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Influence of topography on the running-in of water-lubricated silicon carbide journal bearings
WEAR ELSEVIER
Wear20t ( 1996~ I,-9
Influence of topography on the running-in of water-lubricated silicon carbide journal beatings P. Andersson a j. Juhanko b, A.-P. Nikkil~ic, p. LintulaC,t a VTTTeehnical Research Centre of Finland. Mn,!~t::c::,.r!.':.gTcc~,mu~usy,~lOlBOXI /02, FIN'02044 VITEspo0. Finland b Itelsinki University of Technology, l~boratory of Machine Design, Otakaari 4A, FIN.02150 E.voo, Finland c Tampere [Jniversityof Teclmology, Institute of Materials ScieNce, P.O. Box 589. FIN-33101 Tampere, Finland
Received6 June1994;accepted5 October1995
Abstract The study summarizesan experimentalinvestigationon the running-inof water-lubricatedjournal beatingsand shaft sleeves madeof l0 differentcommercialsilicon carbidematerials, in each test the sliding surfacesbecamepolishedand the sliding conditionswere transformed from boundary or mixed lubricationto full film lubrication within a rather short sliding distance. The initial surface roughness strongly influencedon the initial dynamiccoefficientof friction,and on the running-indistance requited to reduce the coefficientof frictionfrom a level typical of boundaryor mixed lubricationto a level typicalof full film lubrication.The initial surfaceroughnessof the shaft sleevewas more slowly removed than that of the beating, and thereforethe surface quality of the shaft rather than that of the bearing governedthe running-indistancerequiredfor the polishing.The initial surfacetoughnessoftbe bearingpracticallyonly influencedthe dynamiccoefficient of friction at the beginningof the running-inprocedure.In comparisonwith the influenceof the surface roughnessvarialions,the material propertiesof the siliconcarbidesstudiedhad a limitedinfluenceon the running-inbehaviour,whichwas principallyrelatedto a tdbochemical surfacepolishingprocess.
Keywords:Ceramics; Journalbearings;Water lubrication
1. Introduction Silicon carbide (SIC) is currently one of the most widely used advanced ceramics. Within the field of tribology SiC is used in applications such as nozzles, mechanical face seal tings and journal bearings. The reliability of certain pumps, for instance, depends on SiC-based journal bearings lubricated with an aqueous pumping media, tn a previous experimental study on ceramic journal bearings and shaft sleeves under initial conditions of boundary lubrication in water, it was found that the load-carrying capability of SiC-based ceramics is superior to that of alumina, zirconia and silicon nitride [ 1], which result supports the choice of SiC as the material for water-lub•ated journal bearings. The present paper is focused on the requirements considering the type of • SiC-based material and the surface quality for a minimized running-in distance. Sintered SiC bodies are produced at temperatures of 19002100 °C in vacuum, and reaction bonded silicon carbides at Presentaddress:VT'[TechnicalResearchCentreof Finland,M~ufacluringTechnology,P.O.Box 17031,FIN-33101Tampere,Finland. 0043-16481961515.00 0 1996 ElsevierScience S.A, All rightsreserved 3'SD10043-1648 (95) 06815-b
about 1500--1(300°C in vacuum. At high temperatures the chemical compound SiC is metastable in oxygen, hence easily undergoes ttiboehemical wear when being slid against a hard solid in air or water. In most cases the oxidation of SiC produces silicon dioxide (silica, SiO2), which in dry contacts has the ability to form tribofilms on the sliding surfaces [2]. The solubility of SiO2 in water is rather tow, but under mild sliding conditions in water the silica reaction product formed on SiC surfaces is dissoluted into the water, The mild tribochemtt.,: polishing and the lack of loose, hard wear par. tides in the sliding interface promotes the formation of smooth sliding surfaces, The solution formed in the sliding interface is a silicic acid, which is neutral and which has a similar lubricity to pure water. A running-in procedure with SiC test pieces in water has been found to result in the rapid smoothening of the sliding surfaces, and a decrease in the coefficient of friction due to improved conditions for fluid film lubrication [3-5], Full film lubrication by water, which has a low viscosity, requires an extremely fine surface quality and a correct geometry. For water-lubricated SiC sliding surfaces, which are difficult and expensive to polish by ordinary
2
P. Anders$on etal. / Wear 201 (1996) 1-9
REAC,TtON FORCEOF
~o)
~.~
....
~. . . . .
~~Lo~
........... L~ !~°-(b) Fig.I.One of thetwo tribotestersusedin thepment studyas (a) a gener~lview,(b) a generalschematicwithtl~testbearingassemblymasked witha T, ~nd (¢) a detailedschemmic rcprescn~ion(photo:E. MakkonenlvTr). workshop pre_.ctic¢,the abilityto undergo tribochemical polishing is therefore essential.
Further ttibological benefits of SiC ate the high hardness that makes ,he material resistant to abrasion by third bodies and the good thermal conductivity that reduces the detrimental ¢ffecL~of lubricant heating and supports the thermal shock resistance of the material. The present study concerns the results of 36 running-in experiments with silicon carbide journal beatings and shaft sleeves in water. The test parameters chosen provided mitd conditions of boundary or mixed lubrication at the beginning of each test. The ttibological behaviour of 10 different commercially available materials during running.in was studied regarding the influence of various surface roughnesses and material properties, and for that pu~ose test pieces of authentic size were used. After the running-in investigation the samples were studied by load-carrying experiments or by e~p~riments under intermittent rotation [ 6,7].
2. Experimental procedure
bearing housing was loaded by dead weights, and in order to allow friction force measurements it had the ability to slightly turn around and move along the shaft, The friction force was measured with a force transducer. The temperature on the outer surface of the loaded sector of the beating was measured with a thermocouple. The lubricant was de-ionized water, and it was supplied by a diaphragm pump through a continuous-filament polypropylene depth filter with a nominal retention rating of I p.m. Fig. 1 shows one of the two journal bearing testers used in the present study.
2.2. Specimens The present work concerns the behaviour of 10 different commercially available SiC-based ceramics in a running-in procedure. Seven of the materials were sintered silicon carbides ( SiC I--,SiC7), two were silicon.infiltrated silicon car. bides (SiSiCI, SiSiC2) and one was a SiSiC matcriai containing additional free graphite (C-SiSiC), The materials SiCI-SiC6 were a silicon carbides while the material SiC7 was a r-SiC. Each set of samples to be tested consisted of a
2.1. Apparatus
O30/~40 x 40 mm shaft sleeve and a I~40/O56 X 20 nun journal b~aring, both of the same designation of siliconcarbide.Two, three or mostly four specimen setsof each material
The experiments employed two almost identical journal bearing testers previously used for similar lubricated and unlubricated tests [ 1,2 ]. The test set-up consisted of a silicon carbide shaft sleeve on a steel shaft to be rotated, and a silicon carbide journal bearing fixed into a bearing housing. The
were investigated, The different materials were chosen in order to prtl'Jide material properties that would cover the range representative of the silicon carbide materials commercially available at present. Some of the mechanical properties of the specimens, as measured from plate samples of
P. Andersson eta/. ~Wear201 (1996) i-9
Table 1 Mechanical propertiesand carboncontentof the ceramics studied
Material
Flexuralstrenglh
Hardness, HV3
[MPa] SiCI SiC2
SiC3 SiC4 SiC5 SiC6 SiC7 SiSiCI SiSiC2 C-SiSiC
Fracturetoughness
Grainsize
Porosity
[MPa m '/~]
[l~m]
[%]
Content of fleecarbon lwt.%]
340
3000
3.7
4
i.3
7.4
440
3000
3.3
5
3.4
4.6
550 500 200 280
6.6 3.7 3,3 3,5 3,0 3,9 3,5
2 4 15
480 260
3100 3300 2700 2800 3200 2100 2500
2.1 2,0 4.3 6.4 2.5 < 1.0
0 2,6 161 1,2 1,7 0
120
2400
2,9
480
4
2 I0 20 15
the same material designations, are presented in Table i, together with the carbon contents of the materials. Except for the material properties given in Table 1, the materials were characterized regarding their sintering additives including the aluminium content, the content of free carbon, the content of free silicon, the polytypes [4H, 15R, 6H, 3C], the density, the grain size and the pore size and distribution. As none of these features proved to be of relevance for the running-in performance, they were omitted from the present paper.
< I.O
3.0
o
27.5
The materials comprised pores of 1-5 tLmmean size, while the largest detected pores of the materials SiC5 and SiC6 were 34 and 25 ixm, respectively. The flexural strength was determined by four-point bending tests from five samples of each material, the fracture toughness by single edge notched beam (SENB) tests from five samples of each material, the grain size by the line intersect method and the porosity by the Archimedes method from a sample of each material. All the specimens investigated had been commercially produced according to a drawing in common, shown in Fig. 2.
Table2 Meanvaluesof distinctivesurfaceroughnessparametersfor (a) Ihnshaftsleevesand (b) the journalbearings Material (geometry)
Surfaceroughnessparameters Rt
Rp
R~
Rpt
Rv
Rvk
R~
SiCI
(a)
0.50
1.81
1.46
0,46
2.33
0,81
- 016
SiC2
(b) (a )
0.52 0.07
1.57 0.25
1.12 0.16
0.48 0.04
3,46 0.89
0,95 019
-0,80 - 2,55
SiC3
(b) (a) (b)
0.13 0.05 0.I0
0.41 O.I 7 0.48
0.30 O.I I 0.27
0.08 00~ 0.16
2.32 I/',6 2.20
0,44 0.23 0,53
-4.15 - 4.98 -415
SiC4
(a)
0,50
1.87
1,45
0,57
217
0,%
-0,15
(b)
0.41
0.93
0,81
0.23
1,32
0,56
-0,I0
(n)
0,36
2.43
0,96
019
6.59
2,65
- 3.53
(b) (a) (b) (a) (b)
0.24 0,67 0.48 0,06 0,07
0,99 2.27 1.46 0,25 0.34
0.52 1,67 0.% 0.15 0,19
0.29 0,54
0.22 0,05 0,04
5.79 8.11 7,22 1.13 1,39
0,90 3,25 2,22 0.38 0.31
-4,83 - 2,67 - 3,53 - 4.08 - 3,65
(a)
0.49
1.66
1,31
0.49
2.25
0.78
- 0,23
(b)
0,18
0.57
0,45
0.12
1.17
0.35
- 035
(a)
016
0.93
0.75
0,29
1.28
0,53
-0.38
(b)
0.24
0,56
-0.85
017
2.23
0,73
- 1.85
(b)
0.34
0.63 0.60 0,66
1.61
0.25
0.87 0.76 1.04
0,20
(n)
0.20
2.73
1.06
- 1,55
SiC5 SiC6 SiC7
SiSiCI SiSLC2 C-SiSiC
S,: arithmeticaveragesurfaceroughness. Sp: profilemaximumheightabovethe mesa line. R~; meanof R~valueswithinthe ~.~essmentlength. Rp~:parameterfor the densityand heightof the highestasperities. R,: maximumdepthof the profilebelowthe meanline. R,~:parameterfor the densityand depthof the deepestvalleys. R.~:skewness, or amplitudedistributionasymmetryaboutthe meanline.
4
P, AMersson etal,/Wear20l (1996) 1..9
b Fig. 2. (a) Journalbearingand (b) shah sleevespecimens. Because the specimens had been manufactured in different workshops from different materials, significant variations occurred in the as-received surface quality from one grade of silicon carbide to another. Before testing, the surface roughness of all specimens was measured by stylus profilometry using Rank Taylor-Hobson Form Talysurf PC equipment and the roundness of the bearings was measured by stylus profiiometry using Rank Taylor-Hobson Talyrond 73-PC equipment. The deviations from roundness of the as-received bearings ranged from 0.6 to 8.0 p.m. The most distinctive parameters describing the topography of the journal beatings and shaft sleeves from different silicon carbides are presented in Table 2, as mean values representing two, three or mostly four test pieces of the stone material and geometry.
but mostly two pairs of new sleeves and bearings of each material were investigated, making a total of 36 tests. The measured initial clearances between the shaft sleeves and the bearings ranged from 25 to 60 p.m. The eccentricity in the rotation of the shaft sleeve was in the range 7-33 ~m, with the exception of material SiSiC 1, which showedeeeentrieities of up to 70 p.m. Before a test was started, a journal bearing and a shaft sleeve were installed in the tribotester, and the lubricating-water feed and the specimen alignment were checked, The water pressure was held between 3.5 and 6 bar, which gave a water feed rate of 1.7-10.8 × 10 -6 m 3 s- t. Deionized water was used as lubricant in all tests. A rotational speed of 192 rpm, which equals to a sliding velocity of 0.4 m s- i was set and after that a load of 530 N was applied. The test duration was 42 rain and the sliding distance 1000 m. After the tests the topography of the sliding surfaces of the sleeves and the beatings was measured by stylus profilometry, and the sliding surfaces were investigated by optical microscopy. In each test the temperature of the journal bearing housing remained within a few degrees above the test room temperature.
3. Results In every test the highest dynamic friction force was recorded at the beginning of the sliding procedure, and in the present work the initial dynamic coefficient of friction is called ~ . The initial level and the decrease rate of the coef-
2.3, Test procedure Prior to testing, the samples were cleaned in an ultrasonic bath. With each of the two test equipments at least one pair,
0.6
0.6
(b)
g 0.4 1
0,4
0.:
0.2
5O0 Slidingdistance
L0O0 [m]
500 Slidingdistanc~
0.6
0.4 m
0.2----
~'~ ..............
500 1000 Slidingdistance [rnl Fig. 3. Frictiongraphsfor the materials(a) SIC6,(b) SiSiC2and (c) C-SiSiC,
IO00 Ira]
P. Andersson et al. / Wear 201 (1996? !-9
Table 3 The initialdynamic coefficientsof friction(~) and the running-in distance (S,,)of each lest
Material
Initialcoefficient of friction
Running-indistance(m)
Machine I
Machine2
Pair I
Pair 2
SiCI SiC2 SiC3
0.34 O.16 0,06
0,4 0.0-/ 0.03
SiC4
0.35
-
SiC5 SiC6 SiC7
0.22 0.48 0.05
0.21 0,03
SiSiCl
0.35
SiSiC2 C--SiSiC
0.35 0.18
Machine I
Pail I 0,2-/ 0,05 0.26
Machine2
Pair 2
Pair L
Pair 2
Pair I
Pair 2
0.29 0.1 0.26
303 95 I10 295 I-/0
284 208 68 152
274 36 115 324 122
504 4-/ 72 -
-
504
338
- 0.2"/ O.I 3
-
- 0.32 0,01
0.2-/ 0,04
152
5-/
49
l1
II
0.35
0.3
0.34
189
273
648
432
0.29 0.09
0.26 0,16
0.18 0,12
265 5-/
284 114
353 173
187 115
ficent of friction varied from one pair of specimens to another. Three examples of friction curves are presented in Fig. 3. From its initial level, P.o,the dynamic coefficient of friction
nations o f P.o and Ss, is presented in Fig. 4 ( a ) , the c o m b i n a -
tions of the shaft sleeve Ra-values and the/.Co-values are presented in Fig. 4(b) and the combinations of the hardness of the materials and the S=-values in Fig. 4(c). The shaft sleeves obtained polished patches or lines spread around the periphery of the sleeve within a 20 mm or less wide band, In each bearing the polishing was concentrated to an area on the loaded sector of the sliding surface, The polished proportions of the bearing surfaces appeared as a uniform surface, as polished patches or as lines covering areas
decreased asymptotically with increasing sliding distance towards a level of about 0.01 well before approaching the end of the test. The distance at which the coefficent of friction stabilized at its final value of about 0,01 was defined as the running-in distance, calted S,Lin the present work, for the pair of specimens. The ~ and S,, values recorded for the different sliding pairs are presented in Table 3, A plot of the combi-
0.5
0.5
q,
(b)
Po
0.4
0.4
¢
•
li%
o
+
o
~G.
~,
o,e
,
e,
0.3
0.3
•
•
¢4~
•
o
++
0.2
o
•
0,2 o
o
$
o
.o ~
i.o~
o •
-
o
9
I
I
20O
400
I
I
--
800
,D
0
tO00
0
I
I
I
0.2
0.4
0,6
S.,t [m] ,
•
,
.
0,8 R= [pm]
,
.
,
.
,
,
,
.
(c)
.
,
'
• •.
4,.,
....
*.~
. . . . . . . . . . . . .
+ ,z,
.
.
400
0
.
.
,
,
'too. . . . . .
.
,
~
,
,
,
'. . . . . . . ". . . . . . '. . . . . ?." . . . . . . ,,,,.. +*. . . . . . . . i
.i .........
i
i
i+
~[
Fig. 4, The ~l~,tionship (a) between the initial dynan~c cnefficicn! of friction ,u,oand the running.in distance S.,, (b} between/,~ and the surface roughness R,-values of the shaft sleeves and (c) between the hardness of the materi,'ds mid the S.-va]ues.
6
P. Anderssonetal. ~Wear201(1996)1-9
of 1-5 cm2 size, as shown by the examplesin Fig. 5. Most of thejournal bearingsturned out to have a slightlysmallerinner diameter at the center than at the edges and this explainswhy most of the polished areas of the bearings were elliptical or rectangular with rounded corners. The microscopy of the potishcd areas revealed features indicating mild wear, mainly. A significantreduction in the surface roughnessprofileheight was obviousfrom the profile measurements.However,due to the great local variationsin the roughness,any significantvalues that wouldrepresentthe surface roughnesses or the wear volumes were not obtained after the running-in. The friction measurementresults in Table 3 wereanalysed regarding the mechanical properties and chemical composi. tion of the respective materials, and selected surface roughness parametersof each of the 36journal bearingsand the 36 shaft sleeves used in the respective tests. The correlation betweenthe frictionalbchaviourand the materialand surface parameters was analysed qualitatively by plotting graphs of combinationsof friction results and material properties and surface roughness parameters, and quantitativelyby means of the correlation coefficient function of an EXCEL table calculationprogram.The correlationcoefficientdescribesthe relationship between two properties given as two array cell ranges. The con'elation coefficient can have any value between - 1 and + 1, being close to + 1 or - 1 for a good positive or negative correlation and close to 0 for a p~r correlation. An overviewof the most significantresultsof the correlation analysis is presented in Fig. 6. In addition to the correlation data presented in Fig. 6 several other surface roughness parameters and material properties, as such and inserted in a number of mechanicalwear models, were inves. tigated regarding their correlation with the friction results, but due to their poor correlation they were omitted from the present paper.
Ss, values, while the variations in the material properties within the range studied seemed to be of less significance, see Fig. 4(b)-(c) and Fig. 6(a)-(b).
4.1. Factors influencing the initial dynamic coefficient of friction The initial dynamiccoefficientof friction,Pc, reflectedthe initial roughness of the mating surfaces, particularly the height and density of the asperities and their ability to penetrate the lubricant film and cause boundary or mixed lubrication by plowing on the countersurface. In Fig. 6(a), the surface roughnessparameterRpm,which expresses the mean height of the asperities, the parameter Rpk,which expresses the height and the density of the asperities, and in particular the arithmetic average surface roughness Ra, show a good correlationwith the initial dynamicfriction; when thesethroe parametershad low values the ~ value was also low. As seen in Fig. 4(b) the initial coefficientof friction remainedpattie. ularly low for shaft sleeveswithR~-valuesof 0.3 ~m and less. The Ri, value, which represents the highest singular peak within the measurementlength, and the R,, and R,,kvalues, whichrepresentthe valleys in the surfaceprofile,had a rather low correlation to the initial dynamic coefficientof friction. As obviousfrom Fig. 6(b), the friction measurementresults did not significantlycorrelate with the materialpropertiesor the contentof free carbon, The highestcorrelationcoefficient in Fig. 6(b) r~.presentsthe data in Fig. 4(c), which shows a slightly negativetrend with a large scatter. As seen in Fig. 3, the coefficient of friction decreased asymptotically.The decrease was controlled by the increase in the surface smoothness and the size of the smoothened, load-carryingarea~both factors improvingthe conditionsfor fluid filmlubrication.The improvedconditions for fluid film lubriction eventually reduced the need for further frictional, or polishing, work. The system stabilized at a friction level correspondingto full flint lubricationand no further wear,
4. Discussion
4.2. Factors influencing the running.in distance During the running-in the tribocouple was subjected to wear until it formed a sliding interface of sufficientsize and smoothnessfor providing conditions of full film lubrication. The conditions for hydrodynamic lubrication were in each case reached well before approaching the complete sliding distance of 1000 m. After the running-in each surface was more or less unique in appearance, following the roundness errors, the surface undulationsand the roughnessof the mating surfaces that in turn reflectedthe material propertiesand the respectivesurfacefinishingparametersthat had been used in the manufacturing. The observationson the surfacepolishingand the decrease in the coefficientof friction during initial sliding are in good agreement with the results of previous studieson silicon carbide in water [ 1,3-5,8]. The friction and wear behaviourof the sleevesand bearingsstronglyreflectedtheir initialsurface quality; a lower initial surface roughnessled to lower/.toand
As evident from Fig. 4(a), the running-in distance Sst showed a strong correlation with file imtiat dynamic coefficient of friction. A sliding interface with a higher surface roughness required a longer sliding distance before its surfaces were smoothenedsufficiently for full film lubrication and low friction, Therefore it was not surprising that, as shown in Fig. 6(a), the surface roughness parameters Rpm, Rpkand Ra gave the best correlationwith the running-indislance. As observedconcerningP.o,no significantcorrelation between the running.in distance and the material properties was found.
4,3. Sulface roughness vs n~aterialproperties The low correlationbetweenthe mechanicalpropertiesand the running-inbehaviourof the materials is due to the polish-
P. Andersson et al. / Wear201 f1996J 1-9
a
Beadng1"
~
¢
7
~eeve J,
Bearing 1"
Shaft sleeve ~,
Fig, 5, Macroscopic photographs showing different polished patterns on Ihe ,,vaatsurfeze.sof beatings (upper) and shaf~sleeves (tower) made of Ca) SiCI, (b) SiC7 and (c) SiSiC]. The different dcpdzsand directions of the undolations oftha initial surfacesate clearly indicated by the polished patterns, The shaft sieves a~ positioned with their axis of rotation horizontally in the fi~tas.
ing wear of silicon carbide in water being mainly of tribochemical, a.~d not principally of mechanical nature. The running-in of silicon carbide in water is a controlled removal of surface profile asperities. The surface polishing process lasts until conditions of full film lubrication arc achieved. As found in previous studies, the wear process involves the oxidation of the chemical compound SiC (and the oxidation of a Si phase, if present) into $iO2, which is dissolved into the water forming a weak silicic acid. It is possible that various silicon-based aqueous solutions are formed in the tribochemical process. The tribochemicaI process fades out when the frictional power generation decreases due to improved conditions for fluid film lubrication. In the present tests the polishing ended after sliding distances ranging from about 10 to 650 m, Only when it is sufficiently high may the content of free carbon slightly reduce the initial dynamic friction P.o.This is indicated by the friction values in Table 3 for the carbon-rich
materials C-SiSiC and SiCS, which arc slightly below the levels in Fig. 4(b) that would correspond to the respective surface roughnes~es. A high porosity, particularly that of SIC6, seems to increase the coefficient of friction by increasing the surface roughness.
4.4. The initial dynamic coefficientoffriction vs the running.in distance In file present journal bearing geometry, which consisted of a rotating shaft in a stationary bearing, only a small part of the bearing surface was involved in the u'ibologicalcontact. The shaft sleeve, being the rotating counterpart, was involved along its entire periphery. The volumetric wear rates of the two silicon carbide bodies of the same material designation that ar¢ sliding on each other in water) are generally of the same order of magnitude. As the journal bearing area involved in the sliding interface was significantly smallerthan
P, Anderssan et aL/ Wear201 (1996) 1-9
8
5. Conduslons
t.0 0 Po lisa
(a)
0.8 1
.~ 0.6' 8 0.4'
0.2, 0,0
Shaft sleeve Journal bearing ' Surface roughness parameters a ~om s,,
(b)
0.2
°'t -0.2
-0.4
Fiexural Hardness Kzc Porosity Frc¢carbon strength content Fig. 6. The correlation of the initial dynamic coefficient of friction ~ and the running-in disumceS. with distinctive {a) surfaceroughnessparameters and (b) material properties,
the shaft sleeve area involved, the wear depth at the bearing became significantly greater than that of the shaft sleeve. In fact, when the friction mechanism stabilized the polishing of most of the shaft sleeves was still rather incomplete. The correlation coefficients shown in Fig. 6(a) reflect the influence that ,2xc rougi'mess parameters of the shaft sleeve and bearing have on the initial friction and the running-in distance, As seen in Fig, 6(a), the surface roughness of the bearing has a much smaller influence than the surface rough. ness of the shaft sleeve on the running-in distance, Another point of interest obvious from Fig. 6(a) is that the correlation between the surface roughness of the bearing surface is much higher regarding the initial friction ~ than regarding the running-in distance. The initial surface roughness of the bearing is rapidly erased, as its influence is limited to the initial dynamic friction. The roughness of the shaft sleeve influences the sliding conditions for a longer duration, as the asperities are more slowly removed from the large surface of the shaft sleeve, and therefore the correlation coefficient is particularly high between the initial surface roughness of the shaft sleeves and the running-in distance, Sst.
In each of the 36 running.in experiments with silicon earbide journal bearings and shaft sleeves lubricated with water the sliding surfaces became polished and the lubrication mt.chanism was transforuled from boundary or mixed lubrication to full film lubrication within a rather short sliding distance. The tests showed the influence of the initial surface roughness on the initial dynamic coefficient of friction and on the running-in distance required for the polishing necessary to reduce the coefficient of friction to a level of ahem 0.01. The surface roughness of the shaft sleeve was more slowly removed than the surface roughness of the bearing, and therefore the shaft rather than the bearing influenced the running-in distance. In particular, short running.in distances were obtained when the shaft sleeves of the couples had an R~-value of 0.3 p,m or less, The surface roughness of the bearing practically only influenced ff,e dynamic coefficient of friction at the beginning of the running-in procedure, The material properties of the silicon carbide-based materials studied had little or no influence on the running-in behaviour, which was principally related to a tribochemical surface polishing process.
Acknow!edgements The authors are grateful for the financial support from the Technology Development Centre of Finland (TEKES), Safematic Oy and A. Ahlstrom Corporation, Pump Industry. 3'. Knuutila of VTT is acknowledged for the geometrical raeasurements of the specimens. Colleagues at V'IT and Helsinki University of Technology are acknowledged for their assistance in the performing of the experiments.
References [I]P, Andersson and P. Linlula, Load.carrying capability of water. lubricated ceramicjournal bearings, Tribal, lnt,, 27 ( 1994! 315-321. [2] P. Anderssonand A. Blomberg,Instabilityin the tribochemicalwearof silicon carbide in unlubricatedslidingcontacts, Wear, 174 (1994) i-7. [ 3 ] S. Sasaki, The effects of the surroundingatmosphc,: on the frictionand wear of alumina, zirconia,siliconcarbide and siliconnitride, Wear, 134 (1989) 185-200, [4] H Tomizawa and T, Fischer, Friction and wear of silicon nitride and silicon carbide in water: hydrodynan~iclubncaxionat low sliding speed obtained by tribochemical wear, Am. Sac, tubr, F.r:8,n8 Tr~,:.s,, 30 (1987) 41,-46. [5l P. Andersson. The transition from boundary to hydrodynamic lubrication of silicon.based ceramicssliding in water, Feet. 5tlJNordic Syrup. on Tribolagy,Nordtrib'92, Helsinki, 8-11 June 1992,Vol, I, pp. 49-56. [Tribologia, Finn, J, TribaL, 11 (1992) 2.] [6] J. Juhanko, A.-P, Nikkil~ian0 P. Lintula, Wear characteristicsof waterlubricated SiC journal beatings under high toads, Prec. 6th Nordic Symposiura on Tribology, NORDTRlB'94, Uppsata, 12--15June 1994,
Uppsala University,Uppsala, 1994, Vol. 2, pp. 349-356. [7] P, Andersson, A.-P, Nikkilli and P, Lintula, Wear characteristics of water-lubricated SiCjournal bearings in intermittent motion, Wear, 179 (1994) 57-62,
P, Andersson el al, / Wear 201 (! 996) I-9
[81 Ph, Maurin.Perrier,J.P,Farjaudonand M. Cartier,Frictionand wear behaviottroflubricatedceramicjournalbeatings,Proc, int. Conf. Wear of ~?aterials. Orlando. Florida, 7-11 April 1991, ASME,N©wYork, 1991.Vol.2. pp.585-588.
Biographies P. Andersson: received his B,Sc.(Eng) degree in mechanical engineering from the Swedish Institute of Technology, Helsinki ~n 1981, and worked after that with machine design. In 1987 he received his M.Sc.(Eng) degree in mechanicalengineering from Helsinki University of Technology. Since then he has been doing tribological research at the Technical Research Centre of Finland (VTI'), mainly on unlubricated and water-lubricatedceramic slidingcontacts and other topics from industrial enterprises. J. Juhanko: received his M,Sc.(Eng) degree in Mechanical Engine'-ring from Helsinki University of Technology in 1992. is research interest.~include advanced ceramics and paper ~.chnology,He presently works as a postgradua~,e Reset: ,.. Scientist on roll dynamics and bearingapplications. A,-P. Nikkil~i:graduated with a M,Sc.(Eng) degree in processing ceramic grinding wheels used in the pressurized ground wood process, from Tampere University of Technol-
9
ogy (TUT), in 1984. Afterwards, he continued his work at TUT as a Research Scientist, studying mechanicalproperties and especiallythe fatigue properties of silicon nitride atroom temperature, ar.d completing a Licentiate in Technology degree on mechanical properties on silicon-based ceramics in 1990, During 1989-1991 he tested and produced ceramic hot gas filters, in 1992 zinc titanate sorbent and in 1993 he was involved in studies on the wear properties of silicon carbides. From August 1994 to July 1995 he was Deputy Associate Professor o~'Ceramic Materials at TUT. In 1995 he evaluated the long-time behaviour and susceptibility of commercial hot gas filters, P. Lintula: graduated with a M,Sc,(Eng) in New Ceramic Materials and their Applicability to Mechanical Face Seals in 1986 from Tam.re University of Technology (TUT) in Finland. Afterwards, he continued his work at TOT as a Research Scientist, characterizing ceramic materials and completing a Licentiate in Technology degree on Phase Transformations in Zirconia in 1989. Subsequent research projects involved the development and characterization of ceramic tribomaterials for water-lubricatedjournal bearings and mechanical face seals. Since 1992 he has worked at the Technical Research Centre of Finland (VTI') in Tampere, At VTI', his research focuses on th. production of wear and corrosion resistant composite materials by self-propagating high.temperature synthesis,
Wear20t ( 1996~ I,-9
Influence of topography on the running-in of water-lubricated silicon carbide journal beatings P. Andersson a j. Juhanko b, A.-P. Nikkil~ic, p. LintulaC,t a VTTTeehnical Research Centre of Finland. Mn,!~t::c::,.r!.':.gTcc~,mu~usy,~lOlBOXI /02, FIN'02044 VITEspo0. Finland b Itelsinki University of Technology, l~boratory of Machine Design, Otakaari 4A, FIN.02150 E.voo, Finland c Tampere [Jniversityof Teclmology, Institute of Materials ScieNce, P.O. Box 589. FIN-33101 Tampere, Finland
Received6 June1994;accepted5 October1995
Abstract The study summarizesan experimentalinvestigationon the running-inof water-lubricatedjournal beatingsand shaft sleeves madeof l0 differentcommercialsilicon carbidematerials, in each test the sliding surfacesbecamepolishedand the sliding conditionswere transformed from boundary or mixed lubricationto full film lubrication within a rather short sliding distance. The initial surface roughness strongly influencedon the initial dynamiccoefficientof friction,and on the running-indistance requited to reduce the coefficientof frictionfrom a level typical of boundaryor mixed lubricationto a level typicalof full film lubrication.The initial surfaceroughnessof the shaft sleevewas more slowly removed than that of the beating, and thereforethe surface quality of the shaft rather than that of the bearing governedthe running-indistancerequiredfor the polishing.The initial surfacetoughnessoftbe bearingpracticallyonly influencedthe dynamiccoefficient of friction at the beginningof the running-inprocedure.In comparisonwith the influenceof the surface roughnessvarialions,the material propertiesof the siliconcarbidesstudiedhad a limitedinfluenceon the running-inbehaviour,whichwas principallyrelatedto a tdbochemical surfacepolishingprocess.
Keywords:Ceramics; Journalbearings;Water lubrication
1. Introduction Silicon carbide (SIC) is currently one of the most widely used advanced ceramics. Within the field of tribology SiC is used in applications such as nozzles, mechanical face seal tings and journal bearings. The reliability of certain pumps, for instance, depends on SiC-based journal bearings lubricated with an aqueous pumping media, tn a previous experimental study on ceramic journal bearings and shaft sleeves under initial conditions of boundary lubrication in water, it was found that the load-carrying capability of SiC-based ceramics is superior to that of alumina, zirconia and silicon nitride [ 1], which result supports the choice of SiC as the material for water-lub•ated journal bearings. The present paper is focused on the requirements considering the type of • SiC-based material and the surface quality for a minimized running-in distance. Sintered SiC bodies are produced at temperatures of 19002100 °C in vacuum, and reaction bonded silicon carbides at Presentaddress:VT'[TechnicalResearchCentreof Finland,M~ufacluringTechnology,P.O.Box 17031,FIN-33101Tampere,Finland. 0043-16481961515.00 0 1996 ElsevierScience S.A, All rightsreserved 3'SD10043-1648 (95) 06815-b
about 1500--1(300°C in vacuum. At high temperatures the chemical compound SiC is metastable in oxygen, hence easily undergoes ttiboehemical wear when being slid against a hard solid in air or water. In most cases the oxidation of SiC produces silicon dioxide (silica, SiO2), which in dry contacts has the ability to form tribofilms on the sliding surfaces [2]. The solubility of SiO2 in water is rather tow, but under mild sliding conditions in water the silica reaction product formed on SiC surfaces is dissoluted into the water, The mild tribochemtt.,: polishing and the lack of loose, hard wear par. tides in the sliding interface promotes the formation of smooth sliding surfaces, The solution formed in the sliding interface is a silicic acid, which is neutral and which has a similar lubricity to pure water. A running-in procedure with SiC test pieces in water has been found to result in the rapid smoothening of the sliding surfaces, and a decrease in the coefficient of friction due to improved conditions for fluid film lubrication [3-5], Full film lubrication by water, which has a low viscosity, requires an extremely fine surface quality and a correct geometry. For water-lubricated SiC sliding surfaces, which are difficult and expensive to polish by ordinary
2
P. Anders$on etal. / Wear 201 (1996) 1-9
REAC,TtON FORCEOF
~o)
~.~
....
~. . . . .
~~Lo~
........... L~ !~°-(b) Fig.I.One of thetwo tribotestersusedin thepment studyas (a) a gener~lview,(b) a generalschematicwithtl~testbearingassemblymasked witha T, ~nd (¢) a detailedschemmic rcprescn~ion(photo:E. MakkonenlvTr). workshop pre_.ctic¢,the abilityto undergo tribochemical polishing is therefore essential.
Further ttibological benefits of SiC ate the high hardness that makes ,he material resistant to abrasion by third bodies and the good thermal conductivity that reduces the detrimental ¢ffecL~of lubricant heating and supports the thermal shock resistance of the material. The present study concerns the results of 36 running-in experiments with silicon carbide journal beatings and shaft sleeves in water. The test parameters chosen provided mitd conditions of boundary or mixed lubrication at the beginning of each test. The ttibological behaviour of 10 different commercially available materials during running.in was studied regarding the influence of various surface roughnesses and material properties, and for that pu~ose test pieces of authentic size were used. After the running-in investigation the samples were studied by load-carrying experiments or by e~p~riments under intermittent rotation [ 6,7].
2. Experimental procedure
bearing housing was loaded by dead weights, and in order to allow friction force measurements it had the ability to slightly turn around and move along the shaft, The friction force was measured with a force transducer. The temperature on the outer surface of the loaded sector of the beating was measured with a thermocouple. The lubricant was de-ionized water, and it was supplied by a diaphragm pump through a continuous-filament polypropylene depth filter with a nominal retention rating of I p.m. Fig. 1 shows one of the two journal bearing testers used in the present study.
2.2. Specimens The present work concerns the behaviour of 10 different commercially available SiC-based ceramics in a running-in procedure. Seven of the materials were sintered silicon carbides ( SiC I--,SiC7), two were silicon.infiltrated silicon car. bides (SiSiCI, SiSiC2) and one was a SiSiC matcriai containing additional free graphite (C-SiSiC), The materials SiCI-SiC6 were a silicon carbides while the material SiC7 was a r-SiC. Each set of samples to be tested consisted of a
2.1. Apparatus
O30/~40 x 40 mm shaft sleeve and a I~40/O56 X 20 nun journal b~aring, both of the same designation of siliconcarbide.Two, three or mostly four specimen setsof each material
The experiments employed two almost identical journal bearing testers previously used for similar lubricated and unlubricated tests [ 1,2 ]. The test set-up consisted of a silicon carbide shaft sleeve on a steel shaft to be rotated, and a silicon carbide journal bearing fixed into a bearing housing. The
were investigated, The different materials were chosen in order to prtl'Jide material properties that would cover the range representative of the silicon carbide materials commercially available at present. Some of the mechanical properties of the specimens, as measured from plate samples of
P. Andersson eta/. ~Wear201 (1996) i-9
Table 1 Mechanical propertiesand carboncontentof the ceramics studied
Material
Flexuralstrenglh
Hardness, HV3
[MPa] SiCI SiC2
SiC3 SiC4 SiC5 SiC6 SiC7 SiSiCI SiSiC2 C-SiSiC
Fracturetoughness
Grainsize
Porosity
[MPa m '/~]
[l~m]
[%]
Content of fleecarbon lwt.%]
340
3000
3.7
4
i.3
7.4
440
3000
3.3
5
3.4
4.6
550 500 200 280
6.6 3.7 3,3 3,5 3,0 3,9 3,5
2 4 15
480 260
3100 3300 2700 2800 3200 2100 2500
2.1 2,0 4.3 6.4 2.5 < 1.0
0 2,6 161 1,2 1,7 0
120
2400
2,9
480
4
2 I0 20 15
the same material designations, are presented in Table i, together with the carbon contents of the materials. Except for the material properties given in Table 1, the materials were characterized regarding their sintering additives including the aluminium content, the content of free carbon, the content of free silicon, the polytypes [4H, 15R, 6H, 3C], the density, the grain size and the pore size and distribution. As none of these features proved to be of relevance for the running-in performance, they were omitted from the present paper.
< I.O
3.0
o
27.5
The materials comprised pores of 1-5 tLmmean size, while the largest detected pores of the materials SiC5 and SiC6 were 34 and 25 ixm, respectively. The flexural strength was determined by four-point bending tests from five samples of each material, the fracture toughness by single edge notched beam (SENB) tests from five samples of each material, the grain size by the line intersect method and the porosity by the Archimedes method from a sample of each material. All the specimens investigated had been commercially produced according to a drawing in common, shown in Fig. 2.
Table2 Meanvaluesof distinctivesurfaceroughnessparametersfor (a) Ihnshaftsleevesand (b) the journalbearings Material (geometry)
Surfaceroughnessparameters Rt
Rp
R~
Rpt
Rv
Rvk
R~
SiCI
(a)
0.50
1.81
1.46
0,46
2.33
0,81
- 016
SiC2
(b) (a )
0.52 0.07
1.57 0.25
1.12 0.16
0.48 0.04
3,46 0.89
0,95 019
-0,80 - 2,55
SiC3
(b) (a) (b)
0.13 0.05 0.I0
0.41 O.I 7 0.48
0.30 O.I I 0.27
0.08 00~ 0.16
2.32 I/',6 2.20
0,44 0.23 0,53
-4.15 - 4.98 -415
SiC4
(a)
0,50
1.87
1,45
0,57
217
0,%
-0,15
(b)
0.41
0.93
0,81
0.23
1,32
0,56
-0,I0
(n)
0,36
2.43
0,96
019
6.59
2,65
- 3.53
(b) (a) (b) (a) (b)
0.24 0,67 0.48 0,06 0,07
0,99 2.27 1.46 0,25 0.34
0.52 1,67 0.% 0.15 0,19
0.29 0,54
0.22 0,05 0,04
5.79 8.11 7,22 1.13 1,39
0,90 3,25 2,22 0.38 0.31
-4,83 - 2,67 - 3,53 - 4.08 - 3,65
(a)
0.49
1.66
1,31
0.49
2.25
0.78
- 0,23
(b)
0,18
0.57
0,45
0.12
1.17
0.35
- 035
(a)
016
0.93
0.75
0,29
1.28
0,53
-0.38
(b)
0.24
0,56
-0.85
017
2.23
0,73
- 1.85
(b)
0.34
0.63 0.60 0,66
1.61
0.25
0.87 0.76 1.04
0,20
(n)
0.20
2.73
1.06
- 1,55
SiC5 SiC6 SiC7
SiSiCI SiSLC2 C-SiSiC
S,: arithmeticaveragesurfaceroughness. Sp: profilemaximumheightabovethe mesa line. R~; meanof R~valueswithinthe ~.~essmentlength. Rp~:parameterfor the densityand heightof the highestasperities. R,: maximumdepthof the profilebelowthe meanline. R,~:parameterfor the densityand depthof the deepestvalleys. R.~:skewness, or amplitudedistributionasymmetryaboutthe meanline.
4
P, AMersson etal,/Wear20l (1996) 1..9
b Fig. 2. (a) Journalbearingand (b) shah sleevespecimens. Because the specimens had been manufactured in different workshops from different materials, significant variations occurred in the as-received surface quality from one grade of silicon carbide to another. Before testing, the surface roughness of all specimens was measured by stylus profilometry using Rank Taylor-Hobson Form Talysurf PC equipment and the roundness of the bearings was measured by stylus profiiometry using Rank Taylor-Hobson Talyrond 73-PC equipment. The deviations from roundness of the as-received bearings ranged from 0.6 to 8.0 p.m. The most distinctive parameters describing the topography of the journal beatings and shaft sleeves from different silicon carbides are presented in Table 2, as mean values representing two, three or mostly four test pieces of the stone material and geometry.
but mostly two pairs of new sleeves and bearings of each material were investigated, making a total of 36 tests. The measured initial clearances between the shaft sleeves and the bearings ranged from 25 to 60 p.m. The eccentricity in the rotation of the shaft sleeve was in the range 7-33 ~m, with the exception of material SiSiC 1, which showedeeeentrieities of up to 70 p.m. Before a test was started, a journal bearing and a shaft sleeve were installed in the tribotester, and the lubricating-water feed and the specimen alignment were checked, The water pressure was held between 3.5 and 6 bar, which gave a water feed rate of 1.7-10.8 × 10 -6 m 3 s- t. Deionized water was used as lubricant in all tests. A rotational speed of 192 rpm, which equals to a sliding velocity of 0.4 m s- i was set and after that a load of 530 N was applied. The test duration was 42 rain and the sliding distance 1000 m. After the tests the topography of the sliding surfaces of the sleeves and the beatings was measured by stylus profilometry, and the sliding surfaces were investigated by optical microscopy. In each test the temperature of the journal bearing housing remained within a few degrees above the test room temperature.
3. Results In every test the highest dynamic friction force was recorded at the beginning of the sliding procedure, and in the present work the initial dynamic coefficient of friction is called ~ . The initial level and the decrease rate of the coef-
2.3, Test procedure Prior to testing, the samples were cleaned in an ultrasonic bath. With each of the two test equipments at least one pair,
0.6
0.6
(b)
g 0.4 1
0,4
0.:
0.2
5O0 Slidingdistance
L0O0 [m]
500 Slidingdistanc~
0.6
0.4 m
0.2----
~'~ ..............
500 1000 Slidingdistance [rnl Fig. 3. Frictiongraphsfor the materials(a) SIC6,(b) SiSiC2and (c) C-SiSiC,
IO00 Ira]
P. Andersson et al. / Wear 201 (1996? !-9
Table 3 The initialdynamic coefficientsof friction(~) and the running-in distance (S,,)of each lest
Material
Initialcoefficient of friction
Running-indistance(m)
Machine I
Machine2
Pair I
Pair 2
SiCI SiC2 SiC3
0.34 O.16 0,06
0,4 0.0-/ 0.03
SiC4
0.35
-
SiC5 SiC6 SiC7
0.22 0.48 0.05
0.21 0,03
SiSiCl
0.35
SiSiC2 C--SiSiC
0.35 0.18
Machine I
Pail I 0,2-/ 0,05 0.26
Machine2
Pair 2
Pair L
Pair 2
Pair I
Pair 2
0.29 0.1 0.26
303 95 I10 295 I-/0
284 208 68 152
274 36 115 324 122
504 4-/ 72 -
-
504
338
- 0.2"/ O.I 3
-
- 0.32 0,01
0.2-/ 0,04
152
5-/
49
l1
II
0.35
0.3
0.34
189
273
648
432
0.29 0.09
0.26 0,16
0.18 0,12
265 5-/
284 114
353 173
187 115
ficent of friction varied from one pair of specimens to another. Three examples of friction curves are presented in Fig. 3. From its initial level, P.o,the dynamic coefficient of friction
nations o f P.o and Ss, is presented in Fig. 4 ( a ) , the c o m b i n a -
tions of the shaft sleeve Ra-values and the/.Co-values are presented in Fig. 4(b) and the combinations of the hardness of the materials and the S=-values in Fig. 4(c). The shaft sleeves obtained polished patches or lines spread around the periphery of the sleeve within a 20 mm or less wide band, In each bearing the polishing was concentrated to an area on the loaded sector of the sliding surface, The polished proportions of the bearing surfaces appeared as a uniform surface, as polished patches or as lines covering areas
decreased asymptotically with increasing sliding distance towards a level of about 0.01 well before approaching the end of the test. The distance at which the coefficent of friction stabilized at its final value of about 0,01 was defined as the running-in distance, calted S,Lin the present work, for the pair of specimens. The ~ and S,, values recorded for the different sliding pairs are presented in Table 3, A plot of the combi-
0.5
0.5
q,
(b)
Po
0.4
0.4
¢
•
li%
o
+
o
~G.
~,
o,e
,
e,
0.3
0.3
•
•
¢4~
•
o
++
0.2
o
•
0,2 o
o
$
o
.o ~
i.o~
o •
-
o
9
I
I
20O
400
I
I
--
800
,D
0
tO00
0
I
I
I
0.2
0.4
0,6
S.,t [m] ,
•
,
.
0,8 R= [pm]
,
.
,
.
,
,
,
.
(c)
.
,
'
• •.
4,.,
....
*.~
. . . . . . . . . . . . .
+ ,z,
.
.
400
0
.
.
,
,
'too. . . . . .
.
,
~
,
,
,
'. . . . . . . ". . . . . . '. . . . . ?." . . . . . . ,,,,.. +*. . . . . . . . i
.i .........
i
i
i+
~[
Fig. 4, The ~l~,tionship (a) between the initial dynan~c cnefficicn! of friction ,u,oand the running.in distance S.,, (b} between/,~ and the surface roughness R,-values of the shaft sleeves and (c) between the hardness of the materi,'ds mid the S.-va]ues.
6
P. Anderssonetal. ~Wear201(1996)1-9
of 1-5 cm2 size, as shown by the examplesin Fig. 5. Most of thejournal bearingsturned out to have a slightlysmallerinner diameter at the center than at the edges and this explainswhy most of the polished areas of the bearings were elliptical or rectangular with rounded corners. The microscopy of the potishcd areas revealed features indicating mild wear, mainly. A significantreduction in the surface roughnessprofileheight was obviousfrom the profile measurements.However,due to the great local variationsin the roughness,any significantvalues that wouldrepresentthe surface roughnesses or the wear volumes were not obtained after the running-in. The friction measurementresults in Table 3 wereanalysed regarding the mechanical properties and chemical composi. tion of the respective materials, and selected surface roughness parametersof each of the 36journal bearingsand the 36 shaft sleeves used in the respective tests. The correlation betweenthe frictionalbchaviourand the materialand surface parameters was analysed qualitatively by plotting graphs of combinationsof friction results and material properties and surface roughness parameters, and quantitativelyby means of the correlation coefficient function of an EXCEL table calculationprogram.The correlationcoefficientdescribesthe relationship between two properties given as two array cell ranges. The con'elation coefficient can have any value between - 1 and + 1, being close to + 1 or - 1 for a good positive or negative correlation and close to 0 for a p~r correlation. An overviewof the most significantresultsof the correlation analysis is presented in Fig. 6. In addition to the correlation data presented in Fig. 6 several other surface roughness parameters and material properties, as such and inserted in a number of mechanicalwear models, were inves. tigated regarding their correlation with the friction results, but due to their poor correlation they were omitted from the present paper.
Ss, values, while the variations in the material properties within the range studied seemed to be of less significance, see Fig. 4(b)-(c) and Fig. 6(a)-(b).
4.1. Factors influencing the initial dynamic coefficient of friction The initial dynamiccoefficientof friction,Pc, reflectedthe initial roughness of the mating surfaces, particularly the height and density of the asperities and their ability to penetrate the lubricant film and cause boundary or mixed lubrication by plowing on the countersurface. In Fig. 6(a), the surface roughnessparameterRpm,which expresses the mean height of the asperities, the parameter Rpk,which expresses the height and the density of the asperities, and in particular the arithmetic average surface roughness Ra, show a good correlationwith the initial dynamicfriction; when thesethroe parametershad low values the ~ value was also low. As seen in Fig. 4(b) the initial coefficientof friction remainedpattie. ularly low for shaft sleeveswithR~-valuesof 0.3 ~m and less. The Ri, value, which represents the highest singular peak within the measurementlength, and the R,, and R,,kvalues, whichrepresentthe valleys in the surfaceprofile,had a rather low correlation to the initial dynamic coefficientof friction. As obviousfrom Fig. 6(b), the friction measurementresults did not significantlycorrelate with the materialpropertiesor the contentof free carbon, The highestcorrelationcoefficient in Fig. 6(b) r~.presentsthe data in Fig. 4(c), which shows a slightly negativetrend with a large scatter. As seen in Fig. 3, the coefficient of friction decreased asymptotically.The decrease was controlled by the increase in the surface smoothness and the size of the smoothened, load-carryingarea~both factors improvingthe conditionsfor fluid filmlubrication.The improvedconditions for fluid film lubriction eventually reduced the need for further frictional, or polishing, work. The system stabilized at a friction level correspondingto full flint lubricationand no further wear,
4. Discussion
4.2. Factors influencing the running.in distance During the running-in the tribocouple was subjected to wear until it formed a sliding interface of sufficientsize and smoothnessfor providing conditions of full film lubrication. The conditions for hydrodynamic lubrication were in each case reached well before approaching the complete sliding distance of 1000 m. After the running-in each surface was more or less unique in appearance, following the roundness errors, the surface undulationsand the roughnessof the mating surfaces that in turn reflectedthe material propertiesand the respectivesurfacefinishingparametersthat had been used in the manufacturing. The observationson the surfacepolishingand the decrease in the coefficientof friction during initial sliding are in good agreement with the results of previous studieson silicon carbide in water [ 1,3-5,8]. The friction and wear behaviourof the sleevesand bearingsstronglyreflectedtheir initialsurface quality; a lower initial surface roughnessled to lower/.toand
As evident from Fig. 4(a), the running-in distance Sst showed a strong correlation with file imtiat dynamic coefficient of friction. A sliding interface with a higher surface roughness required a longer sliding distance before its surfaces were smoothenedsufficiently for full film lubrication and low friction, Therefore it was not surprising that, as shown in Fig. 6(a), the surface roughness parameters Rpm, Rpkand Ra gave the best correlationwith the running-indislance. As observedconcerningP.o,no significantcorrelation between the running.in distance and the material properties was found.
4,3. Sulface roughness vs n~aterialproperties The low correlationbetweenthe mechanicalpropertiesand the running-inbehaviourof the materials is due to the polish-
P. Andersson et al. / Wear201 f1996J 1-9
a
Beadng1"
~
¢
7
~eeve J,
Bearing 1"
Shaft sleeve ~,
Fig, 5, Macroscopic photographs showing different polished patterns on Ihe ,,vaatsurfeze.sof beatings (upper) and shaf~sleeves (tower) made of Ca) SiCI, (b) SiC7 and (c) SiSiC]. The different dcpdzsand directions of the undolations oftha initial surfacesate clearly indicated by the polished patterns, The shaft sieves a~ positioned with their axis of rotation horizontally in the fi~tas.
ing wear of silicon carbide in water being mainly of tribochemical, a.~d not principally of mechanical nature. The running-in of silicon carbide in water is a controlled removal of surface profile asperities. The surface polishing process lasts until conditions of full film lubrication arc achieved. As found in previous studies, the wear process involves the oxidation of the chemical compound SiC (and the oxidation of a Si phase, if present) into $iO2, which is dissolved into the water forming a weak silicic acid. It is possible that various silicon-based aqueous solutions are formed in the tribochemical process. The tribochemicaI process fades out when the frictional power generation decreases due to improved conditions for fluid film lubrication. In the present tests the polishing ended after sliding distances ranging from about 10 to 650 m, Only when it is sufficiently high may the content of free carbon slightly reduce the initial dynamic friction P.o.This is indicated by the friction values in Table 3 for the carbon-rich
materials C-SiSiC and SiCS, which arc slightly below the levels in Fig. 4(b) that would correspond to the respective surface roughnes~es. A high porosity, particularly that of SIC6, seems to increase the coefficient of friction by increasing the surface roughness.
4.4. The initial dynamic coefficientoffriction vs the running.in distance In file present journal bearing geometry, which consisted of a rotating shaft in a stationary bearing, only a small part of the bearing surface was involved in the u'ibologicalcontact. The shaft sleeve, being the rotating counterpart, was involved along its entire periphery. The volumetric wear rates of the two silicon carbide bodies of the same material designation that ar¢ sliding on each other in water) are generally of the same order of magnitude. As the journal bearing area involved in the sliding interface was significantly smallerthan
P, Anderssan et aL/ Wear201 (1996) 1-9
8
5. Conduslons
t.0 0 Po lisa
(a)
0.8 1
.~ 0.6' 8 0.4'
0.2, 0,0
Shaft sleeve Journal bearing ' Surface roughness parameters a ~om s,,
(b)
0.2
°'t -0.2
-0.4
Fiexural Hardness Kzc Porosity Frc¢carbon strength content Fig. 6. The correlation of the initial dynamic coefficient of friction ~ and the running-in disumceS. with distinctive {a) surfaceroughnessparameters and (b) material properties,
the shaft sleeve area involved, the wear depth at the bearing became significantly greater than that of the shaft sleeve. In fact, when the friction mechanism stabilized the polishing of most of the shaft sleeves was still rather incomplete. The correlation coefficients shown in Fig. 6(a) reflect the influence that ,2xc rougi'mess parameters of the shaft sleeve and bearing have on the initial friction and the running-in distance, As seen in Fig, 6(a), the surface roughness of the bearing has a much smaller influence than the surface rough. ness of the shaft sleeve on the running-in distance, Another point of interest obvious from Fig. 6(a) is that the correlation between the surface roughness of the bearing surface is much higher regarding the initial friction ~ than regarding the running-in distance. The initial surface roughness of the bearing is rapidly erased, as its influence is limited to the initial dynamic friction. The roughness of the shaft sleeve influences the sliding conditions for a longer duration, as the asperities are more slowly removed from the large surface of the shaft sleeve, and therefore the correlation coefficient is particularly high between the initial surface roughness of the shaft sleeves and the running-in distance, Sst.
In each of the 36 running.in experiments with silicon earbide journal bearings and shaft sleeves lubricated with water the sliding surfaces became polished and the lubrication mt.chanism was transforuled from boundary or mixed lubrication to full film lubrication within a rather short sliding distance. The tests showed the influence of the initial surface roughness on the initial dynamic coefficient of friction and on the running-in distance required for the polishing necessary to reduce the coefficient of friction to a level of ahem 0.01. The surface roughness of the shaft sleeve was more slowly removed than the surface roughness of the bearing, and therefore the shaft rather than the bearing influenced the running-in distance. In particular, short running.in distances were obtained when the shaft sleeves of the couples had an R~-value of 0.3 p,m or less, The surface roughness of the bearing practically only influenced ff,e dynamic coefficient of friction at the beginning of the running-in procedure, The material properties of the silicon carbide-based materials studied had little or no influence on the running-in behaviour, which was principally related to a tribochemical surface polishing process.
Acknow!edgements The authors are grateful for the financial support from the Technology Development Centre of Finland (TEKES), Safematic Oy and A. Ahlstrom Corporation, Pump Industry. 3'. Knuutila of VTT is acknowledged for the geometrical raeasurements of the specimens. Colleagues at V'IT and Helsinki University of Technology are acknowledged for their assistance in the performing of the experiments.
References [I]P, Andersson and P. Linlula, Load.carrying capability of water. lubricated ceramicjournal bearings, Tribal, lnt,, 27 ( 1994! 315-321. [2] P. Anderssonand A. Blomberg,Instabilityin the tribochemicalwearof silicon carbide in unlubricatedslidingcontacts, Wear, 174 (1994) i-7. [ 3 ] S. Sasaki, The effects of the surroundingatmosphc,: on the frictionand wear of alumina, zirconia,siliconcarbide and siliconnitride, Wear, 134 (1989) 185-200, [4] H Tomizawa and T, Fischer, Friction and wear of silicon nitride and silicon carbide in water: hydrodynan~iclubncaxionat low sliding speed obtained by tribochemical wear, Am. Sac, tubr, F.r:8,n8 Tr~,:.s,, 30 (1987) 41,-46. [5l P. Andersson. The transition from boundary to hydrodynamic lubrication of silicon.based ceramicssliding in water, Feet. 5tlJNordic Syrup. on Tribolagy,Nordtrib'92, Helsinki, 8-11 June 1992,Vol, I, pp. 49-56. [Tribologia, Finn, J, TribaL, 11 (1992) 2.] [6] J. Juhanko, A.-P, Nikkil~ian0 P. Lintula, Wear characteristicsof waterlubricated SiC journal beatings under high toads, Prec. 6th Nordic Symposiura on Tribology, NORDTRlB'94, Uppsata, 12--15June 1994,
Uppsala University,Uppsala, 1994, Vol. 2, pp. 349-356. [7] P, Andersson, A.-P, Nikkilli and P, Lintula, Wear characteristics of water-lubricated SiCjournal bearings in intermittent motion, Wear, 179 (1994) 57-62,
P, Andersson el al, / Wear 201 (! 996) I-9
[81 Ph, Maurin.Perrier,J.P,Farjaudonand M. Cartier,Frictionand wear behaviottroflubricatedceramicjournalbeatings,Proc, int. Conf. Wear of ~?aterials. Orlando. Florida, 7-11 April 1991, ASME,N©wYork, 1991.Vol.2. pp.585-588.
Biographies P. Andersson: received his B,Sc.(Eng) degree in mechanical engineering from the Swedish Institute of Technology, Helsinki ~n 1981, and worked after that with machine design. In 1987 he received his M.Sc.(Eng) degree in mechanicalengineering from Helsinki University of Technology. Since then he has been doing tribological research at the Technical Research Centre of Finland (VTI'), mainly on unlubricated and water-lubricatedceramic slidingcontacts and other topics from industrial enterprises. J. Juhanko: received his M,Sc.(Eng) degree in Mechanical Engine'-ring from Helsinki University of Technology in 1992. is research interest.~include advanced ceramics and paper ~.chnology,He presently works as a postgradua~,e Reset: ,.. Scientist on roll dynamics and bearingapplications. A,-P. Nikkil~i:graduated with a M,Sc.(Eng) degree in processing ceramic grinding wheels used in the pressurized ground wood process, from Tampere University of Technol-
9
ogy (TUT), in 1984. Afterwards, he continued his work at TUT as a Research Scientist, studying mechanicalproperties and especiallythe fatigue properties of silicon nitride atroom temperature, ar.d completing a Licentiate in Technology degree on mechanical properties on silicon-based ceramics in 1990, During 1989-1991 he tested and produced ceramic hot gas filters, in 1992 zinc titanate sorbent and in 1993 he was involved in studies on the wear properties of silicon carbides. From August 1994 to July 1995 he was Deputy Associate Professor o~'Ceramic Materials at TUT. In 1995 he evaluated the long-time behaviour and susceptibility of commercial hot gas filters, P. Lintula: graduated with a M,Sc,(Eng) in New Ceramic Materials and their Applicability to Mechanical Face Seals in 1986 from Tam.re University of Technology (TUT) in Finland. Afterwards, he continued his work at TOT as a Research Scientist, characterizing ceramic materials and completing a Licentiate in Technology degree on Phase Transformations in Zirconia in 1989. Subsequent research projects involved the development and characterization of ceramic tribomaterials for water-lubricatedjournal bearings and mechanical face seals. Since 1992 he has worked at the Technical Research Centre of Finland (VTI') in Tampere, At VTI', his research focuses on th. production of wear and corrosion resistant composite materials by self-propagating high.temperature synthesis,