Mechanical behaviour of granular ceramic films during drying

A capacitive deflection measurement has been applied to investigate the stress formation during drying of granular ceramic films. The films were prepared from aqueous zirconia suspensions and cast via spin coating on a 3 " Si wafer. Drying Stresses (<(sigma)over bar> approximate to 10(6) Pa) were recorded for different film thickness and relative humidity. The sensitivity of the stress measurement is better than 10 kPa, while the time resolution has been reduced to 10(-2) s. The relative humidity was controlled using a climate chamber. The Young's modulus of the green films (E=45 GPa) was determined using ultrasonic surface waves excited by short laser pulses. The drying process appears to be separated into three different drying regimes characterized by the rate of stress formation. In the first stage stress-free drying is observed which becomes more pronounced for thicker films. In the second stage the rate of stress formation is the largest and nearly constant with time. It decreases with increasing thickness. The third period starts with a turnover to a lower rate of stress formation which decreases with time until the stress reaches a saturation level. A 1-d model of the drying induced stress formation in a porous, visco-plastic film was applied and solved numerically by a Finite Differences Method. The model predictions are in good agreement with the experimental results. A microscopic model for the stress formation at the particle neck region explains the large stress increase in the drying body when a constant contact zone at the necks has been formed after a period of free shrinkage of the liquid pendular rings.