Investigation of the elementary deformation mechanisms in nanocrystalline Pt thin films

Details

Supervisors

Idrissi, Hosni

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil mécanicien, à finalité spécialisée

Abstract

enA growing enthusiasm for thin-film metal has emerged for its noticeable properties. Especially, they are widely used in Micro-Electro-Mechanical Systems (MEMS) for their reliable electrical conductivity and favourable mechanical properties. This results from nano-grain sizes, activating specific deformation mechanisms. Among metals, Platinum (Pt) has shown a high catalytic activity for Hydrogen (H2) production, especially in thin films, where the properties are enhanced. However, thin films are not stable, and their behaviour affects the resulting properties. Indeed, those structures evolve with time, and creep may affect their catalytic properties. The underlying mechanism must then be studied. The FNRS PDR WEAVE - SUSTRAINABLE aims to qualify the deformation of Pt thin films and evaluate the impact on properties. In this context, this work constitutes a part of this project, tackling the characterisation procedure of Pt thin films evolution under deformation, which is not well known today. A Digital Image Correlation (DIC) method is employed to link local deformation of the film to its microstructure. The DIC technique offers a displacement map. Furthermore, the grain morphology can be obtained through Electron Backscatter Diffraction (EBSD) or Transmission Electron Microscopy (TEM). Superposing the two maps highlights the deformation mechanism activated. This work focuses on obtaining an efficient DIC on Pt thin films. For this aim, the procedure followed in a similar work on Aluminium (Al) thin films is transposed to Pt. Thus, a Lab-On-Chip (LOC) setup is used to apply a deformation. LOC is a device created at UCLouvain, applying a tensile strain on a film by releasing internal stresses of the deposited materials. Based on the Al work, Indium speckles were used for the speckle pattern required in the DIC technique. However, due to a contamination of Pt surface, the Indium speckle pattern was challenging to obtain. As the source of this contamination is unknown, several approaches to clean the surface were tried. Once the procedure used in Al work is adjusted to Indium speckles on Pt, DIC has been successfully applied to Pt thin films. The obtained map highlights the localisation of strain. Then, the local strain value has been compared to the macro strain of the sample to assess the reliability of the method. Macro strain is the quantity obtained from direct measurement of the average displacement. The deformation reached is the force equilibrium point between the Pt dog-bone specimen and the attached actuator pulling on the sample. It has been shown that, although the thin film presents a macro-strain, this deformation is not uniform over the sample. Nano-DIC allows to acquire a 2D map of strain locally from the gauge length of the specimen. This strain map unravels the uniformity of the applied strain and quantitatively allows to indicate the mechanical response of the material to uniaxial tension, highlighting events such as strain localisation due to plastic deformation and microstructural evolution. These local deformation maps will then be used in future work to correlate with the nano-crystalline microstructure of Pt acquired separately via advanced microscopy techniques to study which plasticity mechanisms are active during deformation. DIC make those localisations appear, something that global deformation alone does not reveal. This work confirms the presence of localisations on Pt thin films. At present, only speculations can emerge from those structures. The grain map should be obtained to confirm or refute these hypotheses.