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Friction studies of ion beam assisted carbon nitride coating sliding against diamond pin in water vapor
WEAR ELSEVIER
Wear 217 t 1998) 307-311
Friction studies of ion beam assisted carbon nitride coating sliding against diamond pin in water vapor Dong F. Wang *, Koji Kato Luboratory r?fTrihologs~ School o[ Meclumic~d Engineering. Tohoku University. Sendal ~80-77. Japan Received 16 October 19q7:accepted 2 January 1998
Abstract Amorphous carhon nitride coating of thickness 500 nm was deposited onto single crystal silicon using an ion beam assisted depositiou meth(gl ( IBM ). Both the friction coefficient and the friction process of amorphous carlxm nitride coating sliding against diamond pin wcrc measured with a pin-on-disk trihotester and in situ observed with an environmental scanning electron microseope (E-SEM), respectively. In the sliding tests, the amorphous carbon nitride coating gave an initial friction coefficient of 0.02. rising to around 0.08 wben the normal load was con~cutively changed from I0 until 150 mN at a fixed sliding speed of I0 p.m/s. However. an increasing tendency of measured friction coefficients can also be observed when relative humidities varied by up to ahout 80%. The Bowden-Tahor model of friction is shown to be not exactly in accordance with measured friction coefficients for the plastic deformation regime, although the adhesion model for the no plastic deformation regime still needs further studies. © 1998 Elsevier Science S.A. All rights r e , f r e d .
Kt:vwords: Friction: Sliding in water valxw:Armlrphouscarbon nitrkh:coating: Single crystal silicon: Ion beam assisteddeposition ( IBM )
!. I n t r o d u c t i o n Microelectromechanical systems, particularly surface micromachines, often include smooth and chemically active surfaces. Recent studies have revealed a profound influence of friction on the whole pert'ormauee including efficiency. power output and steady-state speed of silicon microdynamic devices [ 1-4 I. But the friction of silicon appeared to be not very satisfactory. When silicon slid against pure metals, high friction coefficients o f 1.0-3.0 were measured by Mishina [ 5 I, whereas a steady and relatively low friction coefficient of about 0.21 was obtained by Xu and Kato [6J when it slid against a diamond in water vapor. Although DLC films show potential as a friction reducing coating for microdynamic devices [ 7 - 9 1 , nitrogen incorporation in DLC increases the fraction of sp: carbon bonds and may become good competitors for DLC for a wide range of sliding elemenLs. ~ A value o f 0. I-0.5 between carbon nitride deposited by different techniques with different nitrogen contents up to 34% and Si3N4 or steel materials has also been preliminarily reported in friction measurements 110-12 I. This paper, however. Ibcu.ses * Correspondingauthor. Tohoku University.School of MechanicalEngineering. Faculty of Engineering.Sendal 980-77. Japan. t A. Grill. Tribology of dianvond-likecarbon and related materials: an updated ncvicw,unpublishedresearch report. 0043-1648/98/$19.00 © 1998El~vicr Science S.A. All rights reserved. PII s0n43-1648 t 98 ) oo 144-6
on the effects of increasing normal load, plastic deformation and relative humidities on friction o f an a r n o , s carbon nitride coating with IO% nit;ogen content (hereafter refo'red to as a - C N , j ) generated in sliding contact with a diamond pin using an environmental .scanning electron microscopo ( ESEM). in which a pin-on-disk tribotester is installed.
Experimental 2. I. Coatblg preparalions An ion beam assisted deposition { IBM) system, as.shown in Fig. I. consisting of a cryogenically pumped chamber, a sputter deposition .source, a low- and high buckler-type ion source and a substmte bolder, was used in this work. A highpurity carbon target (99.999%) was mounted on the cathode, and nitrogen with the addition of argon was used as the bombardment gas. Prior to actual deposition, single crystal silicon was cleaned in situ by bombardment-etchiog for IO min with I KeV and 100 p.A/cm-" nitrogen ions to remove any residual contaminants. -and then held at ambient temperature during deposition. The a-CNo t coating was grown to a necessary thickness o f 500 a m to prevent substmte wear under the m a x i m u m Hertz contact pressure. Typical deposi-
D.b Wang. K. Kam / Wear 217 ( 19981307-31 I 2.2. Sliding tests
, .~,~ f ~ i,.~ o . or s~,~......_...q~.__~r X~-"" #vl~ I ~ Arions / mt.~r / 9 ¢ , ~ / ~ / ~
A pin-on-disk tribotester, installed in the chamber of an ESEM, as shown in Fig. 2, was used to measure the friction and makes possible in situ observation of friction process and the control of relative humidity. The coating specimen was mounted on the disk. A diamond pin of radius I0 #m was employed since a rigid and small wear (or no wear) material is necessary to keep the contact condition steady. The normal lead was consecutively varied up to 150 mN, corresponding to an estimated maximum Hertzian pressure of about 15 GPa. And both the normal load and the friction force can be measured simultaneously with strain gauges during the tests, in order to make the present work comparable to our previous study on silicon 161. a liner sliding speed of 10 # m / s was chosen and a relative humidity of about 24% was controlled during most of the tests.
:i 99~ ' ~ ~
p u ~ uu~t
Fig. I. Schematicillustration of an ion beamassisted dc ~sition (IBM) system.
~Dctector
Strain gauges
3.
Results and d i s c u s s i o n s
3. I. hafluence of&creasing normal load on friction
Disc
The friction coefficient as a function of increasing normal load is shown in Fig. 3. At about 10 mN, the friction coefficient was measured as 0.02. This was followed by a rapid rise to a relatively steady value of 0.06 at around 40 mN, and almost no plastic deformation can be clearly ob~rved at the contacted surface until the load exceeds a critical value of about 50 raN. The friction coefficient also reached its maximum value of about 0.08 at the maximum normal load of 150 mN. Actually, load effect on friction has been an interesting aspect of the tribological study of thin hard coatings, but few
Relative humidity: 0.4% - 80~ Fig. 2. Schemeof a pin-on-disktribotesterinstalled in an environmental scanningelectronmicroscopet E-SEML |ion conditions are: sputtered ions energy I KeV, ion current 100 mA. assisted ions energy I KeV and ion beam current density 40 #A/cm_,.
~L
0.12
o.1
,
0.08 ~
0.06
•. k
0.04
C.~
0.02
A
.
0
0
20
40
60
Normal
so
too
120
~40
t60
load W, mN
Fig. 3. Influence , f m~nal hind on ii'iclkm coc|'ticient and in ~itu oh~rvalions of various contact Ixfints of a typical friction process. Four images are clnrc.,q~mling to the fimr conlacl pusitions A. B. C and D at 5(). gS. I I() and 13n inN. respectively, where image A showing no plastic deformation, image B. C "and D showing pl~,,ti¢ dc|ocnT~lion and ahm)M no wear debris ( plem,c rel;er to Sc,:tion 3.2 |.
D.F. Wang. K. I(am / Wear 217 ¢1998)307-311
309
(a)
J Fig. 4. Groove measuringusin an atomicforce micro.~ope (AIM) studies have been devoted to the effect of consecutive load on friction process especially for DLC and related thin hard coatings. A fixed load is usually the caR. And the main thing which sets the present friction studies apart from the traditional ~ m t c h tests is observing the dynamic variation in friction process rather than determining the critical load. Therefore, a further detailed study is needed for the insignificantly dynamic variation even with increasing load in measured friction coefficients of 0.02-0.08. which is comparable to diamond, and much better than silicon, reported by our previous work as 0.07-0.23 161 under identical test conditions. 3.2. Influence o f plu.vtic deformation on friction
E-SEM images of various contacted points of a typical friction process on a-CNo, coating are a l ~ shown in Fig. 3 above the friction profile. We note that almost no wear debris can be Ibund as groove was forming, and the radial cracks along the groove length always occurred after the unloading of the diamond pin. The former is confirmed by a typical groove measured with an atomic force micro~ope (AFM). As shown in Fig. 4A the groove is believed to be formed by plastic deformation due to the sliding of the diamond pin and almost no wear loss happened. And the latter may be explained based on a recent theoretical analysis [ 131 that radial cracks can be induced during unloading in the case of a sharp indenter. Therefore. a propo~d loading friction process can he summarized into two simple regimes: "no plastic deformation' and "plastic deformation'. For the plastic deformation regime, the gradual increase in friction is believed to be mainly associated with plastic deformation. But for the no plastic deformation regime, adhesive factor may play an important role. The Bowden-Tabor model [ 14.15] of friction may provide a possible understanding for the pre~nt measured results. The frictional force can be expressed as 2a 3
F=F~+F~= ~ n + A S
(J)
where a is the radius of contact, R the radius of the pin, Hthe hardness of coating, A the area of contact and S the shear
(a)
and the correspondingposhionon the coaling.qtrface( b ) .
strength of the interface. The friction coefficient, /7,, is obtained by dividing both sides of Eq. ( I ) by the normal load, W. Suppose the adhesive term is to remain unchanged for the plastic deformation regime, the friction coefficient is then given by F 2 [ 2 \ l'5 I t = -~ --- ~ !z s WO.~H-O.5+ ~,~
~2~
where/.t~ is the constant adhesive term. If Ihe estimated value of//, 32 GPa, b,lsed on the real groove measuring, file normal load and the measured friction coefficient, can he substituted back into this formula, a compari,qm between the ~ r e d results and three fitting curves can he drawn as .~own in Fig. 5. However, for the no plastic deformation regime the plowing term can he ignored, the friction coefficient is then given by S ~t,,=nSlW= -~
(3)
where P is the mean pressure of the comact. This formula slates that the friction coefficient depends independently on the shear strength and the pressure. Therefore, all those factors of coating parameters and running conditions, especially the normal load and relative humidity in present study, will have effects on the friction cocfficienLs for the no plastic deformation regime. o.!
~
-
o.o8
~ O.INb
O
20
40 60 80 tOO J20 I4Q 160 Norned Joand W, u ~ Fig. 5. A comparisonbetweenthe measuredresultsand three fillingcurves by Bowth:n-Tabormodelof friction.
310
D.b: Wang. K, gato / Wear217 (1998) 307-311
3.3. h ~ u e n c e o f relative humidlY." on friction
The friction coefficient at normal load of 50 raN, measured as 0.06. gradually increased with increasing humidities and reached values in the range of 0.07 to 0,08 at relative humidities close to 80,%, as shown in Fig. 6. The trend of increasing friction with humidity is possibly attributed to the saturation of microcontacts with water vapor molecules, causing increa~d bonding and thus increased shear strength across the interface, especially for the no plastic deformation regime. However, the friction coefficients remained almost unchanged for a large range of relative humidities for the normal loads of both 100 and 150 mN, where plastic deformation is believed to play an important role rather than adhesive factor. Generally speaking, carbon nitride coatings possess better thermal and oxidative stability than carbon coatings. Xiong et al. [ 161 synthesized carbon nitride coatings using laser ablation of a graphite target in a nitrogen atmosphere. Compared with amorphous diamond coatings prepared by the .same method, carbon nitride ~.oatings are chemically inert. Infrared spectro~opy studies by Li et al. [ 17] also showed ",hat the C-N band in carbon nitride coating using dc unbalanced magnetron sputterin8 at 22 l0 c m - t remained intact upon heating in air at 623 K for 30 min and disappeared only when heated in air to 873 K for 30 rain, whereas early work by Dugger et al. [ 181 showed that carbon coatings were literally burned away under the latter conditions. The better thermal and oxidative stability may be attributed to the type of C-N bonding in c',n'bon nitride coatings. On the other hand. in cases of nitrogen, argon, etc., or vaccum, carbon coating shows very low values of friction coefficienp, about 0.05 or less. But humid or oxygen containing atmosphere increases friction coefficient beyond the values of 0.6 119]. in present studies, however, only pure water was introduced into the chamber of the E-SEM, while not only water vapor but also oxygen, etc.. is contained in general humid atmosphere. Therefore. it is also relatively understand-
¢ . 1
. . . . . . . .
40
,
,
. . . . . . .
~ d p ~ no~' T~,~ d a-CNL,: ~
• 095 "
i
20
60 . . . . . . . .
,
80 . . . . . . . .
,
100 . . . . . . . .
I__ ........ O.O8
I
,
• •
o.s
t
i t.$
i z
i 2.s
i 3
KPa Fig.6. lnflueP~..eof relati~.: humidit)on fiiction co,zfliciem.The ~.aLer~ra~)r pressure is Ibr comparisonsince only pure ','.ater ".apor is intavducedinto the chamber ( plca-,.crel~rto Section3.3 t. ~ r e
of' w a t e r
vapor
P,
The influence of the increasing load, the plastic deformation and the relative humidity on the friction of a-CNo.~ coating of 500 nm, deposited by an ion beam assisted deposition (IBM) and sliding against a diamond pin, has been studied and discussed. We may summarize that ( I ) the friction coefficients stay in the values of 0.02 to 0.08 at the loads of l0 to 150 mN with 24% relative humidity; nevertheless, the value increases from 0.06 to 0.07-0.08 at the load ofSO mN when controlling the relative humidity to vary up to about 80%. (2) The Bowden-Tabor model of friction is shown to be not exactly in accordance with measured results for the plastic deformation regime, although the adhesion model for the no plastic deformation regime still needs further studies.
Acknowledgements One of the authors ( Dang F. Wang) would like to thank Zhejiang University, Chinese Education Committee and Japanese Ministry of Education, Science and Culture, for the award of a scholarship as well as granting him to study and undertake this research at Tohoku University• Sinecure thanks are also given to Prof. Zhi.Y. Mao for his continuous encouragement. Assoc. Prof. and Dr. Noritsugu Umehera and Mr. Koshi Adachi for their helpful discussions, and Dr. Junguo. Xu. Mr. Ling Zhou and Mr. Hiroyuki Koida for their experimental assistance.
[ I ] R.S.Muller.Mkrodynamics.SensnrsandActuatotsA21-A23(1990) i-8.
,
m - - . ~ - too mN t~ N
~
4. Summary
References
Relative humidity RH, % 0
able that the present relative humidity controlled by the pressure of the water vapor did not influence the measured friction coefficients of coating seriously, while conducting friction studies in air may increase the friction coefficient due to the existence of oxygen.
[ 2 [ KJ. Gabriel.F. Behi. R. Mahadevan.M. Mehregany.In-situfriction and wear mechanismsin integratedFflysiliconmechanisms•Sensors and ActuatorsA2 I-A23 (1990) 184-188. 131 K. Done. W.H. Ko. G.M. Michal. A preliminarystudy on friction measurementsin MEMS.Pa~c.6th Int. Conf. SolidState Sensorsand Actuators.Transducers'91. San Fransisco.CA. 24-27 June 1991.pp. 213-216. 14l K. Kom'.o):,~alos.Surbceengineeringandmicrotribologyformicrt~lectromechanicalsystems.Wear 200 (1996) 305-327. 151 H. Mishina.Frictk~nand wear of semiconductorsin sliding contact v.-ithpure metal.Triho. Im. 21 ( 2 ) ( 1988) 76-82. 161 J Xu. K. Kato. Microwear mechanismsof silicon sliding against diamond in water vaF~r.Tribo. Transuc. STLE 39 (3) (19%) 621020. 171 S. Suzuki.T. Matsuura. M. Uchizawa,S. Yura, H. Shibata,Friction and wear studies on lubricantsand materialsapplicableto MEMS. Pr,s:. IEEEMicro ElectroMechanicalSystems.Nara.Japan. 30 January-2 February 1991.pp. 143-147.
D.F. Wang. K. Kato / Wear 217 ¢1998) 307-311
igl K. Den, W.H. Ko. Application of diamondqike carbon film for microdynamic devices, Prnc. IEEE Solid-State Sensor and Actuator Workshop. Hilton Head Island. $C. 22-25 June 1992. pp, 9g.-IOI. 191 M.R. Houston. R.T. Howe. K. Komvopoulos. R. Maboudian. Diamond-like carbon films for silicon passivatiou in microclectron',echanical devices. Mater. Res. Soc. Syrup. Proc. 383 (1995) 391-402. [ I0] R. Prioii. S.I. Zanctte. A.O. Caride. D,F. Franceschini.F.L. Freile Ir.. Atomic force microscopy of amorphous hydrogenated carbon-nitrogen film deposited by radioffrequcncy-plasma decomposition of methane-ammonia gas mixtures. J. Vac, Sci. Technol. A 14 (4) (1996) 2351-2355. [ I I I E.C. Cutiongco. D. Li, Y.W. Chang. C.S. Bhatia.Tribological behavior of amorphous carbon nitride overcoats for magnetic thin-filmrigid disks. J. Tribo. 118 (1996) 543-548. [ 12J A. Khurshudov. K. Kato. D. Sawada. Mierotribologieal characterization of carbon nitride coatings. Tribo. Lett. 2 (1996) 13-2 I. I 13 ] K. Hayashi, F. Yuan. Elastic-plastic analysis of stresses and initiation
of cracks in a ceramic coaling under indenruion by an elasfic sphere, J. Tfibo.. in press. [ 14] F.P. Bowden. D. Tabor, The friction "andlubricationof solids, Part I, pp. 110-121. 115 ] F.P. Bowden. D. Tabra', The friction and lubrication of ~fids. Part 2. pp. 158--185. [ 16 ] F. Xiong. R.P.H, Chang. C.W. White, Structure and properties of aC:N filmswepared by pulsed laser depusifiou,MRC Sympo~;iumIh'oc. 280 (1993) 587-592. 117l D. LL X. Lin. V.P. Drm:id. Y.W. Chung, M.Y. Chen. M.S. Woug. W.D. SprouL Synthesis and characterizatiou of super-wear-msistam carbon nitride coatings. Diamond Films Technol, 4 (2) (1994) 99III. [ 18 [ M.T. Dugger. K..I. Wahl. Y.W. Chung. B. Bhusha. W. Rothschild.in: Y.W, Choug, H.S. Cbeng ( Eds. ). Advancesin EngineeringTribology. STLE-SP31. STLE. Park Ridge. 1991. p. 43. 1191 A. Grill. Review of Ihe trilmlogy of diamond-like cartwm. Wear 168 419q'3 J 14.'~-153.
Wear 217 t 1998) 307-311
Friction studies of ion beam assisted carbon nitride coating sliding against diamond pin in water vapor Dong F. Wang *, Koji Kato Luboratory r?fTrihologs~ School o[ Meclumic~d Engineering. Tohoku University. Sendal ~80-77. Japan Received 16 October 19q7:accepted 2 January 1998
Abstract Amorphous carhon nitride coating of thickness 500 nm was deposited onto single crystal silicon using an ion beam assisted depositiou meth(gl ( IBM ). Both the friction coefficient and the friction process of amorphous carlxm nitride coating sliding against diamond pin wcrc measured with a pin-on-disk trihotester and in situ observed with an environmental scanning electron microseope (E-SEM), respectively. In the sliding tests, the amorphous carbon nitride coating gave an initial friction coefficient of 0.02. rising to around 0.08 wben the normal load was con~cutively changed from I0 until 150 mN at a fixed sliding speed of I0 p.m/s. However. an increasing tendency of measured friction coefficients can also be observed when relative humidities varied by up to ahout 80%. The Bowden-Tahor model of friction is shown to be not exactly in accordance with measured friction coefficients for the plastic deformation regime, although the adhesion model for the no plastic deformation regime still needs further studies. © 1998 Elsevier Science S.A. All rights r e , f r e d .
Kt:vwords: Friction: Sliding in water valxw:Armlrphouscarbon nitrkh:coating: Single crystal silicon: Ion beam assisteddeposition ( IBM )
!. I n t r o d u c t i o n Microelectromechanical systems, particularly surface micromachines, often include smooth and chemically active surfaces. Recent studies have revealed a profound influence of friction on the whole pert'ormauee including efficiency. power output and steady-state speed of silicon microdynamic devices [ 1-4 I. But the friction of silicon appeared to be not very satisfactory. When silicon slid against pure metals, high friction coefficients o f 1.0-3.0 were measured by Mishina [ 5 I, whereas a steady and relatively low friction coefficient of about 0.21 was obtained by Xu and Kato [6J when it slid against a diamond in water vapor. Although DLC films show potential as a friction reducing coating for microdynamic devices [ 7 - 9 1 , nitrogen incorporation in DLC increases the fraction of sp: carbon bonds and may become good competitors for DLC for a wide range of sliding elemenLs. ~ A value o f 0. I-0.5 between carbon nitride deposited by different techniques with different nitrogen contents up to 34% and Si3N4 or steel materials has also been preliminarily reported in friction measurements 110-12 I. This paper, however. Ibcu.ses * Correspondingauthor. Tohoku University.School of MechanicalEngineering. Faculty of Engineering.Sendal 980-77. Japan. t A. Grill. Tribology of dianvond-likecarbon and related materials: an updated ncvicw,unpublishedresearch report. 0043-1648/98/$19.00 © 1998El~vicr Science S.A. All rights reserved. PII s0n43-1648 t 98 ) oo 144-6
on the effects of increasing normal load, plastic deformation and relative humidities on friction o f an a r n o , s carbon nitride coating with IO% nit;ogen content (hereafter refo'red to as a - C N , j ) generated in sliding contact with a diamond pin using an environmental .scanning electron microscopo ( ESEM). in which a pin-on-disk tribotester is installed.
Experimental 2. I. Coatblg preparalions An ion beam assisted deposition { IBM) system, as.shown in Fig. I. consisting of a cryogenically pumped chamber, a sputter deposition .source, a low- and high buckler-type ion source and a substmte bolder, was used in this work. A highpurity carbon target (99.999%) was mounted on the cathode, and nitrogen with the addition of argon was used as the bombardment gas. Prior to actual deposition, single crystal silicon was cleaned in situ by bombardment-etchiog for IO min with I KeV and 100 p.A/cm-" nitrogen ions to remove any residual contaminants. -and then held at ambient temperature during deposition. The a-CNo t coating was grown to a necessary thickness o f 500 a m to prevent substmte wear under the m a x i m u m Hertz contact pressure. Typical deposi-
D.b Wang. K. Kam / Wear 217 ( 19981307-31 I 2.2. Sliding tests
, .~,~ f ~ i,.~ o . or s~,~......_...q~.__~r X~-"" #vl~ I ~ Arions / mt.~r / 9 ¢ , ~ / ~ / ~
A pin-on-disk tribotester, installed in the chamber of an ESEM, as shown in Fig. 2, was used to measure the friction and makes possible in situ observation of friction process and the control of relative humidity. The coating specimen was mounted on the disk. A diamond pin of radius I0 #m was employed since a rigid and small wear (or no wear) material is necessary to keep the contact condition steady. The normal lead was consecutively varied up to 150 mN, corresponding to an estimated maximum Hertzian pressure of about 15 GPa. And both the normal load and the friction force can be measured simultaneously with strain gauges during the tests, in order to make the present work comparable to our previous study on silicon 161. a liner sliding speed of 10 # m / s was chosen and a relative humidity of about 24% was controlled during most of the tests.
:i 99~ ' ~ ~
p u ~ uu~t
Fig. I. Schematicillustration of an ion beamassisted dc ~sition (IBM) system.
~Dctector
Strain gauges
3.
Results and d i s c u s s i o n s
3. I. hafluence of&creasing normal load on friction
Disc
The friction coefficient as a function of increasing normal load is shown in Fig. 3. At about 10 mN, the friction coefficient was measured as 0.02. This was followed by a rapid rise to a relatively steady value of 0.06 at around 40 mN, and almost no plastic deformation can be clearly ob~rved at the contacted surface until the load exceeds a critical value of about 50 raN. The friction coefficient also reached its maximum value of about 0.08 at the maximum normal load of 150 mN. Actually, load effect on friction has been an interesting aspect of the tribological study of thin hard coatings, but few
Relative humidity: 0.4% - 80~ Fig. 2. Schemeof a pin-on-disktribotesterinstalled in an environmental scanningelectronmicroscopet E-SEML |ion conditions are: sputtered ions energy I KeV, ion current 100 mA. assisted ions energy I KeV and ion beam current density 40 #A/cm_,.
~L
0.12
o.1
,
0.08 ~
0.06
•. k
0.04
C.~
0.02
A
.
0
0
20
40
60
Normal
so
too
120
~40
t60
load W, mN
Fig. 3. Influence , f m~nal hind on ii'iclkm coc|'ticient and in ~itu oh~rvalions of various contact Ixfints of a typical friction process. Four images are clnrc.,q~mling to the fimr conlacl pusitions A. B. C and D at 5(). gS. I I() and 13n inN. respectively, where image A showing no plastic deformation, image B. C "and D showing pl~,,ti¢ dc|ocnT~lion and ahm)M no wear debris ( plem,c rel;er to Sc,:tion 3.2 |.
D.F. Wang. K. I(am / Wear 217 ¢1998)307-311
309
(a)
J Fig. 4. Groove measuringusin an atomicforce micro.~ope (AIM) studies have been devoted to the effect of consecutive load on friction process especially for DLC and related thin hard coatings. A fixed load is usually the caR. And the main thing which sets the present friction studies apart from the traditional ~ m t c h tests is observing the dynamic variation in friction process rather than determining the critical load. Therefore, a further detailed study is needed for the insignificantly dynamic variation even with increasing load in measured friction coefficients of 0.02-0.08. which is comparable to diamond, and much better than silicon, reported by our previous work as 0.07-0.23 161 under identical test conditions. 3.2. Influence o f plu.vtic deformation on friction
E-SEM images of various contacted points of a typical friction process on a-CNo, coating are a l ~ shown in Fig. 3 above the friction profile. We note that almost no wear debris can be Ibund as groove was forming, and the radial cracks along the groove length always occurred after the unloading of the diamond pin. The former is confirmed by a typical groove measured with an atomic force micro~ope (AFM). As shown in Fig. 4A the groove is believed to be formed by plastic deformation due to the sliding of the diamond pin and almost no wear loss happened. And the latter may be explained based on a recent theoretical analysis [ 131 that radial cracks can be induced during unloading in the case of a sharp indenter. Therefore. a propo~d loading friction process can he summarized into two simple regimes: "no plastic deformation' and "plastic deformation'. For the plastic deformation regime, the gradual increase in friction is believed to be mainly associated with plastic deformation. But for the no plastic deformation regime, adhesive factor may play an important role. The Bowden-Tabor model [ 14.15] of friction may provide a possible understanding for the pre~nt measured results. The frictional force can be expressed as 2a 3
F=F~+F~= ~ n + A S
(J)
where a is the radius of contact, R the radius of the pin, Hthe hardness of coating, A the area of contact and S the shear
(a)
and the correspondingposhionon the coaling.qtrface( b ) .
strength of the interface. The friction coefficient, /7,, is obtained by dividing both sides of Eq. ( I ) by the normal load, W. Suppose the adhesive term is to remain unchanged for the plastic deformation regime, the friction coefficient is then given by F 2 [ 2 \ l'5 I t = -~ --- ~ !z s WO.~H-O.5+ ~,~
~2~
where/.t~ is the constant adhesive term. If Ihe estimated value of//, 32 GPa, b,lsed on the real groove measuring, file normal load and the measured friction coefficient, can he substituted back into this formula, a compari,qm between the ~ r e d results and three fitting curves can he drawn as .~own in Fig. 5. However, for the no plastic deformation regime the plowing term can he ignored, the friction coefficient is then given by S ~t,,=nSlW= -~
(3)
where P is the mean pressure of the comact. This formula slates that the friction coefficient depends independently on the shear strength and the pressure. Therefore, all those factors of coating parameters and running conditions, especially the normal load and relative humidity in present study, will have effects on the friction cocfficienLs for the no plastic deformation regime. o.!
~
-
o.o8
~ O.INb
O
20
40 60 80 tOO J20 I4Q 160 Norned Joand W, u ~ Fig. 5. A comparisonbetweenthe measuredresultsand three fillingcurves by Bowth:n-Tabormodelof friction.
310
D.b: Wang. K, gato / Wear217 (1998) 307-311
3.3. h ~ u e n c e o f relative humidlY." on friction
The friction coefficient at normal load of 50 raN, measured as 0.06. gradually increased with increasing humidities and reached values in the range of 0.07 to 0,08 at relative humidities close to 80,%, as shown in Fig. 6. The trend of increasing friction with humidity is possibly attributed to the saturation of microcontacts with water vapor molecules, causing increa~d bonding and thus increased shear strength across the interface, especially for the no plastic deformation regime. However, the friction coefficients remained almost unchanged for a large range of relative humidities for the normal loads of both 100 and 150 mN, where plastic deformation is believed to play an important role rather than adhesive factor. Generally speaking, carbon nitride coatings possess better thermal and oxidative stability than carbon coatings. Xiong et al. [ 161 synthesized carbon nitride coatings using laser ablation of a graphite target in a nitrogen atmosphere. Compared with amorphous diamond coatings prepared by the .same method, carbon nitride ~.oatings are chemically inert. Infrared spectro~opy studies by Li et al. [ 17] also showed ",hat the C-N band in carbon nitride coating using dc unbalanced magnetron sputterin8 at 22 l0 c m - t remained intact upon heating in air at 623 K for 30 min and disappeared only when heated in air to 873 K for 30 rain, whereas early work by Dugger et al. [ 181 showed that carbon coatings were literally burned away under the latter conditions. The better thermal and oxidative stability may be attributed to the type of C-N bonding in c',n'bon nitride coatings. On the other hand. in cases of nitrogen, argon, etc., or vaccum, carbon coating shows very low values of friction coefficienp, about 0.05 or less. But humid or oxygen containing atmosphere increases friction coefficient beyond the values of 0.6 119]. in present studies, however, only pure water was introduced into the chamber of the E-SEM, while not only water vapor but also oxygen, etc.. is contained in general humid atmosphere. Therefore. it is also relatively understand-
¢ . 1
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The influence of the increasing load, the plastic deformation and the relative humidity on the friction of a-CNo.~ coating of 500 nm, deposited by an ion beam assisted deposition (IBM) and sliding against a diamond pin, has been studied and discussed. We may summarize that ( I ) the friction coefficients stay in the values of 0.02 to 0.08 at the loads of l0 to 150 mN with 24% relative humidity; nevertheless, the value increases from 0.06 to 0.07-0.08 at the load ofSO mN when controlling the relative humidity to vary up to about 80%. (2) The Bowden-Tabor model of friction is shown to be not exactly in accordance with measured results for the plastic deformation regime, although the adhesion model for the no plastic deformation regime still needs further studies.
Acknowledgements One of the authors ( Dang F. Wang) would like to thank Zhejiang University, Chinese Education Committee and Japanese Ministry of Education, Science and Culture, for the award of a scholarship as well as granting him to study and undertake this research at Tohoku University• Sinecure thanks are also given to Prof. Zhi.Y. Mao for his continuous encouragement. Assoc. Prof. and Dr. Noritsugu Umehera and Mr. Koshi Adachi for their helpful discussions, and Dr. Junguo. Xu. Mr. Ling Zhou and Mr. Hiroyuki Koida for their experimental assistance.
[ I ] R.S.Muller.Mkrodynamics.SensnrsandActuatotsA21-A23(1990) i-8.
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4. Summary
References
Relative humidity RH, % 0
able that the present relative humidity controlled by the pressure of the water vapor did not influence the measured friction coefficients of coating seriously, while conducting friction studies in air may increase the friction coefficient due to the existence of oxygen.
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