Origins of friction and wear of the thin metallic layer of metal evaporated magnetic tape

The origins of friction and wear of the thin metallic layer of metal evaporated (ME) magnetic tapes were determined at an asperity level using a modified commercial video cassette recorder (VCR) as a tape transport, magnetometer and tribometer. The VCR was instrumented to measure friction force between the rotary heads and tape, head-to-tape spacing to nanometer vertical resolution, and sub-microsecond duration signal dropouts caused by loose wear debris particles in the head-to-tape contact interface. ME tapes with various surface texture and the same lubricant thickness were studied, as well as tapes with various lubricant thickness and the same surface texture. To measure the very subtle origins of wear, tapes were recorded and then played back only one time, and surface topography was measured with an atomic force microscope to see If the tapes were worn. For fixed asperity size, increased asperity areal density or more numerous asperities reduced the plastic deformation that was observed at low asperity areal densities. Having more asperities increases the load bearing area near the highest point on the tape. This prevents high strain and associated plastic deformation of high asperities. Thus, spreading the load over more asperities reduces plastic deformation, and deformation becomes primarily elastic. Tapes that exhibited plastic deformation had higher friction force than the tape that exhibited elastic deformation. A simple model was developed which suggests that high friction force is not due to the energy required to plastically deform the asperities, but arises from high real area of contact during sliding for the tapes that plastically deform which is consistent with plasticity theory in the presence of a tangential force. Tapes that plastically deform do not exhibit higher signal dropout frequencies, and this implies that plastic deformation is not associated with loose wear debris particle formation. For smooth tapes with high isolated asperities, cohesive material failure of asperities due to shear stress brought on by friction force resulted in loose wear debris generation, high plowing component to friction force and interface instability. A thicker lubricant layer reduced friction force by reducing the shear strength of adhesive contact junctions. (C) 1999 Published by Elsevier Science S.A. All rights reserved.