Stress heterogeneity of thermally grown polycrystalline nickel oxide layers

Mechanical properties of NiO films developed at approximately 800 °C in air for 26 h on a 99.9% pure polycrystalline Ni substrates have been investigated by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The oxide layers, 8 μm thick, exhibit a duplex structure with a slight {1 1 1} texture. The average stresses in the oxide films have been deduced by X-ray diffraction to be 330 ± 35 MPa in compression. Detailed analyses of the oxide films performed by atomic force microscopy show surface undulations on the nanometer scale located in some of the grains. These well-defined periodic structures are due to high growth stress heterogeneity from grain to grain in the oxide layer. From a linear analytical model, it is deduced that growth stresses as large as 4 GPa can be developed locally during the high temperature oxidation process as a result of the strong in-plane anisotropy.