Effect of Unfired Tape Porosity on Surface Film Formation by Dip Coating

Thin ceramic layers have been fabricated by dipping green tapes of alumina, formed by the doctor-blade casting method, into aqueous slurries containing mixtures of alumina and either unstabilized zirconia (MZ–ZrO2) or mullite. It was observed that the formation of a thin layer on the surface of the tape is governed by both liquid entrainment and slip-casting mechanisms, and was accelerated by increasing the withdrawal rate, immersion time, or volume fraction of solids in the slurry used for dip coating. By modifying these parameters, layers as thin as 2 lm and as thick as 108 lmwere easily formed. Layer formation was found to be strongly influenced by the structure of the tape surface. Layers formed on the top surface of the tape were found to be as much as 48% thicker than those formed on the bottom surface. This difference appears to be related to the smaller amount of porosity on the bottom surface of the tape. Evidence suggests that the polymer binder, used for doctor-blade casting, concentrated on the bottom of the tape as evaporation occurred from the top surface. The lower porosity on the bottom reduced the casting rate during dip coating and produced significantly thinner layers, relative to the top surface.