DRYING OF GRANULAR CERAMIC FILMS .2. DRYING STRESS AND SATURATION UNIFORMITY

Films composed of ceramic particles were observed during drying. The f ilms were prepared from 20 vol% aqueous dispersions of alpha-alumina a nd alpha-quartz and were free of any organic binder. Conditions for un iform film saturation during drying were established by consideration of a liquid transport model and by direct observation of the drying fi lms. Drying stresses were measured in situ by a substrate deflection m ethod based on an optical interference technique. Simultaneous stress and weight measurements were used to determine stress as a function of saturation. The maximum stress occurred near 100% saturation and was approximately 2 and 1.1 MPa for films produced from 0.35- and 0.68-mum particles, respectively. The maximum stress decreased from 2 to 0.9 M Pa for films produced from the 0.35-mum particles when 0.005 wt% surfa ctant was added to the slurry. The surfactant decreased the liquid sur face tension from 72 to 32 dyn/cm. These observations are direct evide nce of the effects of capillary tension on the state of stress in a ce ramic body. Mechanical properties of the green ceramic films were esti mated by use of a linear elastic fracture model. Knowledge of the crit ical cracking thickness and maximum stress in the film was used to est imate the fracture resistance of the granular film. The fracture resis tance values are approximately 0.02 and 0.007 MPa.m1/2 for films produ ced from alumina and silica, respectively. The difference in mechanica l behavior of the silica and alumina films is similar to that expected by the difference in Hamaker constants between the two materials.