Micro/ macroscopic elastic constants of thin films determined by using Impulse Excitation Technique and X-ray diffraction
The International Conference on Surfaces, coatings and interfaces (Surfcoat), Incheon-Seoul Korea, 28-30 March 2018
Many methods were developed to obtain the elastic constants of coated materials, which are classified into two groups: static and dynamic techniques. The elasticity of thin films may be different from this of bulk materials and de-pends on the elaboration technique. The ana-lytical models, reported in the literature, used to determine the Young’s modulus of a coat-ing are based on two different theories: the flexural rigidity of a composite beam and the classical laminated beam theory (CLBT). The problem is that the validity of these models depends on coating’s thickness and physical properties of both coating and substrate. This work presents a new methodology on the determination of the elasticity constants of thin films using the impulse excitation tech-nique (IET). It is non-destructive technique with high precision and with which the elastic constants are easy to measure. The tests rely on the measurement of the sample resonance frequency before and after deposition. This study was carried out on pure metallic thin films of aluminum and tungsten deposited on glass and steel substrates by DC magnetron sputtering. The thickness of these films was in the range 3-6 µm deposited on substrates of 1 mm thick. Many experimental tests were em-ployed: IET, Nanoindentation, SEM and XRD. Finite element model (FEM) was developed on ABAQUS software, and then the comparison with the analytical approaches allowed to iden-tify the limits and drawbacks of each one. The best model to determine the film Young’s modulus was identified. A new analytical model was developed to de-termine the macroscopic shear modulus of a coating. The model was numerically (FEM) and experimentally confirmed by studying tungsten films deposited on glass substrates. The choice of tungsten was done because it is well known that its elastic behavior at crystal scale is ideally isotropic. In our study, we focus also on the determina-tion of the microscopic elastic constants of tungsten coating revealing the presence of two phases: W-α and W-β. These phases are respec-tively stable with a CC structure and a meta-stable phase with a cubic structure. The goal is to determine the elasticity constants of the W-β phase not sufficiently known in the litera-ture. The macroscopic elastic constants, pole figures made by X-ray diffraction and the crys-talline orientations of the two phases were de-termined. The relationship between strain and stress has been established using the self-consistent model. This relation is a function of the elasticity constants of the W-β phase, the W-α phase and the macroscopic elastic con-stants of the tungsten film.