Insights into microstructure formation and thermal stability of aluminum borate whiskers: Effects of precursor powder

Aluminum borate whiskers (ABOws) have been widely utilized as a secondary phase for fabricating ceramic composites. The morphology and thermal stability of ABOws play a critical role in achieving their optimal performance. In this study, two precursor powders were synthesized via chemical precipitation and mechanical mixing, followed by the induction of a solid–liquid reaction at elevated temperatures to form ABOws. For ABOws prepared from chemically precipitated powder, a metastable intermediate Al 2.1 B 0.9 O 4.5 phase was first formed at 850°C, which subsequently transformed to Al 4 B 2 O 9 at 1000°C and further evolved into needle‐like Al 18 B 4 O 33 with high aspect ratio at 1150°C. In contrast, for ABOws prepared via mechanically mixed powder, the formation of Al 4 B 2 O 9 began at 850°C, and it fully converted to rod‐like Al 18 B 4 O 33 at 1150°C. The morphology differences of those two types of ABOws were ascribed to their different nucleation routines. Homogenous and heterogeneous nucleation occurred in the chemically precipitated amorphous and mechanically mixed powders, respectively, leading to the formation of ABOws with different morphologies. The two types of ABOws demonstrated phase composition stability up to 1400°C. However, pronounced morphological changes were observed at 1200°C. These findings offer valuable insights into the microstructure formation process and thermal stability of ABOws with tunable morphologies.