Mechanical and microstructural characterisation of oxide films damage

Three point-bending tests with simultaneous observations by scanning electron microscopy were performed on NiO films developed at 800 °C in air for various times (i.e. oxide thicknesses) on industrial nickel. The oxide film always consists of two layers, a columnar one due to cationic growth and an equiaxed one due to anion diffusion. During bending, in-situ observations demonstrated that cracks began to extend through the whole oxide film at a critical strain. For higher strains, crack networks appear and distance between cracks tends to a constant value. Finally, spallings are observed at the equiaxed-columnar oxide interface. An apparent mode I fracture toughness parameter of the oxide film was calculated and the mechanical behavior and parameters of the system were related to the morphological evolution of the oxide film with oxidation time and to the initial mode of preparation of the nickel.