Microstructure evolution of thermally grown Al2O3 on NiCrAlY bonding coating for high-temperature thin-film sensors

Abstract

Thermal stability is an important aspect of thermally grown alumina (TGA) for high-temperature thin-film sensors on nickel-based superalloys. In this paper, a thermodynamic coupled model was used to calculate the vacuum diffusion and the oxidation of aluminum atoms in NiCrAlY bonding coatings (BCs). The calculation results show that the thickness of the outer Al-rich β-NiCrAl layer gradually increases during the vacuum diffusion process and reaches the maximum at 1050 °C for 6 h, while the thickness of NiCrAlY BCs is 15 µm. The scanning electron microscope results show that the surface of TGA is mainly composed of metastable alumina (θ and γ-Al2O3) at high oxygen partial pressure (1 ×105 Pa). At lower oxygen partial pressure (10–1 ×103 Pa), the TGA is mainly composed of equiaxed α-Al2O3. The experimental results show that the oxygen partial pressure is the main factor determining the crystal phase of TGA. The energy-dispersive X-ray spectroscopy shows that the TGA thickness is about 500 nm after 12 h of oxidation at 950 °C and 100 Pa, which are consistent with the calculation results. Finally, to verify the improvement of thermal stability of the TGA, Pt/Pt-10%Rh thin-film thermocouples (TFTCs) with Al2O3 insulating layer, TGA and NiCrAlY BCs were fabricated on the surface of the turbine blade by magnetron sputtering. The insulating layer has an excellent insulation resistivity (exceeded 2 ×109 Ω·cm at 1000 °C) during three consecutive high-temperature thermal cycling tests. This result suggests that the TFTCs with thermally grown α-Al2O3 possesses have excellent reliability and stability at high temperature.