ROLE OF SURFACE ASPERITIES ON DURABILITY OF METAL-EVAPORATED MAGNETIC TAPES

The role of the surface asperities of a metal-evaporated magnetic tape on its durability in the still mode of VTR's was investigated. It had been already found that a thin metal magnetic layer of a metal-evaporated tape is either shear-fractured (debonded) at the interface between itself and the base film, or in the subsurface of the base film in the previous paper. 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 were 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 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.