A case study in striation prevention by targeted formulation adjustment: Aluminum titanate sol-gel coatings

By careful consideration of the physical and chemical effects responsible for striation defect formation during spin coating, it is possible to adjust a coating solution's recipe to prevent these coating defects from forming. In this paper we present a specific "case study" to illustrate a useful reformulation strategy. Aluminum titanate (Al₂TiO₅) coatings were made via sol-gel chemical processing. An early version of our sol-gel recipe resulted in coatings that were severely striated and therefore unacceptable for their desired application, even though they had formed the correct crystalline phase during subsequent firing. The present paper examines that specific sol-gel recipe and illustrates a protocol for improving the final coating flatness by selecting a solvent additive that can influence the surface forces during drying. Coatings deposited with the modified solution were smooth and free from striations. The coating morphology was characterized by optical surface profilometry and crystallinity by X-ray diffraction. The widely applicable theory of targeted formulation adjustments to help eliminate striations during spin coating is discussed in light of the current experiments.