Current Trends in the Development of Suspensions and Liquid Precursors for Thermal Spraying: Case of Zn₂TiO₄

When compared with conventional thermal spraying processes, thermal spraying of suspensions allows to produce coatings with outstanding properties in terms of microstructure, surface topography, and phase compositions, as well as mechanical, electrical or tribological requirements. The use of suspensions as feedstock results in an almost unlimited flexibility in terms of chemical composition of the sprayed coatings. Moreover, thermal spraying of suspensions is a promising technique for processing expensive raw materials. Zn₂TiO₄ coatings are only one example where the high costs of blended oxide powders as feedstock material hinders the market introduction, whereas outstanding electrical properties and photocatalytic activity of thermally sprayed Zn₂TiO₄ coatings are of great interest for various industrial applications. In this work, single oxide ZnO and TiO₂ raw materials as well as a Zn₂TiO₄ feedstock powder were used to develop tailored aqueous suspensions suitable for thermal spraying. To follow the formation of the compositions in the system ZnO-TiO₂, differential thermal analysis (DTA) and thermal gravimetry (TG) measurements were performed. Preparation routes of stable suspensions with low sedimentation rates, low viscosity and good flowability are discussed. Exemplary microstructures and phase compositions of sprayed coatings are shown. In all sprayed coatings, the Zn₂TiO₄ phase has been formed during Suspension High Velocity Oxygen Fuel Spraying (S-HVOF). This work demonstrates the potential to develop appropriate cost-efficient suspension feedstocks from single oxide raw materials to obtain Zn₂TiO₄ coatings.