The structure and morphology of oxide films during the initial stages of titanium oxidation

Transmission electron microscopy of detached oxidation films showed that thin films of rutile formed initially of uniform thickness, replicating the metal structure. The replicated metal grains, hereafter called oxide grains, were either polycrystalline on thermally etched facets of the metal, or pseudo-mono-crystalline, consisting of low angle boundaries and subgrains. Subsequently an overgrowth of rutile nucleated on the oxide grains. The overgrowth of single-crystalline, heavily micro-twinned oxide, grew both laterally, enclosing "islands" of matrix oxide, and inwardly, consuming the initial film. Complete coalescence resulted in single crystal oxide grains corresponding to each metal grain. Two networks could be distinguished by contrast variations upon tilting. The first, which was most prevalent, nucleated within the oxide grains and only rarely crossed grain boundaries, whereas, the second nucleated at grain boundaries and grew across the grains. Oriented networks formed on faceted grains, growing along the faces rather than crossing ridges. Random networks were generally associated with pseudo-monocrystalline grains which formed on unfaceted metal grains. The characteristics of both matrix and networks during heating are discussed, and the role of time and temperature on network nucleation and growth is considered.