High-temperature oxidation resistance of YbAlO/Si3N4/Al2O3 composite films

As aerospace technology advances, thin film thermocouples (TFTCs) must work reliably at temperatures exceeding 1300 degrees C. Therefore, TFTCs require not only a stable sensitive layer but also a protective layer with robust oxidation resistance. The existing protective layers are challenging to meet the requirement. Consequently, this paper proposes the YbAlO/Si3N4/Al2O3 composite films as a protective layer, which was sputtered and subsequently annealed in nitrogen at 1300 degrees C to form Yb-alpha-SiAlON. The 18O2 diffusion experiments were conducted. The results of time-of-flight secondary ion mass spectrometry (ToF-SIMS) indicated that the diffusion coefficient (D) of the YbAlO/Si3N4/Al2O3 composite films is 3.44 x 10-16 cm2/s at 1300 degrees C. This is much lower than that of the YAlO/Si3N4/Al2O3 composite films(5.69 x 10-15 cm2/s), which is frequently used in the present TFTCs. The diffusion activation energy (Delta E) of interstitial O in Re-alpha-SiAlON has been calculated by the first principles Nudged Elastic Band (NEB) method. The calculated results showed that the Delta E of Yb-alpha-SiAlON is higher than that of Y-alpha-SiAlON, which is consistent with our experimental data. Finally, the D value of the YbAlO/Si3N4/Al2O3 is only 6.93 x 10-15 cm2/s at 1500 degrees C, indicating that YbAlO/Si3N4/Al2O3 composite films will be a better choice for protective layer at temperatures exceeding 1300 degrees C.