Effects of Additives on Wear Mode and Morphology of Wear

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Wear, 143 (1991) 119-135 119

Effects of additives on wear mode and morphology of weardebris generated in the lubricated sliding of steel

T. Akagak i( Ts u n m k a Te c h n i c a l C ol l eg e, T m r u o k a , Ya m a g a t a 99 7 ( J a p a n )

K. Ka to( To h o k u U n i v e r s i t y, S e n d a i , M i y a g i 9 80 ( J a p a n )

(Received April 27, 1990; revised Augu st 30, 1990; accepte d Sep tem be r 14, 1990)

A b s t r a c t

The predomin ant wear mechanism s were studied as a function of contact pressure andsliding velocity in thin-Shn-lubricated sliding und er conditions of concentrated contactwith a hard chromium steel baII from the view point of additives. Three kind s of lubricantswere u sed in this study: (1) allcylnap hth alene oil (base oil), (2) b ase oil plu s 0.044 wt.%stearic acid and (3) b ase oil plu s 1.0 wt.% tricresyl ph osph ate (TCP). The result s werearranged in the form of a wear m ode diagram. The effects of additives on the d iagramand morphology of wear deb ris generated in each wear mode regime w ere studied anddiscussed. It was found that add itives contributed to the ability to achieve mild flowwear. The d egree of contributi on was as follows: TCP > stearic acid > bas e oil. The sha peof wear d ebris generated did not dep end on the kind of additive in each wear m oderegime; h owever, th e size was str ongly depen dent on th e kind of additive.

1 . I n t r o & u c t i o n

Recently, requests for failure diagnosis of machinery have increased andmany monitoring techniques, based on wear debris analysis, have beendeveloped. Among th em, ferr ogra phic an alysis is a very useful met hod becau seit provides a great dea l of inform at ion on wear m ode, degree of dam age,damaged parts etc. through wear debris analysis [l-3]. At present there aremany reports which have dealt with the possibility of application to practicalma chin es [4-6] an d compar isons between ferr ogra phic a na lysis an d otheranalyses [ 7-91. However, there are few reports which have clarified therelationships am ong th e friction conditions, wear m ode, wear ra te an dmorph ology of wear debr is in deta il [ 10, 111. It is very important that thiskind of information be available in order to predict correctly the possibilityof failure in the future.

In order to describe the predominant wear mode and each possibleregime, m an y wear mode diagram s have been reported in th e litera tu re, i.e.an abrasive wear mode diagram [ 12, 131, a wear m ap for un lubricated slidingof steel [ 14, 151, a fretting m ap [ 161, a wear mode diagra m for lu bricatedsliding of steel [ 17, 181, a wear m ap for un lubricated sliding of Al-S alloys

0043-1648/91/$3.50 0 Elsevier Sequoia/Printed in The Netherlands



[ 191 an d a ~~~~t ~~n d@ra m for ~~b~~~ted ~~n ce~t~ ~ed sliding con ta ctj20, 21 j= These wear mode dr ains ar e useful for ~~der~t ~~rn g wearmecha nisms. However, ther e is litt le work which ha s re l a t ed wear debrisgenera ted to these wear mode diagrams in detail.

Xn previous work, the wear mode diagra m an d effects of ha rd ness onth e diagram were st udied in lubricat ed sliding of car bon steel [ 12, 181. InMS st ud y, effects of a dditives on th e diagra m were stu died Wth a micr oscopicappr oach an d discussed. ~i~e~ore, based on the wear mode ~a~a rn ~wear debris gener at ed in each wear mode was stu died and classified fromth e view point of ~d~~ves~ ~e~at~ons~pa e~een rn ~~~~~o~ of wear debris,wear m ode an d wear r &e were discussed.

Experiments were car r ied out u sing a pin-on-disk test ma c~e (I?& 1).The pin an d d&k were not deta&ed from the apparat us d-g th e measur ementof wear volum e an d scam&~ elect ron micr oscope @EM) obser vat ion , Th ere-fore, the pm rubbed exactly on the same track of the disk in repeatedrota tions, The sh apes a nd s izes of th e specimens a r e ah own in F ’ig, 2. Thepin was a h igh ca fe bear ing st eel ba ll @WI) of d&meter 3-2 mm v&ha flat sur face. Its app~ent cont act ar ea was O-35 mm ’ an d its ~~~~ was

863 HV (load: 4.9 N). The diik wa s 0.45 wt .% carbon st eel (S45C) an d itshardness was 222 H V (load: 4.9 N). The chemica l ~urnpo~~~o~ f th e specimensis sh own in Table 3. After ~~~ an d b&&g+ both specter s were finish edto a sur face roughn ess of U.09 pm R-. ~ub~c~~ used in th is stu dy ar esum ma rized in Table 2. Thr ee k&da of lubrican ts were used: (1) alkyht a-ph ~~en e oif (base oil, 23-35 cSt at 38 “C), (2) base oil plu s U.044 wt.%st ear ic acid an d (3) base oil plus 1.0 wt.% tr icres yl ph osph at e (TCP). The

pin specime n; 2, diskbearing.



121

TABLE 1

Chemical composition (wt.%) of specimens

Material C Si Mn P s Cr

SUJ 1 1.01 0.25 0.30 0.020 0.005 1.05s45c 0.45 0.23 0.73 0.026 0.014 0.10

TABLE 2

Lubricants used in this study

1 Alkylnaph thalene oil (base oil)

2 Base oil plu s steak acid (0.044 wt.%)3 Base oil+ TCP (tricresyl ph osph ate) 1.0 wt.%

TABLE 3

Friction conditions

Slidin g velocity (m s-l) 0.25-1.0Con tact pressu re (MPa) 19.6-156.8Sliding distance (pass) 8X 104 (5 km)

oil was used after filtering with a filter of 1 pm size to a void a mixtu re ofother debris. The oil was supplied to the disk at a flow rate of 0.5 ml min-’using a microtube pum p.

The friction condition is summarized in Table 3. The sliding velocityan d cont act pr essu re were var ied in th e ra nges 0.25-1.0 m s-’ an d 19.6-156.8MPa respectively. The total num ber of passes am oun ted to 8 X 104, whichcorr esponded to a sliding distan ce of 5 km. Room tem perat ur e an d relativehu midity were kept at 25 “C an d 40%, resp ectively. All specimens were

cleaned using trichloroethan e in an ultra sonic clean er.Wear loss was measured only on the disk. The profile of the wear scar

of th e disk was record ed with a pr ofilomet er perp endicular to the slidingdirection at each sta ge of the test . A schema tic pr ofile is shown in Pig. 3.The wear volum e wa s calculat ed from th e cha nge of cross-sectiona l ar eamea sur ed with a planimet er. The measu remen ts of cross-sectiona l ar ea werecon du cted a t four differe nt loca tions in th e wea r sca r, i.e. O”, 90”, 180” an d270”. The wear volume was the average value of these four measurements.The specific wear ra te was calculat ed from th e slope of th e linea r plot ofwear-time dat a obtained dur ing steady stat e wear.

Wear scar s of pin an d disk specimens were observed an d analyzed witha SEM an d a wavelength disper sive X-ra y an alysis facility (WDX). Wear debriswas collected from th e lubrican t by usin g a ferr ogra phic techn ique (22, 231an d observed an d an alyzed with th e SEM an d WDX.



Wear Scar Area ; A

A=Ai-(Az+As) (mm’)

Wear Volume ; AV

4AV = yif; Ai ( D q 19 mm 1

= 59.69 Ti i iWTf3)

Fig. 3. Schemat ic p rofile of wear scar of disk : AI, app arent w ear scar are a; A z an d AS, th eareas of both side ridges; A , net wear scar area; A, average value of four measurements ofwear scar area; V, wea r volume; D, diameter of wear scar.

3 . R e s u l t s

3.1 . Wear character is t icsFigure 4 s h ows the re la t ion sh ips between con tact press u re , s l i d i n g velocity

and specif ic wear rate ( three-dimensional display) obtained for three kindsof lu brican ts . Th ere is a region where sp ecific wear ra te is very sm all an dof the ord er of 10 T8 (m m 3 Nm m i) at h igh s liding velocity an d low conta ct

pr ess u re in th e ba se oil, a s sh own in Fig. 4(a). At low s liding velocity an dh igh cont act p res su re, th ere is a region where sp ecific wear ra te is relativelylar ge an d of th e order of lo-” (m m 3 Nm - ‘). Fu rth er, th ere is a region wh eresp ecific wear ra te is larger then 10 e4 (m m 3 Nm -I). Th u s there are threeregion s wh ere th e s pecific wear ra te is com pletely diReren t in t h e ba se oil.In the base o i l p lus 0 .044 wt .% s tear ic ac id there is a region where thesp ecific wear r a te is of th e order of lOa6 (m m 3 Nm-“) on ly at low slidin gvelocity an d h igh conta ct pres su re an d the wear ra te of anoth er region isof th e ord er of lo-’ (m m 3 Nm-‘), as sh own in Fig. 4(b). Thu s th e regionwhere th e sp ecific wear ra te is s m al l becomes wider by a dding s tea r ic ac id .In t h e bas e oil plus 1.0 wt.% TCP th e region where th e sp ecific wear ra teis sm all an d of th e order of 1 0e8 (m m 3 Nm -‘) becom es even wider, a sshown in Fig. 4(c).

Thus addit ives contr ibute to the increase in the abil i ty to achieve a lowwear ra te. Th e degree of cont r ibu tion is as follows: TCP > s tea ric acid > ba seOil.

3 .2 . Ckzs i$icatitm of w ear m odeIn order to clar ify th e pred o~~t wear m ode, th e wear sca r of each

disk specimen was observed wi th the SEM.Figure 5 sh ows th e scan n in g e lect ron m icrograp h s of wear sca rs genera ted

in th e ba se oil. Figs . 5(a)&(c) sh ow th e sca n n in g electron m icrograp h scorres pon din g to a sp ecific wear ra te of th e ord er of lo-’ (m m 3 Nm -‘).Th e wear sca r is sm all an d very sm ooth, as s h own in Figs. 5 (a) an d 5@).



123

V (m/s)

9.6

/

,’

I

: t

/ / / / /0.5 10

0

V (m/s)

Stearic Acids45c

10-E

10-0 E

1 n’

E

10-7 p

0 0.5 10 uV (m/s)

F@. 4. Three d imen sional display of wear chara cteristics: (a) base oil; (b) bas e oil plu s 0.044wt.% stearic acid; (c) base oil plus 1.0 wt.% TCP.

A film y layer is extru ded in th e m ild plough in g process of su rface as perit iesin a d irection perp en dicu lar to th e s liding direction, a s sh own in Fig. 5(c).Th e film y layer will s epa ra te an d becom e loose filmy wear deb ris . Th is kindof wear h as been n am ed mild flow wear [17 , 18 , 24 1. Figu res 5(d)-5(f)sh ow the s can n in g e lect ron m icrograp h s in th e case wh ere the s pect ic wear



Fig. 5. Sca nn ing electron m icrograph s of wear sca rs gen era ted in th e ba se oil: (a)-(c), (d)-(f)an d (g)-(i) corres pon d to th e sp ecific wea r rat es which a re of th e order of lo-*, 10m 6 an d10m 4-10 -’ (m m ” Nrn- ‘) res pectively, (h ) an d (i) ar e th e enla rgemen ts of th e region s indicat edby th e ar row tips in (g) and (h) resp ectively. Experim ent al cond itions ar e as follows: 0.5 ms-‘, 58 .8 MPa for (a)-(c); 0.2 5 m s- l, 15 6.8 MPa for (d)-(f); 1.0 m SC’, 15 6.8 MPa for(g)-(i). The ar row ind icates th e relat ive d irection of m otion of th e coun terface.

ra te is of the order of lo-” (mm 3 Nm - ‘). The wear s car is lar ge an d rela tivelyrough , as sh own in F ig. 5(d) an d 5(e). Man y con cent r ic plough ing grooves,which are probably formed by the severe ploughing action of work-hardenedasper ities and wear debris, ar e comm on. Many filmy layers an d plate-likelayers ar e form ed in th e severe ploughing pr ocess, as shown in Figs. 5(d)an d 5(e). These la yers an d th e ridges of grooves will sepa ra te a nd becomeloose large filmy an d plat e-like wear debris. Th is kind of wear h as been



125

named severe flow wear [ 17, 181. Figu r es 5(g)-5(i) sh ow scan nin g elect ronmicrogra phs for th e case wh ere t he specific wear r at e is larger than 10e4(mm 3 Nm -‘). In th is case, hea vy vibrat ion occurr ed an d th e driving motor

(40 W) stopped. Ther efore, wear in th is case was cat as tr oph ic wear , leadingto seizur e. The wear scar is extr emely r ough, as shown in Fig. 5(g). Lar gewedge-like agglomera tes, which ar e more th an a few hundr ed m icrometer sin size, ar e frequ ent ly observed on t he wear scar, a s shown in Fig. 5(g).They were also observed on t he wear scar of the pin, wher e th ey formedby tr an sfer of disk ma ter ial. These a gglomer at es cons ist of ma ny plat e-likelayers. Cha ra cter istic pa tt ern s, possibly formed dur ing st ick-slip motion, ar eobserved in th e rea r of th e agglomera tes, as shown in Fig. 5(h). Man y sh eardimples ar e observed in ar eas wher e stick-slip motion occur s, as shown inFig. 5(i). These observations show tha t severe adh esion an d separa tion ar epredominan t. Therefore, it is concluded th at adhesive wear is predominan tin th is case. Thus th ree types of wear m ode ar e observed in th e base oil,accordin g to the ma gnitu de of th e specific wear ra te.

Figure 6 shows th e scan ning electr on microgra phs of wear scars generat edin th e base oil plus stea ric acid. Figures 6(a)-6(c) show those corr espondingto a specific wear ra te of th e order of lOma (mm 3 Nm -‘). These show feat ur essimilar to those in th e base oil, i.e. a fllmy layer is extru ded by th e mildplough ing action (Fig. 6(c)). Thu s mild flow wear is pred omin an t when st ear icacid is mixed with th e base oil. Figur es 6(d)-6(f) show th e scann ing electron

microgra ph s correspondin g to a specific wear ra te of th e ord er of 10e6 (mm3Nm -‘). These a lso show feat ur es similar to th ose in th e base oil, i.e. m an yconcent ric plough ing grooves ar e comm on an d ma ny filmy an d plat e-likelayers are extru ded by th e severe ploughing action, as shown in Figures6(d)-6(f). Thu s severe flow wear is pred omin an t as in th e base oil.

Figure 7 shows th e scan ning electr on microgra phs of wear scar s genera tedin th e ba se oil plu s TCP . F igur es 7(a)-7(c) an d 7(d)-7(f) sh ow th e scan n ingelectr on microgra ph s correspondin g to specific wear ra tes of th e ord er of10e8 an d 10V6 (mm 3 Nm -‘) resp ectively. These wear scar s are also similarto th ose in th e base oil. Ther efore, mild flow wear an d severe flow wear ar e

also predominant in this case.Figure 8 shows the results of X-ray line analysis of wear scars generated

in the base oil plus 1.0 wt.% TCP. Figures 8(a) and 803) show the resultsof analysis in the mild flow wear regime and the experimental conditionscorrespond to point s F an d H in Fig. 9(c) resp ectively. Figur e 8(c) showsth e result in severe flow wear an d the experimenta l condition corr espondsto point G in Pig. 9(c). These r esu lts show t ha t th e concent ra tion of phosph oru sis rich in th e dark er region of the wear scar. The region looked blue orbrown with an optical m icroscope. The num ber of th ese regions increas edwith th e increase in th e cont act pressu re, as shown in Figs. 8(a) an d 8(b).They increas ed st ill more in th e severe flow wear r egime, as shown in Pig.8(c). These d ar ker regions, wh ich look like black-an d-white wave-like pat ter nsor porous pa tt ern s on th e wear scar (Figs. 8(d) an d 8(e)), ar e rea ction layer sgenera ted by th e chem ical reaction between iron a nd phosphoru s. It is well



Fig. 6 . Scann ing electron micro~ap~ of wear sca rs genera ted in th e bas e oil plus 0 .044 wt.%st ear ic acid. (a)-(c) an d (d)-(f) Correspon d to the specific wear ra tes which are of th e orderof 10 --’ an d 10 m e (mm” Nm- ‘) respectively. Experimental conditions are as follows: 0.5 ms-‘, 78 .4 MPa for (a)-(c); 0.25 m s-‘, 156 .8 MPa for (d)-(f). The arr ow ind icat es th e rela tivedirection of motion of the counterface.

kn own th at these rea ct ion produ cts in crease with an increas e in th e sever ityof the fr ict ion cond it ion . Th e resu lt obta in ed in th is s t u dy a grees well withthis fact . Thus i t is expected that chemical wear (separation of reactionlayers ) occu rs together with m ild flow wear an d severe flow wear in th e bas e

oil plu s TCP. The s ize of wear deb ris genera ted b y sepa ra tion of rea ctionlayers in Figs . 8(d) an d 8(e) is es t im at ed to be very sm all (less th an 1 pmin s ize). In genera l, wear deb ris , which is less th an 2 0 pm in s ize for m ildflow wear and less than 90 pm in s ize for severe f low wear, is common



127

Fig. 7. Scann ing electron m icrograph s of wear scars genera ted in the base oil plu s 1.0 wt.%TCP. (a)-(c) and (d)-(f) Correspon d to the sp ecific wear rates w hich are of the order of lo-’and 1O-‘-1O-6 (mm’ Nm-I) respectively. Experim enta l condition s are as follows; 0.5 m s-l,78.4 MPa for (a)-(c); 0.25 m s- I, 156.8 MPa for (d)-(r). The arrow in dicates the relativedirection of motion of the counterface.

[ 17 , 1 81 . Th erefore, al thou gh ch em ical wear occu rs in lu bricated s l iding wearin the oil containing addit ive, i t is the secondary wear mode.

Th u s, if th e sp ecific wear ra te is of th e order of s imilar m a~tu de, th epredom in an t wear m ode is essen t ia l ly th e sa m e wheth er or n ot ad di t ives are

mixed in the base oil .

3 .3 . Wea r m od e d i a g r a mTh e p redomina n t wear m odes were p lot ted in a wear m ode d iagram for

th e thr ee kin ds of lu br ican ts . F’igu res 9(a)-9(c) ar e diagra m s which sh owth e predomina n t wear m ode an d each poss ib le region. The d iagram re la testh e pr edominan t wear m odes to s l id ing velocity an d con tact p ress u re . Assh own in Fig. 9(a), th e predom in an t wear m odes clearly fall in to three sepa ra teregions of the wear mode diagram for the base oil . For the base oil pluss tear ic ac id , the bou n da ry cu rve between th e m ild flow wear regim e an d thesevere flow wear regim e s h ifts towards low s liding velocity an d high conta ctpres su re , as sh own in F ig . 9(b). Fur ther , the boundary curve between themild flow wear regim e a n d the ad h es ive wear regim e a lso sh ifts up wards alittle. Th u s th e region of mild flow wear becom es wider b y add in g s tea ric



128

OriginalSurface

Fig. 8. WDX analy sis of wea r scar gen erated in th e ba se oil pl us 1.0 wt.% TCP: (a)-(c) SEMimage an d P Ka line analysis. The scannin g electron micrographs of (d) and (e) are theenlargem ents of dark er regions on the wear scar, wh ich are rich in ph osph orus. The arrowind icates the relative direction of motion of the coun terface.

ac id . For t he bas e oi l p lus TCP, th e region becomes even wider, a s sh ownin Fig. 9(c).

Thu s i t i s c lear th a t addi t ives contr ibut e to th e abi lity to ach ieve m ild

flow wear . Th e d egree of contr ibu tion is as follows: TCP > st ear ic a cid > ba seO i l .

3 .4 . Wear a%risBased on th e wear m ode d iagram s, wear debris genera ted in each region

of th e d iagram for th ree k ind s of lu br icants was observed with a fe rrograp hictechn iqu e an d th e SEM [22, 231. The s ize of wear debris was mea su redfrom t he scan n ing e lec t ron micrograp hs a t th e in let pos i t ion of th e prec ipi ta tedpla te of wear debris . Th e nu mb er of wear debris p ieces mea su red was 40-50for each tes t .

Figure 10 sh ows the scan ning e lec t ron m icrograp hs of wear debrisgenera ted in th e m ild flow wear regim e, i .e . th e sp ecific wea r ra te is of th eorder of 10 -8 (m m 3 Nm - ‘). Th ese wear debr is were genera ted u n der th eexperim enta l condi t ions which correspond to point s A, D an d F in th e wear



129

,200 ‘\m

‘\,* Adhesive Wear

L$ AB A UC

ESevere A 0

- :;Ir

Flow WearA CJ100 A 0

A 0 0

0 8A0 Mi?d o

0 o MOW Wear

0 0.5 1.0V (m/s)

(4

Mild Flow Wear

0 -v (ill/s) .

@I

AdhesiveSevere Wear

FLOW \.Wear ‘*=_

AE o o‘--su_I

E

P ./ A 0 000 Mild Flow We%A OD o 0

0 0 0 0

I I

0 0.5 1.0V (m/s)

Cc)

Fig. 9. Wear m ode d iagram in lub ricated slidin g friction: (a) base oil; (b) base oil plu s 0.044wt.% stearic acid ; (c) ba se oil plu s 1.0 wt.% TCP. In the wear m ode d iagram (c), mild andsevere flow wear regimes are accompan ied by chem ical wear (separation of reaction layers).

mode diagrams shown in Pig. 9. Figure 11 shows the particle size distributioncur ve which corr esponds to th e scan ning electr on m icrogra ph s in Pig. 10.As shown in Pigs. 10(a)-10(i), filmy wear d ebris is ma inly genera ted whet heror not additives are mixed into the base oil. Most of them are less than 20pm in size for the base oil, 22 pm in size for stearic acid and 15 pm in

size for TCP. The par ticle dist ribu tion curve for th e base oil is similar toth at in stearic acid. These cur ves a nd th at for TCP, however, ar e different ,as shown in Pig. 11. Thus the shape of wear debris is almost the samewhether or not additives are used. However, the sizes are different, becausethe characteristics of plastic flow surface layers may be affected by the kindsof additives (25).

Figure 12 shows the scanning electron micrographs of wear debrisgenerated in the severe flow wear regime, i.e. the specific wear rate is ofth e ord er of 1O-6 (mm 3 Nm -‘). These wear debris are genera ted u nder th eexperimen ta l cond itions which corr espond to point s B, E and G in th e wearmode diagr am s shown in Pig. 9. F’igure 13 sh ows th e pa rt icle size distribu tioncur ves which corr espond to th e scan ning electron microgra ph s in Pig. 12.In th e severe flow wear regime, ma ny types of wear debris a re genera ted,a s sh own in P igs. 12(a), 12(d) an d 12(g). Amon g t h em , lar ge filmy (Pigs.



Fig. 10. Scan ning electr on micrograph s of wear debris genera ted in th e mild flow wear r egime:(a)-(c) bas e oil; (d)-(f) bas e oil plu s 0.044 wt .% st ea ric acid; (g)-(i) bas e oil plus 1.0 wt.%TCP.

12 (b), 12 (c), 12 (e), 12 (f), 12 (h ) an d 12 (i)) an d plat e-like wear d ebr is (Figs .12 (b), 12 (d) a n d 1 2(h )) ar e comm on in the thr ee kind s of lu brican ts . Inpa rticular , long wear deb ris , i.e . wea r deb ris with h igh a sp ect rat io, is eviden t,as sh own in Figs. 12 (b)-12(d) an d 12(i). Thu s, wear d ebris genera ted inth e severe flow wear regime is m eren t from th at in th e mild flow wearregime. The shape of wear debris generated in the severe f low wear regimeis s im ilar for th e th ree kind s of lu brican ts . Th e pa rticle s ize dis tr ibu tion ,

h owever, differs com pletely, as s h own in F ig. 13 . Th e pa rticle s ize of d ebrisgenerated in the base o i l and the base o i l p lus s tear ic ac id covers a wideran ge (in clu ding above 10 0 pm ). Wear debr is gen erated in the bas e oil p lu ss teak acid is larger than th at in th e bas e o il. However, wear d ebr is gen erated



131

0”tl; 0.05tz 0.5 m/s._ 58.8 MPa5

B0 50 100

Fig. 11. Particle size distribution of wear d ebris generated in the mild flow wear regime.

in t h e base oil plus TCP is less than 40 pm in size and smaller than inother lubricants. Figure 14 shows the scanning electron micrographs of weardebris generated in adhesive wear in the base oil. These wear debris weregenerated un der th e experiment al condition which corr esponds to point Cin Pig. 9. Large plate-like and block-like wear debris more than 100 pm insize ar e generated by severe adh esion an d separ at ion, as shown in Pigs.14(a) an d 14(b). Thu s th e sh ape a nd size of debris ar e str ongly depen denton the wear mode in each lubricant. The shape of debris generated in eachsimilar wear m ode is indepen dent of th e kind of additive; however, th e sizeis str ongly dependent on it.

Figure 15 shows the X-ray line analysis result of wear debris generatedin the mild flow wear regime using the base oil plus TCP. These resultsshow tha t th ere are two types of wear debris, th ose r ich in phosphorus an dth ose not rich. Relat ively large debris an d wear debris generat ed in th e severeflow wear regime were not r ich in phosphorus. Thus, wear debris generat edin the base oil plus TCP is not always rich in phosphorus.

4. Discus s ion

In Pig. 9, th e locat ion of each possible region in th e wear m ode diagra mchanges, depending strongly on the lubricant, i.e. the kind of additive. Thusa wear m ode diagram can be u sed to chara cterize lubr ican ts with respectto their ability to achieve a mild flow wear condition.

The specific wear rate is summarized in Table 4. Prom Pig. 4, Table 4an d P ig. 9, the specific wear ra te is of similar ma gnitude if th e predominan twear mode is almost the same, whether or not an additive is used. Therefore,it might be said that mixing an additive to the base oil does not alwaysmean a decrease in wear r at e, but, ra th er, an increase in th e ability to a chieveth e low wear r at e cond ition, i.e. t he m ild flow wear r egime.

Pr om P ig. 4(c) an d Table 4, the specific wear ra te in th e severe flowwear regime using th e base oil plus 1.0 wt.% TCP is a litt le sma ller tha nth at in the base oil and the base oil plus 0.044 wt.% stear ic acid. It mightbe becau se reaction layers prevent th e sudden t ra nsition from a mild condition



133

D (wn)

Fii. 13. Particle size distribution of wear d ebris generated in the severe flow wear regime.

Fig. 14. Scannin g electron micrographs of wear d ebris generated in the adh esive wear regime.

Fig. 15. WDX anal ysis of wear deb ris genera ted in bas e oil plu simage and P Ka line analysis.

1.0 wt .% TCP: (a)-(b) SEM

debris int o th e wear su rface a nd accordin gly th e am oun t of plas tic flow ofsur face layers. Therefore, it might be said tha t TCP has the effect genera tingrela tively fine wear debris. In th is case, t he am oun t of genera tion of weardebris in the severe flow wear regime is much larger than that in the mildflow wear regime, a s shown in Figs. 10 and 12. Ther efore, when TCP ha s

Effects of Additives on Wear Mode and Morphology of Wear

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