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The Lubrication of Steel by Electroplated GoldRiitsu 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 (1967) The Lubrication of Steel by Electroplated Gold, A S L ETransactions, 10:2, 115-123, DOI: 10.1080/05698196708972171

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ASLE TRAXSACTIONS 10, 115--123 (1967)

The Lubrication of Steel by Electroplated Gold

By RIITSU TAKAGP and T UNG LIU2

Sl iding friction bet ween an lh in. spherical steel rider and a gold-plat ed steel flat !Casmeasured for various loads (200-2000 am), gold film thi cknesses (0.1-10 Jl) and steel com ­binations (440C and 52100). Coefficient of friction recorded in each run for 100 traverseswas found to be most sensitiv e to th e rider ma terial (0.1-0.3 wit h 440C rider and 0.1-0.6with 52100 rider, as compared to 0.6-0.7 for unlubricated steel) . W ith a 52100 rider, as th enumber of traverses increase thr ee distinct types of frictional behaviors were found : (A)slowly decreasing, (B) rapid ly decreasing, and (C) increasing (some times a minimum wasfirst reached) . Th eoreti cal coefficient of friction (0.1) was onl y observed with 'Type E'results. With a chromium rich 440C rider, friction was always low presumably due to it slow affinit y to gold .

GIMBAL

Introduction

IN recent years, th ere is an increasing need for a lubri­cant to withstand high temperatur e and high vacuumenv ironments. T o rep lace conv entional lubricants, softmetal films have frequently been considered. Thelubricating action of soft metallic films, such as leadand indium, on hard stee l substrates have beendemonstrated by Bowden and T abor (1 ) . Mo st softmetals are un for tun ately poor lubricants, low melting,and not resistant to oxidati on. One outstanding ex­ception , gold, is soft , inert, and high melting. T hinfilms of gold have been regarded as one of th e mostpromising candida tes for lubrication in high temper­ature and high vacuum env ironments (2, 3) .

Con sid erable in terest has been shown tow ard lubr ica ­tion by gold films as may be demonst rated by severa lrecent publications. Evans and F latley (4-6) made anextensive study of the service life of gold-p lated min­iature ba ll bear ings at high speeds, at which ro llingcontact predominates. Peterson et al. (7) explored t hepotenti al of gold film for sliding lubrication with anickel based alloy . Hopkins and Gaddis (8) usedO.OOl -in ch gold plating on stee l as the standard forevaluat ing solid lubrican ts, qu alifying only thoseyielding less sliding fri ction. T hey obtained frictioncoeffi cients of 0.1-0.3 under atmospheric condit ionsand slight ly less under vacuum env ironments in th e10-6 to rr region . SpaIvins and Buckley (9) showedthe effectiveness of vacuum-deposited gold film onnickel and nickel a lloys as lub rican ts. The importan ce

Presented as an American Society of Lubrication Engineer sPa per at the Lubrica tion Co nference held in Minneap olis,Minnesota, October 18-20, 1966.

1 R esearch P hysicist , R esearch Insti tute, Un iversity of Day­ton, D ayton, Ohio 45409.

' Materials Engineer, Air Force Materia ls Labora tory .Wright-Patterson AF B, Ohio 45433.

© 1967 by American Society of Lubrication Engineers.115

of surface cleaning was thoroughly demonstrated. Inthe sliding of gold metal , the effect of work hardeningon friction was particularly pointed out (10) . T hemet al t ra nsfer and wea r pro cess in sliding has alsobeen reported (11) .

The work reported here consists ma inly of recipro­cating sliding of a rid er with a spherica l ti p on a gold­plated flat stee l surface. The purpose of this work isto find th e effectiveness of gold-plat ing in reducing thecoefficient of fri ction between two stee l spec imensunder various conditions. By studying the cond itionsund er which th e gold films rupture, a better under­standing of th e lubricating mechanism of plated goldma y be obt ain ed. Two kinds of bearing steels, 52100and 440C stainless , were chosen for this inv esti gat ion.

Experimental methods

Th e experiment al work consisted of the preparationof specimens, the measurem ent of sliding fri ct ionforce, th e observation of wear t racks, and some hard­ness measurements.

The dat a on sliding frict ion were obtain ed from anappa ratus shown schemat ica lly in Fig. 1. This ap-

STRAIN GAGES

~-~t

LOAD

FIG. 1. Schematic of reciprocatin g sliding frict ion tester .

pa ratus was a mod ified Bowden-Leben machine . Thespecimens consist of a rider , which in the currentwork was restricted to an VB -inch sphere, and a flat.The rid er was held to a spring loaded arm with apivot in the middle. The ba ck end of the arm wasdriven hydraulically with an automat ic direction re-

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116 Rursu TAKAGI AXD T UXG Lru

versing switc h to repeat traverses of a preset ampli­tude. In th e fron t part of th e arm, i.e., between th erider holder and the pivo t, th e ann was composed oftwo flexible members, one each vertica l and horizon tal.Two sets of four ann st ra in gages were attached tothe flexible members of appropr iate sti ffness for thenorm al load and fri ction force. The st ra in gages wereconnected to a two-channel recorder provided withcarrier amplifiers. The load and friction force werethus continuously recorded during th e sliding frictionexperiments .

Throughout th e ent ire experiment, a sliding speedof 0.17 mm / scc was used. The rid er was held tightly ina chuck to prevent rolling, and was driven recipro callywith a st roke length of 5.5 mm . T he normal load wasfound to be substanti a lly constant with a maximumvariation of ±50/0. The fri ction forces were cal culatedfrom record ed data showing the avera ge of two trav ­erses (one in each direc tion).

T he riders used were commercially availabl e sphereswith surface roughness less than 1 fLinch. Most of th eflat specimens used were th e top faces of speciallymade Timken type test -cups. These sur faces werefurther hand poli shed with 4/0 emery pap er to asurface roughness of th e order of 1 p.inch. T hese cupswere plated by a commercia l process" with pure gold(99.99+% ) with a specified hardness of 80-85 DPH(diamond pyram idal hardness number) . The hardnessof several specimens with lO-fL gold film was checkedon a micro -hardness test er with 5-20 gm of load. T heaverage of measur ed results is 78 DPH. A flat sheetof pure gold of lh-mm thi ckn ess was also used forcomparison . For hardness measurements, th e riderspecimens were first held in a chuck and hand polishedwith 4/0 emery paper to obt ain a flat sur face suit­ab le for th e ind entation. Th e data obtained from amicrohardness test er ar c given in T ab le 1.

Th e specimens were immediately removed from thebath, rin sed with disti lled water , dri ed in a st reamof clean air and placed on the fri ction apparatus. Allth e tests were condu cted at the temperature range of75-85 F and at a relative humidity of 30-500/0.

Th e abov e described cleaning procedure was adoptedfor all sliding experiments. Alt ern ative cleaningmethods were found to lead to larger discrepancies insliding friction data.

The sliding fri ction forces were recorded for 100traverses (50 in each direction ) in most cases. Thefr iction force as recorded from th e output of thestra in gages attached to the vertica l flexible memberis directionally sensitive . In two successive traverses,the output reversed its polarity and the shift in re­corded trace represented th e sum of frict ion forces inboth dir ection s. Th e data thus calculated were theaverage coeffic ients of friction of two adjacent trav­erses and were reported as those of the second paths.(For example th e report ed figure of the fiftiethtraverse is actually the average obtained from thefort y-ninth and th e fiftieth paths) . The sliding fric ­tion between riders of two stee ls (52100 and 440Cstainless) and gold-plated steel of th e same two vari­et ies were measured und er loads of 200, 500, 1000and 2000 gm, over a gold film th ickn ess range of 0.1­10 fL. Sliding fri ction data were also obtained withsteel riders on unlubricated steel and Y2 mm puregold sheet , and with copper rid ers on gold sheet andgold-pla ted steel flat s. Th e reproducibility of resultsis illustrated by Fig. 2, in which th e data of th e tworun s were obtained wit h a t ime interva l of threemonths.

Microscopic observations of wear scar and weartrack of the rider and t he flat , respectively , were mad ea fter th e runs. Theil' appearances and some quanti -

Prior to the sliding friction experiments, t he speci­mens were washed with ace tone and isopropyl alcoholand degreased by cathodic cleaning in 10% trisodiumphosphate solut ion at a current density of 1 ina / em"for one minute and 10 ma/ cm" for another 15 seconds.

Mat erial

o

•

20

z§ 0.6~~~~t-__~__""'~__~ ~~ ~0::

~04l~l.--_Q-_......_-e--~'=! 0 .2uu,u,WouoL-__---L ..l..-__--l -'- .l.....-'

o

NUMBER OF TRAVERSES, n

BO:,illlII :'i'ii lli !)ll

4G

Hardness(D I' II )

.">2100440C Sf;Copper.'>2100440C ssGold

T ABLE 1l!ardness of Specim ens

Specimen

1IS-inch rider~ ;--r ~- i .

Flat

3 Ni ckel st riking is par t of th e process. Th e effect of thisvery thin layer of nickel is considered as an integrul par t ofthat of the gold-pla ting.

FIG. 2. J-I! curves for 52100 riders slid ing on 5-11 thi ckgold-plate d 52100 fla ts, showing the degree of reproducibi lityof th e curves.

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The Lubricati on of Stee l by E lectroplated Gold 117

tative measur ement of wear track width were foundto be significant in the inte rpretation of experimentalresults an d are discussed as individual cases arose.

Results

results are arranged according to the material com­binations and are described in the following pa ra ­graphs. In all four combinations, loads of 200, 500,1000, and 2000 gm were used; th e film thickne ssesra nged from 0.1-10 p.. .

0.8.--------------------,

NUMBER OF TRAVERSES. n

GOLD THICKNESS(ft)

Z 0 .1 00 5 6I- 10 00

~GOL D SHEET 'iJ0::

u, .... ......u, 0.20 k 6I- A

Z Q,Q.,., or.~

W ..... .....0u,u,w00

I I I00 20 40 60 80 100

NUMBER OF TRAVERSES. n

440C STAINLESS RIDER ON GOLD-PLATED440C STAINLESS FLAT

T he sliding friction coefficients became constantbetween 0.1-0.2 with repeated sliding. T ypic al resultsobtained from 200-gm load is shown in Fig. 4. Fo r aload of 1000 gm and less, no noticeabl e cha nge wasobserved after the first ten t ra verses . Under a load of

0.4,---------------------,

1008060

52100 ON 52100 +52100 ON 440 C 0440 C ON 52100 0

4 4 0 C O N 440 C 'iJ

4 0

o

~o

20

OL__-L__---l ..L-__--L.__---'.......

o

zoI­o0::u,

~ 0.4

I-Zwo 0 .2u,u,woo

FIG. 3. f-n curves for steel riders sliding on un lubricatedstee l flats. Load 500 gm .

STEEL ON STEEL '

The sliding fri ction between the four steel combina ­tions (52100 and 440C, as r iders and flats) are sim­ilar (f = 0.&--0.7) . T ypi cal results with 500 gm loadare illu strated in Fig. 3. A slight trend of decreasing

coefficient of fri ction with increasing load was noticed.The size of t he wear scars of th e rid ers measur ed after100 t ra verses, however, differed considerably for dif­ferent materi al combinat ions (Table 2) .

T A B L E 2Slidin g W ear Data of Unlubricated S teel Combinations

Wear und er a load (gm)>

FIG. 4. f-n curves for 440C riders sliding on gold-plated440C flats and a gold sheet. Load 200 gm.

2000 gm, slow change in friction continued in somecases to about 80 paths. A trend of lower frict ion witha th inner film was genera lly observed. At higher loads,however, th e effect of film t hickness tended to beirr egular.

200 500 1500

Rider F lat v d v d v d

52100 52100 Ii (0 .274) 35 (0 .326) 90 (0 .4 13)r52100 440C 17 (0. 273) 35 (0 .326) 74 (0 .393)

440C 52100 0 .9 ' (0 . 130) 3 .4 (0 . 182) 11 (0 .246)440C 440C 3 .2 (0 . 180) 8 .7 (0 .230) 25 (0 .300)

a V = wear volume (10- ' mm 3); :d = wear scar diamet er (mm).

STEEL ON GOLD-PLATED STEEL

The sliding behavior of st eel rid ers on gold-platedsteel varied considerably. Rider materials appea red tobe the most dominating variabl e. T he effects of sub­strate mat er ial, gold film thickness, and load werealso observ ed and shown to be interdependent . The

440C STAINLESS STEEL RIDER ON GOLD-P LATED52100 STEEL FLAT

T he sliding frict ion obtained with this materi alcombina tion is strongly affected by film thicknessbut only slightly by the loads. At a gold film thick­ness of 0.2 p.. or less, fri ction coefficient decreasedrap idly in th e first 10 traverses to a val ue about 0.1and stayed constant. With t hicker films, the co­efficient of friction in most cases did not appear toreach constant values at the one-hundredth traverseand remain ed at higher values ra nging between 0.15and 0.27. Fi gure 5 shows the typical results aga in inthe form of friction coefficient versus number of t rav­erses (f-n) curve, obtained with a load of 500 gm.At th e film thi ckness of 0.5 p.., a curve with inter­mediate behavior is illust rat ed.

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118 RnTSU TAKAGI AND TUNG Lro

0.8.----------------------,

04r----------------------,

Film thickness(fl) 200 500 1000 2000

10 A :\ A A!i A A A B2 A :\ B CI A B C C0 .5 B C C C0 .2 B C C C0 .1 C C C

Load (gm)

52100 RIDER ON GOLD-PLATED 440C STAINLESS FLAT

With 440C stainless subst rates, the genera l cha rac ­teri stics of f-n curves differ only slight ly from tho seobtained with 52100 subst rates except somewhathigher friction was observed . Typical results obtainedwith 500-gm load are shown in Fig . 7. In particular ,the extr emely low friction coefficient of about 0.10 asobtained with 0.5 fL or thinner gold film on 52100substrate at a load of 200 gm was never observ edwith 440C substrate .

The width of the wear track on the flat specimenafter the sliding experiments was also found to varyconsiderably with the material combination. Figure 8shows the track width under 2000-gm load at various

" B, with 52100 flat , C, with 440C flat.

tr end was not ma intained at the higher loads wherethe f-n curves are no longer of T ype A. T his behavioris also illustrated by F ig. 2.

During the first severa l traverses (up to 20) , con­siderable variations in fr ict ion were observed in mostcases. No general ru le could be found to correlate thebehaviors during the se period s. Subsequently, threebasic typ es of f-n curv es, viz. A, B, and C, wereobserved. Each type was found to associate with aspecific wear track appearance. Type A is a graduallydecreasing curve accompanied with a dull wear trackin which the deformation is restricted to the goldfilm. Type B is a rap idly decrea sing curve with awear track composed of a shiny central portion anddull edges with no visible rupture of the gold film.Type C is a curv e hav ing a minimum or increasingthroughout, and rupture of the gold film in the weartrack is evident . Type C curve exhibits a minimumor increases throughout. It is associated with a weart rack containing visibly ruptured ar eas in the midd leof the shiny portion of the track. In extreme cases,these areas became connected to form a continuou sdark band flanked by shiny edges. All the f-n curvesfor the slidings of 52100 stee l riders are classified andlisted in Table 3.

TABLE 3General Shape of f-n Curves for 52100 St eel Rider Sl id ing on Gold­

Plated 52100 or 440C S tainless St eel Flat, where A: GraduallyDecreasing, B : Rapidly Decreasing , and C: Increasiruj

f rom the S tart or after Passing a Minimum

100

100

g

eo

eo

LOAD (g)

200 0500 t::.1000 0

2000 'iJ

60

60

0. 1 00 .5 l::J.5 0

GOLD SHEET 'iJ

GOLD THICKNESS Cft)

40

40

20

20

NUMBER OF TRAVERSES, n

NUMBER OF TRAVERSES , n

Lo -"O---o--

OL-__---L ...L-__---.Jl.--__....L.. ~

o

z0f-U

0::u,

u,0f-ZW

~ 0.2u,u,W0U

00

zoi=u0::u,

u,of­ZWUu,u,Wou

steel. Disregarding the first 20 traverses, it was noticedthat with 200- and 500-gm loads, the f-n curves de­scended gradua lly (Type A) . With the load increa sedto 1000 gm the rate of descent became much morerapid (Type B). At the highest load of 2000 gm aminimum was reached after which the f-n curv eascended (Type C) . It was also observed from theType A curves that f increased with the load. Thi s

52100 RIDER ON GOLD-PLATED 52100 STEEL FLAT

Considerably more variation in friction was ob­served with 52100 steel rid ers as compared to 440C.Three general types of f-n curve s were observ ed. Thesemay be illustrated by the typical results, shown inFig. 6, obtained wit h 2-fL gold film-plated on 52100

F IG. 5. I-n curves for 440C riders sliding on gold-plat ed52100 flats and a gold sheet. Load 500 gm.

FIG. 6. f-n curves for 52100 riders sliding on 2-11 thi ckgold-plated 52100 flat s, showing three basic forms of th ecurves.

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The Lubrication of Steel by Electroplated Gold 119

Ga L a T HICK NES S (f') ~

0 .1 0I D.

10 0

0.8 '

Z0~0

a:::LL

LL0

I-ZW0 0.2LLLLW00

00 2 0 4 0 60 80 100

plated 440C st ainless stee l flat , the mm imum trackwidth is at a film th ickn ess of 2 fL. Observation of thecent ra l portion of the Type C wear tracks on bothth e 440C and the 52100 subst ra tes under microscoperevealed numerous dark spots and loose wear debris.With a 52100 steel rider and a thin gold film, the darkspots frequently form an almost continuous band. Itswidth increa sed with increasing deviation of totalwear track width from th e corresponding trackform ed with a 440C rider.

STEEL RIDER ON GOLD SHEET

The sliding fri ctional behaviors of steel riders on agold sheet are shown in Fig. 9. Con siderably higherfriction were observed with 52100 than with 440C

0.8,....---------------------,

NUMBER OF TRAVERSES, n

LOAD l Q)

10080

RIDER52100 440 C

60

500200 0

4020

zoI­oa::LL

LLoI­Z!:!:!oLLLLWoo

NUMBER OF TRAVERSES , n

film thickness. Of the four combinations, the curve of440C on gold -plated 440C was omitted as it lay veryclose to that of 440C rider on plated 52100 subst rate.With 440C rider, the track width curve is almostparallel to that calculated from an lJs-inch sphere incont act with a flat surf ace (both rigid) covered witha fluid film (dotted curve in Fig. 8). With a filmthickness of 2 fL or more, little difference in trackwidth was found between the results obtained withthe two riders. However with thinner films, consider­ably wider wear tracks were found with 52100 steelriders than with 440 riders. In particular, with gold -

FIG. 7. I-n curves for 52100 rid ers slid ing on gold-p lated440C flat s. Load 500 gm .

FIG. 9. I-n curves for stee l r iders sliding on a gold sheet.

2000

0 .750 .70

0 .520 .50

1000

Load (gm)

500

0 .370 .38

200

0. 250 .22

52100440C

Rider

COPPER RIDER

A copper rid er sliding on a pure gold sheet yie ldeda high friction coefficient of about 1.9 with consider­able fluctuations (Fig. 10). Microscopic observationsshowed that both th e wear sca r and the wear track

T ABLE 4W ear Track W idth , in mm, on Gold Sheet aft er 100 Traverses of

Sliding with St eel Riders

riders. The results are consistent with the data ob­tained with thick gold-plated steel flat s. T he weartrack width varied considerably with the loads butapparently was not affect ed by th e rider material,T able 4.

20 .50 .2

o5 2100 ON 5210 0

OL-__....L_ _ ---JL...-_---L.__.L.-__--'-__~

0 .1

GOLD PLATING T HICKNESS, f-

0.4EE

I 0.3I-0

3:y;

0,20<!a:::I-

a::: 0.1<!W3:

FIG. 8. Track widths on gold-plated flat s afte r 100 trav­erses of rid ers vs , gold film thickn ess. Load 2000 gm .

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120 RUTSU T AKAGI AND T UNG Lru

F IG. 11. / at n = 100 vs, gold film thi ckness for 440Criders sliding on gold-plate d steel flats.

e•

10

o

52

o

o [). 0 'V

• • • •2 00 5 00 100 0 2 0 0 0LOAD ( )

5210 0 SU BSTRATE440 C SUBSTRATE

0 2 0 .5

GOL D PLATIN G THICKNESS. f-

as compared to the case of ind ium describ ed byBowden and Tabor (1, p. 115) .

The pronounced differences in lubricating behaviorbetween th e gold-plated steels is unlikely due to th eslight difference in mechanical properties. AlthoughTable 1 showed that the 52100 stee l has a 570 DPHas compared to a 790 DPH for th e 440C stainlessstee l, thi s difference may only account for a sma lldiscrepancy due to the contact ar ea explained by th ethin-film lubrication th eory of Bowd en and Tabor(1). The change in the general characters of the f-ncurve when the 52100 rid er is replaced with a 440Cstainless one clea rly indi cates a shift of the fri ctionmechanism. Even with a 440C rid er, such indicationmay be obtained by plotting th e terminal coefficient offriction versus film thickness as shown in Fig. 11.Below 0.5-t-t film thickness, 52100 and 440C stainlesssteel substrates yield comparable results, while athigher film thi ckness, positively deviated points ar emade up mainly from data obtained with 52100 stee lsubstrates.

Observation of th e wear scar revealed that 52100steel riders as a rul e, picked up a consider ableamount of gold after sliding as shown by th e colorchange. The 440C stainless steel rid er scars onlyshowed very faint golden color with high loads. It isevident that with 52100 ride r, part of the slidingeventually took plac e between two gold-plated stee lbodies. This mechanism naturally lead s to the high ercoefficient of friction. Comparing thi s with th e dataobtained with copper rid ers where th e coefficientsof friction are much higher, it is logical to regard thatthe action of 52100 rid ers lies somewhere betweenth at of copper and 440C stainless steel riders. Theapp earance of the wear scars of copper riders pointedstrongly to thi s explanation . The wear scar app earedto have several irregularly sha ped golden -coloredpatches. Large agglomerates of gold apparentlyadh ered tightly to the rider such that the shape of

Z o.3r---------------------,oI-U

~ 0.2

1.1..oI-Z 0 .1W

U1.1..1.1..~O L.......l---'--------J---l----'-------l---.J-JU 0.1

zoI­Ua::1.1..

1.1..oI­Zw~1.1..1.1..WoU

NUMBER OF TRAVERSES, n

were rough. Large agglom eration of the loose weardebris was, however, ab sent.

Copper sliding on 0.1- or I-t-t thick gold-plated stee lsurfaces gave, besides much lower coefficient offric tion on the whole, lesser fluctuations of f-ncurves at higher n. A large area with gold- coloredagglomeration was observed on each wear scarsuggest ing th at the sliding eventua lly took placebetween gold and gold.

Discussion

The sliding coefficient of frict ion between cleanedsteel surfaces is in th e 0.6-0.7 range th ough th eamount of wear vary considerably depending on th etypes of steel used (Table 2). Gold-plating un­doubtedly has lubricating effect as the terminalvalues (at one-hundredth traverse) of th e coef­ficient friction are a lway s below 0.6. In particular,with 440C stainless steel riders, thi s value never wasobserved to exceed 0.32. Considerable differen cesexisted, however , between different materials, load ,and film thickness. Very large changes in fri ctionduring running has been observed mostly with 52100rider on gold-plated 440 flat s. A transient valu e ofcoefficient of friction as high as 0.9 was observed.Best lubrication was only obtain ed with very thinfilms where the friction coeffic ient may reach around0.1. In these instances, no rupture of the gold filmwas observed over the wear tracks. Figure 11 showsthe effect of film thi ckne ss on the coefficient offriction. The lowest value was obtained with 0.1- t-tfilm, yet without the indication of reaching a minimum

FIG. 10. /-n curves for copper riders sliding on gold-plated52100 flats and a gold sheet.

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T he Lubrication of Steel by E lectroplated Gold 121

TABI.E 5Observed Lowest Friction Coefficients

occurred, on th e gold film of thickn ess 0.1 fL, und erthe lowest load for 52100 stee l rid ers to avoid filmbreakdown whil e for 440C stainless steel rid ers nodefini te load dependence was found . It is expectedfrom t he standpoint of the differen ce in properti es ofthese two steels described in t he preceding pa ra gra ph,440C stainless stee l is favored as a rider material and52100 steel as the film substrate in order to minimizet he gold film rupture. As a matter of fact, the lowestcoeffic ient of friction observ ed (0.092) was obt ained

th e sliding interface no longer followed the conto urof the copper rid er. Simila r situations were noti ced int he slid ing of copper on lead films repor ted earlier(12) . It was observed that und esirable agglomerationswere formed to obstruct the formation of a thin filmwhich would lubricate effect ively.

The difference in behavior between different ridersmay be ascribed to the differe nce in t heir affinity togold. Gold is kn own to alloy with copper rathereasily , and may be pla ted on 52100 steel directly .440C stainless steel, on the other hand , has a veryhigh chro mium content of 16-18% as compa red to 1.3­1.6% for 52100 steel whil e carbon contents of the twosteels are compa ra ble. The chromium form s a thin butvery adherent layer of oxide upon exposure to airand renders th e steel stainless. The existence of t hisfilm mak es it hard for gold to adhere to the sur face .In electroplating , exte nded pro cedures includingnickel-striking are essentia l for plating gold on 440Cstainless steel. SpaIvin s and Buckley (9) showed thatad hesion between gold film and nickel may beimproved by prior ion bombardment on nickel therebyenha ncing the lubricatin g action of the film.

It was pointed out for a 52100 stee l rid er slidingon a 52100 steel flat that th e lowest stable final fric tioncoefficient for sliding without apparent breakdown ofthe gold film is close to 0.10. Although this lowest fis dir ectl y observed only when the gold film isext remely thin , a suggestion was pro posed for athicker gold film that th e same lowest friction tak esplace in t hose areas where the bottom of the slidingball and the subst rate stee l is separated by anext remely thin sheet of continuous gold film so thatt he overall observed fri ction force, summed up allover the contact areas, is partly lowered. Observedlowest values of th e fri ction coeffic ient for the twokinds of stee ls used ar e listed in T abl e 5. They

Substrate

52100440C

52100

0 .1020. 161

Rider

440C

0 .0!120 .097

for 440C stainless steel ba ll sliding on a gold-plated52100 steel substrate.

If one assumes that the the ory of thin metallic filmlubrication of Bowd en and T abor be applicable to th elowest friction discussed here, the ratio of the shearst rength s of the gold film to the hardness p of th esubst rate steel should be equa l to 0.092. I gnoring thework hardening effect which increases both hardnessan d shear strength and putting into p the hardnessof 52100 stee l, 570 kg/rum", one gets s = 52 kg /mm-.T his shear strength is two-thirds of the hardness ofthe gold film used 78 kg/rnm", which is a reasonabl era tio for gold.

Conclusions

The sliding friction between a stee l rid er and agold-p lat ed steel subst ra te was measur ed underat mospheric conditions. Bo th the rid er pis-inchsphere) and the fla t specimens were ma de of 52100steel and 440C stainless steel to provide four mat erialcombinations. The sliding frictio n experiments werecar ried out in reciprocatin g motion over a st rokelengt h of 5.5 mm with a speed of 0.17 nun / sec andund er a load of 200-2000 gm. The flat specimenswere elect roplated by a commercia l pro cess with puregold to a thi ckness between 0.1-10 fL. The fri ct ionforce was record ed for 100 traverses in each runand ca lcula ted as th e coeffi cient of fri ction . Theexper imental resul ts led to the following conclusions :

1. The sliding fr iction between stee l bodies maybe effectively reduced by gold-p lat ing .

2. The lubrication of sliding steel by gold-pl atingare effected by several factors, most importan t ofwhich is the r ider materi al. Wi th a 440C stainlessstee l rider sliding on a gold-pla ted fla t , much lowerfriction was usually obtain ed as compared to th ecase with a 52100 rid er .

3. The f-n curves obtained with 52100 rid ersshowed three basic types : (a) Friction decreasedgradua lly . (b) Friction coefficient decreased rapidlyto a ra ther low and steady va lue, somet imes reachingabout 0.1 corresponding to the estima ted value fromthin film th eory. (c) A gra dua l increase in friction,frequently after a mini mum was first reach ed, wasobtained whenever a rupture in the gold film occurred.

4. To achieve optimum performanc e of gold aslubri can t , th e film must be as thin as possibl e yetnot ruptured by th e sliding action .

ACKNOWLEDGMENT

Th e authors appr eciat e the skilful technica l assistance ofR. J . Miller.

REFERENCES

1. BOWDEN, F. P ., and TABOR, D., "The Fr iction and Lubri­cation of Solids," Part I , p. 111. Clarendon Press, London,1954.

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122 RIITSU T AKAGI AND Tuxo L ID

B. BENZING, R . J ., "Solid Lubrican ts," In "M odern Ma le­rials" Vol. 4, p. 243. Academic Press, New York , 1964.

.'3 . BRAITHWAITE, E . R. , "Solid Lubri cants and Surfaces,"p. 212. Pergamon Press, New York , 1964.

4. E\'AKS, H. E ., and F LATLEY, T . W., "B earin gs for VacuumOperat ion, R etain er Material and D esign," N ASA T ND-1339, May 1962.

5. EVANS, H . E ., and FLnLEY, T. W., "H igh Speed VacuumPerforman ce of Gold Pla ted Miniature Ball Bearin gswit h Various Ret ainer Mat er ials and Configurations,"N ASA TN D-B101 , Dec. 1963.

6. F L.ULEY, T. \V., "H igh Speed Vacuum Perfo rmance ofMiniature Ball Bearin gs Lub ricated with Combina tionsof Barium, Gold , and Silver Films," N ASA T N D-:?304,June 1964.

7. PETEHSOX, 1\1. B., MURRAY, S. F., and FLOHEK, J. J .,

"Consideration of Lubrican ts for T emperatures above1000F ," ASLE T rans. 2, 225 (1960).

S. HOPKIXS, V., and GADDIS, D. , "Friction of Solid FilmLubricants Being Developed for Use in Space En viron­ments," Lubrication Eng. 21, 52 (1965) .

.'J. SPAL\'IXS, T ., and BUCKLEY, D. H ., "Vapor-D epositedThin Gold Films as Lubricants in Vacuum (10-11 mmHg)," N ASA T N D-3040, Oct. 1965.

10. BOWDEX, F . P. , and TABOR, D., "The Fri ction and Lubrica­tion of Solids," Part II, p. 39. Clarendon Press, Lon don,1964 .

11. AXTLER, M., "Wear, Friction , and Electri cal Noi se Ph e­nom ena in Severe Sliding Systems," ASLE Trans. 5, 297(1962) .

12. T snv .v , Y., and TAKAGI, R ., "Lubricating Proper ties ofLead Film s on Copp er," IVear 7, 131 (1964).

DISCUSSION

TALIVALDIS SPALVINS (NASA, Lewis R esearch Center, Cleve­land, Ohio):

The authors are to be commended for publishing this veryint eresting and informative pap er. Th e following comm ent sand qu estions arose from reading thi s paper :

Gold-deposited films on meta l surfaces to be used as alubricant is of particular interest in the high-temp eratureand high-vacuum environment . One of the important param­eters for durability and strength of such a film is th e adhe­sion between the film and th e substrate. Therefore, filmswhich would be used in such an environment might bedeposit ed by vacuum deposition methods (va cuum evapora­tion or sputtering) rat her than electrodeposit ion. In elect ro­deposition many impurities may be introduced either fromthe pla ting solution itself or th e gases produced during thisprocess. This will redu ce the adh esion pro perti es, and in manyinsta nces peelin g or blistering of the film has been ob­serve d. How ever, electroplat ing is a sufficiently good methodif th e films ar e used at atmospheric conditions and roomtemperature.

I n T able 1, th e difference in hardn ess for lh-inch 52100 rideris 905 DPH, but for the 52100 flat , it is 570 DPH. This is anappreciable difference in hard ess for th e same material , a lmostby a factor of 2. On th e other hand, the hardness of a 440Cflat is 790 DPH, which falls between the two different hard­ness va lues given for th e 52100 stee l. Wha t is the reason forselecting two different hardness values for the same mat erial?

What explanation do th e aut hors have for Fig. 7, wherethe 52100 rider is sliding on 1 p. thi ck gold on 440 C flat ata load of 500 gm? There is a steep rise in the coefficient offri ction, and at 20 traverses the coefficient of friction is abou t0.8. If th e values of the coefficient of friction are taken fromF ig. 3 (52100 rid er on 440 C flat) and Fi g. 9 (52100 rider ongold sheet) at 500 gm and 20 traverses, one obtains th ecoefficient of fri ction about 0.6 and 0.3, respecti vely. Thi sindi cates th at th e coefficient of frict ion is higher for a lubri ­cate d material rather th an an unlubricat ed one.

It has been shown that th e coefficient of friction decreaseswith the film thickness. In th e summary it is stated th atth e optimum performance of gold as a lubricant should beas thin as possible. It would be interesting to know anapproximate minimum thi ckness of gold which gives thi soptimum performance.

H. E. ,EVANS AND C. E. VEST (Goddard Space Flight Center,Greenb elt, Maryland):

It is encouraging to see work still being pursued in thi sarea since it is felt that thi s method of lubrication has ap­plication in special areas where high temperature, radiation,low pressure, and electrical properties are important . Thisapproach to lubrication for space use was actively studiedduring th e early years of spa ce explora tion . How ever, th eneed for long life, continuous operation of space com­ponent s lead most early inv estigators away from thi s ap­proa ch.

In read ing over th e pap er several questions arise dealingwith preparation of test specimens and th e actual platingprocess. Are th e specifications for th e comm ercial plating ofgold availab le or is th is a proprietary process? Were all testpieces processed at the same tim e or did plated parts comefrom various batches? Since plating is such a critical elementin obtaining good film adh eren ce and thickness, is rigidquali ty control maintained? Were th e alt ernate cleaningmeth ods commerc ial or special labora tory procedures ?

Th e paper states th at a film thi ckness of 0.1 p. was ob­tai ned . H ow was thi s obt ained and is thi s a reprodu cibleprocess?

Ph enomena of initial increase in friction at start-up , fol­lowed by a decrease and th en steady-state operation, cor­relates with th e early work we did with gold-plated ba llbearin gs. Our results consistently showed th e increase th endecrease followed by steady-state operation. We attributedthi s to th e effect of th e ploughing term in th e Bowden andTabor thin film an alysis (1) .

A. R . SI'F:XCER (St aff M etallurgist , Th e Bendix Corp., R esearchLaboratories Division, Southfield, M ichigan) :

Th e aut hors are to be particularl y commended for thisnew evidence that gold films of widely varyi ng thickness areeffective in lubri cating hardened ty pe 440C stainless steel inair. Can they add add itional inform ation on th e relativethicknesses of nickel strike and gold plate at th e 0.1 p. goldthi ckness level ? At th e 0.1 p. level th e total plated film is onlyabout 4 millionths of an inch thick. T his would be roughlyequivalent to 1.2 mg of gold per square inch if only goldwere used. It seems reasonable to suggest that the nick el

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The Lubrication of Steel by Electroplated Gold 123

strike might be a significant part of the total plati ng thick­ness in this case .

A UTH ORS' CLOSURE :

The important critical comments concerning the basic ap­proa ch of this work ar e highly app reciat ed.

T o Mr. Spa lvins: In the present work the electrodepositi onwas exclusively used to prepare the gold film. As pointed outby Mr. Spa lvins, vacuum deposition is also a promising meth odfor gold film preparation. However we have no experience ofusing vac uum-dep osite d gold film for sliding experiment. In­formation of experiment s with vacuum-deposite d film iscerta inly welcome.

In our sliding experiments wit h an Alpha T ester using gold­plat ed Timken type races of 52100 steel with two widely di f­ferent hardnesses, no definit e effects were observed in bothfrict ion coefficient and wear. No work was attempted to studythe influence of substra te hardness in th e series of experimentsdescrib ed in this pap er.

I n the initial 20 t raverses or so , frequ ently th ere was largevari at ion in the friction cur ve due to factors such as tra ceimpuri ty and other unknown origins. T herefore we did notattempt to derive any info rmation from th e discussions of thisregion. But it is true, as pointed by Mr. Spa lvins, t hat some­times friction coefficient was higher when gold film was pres­ent than no film at all.

As Fig. 11 shows, no distinct optimum th ickness exists for

440 C slider on gold film down to 0.1 J.l, where no rupture wasobserved at all. For 52100 riders, T able 3 shows that optimumthickness depends upon the applied load . Ho wever, it is ex­pected that beyond 100 traverses, rupture migh t take placefor some cases. In oth er words, t he optimum film thi cknessmight also depend upon the number of trave rses needed.

T o M r. Evans and Mr. Vest: As pointed ou t by th e discus­SO l'S. t he app lication of gold film for lubr ication is limi ted .Bu t it seems st ill worthwhile to furt her investigate th e specificrang e of possible application in sliding. Gold plating wasdone by a large reliab le commerc ial source in a single batch.It is true that th e precise detai ls of this proprietary pro cess isnot known, but no evidence could be detected pointing to anydiscrepancy in th e experiments th at may be attribut ed toinconsisten t electrodeposi tion . Specimens of 0.1 J.l thick filmis the pla ter 's expression . "Alternat e cleaning met hods" meantthe various cleaning procedures tried in th e labora tory be­fore reaching apparently consistent results .

Th e discussion is limited to the steady sta te of slidingfilm before and after th e film rup ture. Other factors such asinitial t ransfer of gold to the rider , extra large asperi ti es, aswr- ll as the ploughing term , are perhaps playing a major rolein the process reaching this steady state as point ed out bythe discussors,

T o Mr. Spencer: In a subsequent work , it was found thatnickel plat ing on steel could only increase the friction andwear. Th e effect of nickel st riking does not seem to be asimple addition of a nickel layer .

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