Fretting & Surface Treatments

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N A S A T E C H N I C A L

M E M O R A N D U M

NASA TM X-73591

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FRETTING OF AlSl 9310AND SELECTED FRETTING

RESISTANT SURFACE TREATMENTS

by R . C . Bi l l

Lewis R e s e a r c h C e n te r

Cleveland, Ohio 44135

TECHNICAL PA PER o b e p resen ted at the Annua: M,:eting of th e

An - erican Fociety of Lubricatw n Engine ers



FRETTING OF AISI 9310 AND SELECTED FRETTIiJG

RESISTANT SURFACE TREATMENTS

by R. C. B i l l

Natio nal Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

ABSTRACT

Frettixig wear exp eriments were conducted wi th uncoated AISI 9310

mating surf ace s, and with combinations inco rporat ing a s e l e c t e d c oa t i ng

t o one of t h e mating sur fa ce s. Wear measurements and SEM observat ions

ind ica ted t ha t sur f ace f a t igue , a s made ev ident by spa l la t ion and sur f ace

crack formation, i s an important mechanism i n promoting f r e t t i n g wear t o

uncoated 9310. Increas ing humidi ty res ul te d in accel erated f r e t t i ng , and

a ve ry no t i c e a b l e d i f f e r e nc e i n na t u r e of t he f r e t t i ng de b r i s . Of t h e

coa tin gs evaluated, aluminum bronze with a poly ester ad di ti ve was most

e f f e c t i v e a t reducing wear and minimizing f re t t i n g damage t o t he mating

uncoated su rfa ce , by means of a se l f - lub r ica t in g f i lm th a t developed on

th e fr e t t i n g surfac es. Chromium pl at e performed a s an e f f e c t i v e p r o te c -

t i ve c oa t i ng , i t s e l f r e s i s t i n g f r e t t i n g and no t a c c e l e r a t i ng damage t o t he

uncoated surface.

INTRODUCTION

F r e t t i ng i s commonly observed on the assembly interfaces of mechan-

i c a l power transmission components. Typi cal examples include sp li ne

couplings ( r e f . 1 ), be ar ing hous i ng i n t e r f a c e s ( r e f . 2), and gea rlf lange

int er fa ces . The f r e t t i n g act ion can be caused by shaf t misal ignments a s



in th e case of sp l i ne wear , or by mechanical s t r a i n dif fere nce s between

mating components, as fo r gea r l f lange in te r f a ces . I n any event , the

occur r ence of f r e t t i ng is marked by su rf ac e damage th a t may i nc lu de

c ra ck i n i t i a t i o n ( r e f . 3), p i t t i n g ( r e f . 4) , and de br is generation. The

consequences often include degradation of ca nponen t f a t i gue l i f e , l o s s o f

c r i t i c a l a ssembly to le r ance , and fou l ing of moving components by debris.

Both th e exte nt of f re t t i n g damage and th e mechanisms of f r e t t i n g ar e

af fected by a number of fa ct or s including s l i p ampli tude, re la t i ve humid-

i t y , temperature , f r e t t in g f requency, normal load, and the mat er ia ls cm -

p r i s in g t h e f r e t t i n g p a ir .

I nc re a s ing s l i p a m pl it ude i nva r i a b ly r e s u l t s in i nc r e a s e d f r e t t i ng

wear, w i th a t r a ns i t io n ampli tude id en t i f ie d above i n which g rea t ly acce l -

erated wear i s observed ( refs . 5 and 6) , and ch ar ac te r i s t ic s of unidirec-

t io n al sl id in g wear mechanisms predominate. Under fr e tt in g condi tio ns i n

which f a t igu e c rack in i t i a t i on i s a c once rn , a c r i t i c a l s l i p am p li tude

( lower than th e wear t r an s i t i on ampli tude) th at causes a maximum degra-

da t i on i n f a t i gu e l i f e of t he f r e t t i ng components i s a lso r epor ted ( r e f s . 7

and 8) .

Relat ive humidity i s known t o in f luenc e the f re t t in g of s t e e l com-

ponents , a s i l lu s t ra te d by th e exper imental re su l t s of Feng and Uhlig

( r e f . 9) . Waterhouse ( ref . 10) suggests tha t re la t i ve humidi ty inf l u-

ences th e ox ida t ion of i ron wear debr i s , r e su l t in g i n sof te r deb r i s fo r

higher le ve ls of humidity and reduced fr e tt in g damage.

Feng and Uhlig al so stud ied t he infl uenc e of fr e t t i n g frequency on

wear of mild s t e e l , and observed reduced wear with increased frequency



up t o ab out 17 Hz. ;or highe r freq uenc ies, l i t t l e change i n f r e t t i ng Gear

w a s observed. Feng and Uhl ig a t t r i bu t e t h e i r r e su l t s t o a su r face co rro -

sio n mechanism. Hurricks (ref . 11) poi nts out th at othe : a c t o r s may be

s ign i f i c an t i n fr equency experiment s, i nc lud ing s t r a in r a t e s en s i t i v i t y of

adhering junct ions, and stress co r ros ion assoc i a t ed wi th su r face f a t i gue

cracks.

There i s considerable debate as t o t he r e l a t i ons h ip be tween f r e t t i n g

wear exper imell rs and f re t t in g fa t ig ue s tudies . I t is conroonly held that

f r e t t i n g acce l e ra t ed f a t i g u e i s brought on by the ea r ly i n i t i a t io n of sur -

face cra cks where s l i p is occ urr ing . Nichio'ka and Hirakawa (r e f. 12) a t -

t r i bu t e c rack i n i t i a t i on t o t he combinat ion of con t ac t s t r e sse s superim-

posed on t h e a l t e r n a t i n g f a t i g u e stresses, with cracks being in i t ia ted

where f r i c t i o n induced stresses a r e highe st. Where pi tt in g and nonpropa-

gat ing cracks ar e observed, i n i t i a l crack growth under f r et t i ng condi t ions

is i n s u f f i c i en t t o p ro vi de c r i t i c a l s i z ed c racks t h a t may propagate under

the a l t e rna t i ng f z t t gue s t r e ss a f t e r g rowing ou t s ide t he s t r e ss concent ra -

t ion zone of t t e contact reg ion . Th i s s t rong ly sugges t s t ha t t he p i t t i ng

and s ubsur face crack propagation leading t o fr e tt i ng wear of many mate-

r i a l s ( r ef . 4 and 13) i s r e a l l y the same mechanism a s th at leading t o th e

format ion of propagating f at ig ue cr acks when al te rn at in g microscopic

fa t i gue s t r e sse s a re super imposed on t he con tac t s t r es s s t a t e .

The purpose of th is inv est iga t io n Is t o evaluate some surfac e t re at -

ment and coat ing combinat ions pa rt ic ul ar ly applic able t o fr et t i ng encoun-

tered a t gear l f lange In ter face s . While a major concern i s t h e p o t en t i a l

f o r i n i t i a t i n g f a t i g u e c r ack s, t h e i n v es ti g a t io n i s p r im ar il y a f r e t t i n g



wear stud y. However, th e f r e t t i n g wez- measurements a r e he av ily supple-

mented bv microscopy s tu d i e s t o de te rmine t he e x t en t t o which p i t t i n g and

c r a ck i n i t i a t i o n a nd p r op a ga t io n c o n t r ib u t e d t o t h e wear. The basel ine

m a t e r i a l w a s untreated AISI 9310, a s t a nd a r d g e a r s t e e l . S u r fa c e n i t r i d e

and su r fa ce carbu r ize t reat ment s were evaluated, and t he co at i ngs exam-

ined i nc luded e l ec t rop l a t ed C r , el ec tr op la te d Ag, plasma sprayed Al-bronze

a d polyimide.

APPARATUS

A s ch em at ic d ia gr am o f t h e f r e t t i n g r i g i s shown i n f i gu re 1. Linear

o s c i l l a t o r y m ot io n is provided by an e l ec t romagne t i ca l l y d r iven v ib r a to r

wi th t he f r equency con t ro l l ed by a v a r i a b l e o s c i l l a t o r . P ea k t o pe ak

f r e t t i n g a m p l i t u d e i s monitcred by means of a capacitance proximit] probe.

The load i s appl ied t o the specimens by placi ng prec is ion weights on a pan

which i s hung from the load arm.

The f r e t t i n g spec imens con s i s t of an upper, s t a t i on a ry , 4.76-

m i l li m e te r - ra d i us , h e mi sp k er i ca l t i p i n c o n t a c t wi t h a l ow er f l a t s u r f a c e

which i s d r i v e n oy t h e v i b r a t o r .

A dry a i r environment was provided by f lowing a i r through an absorp-

t i on dr ie r and then in to th e exper imental chamber . In th i s way mois ture

content was kept i n the range 10 t o 100 pa r t s per mi l l io n. When a

mois ture-sa tura ted envi~onmentwas de si re d, th e a i r was bubbled through

a wa te r- fi lle d column and then blown In to t h e chamber. Intermediate

humidi ty Levels ar e achieved by combining dry a i r and sa t ura ted a i r f lows,

and monr tor ing r e l a t iv e humidi ty .



PROCEDURE

The prepara t ion of the specimen sur face s befor e a f r e t t i ng exper i -

ment depended on the type of coa t in g or s urf ace t rea tment ap pl ied t o th e

surface .

The bar e AISI 9310 s ur f ac es were hand lapped with le vig at ed alumina

and the n washed i n ta p water with a pol is h ing c l o t h t o remove th e a lumina .

Pollowing the washing, th e specimens w e r e r i n s e d i n a b s o l u t e e th a n o l ,

r i n s e d i n d i s t i l l e d w a t er , a nd t h en set a s i d e t o d ry.

The f l a t plasma-sprayed su rf ac es were machine lapped, wi th approxi-

mately 75 t o i05 micrometers ( 3 t o 5 mi ls ) of co ati ng being removed ir,

th e process. In t h i s way th e very rough as-sprayed co ati ng was smoothed

s o t h a t , disregarding th e sur face pores, th e root-mean-square roughness

of th e co at in g was abou t 0.5 micrometer (20 uin.). The lapped su rf ac e was

t h en u l t r a s o n i c a l l y c l e an e d i n a b s o l u t e et h a n o l, r i n s e d i n e t h a n o l, r i n s e d

i n d i s t i l l ed wa ter , and a llowed t o dry .

The su rf ac es with polymer-bonded co at in gs and :hose wit h th e va ri ou s

surfa ce t rea tments were washed i n tap water , r i m ed in e tha nol , and r inse d

in d i s t i l l e d wa te r befo re be ing sub jected tr, f r e t t i n g .

Following the sur fa ce prepa ration treatm ent, th e specimens were as-

sembled in t o the gr ip s according t o the des i red combinat ion . The te s t

chamber was then purged wit h t he s elec ted atmosphere fo r 15 minutes.

The f re t t in g exposure was in i t ia te d by adding the required weight t o

the load pan t o ' . r i ng the con tac t fo r ce to the de s i red l eve l , u sua l ly

1.47 newtons. The amplitude of th e fr e t t i n g motion was 35 micrometers

(0.0014 i n . ) , and t he frequency of t he f r et t i n g motion was 16351 hertz.

The standard dura t ion of th e f r e t t i ng exposure w a s l o 6 cyc les .



F ol lo wi ng e a c h f r e t t i n g e x pe ri me nt , t h e f r e t t i n g s c a r s o n bo t h s u r -

f a c e s were photomicrographed t o record t he s iz e and fea tu re s of th e wear

sc a r s and t h e debr i s accumul at ion. The l oose de br i s xas t hen r i nsed o f f

w i t h e t hy l al coho l , and a l ight - sec t ion microscope w a s used t o measure

th e maximum de pth and diam ete r of t h e wear s c a r s o n b o t h s u r f a c e s . Wear

volumes were ca lc ula te d by applying sph er ic a l cap approximat ions t o th e

wear scar geometry.

In p r i nc i p l e , t h e l i gh t - s ec t i on measurement t echn i que co ns i s t s of

di re c t in g a p lane beam of l i g ht obl iq uely (wi th a 45' angl e of inc ide nce)

a t t he spec imen su r f ace . The r e f l ec t ed l i g h t beam i s viewed throu gh an

op t i ca l microscope . I f a p l ane su r f ace i s viewed, th e l i g h t beam appe ars

a s a s t r a i g h t l i n e a c r o s s t h e f i e l d of v ie w; i f a h e m i sp h e r ic a l s u r f a c e

i s viewed, t he l i g h t beam appe ars a s a smooth curve. Wear or s ur fa ce

d i s t r e s s d u e t o f r e t t i n g a c t i o n shows up a s d e v i a t i o n s fro m t h e smooth

pr o f i l e of t h e su r round i ng su r f ace . With t he su r round i ng su r f ace p r o f i l e

used a s t h e datum, wear de pt h measurements may be made by means of a

bu i l t - i n cro ssh ai r system. The diameter of th e wear a r e a may be measured

by manipula t ion of an indexed micrometer s ta ge t r an sl at io n. Wear dep th

measurements a re accu ra te t o wi th in 0. 5 microm eter, and diam eter measure-

men ts a r e ac cura t e t o about 20 micrometers. The sma lle st wear sc ar s ex-

amined i n t h i s i n ves t i g a t i on had a nomi na l dep t h of about 0.5 micrometer

and a diameter of roughly 80 nicrometers . Thus, these wear sca rs repre-

s e nt e d t h e p r a c t i c a l l i m i t o f r e s o l u t i o n f o r t h e l i g h t - s e c t i o n t e c hn i q u e,

wit h an un ce rt ai nt y i n wear volume, re 3u lt ln g from measurement of about

50 per ce nt. The wear volume un ce rt ai nt y due t o measurement t ech niq ue s for

t he t yp i ca l wear s ca r s i n t h i s i nves t i ga t i on was o f t he o rde r o f 10 pe rcen t .



MATERIALS

The primary material that served as the subject of t h is investiga-

tion was AISI 9310 steel , t he nominal composition of which is given in

t a b l e I. The 9310 steel was in a cold drawn, annealed con di tion , and

the hardness was measured t o be 12 on the Rockwell C scale .

Selection of coating s and surface treatments rptudied i n t h i s eval-

uation q s based on many considerations. The carburized and nitride

surface treatatents were included because they a re st andard treatment

ofte n appl ied t o gear components, and t h e i r performance would n at ur al ly

be of inte res t. The surface pla ti ngs sele cted are ty pi ca l of approach-

es commonly employed t o reduce f r e t t i n g wear. Aluminum bronze-polyester,

and polyimide surf ace co ati ng s were included because of t h e i r good per-

formance in frettingwear

s tud ie s of Ti-6A1-4V su rf ac es (r ef . 14).

Sev era l of the AISI 9310 specimens were carb uriz e case hardened t o

a dep th of 500 t o 700 micrometers (0.020 in . t o 0.030 i n. ), and th e

hardness of th e carb urized su rfa ce was 53 on t he Rockwell C scale . A

surf ace n i t r i d e treatment was applie d t o othe r specimens, and th e af-

fec te d dep th was approximately 500 micrometers (0.020 in .). The hard-

ness of th e nit ri de d su rfa ce was 42 on th e Rockwell C scale .

Two elec trop lat ed surf ace coat ings were evaluated i n t h i s study.

Chromium p la te was appl ied t o a thi ckn ess of 12 t o 25 micrometers

(0.0005 in . t o 0.001 in. ) , and si l ve r pl at e was applied t o a thickness

of roughly 12 micrometers (0.0005 i n. ). Both surfaces were cleaned of

oxides and etched prior t o plating . The si lv er pl ate was applied to a

palladium f la sh which was f i r s t put down on th e 9310 st e e l su rfac e.



The aluminum bronz e-polyester coati ng was appl ie d by a plasma spr ay

techn ique. Composition of th e aluminum bronze wa s copper-10 percent

aluminum, which was cospreyed wi th 10 perce nt (by volume) of an aromat ic

polyester . The thickness of the coat-, as sprayed, wa s 200 t o 250

micromet e r e (about 0.010 in . ) .The polyimide coating w a s solution-sprayed onto th e 9310 sur face

according to the procedure described in d e t a i l i n reference 15. Brief ly,

the coatings were applied t o a fi n a l thicknes s of about 20 micrometers,

baked f o r 1 hour a t 100' C to vo la t i l i ze the thinner , and then baked fo r

1hour a t 300' C t o cu re th e polymer. The choic e of polyimide was based

on i t s combination of mechanical and low fri ct io n and wear (r ef. 16).

RESULTS AND DISCUSSION

Fretting of AISI 9310 Steel

The ef fe ct of amplitude on th e f r e t t i n g wear of uncoated AISI 9310

i n con tact with uncoated AISI 9310 i s shown in figure 2. Up t o an ampli-

tude of about 25 micrometers, th e f r e t t i n g wear volume remains a t a

near ly constan t, r el at iv el y low value. In the 30 to 35 micrometer ampli-

tude range, a t r a ns i t ion i s observed beyond which t he wear i nc re as es

li ne ar ly with increased amplitude. This observation is i n q u a l i t a t iv e

agreement with th e re su lt s of Halliday and Hirst on mild s t e e l ( ref . 5 ) ,

but they observed a tr an s it io n to occur at about 70 micrometers. Mate-

r i a l , frequency, and loading differe nces might account for t h i s

disagreement.

SEM st ud ies show th at , though the wear r at e does begin t o i ncre ase

i n t h e 25 to 35 micrometer amplitude range, the fr e t t i n g mechanisms



causing surface darnage are s t i l l qua l i t a t ive ly s imi la r to those predom-

i n a t i n g at lower amplitudes. Figure 3 summarizes th e typ es of su rfa ce

damage observed as th e f r e t t i n g ampli tude was increased f rom 7.5 m i -

crometers up t o 62 micrometers. The su rf ac es examir.ed showed si g n s of

c rack in i t i a t i on and "exfo l ia t ion . " Thus, fo r appl ica t io ns i n which

one of t h e matin g components is s ub j ec t t o s i gn i f i c a n t f a t i g ue loa d ing ,

va r i a t i ons i n s l i p a mp l it ude al one ( a t l e a s t above 7 .5 micrometers)

would not be expected t o have an important ef fe ct on observed f re t t i ng

f a t i gue l i f e . F r e t t i ng f a t i g ue r e s u l t s ge ne r a l l y show t ha t f a t i gue

l i f e i s i n f a c t n eg l ig ib ly a f f ec te d by s l i p ampli tude beyond the 5 t o

10 micrometer range (re f. 6) .

Measured wear volume a s a f un ct io n of f r e t t i n g exposure (number of

cyc les ) i s shown i n f ig ur e 4. Beyond 3 x1 0~ yc les , the f r e t t in g wear

volume i s roughly pro port ion al t o (number of cycles)1/2. This type of

propor t io na l i ty impl ies th a t the ins tan taneous wear r a t e is i nve r s e l y

prop orti ona l t o t ne accumulated wear :

0

where V i s th e wear r a t e , V i s th e in stan tan eou s wear volume, N the

number of cycles, and t th e time of f re t t l n g exposure. Thus, the

f r e t t i ng wear process for 9310 s t e e l does not foll ow an Archard ty pe



wear equation, ge nera lly de sc ri pt iv e of adhesive wear. Rather, i t is

con sist ent with a s urfac e fa ti gue mechanism a s th e wear r a t e c ontrol-

ling process. Fr et ti ng wear of 9310 s te e l, under th e condi tion s imposed

in th i e s tudy, i s envisioned t o proceed v ia fat igue induced spal lat i on

of ma ter ial (ac celerated by corro sive mechanisms), with the fr ic ti on a l

shear stress a t the surface a s the primary driving force. The surface

f r i c t i o n a l s h ea r stress is given by

where L is the applied load, A the instantaneous area of contact

(wear sca r surf ace area ), and p i s the coeff icient of fr ic t io n. The

contact load L i s assumed t o be uniformly di st rib ut ed over the en ti re

contact ar ea (with the assi stan ce of f in e accumulated debri s). The

number of cy cle s (NC) requi red t o cause a spa ll at io n is approximated by

If each spal lat ion i s of a shallow dept h d, and of approximately the

same volume, hen

0 fv a -, f = frequency

Nc

provided m 1 The approximation m 1 is reasonable since the

spall.%tionsare caused by low cycle fa t igu e with corrosion as a contri -

butory elfect . I t should be emphasized th at t h i s argument, not bei ng a

so lid if-and-only-if argument, does not exclude th e po ss ib il it y of othe r



mechanisms, i n pa rt ic ul ar corros ion and thr ee body abras ion. The micro-

graphs i n f igu res 3 and 5 show sig ni fic ant sur face cracking and spa l l a -

t i o n up t o a t l e a s t lo6 cyc les, i nd ica t ive o f fa t igue , wi th a f i ne r

s ca le process, pos sib ly corr osi on combined with 3 body abra sio n (no str i -

6at io ns or scra t ches can be seen) a f t er 12x10 cycles .

Fr ett in g experiments conducted i n moisture satur ated a ir produced

r e s u l t s d i c t i n c t l y d i f f e r e n t from t h ose s ee n i n d r y a i r . F i r s t , t h e

wear volume is increased by a f ac to r of from 5 t o 8 a f t e r l o 6 f r e t t i n g

cycles. Second, adherent build ups of mat eri al, ap par entl y oxidized de-

b ri s, ar e observed i n and around the contact a reas a s may be seen in

ffgure 6 . I t i s apparent tha t the presence of moisture s ig ni f ica nt l y

a l t e r s t he rheo log ica l p rope r t i e s of the debr i s causing i t t o become

"sticky" and cohesive, forming th e observed laye red dep osi ts. Water-

house (ref . 10) addresses the inf luence of moisture on the oxidati on of

iro n based deb ris , with debris generated i n th e presence of moisture be-

ing sof t e r than tha t generated i n d ry a i r . I t i s f e l t t h a t t h e ac ce le r-

ated wear observed fo r t he case of AISI 9310 s t e e l when f re t t ed in sa t-

u r a t e d a i r is primar ily the re su lt of more rapid corrosion. Experiments

conducted i n a i r a t a co ntroll ed 40 percent r el at iv e humidity showed a

fr et t in g wear volume of 20 t o 25x1 0-~m3, l i g h t l y h igher than tha t

measured for dry a i r .

Performance of Coat ings and Sur fac e Treat ments

The re su lt s of fr et ing experiments involving coated or t r e a ted

AISI 9310 s t e e l i n con tact with un treate d 9310 ar e summarized i n f ig-

ure 7. Fr ett ing was conducted in both dry a i r and moisture sa tur ated



a i r . For purposes of performance evaluat ion, the f r e t t i n g wear of un-

t rea ted 9310 s t e e l against untrea ted 9310 s t ee l in dry a i r and sa tu ra ted

air is indica ted on the f igure .

No wear was measured on the carburi zed 9310 sur fa ce a f t e r f r e t t i n g

against untreated 9310 steel. However, extensi ve tra ns fe r t o th e car-

burized surfac e took place with t he f in a l contact s i t ua t ion being one of

9310 s teel against 9310 steel. Wear t o the untreated surf ace was indis-

tinguis hable from baselin e wear uuder dry a i r conditions. Figure 8 shows

th e presence of conside rable oxidized d eb ri s embedded I n t h e f r e t t i n g

surface, with the presence of f i ne surfac e cracks Indicating i nci pien t

spa l la t ion . S t r i a t io ns a l so v is ib l e on the sur face suggest tha t abra-

sio n by embedded de br is par ti cl es was a l so contribut ory. Under sat ur-

a ted a i r conditions, wear t o the untrea ted surface was 3 t o 4 t imes the

bas eli ne. Again, ver y heavy metallic t r ans fe r to the ca rbur ized sur face

occurred. I t is proposed t hat under f re tt in g conditio ns the hard car-

burized mat eria l could more rea dil y displ ace the prot ect ive surf9:e

oxides in the presence of moisture, and promote more extensive adhesion

between t he su rfac es than took place under dry a i r co ndi ti i~ ns. The un-

tr ea te d ma te ri al , be inb th e weaker of th e two, would then be expected

t o t r ans fe r t o the ca rbur ized sur face .

The results obtained with nitr ided 9310 s t ee l i n contac t with un-

treated 9310 s teel al so showed transf er of mater ia l t o the ni t r ide d s u r -

f a c e i n a l l cases wheu fr et ti ng was conducted in dry a i r . Wear t o t he

untrea ted surface was s l ig ht ly higher than the dry a i r base l ine . Under

sa tura ted a i r condit ions however, t ransfer t o the ni t r ide d surface was



seen about 50 percent of th e t h e , wfth wear occ urr ing otherPsloe. Fret-

ting wear to the unrreated surface i n saturated a i r was much lower than

the corresponding baselbe.

Low wear t o t he hard c h r d t ? ? l a te was observed after fret t ing in

dry a i r , and some tr an sf er of 9312 s t e e l to the chromlllsl pla ted sur fac e

took place. Wear t o the uncoated s ur fa ce was slightly higher than the

dry a i r baseline. When f r e t t h g was c d u c t e d i n saturated air however,

the chroeaium plate was observed t o w e a r , aad wear t o the uncoated sur-

face was reduced. The f r e tt ed chrmnillsl p la te showed a nearly featureless

surface, while oa the mating 9310, "Islandsn of transferred c h r d u a

could be seen (fig. 9) . It is hypothesized that the traasferred chram-

1- wa s In an d i z e d form, and be- embedded in the uncoated 9310

steel surfac e, promoted a 2-body abra sive a c t i m t o the chromitea plate.

Under dry a i r f re tt i ng conditions, Ag plate performed as a eacri-

f i c i a l coating, i t se lf undergo- wear while the opposing uncoated sur-

fa ce showed conside rably reduced f r e t t b g damage compared t o t he base-

l ine. SEn and x-ray dispers ion analys is indicated t ha t a th io, non-

uniform deposit of Ag was present on the uncoated 9310 steel surface

afcer f re tt in g In dry a i r a s may be seen in f lgur e 10. Frett ing under

a moisture satu rated a i r environment did not re su lt in a s ignificant

difference in th e performance of th e pl at e; it s t i l l worked a s a

sa c ri fi c ia l coating, but more wear t o the uncoated 9310 st e e l surface

was seen than under dry a i r condit ions. Considerably gr ea te r experi-

mental scatter occurred.



Like the Ag plate, the altlsliwrm bronze -polyester performed a s a

sa cr i f i c i a l coating with results similar t o those described in refer-

ence 14 f o r the f r e t t i n g of Ti-6Al-4V. W e a r t o t he uncoated 9310

s t e e l su r face was reduced by nearly an order of amgnitude ia both dry

iad saturated air. 14icroscopic exam hation of t h e uncoated surf ace

(fig. 11) revealed that a th in adherent fi lm a c c d a t e d most of the

fre t t in g ac t ion . Based on x-ray disp ers ion cmalysls of th e f r e t te d re-

e o n on t he uncoated specimen, it was concluded (by elhlnatlon) that

the adherent film nust be mostly polyester. The f re tt in g damage fea-

tur es vis ib le on ei th er surf ace were of a benign nature-absent were

fa t igue spa l l s and surface cracks.

The polylmlde coa ting (evalua ted ag ai as t Ti-6Al-4V I n ref . 15)

resul ted An marginally reduced wear t o t he uncoated AISI 9310 steel

mating surface, but i t se lf a sa c ri fi c ia l coating, underwent rat her rapid

wear. Microscopic d n a t i o n of t he f r e t t ed sur faces r evea led ox id ized

debris embedded in th e polyLmide coa ting , and evidence of abras ive wear

t o *h e uncoated surface. I t wa s thus concluded that the predominant

we a r laechanism t o t he uncoated s ur fa ce was 2-body abrasion, with gradua l

disin tegr ation of the coating as i t became overloaded with debris.

mcLUs1ms

From the fretting studies conducted on 93-0 ste el , and the assess-

ment of s ev er al candidate "anti-f ret " coating s applied t o one of the

mating surfaces, the following conclusious ar e d r m :

61. Up to a t l eas t 10 cycles, the f re t t in g of AISI 9310 s t e e l i s

dominated by a surface spallation mechanism caused by localized fatigue

in the contact area.



2. The presence of moisture signif icantly increases the f re t t ing

wear of 9310 steel, and alters the d is t r ib ut ion of f r e t t i ng debr is .

3. From th e standpoint of reduced fr e t t i ng w e a r t o t he uncoated

surface the best coatings were si l v er pla te, and aluminum bronze with

polyester, each resu lt in g in roughly an ord er of magnitude reductio n i n

wear t o the uncoated mating surface under a l l test conditions.

4. Chraniu~i l a te , i t s e l f t h e -st f r e t t i n g wear res i s tan t of the

coa tla gs evaluated, markedly reduced fr et ti n g wear t o th e uncoated mating

surface in saturated air , with no significant change in wear t o the un-

coated surface i n dry air.

rtEmmxEs

1. Valtierra, M. L., P a r k d s , A., axad Ju, P. H., "Spline Wear i n Je t

Puel Environment," Lubri. Eng. 31, 136 (1975).

2. Sibley, L. B., "Discussion of Paper: Fr et ti ng i n Ai rc raf t Turbine

Engines," AGR#Kl CP-161, Advisory Group fo r Aerospace Research and

Development, 5-14 (1974).

3. Goss, G. L. and Hoeppner, D. W. , "Characterization of F ret tin g Fatigue

Damage by SEU Analysis," Wear 24, 77 (1973).

4. B i l l , R. C., " F r e t t i q Wear i n Titanium, bn el -4 00 , and Cobalt-25-

Percent Molybdenum Using Scanning Electron Microscopy," NASA TN

D-6660 (1972).

5. Balliday, J. S., and airst, W. , "The Fr et ti ng Corrosion of Mild Steel ,"

Proc. ROY. SOC. (London) 236A, 411 (1956).

6. Waterhow, R. B., "F re tt in g Corroeion," Pergamon Pr es s Ltd ., Oxford,

113 (1972).



7. Waterhouse, R. B., "Fretting Corrosion," Pergamon Press Ltd, Oxford,

141 (1972).

8. Nishioka, K., and Eayakawa, K., "Fundamental Investigation of Fretting

Fatigue (Part 5, The Effect of Relative Slip Amplitude)," Bull. JSfE

12, 692 (1969).

9. Feng, I., and Uhlig, 8. H., "Fretting Corrosion of Mild Steel in

Air and Nitrogen," J. Appl. Hech. 21, 395 (1954).

10. Waterhouse, R. B., "Fretting Corrosion," Pergamon Press Ltd., Ogford,

126 (1972).

11. Hurricks, P. L., "The Mechanism of Fretting - A Review," Wear 15,

384 (1970).

12. Nishioka, K., and Hayakawa, K., ''Futadanmtal Investigations of Fretting

Fatigue (Part 3, Some Phenomena and Mechanisms of Surface Cracks),"

Bull. JSHE 12, 397 (1969).

13. Waterhouse. R. B., and Taylor, D. E., "Fretting Debris and Delamina-

tion Theory of Wear," Wear 29, 337 (1974).

14. Bill, R. C., "Selected Ftetting Wear Resistant Coatlrngs for Titanium-

+Percent Aluminum - 4-Percent Vanadium Alloy," NASA TN D-8214 (1976).

15. Fusaro, R. L., and Sliney, 8. E., "Graphite Fluoride as a Solid

Lubricant in a Polyimide Binder I' NASA TN D-6714 (1972).

16. Jones, W. R,, Jr., Hady, W, F., and Johnson, R. L., "Friction and

Wear of Poly (amide-imida), Polyimide, and Pyrrhone Polymers at

260" C in Dry Air," NASA TN D-6353 (1971).



TABLE I . - ~ O S I T I O N F AISI 9310 STKEL

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Fretting & Surface Treatments