head–disk contact detection in the hard-disk drives

Wear 255 (2003) 1314–1322

Head–disk contact detection in the hard-disk drives

Andrei Khurshudov∗, Peter IvettIBM, Storage System Division, 5600 Cottle Road, Bld. 28/C202, San Jose, CA 95193, USA

Abstract

The increasing capacity of current hard-disk drives places a great importance on reliability, as tens of gigabytes of data can be lost in aninstant due to a single tribological failure event. To improve the reliability of the head–disk interface there is a need to predict, measureand monitor any interactions during manufacture.

This paper discusses some different techniques that can be used for head–disk contact detection. These techniques range from a traditionalacoustic emission method to some drive specific tests, such as thermal asperity (TA) detection and variable gain amplifier (VGA) signalmeasurements. The paper also discusses the merits of these techniques and ways to theoretically predict contact.© 2003 Published by Elsevier Science B.V.

Keywords:Head–disk contact; Magnetic hard-disk drive; Detection techniques; Reliability

1. Introduction

The areal recording density on rigid magnetic disks con-tinues to increase, consequently, so do the challenges asso-ciated with producing a robust head–disk interface (HDI).Smoother disk surfaces, reduced carbon overcoat thickness,and lower flying heads increasingly stress the HDI and im-pact tribological reliability. At the same time, drive reliabil-ity requirements remain unchanged or increase with time.For example, the industry-standard reliability demonstrationtest (RDT) requires no more than about 1% of drives fail-ures over 1000 power-on hours for a population of 1000server-class (100% duty cycle) drives. This means that of1000 drives reading and writing constantly for almost 42days, only about 10 drives are allowed to have even one ofthose so-called hard errors or unrecoverable errors, whichresult in data loss. Clearly, this requirement is hard to meetwithout having a robust and predictable HDI.

One positive thing about the HDI of the magnetichard-disk drive (HDD) is that if there is no slider (orhead)–disk contact then there is almost no danger of tri-bological problems and following errors. However, theslider–disk contact can be caused by numerous mechanisms:

• Drive particulate contaminants[1] or wear products[1],when a particle gets stuck between the slider and the disk.

• Disk lubricant via formation of liquid bridges[2], dynam-ically established roughness[3,4], or lubricant dewettingmechanism[5].

∗ Corresponding author. Tel.:+1-408-578-1652; fax:+1-408-256-7842.E-mail address:[email protected] (A. Khurshudov).

• Disk corrosion[1].• And finally, by direct slider–disk interaction.

The consequences of this interaction range from “softmagnetic errors” (see below) to “thermal asperities” (see be-low) and to severe media damage, that can lead to a com-plete mechanical and magnetic drive failure.

Besides causing unrecoverable (“hard”) errors, slider–diskcontacts in the HDD may cause the so-called “soft” errorsor events where multiple-retries are required to retrieve theinformation stored on the disk. Moving particles on the diskor slider, pools and droplets of the lubricant, non-repeatabledisk run-out, and other phenomena may cause this typeof error, but they do not necessarily threaten drive’s relia-bility. However, they may strongly affect its performance(since retries lower the rate at which the “useful” data istransferred).

The purpose of this study is to test and compare differenttechniques for slider–disk contact detection using work-ing HDDs. The techniques used in this study range fromtraditional acoustic emission (AE) measurements to somedrive-specific techniques such as variable gain amplifier(VGA) signal detection and thermal asperity (TA) detectiontechniques.

2. Experimental

All experiments in this study were so-called “drive-level”experiments, where a fully functional working driveequipped with an AE sensor was “questioned” via the SmallComputer System Interface (SCSI, pronounced “scuzzy”)

0043-1648/03/$ – see front matter © 2003 Published by Elsevier Science B.V.doi:10.1016/S0043-1648(03)00201-1

A. Khurshudov, P. Ivett / Wear 255 (2003) 1314–1322 1315

bus in order to obtain information about its status. IBMHDDs inside computer-controlled low-pressure chamberswere used in this work. In most experiments, slider–diskclearance was intentionally reduced to promote contacts.The main method of bringing the slider, which was initiallyflying at the nominal flying height of 22–23 nm, into con-tact with the disk (or reducing the slider–disk clearance),was an air pressure reduction.

The AE techniques[6] used in this study are based onmeasurements of elastic stress waves propagating in solidsfrom such sources as zones of elastic or/and plastic defor-mation, cracks, etc. The AE sensor, which mainly consistsof a piezo-electric crystal (PZT) crystal, is attached near thesource of elastic waves. The signal is usually conditioned(amplified and filtered) to obtain the highest signal-to-noiseratio and to “target” some specific frequency range of inter-est. In our experiments, AE sensor (from Physical AcousticsCo.) was attached to the actuator screw on the top cover ofthe drive or to the actuator arm of the upper suspension in-side the drive. The signal was amplified by 40 dB and filteredout below 600 kHz and above 3 MHz to “view” the sliderbody resonance frequencies (which start from∼1.2 MHz).In the case when AE sensor was attached to the actuator armof the upper suspension, an opening was cut in the cover ofthe drive in order to fit-in the AE sensor and to physicallyattach it to the suspension. The latter technique is, however,intrusive and the results we have obtained could have beenaffected to an unknown extent.

The VGA signal is the signal coming from the internaldrive circuitry, which conditions the raw read-back signalfrom the disk in order to keep its amplitude within someoptimal range. For example, if the read-back signal is too

Fig. 1. The drive’s so-called “scope trace” showing the TA event. The base line of the raw read-back signal “takes a quick dive” when the MR sensortouches the disk and also recovers quickly after the sensor cools down. The entire event takes about 0.2 us time.

weak (e.g. the magnetic head is flying too high), then theVGA circuit increases the signal amplification (or gain). Ifthe signal is too strong (e.g. the head is flying too closeto the disk), the VGA circuit will reduce signal amplifica-tion. Therefore, the VGA signal is roughly proportional (butnot linearly proportional) to the magnetic spacing, and thechange in VGA signal can be often only attributed to flyingheight change.

The TA is an event registered when the magneto-resistive(MR) read element of the slider touches the disk surface.Since this contact takes place at high speed, then some el-ement heating takes place, and the base line of the rawread-back magnetic signal moves away from its normal po-sition (seeFig. 1). This base-line shift occurs very quickly(in the order of a fraction of a millisecond) and disappearsas quickly with cooling of the MR stripe. However, the tem-perature rise can be so high that the MR stripe electrical re-sistance can change far beyond the expected range therebyincreasing (or decreasing—depending on the circuit polar-ity) the signal beyond its saturation threshold making it un-readable to the drive’s circuitry. This type of error could beintermittent (recoverable, or “soft”) or permanent (“hard”).TA events have a degrading effect on drive performance andreliability. However, the TA-detection technique itself is agood way of studying slider–disk interactions, or, more pre-cisely, contacts between the MR element and the disk sur-face.

In this study, all of the three above techniques were usedto investigate slider–disk contacts. Many of the tests thatinvolved high-speed low air-pressure contacts between theslider and disk were destructive and, therefore, new driveswere used in each of these tests.

1316 A. Khurshudov, P. Ivett / Wear 255 (2003) 1314–1322

Fig. 2. AE measurements done on the drive with the opening in the top cover indicate the predominant slider–disk contact to occur at about 0.22–0.30 atm.The measurements also show some increase in AE activity starting from∼0.5 atm.

3. Results and discussion

Fig. 2shows the results of AE measurements done on theone drive with the opening in the top cover. The AE signalstays relatively unchanged while the air pressure is slowlyreduced down to∼0.5 atm (∼50 kPa), when some increasein AE activity is observed. When the pressure is reducedeven further, the signal starts increasing rapidly indicatingthe transition from “occasional” contacts (when the slider in-frequently touches isolated disk locations) to “predominant”contact, when the slider starts rubbing on the surface of thedisk. These results indicate that the predominant slider–diskcontact occurs for this drive at about 0.22 atm (∼22 kPa) to0.30 atm (∼30 kPa).

This type of measurements using AE has both advantagesand disadvantages. The main advantage is that the AE sensoris mounted as close to the HDI as possible, which keepssignal losses (both signal power and frequency content) to aminimum. One disadvantage is that the measurement is donefor one HDI only, while other HDIs in the drive are too faraway from the sensor and their contribution to AE signal istoo small to extract any useful information about their stateduring the test. Another disadvantage is, of course, the needto cut the opening in the drive cover to mount the sensoronto a flexible slider suspension arm, which may change theslider–disk interaction in some uncontrollable and unknownway (e.g. increase the suspension pre-load, twist and bendthe suspension, change the flying height of the slider, etc.).

Fig. 3shows the results of AE measurements done on theclosed drive with the AE sensor attached to the voice-coilactuator securing screw on the top of the drive cover.Again, the “predominant” slider–disk contact occurs in therange of about 0.25–0.30 atm (or∼25–30 kPa). The mea-

surements also show some change in AE response startingfrom ∼0.4 atm (∼40 kPa), which may be an indicator ofincreasing “occasional” contacts.

Fig. 4 shows AE measurements on five closed drives in-dicating that the predominant slider–disk contact in thosedrives takes place at about 0.25–0.33 atm. AE signal activityfor two or three of these drives starts increasing almost im-mediately as the air pressure is reduced, which may be an in-dicator of the occasional contacts from∼0.9 atm (∼90 kPa).

The above measurements indicate that there appearsto be two distinct types or states of slider–disk contact:“occasional”, when the slider and disk do not interact veryfrequently and the contacts take place only at a few isolatedlocations on the disk, and “predominant”, when the slideris rubbing on the disk (while still being partially supportedby the air-bearing). The question is how to better evaluatethese two different states of contact experimentally? Forthis purpose, VGA and TA measurements could be useful.

Firstly, let us study the case of “predominant” contact.Fig. 5presents VGA measurements at the very outer diame-ter (OD) of the fully functional closed drive. The most inter-esting part of this curve, which approximately represents achange in the magnetic spacing between the slider and diskwith the decreasing air pressure, is the range where the datacurves start forming a “knee” at low pressure. The reasonfor this “knee” is that the slider starts making high-speedcontact with the disk, and the friction force applies a torqueto the slider in such a way that it pitches forward thus liftingthe trailing edge, with the magnetic element. Therefore, thedistance between the element and magnetic layer increases,reversing the trend with decreasing air pressure. The pointof the physical touchdown can be associated with the curveminimum for each slider in the drive. Apparently, different


Fig. 3. AE measurements done on the closed drive with the AE sensor attached to the voice-coil actuator screw on the top of the cover indicate thepredominant slider–disk contact to occur at about 0.25–0.30 atm. The measurements also show some increase in AE activity starting from∼0.4 atm.

sliders have different initial flying height and slightly dif-ferent pressure-sensitivities, which result in a range of thecontact pressures.

The measurements inFig. 5indicate that the “predominant”slider–disk contact—or the initial slider touchdown—tookplace at about 0.20–0.28 atm. Since the test is on a sin-gle data track, the occasional contacts are too few to bereliably detected by this technique. But, VGA signal mea-surements can reliably estimate a transition to slider–disksliding.

Fig. 4. AE measurements on five closed drives indicate that the predominant slider–disk contact in those drives takes place at about 0.25–0.33 atm.Occasional contacts start from∼0.9 atm in 20–30% drives.

When this technique for detection of the “predominant”contact is established, let us try to use the TA scan as a wayto investigate individual or “occasional” contacts.

Fig. 6 shows a full-surface TA scan for all head–disk in-terfaces inside the drive made at two air pressures: at 1 atm(circles) and at 0.7 atm (+). This figures show the coor-dinates of detected TA events using sectors and cylinders,which are the logical coordinates used to locate the datain the drive. Numbering of the cylinders in a drive startsfrom the OD of the disks and continues towards the inner


Fig. 5. VGA measurements are done at the very OD of the disks in the drive. The measurements indicate that the predominant slider–disk contact—orslider touchdown—takes place at about 0.20–0.28 atm. Occasional contacts cannot be reliably detected by this technique.

diameter (ID) of the disks. Assigning numbers to the sec-tors is less straightforward and falls outside the scope of thispaper, but is done in the circumferential direction for thedisks.

The main observations fromFig. 6 are

(a) There are more TA events (contacts) at 0.7 atm than at1 atm, which correlates with the predictions of increas-ing number of “occasional” contacts from the AE mea-surements.

(b) Only a few of these TA events arerepeatable atboth pressures, which may be an indicator of a few

Fig. 6. A full-surface TA scan for all head–disk interfaces inside the drive made at two air pressures: at 1 atm (circles) and at 0.7 atm (+). Horizontalaxis shows sectors of the TA events, and the vertical axis shows cylinders.

things. For example, these TA events can be causedby moving particles, by lubricant droplets, lubricantroughness effects, lubricant pooling, etc., which couldcause the same results. This could be also a result ofa non-repeatable disk run-out, which prevents repeat-able contacts at the very same locations. All of thesemechanisms were already mentioned in the introduc-tion and are studied in several previous publications[1–5].

If the TA technique is used to study both the “occasional”contact and the “predominant” contact, and the drive


Fig. 7. Full-surface TA scans for all head–disk interfaces inside the drive at different air pressures: at 1, 0.7, 0.4, 0.30, 0.25, and 0.20 atm. At 0.20atm,the number of TA events becomes very large. Horizontal axis shows sectors of the TA events, and the vertical axis shows cylinders.

(a different one) is pumped-down, then the following picturewill emerge (seeFig. 7).

Some TA events will appear and disappear until the pres-sure becomes substantially low—about 0.4 atm, which cor-responds to about 8 nm loss in the slider’s flying heightrelative to it flying at 1 atm. The range of pressure from 1to 0.7 atm represents (for this specific drive) the range ofnon-repeatable “occasional” slider–disk contact. When thepressure is reduced even further, more and more contactstake place and more and more of those contacts become re-peatable TA events. In the pressure range of 0.20–0.25 atm,the number of TA events becomes very large indicating tran-sition to “predominant” slider–disk contact.Fig. 8 re-plotsthe data fromFig. 7 in a way similar to that inFigs. 2–5.

A summary of the merits of all three techniques is shownin Table 1. It seems that all the techniques agree well ontheir predictions.

Table 1

Technique Predominant contact (atm) Occasional contact (atm) Best application

VGA 0.20–0.28 N/A Predominant contact detectionTA 0.20–0.25 0.25–1.0 Occasional contact detection, but can be used for bothAE 0.25–0.33 0.33–0.9 Both, but less accurate

It is interesting to find a good method ofpredicting theabove-presented experimental results using some kind oftheoretical or empirical model. For example, one way is toanalyze the lubrication modes (e.g.[7,8]) for a given drivedesign at different pressure levels and see if this techniquecan predict the predominant or occasional contact mode.The main parameter used ([7,8], etc.) to estimate the lubri-cation regime in a tribological system is the dimensionlessparameterΛ, which is the ratio of the thickness of the filmh (liquid or gas) to the equivalent r.m.s. roughness of bothcontacting surfaces, Rq= (Rq2

1 + Rq22)

0.5.Using Λ parameter, three major regimes of lubrication

can be defined in general as follows:

• Boundary lubrication: the entire load is carried by thesurface asperities,Λ < 1. Friction force in this mode ishigh.


Fig. 8. This figure re-plots the data fromFig. 7 in a way similar to that inFigs. 2–5. This plot indicates that the predominant slider–disk contact takesplace at about 0.20–0.25 atm. Occasional contacts for this drive start at 1 atm, while the repeatable occasional contacts start at 0.4 atm. Horizontalaxisshows sectors of the TA events, and the vertical axis shows cylinders.

• Thin-film lubrication (or mixed lubrication): part of theload is carried by the asperities and the rest is carried bythe film (air-bearing generated air film), 1< Λ < 3–5.When is this regime, the friction is rising rapidly fromvery low (flying) to high (sliding).

• Thick-film lubrication: the total separation of asperitiesby the (air) film is many times larger than the size of thesurface roughness. The (air) film predominantly carriesthe load,Λ > 5. In this mode, the friction is usually verylow.

If this approach is used for the HDD design discussed inthis work, then the main lubrication regime predicted for al-

Fig. 9. The dimensionless slider flying height (Λ) is shown as a function of air pressure. The flying height curve intercepts the estimated upper boundaryof the mixed lubrication regimeat about 0.18 atm.

most the entire pressure range would be thethick-film lubri-cation regime, as shown inFig. 9. It has to be mentioned thatthe majority of HDDs are designed to operate in the pres-sure range from 1 to 0.7 atm (or∼70 kPa, which equals thepressure at the altitude of∼10,000 feet). Our estimation (seeFig. 9) for this pressure range shows that theΛ parameterchanges from 13 to 16 indicating that the slider–disk sepa-ration is about 13–16 times larger than the equivalent r.m.s.roughness of both surfaces, which means a complete separa-tion of the surfaces. An attempt to predict the ‘predominant’contact using this approach yields a quite satisfactory re-sult: according to theΛ-based approach, the transition froma thick-film lubrication regime to a thin-film lubrication (or


Fig. 10. The slider flying height is shown as a function of air pressure. The flying height curve intercepts the upper boundary of themixed lubricationregime(TOH level) at about 0.17 atm.

mixed lubrication) should take place at the air pressure aslow as ∼0.18 atm, which is not far off the experimentallyobserved transitional range of 0.2–0.3 atm. Therefore, onemay conclude that theΛ-based approach may be used forestimations of slider–disk separation in the HDDs. However,we do not believe that this conclusion is a general one sincethe disk and slider roughness are just two of many param-eters affecting the slider–disk clearance. For example, thismodel does not consider possible effects of the disk wavi-ness, slider air-bearing design (shape), lubricant dynamicroughness, and some other parameters that are known to af-fect the clearance. Therefore, even if theΛ-based approachin this work was able to predict slider–disk contacts rela-tively well, we believe that this is rather coincidental thanreal, and another model may be required to support theΛ-based model.

Another way to predict a transition to the “predominant”contact is to use a so-called “take-off height” (TOH) ofthe disk (or the “glide-height avalanche” value), whichis routinely measured for the rigid magnetic recordingdisks.

The TOH is the lowest flying height the slider couldachieve on a given disk and is, therefore, an empirically ob-tained parameter, which takes into account disk waviness,lubricant dynamic roughness, etc. However, the slider usedin this test is not the very same slider used in the drive,but is a specially made slider equipped with the PZT sen-sor mounted directly onto the slider. This provides for veryhigh sensitivity to slider–disk contacts and, in combinationwith an elaborate calibration procedure, allows for detectingTOH values down to a few nanometers. Unfortunately, thesliders used in the drive are not equipped with any sensorsbut magnetic read and write heads.

Fig. 10 shows the model based on the TOH approachthat predicts the transition from thick-film lubricationto the mixed lubrication at about 0.17 atm, which is inperfect agreement with theΛ-based model, and also isin a good agreement with the experimental results. Itseems that the best approach to predicting the transitionto ‘predominant’ contact or mixed lubrication regime inthe HDD is to use both models to obtain the whole rangeof air pressures. Apparently, the models discussed abovegive a good estimate of when the slider starts touching thedisk.

Unfortunately, predicting the ‘occasional’ contacts is notas straightforward as predicting the ‘predominant’ contact.Slider–disk interaction at high pressure (and larger clear-ances) is very statistical in nature and also is very disk andslider specific. This means that some isolated disk defectsor some very low-flying sliders (or the combination of both)may produce lots of occasional (and often repeatable) con-tacts even when the model predicts large slider–disk clear-ance. Also, the variation in ‘occasional’ contacts from driveto drive is usually very large, while the transition to the‘predominant’ contact is more-or-repeatible for the samefamily of drives.

4. Conclusions

1. Two different slider–disk contact modes are discussedin this study: the ‘occasional’ contact mode, when thecontacts (repeatable or not) take place at some isolateddisk locations only and the ‘predominant’ mode, whenthe slider is rubbing on the disk (while still being par-tially supported by the air-bearing). The changes of the


slider–disk interface were investigated while the transi-tion from one mode to another is forced using decreasingair pressure.

2. Three different experimental techniques for detectingslider–disk contacts in the HDDs were demonstrated andcompared in this work. While all three measurements—AE, VGA, and TA scans—correlated well on predictingthe transition to ‘predominant’ slider–disk contact in thepressure range of 0.2–0.3 atm, they (AE and TA scan)also predicted ‘occasional’ slider–disk contacts for al-most the entire operating pressure range of the drive.Further analysis of TA scans shows that the great ma-jority of high-pressure contacts are non-repeatable innature.

3. Two models were discussed, compared and found to becorrelating relatively well with each other (and with theexperimental results) in predicting the pressure when the‘predominant’ slider–disk contact would first occur. Useof both models for the drive analysis is recommended inorder to increase the accuracy of predictions.

References

[1] A. Khurshudov, R.J. Waltman, Tribology challenges of modernmagnetic hard disk drives, Wear 251 (2001) 1124–1132.

[2] A. Khurshudov, P. Baumgart, R.J. Waltman, In-situ quantitativeanalysis of nano-scale lubricant migrations at the slider–disk interface,Wear 225–229 (1999) 690–699.

[3] A. Khurshudov, R.J. Waltman, The contribution of thin PFPElubricants to slider–disk spacing, Tribol. Lett. 11 (3–4) (2001) 143–149.

[4] R.J. Waltman, A.G. Khurshudov, The contribution of thin PFPElubricants to slider–disk spacing. 2. Effect of film thickness andlubricant end groups, Tribol. Lett., submitted for publication.

[5] R.J. Waltman, A. Khurshudov, G.W. Tyndall, Tribol. Lett., in press.[6] A. Khurshudov, F.E. Talke, A study of sub-ambient pressure tri-pad

sliders using acoustic emission, ASME Trans., J. Tribol. 120 (1996)54–59.

[7] H.S. Cheng, Lubrication regimes, ASM Handbook, vol. 18, Friction,Lubrication, and Wear Technology, ASM International, 1992, pp.89–97.

[8] G.-L. Chen, J. Huan, The effect of lubricant–roughness ratio, lambda(Λ), on tribological performance of magnetic thin film disks, IEEETrans. Magn. 29 (6) (1993) 3936–3938.

head–disk contact detection in the hard-disk drives

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