The Effect of Disk Roughness on the Wear of Contact Recording Heads

M. Suk

storage Systems Division, IBIVI, San Jose, CA 95193

The Effect of Disk Roughness

on the Wear of Contact

Recording Heads

In conventional disk files, the slider is supported by an air-bearing when the disk is rotating at its designed speed. With the continued reduction of magnetic spacing in order to increase the areal density, a natural extension of the traditional recording system is contact recording. We investigate the wear of the contact recording head designed for such a class of rigid magnetic disk files where the read/write element carrying slider is intended to remain in continuous contact during every phase of the disk drive operation. In particular, we study the effect of disk roughness and load on the wear rate of the recording head. It is observed that the wear rate is proportional to initial interfacial load, however the observation cannot be extrapolated beyond the loads studied in the paper. The experimental observations agree well with expecta-tions for a system where an abrasive wear model applies. We also show that the wear rate is predominantly governed by the existence of isolated asperities that lie well outside of three standard deviations of the disk surface roughness.

I n t r o d u c t i o n

Demand for higher capacity storage systems has led to reduc-tion of flying height of convenreduc-tional slider/suspension systems, where the slider is in contact with the disk surface only during start and stop phases of the disk drive operation. When the disk is rotating at its design speed, typically above 5400 rpm, the slider is supported by an air-bearing and normally does not contact the disk surface. With continued reduction of magnetic spacing in order to increase the areal density, a natural extension of the technology is a system where the slider remains in contact with the disk surface during all phases of drive operation. Al-though contact recording systems have existed in tape drives for many years, application to rigid disk drives was first intro-duced by the Censtor Corporation in 1988.

The first version of the contact recording assembly, referred to as the "Flexhead," was primarily designed for perpendicular recording systems (Fig. 1). The Flexhead differs drastically from a conventional slider/flexure/suspension system in that the slider is integrated in the suspension with no flexure. The mass of the slider/suspension system is less than 50 mg and the dimension of the slider, or the wear pad, is about 30 /^m by 15 p.m.. The height of the pad is about 5 jim. Some mechani-cal and magnetic properties can be found in Hamilton et al. (1991).

The Flexhead offers several key advantages over conven-tional sliders due to its size and weight. Its small size allows an increase in the disk packing density, i.e. for a given size disk file, more disk platters can be integrated into a file. Hence, given the same areal density for both the contact and conventional recording systems, the total storage capacity of a disk file utiUz-ing the Flexhead would be greater. The low mass of the inte-grated system means that the disk file would be far less suscepti-ble to disk surface damage due to impulsive loading. Further-more, since the interface load is usually held below 1 mN and the contact area is very small, the level of stiction at the interface is also quite low. Consequently, the motor torque requirement during the start-up phase of a disk file can be significantly relaxed. This is extremely important, since the torque output of

Contributed by the Tribology Division for publication in the JOURNAL OF TRIBOLOGY. Manuscript received by the Tribology Division February 1, 1995; revised manuscript received August 20, 1995. Associate Technical Editor: A. K. Menon.

motors for many small disk files used in portable systems is limited due to spindle size and available power.

Some of the disadvantages of contact recording heads like the Flexhead include conformity between the wear pad and the disk surface due to the lack of a flexure, and handling difficulties due to its size. Additionally, even if one assumes that the wear debris generated by the head and disk does not degrade magnetic performance of the interface, the life of the file is limited by the amount of pad that can be worn before the magnetic signal disappears.

In designing contact recording systems, the pad size must be made as small as possible in order to minimize spacing loss due to conformity issues. The pad must also be made durable so that the wear rate can be controlled. A further necessary requirement is low mass contact recording heads to avoid cata-strophic head-disk interface failures (Yasuda and Kaneko, 1988).

Since the introduction of contact recording heads by Censtor Corp., some important issues have been addressed. Donavon and Bogy (1992) showed that the wear pad always remained in contact independent of the direction of the disk rotation for speeds below 24 m/s with the calculated interface load of 200 mN. Muraoka and Nakamura (1993) studied spacing loss due to possible aerodynamic lift and issues related to magnetics but not tribology. Shinohara and Takahashiu (1993) concluded that a perpendicular contact recording head can be used to achieve aerial densities of greater than 1 Gb/in^. Some aspects of wear have been studied by Ohta et al. (1993), but the main focus was on the relationship between component vibration character-istics and wear. Some issues in contact recording have been discussed by Bogy (1993), but they do not specifically address the wear of the contact recording heads.

In this paper, we study the wear of contact recording heads for an integrated system similar to the Flexhead, but designed

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Fig, 2 IBM version of the contact recording head

and fabricated by IBM (Fig. 2 ) . In the IBM version of the wear pad/suspension assembly, the wear pad size is about 35 /.tm by 35 ^m. The suspension and the pad was fabricated for mechani-cal testing only with the wear pad overcoated with a film of wear resistant carbon. Our study is concentrated on tribological aspects of the wear-pad/disk interface and does not address the magnetic performance or manufacturability issues. We show that the wear rate is proportional to load and, to first order, is governed by the peak-to-mean value of the disk surface roughness.

Description of Experiment

The contact recording head, hereafter referred to as the wear pad, is first examined under a microscope to verify the integrity of the structure. The pad is then imaged with a Tapping-Mode Atomic Force Microscope (AFM) to determine the initial sur-face characteristics. Next, the pad height is measured with a contact surface profllometer with the vertical range of 10 fxm. Since the pad height is around 8 /xm, the full height of the pad with respect to the bottom surface of the suspension can be measured. A typical profllometer trace is shown in Fig. 3. The side wall of the wear pad can also be accurately profiled with the AFM (Fig. 4 ) . The initial profile is stored and is later compared to the respective post-test profilometry measurement to determine the pad wear height.

Figure 5 shows the equipment layout used in determining the interfacial load. The suspension is mounted on a stainless steel plate, which in turn is mounted onto an aluminum block. The aluminum block is mounted onto an XYZ-translation stage with roll and pitch adjustments. The wear pad is positioned over a disk resting on top of a scale which has 0.()1 mg resolution. Due to the surrounding environment, however, the LED output from the scale fluctuates by ± 2 mg.

A video camera with a beamsplitter attached to the end of the lens is positioned over the top surface of the wear pad. The image of the pad can be viewed on a monitor connected to the video camera. A laser beam is directed into the beamsplitter deflecting the laser beam down onto the suspension (Fig. 6). The suspension partially reflects the incident light but also allows some light transmission. The transmitted component of the light reflects off the disk, and the two reflected beams can be seen on an image plane.

0 20 40 Scan Length (/Lim)

Fig. 3 Typical profitometer trace of the wear pad (top of the figure contacts the disk surface

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Fig. 4 Side wall of the wear pad measured with an atomic force micro-scope

Any roll in the suspension with respect to the disk is removed by adjusting the stage so that the two reflected light spots on the image plane are coUinear with respect to a vertical line (Fig. 6). By measuring the separation distance between the two reflected beams, the pitch angle of the suspension with respect to the disk surface can be determined. After noting the vertical separation distance of the two beams, the suspension is lowered onto the disk until the relative pitch angle is around 0.3 degrees. At this point, the reading on the scale is recorded and the suspen-sion is raised off the disk. If the scale reads a higher load than the desired interface load, the pitch angle of the suspension is reduced, and it is lowered onto the disk once again. This process is repeated until the desired load is realized; the final relative pitch angle is then recorded.

With the translation stage moved out of position, the scale is replaced by a spindle motor. The base of the motor is bolted down onto the table and a disk is mounted on the spindle. The suspension is again positioned over the disk and the camera is positioned over the suspension. The stage is adjusted so that the relative pitch angle between the light beams reflecting off the suspension and off the disk surface matches the previously recorded value. The suspension is then lowered until the relative pitch angle is again around 0.3 degrees.

Due to the initial angle of 0.3 degrees, the reflected light beams from the disk and slider result in interference fringes, which can be seen on the monitor through the video camera. As the pad wears, the spacing between the disk and the suspension changes altering the fringe pattern. By videotaping the interfer-ence pattern, the pad wear can be estimated to at least within 100 nm. The 100 nm uncertainty is the result of blurry fringe lines. After the test, the wear pad height is remeasured with the profllometer and the result is compared to the initial profile measurement. By superimposing the two profile measurements, the final wear height can easily be determined. Some examples are presented in the Results and Discussion section.

All the measurements were taken using 65 mm untextured supersmooth thin film disks with carbon overcoat and a fluoro-carbon lubricant of 1-2 nm thickness. The disks for initial tests were not exposed to any tape burnish process as to avoid scratching of the disk surface, but they were burnished with a

Disk either on lop of Melller 5010 scale or clunfwd on n spindle.

Image Plane

Disk

Fig. 6 Schematic showing how the laser light is used to determine the relative pitch angle between the disk and the suspension

slider flying at approximately 20 nm. The disks were character-ized with the AFM by imaging areas of 20 jj.m by 20 fim at four different regions of the disk separated by 90 degrees around the disk.

To minimize the effect of disk variability, most of the wear measurements were performed in continuous seek mode. The suspension was linearly actuated from 24 mm radius to 29 mm using a linear actuator. The actuation speed was set so that the one complete access cycle took about 4 seconds. It was assumed that covering 5 mm of the disk would effectively reduce scatter in the data due to variability between disks. The spindle was rotated at 3600 rpm resulting in linear speeds ranging from 10 m/s to 14 m/s. The video recorder was triggered so that it would record only when the slider was within the optical view of the camera.

Results and Discussion

The wear volume of contact recording heads as a function of time for three different interface preloads of 390, 490, and 590 jiN is shown in Fig. 7. The wear volume reaches some steady-state level about two to three days into the test, but it is apparent that the majority of the pad wear occurs within one day. Similar observations have been made as early as 1962 by Mulhearn and Samuels (1962) for systems where an abrasive wear model applies. Hence, the wear volume, V, can be repre-sented by an equation of the form

V = Vf(l - «-""),

where Vf the total wear volume, /3 is a constant, .s is the sliding velocity, and t is the sliding time.

In our tests, the disk functions as the abrasive surface where the degree of its "abrasiveness" is related to the surface roughness. Initially, asperities on the disk surface, functioning as a cutting tool, cut into the softer wear pad removing a large amount of material. However, with the wear pad repeatedly passing over the same regions of the disk surface, the asperities on the disk become blunt and the abrasiveness of the disk

sur-2000 + 590 uN o 480 (Oi X 390 ixH 100 300 300 Time (hrs)

?i-Fig. 8 A post-test AFIVI image of the wear pad showing crowning and cambering of the pad

face is effectively reduced, resulting in a reduced wear rate of the pad. This simultaneous wear of the pad and the disk is the primary reason for the exponential form of the above equation in agreement with the results presented in Hamilton et al. (1991). Thus, the initial wear-in period depends on the roughness of the disk.

Constant wear rate would be observed if the amount of abra-sive on the harder surface did not significantly change during the measurement period. Although simulating this condition with a rotating disk is impossible, it is possible with tape sys-tems. In fact, Forrest et al. (1993) have observed that a re-cording head sliding on a continuous supply of fresh tape wore much faster than a head sliding against a loop of tape.

It is clear that an abrasive disk that does not change is not acceptable in most cases where the main objective is to mini-mize the wear of the interface and not just the wear of one of the two mating surfaces. On the other hand, if the disk overcoat were much softer than the wear pad, then the disk overcoat may wear at an unacceptable rate. Hence, it is extremely critical that the surface characteristics of the two mating surfaces be "matched" in order to optimize the durability of the interface. In general, the wear characteristics of a particular material, A, cannot be optimized without knowing its mating surface's durability properties on material A.

An AFM image of the wear pad taken after sliding over the disk surface in track seeking mode for about 160 hours is shown in Fig. 8, where the pad surface resembles that of a section of a sphere. In addition, no visible grooves that would have been introduced by asperities on the disk are found on the pad surface (Rabinowicz, 1965). Both observations are expected since the slider was actuated radially over a 5 mm band of the disk surface while the disk was rotating at 60 Hz. Cutting lines along the surface of the wear pad would be expected only if the pad were forced to follow a single track. An AFM image of the pad after

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Fig. 7 Wear volume versus time for three different interface loads (390 AiN, 490 iM, and 590 juN)

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Fig. 10 A three-dimensional AFM image of the disk surface before i test

a track following test shows the expected tangential grooves on the pad surface (Fig. 9 ) .

The blunting of asperities or smoothening of the disk surface can be implicitly observed by studying the surface characteris-tics of the disk before and after a test. Over a virgin area of the disk surface, the probability of tall asperities is expected to be greater than in regions where the wear pad has rubbed for a period of time. Four 20 X 20 jim areas of the disk immediately next to the worn area were imaged with the AFM, and four adjacent worn areas were also imaged—the images were taken from a disk used in a 590 //N load test for 160 hours. Figures 10 and 11 show the three-dimensional AFM image for the un-worn and un-worn areas, respectively.

Dramatic differences can be clearly seen from Figs. 10 and 11, but the histogram of the surface roughness reveals more tangible information. Figure 12 shows the disk surface roughness histograms for the same four regions of the disk shown in Fig. 10. Likewise, histograms for the worn areas are shown in Fig. 13. Comparison of these two figures show that the tall asperities have been worn away. We can now deduce that the wear of the pad is predominantly governed by isolated asperities and less associated with asperities in the l-cr range or even in the 3-(T range. The average rms values for data shown in Figs. 12 and 13 are 1.6 and 1.0, respectively. Note that the surface roughness statistics below the mean line contribute minimally to the wear process, but may slightly affect the final wear rate since the statistical nature of the disk surface is related to the amount of wear debris that can be lodged between asperi-ties.

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Fig. 12 Histograms of the surface roughness of a virgin region of the disk at four different locations of the disk separated by 90 degrees. The existence of tall asperities is easily observed.

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- 2 0 - 1 0 0 10 20 Surface Height (nm)

- 2 0 - 1 0 0 10 20 Surface Height (nm)

Fig. 13 Histograms of the surface roughness of worn regions of the disk shown in Fig. 12. The data were taken after sliding on the disk in track seeking mode with 590 iM load for about 160 hours.

Since the abrasiveness of the worn area of the disk was significantly reduced, it was hypothesized that the wear volume of a new wear pad sliding in the worn region of the disk should be much smaller. To verify this hypothesis, a new wear pad was continuously accessed over the worn region of the disk surface (shown in Fig. 10) for about 142 hours. The surface profiles before and after the test are shown in Fig. 14. The final wear volume was determined to be 160 ^ m \ about an order of magnitude less than when a wear pad was slid over a virgin area of the disk. This trend is consistent with that observed by Forrest et al. (1993).

Next, the wear of the pad on a disk with a smaller number of isolated asperities was measured. The histogram of the surface roughness of four regions of the disk adjacent to the accessed region is shown in Fig. 15. The test was run with 590 /.tN load for 130 hours. Other experimental parameters were kept the same as previous tests. The wear from running on the surface characterized in Fig. 15 is much less than wear on the surface characterized in Fig. 12 as is expected. The wear volume was about 200 fim^, which is 8 times smaller than that of disks represented in Fig. 10. The wear of some asperities can be seen from the post test histogram of the disk surface (Fig. 16). Even

20 40 Scan Length (pm)

60

Fig. 11 An AFM image of the disk after testing in track seeking mode with 590 fM of load for about 160 hours

- 3 0 - 1 0 0 10 SO Surface Height (nm)

- 2 0 - 1 0 0 10 20 Surface Height (mn)

Fig. 15 Histograms of tlie surface roughness of tlie smootlier disk. The data are from a virgin region of tlie disl< at four different locations. The existence of isolated asperities can be observed.

with this smoother disk, the wear process appears to be due to an abrasive wear mechanism.

Another measurement was taken on a disk that had a some-what higher rms value than the disk described above (Fig. 15), but with a slightly higher peak-to-mean surface height. The wear volume as a function of peak-to-mean surface height for the three sets of disks is shown in Fig. 17. A similar plot, but as a function of rms is shown in Fig. 18—lines connecting data points in Figs. 17 and 18 are only to guide the eyes. Strong correlation between wear and peak-to-mean surface height and a weaker one between wear and rms roughness also supports the notion that isolated asperities are the main cause of pad wear. Note that a good correlation between wear rate and rms roughness is feasible if the disks have a minimal number of isolated asperities.

Finally, the linear dependence of the total wear volume on load is easily seen from Fig. 19, where the total wear volume is plotted as a function of load. Again, this linear dependence is consistent with sliding systems experiencing abrasive wear. Although the total wear volume appears to be linearly

propor-1.2 1.4 1.6 rms rouglmess (nm)

1.8

Fig. 18 Wear volume versus rms roughness for three sets of disks

tional to load, extrapolating above 590 /xN and below 390 /.tN may not be valid. Some experiments were run at 780 iiN load, but the wear pad structure repeatedly broke apart from the sus-pension. Below 390 f^N, the wear pad may lift off from the disk due to hydrodynamic forces. Figure 20 shows the arm mounted acoustic emission rms value as a function of disk speed for a 250 pN interface load. It is clear that an air-bearing begins to build around 3 m/s and probably separates the wear pad from the disk at around 10 m/s as evidenced by lack of acoustic emission energy detected at speeds above 10 m/s. The AE signal for loads above 390 /xN showed virtually no air-bearing effect. Summary and Conclusion

In this paper, we study the wear characteristics of a contact recording head that is an integral part of a recording head/

suspension assembly. The head/suspension assembly resembles that of the Flexhead by Censtor Corporation, but was designed and fabricated by IBM. We examine the effect of load and the disk surface finish on the wear rate of the pad. The wear of the pad and the surface characteristics of both the disk and the head are measured using both a single-trace contact surface profllometer and a Tapping-Mode Atomic Force Microscope.

- 2 0 - 1 0 0 10 20 Surface Height (nm)

- 2 0 - 1 0 0 10 20 Surface Height (nm)

Fig. 16 Histograms of the surface roughness of the disk immediately next to the unworn regions of the disk shown In Fig. 13. The data were taken after sliding on the disk in track seeking mode with 590 fiN load for about 130 hours.

1800

600

Fig. 19 The total wear volume as a function of load for three different loads. These data are derived from the data shown in Fig. 7.

2000

4 e 8 10 12 14 Peak-to—Uean Surface Height (nm)

Fig. 17 Wear volume versus peak-to-mean surface height for three sets of disks

6 10 Speed ( m / s )

We determined that the final wear volume is proportional to the initial interfacial load with some limitations which confine the results to the range of loads studied. Below 390 yuN and above 10 m/s linear speed, the wear pad is likely to separate from the disk surface. We also determined that the wear of the pad follows that of an abrasive wear system, where the wear rate is expected to be constant if the abrasiveness of the disk remains constant. The observed exponential decay in the wear rate is explained by the fact that the abrasiveness of the disk decreases as the wear pad continuously wears the peaks of the disk surface. We conclude that to first order the wear rate is related to the peak-to-mean value of the disk surface rather than the rms roughness, and that by removing most of the isolated asperities, the wear rate can be minimized.

Acknowledgment

The apparatus to determine the load was developed by B. Hiller and W. Gill. I would like to thank Tony Wu and Singh Bhatia for providing the disks, and Clint Snyder and Robert Fontana for providing the contact recording heads. I would also like to thank Robert Schramm and Alan Klenk for their help in running the experiments.

References

Bogy, D. B., 1993, "Some Critical Tribological Issues in Contact and Near-Contact Recording," IEEE Trans Magn, Vol. 29, No. 1, Jan., pp. 230-234.

Donavon, M., and Bogy, D. B., 1992, "Head-Disk Interface Dynamics of the Censtor Micro Flexhead Contact Recording System," M.S. Degree Plan II Project No. 92-001, Computer Mechanics Laboratory, University of California at Berke-ley, Mar.

Forrest, D., Matsuoka, K., Tse, M-K, and Rabinowicz, E., 1993, "Accelerated Wear Testing Using the Grit Size Effect," Wear. Vol. 162-164, Apr., pp.

126-131.

Hamilton, H., Anderson, R., and Goodson, K., 1991, "Contact Perpendicular Recording on Rigid Media," IEEE Trans Magn., Vol. 27, No. 6, Nov., pp. 4 9 2 1 -4926.

Mulhearn, T. O., and Samuels, L. E„ 1962, "The Abrasion of Metals: A Model of the Process," Wear, Vol. 5, pp. 478-498.

Muraoka, H., and Nakamura, Y., 1993, "High-Density Recording Characteris-tics of Perpendicular Magnetic Recording with Sliding-Contact Hard Disk Sys-tem," Electronics and Communications in Japan, Part 2, Vol. 76, No. 4, pp. 106-112.

Ohta, Y., Rokugawa, A., and Takahashi, J., 1993, "An Effect of Vibration Characteristics of Head/Disk on Durability,'' Japanese Applied Magnetic Society

Academic Lecture Series, Vol. 17.

Rabinowicz, E., 1965, Friction and Wear of Materials, Wiley, New York, NY, p. 167.

Shinohara, M., and Takahashiu, M., 1993, "High Density Recording by Contact Recording," Japanese Applied Magnetic Society Academic Lecture Series. Vol. 17.