Effect of Alumina Additions on Microstructural Aspects of the β to α Transformation in Tantalum (V) Oxide

Tantalum (V) oxide (Ta 2 O 5 ) has potential applications as part of an environmental barrier coating system for Si 3 N 4 ‐based turbine components. However, at elevated temperatures, Ta 2 O 5 undergoes a phase transformation from the orthorhombic (β) phase to the tetragonal phase (α), which is undesirable because of the associated volume change. The purpose of the present work was to study the effect of alumina additions (0–5 wt%) on the β to α transformation temperature, and associated modifications to the Ta 2 O 5 microstructure. Sintered microstructures were characterized using SEM (scanning electron microscopy), and XRD (X‐ray diffraction) was used to identify the phases present at room temperature. It was found that for undoped Ta 2 O 5 , transformation of the low‐temperature β‐phase begins at ∼1300°C, and leads to extensive microcracking of the sintered sample. For samples containing alumina, an increase in the transformation temperature was observed. The solubility limit of alumina in Ta 2 O 5 was between 1 and 3 wt%; for samples in which this was exceeded, the AlTaO 4 second and phase particles were seen to be highly effective at inhibiting grain growth.