FRICTION INDUCED IRREVERSIBLE STRETCHING OF SUBSTRATE FILMS BY RECOR- DING WITH A CATAMARAN GLIDER ON THIN FILM FLOPPY DISKS

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1 Journal of the Magnetics Society of Japan Vol. 15 Supplement, No. S2 (1991) 1991 by The Magnetics Society of Japan FRICTION INDUCED IRREVERSIBLE STRETCHING OF SUBSTRATE FILMS BY RECOR- DING WITH A CATAMARAN GLIDER ON THIN FILM FLOPPY DISKS Hans-Peter SCHILDBERG and Hartmut HIBST BASF Aktiengesellschaft, Ammoniaklaboratorium, D-6700 Ludwigshafen, Germany The tribological interaction between a commercial floppy disk catamaran glider and flexible magnetic thin film media is investigated on a tribometer under floppy disk drive conditions. Apart from the usual wear scars which develop in the metallic thin film due to abrasion and microploughing, one observes elevations of the medium (up to 15 Ilm) along the track of the head. The profile of these elevations in directions perpendicular to the track reflects the two rails of the catamaran glider and strongly depends on the thickness of the polymer substrate, its elastic properties and on the frictional force between head and medium. The observations are explained by the distribution of shear forces in that region of the polymer substrate which is directly underneath and besides the rails of the sliding R/W-head. Consequences for magnetic recording, the selection of substrate films and the head/tape interface are considered. INTRODUCTION Magnetic thin film media on flexible substrates allow for substantially higher storage densities than particulate media. However, severe tribological problems are encountered with the employment of these media in recording devices of the helical scan or floppy drive type. For recording with the former type one has recently managed to successfully employ a Co-Ni-O thin film tape (Video Hi8). For the latter type, where the tribological requirements are much stricter, one has not yet succeeded in introducing a commercial thin film floppy. In this paper we report about a new, detrimental tribological phenomenon which manifests itself by irreversible deformations of the substrate film along the track of a floppy head due to the frictional forces between head and medium. EXPERIMENTAL A conventional tribometer is used to investigate the interaction between head and thin film medium (R/W-head: commercial Mitsubishi catamaran glider from 5.25" floppy drive, 135 tpi; head load: 0.1 N; spinning speed: 300 rpm). The media (Cr/Co-Ni-Cr or Co-cr deposited on PI or PET, thickness: 9 to 50 Ilm) were lubricated with Fomblin-AM2001 by dip coating. After head passes the polymer substrate film exhibits distinct deformations parallel to the track (fig.1).the highest elevations are observed close to the edges of the two rails of the catamaran glider (fig. 2). For thinner films the bumps come out more clearly resolved. It is remarkable that these elevations encompass the entire foil (fig. 3). The distinct drop of the friction coefficient in the interval of loo to 300 revolutions (fig. 4) indicates that the deformations develop during the first 300 revolutions. Thereafter the contact area between the rails of the head and the lubricated medium is substantially reduced and a much lower friction coefficient is obtained. At this stage any attempt to record onto the medium or to retrieve old information is absolutely hopeless, because the distance between the pole tips of the R/W-head and the surface of the thin film medium is of the order of several micrometers. INTERPRETATION Due to non-zero friction between head and medium the part of the medium directly under the rails of the catamaran glider is dragged in direction of head movement until the shear forces that develop immediately in the film attain a value capable to counterbalance the frictional force acting on the film beneath the rails (fig. 5). For a detailed explanation we resort to a onedimensional model (fig. 6). We imagine the medium as being of infinite length in x-direction and of large width in y-direction. Furthermore, to model the elastic properties of the medium (which constitutes a continuum in the theory of elasticity), we regard it as being composed of rigid stripes of infinitesimal width, oriented in x-direction. Between two adjacent stripes we imagine springs that allow for a relative displacement in x-direction (shear). The outermost stripes in y-direction are regarded as being fixed in space (i.e. cannot be displaced in x-direction). A rail of finite width in y-direction and infinite length in x- direction is placed onto the medium. With the rail being still at rest we draw a line across the medium to mark the original x-position of all stripes. After the head has started sliding at constant speed in x-direction, the x-position of the stripes will change as indicated by the position of the

2 a) a) l E ::1.: bt.:1 ;: so '" j25. OlD SOU.O (10 2$ relative distance ill..,adial direction [Illll] C 2.o ,-,-, ,...,...,-., ,.-,-,--..--,--.-,,...,!4.0.l!.;;03.0 -; 2.0.t;.g 1.0 o.o,f;:/7--'--'-";;ioo<l;;;;.oi500;;;;.o--':"-";/ ""'.o/':":"':"";;;2500i;:o.o--';3000;;;i;:o'-.o 0.0 relative distance in radial direction {pm}, Fig. 2: Scans with a stylus profilometer over the surface of the media shown in fig. 1. The horizontal scale of the schematic cross section of the catamaran glider (a) is identical to the horizontal scale used for the profiles (b) and (c). c) 65.0 metallized side of PI-foil (top side) 'i' : Polyimide bulk material, 50 m thick 15.0 R129mm Fig. 1: Optical micrograph of the surface of thin film samples after head passes performed with a catamaran glider at different radii R. Apart from the usual small wear scars one observes distinct elevations in the medium. (a): catamaran glider. (b): PI(Upilex R, 50 Ilm)/C<ry8Cr22(192nm) (c): PET(36VXR502, 9Ilm)/Cr(200nm)/ C062.5Ni30Cr7.5(50nm) rear side of PI-foil toe/ative distance in radial direction [um] Fig. 3: Cross section of the medium shown in fig. l(b) as determined by two scans with a stylus profilometer on the top and rear side of the medium

3 number DJ Tel'oJution Fig. 4: Friction coefficient recorded for the sample shown in fig. l(b) as function of revolutions. Note the unusual scale of the horizontal axis. under the rail. The shear forces between stripes 2 and 3 and between stripes -2 and -3, i.e. F s,2 and F s,-2, are of course smaller because they only need to counteract the forces Ff exerted on the stripes enclosed between stripes 3 and -3. Exactly under the centre the shear forces are zero, because stripe 1 and - 1 exhibit no displacement relative to each other. Thus the shear forces in the substrate film linearly increase from a point under the centre of the rail to its edges. The slope of this linear function is independent of the width of the rail but only depends (linearly!) on the friction coefficient. Consequently, the broader the rail, the larger the shear forces in the film under the edges. Outside the rail, i.e. starting with stripe 4 and - 4, the shear forces stay constant in the purely one dimensional model. However, in reality the rail of the head is only of fmite length. This is why in real systems the shear forces decay to zero with increasing distance from the track. Their maximum value is thus attained under the edges of the rail. Any shear force causes the medium to be stretched. Upon relaxation after the head has passed by, the polymer substrate film never fully contracts to its original shape but exhibits some irreversible Fig. 5: Due to friction between the rail of the R/Whead and the surface of the thin film the medium is dragged in direction of the sliding rail (curved arrows). line segments on each stripe (fig. 6). This displacement is due to non-zero friction between rail and medium which causes the surface of each stripe under the sliding rail experiencing the same force per unit length F f dragging the stripe in x-direction. Of course, the surfaces of all9ther stripes not covered by the rail do not experience this force. The forces Ff will be counterbalanced by shear forces F s that act between adjacent stripes. (Here we define shear force as integral of the shear stress over the thickness of the substrate film. The resulting unit is hence force per unit length). The shear forces between the first stripes outside the rail (nos. 4 and -4) and the outermost stripe under the rail (nos. 3 and -3), i.e. F s,3 and F s,_ 3, have to counteract all forces Ff acting on the stripes F s, -s s; F s, -4 'd F s, -4 S; F s, -3 F s,-3 = 3 F f.. F s,-2 = 2 F f Fs,_1 =lf f F s,o =0 F s,l = 1 F f Fs,2 =2F f F s,3 =3F.. F s,4 S;F s, 3 F s,5 S;F s,4 J.. x direction of head movement relative to the tape Fig. 6: One dimensional model to explain the elevations in the substrate film due to shear stress induced by friction between head and medium

4 component of stretching. In a cyclic process as in a floppy drive (which corresponds to our tribometer) these components accumulate. Because the shear forces attain their highest values at the edges of the rails, the irreversible components of streching will also be largest in these region. Since the medium far away from the circular track remains unchanged in its lateral dimensions, the substrate film bends upwards along the track to accommodate the lateral increment. This upward direction is due to the fact that the shear stress is not constant over the thickness of the substrate film, but actually decreases with distance from the surface and thus a correspondingly less lateral increment deep in the fllm is brought about. (The absolute difference between the shear stress close to the surface and close to the rear side of the film will be rather small, because the thickness of the common substrate fllms (9 to 75 f.1m) is very small compared to the width of the rail). Bending upwards takes advantage of this variation over the thickness of the film, because the curvature in the highest point of the peaks requires the largest lateral stretching at the surface and less lateral stretching or even compression at the rear side. CONCLUSION Since the friction induced irreversible deformations of the substrate fllms are caused by irreversible stretching, the following measures would alleviate this problem: a) use of substrate fllms with higher E-moduli b) use of thicker substrate fllms c) use of heads with narrower rails d) realization of lower friction coefficients a) The stiffer the substrate film, the smaller are in general the irreversible components when being stretched. Fig. 7 shows two samples that were prepared and investigated under identical conditions. The sample with twice the large E-modulus exhibits elevations only about half as high. b) The magnitude of the shear stress between two adjacent stripes in our one dimensional model is of course inversely proportional to the thickness of the substrate fllm, because the integral of the shear stress over the thickness must be equal to a force per unit length solely determined by the friction coefficient and the position in y-direction (fig.6). This means that media based on thicker substrate films (all other parameters assumed equal, which entails in particular identical friction coefficients) are less prone to the irreversible deformations discussed in this paper. Alternatively one could say that if we intend to produce a thin fllm medium with the substrate film 20.0 r--r-.,.-,--,..-..,.--r-,...,-.,.-.-,..--r-.., ;-"'-'--"'-",,--, i",..",,,,,,,,,i '.,J :: ] 1: 5.0 PI (Upilex R), 501lm, E modulus: 3400N/mm L...=::::..-'-..l..-.J.-'---'--.J...L-'--'--.J.-...L--"-:::::.=-... loooo.o relative distance in radial direction [urn] PI (Upilex S), 50llm, ' 15.0 E-modulus:..:::. 6200N/mm2?o ] ',::: 5.0 '" : III III 1Ii, :. Ill" \,\ \I O..LO -===:i::...j'-25..l.00-.d.j.-'---'--.j.5-dd.ld-.d'--'---'---'::75::00.0:"-=1-:-: relative distance in radial direction [urn] Fig. 7: Comparision of the change of the surface proflle of two thin film media [PI(50f.1m)/Cr(200nm)1 C062.5Ni30Cr7. 5 (50nm) ] after revolutions with a catamaran glider. The media differ only by the E-moduli of their substrate fllms. The magnetic coating, the lubricant (Fomblin AM2001) and the experimental conditions are identical. thickness being reduced by a factor of 2, say, we should take care to get the friction coefficient down by the same amount in order to keep the shear stress at the same level as for the thicker medium. c) Since the maximum shear force occurring depends linearly on the width of the rail, the need for employing heads with rails as narrow as possible is obvious. " d): Since the slope of the function describing the linear increase of the shear forces in the substrate fllm depends linearly on the friction coefficient, lubricants and surface topography of the media (e.g. nodules as for ME-tape) have to be optimized for yielding friction coefficients as low as possible. The simplest way to reduce friction, namely by increasing the Ra-value of the medium, is not operational for thin film media, because it entails increasing distance loss and thus jeopardizes the goal of higher storage densities we just attempted to achieve by switching from particulate to thin film coatings

5 The reasons for commercial particulate floppy disks not exhibiting the elevations described in this paper ly in the thickness of their substrate film (75 instead of 50 m) and in their low friction coefficient of about For the sample of fig. 1 b the initial friction coefficient was close to 1.0, probably due to the extreme smoothness of the substrate film (Ra = 3 nm). SUMMARY The circular elevations observed in thin film floppy disks when recording with a catamaran glider are due to friction induced shear forces in the substrate film. These forces increase linearly from a point under the centre of the rail to its edges. The slope defining this increase only depends on the friction coefficient in a linear manner. The maximum shear forces and hence maximum components of the induced irreversible stretching in the substrate film occur under the edges of the rails. The increment in the lateral dimensions of the substrate film along the track of the head is accommodated by forming elevations. These deformations are fatal to recording or retrieving data. They can be reduced by using substrate films with higher E-moduli, by increasing the thickness of the substrate films, by realizing lower friction coefficients and by using heads with narrower rails