New method for the measurement of stress in thin drying gel layers, produced during the formation of ceramic membranes

Abstract

A new method has been developed for the measurement of stresses in thin gel layers during the drying process. The gel layers under investigation consist of boehmite, and are produced during the formation of γ-Al2O3 ceramic membranes. The method is based on the principle of the cantilever beam. Detection of beam deflections is carried out with a laser displacement meter, using a 760 nm infrared laser. Deflections of the beam can be measured continuously during the drying process, making it possible to monitor the stress in the layer also continuously. Examples illustrate that stresses may be as high as 180 MPa. Cracking of gel layers most likely manifests itself by a marked decrease in the stress. Uncracked gel layers may be stressed and subsequently unloaded by slowly lowering and increasing the relative humidity. An increase of temperature at constant air flow and relative humidity leads to an increase of the stress. Lowering the rate of air flow over the sample lowers the stress which is built up. The method which is described here can very probably also be applied to other coating materials, which are dried on rigid supports.