Understanding passivation potential and early-stage oxidation mechanisms from the intermediate temperature oxidation of CrMoNbTaW based sputtered alloys

This research investigates the oxidation behavior of CrMoNbTaW thin films at intermediate temperatures (300 °C–500 °C) during in-air annealing, emphasizing the passivation potential of the protective oxide layer formed at the onset of oxidation. The effects of Cr alloying in MoNbTaW and Al capping on oxidation were analyzed to understand their protective potential. Alloying Cr promotes the development of a continuous oxide layer that acts as a diffusion barrier, increasing the oxidation resistance compared to MoNbTaW. The CrMoNbTaW oxide layer exhibited increasing thickness with annealing temperature and is amorphous. In contrast, applying an Al capping layer to CrMoNbTaW thin films results in a kinetically preferred compact nanocrystalline Al₂O₃ layer on surface along with mixed oxide sites that significantly reduces inward oxygen diffusion. Nanoindentation showed the amorphous mixed oxide layer on CrMoNbTaW to be notably softer and more compliant than the underlying metal. On the other hand, Al-coated films had variations in mechanical properties on top surface due to Al₂O₃ layer but were more protective of base materials hardness and modulus than the uncoated samples. Under identical annealing conditions, both Al-coated and uncoated CrMoNbTaW oxidized rapidly, but exhibited distinct oxidation mechanisms.