Atom-probe study of the initial stage of selective oxidation of Ni from the Cu-Ni alloy system

The initial oxidation stages of Ni and Ni-Cu alloys have been studied using the atom-probe field ion microscope (AP-FIM). Oxide layers were formed in situ in the FIM chamber on the clean metal surfaces at 1 × 10^-4 Torr (10 ^-2 Pa) O₂ at 870, 970 or 1070 K for less than l min. The atom-probe analysis on the {111} and {100} surfaces of these oxidized alloys showed that only NiO formed on all of the alloys for this relatively low oxygen partial pressure, although incipient Cu₂O formation could not be ruled out in a few samples of the Cu-rich alloys. The concentration of oxygen in the NiO fluctuated layer-by-layer in the 〈111〉 direction as expected for its NaCl-type structure. Ne field ion images and atom-probe analyses of the oxidized samples revealed a sharp NiO/metal interface for pure Ni and for the Cu-rich alloys, and a diffuse interface extending over a few atomic layers for the Ni-rich alloys. The NiO and Ni were cube/cube oriented.