Date of Award
Doctor of Philosophy (PhD)
Two model systems, Co-Rh and Ni-Co, were used to study surface segregation and oxidation phenomena on binary alloys.
Cobalt was found to sputter preferentially from five Co-Rh alloys ranging in composition from 19.3 to 84.7 at % Rh when irradiated by 4 keY Ar⁺ ions. Surface segregation experiments on this system in the temperature range 623 to 1123 K showed that cobalt was enriched on the surface of a 84.7Rh alloy while rhodium enrichment was determined for a 19.3Rh alloy. No conclusive segregation could be measured on the surface of a 47.8Rh alloy. The variation of surface concentration with temperature indicated enthalpy values of 1 ± 1 kJ mol‾¹ and -3 ± 7 kJ mol‾¹ for the 19.3Rh and 84.7Rh alloys, respectively. Reasonable agreement was found between these measured enthalpy values and those calculated from the combined bond breaking and lattice strain models of surface segregation.
Oxidation experiments were performed on (110) Ni-3.86Co and Ni-4.0Co polycrystalline alloys at temperatures in the range 673 to 1073 K and a pressure of 5x10‾³ Torr. The effects of surface pretreatment and orientation of the alloy surfaces were correlated to differences in oxide morphology and growth rates. Single crystal specimens which were chemically polished, annealed in vacuum and initially exposed to oxygen at room temperature showed a tendency to form (100) (NiCo)O ⎮⎮ (110) Ni-3.86Co. These layers also exhibited the slowest growth rates and greater enrichment of CoO from the inner to outer oxide surfaces. Single crystal surfaces prepared by chemical polishing, annealing in vacuum and initial exposure to oxygen at the reaction temperatures developed additional oxide orientations parallel to the underlying alloys. Finally, the polycrystalline Ni-4.0Co specimens which were prepared by mechanical polishing followed by exposure to room temperature oxygen developed polycrystalline oxide films. These types of structures were associated with higher rates of oxide growth and lower enrichments of CoO from the inner to the outer oxide surfaces.
The experimentally measured composition profiles were analysed according to the Wagner(⁶) model of oxidation, modified to include the effects of short circuit grain boundary diffusion. Effective alloy interdiffusion coefficients were calculated which showed that the rate of transport in the alloy zone beneath the growing oxide layer was enhanced relative to that expected solely on the basis of volume diffusion in this phase. The results were rationalized according to a dislocation model for the accomodation of plastic strain and vacancy annihilation in cation conducting scales(¹⁴⁹).
Ellison, Keith Anthony, "Surface Segregation on Co-Rh and Oxidation of Ni-Co Alloys" (1990). Open Access Dissertations and Theses. Paper 3513.