Determination of biaxial stress and strain states of fibre-textured thin films by X-ray diffraction measurements

This paper reports a study on the determination of the internal stresses and elastic strains in thin films. The determination of these stresses and strains in thin films, especially those with strong fibre textures, is challenging. The method used is a combined technique of Bragg-Brentano and Asymmetric X-ray diffraction (XRD) configurations referred to as BBA-XRD. This method allows the use of conventional powder X-ray diffractometers to determine isotropic and anisotropic in-plane and out-of-plane strains of textured thin films. In this study, the internal stress and lattice strains of niobium (Nb) thin films deposited on nickel-titanium (NiTi) substrates in both as-deposited condition and after in-plane uniaxial tension were analysed using this method. The results obtained were verified by comparing with synchrotron grazing angle XRD measurements. The isotropic in-plane strains of the deposited Nb thin films were determined to be ε_x=ε_y=−1.73% and the out-of-plane strain was ε_z=+2.28%. After a uniaxial tensile in-plane deformation of the NiTi substrate by 6.1 % in the y direction, the anisotropic strain state of the film changed to ε_x=−2.03%, ε_y=+0.80% and ε_z=+0.80%. This paper also presents the specific conditions required for the BBA-XRD method to be applied to hexagonal, tetragonal, and rhombohedral crystal systems.