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The tribology of flexible magnetic recording media
Journal of Magnetism and Magnetic Materials 155 (1996) 312-317
ELSEVIER
Invited paper
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Journalof amnadg netlsm magnetic J R materials
The tribology of flexible magnetic recording media J.L. S u l l i v a n
*
Electronic Engineering and Applied Physics, Aston Unit'ersit3", Birmingham, B4 7ET. UK
Abstract The tribology of magnetic recording systems is different to conventional tribology in many important aspects and an understanding of this branch of the science is still in its infancy. This paper reviews some of the basic laws of friction, lubrication, heat flow, contact and wear as they apply to flexible media/head contacts. From this review, the surface and subsurface parameters having the most influence on the future development of media and systems are identified.
1. Introduction Due to continued pressure for more data stored in ever decreasing package sizes, the data densities demanded of magnetic media have increased monotonically. With this increase, the recording wavelength is reduced and the trend is to thinner magnetic coatings, smaller head/media spacings and smaller head dimensions. This has the effect of producing acute tribological contact problems in flexible media where in order to reduce spacing losses and achieve projected storage densities, continuous contact is essential. It is the physical/mechanical contact problems which are now and will increasingly in the future be significant limiting factors in dynamic magnetic storage technology development. In the past tribological problems which have occurred have been solved largely by a combination of good design, lower contact pressures, harder head materials, more wear resistant magnetic layers, better lubricants and by natural tribological laws. For example, a simple law describing the wear of materials under a wide variety of conditions is the Archard wear law [1], which may be stated as: o ) = k W / p m, where w is the wear rate, the volume of material removed per unit distance of sliding, W the applied load, Pm the flow pressure, or hardness and k a constant of proportionality sometimes known as the wear coefficient. Thus, increasing head and media hardnesses will decrease wear as will reduction in head loads. Here, however, technology is further helped by nature, in that k does not remain constant [2], but may decrease by orders of magnitude at very low loads. Conversely friction coefficient may increase at similar low loads, particularly with very smooth surfaces. As a consequence of this dynamic and static
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friction problems arise under low load conditions which are not noticed at higher loads. 2. Particulate media The vast majority of media in use today consists of magnetic pigment [3] bound together by means of a polymeric resin. The magnetic layer in contact with the read/write heads will have the tribological properties of a filled polymer, complicated by the addition of head cleaning, wetting, cross linking and anti-fungal agents, solvents, carbon black and lubricant. In terms of the tribology of the system the most important part of this mix is the binder, typically a co-polymers such as polyurethane-polyester, although the effect of other components must be recognised. Polymers may exist in crystalline of amorphous states and in the amorphous state may be either glassy, rubbery of viscous. Polymeric binders normally consist of two phases, a 'hard' glassy phase to give the system mechanical integrity and an a 'soft' rubbery phase to provide the flexibility required of the media. At the media/head interface the filled polymer is in moving contact with a hard (ceramic/glass/ferrite) surface. Before considering specific aspects of friction and wear, it is necessary to consider the nature of this contact. The real area of contact is dependent on the physical properties of the surface and since contact will only occur at asperity tips, is generally very much lower that the apparent area of contact. When contact occurs, these asperities may deform either plastically or elastically and it is important to know the mode of deformation before wear and friction can be understood. Bushan [4] adapted the work of Greenwood and Williamson [5] and defined a plasticity index ~ tbr polymers such that: q,= E * / Y ( , ~ / r , ) '/-~
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J.L. Sulliran /Journal of Magnetism and Magnetic Materials 155 (1996) 312 317
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Friction is an important parameter in the design of magnetic recording systems. With miniaturisation and the use of low power drives, friction which is too high can cause speed variations or failure of drive motors and can lead to high surface temperatures.
sition temperature and toughness. The above analysis and practical experience shows that a rougher surface produces lower frictional forces, however, due to the need for mechanical durability and to reduce spacing losses to a minimum, surfaces need to be as smooth as possible. Experimental observations show an increase in coefficient of friction with running time, at least over the first few passes. This is to some extent due to the surface becoming smoother, but this is not entirely consistent with the assumptions that contact is elastic. Our findings from scanning electron micrographs of worn surfaces [9] and from extensive surface analyses show that not only does the surface roughness decease, but the polymeric binder in the outer layers changes chemically. Assuming the contacts to be mainly adhesive, the changes may be sufficient to influence the frictional force.
4. D y n a m i c friction
5. Static friction - Stiction
The force resisting motion between two bodies consists of an adhesive component plus a deformation component (plastic deformation or ploughing term plus a hysteresis loss term). Considering the viscoelastic nature of the magnetic layer, the nature of the contact and the surface topography the total deformation component is negligible and friction is due to adhesion [8]. Thus the frictional energy is dissipated in the contact region and the frictional force may be written as:
Static friction, the force required to initiate motion, is generally higher than dynamic friction, that required to maintain it. In systems where the head is parked in contact with media this difference is particularly pronounced and can lead to high static friction and a phenomena known as stick-slip, which results in a jerky and uneven motion of the tape or disc. There are a number of reasons for this high initial friction. The magnetic layer is in a viscoelastic state at normal operating temperatures, where strain is rate dependent. Thus the real area of contact increases with time, hence, increasing the initial friction value. In addition, Van der Waal forces operate between the smooth surfaces of media and head [10] and these increase the surface area above that predicted by theory. Both effects are more obvious when the system is stationery and has been for a length of time. Very high static friction forces (and other problems) are most apparent under conditions of high humidity [I I], possibly due to meniscus effects [12] a n d / o r water plasticisation of the polymer [13]. The meniscus effect is possible since high friction is also observed if too much lubricant is present on the surface. It is more likely, however, that increased moisture levels instigate hydrolysis and plasticization takes place [14]. To overcome these problems polymer properties of high complex modulus and high hardness are required with an additional requirement for the binder to be hydrophobic.
where E* is the complex modulus of elasticity, Y the yield strength or hardness, cr the standard deviation of asperity heights and r~ the mean radius of those asperity peaks. If t) < 1.8 the contact is elastic and if ~0> 2.6 the contact plastic. Bushan [6] and Bushan and Doerner [7], by insertion of measured values into the above equations, found that typical head media contacts are elastic. From this the real area of contact is A r = 3.2 W/qJY. 3. Friction of particulate media
F ~ rAr where r is the interfacial shear stress and is not constant, but will increase with pressure and, in general, fall with temperature within the operating range of the media. Friction will also be speed dependent due to the time dependence of the elastic properties of the polymer and the increase in temperature with increase in velocity. With particulate media, a fatty acid/fatty acid ester type lubricant is normally incorporated into the bulk of the magnetic coating. The coating acts as a reservoir to preserve and replenish a surface lubricant film of about 2 or 3 molecular layers thick, and coverage as low as 10%. Taking into account the presence of the lubricant, the frictional force can be written as: F=A,{r(l-c~)+r,c~} where 7~ is the shear stress in the lubricant and c~ the fractional areal lubricant coverage. The lubricant acts as a classical boundary film, but will also produces some plasticisation at the polymer surface. Polymer surfaces, however, have low surface energy and this together with the low lubricant coverage means that the frictional force will not change substantially with lubrication. Hence, friction is largely determined by the properties of the magnetic layer. Thus a high hardness, high complex modulus material should be used to reduce friction, with due regard to other important material properties such as a suitable glass tran-
6. Interfaciai forces
In the absence of plastic or irrecoverable deformation, the force operative between the contacting asperities is adhesive. This adhesive force may be physical, involving Van der Waal bonds [15], or chemical involving electrostatic, ionic or co-valent bonding. Interfacial Van der Waal bond strengths [16] could be significant and co-valent
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J.L. Sullivan/Journal of Magnetism and Magnetic Materials 155 (1996) 312-317
bonding occurs between metals and polymers [17]. However, at polymer ceramic contacts ion-dipole bonds are most likely to give rise to the strong adhesive junctions. Within polymers atoms are bound in the chains by co-valent bonds (about 10 e V / a t o m ) and chains to chains by dipole-dipole interactions (about 0.2 eV/atom). Assuming ion-dipole attractions, head/medium adhesive junctions will be strong enough to produce shearing within the bulk of the magnetic layer. This process results in wear and transfer of material from media to head.
7. Transfer films During running transfer layers, which cannot be removed by solvent washing and variously described as friction polymer or brown stain, are invariably formed on the head. Loose debris is also produced and causes 'head clogging'. A major function of the head cleaning agent (HCA) is to limit the transfer layer thickness and reduce spacing losses. (A further function is to act as a bearing surface and effectively protect the soft media against wear). The HCA does not materially affect head clogging. Transfer films can substantially effect the friction and wear properties of junctions [18]. With polymers such as those used in binder systems which are non-linear and of relatively low crystallinity, transfer occurs as a discontinuous lumpy film with lumps of material varying from sub-micron to a few microns in diameter [19]. Once this layer has been formed contact is then substantially between the layer and the media. The description of the layer as a friction polymer [20], implies some chemical interaction, however, in extensive studies of such films we have found little evidence for this. Films consists mainly of iron oxide, which is consistent with particles being transferred directly from media to head due to adhesive wear and held in place by ion-dipole interactions. These transfer films are not formed for relative humidities of above about 45%, where water saturation of active sites on the head prevents the formation of adhesive bonds. The formation of transfer layers may be beneficial in reducing friction and wear of the head, but may introduce unacceptable spacing losses. Loose debris formation is always detrimental. Osaki et al. [21] describe head clogging as due to debris agglomerates picked up by the tape and transferred to the head where they accumulate in the gap region producing spacing losses. We have also seen such effects [22] which can also cause gap damage affecting high frequency response of the system.
8. Surface and contact temperatures Bushan [23,24] predicted temperature rises for typical head/tape contacts of 7 to 10°C, although he concedes that if magnetic particles are exposed the contact temperatures can range from 600 to 900°C. There is some correla-
tion of these results with IR measurements [25]. Our calculations, however, using a method due to Mekala [26], suggest contact or hot spot temperatures much higher than these. In this model the heat generated due to friction, is equated to the sum of the heat lost due to conduction into the head surface, conduction into neighbouring asperities and that due to forced convection to the air. From atomic force micrographs the surface was modelled into a network of nodes and at each of the nodes a conical asperity with random Raleigh distributions of volumes and base radii was assumed. From this model maximum contact temperatures for a particular linear tape system were calculated to be over 100°C. If all the load is carried instantaneously by one asperity, and we have evidence to show that this may occasionally occur, then very high temperatures are predicted and this can have profound effects on the mechanisms of wear and friction.
9. Wear mechanisms in h e a d / m e d i a contacts Wear is classified as adhesive, fatigue, abrasive, corrosive or erosive, with perhaps a few subsets of these wear forms. The mechanisms are not mutually exclusive. Forces between head and media are largely adhesive and these forces lead to transfer film and debris particle formation. Adhesive wear can be reduced by introducing a boundary lubricant between the opposing surfaces and, in this case, by reducing the polar molecule content of the binder. Sullivan and Sharma [27] found diskette lifetimes reduced by two orders of magnitude without surface lubrication, although friction values were relatively unaffected. When two solids are in sliding contact, maximum stress occurs not at the asperity junction, but below this surface. The depth depends on the radii of asperity contact (which is load dependent) and the coefficient of friction [28]. When an isolated asperity on either head or tape approaches an opposing asperity the subsurface is in compression, as the asperities separate it is in tension. Hence, for repeated contact, the area of subsurface immediately below the asperity contacts is subject to cyclic stressing and fatigue cracks will eventually appear. These cracks propagate under further cyclic stressing until wear particles are produced. There are two forms of abrasive wear; two body abrasion, when a hard rough surface slides over a softer one resulting in ploughing action and three body abrasion, when a hard particles becomes entrapped between two sliding bodies causing the same effect. An essential requirement is that plastic deformation should occur beneath a penetrating asperity or abrasive particle. From the work of Halliday [29], plastic deformation and hence abrasive wear will only occur in polymers if the opposing hard surface is very rough, ~ 12 Ixm, a roughness not experienced in magnetic media or heads. Thus two body abrasive wear should not occur and three body only if large agglomerates of debris become entrapped between head and
J.L. Sullican /Journal of Magnetism and Magnetic" Materials 155 (1996) 312-317 media. This latter effect is almost certainly the origin of scratches seen on tape and diskette surfaces after prolonged use. Abrasive wear rate falls substantially with particle/ asperity size below a critical dimension [30]. When the diameter is below about 1 Ixm [31] conventional abrasion is not operative, but microabrasion or polishing wear occurs [31 ] where the hardness of the abrading particles need not fulfil the Richardson criteria [32], but can be relatively low. For magnetic media, particle sizes greater than 1 p.m are not common and hence polishing is probably the most important mechanism in head wear. Conventional polishing normally occurs when small hard particles imbedded into an elastic backing medium are passed at high speed and low load over the surface to be treated. This is exactly the sitt, ation which occurs in head/media contact. For polymeric materials the abrasive wear rate is proportional to 1/Se [33], where S is the breaking strength of the material and e the elongation to break. The addition of the magnetic particles to the polymer will increase S, but reduce e and hence the filled polymer is no more resistant to abrasion than the polymer itself. For head media interactions the major corrosive elements are supplied by the atmosphere; oxygen can produce oxidative degeneration of the polymer surface and oxidation of elements of the head: and water vapour can produce hydrolysis and chain scission in the polymer and hydroxide formation on ceramics, glasses, ferrites and metals. Water vapour is a major cause for concern in head/media interactions, where high humidity can result in high friction, stiction and high wear. Elements present in particulate tapes also may possibly produce corrosive reactions, for example, phosphates from cross linking agents could effect metal elements in MIG or MR heads. We have. however, found no evidence of this, although there is some circumstantial evidence that CI from degeneration of CI containing polymeric binders may under some circumstances produce some corrosion. Erosive wear is the removal of material from a surface by the impact of solid particles or fluids. In the case of head/media interfaces only erosion due to solid particle impact may be of interest and then only to explain one particular phenomena. In certain heads, differential wear is observed where the metal is worn away at a greater rate than the surrounding ceramic, resulting in removal of metal to a depth where contact with the media cannot possible occur. It is possible that particles entrapped in the area produce erosive wear. The amount of wear will depend on the kinetic energy, particle density, impact angle and particle size [34,35]. The same material properties are required for the resistance to erosive wear as for abrasive wear. 10. Particulate media wear
Sharma [9] and Sullivan and Sharma [27] studied the durability of particulate diskettes under a variety of condi-
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tions of lubrication. Extensive optical microscopy, SEM and XPS examinations were employed to isolate the mechanisms of wear. It was found that wear proceeded in three stages; initial smoothing of a wear track leading to plastic deformation with little or no material removal: removal of laminar particles or flakes of thickness much less than the magnetic coating and finally a catastrophic phase where large areas of the wear track were destroyed. Usually wear will not progress further than stage one, but this stage is often visible to the naked eye in a well used media. Other investigations on both tape and diskettes have shown that the processes leading to wear and to ultimate catastrophic failure of all such media is similar. Although contact should be elastic, high localised contact temperatures can lead to polymer softening and plastic flow in the first stage. The second stage is the result of continuous cyclic stressing of the media which leads to the initiation of sub-surface fatigue cracks which grow to the surface and produce laminar wear particles, a process analogous to that described by Suh [36] for metals. Wear debris then becomes entrapped between the head and media producing three body interaction and an avalanche failure mechanism. These findings are partially confirmed by recent work of Osaki et al. [37]. The importance of adhesion and fatigue in polymer wear has been described by Omar et al. [38]. 11. Head w e a r and differential head wear
Recording heads consist of a body of material which is hard. ceramic, ferrite or a combination of these and with, for example, MR heads or MIG heads some metal element. Allowable wear may be small over the life of the component. Head materials generally have high elastic modulus, high fracture strength, high chemical stability and high thermal stability, but they are also very brittle, lnterfacial adhesion is low and consequently the shear strength of the junction is low. thus adhesive wear of these surfaces is very unlikely and indeed we have never found any evidence for adhesive transfer from head to media. The most common form of wear in these materials is due to abrasion/micro-abrasion and fatigue, although they are not ideally brittle and plastic flow can occur. Reaves and Sullivan [22] studied the wear of VHS video heads of manganese zinc ferrite and glass. The wear rate of these heads was dependent on the abrasivity on the media. Chromium dioxide tape (Knoop hardness, 225 Mpa) produced a wear rate an order of magnitude higher than an iron oxide tape (hardness 118 Mpa) of similar particle loading. For iron oxide tapes with increasing levels of HCA, wear increased almost linearly with concentration [39]. In the case of the chromium dioxide tape the worn surface was very smooth and typical of a polishing wear process. With the iron oxide tapes in addition to the general smoothing of the head surface, scratches and groove with sizes of up to about 0.3 ~m wide were evident on
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J.L. Sulliran / Journal of Magnetism and Magnetic Materials 155 (1996) 312-317
head surfaces caused by plastic deformation. However, no particles exist within the media large enough to cause deformation of that magnitude [12]. Hence, they are either due to three body abrasion due to agglomerates of particles from the media (Reaves and Sullivan used transmission electron microscopy to show that head cleaning agents agglomerates were present in the magnetic layers) or due to isolated large asperities in the media surface similar to those identified by Hahn [40]. In either case all the load would be supported momentarily on one contact and this could lead to very high temperatures being generated. This would easily explain the plastic deformation observed. The mechanisms outlined above are operative in glass, ceramics and ferrites, with ferrite single crystals, however, there is the additional effect of crystal orientation. The wear in different crystallographic directions in ferrites is highly anisotropic and the anisotropy is dependent on the magnetic particle [41]. For example, the effect is more pronounced for iron oxide than for chromium dioxide. Wear rates of heads are important, but of equal importance are differential head component wear rates which introduce spacing losses. When using a combination of different materials in recording heads it is very difficult to exactly match wear rates. The final choice of materials is usually a compromise, but even with careful choice of materials, differential wear still occurs. An example already referred to is the hollowing out of the MIG heads and the reason for this is still under investigation by ourselves and others. Other forms of differential wear occur in linear recording heads which have elements of ceramic, glass and ferrite. Under conditions of high humidity the ceramic may wear at a rate greater than the other two elements. The effect of humidity on ceramics is a very complex problem [42]. There may be a tribo-chemical reaction which produces hydroxides, which may either increase or reduce wear. In the case of heads we examined, which used a two phase titanate ceramic, enhanced ceramic wear was not due to a chemical, but to a mechanical process caused by water molecule ingress. This effect is due to the non-compressibility of water and is well known in the pitting of metals [43]. Sintered ceramic materials are always porous. If the grain size is small, water under hydrostatic pressure can enhance crack formation and propagation in the ceramic surface. This leads to increased rates of removal of the surface due to microfatigue which enhances polishing wear. The problem can be reduced by using more abrasive tapes and suffering higher overall wear [44], but this could result in an unacceptable reduction in head life, or the problem can be overcome by using hard coatings on heads [45], but in many applications the resulting spacing loss would not be acceptable. 12. Metal evaporated (ME) media The majority of this work has been devoted to particulate media and this equitably reflects the usage, however,
ME media which has higher effective magnetisation is of increasing importance. Flexible ME media presents its own unique tribological problems. Unlike particulate media, surface coverage of the lubricant must be complete, must remain so for the projected lifetime of the media, must afford complete corrosion protection and should not be more than about 5 nm thick. ME media does not have the advantages of porosity and surface chemistry of the particulate media, thus the demands on the lubricant present a severe challenge. In addition the thin metal films are far less compliant than particulate filled polymer films and should not be subject to strain if cracking and catastrophic failure are to be avoided. 13. Friction and wear of ME media ME media relies on a complete boundary lubricant film for its successful operation. The surface of the evaporated metal film is oxidised and sometimes carbon coated to give additional protection in the case of momentary breakdown of the boundary lubricant. This provides a low surface energy contact which can survive occasional head/media asperity interaction, but the lubricant must be capable of self healing any breach if high wear is not to ensue. Friction is almost totally accounted for by the force required to shear the boundary lubricant film. Considering the plasticity index of Greenwood and Williamson [5] and the surface roughness of the media and heads, the deformation of the asperity contacts on the tape should be elastic-plastic and the dominant wear mechanism at both media and head is one due to fatigue. Hempstock and Sullivan [46] have studied the wear of commercial ME Hi-8 video tape, correlated this to signal performance and identified the wear mechanism. Little wear is seen on the sample surfaces for relatively long running times. Wear particles are then seen to appear corresponding to the start of surface delamination at which point a substantial increase in signal errors is observed. The process proceeds until catastrophic fatigue cracking and delaminative failure of the film occurs. This corresponds to depletion of lubricant, measured by XPS, in the area of the scar. The mechanism is classic Sub [36] delamination. A gradual breakdown of the lubricant film allowing head-metal contact to occur will greatly accelerate the process. 14. Conclusion Much of the pioneering work in tribology may be used to elucidate mechanisms of friction and wear in these systems, however, the use of classical ideas does not mean that the phenomena have been or can be adequately explained in these terms and a great deal of speculation still remains. As far as tribology of these systems is concerned the industry is in a critical stage where good fortune can no longer be relied upon. There is an urgent need for more
J.L. Sullil'an / Journal ¢?/'Magnetism and Magnetic Mate rials 155 ( 19961 3 / 2 -317 effort to be m a d e in u n d e r s t a n d i n g the basic m e c h a n i s m s of contact in t h e s e very c o m p l e x s y s t e m s in order that projected recording densities m a y be achieved in practical situations.
References [1] J, Archard, J. Appl. Phys. 24 (1953) 981. [2] E. Rabinowicz. Proc. Conf. on Engineering Materials for Advanced Friction and Wear Applications. ASME, Gaithersburg, (1988) 209. [3] M.P. Sherrock, IEEE Trans. Magn. 25 (19891 4374. [4] B. Bashan, J. Trib. Trans. ASME 106 (1984) 26. [5] J.A. Greenwood. J.B.P. Williamson, Proc. Roy. Soc. A 292 (1966) 3006; B, Bushan, ASLE Trans. 28 (1985) 181. [6] B, Bashan, ASLE Trans. 28 (19851 181. [7] B, Bushan, M.F.J. Doerner, Trib. Trans. ASME 111 (19891 452. [8] B,J. Briscoe, D. Tabor, in: Polymer Surfaces, eds. D.T Clarke, W.J. Feast (Wiley, New York, 19781. [9] P,J. Sharma, Factors Aflecting the Durability of Floppy Diskette Media and Mechanisms of Wear, Aston University. Birmingham (1989). [10] 11. Tabor, Advances in Polymer Friction and Wear, ed, LH.Lee, 5A (Plenum, New York, 19741 5. [11] R.F. Hegal, Tribology Trans, 36 (1993) 67. [I 2] E. Rabinowicz, Friction and Wear of Materials (Wiley. New York. 19651. [13] R.L. Bradshaw. B. Bhushan, C. Kalthoff. M. Warne, ASLA Trans. 27 (19861 207. [14] M. Edge, N.S. Allen, W. Chen, C.V. Horie. European Poly mer J. 29 (1993) 1031. [15] B.V, Deryagin, 1.I. Abrikosova, J. Exp. Theor. Phys. 3 ( 19561 819. [16] J.N. Israelachvili, D. Tabor, Proc. Roy. Soc. A 331 (19721 19. [17] D.H. Buckley, W.A. Brainard, Advances in Friction and Wear of Polymers, 5B (Plenum, New York, 1974). 315. [18] P.M. Dickens. J.L. Sullivan, J.K. Lancaster, Wear 119 (19861 6525.
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[19] K. Tanaka, I. Uchiyama, Wear 23 (19771 153. [20] J.L. Lauer, W.R. Jones, The Tribology and Mechanics of Magnetic Storage Systems, eds. Bhushan and Eiss, Vol. 3 (ASLE, IL), 14. [21] H. Osaki. J. Kurihara, T. Kanon, IEEE Trans. Magn. 30 (19881 1491. [22] L.M. Reaves. J.k. Sullivan, Tribology Internal, 23 (19901 251, [23] B. Bushan, J. Trib. Trans. ASME 109 ( 19871 243. [24] B. Bashan, J, Trib. Trans. ASME 109 (19871 252. [25] R. Gulino, S. Blair. W.O. Winer, B. Bhushan..I. Trib. Trans. ASME 108 (19871 29. [26] D. Mekala. private communication. [27] J.L. Sullivan, P.J, Sharma, J. Phys. D 25 (19921 321. [28] G.M. Hamilton, L.E. Goodman, J. Appl. Mech. Trans ASME 33 (19661 371. [29] J.S. Halliday, Proc. lnst Mech. Eng. 169 (19551 221, [30] E. Larsen-Basse, Wear 19 (19721 35. [31] E. Rabinowicz, A. Mulls, Wear 8 (19611 381. [32] R.D.C. Richardson, Proc. Inst. Mech. Eng. 182 (1967)410. [33] J.K. Lancaster. Plastics and Polymers 41 (19731 112. [34] I. Finnie. W e a r 3 (19601 87. [35] S. Aoyama, M. Kishimoto. IEEE Trans. Magn. 27 (1991) 791. [36] N.P. Suh, Wear 44 (19771 I. [37] H. Osaki, E. Oyangi, H. Aomuma, T. Kanou, J. Kurihara, IEEE Trans. Magn. 28 (19921 76. [38] M.K. Omar, A.G. Atkins. J . K Lancaster, .I. Phys. D 19 (19861 177. [39] L.M. Reaves, J.L. Sullivan. J. Phys. D 25 ( 19921 328. [40] F.W. Hahn. IEEE Trans. Magn. 20 (19841 918. [41] S.E. Kadijk, A. Broese van Groenou, Wear 139 (19901. [42] T.E. Fischer. W.M. Mullins, J, Phys. Chem. 96 (19921 5690. [43] J.L. Sullivan. M,R. Middleton, ASLE Trans. 28 ( 19851 431. [44] A. Broese van Groenou, H.J.J.C. Meulenbroeks, M. de Jong. 1EEE Trans. Magn. 26 (19901 153. [45] V. Zieren, M. de Jong. A. Broese van Groenou, J.B.A. Zou. P. Lasinski, G.A.S.M. Theunissen. IEEE Trans. Magn. 3(I (1994) 340. [46] M.S. Hempstock, J.L. Sullivan. J. Magn. Magn. Mater. 155 (19951 323 (these Proceedings),
ELSEVIER
Invited paper
~
Journalof amnadg netlsm magnetic J R materials
The tribology of flexible magnetic recording media J.L. S u l l i v a n
*
Electronic Engineering and Applied Physics, Aston Unit'ersit3", Birmingham, B4 7ET. UK
Abstract The tribology of magnetic recording systems is different to conventional tribology in many important aspects and an understanding of this branch of the science is still in its infancy. This paper reviews some of the basic laws of friction, lubrication, heat flow, contact and wear as they apply to flexible media/head contacts. From this review, the surface and subsurface parameters having the most influence on the future development of media and systems are identified.
1. Introduction Due to continued pressure for more data stored in ever decreasing package sizes, the data densities demanded of magnetic media have increased monotonically. With this increase, the recording wavelength is reduced and the trend is to thinner magnetic coatings, smaller head/media spacings and smaller head dimensions. This has the effect of producing acute tribological contact problems in flexible media where in order to reduce spacing losses and achieve projected storage densities, continuous contact is essential. It is the physical/mechanical contact problems which are now and will increasingly in the future be significant limiting factors in dynamic magnetic storage technology development. In the past tribological problems which have occurred have been solved largely by a combination of good design, lower contact pressures, harder head materials, more wear resistant magnetic layers, better lubricants and by natural tribological laws. For example, a simple law describing the wear of materials under a wide variety of conditions is the Archard wear law [1], which may be stated as: o ) = k W / p m, where w is the wear rate, the volume of material removed per unit distance of sliding, W the applied load, Pm the flow pressure, or hardness and k a constant of proportionality sometimes known as the wear coefficient. Thus, increasing head and media hardnesses will decrease wear as will reduction in head loads. Here, however, technology is further helped by nature, in that k does not remain constant [2], but may decrease by orders of magnitude at very low loads. Conversely friction coefficient may increase at similar low loads, particularly with very smooth surfaces. As a consequence of this dynamic and static
* Email: [email protected].
friction problems arise under low load conditions which are not noticed at higher loads. 2. Particulate media The vast majority of media in use today consists of magnetic pigment [3] bound together by means of a polymeric resin. The magnetic layer in contact with the read/write heads will have the tribological properties of a filled polymer, complicated by the addition of head cleaning, wetting, cross linking and anti-fungal agents, solvents, carbon black and lubricant. In terms of the tribology of the system the most important part of this mix is the binder, typically a co-polymers such as polyurethane-polyester, although the effect of other components must be recognised. Polymers may exist in crystalline of amorphous states and in the amorphous state may be either glassy, rubbery of viscous. Polymeric binders normally consist of two phases, a 'hard' glassy phase to give the system mechanical integrity and an a 'soft' rubbery phase to provide the flexibility required of the media. At the media/head interface the filled polymer is in moving contact with a hard (ceramic/glass/ferrite) surface. Before considering specific aspects of friction and wear, it is necessary to consider the nature of this contact. The real area of contact is dependent on the physical properties of the surface and since contact will only occur at asperity tips, is generally very much lower that the apparent area of contact. When contact occurs, these asperities may deform either plastically or elastically and it is important to know the mode of deformation before wear and friction can be understood. Bushan [4] adapted the work of Greenwood and Williamson [5] and defined a plasticity index ~ tbr polymers such that: q,= E * / Y ( , ~ / r , ) '/-~
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 6 9 7 - 4
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J.L. Sulliran /Journal of Magnetism and Magnetic Materials 155 (1996) 312 317
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Friction is an important parameter in the design of magnetic recording systems. With miniaturisation and the use of low power drives, friction which is too high can cause speed variations or failure of drive motors and can lead to high surface temperatures.
sition temperature and toughness. The above analysis and practical experience shows that a rougher surface produces lower frictional forces, however, due to the need for mechanical durability and to reduce spacing losses to a minimum, surfaces need to be as smooth as possible. Experimental observations show an increase in coefficient of friction with running time, at least over the first few passes. This is to some extent due to the surface becoming smoother, but this is not entirely consistent with the assumptions that contact is elastic. Our findings from scanning electron micrographs of worn surfaces [9] and from extensive surface analyses show that not only does the surface roughness decease, but the polymeric binder in the outer layers changes chemically. Assuming the contacts to be mainly adhesive, the changes may be sufficient to influence the frictional force.
4. D y n a m i c friction
5. Static friction - Stiction
The force resisting motion between two bodies consists of an adhesive component plus a deformation component (plastic deformation or ploughing term plus a hysteresis loss term). Considering the viscoelastic nature of the magnetic layer, the nature of the contact and the surface topography the total deformation component is negligible and friction is due to adhesion [8]. Thus the frictional energy is dissipated in the contact region and the frictional force may be written as:
Static friction, the force required to initiate motion, is generally higher than dynamic friction, that required to maintain it. In systems where the head is parked in contact with media this difference is particularly pronounced and can lead to high static friction and a phenomena known as stick-slip, which results in a jerky and uneven motion of the tape or disc. There are a number of reasons for this high initial friction. The magnetic layer is in a viscoelastic state at normal operating temperatures, where strain is rate dependent. Thus the real area of contact increases with time, hence, increasing the initial friction value. In addition, Van der Waal forces operate between the smooth surfaces of media and head [10] and these increase the surface area above that predicted by theory. Both effects are more obvious when the system is stationery and has been for a length of time. Very high static friction forces (and other problems) are most apparent under conditions of high humidity [I I], possibly due to meniscus effects [12] a n d / o r water plasticisation of the polymer [13]. The meniscus effect is possible since high friction is also observed if too much lubricant is present on the surface. It is more likely, however, that increased moisture levels instigate hydrolysis and plasticization takes place [14]. To overcome these problems polymer properties of high complex modulus and high hardness are required with an additional requirement for the binder to be hydrophobic.
where E* is the complex modulus of elasticity, Y the yield strength or hardness, cr the standard deviation of asperity heights and r~ the mean radius of those asperity peaks. If t) < 1.8 the contact is elastic and if ~0> 2.6 the contact plastic. Bushan [6] and Bushan and Doerner [7], by insertion of measured values into the above equations, found that typical head media contacts are elastic. From this the real area of contact is A r = 3.2 W/qJY. 3. Friction of particulate media
F ~ rAr where r is the interfacial shear stress and is not constant, but will increase with pressure and, in general, fall with temperature within the operating range of the media. Friction will also be speed dependent due to the time dependence of the elastic properties of the polymer and the increase in temperature with increase in velocity. With particulate media, a fatty acid/fatty acid ester type lubricant is normally incorporated into the bulk of the magnetic coating. The coating acts as a reservoir to preserve and replenish a surface lubricant film of about 2 or 3 molecular layers thick, and coverage as low as 10%. Taking into account the presence of the lubricant, the frictional force can be written as: F=A,{r(l-c~)+r,c~} where 7~ is the shear stress in the lubricant and c~ the fractional areal lubricant coverage. The lubricant acts as a classical boundary film, but will also produces some plasticisation at the polymer surface. Polymer surfaces, however, have low surface energy and this together with the low lubricant coverage means that the frictional force will not change substantially with lubrication. Hence, friction is largely determined by the properties of the magnetic layer. Thus a high hardness, high complex modulus material should be used to reduce friction, with due regard to other important material properties such as a suitable glass tran-
6. Interfaciai forces
In the absence of plastic or irrecoverable deformation, the force operative between the contacting asperities is adhesive. This adhesive force may be physical, involving Van der Waal bonds [15], or chemical involving electrostatic, ionic or co-valent bonding. Interfacial Van der Waal bond strengths [16] could be significant and co-valent
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bonding occurs between metals and polymers [17]. However, at polymer ceramic contacts ion-dipole bonds are most likely to give rise to the strong adhesive junctions. Within polymers atoms are bound in the chains by co-valent bonds (about 10 e V / a t o m ) and chains to chains by dipole-dipole interactions (about 0.2 eV/atom). Assuming ion-dipole attractions, head/medium adhesive junctions will be strong enough to produce shearing within the bulk of the magnetic layer. This process results in wear and transfer of material from media to head.
7. Transfer films During running transfer layers, which cannot be removed by solvent washing and variously described as friction polymer or brown stain, are invariably formed on the head. Loose debris is also produced and causes 'head clogging'. A major function of the head cleaning agent (HCA) is to limit the transfer layer thickness and reduce spacing losses. (A further function is to act as a bearing surface and effectively protect the soft media against wear). The HCA does not materially affect head clogging. Transfer films can substantially effect the friction and wear properties of junctions [18]. With polymers such as those used in binder systems which are non-linear and of relatively low crystallinity, transfer occurs as a discontinuous lumpy film with lumps of material varying from sub-micron to a few microns in diameter [19]. Once this layer has been formed contact is then substantially between the layer and the media. The description of the layer as a friction polymer [20], implies some chemical interaction, however, in extensive studies of such films we have found little evidence for this. Films consists mainly of iron oxide, which is consistent with particles being transferred directly from media to head due to adhesive wear and held in place by ion-dipole interactions. These transfer films are not formed for relative humidities of above about 45%, where water saturation of active sites on the head prevents the formation of adhesive bonds. The formation of transfer layers may be beneficial in reducing friction and wear of the head, but may introduce unacceptable spacing losses. Loose debris formation is always detrimental. Osaki et al. [21] describe head clogging as due to debris agglomerates picked up by the tape and transferred to the head where they accumulate in the gap region producing spacing losses. We have also seen such effects [22] which can also cause gap damage affecting high frequency response of the system.
8. Surface and contact temperatures Bushan [23,24] predicted temperature rises for typical head/tape contacts of 7 to 10°C, although he concedes that if magnetic particles are exposed the contact temperatures can range from 600 to 900°C. There is some correla-
tion of these results with IR measurements [25]. Our calculations, however, using a method due to Mekala [26], suggest contact or hot spot temperatures much higher than these. In this model the heat generated due to friction, is equated to the sum of the heat lost due to conduction into the head surface, conduction into neighbouring asperities and that due to forced convection to the air. From atomic force micrographs the surface was modelled into a network of nodes and at each of the nodes a conical asperity with random Raleigh distributions of volumes and base radii was assumed. From this model maximum contact temperatures for a particular linear tape system were calculated to be over 100°C. If all the load is carried instantaneously by one asperity, and we have evidence to show that this may occasionally occur, then very high temperatures are predicted and this can have profound effects on the mechanisms of wear and friction.
9. Wear mechanisms in h e a d / m e d i a contacts Wear is classified as adhesive, fatigue, abrasive, corrosive or erosive, with perhaps a few subsets of these wear forms. The mechanisms are not mutually exclusive. Forces between head and media are largely adhesive and these forces lead to transfer film and debris particle formation. Adhesive wear can be reduced by introducing a boundary lubricant between the opposing surfaces and, in this case, by reducing the polar molecule content of the binder. Sullivan and Sharma [27] found diskette lifetimes reduced by two orders of magnitude without surface lubrication, although friction values were relatively unaffected. When two solids are in sliding contact, maximum stress occurs not at the asperity junction, but below this surface. The depth depends on the radii of asperity contact (which is load dependent) and the coefficient of friction [28]. When an isolated asperity on either head or tape approaches an opposing asperity the subsurface is in compression, as the asperities separate it is in tension. Hence, for repeated contact, the area of subsurface immediately below the asperity contacts is subject to cyclic stressing and fatigue cracks will eventually appear. These cracks propagate under further cyclic stressing until wear particles are produced. There are two forms of abrasive wear; two body abrasion, when a hard rough surface slides over a softer one resulting in ploughing action and three body abrasion, when a hard particles becomes entrapped between two sliding bodies causing the same effect. An essential requirement is that plastic deformation should occur beneath a penetrating asperity or abrasive particle. From the work of Halliday [29], plastic deformation and hence abrasive wear will only occur in polymers if the opposing hard surface is very rough, ~ 12 Ixm, a roughness not experienced in magnetic media or heads. Thus two body abrasive wear should not occur and three body only if large agglomerates of debris become entrapped between head and
J.L. Sullican /Journal of Magnetism and Magnetic" Materials 155 (1996) 312-317 media. This latter effect is almost certainly the origin of scratches seen on tape and diskette surfaces after prolonged use. Abrasive wear rate falls substantially with particle/ asperity size below a critical dimension [30]. When the diameter is below about 1 Ixm [31] conventional abrasion is not operative, but microabrasion or polishing wear occurs [31 ] where the hardness of the abrading particles need not fulfil the Richardson criteria [32], but can be relatively low. For magnetic media, particle sizes greater than 1 p.m are not common and hence polishing is probably the most important mechanism in head wear. Conventional polishing normally occurs when small hard particles imbedded into an elastic backing medium are passed at high speed and low load over the surface to be treated. This is exactly the sitt, ation which occurs in head/media contact. For polymeric materials the abrasive wear rate is proportional to 1/Se [33], where S is the breaking strength of the material and e the elongation to break. The addition of the magnetic particles to the polymer will increase S, but reduce e and hence the filled polymer is no more resistant to abrasion than the polymer itself. For head media interactions the major corrosive elements are supplied by the atmosphere; oxygen can produce oxidative degeneration of the polymer surface and oxidation of elements of the head: and water vapour can produce hydrolysis and chain scission in the polymer and hydroxide formation on ceramics, glasses, ferrites and metals. Water vapour is a major cause for concern in head/media interactions, where high humidity can result in high friction, stiction and high wear. Elements present in particulate tapes also may possibly produce corrosive reactions, for example, phosphates from cross linking agents could effect metal elements in MIG or MR heads. We have. however, found no evidence of this, although there is some circumstantial evidence that CI from degeneration of CI containing polymeric binders may under some circumstances produce some corrosion. Erosive wear is the removal of material from a surface by the impact of solid particles or fluids. In the case of head/media interfaces only erosion due to solid particle impact may be of interest and then only to explain one particular phenomena. In certain heads, differential wear is observed where the metal is worn away at a greater rate than the surrounding ceramic, resulting in removal of metal to a depth where contact with the media cannot possible occur. It is possible that particles entrapped in the area produce erosive wear. The amount of wear will depend on the kinetic energy, particle density, impact angle and particle size [34,35]. The same material properties are required for the resistance to erosive wear as for abrasive wear. 10. Particulate media wear
Sharma [9] and Sullivan and Sharma [27] studied the durability of particulate diskettes under a variety of condi-
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tions of lubrication. Extensive optical microscopy, SEM and XPS examinations were employed to isolate the mechanisms of wear. It was found that wear proceeded in three stages; initial smoothing of a wear track leading to plastic deformation with little or no material removal: removal of laminar particles or flakes of thickness much less than the magnetic coating and finally a catastrophic phase where large areas of the wear track were destroyed. Usually wear will not progress further than stage one, but this stage is often visible to the naked eye in a well used media. Other investigations on both tape and diskettes have shown that the processes leading to wear and to ultimate catastrophic failure of all such media is similar. Although contact should be elastic, high localised contact temperatures can lead to polymer softening and plastic flow in the first stage. The second stage is the result of continuous cyclic stressing of the media which leads to the initiation of sub-surface fatigue cracks which grow to the surface and produce laminar wear particles, a process analogous to that described by Suh [36] for metals. Wear debris then becomes entrapped between the head and media producing three body interaction and an avalanche failure mechanism. These findings are partially confirmed by recent work of Osaki et al. [37]. The importance of adhesion and fatigue in polymer wear has been described by Omar et al. [38]. 11. Head w e a r and differential head wear
Recording heads consist of a body of material which is hard. ceramic, ferrite or a combination of these and with, for example, MR heads or MIG heads some metal element. Allowable wear may be small over the life of the component. Head materials generally have high elastic modulus, high fracture strength, high chemical stability and high thermal stability, but they are also very brittle, lnterfacial adhesion is low and consequently the shear strength of the junction is low. thus adhesive wear of these surfaces is very unlikely and indeed we have never found any evidence for adhesive transfer from head to media. The most common form of wear in these materials is due to abrasion/micro-abrasion and fatigue, although they are not ideally brittle and plastic flow can occur. Reaves and Sullivan [22] studied the wear of VHS video heads of manganese zinc ferrite and glass. The wear rate of these heads was dependent on the abrasivity on the media. Chromium dioxide tape (Knoop hardness, 225 Mpa) produced a wear rate an order of magnitude higher than an iron oxide tape (hardness 118 Mpa) of similar particle loading. For iron oxide tapes with increasing levels of HCA, wear increased almost linearly with concentration [39]. In the case of the chromium dioxide tape the worn surface was very smooth and typical of a polishing wear process. With the iron oxide tapes in addition to the general smoothing of the head surface, scratches and groove with sizes of up to about 0.3 ~m wide were evident on
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head surfaces caused by plastic deformation. However, no particles exist within the media large enough to cause deformation of that magnitude [12]. Hence, they are either due to three body abrasion due to agglomerates of particles from the media (Reaves and Sullivan used transmission electron microscopy to show that head cleaning agents agglomerates were present in the magnetic layers) or due to isolated large asperities in the media surface similar to those identified by Hahn [40]. In either case all the load would be supported momentarily on one contact and this could lead to very high temperatures being generated. This would easily explain the plastic deformation observed. The mechanisms outlined above are operative in glass, ceramics and ferrites, with ferrite single crystals, however, there is the additional effect of crystal orientation. The wear in different crystallographic directions in ferrites is highly anisotropic and the anisotropy is dependent on the magnetic particle [41]. For example, the effect is more pronounced for iron oxide than for chromium dioxide. Wear rates of heads are important, but of equal importance are differential head component wear rates which introduce spacing losses. When using a combination of different materials in recording heads it is very difficult to exactly match wear rates. The final choice of materials is usually a compromise, but even with careful choice of materials, differential wear still occurs. An example already referred to is the hollowing out of the MIG heads and the reason for this is still under investigation by ourselves and others. Other forms of differential wear occur in linear recording heads which have elements of ceramic, glass and ferrite. Under conditions of high humidity the ceramic may wear at a rate greater than the other two elements. The effect of humidity on ceramics is a very complex problem [42]. There may be a tribo-chemical reaction which produces hydroxides, which may either increase or reduce wear. In the case of heads we examined, which used a two phase titanate ceramic, enhanced ceramic wear was not due to a chemical, but to a mechanical process caused by water molecule ingress. This effect is due to the non-compressibility of water and is well known in the pitting of metals [43]. Sintered ceramic materials are always porous. If the grain size is small, water under hydrostatic pressure can enhance crack formation and propagation in the ceramic surface. This leads to increased rates of removal of the surface due to microfatigue which enhances polishing wear. The problem can be reduced by using more abrasive tapes and suffering higher overall wear [44], but this could result in an unacceptable reduction in head life, or the problem can be overcome by using hard coatings on heads [45], but in many applications the resulting spacing loss would not be acceptable. 12. Metal evaporated (ME) media The majority of this work has been devoted to particulate media and this equitably reflects the usage, however,
ME media which has higher effective magnetisation is of increasing importance. Flexible ME media presents its own unique tribological problems. Unlike particulate media, surface coverage of the lubricant must be complete, must remain so for the projected lifetime of the media, must afford complete corrosion protection and should not be more than about 5 nm thick. ME media does not have the advantages of porosity and surface chemistry of the particulate media, thus the demands on the lubricant present a severe challenge. In addition the thin metal films are far less compliant than particulate filled polymer films and should not be subject to strain if cracking and catastrophic failure are to be avoided. 13. Friction and wear of ME media ME media relies on a complete boundary lubricant film for its successful operation. The surface of the evaporated metal film is oxidised and sometimes carbon coated to give additional protection in the case of momentary breakdown of the boundary lubricant. This provides a low surface energy contact which can survive occasional head/media asperity interaction, but the lubricant must be capable of self healing any breach if high wear is not to ensue. Friction is almost totally accounted for by the force required to shear the boundary lubricant film. Considering the plasticity index of Greenwood and Williamson [5] and the surface roughness of the media and heads, the deformation of the asperity contacts on the tape should be elastic-plastic and the dominant wear mechanism at both media and head is one due to fatigue. Hempstock and Sullivan [46] have studied the wear of commercial ME Hi-8 video tape, correlated this to signal performance and identified the wear mechanism. Little wear is seen on the sample surfaces for relatively long running times. Wear particles are then seen to appear corresponding to the start of surface delamination at which point a substantial increase in signal errors is observed. The process proceeds until catastrophic fatigue cracking and delaminative failure of the film occurs. This corresponds to depletion of lubricant, measured by XPS, in the area of the scar. The mechanism is classic Sub [36] delamination. A gradual breakdown of the lubricant film allowing head-metal contact to occur will greatly accelerate the process. 14. Conclusion Much of the pioneering work in tribology may be used to elucidate mechanisms of friction and wear in these systems, however, the use of classical ideas does not mean that the phenomena have been or can be adequately explained in these terms and a great deal of speculation still remains. As far as tribology of these systems is concerned the industry is in a critical stage where good fortune can no longer be relied upon. There is an urgent need for more
J.L. Sullil'an / Journal ¢?/'Magnetism and Magnetic Mate rials 155 ( 19961 3 / 2 -317 effort to be m a d e in u n d e r s t a n d i n g the basic m e c h a n i s m s of contact in t h e s e very c o m p l e x s y s t e m s in order that projected recording densities m a y be achieved in practical situations.
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[19] K. Tanaka, I. Uchiyama, Wear 23 (19771 153. [20] J.L. Lauer, W.R. Jones, The Tribology and Mechanics of Magnetic Storage Systems, eds. Bhushan and Eiss, Vol. 3 (ASLE, IL), 14. [21] H. Osaki. J. Kurihara, T. Kanon, IEEE Trans. Magn. 30 (19881 1491. [22] L.M. Reaves. J.k. Sullivan, Tribology Internal, 23 (19901 251, [23] B. Bushan, J. Trib. Trans. ASME 109 ( 19871 243. [24] B. Bashan, J, Trib. Trans. ASME 109 (19871 252. [25] R. Gulino, S. Blair. W.O. Winer, B. Bhushan..I. Trib. Trans. ASME 108 (19871 29. [26] D. Mekala. private communication. [27] J.L. Sullivan, P.J, Sharma, J. Phys. D 25 (19921 321. [28] G.M. Hamilton, L.E. Goodman, J. Appl. Mech. Trans ASME 33 (19661 371. [29] J.S. Halliday, Proc. lnst Mech. Eng. 169 (19551 221, [30] E. Larsen-Basse, Wear 19 (19721 35. [31] E. Rabinowicz, A. Mulls, Wear 8 (19611 381. [32] R.D.C. Richardson, Proc. Inst. Mech. Eng. 182 (1967)410. [33] J.K. Lancaster. Plastics and Polymers 41 (19731 112. [34] I. Finnie. W e a r 3 (19601 87. [35] S. Aoyama, M. Kishimoto. IEEE Trans. Magn. 27 (1991) 791. [36] N.P. Suh, Wear 44 (19771 I. [37] H. Osaki, E. Oyangi, H. Aomuma, T. Kanou, J. Kurihara, IEEE Trans. Magn. 28 (19921 76. [38] M.K. Omar, A.G. Atkins. J . K Lancaster, .I. Phys. D 19 (19861 177. [39] L.M. Reaves, J.L. Sullivan. J. Phys. D 25 ( 19921 328. [40] F.W. Hahn. IEEE Trans. Magn. 20 (19841 918. [41] S.E. Kadijk, A. Broese van Groenou, Wear 139 (19901. [42] T.E. Fischer. W.M. Mullins, J, Phys. Chem. 96 (19921 5690. [43] J.L. Sullivan. M,R. Middleton, ASLE Trans. 28 ( 19851 431. [44] A. Broese van Groenou, H.J.J.C. Meulenbroeks, M. de Jong. 1EEE Trans. Magn. 26 (19901 153. [45] V. Zieren, M. de Jong. A. Broese van Groenou, J.B.A. Zou. P. Lasinski, G.A.S.M. Theunissen. IEEE Trans. Magn. 3(I (1994) 340. [46] M.S. Hempstock, J.L. Sullivan. J. Magn. Magn. Mater. 155 (19951 323 (these Proceedings),