644 Surfaceand CoatingsTechnology,68/69 (1994)644—650

Tribologicalperformanceof thin film amorphouscarbonovercoatsformagnetic recordingrigid disksin various environments

BharatBhushan,JuaiRuanComputerMicrotribologyand Contamination Laboratory, DepartmentofMechanical Engineering, TheOhio State University, Columbus,OH43210—1107,USA

Abstract

An ultrahighvacuumtribotesterequippedwith Augerelectronanalyzerandmassspectrometeris usedto studythefriction andwearbehaviorof amorphouscarbon coatedmagneticthin film rigid disks. The testsincludecontinuous slidingofAl

203—TiC slidersagainstlubricatedandunlubricatedpolishedandtextureddisks.Operatingenvironmentis foundto playan importantrole in thetribologicalperformanceof thecarbonovercoat.Wear lives were longestin dry nitrogenand argonenvironmentsascomparedwith ambientenvironment(with oxygenandwatervapor),in agreementwith someofthepreviouslyreportedresults.The wearlives of thecarboncoatingsin vacuumwere inferior to thosein other environments.In comparison,surfaceroughnessand lubricationplay a lesssignificantrole in friction andwearfor thediskstested.Based onthis study,we concludethatpoorwearlife in vacuumresultsfromintimateslider-to-diskcontact.The longlife in dryAr andnitrogenascomparedwith Ar + 02 andambientenvironment, resultsfromtheabsenceoftribochemicaloxidation prevalentin theoxidizingenvironment.

1. Introduction overunlubricatedamorphouscarbon(a-C)coateddisks,friction was relativelystableand small when the disks

For magneticdisk drives, the headslider surfaceis wereoperatedin nitrogen.Friction increasedquickly asdesignedsuchthat a hydrodynamicair bearingis devel- soon as the disks were exposedto oxygen. Thereforeopedundersteadyoperatingconditions.However,physi- Marchonet al. [3] concludedthat tribochemicaloxida-cal contact occurs duringstarts and stops [1]. The tion is an importantfactorfor diskfailure in the ambienthead—disk separationis typically about 100 nm. The air environment.According to Marchon et al. [3], thecurrentinterestin the computerindustryis in ultrahigh friction increasein air or oxygen isdueto the smoothen-density (e.g. about 10 Gbits in

2) magnetic recording. ing of thea-Csurface(increasingtherealareaof contact)To achievesuchhigh recordingdensities, it isnecessary as aresult of tribochemical wearof the surfacecarbonto reducefurther the head—disk separation.Reducing atomsin atmospherescontainingoxygen.the separationwill inevitably increasethe probability Stromet al. [4] measured friction betweenAl

203—TiCof direct contact between slider and disk surfaces. slidersand unlubricatedtextureddisks in various gasFurthermore,the thicknessof protectivecoatingon the environments.They observedthat for a-C coateddisksdisk surfacesneedsto be reducedfurther to increasethe in dry oxygen or in l%—2% relative humidity (RH)recordingdensity. Morefrequentcontactbetweendisks environments,friction increased rapidly.In other dryand slidersand reducedthicknessof protectivecoating gases (nitrogen, argon, and helium) or low humidityrequirethat the tribological performanceof the coating (0.5% RH) environments,friction was stable.When amaterialbe significantlyimproved. 4% humidity is addedto the gases,friction increased

It is known that many factorssuch as theroughness rapidly in all gases.For Zr02 coated disks, friction inand lubricationof disk surfacesand operatingenviron- all gasenvironmentswas stable.Their results areconsis-ments can significantly affect the friction and wear tent with the observationmadeby Marchonet al. [3].behavior of thin film protective coatings [1—10]. Duggeret al. [5,6] andWahi etal. [7,8] observedthatRoughnessis important becauseit affects the contact the wearlife of a lubricated a-Ccoateddisk slid againstareabetweena slider anddisk surface[1,2]. To reduce Mn—Zn ferrite hemisphericalpins is slightly longer inthecontactarea, disksarecommonlytextured.Lubricant humidair (50% RH) than thatin dry airfollowedby dryfilms are usedto reducethe adhesion.Operatingenviron- nitrogenandvacuum.Thereforehumidity and ambientment is important becausetribochemical reactioncan air seemto be an advantage.In their experiments,theytakeplace at the disk andslider interface.For example, used ahemisphericalpin insteadof a flat slider which ledMarchon et a!. [3] haveobservedthat when a slider to wearat very highinterfacial pressures.As a result,the(both Mn—Zn ferrite and CaTiO3 ceramicsliders) slid wearmechanismis believedto be ofmechanicalabrasion

0257—8972/94/$7.00 © 1994— Elsevier ScienceS.A. All rights reservedSSDI 0257-8972(94)08106-9

B. Bhushan,J. Ruan / Tribologyof thinfilm a-C overcoats 645

ratherthantribochemical.Therefore,oxygenand humid- ments on friction and wear at the disk—slider (pin)ity are beneficialto wear [1]. interfaces. Wehave builtanultrahigh vacuumtribotester

Miyoshi [9] reportedthat, for silicon nitride hemi- (friction and wear tester)to investigate systematicallysphericalpins in sliding contactwith amorphoushydro- the role of environments on friction and wear ofgenatedcarbon coatings,the coefficient offriction is disk—slider interface. In thispaperwe presentdataforhigher in humid air than that in dry nitrogen,and the sliders slidagainst polished and textured disks, bothlife time of the films is shorterin humid air than in lubricated andunlubricated,in various environmentsnitrogen.Thus, accordingto Miyoshi [9], “water vapor (dry argon, dry nitrogen, argonwith oxygen, ambientgreatlyincreasedthe friction” and reducedthe life time air, andvacuum).of the coatings,which is not consistentwith the Duggeret al.andWahi et al.’s data[5—8]. 2. Experimentalapparatusand procedures

It thereforeappears thatthe exact role of environ-mentson friction andwear of amorphouscarbonfilms The experimentwasconductedin a home-builtultra-is not understood.Clearly, furtherwork is necessaryfor high vacuumtribotester.The vacuumsystemincludesaabetter understandingof theeffect of operatingenviron- rotary pump and a turbomolecularpump, capableof

Unlubricatedpolished Unlubricatedtextured ,

0.6 0.6 1Ar+0

2 ,i ‘‘b

fi 0.44-0 C Vacuum

4)•0

0.2 : Ambient ~ 0.4oo 0.L

0 I I 0— I I

0.1 1 10 100 0.1 1 10 100

Numberof revolutions(xl000)Numberof revolutions(xl 000)

(a) (a)

Lubricatedpolished Lubricated textured0.6 0.6-

4 •I

4 ilti~IIk ‘ Vacuumo ;‘~ flh’~ CC0 + 02~ 0.4 Ar+O,, ~ 0.4-o - ~I

C~ 1. — / Ambient ArCl Ambient~0

~ 0.2-~ 0.2 0 N2

/

N20 I o. I

0.1 1 10 100 0.1 1 10 100

Numberof revolutions(xl 000) Numberof revolutions(xl 000)

(b) (b)

Fig. 1. Coefficientof friction profiles as afunctionof numberof disk Fig. 2. Coefficientof friction profiles asa function of disk revolutionsrevolutionsfor polisheddisks invariousenvironments:(a)unlubricated; for textured disks in various environments:(a) unlubricated;(b) lubricated. (b) lubricated.

646 B. Bhushan,J. Ruan/ Tribologyof thin film a-C overcoats

Table 1Roughnessesandlife times invarious environmentsof four disks

Disk type r.m.s. Approximatenumberof revolutionsafterwhich disk failureocurred(trackcondition)roughness

Vacuum Argon+ Ambient Nitrogen Argonoxygen

Unlubricated, 1.9 nm 1000 2000 8000 90000 100 000polished (severe) (visible) (faint) (faint) (faint, discontinuous)Lubricated, 2.1 nm 2000 6000 30000 150 000 80000polished (severe) (easily visible) (faint) (faint) (faint)Unlubricated, 6.4 nm 1000 8000 14000 100000 120 000textured (severe) (severe) (faint) (faint) (faint)Lubricated, 6.2nm 1000 4000 22000 130000 120000textured (very visible) (severe) (faint) (faint) (very faint)

reachingto 1 x 10~Torr within 4 h of pumpingwithout vacuumor air, the chamberwas initially evacuatedtohaving the systembakedandto less than 1 x iO~Torr less than 2 x 10-6 Torr (roughly 1 h of pumping) andwith a very mild baking (about 100°C).Details of the then back filled with the specified gases.The chamberset-upwill be describedlaterelsewhere, wasexposedto air for ambientair tests.

For the experiments,the disk was rotatedby a d.c. Thedisk samplesconsistof magneticdisksmadewithmotorwhich is controlledwith a closed-loopcontroller textured and polishedaluminium substrates,with andfor a constant rotatingspeed.Rotationalspeedof the without lubricant film. The roughnessesof the disksaremotor for the testswas200 rev min’, correspondingto listed in Table1. The disks aremadeof Al—Mg alloya relativelinear speed of0.5—0.8m s’ at diskdiameters substratewith a 10—20 J.tm thick electrolessplatedNi—Pof 50—75 mmrespectively. Anormal loadof 100 mN was coating, an about 75 nm thick magnetic coatingapplied to the sliders.The sliders were incontactwith (Co79Pt14Ni7), and an about 20 nm thick a-C (alsodisks attheseconditions.Bothnormal andfriction forces referred to as diamondlike carbon) coating.Detailedwere measured usingmetal (constantan)foil strain chemicalanalyses[11—13] show that thesecarboncoat-gauges.Thesensitivityforbothnormal andfriction forces ings areamorphous carbon,which consistof sp

2-bondedis about 1 mN. Experimentswere conductedin argon, clustersand sp3-bondedrandomnetworks. The lubri-nitrogen,argon+ oxygen,ambientair, andvacuum.For cateddisks also havea 2 nm thick perfluoropolyetherthevacuumexperiments,thechamber pressurewasabout (Z-DOL) lubricantcoating.Bothmacro- andmicroslid-2 x iO~Torr (the vacuumsystemwasnot baked).For ers (70% of the full size macrosliders)were used.Theexperiments conducted in environments otherthan results are verysimilar for both sliders.Therefore,we

__ __ __ c:lAr+o2

Unlub. Lub. Unlub. Lub.polished polished textured textured

Fig. 3. Life times offour disksin differentenvironments.

B. Bhushan,J. Ruan / Tribologyof thin film a-C overcoats 647

will only presentdataobtainedusingmicrosliders.The vary from test to test by asmuch by a factor of 2. Figs.sliders’ material is Al203—TiC (70—30 by weight). The 1 and 2 show the coefficient offriction profiles as ar.m.s. roughnessof the sliders wasabout 1.5 nm. function of disk revolutions for polishedand textured

disks respectivelyand for unlubricated(Figs. 1(a) and3. Results 2(a)) and lubricated(Figs. 1(b) and 2(b)) disks. A

summary of life times for different disks is shown inTable1 summarizesthewearlivesandopticalobserva- Fig. 3. We observethat the life times of all four disks

tions for au. disks. Wehavedefinedwear life as thetest are theshortestin vacuum, longestin dry nitrogen orduration at which friction starts to increase rapidly. in dry argonenvironments,and are intermediatein theWearlife of an interfacein a given environmentcould ambientair or in argonplus oxygen.The vacuumresults

Sliding di.ection~’ .

Unlubricated Lubricated 50 JIm

Argon

Nitrogen

Ambient air

Argon + oxygen

Vacuum

Fig. 4. Optical micrographsof polished, unlubricated(left) and lubricated (right) disks after sliding against A1203—TiC sliders in variousenvironments afteronsetof thedisk failure.

648 B. Bhushan,J. Ruan / Trihology of thinfilm a-C overcoats

are consistentwith that reportedby Duggeret a!. [5,6].The nitrogenandambientair results areconsistentwith Slidingdirection ~o ~rn

thoseobtainedby Marchonet al. [3], Stromet a!. [4]andMiyoshi [9]. Theresult in argon+oxygenenviron-ment is also consistentwith Marchon et al. [3]. Lifetimesof lubricateddisks in the ambientair are longer _______________________________________________________

than those of unlubricated disks [1]. Lubricationappears to have a less consistent effect in otherenvironments.

Fig. 4 shows theoptical micrographsof wear tracksof the polisheddisks (left, unlubricated; right,lubricated) Argonafter the tests. Itcan be seenthat the wear tracksare ~ -__________________

extremelymild for dry nitrogenand dryargonenviron-mentsand the major featureof thesetracksare minorscratchmarks. Thesescratchmarksare probablydue tothe presenceof debris at the slider—disk interface.Inargon with oxygen and in the ambient air, the weartracksalso havescratch marks,but the unique featureof these weartracks is a “white” band as seen in themicrographs.The wear tracksgeneratedin vacuumare Nitrogenmarkedby severeploughingdamageto the disksurfaces. yr~An enlargedview of the weartrack in vacuumis shownin the scanningelectron microscopy (SEM) image ofFig. 5(a). Thefigure clearly shows thedebrison the disk _______________________________________________andalso theploughingwear.Damageto the slidersafterrunning against lubricated polished disks in differentenvironmentsis shown in the optical micrographsinFig. 6. Since thedifference between slidersrun against

Ambient airlubricated andunlubricateddisks is small,only those

Argon + oxygen

i~’~gVj1~(a) .. I

* ,:

Vacuum

Fig. 6. Optical micrographof worn slidersnear their leading edgeafterrunning againstpolishedlubricateddisks invariousenvironmentsafteronsetof thedisk failure.

run againstlubricateddisksare shown. Depositson the(b) sliderscanbe seen very clearly.Debris accumulationsin

Fig. 5. SEM imagesshowingworn areasof a polishedlubricateddisk dry nitrogen,dry argon, and vacuumenvironmentsallslid againstan AI

2O3—TiC sliderin vacuum: (a)worn disk; (b) worn appearto be similar to eachother,whereasthosein theslider, ambient air and in argon+oxygen are also similar.

B. Bhushan,J. Ruan/ Tribology of thin film a-C overcoats 649

Fig.5(b) (SEM image) shows anenlarged view of a -250

deposit on a slider run in vacuum. Energy-dispersedX-ray (EDX) spectroscopyhas beenused to studythechemicalcomposition of the debris on both the disksand sliders. As shown in Fig. 7, the debris showninmaterial transferredfrom disk to the slider. Because -. - -— -~

Fig. 5(b) on the slidersurface consistsof Pt, Ni, andCo,EDX spectroscopy probesa relatively largedepthof thesamples(tensof microns),verysmallamountsof material _______________________________________

250 Itransfer from slider to the disk surface cannot be 25 50

detected.Since magnetic materialsare found on the (a)sliders, this indicatesthat wear took place in themag-netic layerof the disks. Thisis alsoconfirmedby atomicforce microscopeimaging of the wear tracks as shownin Fig. 8, which indicatesthat the wear trackgeneratedin vacuumis asdeepasabout 150 nm, deeperthan themagneticlayer. EDX analysisof sliders testedin oxidiz-

0

ing environmentshows isolated incidenceof disk mate-na! transferto the slider surface.

~-5O.0We stopped the tests for observation of the disk -

0damageduring sliding before the failureoccurredand Ill

25.0observedthat damagewasaccumulative.We alsonotedthat scratching(ploughing)takesplace at discreteloca- 0

tions on the disk surface for each revolution the disk orotatesunder the slider, but the scratch depth (in theworn region)after ashorttestingperiodwascomparable 50.0

JIMwith that of a much longertest. The densityof scratch (b)(e.g.scratch lengthsperunit area)continuouslyincreasesas afunction of disk revolution, as evidencedin Figs. Fig. 8. Atomic forcemicroscopeprofiles of a worn regionof a polished

lubricateddisk slid againstan A12O3—TiC sliderin vacuum:(a) two-9(a) and 9(b) for a polished diskslid againsta slider in dimensionalprofile; (b) three-dimensionalprofile.

the air after5000rev and8000rev respectively.Thewearlife was about 10 000 revolutions. Scratchdensity inFig. 9(b) is higher than thatin Fig. 9(a). The scratchis primarily as aresult of the ploughing effect. The reasonmost likely due to the presenceof debris on the disk for a severeploughingin vacuumis probably that, sincesurface, there are very fewforeign moleculeson the disk and

From the resultspresented here,it canbe concluded slider surfaces, thedisk and slider materials are inthat differentwearmechanismswereeffectivein different intimatecontactandwould form strongchemicalbonds.environments.In vacuumenvironment,wear takesplace Because of therelativemotion between disksandsliders,

the contactpoints are instantly torn apart,resultinginmaterial removalfrom the disks or sliders. However,

Pt since the slider material is much harder than diskmaterial, weartakesplace primarily on disks. In otherenvironments,gas molecules exist on the diskandslidersurfaces.Thesemolecules(even one monolayeron thedisk surface) act as a bufferlayer and canpreventthedisk andslidermaterialsto form strongchemicalbonds.M

Therefore wearis much smallerin theseenvironmentsC

_Nithan in vacuum. However, at isolated contact pointswherecontactpressurebecometoo high, e.g. becauseof

C ________I debrisalreadygeneratedon the disk surfaces,wearcan

0 2 4 6 8 lOKeVstill take place.However, in general,wearrateis much

X-rayenergy smallerthan thatof vacuumenvironment.When the results in ambientair and that in argon

Fig. 7. EDX spectraof depositson an A1203—TiC slider after slidingagainsta lubricatedpolisheddisk in vacuum.Spectraweremeasured plus oxygen werecomparedwith thosein dry nitrogenin the regionsindicatedby anarrow in Fig. 5(b). and dry argon, it becomesclear that chemical effects

650 B. Bhushan,J. Ruan / Tribologyof thin film a-C overcoats

also influence thefriction andwearperformanceof disks.The disklife time in argon+ oxygenor in ambientair ismuch shorterthan thatin dry argonor in dry nitrogen.We observe “whitish”bands on wear tracksgeneratedin air or in argon+oxygen,not in otherenvironments.Thesebandswere continuousover thewear track. It isreasonableto believethat thesebandsweremost likely

________________________________________________ dueto oxidation,probablyan effect similar to chemicalpolishing, resulting in nanoscale smoothingof the disksurface[3,4,9].

Acknowledgment

We thank S. Sun for assistancein the tests.

a References

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Devices,Springer,New York, 1990.[2] H. Tian andT. Matsudaira,J. Trtbol., 115 (1993)28.[3] B. Marchon,N. Heiman and MR. Khan, IEEE Trans. Magn.,

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[10] B. Bhushanand S. Venkatesan,J. Mater. Res.,8 (1993) 1611.[11] M. Rubin, C.B. Hopper,N.H. Cho and B. Bhushan,.1. Mater.

b Res.,5 (1990)2538.[12] N.H. Cho,KM. Krishnan, D.K.Veirs,M.D. Rubin,C.B.Hopper,

Fig. 9. Optical micrographsof a polisheddisk slidagainstA12O3—TiC B. BhushanandD.B. Bogy, J. Mater. Res.,5 (1990)2543.

slidersin ambientair for (a)5000rev and(b) 8000 rev. Onsetof failure [13] B. Bhushan,A.J. Kellock, N.H. Cho and J.W. Ager, .1. Mater.

occurred afterabout10000passes. Res.,7 (1992) 404.