Influence of Feedstock Powder Modification by Heat Treatments on the Properties of APS-Sprayed Al₂O₃-40%TiO₂ Coatings

Aluminum titanate (Al₂TiO₅) is a congruently melting compound in the binary Al₂O₃-TiO₂ system, which decomposes below 1200 °C. Its properties (e.g. thermal conductivity, CTE) differ significantly from those of Al₂O₃ and TiO₂. Thus it is of special interest to study the stability of Al₂TiO₅ in the spray process and its influence on the coating properties. A commercial fused and crushed Al₂O₃-40%TiO₂ powder, which was found to be substoichiometric, was selected as the feedstock material for the experimental work, as the composition is close to stoichiometric Al₂TiO₅. Part of that powder was heat-treated in air at 1150° and 1500°C in order to vary the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including metallographically prepared cross sections. A powder having Al₂TiO₅ as the main phase was not possible to be prepared due to inhomogeneous distribution of Al and Ti in the original powder. Plasma spraying was performed with a TriplexPro-210 (Oerlikon Metco) using Ar-H₂ and Ar-He plasma gas mixtures with 41 and 48 kW plasma power. Coatings were studied by XRD, SEM of metallographically prepared cross sections, and microhardness HV1. Moreover, the results show a clear influence of the Al₂TiO₅ content in the feedstock powder on the phase composition of the coatings.