Effect of type of solvent alcohol and its molar proportion on the drying critical thickness of ZrO2–3 mol% Y2O3 films prepared by the sol–gel method

The effect of the type of the solvent alcohol and its molar proportion on the drying critical thickness of 3 mol% Y2O3-stabilized ZrO2 (YSZ) sol–gel films was examined experimentally and modeled analytically. To this end, zirconium n-propoxide was dissolved using different solvent alcohols (ethanol, methanol, and propanol) and different molar proportions of propanol (alcohol-to-alkoxide molar ratios of ~ 6.5, 11.5, or 16.5), and the resulting sol–gel solutions were dip-deposited at different substrate withdrawal rates until cracking occurred after drying at 100 °C. It was found that the drying critical thickness, i.e., the maximum thickness that can be reached without film cracking, was greatest when the solvent alcohol was propanol, despite the tensile strength of the gelled network being the lowest. The use of ethanol and methanol as solvent alcohols results in sol–gel films with greater strength but thinner drying critical thicknesses. The analysis of the results using a drying stress model showed the drying critical thickness to be conditioned by the balance between the evaporation rate and viscosity of the sol–gel solutions, and the experimental data indicated that the predominant factor to take into account when choosing the solvent alcohol is the evaporation rate of the solution. It was also found that the critical thickness increases with increasing molar proportion of solvent alcohol, again despite the decreasing tensile strength of the gelled network. The analytical modeling of the drying stresses indicated that, once the solvent alcohol has been chosen, the determining factor in the drying critical thickness is the viscosity of the sol–gel solutions. Implications for obtaining thicker, crack-free, sol–gel films of YSZ or other oxide ceramics during drying that have greater tensile strength are discussed.