REPORT 1254

CONTENTS

SUMMARY ---------------------------------------------------------------INTRODU~ION-------------------------:-------------------------------THEORY -----------------------------------------------------------------APPARATUS AND PROCEDURE ------------------------------------------

Appmatw -------------------------------------------------------------Matitik and %otim -------------------------------------------------

RESULTS AND DHCUSSION ---------------------------------------------Clean Steel on Clean S&l-----------------------------------------------Nat&llyFormed Stiaw FW------------------------------------------

fionofide ti-----------------------------------------------------MkwHanmw ti--------------------------------------------------Nickel otideti------------------_--1-----------------------------Laoquerorvarniah ~m---------------------------------------------

Wfomti Fflm --------------------------------------------------------fionofide ti-----------------------------------------------------CMotida mdstid&-----------------------------------------------MeWosoaps ------------------------------------------------------Wtidlubnmnti -----------------------------------------------------Niokelofide --------------------------------------------------------Lwquemor vtihm -----------------------------------------------Pbti@-----------------------------------------------------------

Other Methodaof Lubdmtion by tiU&-----------------------------------Bonding ofsolid lubrioanti-------------------------------------------'Mo~aasoUd lubnmnt ---------------------------------------------Other soti&=lubtimnti --------------------------------------------

SUMMARY OF~ULTS -------------------------------------------------APPENDIXES

A—ADHESION THEORY OF FN~ION ------------------------------B-FILM PREP~ATION ---------------------------------------------

REFERENC~------------------------------------------------------------TA13LE L—PHYSICAL AND CHEMICAL PROPERT13ZS OF SOLID LUBRI-

C~TS ----------------------------------------- .-.---------------------91

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REPORT 1254’: “’

FRICTION,WEAR, AND SURFACE DAMAGE OF METAiS-AS AFFECTED BY SOLID SURFACEFILMs’

By EDMONDE. BISSON,ROBEIRTL. JOHNSON,MAX A. S~BT, and DOUGLASGODFEBY

SUMMARY

As predicted by friction theo~, acperiments hnixl thutjriction and surface dumage of nwtali can be redd by solidw.qfme Ji?ms. The ability of mata+i% i%form &urfacejilm..sthai preveni welding wa a very importan$ fuctor in wearof dry and boundary lubricded su.rfmxx. Films of ‘~aphiiiccarbon on cud irons, IWO on ni.ckeJd?.eye, and PeO and3’e304 on ferrom -“ale were found to be berw$iiu.?.Abrdve $lnw eueh m .F603 or MoO, were de$nii%?ydetimental, It appears that the importmw of & ji.lms tofriction and wear procewx h not beenftiy appreciated.

5!’?MchamiculreacttiflnM FeS and Fem were bothe~eetivein the preutintion of wface we.?ding. Be-cause of b lowersh-eurstrength; FeG was more e$ec$ti t-km FeS in reducingjriction.

Solid lubricants Wh w molybdmwm did? and graphitewarrant cureftd corwidemiion for u8e under severe cimdii%nssuch @ high temperatwx~. Surface jilnw of those mataii%,werefound i%hunebenejhid resw?tein friction stwdia at htihloti and high slti<ng udoeiiieg. -Bonding nwchaniametudtislwwed tka$ boti MoS1 and graphti can de bonded to manysurfaces by we of a rtxrin-forming liqwid. Roi?Jingcimlu.etbeuring experim.enisshowed that efective lubrication.& bemaintained at tempera$ureeof 1000° F by injectiun of minuteamounts (0,001 lb/rein) of i!liae solids. In the presence ofoqigen, MoS2 oxidizes i%M003 at a wry low rate ai 760° F, theprocimsbecoming rapid a-ttemperatuxeeabove ~060° “F.

INTRODUCTION ...”- -<

This paper is bas~d on NACA reikaxch datq obtainedduring 4 series of investigations in the field of lubrication,friction, and wear conducted from 1946 to 1954. Each in-vestigation in the series had specific objectives, but all weredirected toward determination of the role of surface iihnsin frktion and war.

As is VWJIknown, the presence of a contaminating filmbetween sliding surfaces can have a marked effect on friction,wear, and surface damage. (A contaminant is defined asany material other than those comprising the sliding sur-fnces) Some contaminants are beneficial (lubricants), whileothers are dehirnentd (abrasives). Considerable evidence(refs, 1 to 8) ideates the importance of solid surface iibnsto the. comparability of sliding surfaces. Physical andchemical surface changes have been associated with satis-factory and with unsatisfactory operation of metallic sliding

surface9 (ref. 2). As an example, ‘%un-in” has long b~enknown to be eifeotive in improving the performance and theIoad-ca@ng capacity of surfaces. Among the factors thatlead, tQ satisfactory performance ‘are the formation andmaintenance of certain beneficent solid surface h.

Such solid films are commonly formed on sliding surfaceby use of extremeprcwure additives in lubricants; che@aliilms of low shear strength or of low melting point are formedby reaction between additives and the sliding surfaces underconditions of extreme pr&sure and temperature. Thsori~for the mechanism of extreme prexwre lubrication are dis-cussed in refersgces 3 and, 8.

A complete study of the mechanism of aotion of sofidsurface films and their effect on friction and surface damageto ‘sliding surface9 encompass- a large nmnber of factors”.Some of the fundamental factors are d.@cusse~ in theT~ORY section of this report; factors that are importantin the use of solid ilms have been, stqdied experimentally,and. the results obtained are p$~ented herein.

The purpose of this report ti to iho~ the relation of s~fii+films to the friction, wear, and surface damage of slidingmetak Study of the role of surface iilms requhw ccm-sideration of how the & h formed, The ti maybe (1~formed naturally, (2) preforme~ or (3) formed by-othermethods having practical value.

A modified version of this paper was submitted by E. E.Bisson to the University of Florida, College of Engineering,as a thesis in partial f@illrnent” of the requirements for theprofessional degree ‘of Mechanical Engineer, May, 1954.

. THEORY .

&M,@is of the effects of solid surface iilms should includea review of the generally accepted theory of dry friction.This theory predicate adhesion between contacting surfaces.(It is of Wmrest to note that, as early as “1785, Coulomb(ref. 9) recognized that “cohesion” between solids was a fkc-tor, although he bdievsd “coh-=ion” contributed a relativelysmall part of the total fiction force.) The adhesion theoryof friction (appendix A) was independently advanced byMerchant in this country (ref. 10) and by Bowden and Taborin England (ref. 8) at about the same time (1940 to 1942).

The adhesion theory of friction predicates the contact of“surfaces through asperities; this theory proposes that frictionforcq ~. equal to the sum of two terms: a shear term and aploughing or a roughnew term. Sinoe the contact of surfac~

~Supei%ch NAOA TN 3444,“Frlotkm, Wmr, nndSmfim Damaged MetalsM AfXecW by Mld Sarfma Ffhq” by Edmond E. Bksm, Robert L. Jolmwn, Max A. Swibrt, and

Douglas@dhOy, 18S5.,..

93

94 RDPO13T1254—NATIONAL ADVISORY COMMITl!EEIFOR AERONAUTICS

is through surface aspexitks, it haa been postulated (ref. 8)that plastic flow occurs at the points of contact until the realcontact area A is great enough to support the load. Thearea A then equals W/p, whexe W is load and p is flowpr~ure (yield strength in compression). The initial surfacecontwt stress under time conditions is not a low value,but is equal to the flow pressure. Pressure welding occursat these highly stressed contact points, and the weldedjunctions must shear before relative motion can take place.The force required to shear them is one of the componentsof the fiction force, generally willed the “shear” term.

Appen&~ A shows the development of the equations ex-pressing the adhesion theory by Bowden and Tabor and byMerchant. Under certain conditions (as when the surfacesare quite clean), the shear term may be the more importantof the two, and the second term may be neglected. Inappendix A, surface contact pressure is represented byBowden and Tabor as the symbol p for flow pressure and byMerchant as the symbol H for preswre surface hardness.These two pressures are the same.

For cases in which the ploughing term is negligible,coe.tlicientof fiction p (defined as ratio of fiction force toapplied load) is shown (appendix A) to be equal to the ratioof shearstrength to flow pressure, that is, P=s/p. Reductionin i%ictioncoefficient for this case therefore depends on reduc-tion in the value of ratio of shear strength to yield strength.The ratio can be reduced by: (1) decreasing shear strength,(2)increasing yield strength, or (3) a combination of (1) and (2).Obtaining both conditions in one material is practicallyimpossible because those materials with low shear strengthhave low yield strength and vice versa. Howevw, by theuse of low-shear-atiength h (with thiclm-es as small asmillionths of an inch) on hard base materials, both desirableconditions may be obtained (ref. 8, p. 112). Thus, low shearstrength is obtained without appreciable decrease of the yieldstrength of the combination. The load will thus be sup-

ported through the film by the hard base material, while shearoccurs within the soft thin illrn. These low-shear-strengthiilrns can be of the following types: chemical reaction prod-ucts (oxides, chlorides, sultides), metals, fluid lubricank, andso forth.

Among the properties other than shear strength, flowpressure, and hardness that are important in the action ofsolid films are: crystalline structure and orientation, chemicalcomposition, chemical activity, and adsorption.

An important function of surface iilrns (frequentlyneglected in analyses of their action) is to act as a contami-nant for the reduction of welding or adheaion botwmnrubbing surfaces; contaminants reduce welding by reducingc@act of clean metal to clean metal. The better the bondbetween iihn and surface, the g-renterthe resistance to rup-ture of the iihn, and, consequently, the greater the protectionfrom welding.

The discussion of this section shows that the bnaic roquiro-ment for reduction of friction, wear, and surface damage ofsliding surfacea is the presence of a lowshear-stmngth con-taminant. This contaminant serves tmo functions, eitheror both of which maybe influential: (1) to reduce the amountof welding at the surface asperities, and (2) to decrease thoshear force. In order to carry out the second function, thecontaminant must be a material of lower shear strength thanthe softer of the two materials which are sliding one againstthe other (appendix A).

APPARATUSAND PROCEDUREAPPARATUS

In the investigations reported, five difTerent pieces ofapparatus were used. These are identified as A, B, C, D,and E; general descriptions of them are given in the followingparagraphs, and detailed dewaiptions in the references noted.Data presented in this report are identified in the variousfigures according to the apparatus used.

Apparatus A; for kinetio friotion (ref. 11).—Most of the

RcM-pdbn

Fmm I.—Kinetic-friction apparatus; apparatusA (ref. 11).

FRICTION, WDAR, AND SUT4FACEDAMAGE OF METALSAS AFFECTEDBY SOLID SURFACEFILMS 95

experimental friction and wear studies were conducted withtlmkinetic friction apparatus shown@ figure 1. The princi-pal elements of the apparatus axe the specimens, which arean Asticdly restrained spherical rider (either j4- or %~in.radius) and a rotating disk 13 inches in diameter. The rideris loaded by a weight applied along its vertical axis. Frictionforce between the rider and the disk is measured by fourstrain gagea mounted on a beryllium-copper dynamometerring. The coefficient of friction is computed by dividing themeasured friction force by the applied normal load: P=F/W.A radial-feed mechanism, when operating, causes the riderto traverae a spiral track on the rotating diek; the rider thenslides on virgin surface of the disk. Where solid surfacefilms were investigated, they were usually applied to the diskspecimen before testing.

Apparatus B; for kinetio friction (ref. 12).—In some of theendurance runs to check the wear or life of bonded films, anapparatus similar in principle to apparatus A was used. ItdifTersfrom apparatus A in that no radial traverse takesplace; the rider thus slides in the same wear track on thedisk specimen at all tirncs. It is a modification of theapparatus described in detail in reference 12. The sphericalrider specimens have a radius of %8inch and the disk speci-mens are 2% inches in diameter.

Apparatus C; for statio friction (ref. 13).—The principalelements of the static-friction apparatus are a rider wmrn-bly, in which three %-inchsteel balls are rigidly clamped, anda stationary horizontal plate on which the rider amembly issupported by the balls. The applied load was distributedequally among the three balls, and the rider assembly wasmoved over the plate by application of a horizontal tuggingforce. Friction-force measurements were made with straingages mounted on a beryllium-copper dynamometer ringbetween the tugging force and the rider assembly. Forcewas applied slowly, elastically extending the dyimmometerring until the force was great enough to overcome staticfriction and slip of the carriage occurred. Initial slip of therider assembly was determined by observation through atelescope equipped with cross hairs. When motion was ob-mrved, an indicating mark was made on the time-forcerecord of the potentiometer chart.

Apparatus D; for reciprocating sliding (ref. 2).-some ofthe enrlier studies of naturally forming surface iilms weremade with a reciprocating slider mechanism. It appliednormnl load to two contacting surfaces, and slowly recipro-cated one of the surfaces.

Apparatus E; for kinetic friction (ref. 14).—Experimentson solid lubricants, used in loose powder form, were con-ducted on an apparatus similar in principle to apparatus A.A ring with three equally spaced axial nodes was used as therider specimen. The nodes were ground with a cylindricalface of 2-inch radius, parallel to the ring radius, so that intheory, line contact existed between each of the three slidingnodes and the disk. The rider specimens were of SAE 4620steel, case hardened to Rockwell C-62. The disk specimenswem of SAE 1020 steel.

MATERIALS AND PROCEDUZtE

Most of the studies have been made with both specimensof steel, with the films under study applied to the disk

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FIGUED2.—Friction at high sliding velocities of steel on steel (ref. 11).Apparatus A; radius of spheriml rider specimen, H inbh.

specimen. Some studies were made with a nonfemousalloy, such as one of the various nickel or copper alloys, asthe spherically tipped (%6-in. radius) rider specimen. Solidiilms imwtigated included various oxides, snliides, andchlorides of iron, as well as materials such as molybdenumdisulfide and graphite. Formation of various preformedfilms is described in detail in appendix B.

In most cases, the specimens were a sphere and a flatsurface so as to enable calculation of both initial contactarea and initial contact stress by the Hertz equatiow. Thefiction data presented are typical of data obtained in manyruns. The limits of experimental error in the friction valuespresented were not uniform among all experiments becauseof the difficultiesin maintaining absolute control of ilm thick-ness. In all but isolated cases,however, the maximum mperi-mental error in friction coefficient, based on reproducibility,was +0.03. In most cases, it was considerably less than0.03. For compmative purposes, a load of 269 grams wasused in obtaining most of the data presented. This loadproduces an initial Hertz surface stress (126,000 psi) that isin the range of contact stirewx commonly pre9ent in air-craft engine components that require lubrication. Accord-ing to reference 15, this stress is within the range of normalstresses (69,000 to 282,000 psi) for turbine-engine rollingcontact bearings. At the same time, surfaces in contactunder relatively light load and having a large apparent areaof contact can have high stresses at localized contact areas(the contacting asperities). Even with lightly loaded sur-faces, local pressure at these small points of contact is equalto the flow pressure of the materials (ref. 8) and causesplastic flow at these points.

RESULTSAND DISCUSSIONCLEANSTEELON CLEAN STEEL

For steel+n+teel surfaces cleaned by outgwing at 1000° Cin vacuum, fiction coefficients in vacuum ss high a-s3.5 havebeen measured (ref. 16). These surfaces were considered tobe free of ordinary contaminants and most of the oxide-s.For steekm-steel surfaces cleaned in air by the methodpresented in appendix B, mtinm friction coticient inair is 0.S4 (fig. 2). The surfaces operating with a frictioncoefficient of 0.54 were known to have a h of FeSOAapprox.i-

96 REPORT 1254—~ATION~ ~VISOqY COMMf’ITEE -FOR A13RONA’OTICS

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FIQWEE3.-Surfacee of rider specimepsafter tests of figure 2.

I IContinwxrs sliding on

same track

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FICWRE 4.-Recording potentiometer traoing showing effect of high-

velocity sliding over a continuous path (without raQid_traverse) oncoefficient of kinetic friotion. Blaok wear debris (ferrous oxida~eO) was visible at beginning of lower stable friction value (ref. 11).Apparatus A; unkbricated steel; loa~ 269 grams;- sliding velooity,4000 feet per minute; radius of spherical rider epecimq ~ inch

rnatdy 25 A thick (about 10-7 in.) (reb. 17 ‘and 18). Assliding velocity is increased, friction coefEQent is relativelyconstant at a value of 0.54 up to about 1600‘feet per min-

ute; above this value, a down{ard trend can be obwrvocl,For steel-on-steel surfwea lubricated with either oleic acidor “SAE 10 lubricant, friction coefficient was approxinmtoly0.10 at minimum sliding velocity, decreasing to 0.06 at 6600feet per minute (fig. 2). For these results (discussed in morodetail in ref. 11) the classical lame of friction are conilrmocl:friction coefficient is essentially independent of load md, atthe lower speeds, is independent of sliding velocity,

Surfa”m appearance of the ball specimens from the ch+yand -the lubricated runs of iignre 2 is shown in figure 3. Thophotctmicrograpbs show that surface failure by welding oc-curred with the dry specimen; no surface failure (welding) isevident for the lubricated specimens but ploughing is appw-ent. The-welding for dry steel was extensivb, resulting illappreciable “tearing-out” and metal transfer,

NATURALLYFORMEDSURFACEFILMS

Iron oxide films.-In figure 2, the downward trend offiction coefficient for the dry steel specimens at the highersliding velocities is not @nsidered a violation of one of thaclassical fiction laws (fiction is independent of velocity);rather, it is considered the result of other variables entmin~into the mechanism. The downward trend is consideredprimarily a resultiof formation of beneficial iron oxide filmsof appreciable thickness; formation of oxide films would boaccelerated at higher sliding velocities because of higher rateof heat generation at the sliding sufaces resulting from thegreater release of frictional energy. In partial confirmationof this concept, when the steel slider was permitted totraverse the same wear track on the disk (fig. 4), FeO wasidentided (by X-ray diilraction) as the chief constituent in

FRICTION,WIL$R,-~ SURFACED#L4GE. ~F WTW%@_.@?@WK!W-W. WJ.@ .sWA~ F- 97

the wear debris; there wawwa incident reductioxriq &ictioncoefficient from 0.38 to 0.!+4. It is reascmqbl.eto assumethat the FeO filn-wm being-formed at W,@$; d “ g the

T30-second traverse of the same track on the disk, ti e ii.lmattained a thickqess+uch -that, -it bgcame visibl~ to. theunaided eye. Tlie &-&dua~decrease m,%ictio~ ‘co cientFcould b~ th~ resdt- of- the gwdu+-increasb”in iilin tk@ness.Inhsmuch as’ film thickness growth $ Iin+ed’ ~y wear andrupture, friction coefficient ivo-id-be-expec@-d-tt -jeach ast~ble value m film thickness stabilizes. NO measqreme..tof the film thickness could be made miir-~e- dynamic coP-di~ons; the ~appearance of’ the ,w~ track was, ho~ev:r,_——somewhat siinihw to wear tr&””pm~wT on preptiedoxide. &s approximately 1000 !A thick. ‘ ~ ~

It is IwDothcsized that, &dti-&~E—mnditions of vaii-. . ,,.ablo severity for spec@egs., of_steel oh steel, s&facJ ,oxid@form in the order l?eO to ~q04 tq FegOS. Oonsi&rable coh-firrning widence (refs,_2, 4&5; ~6.,JQ,-and. 20~h&s theimportmwe qf oxides in the reduction of f+ction, w+, andsurface damage of didihg surfacis. ~.On mo$hni-in=wfgces,oxides have been identified-’’(refs..-2, l?. and 20). ~Figure6, bawd. on references z and 1-9‘%hotis (quali$divel~ only)the estimated tioun~ of -FeSOIwd= .~.~Oa-_prese~taftervprious degrees of run-in.- As shown, the. amount- of.,FeaOlincreases to a mtium for the’ “fully run-in” condition;after further operation at higher load, less FegOl is present.On the other hand, F%OSshows a sl@ht increase with degieedf run-in to the “fully run-in” point, after -whicha markedincrease in l?elOs is observed with failure of the surfaw%.The surface iilms were idrhtified by X-ray diffraction. kreference 19, it is’ shown that, at the minimum wear iate(which is usually wociated with atttient of a “wellrun-in” condition), the amount of Fes04 is large; at themaximum wear rate (iiplying incipient surface failure), tha~mouut of Fes04 is lower and ~the amount of Fe20s k largerLhunat the minimum wear rate. These data may”also show”why there has been considerable contrave~ ii publjshettliterature about whether oxid@ of iron am good- or bidfrom considerations of friction and wear. This controversywas an incentive for the NACA to obtain fundamental infor-mation on the specific oxides of iron: Fe301and F%OS. “

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hcwrm 5,—Effeot of run-in on formation of natural like of Fe30A’and l?e~Os. Datu me qualitative chd areIestimated from X-ray,dMractionresultsof reference2 (apparatusD) and from referen@ 19..,.

-em~hwith thimplications of_figura5,3@@ (ref.20j states that iron oxide layera are dbtected on most run-insurfacss--He in@@s (&. dOi&@mpbell,-;~~ ,6>that oxidesplay a p,art@ the mechaijiqm of friction by affecting t$e ratioof ~ded-tc-imweld+ ar&.”J H&,also-imlicit~- hat-~jtiouso~des ~an tie forme{by exctiive 16ad.i&. !! his Oint is

Ycofimedtik-fi~e+,wti~bh-~e oxide~ F&03 appea+s on the surface @der “surface failure”con.titi.o@= – ——. – .—– —–, ,,l.,,:

Recent’, experiments on the influence of a&orbe~ @ms—-.-weFe–Condu@tEd-%y Bo-fden m-d-Young @ef. ‘[671~ Tlieir~erimrmts, shotied the very marked effect of: o.sjgen inrediici@- ti-cti6ti”~or iio-n-iion co~-inations itliath—idbeenthoroughly outgassed at 1000° C in vacu~ (10-6 mm Hg).Th;surfgc& ‘wed, had a coifficidnt of hictfin of 0.4 beforeoutgassihg:= Removal of con@n&natd+g films, inc~uding>heoxides, incre+sed ccr$kient of fricti~n b 3.5, aqd’ led to

dcornrJet@&iiye- und&_;&@in condition.. _ A.. !: ,ion ofoxygen to th~ vacu&n c.ly@ber reduced friction coefficientfrom_3.5 &l.2.;. with Q~gen:yr&enti@rnplet&3*~e nolonger oyxrr~d>-+._j_ “.; ,~-’--- ,,

Dah.of. reference-21’&oi@hat ~~clndon--of oqgen borncl~n specimens-b~use .oLaJ~blan&~_ii.iiii>edium, such as—— --.——____highly- purified-~tane, for-st+&oni”tee~e6iiens-praducesa bi-gh frictiori coefficient; fiese data are shown in figure 6.&, s&ling velocity increases above 2000 feet per minute,&iction. coefficient shows a dowmvard trend “’for, the ~yspdrgeris, but sho+s a very marked upward trend-for similarspecimens immersed in cetane, reaching a value greater than1.0. The increase in fiction is probably due to the absenceof the beneficial oxide fibs’; their formation would be pre-vented by the cetane, which excludes olkygen. Figure 6shows: that at low sliding’ velocities friction coefficient isl’ower for specimens with cetane than for dry specimens; thecetane may’ Act as a contaminant behveen the surfaces,This benetidial effect might ba &pected to be greater at lowthin at high sliding .mlocities bec5use of 10WW*surface tem-perature+. $%niilm”results-on the action of benzene as a“blanketing” medium in metal cutting experimemk amreported by Ernst and Merchant <ref: 22); they found. that‘,, ,, ) ,, ,

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l?cci~’ 6.-Data Bhoivingeffecb‘of cetane ‘as blanketing medium (to’restrict availabtity of oxygen to specimen surfaces) on frhtion ofsteel against &eel (ref. 21). @pmtus A; load, 269 gmms; radius.Of sp@ic&ridar specimenl ~ inch. , . _ -

_—. —.

98 REPORT 1254—NATrONAL ADVISORY COMMITTEEFOR AERONA~CS

Timq m“n

FIQurm 7.—VJear of several rnhterials sliding against hardened SAE52100 steel without lubrioant (ref. 23). Apparatm A; ddingvelocity, 5000 feat per minute; loa~ 50 grams; radius of spherioalrider speoime~ fio inch- ‘

friction coefficient was increased in the presence of benzeneand decreaeed in its absence.

Thus, there is considerable evidence indicating the funda-mental and important effect of oxides on the friction andsurface damage of sliding metals. Not all oxides, however,are beneficial; detrimental oxide iihns can be produced undercertain conditions (refs. 2 and 20). Additional discussionof the effect of speciiic oxides is given in the section Pre-formed FihnB.

Miscellaneous fllrns.-In some practical applications,surface f31mshave a marked effect on fiction, w-ear,.anddamage of sliding surfaces (refs. 23 to 25). Most of thebearings employed in turbine-we aircraft engines are rollingcontact bearings (ref. 23). One of the principal sources offailure in such bearings has been the cage (separator orretainer). & discussed in reference 23, most cage failuresare cwsed by faulty lubrication at the cag~locating surface.This location is particularly susceptible to failure be~usethe surfaces are in pure sliding at relatively high slidingvelocitie9; also, the conilguration is such that it is diflicultto maintain a lubricant fi between the sliding surfaces.These conditions reduce lubrication to extreme boundaryconditions, and excessivemetallic adhesion frequently occurs.One means of reducing the severity of this problem is use ofa cage material that hae less tendency to adhere to steel(under marginal conditions of lubrication) than the materialsin current use. The NACA approach wae to study thefriction and wear properties Of various materials, both dryand lubricated; the results were analyzed to determine theiihn-forming properties of the severalmaterials. The studies

.48 Xl~3I I I

x Beryllium nickel.A BerylGum rickd

(Iwot-treoted)y Disco

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n Inconel (cost)A Nimonic 80

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o 40 80 120 . 160 200Time, min

FIQURDS.—’Wear of several matenale sliding against hardensd SA1352100 steel without Iubrioant (ref. 25). Apparatus A; slidingvelooity, 5000 feet per minute; load, 50 grams; radiue of sphwfoalrider speaimeq ~e inch.

of reference 1 show that the mechanism of lubrication pre-vailing in bearing materials containing low-shear-strmgthcomponents and in cad iron containing graphite involvoethe smearing of the low-shear-strength phase ever a hardmatrix.

Results of the NACA study of cage materials am describedin detail in reference 23. The investigation covered tlmsliding against SAE 52100 of materials such w bronze (con-ttig 7.6 percent lead), beryllium copper, Nichromo V,nodular iron, and gray cast iron. Figure 7 shows somo ofthe wear data obtained at a sliding velocity of 6000 feet perminute and a load of 50 grams. (For these experiments,the rider specimen, Jirin. radius, was of the various mate-rials and the disk specimen was of SAE 52100 steel hardenedto Rockwell C-00. Size of the wear spot on the ridm wasmeasured, and wear volume was calculated from this mme-urement.)

From this investigation, ability of materials to form surfacoiilms that prevent welding appears to be a most importantfactor in both dry friction and l-wmdary lubrication (ref.23). The surface films formed vnm -lerivedfrom within thostructure of the various materials (e. g., graphitic carbon inthe caet iron and lead in the bronze). Under both dry andlubricated conditions, monel, Nichrome V, and bmylliumcopper formed films believed to be nickel oxides. Whenpresent, the iihns improved the performance of these mate-rials. In this investigation, the fdms apparently had agreater effect on reduction of surface damage and of wmrthan on reduction of friction. The friction coefficient withthe best materials of this investigation, the cast irons, wasrelatively high (approximately 0.5).

FRICTION, WTMR, AND SURFACEDAMAGE OF METALSAS AFFECI’ED BY SOLID SURFACEI?H&s 99

Niokel oxide films.-References 24 and 25 report aninvestigation of the wear and sliding friction properties ofnickel alloys operated against various steels. Several monelmotd alloys and other nickel alloys that have good high-immpemturephysical properties (such ss Inconel and Nim-onic 80) were investigated. As in the studie9 of reference23, it wcs found that desirable performance characteristicsrmd absence of extreme rnsse welding of all the materislsstudied could be associated with the development on thesliding surfaces of a naturally formed b. The ti wssidentified ss nickel oxide NiO by X-ray difhaction. Wemdata for some of these alloys at a sliding veloci~ of 5000feet per minute and a load of 50 grams me shown in figure8 (note that the wear scale for figs. 7 and 8 are diilerent).On the bssis of wear and friction properties, csst Inconelpreformed very well in these experiments and comparesfavorably with nodular iron (which proved better thanbronze, ref. 23). Nimonic 80 also showed promise as apossible cage material. Both of these nickel alloys haveconsiderably better high-temperature properties than downodular iron.

Other studies showing the important effect on wear ofnickel oxide are described in the section Preformed I?ilms.

Wear and tilction experiments were conducted with someof the cage materials, listed in figutw 7 and 8, sliding againstdisks of tool steels (molybdenum and tungsten types). Thewear and friction results showed trends similm to those fortests against 52100 disks (figs. 7 and 8).

Laoquer or varnish films.-Wlth reciprocating engines,good wear performance of piston ring-cylinder barrel com-binations hss been associated with formation of a very thinlacquer or varnish film on the surfaces. It has also beenshown (ref. 26) that, for a journal bearing employing alubricant of relatively poor lubricating ability or “iihnstrrmgth” (a silicone), formation of a lacquer film providesa margin of safety and incresses the load-carrying capacityof tho bearing.

The aircraft turbine engine is generally characterized byhigh operating temperatures; these high temperaturesaccelerate the process of oxidation, polymerization, anddecomposition of the lubricant. These processes of degradwtion of lubricant result in formation of lacquer films on hotsurfaces such w bearings. Because these iihns are naturallyoccurring, a fundamental evaluation of the role of such fdmsin friction and surfaco damage would be worth while.Their influence on surface damage is particularly importantin the cnse of the aircraft gss-turbine engine, which hss abearing or bearings operating dry for a short time afterstart of the engine. This condition is the result of the“high-temperature soak-back” of the turbine bearing aftershutdown of the engine (refs. 27 and 28). Since the flow ofcnolant (lubricnnt) stops with shutdown, the bearing tem-pmature incresses because of the large reservoir of heat inadjacent large metal masses (such ss the turbine wheel,immediately adjacent to the turbine bearing). Both Hunt(ref. 27) and Gurney (discussion, ref. 28) show that bearingtemperatures on the order of 500° 1?prevail under the soak-back condition. When the engine is restsxted after aperiod of shutdown, the bearings operate without lubricant

for a short time until lubricant flow is established. Duringthis initisl period, the bearing parts (including the cage) areoperating under conditions of dry friction. Because thecage hss been a principal source of bearing failures and thesecage failures have been established as lubrication failures(ref. 28), a study wss made of the fiction and surface-damage charactistiw of iilms formed on steel surfaces by de-composition of severed types of lubricant. These results arereported in the following section under Lacquers or varnishes.

PRRFORMRDFILMS

Studies were made of a number of preformed solid surfacefilms of the type formed: (1) naturally (as in the case ofoxides), (2) by chemical reaction of surfaces with chemicallyactive additives (extreme pressurelubricant additives), or (3)from solid lubricants (such as molybdenum &sulfide andgraphite) that function m supplemental lubricants (ref. 29).Formation of the various preformed films is described indetail in appendix B.

Iron oxide illms.-Because of the importance of ironoxides, data were obtained on films (1200 A thick) of thespecific oxides F~04 and FqOs. These data, shown infigure 9, are from reference 30. The fiction coefficients, aswell as visual examination of the surfaces, show that Fe~04can be quite beneficial in decreasing friction and in preventingsurface damage. In comparison, F%O~showed high frictionand excessive welding and surface damage. Figure 10shows a compm-isonof the rider specimemsafter investigation;surface prdkction was much better with the F~O,, whichlargely prevented surface welding and metal transfer.

Confirming evidence of the importance of the speciiiciron oxide is available in references 31 and 32. Fretting(fretting corrosion) is a concentrated and severe form ofwear. With specimens of steel against glss-s (ref. 31), thefirst wear particles produced during fretting were extremelysmall and apparently virgin materisl. These particlesoxidized when exposed to air. Viiual observations (ref. 31)showed color changea that suggested successive oxidation of

0

IhGmm 9.—l?riotion at high sliding velooitiex of dry unlubricatedsteel against steel with no W and with preformed films of F%04and Fe:@ approximately 1200 A thiok (ref. 30). Apparatus A;radius of spherioal specimen, ?4inch.

~()() -’:, .” REPORT 1254-NATIONAL tith130R”Y COMltTITE E FOR AEROtiA@l%26 ‘

~,j.,_.. ,, . ----- :-, .. ,, -’,

.1, ,,, ,,. , 1,,’ ”-,1 ~~!(a)~GUEE,,}&+lrfaC+914 ,,; , ,, .,, ,, ,,.

z-. .. ...

-. ;

...,. . ..

:,.. ‘“

. . .

... . .. .. .

. . .-.

. .,.

..-.+.. ..

,---.-.’

.“.; -’ ... .. :.

.-, .. ..-‘..-

-~.

Fe to l?eO, ‘to F%O& mid flnally”to F%O,, ivhieh .ti ‘h~d-andhighly. bb~ivi. I$ictio,n cmflicjenf” of’ ‘“po~derq’ F~Qs+bmphcts” reciprocated agahst each other w=’ r$ativer~@M ant (after a few hundied cycles) af ~aYalue jf approki-

‘mnt/Jy 0.5 (ref. 32). T@ fiction coeffififit ‘bf steel ‘Oh

‘steel (0.6) was -hot too- di.ilerent from that wi~ the ,FeiOg“compacts:’ With powdered’ FeSO, ‘compacts :ecipmcafigagaindt each other, friction cocfiicitit ~as at tlrs~low -(0.3),btit giadually increased until, ‘at 600 “c-y&l&,’tie wltie ivtis0.5;tlm

T

miythafirtirwFeXkcompactrjkmnkiation

an@ che .MJ imalysi@of the deb~ on ~the syrface;of theF+Oicornpacts xhowed-tha~Fe@,--wtw&s~ Thti thefr?tting in all t+ree ca~es,(l)steel oh stee$ (2) F40S on!F%Oa;m:d- (3~Fea0~—lotiFe@4~ f~ranmunbWoFcycl@a; .es-sentiallyi that of Fe@ agai& Fl%Os~s,mgg~ted -by ~he

~ y.mpasured-valu ‘ *f–~tion_*~cient=-,--, .‘Zye data, ~ell ,$L’tie data on naturally O’CC+ &n

OXIe+il+ns‘previousl@iscussed~mpha&ze*_impoitaimeof ~k-n, oxide’ films tp .frktion, wear, ~d su+ce d&rn&e.E&ectm~lubri ation%ndw,conditionso&x&ernmbo@d-@y

idl+ication (w tie etal-to-met~ contact takes pl+we}-isveryoften-afunctio&f~wurq crf%he,oxide+lms-p @t. I

r~eq snrfaces~are l~bricated .ti’th fatt~ acidd, oxidl fibnsmu+ contribute I!C-th=fomnationm~-l.metalliwap bad the .w+rface.~This point & developed ~ the section ~Netal-li~So.api_ _,—‘-: ‘Chlorides ‘~”d sul$ldes~-!l%e. ~ectia~sm of the actionof extreme pre~ure lubricants is considered, to_be one ofchemical ,ywction, between active” additiv~ aqd we metidsurfaces. Com~ounds; of. chlorine; sulfur, or phosphorousare formed, depending upon the tjpe of additive used.

,-7-’ -.-,”..: .–-, !-” :-t ;, ‘, . . . .. .,-, ,,, , ,,, 1,-, ,, 1,,: -, L “ if,

.,k) JW3., r, !.,,, ,,, , ,,, ,,, J

ecimene aft.ertegts’ of fi&&9~’ ‘: ,, ~ f ; ,..! ,,

.:60 . ,“

-,, --—_ -—L. ._. —1 I ,, :, ,’:, ,, 1,, ,

“2- -

.\. - l’!

\

; %- ‘“ ‘,’ ‘ ‘. > \\’; ) 1“

1’ ,$; I

“ g .40 .! 1,,:,

9 ‘., J-.

Film I ,< 1’1

l!?I )

= ‘. +0$

%’ ‘- ‘ ‘“’,, -,--- -,

None < “

z .20 ,, if~

.,g ‘, ,, ,,,=alG

,, ,’ I.,

I0’ 1600. 3200 14800’J ‘ 6400 ‘KHM

!-ii - Sliding vekxjty, ft/n@, , ,,,, , 1, ,1,,,

l?hrm !tl.-Frictiori at’ high. sliding vblooit~es of steal against ‘steel,.tith no” ti and’ Ivith’ phformed filti ‘of FeC12’did’ Fc~ tififiro?!l-

.’ matel~ ,1000 A thibk (ref. 30). App~ratu$ A; r~diu$ of $phorloril,, rider specimen, ~ inoh.

,,, -, .,, , ,,, ,

R~”ults of an investigation (ref. 30) on preformed chlo~idhand sul.iidefilms approximately 1000 A thiok hre shown in

‘figure 11. Vis@l extiation of the’ surfaces ‘showed thatboth F& krid FeClr are effective h preventing excessivo-surface damage; FeC19is much more effective than l?eS in‘redticihg &iction. The difference iri friction coefficim t&msponds to the difference in sheai ‘strengths of the Wodornpounds. ,If the reaction products from extreme-pressurd-lubF@nii ‘additives of the- chlorine and sulfur types ‘are-@nidy FeCl*’ and F+, respectively, tie effectivemws of‘the adti”tive containing chlohe should ~e apprecitiblysup&ior. The superiority of chloride films over’ sulfide fih~

FRICTION, WEAR, AND S@RFA@l DAMAG$EOF-lA@liULS ‘M ‘=CH!3D BY “SOLIDSUBFACEFH.MS 101

is confirmed by Bowden and Tabor (ref. 8), who foundsimilar differences in friction coefficient.

Effectiveness of sulfidefihns in reducing friction has beenshown to be very much a function of h thickness (refs.(i rmd 13). Campbell shows that, on copper, friction co-efficient decreases as sulfide film thickness increases up toabout 1000 A. Results obtained by Greenhill (ref. 33)indicate that sulfide films on steel, copper,- or silver showedmasimum reduction in friction when the thiclmess exceededabout 1500 A. Under the conditions of experiments withsulfide films of FeS on steel reported in referance 13, athickness of approximately 6000 A is required for minimumfriction coefficient, although sutfhce welding was effectivelyprevented by films 4500 A thick. These data are shown infigure 12. Since the data of figure 11 are for an FeS iilm ofapproximately 1000 A where welding was effectively pre-ventcd, the value of 45OOA for the studies of reference 13appears to be peculiar to the conditions under which it wiisobserved.

—There is rather general acceptance atMetallia soaps.the present time of the theory that boundaiy lubrication bymuterirdssuch as fatty acids is effectwdprimarily by chemi-sorption rather than physical adsorption. Lubrication ismost effective, therefore, when there is some reaction of thefatty acids with the metals to form a metallic soap (refi, 8and 34). Reference 8 shows that lubrication with fattyacids can be maintained at temperatures considerably abovethe melting temperature or resorption temperature of thefatty acids; in fact, the temperature at which lubricationfailure takes place correlates remarkably ~elt -with themelting temperature of the metallic soap that is believdto be formed in the chemisorption of the fatty acickifon’tliesurface. .It has been shown (ref. 8) that the formation of ametallic soap is very markedly affected by ‘the presetice‘orabsence of oxide films. In experiments on skel, the r~ultsof reference 35 show that the type of oxide film is also ex-tremely important. With a film of F%OS on the surface,stearic acid in cetane was effective as a lubricant at slidingvelocities only up, to 3000 feet per minute; with a film ofFe~04on the surface, the same cm.mentrationof stemic acidin cetane was effective at sliding velocities higher than 7000feet per minute, —

Solid lubric.ants.-Because solid lubricants are frequentlyused as supplemental lubrimte, they must be eilective underconditions under which liquid lubricants become ineffective(ref. 29). .Jn orckzto .ba effectim,.a solid lubricant nyst be

_, -_<.8 < —% . :.&

----$.6%a

B.4

—z8 ‘-.20 m 12$03. 16~

FeS f&%&cness, A

FIWRD 12.—Effect of thickness of W of FoS on statio coefficient of.friction of steel agafnst steel (r& 13). Apparatue C; load, 2400cprims;radius of sphericnlrfder specimen, % inoh: ,-

.60

._ ___ --—- --.

—2

%x

5- ~\,

N~ :40 —, — ‘ ‘— ‘— ,—- “,’, ,“

.&

\..Q < Lubricont~ . .. -

\=

I=-- _ Nme ‘

z~ .~ –— --–- --- -–- -- — --- . -- IGm@ite—

%~— - Ia — 7--- — —— . MoS2 —z --- --- --- _ -- ___

WE 10oil. —— .

0 1600 3200 ~ 48@3 64m 8000Sl~~_ velocity, ft<min—— ——— .—— —. ——

FrQUEE 13.—Friction at high eliding velodties of steel against steel@w.@ormed.ti. .of MoS and graphite (ref. 30). ApparatusA; radius of spherioalrider specimen, H inoh.

mamta@iin-p]ace at = w=t=c~sfiwe (ref. 36); that is,it must .be strongly --adherent -to - the surf~ce and the hmuiWbZ-w-itinuous. & indicated in reference 36, a “’sslf-

_——

repairin#’ film is the most effective means of maintaining afilm at the surfam”. Otdq .types-of~. _~e, however,effective for limited periolls of time.

Ii-addiiloii b– th6 ret@6riii-th= “thtilubricant’~ bestrongly adherent to the surface, solids for use in the &titmeet oth–~—r-quirm-en-ts; Ma&iriid.s‘for this applicationshould have the following physical and chemical properties:(I) ‘~hmeltirig– point,-b-e-cmmiso~”the high “temperaturesinvol~ed, (2) low hardness, because low hardness in a rna-t@:d-b--iciatwl with:~ow sh~~ stimgt@+i+d- coti-equentlylow friction (see appemhx A), -(3) Iaminar structure (somelayer-lattice materials have been quite effective as solidlubricants), (4) limited solubility, (5) Iimited chemical re-activity, and (6) surface adher~c+. The five materialsihd~~dedin “table I should be relatively effective in redgckg.f&c$ion aqde wear. On the basis of their properties, M&and graphite were indicated to be the materials of mostinterest.

Campbd”:(ref. 6) discussed the desired properties of solidlubricants and”hcluded, in addition to the mentioned points,the following: (1) The film should have good elastic proper-ties; and (2) for a apeciiic application, cerkin other proper-ties mayl be requi,red“of -the solids: high electrical conduc-tivity, high thermal conductivity, corrosion inhibition, lowdensity, small. particle size, and freedom from abrasivecontfgninants. The requirement of freedom from abrasivecontaminants is extremely important and is discussed later.

& shown in table I, both graphite and Mo& have excellentproperties horn the standpoint of posswing high meltingpoint, laminar structure, low hardness (which correspondsto low shear strength), and relative chemical insrtness.Since both of these materials have a laminar structure, sheaiwithin the material (in the plane of the laminae) shouldreadily occur. Experimental friction results with MoSS andgraphite are shown in figure 13; these data are discussed iDdetail in reference 30. At room temperature and low speed,there is little choice between the two materials. At thehigher speeds, the friction codicient of iMo& is lower.Visual observation of the rider specimms indicated that some

.—— —--- . . ..— ———— —-. —

102 REPORT 1254—NATION& ADVISORY COMMITTEE FOR AERONAlJ!lX!8

I .0 xIO=

Q

8 {

o Untreatedspechen run in air

&q %ehKIFXE%WJy rUnrn Oirat KKX3 OF–~ CWe film chemically p%forrmd

~ .6 {.

s

;L \;

.4~

e\

.2-

A\

Q.

0( a) ‘-- --- - --- __ __ .— L .P

Temperature, “F

(a) Wwlr.(b) Friction.

hNJRE 14.—~ear and friotion of cast .bconel (rider) sliding againstNI-10 tool steel (disk) unlubricated. Apparatus B; sliding velocity

120 feet per minute; loa& 1200 grams; radius of spherical riderspeaimen,$&inoh.

welding had occurred with the graphite film and none withthe MoS8 film. Occurrence of welding with the graphitefilm may have been afected by manner of iilm formationor by humidity. An important difhrence between thegraphite and the Mo% films is the manner in whichthe two films wexe formed; the graphite il.lm wasn rubbed film and may not have adhered well or beencompletely continuous on the surface. The MoSS filmwas a continuous film, bonded to the surface by the cornsyrup method described in appendix B. The second factorinvolves the presence or absence of adsorbed water iilms.Considerable research (conducted by Savage, Ramadanoff,Campbell, and others, and reviewed by Campbell, ref. 6)has shown that the presenca of adsorbed watar or oxygenfilms promotes ease of slip between adjacent crystal planesof graphite. Thus, in low humidities or in a vacuum, wearand fiction with graphite are very high. Campbell alsostates that, if the graphite is properly run-in under normalconditions so that sufficient water is adsorbed, a high degreeof orientation is produced x ith the slip layers parallel to thesupporting surface. After such run-in, wear is very low

even in relative humidities as low as 0.4 percent. Withthe NACA rubbed flhns, no run-in was attempted; thus,sliding uuder severe conditions (such as the higher sliclingvelocities of &g. 13) generated considerable frictional heatat the contacting surfaces. This heat may havo deaorbedany water iihns at the surface, and consequently reducedthe lubricating effectiveness of the graphite.

Wlckel oxide.-It was observed (refs. 24 and 25) thatgood performance characteristics (low wear and preventionof surface damage) of nickel alloys were obtained when asurface iilrn of NiO was present; when no surface film waspresent, poor performance was obtained.

Research at this laboratory has shown that, with increamin temperature, wear of Inconel decreasea. This result wouldbe expected, since an oxide iilm could form more rendily atthe higher temperatures. The reduction in both wear andfriction with increase in temperature is shown in figure 14.The data show that wear at temperatures between 600° and1000° F is approximately one-twentieth that at 75° l?, Theeflect of the film on wear was checked with two types ofexperiment: In the first, specimens with preformed NiO illmswere tested; in the second, an attempt was made to preventthe natural formation and repair of the oxide film by limitingthe availability of oxygen to the specimens.

Films were preformed by two methods. One film was pre-formed on a cast Inconel specimen by making a wear run inair at 1000° F; a room-temperature wear run was thenmade with this same specimen. As indicated in figure 14 (a),the wear rate was approtiately one-tenth of that obtainedwith an untreated specimen at room temperature. A filmwas preformed on a second cast Inconel specimen by heatingit in molten caustic (NaOH). The specimen was then run atroom temperature, and the data (fig. 14 (a)) show that wearwas again approximately one-tenth of that obhined undersimilar conditions with an untreated specimen of the samematerial. The experiment involving oxygen availability wasmade with cast Inconel at 1000° F in an atmosphere of argon.Although some oxide was undoubtedly present (because airwas present as a contaminant), wear was higher by a factorof 4. These results point to the beneficial effect of the nickeloxide film on both wear and prevention of surface damage,

Lacquers or varnishes.-The study on lacquers andvarnishes (referred to in the section Naturally Formed Sur-kweFilms) was made on friction specimens to which had been>ppliedlacquer or varnish fihns from decomposition of several

i .4

$g~ .2%

$3v

o la 3mo mm m aocmSlldkq velocity, ft/S8C

hUItE 15.—Effect of sliding veloaity on kinetio friction of steel_ s*1 lub~uted with silicone fluid and decomposition filmsformed from a silicone (ref. 37). Apparatus A; load, 269 gmms;initial Hertz surface stress, 126,000 psi; radius of spherioal ridor ‘specimen. jf inoh.

FRICTION, WJ3AR,AND SURFACEDAMAGEOF MJ3’I’ALSAS AFFECTEDBY SOLID SURFACEFIZMS 103

types of lubricant. These lubricant types included ordinarypetroleums and synthetic lubricants of the diester, glycol,and silicone types. Results of this investigation are reportedin ddail in reference 37.

In generrd, the data of reference 37 show that the de-composition products reduced friction and surface damage ofslider surfaces when compared with dry, clean steel surfaces.The effect of surface films in reducing friction and protectingthe surfaces is strikingly demonstrated in figure 16, whichshows friction coefficient under three conditions: (1) with sili-cone (rLpoor lubricant) alone, (2) with a silicone decompo-sition film, and (3) with silicone fluid over the decompositionfilm. The marked reduction in friction with decompositionfilm is apparent. Furthermore, with the fluid alone, surfacedamage was severe; appearance of the rider was similar tothat shown in figure 3 (a). With the decomposition film aloneand with the fluid over its decomposition film, damage to thesurfaceawas very slight; rider appearance was similar to thatof figure 3 (b).

Platings,-Bowden and Tabor (ref. 8) have shown thatplated films of soft metals can produce extremely low frictioncoefficients, provided the underlying metals do not contacteach other through the fi. This finding implies a limitingfihn thickness befow which protection to the surfaces isinadequate. Bowden and Tabor found that the limitingthickness was of the order of 10-6 centimeter for iridiumfilms. Other metals such as lead, silver, cadmium, andcopper have been used as fiction-reducing platinge. Thedata of reference 8 (pp. 113–114 and 95-97) confirm thetheory (appendix A) that the lower the shear strength of thefilm material, the lower will be the value of friction. Theirresults showed that “. . . friction of copper>lead>indiumand that the values of friction . . . are roughly proportionalto the shear strengths of these metals.” From frictionresults obtained with a steel slider sliding on surfaces of steel,copper, lead, and iridium, Bowden and Tabor calculatedshear strengths of these materials. These values are givenin the following table (ref. 8, p. 97); measured shear strengthsof the pure metals are included for comparison:

I Shmr Skength,@mm*I I

I h{etal Oahlakdfrom frlatkm

m~

In n practicnl application involving the cages of rollingcontact bearinga for aircraft gas-turbine engines, silver haabeen used (ref. 38) as a plating material. Its use preventedrecessive metal transfer and “pickup” of the bronze cagematerial to the steel bearing materird during operation afterhigh-temperature soak-back.

OTHER METHODS OF LUBRICATION BY SOLIDS

In practice, solids may be used to lubricate by twomethods: (1) inclusion of the solid aa a minor constituent ofthe materiale to be lubricated; this method provides lubrica-tion by formation of a film on the surface from within the

Loo%g

t-

Kc-bj—zfD—Ill

11111

““”EEKGlycwol B=i=a-F,. , 1resin 1 ,

?i1 1 1

I i II [!1 I[ I I I

r1I I 1

cull SynJp a f If I ! Iresin

I,

1 1 t

I I , I I JI I I 1

Codmlumplote. ! : I , ~

Wvef pmCleon steel

,iii

bnnhle’ fohlre ~ ~I I I I I I I 1

0246810 t2141618 20222426Time, ml.

I I I 11000 5000 lop b,COO 20$XK)

Number of cycles

l?ImJBE 16.—Enduranw tests showingtotal number of uyclesto failurafor eeveral rum of solid-lllm Iubricantaof Mo% using four @Terentresin-forming liqtick. Cadmium-plated, silver-plated, and ohmunlubricatedspeoimeusinoluded for compm-son. F]ti thioknessee,0.0002 to 0.0005 inti Failure was determined by high friotionforce and chatteringof speoimens(ref. 40). Apparatus B; radius ofsphericalspecim~ ~ inoh.

structure of the material, and (2) preformation of a surface~ and constant repair of this film. In method (l), thebearing material can be made by powder metallurgy with asolid minor constituent that is an excellent lubricant. Withmethod (2), the surfaces may have a preformed lubricantfilm, and some external means may be provided to replenishthe material so as to maintain a continuous film.

Bonding of solid lubricants.-Referenceg 39 and 40cover investigations made to determine the mechanism ofbonding of Mo& and other solid lubricants to variousmaterials and to determine the friction and wear character-istic of such bonded films. It is shown in reference 3!3 that,when a solid powder suspended in a resin-forming liquidvehicle is applied to a hot surface, the liquid decomposes orpolymerizes to resins that bind the particles of powder to-gether and to the surface. Powders can be bonded in thisway to materials such aa steel, aluminum, brass, stainlesssteef, or glass. The resin-forming liquids that were investi-gated are: asphalt-base varnish, sificones, glycerin, ethyleneglycol, polyglycol ether, and corn syrup. Solid lubricantsbonded by this method were Mo%, graphite, and F%O,.Electron’ diffraction studies (ref. 39) of a Mo% film bondedto steel showed no evidence of chemical reaction betweenthe Mo% and the steel. As a part of the bonding inv@iga-tion, rubbed films were also studied. Rubbing of Mo%and graphite was shown (refs. 39 and 30) to produce orienta-tion of the materials.

104 REPORT 1264—NATIONAIJ ADVISORY CO~E FOR AERONAUTICS

,0-5 . .. —.—.‘o

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—.— — —

——— —.

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— —

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— . .—,.&z. -:. -

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8Z-—

,-

0 5 10 Is ,20 2 5— :.%. =—— —- ~ —--MoS_ntmtion+?rcenW--wi9ht

:,.,..:--- ,.-:,. ,: -,_. ,-..

FIGURE 17.—Wear and”fricti”on-of ho~pressed btig material con-—...—— ——Tti@i@iiSXmXIOr specmmnc {ox mlii~w-ivas measti)

were composed of 5 percent” copper (by weight), 95 to 60 percentsilver, and O to 35 percent Mo% D@k speoimene WE low steel(ref. 41).. Apparatus A; #Ming veloci~, 5000 feet per mfnute;loa& 519 grams; time 60 minutes; rachs of spherioal specimen,s, inch.

The studies of reference 39 ‘showed that resin-formingfluids on steel surfaces chemically ieduc% Fe,O, to FeO,.This conclusion was reached from elec@n-distraction studiwof a hIo% Wn bonded’tith ‘corn syrup as w~-as from chemi-cal a.dilyses of mmiou$ *ures of MoS2,’ corn’ syrup, -iron,add FEKA. Prwence @ tie bond~ Mo% ~ of EGO1rcitherthan Fe20s (the oxide originally pre+tit on the su.ifacebefore the Mo& @n was bonded) should be advant&eotibecause of the better fiction and surface protecting prop-ertiesof F%O1(fig. 9 and ref. 30)..

MoSZ as a solid lubricsnt.-The stu&s of ~refererice“40hhowed that solid lubricants can be applied by a practicslbonding method. The reported friction andenduragc~ data,obtained under the severe conditions of’ completely dryoperation and high surface stre&s, showed that solid-filinlubri@ts (from 0.0,002to 0.0005 in. thick) of MoSZbondedwith v’srious resins (iic.luding corn syrup) possksed goodlubricating qualities (fig. 16). For timparison purposes,figure 16 prksents results for clean std specimens @d forsp&i.menswith a 0.0005-inch plating of cdn.ium or of silver.- b’ invmtigation ivtis made (r6f. 41) to study the lubri-cating effectiveness of Mo& included-as i minor constituentin materials mad~ by powd~metallurgy techriiques; suchbearing materials might be capable of. op&ati& success-fully even under extreme boundary-lubrication conditions‘(possibly even completely dry). For these experiumgts, therider specimens were powder-metallurgy compacts of:w’pper, 5 percent (by.,weight); +lver, 95 “h-60 percent; agdM&, o to 35 perwit: Values of friction and ywar of thesematerials are shown in figure 17. W&r wti dd ermined

FIc+wRE 18.—Effeot of sliding velooity on coefficient of friotion for

various mkturee of Mo% and a silicone fluid (a poor boundary lubd-

cant) (ref. 43). Apparatus A; load, 269 gmms; radius of sphwjcalrider, )f inch. Solid film of Mo& formed by oom syrup m~thoddescdbed in appendix B.

‘,

‘ An-mm! of oil in m“xfurewith wIfkd’ Mo%, percent by @qhl

l?mtrnn 19.-Effe@ of mixtures of oil @&e,. mediohml) and Moi% onfriction caeffloient (ref. 44). Apparatup E; load, 40 pounds; slldlnvelocity, 6 fwt per m-nub; radius of oylindricnlrider speoimotr,1inclwk; steel on steel.”

,, .,,

t’horn .th~measured wear spot on the rider specimens. ~hesodata show that tiiction .coeflicient decreased progiwsivelywith increase in’ concentration of MoSZ, With wear, how-ever] there was a deiihite optimum in Mo% concentration.High W&W at low concentrations probably resulted fromlack of eflective lubri”bation; high wear rtit~high concontrn-tions probably resulted born lack of phytical strength of thematerial. Sincethe specimens~were operated withoutatarnal lubrication, such materials show some promise forsevere conditions of load, speed, and lack of lubrication. Inthese experi@Zm@ wkliig (is observed visually) was absentfor all compacts that contained more than 5 percent MoSZ.The materi@s apparently formed an effective lubricatingfilm on the s@aceTj E-tixuisferof solid lubrican~from withinthe structur~ of the materials. .

An investigation of lubri&tion by preformation of a surfacefilm and cofi=t~~epsir of ‘this film is reported ‘in referenco42. The solid lubricant waa supplied to the specimens withan “air-mist” system; the same effect could undoubtedlyhave been produced if the solid w-em supplied in a volatiloliquid carrier. Preliminary im-estimationshad shown that,with introduction of powdered. M& in a rolling contaotbearing, a continuous film of Mo& was formed on tho con.tncting surfaces by the rolling action of the bearing com-ponents. It is possible (ref. 42) to operate conventional~olling contact bearings so lubricated either at high tempera-tyrw. (approaching 1000° F) or ‘at high speed (to DN valuesof 900)000, equivalent’ to a iotative speed of approximately12,000 @m for a 75-mni-bore bearing).

FRKYCION, WEAR, _ SURFACE-DAMAGE” OF M&l?ti’-AS -ti-tifi-D-B~ S“O-d %tiACE FILMS 105

0’; I -2 3 -’, 4 5S& mnkmt, ~cmt by weight

FIGURE 20.-Effect of silica additions’ to MoSj on WW5 and friotion ofstcml specimens (ref. 44). Apparatus E; load, 40 pounc@ slidingvolooity, 6 feet per minutq duration, 6-hours; radius-of cylindricalri~or sp~cimen, 2 inohes; steel on steoL ,—,, :’, ,“1, ‘., .

I Mktlllk% Or 13USp011SiOllS Of’ SOhh iIl liquid tiel% IUayhOtat first consideration seem to fall in the clam of &olidsurface-film lubricants, as discu&ed in this ~feport. Materiid sosuspended muy, however, provide lidmicatioh by forming asolid film at the sliding sutfacd. I The ‘“studiespresentedin appendix E of referem% 43 shoived that low ~frictionrmd complete protection of the - surfaces: frcm.i “fielding(CS obseuved visually) could be obtained with mixtuiw” ofMo& in ‘a silicone fluid. These resd”ts vvere”obtained withconcentrations of 18 and 60 percent Mo&” (@~ 18).” Thesilicone ‘fluid by itself was” aq extrem61y poor ~boundarylubricant for the steel+n-eteel specimens’ of thei6 “experi-ments. Surface welding was prevented for Mo& concentra-tions ns 10* as 5 percent. It should be recognized; however,that these particular mixtures were subject to settling, sincethey were not colloidal suspensions; such settling could bedetrimental in a practical mechanism. I ~” -

Mktures of Mo& and a lubricant (white oil) producedfriction coefficients (ref. 44) lower than thos~ oblhined with

,,, ,14

,’,1

;,, ,

( G Id? ,,...-H++E ! I I I J./

,,I A ,,

4,, , ,,, ,,,1,

S03 850 900 %0 IOOC tOW IIOQ 1153,,, Teriipemture, T ‘

TKhJRE 21.—Oxidation oiiatiuteristics of Mo&’ ~ ~. ‘(Data ‘f~m,,, ,, ,,, , ref. 46,) ‘.’ ‘“ ‘

-—~60 -- . .

\ > ~-, ——. ——i ‘- —--- ----- .- .8: ‘.

I “ “;+2 ;:?’~ ><( ‘-<—– —-- — ——\ —, —:

,, ;, ~_\ ‘ _ __ —__ __ ––.$-:4”0—, — – :,-—..9 ! --—*,— —— ‘- -‘1::’;.–, -- —’--?,,

---Dry steel.:..:!. ._. _-_: _-.. ..–l_, ,:<,L —— --

I ‘- “—. ——. . ...”_/;’; ;, _=/ ._ ’=’ .::.1 -’: =:: -: ;%1

-:’7-=; --_-i_:, _ ..’ _:_ ‘ z -,,=oxi%ed ~ (M003)- - ___ ‘,— -- —

----. . .4 .. . . . ---=,-, ,’, ,.

‘~. -M;-<-- ----

.,“1 ,, ~.,

, :0 2om. 40(: 600C ,’601xISiding vebcity, ft/min . . ..

FIGURE22.—Friotion at high sliding velocities of steel against steelwith preformed ti of rqbwd MoG and os@@ed Mo% (~f. 46).Appdratus A; ryliu$ of spherical rider speoiqen, X inoh. ‘‘ “

,, , ,, ,, /,”,

ieithe~ the MO& done or the oil alone @gi 19). The lowesiConcentiatiomof ~Mo& required for minimum friction ~c.6efFi-cientia approximately 10 percent. :-. ~~ ‘I ‘I’ ‘ “’.’ - & “~revioiudy.discussed, contaminants may be importantin Ithe: use- of solid lubricants: ‘An investigation. of theseeffects, -.wing Mo’& a.i the solid lubricant,” k- described inm&renke&44 ‘aid-. 14. The .dudib of reference 44 rev&Ieil-that nittially pr&ent contaminants, such as silicai mightincrdiid wbar considerably, even though fiction coefficientmiis irm+ased only slightly (fig. 20).” Moisture has ELverydetrhhentd ~effect cm ‘lubri”mtiotiwith Mo!%; botb frictionand (xv-m i.m&iw6 as ‘“moisture-chntent in&a& (ref. 14)..Moistr@e in IMo& also“foti- acids ~tliat may’ be ccirosive.-Purity of. the Mo%. k therefore of “considerableimportam%.

Campbell (ref. 6) states: “When chemickl ‘con~ersionc@at@~ qre used in combination with other fiolid lubric@s,a surfac~ is produced, $vhichoutwears by a large -f@ctor~itherlubricant alone.”: ,13@ data”.show-,thtit marked, in”me~ inlife (m-measured by cyclewto failure) could be obtained with,q gap~~ film ov~ a phosphate surface; the phosphatetreatment was presumed to improve adherence of th~ lubtir

,wmt, ~h. 13aryell: and MJne (ref.- 45), showed, similrq-improv~enta (reduction in .scu.fthg wear and igerease ,tisdz~e load) by the--use of lMo& in . association: -withphosphate surfaws. -;. .,

,-I?IGURE 2h’.~phemat~c ~pre&&tion of rider-disk combination, with

tino<&zed - and’ ‘with okidized ‘XIo&’ films.

106 ‘REPORT 1254—NATIONAL ADT180RY COMWTCEEl FOR AERONA’0TI08

Fxc+mm 24.-Effect of MoQ on friction of steel against steel at highsliding velocities (ref. 46). Apparatus A; mdiua of spherioal riderspacimen, ;{ inch.

Since Mo& appeared promising in the investigations atroom tarnpexature, it TVSSconsidered ss a high-temperaturelubricant. An X-ray diffraction investigation of its chemicalstability at high temperatures was accordingly made (ref.46). This investigation showed that in vacuum there is nophase change of the Mo& at temperatures below 1000° F.In the presence of the oxygen, however, Mo& wss found tooxidize to molybdenum trioxide Mo03 at a very low rate at750° F; the rate of oxidation increases steadily with increasein temperature ss showri in figure 21. Since M003 is abra-sive, the fkiction eharaetwistics of both an oxidized iilm ofhfoa and a rubbed film of pure MoO, were studied. Theresults are shown in iigure 22. The data show that MoOS isa very poor lubricant and its use resulted in high friction ande.xccseive vmlding. The “oxidized” Mo& film, however,showed results remarkably similm to those for the unoxi-dized Mo% film.

An explanation of the mechanism of action of the M& inthe “oxidized” condition must emsider the actual surfacefilms in both the oxidized and the unoxidized conditions.Sketches of the two surfa& films, based on an hypothesisthat esplains the mechanism, are shown in figure 23. Whileconditions in these experiments were intended to give as nearcomplete oxidation as possible, undoubtedly some fraction ofthe Mo& remains unoxidized; even though thk’ fraction isestremely small, it acts as an eilective solid lubricant at thesurface. The & immediately adjacent to the surfaceappears to be that part which produces the beneficial results.

studies were made (ref. 46) of a film of pure MoO, appIiedto a clean steel disk by an evaporation technique; the MoOSwas condensed fkom the vapor state on a steel disk until afilm approximately 0.003 inch thick wss present. Frictioncoefficients obtained with this film (upper curve of fig. 24)were higher than those for clean steel on clean steel at alIsliding velocities. Studies were also made (ref. 46) of anevaporated MoOS fi applied to a steel disk on which therewas a thin Nm of mixed iron oxides and esrbon; this bssefilm was produced by painting the surface of the hot diskwith corn syrup. & previously discussed, the corn syrupreduces FqOS to Fe30{ at high temperatures (ref. 39).Friction with the evaporated MoOS film applied over the

mixed iron oxides and carbon is shown in figure 24; the datashow that fiction coefficient with this film is higher than thatfor a bonded Mo& iilm, but lower than that with eitherMoOS on clean steel or for clean, dry steel. This result againillustrates the importance of iron oxides to the frictionmechanism; the reduction in friction (as compared with eitherMoOS on clean steel or with clean steel) is probably the resultof surface protection by the Fc304 ~. - -

The studies of reference 31 suggest that fretting damage ismused bssicdly by high adhesive forces; the adhesiontheoryof friction predicts that a solid surface iilm of low shearstrength should be effective in the mitigation, or inhibition,of fretting. Beeause of the effectiveness of Mo& as a solid-iilm lubricant, it wss checked (ref. 47) for its effectiveness asan inhibitor of fretting. Data obtained that show Mo& tobean effective fretting inhibitor are presented in the followingtable; for comparison purposes, data for chmn specimens andfor speoimens lubricated with mineral oil are included:

I Spdmelul I sorfw4 flhn s%do~fr%-Ung

These results show that a surface film (even mineral oil)delays the start of fretting; this tiect is believed to resultfrom reduction of the adhesive forces responsible for fretting,Of the various methods of applying Mo&, bonding of theiilm wss olearly superior, probably because of the strongadherenee of the bonded film to the surface.

Other solids as lubricants .—As previously stated, graphiteis a solid that has many of the properties desired of a solidlubrimnt for use at high temperature. In consequence,graphite was included in a previously unreported investiga-tion in which lubrication of bearings at high temperaturewas studied; method of application of the graphite wasessentially the “air-mist” system of reference 42. Theinvestigation showed that lubrication of a 20-millimeter-boreball bearing operating at 2500 rpm with graphite was suc-cwsful at temperatures to 1000° F. Good results were ob-tained with both dried and undried graphite. In contrastto the harmful (abrasive and corrosive) products from oxidat-ion of Mo&, the oxidation products from graphite are‘(harmless” (ref. 6). As indicated in reference 6, “graphitestarts to oxidize to carbon dioxide at 360° C (662° F).”

Boyd and Robertson (ref. 48) investigated a number ofsolid lubricants at extremelyhighpressure(to 400,000lb/sq in.)and found that MoS1 rmd graphite were among the beat ofthe various lubricants included in their investigation. Itis also possible that oxidw of several metals used in thepowdered form might effectively lubricate.

There have been several practical instancea at the Lewislaboratory that have proved the effectiveness of solid lubri-cants where extremely severe operating conditions are in-volved. For example, a large number of air-handling wdveaare required to operate in a corrosive atmosphere at ambienttemperatures up to 600° F. An attempt to operate the

FRICTION, WEAR, AND SURFAC13 DAMAGE OF METAI.JSAS AFFECrED BY SOLHI SURFACE FILMS 107

valves with a bearing combination of a relatively hardstainleassteel journal and a hard stellite bearing resulted inbearing seizure. A bearing was designed that had aninterrupted surface with the surface interruptions filled withrLsolid lubricant. Dry bearings of this type have operatedsatisfactorily over a period of time at temperatures up to600° F.

SUMMARYOF R13+3ULTS

Under extreme boundary-lubrication conditions, wheremetal-to-metal contact take9 place, the generally acceptedtilction theory (according either to Bowden and Tabor or toMerchant) predicts that friction and tendency to surfacefailure (by welding) of rubbing metals can be reduced rela-tively simply; this reduction can be accomplished by a re-duction of the ratio g/p wheres is shear strength of the softerof the two contacting materials and p is flow (yield) pressureof the softer of the two contacting materials. The mostpractical mems of reducing the ratio s/p is to reduce the valueof s. The use of thin low-shear-stiength i31rnson hard basematerials resndtsin a reduction ofs with negligible reductionof p, Thus any low-shear-strength material (for example,some oxides, sulfides, plated films, liquid lubricants, etc.)that acts as a contaminant between sliding surfaces shouldbe effective in reducing fiction and surfau failure. Exper-imental inveatigntions by the NACA produced the followingresults, which are consistent with this view:

Experiments with iron oxides showed that FeO and F%O,are generally beneficial, while F%OSis harmful. With steelspecimens in sliding, exclusion of oxygen by use of a blanket-ing medium prevented the formation of the beneficial oxidwand permitted extensive surface welding. In fact, the resultsof these and other investigations indicate that, with manymetals and particularly ferrous alloys, effective lubricationis very often dependent on the presence of an oxide film thatcan serve either as a solid lubricant film itself or as a means ofobtaining a metallic soap iilm (by reaction with fatty acids)which serves as the solid lubricant. The importance ofoxidea to the friction and wear processes has not always beenfully appreciated.

Chemical reaction films such as FeS and FeCIZwere effec-tive in preventing surface welding; FeCIZ was much moreeffective than F& in reducing friction (for example, at 3000ft/min sliding velocity, friction coefficient jLwas approxi-mately 0.13 for FeC12and 0.40 for FeS). This difhrence infriction corresponds to the difi%rencein shear strengths of thetwo compounds.

Wear studies showed that prevention of surface damageand maintenance of low wear could be associated with theformation of naturally occurring surface films on one or bothof the sliding specimens; for cast irons, the surface film con-sisted of gmphitic carbon and for various nickel alloys, thefilm consisted of the nickel oxide NiO. In one investigationwith cnst Inconel sliding against hardened M–10 tool steelat temperaturesranging from 75° to 1000° F, a very markeddownward trend of wear with increase of temperature wasobserved. It is speculated that tbia downward trend -iv=the result of formation of beneficial nickel oxide(s) at thehigher temperature. In this same investigation, wear wasreduced by a factor of 10 through pretreatment of the InconeJ

specimen to form a nickel oxide surface ti. It was potibleto increase wear by limiting formation of the oxide film.This result was obtained by conducting tests at 1000° F inwhich the availability of oxygen was restricted by displacingthe air surrounding the specimens with argon; under theseconditions, wear increased.

Investigations of solid surface b of the lacquer or var-nish type (preformed from decomposition of several petrol-eum and synthetic lubricants) showed that such tllms re-duced friction and surface damage considerably, comparedwith dry clean steel surfaces. Furthermore, when a fluidsilicone ti is present over its lacquer or varnish b, tiereduction in both friction and surface damage is quite markedas compared with the silicone iilm alone.

Various solids were considered for use as solid-film lubri-cants; of those considered, Mo& and graphite seemed bestfor use under severe operating conditions such as hightemperatures, high loads, or any of the various operatingconditions leading to extreme boundq lubrications. In-vestigations of these solids under high contact presures,high temperatures, or high sliding velocities indicated that,in general, beneficial results were obtained with each ma-terial in the lubrication of steel on steel. These materialswere therefore studied extensively with respect to: (1)methods of bonding to surfaces, (2) lubrication of rollingcontact bearings, and (3) for Mo&, stabili~ at l@h tem-perature.

Studies of the mecbmiam of bonding showed that bothMo& and graphite could be bonded to most surfaces by theuse of a resin-forming liquid vehicle. The k so bondedwere quite effective in preventing damage and reducingfriction over limited periods of time. The MoSg and graphitewere used to lubricate rolling contact bearings at high tem-perature and provided eilective lubrication up to 1000° F.This application required a solid lubricant iihn to be formedand maintained (repaired) by continual injection of smallamounts (approximatdy 0.001 lb/rein) of the solid lubricant.

X-ray *ction studies of Mo& showed no phase changesbelow 1000° Fin the absence of oxygen. Electron and X-raydiffraction experiments show that in the presence of oxygenMo& oxidizes to molybdenum trioxide MoO*. The rate ofoxidation increases steadily with ncrease in temperature:it is low below 750° F and becomes high at temperatureabove 1050° F. Presence of a very small amount of un-oxidized M& in oxidized bonded Mo& iilms will, however,maintain low friction and provide good protection fromsurface damage.

The general results of these investigations show that thepresence of certain low-shem-strength solid surface *can markedly reduce friction, wear, and surface damage ofrubbing metals operating under conditions of extreme bound-ary lubrication (where metal-to-metal contact takes place).

LEWISFLIGIPPPROP-ION LABORATORYNATIONALArmrsorw Commrmm FORAERONAUTICS

CLEVELAND,OHIO,Felwuury10,1956

108 REPORT 125fiNA~ON~ A?~ORy COMMPITEEl FOR AERONAUTICS.-

APPEND~ A,’, ,,,, ADHESION T~ORY OF FRICTION

,, Bow-den and Tabor (ref. 8) ~ I When th6 roughness texm is negligible,,,

Friction, F= S+ P= Shear+ Plo&#ing .”

@’..A&p’p.,,

A=Load . W

Flow pressure=~’

~=Frictiion_~_As_A’p,, Load –w-w w

Whti the ploughing term is negligible;,.,

,,-!

M_erchant (ref. “1O)

~s#=&Tan’O=Shear+Roughr&s

,.,. ,. . !

,,

,4:-~.,, ..- ,.. . .–

s Shear stiength I‘=Z?=Pressure surface hardness

where

friction forcesh&r forceploughing forcesh~ areashear strength,ploughing areaflow pressure (in certairi cases equivalent to yiolcl

strength in compression)load .,average angle that planes of slip make with gmmral

plane of surfacepresswe surface hardness (in certain cases equivalent

to yield strength in compression)

APPENDIXB

FILM PREPARATION

“ Tie prep~at~on of various preformed films@ d’kwribed inconsiderable detail’ ip references 13, 30,.37, 40,- and 46;’scondensed version of th’e procedure for each Speciiic b isincluded in thk’ appendix.‘ The iinishing and cleaning procedure” snmmmized here isdescribed in detail in reference 11. Each disk ~aa subjected’to a similar *g, grinding, and lapping pro~dqre, whichVm followed by a cleaning procedure essentially w follows:

(1)(2)(3)

; (4j~ ‘(5)(6)(7)

.,

De&ease in low-aromatic cleaning naphtha’Abrade with 3/0emery paperWash with mixture of- 50 percent benzene and” %0

peiwent ac8toneScrub with levigated alumina -,Rinse with waterRi.&e with 190 proof ethyl alcohol ; :Dry in clean, warm air

FORMATION OF FaO{ FILM (REF. m)

A film of FeSO,was formed by heating steel in a restrictedosygen supply; the disk was heated to 375°0 (707° l?) in ametal bell jar in which the air pressure was 0.1 miUimeterofmercury. After 30 minutes in the vacuum furnace, inter-ference colors were obtained which indicated a thickness inthe order of 1200 A. The steel w-as cooled to room tem-perature in the furnace under the same low preixmre.

FORMATION OF FmOI FILM (ERF. S@

A &n of the oxide I?%OS,which contains the greatestamount of oxygen of the iron oxide ti investigated, wasprepared by heating a steel disk to 350°C (662°F) in clean,dry air at atmospheric pressure. The disk was removed

.,.

horn the ,source of h~ai when the ipterfqrence colors on tho,,

surface indicated thiqlmess about 1200A.

FORMA~ON ‘OF FeCll FILM (REF. w)

A iilm of Fd71, was formed by exposing a warmed, clodnsteel disk to the hot vapors composed of an azeotropicsolution of hydrochloric acid and water. The warm diskwas placed b a hot gas chamber at ‘100° C (212° l?), whichw-as then partly evacuated. when a valve was’ opened fLtthe “inlet to the chamber, a ‘vapor composed of 20 p~rcmtwater and 80 percent hydrochloric acid could envelop thedisk: -A 2-miuute &posure ww sufficient to form m thin,’white, uniform coating believed (on basti of interferenhocolors) to be approximately 1000 A thick.

FORMATION OFFeSFILM

~etic friction experiments of figure 11 (ref. SO),—A&of FeS waa formed by exposing a heated disk to hydrogop,stide Hs gas. The clean steel disk was placed in a yacuumfurnace at room temperature and the air presspm rcducecl,to 0.25 rnill@neterj of mercury. Hydrogen sultide was ad-mitted Wd the pressure again reduced. This procws wasrepeated, until the chamber was purged of air. The rliqkwas then hinted ,to 350° C (662° l?) and an additionalamount of H$ ,was admitted. The film formed rapidly ~ndthe furnace was @lowed to cool while the low pr~ure wasmaintained. The fi thiclmess was believed (oD basis ofinterference colors),@ be approximately 1000 A.

Static ,tiotion experiments of flgnre 12 (reft 13)=—.l?ihusof J?eSwere formed on the rider specimens (balls) by heatingin an atmosphere of ,H2S. Thickness of the films was calcu-

FRICTION, WEAR, AND SUR?tA~ DAMAGE OF MDTQS, AR wCITOD- BY SOLID SURFACE FILMS 109..

Iated from weight gain and was checked by means of-themoditled chemical spot test, as discussed, in reference 13.#l weight gain was awweil to ,be &used by the additionof sulfur, which was stoichiometrically”&mbined with ironto form iron sulflde. ,.- .,.

FORMATION OF blo% ~,M

‘ Corn syrup resin for film of figure 1S (ref. ‘SO).-Nolyb-denum disuliide MoS, powder was mixed intw a smoothpaste with an organic binder (commercial corn syrup). Themkture was painted on a steel disk, the tempamture of whichwas 350° C (662° F). After the disk cooled, loosely adheringMoS, was scraped off with a straightedge until the dihn wasthin (appro.xinmtely 0,006 in.), tenacious, gray-black, andvery smooth. The disk was tihed by light abrasion with3/0 emwy paper, followed by washing ,in 190 proof ethylalcohol.

Corn syru~ resin for film of llgure “16 .(r6f,, 40).—Equ~parts by weight of Mo& and corn syrup mire mixed to apaste, Tho disk was pmheded to 250° to 300°0 (4S2° to’572° l?) tmd the paste applied to, the disk. After the diskhad coded and tlie exc~- was scr@d off, the surface ,wasrubbed with fine steel wool’ and burnished with”soft, cleancloth to form a iilm of thickness from 0.0002 to 0.0005 inch.- Asphalt-base resin (ref. 40).-A mixture of 1 part (byweight) Mo& and 2 parts (by ‘Weight) asphalt-base -vtihthinned 60 percent (by volume) with naphtha was brushedon a clcrm steel disk; the &n was then air ~ied at .roo’m’temperature until tack free. This film will become suiiicien~ly hard (equivalent to hardness obt&ed when cured for 3 hrat 1500 C) if enough time at room temperature is allowed.Tlm disk was then rubbed with steel wool ind burnished withp soft clean cloth to form a film of thickn+ from 0.0002~00,0006 inch.

Silicone varnish resin iref. 40).-A mixhma of 1 part (byweight) hloSi and 2 parts (by weight) silicone wyriish thiineclzo percent (by volume) with Wlerie was brushed on a:cleansteel disk and dried under infrared rays untiI firm. The filmwas cured by heating at 200° to 250° C (392° to 482° l?) for3 houm. The disk was then scrubbed with steel wool andburnished with soft clean cloth to foriri a @m thicknwa from0.0002 to 0.0005 inch.

Qlyoerol resin (ref. 40) .-A mixture of l-part (by w@ght)Mo& tmd 2 parts (by weight) glycerin preboiled (to about.one-fifth its original volume) wasrubbed on a clean steel disk.Tho iilm was then tiled either by (a) heating to 250° to 300°C (482° to 572° F) and repeating application. of mixturewith rubbing until film covered surface completely and con-tinued heating until dry, or (b) drying undti inbred until.film was firm, then cured by heating for 3 hoti at 250° C(482° J?). The diqk was then scrubbed with he steel woolmd burnished with soft clean cloth to form a Elm of thick-ness from 0.0002 to “0.0005 inch. ,,

FORMATION OF OXIDIZED MoSj FOR FILM OF FIQtiE “2 (REF.- 46),—

A disk, on which had been formed a flhn of MoSg in ac-eorcla,ncewith the method of reference 30, was placed in a

mu.illefurnace and heated Z hour at 1000°1’. This treat-ment produced a top layer primarily of M003, with an under-lfig, @in lbyer of r&ed Mo&, tion oxides, and carbon.

... . .EOkklATIONOF BfoOa

Rubbed ,MoOSfor lllms of figure. 22 (ref. 46).~arti6,&ystaWie MoO* Wasrubbed on th~flat disk to produce therubbed M003, film. .Th.is rubbing essentially resulted infilling the microscopic valleys of ‘the steel surfaces. ~-

Evaporated MoOa for ~ of figure, 24 (ref. 46).~TheiMoOawas evaporated orito ‘the surface of two disks; Oneof tli~e diskswas clean; the second had a fihg of mixed ironoxides and carbon on the surface. :. The mixture of ironoxides and caibon mm-obtained by painting the surface of ahot st@ disk with corn “syrup only. ,Evaporation of MoO,was done by hating molybdic acid HiMoOl in a crpcible:during heating, the acid dehydrated and becanie MoOJ,which melted, evaporated, then condensed cm the cool disksurface immediately above the crucible. The films’ wereapproximately 0.003 ‘inch thick. Details of the film prepa-ration are given in reference 46. .’

FOBMA’HON OF GRAPHITE FILhl, ~. M).

Graphite was deposited on&e surface of thi steel disk bycompressing flaky graphiteinto a&e, which was then pressedagaiist the rotating disk. The reimltingfilm, while not com-pletely chtinuous, was estimated to be less than 0.000$,.inch thick.

,, , FORMA~ON OF NICKEL OXIDE ‘

‘Nickel oxide films’ &re preformed by two methods. Inthe first, the fibn was preformed Oria cast IncogeI specimenby running a sliding friction wqmriment in air at 1000° F;previous sliding friction experiments with-niclnil alloys hadshowri that.NiOw_w forma on the iiu.rface(refs. 24 and 25).In tie second method, the film was preformed by heating acast Inconel specimen in ~olten caustic NaOH. Exactthick&s of tliese nickel oxide*. is unknofi.

FORMA~ONOFLACQUBRSOEVAB~~ (REF.m

The lacquer or varnish fi were formed by heating thedisk specimen in air. Before heating, a thin iilm of fluidlubricant was unifordy deposited on the disk, surftie; inmost cases, the quantity of fluid was approtiately 1 cubiccentimeter and the fluid was uniformly applied as a fie mist.

Decomposition of the fluid lubricant was accomplished byheating the “dish ti temperatures slightly higher than thoseat -which the fit visible vaporization of the “lubricant oc-curred: The tempwat&s were maintained as long as 14hours. :

PLATINGS

Cadmium plating.-The ca&nium plate was applied usinga le@ anode in ‘complex cyanide of cadmium bath. Ththichias of the plate was 0.0005 inch.

Silver, plati.ng.-The qilv~ plate was applied using rssilver anode in. a silver cyanide bath with aerosol as abrightener. The thickness of the plate was 0.0005 inch.

110 REPORT 1254—NATIoNAL ADVISORY COMblZJTDD FOR AERONAUTICS

REFERENCES

1. Bowden, F. P., and Tabor, D.: The Lubrication by Thin MetallicFilros and the Action of Bearing Metals. Jour. AppL Phys.,vol. 14, no. 3, Mar. 1943, pp. 141–151.

2. Goo& J. N., and Godfrey, Douglas: Changes Found on Run-Inand Scuffed Surfaces of Steel, Chrome Plate, and Cast Iron.NACA TN 1432, 1947.

3. Prutton, C. F., Turnb~ Davi~ and Dloughy, George: Meoh-am-sm of Action of Organic Chlorine and Sulfur Compounds inExtrem&Preswre Lubrication. Jour. Inst. Petr. (London), vol.32, no. 266, Feb. 1946, pp. 90-118.

4. Hughes, T. P., and lvhittingham, G.: The IMuence of SurfaceFihna on the Dry and Lubricated Sliding of Metals. Trans.Faraday Sea, vol. ~VIII, no. 249, pt. 1, Jan. 1942,pp. 9-27.

5. lVebb, lVellsA.: The Intluenceof Iron Oxide on JVearof RubbingSurfacea. Scien~ vol. 99, no. 2575, May 5, 19Q pp. 369-371.

6. Campbell, ~. E.: Solfd Lubricants. Lubrication Eng., VOL 46,no. ~ Aug. 1953, pp. 19S-200.

7. Tingl~ E. D.: The Importance of Surface Oxide Fflma in theFriction and Lubrication of Metale. Trans. Faraday SoU., vol.46, no. 326, pt. 2, Feb. 1950, pp. 93-102.

8. Bowde~ F. P., and Tabor, D.: The Friction and Lubrication ofSolids. Clarendon Press (0.sford), 1950.

9. Cadomb, M.: Theorie d= machinea dmple. Memoime de mathe-matique et de physique. 1785, VOL10, pp. 161–334.

10. hferchant, hl. E.: The Meahaniem of Statio Friction. Jour. APPLPhye., VOL11, no. 3, hfar. 1940, p. 230.

11. Johnson, Robert L., Swilmrt, Max A., and Bisaon, Edmond E.:Friction at High Sliding Velocities. NACA TN 14Q 1947.

12. hfurray, S. F., and Johnson, Itobert L.: Effects of Solvents inImproving Boundary Lubrication of Steel by Silicmw. NACATN 2788, 1952.

13. Levine, Erva C., and Peterson, MarahallB.: Formation of StideFilmE on Steel rind Effect of Such Fihna on Statio Friction.NACA TN 2460, 1951.

14. Peterson, MarshallB., and Johnson,Robert L.: Friction and ~earInvestigation of hlolybdenum Dhtide. I—Effect of Moisture.NACA TN 3055, 1953.

15. Macks, E. Fre& PreEminarYIrm%tigation of Needle Bearings ofl}GInch Pitch Diameter at Speeds to 17,000 RPM. NACATN 1920, 1949.

16. Bowde~ F. P., and Young, J. D.: Friction of Clean Metals mdthe Influence of Adsorbed Films. Proc. Roy. Sot. @ondon),ser. A, VOL208, Sept. 7, 1951,pp. 311-325.

17. H--s, Jay C.: Ffhnaand Surface CleanlinW. Metal Finishing,VOL+ nos. 8-9, Aug. and Sept. 1946,pp. 328-333, 386-388.

18. Nelson, H. R.: The Prhnary Oxide Fihn on Iron. Jour. Chem.Phye., vol. 5, no. 4, Apr. 1937, pp. 252-269.

19. Pomey, J.: Friction and ~ear; Translation of “LO Frottementet L’Usure.” Rapport Tedmique no. 36, Office NationaleD’Etudes et de ReoberohesA6ronautiqu~ 1948. Travaux duGroupement Francais pour le D6veloppement d- RechercheaA&onautiquea (G. R. A.). (Available in English translation= NACA TM 1318.)

20. Finch, G. T.: The Structure of Sliding Surfacea. Engineering(London), vol. CLIX, no. 4131, Mar. 16, 1945,p. 215.

21. Johnson, Robert L., Sw&er$ Max A., and Bfason, Edmond E.:Friction at High Sliding Velodti~ of Surfaces Lubricated withSulfur as an Additive. NACA TN 1720, 1948.

22. Ernst, Hans, and Merohan~ M. Eugwe: Chip Formation, Fric-tion and Finish. The CincinnatiMilling Machine Co., Aug. 24,1940.

23. Johnson, Robert L., Sw&ert, Max A., and Biason, Edmond E.:Inwatigation of IVenr and Friotion Properties under Sliding

Conditions of Some Materials Suitable for Cages of Rolling-Contact Bearings. NACA Rep. 1062, 1952. (SuporsodesNACA TN 2384.)

24. Johnso~ Robert L., Swikert, Max A., and Bisson, Edmond IL:Wear and Sliding Friction Properties of Niokel Alloys Suitedfor Cages of High-Temperature Rolling-Ckmtaot Bearfnge.I—Alloys Retaining Mechanical Properties to 600° F. NACATN 2758,1952.

25. Johnson,Robert L., Stiert, Max A., and Bisson,EdmondE.:~ear and SlidingFrictionPropertiesof NiokelAlloysSuitedfor Cagea of High-TemperatureRolling-ContaotBearfnga.11-Alloys RetainingMechanicalPropertiesabove 600° ~.NACATN 2759,1952.

26. Brophy, J. E., M]ltz, R. O., and Zieman,}V. A,: Dlmethyl-Silicone-PolymerFluidsandThefrPerformanceGharaoterietksin UnilatirrdlyLoaded JournalBearings. Trans.A.S.M.E.,vol. 68,no. ~ May 1946,pp. 36E-360.

27. Hunt, Kenreth C.: PetroleumRequirementsof Brftieh GasTurbines. II-Lubricants. SAE Jour.,vol. 69, no. 11, Nov.1951,pp. 20-21.

28. ~~lcook, Donald F., and Jones,FrederiokC.: ImprovedHigh-SpeedRollerBearings. LubricationEng,, vol. 6, no. 4, Aug.1949,p. 184;diacwsionby DanielGurney.

29. Bkaon,E. E., and Johnson,R. L.: NACA FriotionStuclhsofLubricationat High SffdingVelocities. Lubrication13ng.,VOL6, no. 1, Feb. 1950,pp. 16-20.

30. Johnson,RobertL., Godfrey,Dougl~ and Bisson,EdmondE,:Friotfonof Solid Fihnaon Steel at High SlfdingVelodtks.NACATN 1678,1948.

31. Godfrey,Douglas:Investigationof Frettingby MioroscopioOb-servation. NACA Rep. 1009,1951. (SupenwdesNACA TN2039.)

32. Bailey,John M., and Godfrey,Douglas:G%efHdentof Frlotlonand Damage to ContaatArea During the Early StagesofFretting. 11-Stee3 Iron, Iron Oxide, and GlaeaCombjnwtions., NACATN 31441964.

33. Greenhi~E. B.: The Lubricationof Metalsby CompoundsCon-tainingSulfur. Jour. Inst. Petr. (London),vol. 34, no. 297,&Pt. 1948,PP.6S9-669.

34. ZhrOaII,JV.A.: PresentProblemsandFutureTrendain Lubrica-tion. NRL Rep. 4080,Chem.Div., NavalRes. Lab., Vlaeh-ington(D. C.), Nov. 26, 1952.

35. Johnso~RobertL., Peterson,MarshallB,, andStikert, MaxA.:Frictionat High SlidingVelocitiesof Otide Fjhns on SteelSurfaces Boundary-Lubricatedwith Stearlo-AoidSolutions.NA CA TN 2366, 1951.

36. Bisso~ Edmond E.: The Influence of Solid Surface FMna on thoFriction and Surface Damage of Steel at High Sliding Velooltics.Lubrication Eng., VOL9, no. 2, Apr. 1953, pp. 75-77.

37. Johnson, Robert L., Godfrey, Douglas, and Bkson, Edmond E.:Friction of Surface Fihna Formed by Decomposition of CommonLubricants of Several Types. NACA TN 2076, 1950.

38. Wiicock, Donald F., and Jon% Frederiok C.: Improved High-Speed Roller Bearings. Lubrication Eng., vol. 6, no. 3, JuncI1949, pp. 129-133.

39. Godfrey, Douglas, and Biwon, Edmond E.: Bonding of Molyb-denum Disultide to Various Materials to Form a Solid Lubricat-ing Film. I—The Bonding Meohanism. NACA TN 20%,1952.

40. Qodfrey, Douglas, and Bisson, Edmond E.: Bonding of Molyb-denum Dieul13deto Various Materials to Form a Solid Lubrloa&ing Film. 11-IMotion and Enduranoe Charaoterletim of lWmsBonded by Praotkal Methods. NACA TN 2802, 1952.

FRK71?ION,WEAR, AND SURFACE DAMAGE OF METALS AS AFFECTED BY SOLID SURFACE FHMS 111

41. Johnson, Robert L., Swikert, Max A., and Bkaon, Edmond E.: taminants and Method of Application. NACA TN 3111, 1954.Friction and Wear of Hot-Pressed Bearing Materials Containing 45. Barwe~ F. T., and Milne, A. A.: Use of Molybdenum DisuMdeMolybdenum Dimlfide. NACA TN 2027, 1950. in Association with Phosphat.ed Surfaces. Scientific Lub., vol.

42, Mnoks, E. F., Nemet.h, Z. N., and Anderson, W. J.: Prelhninmy 3, no. 9, Sept. 1951, pp. 9-14.Instigation of Molybdenum Disulfide-Air-Mist Lubrication 46. Godfrey, Douglas, and Nelson, Erva C.: Oxidation Characteristicsfor Roller Bearings Operating to DN Values of 1X I@ and Ball of Molybdenum DisuMde and Effeot of Such Oxidation on ItsBearLngaOperating to Temperature of 1000° F. NACA RM Role as a Solid-Film Lubricant. NACA TN 18S2, 1949.E51G31, 1951. 47. Godfrey, Douglas, and B&on, Edmond E.: Effediveneas of

43. NACA Subcommittee on Lubrication and Wear: Review of Cur- Molybdenum DisuKide as a Fretting-Corrosion Inhibitor.rent and Anticipated Lubricant Problems in Turbojet Enginw NACA TN 21S0, 1950.NACA RM 51D20, 1951. 48. Boy& John, and Robertaon, B. P.: The Friction Properties of

44. Peterson, Marshall B., and Johnson, Robert L.: Friotion and Wear Various Lubricants at High Preawuw. Trans. A. S.M. E., vol.Investigation of Molybdenum DisuMde. 11-Effects of Con- I 67, no. 1, Jan. 194S, pp. 5i-56; discussion, pp. 56-59.

TABLE I.—PHYSICAL AND CHEMICAL PROPERTIES OF SOLID LUBRICANTS

Crystalline propefies~olhy,g Hardnesa,

t%lubility

Material Chemical“c mho

6tNCtUlW Form Orientation In water Other reaotivi~solvents

Molybdenum disultide Mo&- 11ss 1.0-2 .5----- Hexagonal ----- Laminar---- Random---- InsoluMe___ HfiQ, Mediukn.

Tungstm distide ‘iV&------ 1250 h. MediumFriable----- Hexagonal ----- Laminar---- None------ Insoluble___ H~F~

Graphite C----------------- 3527 l.o-2.0----- Hexagonal ----- Laminar---- Hihlyre- Imjoluble--- None---- I&lw

t%rdbad iOdidOPbIl ------------ 402 ------------ Hexagonal ----- Laminar---- None------ Slightly____ KI-----_ gfe:Silver sulfate Ag,SOl--------- 652 ------------ Orthorhombic--- ------------ None------ Slightly---- HNO&

H,S@.

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