The Cracking of Sol-Gel Films During Drying

Abstract

The cracking behavior of acidic silica sol-gel films during drying and low temperature heat treatment was studied. Films that cracked during drying exhibited a variety of unusual crack morphologies including sinusoidal cracks and parallel crack pairs. The effects of the water content of the sol and temperature on the critical thickness above which cracking of the films occurred were determined. The critical thickness decreased with increasing water content, most likely because the surface tension of the liquid in the pores of the gel increases with water content and because sols with low water concentrations did not gel until the solvent had evaporated. To determine why the critical thickness decreased with temperature, thermal analysis and shrinkage measurements of both constrained and unconstrained films were performed. Thermogravimetric analysis indicated that rapid weight loss occurred in the temperature region where the rapid decrease in critical thickness occurred. A small amount of shrinkage of the films also occurred in this region. Finally, shrinkage measurements of films debonded from the substrate indicated that shrinkage was very anisotropic with nearly all of the shrinkage occurring in the plane of the film and very little in the thickness direction.