Ceramic Couples Lubricated with Hexadecane

Ceramic Couples Lubricated with Hexadecane

635 (;. Bartelt Hundesanstalt fur Materialforachung und -priifung (BAM) I'nter den Eichen 87, 12200 Berlin, Germany and SiC/SiC Sliding friction an...

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635

(;.

Bartelt

Hundesanstalt fur Materialforachung und -priifung (BAM) I'nter den Eichen 87, 12200 Berlin, Germany and SiC/SiC Sliding friction and wear of ZrO2/ZrO2. A1203/A1203, Si,N,/Si,N, couplea were studied for a wide range of sliding speeds using pure hexadecane and solutions of additives in hexadecane as lubricants. With Zr02/Zr02 and Al,O,/Al,O, couples lubricated with pure hexadecane, transitions from mild to severe wear occurred when the sliding speed was increased. Severe wear of the Z r 0 2 / Z r 0 2 couple iS protlably caused by high frictional heating due to the low thermal conductivity of zirconia. I n contrast, the transition to severe wear of the lubricated Al,O,/Al,O, couple is determined by intergranular fracture. When fatty acids or zinc dialkyl dithiophosphatea were added to the base lubricant hexadecane, the wear transitions of the ZrO,/ZrO, and Al,O,/Al,O, couples were significantly changed. With Si,N,/Si,N, and SiC/SiC couples which were lubricated with pure hexadecane no transition to severe wear occurred at higher sliding speeds. In comparison to dry sliding, friction and wear were remarkably reduced by lubrication with pure hexadecane. For the operating conditions studied, friction and wear were only moderatly affected by the additives. 1.

INTRODUCTION

Advanced ceramics a r e increasingly being uaed as materials for engineering applications such as roller bearings, journal bearings, seal rings and parts for iriternal combustion engines. The advantages of the ceramic materials a r e their high hardness. high thermal stability. chemical inertness and, compared to metals, their low density. On the debit aide the brittleness of the ceramica is accepted. Since ceramics, compared to metals. have different physical and chemiccal properties, tribological behaviour which differs from that of metals has to be expected for boundary and mixed lubrication. Due to the variable composition of the ceramics, different friction and wear

characteristics have to be expected for the different ceramic materials as well. In order to improve the understanding of the mechanism which determine friction, wear and lubrication, the tribological behaviour of ceramic/ceramic sliding couplea with the frequently uaed ceramics ZrO,, A1,0,, Si,N, and S i c was studied. The experiments were carried out for lubrication with hexadecane and aolutiona of additives in hexadecane.

2.

EXPERIMENTAL PROCEDURES

The tribological behaviour of lubricated Z r 0 2 / Z r 0 2 , A1,0,/A120,. Si,N,/ Si,N, and SiC/SiC couples was studied with the help of a four ball machine (model VKA 110, manufactured by Hama Press- und Maschinenbau, Hamburg, Ger-

636 many) wing rpherer with a diameter of 1.27 cm. An the three lower rpherer were

fiied and the upper rotated, rlidhg motion resulted. Friction and wear of the ceramic coupler were rtudied for a rliding rpeed range from 0.002 to 0.S8 m/r, applying a force of LOO N per contact for a rlidhg dirtance of 72 m (partly for 1000 m) at room temperature. Pure hexadecane and solutiom of long chain fatty a c i b or zinc dialkyl dithiophorphster in hexadecane were wed M lubricantr. = W/(F I) were Wear coefficient8 determined an a measure of wear (G: wear coefficient in mm3/N m, Wv:wear volume in mm3, F: force in N, 8: rlidhg distance in m). The wear volumer determined for the lower rpherer were calculated on the basin of the wear rcar diameters and profilometric mearuremcntr. For ZrO,/ZrO, coupler at high rliding rpeede the lower rpherer could not properly be f i e d ro that no wear mars were formed 1u wear marb. The wear coefficientr ertimated are given together with the regular valuer in the figurer ( f i i e 1, f i e 3 and f i e 4 ) labelled with "ertim.". The determination of wear volumes for Al,O,/Al,O couples in the low wear region (fiure 8 j war lesr accurate than for the other valuer, an the wear volumer were low and the rhape of the wear warn war difficult to determine. The frictional torque or the frictional force for a paritcular lever arm war measured with a force tranrducer (Hottinger Baldwin Mesrtechnik GmbH, Darmrtadt, Germany). The numbers in the figures refer to the mean values of the coefficients of friction for rlidhg distances from 62 to 72 m or for the longer rliding distancer to mean valuer for distancer from 900 to 1000 m. When the coefficientr of friction could not be determined

due to accidental problem with the equipment or due to problem with frictional vibrotionr which occurred with Zr02/Zr0, coupler at &her slidins rpeeb no valuer for the coefficientr of friction are given in the figures. The zirconium dioxide employed war commercial material which was stabilized with Y,O,. With the help of remiquantitative X-ray fluorercence analyeia an yttrium content of 4.7 wt.% was determined. An zirconium occurr naturally with haihium, a hafnium content of 1.6 wt.% war not rurpriring. Only tetragonal zirconia, but no cubic or monoclinic zirconia, war detected by m e m of X-ray diffraction annlyrir. It WM ertimated that the content of cubic zirconia war leas than 510% and the content of monoclinic zirconia lerr than 1%. Both batcher of the commercial aaluminum oxide had, according to the manufacturer, an aluminum oxide content of 99.7 %. X-ray fluorercence revealed that the lrt aluminn batch had a magnerium content of 0.2 wt.X and the 2nd batch a magerium content of 0.7 wt.%. The commercial rilicon nitride ramplea contained MgO ar a rinter additive. With the help of X-ray fluorercence, a magnerium content of 0.5 wt.X war determined. Tungrten was found by X-ray fluorercence in 1.9 wt.% but could not be detected in X P S experimentr. The material war hot iaortatically prerrured rilicon nitride according to the manufacturer. A phare comporition of 80-85% 6-rilicon nitride and 15-20% a-rilicon nitride war determined by meam of X-ray diffraction analyrir. Silicon carbide war, according to the manufacturer, material which had been sintered and subrequently demified by hot irortatic prerring. The chemical comporition WM given M 98.5 w t . X silicon car-

637 bide, 0.3 wt.% aluminium, 1.0 wt.X free carbon, no free silicon and traces of nitrogen and oxygen. X-ray fluorescence confirmed that the sampler contained 0.3 wt.% aluminium. The other elements with lower atomic numbers than that of fluorine were not detectable by X-ray fluorercence. The material was pure hexagonal a-rilicon carbide accord& to the manufacturer. The ceramic rpheres and parts of the equipment which were in contact with the lubricant8 were cleaned by rinsing with toluene. ethanol, acetone and three times with n-hexane. Hexadecane, obtained from Jamsen Chimica, Bfiggen, Germany, waa used in "99%"quality as the base lubricant. It was purified by dry column chromatography Over aluminium oxide powder ("Aluminiumoxid 90 aktiv, neutral (Aktivitatsstufe I)", 70-230 mesh ASTM, obtained from E. Merck, Darmtadt, Germany). The viscosity war found to be 3.34 mPa s at 20 OC.

Zinc dialbl dithiophosphates, which were wed aa antiwear additivea, were synthesized according to A. D. Brazier et al. (13). The melting point of zinc di-nbutyl dithiophorphate waa found to be 40 OC which in identical with the value reported by P. Cann et al. (IS). Zinc dihobutyl dithiophoephate had a melting point of 113 OC which ir 1 OC higher than the melting point reported (15). No comparieon waa available for the melting point of 36 OC determined for zinc di-n-dodecyl dithiophosphate. Palmitic acid and rtearic acid were obtained from E. Merck, Darmtadt, Germany, in "GC 99%" or "GC > 99%" quality. A Karl Fischer coulometer, model 652, manufactured by Methrohm, Herinau, Switzerland, was wed for the water determinations. The author would like to

thank Prof. K.-H. Reichert, Tecbnirche Univeriitiit Berlin, for the opportunity to w e the Karl Fhcher coulometer. Semiquantitative X-ray fluorescence analyris was carried out at fracture rurfaces of the ceramics wing a requential spectrometer, model SRS 303, manulactured by Siemens, Karlsruhe. Germany. The author would like to thank Dr. R. Uttech and Dip1.-Ing. M. Michaelh, Bundesanntalt & Materialforrchung und - p r i m , laboratory 10.55, for the meaaurements. X-ray diffraction was carried out with polirhed rampler of the ceramics. A spectrometer, model D 5000, manufactured by Siemens, WM wed for the analyses. The mearurementr were taken for a range from 10 to 130° (2 e ) with a positionsensitive detector wing the Cu-K, line. The author would like to thank Dr. B. Peplineki, Bunderanatalt fir Materialforschmg und -priiRmg, laboratory 10.33, for the measurements and their interpretation. X-ray photoelectron rpectroscopy (XPS) war carried out with a amall spot XPS rpectrometer SSX-100, model 206, manufactured by Surface Science Instr./ F ~ o MUSA. , The rpectrometer WM operated with monochromatic X-rayr which had an energy of 1486.6 eV (Al-Ka line). In order to avoid charging the Sic surfaces were flooded with electronr of 2 eV at an argon prersure of 2 x 10-5 pa. me photoelectrons were alowed down with constant analyzer tranrmimson energy before entering the analyzer. The hydrocarbon C l a carbon peak with an energy of 284.6 eV waa wed M the rtandard peak. The author would like to thank Dr. T. Gross and Mr. D. Treu, Bundesanetalt flir Materialforrchung und -priifimg, laboratory 5.33, for their valuable advice and the meaaurementr. Profilogram were carried out using

638

an instrument manufactured by Hommelwerke, Villingen-Schwennjnnen, Germany, model T 20 S. The author would like to thank Dipl.-U. J. Schwenzien, Bundesanrtalt ftir Materialforrchung und -priif'uag, laboratory 5.24, for the meanurementr. Scelectron micrograph were taken from gold rputtered rurfacer, rince the electric conductivity of the ceramics was low. The micrograph were taken with a model S 180 microrcope, manufactured by Cambridge Imtr., U.K. The author would like to thank Mrs. S. Benemann, Bundesanetalt f i r Materialforrchung und -priifbng, laboratory 5.33, for the microgr aphe.

3. RESULTS AND DISCUSSION 3.1. Zirconium dioxide oauplw

The wear of the ZtO,/ZrO, couple ia given in f i e 1 for lubrication with pure hexadecane at different sliding rpee6. The result8 nhow that a clear tramition from a mild wear region to a revere wear region occurr at a rliding speed of 0.07 d r . The coefficients of friction were found to be about 0.6. This wear transition, briefly outlined earlier in (7), agrees well with the rerultr obtained by M. Woydt (80, 81), who found similar wear transitions for the dry eliding of ZrO,/ZrO, . As pure zirconia urually forms monoclinic, tetragonal or cubic phases, M. Woydt studied materials with different phare compositions. Although the resultr for the cubic material (Y203 stabilized), the materials rich with tetragonal zirconia (Y203 stabilized) and the material conrioting of about 50 % tetragonal and 50 % cubic zirconia (MgO rtabilized) were not identical, all materials showed tramitiom to high wear when the sliding speed was increased. S. W. Lee et al.

(53) carried out experiments wing a ballon-three-flatr wear tester with a rteploading procedure for tetragonal zirconia (Y,O, rtabilized). The wear maps determined rhowed that the wear behaviour wan affected by the rurromding media (air,water, paraffm oil and a formulated oil) but that traneitiom to high wear could not be prevented under revere operating conditions. The resultr obtained by M. Woydt (80), who carried out X-ray-diffraction aualyser of wear particler and wear rcars end trammiorion electron microscopy of wear particles indicate that the p h e composition of zirconia is changed when zirconia in worn in the ievere wear region. Similar results were obtained by W. M. Rainforth and R. Stevem (60)for Zr02/rteel under dry ambient conditions. It ie asrumed by M. Woydt (80, 81), ar well ar by V. Aronov (6). that high frictional heating lea& to phase tramformation which in turn l e a 6 to volume and rtrers changer which accelerate wear. The Ntface heating which taker place on the rurface of E l i d i n g microcontacts war calculated according to D.KuhlmamWilsdorf (49-51), who based her work on the model of H. Blok (11, 12) and J. C. Jaeger (16, 41). The temperature increase above ambient temperature ia given in figure 5 for ceramic/ceramic coupler at different r l i k speede. The calculations were carried out for elastic behaviour, a radium of curvature of the microcontacts of 100 pm, a force of 100 N, a coefficient of friction of 0.1 and valuer of the material propertier which refer to room temperature (table 1). An W.0. Wmer et al. (34, 59) found the number of microcontactr for a steel pin eliding over a raphire dirk to be approximately 7 and ar there M no other experimental rerult available, the calculatiom were done assuming the

639

*.**

estlm. estlm.

Pure hexadecane

* 0.47

**/

/

** F-e 1. Wear coefficiente of the ZrO,/ZrO, couple lubricated with pure hexadecane at different eliding speede. The experiments were carried out with a four ball machine applying a force of 100 N per contact for a rliding dintance of 72 m at room temperature. The numbers correspond to the coefficients of friction.

**

0 . 5 7 1 0.63

OF/-*

*

4

1

0.57

0.01

0.001

0.1

1

Sliding Speed h / s )

4

AAA VVV

Hsxadecane

+ 0.1 molX stearlc acld

+

0.001

I

Hsxadecane

f

F-e 2. Wear coefficients of the Zr02/Zr02 couple at different sliding speeds. Solutione of fatty wide in hexadecane were u e d M lubricante. The experiments were carried out with a four ball machine applyha a force of 100 N per contact for a eliding distance of 72 m at room temperature. The numbers correspond to the coefficientr of friction.

1.0 molX palrnltlc acld

0.01

0.1

Sllding Speed (rn/s)

1

...

000 Hexadecane

t 0.1 molX zinc dllsobutyl dlthlophosphate

estlrn. 0

Hexadecane t 1.0 molX zlnc dllsobutyl dlthlophosphate

-E

.

z

1

m

E

-E

0 .‘

0.19

/

-

0 w-

W 0

/

0 0.57

L

10-8 10-9

a -..

4

0

023,).

-..__,_

0.31

0.43

:/

B%lS 02’

1 0.0 1

0.001

4

1

-E

0.43

1

0.1

Sliding Speed h / s ) 000 Hexadecane

est’rn’

mol% zinc dl-n-dodecyl dlthlophosphate t 0.1

000

8

Hexadecane t 1.0 molX zinc dl-n-dodecyl dlthlophosphate

Figure 4. Wear coefficiente of the Zr02/Zr02 couple at different eliding speede. Solutiona of zinc di-n-dodecyl dithiophoaphate in hexadecane were used a8 lubricants. The experimente were carried out with a four ball machine applying a force of 100 N per contact for a sliding dietance of 72 m at room temperature. The numbers correepond to the coefficients of friction.

z

1

m

E

E

I

Y

.

:

0 0

.’ / L

m

Figure 3. Wear coefficients of the Zr02/Zr02 couple at different sliding apeede. Solutiona of zinc diisobutyl dithiophoephate in hexadecane were used as lubricante. The experimente were carried out with a four ball machine applying a force of 100 N per contact for a eliding dietance of 72 m at room temperature. The numbere correepond to the coefficients of friction.

.--7

u.11

0.00 1

0.01

0.10

0.1

Sliding Speed h / s )

1

641

number of microcontacts to be 7. The results in f m e 5 show that the temperature increases are significantly higher for ZrO, than for A1,0, and Si,N, which in turn are higher than the values for Sic. Due to the mathematics and the material properties these differences can be traced back to different thermal conductivities which differ partly by more than one order of magnitude (table 1). If the effect of temperature on the properties of the materials is neglected, the temperature increase is proportional to the coefficient of friction. For a sliding rpeed of 0.07 d a , where the transition to severe wear was observed (figure l), one obtaine a temperature increase of 188 K for a coefficient of friction of 0.1 (figure 5). For a coefficient of friction of 0.6 the sixfold value of 1130 K is then arrived at. As the precision of the temperature calculations is limited, eg. due to the uncertain assumption for the d e r of microcontacts, the phase transition cannot be identified on the banis of the temperature calculations. Furthermore the pressure dependence of the phase transition has to be taken into account. Although the pressure dependence can be estimated according to Clausiue-Clapeyron (10, 57) for known enthalpy and volume change of the phase transition, thin involves a hrrther uncertainty. In addition, siuter additives are used for the production of commercial zirconia, so that the phane diagram for the particular sinter additive and its concentration hae to be known. As the temperature calculations according to D. Kuhlmann-Wilsdorf assume dry sliding, the results cannot be transferred to lubricated sliding under all circumstances. Since the coefficients of

friction are, at Ieant for ZrO,/ZrO, and Al,O,/Al,O, (section 3.2.) lubricated with pure bexadecane, comparable to dry sliding, OM can rule out for these couples and the given operating conditions that the contact of the arperities ia significantly affected by the lubricant. The thermal conductivity of paraffm oils ia approximatly 0.15 J/s m K which is one order of magnitude lower than the thermal conductivity of ZrO,. The heat conduction into the oil should therefore not contribute too much to the Overall heat flux. The wear of the ZrO,/ZrO, sliding couple for lubrication with rolutiona of long chain fatty acida in hexadecane at different sliding epee& ia given in f w e 2. The coefficientr of friction are significantly lower for hexadecane containing fatty a c i b than for pure hexadecane. The transition to severe wear L observed for lubrication with solutions of fatty aci& in hexadecane as well as for lubrication with pure hexadecane. Ae longchain fatty a c i b consist of a carboxylic group and a long hydrophobic hydrocarbon rest, the molecule has a pronounced polar and a pronounced nonpolar part. Thus aborption of fatty aci& from rolutions of fatty acids in nonpolar solvents has to be expected at the rolid/liquid interface for polar or ionic solids. Thin waa shown experimentally for stearic acid in cyclohexane which was adsorbed on the ionic surface of aluminium oxide (46). The decreane of the coefficient of friction in the mild wear region from 0.6 for lubrication with pure hexadecane to approximately 0.12 for lubrication with ~ 0 1 U t i oof~ long chain fatty acids in hexadecane is probably caused by adaorption layers of fatty acid molecules on the surface of the zirconia sampler. Ae fatty aci& are not able to form reaction layers due to decomposition and an no reaction with the surface ma-

642

2500

-g

2000

1 I

-

I I I I I I I I I

Q

a

z

(II

0

E

za

Figwe 5. Frictional temperature increaoe for ceramic/ceramic coupler at the rurface of the microcontach at different rliding ~ p e e b .The calculatiom were made according to D. Kuhlmann-Wilrdorf, (UIUmina elartic behaviw, a tadiw of curvature of 100 pm, a d e r of microcontactr of 7, B force of 100 N and a coefficient of friction of 0.1. Material propertier which refer to room temperature were uaed for the crlculat iom.

1500 -

I I

I

I

44

a

L

E

1000 -

0

l-

500 -

___---

4

4-

0

1

I

,11111,

I

I

1 11

I

I

I111111

1

I

I

I'

Sliding Speed h / s )

Table 1. Material properties of the ceramicr at room temperature. Ceramic

Specific Heat

Vickerr

Young'r

Hardaesr

Modulw [N m-*] [J ''gk

[N m'2]

K"]

Dmity ckg

m-31

Thermal Conductivity [W m-l K-']

Poirron'r Ratio

[-1

~~

SIC

2.4 x 10"

4.3

x

10"

700

3200

110

0.16

Si,N,

1.6

x

10"

3.1

x

10"

700

3250

35

0.29

*1*03

1.6

x

10"

3.8

x

10'l

900

3930

30

0.22

ZrO,

1.2

x

10"

2.0

x

10"

400

6050

2.5

0.30

643 terial ie expected. tramition from the mild to the severe wear region cannot be prevented by the fatty acids. For the dry gliding of ceramic/ceramic couples, the adsorption of water (33, 52) or other molecules (64), which were allowed to enrich the surrounding gas atmosphere, was found to affect the tribological results significantly as well. Wear of the ZrO,/ZrO, couple iE given in f i e 3 and f m e 4 for lubrication with solutions of zinc diisobutyl dithiophoaphate and zinc di-n-dodecyl dithiophosphate in hexadecane at different sliding speeds. At low sliding speeds friction and wear t more effectively reduced by zinc di-n-dodecyl dithiophoephate than by zinc diiaobutyl dithiophosphate. This hi probably caused by better adsorption of zinc di-n-dodecyl dithiophosphate, since longer hydrocarbon rests generally promote adeorption. At higher sliding speeds considerably increased surface temperatures at the microcontacts due to the low thermal conductivity of zirconia (figure 5) have to be taken into account. At higher temperatures the decomposition of zinc dialkyl dithiophosphates takes place, which yields a variety of different products (13, 19, 24. 54). The effect of the zinc dialkyl dithiophosphates ia probably cawed by the formation of reaction layers due to the decomposition of the zinc dialkyl dithiophoephatee, since corresponding results were found with metallic sliding couples (8, 35, 67). As the thermal conductivity for steel ia higher than for zirconia, higher frictional heating i~ expected with zirconia coup1ea. The wear traneitiona of the ZrO,/ ZrO, couple are reduced better by zinc di-n-dodecyl dithiophosphate than by zinc diiaobutyl dithiophosphate. This might be caused by better adsorption subsequently

followed by decompoaition. A thicker reaction layer due to the longer alkyl chain might affect the tribological results a8 well. The better results for the higher concentrations for both of the zinc dialkyl dithiophosphates are probably caused by higher reaction rates. Varioua author8 (2, 28, 63, 66) found wear coefficients from 10-8 to lo-’ mm3/ N m for ZrOp/Zr02 sliding couples lubricated with pure p a r a f f i oils. These rer u l t s obviously correspond to values in the mild wear region. The wear coefficients are higher for low sliding distances (S. W.Lee et al. (53) and figure 1) since a pronounced dependence of the wear coefficient on the sliding distance was found by M. Woydt (80) and T. E. Fischer et al. (28). High wear iE reported for severe operating conditions by K. F. Dufrane et al. (25), W. A. Glaeser (discussion to V. Aronov (6)) and S. W.Lee et al. (53) for lubrication with paraffhic lubricants that were pure or contained additives. In addition hot spots were observed by K. F. Dufrane et al. and W. A. Glaeaer. The coefficients of friction found in thia study are as high as those found for dry sliding (14, 30, 69, 70). They are higher than the coefficients of friction reported by different authors (28, 36, 63) for Z r 0 2 / Z r 0 2 lubricated with pure paraffin oil. These differences are probably caused by different purification procedures for the p a r a f f i oil. the equipment and the ceramic sampler since friction and wear can easily affected by low amounts of surface active compounds. Thie is clearly ahown when the reeulta for lubrication with pure hexadecane are compared with those for lubrication with 0.1 mole% stearic acid in hexadecane ( f i e 1 and 2). H.Ishigaki et al. (40) and K.-H. Zum Gahr et al. (84) found that the grain size of zirconia affected not only the mecha-

644

nical but alro the tribological propertier.

The wear of Al,O,/Al,O, lubricated with pure hexadecane is given in fiiure 6 for different eliding rpesde. The results show that the friction and wear coefficients are high for the operating conditions studied and that the results for the two batches of alumina do not differ rignificantly . Transition8 from mild to revere wear were reported by D. E. Deckmanu, S. Jahanmir and S. M. Heu (22, 32, 39, 76, 77) for dry rlidhg ar well an for lubricated alidm with a pure paraffin oil. The tranritions were found M a h c t i o n of the rliding speed, contact load and partly M a h c t i o n of the rlidiug distance and the ambient temperature. The experiment8 by S.-S. Kim et al. (45) and H. Kim et al. (44) who primarily rtudied wear for different sliding dirtancee confirmed these results. K.-H. Habk and M. Woydt (37, 79) studied dry sliding for different eliding rpeeda and ambient temperatures. They found transitions to severe wear which were promoted when the rliding epeed or ambient temperature war increased. Similar result8 were found by L. Eepoaito et al. (26). S.-J. Cho et al. (18) carried out experimente for Al,O,/Si,N, coupler with different grain sizer of alumina lubricated with a pure paraffin oil. The rerulta showed that the tramition to revere wear changed with the grain size of the alumina material empolyed. The work of K.-H. Zum Gahr et al. (84), A. K. Mukhopadhyay et al. (58) and M. Miranda-Martinez et al. (56) confirmed that the wear of alumina depends on the grain rize. The quality of the alumina rampler with which the above otudier were carried

out war reported ar 99,O X , 99.5 %, 99.7 X and 99,99 X alumina (45, 22, 79, 84) or high purity alumina wae stated to have been wed (18, 44). The alumina purity of the sampler used for the author’s experimentr war comparable to there qualities (rection 2.). For zirconia-toughened alumina wear testa with Al,O,/Si,N, couples were carried out by C. He et al. (38) wing a purified paraffm oil for lubrication. It war ahown that the loads at which tramitions to the revere wear region occurred could clearly be shifted with increasing zirconia content to higher loadr. Thin effect war reported for zirconia contenti up to 20 ~01%. A vdue of 6.5 K wae calculated for the temperature increase due to frictional h e a t h according to D. Kuhlmann-Wilsdorf for a r h b g rpeed of 0.005 d r , a coefficient of friction of 0.5 and for the other arrumptiom made for the calcula5. An the experitiom presented in f-e mental rerultr for lubrication with pure hexadecane (ligure 6 ) demonstrate that wear of the Al,O,/ Al,O, couples for low rliding speeds already corresponda to valuer of the ievere wear region, the mechanism of the wear transition in obviouly not determined by frictional heating. With the help of scanuing electron microrcopy D. E. Dtctmann et al. (22) rtudied material which wan worn in the mild and in the revere wear region. The micrograph from rcarr in the mild wear region rhowed only rome localized regions of fracture wherear the micrograph from severe wear experiments rhowed that extenrive intergranular fracture had occurred. The wear coefficientr for A1,O / Al,O, couples (let batch of Al,O,fwhich were lubricated with rolutiona of palmitic acid in hexadecane are given in f m e 7 at different rliding epee&. Although the

645

Pure hexadecane, 1st batch Al 0

tt*

Pure hexadecane. 2nd batch All 0 3

F m e 6. Wear coefficient8 of A1203/A1203 coupler lubricated with pure hexadecane at different r l i h rpeeb. The experiments were carried out with a four ball machine applying a force of 100 N per contact for a rliding dietance of 72 m at room temperature. The numberr correspond to the coefficients of friction.

033 - m

0.00 1

0.01

0.1

1

Slldlng Speed (m/s)

vvv

Hexadecane + 0.1 molX palrnidc acid 1st batch A12 0 3

Figure 7. Wear coefficientr of the Al,O,/Al,O, couple at different eliding rpeeb. Solutiom of palmitic acid in hexadecane were wed as lubricantr. The experimentr were carried out with a four ball machine applying a force of 100 N per contact for a rliding dirtance of 72 m at room temperature. The number8 correrpond to the coefficientr of friction.

V V V Hexadecane

+

1.0 molX paimldc acid

1st batch Al

L

; a

O3

10-71 1o-8

1 .

10q9 0.001

0.0 1

0.1

Slldlng Speed (m/s)

1

646

coefficients of friction are, compared to lubrication with pure hexadecane, reduced by palmitic acid, the wear coefficients remain high. J. J. Kipling and E. M. Wright (46) brought a solution of stearic acid in contact with the large surface of alumina powder. Since thia caused the rtearic acid concentration of the solution to decrease one can assume that adsorption of stearic acid on the alumina surface occurred. The low coefficients of friction for lubrication with solutions of palmitic acid in hexadecane are therefore expected to be caused by adsorption layers. P. Studt’s experiments (71, 72, 73), in which the coefficients of friction were determined for lubrication with solutionn of fatty acids and zinc dialkyl dithiophosphates depending on the alkyl chain length, led to the same conclusion. When the additive molecules had an alkyl chain of six or more carbon a t o m the coefficients of friction were significantly lower than those for additive molecules with short alkyl chains. For molecules with a polar and a nonpolar part, enhanced surface activity occurs M a rule at chain length of six or more carbon atoms. Thus P. Studt’s results also support the idea that the effect of the additives is cawed by adsorption layers. The wear coefficients for lubrication with solutions containing zinc diiaobutyl dithiophosphate and zinc di-n-dodecyl dithiophoshte are shown in figure 8 and tigure 9 for different sliding speeds. Alumina from the 2nd batch was used as material. For lubrication with both zinc diisobutyl dithiophosphate concentrations the same transitions to severe wear occurred. In contrast, the rerults for the two different concentrations of the zinc di-n-dodecyl dithiophoephate solutions differ considerably. Whereas wear lubricated with the 1.0 mole%solution corres-

ponb to the severe wear regioh for the whole sliding speed range, wear of cxperiments lubricated with the 0.1 mole% solution decreases fuat with increasing slidiug weed and then suddenly increases to values of the severe wear region. A frictional temperature increase of 72 K was calculated according to D. Kuhlmann-Wilsdorf for a force of 100 N, a sliding rpeed of 0.15 m/s, a coefficient of friction of 0.15 and the assumptions made for the calculations given in figure 5. An the experiments for the A1,0,/ Al,O, couples were carried out for aliding rpeeds up to 0.15 m/e thermal decomposition of the zinc dialkyl dithiophosphates might only be possible in the upper rliding q e e d range studied. But the friction and wear data indicate no change of the effect of the additive8 for higher sliding rpeedr. Since the zinc dialkyl dithiophosphate molecules consist of polar and nonpolar functional groups, the low cotfficientr of friction are probably cawed by adsorption layerr. Scanning electron micrograph were taken from wear scars of ramples lubricated with solutions of the additives in hexadecane or with pure hexadecane. The micrographs showed fracture surfaces which covered decisive parts of the wear @carsparticularly in the severe wear region. These results for lubrication with solutions containing additives agree well with results reported by D. E. Declunan et al. (22) for lubrication with pure paraffin oil. T. A. Michalske et al. (55) determined the crack velocity of singlecrystal a-aluminum oxide propagating on the (1012) plane in the surrounding media water, ammonia, hydrazine. and acetonitrile. The experiments showed that the crack velocity which WM determined for different stress intemities K, changed considerably with the surrounding medium.

647

Hexadecane + 0.1 molX zlnc dllrobutyl dmlophocphate 2nd batch A 1 2 0 3

-

F i e 8. Wear coefficients of the

Hexadecane + 1.0 molX zlnc dllcobutyl dlthlophocphate 2nd batch Al 203

E

z \

A1,03/A1,0, couple at different rliding apeeb. Solutions of zinc diirobutyl dithiophosphate in hexadecane were wed as lubricants. The experiments were carried out with a four ball machine applying n force of 100 N per contact for a sliding dirtance of 72 m at room temperature. The numberr correqond to the coefficients of friction.

m

-E E

0.18

c

c

CP 0-10

0.14 W' 0.14

0

I I

0

0

I

i

0.0 1

0.00 1

0.1

1

Sliding Speed (rn/s) 1

000 Hexadecane + 0.1 molX zlnc dl-n-dodecyl dlthlophorphate 2nd batch A 1 2 0 3

0.0

::it 8,

;

"..0.12

L

A

;

Figure 9. Wear coefficients of the Al,O,/Al,O, couple at different diding speeds. Solutions of zinc di-n-dodecyl dithiophosphate in hexadecane were wed as lubricants. The experimentr were carried out with a four ball machine applying a force of 100 N per contact for a sliding distance of 72 m at room temperature. The numbers correrpond to the coefficients of friction.

Hexadecane + 1.0 molX zlnc dl-n-dodecyl dlthlophorphate 2nd batch Al O3

0.0.10

'0 0' '

!

;

0.1 1

0.001

0.0 1

0.1

Sliding Speed (rn/sl

1

648

Althowh the wear mechanirm r e e m to be rometimes complex, ar can be reen from fiiure 9 , one can state that adsorption and intergranular fracture, which in earily affected by the surrounding medium, play an important role for the friction and wear mechaniam of lubricated AI,O,/A1,0, coupler. The wear coefficients in the mild wear region for different surrounding media and operating conditions cover the range from to about lo-' mm3/N m (22, 32, 39, 44, 75, 76). For the severe wear region wear coefficients from mm3/N m were reported. These to values are in agreement with the resultr given in r i e s 6 to 9 . The deviations for the same surrounding media might be caused by the effect of grain size, different geometrical arrangements and partly different operating conditions. The coefficients of friction reported by different

+*+

groups (22, 32, 39, 63, 82. 83) for lubrication with pure paraffii oil are for the moat part lower than the values given iu r ? e6. There differencer, as well as the differencer for the ZrO,/ZrO, couples, ate probably cawed by different purification procedurer.

3.3. Sil&n nitri& ampler The wear coefficientr of Si,N,/Si,N, coupler lubricated with pure hexadecane are given in figure 10 for different sliding epee& and the eliding dirtancer of 72 and lo00 m. An figure 10 shows, no tramition to severe wear taker place with increaring eliding rpeed. Inntead a alight decrease of the wear coefficients and coefficientr of friction can be found. The wear coeff'icientr do not differ within the error limits for a eliding dirtaxwe of 72 and 1000 m, i. e. the wear volume ir proportional to

Pure hexadecane. 72 rn slldlng distance

***

Pure hexrdecane. 1000 rn slldlng distance

F i e 10. Wear coefficients of the Si3N,/Si3N, couple lubricated with pure hexadecane at different eliding rpeeda for 72 and 1000 m rliding distance. The experiments were carried out with a four ball machine applying a force of 100 N per contact at room temperature. The nunberm correspond to the coefficientr of friction.

1o-6

4

L

020

*+

Q

;

0.17

0.17

*

0.08

1o-' 0.00 1

0.01

0.1

Sliding Speed h / s )

1

649 the slidhg distance. Wear of Si,N,/Si,N, couple~lubricated with pure paraffm oil was determined for increasing contacts l o a 6 by R. S. Gates, S. M. Hsu and E. E. Klaua (31, 32). The reaults showed only the first signe of a tramition to severe wear. Since tramitione to severe wear occurred for ZrO,/ZrO, and Al,O,/Al,O, couples with increasing sliding speed and contact load (sectione 3.1. and 3.2.) the results for lubricated Si,N,/Si,N, couples are more favourable by far. R. S. Gates et al. (31) and S. Jahanmir et al. (42) determined wear coefficients of approximatly and 4 x lo-’ mm3/N m for Si,N,/Si,N, COU~ICE lubricated with pure paraffin oil. These values are in agreement with those presented in f m e 10. A small wear coefficient of 2.4 x 10’’ mm3/N m waa found for a Si,N,/Si,N, couple lubricated with paraffiin oil at room temperature. But a higher wear coefficient of 3.6 x mm3/N m was determined for 150 OC. The coefficients of friction which were determined in the author’s experiments cover a range from 0.1 to 0.2. Theme results agree well with the results reported (2, 23, 31, 42, 47, 78) for sliding motion and fretting lubricated with paraffin oil contain& no additives. Coefficients of friction were determined for dry sliding of Si,N,/Si,N, couples at different sliding speede and ambient temperatures using Si,N, of reveral manufacturers (1, 17, 27, 37, 68). As values of 0.6 were found the coefficients of friction are significantly higher than the corresponding values for lubrication with pure paraffin oil. The wear coefficients for dry sliding at room temperature and higher contact pressures were, at to lo-, mm3/N m, also coneiderably higher than those for lubri-

cation with prraffm oils containing no additives. The rdoorption of lubricant molecules 011 the relatively nonpolar silicon nitride surface i~ &cursed by S. Jahanmir et al. (42) as a potential reason for the low coefficients of friction for lubrication with pure paraffin oil. Results (42) achieved by scanning electron microscopy, transmission electron microscopy and Auger electron epectroacopy indicate that the surface material ir inhomogeneous and comhts partly of oxidized compounds. The degree to which lubricated friction and wear is affected by oxide material, which was also detected on silicon nitride surfaces after dry sliding (21), however, cannot be arsessed 011 the bash of these resulta. Figure 11 shows the friction and wear of Si,N,/Si,N, couples lubricated with rolutione of additives in hexadecane at a sliding rpeed of 0.005 and 0.58 m / s . The t e d t s for sliding apeeds between 0.005 and 0.58 m/r were in between those presented h f i e 11. The experiments were carried out with 1.0 mole% solutions of palmitic acid, zinc di-n-butyl dithiophosphate, zinc diieobutyl dithiophosphate or zinc di-n-dodecyl dithiophoshate in hexadecane. An the results show, friction and wear are only moderately affected by the additives. An variow authors (4, 5, 29, 74) found very low coefficients of friction for Si,N, and S i c couples lubricated with water, the water content for pure hexadecane and solutions of 1.0 mole% palmitic acid in hexadecane w a ~determined using Karl Fischer coulometry. Pure hexadecane which had been in contact with laboratory air at room temperature had a water content of approximately 20 ppm and the solution of palmitic acid in hexadecane had a water content of about 29 ppm. When the oil phase had been brought into

650

without additive zinc diisobutyl dithiophosphate zinc di-n-butyl dithiophosphate zinc di-n-dodecyl dithiophosphate palmitic acid

F i e 11. Coefficientr of friction and wear coefficientr of the Si3N,/Si3N, couple lubricated with rolutionm of additive8 in hexadecane at a rliding rpeed of 0.005 and 0 3 8 m/r. Pure hexadecane, 1.0 mole% rolutione of palmitic acid, zinc di-n-butyl ditbiophorphate, zinc diiaobutyl dithiophorphate and zinc di-n-dodecyl dithiophorphate in hexadecane were wed M lubricantr. The experiments were carried out with a four ball machine applying a force of 100 N per contact for a rliding dirtance of 72 m at room temperature.

65 1

contact with water a water content of 43 ppm wae determined for hexadecane and a water content of 60 ppm wae determined for the 1.0 mole% eolution of palmitic acid in hexadecane. The reaulte thue show that the water concentration ia low in pure hexadecane and only little increaeed by the preaence of palmitic acid. As zinc dialkyl dithiophoephatee interfere with the Karl Fiacher reaction (62, 65) no water determination wae carried out for zinc dialkyl dithiophoephate eolutions. J. J. Habeeb et al. (36) determined friction and wear for Si,N,/Si,N, couplee lubricated with a eolution of lauric acid in paraffin oil. The reaulte showed that wear hcreaeed compared to lubrication with pure paraffin oil. T h i ~ie in agreement with the data given in f w r e 11. In contraat, a decreaae of friction and wear was found by S. Jahanmir et al. (42) wing atearic acid an lubricant additive. Ae differences are not expected for long chain fatty acidn which differ only little in chain length, the reaeon for the varying results ia unclear. The different reedte are probably caued, apart from different operating conditions, by coincidental effect8 euch ae different purification procedures. Z b c dialkyl dithiophoephatee ae lubricant additives were wed by various groups (9, 31, 36, 78) for paraffii oil lubricated Si,N,/Si,N, couplea. As commercial zinc dialkyl dithiophoephatee without apecification of the alkyl reate (9, 31, 36) were employed, a direct comparison with the reeulte preeented in firgure 11 ie difficult. In addition, the experimente of J. Wei et al. (78) were carried out using a reciprocating wear teeter and the reeulte of other authors (9, 31, 36) were obtained for eliding motion under different operating conditiona. Ae zinc dialkyl dithiophoephatee are expected to have an

effect at higher contact temperatures, a clear picture can only be arrived at if a broader range of operating conditions and ambient temperaturea ia studied with well characterized pure compounde. 3.4. Silicon carbide coupler

The wear coefficiente of the SiC/SiC couple lubricated with pure hexadecane are given in f i e 12 for different sliding speeds and the Eliding dietancee of 72 and 1000 m. Figure 12 ahows that no traneition to eevere wear OCCUTE with increaeing eliding epeed. Two values of the wear coefficient8 are coneiderably higher than the others, but they are obviously exceptions. Ae the coefficiente of friction are approximately 0.10 they are lower than thoae of the other ceramic couplea etudicd. For SiC/SiC couples lubricated with pure paraffin oil R. S. Gate8 et al. (ref. (32), f m e 50) found only fvet a i g of ~ a tramition to aevere wear with increaring contact loads. Since no tramition with increaeing Eliding epeed wan determined either these fiidinga are comparable to thoee for lubricated Si3N,/Si3N, couplee. With a pin-on-disk tribometer P. Salonen et al. (63) found the pin wear to be 4 x lo-' mm3/N m and the disk wear to be 4 x lo-' mm3/N m for a SiC/SiC couple lubricated with a pure paraffii oil. AE the wear coefficiente were determined for a sliding dietance of 14000 m, the difference8 to the valuee given in figure 12 are probably due to wear depending on the eliding distance. P. Salonen et al. (63) reported coefficiente of friction of 0.10 and K. Demizu et al. (23) valuee of 0.12 for reciprocating motion lubricated with pure paraffim oil. These resulte agree well with those preaented in f i w e 12. In contrast to lubricated SiC/SiC rlidins couples, friction and wear under dry

652

Pure hexadscane. 72 rn slldlng dlstance

*+t

+**

Pure hexadscane. 1000 m slldlng dlstancs

F w e 12. Wear coefficients of the SiC/SiC couple lubricated with pure hexadecane at different eliding epee& for 72 and 1000 m rliding distance. The experiment8 were carried out with a four ball machine applying a force of 100 N per contact at room temperature. The nwnberr correapond to the coeilicientr of friction.

a

"1

i

1o-8

0.001

*

*

*

0.1 1 0.12

0.13

0.10

0.09

*+ 0.08

0.10 0.12

0.08

0.0 1

Slldlng Speed

0.1

1

Im/s)

elid- are significantly higher ((37). (79) and (43) quoted from (52)). Coefficients of friction of approximately 0.4 and wear coefficiente of at leatit m3/N m were reported for different eliding rpeeda and ambient temperaturee. Thus friction and wear of the nonionic SiC/SiC and Si,N,/Si,N, ceramic couples were, unlike the ionic ceramic couples. eaeentiolly reduced by lubrication with pure paraffin oil. Wear ecare of eilicon carbide swfacea were studied with the help of X-ray photoelectron spectroscopy (XPS). he eurfaces etudied had been worn in experimente with hexadecane lubricated Sic/ S i c couplea at a sliding apeed of 0.58 d a , applying a force of 100 N per contact for a sliding dirtance of 1000 m at room temperature. The reaulte revealed that considerable chemical change6 had taken place on the aurfacee. On the basis

of the binding energiee the reaction pro-

duct6 probably have to be aseigned to partly and completely oxidized eilicon carbide. Whether the tribochemical products are hydroxide or oxide compound6 cannot be judged on the bash of the XPS spectra. Plastically deformed material, which war obviously wear debris, was observed on ecmnhg electron micrographs of worn silicon carbide eurfaces. The XPS spectra rhowed that, compared to micrographs of aurfaces from which the wear debris had to a large extent been removed, the plartically deformed debrie wae oxidized material. A8 the exact compoeition of the tribochemical reaction products and their tribological behaviour ia not known, a T i 1 conclusion regarding Priction and wear of lubricated SiC/SiC couples ir not poreible. But the potential impact which oxidized compounds might have on tribological behaviour should be

653

without additive zinc diisobutyl diihiophosphate zinc di-n-butyl dithiophosphate zinc di-n-dodecyl diihiophosphate palmitic acid

Figure 13. Coefficientr of friction and wear coefficientr of the SiC/SiC couple lubricated with solutiom of additives in hexadecane at a sliding rpeed of 0.005 and 0.58 &ti. Pure hexadecane, 1.0 mole%solutions of palmitic acid, zinc di-n-butyl dithiophorphate, zinc diirobutyl dithiophosphate and zinc di-n-dodecyl ditbiophorphate in hexadecane were wed an lubricants. The experiments were carried out with a four ball machine applying a force of 100 N per contact for a rliding diutance of 72 m at room temperature.

654

taken into account when the friction and wear mechanism of lubricated SiC/SiC couplea are diecussed. The coefficiente of friction and wear coefficienta of SiC/SiC couplea lubricated with aolutiom of additivea in hexadecane are given in f w e 13 for the Eliding apeede of 0.005 and 0.58 m/a. The reaulta for Eliding apeeds between 0.005 and 0.58 m/a lay between thoae preaented in figure 13. The experiment0 were carried out with 1.0 mole% aolutiona of palmitic acid, zinc di-n-butyl dithiophoaphate, zinc diisobutyl dithiophoaphate or zinc di-n-dodecyl dithiophoaphate in hexadecane. Figure 13 ahowa that friction and wear which are already low for lubrication with pure hexadecane are only alightly affected by the additivea. P. Studt (71, 72, 73) determined the coefficienta of friction for lubricated Sic/ S i c couplea with a ephere-on-disk tribometer for a Eliding apeed of 0.005 &a, applying a force of 10 N for a eliding dietance of 100 m at room temperature. Fatty acids and zinc dialkyl dithiophoaphatee with different alkyl chain length were used aa the additivea. The reaulta showed that the coefficienta of friction did not depend on the alkyl chain length of the additivea and that theac coefficients of friction had almost the aame value a8 the coefficient of friction determined for lubrication with pure hexadecane. Correeponding reaulta were found by K. Demizu et al. (23) for reciprocating motion of SiC/SiC couplea lubricated with eolutiona of trialkyl phoaphitea of different alkyl chain length in paraffin oil. The reaulta for SiC/SiC coupler lubricated with aolutiona of zinc dialkyl dithiophosphatea were achieved at room temperature. A8 no significant frictional heating due to the low thermal conductivity ia to be expected for ailicon carbide,

no effect of the additivea cawed by reaction layera ir to be expected for moderate ambient temperaturea.

The author would like to thank Mra. B. Zimmermanu for her aaaiatance and the Deutache Forachungagemeinechaft for their fmancial aupport.

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