1997 wear 210 fretting oil kalin

8/12/2019 1997 Wear 210 Fretting Oil Kalin

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W E RE L S E V I E R W e a r 2 10 ( 1 9 9 7 ) 2 7 - 3 8

W e a r m e c h a n i s m s in o i l l u b r i c a te d a n d d ry fr e tt in g o f s i l i c o n n i tr id eaga ins t bear ing s t ee l contac t s

M . K a l i n a , .I. V i ~ i n t i n * , S . N o v ~ k b , G . D ra W l 6 bUniversi ty of Ljubl jana, Fa cul ty of Mec hanical Engineering, Bo~i~i t~ 'era 8, IO00 Ljubl jana. Slovenia

b Jo~e fSte f im Inst i tute . Jamo va 39, I000 I .j .~pl jana, SloveniaRece ived I I June 1996: r evised 24 Ma rch 1997: accepted 7 Ap r i l 1997

gb s t rac tThe w ear and fr iction behavio ur of s ilicon nitr ide against bearing steel was investigated under lubricated and dry freuing conditio ns as afunctio n of amplitude and test duration. Tests were perform ed on a high frequency fretting tester . Silicon nitr ide bearing balls were used asthe upper oscillating specimen s while the lowe r s tationary flats were s tandard specimens of bearing steel. Amplitude s in the intermediate 5 to

50 p,m range and a test duration from l0 to 360 m in were s tudied. In lubricated cond itions a commercial lubricant, ISO VG 220 . was used.Light m icroscopy , scanning e lec tron microscopy ( SEM ) , energy d ispers ive spec troscopy ( EDS ) , Auger spec troscopy (AE S) andtransmiss ione lec tron microscopy (TEM ) were employed to de te rmine the wear mechanisms .

Under lubr ica ted condi t ions t r ansi t ion f rom h igh to low wear vo lumes was recognised with inc reas ing ampli tude . At lower ampli tudes andin ~e ea r ly s tage of f r e tt ing tes ts a t modera te am pli tudes , mechanica l wear domina ted . Cracks on the s t ick - s l ip boundary and spa l l ing of athin tr ibolay er was observed . Un der these condition s the highest wear in lu bricated fretting was obtained . In the f inal s tage of fretting tests atmodera te ampli tudes , and f rom the beginning a t h igher ampli tudes, t r ibochemica i wear is sugges ted as the dom inant wear form. A 0 .2 l tmth ick t r ibo layer was observed on the contac t , conta in ing inc lus ions with d if fe ren t Fe and Si conten ts . A very h igh concentra t ion of ca rbon ,formed by oil de gradation, was also determ ined in this layer , confirm ing the cr itical inf luence of oil on the wea r behaviour .

Quite a d if fe ren t wear mechanism is p roposed for d ry f re t t ing condi t ions . Resu l ts o f A ES ana lys is showed a layer an order of magni tudeth icker than in lubr ica ted f re tt ing , a lso having a r emarkably d if fe ren t chemica l compos i t ion . TEM ana lys is conf irmed tha t the reac t ion layerconsisted of a s ilica-r ich amorpho us phase conta ining small inclusions of Fe2 0, and Fe~O.~. In contrast to lubricated conditions, where thelayer created was ductile , in the case of dry fretting the layer was br ittle . The continu ous process of forming and spalling the br ittle tr ibolaye rcaused much h igher wear r a tes and wear losses than under lubr ica ted f re t t ing condi t ions . No t r ans i t ion in w ear behavionr was observed aswas the case in lubricated fretting. 1997 Elsevie r Science S.A.Ke~a,'ords: F r e t t ing w e a r : S i l i c on n i t ri de : B e a r i ng s t e e l : C e r a mi c s : O i l : L ubr i c a t e d c ond i t i ons : D r y c ond i t i ons

1 I n t r o d u c t i o n

H y b r i d c e r a m i c b e a r i n g s a r e f i n d i n g i n c r e a se d a p p l i c a t i o n si n m a c h i n e t o o l s p i n d l e s a n d a e r o s p a c e v e h i c l e s . T h e y h a v es t e e l r i n g s a s s e m b l e d w i t h c e r a m i c b a l l s , H o t i s o s t a t i c a l l yp r e s s e d ~ , il ic on n i t d d e w a s f o u n d t o b e t h e m o s t a d v a n t a g e o u sm a t e r i a l f o r t h e c e r a m i c r o l l i n g e l e m e n t s . T h e a d v a n t a g e s o fh y b r i d b e a r i n g s a r e m o s t p r o n o u n c e d i n h i g h - s p e e d a p p l i c a -t i o n s b e c a u s e o f r e d u c e d c e n t r i f u g a l f o r c e , s i n c e t h e d e n s i t yo f c e r a m i c b a l l s i s 6 0 l o w e r t h a n t h a t o f st e e l b a l l s, i t i sk n o w n t h a t t h e s t r e n g t h a n d c o r r o s i o n r e s i s ta n c e o f c e r a m i c

* C or r e s pond i ng a u t ho r . T e l . : +3 86 61 125 9200 : f a x : +3 86 61 125188.

a P r e s e n t e d a t t he 11 th I n t e r na t i ona l C onf e r e nc e on W e a r o f M a t e r ia l s .S a n D i e go , C A , U S A , 20 - 23 A pr i l 1997,0043 - 1648 / 97 / 17 . 00 1997 E l s e v i e r S c i e nc e S .A . A l l r i gh t s r e s e rve dP i l S 0 0 4 3 - 1 6 4 8 ( 9 7 ) 0 0 0 8 2 - 3

m a t e r i a l s i s m a i n t a i n e d o v e r a m u c h g r e a t e r t e m p e r a t u r er a n g e t h a n f o r t y p i c a l b e a t i n g s t e el . T h i s a l l o w s h y b r i d b e a r -i n g s t o o p e r a t e a t o v e r 5 0 0 C , a n d a l l - c e r a m i c b e a r i n g s a to v e r 1 0 0 0 C [ I - 3 ] , w h i l e t h e l i m i t i n g t e m p e r a t u r e f o r c o n -v e n t i o n a l b e a r i n g s i s s i g n i f i c a n t l y l o w e r . M a n y r o l l i n g b e a r-i n g s a r e s u b j e c t e d t o m a r g i n a l l u b r i c a t i o n w h i c h c a u s e si n t e r a c t i o n s o f t h e a s p e r i t ie s o n m a t i n g s u r f ac e s , l e a d i n g t om i c r o p l a s t i c d e f o r m a t i o n , m i c r o p i t t i n g a n d f i n a l ly b e a r i n gf a i l u r e [ 4 1 . C e r a m i c s h a v e a h i g h y i e l d s t r en g t h a n d c a np r e v e n t s e v e r e d a m a g e o n r o l l i n g s u r f ac e s w i t h m a r g i n a ll u b r i c a t i o n o r i n o t h e r c a s e s o f l u b r i c a n t s t a rv a t i o n .

O n e o f t h e w e a r f o r m s w h i c h s i g n i f i c a n t l y l i m i t s t h e o p e r -a t i n g l i f e o f r o l l i n g - e l e m e n t b e a r i n g s a n d o t h e r m a c h i n e c o m -p o n e n t s a n d s t r u ct u r e s i s a l s o f r e tt i n g w e a r . i n r e c e n t d e c a d e ss t u d i e s o f d i f f e r e n t as p e c t s o f t h e s u b j e c t h a v e b e e n m a d e .b u t q u i t e d i f f e r e n t a n d i n c o m p a t i b l e r e s u l t s h a v e b e e n


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28 M. Ka l in et al . / Wear 210 (1997) 27-3 8obtained. The main reasons for the lack of a s t r ingent defi-n i t ion o f fret t ing are the comp lexi ty of the process i tself , theeffects of the prevai l ing con tact condi t ions , the m aterials incontact and the surrounding environment . Although ceram ic-metal contacts are o f great in terest and o f great potential forthe fu ture, the l i terature avai lable on fret t ing wear for suchmaterial combinat ions is s t i l l very l imited 15-7]. We havenot tbund any publ ished paper about o i l - lubricated fret t ingfor s teel--ceramics contacts . O n the o ther hand, more s tudiesof ceram ic-s teel pairs were made under various s lid ing con-d i t ions [8 -171 .

Engineering ceramics have been t radi t ional ly consideredto be part icularly res is tant to chemical wear. How ever, i t i sbecom ing clear that the in teractions of the ceram ic with envi-ronme nt and the countefface p lays a cri tical ro le in determin-ing wear perform ance [ 18-261. A tr ibochemical react ioninvolving Si3N4 was fi rst reported by Fischer and Tom izaw a[ 181 fo r s l id ing of s i l icon ni t r ide ag ainst i tself under ambien tcondi t ions . According to these researchers , the react ioninvolve s oxidation of Si3N4 to an am orph ous hydra ted SIC),_,resul t ing in a s ignificantly low er fr iction coefficient and wearrate. Under dry condi t ions , the wea r occurs by m icrofracture,while at moderate levels of humidi ty i t occurs by the cumu-lat ive effect of microfracture and t r ibochem ical in teract ion .Gee and B utterfield [ 191 reported the formation o f s i l iconoxynitr ide in addi t ion to hydrated s i l icon oxide in bal l -on-ring s l id ing wear tes ts . They claim ed that the wea r of s i liconni tr ide great ly depends on s l id ing speed s ince the amount ofheat generated and hence the temperature at the in terface isd irect ly related to the s l id ing speed. The tem perature gener-ated in the contact is more or less an es t imation, based oncalculat ions 127-301 or experimental evidence [30--321,s ince in both cases many problems are s t i l l unsolved. Forexam ple, Griff ioen et at . [ 31 ] m easured tem per atu~ s as h ighas 2700 "12 in a s i l icon n i t r ide-sapphire s l id ing system. O nthe o ther hand, Weick e t al . [321 detected a surface temper-ature r ise of only 130-140 K in unlubrieated fret t ing contactsof d ifferent ceramic materials , which agreed well also witht~ ir theoret ical calculat ions , but the serious problem of deter-mining the teal contact are a was pointed out . In the researchof Ishigaki et al . [ 33] a t r ibological ly induced phase t rans-form ation of x-Si3Na to ~-S i3N 4, which normally occurs at1550 C, was reported . The y sug gest the temperature as thefactor responsible for the t ransformation. Don g and Jahanmir[ 231 class if ied the wea r process in to d ifferent regions , char-aeterised by certain levels of fr ict ion and wear rates as afunct ion of load and temperatures between 23 C and 1000C. They recognised microfracmre (high loads), tribochem-ical react ion ( 23-40 0 *C), select ive oxidat ion (400--700 C ),formation of crystal l ine precipi tates (700 -90 0 C) and oxi-dat ion (above 900 (2) as the control l ing wear mechanismsin dry re ciproca ting sliding o f Si.~Na against itself. Also , awear mechanism transi t ion zone was reported as the load ortemperature was varied . Evident ly , temperatures p lay animportant ro le in the w ear behav iour of s i l icon ni t r ide, espe-cial ly at h igher values where chem ical processes are involved.

Similarly , in s l id ing tes ts of ceramics against metals , t t i -boehemical interactions have been reported either with thea tmosphere o r wi th the mat ing meta ls [7 - 15 ] . A speed - andtemperature-dependence of react ion products and their effecton were" behaviour has been recognised, but w ith contradic-tory resul ts . I t w as reported that increasing speed could causelower [8 ,161 or h igher [9 ,141 w ear rates . Also , the effect oftemperature and creat ion o f a react ion layer could cause lowerwea r rates [ 5 ,6 ,11,16,34 ]; on the o ther hand, in refs . [ 3 ,14 ]h igher wea r was obtained. In some s tudies 11,341 lowerwear rates were obtained due to softening of the layer ath igher temperatures . According to the es t imated high tem-perature s at the tribocontac t [ 8,141, in lubricated contac t thereact ion of s i l icon ni t r ide and s teel could al~ be expected .Nam ely, the onset temperature of react ion between Si.~Naandiron was reported to be 1095 (2 [241, and F e(S i) so l idsolution w as found to be present above th is temperature [ 25 l .In al l o i l - lubricated tes ts considerably lower wear wasobtained [8-10,16] than in dry tes ts , but the detai led mech-anisms of the wear were not in troduced. The t r ibochemicalwear mechan i sm was o f ten accompan ied by mater ia l t rans fe rand adhesion, most ly from softer s teel to the ceramic speci-men [ 16 ,17 ] . These f i lms / l ayers w ere somet imes re fe r red toas protect ive and reduced wear [9 , 31 , but sometimes causedhigher wea r [ 10 ,17]. In sum mary , the presence o f tr iboch-emical f i lms on ceram ic and m etals under s l id ing or fret t ingcondit ions were reported m any t imes. Although their s t rongdependence on environmental and operat ing condi t ions wasconfirmed, detai ls of the mec hanism s and chemical react ionsare s t i ll not unders tood. T he s i tuat ion is complicated by theman y influent ial param eters and variat ions in tes t condi t ions .The purpose of our work was evaluat ion of the fret t ingwear behaviour of the d ifferent material combinat ions usedin bal l bearing appl icat ions , under the same tes t ing condi-t ions . In previous papers [35,361 we have reported on theeffect of the ampli tude and tes t t ime on the fret t ing wear ofsilicon nitdde against silicon nitride and bearing steel againstbearing s teel contacts in o i l - lubricated and dry condi t ions .The p resen t work w as focused on the f re tt ing wear mecha-nisms in s i l icon ni t r ide against bearing s teel contacts und erthe same condit ions as were used in our previousinvestigations.

2 . E x p e r i m e n t a l2 . . O p e r a t h l g p r o c e d u r e s

Experimen ts were done in a comm ercial ly avai lable h igh-frequency fret t ing mac hine. A detai led descrip t ion of thetes ter has been reported elsewh ere [ 35 ] . B asical ly i t consis tsof a s tat ionary base, a holde r which is driven in such a wayas to produce a l inear osci l lat ing motion, a loading cel l and acomputer-base d regulat ion system. The low er, f lat specimensmade from beat ing s teel were f ixed in the base, while theupper specimens, i .e. silicon nitride balls, were fixed in the


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M. Kalin et al. / Wear 210 1997 t 27-3:; 2 9

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I - 88 hi p = 2 4 0 0 M P a )........... ~ 7 0 0 HV p o l i s h e d )I s s0~ lv

1Q, ~ ' .A_ 0.054. . . . .m .~ ~ : . Lubricant SO VG 220, n 1 , i' , 0 ,2632m m

Fig. I . Sch ematic d iagram of the test condit ions.os c i l l a t ing ho lde r . T he re la t ive hu mid i ty o f the a i r was 50- -60 ( amb ien t cond i t ions ) .A norma l fo rce o f 88 N was app l i ed th rough the load ingce l l a t each con tac t . T he f requency o f os c i ll a t ion was 21 0 Hz .in o rde r to de te rmine the e f fec t o f a pa r t i cu la r ampl i tude onwear behav iour , the ampl i tude was inc reas ed a t f ive l eve l sbe tween 5 p , m and 50 p . m. At each ampl i tude t e s t s weres topped 6 t im es in the range f rom 10 to 360 min . T he cor re -spon ding nu m ber o f cycl es wa s 0.1 10~ '-4.5 x i0 ~' and therange o f s l id ing d i s t ances 2 -900 m. E ve ry t ime the t e s t wass topped , a new pa i r o f s pec im ens w as us ed and a new te s twas pe r fo rm ed . T es t s w ere repea ted 3 - -4 t imes a t each ampl i -tude and t e s t dura t ion to ens ure repea tab i l i ty o f the re s u l t s .Due to the cons tan t f requency o f os c il l a t ion , s l id ing ve loc i tyinc reas es wi th inc reas ing ampl i tude f rom 0 . 0042 m s -~ to0 . 04 2 m s - I .

Be fore the t e s t . s pec imens were u l t ra s on ica l ly c l eaned inca rbon t e t rach lo r ide . A s ma l l amo unt o f o i l was s p read onthe s u r face o f the f l a t s pec imen in lubr ica ted cond i t ions . Du r-ing the t e s t . no add i t iona l o i l was s upp l i ed on the con tac t .Af te r the t e s t . t he s pec im ens w ere c leaned to remo ve re s idua lo i l and worn pa r t i c le s . A s chem at ic d iagram of the t e s t con-d i t ions i s s hown in F ig . I .

2.2. SpecimensT h e m a t e ri a l o f t h e f la t s p e c im e n s w a s A I S 1 5 2 1 0 0 b e a t i n g

s tee l . S pec imens were mach ined f rom rod s tock and werehea t t rea ted to a ha rdnes s o f approx ima te ly 850 HV. T hes ur face was g round to a roughnes s R . , =O. 05 p . m andRm.~ = 0 . 44 p , m. Dimens ions o f the s pec imens w ere 0 2 4 7 m m .

Ceramic s pec imens were s t anda rd bea r ing ba l l s wi th ad iame te r o f 9 . 525 mm hav ing a h igh ly po l i s hed s u r face f in i s h .No ev idence o f po l i s h ing s c ra tches o r poros i ty was found onthe surface . The materia l was hot isos ta t ica l ly pressed S i~N.~( N B D - 1 0 0 o b t a i n e d f r o m C e r b e c C o r p ., E a s t G r a n b y , C T ) .

T he g ra in s i ze o f s i l icon n i tdd e w as abou t I p .m, i t s ha rdnes swas 1700 HV and f rac tu re toughnes s , K , c , 5 . 4 M P a m '/2 .In lubr ica ted t e s t cond i t ions commerc ia l o i l IS O VG 220wi th a v i s cos i ty index o f 85 w as us ed .2 .3 . Methods o f ana lysi s

T he worn s u r faces o f the f l a t and the ba l i we re f i r s t exam -ined unde r a l igh t mic ros cope . Diam e te rs o f we a r . sca rs ind i rec t ions pa ra l l e l and pe rpend icu la r to the s l id ing d i rec tionwere m eas ured . T hey were a lm os t o f the s ame s i ze , since thed i s p lacement ampl i tude was ve ry s ma l l . Wear vo lume , i . e .to ta l wea r loss f rom the ba l l and the f l a t , was ca lcu la ted f romthe equa t ion p ropos ed b y Kla f fke | 51 (R : rad ius o f the ba ll :d t : d i ame te r o f the we a r . sca r; A: amp l i tude ) :w=T rd~, d~ 2A2d,8 8R 3 8RS ince the ma t ing ma te r i a l s in con tac t have qu i t e d i f fe ren tmechan ica l p rope r t i e s, on s om e coup le s w ea r los s was de te r -mined s epa ra te ly on the ba l l and on the f l a t , u s ing p ro f i lo -me te . Res u l t s s howed tha t the wea r on s t ee l f l a t s was 2 -3, imes g rea te r than tha t o f the ce ram ic ba l l s fo r a ll ampl i tudesus ed . Neve r the le s s , the to tal w ea r vo lum e o f a pa i r ag reedve ry we l l wi th the to ta l wea r vo lum e ca lcu la ted by the p ro -pos ed equa t ion . Di f fe rences were be tween 5 and 20 .B e c a u se o f t h e l a r g e n u m b e r o f s p e c im e n s a n d t h e v e r y t i m e -cons uming p ro f i lome t ry p rocedure , to ta l wea r los s was ca l -cu la ted by the equa t ion and i s p re s en ted a s an ave rage o f a l lrep re s en ta t ive re s u l t s f rom the t e s t s unde r the s ame cond i t ionsand tes t durat ion.

In o rde r to de te rmine how the na tu re o f the wea r p roces sdepends on the t e s t t ime and ampl i tude o f os c i l l a t ion , wea rs ca rs were examined us ing s cann ing e lec t ron mic ros copy( S E M ) a n d e n e r g y d i s p e r si v e s p e c t ro s c o p y ( E D S ) . O n s o m es p e c im e n s A u g e r e l e c tr o n s p e c t r o sc o p y ( A E S ) a n d t ra n s -m i s s io n e l e ct r o n m i c r o s c o p y ( T E M ) w e r e e m p l o y e d t o c o n -f i rm s ome hypo thes es tha t were s ugges ted by p rev ious


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Amplitude Im ]F i g . 2 . W e a r r a t e o f b a l l a n d f l at s p e c i m e n s a s a f u n c t i o n o f t i m e f o r d i ff e r e n t a m p l i t u d e s o f o ~ i l l a t i o n u n d e r ( a ) l u b r ic a t e d c o n d i ti o n s , ( b ) d r y c o n d i t i o n s .W e a r l o s s o f b a ll a n d f l at s p e c i m e n s a s a f u n c t i o n o f a m p l i t u d e u n d e r ( c ) l u b r i c a te d c o n d i t io n s . ( d ) d r y c o n d i t i o n s .a n a l y s e s ( S E M , E D S ) . T h e a r e a o f A E S a n a l y s i s w a s ap p r o x -ima te ly 100 ~ m 2 .3 . R e s u l t s a n d d i s c u ss i o n3 1 W e a r r a t e a s a f u n c t i o n o f t e s t t i m e

In F ig . 2 ( a ) i t c an be s een tha t the h ighes t w ea r ra t e inlubr ica ted cond i t ions was ob ta ined : 'n the ea r ly s t age o f f re t-r ing . Wi th fu r the r f re t t ing the wea r ra t e dec reas ed . Af te r ace r t a in pe r iod the we a r ra t e reached a s t eady-s ta te va lue andbecame nea r ly cons tan t .

F i g . 2 ( b ) s h o w s w e a r r a te a s a f u n ct i o n o f t i m e i n d r yf reu ing cond i t ions . A rap id inc reas e o f wea r ra t e can beobs e rv ed a t ~ be beg inn in g o f the t e s t . Af te r the in i t ia l pe r iod ,s t ab le cond i t ions were ob ta ined and the wea r ra t e rema ineda lmos t co ns tan t to the end o f the te s t a t al l ampl i tudes . T h i swas a l s o the ca s e in lubr ica ted cond i t ions , bu t the s t age o funs e t t l ed wea r ra t e l a s t ed fo r a longe r pe r iod than in d ryf re t t ing . I t i s ev iden t tha t the va lue o f the wea r ra t e w as mu chh ighe r in d ry than in lubr ica ted cond i t ions .3 2 W e a r l o s s a s a f i m c t i o n o f a m p l i t u d e

C o m p a r i s o n b e t w e e n t h e d i a g r a m s o f w e a r l o s s a s af u n c ti o n o f a m p l i t u d e f o r lu b r i c at e d ( F i g . 2 ( c ) ) a n d d r y(F ig . 2 (d ) ) cond i t ions ind ica te s s ign i f i can t ly d i f fe ren t wea rbehav iour . W hi le the wea r los s inc reas ed a lmos t l inea r ly wi thinc reas ing ampl i tude in d ry cond i t ions , obv ious peak v a lues

be tween 20 ~ m and 30 Ixm amp l i tude were de tec ted in lubr i -ca ted f re t t ing . L ow er and h ighe r ampl i tudes re s u l t ed in lowe rw e~ los s . Als o , i t mus t be po in ted ou t tha t the wea r los s ind r y c o n d i t io n s i s a b o u t t w o o r d e r s o f m a g n i t u d e h i g h e r t h a nin lubr ica ted cond i t ions . App a ren t ly no t on ly the p re s ence o fo i l bu t a l s o the ampl i tude o f os c i l l a t ion a f fec t s the wea rbehav iou r in s e lec ted cond i t ions . S imi la r wea r behav iou r wasa l s o obs e rved in a p rev ious inves t iga t ion | 35 ] w here f re t t ingwea r in d ry and lubr ica ted S i3N4 aga ins t S i3N4 con tac t s w ass tud ied unde r the s ame con d i t ions a s were us ed in the p re s en twork .

F ur the r ana lys es were pe r fo rmed to e luc ida te the wea rmechan i s ms and p roces s es tha t were dominan t fo r s e lec tedma te r i a l combina t ion and t e s t cond i t ions .3 3 S EA 1 a n d E D S o b s e r v a t i o n s3 3 I L u b r i c a t e d c o n d i t i o n s

F ig . 3 (a ) s how s the wea r s ca r on a ce ramic ba l l a f t e r 360ra in o f f re t ting a t 5 p . m ampl i tude . T he typ ica l appea rance o fs t i ck -s l ip co nd i t ions can be obs e rved . W hi le in the sl ip reg ionc racks o r i en ted pe rpend icu la r to the s l id ing d i rec t ions a reev iden t , the re i s a lmos t no damage in the s t i ck zone . T hes ame wea r p roces s was p re s en t on the s t ee l p la te . A de ta i lf rom the s l ip reg ion on the s t ee l wi th c racks i s s hown inF i g . 3 ( b ) . A t a n e a r l ie r s t a g e o f f re u i n g a t t h e s a m e a m p l i -tude , s imi la r wea r behav iour w as de te rmined , excep t tha t thenumber and the ex ten t o f c racks were s ma l le r . T he re fo re ,


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M. Kalin et a l . / Wea r 210 t 1997 27 -38 31

Fig. 3. SEM microg rams of the w ear scars created in lubrica,.ed *, rettingafter 360 min at 5 Ixm amp litude on ( a) Si3N4 ball, (b ) steel plate.m e c h a n i c a l w e a r i n t h e f o rm o f c r a c k s w a s t h e d o m i n a n t w e a rm e c h a n i s m a t th i s a m p l it u d e .

A f t e r o n l y 1 0 ra i n o f t e s ti n g a t 2 5 p . m a m p l i t u d e , a t ri b o -l a y e r w a s o b s e r v e d o n b o th m a t e ri a l s. F i g . 4 ( a ) a n d ( b ) .A c c o r d i n g t o E D S a n a l y s i s it w a s c o m p o s e d m a i n l y o f Si an dF e . F r o m F i g . 4 ( a ) a n d ( b ) i t c a n b e s e en t h at th e l a y er w a sb r i t t l e a n d s p a l l e d d u r i n g t h e p r o c e s s , w h i c h i m p l i e s a r e l a -t i v e l y i n t e n s e w e a r lo s s . T h i s o b s e r v a t i o n a l s o a g r e e s w i t h

t h e h i g h w e a r r a t e i n th e e a r l y s t a g e o f l u b r i c a t e d f r e t t in g a t2 5 p ,m a m p l i t u d e , p l o t t e d i n F i g . 2 ( a ) . T w o d i f f e r e n t m e c h -a n i s m s o f f o r m a t i o n o f t h e t r ib o l a y e r c o u l d b e s u g g e s t e d : ( 1 )s i m p l e t r a n s f e r o f t h e m a t e r i a l a n d ( 2 ) t r i b o c h e m i c a l r e a c t i o nb e t w e e n t h e c e r a m i c a n d m e t a l , i f t h e t r a n s f e r o f t h e m a t e r i a lw a s t h e m e c h a n i s m , t h e l a y e r o n t h e c e r a m i c b a l l w o u l dc o n t a i n n o s i l i c o n , o r c e r a m i c m a t e r i a l w o u l d b e p r e s e n t a st h e f r a g m e n t s i m p r e s s e d i n t o t h e s te e l , b e c a u s e o f t h e m u c h

~i i i i i i i : :i : i i : : : : : : :

Fig. 4. SEM microg raphs of the layers created in lubricated fretting after 10 min at 25 p.m amplitude on (a ) Si~N.~ball . (b) steel plate . SEM micrographs ofthe reaction layers crealed in lubricated fretting after 360 rain at 25 p.m amplitude on (c) Si,N4 ball, (d) steel plate.


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32 M. Kalin et a l / Wear 210 1997) 27-38

~ ~ . . i . , .

. . . .. . i iI I I I I I Ii , ll ::I:I:: : i i : ii i iii i ii i i i i : i i : : i i i i i i i i i i

Fi g . 5 . SE M m i c r o g r a p ~ o f l h e l a y e r s c r e a t e d i n l u b r i c a te d fr e t t i n g a f t e r I 0 r a i n a t 5 0 I tm a m p l i t u d e o n a ) S i g N 4 b a l l , b ) s t e e l p l a t e . SE M m i c r o g r a p h s o ft h e l a y e r s c r e a t e d i n l u b r i c a t e d f r e t t in g a ft e r 3 6 0 m i n a t 5 0 I ~m a m p l i t u d e o n c ) S i ~4 b a l l , d ) s t e e l p l a t e .

highe r ha rdness and b r i t tl eness o f ce ramics . S ince the l aye rexh ib i t s a homogeneous composi t i on con ta in ing i ron ands i l i con , t he second mechan i sm seems to be more p robab le .The mos t i n f luen t ia l wea r m echan i sm a t t he g iven condi t i onswas the fo rma t ion and spa l l ing o f t he l aye r i n t he con tac t.

Af t e r 360 min o f f re t t ing a t 25 Ixm ampl i tude a spec i f i ct r i bo l aye r was fo rmed ove r the wea r sca r on the worn ce ramicsur face , shown in F ig. 4 ( c ) . As can be seen , t he l aye r con-ra ined whi t e i nc lus ions . Accord ing to EDS ana lys i s ( seespec t ra a tt ached to F ig . 4 (c ) ) t he whi t e pa r t i c le s con ta ineda h igh co ncen t ra t i on o f i ron , i n con t ra s t t o t he da rk ma t r ixwhich co ns i s t ed o f on ly a s i l i con-based phase w i th no i ron .Thi s suppor t s t he hypo thes i s t ha t t he whi t e pa r t ic l e s a re chem-ical react ion pro ducts, s ince stee l wear par t ic les could not beemb edded in t he ce ramic due to t he i r mechan ica l p rope r t ie s .The l aye r on the s t ee l wea r sca r showed s igns o f smear ingand p l a s t ic de forma t ion , imply ing the duc t i le cha rac t e r o f t hel aye r , a s seen in F ig . 4 ( d) . Th i s i s qu i t e d i f fe ren t in appea r -ance from the ear ly stage of the test . Bri t t leness of the t r ibo-l aye r a t t he beg inn ing o f f re t t i ng and more d uc t i le behav iourat the f inal stage suggest a t ransi t ion in wea r behavio ur fromseve re mechan ica l wea r w i th a h igh wea r ra t e t o t he more'p l a s t i c - type ' o f wea r , w hich l ed to a l ow er wea r ra t e i n t hefinal stage.

Acco rding to the observ at ions i l lust ra ted in Fig. 4 , weassume tha t t he con tac t cond i t i ons i n t he l a t e r s t age were

seve re enough to p romote t r i bochemica l r eac t ion be tween thema te r i a l s i nvo lved . We suppose tha t t he duc t i le l aye r fo rmedin the con tac t r educed the wea r ra t e due to i t s lower v i scos i ty .Tr ibo laye rs t ha t r educed wea r ra t e s we~ obse rved a l so i ns l i d ing t e st s o f ce ramic -s t ee l [8 ,11 ,13] o r ce ramic -ce ra miccouples [19-22] , bo th i n l ubr i ca t ed o r d ry cond i t i ons .K la f fke [5 ,6 ] r ecogni sed tha t " the fo rma t ion o f su r face l ay-e r s can change the t r i bo log ica l beha v iour d ra s t i ca lly and i sin f luenced by seve ra l pa rame te r s o f t he t e s t cond i t i ons " . Hesugges t ed tha t , i n a more re f ined mode l o f wea r behav iour ,chemica l r eac tion , spo t t empera tu re s , t he rma l conduc t iv i ty ,and phase t r ansforma t ions shou ld be t aken in to account . Veryfew re sea rche rs have s tud ied ce ramic -s t ee l s l i d ing pa i r slubr i ca t ed w i th o i l s [8 -1 0 ,1 6] . In Ref . [8 ] where S i 3 N 4ce ramics were s tud ied , a dec rease i n wea r ra t e was ob ta inedat high temperatures. I t was suggested that protect ive f i lmforma t ion th rough t r i bochemica i i n t e rac t ions be tween thema te r i a l s and the o i l a t h ighe r t empera tu re s , accompanied bythe h igh s l i d ing ve loc i ty , was re spons ib l e fo r such behav iour .

The t r i bo l aye r c rea t ed on the ce ramic ba ll a f t er 0 ra in o ff re tt i ng a t 50 I~m ampl i tude i s p re sen ted in F ig . 5 ( a ) . Thelaye r , which con ta ined inc lus ions w i th a sma l l amou nt o f Fe ,cove red m ost o f t he wea r sca r and on ly a few sma l l a reas a tthe very edge of the scar were of the or iginal si l icon ni t r idema te r i a l , t yp i ca l ly w i th c racks ( see uppe r r i gh t a rea o fF ig . 5 (a ) ) . The t r i bo l aye r on the s t ee l sample con ta ined a


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M. a l i n eu aL ~We ar 210 1997) 27-38 33

. . = ~ . . . . . ~ . . . . k ~

..

..

, ~.~,~

~_-= : . . . . z= . . -~ ~ , . ~. ~. b -

F i g . 6 . S E M m i c r ogr a phs o f w e a r s c a r on a ) S i. ~N 4 ba l l a nd b ) s t e e l p l a t e c r e a t e d a f te r I 0 m i n o f d r y frettinga t 5 ~ m . S E M mkrogt phs of t he l a ye r s c r e a t e don c ) S i 3N 4 ba l l a nd d ] s t e e l p l a t e a l t e r 360 r a in o f d r y f r e t t i ng a t 5 p . m .

ce r t a in amo unt o f S i . F ig . 5 ( b ) s hows ev :. dence o f p la s t icde fo rm a t ion and s m ear ing o f the l aye r on the s t ee l p la te . T h i simpl ie s tha t the l aye r w as duc t i l e .

W ear s ca rs a f t e r 360 ra in o f t e s t ing a t an ampl i tude o f 50i~ m s howed a s imi la r appea rance . T he wea r s ca r on thece ram ic ba l l w ~ pa r t ly cove red wi th a laye r con ta in ing inc lu -s ions r ich in iron (F ig . 5 ( c ) ) , a s was a l s o obs e rved a t 25 p .mampl i tude and a t an ea r l i e r s t age a t the s ame ampl i tude . Onthe o the r hand , the s u r face o f the wea r s ca r on the s t ee ls p e c i m e n w a s m o s t l y c o v e r e d w i t h a n a p p a r e n t l y h o m o g e -neous t r ibo laye r , s hown in F ig . 5 (d ) . A ccord ing to E DS ana l -ys i s the l aye r con ta ined a h igh amou nt o f S i f rom the ce ramicba l l , p re s umably due to the t r ibochemica l reac t ion be tweenthe ma t ing ma te r i a l s . T he obs e rved c racks were mos t p rob-ab ly c rea ted dur ing coo l ing a f t e r the t e st .

F rom the re s u l t s o f S E M and E DS ana lys es i t c an be p ro -pos ed tha t a comb ina t ion o f mechan ica l and t r ibochemica lwea r in f luenced the wea r b ehav iou r even in the f i r s t s t age o ff re t t ing a t the h ighes t ampl i tude us ed . S ince h ighe r ampl i -tudes a t cons tan t f requency mean a h ighe r s l id ing ve loc i ty , iti s rea s onab le to conc lude tha t the t empera tu re inc reas es wi thampl i tude . Hence , p robab ly due to h ighe r t empera tu re s , aduc t i l e l aye r w i th low v i s cos i ty i s c rea ted on the s t eel s pec i -men , s o tha t lower wea r ra t e and lower wea r los s (F ig . 2 ( a ) ,( c ) ) w e r e o b ta i n e d a t 5 0 p m t h a n a t 2 5 p m a m p l i tu d e . As imi la r we a r t rans i tion was obs e rved in s l id ing t e s ts o f S iaN 4i n h u m i d a i r b y G e e a n d B u t t e r f i e l d [ 1 9 ] - - r e l a t e d t o

inc reas ed s l id ing s pee d- - and in un lubr ica ted f re t t ing te s t s byM e l a n d r i e t a l. [ 2 2 ] a n d B i ll [ 3 7 ] - - r e l a ~ d t o i n c r ea s e dt e m p e r a tu r e . A u i a [ 3 8 ] e x p l a i n e d s u c h a d e c r e a s e i n w e a rlos s by non-monoton ~ c va r i a t ions o f the dampin g capac i ty o fthe v i s coe la s t ic m a te r i a l obs e rved in the con tac t which pas s esth rough a max imu m w i th inc reas ing t empera tu re . T h i s exp la -na t ion i s a l s o cons i s t en t wi th our observ tions o f t h e m o r educ t i l e cha rac te r o f the reac t ion l aye r ,3.3.2. D o condi tions

As in the ca s e o f lubr ica ted f re t t ing , a t 5 p , m ampl i tudetyp ica l s t i ck and s l ip reg ions on the ce ramic and o n the worns teel s u r faces w ere a l s o ob ta ined in d ry f re t ting . In F ig . 6 ( a )and (b ) wea r s ca rs a fmr on ly IO ra in o f the t e s t a re s hown.In the s ti ck reg ion a lmos t no dam age can b e obs e rved , e i the ron ce ram ic o r on the s t ee l s pec imen , in con t ra s t to lubr ica tedcond i t ions , where in the s l ip reg ion on ly mechan ica l wea rwas de tec ted a t the s ame t e s t t ime and ampl i tude , a we l l -de f ined l aye r was p re s en t in d ry f re t t ing . T he l aye r on thece ramic ba l l was b r i t t l e and s pa l l ed , a s i s s hown in de ta i l i nF ig . 6 ( a ) . F rom the appea rance o f the uncove red s u r face ,a l s o s een in the da rk a rea , we s uppos e tha t the ce ramic s u r facew a s m a i n l y m e c h a n i c a l ly u n d a m a g e d . E D S a n a l y s i s o f t h elaye r conf i rmed the p re s ence o f S i and F e , which i rd ica te stha t t r ibochemica l reac t ion took p lace , a s was a l s o p ropos edb y o t h e r a u t h o r s f o r d r y s li d i n g o f c e r a m i c - s t e e l p a i r s [ 9 -1 2 , 1 4 - 1 6 ] .


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34 M. Kalin et aL / Wear210 997) 27-38

Fig . 6 c ) and d ) show a l aye r on the worn ce ramic andstee l surfaces, respect ively , crea ted af ter 360 ra in of f re t t inga t 5 tLm ampl i tude . The l aye r cove red a g rea t pa r t o f t he wea rsca rs on bo th ma te r i a ls . F ig . 6 c ) shows a smooth , appa ren t lyhomo geneous l aye r w i th c racks on the ce ramic ba ll . The ED Sspec t rum on the sam e f igure i nd i ca te s a h igh amoun t o f Feand Si i n t he l aye r , suppor t ing the hypothes i s o f t r i bochemica lreac t ion a s t he ma in m echan i sm o f wea r .

W e a r m e c h a n i s m s a t 2 5 t t m a n d 5 0 t t m a m p l i tu d e s w e r eve ry s imi l a r , excep t t ha t t he seve r i t y o f wea r a t 50 t tm ampl i -t ude was g rea t e r t han a t 25 p ,m ampl i tude . Th i s co inc ideswi th t he a lmost l i nea r i nc rease o f w ea r l oss w i th i nc reas ingampl i tude w i thou t any t r ans it i ons o r s ign i f ican t changes i nwea r behav iour F ig . 2 d ) ) , whi l e i n l ubr i ca ted condi t i onsremarkab le changes were recognt sed Fig . 2 c ) ).

R igh t f rom the beg inn ing o f t he t e s t a t 50 t tm ampl i tude at r i bo l aye r was fo rmed o ve r t he en t i l e wea r sca r on each spec -imen , F ig . 7 a ) and b ) . The l aye r seems to be t h icke r t hanin the p rev ious ca s es and spa l l ed in t ens ive ly dur ing f re t t ing .A t t he end o f t he t e s t t he worn sur faces d i sp l ayed a s imi l a rappea rance . The seve re cond i t i ons i n f luenced the s t eel spec -imen to a much grea t e r ex t en t t han the ce ramic ba l l , a s wasa l so conf i rmed by p rof i l ome t ry measurement s . M ost p roba -

b ly t he F e - and Si -con tain ing t r i bo l aye r was gene ra t ed p ref -e ren t i a l ly on s t ee l and adhe red to t he ce ramic spec imen . Af t e r360 ra in of f re t t ing the layer shows a f ibre- l ike appearance ,a lso demonstra t ing i t s br i t t le nature , Fig. 7 c) . As shown inthe EDS spec t rum a t t ached to F ig . 7 d ) , t he l aye r con ta insh igh amoun t s o f i ron , s i li con and oxy gen , which aga in impl i e st r ibochemica l wea r . A ccord ing ly , t he p rac t i ca l ly undamag edce ramic sur face unde r t he l aye r sugges t t ha t mechan ica l wea rprocesses w ere no t i nvo lved s ign i f i can t ly .

The re su l t s o f SEM and EDS ana lyses imply the rap idforma t ion o f t he t r i bo l aye r a t 25 an d 50 I~m ampl i tude , p rob-ab ly due to an inc rease o f t empera tu re i n t he con tac t , Incon t ra s t t o t he wea r mechan i sm in l ubr i ca t ed condi t i ons ,where i ron- and si l icon-conta ining inclusions were detec ted,the t r i bo l aye r fo rmed unde r d ry cond i t i ons seems to be mo rehomog eneous . In d i s t inc t ion to t he l aye r fo rmed in l ubr i ca tedcondi t i ons , i t con ta ined a h igh concen t ra t i on o f oxygen ,imply ing ox ida t ion a s one o f t he ma jor chemica l p rocessesinvo lved . Hence fo rma t ion and spa l l ing o f t he b r i t tl e tr i bo-l aye r con t ro l l ed the wea r be hav iour dur ing the w hole t e s t i ndry condi t i ons . Th i s r e su l t ed in t he h ighes t wea r l oss

F i g . 2 d ) ) o b t a in e d i n o u r w o rk .

Fi g . 7 . SE M mi c rogra phs o f a ) t he l a ye r on S i gN4 ba l l a nd b ) t he we a r s c a r on s te e l p l a te c re a t e d i n d ry f r e tt i ng a f t e r 10 mi n a t 50 t i m a mpl i tude . SE Mmicrog raphs of the layers c rea ted in dry fre t ting a f te r 360 ra in a t 50 tam amp l i tude on c ) Si.~N4 ba l l . d) s tee l pla te .


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M. Kalin et al. Wear 210 1997) 27-38 35100

8i

s o. 2r

40

ord 2oO l

0

(a)

1.2 c )I

o . 0 . 8

.....0.4

0.2

Lubricated

4O 6OT i m e [mini 80 100

~ F e.--v--O- o - C---a~ Si~ C a~ N- - + - s

[DO , ,- ,(b)=::Fe--*--Og so Dr y ---s--C

-a - -5 i- - I , - N

( o - ~ K _ A0 4O 8O 129 I0 0 200

ime [ m i n i

w v0 O

L u b r i c a t e d

40 60 80 180 200

, . 2 / d )= 1 1 Dry

.,~ O,z~(. . . . . . . siE'e L ~ o .eo . 40.2

0 .100 0 40 80 120

T i m e m i n l T i m e [ m i n iFig. 8. AES spectra of a depth profile across the tribochemical layer created on a steel plate after 360 rain at 50 ~m amplitude in (a) lubricated fretting, (b)dry fretting: Si/Fe ratio as a function of etching time in (c ) lubricated fretting, (d) dry fretting: 100 min = I I~m.3.4. AES and TEM analyses

In o rde r to ob ta in more de ta i l ed in fo rma t ion on the com-pos i t ion o f the t r ibo laye r , AE S an a lys i s o f the wea r s ca rs onthe s t ee l p la te s a f t e r 360 ra in o f lubr ica ted and d ry f re u ing a t50 p . m was pe r fo rmed . T he a rea o f ana lys i s was approx i -ma te ly 100 p ,m 2 , s o tha t a rep re s en ta t ive comp os i t ion o f them a t e r ia l w a s e x a m i n e d . F i g . 8 ( a ) a n d ( b ) s h o w th e A E Sdep th p ro f i l e ac ros s the tdbo la ye r .3.4.1. Lubricated conditions

Us ing the dec reas e in S i concen t ra t ion on the s t ee l p la te a sa c r i t er ion , the th icknes s o f the l aye r p roduced in lubr ica tedcond i t ions i s cons ide red to be abou t 0 . 2 p ,m, F ig . 8 ( c ) . T heconcen t ra t ion o f oxyg en in the l aye r i s neg l ig ib le , whi l e are la t ive ly h igh amou nt o f ca rbon w as de tec ted in the ma te r i a lbe low the s u r face , i. e . mu ch h ighe r than can be d i s s o lved inF e . T he s hape o f the ca rbon peak impl ie s tha t s ome o f theca rbon ex i s t ed in e l ementa ry fo rm, which exp la ins i t s h ighconcen t ra t ion in the l aye r . T he p re s ence o f ca rbon co u ld beas c r ibed to o i l degrada t ion p roduc t s d i f fus ed in to the i ron , a sw a s a l s o s u g g e s t e d b y M o r i m o t o [ 8 ] . A s e v i d e n t f r o mF ig . 8 (c ) , the S i :F e a tomic ra t io on the s u r face o f the wea rs c a r is a b o u t l : l . F r o m t h e c o m p o s i t i o n o f t h e la y e r w h e r ea lmos t no n i t rogen was p re s en t , i t c an be conc lud ed tha t thelaye r does no t con ta in f rag ments o f S i3N, , pa r t i c le s embed dedin s t ee l, bu t i s the re s u l t o f t r ibochemica l reac t ion be tweence ramic and s t ee l . T h i s a l s o favours the pos s ib i l i ty o f thepres ence o f a s o lid so lu t ion F e ( S i ) which was found byVleugels e t a l . [24] a t 1095 (2.

3.4.2. Dr), condi tionsF ig . 8 (d ) s hows tha t the th icknes s o f the l aye r in d ry f re t -

t ing cond i t ions i s nea r ly 2 p .m. T he t r ibo laye r con ta ined ah igh am ount o f S i , wh ich a l s o agrees w i th the re s u l t s o f E DSana lys i s . F rom F ig . 8 (d ) i t c an be s een tha t the S i :F e ra tioon the s u r face i s aga in : 1 , which cou ld s ug ges t the p re s enceof an F eS i phas e , a s in the ca s e o f lubr ica ted f re t t ing . How-eve r , the h igh oxyg en concen t ra t ion in the en t i re dep th o f thelaye r impl ie s tha t the p re s ence o f s i l icon ox ide and i ron ox ideswould be more p robab le .

T he re s u l t s o f AE S ana lys i s s how a rem arkab le d i f fe rencei n t h ic k n e s s ( b y a n o r d e r o f m a g n i t u d e ) a n d i n c o m p o s i ti o nof the reac t ion l aye rs p roduced unde r lubr ica ted and d ry f re t -t ing cond i t ions , bu t the u l t ima te ans w er a s to the compos i t ionof the reac t ion p roduc t s w as no t ob ta ined . In o rde r to iden t i fythe phas es p re s en t in the l aye r , X-ray ana lys i s was employ ed ,bu t s ince the wea r s ca rs were s ma l l , rough and non-p la in ,re s u l t s were no t s a t i s fac to ry . T o ob ta in unequ ivoca l re s u l t son the phas es p roduced , T E M ana lys i s was the re fo re pe r -fo rmed on s e lec ted samples . F ragm ents o f reac t ion l aye rwe ret rans fe r red to a ho l low ca rbon c oa ted Cu g r id and inves t iga tedus ing a J E O L 2000 F X t rans mis s ion e lec t ron mic ros cope wi tha t ta c h e d U T W S i ( L i ) E D S d e t e c t o r f r o m L i n k .

T E M a n a l y s i s o f t h e t r i b o l a y e r c r e a t ed a f t e r 3 6 0 r a i n o fd ry f re t t ing a t 50 p .m am pl i tude s h owed tha t the l aye r con-s i s t ed o f a s i l i ca - r i ch amorphous phas e con ta in ing s ma l li n c lu s i on s o f ir o n o x i d e s ( 5 - 2 0 n m ) . I n F i g . 9 ( a ) - ( f ) d a r kf i eld mic rographs , co r re s p ond ing s e lec ted a rea e lec tron d i f -f r a ct i on ( S A E D ) p a t te r n s a n d E D S s p e c tr a o f t w o d i f f e re n t


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3 6 M. galin et al. /W ea r 210 1997) 27-38

I + 1 1 + lo o 1 1lE , qm N ) 0 . 0 2 . 0 4 . 0 6 . 0 8 . 0 E keY) 0 . 0 2 . 0 4 . 0 6 . 0 8 . 0

F i g , 9 . a ) . d ) D a r k f ie l d t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s : b ) , e ) c o r r e s p o n d i n g s e l e c t e d a r e a electron d i f f r a c t i o n p a t t e r n s : c ) , f ) E D S s p e c t r a o f t w od i f f e re n t a r e a s o f t h e t d b o l a y e r .

areas of the t r ibolayer are shown. In Fig . 9 a) crystal l inepart icles up to 5 nm in s ize were observed. From the SAEDpattern of th is area Fig . 9 b) ) we found that the part iclesare rhombohedra l Fe203 hema t i t e ) . F rom the EDS spec t raFig . 9 c) ) , and from the uniform contras t of the matrix inFig . 9 a) , we can conclude that Fe203 part icles are sur-rounded by an amorphous S iO, -dch phase g las s ) . Ni t rogenwas not found in any s ignificant concentrat ion b ut we founda small amount o f chrom ium w hich orig inated from the s teel .A s imilar s i tuat ion is d isplayed in Fig . 9 d), e) and f) ,where the region with up to 20 nm large cubic Fe304 ma g-net i te) part icles embed ded in a s i lica-rich am orphous phaseis presented. This confirms that a tribochemical reaction, i .e.oxidat ion, took place. Si3N4 was not detected in the layer,confirming that the layer is a t r ibochemical react ion productand that oxidat ion control led the w ear behaviour. Such resul ts

also agree with o ther invest igat ions of ceram ic-s teel couples[7 ,9 ,11 -161 .3 .5 W e a r m e c h a n i s m s3 .5 . I . L ub r i ca t ed cond i t i ons

Two different wear mechanisms are proposed for s i l iconnitride against bearing steel in selected test conditions. Atlower amp li tudes and in the early s tage of fret t ing at moderateampli tudes , mechanical we ar dominated . Crac ks on the s t ick-s l ip boundary and spal l ing of a th in t r ibolayer were observed.Under these condi t ions the h ighest wear rate in lubricatedfretting was obtained.In the final stage of the fretting test at mod erate am plitudes,and fight from the beginning at higher amplitudes, tribochem-ical wear is suggested to be the dominant form of wear.


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M . K a l b r e l a l . / W e a r 2 1 0 f1997J 2 7 - 3 8 37

T he t r ibo laye r appea red a s a nonho mog eneous ma te r i a l , con-ta in ing inc lus ions wi th d i f fe ren t F e and S i con ten t. W e s up-pos e tha t the h ighe r con tac t t empera tu re gene ra ted byinc reas ed s l id ing ve loc i t i e s acce le ra ted chemica l reac t ionbe tween F e f rom the s t ee l, S i f rom s i l i con n i t r ide and o i l . Onthe bas i s o f AE S ana lys i s the l aye r was on ly 0 . 2 p . m th ickand , accord ing to i t s ve ry low n i t rogen con ten t , d id no t con-rain S i 3 N 4 , s uppor t ing the a s s umpt ion tha t the l aye r i s a t r i-bochemica l reac t ion p roduc t . A ve ry h igh concen t ra t ion o fca rbon , fo rmed by o i l degrada t ion , was found in the l aye r .T h i s impl ie s the c r i t i ca l in f luence o f o i l , no t on ly a s a lubr i -can t , bu t a l s o a s an ac t ive e lement in fo rm ing reac t ion p rod-uc t s . T he t r ibo laye r was duc t i l e , p re s umably due to a h ighcon tac t t empera tu re unde r thes e cond i t ions . T rans fo rma t ionf rom a mechan ica l wea r p roces s , i . e . f rac tu re, to t r ibochem -ica l wea r w i th s mear ing and p la s t i c de fo rma t ion o f the l aye roccur red , w hich s ign i f i can t ly reduced the wea r ra t e aad w ea rloss.3.5.2. DO conditions

A d i f fe ren t wea r mechan i s m was found fo r d ry f re t t ingcond i t ions . Res u l t s o f AE S an a lys i s s howed a l aye r an o rde ro f magn i tude th icke r than in lubr ica ted f re t t ing , a l s o wi th aremarkab ly d i f fe ren t chemica l compos i t ion . T E M ana lys i sconf i rmed tha t the reac t ion l aye r cons i s t ed o f a s i l i ca - r i chamorphous phas e con ta in ing s ma l l inc lus ions o f the i ronoxid es Fe_,O~ and Fe~O4. No Si3N4 was prese nt in the layer.T h i s co nf i rms tha t the ox ida t ion o f s t ee l a s we l l a s o f s i li conn i t r ide was the con t ro l l ing t r ibochemica l p roces s unde r d ryf re t t ing cond i t ions .

Under d ry f re t t ing cond i t ions the l aye r was fo rmed f romthe ve ry beg inn ing o f the t e s t a t a l l ampl i tudes us ed . T h i ss ugges t s h ighe r t empera tu re s and a l s o a more rap id inc reas eof the t empera tu re than in lubr ica ted f re t t ing , in con t ra s t toh tb r i ca ted cond i t ions , w here the l aye r c rea ted w as duc t i l e , inthe cas e o f d ry f re t t ing the l aye r was more b r i tt l e . T he con-t inuous p roces s o f fo rming and s pa l l ing o f the b r iu le t r ibo-laye r caus ed mu ch h igh e r wea r ra t e s and wea r los s es thanunde r lubr ica ted f re t t ing cond i t ions . In con t ra s t to lubr ica tedf re t t ing , no t ransi t ion in wea r beh av iour w as obs e rved , re s u l t -ing in monoton ica l p rogres s o f wea r los s.4 C o n c l u s io n s

F re t t ing wea r mech an i s ms o f s il i con n i t r ide aga ins t bea r ings tee l were g rea t ly a f fec ted by the p re s ence o f o il and theampl i tude us ed .

T w o d i f fe ren t wea r mechan i s ms a re p ropos ed fo r s i l iconni t r ide fre t ted agains t bearing s tee l in lubricated tes t con-d i t ions. A t lower am pl i tudes and in the ea r ly s t age o f f ret -t ing t e s t s a t modera te ampl i tudes , mechan ica l wea rdomina ted . Under thes e cond i t ions the h ighes t wea r t a rein lubr ica ted f re t t ing was ob ta ined . In the f ina l s t age o ff re t t ing a t mod era te am pl i tudes , and r igh t f rom the beg in -n ing a t h ighe r ampl i tudes , a t rans i t ion f rom h igh to lowwear occur red . A low-v i s cos i ty p ro tec t ive l aye r fo rmed in

the con tac t , p robab ly due to the h ighe r con tac t t empera -tu re s gene ra ted by h ighe r s l id ing s peeds un de r thes e con-d i t ions , thus reduc ing the wea r ra t e and wea r los s . T helaye r c rea ted was approx ima te ly 0 . 2 p , m th ick and con-ra ined a h igh concen t ra t ion o f ca rbon due to o i l decom-pos i tion . T r ibochem ica l w ea r i s s ugges ted a s the dominan twea r fo rm.Much h ighe r wea r ra t e s and wea r los s es were ob ta inedunde r d ry than u nde r lubr ica ted f re t t ing cond i t ions . Und e rdry cond i t ions a l aye r an o rde r o f magn i tude th icke r wi thchemica l comp os i t ion d i f fe ren t f rom tha t in lubr ica ted f re t -t ing fo rmed f rom the ve ry beg inn ing o f the t e s t a t a l lampl i tudes us ed . T E M ana lys i s conf i rmed tha t the reac tionlaye r cons i s ted o f a s i l i ca -r i ch amorpho us phas e con ta in ings ma l l inc lus ions o f the i ron ox ides F e :O 3 and F e~O4. NoS igN 4 was p re s en t in the l aye r , conf i rming an in tens e ox i -da t ive t r ibochemica l reac t ion . i 'he ox ide l ay e r was b r i t t l es o tha t s pa l l ing and f rac tu re took p lace ex tens ive ly . Not rans i tion in wea r behav io ur was obs e rved .

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B i o g r a p l d e sM i t j an K a l in g r a d u a t e d in M e c h a n i c a l E n g i n e e r i n g f r o m t h eF a c u l t y o f M e c h a n i c a l E n g i n e e r i n g , U n i v e r s i t y o f L j u b l ja n a ,i n 1993 . He rece i ved h i s M. Sc . degree i n 1996 . At pre sent ,h e i s a P h . D , s t u d e n t a n d A s s i s t a n t f o r M e c h a n i c a l P o w e rT r a n s m i s s i o n a n d T r i b o l o g y a t t h e F a c u l t y o f M e c h a n i c a lEng i neer i ng . Hi s r e search f i e l d i nvo l ve s t r i bo l ogy , ro l l i ngc o n t a c t m e c h a n i c s , r o l l i n g b e a r i n g s a n d f r e t t in g w e a r w i t hspec i a l i n t ere st i n ceram i c mat er i a l s .J o ~ e V i ~ i n ti n r e c e iv e d h i s B . S c . d e g r e e i n 1 9 7 2 , M . S c . d e g r e ei n 1 9 7 5 a n d P h . D . in 1 9 7 8 i n M e c h a n i c a l E n g i n e e r i n g fr o mt h e F a c u l t y o f M e c h a n i c a l E n g i n e e r i n g , U n i v e r s i t y o f L j u -b l ja n a . F r o m 1 9 7 2 - 1 9 8 0 h e w o r k e d a s A s s i s t a n t a t t h e F a c u l t yo f M e c h a n ic a l E n g i n e e r in g a n d f ro m 1 9 8 0 - 1 9 8 7 a s D i re c to ro f t he Ins t i t u t e f or Research i n Mat er i a l s and S t ruc t ures i nL j u b l ja n a . H e w a s e l e c t e d t o A s s i s t a n t P r o f e s s o r in 1 9 8 4 a n dt o A s s o c i a t e P r o f e s s o r i n 1 9 8 9 f o r T r i b o l o g y a n d M e c h a n i c a lP o w e r T r a n s m i s s i o n a t th e U n i v e r s i ty o f L j u b lj a n a . H i s p r i n -c i pa l r e search i n t ere s t s are in t he f i e l d o f t r i bo l ogy and t ech-n i c a l d i a g n o s t i c s . H e h a s a u t h o r e d a n d c o - a u t h o r e d m o r e t h a n7 0 p u b l i c a t io n s . T o g e t h e r w i t h c o - a u t h o r s , h e h a s p u b l i s h e dm o r e t h a n 1 4 0 t e c h n i c a l r e p o r t s. D r V i ~ i n t in o r g a n i s e d t h eI n t er n a t io n a l C o n f e r e n c e o f T r i b o l o g y i n L j u b l ja n a 1 9 8 6 )a n d w a s a m e m b e r o f t h e o r g a n is i n g a n d s c ie n t if ic c o m m i t t e e sf or s evera l na t i ona l and i n t ernat i ona l conf erences on t r i bo l -o g y a n d m e c h a n i c a l e le m e n t s . H e i s H e a d o f t h e C e n tr e o fT r i b o l o g y a n d T e c h n i c a l D i a g n o s t i c s a n d P r e s i d e n t o f t h eS l o v e n ia n S o c i e t y o f T r i b o l o g y .S a~ ia N o v a k s h e j o i n e d t h e D e p a r t m e n t o f C e r a m i c s a t t h e J .S t e f an Ins t i t u t e , Un i vers i t y o f Lj ub l j ana , S l oven i a , i n 1975 .S h e r e c e i v e d h e r B . S c . d e g r e e i n c h e m i s l r y f r o m t h e U n i v e r -s i t y o f L j u b l j a n a , D e p a r t m e n t o f C h e m i s t r y a n d C h e m i c a lT e c h n o l o g y , i n 1 9 8 1 , h e r M . S c . i n 1 9 8 7 a n d P h . D . in 1 9 9 6 .At present she i s work i ng a s a r e search as soc i a t e a t t he J .S t e f an Ins t i t u t e . Her f i e l d o f spec i a l i sa t i on i s proc es s i ng o fa d v a n c e d s tr u ct u ra l c e r a m i c s a n d s t u d y o f w e a r o f c e r a m i cmat er i a l s i n d i f f erent app l i ca t i ons .G o r a n D r a~ i~ w a s b o r n i n J e s e n i c e S l o v e n i a ) i n 1 9 5 7 . A f t e rs e c o n d a r y s c h o o l h e m a t r i c u l a t e d a t t h e U n i v e r s i t y o f L j u -b l ja n a , D e p a r tm e n t o f C h e m i s t r y a n d C h e m i c a l T e c h n o l o g y ,f r o m w h e r e h e g r a d u a t e d i n 1 9 8 0 . I n t h at y e a r h e j o i n e d t h eD e p a r t m e n t o f C e r a m i c s a t t h e J . S t e fa n I n s ti tu t e , U n i v e r s i t yo f L j u b l j a n a, w h e r e h e w o r k e d i n t h e f i e ld s o f t h e r m o l u m i -n e s c e n c e d o s i m e t r y , p r o t o n i c h u m i d i t y s e n s o r s a n d l e a d -b a s e d r e la x e r s. I n 1 9 8 5 h e r e c e i v e d a m a s t e r d e g r e e a n d i n1 9 9 0 a d o c t o r o f c h e m i c a l s c i e n c e d e g r e e f r o m U n i v e r s it y o fLj ub l j ana . At pre sent he i s work i ng a s a r e search as soc i a t e i nt he Cera m i cs Dep ar t me nt a t t he J . S t e f an Ins t i t u t e . Hi s pre sentpro f e s s i ona l f i e l d o f i n t ere s t i s r e l a t ed t o i norgan i c chemi s t rya t h i g h t e m p e r a t u r es , n o t a b l y t h e d e v e l o p m e n t a n d c h a r a c -t er i sa t i on o f c erami c mat er i a l s w i t h spec i a l proper t i e s . Hes p e c i a l i s e s i n e l e c t r o n m i c r o s c o p y a n d m i c r o b e a m a n a l y s i st e c h n i q u e s a n d i s a r e g u l a r c o n tr i b u t o r t o p r o f e s s i o n a l j o u r -na l s and i n t ernat i ona l me e t i ngs .

1997 wear 210 fretting oil kalin

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