Stress Development in Hard Particle Coatings in the Absence of Lateral Drying

Stress development in coatings prepared from aqueous suspensions of hard particles was monitored using a modified cantilever deflection technique that suppresses lateral drying fronts. Without complications from lateral drying, stress development was found to be negligible until a critical amount of drying when stress increases dramatically to a maximum and then falls more slowly as drying completes. In addition, the maximum stress measured for coatings prepared on the modified cantilevers was more than double the maximum stress for coatings measured on standard cantilevers where lateral drying fronts were present. By achieving drying uniformity, direct correlations were made between the microstructure of the coating and the measured coating stress. Cryogenic scanning electron microscopy (cryoSEM) was used to image the microstructure of aqueous alumina and silica particle coatings during the intermediate stages of drying. CryoSEM revealed that stress increases dramatically once the particles form a saturated close-packed network, and that stress decays as water evaporates from the pore structure.