| Abstract | The influence of various reactive-element (RE) oxide coatings (Y₂O₃, CeO₂, La₂O₃, CaO, HfO₂, and Sc₂O₃) on the oxidation behavior of pure Cr, Fe-26Cr, Fe-16Cr and Ni-25Cr at 900ºC in O₂ at 5 x 10⁻³ torr has been investigated using the ¹⁸O/SIMS technique. Polished samples were reactively sputter-coated with 4 nm of the RE oxide and oxidized sequentially first in ¹⁶O₂ and then in ¹⁸O₂. the effectiveness of each RE on the extent of oxidation-rate reduction varied with the element used. Y₂O₃ and CeO₂ coatings were found to be the most beneficial, whereas Sc₂O₃ proved to be ineffective, for example, for the oxidation of Cr. SIMS sputter profiles showed that the maximum in the RE profile moved away from the substrate-oxide interface during the early stages of oxidation. After a certain time the RE maximum remained fixed in position with respect to this interface, it's final relative position being dependent on the particular RE. The position of the RE maximum within the oxide layer also varied with the substrate composition. For all coatings ¹⁸O was found to have diffused through the oxide to the substrate-oxide interface during oxidation, the amount of oxide at this interface increasing with increasing time. The SIMS data confirm that for coated substrates there has been a change in oxide-growth mechanism to predominantly anion diffusion. The RE most probably concentrates at the oxide grain boundaries, generally as the binary oxide (RE)CrO₃. Cr³⁺ diffusion is impeded, while oxygen diffusion remains unaffected. |
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