Drying of Granular Ceramic Films: II, Drying Stress and Saturation Uniformity

Abstract

Films composed of ceramic particles were observed during drying. The films were prepared from 20 vol% aqueous dispersions of a-alumina and a-quartz and were free of any organic binder. Conditions for uniform film saturation during drying were established by consideration of a liquid transport model and by direct observation of the drying films. Drying stresses were measured in situ by a substrate deflection method 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-ym particles, respectively. The maximum stress decreased from 2 to 0.9 MPa for films produced from the 0.35-pm particles when 0.005 wt% surfactant was added to the slurry. The surfactant decreased the liquid surface tension from 72 to 32 dydcm. These observations are direct evidence of the effects of capillary tension on the state of stress in a ceramic body. Mechanical properties of the green ceramic films were estimated by use of a linear elastic fracture model. Knowledge of the critical cracking thickness and maximum stress in the film was used to estimate the fracture resistance of the granular film. The fracture resistance values are approximately 0.02 and 0.007 MPam'O for films produced from alumina and silica, respectively. The difference in mechanical behavior of the silica and alumina films is similar to that expected by the difference in Hamaker constants between the two materials.

Colloidal Processing of Ceramics

“…Removal of the liquid vehicle required for colloidal processing often leads to problems with dimensional control, segregation, and cracking. 3,43,[132][133][134][135][136][137][138][139][140][141] In recent work, the drying behavior of ceramic layers derived from charge-stabilized colloidal suspensions, 135,136 tape-casting 133,134,137 and gelcasting 142 suspensions, and colloidal suspensions of varying stability 3 has been studied for the purpose of characterizing drying stress and structural evolution.…”

Earthen construction: an increase of the mechanical strength by optimizing the dispersion of the binder phase

“…However, different refinements [23,44] lead to results sufficiently close to use this conceptual simple model in order to obtain a first order approximation of the mechanical behavior. It initially describes the behavior of spheres of unique diameter submitted to Van der Waals attraction, but has also been extended to assemblies of particles submitted to other forces such as capillarity [12]. This simple model can be applied to a construction material, considered as aggregates, linked by a binder phase (cement, clays, plaster…).…”

Plasticity and Fracture in Drying Colloidal Films

“…which is frequently used to model pressure variations in drying films [16][17][18][19], and where the terms on the righthand side are usually taken to be constant across the saturated film (regions 3 and 4 in Fig. 1).…”