The tribology of laminated magnetic recording heads

Abstract

This thesis investigates the mechanisms that lead to pole tip recession (PTR) in laminated magnetic recording heads (also known as "sandwich heads"). These heads provide a platform for the utilisation of advanced soft magnetic thin films in practical recording heads suitable for high frequency helical scan tape recording systems. PTR results from a differential wear of the magnetic pole piece from the tape-bearing surface of the head. It results in a spacing loss of the playback or read signal of 54.6dB per recording wavelength separation of the poles from the tape. PTR depends on the material combination used in the head, on the tape type and the climate - temperature and relative humidity (r.h.). Five head materials were studied: two non-magnetic substrate materials- sintered multi granular CaTi03 and composite CaTi03/ZrTi04/Ti02 and three soft magnetic materials- amorphous CoNbZr, and nanocrystalline FeNbSiN and FeTaN. Single material dummy heads were constructed and their wear rates measured when cycling them in a Hi-8 camcorder against commercially available metal particulate (MP) and metal evaporated (ME) tapes in three different climates: 25°C/20%r.h., 25°C/80%r.h. and 40°C/80%r.h. X-ray photoelectron spectroscopy (XPS) was used to examine changes the head surface chemistry. Atomic force microscopy (AFM) was used to examine changes in head and tape surface topography. PTR versus cycling time of laminated heads of CaTi03/ZrTiO4/Ti02 and FeTaN construction was measured using AFM. The principal wear mechanism observed for all head materials was microabrasion caused by the mating body - the tape surface. The variation in wear rate with climate and tape type was due to a variation in severity in this mechanism, except for tape cycling at 40°C in which gross damage was observed to be occurring to the head surface. Two subsidiary wear mechanisms were found: third body scratching (all materials) and grain pullout (both ceramics and FeNbSiN). No chemical wear was observed, though tribochemical reactions were observed on the metal head surfaces. PTR was found to be caused by two mechanisms - the first differential microabrasion of the metal and substrate materials and which was characterised by a low (~10nm) equilibrium value. The second was by deep ploughing by third body debris particles, thought mainly to be grain pullout particles. This level of PTR caused by this mechanism was often more severe, and of a non-equilibrium nature. It was observed more for ME tape, especially at 40°C/80%r.h. and 25°c/20%r.h. Two other phenomena on the laminated head pole piece were observed and commented upon: staining and ripple texturing.