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Surface analysis of boundary-lubricated spherical roller thrust bearings
WEAR Wear 215 (It)~8) 156-164
ELSEVIER
Surface analysis of boundary-lubricated spherical roller thrust bearings Ulf Olofsson *, Senad Dizdar Ala('kim' Eh'met~lx. i)e/mrtment ~f Mai'hirl~" De.~igo. Rro'al hs,~tirut¢ ~?f T¢('lmoh,gy. S- I00 44 Srockhohu. Swe~k.~l Received 28 April iq97; accepled 31 October [q97
Abstract
B~mnda~'-lubricated spherical roller thrust bearings have been experimentally studied, Quantitative depth profile analysis of boundary layers by glow discharge optical emission spectroscopy (GD-OES). three-dimensional surface topography measurements and frictinn measurements were performed. The results show that bath the boundary layer and the wear of the cont~lcting surfaces were affected in a statistically :~ignilicant way by a change of the lubricating oil. Parametern fi)r the characterisation of the boundary layer structure enrichment by trib~:hemicaUy active chemical elements have been suggested, ~_~t998 Elsevier Science S.A. Krv,~,rd.w Spherical roller thru~,l I'~.,aring~,-Boundary lubrication: Bound:try htyers: Wear: SurRice topography: Quantilati'¢e deplh prolile an:dy.~is: GD-OES
I. Introduction In ball bearings as well as spherical roller bearings there is always sliding present in the contact due to the curved contact surfaces i see Hailing I ! l and Kellstrbm 121 I. In Fig. I the slip directions are presented for an elliptical contact. The arrows shown in the contact represent the directions of slip when the hall is rolling into the paper, There are two points along the contact where the sliding vehvcity is zero. At all other points along the contact, sliding is present. Olofsson 131 characterised the wear in I'~mndary-lubricated spherical roller thrust bearings. Outside the zero sliding points there were signilicanl changes in the washer pn)liles due to wear. The.se changes greatly affect the fatigue life of the bearings (see Olofsson 141 ). Olofsson and BjOrklund 151 used 3-D surface measurements and analysis to chan~clerise the differeat wear mechanisms that occur in boundary-lubricated spherical n~ller thrust bearings. In Ref, [ 6 l, Oiofsson used a fractional factor experiment, series to study how the contacting ~urtace.,, of the bearing were affected by the rotational speed, the load. the hardness of the contacting surfaces and the type of lubricant, The results show that the fatigue life and the amount of wear on the washer surlaccx were affected by the type of lubricant and the a~tational speed. A change of the load or a change of the hardness of the contacting surfaces did not influence the ,~'ear or the fatigue life of the contacting ~,urt-aces. " ('.,rle-,ponthng uulh~r on4~.-164~/~.JS/$19AlO ,- I*Y.,*~l'l~'~ict Science S.A. All rigllt', ~¢,,cr~ed PI! S 0 0 4 3 - 1 6 4 H ( t ) 7 1 o n 2 5 9 - 7
~ro dicing points Fig. I. Slip line', in ;~tl elliptical ~.'JJtlttu¢l,
Wear in boundary-lubricated rolling and sliding contacts could be significantly reduced by a tribochemical (chemical reactions controlled mostly by the friction heat ) modification of tile boundary layer structure as well as by insuring proper loading conditions for an "in situ' dynamic equilibrium between continuous failure and (re) formation of the boundary layers, The aim of this investigation is firstly, to study the tfibochemical modilication of the boundary layers obtained by two conmtercial lubricants under different loading conditions of bound,,-y-lubricated spherical roller thrust bearings, and secondly, In study how the structure of the formed boundary layers is correlated to the wear and coefficient of friction of the contacting surfaces. 2. B o u n d a r y lubrication a n d b o u n d a r y layers Boundary lubrication (see reviews of Chichos [7] and Goodfrey 181 I is according to ISO 4378 delined as " a type
157
u. olot~l.son. 5. Di:&tr/Wear 215 (i~)9~¢J 156-164
of lubrication in which friction and wear between two surfaces in relative motion are determined by the properties o f the surface and by properties of lubricant other than bulk viscosity.'" The tribophysical and tribochemical properties of a lubricant are expressed in the ability to form the very thin boundary layers ( from a few nanometers up ton few micrometers ) on the contact surfaces by the action of particularactive substances (additives). In the lubricated sliding and rolling contact situations when full film lubrication fails or cannot be formed, the boundary layers act as a boundary lubricant, i.e., they are easily sheared and successfully separate the contacting surfaces. The boundary layers formed by physical or chemical adsorption of hydrocarbon molecules at the surlace cannot be exposed to a higher contact loading because of their expressed instability under high temperature, approximately over 150°C. in contrast, the boundary layers formed by tribochemical reaction between additives and the metallic contact surface are more stable, are formed only under high loading, and can be exposed to even higher contact loading conditions i 7 I. In fact. these I'~mndary layers are a mixture of products of tribochemical reactions between the metal oxide surface and the lubricant, especially tribochemically active substances based on P. S, N. Zn. Ca, CI. etc. An unavoidable and dominating component of the boundary layers formed by trib~x:hemical reactions are oxides at the metal surfaces. Sullivan 19.101 has written a general review concerning the existence and role of oxides in wear reduction and protection against the transition to severe wear. as well as factors affecting the dynamic equilibrium of continuous failure and ( re )formation of the oxides in the boundary layer. The influence of the boundary layer structure on the friction and wear properties of boundary-lubricated contacts has been widely reported, but not comprehensively explained, The standards for lubricant testing accepted by national and international standard bureaux, as well as by the major oil companies have mostly been empirical and experimental in nature. Published models in the lield of boundary htbrication deal in different ways with the influence of the structure of the boundary layers formed on the contact surface. The mtxlel developed at IBM by Bayer et ai. ( sec review by Beerbower I I I I ) takes the shear stress in the contact area as the shear stress of the base (volume) material, Rowe-Kingsburry's adsorption-desorption model I I I I concerns reversible adsorption-4.1esorption of the boundary layers but does not deal with irreversible tribochemical princesses, such as oxidation and surface reaction with the additives, which are of key importance under regimes other than mild wear. Corrosive inodels lbr very particular areas of boundary lubrication. such as those of Tao, Kreuz and Fein I I I I. approach the modelling of boundary lubrication by way of irreversible tribochemical processes on the worn contact surface. The kinetic temperature model suggested by Matveevsky and Buyanovsky I 121 concerns the forming (by tribochemical reaction ) and failure ( transition to seizure ) temperatures of boundary layers. According to the authors, this model has a
reasonably good agreement with experimental results. A model concerning adsorption-desorption and oxidation in the boundary layer applied to a particular tribopair has been presented by Sullivan I 131. However, the published models include some variables which cannot be evaluated with very high confidence I 11-131, such as the energy of adsorption in the adsorption-desorption model or activation energies in the kinetic temperature model. A model of the oxide layers in friction processes is proposed by Zhao et al. I 14 i and Zhao and Liu ! ! 5 I- According to this model, the boundary layer consists of three sublayers see Fig. 2 ): ( 1) friction reduction layer, 4 II ) diffusion and reduction layer and ( Ill ) stable oxidised layer.
3. Experimental The experiments presented in this report have been described previously (see Olofsson 16l ). Spherical roller thrust bearings ( SKF 29416 E) were c h u r n as test objects and 16 tests were pertbrmed using two bearings in each test. The number of revolution.,; Ibr each test was 30 004), Spherical roller thrust bearings consist of lbur parts: the shaft washer. the housing washer, the cage assembly and the rollers. In this study, only the housing washer surface is analy~d, To be able to study how the boundary layers, the wear and the coeflicient o f friction depend on the rotational speed, the load. Ihe hardness and the type of lubricant, an asymmetric fractional factorial experimental design ( see Addelman 1161 and Margolin [ 171 ) was c h u r n . In this study, four factors arc varied at two levels and one factor is varied at three l e v e l s l a s described in Table I. In Table 2 the running purameters arc presented. Note that both lubricants have the same viscosity 168 mm-'/s) at 40°C. At IOO°C the vi~osily is 9 tom-'Is and i I tom-'Is for the mineral oil and the synthetic oil respecti rely. Both lubricants pass the 12th step in the FZG wear test according to DIN 51354. For standard spherical riffler thrust bearings, the washer hardness and the roller hardncss is 720 HV and 750 HV respectively. The difference in hardness between a standard washer and a special hardened washer was about 30 HV. For the rollers, the hardness difli~rence was about IO HV.
T
0i,¢~'~ t:~0wIre surface Fig. 2. Oxide layer model of the l~mndary layer,fnlm Zh:,o Cl ill.1141.
t5,~
U. Oh~[sson. S. Di:dar / Wear 215 ¢i~t~,~¢~156-164
Tabh: I Dc~,ign nmlrix fi~¢a~,ymm¢lricaf fraclinnal fau'lnri:d denign Te,,I no. 1
2 3 4 5 6 7 N tl in !I 12 13 14 15 [6
Rotational :,wed
Hardness t washer.,. I
H:,rdnesn ( roller ~
Load
Lubricant
low middle middk" high low middle middle high lms middle Middle high low middle middle high
h~w Ires Io~ loss high high high high low Io~ low Io~.~,' high high high high
low k,¢,' low I~w I~,~,~,' Inw lily,' le,,,v high high high high high high high high
low high high low hiEh low low high low high high low high low low high
low low high high high high low low high high low 1u¢, low low high high
Table 2 Ruiming pa:ameletn in lilt." as~ iii1|1c1ri¢ fra¢liollal I~1¢Io1tle~,igll Lov.
Middle
High
Retail*real ,,l'x:cd ~rpll ~.| Ilardrning x~a,her,. i|ardenmg rolicr~,
4 ,,landard :4a n d;.|td
21)
I.uhricam
SHII.I.'rF.IJ.US hSS ( millcr;d-tfibh:tscd I
-
I04) special Sl'~'cial 220 1~" )P, II, SHC 526 1,.ynthetic-oibba.,,¢d }
The asymmetr).' of this factorial design comes from the mixed levels of the investigated factors. By nlea,lS of factorial design, it is tx~ssible to study qualitative variables (type of lubricant I a~, well as quantitative variables ( nornml load and rotational speedL A factorial design makes it possible to calculate an average value, as well as the main effect of increasing, for instance, the normal Ioati. Moreover. it is pos.,,ible to calculate the effect of how two or nlore factors interact. such as changing both normal load and type of lubricaqt. An effect is defined as the diflL'rence in response on moving from a low level to a higher level. Often there is a certain hierarchy in terms of absolute magnitude. Main effects are often greater than tv,'o-faclor interactions, which in turn are often greater than three-factor interactions. If one is not interested in higher order interne)tiros or believes thai they can properly be disregarded, fractional factorial design can be used. Fraction:d factorial design requires I'ew'cr runs than full factorials, but fev,er interaction effects can be calculated. A full factorial design in tyro variables varied at two levels and one variable va,ried at three levels requires 48 tests. Compare thb, with the actual desigr ",,here t6 tests were run. In this ,.tud v onl~ the main effects are considered. In order tojustil~ the assumption of independent errors, tile spe¢ime)l.,, w e r e tested in randinn order. 'X'ates" algorithm I see Montgonlery I 181 ) was used when calculating the mean lexels and the main ctlL'cts. Note that
two main effects can be calculated from the rotational speed. which has three levels. R~,,, is the effect from moving from the |ow level to the middle level and R,,h is the effect from moving from the middle level to the high level. Three-dimensional surface roughness measurements were performed after the tests. Here the test equipment is a Rodenstock RM 6(~). which is an optical ibcusdetection instrument. The verilication of the optical focus detecting instrument is presented in a separate repurt (see Ololgson and BjOrklund
11')1), To qualitatively characterise the wear, a visual and parametric characterisalion technique was used. There are 14 3D surface parameters proposed in Ref. 1201. Besides the above mentioned 3-D surface parameters Olofsson and BjOrklund [ 51 used the 3-Dequivalents of the five parameters cv:duated froth the Abbot curve? to characterise the wear of worn spherical roller thrust bearings. Based on these results, two parameters were considered appropriate in this study, the properties of which are summarised below. The first parameter. S,e describes th:" amplitude distribution. S,~ is the root mean square value of the surface departures from the mean plane within the sampling area and gives a measure of how ' DIN. Mea~,urcmcnt of ~,ur[ac¢ roughn~;~; param,JIcr~, N~. Rs,~. R.~. M~..
M,, I'{Pr the ,.le~cription t)l the ina[¢ria[ F~rtion in the roughness. Gcrn~.an Standard. DIN 4776. ( I '~t~l I
U. 01o1~.~o.. S. D i : d u r / Wear 21511998j I.¢16-164
the roughness is affected by the different independent variables. The second parameter. Sd, is a hybrid parameter which combines wavelength and amplitude. S0¢ is the developed interracial area ratio of the surface. This is the ratio of the increment of the interracial area of a surface over the sampling area. According to Dor,g and Stout [211 this parameter can reflect the transition of the tribological process when a surface is worn. For a fine surface manufactured by honing, or grinding. the developed interracial area ratio is usually smaller than I C,~. For rough surfaces manufactured by turning or shaping, the values of the developed interracial area ratio may be larger than IOe~. Before evaluating the surface parameter.,;, the curved tbrm of the original surfaces were removed by least square fitting to a second degree polynomial. The surfaces were also filtered through a median filter. The coefficient of friction was measured in a ball-on-flat tribometer (see Hagman and Bjt~rklund 1221 I. A ball with diameter 5 mm. made of ball bearing steel, was pressed against the washer surface and the coefficient of friction was measured under lubricated as well as unlubricated conditions. The sliding amplitude was 5/.tm and the sliding veloeity was O. i mm/s. A normal load of 5 N was used. giving a Hertzian maximum pressure of approximately 1200 MPa. The maximum normal pressure for the bearings during tests was also about 1200 MPa. The quantitative depth profile analysis of the boundary layers has been obtained by glow discharge optical emission spectroscopy. GD-OES. The physical and chemical background of the quantitative depth profile analysis by GD-OES analysis can be lbund in the works of Hocquaux 1231 and Bengtsson 124 I. The analysis will be briefly presented with the help of a schematic diagram ( .see Fig. 3 ) of a glow discharge source (typically Grimm glow discharge lamp). The material of the sample surface is removed by cathodic sputtering in an atmosphere of a noble gas carrier (typically argon ). The atoms released from the surl~ce collide with each other and come into an excited state, producing a cathtxlic glow and resulting in plasma. The characteristic spectral lines for the elements removed from the surface as well as for the carrier gas are emitted from the plasma and measured by a fast. multi-channel optical spectrometer as a function of the sputtering rate. Then, inlbrmation about the depth variation of the elements divided by area of sputtering is obtained by the integration of the element signals at a very high sampling A t - inlet ~ . ~
/ - - - -
159
rote and is presented in a diagram. In addition, the technique provides simultaneous registration of all chemical elements of the boundary layer, and has approximately one order of magnitude better sensitivity and relative precision of the registration and quantification compared to X-ray photoelectron spectroscopy XPS (ESCA) or Auger electron spectroscopy AES. A very short time ( typically i00 s) is needed for analysis. leading to a high sample throughput and a relatively low cost per analysis~ Unfortunately. this spcctro.~opy provides no direct information about chemical bonds in the boundary. layer but valuable conclusions can be implicitly drawn. When the GD-OES analysis is applied to h~mndary layers. a phenomenon called prel~renlial sputtering can appear. The
o,
"1
Depth profile (nm)
Fig. 4. P;.traffleterr, u-.ed in the ail~dyr.i~ Of re'.,ullr, ttheained by the quanlilalive depth prolilc .'maly sb, by ( iD-OES. D,, ( nm "lis the depth o f friction* rcducli~,¢ ~+xid¢ layer--the depth of the cro~,sing poinl of the oxygen and iron clmccnU'alitm curv¢~, , r o.~.y~212ilcnfichng:li[ dcplh hi Ire h,undary laycr~ and/)a, ( nail ) is the total pznetration t enriehtllelg ) &'pth of the aftra:heroically active ckrffu:nl> ~TCHAEL i.e.. sum . f deplhs '.'~.hen their 9.ei.=.hl ¢or~en.,ration is d.:crea.~d to I',~" in the hmndary layers.
Vacuum pump • -- Insulator
Sample
O - ling Vacuum pump
~---~
"~-
Cooling
Fig. 3. Schetnat[¢ diagram ol a Grimm glow di.~,'harg¢ lamp. trom tlocquaux
123t.
Fig. 5. Photograph of measuredImusing w~L~her.The reclan~.lem.',rk~ the ltd,'alton and the .size ~1"Ihc mea..,urcd arcs. The cot~rdlnalu ~,y:,lcm .,,how.,,Ihe measurement directions in the residual term plot~,.
U. Oh!l~:~.n. S, l)iz~hlr/Wear 215 ( 19981 156-164
I(vl)
order of sputtering and the sputtering rate of atoms in the boundary layer can depend on their of chemical environment. as well as on the distribution of the compounds within the boundary layers. Therefore, the results of quantitative depth analysis of the boundary layers should be taken as a lirsl order approximation. Dizdar and Andersson 1251 have observed, using quantitative depth prolile analy.sds by GD-OES, that even at very low sliding velocity the friction coefficient is greatly influenced by presence of the tribochemicali~, active substances in the boundary layer. A correlation was observed between the phenomenology of the structural changes and changes in the wear properties. in order to describe this correlation, the Iollowing parameters are suggested ( see Fig. 4). ( I ) D,~ (rim) as the depth of friction-reductive oxide layer--the depth ~,f the crossing point of the oxygen and iron concentration curves or oxygen enrichment depth in the boundary layers. (21 DI: (nm), the total enrichment depth of the tribochenfically active substances, calculated as the depth when
the sum of weight concentrations o f the chemical elements are decreased to I C~ in the boundary layers. In this study tribochemically active elements are phosphorus, zinc and sulphur. The mechani:~m of the surface enrichment by the tribechemical active substances is complicated, and many different chemical substances are used in order to ensure the enrichment. Theretbre, it may be reasonable to take the resulting effect, the total enrichment depth of the chemical elements of the tribochemically active substances, as a parameter. Usual steel impurities like Mn and Si are normally below 0.30% Ibr ball-bearing steel. Fig. 5 is a photograph of a tested housing washer. The part of the surface that is investigated with the different analysis Iools is marked with a rectangle, The two circular bands on the housing washer surface are the zero sliding points and thus the investigated surface is midway between the two zero sliding points. This part of the surface is subjected to wear (see Ololgson 131 ). Two 3-D surface measurements were Test 1
100
New 100
75
75
'
~
Fe
o< tO
.--o-- N ~ p
1-
"--"-- S r
o
(,~
Ii
-----ca
J=
._m (1)
"~
50
O C
25
25
j
0 0
25
50
75
100
.
I
2,5
.
I
50
.
|
75
.
|
100
Depth profile (nm)
Depth profile (nm)
l:tg, t), rcsult~ Irllzll (~D-OI-S and 3-l) hurl'ace IIK'a.~urt'llI¢lll~., |1¢~ ~ln~'orlt ~LIrILI~¢. I),j = _~ hill, 1~11 ~4 IIIII. "~',i = ()'t)t pIlL -~)'d,= I(}.~',;. ~ZI =O. L2,
Fig. 7. Rc,'.uh~. f r o m GD-OI:'S and 3 - D surface meusurcill¢lflS. "1"¢~! I. rola(iongd P,pced 4 rpm, lubricaling oil Shell TeUu,,, (~8S. I ) . = II nm. D i = 2 7 iin|, ~,~= ~).l~'+ p.m. S,h = U.4',+. ~t +~0.20, ltq = ( L ] 2 .
161
U. Ohg¢,wm. S, Dizdar /'Near 215 (19<)8) 156-164
performed on each housing washer. The surface was measured with a IO ,am sampling interval and 128× 128 da~a points in trace and onhogonal trace direction::. The test samples for analysis with G D - O E S were first cut out from the same radius of the washers in the form o f 7 × 7 × 7 mm cubes, degreased carefully with trichlorethylene and acetone in an ultrasonic bath. The cut pieces were then embedded in a low melting temperature (about 150°C) casting alloy in order to form disks suitable for analysis on a LECO G D S 750 spectrometer. The test samples for friction measurements were also cut out from the wa:+hers in the form of 7 × 7 × 7 mm cubes. Before the test. the pieces were d e g r e a ~ d carefully with trichlorethylene and acetone in an u l t r a ~ n i c bath, For the lubricated tests, the same oils as were u ~ d during the bearing tests were applied to the corresponding test pieces. Thus, the lubricant based on mineral oil Shell Tellus 68 S was applied to pieces that were tested with that mineral oil and the synthetic oil Mobil SHC 526 was applied to test pieces that were tested with that synthetic oil. T w o friction measurement were performed on each housing washer.
4.
Results
The results from the quantitative depth profile analysis by GD-OES- and 3-D surface topography measurements for an unworn housing washer are presented in Fig. 6. Figs. ( 7 ) ( i O) p r c ~ n t results from test I, 9, 12 and 4 respectively. The differences between test I and test 9 are the typ¢ o f lubrk-ant and the roller hardness. Olofsson 16] showed that in this ease a change of the roller hardness did not influence the wear and fatigue life of the beatings. The difference in the wear shown in the residual plots are due ¢o the two types of lubricant, it is interesting to notice the relatively high concentration o f phosphorus (P) and zinc ( Z n ) in test 9, where the syntheticoil-based lubricant was used. Both tests performed at low rotational speed and with a synthetic-oil-based lubricant produce relatively deep layers of P and Zn. and at the .same time the surface was relatively unaffected by wear ( test 5 and test 9). These results can be compared with the rgsults from the two test~ performed at low rotational speed and with the
Test 9
100
T e s t 12
100
75
75
+--=--- Fe ---o--
.<
+
.5 50
elL1
8 6
.--~--
p
~ S
50
-
Cr
Ca
O
0
0
---~-- N
---~--- Zn
25
2.~
0
25 50 75 Depth profile (nm)
25
1O0
Depth
g,
50 prtite
75
100
(nm)
g
"8
t
.~ t/J
°
W
-j
Fig. 8. Results from GD-OES and 3-D surface measaremems. Tcs! 9. rotational speed 4 rpm. lubrLcatim 2 ~fi|Mobil SHC 526. Do = 14 nm. l)l = I.L7
Fig, 9, Rcsuil.~ from GD-OES :rod 3-D surl'~c mca..,urem~m~.. Te,~t 12. ndatitmul ~i:~¢d IlK) rpm, luhrieating oil Mobil SHC 526. !.)+) = 9 nm, DI: = 18
nm. S,, = n..x7 #m. s,, = 4.(F/+ + p.,. = o. 12. ,ul = O. 14.
nm. S. I = n.57
p.m. S,~, =
5..J,c/~./~c = O. 14. ~ , = i L 15.
U. Olof~'son S. Dizdar / Wear 215 (1998j 156-164
162
"t'able 3
Test 4
100
Experimentally determined parameters f r o m quantitative depth profile analysis. 3-D surface analysis and friction measurements
75
---o--- N ~ p
g o ~
¢-
50
8 8
• Cr ~ C a
e.
,z-
25
.
o
s
.
25
•
.
t
50
75
.
Test no.
I)c~ (nml
Dr. (nm)
Sq
S,i,
(/J.m )
(~)
[ 2 3 4 5 ~• 7 8 9 It} II 12 13 14 15 16
I1 8 16 7 12 9 7 6 14 8 9 ~; I1 7 II 8 5
27 28 196 I1 139 26 23 14 147 32 28 18 51 20 42 19 4
0.06 O, lU ().69
0.4 0.7 6.2 3.8 2.2 5.4 2.6 4.2 4.l) 2.9 0.6 5.4 0.4 2.8 2,4 5.6 I 1).2
New
0.42 0.27 0.54 n.30 0.46 0.47 0.33 0.10 0.57 0.n4 0.33 0.26 (I.61) 0,91
p,,
~l.
0.20 0.12 0.14
0.12 (I. 12 0.14
O. 13 0.16 0.14 0.09 0.14
O. 15 (I.14 0.14 n. 12 0,12
0.12
0.14
0.I I
().15
0.11 0.14 II. I l a. I " (1.12 0.14 I). 12
n.13 0.15 n.15 (I.13 0.15 0.13 -
100
Depth profile (nm)
8
*d Fig. I0. Results from GD-OES and 3-D surface measurements. Test 4, rotational ~,pecd I ~ l r p m , lubricating oil Shell Tellus fibS. 1)~, = 7 ntl), Dj = I l n m . S, I = 0 . 4 2 ~ m . S,, = 3.8q~. P r = n. 1.1. b h = O. 15.
mineral-oil-based lubricant (test I and test 13). Here the surlace asperities were worn down to an almost mirror smtx~th surface and the low concentration of P and Zn persists in the results from the GD-OES analys~s. In test 12 and test 4 the rotational speed was at the high level and the mineral-oilba.~d and the synthelic-oil-ba,~ed lubricant ~,rere used respec-
tively. The results from the 3-D surface topography measurements show that the surface is relatively unaffected by wear and the GD-OES results show that low concentration of P and Zn enrichment persist in both cases. In this case, the high level of the rotational speed leads to a higher film thickness and thus fewer asperity interactions and less wear. The dynamic equilibrium of continuous fidlare and reformation of boundary layer seem to be obtained in combination of the low rotational speed and the tribochemicM properties of the synthetic-oil-based lubricant. Table 3 presents results from the parameter character(sat(on of the boundary layers, the worn surlaces and the coefficient of friction. In Table 4 the resnlts from the factorial analysis are presented. A presenled main effect is a change in the response when moving from a low level to a higher level For example S, is increased by 0.2 p,m when we change the lubricating oil from the low level (Shell Tellus 68S) to the high level (Mobil SHC 526). One can also interpret the calculated effects as a rank of this specific parameter influence on the response. The greater the effect, Ihe more the response
Table 4 Ro.ull~. f r o m Iattorial Lle,,ign I'z+r p~rarmzlq+~, f r o m quazllitativ~: depth prolile anai)'.~i~,. 3 - I ) surf.'u:¢ analyst.', and friction mcasurctncnts
Mean Vek~:ily Io~- ~ middk. Vch~:ity m i d d i c ~ h i g h H a r d e n i n g v,a,,,her l o w ~ high H a r d e n i n g n~ller l o w ~ hitch [.x;ad l o w - , high I.uhricanl l o w ~ high
"t Sl~i~,licall)" .,,ignilieallt main ¢1"t~'¢1.
D,, t nnl I
Di Inm I
ij.Cp - -.2.4" - 2. I * - 1.4 I). I nA
-
2. t *
.~ 1.9 399 35.S I 9.[t 13, I 23.8 5().0"
.~;,~ t MAn )
S,+, { G I
P-r
P-I
n.35 O. 13 O, Ifl O,IX)9 - 0,017 - n.04.s
3.2 1,34" 1.67" 0.22 - O. It} - n,50
(), 13 - 0.nl 5 0.t~.)8 - 0.(H}8 - n.n[7 - 0.{))4
0. t 4 - O.(H)3 O.O(ll - ().(X|2 O.(X)9 0.(XX)
0.20 ~
13)4 *
().(X)4
0.012"
[I. Oh~l~'sott. S. Di:.dar l Wear 215 r 1~9,% 156-164
is influenced by this particu:ar effect. Two methods were used to check if the results were .statistically significant. For the 3D surface topography parameters and the coefficient of friction replicate measurements were performed and the results from the analysis were considered statistically significant if the calculated value was larger than twice the standard devi,lion. Another test was used for the two parameters in the quantitative depth profile analysis by GD-OES. Since no replicate runs were performed in this analysis, the estimates of the effects were ploaed on normal probability paper. The effects that are negligible will tend to fall on a straight line on this plot. whereas signilicant effects will not lie along the straight line (see Montgomery 11811. It is interesting to notice that a change of the type of lubricant have a statistically signilicant effect on five of the six investigated parameters. That is, the tribochemical contents of the boundary layers (Do and De.). the surface roughness (S, and S,j,) and the coefficient of friction under lubricated conditions ( #t. } were all affected in a statistically significant way by a change of the lubricating oil. Also a change of the velocity influenced the tribochentical contents of the boundary layers and the surthce roughness in a statistically significant way. For the coefficient of friction under unlubricated conditions ( # v there were no significant calculated main effects. One possible interpretation of this result is that the tribochemical reactions in the boundary layers do not affect the coefticient of friction in a statistically signilicant way. This in contrast to the wear of the surface asperitie:: that was greatly reduced when high concentratimJs of phosphorus (P) and zinc ( Zn ) were measured in the boundary layers.
$. Conclusions The inllucncc of two commercial lubricants on the surface topography (asperity wear}, friction and boundary layer properties of boundary-lubricated spherical roller thrust bearings have been investigated. By combining results front GDdES analysis. 3-D surface topography measurements and iYiction measurements the Iollowiug conclusions can be drawn from the experimental results. ( I ) At low rotational speed the synthetic-oil-based lubricant (Mobil SHC 526) builds up a boundary layer with relatively high concentration of P and Zn. (2) The mineral-oil-based lubricant ( Shell Tellus 68 S } shows no tendency to lbrm boundary layecs with relatively high concentration of P and Zn at !ow rotational speed. (3) The factorial analysis shows that both the boundary layer structure and the w0ar of the surface asperities of the contacting surfaces arc affected in a statistically significant way by a change of the lubricant. (4) The suggested parameter set for the charactcrisation of the boundary layer structure enrichment (Do and D;=) together with 3-D surface parameters as S, and S,;, proved to be very useful when characterising the surface topography
163
and ~.ribochemical changes in boundary-lubricated spherical roller thrust bearings.
AckJ~owledgements The authors wish to thank Dr. Arne Benglsson at The Swedish Institute for Metal Re.arch in Stockholm forexlensire work on conducting the GD-OES analysis. Profes.,a~r SOren Andersson and Dr. Stefan Bjtirklund made many valuable comments on this manuscript. This work was financially supported by the Swedish R e . a r c h Council for Engineering Sciences (TFR). the Natmnal Board for Industrial and Technical Development ~NUTEK). H~igglunds Drives AB and SKF Sverige AB.
Appendix Nrmw/lclcHttrd
D~.: total enrichment depth of the tribochemically active elements ( phosphorus, zinc and sulphur) in the boundary layers Do oxygen enrichment depth in the boundary layers or depth of the crossing point between the oxygen and iron concentration curves H r effect of changing the roller hardness from the low level to the high level H,, effect of changing the washer hardness from the low level to the high level L., effect of changing the load from the low level to the high level L,, effect of changing the type of lubricant R=,,, effect of changing the rotatitmal speed from the low level to the middle level R,,,, effect of changing the rotational speed from the middle level to the high level Sq r~x~t mean square deviation of surface S,;, developed interracial area ratio /z~. coefficient of friction under lubricated conditions /.t;. coefficient of friction under unlubricaled conditions
References t I t J+ Hailing. The microslip helWCCn a hall and il.,, track in hall-thrm, I bearings, hmrna! of Basic Engineering. March i lq~b6i, 213-22n. [21 M. K,:ll.,,~riim. R~dling conlact guidance {ff mi|crs in spherical roller bearings,. ASME Paper 7q-LUB-23. A S M E / A S L E Lubricathm Con[:~rence. ()ctoh~:r ( 1970 ). 131 U. Olofsson. C'haractcrisatio~ ~'wear in I~madary luhricalcd sp~.ricaJ ro}[¢r Ihru.,~t hearing.',, ac¢cplcd fi~r prcscnlali*m in Wear 21)8 ( 1¢,'97) 194-203. [41 U. Ohd~son. Faziguc iife rcduclion due h~ wcar in ~mndary lubficaicd spherical roller thru.,,I hearings. Wear 207 I 1~,~)7~ 74-78.
t fi4
If. Oh~s'sn~;. X Di:dar/Wear 215 t1998~ 156-164
[51 U. Olofsson, S. Bjiirklund. 3-D surface analysis of worn spherical roller thrust beatings, Metrokrgy and properties of Engineering Surfaces 7th lmemational Conference. (ibtel'mrg. Sweden. 2-4 April t 1997).
16 b U. Olol:sson. An experimental investigation of wear and finigue liJe n:duction of hmndary lubricated spherical roller thrust bearings. accepted for presentatkm in Trilxflogy International ( 1997 ). [ 7 ] H. Chicht~s. Tril~flogy--A System Approach to the Science and Technology of Friction. Lubrication and Wear. Elsevier. Amsterdam (1978). l SI D. Gcatt~y, Boundary lubrication, in: P.M. Ka, |nterdisciplinao' Approach [o Friction and Wear: Proceedings of NASA Symia~sium ( 1967 ). San Antonio. TX: NASA, SP- 18 I. Washington ( 1968 ). I gl J . L Sullivan, The r~fle of .xides in the protection or tribological surt/u:es: Part I. IMchE J. Mech. Eng. CI43 (1987) 283-291. I lO[ J.L Sullivan, The role of oxides in the protection of ~ribotogi,'al sud~'es: Pan 2. IMchE,l. Mech. Eng. C 144 (1987) 293-3|)1. I l I [ A. Beerl~wer. A critical sulx,cy of mathematical m~slel for boundary lubrication, ASLE Trans. 14 i 1971 t 90-1{)4. [t21 R.M. Matveevsky. I.A. Buyano,.sky, Principal characteristic of Ixmnda~' lubrication. ASLE Trans. 30 ! 1987 ) 526-53{). ] i31 J.L. Sullivan. BoundaD' lubrication and oxidalional wear, J. Phys. D, AppL Phys. 19 (19~6) 1999-2Ul I. [ I41 Y W. Zhao, LJ. Liu, L.Q. Zheng. The oxide film and oxide coating on steels under brenda D' lubrication. Rrtx:ecdings front t6lh LeedsLyon Symlx~siumon Trihdogy t 1989l 305-31 I. I 151 Y.W. Zhao, JJ. Liu. The nature of the friction transition as a I'wtction . r load and speed h*r the steel/steel system. ASME/STLE Tribol. Trans. 33 t 19tJ()) 6484)52. I 16 ] S. Addclman, Orthogonal main-efl~zct plans fi,r asymmetrical factorial expert.hems. Techuomctrics 4 ! 1~2 ) 21-46. [17] B.H. Margolin. Systematic method~ I.r analysing 2"Y" factorial experitm'nts with applications. Technometrics 9 ( 196.7) 245-259. I 181 D.C. MontgomeD'. Design and Analysis of Experiments. Wiley, New York ( 1991 t. l l9] U. Olol\sson. S. Bj~rklund. Verilication of a noncontacting threedimensional surface roughness, nlea.surcment system. TRITA-MMK Ib'q~:2. Royal Institute of Technoh)gy. Stockholm. Sweden (1996.). 12hi K..I. Stout PJ. SullKan, W.P. D(mg. E. Mainsah, N. Luo. T. Malhia. H. Zahouani. The Development of Methods for the Characteri~,ation
121 I
1221
1231 1241 [ 251
of Roughness in Three Dimensions. Publication no. EUR 15178 EN of the Commission of the European Communities. ISBN 0 7044 1313 2 ( ~993l. W.P. I~mg, K.J. Stout. An integrated approach to the character|sat|on of surlhce wear: l. Qualitative character|sat|on. Wear 18 I- 183 ( 1995 ) 7(~).-716. L. Hagman. S. Bj(wklund, Micro-slip measurements in contacts subjetted to normal ~'md tangential loading. 7th Nordic Symposium on Tribology Nordttib96, Bergen ( 1996 I. H, H(n:quaux, Thin iilm analysis, in: R.K. Marcus (Ed. L Glow Discharge Spectroscopies. Plenum. New York ( 19{)3). A. Benglsson. Quanlitative depth prolile analysis by glow discharge. Spectrochim. Acta 49B ( 1994l 411-429, S. Dizdar. S. Andersson. Friction characterisadon ofl~mndary layers at very Jo.w sliding velocity. 7th Nordic Symposium -n Trihology Nordtrib96. Bergen (1996).
Biographies UIf Olofsson received in 1987 a MSc degree from the Royal ]nslitute of Technology, Stockholm. Sweden. Between 1987 to 1994. he was employed as a research engineer at the Swedish National Testing and Research Institute. He received in 1994 a Licentiate of Engineering degree at Cha|mers University of Technology and in 1996 a PhD at the Royal Institute of Technology. Presently. he is a research engineer at the Department of Machine Design, the Royal Institute of Technology, Stockholm, Sweden. Senad Dizdar received in 1989 a MSc degree in Mechanical Engineering from University in Mostar, Bosnia and Herzegovina, and in 1997 a Licentiate of Engineering degree from the Royal Institute of Technology. Stockholm, Sweden. At present time, he is a PhD student-researcher at the Department of Machine Design. the Royal Institute of Technology. Stockholm, Sweden.
ELSEVIER
Surface analysis of boundary-lubricated spherical roller thrust bearings Ulf Olofsson *, Senad Dizdar Ala('kim' Eh'met~lx. i)e/mrtment ~f Mai'hirl~" De.~igo. Rro'al hs,~tirut¢ ~?f T¢('lmoh,gy. S- I00 44 Srockhohu. Swe~k.~l Received 28 April iq97; accepled 31 October [q97
Abstract
B~mnda~'-lubricated spherical roller thrust bearings have been experimentally studied, Quantitative depth profile analysis of boundary layers by glow discharge optical emission spectroscopy (GD-OES). three-dimensional surface topography measurements and frictinn measurements were performed. The results show that bath the boundary layer and the wear of the cont~lcting surfaces were affected in a statistically :~ignilicant way by a change of the lubricating oil. Parametern fi)r the characterisation of the boundary layer structure enrichment by trib~:hemicaUy active chemical elements have been suggested, ~_~t998 Elsevier Science S.A. Krv,~,rd.w Spherical roller thru~,l I'~.,aring~,-Boundary lubrication: Bound:try htyers: Wear: SurRice topography: Quantilati'¢e deplh prolile an:dy.~is: GD-OES
I. Introduction In ball bearings as well as spherical roller bearings there is always sliding present in the contact due to the curved contact surfaces i see Hailing I ! l and Kellstrbm 121 I. In Fig. I the slip directions are presented for an elliptical contact. The arrows shown in the contact represent the directions of slip when the hall is rolling into the paper, There are two points along the contact where the sliding vehvcity is zero. At all other points along the contact, sliding is present. Olofsson 131 characterised the wear in I'~mndary-lubricated spherical roller thrust bearings. Outside the zero sliding points there were signilicanl changes in the washer pn)liles due to wear. The.se changes greatly affect the fatigue life of the bearings (see Olofsson 141 ). Olofsson and BjOrklund 151 used 3-D surface measurements and analysis to chan~clerise the differeat wear mechanisms that occur in boundary-lubricated spherical n~ller thrust bearings. In Ref, [ 6 l, Oiofsson used a fractional factor experiment, series to study how the contacting ~urtace.,, of the bearing were affected by the rotational speed, the load. the hardness of the contacting surfaces and the type of lubricant, The results show that the fatigue life and the amount of wear on the washer surlaccx were affected by the type of lubricant and the a~tational speed. A change of the load or a change of the hardness of the contacting surfaces did not influence the ,~'ear or the fatigue life of the contacting ~,urt-aces. " ('.,rle-,ponthng uulh~r on4~.-164~/~.JS/$19AlO ,- I*Y.,*~l'l~'~ict Science S.A. All rigllt', ~¢,,cr~ed PI! S 0 0 4 3 - 1 6 4 H ( t ) 7 1 o n 2 5 9 - 7
~ro dicing points Fig. I. Slip line', in ;~tl elliptical ~.'JJtlttu¢l,
Wear in boundary-lubricated rolling and sliding contacts could be significantly reduced by a tribochemical (chemical reactions controlled mostly by the friction heat ) modification of tile boundary layer structure as well as by insuring proper loading conditions for an "in situ' dynamic equilibrium between continuous failure and (re) formation of the boundary layers, The aim of this investigation is firstly, to study the tfibochemical modilication of the boundary layers obtained by two conmtercial lubricants under different loading conditions of bound,,-y-lubricated spherical roller thrust bearings, and secondly, In study how the structure of the formed boundary layers is correlated to the wear and coefficient of friction of the contacting surfaces. 2. B o u n d a r y lubrication a n d b o u n d a r y layers Boundary lubrication (see reviews of Chichos [7] and Goodfrey 181 I is according to ISO 4378 delined as " a type
157
u. olot~l.son. 5. Di:&tr/Wear 215 (i~)9~¢J 156-164
of lubrication in which friction and wear between two surfaces in relative motion are determined by the properties o f the surface and by properties of lubricant other than bulk viscosity.'" The tribophysical and tribochemical properties of a lubricant are expressed in the ability to form the very thin boundary layers ( from a few nanometers up ton few micrometers ) on the contact surfaces by the action of particularactive substances (additives). In the lubricated sliding and rolling contact situations when full film lubrication fails or cannot be formed, the boundary layers act as a boundary lubricant, i.e., they are easily sheared and successfully separate the contacting surfaces. The boundary layers formed by physical or chemical adsorption of hydrocarbon molecules at the surlace cannot be exposed to a higher contact loading because of their expressed instability under high temperature, approximately over 150°C. in contrast, the boundary layers formed by tribochemical reaction between additives and the metallic contact surface are more stable, are formed only under high loading, and can be exposed to even higher contact loading conditions i 7 I. In fact. these I'~mndary layers are a mixture of products of tribochemical reactions between the metal oxide surface and the lubricant, especially tribochemically active substances based on P. S, N. Zn. Ca, CI. etc. An unavoidable and dominating component of the boundary layers formed by trib~x:hemical reactions are oxides at the metal surfaces. Sullivan 19.101 has written a general review concerning the existence and role of oxides in wear reduction and protection against the transition to severe wear. as well as factors affecting the dynamic equilibrium of continuous failure and ( re )formation of the oxides in the boundary layer. The influence of the boundary layer structure on the friction and wear properties of boundary-lubricated contacts has been widely reported, but not comprehensively explained, The standards for lubricant testing accepted by national and international standard bureaux, as well as by the major oil companies have mostly been empirical and experimental in nature. Published models in the lield of boundary htbrication deal in different ways with the influence of the structure of the boundary layers formed on the contact surface. The mtxlel developed at IBM by Bayer et ai. ( sec review by Beerbower I I I I ) takes the shear stress in the contact area as the shear stress of the base (volume) material, Rowe-Kingsburry's adsorption-desorption model I I I I concerns reversible adsorption-4.1esorption of the boundary layers but does not deal with irreversible tribochemical princesses, such as oxidation and surface reaction with the additives, which are of key importance under regimes other than mild wear. Corrosive inodels lbr very particular areas of boundary lubrication. such as those of Tao, Kreuz and Fein I I I I. approach the modelling of boundary lubrication by way of irreversible tribochemical processes on the worn contact surface. The kinetic temperature model suggested by Matveevsky and Buyanovsky I 121 concerns the forming (by tribochemical reaction ) and failure ( transition to seizure ) temperatures of boundary layers. According to the authors, this model has a
reasonably good agreement with experimental results. A model concerning adsorption-desorption and oxidation in the boundary layer applied to a particular tribopair has been presented by Sullivan I 131. However, the published models include some variables which cannot be evaluated with very high confidence I 11-131, such as the energy of adsorption in the adsorption-desorption model or activation energies in the kinetic temperature model. A model of the oxide layers in friction processes is proposed by Zhao et al. I 14 i and Zhao and Liu ! ! 5 I- According to this model, the boundary layer consists of three sublayers see Fig. 2 ): ( 1) friction reduction layer, 4 II ) diffusion and reduction layer and ( Ill ) stable oxidised layer.
3. Experimental The experiments presented in this report have been described previously (see Olofsson 16l ). Spherical roller thrust bearings ( SKF 29416 E) were c h u r n as test objects and 16 tests were pertbrmed using two bearings in each test. The number of revolution.,; Ibr each test was 30 004), Spherical roller thrust bearings consist of lbur parts: the shaft washer. the housing washer, the cage assembly and the rollers. In this study, only the housing washer surface is analy~d, To be able to study how the boundary layers, the wear and the coeflicient o f friction depend on the rotational speed, the load. Ihe hardness and the type of lubricant, an asymmetric fractional factorial experimental design ( see Addelman 1161 and Margolin [ 171 ) was c h u r n . In this study, four factors arc varied at two levels and one factor is varied at three l e v e l s l a s described in Table I. In Table 2 the running purameters arc presented. Note that both lubricants have the same viscosity 168 mm-'/s) at 40°C. At IOO°C the vi~osily is 9 tom-'Is and i I tom-'Is for the mineral oil and the synthetic oil respecti rely. Both lubricants pass the 12th step in the FZG wear test according to DIN 51354. For standard spherical riffler thrust bearings, the washer hardness and the roller hardncss is 720 HV and 750 HV respectively. The difference in hardness between a standard washer and a special hardened washer was about 30 HV. For the rollers, the hardness difli~rence was about IO HV.
T
0i,¢~'~ t:~0wIre surface Fig. 2. Oxide layer model of the l~mndary layer,fnlm Zh:,o Cl ill.1141.
t5,~
U. Oh~[sson. S. Di:dar / Wear 215 ¢i~t~,~¢~156-164
Tabh: I Dc~,ign nmlrix fi~¢a~,ymm¢lricaf fraclinnal fau'lnri:d denign Te,,I no. 1
2 3 4 5 6 7 N tl in !I 12 13 14 15 [6
Rotational :,wed
Hardness t washer.,. I
H:,rdnesn ( roller ~
Load
Lubricant
low middle middk" high low middle middle high lms middle Middle high low middle middle high
h~w Ires Io~ loss high high high high low Io~ low Io~.~,' high high high high
low k,¢,' low I~w I~,~,~,' Inw lily,' le,,,v high high high high high high high high
low high high low hiEh low low high low high high low high low low high
low low high high high high low low high high low 1u¢, low low high high
Table 2 Ruiming pa:ameletn in lilt." as~ iii1|1c1ri¢ fra¢liollal I~1¢Io1tle~,igll Lov.
Middle
High
Retail*real ,,l'x:cd ~rpll ~.| Ilardrning x~a,her,. i|ardenmg rolicr~,
4 ,,landard :4a n d;.|td
21)
I.uhricam
SHII.I.'rF.IJ.US hSS ( millcr;d-tfibh:tscd I
-
I04) special Sl'~'cial 220 1~" )P, II, SHC 526 1,.ynthetic-oibba.,,¢d }
The asymmetr).' of this factorial design comes from the mixed levels of the investigated factors. By nlea,lS of factorial design, it is tx~ssible to study qualitative variables (type of lubricant I a~, well as quantitative variables ( nornml load and rotational speedL A factorial design makes it possible to calculate an average value, as well as the main effect of increasing, for instance, the normal Ioati. Moreover. it is pos.,,ible to calculate the effect of how two or nlore factors interact. such as changing both normal load and type of lubricaqt. An effect is defined as the diflL'rence in response on moving from a low level to a higher level. Often there is a certain hierarchy in terms of absolute magnitude. Main effects are often greater than tv,'o-faclor interactions, which in turn are often greater than three-factor interactions. If one is not interested in higher order interne)tiros or believes thai they can properly be disregarded, fractional factorial design can be used. Fraction:d factorial design requires I'ew'cr runs than full factorials, but fev,er interaction effects can be calculated. A full factorial design in tyro variables varied at two levels and one variable va,ried at three levels requires 48 tests. Compare thb, with the actual desigr ",,here t6 tests were run. In this ,.tud v onl~ the main effects are considered. In order tojustil~ the assumption of independent errors, tile spe¢ime)l.,, w e r e tested in randinn order. 'X'ates" algorithm I see Montgonlery I 181 ) was used when calculating the mean lexels and the main ctlL'cts. Note that
two main effects can be calculated from the rotational speed. which has three levels. R~,,, is the effect from moving from the |ow level to the middle level and R,,h is the effect from moving from the middle level to the high level. Three-dimensional surface roughness measurements were performed after the tests. Here the test equipment is a Rodenstock RM 6(~). which is an optical ibcusdetection instrument. The verilication of the optical focus detecting instrument is presented in a separate repurt (see Ololgson and BjOrklund
11')1), To qualitatively characterise the wear, a visual and parametric characterisalion technique was used. There are 14 3D surface parameters proposed in Ref. 1201. Besides the above mentioned 3-D surface parameters Olofsson and BjOrklund [ 51 used the 3-Dequivalents of the five parameters cv:duated froth the Abbot curve? to characterise the wear of worn spherical roller thrust bearings. Based on these results, two parameters were considered appropriate in this study, the properties of which are summarised below. The first parameter. S,e describes th:" amplitude distribution. S,~ is the root mean square value of the surface departures from the mean plane within the sampling area and gives a measure of how ' DIN. Mea~,urcmcnt of ~,ur[ac¢ roughn~;~; param,JIcr~, N~. Rs,~. R.~. M~..
M,, I'{Pr the ,.le~cription t)l the ina[¢ria[ F~rtion in the roughness. Gcrn~.an Standard. DIN 4776. ( I '~t~l I
U. 01o1~.~o.. S. D i : d u r / Wear 21511998j I.¢16-164
the roughness is affected by the different independent variables. The second parameter. Sd, is a hybrid parameter which combines wavelength and amplitude. S0¢ is the developed interracial area ratio of the surface. This is the ratio of the increment of the interracial area of a surface over the sampling area. According to Dor,g and Stout [211 this parameter can reflect the transition of the tribological process when a surface is worn. For a fine surface manufactured by honing, or grinding. the developed interracial area ratio is usually smaller than I C,~. For rough surfaces manufactured by turning or shaping, the values of the developed interracial area ratio may be larger than IOe~. Before evaluating the surface parameter.,;, the curved tbrm of the original surfaces were removed by least square fitting to a second degree polynomial. The surfaces were also filtered through a median filter. The coefficient of friction was measured in a ball-on-flat tribometer (see Hagman and Bjt~rklund 1221 I. A ball with diameter 5 mm. made of ball bearing steel, was pressed against the washer surface and the coefficient of friction was measured under lubricated as well as unlubricated conditions. The sliding amplitude was 5/.tm and the sliding veloeity was O. i mm/s. A normal load of 5 N was used. giving a Hertzian maximum pressure of approximately 1200 MPa. The maximum normal pressure for the bearings during tests was also about 1200 MPa. The quantitative depth profile analysis of the boundary layers has been obtained by glow discharge optical emission spectroscopy. GD-OES. The physical and chemical background of the quantitative depth profile analysis by GD-OES analysis can be lbund in the works of Hocquaux 1231 and Bengtsson 124 I. The analysis will be briefly presented with the help of a schematic diagram ( .see Fig. 3 ) of a glow discharge source (typically Grimm glow discharge lamp). The material of the sample surface is removed by cathodic sputtering in an atmosphere of a noble gas carrier (typically argon ). The atoms released from the surl~ce collide with each other and come into an excited state, producing a cathtxlic glow and resulting in plasma. The characteristic spectral lines for the elements removed from the surface as well as for the carrier gas are emitted from the plasma and measured by a fast. multi-channel optical spectrometer as a function of the sputtering rate. Then, inlbrmation about the depth variation of the elements divided by area of sputtering is obtained by the integration of the element signals at a very high sampling A t - inlet ~ . ~
/ - - - -
159
rote and is presented in a diagram. In addition, the technique provides simultaneous registration of all chemical elements of the boundary layer, and has approximately one order of magnitude better sensitivity and relative precision of the registration and quantification compared to X-ray photoelectron spectroscopy XPS (ESCA) or Auger electron spectroscopy AES. A very short time ( typically i00 s) is needed for analysis. leading to a high sample throughput and a relatively low cost per analysis~ Unfortunately. this spcctro.~opy provides no direct information about chemical bonds in the boundary. layer but valuable conclusions can be implicitly drawn. When the GD-OES analysis is applied to h~mndary layers. a phenomenon called prel~renlial sputtering can appear. The
o,
"1
Depth profile (nm)
Fig. 4. P;.traffleterr, u-.ed in the ail~dyr.i~ Of re'.,ullr, ttheained by the quanlilalive depth prolilc .'maly sb, by ( iD-OES. D,, ( nm "lis the depth o f friction* rcducli~,¢ ~+xid¢ layer--the depth of the cro~,sing poinl of the oxygen and iron clmccnU'alitm curv¢~, , r o.~.y~212ilcnfichng:li[ dcplh hi Ire h,undary laycr~ and/)a, ( nail ) is the total pznetration t enriehtllelg ) &'pth of the aftra:heroically active ckrffu:nl> ~TCHAEL i.e.. sum . f deplhs '.'~.hen their 9.ei.=.hl ¢or~en.,ration is d.:crea.~d to I',~" in the hmndary layers.
Vacuum pump • -- Insulator
Sample
O - ling Vacuum pump
~---~
"~-
Cooling
Fig. 3. Schetnat[¢ diagram ol a Grimm glow di.~,'harg¢ lamp. trom tlocquaux
123t.
Fig. 5. Photograph of measuredImusing w~L~her.The reclan~.lem.',rk~ the ltd,'alton and the .size ~1"Ihc mea..,urcd arcs. The cot~rdlnalu ~,y:,lcm .,,how.,,Ihe measurement directions in the residual term plot~,.
U. Oh!l~:~.n. S, l)iz~hlr/Wear 215 ( 19981 156-164
I(vl)
order of sputtering and the sputtering rate of atoms in the boundary layer can depend on their of chemical environment. as well as on the distribution of the compounds within the boundary layers. Therefore, the results of quantitative depth analysis of the boundary layers should be taken as a lirsl order approximation. Dizdar and Andersson 1251 have observed, using quantitative depth prolile analy.sds by GD-OES, that even at very low sliding velocity the friction coefficient is greatly influenced by presence of the tribochemicali~, active substances in the boundary layer. A correlation was observed between the phenomenology of the structural changes and changes in the wear properties. in order to describe this correlation, the Iollowing parameters are suggested ( see Fig. 4). ( I ) D,~ (rim) as the depth of friction-reductive oxide layer--the depth ~,f the crossing point of the oxygen and iron concentration curves or oxygen enrichment depth in the boundary layers. (21 DI: (nm), the total enrichment depth of the tribochenfically active substances, calculated as the depth when
the sum of weight concentrations o f the chemical elements are decreased to I C~ in the boundary layers. In this study tribochemically active elements are phosphorus, zinc and sulphur. The mechani:~m of the surface enrichment by the tribechemical active substances is complicated, and many different chemical substances are used in order to ensure the enrichment. Theretbre, it may be reasonable to take the resulting effect, the total enrichment depth of the chemical elements of the tribochemically active substances, as a parameter. Usual steel impurities like Mn and Si are normally below 0.30% Ibr ball-bearing steel. Fig. 5 is a photograph of a tested housing washer. The part of the surface that is investigated with the different analysis Iools is marked with a rectangle, The two circular bands on the housing washer surface are the zero sliding points and thus the investigated surface is midway between the two zero sliding points. This part of the surface is subjected to wear (see Ololgson 131 ). Two 3-D surface measurements were Test 1
100
New 100
75
75
'
~
Fe
o< tO
.--o-- N ~ p
1-
"--"-- S r
o
(,~
Ii
-----ca
J=
._m (1)
"~
50
O C
25
25
j
0 0
25
50
75
100
.
I
2,5
.
I
50
.
|
75
.
|
100
Depth profile (nm)
Depth profile (nm)
l:tg, t), rcsult~ Irllzll (~D-OI-S and 3-l) hurl'ace IIK'a.~urt'llI¢lll~., |1¢~ ~ln~'orlt ~LIrILI~¢. I),j = _~ hill, 1~11 ~4 IIIII. "~',i = ()'t)t pIlL -~)'d,= I(}.~',;. ~ZI =O. L2,
Fig. 7. Rc,'.uh~. f r o m GD-OI:'S and 3 - D surface meusurcill¢lflS. "1"¢~! I. rola(iongd P,pced 4 rpm, lubricaling oil Shell TeUu,,, (~8S. I ) . = II nm. D i = 2 7 iin|, ~,~= ~).l~'+ p.m. S,h = U.4',+. ~t +~0.20, ltq = ( L ] 2 .
161
U. Ohg¢,wm. S, Dizdar /'Near 215 (19<)8) 156-164
performed on each housing washer. The surface was measured with a IO ,am sampling interval and 128× 128 da~a points in trace and onhogonal trace direction::. The test samples for analysis with G D - O E S were first cut out from the same radius of the washers in the form o f 7 × 7 × 7 mm cubes, degreased carefully with trichlorethylene and acetone in an ultrasonic bath. The cut pieces were then embedded in a low melting temperature (about 150°C) casting alloy in order to form disks suitable for analysis on a LECO G D S 750 spectrometer. The test samples for friction measurements were also cut out from the wa:+hers in the form of 7 × 7 × 7 mm cubes. Before the test. the pieces were d e g r e a ~ d carefully with trichlorethylene and acetone in an u l t r a ~ n i c bath, For the lubricated tests, the same oils as were u ~ d during the bearing tests were applied to the corresponding test pieces. Thus, the lubricant based on mineral oil Shell Tellus 68 S was applied to pieces that were tested with that mineral oil and the synthetic oil Mobil SHC 526 was applied to test pieces that were tested with that synthetic oil. T w o friction measurement were performed on each housing washer.
4.
Results
The results from the quantitative depth profile analysis by GD-OES- and 3-D surface topography measurements for an unworn housing washer are presented in Fig. 6. Figs. ( 7 ) ( i O) p r c ~ n t results from test I, 9, 12 and 4 respectively. The differences between test I and test 9 are the typ¢ o f lubrk-ant and the roller hardness. Olofsson 16] showed that in this ease a change of the roller hardness did not influence the wear and fatigue life of the beatings. The difference in the wear shown in the residual plots are due ¢o the two types of lubricant, it is interesting to notice the relatively high concentration o f phosphorus (P) and zinc ( Z n ) in test 9, where the syntheticoil-based lubricant was used. Both tests performed at low rotational speed and with a synthetic-oil-based lubricant produce relatively deep layers of P and Zn. and at the .same time the surface was relatively unaffected by wear ( test 5 and test 9). These results can be compared with the rgsults from the two test~ performed at low rotational speed and with the
Test 9
100
T e s t 12
100
75
75
+--=--- Fe ---o--
.<
+
.5 50
elL1
8 6
.--~--
p
~ S
50
-
Cr
Ca
O
0
0
---~-- N
---~--- Zn
25
2.~
0
25 50 75 Depth profile (nm)
25
1O0
Depth
g,
50 prtite
75
100
(nm)
g
"8
t
.~ t/J
°
W
-j
Fig. 8. Results from GD-OES and 3-D surface measaremems. Tcs! 9. rotational speed 4 rpm. lubrLcatim 2 ~fi|Mobil SHC 526. Do = 14 nm. l)l = I.L7
Fig, 9, Rcsuil.~ from GD-OES :rod 3-D surl'~c mca..,urem~m~.. Te,~t 12. ndatitmul ~i:~¢d IlK) rpm, luhrieating oil Mobil SHC 526. !.)+) = 9 nm, DI: = 18
nm. S,, = n..x7 #m. s,, = 4.(F/+ + p.,. = o. 12. ,ul = O. 14.
nm. S. I = n.57
p.m. S,~, =
5..J,c/~./~c = O. 14. ~ , = i L 15.
U. Olof~'son S. Dizdar / Wear 215 (1998j 156-164
162
"t'able 3
Test 4
100
Experimentally determined parameters f r o m quantitative depth profile analysis. 3-D surface analysis and friction measurements
75
---o--- N ~ p
g o ~
¢-
50
8 8
• Cr ~ C a
e.
,z-
25
.
o
s
.
25
•
.
t
50
75
.
Test no.
I)c~ (nml
Dr. (nm)
Sq
S,i,
(/J.m )
(~)
[ 2 3 4 5 ~• 7 8 9 It} II 12 13 14 15 16
I1 8 16 7 12 9 7 6 14 8 9 ~; I1 7 II 8 5
27 28 196 I1 139 26 23 14 147 32 28 18 51 20 42 19 4
0.06 O, lU ().69
0.4 0.7 6.2 3.8 2.2 5.4 2.6 4.2 4.l) 2.9 0.6 5.4 0.4 2.8 2,4 5.6 I 1).2
New
0.42 0.27 0.54 n.30 0.46 0.47 0.33 0.10 0.57 0.n4 0.33 0.26 (I.61) 0,91
p,,
~l.
0.20 0.12 0.14
0.12 (I. 12 0.14
O. 13 0.16 0.14 0.09 0.14
O. 15 (I.14 0.14 n. 12 0,12
0.12
0.14
0.I I
().15
0.11 0.14 II. I l a. I " (1.12 0.14 I). 12
n.13 0.15 n.15 (I.13 0.15 0.13 -
100
Depth profile (nm)
8
*d Fig. I0. Results from GD-OES and 3-D surface measurements. Test 4, rotational ~,pecd I ~ l r p m , lubricating oil Shell Tellus fibS. 1)~, = 7 ntl), Dj = I l n m . S, I = 0 . 4 2 ~ m . S,, = 3.8q~. P r = n. 1.1. b h = O. 15.
mineral-oil-based lubricant (test I and test 13). Here the surlace asperities were worn down to an almost mirror smtx~th surface and the low concentration of P and Zn persists in the results from the GD-OES analys~s. In test 12 and test 4 the rotational speed was at the high level and the mineral-oilba.~d and the synthelic-oil-ba,~ed lubricant ~,rere used respec-
tively. The results from the 3-D surface topography measurements show that the surface is relatively unaffected by wear and the GD-OES results show that low concentration of P and Zn enrichment persist in both cases. In this case, the high level of the rotational speed leads to a higher film thickness and thus fewer asperity interactions and less wear. The dynamic equilibrium of continuous fidlare and reformation of boundary layer seem to be obtained in combination of the low rotational speed and the tribochemicM properties of the synthetic-oil-based lubricant. Table 3 presents results from the parameter character(sat(on of the boundary layers, the worn surlaces and the coefficient of friction. In Table 4 the resnlts from the factorial analysis are presented. A presenled main effect is a change in the response when moving from a low level to a higher level For example S, is increased by 0.2 p,m when we change the lubricating oil from the low level (Shell Tellus 68S) to the high level (Mobil SHC 526). One can also interpret the calculated effects as a rank of this specific parameter influence on the response. The greater the effect, Ihe more the response
Table 4 Ro.ull~. f r o m Iattorial Lle,,ign I'z+r p~rarmzlq+~, f r o m quazllitativ~: depth prolile anai)'.~i~,. 3 - I ) surf.'u:¢ analyst.', and friction mcasurctncnts
Mean Vek~:ily Io~- ~ middk. Vch~:ity m i d d i c ~ h i g h H a r d e n i n g v,a,,,her l o w ~ high H a r d e n i n g n~ller l o w ~ hitch [.x;ad l o w - , high I.uhricanl l o w ~ high
"t Sl~i~,licall)" .,,ignilieallt main ¢1"t~'¢1.
D,, t nnl I
Di Inm I
ij.Cp - -.2.4" - 2. I * - 1.4 I). I nA
-
2. t *
.~ 1.9 399 35.S I 9.[t 13, I 23.8 5().0"
.~;,~ t MAn )
S,+, { G I
P-r
P-I
n.35 O. 13 O, Ifl O,IX)9 - 0,017 - n.04.s
3.2 1,34" 1.67" 0.22 - O. It} - n,50
(), 13 - 0.nl 5 0.t~.)8 - 0.(H}8 - n.n[7 - 0.{))4
0. t 4 - O.(H)3 O.O(ll - ().(X|2 O.(X)9 0.(XX)
0.20 ~
13)4 *
().(X)4
0.012"
[I. Oh~l~'sott. S. Di:.dar l Wear 215 r 1~9,% 156-164
is influenced by this particu:ar effect. Two methods were used to check if the results were .statistically significant. For the 3D surface topography parameters and the coefficient of friction replicate measurements were performed and the results from the analysis were considered statistically significant if the calculated value was larger than twice the standard devi,lion. Another test was used for the two parameters in the quantitative depth profile analysis by GD-OES. Since no replicate runs were performed in this analysis, the estimates of the effects were ploaed on normal probability paper. The effects that are negligible will tend to fall on a straight line on this plot. whereas signilicant effects will not lie along the straight line (see Montgomery 11811. It is interesting to notice that a change of the type of lubricant have a statistically signilicant effect on five of the six investigated parameters. That is, the tribochemical contents of the boundary layers (Do and De.). the surface roughness (S, and S,j,) and the coefficient of friction under lubricated conditions ( #t. } were all affected in a statistically significant way by a change of the lubricating oil. Also a change of the velocity influenced the tribochentical contents of the boundary layers and the surthce roughness in a statistically significant way. For the coefficient of friction under unlubricated conditions ( # v there were no significant calculated main effects. One possible interpretation of this result is that the tribochemical reactions in the boundary layers do not affect the coefticient of friction in a statistically signilicant way. This in contrast to the wear of the surface asperitie:: that was greatly reduced when high concentratimJs of phosphorus (P) and zinc ( Zn ) were measured in the boundary layers.
$. Conclusions The inllucncc of two commercial lubricants on the surface topography (asperity wear}, friction and boundary layer properties of boundary-lubricated spherical roller thrust bearings have been investigated. By combining results front GDdES analysis. 3-D surface topography measurements and iYiction measurements the Iollowiug conclusions can be drawn from the experimental results. ( I ) At low rotational speed the synthetic-oil-based lubricant (Mobil SHC 526) builds up a boundary layer with relatively high concentration of P and Zn. (2) The mineral-oil-based lubricant ( Shell Tellus 68 S } shows no tendency to lbrm boundary layecs with relatively high concentration of P and Zn at !ow rotational speed. (3) The factorial analysis shows that both the boundary layer structure and the w0ar of the surface asperities of the contacting surfaces arc affected in a statistically significant way by a change of the lubricant. (4) The suggested parameter set for the charactcrisation of the boundary layer structure enrichment (Do and D;=) together with 3-D surface parameters as S, and S,;, proved to be very useful when characterising the surface topography
163
and ~.ribochemical changes in boundary-lubricated spherical roller thrust bearings.
AckJ~owledgements The authors wish to thank Dr. Arne Benglsson at The Swedish Institute for Metal Re.arch in Stockholm forexlensire work on conducting the GD-OES analysis. Profes.,a~r SOren Andersson and Dr. Stefan Bjtirklund made many valuable comments on this manuscript. This work was financially supported by the Swedish R e . a r c h Council for Engineering Sciences (TFR). the Natmnal Board for Industrial and Technical Development ~NUTEK). H~igglunds Drives AB and SKF Sverige AB.
Appendix Nrmw/lclcHttrd
D~.: total enrichment depth of the tribochemically active elements ( phosphorus, zinc and sulphur) in the boundary layers Do oxygen enrichment depth in the boundary layers or depth of the crossing point between the oxygen and iron concentration curves H r effect of changing the roller hardness from the low level to the high level H,, effect of changing the washer hardness from the low level to the high level L., effect of changing the load from the low level to the high level L,, effect of changing the type of lubricant R=,,, effect of changing the rotatitmal speed from the low level to the middle level R,,,, effect of changing the rotational speed from the middle level to the high level Sq r~x~t mean square deviation of surface S,;, developed interracial area ratio /z~. coefficient of friction under lubricated conditions /.t;. coefficient of friction under unlubricaled conditions
References t I t J+ Hailing. The microslip helWCCn a hall and il.,, track in hall-thrm, I bearings, hmrna! of Basic Engineering. March i lq~b6i, 213-22n. [21 M. K,:ll.,,~riim. R~dling conlact guidance {ff mi|crs in spherical roller bearings,. ASME Paper 7q-LUB-23. A S M E / A S L E Lubricathm Con[:~rence. ()ctoh~:r ( 1970 ). 131 U. Olofsson. C'haractcrisatio~ ~'wear in I~madary luhricalcd sp~.ricaJ ro}[¢r Ihru.,~t hearing.',, ac¢cplcd fi~r prcscnlali*m in Wear 21)8 ( 1¢,'97) 194-203. [41 U. Ohd~son. Faziguc iife rcduclion due h~ wcar in ~mndary lubficaicd spherical roller thru.,,I hearings. Wear 207 I 1~,~)7~ 74-78.
t fi4
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[51 U. Olofsson, S. Bjiirklund. 3-D surface analysis of worn spherical roller thrust beatings, Metrokrgy and properties of Engineering Surfaces 7th lmemational Conference. (ibtel'mrg. Sweden. 2-4 April t 1997).
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1231 1241 [ 251
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Biographies UIf Olofsson received in 1987 a MSc degree from the Royal ]nslitute of Technology, Stockholm. Sweden. Between 1987 to 1994. he was employed as a research engineer at the Swedish National Testing and Research Institute. He received in 1994 a Licentiate of Engineering degree at Cha|mers University of Technology and in 1996 a PhD at the Royal Institute of Technology. Presently. he is a research engineer at the Department of Machine Design, the Royal Institute of Technology, Stockholm, Sweden. Senad Dizdar received in 1989 a MSc degree in Mechanical Engineering from University in Mostar, Bosnia and Herzegovina, and in 1997 a Licentiate of Engineering degree from the Royal Institute of Technology. Stockholm, Sweden. At present time, he is a PhD student-researcher at the Department of Machine Design. the Royal Institute of Technology. Stockholm, Sweden.