Fabrication and Characterization of Multifunctional Bio-Inspired Composites

Abstract

This dissertation concentrates on a comprehensive study on bio-inspired (“Nacre”-like) composite materials. It involves the design and fabrication approaches and characterize properties of composite materials including meta-ceramic brick-and-mortar composites and piezo-polymer matrix ceramic reinforced composites. Hybrid composites of layered brittle-ductile constituents assembled in brick-and-mortar architecture are promising for applications requiring damage tolerance. Mostly, polymer mortars has been considered the ductile layer, however, low stiffness of polymers does not efficiently transfer the shear force between hard ceramic bricks. Theoretical models point to metals as a more efficient mortar layer. However, infiltration of metals into ceramic scaffold is non-trivial, given the low adhesion between metals and ceramics. We report on an alternative approach to assemble brick-and-mortar metal-ceramic composites by using electro-less plating of nickel on alumina micro-platelets, which are subsequently aligned by magnetic field, taking advantage of paramagnetic properties of nickel. The assembled nickelcoated ceramic scaffold is then sintered using spark plasma sintering (SPS). We report on materials and mechanical properties of the composite. The fabricated metal-ceramic composite shows a rising R-curve fracture behavior. The results show that this is a promising approach toward development of damage-tolerant metal-ceramic composites. Hybrid materials of inorganic-organic phases in which each phase provides different functionality are attractive candidates for achieving multi-functionality. Using a layer-by-layer approach, we fabricated sheets of piezoelectric polymer P(VDF-TrFE) reinforced by aligned sub-micron thick platelets of single crystal sapphire. The as-fabricated films were transparent and piezoelectric, exhibited ductility up to ~330%, and tensile toughness of up to 26 J/g. In addition, we investigated the effect of thermal annealing of the polymer on the crystallinity of the polymer phase and its effect on the mechanical and piezoelectric properties of the fabricated films. Thermal annealing resulted in improvement of the elastic modulus and piezoelectric properties of the films. PVDF and its co-polymers piezoelectric polymers in film and nanofiber forms are increasingly used for sensing, actuation and energy harvesting. Given the semi-crystalline structure of these polymers, their electromechanical coupling behavior changes with thermomechanical processing. This research reports on the evolution of the mechanical properties, piezoelectric properties and morphology of P(VDF-TrFE) piezoelectric polymer thin films fabricated by spin- coating during thermal annealing and drawing, studied via tensile test, polarized optical microscopy, X-ray diffraction, polarized FTIR, and piezoresponse force microscopy (PFM). The results show that annealing and drawing process result in 10 and 13 times improvement in the elastic modulus and ultimate strength of the films, respectively. In addition, the piezoelectric constant and electromechanical coupling improves by 30% and more than 17 times, respectively. These changes are accompanied by 65% increase in the percentage of the crystallinity of the semi-crystalline piezoelectric films.