THE INFLUENCE OF CERIUM ION-IMPLANTATION ON CHROMIUM OXIDATION

Over temperatures (750-950-degrees-C) where chromia scale growth on chromium is controlled by short-circuit path diffusion processes, the implantation of 2 x 10(16) cerium ions cm-2 dramatically affected the oxidation behaviour of this metal in oxygen. During 66 h exposures the extents of attack were reduced and scale adhesion was improved. Secondary ion mass spectrometry analysis of chromia scales formed during sequential oxidation in oxygen with different O-18 enrichments showed that, whereas on unimplanted chromium the scale growth was dominated by outward cation transport, on the cerium implanted metal the initial scale development occurred primarily by inward oxidant transport. During the later part of the exposure, however, localized external scale growth also occurred by cation diffusion. The cerium concentrated as a continuous band within the scale near the gas interface but additionally was distributed through the scale at a decreasing concentration with increasing depth. Scanning transmission and transmission electron microscopy revealed that it was present within the grains and also at grain boundaries as small CeO2 and/or CeCrO3 particles. In addition, cerium was segregated along the oxide grain boundaries. A strong and plausible correlation appeared to exist, therefore, between the changes in oxidation properties and the mode by which cerium became incorporated within the chromia scale.