| A small addition of elements with a high affinity to oxygen can have a profound effect on the high temperature oxidation behaviour of many metals and alloys. In order to explain the improvement in oxidation resistance, the research was conducted using Ni-NiO as a model system of cation-diffusing oxides, and Ce as a typical reactive element. Three essential techniques were employed to modify the surface of Ni with Ce and CeO2: ion implantation, sol-gel technology, and reactive sputtering. The improvement of Ni oxidation resistance was assessed by oxygen uptake measurements mainly during the early stages but also for long-term exposures at temperatures between 873 and 1073 K in pure oxygen, both at low and atmospheric pressures. The variety of oxides produced were examined in detail by several advanced techniques including Rutherford backscattering spectrometry, Auger electron spectroscopy, secondary ion-mass spectrometry, transmission- and scanning-transmission electron microscopy equipped with electron and x-ray analyzers, atomic force microscopy, infrared spectroscopy, and x-ray diffraction techniques. In order to provide direct evidence regarding the mechanism of oxide growth, a sequential oxidation using oxygen isotopes 16O2/18O2 was conducted.; After conversion to the form of ceramic coating, superficially applied CeO2 sol-gel significantly reduced the Ni oxidation rate as well as changing the NiO morphology and internal microstructure. The extent of the effect depended on coating thickness, size of CeO2 particles, substrate surface finishing and preoxidation before coating. Under optimum conditions, the reduction in the Ni oxidation rate achieved by sol-gel, reactive sputtering, and ion implantation, was similar. It was found that Ni oxidation resistance is controlled by a well-defined NiO sublayer that is composed of randomly-oriented NiO grains and CeO2 particles. Moreover, in this sublayer, the Ce4+ ions segregate to the NiO grain boundaries. At high temperatures, the Ce4+ ions block the outward diffusion of Ni2+ cations along the NiO grain boundaries while allowing the inward diffusion of O2 anions to continue. The "dynamic-segregation mechanism" is proposed in which Ce ions do not statically block the NiO grain boundaries, but actively diffuse along them. It is suggested that the NiO texture and microstructure, which are primarily influenced by the crystallographic orientation of the Ni substrate, are of critical importance for the stability of the reactive element concentration at NiO grain boundaries over the oxidation time. |
