Phase evolution of plasma sprayed Al2O3−13%TiO2 coatings derived from nanocrystalline powders

Commercial nanosized alumina and titania particles were selected as raw materials to prepare the blended slurry with composition of Al2O3-13%TiO2 (mass fraction), which were reconstituted into micrometer-sized granules by spray drying, subsequently sintering at different temperatures to form nanostructured feedstock for thermal spraying, and then Al2O3-13%TiO2 nanocoatings were deposited by plasma spraying. The evolution of morphology, microstructure, and phase transformation of the agglomerated powder and as-sprayed coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that Al2O3 retains the same α phase as the raw material during sintering, while TiO2 changes from anatase to rutile. During plasma spraying, some α-Al2O3 phases solidify to form metastable γ-Al2O3, and the volume fraction of α-Al2O3 decreases as CPSP increases. However, peaks of the TiO2 phase are not observed from the as-sprayed coatings except for the coatings sprayed at the lower CPSP. As the CPSP increases, nanostructured TiO2 is dissolved easily in γ-Al2O3 or χ-Al2O3·TiO2 phase. After heat treatment, γ-Al2O3 in the coatings transforms to α-Al2O3, and rutile is precipitated.