Wear mechanisms of sandwich-type video heads

Abstract

Differential wear between the various material components of composite magnetic recording heads is observed upon running against the magnetic medium. A developing level difference between the magnetic material and the medium bearing surface leads to signal degradation, exponential with the difference and the signal frequency. Experimental evidence is presented for a three-body erosion mechanism in the case of sandwich heads employed in commercial helical scan, Hi-8 video tape recorders. Two sintered ceramic substrates, CaTiO 3 and composite Zr-doped CaTiO 3 , in combination with three soft magnetic materials, amorphous CoNbZr and the nanocrystalline iron alloys FeNbSiN and FeTaN, have been studied. They have been studied in conjunction with commercial metal evaporated (ME) and metal particulate (MP) tapes. Level difference has been measured by interferometry and AFM (atomic force microscopy), the latter also observing level differences between the various components of the sandwich: soft magnetic metal, ceramic laminations and the metallic bonding layer. AFM shows a pull-out of particles from the ceramic substrate with similar sized particles observed on the track. Scanning Auger micrographs have shown the transfer of ceramic particles on to the track and of metal on to the substrate. Wear rates and wear mechanisms have been identified for the materials in isolation by studying single-material dummy heads. This has been identified as plastic deformation controlled microabrasion, transforming to grain pullout in the crystalline materials in some instances. It is speculated that these pull-outs are responsible for the wear of the soft metallic track when this is out of contact with the tape. The evidence for chemical wear is inconclusive. The involvement of transfer material from the tape to the head is speculated upon.