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Lubrication of Bearing Steels with ElectroplatedGold under Heavy LoadsRiitsu Takagi a & Tung Liu ba Research Institute, University of Dayton, Dayton, Ohio, 45409b Air Force Materials Laboratory, Wright-Patterson AFB, Ohio, 45433Published online: 25 Mar 2008.

To cite this article: Riitsu Takagi & Tung Liu (1968) Lubrication of Bearing Steels with Electroplated Gold underHeavy Loads, A S L E Transactions, 11:1, 64-71, DOI: 10.1080/05698196808972209

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ASI,E TRAXSACTIO:-i S 11, 64-71 (1968)

Lubrication of Bearing Steels with ElectroplatedGold under Heavy Loads

By R IITSU TAKAGP an d T UNG LIU"

Th e lubricating action of electropla ted gold on 52100 steel and 440 C stainless stee l in slidingunder US-kg load was exam ined icith a test er consisti ng of a single mb block loaded aqainsl arot ating disk . The ad vantage obtained with gold plating was th at of low wear while th e co­efficient of sliding /li ct.ion remained high . The wear life of thick film s was much long er ; an e20 J1. film had a wear liie of 150,000 revolu tions, while 1 J1. film s had wear lives of about 3 000revolutions. S ilver, copper, and two gold alloys plat ed to th e appropriate th ickness were ~bleto ex tend th e wear life to seve ral tho usand reuolutions while nickel was not effecti ve at all .Th e failure o] th e plat ed film s was usually marked by a rapid incrcase in wear rat e. W ithth ick gold alloy film s, wear deb ris in th e form of flak es were obtained in addition to irregularshape d particles. The ap pearan ce of the wear tra ck indicat ed that th e gold film underwentconsiderable plast ic deformation .

FIG. 1. Schematic of friction force recording system formod ified lubricant test er.

and sca rs gave limited ind ica tion to the fail uremechanism.

The ma in purpose of this work is to clarify th ewear mechanism of gold films in sliding. A load of68 kg and a speed of 13.2 ern/ sec wa s used to createa moderately severe sliding condition so that most ofthe experiments could be carr ied out until fai lure oc­curred . T he lubricating capability of gold was com­pared with severa l other meta ls and gold a lloys.

Expe rimenta l methods

The sl.ding Iri ct .on and wear measurements werecarried out with a modified commercial lubri canttester. The test specime ns consist ed of a rectangularblock riding on a rotating race coated with a softmetal. The linear sliding spee d obtained with the35-mm outside diamet er race rotating at 72 rpm was13.2 ern/ sec, The normal load was applied by a deadweight thro ugh a lever sys te m having a nominalmechanical advantage of 30. Initial line contact wa s6.35 mm long. The fr iction force wa s measured withthe system shown in Fig. 1. The friction force viadisp lacement of the block was transm itted through

Introduction

BOWDEN a nd Tabor (1 ) established t hat two slidingste el surf aces may be lubrica ted by a thin film of asoft metal such as lead or indium. According to t headhesion theory, th is soft metallic film separa tes thestee l surfaces and provides an easily sheared region,thereby reducing both the adhesive wear and the fric­tion .force. Go ld, bein g soft, chemica lly inert , and highm elting ha s attracted the attention of friction andlu br ica t ion workers for it s potential as a lubri cantunder high vacuum and high temperature environ­ments (2, 3) . Evans and Flatley (4-6) showed t hat itis practica l to use gold plating as a lubr icant for min­iature ball bearing s. No exte ns ive work on th e dura­bility of gold films for slider bearings has been re­ported , howeve r, partly because the coefficient ofslid ing fricti on is usu all y too high to be consideredsuccessfu lly lubr ica ted .

In a n ear lier work (7) the sliding friction of aspher ica l steel r ider on a gold-plated steel fla t wasmeasured . The friction coefficient was found to dependon the materia l of t he rid er, film thickness, appliedload, and to some exte nt on th e subst ra te materia l.Under the most favorab le conditions, the coefficientof fr ict ion fell to about 0.1 which is in agreement withthe va lue estimated from the adhesion theory. Theloa d- speed combination used during th e work wasrelatively mild. The appearances of the wear t racks

Presented as an Amer ican Society of Lubrication Engineerspaper at the ASLE/ ASME Lubrica tion Conference held inChicago, Illinois, October 17- 19, 1967.

1 R esearch I nstitute, University of Dayton, Dayton, Ohio45409 .

2 Air Force Materials Laboratory , " ' right-Pat terson AFB,Ohi o 45433.

64

Proving Ring

Block Holder

ThermocoupleHole

Race

Joint

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Lubrication with E lectrop la ted G old 65

Hardness (D PN ) at 500 gm

T A BLE 1H ardness of R aces an d Blocks

a movable linkage to a horizon t al shaft, which wasguided horizontally by a ball bu sh in g and connectedt o a provin g ring equipped wit h a lin ear var ia ble clif­fer ential transformer (LVD T) . Throu gh t he LVDTamplifier, the fri ction force signa l was recorde d on atwo channel str ip chart recorder simu lt aneous ly witht he temperatu re of t he block . A hi gh tempe rat uregrease was used to lubrica te t he te ster bearings locatednear t he specim ens t o avoid possible contaminat ion ofthe slid ing su rf aces by t he vapo r of the grease.

The r a ces and block s were m ade of eit he r 440 Cstain less or 52100 bea ring ste els. These spe cimens werem ech anically fini sh ed after hea t t reatment. The hard­ness of the correspo nd ing pair of ra ce and blo ckactually used was not a lways t he same, a nd t heaverage values m easured a re shown in T able 1.

a M ea sured with 25 g loa d on 20 I-' film s .

12310030661815

Pla t ingrute

(mg/um p . min )

60- 7015- 2520-5020-302.5-6050-60

Opera ti ngtcm p .

99.99 + Au4 Sb; bn lnn ce go ld2-4 Co, N i, Tn ; balancc go ld99 ,9 Ag99 ,9 Cu99 ,5 Ni

65168209

951613 16

Bath type

AcidCyanideAcidCyani deF luo ro borateConventio nal(p H = 2- 4 )

Metal

(a ) Pl a ti ng Processes

G o ldGold a lloy AGold a lloy BSi lverCopperN icke l

(h ) H ard ness a nd Composi tion of Plated Metals

M e tal H ardness (D PN) " Com position ( % )

T A B L E 2Gold and Other il l etal Platings

G oldGo ld alloy AGo ld a lloy BSi lverC o ppe rNi ckcl

It uce Blo ck

520 61079 0 7907 1.5 680

52 100s"52100h"440 C sta in less

Steel

c 52100 steel of t wo d iffe rent hardne ss was used .

Four m et als-gold , silver , copper, and ni ckel-andtwo gold a lloys were used to electrop late t he racesused in t his work . The conditions of t he pl ating, car­ri ed ou t by a commer cial source, a re given in T able2 (a) . The pret reatmen t of t he specim ens and t hepla t ing processes were care fu lly controlle d. The filmthickness 01 nominally 5- , 10- , an d '20- p. thick mmsof pure gold and gold a lloy A (Table 2 ) pl ated on the52100s steel wer e chec ke d with a magnetic gage andt he v a ri a t ion from t he specified thickness was foundto be less than 30 % . H ardness and composit ions oft he pl ated metals and alloys a re shown in T able 2(b) .The t wo gold a lloys of sim ilar gold content werese lected for t his work.

The m aximum su r fa ce roughness of t he races priorto pl ating was 0.4 fJ. rms. The rubbing surfa ces of theblo ck specimens wer e wet polish ed wi th 4/0 eme rypaper t o a roughness of 0.05 fJ. rms. P r ior t o each run ,the specime ns were t horoug h ly cleane d with acetoneand di stilled water and degr cased cathodically in abath of 10% a que ous solut ion of trisodium phosphate.

The standard op er a ting p ro cedure was t o initiallyapp ly a load of 13.6 kg. After eac h 50 revolutions,t he load was in creased by 13.6 k g, until a full load of68 .0 k g was reache d . This step loading procedure wasselec ted to p ro vide a sm oother run- in and to reducevibrations. To fa cil itate direct comparison, it wasassumed in ca lcu lating the number of revolutions ofwear lif e that t he amount of wear in the initial stageproceeded in proportion to the product of load app lied

and number of revolutions at t ha t load. The fir st ' 200revolutions under partial loads wer e thus convertedto 100 in full load. The t est specimens were a llowe dt o run under 68-kg load with occasional interruptionst o weigh the race and block on an analytical balancesens it ive to 0.01 m g. Since wear rate of the race wasin gene ra l fa r from constant and was negli gibly smalluntil the period 01 r apid destructiv e wear b egan, thet est was term inated when the weight loss of the r aceexceeded I-mg. The number of revolutions correspond­ing t o a I-mg we ight loss was estimate d and reportedas the wear lif e. It was desirable t o have t wo suc hweighings m ade fairly close t o t he I-mg weight lossof the race. Care fu l v isual ob serva tion of the am ountof debris collected on a sheet of white paper placedunder t he running specime ns usu ally enabled on e t ostop t he running at approximately the desired t ime,from whi ch the wear life cou ld be obtained wi th anes t im a ted accuracy of within 5% + 10 revolutions.

Results

The fri ction and wear data of 52100s st eel blo ck ssliding on gold -p la ted 52100s steel r aces a re shown inT able 3, which shows that for a gold film thick nessrange of 0-20 fJ. , the variation of the coefficien t of fri c­t ion is wi thin the range of 0.6-1.0, while tha t of t hewear lif e ranges over a span of four orders or m a g­ni tude. In vi ew of the limited accuracy of film t hick ­ness (error <30%) , the surfa ce roughness of subs t r atesteel races « 0.4 fJ. rrn s ) , as well as the wide devia­tions normally in volved in the life m easu rements,

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66 RIITSU T AKAGI AN D T U N G Lru

T A III.E aFr iction and W ear of Gold-Pla/l'l[ 1;2100 8 Steel

Wear life (rev.Zl mg)

F ilm t hi ckness (It) CoefT. of friction l st run 2nd ru n 3rd run Average

0· ~0.6 5 ;j 20 100 .2 0 . 6 ~ 0 .8 500 noo a50 58a1 0.6 ~ 0 .8 3,000 3,200 2,000 2 ,7335 0 .6 ~ 0 .8 >27,000 > 30,000 > :33,000 >30,000

20 0 .8 ~ 1.0 >27,000 > .50,000 150,000 >42,000

a Unlubr ica te d slid ing ; the da ta under wear life a re really revolu t ions for 1-m g weigh t loss.

phere and the authors appropriately lend significanceonly to the order of magnitude in the wear lives, asmentioned in th e preceding paragraph. Consequently,other test s with 5- and 20-jl. gold films were ter­minated at th e end of a day and only the minima ofthe wear life data were reported. In fact, figures suchas > 25,000 and 150,000 were considered to give wearlives of the sa me order of magni tude.

It has been pointed out tha t th e wear rate was ingenera l fa r from constant . In pra cti ce, the wear lifewas defined as the number of revolutions correspond­ing to an arbitrari ly chosen 1-mg weight loss of th erace. The choice of this weight is not of part icula rimp ortan ce. A typica l experiment sta rt ing with a shortrun-in period was characte rized by ra pidly cha ngingfriction force and block tempera ture. The slidingsys tem subsequent ly settled down to a period ofsmooth run ning with nearl y constant friction forceand block tempera ture. During t his period, th e weightloss of the race was usually less t han 0.2 mg. A suddent ra nsit ion into a ra pid wear period would follow duringwhich both the fri ction force and the block temp era­ture becam e ir regular . It was not un common in a run ,of several thousand revolu tion s th at th e weight lossincreased from 0.1 mg to 10 mg within th e last 100revolu tion s. Weight loss of 1 mg in a gold film cor­responds to only about 0.075- jl. redu ction in avera gethickness, usually an insignificant change in filmthickness.

T he effects of the block and t he race materials onthe sliding of stee l aga inst gold plat ed steel are shownin T abl e 4. Both th e hardn ess of the 52100 stee l raceand the use of 440 C or 52100 steel block had only

200

wa:

I SO :>~a:wQ.::;;w

100 f-

_ WEIGHT LOSS , RACE

o COEFFICIENT OF FRICTION

t:> BLOCK TEMPERATURE

1.2

1.0

z0f- O.Bu;ru,

u,0 0 .6f-zwu 0 .4L;:u,w0u

0 2

00

F ro. 2. Coefficient of fricti on, b lock tem per a tu re , andwei gh t loss of ra ce plot ted aga ins t nu mber o f ra ce rev olu tions .Block : 52100, Hace : 20 Jl thi ck gold -plated 52100.

S- 10

NUMBER OF REVOLUTI ONS

wear life data obtained were considered signifi cantonly in the order of magnitude.

In th e particular experiment using a 20-jl. gold plat­ing on 521oos, the wear life was measured to be 150,000revolutions. At 72 rpm, this life corresponds to a totalrunning tim e of about 35 hr. In fact, there were fiveovernight interruptions during th e process. The pos­sible effect of these interruptions on wear life couldnot be determined unl ess t he experiments were ca rriedout under controlled atmospheres. However, it wasevident t ha t the overn ight stops frequently causedchanges in coefficient of frict ion (F ig. 2). The presentstudy was carried out in the normal lab ora tory at mos-

T A IIu : 4W ear L ite s of Gold Plated S leds

Block 52100 s Block 52 100h Blo ck ·H OCFilm

thickness Race Ra ce R ace(It) 52100s 52100h 52100s 52100h 52100" 52 100h 440 C

° 20 300 46 750 40 330 20 .2 350 1,000 300 1,200 1,IJOO 1,600 1,3001 2,000 2,100 2,500 8,000 3,000 5,000 13,0005 > 33 ,000 32,000 8,000 > 19,000 1n,500 >70,000 >10,000

20 150,000 >30,000 >35,000 > 30,°9° 40 ,000 > 25,000 >10,000

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Lubrica t ion with El ectroplated Gold

T .WI.E 5Jl"cu r ut. L of F ariou « stetnl-Plata l ;; 2 too S led

(a) ;i2J()O Steel Block"

Plating

67

F ilm thic k ness (IL )

0 .215

20

Cocf I. of Fricti on f

Go ld

1.200S,OOO

> IIl ,OOO>:m.ooo

lUi-1 .0

Go ld a lloy A

4004.0()O

liOO10

n.u-0 /

t ;old a lloy B

O. li- O. /

Silve r

aoo100400

;, ,500

0 .5 -0 .7

Co pper

,'iOOGOO!)OO

2 ,200

O. 5-0.G

N ickel

7001002020

0 .5-1 .2

(1)1 440 C St eel Blockh

Plu tiug

Film thickness (IL ) Go ld Gold a lloy :\ <;old a lloy B Silver

0 .2 i.eoo liOO ;jOO :~llO

1 5,000 ;; .000 li.OOO SOO5 > 70 ,000 !l00 14.000 I .GOO

20 > 2;";,000 1.100 !l00 4,!l00

CoelT. of f'rirt ion f o.s-o.u 0 . 5-0. G 0.4-0 . ;' O.;, -O .!l

Coppe r Ni ckel

300 1301,100 501.200 101;03 10

0 .;, - 0 ./ 0.6-1.0

a For unlubri car ed slidi ng, I, = / ;')0 and f = D./;j- O.S.'i.b For unluhricated slid ing, L = ;~;m :1I1l1f = O.7-0.S .

slight effect, if any, on th e wear life while the hard­ness of th e 52100 stee l block had no dete ctable effectat all. H enceforth , no distin ction will be mad e be­tween 52100 steels of two different hardness.

A series of experiments were made to compare goldplating with oth er metals and alloys. Silver, copper,

nick el, and two gold alloys in addition to pur e goldwere plated on 52100 stee l races and run against both52100 and 440 C block s. Th e results are shown inTable 5. Again the friction data fail ed to indicate anysignificant variation , while th e weal' life data indicateda very pronounced effect of film material and thi ck-

FIG. 3. Photogra ph s of slid ing a rea s of 440 C block (upp er), 20-1' thick , metal-pla ted 52100 race (lo wer) pa irs after test . Direc­t io n o f motion of ra ce surface in con ta ct wi th bl ock is shown by a ll a rro w un eac h specime n. Film ma teri al : (a) gold , (b ) silver,( c) copper , (d ) ni ck el , (e) gold nllov A, a nd (I ) gold nll ov B.

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58 H (IT:-;C T.-\ KA t: I A XD T C X G LI C

FlG. 3 (Co nti nued) .

ness. Moder ate improvement of wear life was ob tain edwith 20-p. silve r a nd copper pla t ings , though to a lesserexte nt than gold. Nick el plating was virtua lly worth­less at a ny thickness for weal' p revention . The twogold a lloy platin gs on th e 52100 rac es showed optimumfilm thickness with both ;')2 100 a nd 440 C block s.Beyond a cer ta in point , the wear life redu ced withincreased film thickness.

The wea r sca rs on bot h th e b locks a nd th e ra ceswer e examined a nd photographed (20-p. films arcshown in Fig. 3). In a ll cases except t hat of th e pure

gold (F ig. 3 (a ) ) , th e wc.gh t loss of the race excee ded1 mg. It may be seen tha t th e wear sca rs on theblocks run with gold mid gold a lloy films wer erelatively sma ll. The races plated with gold a lloysreveal ed extended areas where film a ppa rent ly peeledoff rev ealin g t he surface st ruct ur e of th e subst ra tes.A similar sit uati on was a lso evident on the silve r a ndnick el f lms.

A light sect ion microscope was used to examine thesha pe of the wear t rack of the races. I n this instrumenta very narrow slitted planar beam of light wa s pro-

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Lubrication with E lectropl nt ed Gold 69

jected at th e su rfuce of t he ob ject a t an an gle of 45°and the reflect ed beam was viewed from a micro scopepositioned at th e 135° an gle. A fia t surface ga ve animage of a straight horizon tal line while t he shape ofa ny su rf uce irregularity wa s observe d with a vert ica lmagnifica tion of a bout 1.4 t imes la rger t ha n thehorizon ta l magnifi ca tion , as if t he surf ace were sec­ti oned at a 45° a ngle. F'igu re 4 (a) shows th e edge of

FIG. 4 . Li gh t "l'"l ion mi crogra ph " of f'dgl's o f sli ding su r­faces of ra ces. (a) 20-/, thick , go ld-plat ed 52100 1':1(' ( ', s lidingagainst 52100 h lor-k, after 30,000 rovo lu t ions a nd (h) 20-1't hick , go ld a lloy .:\ -p la ted 52100 rn co, slid ing aga inst 440 Cb lock , nftor 1,100 re vo lu t ion s .

a gold film after 30,000 revolution s, The edge ha s beenbuilt up to a heigh t of a lmost 200 I)" t en times that ofth e ori ginal film thick ness , For other film s, no suc hedge bu ild-up was observed. A ty pica l case of golda lloy A is shown in F ig. -l Ih}, after 1,100 revolution s.

Discussion

The cxpt-rimcutu l resul ts on the lubrica tion of steelss lidi ng und er heavy load s by electroplated gold haveshown t hat long wear liv es ma y be obtained with 5­and 20-/l films. The frictional forces on the ot her han dwer e usually higher than even t he case of unl ubri­outed stee ls. In one parti cul ar experiment (previous lymen tioned ) , the wear life (1 mg weigh t loss of thera ce ) wa s 150,000 revolutions, much long er than anyoth er one. The coefficient of fri ct ion wa s mea sured at0 .8-1.0 (F ig. 2) as compared to 0.6 obtained from un-

lubri cated spec ime ns. This high coeffic ient of fri ctionsuggests that t he actua l contact a rea wa s relativelylarge. A thick film was a lso needed for subs tantia limprov emen t of wear li fe. The mechanism of slid ingappeared to be quite d ifferen t from that of a balls lid ing slowly on a fia t , re por ted previously (7), inwhi ch case th e conventiona l thin film theory describedby Bowden a nd T abor {l ) is applicn b le to the t hin(less than 0.5 p.) gold film with a frict ion coefficien tof a bout 0.1. I n t he present case of a fla t sliding on acy lind rica l surface with an initia l line contact per­pendicular to the direction of motion under heavyload with a pprec ia ble speed, a t hin gold film is nolonger durable. Instead , th e beneficia l effect of goldfilm in t his type of sliding is der ived from a thi cklayer approaching t he case of s liding on bulk gold.The plastic defo rmation is, however , restricted to thefilm materia l so tha t no mi croscopi c deformation ofthe subst ra te tak es place.

Three types of wear debris wer e observed. Themost common wa s th e irregu larly sha ped sma ll parti­cles . 'With pure gold films, occasionally need les wer ea lso obtained . These needl es are felt to come from theedge build-up shown in Fig. 4 (a), a fte r bein g shearedby the blo ck edge during slid ing. Wi th 5- and 20 -,lgold alloy film s, thin flakes of moderate size wer efound whi ch apparen tl y peeled off from the substrates(e.g., Fig. 3 {e)) . The mecha nism of this type ofIailure is usually du e to fatigue at t he plating- sub­stra te interfac e.

The 0.2-,), thick nick el pla ted ra ce lost 1 mg in ­abou t 700 revoluti ons {T able 5 (a)) . This is close tothe value obtained with t he unplatcd materials (750re volutions, Table 4). I ncreased film thickness simplyshorte ns wear life. T he poor fr iction and wea r prop ­erty of nickel plating indicates tha t t he wear reducti oncause d by thick elect ropla te d gold is not lik ely dueto the nickel st rik ing used to imp rove the adher enceof gold on stee l. The st rong effect of film thickn ess ont he durabi lity of gold film further suggests that themass of gold away from the contact area plays a moreimportant rol e in absorbing di sturbance to sliding thant hat lying next to th e subst ra te -film interface. Actu­a lly th e radia l di splacemen t of t he gold at the surfaceupon contact is estimated to be less than 1 p.. Byusin g the ha rdness of pure gold 65 k gyrnm" pri or towork hardening, the real contact area on a verythick gold film wa s ca lcula te d to be about 1 mm "under the sta tic load used , 68 kg. The real contartareas estimate d by observation of the disengagedspec ime ns were a lwa ys la rger , usually a bout 2 mrn"for fj - and 20-/l gold films. A slow in crea se in fri ctionwith in crea sin g film t hick ness is illust rated in t hefirst column of T able 3. The forma tion of the enla rge dcontact area is lik ely ca use d by plasti c flow whi ch isesse n t ia lly the phenomenon of junction growth firstreported by M cFru-luno and Tabor (8) . Go ld, bein ghig h ly ma lleable, is capable of undergoing consider -

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70 RIITS U T AKAGI AK D T u xo L IU

a bly more plast ic deform ation without failure th anoth er met al s and a lloys, and should provide longerwear life as a lubricating film.

Visual evidence of plastic flow of gold was a lsoobserved during the long runs. Occasion ally a smalla mount of gold, whi ch was first displaced to th e out­s ide of the wear t rack such as shown in Fi g. 4 (a) andt hen re-entered the contact a rea by slight lateralmo tio n of the block , would subsequently spread outover t he ra ce upon repeated sliding.

Conclusions

The sliding friction and wea r between a steel blockand a stee l race plated with gold and other soft met al swere measured under a load of 68 kg and at a linearspeed of 13.2 em/sec . The pl ating covered a filmt hickness ra nge of 0.2 to 20}-t. The wear life was de­fined as t he number of revolutions corresponding tol-mg weight loss of t he ra ce. Experimenta l results ledto t he followin g conclusions :

1. Go ld plating may improve the wear life, as de­fined above, over th at of un lubricated steel ra ces upt o four ord ers of magnit ude, although t he frictionfor ce rema ins high . Under t he experimenta l condit ionsused , t he sliding mechani sm is different from th at ofother soft metal films.

2. Gold plating as a lubricant is most effect ive withhigh film t hickness (at least 5}-t, probably about 20 }-t .)

3. Silver, copper, and gold a lloys plated to th epr oper thickness may improve th e wear life severa lhundred times, far less effect ive th an gold. Observa -

t ions show t ha t gold films underw ent considera blymore plasti c deformati on th an oth er ma teri al s.

4. Resul ts on gold a lloy films indicate th at th erea re tw o basic fa ilure mechani sms: rubbing off andpeelin g. Wh en a film exceeds t he optimum thickness,rela ti vely large flakes may peel off as wear debris,thus greatly shortening the wear life of th e film.

ACKNOWLEDGME NT

T echnical help of R. J oseph Miller is ackno wledged .

R E F E RE NC ES

1. BOWDEN, F. P. , and T ABOR, D ., "T he F rict ion and Lu brica­t ion of So lids," Part I . p . 111. C larendon P ress, London , 1954.

2. BENZING, R. J ., "Solid Lubrican ts," In "M ode rn Materials,"Vol. 4, p . 243. Academ ic Press, N ew York , 1964.

3. BRAIT HWAITE, E. R. , "S olid Lubricants and Surfaces," p. 212.Pergam on Press, New York, 1964.

4. EVANS, H. E ., and FLATLEY, T. \V., "Bearings for VacuumOpera tion , Retain er M ateri al and D esign," N ASA T N D­1339, May 1962.

5. E VANS, H. E., and F LATLEY, T. \V., "High Speed VacuumPerforman ce of Go ld Plated Miniature Ball Bearings withVarious R et ainer Materi als an d Configurations," NASAT N D-2101, Dec. 1963.

6. FLATLEY, T . W., " High Speed Vacuum Performan ce of Mini­a ture Ball Bearings Lu bricated with Co mbinations of Bar­ium, Gold, and Silver F ilms," N ASA T N D-2304 , June 1964.

7. TAKAGI, R ., and LIU, T. , "T he Lubrication of St eel byE lectroplated Gold," ASLE T rans. 10, 115-1 23 (l96i ) .

8. McFARLANE, J. S., and TABOR, D ., "R ela tion between Fri c­tion and Adhesion," P roc, Hoy . Soc. (Londo n) SCI'. A, 202 ,244 (1950) .

DISCUSSION

R. E . LI-:E, J IL (Gene ral El ectric Co., Schenect ady , N ew Y ork ) :

Becau se of the unique properties of gold, conside ra ble efforthas been expended by va rious inv estigators over the years tocap ita lize on its low shear strength, good conducti vity, highmelt ing point , and resistan ce to corrosion . Unfor tunately , thesedesirable properties arc offset by a number of undesirablecharacteristics , namely , the susceptibility of gold to mechan­ical wear , metal transfer , and welding. This situa tion is fur th erborn e out by the not icable absence of pu re gold electrica lconta cts. The fun ction of gold in electrical contac ts is gen­e rally limi ted to tha t of an alloy additive for the prev ent ionof ta rn ishing , or as a th in protective plating to im prove relia­b ility on low cur rent-carrying contac ts and pins, i.e., drycircuit relays and connectors . To improve the sliding behaviorand lower th e coefficien t of fricti on of gold versus gold inconnector contact applications, a thin film lubricant known asoctadecyla mine-hy dro ch loride (OD A-H C\) was developedseveral yea rs ago (Al).

Co nsiderable work has been carr ied out with gold as a solidfilm pla ting to lubrica te rolling element bea rin gs. These efforts(A 2-A5) have met with varyi ng degrees of success, dependingupon the plati ng process used , clements plated , typ e of test ,test para meters, and the criterion estab lished for judging per­forman ce. Information revealing th e extent to whi ch thin

film gold pla tings are being used on rolling clement bearings,particularly aerospace applicat ions, would be most welcom e.

H owever , the life of gold films in sliding a pplica t ions woulda ppear to be even more limi ted tha n that of rolling a pplica­t ions, since in rolling, a better understanding of the wearmechani sm is requi red to optimi ze film performan ce. Theaut hors are to be congra tula ted for their comprehensive workon gold films under sliding conditions, report ed in this paper,and in a prev ious pap er pub lished by them in 1966. Theyfound the wear life of gold films performing under heavyloads can be increased by contro lling the film thickness some­where bet ween 5 and 20 u , I n our work for the Air Force onheavily load ed rolling clemen t bearings and gears, we alsoachieved fairl y good results with gold films in th e ord er of 5 p.

in thickness. Lik e th e au th ors, we did not run gold againstgold, hu t 52100 steel again st gold, since reducing the inter­ntornic attraction between sliding members tends to minimizeweld ing. As rep orte d by th e authors, we also found evidenceof flak ing and relatively high fri ction . We attributed th e hi ghfriction to th e bu ildup of sma ll islands of loose wear debriswhich were subse quently plasticall y deformed and weld edto the sur face. The possib ilit y of mechani call y working th egold islands ma y also be a contributing factor, althoughto a lesser degree.

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Lubrication with El ectroplated Gold 71

On th e work of the autho rs, severa l questions come tomind whi ch t hey might care to commen t on. (1) D o theyhave any t houghts as to the reason or reasons th e endurancelife of gold a lloy A and B peaked a t a round 1- 5 p. of filmt hickness as op posed to pu re go ld which peaked between 5and 20 p.? (2) Could t hey discuss th eir tech niques for applyingthin films un iformly and measuring film thickn esses downto 0.2 p. in a little more det ai l.

REFERE~CES

Al. SPEHGEL, J ., and GODWIN, E., "A T hin Fi lm Lu br ican t forCo nnector Contact s," I EEE, Vol. PMP-l , N o. 1 (Parts,M ater ials a nd P acka ging) , June (1965) .

A2. EVANS, H . E., and F LATLEY, T . ' V., "H igh Speed VacuumPerfor mance of Gold Plated M ini a ture Ball Bearin gs withVarious R eta iner M aterial s and Co nfigura tions," N ASAR p l. T ND-2101, D ecember, 1963.

AS. LEE, R. E., JR., " Lubrica tion of H ea vily Loaded, LowVelocity Bearings and Gears for Aerospace E nv ironme ntalFa ciliti es," Arnold Engineering D evelopmen t Center R e­port AEDC-T R -65-19; available from DDC or CFSTIas AD 456355, J anu ary, 1965.

A4. LEWIS, P ., M URRAY, S. F., P ETERSON, M , B., and ESTEN,H ., " Lubricant Evaluation for Bearing Syst em s Operatingin Sp atia l E nvironments," ASLE T rans. 6, 67-69 (1963) .

A5. BUCKLEY, D . H., SWIKERT, M ., and J OHNSON, R . L ., " F ric­t ion , ' Year , Evaporation R ates of Various M aterials inVa cuum to 10-1 mm H g," ASLE T rans. 5, 8-23 (1962) .

M . B . P ETEHSON (M echonica l Technology, lnc., Latham , N ewY ork ) :

The authors have shown t hat pure gold films have mu chlon ger lives t ha n several other metals. These in clude goldall oys whi ch are harder than t he pure go ld ; silver which hasver y si milar properties to gold; as well as copper and nickel.The aut hors hav e shown th a t gold films, to be most effecti ve,should be pla te d to thickness of app ro ximately 10 u,

It is interes ting t hat gold should be so much supe rior tosilver , whi ch is similar in m echanical prop ert ies . The authorshave noted t ha t the gold underw ent conside rably more plastic

deformat ion than t he other materi a ls and also th at t he goldappea red to be reus ed . T he fact that it underw ent more de­form ation cou ld be attributed to it s longer life ' how ev er itsreuse in t he film has a lso been shown for MoS,~bonded fiims.Sin ce gold wou ld be th e least likely meta l to form oxide orother contam ina ting films, its may be able to re -nd herc toth e gold film and t hus be reused . If t his is th e reason forits effectiveness, on e would think t ha t it wou ld be betterthan, for example, indium, which would be more du ctil e.

T here is some question as to the meaning of t he 1 mgweight loss. D oes th is mean that the film is beginning to comeoff t he surface , and if the test were continued, it wouldfail ; or does th is mean that the film has on ly worn to thatamount and would ha ve significantly lon ger life if the testwere cont inued ?

AUTIIOHS' C LOSUHE :

In answer to M r. Lee: The fa ilure of 5 p. or thicker goldalloy films was alway s observed with moderate-sized t hinflakes peeling off. It is likely that the adhesion between t hickgold alloys and the substrate steel is easily weakened, prob­ab ly by fa tigu e. T he existence of ma ximum wear life isundo ubtably due to t he two compet ing mecha nisms, namely ,t he above-mentioned peeling and t he gradual rubbing off oft he film . Pure go ld films, on the oth er hand, showed no failuredu e to t he former mechanism. T he elec tropla ting was per­formed by a commercial source according to prop rietary proc­esses. According to th e processor, th e th ickness is controlledby selected plating time and current density and is verifiedby bctascop e.

In an swer to M r. Peterson : I n all of our experiments, itwas noticed that when t he weigh t loss of t he race on ce ex­ceeded 0.2 mg, the wear ra te became very high. No sig­nifican t change in th e results may be expe cted if a weigh t lossof 0.2 or 10 mg was chosen instead of the 1 mg used. Theamo unt of 1 mg weight loss was chosen because at this stage,an average 0.2 p. film was well over 50% worn ou t, as shownby th e appearan ce of dark brown color on t he ra ce sur facesimilar to th at obtained with unlubricated races.

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