Thin film buckle-delamination - Dundurs parameters influence

In recent years, there has been a strong push towards the modern flexible electronic devices, such as foldable phones or wearable biosensors and electronics. These devices are composed of numerous thin layers of metallic and non-metallic materials and they must withstand harsh usage and a large number of bends and deformations. For example, a foldable smartphone should be able to undergo more than 100 000 open-close cycles without any electrical and mechanical failure. Such strict conditions of use bring high demands not only on the used materials, but also on the connection between them. Therefore, an attention to the adhesion between dissimilar materials has to be investigated. One way to quantify the adhesion between thin film and substrate material is the buckling -induced delamination which uses compressive stress in the thin layer to form a buckle and delaminate it from the substrate. However, the nature of the method itself induces normal and shear loading combination at the tip of the delamination crack and it is important to be able to separate these two components. For this reason, a mode-mixity parameter describing the ratio between shear and normal loading has been introduced 30 years ago. However, in such time, there was only a limited number of material combinations used in real applications, therefore it was deemed that the influence of mismatch between material properties of the film and substrate can be taken as constant, regardless of the material parameters mismatch. Contrary to this, recent advances in the field of modern materials science promote the use of strongly dissimilar materials combinations, such as metallic thin films on polymer or ceramic substrates. The introduction of such materials raises a question if the 30 years old simplification can be still used. Recent research shows that it cannot and the material mismatch has to be accounted for. This project will combine the numerical modelling, analytical approaches and experimental measurements of the thin films adhesion in order to quantify the ratio between normal and shear loading for strongly dissimilar materials as a function of the so-called Dundurs parameters. The numerical modelling enables us to check the materials mismatch influence for endless material combinations and experimental techniques will be used to obtain input data as well as validate the models and the research hypotheses, leading to a better understanding of the material delamination processes and, therefore, more durable modern devices. 1

In recent years, there has been a strong push towards the modern flexible electronic devices, such as foldable phones or wearable biosensors and electronics. These devices are composed of numerous thin layers of metallic and non-metallic materials and they must withstand harsh usage and a large number of bends and deformations. For example, a foldable smartphone should be able to undergo more than 100 000 open-close cycles without any electrical and mechanical failure. Such strict conditions of use bring high demands not only on the used materials, but also on the connection between them. Therefore, an attention to the adhesion between dissimilar materials has to be investigated. Before and throughout the project, it was found that there was a large misunderstanding of the simple assumptions related to the buckling-induced delamination method, used for measurement of the real adhesion of thin layers - 30-year-old and wrong interpretation of the results was being used even nowadays with modern, advanced material combinations. The project experimental and modelling work proved the wrong nature of such assumptions and produced a proper way of measurement analysis, depending on what material combination is used in the real device. This refined method of measurement allows for a better understanding of the delamination processes at the science-level, but also adds new tool for a better production of microelectronic and flexible devices, reducing the costs and increasing their reliability and durability. Additionally, the project dealt with few other obstacles, besides its main objective. Most notably with the proper ways of measuring the mechanical properties of thin layers of materials. While dealing with this issue, a more in-depth understanding of the indentation of materials on nanometer scale was investigated and the results of the side-work within this project revealed new discoveries enabling more precise measurements with indentation tools used in both research and industry. More precise measurements help again with the costs of development and final products, enabling more accurate tuning of materials towards higher reliability.