Environmental effects on the streaming mode performance of metal evaporated and metal particle tapes

A commercial Hi-8 video cassette recorder was instrumented to measure friction force between the rotary heads and tape, rms head output, and signal dropouts to sub μs duration. Streaming (play) mode experiments using metal evaporated (ME) and metal particle (MP) tapes were performed at design tension under equilibrium and nonequilibrium conditions inside of an environmental chamber at various temperature and specific humidity (SH=ratio of the weights of water vapor to dry air in the mixture). Interface stability and recording performance at a 0.6 μm recording wavelength were measured to bit level resolution using a dropout counter and changes in rms head output were correlated to changes in head-to-tape spacing using the Wallace equation. ME tape performed best at moderate SH (from 0.009 to 0.013) in the operating temperature range of 15.6 to 32.2°C, whereas MP tape performed best at low temperature. At a given temperature, higher SH increased normal and friction forces and decreased head-to-tape spacing due to spontaneous water meniscus formation between contacting and noncontacting asperities on the head, tape, and debris particle surfaces. Dropout frequency and interface stability were sensitive to both SH and temperature. Humidity dependence was governed by the relative size difference between wear debris particles and spontaneously formed water menisci. Competing mechanisms of increased lubricant mobility and spontaneously formed water menisci of smaller radii of curvature at higher temperature governed temperature dependence. A model based on capillary condensation of water vapor onto surfaces in a sliding contact is developed to explain experimental data. An expression for meniscus force consisting of both Laplace and surface tension contributions is developed for nanometer size contact spots and wear debris particles. The model predicts that meniscus force will increase and head-to-tape spacing will decrease with increasing SH which was observed experimentally. Four lubrication regimes are defined for wear debris particles passing through the contact interface. Both SH and extent of deformation of a particle determine whether menisci will form around the particle, and presence or absence of associated meniscus forces explains trends in dropout frequency data. The model allows determination of meniscus height from which minimum values of Kelvin radius and relative humidity (RH) in the contact interface can be calculated. Proximity and intimate contact of the surfaces increases RH and SH in the contact interface over that maintained in the environmental chamber to near their saturation values