- Email: [email protected]
On tribological problems in magnetic disk recording technology
WEAR ELSEVIER
Wear 190 (1995) 232-238
On tribological problems in magnetic disk recording technology Frank E. Talke University of California, San Diego Centerfor
Magnetic Recording Research, La Jolla, CA 92093-0401, USA
Received 9 January 1995; accepted 5 July 1995
Abstract Critical tribology problems of the head-disk interface are reviewed. Surface topography of hard disks is discussed along with experimental results concerning the friction and stiction behavior of lubricated carbon coated disks. The effect of environmental conditions on the headdisk interface is analyzed together with current techniques to measure the flying height between slider and disk in the nanometer spacing range. The effect of air bearing design on the tribology of the head disk interface is discussed and a critical evaluation of recently proposed approaches towards contact recording is presented. Keywords:
Magnetic disk recording; Surface topography; Head&disk interface
1. Introduction
In the last decades, magnetic recording has become the predominant technology for the storage of digital information in modern computer systems. Magnetic recording is accomplished by the relative motion between a magnetic medium and a magnetic recording head, the latter consisting of a small electromagnet with a gap facing the magnetic medium (Fig. 1) . During writing, a current is applied to the windings of the electromagnet, creating a fringing field at the head gap and magnetizing the magnetic medium according to the direction of the current applied to the head. During reading, the flux from the magnetic medium is intercepted from the head core, inducing a voltage pulse in the coil of the read head
111. To achieve high recording density, it is important that the magnetic head be positioned in close proximity to the magnetic medium. In fact, from the point of view of magnetics, highest signal resolution and storage density is achieved by placing the magnetic element in direct contact with the magnetic medium. Since contact between moving surfaces causes friction and wear, the head-disk interface in magnetic recording disk files is generally designed as a self-acting airbearing. In the first disk drives designed in the late 195Os, the magnetic read-write element was supported by a pressurized air bearing, resulting in a spacing of more than 25 pm between slider and disk. Owing to the need for higher storage densities, and the better understanding of airbearing theory, pressurized airbearings were replaced by self-acting airbearings, resulting in a continually decreasing head-disk spacing. 0043.1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06730-2
A further, important breakthrough in disk drive technology came with the invention and implementation of “in-contact start-stop technology”, generally known as “Winchester” technology [ 21. In this approach, a lightly loaded slider (typically 30-90 mN) is spring-loaded against the disk and the air bearing is established during the spin-up of the disk. Since the head-disk interface must survive several thousand startstop cycles during the life of the disk drive, friction and wear of both the slider and the disk become important considerations in the design of the head-disk interface. At current, typical head4isk spacings in magnetic recording disk drives are on the order of 60 nm (Fig. 2), a spacing roughly onetenth the wavelength of light. With the decrease in spacing between slider and disk, the recording density has increased substantially. In particular,
wite culrerlt Ma~netlzatlan -++j+t
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A
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V
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Fig.
1
0
1
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1
0
1
0
I. Principle of the magnetic recording process.
1
F. E. Talke / Wear 190 (1995) 232-238
Fig. 2. Recording density and head-disk vs. year of shipment.
spacing in commercial
233
disk drives
typical recording densities in 1960 were on the order of 5 kbits in-‘, while in today’s drives ( 1994) recording densities of 500 Mbits in-’ can be achieved (Fig. 3). Because of this density increase, the size of disks has been reduced from a typical disk size of 360 mm diameter in the 1960s and 1970s to 65 or 48 mm diameter at present. With the decrease in disk size, new applications and different operating environments are encountered. In particular, small form factor drives such as are being used in lap top and note book computers are likely to be exposed to humidity and temperature levels exceeding those of standard computer room environments. Furthermore, the number of start-stop cycles occurring in those applications often exceeds by a factor of ten the number of start-stop cycles encountered in conventional hard disk drive applications. Because of the increased number of start-stop cycles and the increased operational demands on the head-disk interface, the need for a detailed understanding of the tribological phenomena at the head-disk interface is apparent and increasingly more attention has been directed towards this goal. Issues related to surface roughness, wear protection of the magnetic medium, preferential wear of the pole tips of the read-write element, etc., need to be studied, along with considerations regarding the effects of humidity, contamination, stiction and slider design. In this paper, we review critical tribological problems of the head-disk interface. We begin with adiscussion of surface topography of hard disks, followed by experimental results concerning the friction and stiction behavior of lubricated carbon coated disks. Following this, we discuss the effect of environmental conditions on the head-disk interface, highlight some of the current techniques to measure the flying height between slider and disk in the nanometer spacing range, and address the effects of air bearing design on the tribology of the head disk interface. Finally, we present a critical evaluation of recently proposed approaches towards contact recording where the spacing between slider and disk is reduced to almost zero.
Suspension Arm
Read/Write Element
Hard Disk Fig. 3. Schematic of the head-disk interface.
the spring loaded slider, the disk, and the gimbal spring supporting the slider. The substrate of current disks is generally aluminum, even though glass and glass ceramics are under investigation. The substrate is coated with a layer of Nip, on which a magnetic layer of 30-50 nm thickness of CoNiCr or CoPtNi is sputtered, followed by a 20-30 nm wear protective carbon overcoat. The carbon overcoat is generally lubricated with a 2 nm thin layer of a perfluoropolyether lubricant to protect the carbon overcoat and the magnetic medium (Fig. 4).
2. The head-disk interface A schematic view of the head-disk interface of a typical magnetic recording disk drive is shown in Fig. 3, indicating
Fig. 4. Cross-section
of thin film recording
disk.
234
F.E. Talke/ Wear 190 (1995) 232-238
Fig. 6. Surface roughness
Wavenumber
of magnetic recording
orc.ntatcon
Fig. 5. Typical Raman signal from disk with carbon overcoat.
The slider material for present day thin film sliders is alumina titanium carbide, a hard ceramic, although other materials such as ferrite and barium titanate have been used. Other, alternative slider materials under investigation are silicon carbides which are likewise very hard and durable materials. It is important to note that the head-disk interface would not survive without the presence of a lubricant layer on the disk. An equally important factor in establishing a reliable headdisk interface is the carbon film on the disk surface. This carbon film, produced by sputtering, is a predominantly amorphous film [ 3,4], consisting of a random network of sp2 and sp3 bonds. A typical Raman spectrum of a carbon overcoat is shown in Fig. 5, indicating the presence of two distinct peaks at 1360 and 1580 reciprocal wave numbers. The peak at 1380 cm- ’corresponds to diamond, or sp3 bonding, while the peak at 1580 cm- 1corresponds to graphite, or sp’ bonding. Many process parameters such as plasma pressure, power level, and plasma composition influence the mechanical properties of the carbon overcoat and its interaction with the slider. In recent years, hydrogenated carbon overcoats have shown best tribological behavior [ 51.
a
of thin film disks
Although magnetic thin film disk surfaces appear to be almost perfectly smooth when investigated under a low power microscope, atomic force or scanning tunneling microscopy investigations show (Fig. 6) that a typical disk surface is rough on a nanoscale [ 61. Deep grooves and high ridges are visible under large magnification, introduced intentionally during texturing of the disk surface prior to sputtering of the magnetic layer. The grooves and ridges are needed to provide a well-controlled amount of surface contact between slider and disk, thereby reducing friction during start-up of the disk drive when the slider is in contact with the disk. In order to use very smooth disk surfaces and still be able to start the disk without excessive stiction force between slider and disk, so-called “zone texturing” is of general inter-
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Fig. 7. Typical zone-textured
12.7nm -4.2nr
disk
est. In this approach, a small annular ring of rough texture and large surface roughness is provided for the area where the slider starts and stops, while in the data zone, where the slider flies over the disk, the surface is made very smooth to allow flying at values of 25-50 nm. A typical zone-textured disk is shown in Fig. 7 consisting of a number of individual peaks on the order of 25 nm in height, separated by a distance on the order of 100 p,m.
4. Friction and wear characteristics 3. Surface characterization
disk
Run-1
of disks
In Fig. 8, the variation of the coefficient of friction during start-up is shown for a very smooth disk, indicating that 8.00 7.00 --
1 4 0
0.02
0.04
0.06
0.08
0.1
llme (sets) Fig. 8. Variation of the coefficient of friction during start-up.
235
F.E.Talke/Wear190(199.5)232-238
friction rises from zero during start-up to a very high peak value, from which it decreases to zero as the drive spins up and hydrodynamic flying of the slider is initiated. The maximum value of the coefficient of friction during the initial start-up phase is a measure of the static friction and is generally called “stiction”. Since the flying height between slider and disk has decreased substantially in recent years, today’s disk surfaces exhibit much lower peak-to-valley roughness (20-40 nm) than in the past. Clearly, this decrease in roughness is accompanied by an increased tendency of disk surfaces towards stiction, and many investigations have been carried out to evaluate the dependence of stiction on head-disk design parameters and disk environmental conditions [ 71. As shown in Fig. 9, the average coefficient of friction between a slider and a lubricated disk increases with the number of start-stop cycles, starting in general at a low value of 0.25 and increasing to a high value on the order of 0.7 or higher after 10 000 cycles. Performing friction measurements on unlubricated carbon-coated disks and MnZn and CaTiO, sliders in air, nitrogen, and oxygen, Marchon et al. [ 81 observed that the increase in the coefficient of friction was related to the presence of oxygen. They suggested that during unlubricated sliding, oxygen is chemisorbed on the carbon surface, leading to the formation of surface oxides. Thermal desorption of these species leads to CO and CO, formation which depletes the surfaces from carbon. This tribochemical wear process requires the presence of dangling bonds on the carbon-coated disk and thermal activation due to frictional heating during sliding. If sliding tests are conducted with lubricated disk surfaces in alternating environments of humid air and dry nitrogen, a dramatic decrease in the coefficient of friction is observed whenever dry nitrogen is introduced in the test chamber (Fig. 10, Ref. [ 91) . If the test is repeated for conditions of 40% relative humidity (Fig. 11) , no change in the coefficient of friction is observed for either lubricated or unlubricated conditions, indicating that relative humidity is one of the key parameters in the friction and wear of carbon-coated disks. Although the tribochemical wear theory is supported by experimental evidence using unlubricated clean disk surfaces, experimental evidence suggests that in the case of lubri-
0.90 0.80 070 LO 3 0.50 5 0.40 f#0.30 $ 0.20 cl0.10
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,
10
1
I
1
20
I
I
30
I
I
40
Disk Ftevotutions (xl 000) Fig. 10. Frictional behavior of lubricated disk in humid air and dry nitrogen, from Ref. 191.
$0.4
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--
Fig. 11. Frictional relative humidity.
behavior
of lubricated
disk in air and nitrogen
at 40%
cated disks the lubricant film acts as a chemical barrier, thereby reducing the occurrence of tribochemical wear in favor of abrasive and adhesive wear.
5. Disk lubricants The lubricants in general use in today’s disk files are perfluoropolyethers. These lubricants are chemically inert and non-reactive. To improve their adhesion and lubrication performance, bifunctionally terminated perfluorocarbons such as Fomblin Z-DOL or AM-2001 are commonly used. The chemical structure of AM 2001 is shown below:
0.00 0
5aIO
100al
1m
2cmO
css cycles Fig. 9.Coefficient
of friction as a function of the number of start-stop cycles
Since the lubricants on the disk surface are exposed to atmospheric conditions, and are applied only once during the
236
Fig. 12. Coefficient polyether (disk S)
F.E. Talke/ War
of friction
for phosphazine
(disk E)
190 (1995) 232-23X
and perfluoro-
lubricated disks. Fig. 13. Pressure distribution
over two-rail slider.
manufacturing process, the most important parameters of a potential lubricant are vapor pressure, chemical stability, and load-carrying capability. Fluorinated hydrocarbons require chlorofluorocarbons as solvents during the lubricant application process. The desire for environmentally more friendly lubricants has led to investigations of phosphazines as potential alternative lubricants [lo]. The chemical structure of phosphazine lubricants is as follows:
Phosphazine lubricants show only a weak increase in the friction characteristics as a function of the number of startstop cycles, and they can be dissolved in hexan and other “environmentally friendly” solvents. A typical friction plot during contact start-stop testing is shown in Fig. 12 for disks lubricated with a phosphazine (disk E) and perfluorinated polyether (disk S), indicating that the coefficient of friction increases substantially more for perfluorinated disks than for phosphazine lubricated disks. Fig. 14. Pressure distribution ment analysis.
6. Slider design For the results shown in Figs. 8-12, alumina titanium carbide sliders were used consisting of two air bearing rails with a front taper (Fig. 3). At high relative velocities between slider and disk, an air bearing is formed which separates slider and disk. The pressure distribution in the airbearing can be
calculated by numerical nolds equation including
=6
over negative pressure slider using finite ele-
solution of the compressible rarefaction effects, given by
Rey-
F.E. Talke / Wear 190 (1995) 232-238
231
1: AIR BEAlUNC3SURFACE, AJO,-liC 2:fwmTmJmALuMINA 3:NbFePOLES Fig. 15. Schematic view of trailing edge of slider-disk interface.
A typical pressure distribution for a two-rail slider and a negative pressure slider, obtained by finite element analysis of the Reynolds equation, is shown in Fig. 13 and Fig. 14, respectively. Since the durability of the slider-disk interface is related to the length of sliding between slider and disk during startstop, it is important to design the slider with the shortest takeoff distance possible. To reduce materials interactions between slider and disk during start-up and landing, attempts have recently been made to use carbon coated sliders. Although sliding of similarmaterials such as a carbon-coated slider and a carbon-coated disk would seem to be undesirable, improved friction behavior of the interface is observed. 7. Pole tip recession and flying height A schematic view of the trailing edge region of the slidersdisk interface is shown in Fig. 15, indicating that several factors contribute to the effective flying height between slider and disk, namely, surface roughness of disk and slider, lubricant thickness, carbon overcoat thickness and airbearing thickness h. In addition, a distance PTR is shown in Fig. 15 which is equal to the height between the air bearing surface and the magnetic pole tips of the thin film head. Since the pole tips are made of permalloy (NiFe) , preferential wear of the soft pole pieces relative to the sputtered alumina (2) and the alumina titanium carbide ( 1) of the slider may occur either during manufacturing of the slider or during its use in the disk drive. Pole tip recession measurements with AIM and optical surface interferometric methods indicate that typical values of pole tip recession are on the order of 10 nm.
8. Slider flying height measurement Owing to the decrease in the slider flying height over the last decade, white light interferometric methods have become inadequate to measure the head-disk spacing since distinct color fringes are absent below 100 nm head-disk spacing. Multiple wavelength monochromatic interferometry must be used to measure the head-disk spacing at those small flying heights. In this approach, the intensity of the interference fringe pattern at arbitrary gray levels is determined and the flying height is calculated from interpolation of the intensity between the peaks and valleys of the intensity distribution. A different method for the measurement of relative displacements of the slider at spacings in the nanometer flying height range is phase demodulated interferometry. Here, the phase difference of light between an initial and final position of the slider is measured relative to the disk, resulting in an absolute measure of the flying height change between the two positions. 9. Discussion Owing to the need for further decreases in the flying height below 60 nm, the following two questions arise: (a) how low can flying heights decrease?, and (b) is it feasible to reduce the flying height to zero, thereby establishing contact recording? In a search for an answer to the last question, several non-conventional approaches to contact or near-contact recording have been proposed in the last few years. These are 1. miniaturized, lightly loaded sliders in contact with the disk
ill19 2. thick film lubricated disks using a liquid bearing instead of an air bearing [ 121, and
238
F.E. Talke/ Wear I90 (1995) 232-238
3. programmable air bearing sliders which fly at close spacing only when needed for recording purposes [ 131. In the contact recording approach of ref., a lightly loaded (400 p_N) miniaturized recording element with a carbon wear pad is placed in contact with the carbon-coated magnetic disk and a controlled low wear rate is obtained as a consequence of the light load. In the approach of Ref. [ 121, a film of a liquid boundary lubricant is constantly re-applied to the disk surface by means of a wicking mechanism, and the head is “flying” on a liquid lubricant film. In the programmable air bearing approach of Ref. [ 131, the read-write element is lowered onto the disk by piezoelectric means only when a read-write function of the head is required. Very little information exists presently in the open literature on any one of the above three “contact recording” or near contact recording approaches. However, the programmable air bearing approach requires large slider dimensions to yield reasonable motion of the read-write element, and, thus, seems to be an impractical approach from the point of view of mass production. Likewise, the liquid lubrication approach of Ref. [ 121 is difficult to implement from the point of view of mass production. The lightly loaded contact recording approach of Censtor (Fig. 15) is a revolutionary step away from conventional air bearing design technology. Here, an extremely lightly loaded head structure (approximately 400 ~_LN)of approximately 25 km linear dimensions is used in constant contact with the disk surface, establishing a maximum output of the magnetic signal. However, since wear of the head and the disk is unavoidable, concerns related to contamination of the head-disk interface exist as well as concerns related to disk and head wear. Continuous contact between slider and magnetic disk requires a detailed understanding of the tribology and materials interactions at the head-disk interface. That is to say, in present day magnetic recording disk drives, in which the slider is supported by an air bearing, approximately 1Ok startstop cycles occur during the life of the drive, corresponding to about 10k contact revolutions. On the other hand, for ‘‘incontact” recording disk drives in which the air bearing is absent, approximately 10 billion contact revolutions occur, requiring that the amount of wear per contact revolution is only a fraction of that of present day head-disk interfaces. Thus, it is apparent that the step toward contact recording is not easy and that a more detailed understanding of the materials and tribology problems of the head-disk interface under “contact recording” is urgently needed. 10. Conclusions and recommendations The tribology of the magnetic disk-head interface depends on a large number of interrelated problems, spanning the area of surface characterization, tribochemical wear, lubricant
analysis, boundary friction, and materials properties of slider and disk. Owing to the constant rate of decrease of flying height, measurement problems of physical quantities such as flying height and pole tip erosion are becoming increasingly more important. Extrapolating from the understanding of the tribology of the head-disk interface as it is known today, one can conclude that it will be extremely difficult, if not impossible, to predict when and if contact recording will ever be a viable alternative to the well entrenched air bearing technology. In fact, it seems justifiable to predict that the air bearing technology, as it exists in today’s disk files, will continue to improve and that spacings on the order of 15 nm will be implemented long before contact recording at zero spacings. Acknowledgements I would like to thank M. Wahl and S. Nadimpalli help in the preparation of this manuscript.
for their
References [ l] C.D. Mee and ED. Daniel, Magneric Recording, Vol. I: Technology, McGraw-Hill, 1987. [2] J.M. Harker, D.W. Brede, R.E. Pattison, G.R. Santana and L.G. Taft, A quarter century of disk file innovation, IBMJ. Rex Dev., 25 ( 1981) 667-690. [ 31 H. Tsai and D.B. Bogy, Characterization of diamondlike carbon films and their application as overcoats on thin-film media for magnetic recording, J. Vat. Sci. Tech. A, 5 (6) ( 1987) 3287-3312. [4] J. Robertson, Amorphous carbon, Adv. Phys., 35 (4) (1986) 317374. [ 51 J.K. Lee, M. Smallen, J. Enguero, H.J. Lee and A. Chao, The effect of chemical and surface properties of hydrogenated carbon overcoats on the tribological performance of rigid magnetic disks, IEEE Trans. Mag.,29(1) (1993)276281. [6] M. Yang and F.E. Talke, Surface roughness investigation of magnetic recording disks using STM and profilometry measurements, Wear, I70 (1993) 15-24. [7] Y. Li, D. Trauner and F.E. TaJke, Effect of humidity on stiction and friction of the head/disk interface, IEEE Trans. Magn. 26 (5) ( 1990) 2487-2489. [8] B. Marchon, N. Heiman and M.R. Khan, Evidence of tribochemical wear on amorphous carbon thin films, IEEE Trans. Msg., 26 (1) (1989) 168-170. [9] M. Yang, S.K. Canapathi, R.D. Bahutson and F.E. TaJke, The frictional behavior of thin film magnetic disks, IEEE Trans. Magn., 27 (6) (1991) 5157-5159. [ 101 M. Yang, F.E. Talke, D.J. Perettie, T.A. Morgan, K.K. Kar and G.E. Potter, Cyclophosphazines as new lubricants for rigid magnetic recording media. to be published in STLE, 1994. [ 1l] H. Hamilton, K. Goodson, C. Baldwin and R. Anderson, Contact perpendicular recording with integrated head/flexure, in TRIB, Vol. 3, Concepts in Contact Recording, ASME, Philadelphia, PA, 1992, pp. 13-23. [ 121 J.U. Len&e, Non-contact recording with a liquid interface, TRIE. Vol. 3, Concepts in Contact Recording, ASME, New York, 1992, pp. 25% 26. [ 131 V.D. Khanna, Fly low only when necessary, TRIB, Vol. 3, Concepts in Contact Recording, ASME. 1992, pp. l-l 1.
Wear 190 (1995) 232-238
On tribological problems in magnetic disk recording technology Frank E. Talke University of California, San Diego Centerfor
Magnetic Recording Research, La Jolla, CA 92093-0401, USA
Received 9 January 1995; accepted 5 July 1995
Abstract Critical tribology problems of the head-disk interface are reviewed. Surface topography of hard disks is discussed along with experimental results concerning the friction and stiction behavior of lubricated carbon coated disks. The effect of environmental conditions on the headdisk interface is analyzed together with current techniques to measure the flying height between slider and disk in the nanometer spacing range. The effect of air bearing design on the tribology of the head disk interface is discussed and a critical evaluation of recently proposed approaches towards contact recording is presented. Keywords:
Magnetic disk recording; Surface topography; Head&disk interface
1. Introduction
In the last decades, magnetic recording has become the predominant technology for the storage of digital information in modern computer systems. Magnetic recording is accomplished by the relative motion between a magnetic medium and a magnetic recording head, the latter consisting of a small electromagnet with a gap facing the magnetic medium (Fig. 1) . During writing, a current is applied to the windings of the electromagnet, creating a fringing field at the head gap and magnetizing the magnetic medium according to the direction of the current applied to the head. During reading, the flux from the magnetic medium is intercepted from the head core, inducing a voltage pulse in the coil of the read head
111. To achieve high recording density, it is important that the magnetic head be positioned in close proximity to the magnetic medium. In fact, from the point of view of magnetics, highest signal resolution and storage density is achieved by placing the magnetic element in direct contact with the magnetic medium. Since contact between moving surfaces causes friction and wear, the head-disk interface in magnetic recording disk files is generally designed as a self-acting airbearing. In the first disk drives designed in the late 195Os, the magnetic read-write element was supported by a pressurized air bearing, resulting in a spacing of more than 25 pm between slider and disk. Owing to the need for higher storage densities, and the better understanding of airbearing theory, pressurized airbearings were replaced by self-acting airbearings, resulting in a continually decreasing head-disk spacing. 0043.1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06730-2
A further, important breakthrough in disk drive technology came with the invention and implementation of “in-contact start-stop technology”, generally known as “Winchester” technology [ 21. In this approach, a lightly loaded slider (typically 30-90 mN) is spring-loaded against the disk and the air bearing is established during the spin-up of the disk. Since the head-disk interface must survive several thousand startstop cycles during the life of the disk drive, friction and wear of both the slider and the disk become important considerations in the design of the head-disk interface. At current, typical head4isk spacings in magnetic recording disk drives are on the order of 60 nm (Fig. 2), a spacing roughly onetenth the wavelength of light. With the decrease in spacing between slider and disk, the recording density has increased substantially. In particular,
wite culrerlt Ma~netlzatlan -++j+t
Read
A
A
voltage
A
V
V ClockPulses Recovefedcode
Fig.
1
0
1
’
1
0
1
0
I. Principle of the magnetic recording process.
1
F. E. Talke / Wear 190 (1995) 232-238
Fig. 2. Recording density and head-disk vs. year of shipment.
spacing in commercial
233
disk drives
typical recording densities in 1960 were on the order of 5 kbits in-‘, while in today’s drives ( 1994) recording densities of 500 Mbits in-’ can be achieved (Fig. 3). Because of this density increase, the size of disks has been reduced from a typical disk size of 360 mm diameter in the 1960s and 1970s to 65 or 48 mm diameter at present. With the decrease in disk size, new applications and different operating environments are encountered. In particular, small form factor drives such as are being used in lap top and note book computers are likely to be exposed to humidity and temperature levels exceeding those of standard computer room environments. Furthermore, the number of start-stop cycles occurring in those applications often exceeds by a factor of ten the number of start-stop cycles encountered in conventional hard disk drive applications. Because of the increased number of start-stop cycles and the increased operational demands on the head-disk interface, the need for a detailed understanding of the tribological phenomena at the head-disk interface is apparent and increasingly more attention has been directed towards this goal. Issues related to surface roughness, wear protection of the magnetic medium, preferential wear of the pole tips of the read-write element, etc., need to be studied, along with considerations regarding the effects of humidity, contamination, stiction and slider design. In this paper, we review critical tribological problems of the head-disk interface. We begin with adiscussion of surface topography of hard disks, followed by experimental results concerning the friction and stiction behavior of lubricated carbon coated disks. Following this, we discuss the effect of environmental conditions on the head-disk interface, highlight some of the current techniques to measure the flying height between slider and disk in the nanometer spacing range, and address the effects of air bearing design on the tribology of the head disk interface. Finally, we present a critical evaluation of recently proposed approaches towards contact recording where the spacing between slider and disk is reduced to almost zero.
Suspension Arm
Read/Write Element
Hard Disk Fig. 3. Schematic of the head-disk interface.
the spring loaded slider, the disk, and the gimbal spring supporting the slider. The substrate of current disks is generally aluminum, even though glass and glass ceramics are under investigation. The substrate is coated with a layer of Nip, on which a magnetic layer of 30-50 nm thickness of CoNiCr or CoPtNi is sputtered, followed by a 20-30 nm wear protective carbon overcoat. The carbon overcoat is generally lubricated with a 2 nm thin layer of a perfluoropolyether lubricant to protect the carbon overcoat and the magnetic medium (Fig. 4).
2. The head-disk interface A schematic view of the head-disk interface of a typical magnetic recording disk drive is shown in Fig. 3, indicating
Fig. 4. Cross-section
of thin film recording
disk.
234
F.E. Talke/ Wear 190 (1995) 232-238
Fig. 6. Surface roughness
Wavenumber
of magnetic recording
orc.ntatcon
Fig. 5. Typical Raman signal from disk with carbon overcoat.
The slider material for present day thin film sliders is alumina titanium carbide, a hard ceramic, although other materials such as ferrite and barium titanate have been used. Other, alternative slider materials under investigation are silicon carbides which are likewise very hard and durable materials. It is important to note that the head-disk interface would not survive without the presence of a lubricant layer on the disk. An equally important factor in establishing a reliable headdisk interface is the carbon film on the disk surface. This carbon film, produced by sputtering, is a predominantly amorphous film [ 3,4], consisting of a random network of sp2 and sp3 bonds. A typical Raman spectrum of a carbon overcoat is shown in Fig. 5, indicating the presence of two distinct peaks at 1360 and 1580 reciprocal wave numbers. The peak at 1380 cm- ’corresponds to diamond, or sp3 bonding, while the peak at 1580 cm- 1corresponds to graphite, or sp’ bonding. Many process parameters such as plasma pressure, power level, and plasma composition influence the mechanical properties of the carbon overcoat and its interaction with the slider. In recent years, hydrogenated carbon overcoats have shown best tribological behavior [ 51.
a
of thin film disks
Although magnetic thin film disk surfaces appear to be almost perfectly smooth when investigated under a low power microscope, atomic force or scanning tunneling microscopy investigations show (Fig. 6) that a typical disk surface is rough on a nanoscale [ 61. Deep grooves and high ridges are visible under large magnification, introduced intentionally during texturing of the disk surface prior to sputtering of the magnetic layer. The grooves and ridges are needed to provide a well-controlled amount of surface contact between slider and disk, thereby reducing friction during start-up of the disk drive when the slider is in contact with the disk. In order to use very smooth disk surfaces and still be able to start the disk without excessive stiction force between slider and disk, so-called “zone texturing” is of general inter-
cl
a.5
4.3
‘
0.0 ,v---
I -4.2u
3
62 Dlrtnnea
lEGi 121 lHicront1
229
U: I_:
Fig. 7. Typical zone-textured
12.7nm -4.2nr
disk
est. In this approach, a small annular ring of rough texture and large surface roughness is provided for the area where the slider starts and stops, while in the data zone, where the slider flies over the disk, the surface is made very smooth to allow flying at values of 25-50 nm. A typical zone-textured disk is shown in Fig. 7 consisting of a number of individual peaks on the order of 25 nm in height, separated by a distance on the order of 100 p,m.
4. Friction and wear characteristics 3. Surface characterization
disk
Run-1
of disks
In Fig. 8, the variation of the coefficient of friction during start-up is shown for a very smooth disk, indicating that 8.00 7.00 --
1 4 0
0.02
0.04
0.06
0.08
0.1
llme (sets) Fig. 8. Variation of the coefficient of friction during start-up.
235
F.E.Talke/Wear190(199.5)232-238
friction rises from zero during start-up to a very high peak value, from which it decreases to zero as the drive spins up and hydrodynamic flying of the slider is initiated. The maximum value of the coefficient of friction during the initial start-up phase is a measure of the static friction and is generally called “stiction”. Since the flying height between slider and disk has decreased substantially in recent years, today’s disk surfaces exhibit much lower peak-to-valley roughness (20-40 nm) than in the past. Clearly, this decrease in roughness is accompanied by an increased tendency of disk surfaces towards stiction, and many investigations have been carried out to evaluate the dependence of stiction on head-disk design parameters and disk environmental conditions [ 71. As shown in Fig. 9, the average coefficient of friction between a slider and a lubricated disk increases with the number of start-stop cycles, starting in general at a low value of 0.25 and increasing to a high value on the order of 0.7 or higher after 10 000 cycles. Performing friction measurements on unlubricated carbon-coated disks and MnZn and CaTiO, sliders in air, nitrogen, and oxygen, Marchon et al. [ 81 observed that the increase in the coefficient of friction was related to the presence of oxygen. They suggested that during unlubricated sliding, oxygen is chemisorbed on the carbon surface, leading to the formation of surface oxides. Thermal desorption of these species leads to CO and CO, formation which depletes the surfaces from carbon. This tribochemical wear process requires the presence of dangling bonds on the carbon-coated disk and thermal activation due to frictional heating during sliding. If sliding tests are conducted with lubricated disk surfaces in alternating environments of humid air and dry nitrogen, a dramatic decrease in the coefficient of friction is observed whenever dry nitrogen is introduced in the test chamber (Fig. 10, Ref. [ 91) . If the test is repeated for conditions of 40% relative humidity (Fig. 11) , no change in the coefficient of friction is observed for either lubricated or unlubricated conditions, indicating that relative humidity is one of the key parameters in the friction and wear of carbon-coated disks. Although the tribochemical wear theory is supported by experimental evidence using unlubricated clean disk surfaces, experimental evidence suggests that in the case of lubri-
0.90 0.80 070 LO 3 0.50 5 0.40 f#0.30 $ 0.20 cl0.10
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,
10
1
I
1
20
I
I
30
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I
40
Disk Ftevotutions (xl 000) Fig. 10. Frictional behavior of lubricated disk in humid air and dry nitrogen, from Ref. 191.
$0.4
a
eo.2
--
Fig. 11. Frictional relative humidity.
behavior
of lubricated
disk in air and nitrogen
at 40%
cated disks the lubricant film acts as a chemical barrier, thereby reducing the occurrence of tribochemical wear in favor of abrasive and adhesive wear.
5. Disk lubricants The lubricants in general use in today’s disk files are perfluoropolyethers. These lubricants are chemically inert and non-reactive. To improve their adhesion and lubrication performance, bifunctionally terminated perfluorocarbons such as Fomblin Z-DOL or AM-2001 are commonly used. The chemical structure of AM 2001 is shown below:
0.00 0
5aIO
100al
1m
2cmO
css cycles Fig. 9.Coefficient
of friction as a function of the number of start-stop cycles
Since the lubricants on the disk surface are exposed to atmospheric conditions, and are applied only once during the
236
Fig. 12. Coefficient polyether (disk S)
F.E. Talke/ War
of friction
for phosphazine
(disk E)
190 (1995) 232-23X
and perfluoro-
lubricated disks. Fig. 13. Pressure distribution
over two-rail slider.
manufacturing process, the most important parameters of a potential lubricant are vapor pressure, chemical stability, and load-carrying capability. Fluorinated hydrocarbons require chlorofluorocarbons as solvents during the lubricant application process. The desire for environmentally more friendly lubricants has led to investigations of phosphazines as potential alternative lubricants [lo]. The chemical structure of phosphazine lubricants is as follows:
Phosphazine lubricants show only a weak increase in the friction characteristics as a function of the number of startstop cycles, and they can be dissolved in hexan and other “environmentally friendly” solvents. A typical friction plot during contact start-stop testing is shown in Fig. 12 for disks lubricated with a phosphazine (disk E) and perfluorinated polyether (disk S), indicating that the coefficient of friction increases substantially more for perfluorinated disks than for phosphazine lubricated disks. Fig. 14. Pressure distribution ment analysis.
6. Slider design For the results shown in Figs. 8-12, alumina titanium carbide sliders were used consisting of two air bearing rails with a front taper (Fig. 3). At high relative velocities between slider and disk, an air bearing is formed which separates slider and disk. The pressure distribution in the airbearing can be
calculated by numerical nolds equation including
=6
over negative pressure slider using finite ele-
solution of the compressible rarefaction effects, given by
Rey-
F.E. Talke / Wear 190 (1995) 232-238
231
1: AIR BEAlUNC3SURFACE, AJO,-liC 2:fwmTmJmALuMINA 3:NbFePOLES Fig. 15. Schematic view of trailing edge of slider-disk interface.
A typical pressure distribution for a two-rail slider and a negative pressure slider, obtained by finite element analysis of the Reynolds equation, is shown in Fig. 13 and Fig. 14, respectively. Since the durability of the slider-disk interface is related to the length of sliding between slider and disk during startstop, it is important to design the slider with the shortest takeoff distance possible. To reduce materials interactions between slider and disk during start-up and landing, attempts have recently been made to use carbon coated sliders. Although sliding of similarmaterials such as a carbon-coated slider and a carbon-coated disk would seem to be undesirable, improved friction behavior of the interface is observed. 7. Pole tip recession and flying height A schematic view of the trailing edge region of the slidersdisk interface is shown in Fig. 15, indicating that several factors contribute to the effective flying height between slider and disk, namely, surface roughness of disk and slider, lubricant thickness, carbon overcoat thickness and airbearing thickness h. In addition, a distance PTR is shown in Fig. 15 which is equal to the height between the air bearing surface and the magnetic pole tips of the thin film head. Since the pole tips are made of permalloy (NiFe) , preferential wear of the soft pole pieces relative to the sputtered alumina (2) and the alumina titanium carbide ( 1) of the slider may occur either during manufacturing of the slider or during its use in the disk drive. Pole tip recession measurements with AIM and optical surface interferometric methods indicate that typical values of pole tip recession are on the order of 10 nm.
8. Slider flying height measurement Owing to the decrease in the slider flying height over the last decade, white light interferometric methods have become inadequate to measure the head-disk spacing since distinct color fringes are absent below 100 nm head-disk spacing. Multiple wavelength monochromatic interferometry must be used to measure the head-disk spacing at those small flying heights. In this approach, the intensity of the interference fringe pattern at arbitrary gray levels is determined and the flying height is calculated from interpolation of the intensity between the peaks and valleys of the intensity distribution. A different method for the measurement of relative displacements of the slider at spacings in the nanometer flying height range is phase demodulated interferometry. Here, the phase difference of light between an initial and final position of the slider is measured relative to the disk, resulting in an absolute measure of the flying height change between the two positions. 9. Discussion Owing to the need for further decreases in the flying height below 60 nm, the following two questions arise: (a) how low can flying heights decrease?, and (b) is it feasible to reduce the flying height to zero, thereby establishing contact recording? In a search for an answer to the last question, several non-conventional approaches to contact or near-contact recording have been proposed in the last few years. These are 1. miniaturized, lightly loaded sliders in contact with the disk
ill19 2. thick film lubricated disks using a liquid bearing instead of an air bearing [ 121, and
238
F.E. Talke/ Wear I90 (1995) 232-238
3. programmable air bearing sliders which fly at close spacing only when needed for recording purposes [ 131. In the contact recording approach of ref., a lightly loaded (400 p_N) miniaturized recording element with a carbon wear pad is placed in contact with the carbon-coated magnetic disk and a controlled low wear rate is obtained as a consequence of the light load. In the approach of Ref. [ 121, a film of a liquid boundary lubricant is constantly re-applied to the disk surface by means of a wicking mechanism, and the head is “flying” on a liquid lubricant film. In the programmable air bearing approach of Ref. [ 131, the read-write element is lowered onto the disk by piezoelectric means only when a read-write function of the head is required. Very little information exists presently in the open literature on any one of the above three “contact recording” or near contact recording approaches. However, the programmable air bearing approach requires large slider dimensions to yield reasonable motion of the read-write element, and, thus, seems to be an impractical approach from the point of view of mass production. Likewise, the liquid lubrication approach of Ref. [ 121 is difficult to implement from the point of view of mass production. The lightly loaded contact recording approach of Censtor (Fig. 15) is a revolutionary step away from conventional air bearing design technology. Here, an extremely lightly loaded head structure (approximately 400 ~_LN)of approximately 25 km linear dimensions is used in constant contact with the disk surface, establishing a maximum output of the magnetic signal. However, since wear of the head and the disk is unavoidable, concerns related to contamination of the head-disk interface exist as well as concerns related to disk and head wear. Continuous contact between slider and magnetic disk requires a detailed understanding of the tribology and materials interactions at the head-disk interface. That is to say, in present day magnetic recording disk drives, in which the slider is supported by an air bearing, approximately 1Ok startstop cycles occur during the life of the drive, corresponding to about 10k contact revolutions. On the other hand, for ‘‘incontact” recording disk drives in which the air bearing is absent, approximately 10 billion contact revolutions occur, requiring that the amount of wear per contact revolution is only a fraction of that of present day head-disk interfaces. Thus, it is apparent that the step toward contact recording is not easy and that a more detailed understanding of the materials and tribology problems of the head-disk interface under “contact recording” is urgently needed. 10. Conclusions and recommendations The tribology of the magnetic disk-head interface depends on a large number of interrelated problems, spanning the area of surface characterization, tribochemical wear, lubricant
analysis, boundary friction, and materials properties of slider and disk. Owing to the constant rate of decrease of flying height, measurement problems of physical quantities such as flying height and pole tip erosion are becoming increasingly more important. Extrapolating from the understanding of the tribology of the head-disk interface as it is known today, one can conclude that it will be extremely difficult, if not impossible, to predict when and if contact recording will ever be a viable alternative to the well entrenched air bearing technology. In fact, it seems justifiable to predict that the air bearing technology, as it exists in today’s disk files, will continue to improve and that spacings on the order of 15 nm will be implemented long before contact recording at zero spacings. Acknowledgements I would like to thank M. Wahl and S. Nadimpalli help in the preparation of this manuscript.
for their
References [ l] C.D. Mee and ED. Daniel, Magneric Recording, Vol. I: Technology, McGraw-Hill, 1987. [2] J.M. Harker, D.W. Brede, R.E. Pattison, G.R. Santana and L.G. Taft, A quarter century of disk file innovation, IBMJ. Rex Dev., 25 ( 1981) 667-690. [ 31 H. Tsai and D.B. Bogy, Characterization of diamondlike carbon films and their application as overcoats on thin-film media for magnetic recording, J. Vat. Sci. Tech. A, 5 (6) ( 1987) 3287-3312. [4] J. Robertson, Amorphous carbon, Adv. Phys., 35 (4) (1986) 317374. [ 51 J.K. Lee, M. Smallen, J. Enguero, H.J. Lee and A. Chao, The effect of chemical and surface properties of hydrogenated carbon overcoats on the tribological performance of rigid magnetic disks, IEEE Trans. Mag.,29(1) (1993)276281. [6] M. Yang and F.E. Talke, Surface roughness investigation of magnetic recording disks using STM and profilometry measurements, Wear, I70 (1993) 15-24. [7] Y. Li, D. Trauner and F.E. TaJke, Effect of humidity on stiction and friction of the head/disk interface, IEEE Trans. Magn. 26 (5) ( 1990) 2487-2489. [8] B. Marchon, N. Heiman and M.R. Khan, Evidence of tribochemical wear on amorphous carbon thin films, IEEE Trans. Msg., 26 (1) (1989) 168-170. [9] M. Yang, S.K. Canapathi, R.D. Bahutson and F.E. TaJke, The frictional behavior of thin film magnetic disks, IEEE Trans. Magn., 27 (6) (1991) 5157-5159. [ 101 M. Yang, F.E. Talke, D.J. Perettie, T.A. Morgan, K.K. Kar and G.E. Potter, Cyclophosphazines as new lubricants for rigid magnetic recording media. to be published in STLE, 1994. [ 1l] H. Hamilton, K. Goodson, C. Baldwin and R. Anderson, Contact perpendicular recording with integrated head/flexure, in TRIB, Vol. 3, Concepts in Contact Recording, ASME, Philadelphia, PA, 1992, pp. 13-23. [ 121 J.U. Len&e, Non-contact recording with a liquid interface, TRIE. Vol. 3, Concepts in Contact Recording, ASME, New York, 1992, pp. 25% 26. [ 131 V.D. Khanna, Fly low only when necessary, TRIB, Vol. 3, Concepts in Contact Recording, ASME. 1992, pp. l-l 1.