Architecture and phases composition of suspension plasma sprayed alumina–titania sub-micrometer-sized coatings

Sub-micrometer-sized Al2O3–TiO2 plasma-sprayed coatings exhibit superior performances compared to micrometer-sized ones. Two routes can be implemented to manufacture such finely structured coatings: i) spraying micrometer-sized agglomerates of nanometer-sized particles which results in a two-scale coating architecture and ii) spraying a suspension of sub-micrometer-sized particles (suspension plasma spraying, SPS). SPS was implemented in this study and Al2O3-base coatings with 13 and 60 wt.% of titania, respectively, were manufactured by spraying a suspension made of a mixture of Al2O3 and TiO2 particles both of 300 nm, average diameter. Coating structural features and phase contents were studied. Results show that the coatings exhibit a very fine lamellar structure with a homogeneous repartition of Al and Ti. Complex phases, made of intermediate Al, Ti, and O oxides, have been also identified. Indeed, coatings formation results from rapid solidification rates and high transient thermal fluxes imparted by the plasma flow to the substrate due to the short spray distance encountered in SPS (in the order of 30 mm) requested by the small kinetics and thermal inertia of sub-micrometer-sized particles.

► Al2O3–TiO2 coatings are manufactured by suspension plasma spraying.
► Al2O3 13 wt.% TiO2 coating is composed of a higher α-Al2O3 phase content than γ one.
► A tialite phase is generated when the suspension is composed of 60 wt.% of TiO2.
► Lamellae composed of Ti and O elements are rather crystallized.
► Lamellae composed of Al, Ti and O elements or Al and O ones are rather amorphous.