Cracking of titania nanocrystalline coatings

Cracking of wet-chemically precipitated nanocrystalline TiO2 films deposited on different platelet-like substrates is investigated. The thicknesses of the films are also in the nanometric range: 60, 140 or 300 nm. Scanning electron microscopy, X-ray diffraction and nitrogen adsorption are employed to determine layer thickness, grain size, and porosity. The synthesis of TiO2 films through an aqueous route affords very good reproducibility of these properties (layer thickness ±5 nm). Mica, SiO2 and Al2O3 platelets are used as substrates, providing different properties with regard to surface charge, structure and roughness. The drying step is found to be responsible for the cracking of some of the TiO2 layers and the evolution of the cracks during sintering is related to shrinkage of the substrate. Film thickness as well as the chemical nature of the substrate influences the cracking of nanocrystalline TiO2 films during the drying step.