Relation between the oxidation mechanism of nickel, the microstructure and mechanical resistance of NiO films and the nickel purity. II. Mechanical resistance of NiO films

The differences in the nickel oxidation mechanism and the microstructure of the NiO films depending on the nickel purity were described in the preceding paper (part I). In this second part, the mechanical characteristics were determined mainly by three-point bending tests performed “in situ” in a scanning electron microscope. In relation with the nature of the oxidation mechanism and the film microstructure differences, specific mechanical behaviours were observed. Nevertheless, regardless of the nickel grade, the oxide toughness evolves with the thickness of the scale and tends towards the value given for bulk NiO. Amongst the differences related to Ni purity, one consists in the fact that spalling occurred mainly at the oxide/oxide interface for the industrial grades, while it appeared at the metal/oxide interface for pure Ni. This fact was related to the scale morphology, i.e. to the oxidation mechanism.