Role of water vapor on the wear of MnZn ferrite heads sliding against magnetic tapes

Abstract

Friction and wear behavior of metallic (mu-metal) and ceramic (MnZn ferrite) head materials sliding against CrO 2 magnetic tape was investigated. Accelerated tape drive tests were conducted in air at various relative humidity levels. Knoop indentation and nano-scratch techniques were used to measure wear. Friction force was measured simultaneously using a strain gage load beam. To investigate static fatigue induced fracture at high humidities, static tests were conducted on MnZn ferrite in an environment chamber. The head specimens were observed optically and with a scanning electron microscope (SEM). The results showed a strong influence of humidity on the tribological behavior of the materials tested. Wear mechanisms of metal and ceramic head materials were found to be different. In the case of mu-metal heads, water was believed to act as a lubricant in the mid-RH (relative humidity) range and to mitigate friction and wear. At high humidities, friction and wear increased dramatically and patches of debris were observed at the head gap. This increase is attributed to formation of menisci bridges at asperity contacts resulting in high adhesive friction and increased abrasive wear due to agglomeration of debris. In the case of MnZn ferrite, friction and wear increased steadily up to 65% RH and then increased catastrophically. The fracture stress of ceramics is known to reduce at high humidities (moisture-assisted fracture or static fatigue). This phenomenon is thought to initiate brittle fracture at the location of surface defects resulting in increased wear. The static tests with MnZn ferrite at high humidity resulted in formation of loose debris, which confirms the static fatigue theory.