Modeling competing fracture for dry transfer of thin films to a flexible substrate




Jain, Shruti, Ph. D.

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Dry transfer printing is where a thin film is transferred from a host substrate to a target substrate by taking advantage of the difference in adhesion between the thin layer and the substrates. This technique can lead to high throughput industrial-scale manufacturing of flexible devices using thin films and 2D materials. A major roadblock for applying the method is an inadequate understanding of how transfer printing depends on the material properties of the substrates and the thin film and their interfacial interactions, which limits the reliability of dry transfer methods. This dissertation provides this knowledge through computational analysis with cohesive zone models. Two approaches are used: 2D finite element simulations and 1D finite difference beam theory models. Both mode I and mixed-mode fractures are simulated. Scaling equations are developed to quantify load, crack length, end rotation, damage zone length and mode-mix as a function of material and interface properties. Competing fracture during the transfer of a 2D material like graphene is simulated with finite element models in ABAQUS®. Two damage initiation criteria are used with cohesive zone models. Interface strength is observed to be the primary factor affecting crack path selection for both damage criteria. Weaker interfaces break first, independent of the fracture energy. However, convergence issues with ABAQUS® are identified. So, fast, robust beam theory models are developed to understand the parametric dependence of crack growth on material and interface properties and captured in a fracture map. In addition to 2D materials, transfer printing of thin films is also studied. Parameters such as interface defects, thin film thickness and interface properties are varied to understand crack growth in thin film transfer. Interface defects modeled as initial crack lengths are observed to be the most significant factor in determining the success of the transfer process. When interface cracks are of equal lengths, fracture energy determines the crack path selection for stiff thin films and thicker films whereas for soft thin films, interface strength determines which interface breaks. A strong correlation between steady state crack tip mode-mix and crack path selection is observed despite using a mode-independent fracture energy.


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