New processes for obtaining advanced materials, in form of coatings, are developing more and more to meet the socio-economic and the environmental needs. Coatings, as protective layers, improve the surface properties, characteristics of materials. They can be used individually or in combination (multiple layers) depending on the customer requirements. Thus, for a reliable and safe operation, knowledge of the mechanical properties and in particular the elastic ones of each individual layer and also the whole composite film performance is important to avoid the failure of structure.

The purpose of this study is to develop the Impulse Excitation Technique (IET) and to determine its limits for measuring the macroscopic elasticity constants (ECs) of in-plane anisotropic multilayer thin films taking into account the effect of architectured layers, error sources and uncertainty of measurement. In the literature and for a designed anisotropic coating, no technique is available to identify the ECs values of each layer and composite film. This difficulty motivates the researchers to innovate new methods for material characterizations.

The IET is based on the analysis of vibrational frequencies created by an impact on a specimen. This is used to characterize the elasticity of films at the macroscopic scale [1-2]. By analyzing the shift of the resonant frequencies between the substrate and the coated substrate, measured by IET, the shear and Young's moduli of each individual film were determined using different analytical models. A series of Finite Element Analyses (FEA) was carried out and a comparison of vibration modes was performed in order to choose the best analytical model able to predict the true values of elasticity constants.

Using this methodology, new models were developed for multi-coated substrate and in-plane anisotropic coatings based on Classical Laminated Beam Theory (CLBT) taking into account the shift of the neutral axis after deposition. By comparing the results of the developed models to those of the numerical models, negligible difference was found (< 0.5%), compared to other models proposed in the literature. Finally, by an inverse method, the ECs of coatings were determined using the new models.

In this work, niobium (Nb)/titanium (Ti) bilayer micrometric thin films were deposited over Stainless Steel (AISI) 316L, Glass substrates and Silicon (Si) wafers by magnetron sputtering, Physical Vapor Deposition technique (PVD). Metallic films (e.g. W and Ti) were deposited by Glancing Angle Deposition (GLAD) to achieve different in-plane anisotropy levels with different glancing angles varied between 10° and 80°. Different pair of perpendicular substrates are used in order to measure the resonant frequencies at two in-plane directions.

For the case of Nb/Ti/AISI multilayer structure, three successive frequency measurements were performed by IET: one for the naked substrate and one after the deposition of each film. For the case of W/AISI with GLAD, the resonant frequencies are measured by IET in the two in-plane directions using the two substrates of each pair. The corresponding ECs were determined, using the new analytical model of multilayer film and the values obtained by IET were compared with the measurements made by Nano Indentation (NI). A good agreement was found and the difference between the static (NI) and the dynamic (IET) techniques is of the order of measurement uncertainty.

This study allowed us first to develop the IET for determining the ECs of each individual film in multilayer structure and secondly to elaborate in-plane anisotropic films at the macroscopic scale allowing the prediction of their ECs by IET.

References:

[1] M.F.Slim, A.Alhussein, F.Sanchette, B.Guelorget, M.Francois,Thin Solid Films,631,172-179 (2017)

[2] M.F.Slim, A.Alhussein, A.Billard, F.Sanchette, M.Francois,Journal of Materials Research,32,497-511 (2016)