Design and Fabrication of Damage Tolerant Hierarchical Materials

Hierarchical, architected materials have the potential to be transformational for a variety of applications, providing components which simultaneously offer the best performance attributes of ceramics, metals, and plastics. Hierarchical materials are materials which concurrently realize functional features on multiple length scales (sub-micron up to a millimeter level). This allows large void space in a material structure to be filled with load bearing members, adding compliance to the material without significantly increasing the density. Researchers have pursued hierarchical material structures for almost 20 years, leveraging inspiration from both nature and architecture, but few have shown success in synthetically re-creating these structures at multiple length scales. Prior attempts to experimentally fabricate hierarchical structures using 3D printing have only produced structures at large (mm) scales while micro-fabrication techniques using expensive custom equipment to produce sub-micron structures do not translate well to high-volume, low-cost production. Palo Alto Research Center (PARC) has invented a novel approach for manufacturing large area hierarchical materials based on electrohydrodynamic film patterning (EHD-FP) that mitigates both of the aforementioned shortcomings. EHD-FP enables the fabrication of hierarchical, architected materials with features at multiple length scales while creating a process which can easily scale up to quickly create large area patterned films at low cost. This talk focuses on evaluating a series of two-dimensional (2D) EHD-FP kagome and triangular truss structures made with ultraviolet (UV) curable polymers. Using the EHD-FP process, films with hierarchical spatial features can be easily and rapidly created with low viscosity UV cross-linked polymers. The EHD-FP process also has the unique ability to infuse these hierarchical features with aligned nanoparticles during the curing phase, providing nanoscale structure. Estimations of mechanical toughness and strength will be presented based on experimental tensile testing results. Experimental results will be compared with modeling expectations of improvements in material properties. A finite element model (FEM) in COMSOL, a commercial simulation package, is used to further explore the impact of varying geometrical parameters and levels of hierarchy in the EHD-FP films in order to recommend an optimal subset of damage tolerant hierarchical materials structures. Our results provide confirmation of some of the underlying mechanics for hierarchical materials, allowing for a path towards transformative, large area hierarchical materials with superior functionality over bulk constituents.