in situ quantitative analysis of nano-scale lubricant migration at the slider–disk interface

Ž .Wear 225–229 1999 690–699

In situ quantitative analysis of nano-scale lubricant migration at theslider–disk interface

Andrei Khurshudov ), Peter Baumgart, Robert J. WaltmanIBM, Storage System DiÕision, 5600 Cottle Road, San Jose, CA 95193, USA

Abstract

A thin layer of lubricant is a critical element of the headrdisk interface needed to improve its tribological durability and to preventmedia corrosion. Local thinning of lubricant with its subsequent breakdown often leads to the immediate failure of the interface. Thispaper is devoted to the in situ quantitative analysis of nano-scale lubricant migration on the surface of a thin-film disk using the Optical

Ž .Surface Analyzer OSA . The calibration procedure, which enables quantitative measurements, is discussed and the technique’scapabilities are demonstrated using specially prepared samples. Two cases of sliderrdisk interaction are analyzed: low-speed, when theslider is dragged over the disk surface, and high-speed, when the slider is flown over the same track for several days. Lubricant migrationphenomena, such as depletion and pooling, are investigated quantitatively to analyze the origination of carbon wear and the mechanismsof interfacial failure. A model of high-speed sliderrdisk interaction involving the dynamic formation of a liquid bridge at the interface isproposed. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Lubricant migration; Head–disk interface; Optical surface analyzer

1. Introduction

Several different techniques have been used in the pastto investigate phenomena on the surface of magnetic rigiddisks such as lubricant migration and degradation, carbonwear, and particle generation. These techniques include the

w x Ž .Kelvin probe 1 , X-ray photoelectron spectroscopy XPSw x Ž . w x2 , secondary ion mass spectroscopy SIMS 3 , photonic

w x w xprobe 4 , ellipsometry 5 , and infrared spectroscopyŽ . w xFTIR 6 . All of these techniques have some limitationssuch as low speed and resolution, or are ex situ techniques.

Ž .The Optical Surface Analyzer OSA was introduced sev-eral years ago in an attempt to achieve high-speed, high-resolution in situ measurements of thin-film lubricant mi-gration and degradation, as well as carbon thickness

w xchanges 7 . This technique was successfully used in stud-ies of lubricant migration and interfacial wear.

This paper extends research in the field of head–diskinterface tribology using OSA. It concentrates on the role

) Corresponding author. Fax: q1-408-256-2410; E-mail:[email protected]

of ‘mobile’ lubricant in providing durability, lubricantpick-up by the air bearing, lubricant migration, and dis-cusses the possibility of a lubricant bridge formation at thehead-disk interface.

2. Experimental method and equipment

Ž .An OSA see Fig. 1 uses P- and S-polarized light tomeasure thickness changes in both, lubricant and carbonlayer of a thin-film disk. Polarized light interacting withthe disk surface results in a combination of absorption,reflection, and scattering. The amount of reflected andscattered light is measured using two photodetectors: PrS

Ž .scattered with an integrating sphere and PrS specular.w xOSA 7 is designed in such a way that S- and P-polarized

light reflectivity will vary in different ways as a functionof thickness of disk lubricant and carbon overcoat. Thin-ning of the lubricant increases the intensity of reflectedS-polarized light, but decreases the intensity of reflectedP-polarized light. The opposite is true for an increase oflubricant thickness. Both, S- and P-reflected light intensi-ties increase in the case of carbon film thinning. Wearparticle formation or surface roughness increase leads to a

0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0043-1648 98 00381-0

( )A. KhurshudoÕ et al.rWear 225–229 1999 690–699 691

Fig. 1. Schematics of the OSA and the disk surface changes, which could be detected with OSA.

decrease in both, S- and P-polarized light specular compo-nents due to increased light scattering.

This is summarized in Fig. 2 called the 2-D histogram.The data in the first quadrant correspond to the completewear of the lubricant plus some wear of the carbon over-coat. Data in the second quadrant correspond to lubricantthinning. Wear and contamination particles and roughnessincrease can be detected in the third quadrant, and lubri-cant pooling will generate data points in the fourth quad-rant. The central part of the 2-D histogram consists of datapoints, shown as a circle in Fig. 2, with values within thenoise level of the system. More distant points mean strongersignals and allow more accurate interpretation. A special

w xsoftware 8 enables trace-back of data-points on the 2-Dhistogram to their original locations on the S- and P-imagesand to find the exact location of wear particles or areas ofcarbon wear and lubricant migration on the disk surface.Data from both S- and P-images are needed simultane-ously to interpret the OSA images.

OSA allows qualitative in situ monitoring of interfacialw xchanges 7 . However, a quantitative analysis is much

more desirable. In order to enable quantitative analysis, thefollowing calibration procedure was used. A set of thin-filmdisks with both nitrogenated and hydrogenated carbon

Fig. 2. 2-D histogram based on THE data of S- and P-polarized lightimages and its interpretation. Marked area in the center indicates intensi-ties within the noise level.

coatings was prepared and the disks were half-dipped intoa lubricant bath containing perfluoropolyether lubricant

˚Ž .PFPE . Thus, several steps of lubricant layers up to 20 AŽ .thick as measured by FTIR were prepared. In the lubri-

cated part of the disk, additional lubricant was removed innarrow bands by wiping with a cloth, saturated in solvent.The thickness removed was measured with FTIR. TheOSA images of the half-lubricated disk surface with twowiped bands are shown in Fig. 3. It can be seen that

Ž .lubricant thinning wiped areas increases the intensity ofS-polarized light and decreases the intensity of P-polarizedlight. In this study S-polarized reflectivity as a function oflubricant thickness was used to calibrate the OSA signalbecause S-polarized light is, in general, more sensitive tolubricant thickness changes, while P-polarized light is

w xmore sensitive to the carbon thickness changes 7 .In order to investigate tribological changes at the

head–disk interface, two types of tests were performed˚using fully lubricated thin-film disks with about 20 A of

PFPE film.Ž .In the first test, low-speed 0.6 mrs dragging of the

slider was utilized to prevent formation of an air bearing.Under these conditions, the normal force is known and

Žequal to the suspension pre-load about 40 mN in this.case . This type of testing allows better control of

sliderrdisk interaction by eliminating local lubricant orCOC damage during occasional high-speed asperity con-tacts, which exist at nominal drive operating speeds. Dragtesting leads to a gradual wearing out of the protectivelayer of lubricant and carbon overcoat under a knowncontact force. In this test, the carbon-coated sub-ambientpressure slider with an elongated central pad was used.

In the second test, a sub-ambient pressure carbon-coatedslider with a central pad near the trailing edge was keptflying over the same track of the disk at 8 mrs for 7 days.

Ž .Elimination of lateral slider movements seek allowsŽspeeding-up interfacial processes lube migration, wear,

. Žetc. while keeping the contact conditions velocity, spac-.ing, sliderrdisk interference, contact force, etc. similar to

that in a drive. The contact force between the flying sliderand the disk is usually unknown.

( )A. KhurshudoÕ et al.rWear 225–229 1999 690–699692

Fig. 3. S- and P-polarized images of the same area of a partially lubricated disk and their correspondent cross-sections. The disk lubricant was partiallyŽ .wiped in radial direction using a solvent see two vertical bands . The width of the image in radial direction of the disk is 3 mm. The top of the image is

Ž .closer to the OD outer diameter of the disk. The image corresponds to 3608 scan of the disk.

Ž .Both the friction force and acoustic emission AEw xsignals 9 were continuously monitored during the test.

AE signal was high-pass filtered below 600 kHz to obtaininformation only about the slider body vibrations due toasperity impacts and to eliminate the effect of low-frequency signals caused by mechanical noise of the sys-

tem, vibrations of the suspension, air bearing vibrations,etc.

The OSA was used to take images of the disk surface atpredetermined times during the test. In the case of lowspeed testing, the slider was always lifted from the surfaceto enable OSA measurements, and loaded again on the


same track after the measurement. The slider positioningaccuracy was about 0.5 um.

3. Results and discussion

3.1. Calibration

Fig. 4 presents the results of the OSA sensitivity cali-bration. In this figure the change in S-polarized lightreflectivity is presented as a function of lubricant thicknessfor both hydrogenated and nitrogenated carbon overcoats.Fig. 4 indicates that there is a linear relationship betweenS-polarized reflected light intensity and the thickness ofthe lubricant and that this relationship is independent ofthe chemistry of the carbon overcoat. With the help oflinear data fitting, this relationship could be describedanalytically as follows

S-polarized light reflectivitysK=Lubricant thickness,1Ž .

where K is the proportionality constant equal in our teststo 0.0022 when the lubricant thickness is given inangstroms. It should be mentioned that this constant Kstrongly depends on the type of the lubricant and may alsovary from one specific equipment to another.

3.2. Low-speed drag testing

Fig. 5 presents OSA images of the same area of the diskŽ .at different cycle rotation numbers. This specific disk

˚initially had about 20 A of partially bonded film of PFPE˚ ˚Ž .lubricant 15 A bondedq5 A free on the CH overcoat.x

Ž .From the first two images 3000 cycles we observe that˚Ž .there is already some lubricant pooling up to 14 A

outside the contact area of the outer rail of the sliderŽ .closer to the outer diameter of the disk . Another location

˚Ž .where even larger pooling up to 54 A is observed is thecenter of the track just under the central rail of the slider.

Ž .The slider used here has a negative camber cross-crownof about 30 nm. No lubricant was observed on the slider

Ž .air-bearing surface ABS after the test. Some lubricantwas observed on the vertical face side of the slider at thetrailing edge. Therefore, these narrow linear zones ofaccumulated lubricant under the central rail are most likelya result of trailing edgerdisk contacts when the slider waspitching back and forth. This may happened both duringthe constant speed sliding and during the slider decelera-tion to a complete stop just before the OSA images weretaken. When the slider contacts the lubricated disk with itstrailing edge, some lubricant pick-up occurs and this lubri-cant could be later found on the vertical face side of theslider at the trailing edge. When the slider touches the disksurface again, some part of this lubricant drops on the disk

Žsurface resulting in the observed lubricant pooling see.Fig. 5 .

More importantly, there is some lubricant depletion˚Ž .;13 A observed in the first two images under the inner

rail of the slider. When averaged over the entire track, this˚lubricant depletion is equal to only about 5 A, but at some

˚local spots it is as high as 13 A, which is more than half ofthe total lubricant thickness.

The second set of images, taken at 40,000 cycles, showsapproximately the same picture, but the scale of observedphenomena is larger. The lubricant pooling is clearly in-

˚creasing from 3000 cycles and reaches up to 90 A at40,000 cycles. The location of this highest lubricant pool isapproximately the same. The lubricant depletion was also

Fig. 4. Calibration results: S-polarized light reflectivity vs. lubricant thickness for CH and CN overcoats.x x


Fig. 5. S- and P-polarized light images of a disk surface after 3, 40 and 43 K cycles of low-speed drag testing. Lubricant pooling, depletion, and carbonwear are observed. The width of the images is 3 mm, the width of the slider is about 1.6 mm. Sliding direction: from right to left. The top of the picture iscloser to the OD of the disk.

continuing under the same inner rail with the maximum˚Ž .lube depletion ;20 A also at exactly the same location

as at 3000 cycles. Lubricant pooling under the central railis less repeatable since the slider could also smear thedrops of lubricant on the disk surface.

The final two images in Fig. 5 show interfacial failurewith carbon wear and particle generation under the inner

rail of the slider at 43,000 cycles. It is worth noticing thatinterfacial failure occurred under the same rail wherelubricant depletion was previously detected. Another thingto notice is that some wear particles can be found as far as1–1.7 mm away from the wear tracks closer to the outeredge of the disk. The test was performed at relatively low300 rpm. Still, the centrifugal force was high enough to


˚ ˚ ˚Ž .Fig. 6. COF, AE, lube pooling and depletion vs. drag cycles for the disk with initially 20 A of lubricant 15 A bondedq5 A free on CN .x

move some of these large particles away from the weartrack.

Fig. 6 shows changes in the AE intensity, the coeffi-Ž .cient of friction COF , averaged over the entire disk track

Žlubricant pooling and depletion values at the locations.discussed above as a function of drag cycles.

The first lubricant depletion value at 3000 cycles is˚about 5 A, which is equal to the thickness of the free

˚lubricant. As can be seen in Fig. 6, about 5 A of freelubricant were completely removed during first 3000–5000cycles. Lubricant depletion varies somewhat around the

˚disk track with an average value of about 5 A. It can also

Fig. 7. S- and P-polarized light images of the disk surface after 16,000 cycles of low-speed drag testing. Lubricant depletion is observed. The width of theimages is 3 mm, the width of the slider is about 1.6 mm. Sliding direction: from right to left. The top of the picture is closer to the OD of the disk.


be seen from Fig. 6, that the AE rms signal was initiallyhigh and fluctuating, but decreased to its lowest values atapproximately the same time as the free lubricant wasremoved. It is not quite clear how closely these twoprocesses are related, but it is possible that free lubricantstayed at the interface for just long enough to assist theinitial interfacial burnishing.

After the initial quick lubricant removal, the depletionwas almost a linear function of the drag cycles resulting inthe continuous thinning of the lubricant. When the averagedepletion reached about 75% of the initial lubricant thick-ness, the lubricant film started to break down locally,

Žcausing rapid interfacial failure between 40,000 and.43,000 cycles , which was also accompanied by an in-

creased AE signal and a decrease in the COF. The AE

intensity increase was caused by a direct interaction be-tween the slider and disk surfaces at the spots of lubricantbreak-down. The AE is, in general, more sensitive to the

w xinterfacial changes 9 than friction. The decrease in COFcould be observed about 3000–5000 cycles after the first

Žlubricant breakdown as measured by AE at ;39,000.cycles and by OSA at ;40,000 cycles , and was caused

by formation of a large number of wear particles on thedisk surface. These particles increased sliderrdisk separa-tion and quickly decreased both apparent and real contactareas between the disk and the slider thus decreasingstiction and friction forces.

Lubricant pooling saturates with the number of dragcycles. Friction was clearly insensitive to lubricant migra-tion on the disk surface and was, probably, controlled by

Fig. 8. S- and P-polarized light images of the disk surface after 5 and 120 h of on-track flying at 8 mrs using sub-ambient pressure slider. Lubricantpooling is observed. The width of the images is 3 mm, the width of the slider is about 1.1 mm. Slider flying direction: from right to left. The top of thepicture is closer to the OD of the disk.


˚ ˚ŽFig. 9. COF, AE, lube pooling vs. time for on-track test with sub-ambient pressure slider flying over the disk with initially 20 A of lubricant 15 A˚ .bondedq5 A free on CN .x

much stronger adhesive forces caused by the liquid menis-cus formed at the sliderrdisk contacts. It is possible, thatthe observed local lubricant pools also caused some lubri-cant depletion at other locations without changing the

resultant meniscus force. The total test duration beforefailure was about 43,000 cycles, but the location of laterlubricant breakdown was already observed during the firstOSA measurement at 3000 cycles. This shows not only the

˚Fig. 10. COF vs. fitted lubricant thickness increase for on-track test with sub-ambient pressure slider flying over the disk with initially 20 A of lubricant˚ ˚Ž .15 A bondedq5 A free on CN .x


sensitivity of OSA, but also that the traces of interfacialfailure can be found at very early stages of sliderrdiskinteraction.

Fig. 7 shows images of a similar disk with fully bonded˚20 A of the same lubricant after 16,000 drag cycles—just

before this interface failed at about 18,000 cycles. Fromthis figure we observe lubricant depletion only, and nolubricant pooling. The only difference between the disks in

˚Figs. 5 and 7 is the presence of 5 A of free lubricant on topof the bonded layer in the former case. It shows that somemobility of the lubricant is needed to provide higherdurability since only a mobile lubricant is capable ofcuring damage to the bonded part. Free lubricant is lesscapable of carrying the slider load and is easily displacedfrom the sliderrdisk contacts. But, even when it wasdisplaced from the contact area at the early stages of thetest, it was still capable of migrating back onto the dam-aged spots and delaying interfacial failure under givenexperimental conditions. If the rate of lubricant displace-ment is equalized by the rate of its backward migration,then an interface with practically unlimited life could beobtained.

3.3. High-speed on-track testing

Fig. 8 shows OSA images of the same area of the diskas Fig. 7 at different time of testing when a sub-ambientpressure slider with a central pad near the trailing edge

˚ Žwas flying over the disk lubricated with 20 A of PFPE 15˚ ˚ .A bondedq5 A free for 160 h. It is a known fact thatsub-ambient pressure sliders often show some lubricantpick-up on the ABS surface or in the ABS cavity aftertesting. Fig. 8 presents in situ evidence of the air bearing-induced suction action on the lubricant film. The lubricant

˚pooling is 2 A high on the disk surface after 5 h of testing.The continuing suction by the air bearing results in forma-tion of a continuous pool of lubricant along the track as

˚high as about 20 A after 120 h. This is direct evidence thatthe air bearing suction is sufficiently strong not only tomove the lubricant laterally, but also move it vertically upto several nanometers. Finally, some lubricant moleculesget separated from the disk and sucked into the negativepressure pockets on the slider surface.

Another phenomenon we would like to introduce in thispaper is the formation of a continuous lubricant bridgebetween the disk and the slider flying at high speed. By theterm ‘continuous’ we mean the bridge, which exists for allor most of the time needed for the slider to complete arevolution. A continuous vertical bridge is most likely aseries of random events occurring at high frequency be-tween the different points of the slider and the disk. Weused in situ measurements of the lubricant film thicknesswith OSA and correlated them with the measured COF.The reasoning in this case is that if a liquid bridge exists atsome point of the test, then the adhesive force, and there-fore, the friction force should correlate with the scale of

this phenomenon. For example, an increasing bridge shouldincrease the meniscus force, which will increase the fric-tion force. If the bridge size is constant, then the frictionshould stay constant.

Fig. 9 presents the acoustic emission rms signal, theCOF and the height of the observed lubricant pool asmeasured in situ by OSA. In order to minimize the scatter,the data-points for the pool height were fitted with asecond order polynomial. The lubricant thickness wassaturating with time. The AE signal saturated soon afterthe beginning of the test. Friction was decreasing untilabout 20 h into the test due to interfacial burnishing.Starting from about 20 h, friction was increasing andstabilized after about 90 h of testing. It is worth mention-ing that the AE signal does not generally correlate withfrictional because they originate from different frequencyranges. The AE signal is sensitive to high frequencyphenomena. Measured friction force contains low fre-quency signal. Both these signals may correlate if a stronglow-frequency process causes both of them. If the low-frequency component of the AE signal is strong enough, itmay saturate the high-pass filter and become detected.

Fig. 11. Model of interaction between the lubricated disk and low-flyingŽ .sub-ambient pressure slider at the beginning of the test a , when the disk

burnishing just starts, and after the burnishing is completed and the liquidŽ .bridge is fully established b .


Starting from about 50 h in Fig. 9, there are severalinstances when frictional and AE signals respond in thesame way: drop and increase together. This means that

Žthere is a common low-frequency of the order of a few.kHz interfacial process causing both of them. Introducing

a continuous lubricant bridge between the slider and thedisk formed between 20 and 50 h of testing, it is possibleto explain these synchronized changes in both signals. Ifthere is a liquid bridge at the sliderrdisk interface, then thecontact force and, therefore, friction are controlled by themeniscus force. If the bridge breaks down temporarily thenboth, meniscus force and friction will drop. Since the AEsignal amplitude is a strong function of the contact force, itshould stay high when the liquid bridge is intact and dropin case of its breakdown, following the friction. This isexactly what we observe in the test shown in Fig. 9.

Ž .Fig. 10 presents the correlation of the fitted see Fig. 9lubricant thickness increase values from Fig. 9 with theCOF measured at the time when the OSA image was takenfor the part of test when the liquid bridge was presumably

Ž .established after 20 h of testing . A fairly good linearcorrelation can be observed. It supports the previous hy-pothesis that a lubricant bridge formed between the sliderand the disk beginning from about 20 h of testing controlsthe magnitude of friction.

Fig. 11 shows a possible model of the sliderrdiskinterface evolution in the test discussed above. Before theburnishing is completed and while lubricant pooling is

Ž .insignificant see Fig. 11a , the contact force at the inter-face depends on disk topography, slider flying height,air-bearing characteristics, and the suspension pre-load.After the liquid bridge is established, the contact loadbecomes dependent on the force of the meniscus formed atthe interface.

4. Conclusions

1. The technique of the in situ quantitative analysis ofnano-scale lubricant migration at the slider–disk interfacewas demonstrated. It was shown that this technique ishighly sensitive to angstrom level interfacial changes. OSAallows not only to investigate in great detail critical interfa-

cial phenomena such as lubricant pooling and depletion,carbon wear and formation of wear particles, but alsoallows to observe directly the pooling of lubricant due tothe sub-ambient pressure air-bearing.

2. The process of lubricant pooling, depletion, and itsfinal breakdown at the head–disk interface was investi-gated using OSA during low-speed drag tests. It wasshown that the free part of the lubricant was almostcompletely displaced during the first 7–15% of the totaltest time to failure. In spite of this, displaced lubricant wasstill working at the interface via the migration mecha-nisms, where it was curing damage to the bonded part ofthe lubricant layer. In the absence of lubricant pooling inthe case of fully bonded lubricant, much shorter durabilityof the interface was observed.

3. Using in situ OSA measurement of lubricant thick-ness changes and their correlation with friction and AEsignals, a model of dynamic liquid bridge formation be-tween the disk and a sub-ambient pressure slider wasproposed. A mechanism of the lubricant pick-up by theslider ABS surface was introduced.

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in situ quantitative analysis of nano-scale lubricant migration at the slider–disk interface

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