Relation between the oxidation mechanism of nickel, the microstructure and mechanical resistance of NiO films and the nickel purity: I. Oxidation mechanism and microstructure of NiO films

The effect of impurities on the oxidation mechanism of nickel and on the mechanical characteristics of NiO films was studied on two industrial nickel grades compared to a pure nickel. In this part, influence of impurities on the oxidation mechanism and on the NiO film microstructure will be detailed. The oxidation mechanism, especially studied at 800 °C in air, was clarified using complementary techniques, SEM and STEM microstructural and analytical investigations, XPS analyses, profilometry, oxygen isotopic exchange and SIMS. Depending on the Ni purity, the morphology of the oxide scale and the porosity amount differs. Duplex oxide layers and internal oxidation are observed on industrial grades, while a simple NiO film grows on pure nickel without internal oxidation. An oxidation mechanism, which differs according to the presence or not of impurities in the nickel substrate, is proposed. The mechanical characteristics will be characterised and discussed in the following paper (part II) in relation with the microstructural modifications induced by changing the nickel purity.