Atomic transport mechanisms in thin oxide films grown on zirconium by thermal oxidation, as-derived from 18O-tracer experiments

Two-stage oxidation experiments using 16O and 18O isotopes were performed to reveal the governing atomic transport mechanism(s) in thin (thickness <10 nm) oxide films grown during the initial stages of dry thermal oxidation of pure Zr at 450 K. To this end, bare (i.e. without a native oxide) Zr(0 0 0 1) and Zr(101¯0) single-crystalline surfaces were prepared under ultra-high vacuum conditions by a cyclic treatment of alternating ion-sputtering and in vacuo annealing steps. Next, the bare Zr surfaces were oxidized at 450 K and at pO2 = 1 × 10−4 Pa, first in 16O2(g) and subsequently in 18O2(g). The 18O-tracer depth distributions in the oxide films were recorded by time-of-flight secondary ion mass spectrometry. It was concluded that the early stage of the oxidation process is governed by oxygen transport to the metal/oxide interface through the lattice and along the grain boundaries of the nanosized oxide grains whereas, on continuing oxidation, only oxygen lattice transport controls the oxidation process. An oxide-film growth mechanism is proposed.