Paper XI (viii) Wear of Oxide Magnetic Recording Disks

Wear Particles - D. Dowson et al. (Editors)

0 1992 Elsevier Science Publishers B. V. All rights resetved.

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Paper XI (viii)

Wear of Oxide Magnetic Recording Disks Y. Miyake, T. Kazama, H. Kataoka and T. Higashiya

Wear of the lubricated Oxide magnetic recording disks slid against a MnZn ferrite pin The wear characteristics of disks was evaluated was investigated by “pin - on - disk” tests. by a profilometer and a Zow uoltuge scanning electron microscope (SEM). The results obtained are as follows : (i) Wear of the magnetic malrix layer on the disk has three distinct Stages. A “ zero - wear ” stage which involves wear of only the Al203 - parlicles, a “ mild - wear ” stage during which wear of the Al203 and magnetic - particles occurs with that of a polymeric binder, and a “severe - wear” stage during which the magnetic matrix layer breaks loose from the Substrate. (ii) The decrease in the read - back signal is consistent with wear and fatigue of the magnetic matrix layer. 1. INTRODUCTION

The Storage capacities of rigid disk drives have increased significantly in recent years. It has been well known that the use of thin continuous film media increases data Storage capacity for magnetic recording disks, compared with that of Oxide particulate magnetic recording media. However, thin films Pose several Problems during practical appli cation, i. e. poor wear - resistance, large friction compared with that of particulate magnetic media (1) (2). Therefore, conventional magnetic media consisting of Oxide particles, such a s y-Fe203 are widely iised for magnetic recording disk drives with large Storage capacities ( 3 ) . Little lias been published to date on the mechanisms of failure of Oxide magnetic disks, especially, on the so - called “ head Crash” phe nomenon. Weiss (4) has postulated that the wear of magnetic disks is of the abrasivetype due to worn slider particles, or loose Al203 - particles present a t the interface between the head and disk. Hamada et al(5) have carried out head Crash experiments to evaluate the strength of different magnetic recording disks. From the extent of damage to the disk caused during the head Crash experiments, the toughness of coatings was estimated qualitatively. And Salehizadeh et al(6) have examined the wear behavior of Oxide magnetic recording disks using a pinon -disk tester and proposed a fatigue failure model. However, these Papers investigated

wear rates, the friction coefficient and the durability of magnetic matrix layer from a macroscopic viewpoint using friction measurements, surface profilometers, and scanning electron microscopes. This report concerns the wear characteristics of Oxide magnetic recording disks from a microscopic viewpoint. The wear characteristics were investigated with pin - on - disk tests by sliding a loaded MnZn ferrite pin on the lubricated magnetic surface. The relationship between the wear of the magnetic matrix layer and the read-back signal have been shown. 2. EXPERIMENTAL

The experiment was carried out by pinon-disk tests. Figure 1 Shows a schematic view of the experimental apparatus. A disk of 9.5 inches in diameter was attached to a spindle hub with a clamp, and was driven by a n induction motor in an a i r circulating closed System (below class 100) to avoid the iiifluence of airborne dust particles. The magnetic matrix layer of the disk is composed of y - Fe203 / Al203 particles dispersed in a polymeric binder, and is coated on an A1-Mg alloy Substrate by spin coating. The average length and width of y-Fe203 particles are 0.5 micron, arid 0.05

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3. RESULTS

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Flg. 1 Schematic View of Experimental Apparatus

micron, respectively. Thickness of the layer is about 0.4 micron, and the surface roughness average Ra is 0.01 to 0.02 micron. The Al203 particles of which the average diameter is 0.7 micron, are added to the binder to improve the disk's wear - resistance. The MnZn ferrite pin was fixed to a holder which was attached to a spring-suspension. The ferrite pin was loaded onto the surface of magnetic matrix layer by lowering t h e Suspension. After pressing the pin onto the disk with a 98 mN load using a vertical micro-Stage, the disk was rotated. The sliding velocity is 20 d s . And perfluoropolyether a s a lubricant is applied on ths disk surface to reduce the friction and to minimize the wear. The radius of curvature of the pin is 19 mm. In addition, the vickers hardness of the Al203 particles and the MnZn ferrite are 2,300 and 650, respectively. As shown in Fig. 1, a read I write head fabricated with MnZn ferrite was positioned in Order to produce a read-back Signal from the magnetic recording disk. The head was Set up on the Same track (on the surface of the magnetic matrix layer) as that on which the pin slid, and the read-back Signals were monitored simultaneously using an oscilloscope. After each test, the surface of magnetic layer was measured by a surface profilometer (Kosaka Laboratory Ltd., MODEL SE-3F) and with a stereoscope using micrographs from a low voltage scanning electron microscope (Hitachi, Ltd., S-900, a magnification of 20,000) to determine quantitatively the degree of wear. Then, the surface and cross-section of worn layer were observed using a n optical microscope and a low voltage scanning electron microscope. All the tests were carried out in a filtered air chamber in which the temperature and humidity were kept a t 20 30 "C, and 45 55 %, respectively.

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Figure 2 Shows the relation between the wear scar depth measured by the profilometer and the number of sliding cycles. As shown in this figure, the wear scar depth of the magnetic matrix layer in the early Stage of the sliding test could not be measured by the profilometer, and wear scars also could not be observed by the optical microscope. Therefore, the damage of the surface of the magnetic matrix layer before 5,000 cycles was found to be very small. However, when the number of cycles exceeds 5,000, the wear scar depth becomes measurable an d increases with increasing sliding cycles. Figures 3 and 4 Show the wear scar profiles and the SEM micrographs of surfaces of magnetic matrix layer, respectively. As shown in Fig. 3 (b), no wear: scar under the pin is observed, and the surface of magnetic layer which experiences sliding (< 5,000 cycles ) is found to be smoother than before the test as shown in Fig. 3 (a) and (b). At 20,000 cycles, the wear-scar depth becomes about 0.05 micron, and at 70,000 cycles, the depth reaches about 0.3 micron, as shown in Fig. 2. The surface profiles a t 20,000 and 70,000 cycles are shown in Fig. 3 (c), and (d), respectively. Figure 4 Shows the SEM micrographs of surfaces of Oxide magnetic matrix layer before and after the sliding tests, corresponding to each wear Profile shown i n Fig. 3. The micrographs of the surfaces before the sliding test, and after the sliding of 1,000 cycles are Fig. 4 (a), and (b), respectively. And the micrographs of the surface after the sliding of 20,000 and 70,000 cycles are Fig. 4 (c), and (d), respectively .

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E'ig. 2 Wear characteristics of disks (Load : BmN, Velocity : 2 O d s )

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Flg. 3 Surface Profile across the Wear Scar produced by the MnZn ferrlte spberical pin ;( a ) d cycles, (b)=1000cycles, (c)=20.000 cycles, and (d)=70.000 cycles

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Fig. 4 Scanning electron micrographs of the surface of magnetic matrix layers ; (a)=O cycles, (b)=1000cycles, (c)=20.O00cycles, and (d)=70.000 cycles.

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Ac shown in Fig. 4 (b), the protruded parts of Al203 - particles only are worn initially. Depth of the wear is less than 0.01 micron as shown in Fig. 2. Wear of the protruded Part of Al203 - particles continues until 5,000 sliding cycles. This Stage of wear can be designated Wear Stage I or the ‘‘Zero - wear” Stage (7). In Wear Stage II , or the mild - wear Stage, from 5,000 to 50,000 cycles in which 1.he depth of wear ic from 0.02 to 0.2 micron, the wear of Al203 and magnetic - particles occurs with that of the polymeric binder. The height of Al2O3particles protruded, decreases with increasing sliding cycles. Further, most of the magnetic particles in the surface layer break into powder, forming needle - like particles as shown in Fig. 4 (c). When the wear scar depth exceeds 0.2 micron, Wear Stage lli is reached, which is called the severe - wear Stage. Here, Al203 particles break loose from the polymeric binder, and/or the magnetic media is separated from the aluminum Substrate. The Profile and the SEM micrograph of this stage are shown in Fig. 3 (d), and Fig. 4 (d), respectively. Figure 5 Shows the relation between the height of Al203 - particles protruding from the surface of the polymeric binder and the number of sliding cycles. The heights of particles were measured in 5 X 5 micron2 areas of scanning electron micrographs. As shown in the figure, the protrusion decreases with increasing number of sliding cycles. The mean height before sliding tests is 0.11 micron (data scatter is wide and ranges from 0.02 micron to 0.15 micron). A i 5,000 cycles, in the “Zero - weai“’ Stage, the mean height of protrusion becomes about 0.05 micron. At 10,000 cycles, in tlie mild - wear Stage, the height becomes very small, and could not measured. From these results, the Al203 - particles are found to be worn to the Same level as that of the polymeric binder. F ig u r e 6 Shows s c a n n i n g e l e c t r o n micrographs of the Cross-section of magnetic disks. Fig. 6 (a) Shows the Cross-section of a magnetic disk before sliding. The magnetic particles are oriented parallel to the Substrate. The mean width of magnetic particles is about 0.05 micron. Figure 6 (b) Shows the crosssection of a magnetic disk wear - scar produced by sliding the MnZn ferrite pin for 20,000 cycles (in the mild wear Stage). Ac shown in the figure, in the upper part of the magnetic matrix layer (1/3 1/2 depth of the thickness from the Surface), the magnetic particles are fractured and are more packed, compared with those in the lower part. The lengths of the magnetic particles are almost the Same as their widths.

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Fig. 6 - Scanning electron micrographs of the crosssection of the magnetic disk ;(a) before süding tat, and (b) fatigue layer produced by the MnZn ferrite pin after 20.000 cycles. a plated layer to ensure nondestruction of the sample during it‘s making.

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Wear of the surface of Oxide magnetic recording disks is closely related to the destruction of the data recorded in the magnetic matrix layer. Figure 7 Shows the relation between the normalized amplitude of the read-back Signal produced by the magnetic head and the number of sliding cycles. The read-back Signal does not Change during the “zero-wear” Stage (Stage I ) in which only the wear of Al203 - particles occurs. The decrease in the read-back Signal during the mild-wear Stage (Stage II) is consistent with the wear of magnetic particles. ’

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2 5 10 20 50 100 Number of Sliding cycles (X103) Fig .7 Relationship between the amplitude (normalized) of the read-back Signal produced by the magnetic head and the number of slldlng cycles. 1

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Figure 8 Shows optical micrographs of the surfaces of MnZn ferrite pins before aiid after sliding tests ( (a) 2,000, (b) 20,000 (CI 50,000 cycles) , Small parallel grooves are Seen in Fig. 8 (b) and (c). Ac the sliding Progresses, many fine grooves are found on the surface of the MnZn ferrite pin. On increasing the number of sliding cycles, the grooves becomes deepsr, and the wear area on the pin iiicreases gradually. At 2,000 cycles and 20,000 cycles as shown in Fig. 8 (a) and (b), the debris including lubricant. had adhered to the surface of the ferrite pin. These are debris of the magnetic matrix layer and the ferrite pin.

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4. DISCUSSION

The function of recording media is to record and to Store data stably. In other words, the important property for recording media is no Change of the magnetic characteristics of yFe203 particles. The degradation of the media is cauced by the wear and the destruction of yFe203 particles, and by the frictional heat involved in demagnetization. Therefore, nondegradation of the media necessitates a magnetic matrix layer with a structure that avoids direct-contact between the surface of the pin and the magnetic particles. For example, non-magnetic material such as A1203 - particles dispersed in a polymeric binder is useful to achieve this end. The magnetic matrix layer is polished generally to control its thickness and surface roughness. When the surface of the magnetic matrix layer is polished with Al203-polishing tape, the Al203 - particles in the matrix layer protrude from the surface of magnetic particles and polymeric binder, because the Al203 - particles are harder than other materials in the matrix layer. Wear of only the Al203 - particles in the Wear Stage I does not infiuence the function of the recording media as shown in Fig. 7 . But, as shown in Fig. 2, at more than 5,000 sliding cycles, wear of Al203 and magnetic particles occurs with that of the polymeric binder, causing the surface of the polymeric binder to turn black (it is carbonized by the heat of friction which is generated by the contact between the pin and the magnetic layer). It is believed that the Stress in the surface of the magnetic matrix layer caused by normal and tangential forces leads to the fracture of the magnetic particles, and that the magnetic layer is degraded gradually, and the magnetic characteristics of the recording media are depleted. The time-length of the “zero-wear” Stage and the mild-wear Stage are dependent on the thickness of lubricant on the magnetic layer. Thus, it is necessary to carefully select the quantity and the type of lubricant used(8).

Fig. 8 Optical micrographs of the surfaces of MnZn ferrite plns before and after sliding tests; (a) 2000, (b) 20.000, and (c) 50.000 cycles.

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5. CONCLUSIONS

The wear characteristics of lubricated Oxide magnetic recording disks were investigated with “pin - on - disk” tests. The test conditionc are as follows : The pin load is 98 mN, material of the pin is MnZn ferrite, the magnetic matrix layer consists of ~ - F l 2 O 3 / Al203 particles i n a polymeric binder, and the sliding velocity is 20 d s . I t is then concluded that ; (i) Wear of the surface of Oxide magnetic recording disks has three Stages, “Zero wear” , mild-wear and severe-wear. (ii) The time-length of the “zero-wear” Stage is found to be dependent on the protrusion of A1203 - particles in the magnetic matrix layer. (iii)During the “zero-wear” Stage i n which the Al203 - particles only are worn, the amplitude of the read-back Signal does not Change, because the MnZn ferrite pin does not contact the magnetic particles dispersed in the polymeric binder. (iv) Wear of both y-F2O3 particles and the polymeric binder begin i n the mild-wear Stage, and the break of the magnetic matrix layer occurs in the severe-wear Stage.

ACKNOWLEDGEMENT The authors wich to thank Mr. M. Hayashi, Mr. M. Imamura and Dr. J. Naruse for their guidance and helpful discussion. We are also thankful to Prof. K. Kat0 of Tohoku Univercity for his excouragement, and Prof. K. Miyake of Teikyo Univercity and Mr. M. Nagaya for t h e i r m a n y h e l p f u l c o m m e n t s on t h e manuscript. REFERENCES (1) SMITH, S. and MEE, P. et al., ‘Fabricatiod Wear Properties of arbon Overlayers’, Trib. and Mech. of Magnetic Storage Systems, STLE SP-27 1989, 93-97 (2) LI, Z., RABINOWICZ, E. and SAKA, N., ‘ The Stiction between Magnetic Recording Heads and Thin Film Disks’ , Trib. and Mech. of Magnetic Storage Systems, STLE SP-27, 1989, 64-70 (3) NARUSE, J. e t al., ‘Large Scale Magnetic Disk Files’, Hitachi Review, 37, 1988, 275282

(4) WEISS, R. D., ‘Abrasive Wear i n Magnetic Disk Recording’, J. Appl. Phys., 50, 3, 1979, 2399-2401 (5) HAMADA, M., ISHIDA, S. and OGAWA, S., ‘Mechanical Strength of Coating Films for Magnetic recording Disks’, Fujitsu Sci. & Tech. J., 1976, 191-212 (6) SALEHIZADEH, H. and SAKA, N., ‘Fatigue failure of Rigid Oxide Magnetic recording Media’, Trib. and M e c . of Magrktic Storage Systems, STLE SP-25, 1988, 94-100 (7) BAYER, R. G., ‘Prediction of Wear in a Sliding System’, Wear, 2, 1968, 319-332 (8) KLAUS, F. E. and Bhushan, B., ‘Lubricants i n Magnetic Media-A Review’, Trib. and Mech. of Magnetic Storage Systems vol. lT, ASLE SP-19, 1985, 7-15