The growth and structure of thin oxide films on cerium ion-implanted nickel
- PDF / 1,505,328 Bytes
- 13 Pages / 612 x 792 pts (letter) Page_size
- 1 Downloads / 161 Views
I.
INTRODUCTION
ION implantation is a reliable and reproducible method of modifying the surface chemistry of materials in ways that are difficult or impossible to achieve through other techniques. For over 2 decades, it has also been used successfully to improve the high temperature corrosion resistance of metals and alloys. Of the many species used, the best results were achieved after implantation of so-called reactive elements, such as yttrium and the rear earths. The influence of reactive element implants on the high temperature corrosion behavior of metals and alloys has been the subject of many studies. The early literature was reviewed in References 1 through 3. For chromia and alumina scales, several different benefits caused by reactive element additions have frequently been reported: benefits such as the reduction of scale growth rates, the increase of scale adherence, the reduction of growth stresses, the prevention of sulfur segregation to the oxide-metal interface, and the enhancement of the selective oxidation of Cr in alloys.[4–7] For nickel oxide, the reduction of the scale growth rate at temperatures lower than 1300 K has been documented as a major improvement.[8–11] Despite the extensive research performed in this area, there is still no general theory to explain this so-called ‘‘reactive element effect.’’ At present, two main hypotheses seem to explain the majority of experimental evidence: (1) inhibition of diffusion processes by reactive element ion segregants at oxide grain boundaries, a hypothesis supported by experiments with chromia[2,12] and nickel oxide; [13,14] and (2) blockage of the cationic reaction step by the reactive element ions segregated at the oxide/metal interface, a hypothesis which has been proposed for chromia scales.[15] In addition, for alumina formers, inhibiting F. CZERWINSKI, Research Associate, and J.A. SZPUNAR, Professor, are with the Department of Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2A7. W.W. SMELTZER, Emeritus Professor, is with the Institute for Materials Research, McMaster University, Hamilton, ON, Canada L8S 4M1. Manuscript submitted December 12, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
growth by changing the phase transformation temperature of unstable aluminas has also been postulated.[16] However, recently reported differences in the effectiveness of surface applied reactive elements for chromia and alumina scales suggest that the mechanism for improving oxidation resistance may be different for various oxides.[17] In this study, we investigated the early stages of oxide formation on Ce-implanted Ni, using a number of analytical techniques to identify the oxide microstructure and the location of the reactive element. The comparison of these results with previously reported characterization of oxide growth on Ni with surfaces modified by CeO2 sol coating[14,18] gives new insights into the mechanism by which Ce reduces the NiO growth rate.
II.
EXPERIMENTAL PROCEDURES
Specimens of polycrystalline nickel, 99.99 pct
Data Loading...
