A Microstructural Zone Model for the Morphology of Sol-Gel Coatings

The thickness and the morphology of dip-coated single sol-gel layers is easily controlled by varying the sol compositions and the deposition parameters. A thorough study of the microstructure of transparent conducting ZnO:Al coatings deposited on fused silica substrates using X-ray diffraction, X-ray reflectometry and transmission electron microscopy cross-sections as well as of In2O3:Sn, SnO2:Sb, ZnO and TiO2 coatings reported in the literature shows that three basic morphologies can be observed: granular, layered and columnar. In multilayer systems they were found to depend essentially on the single layer thickness (SLT) and on the crystallite size determined from the data of thick films, a parameter called the “intrinsic crystallite size (ICS)”. All the results so far analysed are in agreement with a 3-zone model when ICS is plotted against SLT or in a more refined version when q = ICS/SLT is plotted against the homologous temperature Tsintering/Tmelting. Comparison with the Movchan-Demchishin and Polley-Carter models proposed for PVD and CVD coatings, respectively, is presented.