Nanostructured Interphases for Improved Interfacial Adhesion in Structural and Ballistic Composites

Abstract

Fiber reinforced polymer matrix composites are a class of structural materials that have gained high desirability in a wide range of applications over the past few decades. Due to their high specific strength and toughness, low density, and design flexibility, fiber reinforced composites have been preferred over traditional homogenous materials, such as ceramics and metals, in structures and components within the military, aerospace, automotive and marine industries. The two constituents of these composite materials are typically the rigid fibers and compliant polymer resin, acting as reinforcement and matrix phases, respectively. Yet unlike biological multicomponent materials, such as bones, teeth, and bamboo, composites are heterogeneous materials suffering from failure-prone, discontinuous, and discrete fiber-matrix interfaces that limit them from achieving their ideal theoretical mechanical properties. Therefore, improving interfacial adhesion and the load transfer mechanism between the fiber and the matrix, while simultaneously maintaining the structural integrity and light weight of composite structures, is of great importance for the fabrication of high performance composites and has been a long-lasting challenge in the field of composite materials. This dissertation is an effort to experimentally investigate hierarchical and multifunctional fiber reinforced polymer matrix composites with improved interfacial and interlaminar adhesion through the integration of nanoscale interphases and interlayers. As nanotechnology regularly introduces new functional building blocks, many promising and lightweight nano-reinforcement approaches are continuously emerging and integrated into composite materials. Here, the potential and role of chemical interactions between nanomaterials (aramid nanofibers (ANFs) and nanofibrils, zinc oxide nanowires (ZnO NWs), laser induced graphene (LIG)) and fiber surfaces (aramid, carbon, glass), along with their impact on the morphology and adhesion quality of the resulting interphases and interlayers are initially investigated. As a result, it is demonstrated that well-adhered nanostructured interphases and interlayers can be achieved in various fiber reinforced composites through a number of chemical processes, which include fibrilization, physical and electrostatic adsorption, surface functionalization, hydrothermal growth, and laser-induced graphitization, as well as other mechanical approaches, such as transfer printing and spray-coating. Further research is then performed to thoroughly investigate and optimize the effect of the introduced nanostructured interphases and interlayers on the interfacial and interlaminar properties of both fabrics and composites under quasi-static and dynamic loading conditions, all while maintaining their structural integrity, light weight, and flexibility. The obtained results conclusively indicate that aramid nanostructured interphases are capable of enhancing the interfacial shear strength (IFSS) and interlaminar properties of quasi-statically loaded aramid and glass fiber reinforced composites, while also improving the impact response and stab resistance of ballistic protection aramid fabrics. Moreover, ceramic zinc oxide interphases are studied using a novel experimental setup and are shown to allow for the tailoring of composite interfacial properties as a function of the applied strain rate. Finally, ANF and LIG nanostructured interlayers are demonstrated to suppress delamination and improve interlaminar fracture toughness in both aramid and carbon fiber reinforced polymer matrix composites. The nanomaterial reinforced interlaminar regions exhibit improved toughening mechanisms that increase energy absorption, and thus delay catastrophic failure due to delamination in composite structures. The research presented in this dissertation provides a multitude of scalable and efficient approaches for the grafting of nanostructured interphases and interlayers capable of yielding hierarchical and multifunctional fiber reinforced polymer matrix composites with improved mechanical performance and maintained light weight and flexibility.