Abnormal grain growth during rapid annealing of sol-gel alumina thin film deposited on Ni-based superalloy

A ∼50 nm thick alumina layer was deposited on an Ni-based superalloy substrate by a sol-gel method. α-AlOOH particles presented in the layer after drying at 140°C transformed mostly to α-Al2O3 grains within ∼1 min at 1100°C under a low oxygen partial pressure annealing environment. During the same time period, the α-Al2O3 grains grew significantly in the lateral direction, resulting in the aspect ratio of grain diameter to thickness of ∼20. The presence of a preferred orientation in the α-Al2O3 layer suggested that the mechanism for the lateral growth was abnormal. The lateral growth mechanism appeared to become very slow when a critical thickness (∼100 nm) was reached.