Hiroyuki Osaki

Wear mechanisms of metal-evaporated magnetic tapes in helical scan videotape recorders

The wear processes of metal-evaporated magnetic tapes by rotary heads in videotape recorders were investigated by continuous microscopic observations of the rubbing surfaces of magnetic tapes and rotating heads using stroboscopes. It was found that the magnetic layers of the metal-evaporated tapes were worn out by two different wear mechanisms, depending on the contact pressure between the tapes and rotary heads. Under higher contact pressure, the frictional force between the tape and the rotary head increased with an increasing number of head traces, and the magnetic layer was partly peeled by the rotary head as soon as the frictional force exceeded a critical value. Under lower contact pressure, the magnetic layer of the tape was worn gradually in a mild wear process and eventually a part of it fractured because of decreasing mechanical strength with a decrease in its thickness. In each case, the large metal particles from the fractured magnetic layer transferred onto the rotary head, and in turn provided a higher frictional force to the magnetic layer of the tape and initiated a severe wear process. >

Mechanisms of head-clogging by particulate magnetic tapes in helical scan video tape recorders

The head-clogging processes of video heads by particulate magnetic tapes in helical scan video tape recorders were investigated by continuous microscopic observations of the rotating video heads using a stroboscope. It was found that most wear debris of typical tapes is expelled out of the rubbing surface of a video head and piles up on non-contact area of the exit side of the head. The wear debris which forms an agglomerate contacts a tape surface, and is deposited on the tape surface. The following head picks it up on the rubbing surface (contact area) where the magnetic gap is located. A spacing loss occurs with a consequential decrease in reproduced voltage. >

Wear mechanism of particulate magnetic tapes in helical scan video tape recorders

The wear process of particulate magnetic tapes by rotary heads in video tape recorders was investigated. It was found that the plastic flow of the polymer binder in the top layer on the surface asperities (low swellings) of a magnetic layer of a tape caused by head rubbing forms the smoothed surface. The increase in the real contact area on these surfaces asperities by smoothing creates a frictional force increase up to the shear strength of the asperities. When the frictional force exceeds that strength, the asperities are fractured by the shear stress. The wear debris from many fractured asperities gathers into large lumps and retransfers onto the tape surface. The increased frictional force between a rotating head and retransferred lumps of wear debris with an increasing number of head traces causes a larger shear fracture inside a magnetic layer under the retransferred lumps. Wear debris from the scars made by large shear fracture, combined with retransferred lumps, scratches the tape surface as a prow. >

Role of surface asperities on durability of metal-evaporated magnetic tapes

The effect of the surface asperities of a metal-evaporated magnetic tape on its durability in the still mode of video tape recorders was investigated. It has been already found that a thin metal magnetic layer of a metal-evaporated tape is shear-fractured (debonded) at the interface between itself and the base film, or in the subsurface of the base film. Since the fracture always started at a peculiar value of the frictional force for each tape, it was presumed that the fracture is triggered by the frictional force introduced by a rotating video head. The first series of arc-shaped cracks which confirms that the tape surface was subjected to high frictional force by a head directly was found. Furthermore, it was found that the shear fracture (debonding) is dominated by the average value of the frictional force per unit area between the tape surface and the rotating head. Because the asperities on the tape surface reduce the average value of the frictional force per unit area by decreasing the ratio of real contact area to apparent contact area, they can improve the durability of metal-evaporated tapes in the still mode. >

Wear mechanisms of Co-Cr sputter deposited magnetic tapes in helical scan video tape recorders

The wear mechanisms of Co-Cr sputter-deposited tapes for perpendicular magnetic recording by rotary video heads in the still mode of video tape recorders were investigated by using two kinds of experimental apparatus. One is a rotary-head-type friction tester in which the surface of the rotating head can be observed continuously by a microscope with a stroboscope synchronized with the head rotation; the other is a scratch tester in which the scratched surface of a tape specimen can be observed in situ through the transparent hemispheric slider. It was found that the wear debris of the Co-Cr layer transfers onto the head surface, forms the asperities on the head, and scars the tape surface, with the increasing amount of head tracing. A carbon protective layer can prevent the Co-Cr layer from transferring onto the head and being subsequently scarred. However, the carbon protective layer may be peeled by the rotating head when the frictional force increases up to the interfacial shearing strength between the carbon layer and the Co-Cr layer. >

Flexible media : recent developments from the tribology point of view

With the increase in the recording density of hard disk systems, the demand on the increase in the data recording capacity of tape drive systems as back-up systems is increasing. One of the advantages of the tape drive systems is high volumetric recording density, which is obtained by high areal recording density and thin tapes. The areal recording density can be increased by introducing high performance tapes, such as metal evaporated tapes, with superior magnetic characteristics and smooth magnetic surface to reduce the spacing loss. However, a smoother surface often produces a higher friction coefficient, which could cause tape damage by rotary heads and unstable tape runnability in the tape drives. Adoption of MR heads to tape drive systems is also effective in increasing the areal recording density. However, the wear allowance of the MR heads (shield type) is much smaller than that of the inductive heads. Thinner tapes show lower mechanical stiffness in general, which could cause damage to tape edges easily during tape transportation. In the second or later generation of tape drive systems, a thinner tape is often introduced. These thinner tapes should also have the interchangeability of the original thickness tape. New materials for a base film, such as PEN (polyethylene naphthalate) or aramid in which the elastic moduli are larger than those of PET, are required for thinner tapes. It was found that these side effects by the increase in the volumetric recording density can be improved by tribology. The tribological improvements from the drive design side is very important, as well as from the tape design side.

Tribology of videotapes

Abstract Increased recording density in magnetic recording has been achieved by introducing newly developed magnetic tapes, such as metal particulate tapes and metal-evaporated tapes. As a loss in magnetic recording increases with spacing between a magnetic tape and a magnetic head, these are obliged to contact each other. A tape also rubs tape-transportation elements in videotape recorders (VTRs), such as the stationary drum and guide posts. Thus it is not too much to say that the development of new magnetic tapes for practical application depends on tribological research. In this paper, tribological analyses for the improvements of durability and runnability of metal particulate tapes and metal-evaporated videotapes are reviewed.

Recent research of tape/drive tribology

Decreasing track width and tape thickness to increase the volumetric recording density of helical scan-tape-drive systems, which is suitable for higher volumetric recording density, will result in the failure of tracking. The displacement of tape forwarding position, which causes failure of tracking, is caused by static friction coefficient between a tape and a roller guide. It was found that surface roughness and materials of roller guides are very important to reduce static friction coefficient.

Tribology of metal evaporated tapes—for improvement of recording density

Increased recording density in video tape recorders and tape drives for data storage has been achieved by the increase in areal recording density and the decrease in tape thickness. Areal recording density can be increased by introducing high performance tapes, like metal evaporated tapes, with superior magnetic characteristics and smooth magnetic surfaces to reduce the spacing loss. However smoother surfaces often produce a higher friction coefficient, which could result in tape damage by the scanning heads and unstable runnability of tapes in VTRs or tape drives. Also thinner tapes show lower mechanical stiffness in general, which could result in damage of the tape edges during tape transportation. Superior durability and runnability are thus required of high performance tape in addition to magnetic characteristics, in spite of the trend towards smoother surface and thinner tapes. Therefore the development of practical new magnetic tapes requires research into their tribology. It was found that the durability and runnability of metal evaporated tapes with smoother surfaces can be improved by DLC coating, and that the edge damage of thinner tapes can be eliminated by decreasing the static friction coefficient, but not the kinetic one. Though the durability and the runnability of metal evaporated tapes themselves have been improved from the tape design point of view, as mentioned above, further improvement may be expected by integrating tape design with that of the VTR/tribo-elements tape drive design and thus further increasing recording density in the future.

Effect of temperature, humidity and crystal directions on wear of rotary heads

Abstract In the advanced tape drive systems which achieve a higher recording density, the wear of rotary heads should be decreased to obtain a higher reproduced output signal by decreasing the ‘Gap depth’ of heads, keeping enough life time. By using the heads made by two kinds of crystal directions of Mn–Zn ferrite and metal evaporated tapes with DLC coating, temperature and humidity dependences of head wear are investigated. The wear rates of both crystal direction heads increase with the decrease in temperature. The wear rate of a (110) head increases very quickly at low temperature, compared with a (100) head. These temperature dependences of head wear are caused by the temperature dependence of the elastic modulus of the tape and the temperature dependence of the friction coefficient between the tape and the head.