 14(i ).

A liquid-nitrogen-cooled plate type chevron baffle wasalso considered for these experiments. This configura­tion was not , however, used because experimental evi­dence by colleagues showed that it also allowed test­chamber contamination.

The experiences related here were obtained with acommon diffusion pump oil composed of petroleumhydrocarbons. Separate experiments were conductedwith additional diffusion pump oils includin g a siliconeand a sebacate. In no case was back-migration of dif­fusion pump oil completely eliminated.

The inability to completely eliminate the back-mi­gra tion of oil vapors from the oil diffusion pump resultedin the replacement of the oil diffusion pump with anionizati on pump . Thi s pump employs no fluid or vaporsin its operation.

DISCUSSION

M . H . H ABLANI AN AND E. G. J ACKSON (Na tional Research Corpo­ration, Cambridge 42 , Massachusett s) :

We wish to comment particularly on the appe ndix of thi s pap er.It is apparent fro m th e difficult ies th e authors had with diffusionpumps th at th ere is a possibility of oil contamina tion in diffusionpum p systems. H owever, th ere is much evidence ava ilable indi­cating that t he difficult y is not an inh erent one, an d can beavoided by suita ble baffle design and pump dow n techniqu es.Th e authors' success with an ion-mechan ical pump combinatio nshows that backst reaming can be tr apped even at pressures aboveth e ultrah igh vacuum ra nge.

Th e au thors do not give man y deta ils of th e diffusion pum psystem and pu mpin g proced ure, such as th e size and type of th ediffusion pum p, th e pump -dow n sequence and associa ted pres­su res, the presence or abse nce of sepa ra te roughing line, etc. It is

th erefore difficult to determine exactly all possible conditio n;contrib uting to excessive backs trea ming.

T he obv ious difficulty appears to be insufficient trapping. Themajo r difficulties with the tr apping arrangements used by th eauthors are : (1) The main liquid nit rogen cold trap does notensure at least one contact on a cryoge nic surface by molecule;coming fro m any direct ion; (2) All other baffle ar rangement;used above th e main liqu id nit rogen trap are insufficient beca useth ey are either not cooled at all, or cooled only by wa ter.

The spiral liquid nit rogen cold t rap permits molecules to pas,th rough betwee n the spira l and th e uncooled walls. In additionmolecules which happen to travel along pa ths parallel to th espiral surface, bouncing off warm outer wa lls, will pass thro ughth e t raps without contacting th e cooled parts of the t rap . Aftersome of th e oil molecules appear in th e space abov e the cold tra p.

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F riction of Materials in Vacuum 23

it is useless to tr y to sto p th em fro m ente ring th e working cham­ber by optical baffles if those baffle s are not ref rige ra ted .

It is import ant to distingu ish between prima ry ba ckstreamingwhich occurs in a well-defin ed pattern from th e top je t of th epump, and th e molecules re-evaporating fro m the wall s whichcome from random directi on s and canno t be sto pped by un­cooled baffles . A well-d esigned liquid nitrogen baffle mu st ass ureat least one contact on a refrigerated surf ace for molecules comi ngfrom any dir ecti on. In sensit ive expe rime nts it may be desirab leto have a minimum of two or mor e such contac ts. N . M illeronat a recent Am eri can Vacuum Societ y meeting reported th at asystem designed to ensure a t least one contact results in fou r orfive actual contacts fo r th e ave rage m olecu le, and th er eby re­duces the prob abilit y of a molecule passin g throu gh to trulynegligible proportions.

Although heat ed grids for oil decompositi on hav e been dis­cussed in liter ature some time ag o, th eir effect has not been evalu­ated. To be efficient. th ey should be tight . in whi ch case th eywill occupy a lar ge a rea of th e inlet duct and conseq uent ly de­crease the pum pin g speed. There is a possibility th at such heatersmay drive th e p roduct s of oil decomposit ion into th e cha mber.It would be perh aps better to place such ba ffles below th e liquidnitrogen tr ap rather th an above as used by th e aut hors. In addi ­tion, the heat ers in Fig. 14d , c, and h appear to heat th e opt ica lbaffles above th em by radiation a nd th erefor e mak e th ose baffleseven less effect ive . The copper foil t rap s show n in Fig. 14c a reonly effective afte r a th orou gh bak eout and th ey ha ve beenproven only in small glass sys tems. It is not sur prising th erefor ethat they were not helpful as used by th e aut hors.

In addition. it would be desi rable fo r fri ct ion experime nts underspace condi tions to hav e a higher vac uum th an produced in th esystem descr ibed in th e pa per. Even with a liq uid helium con­densing coil. th e aut hc rs achieved only 2 X 10- 7 mm Hz, whi chis about three orders of magnitude higher th an th e ca pa bilit ies ofpresently ava ila ble ultrahigh vacuum syste ms with oil diffusionpumps.

Following a re some references which indica te th at vac uumessentially free of heavier hyd rocarbon s can be obta ined with oildiffusion pumps.

I. "The M easu rem ent of Low Partial Ga s Pressu re with th eMass Spectr omet er " by H . \V. D ra win and C. Brunee (Atlas­Werke AG. Germa ny) from Vn k uum-Fechnik, f\o . 3 (1960) . Thefo llowing parti al pr essures were obta ined with mercur y diffu sionpumps:

Hydrogen. 1 X 1O - !fm m H z : Wat er. 1 X 10- 111mm H <; ; Car­bon monoxid e. 5 X 1O- 1nmI)1 Hg ; Nit rogen , 5 X 1O- 10mm H g ;Carbon dioxide. less th an 1.5 X 1O-!f mrn H u : Hydroca rb ons,less than 1 X 10 - 111 mm H z ; M ercury , less tba n 1 X 10 - 1 (1 mmHg; Total pressur e. less tha n 3.3 X 1O-!f m rn Hz . Using oildiffusion pump s, very simila r results we re obta ined a t total pr es­sure of 3 X lO-!' mrn Hg. The a uthors say: " ... With bothpump types. a bout th e same ultima te pa rti al p ressures can be ob­tained with th e exception of hydrogen . Th e pa rti a l pr essure ofhydrogen wa s a t least ten times higher with th e oil diffu sionpumps compared to th e mercu ry pumps." It may be not ed alsothat reducti on of for elin e pr essure of a diffu sion pum p reducesthe amount of hyd rogen on th e high vac uum side.

2. X. Mi llcr on ( La wrence R adi ati on Lab or atcry, Unive rsityof California, Li vermore. Ca !if.) a t th e First Svmpo sium on Sur­face Eti ects on Spa cecraf t Ma t erial s, Palo Alt o, Calijorn ia (May,1959). " Diffusion Pumps ha ve been dem onstrat ed to be inh er ­ently clean . . .. Fir st , diffu sion pumps, both mercury a nd oil,can be com plete ly tr apped . N o oil o r crac ked hyd rocar bon spec-

trum, and no mer cury, down to par t ial pr essures of 10- 11 mmH g for oil and 1O- 1fl mm Hg for mer cury is de tect ed ."

3. "Mass Sp ectrom et er Investi gations in th e Ult rahigh Vac­uum " by P . J ahn a nd J . Zah rin ger ( M ax-P la nck In st itute forNuclear Physics, Heid elberg) , Vuk uum -Technik , N o. 10, 5, p . 138(June, 1961 ) .

"A very sensitive mass spec t ro me ter-allow ing th e measur e­ment of partia l pr essures below 1O- 1:! torr-was used to inves­ti gate th e compo sitio n of th e resid ua l gas in a va cuum system,as obta ined with oil diffusion pumps plu s nonrefrigerated baffl es.and with an ion-ge tte r pump. The spect rum of th e residual gaswas checked dail y during seve ra l weeks wh ereby differ en t typesof pumps, ba ffl es and pumping fluid s have been used ."

" T he inv esti gat ion s have shown that th e com bina tion two-stagepump plus silicone-ult ra fluid plus aluminum oxide tr ap hasbeen pr ov en suita ble for pr oduction of ultrahigh vacuum . T hepr essure rema ins consta nt for man y weeks a t its initia l valueand th e residu al gases from th e pump a re only methan e an dhydrogen ."

In genera l, experime nts wh ich great ly depend on sur face con ­ditions mu st be cond ucted wi th ve ry careful vacuum design andtechniques, or contamination is possib le with both diffusionpump s and ion-gettering pumps wh en used together with me­chanica l pumps.

A UTH ORS ' CLOSU RE :

The com ments by M essrs. M. H . Hab lanian and E . G. J ack sonon va cuum techn ology with diff usion pumps ar e m ost welc om eeven th ough th ey do not pertain to th e bod y of th e paper . Theappendix to this paper to whi ch th ey refe r was included prima rilyso that other investigators would not have to duplicate thiseffort . It is reasonable to expec t th at the problem of back migr a­tion ca ll be solved . Th e effo rts described in th e appendix , how ever .did not solve the pr ob lem . Rather tha n to spe nd mor e effort,the pr ob lem was avoid ed by usin g an ion pump. The ion pumpedsys tem is not fr ee from prob lems but for tunately th ey a re differ ­ent th an the one of p rim ary conce rn here.

Th e refer ences cit ed by th e discussers a re indicated to sugge stth at sys tems " essentially" free of back migration can be obta ined.The prob lem in friction, wea r, and lubrica tion studies is th atsys tems mu st be absol utely free of orga nic conta mina t ion. Invisits to a number of labora tories no diffu sion pumped sys temswer e observed th a t could be considered absolute ly clean . f\ 0 doubtt hose sys te ms included man y of the design considera tions ad ­van ced by th e discussers. On e labora to ry indicated th at a diffu ­sion pumped sys tem with a zeolite tr ap was completely free ofback m igration . The effect iveness of such a tr ap has not beenveri fied by th e authors, however , it mer it s considera t ion wh ereonly di ffusion pumped sys tems a re available.

One specific po int raised by th e discussers concerne d the highpr essure achieved with liquid helium cryopumping. That pr essure(2 X 10- 7 mm Hg ) was obta ined by design rather th an as aminimum pr essure. Th e point mad e was th at pr essure measu re­ments a lone ar e not an adequa te mean s of definin g space sim ula­ti on . I t is im portant to define molecula r and at om ic species pres­ent in th e cha mbe r a nd to measur e partial pressures. Using liqu idhelium to the limit of it s cry opurnping ca pa bilit ies in th e appa­ra tus, we achieve d pr essures in th e 10 - (1 mm H g ran ge. The pres­sures were measured with a nude hot ca thode ioni zation gagead jacent to th e experimenta l specime ns. Using othe r gage s oftypes more commo nly employe d by others includin g th e discuss­ers, lower va lues would be obta ined .

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