the tribochemical wear of the carbon surface atoms in oxygen-containing atmosphere. Based on indirect evidences, a mechanism involving ox en chemisorption ...
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 26, NO. 5, SEPTEMBER 1990
TRIBOCHEMICALWEAR ON AMORPHOUS CARBON THIN FILMS
Bruno Marchon, Mahbub R. Khan,Neil Heiman SEAGATJ5 MAGNETICS, Fremont, CA 94538, USA P. Pereira Universidadde Los Andes, Merida, Venezuela and
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A. Lautie LASIR-CNRS, 94320 Thiais, France
7
Some physico-chemical taking place at a sliding CaTi03-ceramic/Unlube carbon film interface of a ri id disc drive are described. A major cause of frictional b d d u p is the tribochemical wear of the carbon surface atoms in oxygen-containing atmosphere. Based on indirect evidences, a mechanism involving ox en chemisorption on surface dangling bonds and CO/(!% desorption under the sliding is proposed. Raman data motion of the Read/Write bringing some evidence of the formation of a . graphite transfer film at the interface of some carbon films, are shown. Catalytic activity of the slider ceramic towards oxidation is also demonstrated.
1
primarily responsible for qlider/media frictional buildup [21 . n this paper, a more complete account on this subject wi 1 be given. Catalytic propenies of the oxlde-based slider towards carbon oxidation will also be addressed.
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I " The tribolo of the Head/Disc interface of a Winchester disc drive is a K l d of rowing interest [lJ. Lower flying hei ht and constant need for greater durability in term of numter of Contact/Start/Stop (CSS), necessitates a complete understandin of the physical, as well as physico-chemical processes takng place at the sliding interface. Thin film media are coated with a wear-resistant coatin [2], zirconia [3kor amorphous carbon [4]. materials can Mn-Zn ferrite, Calcium Titanate, or a mixture Alumina/Titanium Carbide.
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In this study, we report some evidence for tribochemical wear between a clean, unlubricated carbon film and a CaTi03-based slider, s stem which covers a wide range of commercial products. %he initial idea was to bring some understanding of the important frictional buildup, often reported for this type of junction [5-61. Physical types of wear such as abrasive wear, adhesive wear and surface fatigue are more familiar types of tribological events which can be isolated as well. They often lead to catastrophic wear, and they are outside the scope of this paper. A great deal of knowledge has been accumulated in the literature concerning the tribology of carbon compounds such as compacted graphite [7-lo], or diamond 111. Some of these studies have focused on the effect o! the surrounding environment on friction and wear properties [9,1O], and it has been suggested that carbon surface functionalities play a crucial role in friction coefficient and wear mtes [ 01. A transfer of a graphite film, with the basal plane arallel to the shear plane has also been documented 127. Carbon surface s ecies lower the surface free energy Ly saturatin dangling &onds. They can thermally desorb in a wide range of temperature from ca. 200C to values beyond lOOOC [13]. Amorphous c bon f i s can be described as a random 3D network of spyand sp carbon bonds [14,15], and is has been recently demonstrated that they form surface oxides as well 161. Some tribolo 'cal pro erties of such films have already been reported [17%0]. Alttough the effect of environment mostly water vapor, has been investi ated [19,20], little i; known about the molecular aspect of their frictional and wear properties. Some preliminary report has focused on this topic: by performing wear tests in controlled environment, we have shown that oxygen adsorption/desorption sequence is
EXPERIMENTAL Special 95mm disc substrates were prepared with roughness average Ra in the range 20-25 Angstrom cutoff 25 microns). Texture profiles were obtained with a encor P1 contact profilometer, e uipped with a 2 micron stylus. Discs were coated with 401A of carbon by DC-magnetron sputtering. Standard Calcium Titanate sliders (9 microinches flying hei ht) with log load were used. Friction measurements were performed on a standard spin-stand, enclosed in a sealed plexi lass box. Drag tests were performed.at 25 RPM near the at a radius of ca. 25mm. High purity gases (N and 0 2 ) were employed. No lubricant was used, and specia? care was taken to avoid deposition of organic contaminants from ambient air. Grazing angle FTIR spectra were obtained on a Nicolet spectrometer, and Raman.data were performed using a Dilor-Omars microprobe. Catalytic experiments were run in a conventional flow reactor, usin a 02/He (1:3) mixture. Polycrystalline gra hite (325 mesa) from Ultra Carbon Corp. was mured with Ca$i03 powder obtained after finely grinding minicomposite heads in an agate mortar. Temperature ramp was set to lC/minute. Reaction rates were monitored by gas (CO/CO2) chromatography.
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8,
s"
50
100
Number of Revolutions Fig. 1:Typical friction buildup observed durin a drag test on unlubricated, smooth (Ra,25Aq carbon coated media with a minicomposite head (log load)
0018-946419010900-2670$01.OO 0 1990 IEEE
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A dramatic change of the frictional properties is observed when these tests are erformed in an inert environment such as nitro en (Figure 6f Minor, if no friction increase at all is detectel for periods of time up to several days, and the usual pattern of friction increase is recovered as soon as the system is laced back in air or oxygen. The addition of water vapor to t i e nitrogen environment (RH=30%) also shows no frictional buildup. A sequence of nitrogen/oxygen environment leads to a frictional pattern such as the one re roduced in Figure 7: friction rises in oxygen and decreases sightly when back in nitro en. Further oxygenlnitrogen exposure leads to a staircase-life m e . Friction in nitro en vs. the one in oxygen for each step of this experiment &ows a strai crossing zero (Figure 8). The magnitude of the f&l% friction coefficient of the 'unction when placed in nitrogen can vary, according to difierent experimental conditions. In the experiment shown in Figure 9 for instance, the friction coefficient goes reversibly from ca. 1.5 to 0.16 when going alternativelyfrom oxygen to nitrogen.
1.5
1.0
0.5
RADIAL
POSITIOW OF THE SLIDER
Fig. 2 Frictional map across disc radius.
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RESULTS Figure 1 shows a typical pattern of frictional buildup between an unlubricated carbon-coated disc of light texture and a minicomposite head CaTiO3, log load). In this drag test performed at 25RPM (6cm/s), initial friction coefficient value is 0.16, and it increases gradual1 to values above 1.2. In this articular example, frictio! coeJicieqt levels off after only 101 disc revolutions. This fnctional bulldup is observed dunng CSS tests as well, and seems to be an intrinsic property of unlubed carbon films. A frictional map, measured radially, of the disc surface after the drag test described above can be obtained by qoving the slider across the disc surface from OD to ID (Figure 2). Three m m m a can be clearly distinguished, c o r r e s p d i n g to three coincidences of the two slider rails with t e dra test tracks. This clearly confirms that the disc surface !as undergone a change leading to higher friction during the drag test. As a matter of fact, a mcroscopic investigation of the friction tracks shows thinning of the carbon layer (Figure 3), associated to a smoothing of the disc surface as shown with a stylus profilometer (Figure 4). Aminteresting fact of this type of experiments is that no wear particles can be seen (Figure 3). Heavier texture [22], or a thin layer of to ical lubricant can slow down this fnctional buildup. Also, carLn films prepared under different experimental conditions can show very different rate of friction buildup, as shown in Figure 5.
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microns
Fig. 4: Stylus profile on wear track after drag test.
Fig. 5: B a g tests on four different carbons prepared under vanous conditions. Special types of carbon films show very good friction performance in CSS tests. In Figure 10 is re roduced static nction vs. number of CSS for two particular Elms of and B. Both discs were of light texture and unlukd. e B shows fnction forces reaching log quite rapid1 Z i o n for film A stays in the 2-4g range .after 4000 CSZ: Under mcroscopic inspection, the ABS rails of the slider involved with carbon A is covered with a black residue Figure l l a Slider in contact with carbon B sta s clean {Figure Micro robe Raman spectrum recordedl on the re. l l a shows a intense peak at ca. black coplpound of i! 1600cm- accompanier with a smaller feature at around 1330cm-l'(Figure 12a). This contrasts wit1 the s ectrum of a standard carbon film composed of broader bands &igure 12b).
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Fig. 3: Wear mark on media after drag test, showing carbon thinning
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NITROGEN
100
DISCUSSION
OXYGEN
200
Fig. 6: Drag test on unlubricated,.smooth(Ra=25A) media, showing the effect of environment.
When ex osed to XeF2 vapor pressure for 2 hours, standard carbon flms also demonstrate a stron improvement of their frictional characteristics. In Figure 18 are reproduced drag tests performed on treated and non-treated surfaces. The latter exhibits rapid frictional buildup such as the one de icted in Figure 1, but the fluorine-exposed surface shows onry a slow friction increase, going from 1.8g to less than 3g in about 150 revolutions.
It is clear that the cause of friction increase durin re eated sliding contacts is a smoothing of the texture, fead?n to geater area of true contact. This is shown unambiguous$ in igures 2 to 4, and this fact is now well documented [22-241. One way of slowing down this process is therefore to start with a heavier texture [22 The origm of the wear process leading to such behavior, kowever, was mostly unknown, or had been assumed to be of abrasive type. However, it is clear by lookin at Figures 1, 3 and 4 that a molecular type of wear is takng place: about a hundred passages over the same disc spot alter the carbon film by less than its fhickness, i.e. about less than 400 Angstrom. Thus, each sliding contact wears the carbon surface by only a few Angstroms, i.e. one or two atomic layers. Also, the fact that no such effect is noticed in nitrogen, together with the absence of macroscopic wear debris on the disc surface (Figure 3) led us to believe that carbon oxidation was mainly responsible for this wear process [21 . As said before, oxy en can readily chemisorb on carbon su aces, saturating the fangling bonds. 131. Thermal desorption of these surface s ecies durin sliiing, to yield CO and/or C02, slowly de Etes the susace from carbon atoms. This particular tribociemical wear mechanism only gaseous products and the interface stays clean (I$$:: 3). As discussed by Fisher in a recent review pa er on tribochemical wear mechanisms [25], contrary to agrasive wear which produces rough surfaces?tribochemical wear leads to smooth surfaces, raising the friction coefficient (Figure 1). The data resented here are therefore in agreement with this earlier stuzy.
d
50
Number of Revolutions
.
Fig. 7: Drag test on unlubricated,smooth (R =25A) media, dunng a environmental sequence of b2/N2 exposure.
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Fig. 9: Another 02/N2 se uence experiment, showing different behamor %om the one shown in Fig. 7.
The temperature at the slider/disc interface during motion has been studied both theoretically and experimentally [26,27]. It de ends. on many unknown arameters such as the thermal con&ctivity of both matenag and the number of contact points. Flash tem ratures of several hundreds of degrees centigrade are goocstimates when sliding s eeds are of the order of lm/s, which is typically encounteretbetween head and disc during start/stop o erations. Such temperatures are be ond the onset of CO and E 0 2 thermal desorption 1131, and tgey are consistent with the proposed mechanism. However, these models fail to account for situations such as the one de icted in Figure 1, where low dragging speeds of 6cm/s and Lss, would generate flash temperatures well below
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Fig. 8: Friction in N2 vs. friction in 0 for each step of the drag test shown in Figure
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3
0
3
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Fig. 10: Contact/Start/Stop experiments on carbons A and B.
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CO/CO2 desorption temperature. These models, however, are macroscopic models p d involve the bulk of the materials. In the present case it is likely that the. energy transfer between slider and carbon surface does not involve more than a few atoms. As discussed by Fisher [25 the whole concept of temperature becomes therefore m e evant, and only a quantum mechanical analysis involving bonding and antibonding orbitals would be appropriate.
4
Fig. 12: a) Microprobe Raman spectrum of black residue shown in Fig. 11%compared to b) spectrum of a typical carbon film.
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Sases there ma be some combinations favoring local, t erma iy activate2 [B] eneration of a graphitic transfer film at the interface, wi& its basal. plane parallel to the interface, bringin the friction coeffiaent to low values such as in Figure 9. A i s has k e n previoyly suggested by Enke [18 to account for low fnction coefficients of a-C surfaces, and it has also been documented for compacted graphite sliders [12 and MoS2 films [29]. In some other cases, the film would not form, and the friction would stay relatively high (Figure 7).
Fig. 11: Slider rail after CSS ex eriment ; bycarbon B. a) carbon A
As noticed in Figures 7 and 8, the friction coefficient
decreases sli htly when the system is back in a nitro en environment.%hs effect had been explained b the fact &at the shear strength of the interface becomes &ferent when oing from an oxide/oxlde to an oxide/carbon junction [21]. h e .ma nitude of the friction drop is however not always redictahe, and in some cases, such as the one reproduced in Fi re 9, friction coefficient reversibly oscillates between 0.B and 1.5 when the system is alternatively placed in nitro en and oxygen. We do not have a definite explanation for &is apparently erratic behavior. However, we suspect that the nature of the top carbon layers pla s an important role in the friction properties of the inteJace when it is placed in inert environment. It is well known that carbon dangling bonds of the p y h i t e prismatic plane [12], or the unhydrogenated diamon (1 1 surface [lo ead to hi shear strength values. On the ot er hand, t l e basal p%e of p p h t e is complete1 saturated, and shows very ubricating properties [ 721. Among all the. parameters i n v g : in the system (texture, carbon, sliding speed, residual
h
'
50
100
50
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Number of Revolutions Fig. 13: a) Drag test on fluorine-treated surface b) Untreated surface.
We have some preliminary evidence for the formation of such yaphitic platelets under the slider rails: as seen in Fi 0, .CS$ tests performed on special types of carbon exhibiting low fnction, led to the formation of a black residue at the interface (Figure lla). Microprobe Raman spectra recorded on this material show two distinct peaks
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("D" and "G" peaks), characteristics of microcrystalline gra;\hite of about 200A size 1301 (Fi re 12). The spectrum is very different from the one o g i n e d from a t pica1 amorphous carbon film, showing two broad, overlapping gands Figure 12b) [31]. This observation could therefore account or the low friction associated with this t pe of carbon. It is not clear however, whether this kind of f&n is suitable as an overcoat for disc drive media, since its a parent ,softness makes it more susceptible to abrasive wear. ginally, it should be emphasized that although the generation of third-body graphite platelets at the interface can account for the observed experiment facts, the above discussion is still very and h pothetical, and a complete understanding of the 0 2 2/H2 /a-CarbonfGraphite system has yet to be obtained.
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NdimentajX
B
As shown earlier (Figure 5 ) , various carbon films can demonstrate very dissimilar reactivity towards tribochemical wear. In a forthcoming paper, aman data will be shown exhibitin greater amount of s$bonds for carbon of slow frictionafbuildu as o posed to the ones showing very rapid friction rise. phis grings additional support for the mechanism ro osed here, since oxy en attacks primarily double b o n g 821. Along the same Tine, another way of inhibiting carbon reactivity would be to block the active sites at the surface [32. This is probabl one of the functions of to ical lubricant molecu&s, especially with functionaf end groups [33]. This is also a probable reason for the improvement of the carbon film tribology when it is sputtered in h drogen-containing argon [34], or wh n !i is sputtefed-e!che$ in oxygen 351. Another way of sa?,:ating the active sites is to ex ose !he carbon film to an atmosphere of fluorine, which is Rnown to oxidize ra hite into graphite fluoride, even at room temperature b6f By letting a disc sit in gaseous XeF2, a t ical fluorination com ound, f r two hours, a characteristic! !C stretching mode at 7 4 4 0 ~ m -is~ detected by FTIR [37], showing the effectiveness of the reaction. Drag tests subsequently performed on this special disc show a substantial reduction of the rate of friction buildu (Figure 13). This brings another element in favor of a tribocgemical wear mechanism.
was indeed catalytic as opposed to stoichiometric by running the reaction at 450C for more than 20 hours: no decrease in activity was noticed (Figure 15). The origin of the catal tic properties of metal oxides have been extensive1 discusseB in the literature [38]. One of their properties, is tieir ability to exist under vanous oxidation states. Th:? is the case, of titanium compounds. Furthermore, the ability of perovskites such as calcium titanate .to include oxygen vacancies [39], would also favor 0 dissociation, often cited as a rate limiting step for oxiiation reactions 401. As a matter of fact, TWO recent apers have confirmec! the catalytic activity of alkaline ear& titanates in general, towards oxidation reactions [41,42].
t
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DE-041
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do
600
BOO 1000 TME(YIN)
1200
1, IO
Fig. 15: Rate of graphite oxidation at 450C vs. time for the mixture CaTi03/Graphite
The beneficial role of topical lubricant in the tribology of the head/disc interface has already been addressed by several authors 33,431. One of its main effects is believed to reside in a reduction of the shear strength, which strongly lowers adhesive and/or abrasive wear rates. In the framework of the tribochemical wear process presented here, we may add some arguments to the list of the benefits of topical lube: firstly, it lays a role as an ener buffer, since its long, branched hylrocarbon chain can %sorb the frictional energy and convert it into vibrational modes, releasing it slowly to the bulk, therefore preventing CO/CO2 desorption. Also, as said earlier, it blocks carbon active sjtes (dan ling bonds) at the surface, preventing oxygen chemisorption.%nally, it also acts as a shield to separate the slider ceramic from the carbon surface, preventing any catalytic activity.
1E-04 OO -?o+Eo
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'
CONCLUSION 450 b) 510
TEMPERATURE(C)
Fig. 14: Rate of graphite oxidation vs. temperature a) CaTi03/Graphite (25%:75%) mixture; b) Graphite
Finally, let us address the issue of catalysis. It is well known that most metal oxides can catalyze carbon gasification reactions 1381. The intimate contact between the CaTiO3based slider with the carbon surface could therefore enhance the tribochemistry. To investigate on this matter, a ty ical gasification reaction of graphite by oxygen has geen undertaken. A finely ground mixture of graphite (75%)-CaTiO3 powder (25% was sub'ected to a flow of 0 2 at increasing temperature )Figure 14\. A 20-fold increase in reactivity is observed for the mixture as compared to yure graphife, demonstrating the catal tic activity of the s ider material towards combustion. Tze decrease in reaction rates for temperatures above 350C is attributed to a loss ,of carbodceramic contact. We also checked that the reaction
We have demonstrated that tribochemistry is mostly responsible for frictional buildup at a clean CaTi03/Carbon film interface. The sequence of events leading to carbon depletion is: oxygen chemisorption-CO and/or CO2 deso The former occurs naturally on the ,surface dan ling%%; and the latter is activated by the sliding motion of the head. This wear process creates smoother surfaces and higher frictional forces. Some evidence for the beneficial formation of a raphite transfer film at the interface of some carbon films %as also been shown. Methods for slowin down the l%ey include frictional buildup process have been dis heavier texture, carbons with higher ratio, active sites blocking, and application of to ical lubricant. Finally, a strong catalytic activity of the CadO3-based ceramic slider towards carbon combustion has been demonstrated.
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This stud probably describes only a small fraction of all the physico-clemical processes taking place at the carbon/ceramic slider interface during the slidin motion. The role of water vapor for instance which is anown to drastically affect friction and wear rates for graphite [44], has not been addressed here. Also, similar catalytic investigations on other
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types of slider materials (e-Zn, Al2O3/TiC) should be undertaken. Further studies using surface-sensitive techmques such as High Resolution Electron Ener (HREELS) [45] and ScanningTunneling should now be undertaken to characterize and elucidate the role of carbon surface functionalities. The former would investigate their nature, dynamics and stability and the latter provides the possibility of directly visualizhg atomic arrangements at the surface.
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[29] M.R. Hilton and P.D. Fleischauer, Mat. Res. Soc. Sym .Proc. Vol. 140,227 (1989). I301 P. Lespade, R. Al-Ahi, and M.S. Dresselhaus, Carbon 427 (1982 31 H. Sela, Surface andboatin Techn. 31,161 (1989). N. Murdie and Hyjazie, 32 E.J. Hi C a r b o n 1 6 8 9 (1989): [33] T. Miyamoto I. Sat0 and Y. Ando Tribolo Mechanics of Magnitic Storage Syhems V,%S?g SP-25.
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US Patent #4,778,582 1988 InternationalPatent # 0 8/05953
k. Watanabe and T. Nakajima, in "Preparation, L,YV,.
The authors want to thank R. Jacobs B. Ho, D. Williams, J. Norton and S. Vierk for preparation of disc substrates, s uttering and tribological evaluations. The also acknowled e Sagon for Raman measurements and J.N. Vo for spectra.
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