Nanomaterial-Based Surface Modifications for Improved Ballistic and Structural Performance of Ballistic Materials

Abstract

Improving the ballistic protection abilities of engineering materials has become increasingly important given the continuous rise in threat levels. Two polymeric fibers heavily used as ballistic materials for their high strength to weight ratio, impact resistance, energy absorption, and wear and abrasion resistance are ultra-high molecule weight polyethylene (UHMWPE) and aramid fibers. These fibers are used both as fiber reinforcement in polymer matrix composites for hard body armor applications, such as helmets and military vehicle armor panels, and as woven fabrics for soft body armor applications, such as bullet proof vests and gloves. Yet, in the case of fiber-reinforced polymer composites (FRPCs), these polymer fibers are known to suffer from poor adhesion to the matrix due to their smooth and chemically inert surfaces, resulting in discontinuous interfaces that limit their FRPCs from reaching their use in structural applications. In addition, when these polymer fibers are woven into a fabric, their resulting low inter-yarn friction causes for easy slipping between neighboring yarns, which reduces structural packing within the woven fabric and results in failure at lower velocity threats. Therefore, both the fiber-matrix interface and the inter-yarn friction of UHMWPE and aramid fibers require considerable improvements for furthering their use and their performance in ballistic and structural applications. Research efforts aiming to improve these properties have mainly relied on the development of fiber surface modification techniques. Particularly, nanoscale surface modifications have gained attraction in recent years due to their ability to simultaneously introduce bonding mechanisms that rely on chemical interactions and mechanical interlocking. However, current approaches are known to compromise the structural integrity of the fibers, its weight, or its scalability to industrial level applications. This dissertation explores the use of three nanomaterial-based surface modifications, primarily aramid nanofibers (ANFs) and zinc oxide nanowires (ZnO NWs) on UHMWPE fibers and laser induced graphene (LIG) on aramid fibers, in order to improve the interfacial and inter-yarn properties of UHMWPE and aramid fibers through simple, fast, and benign processes. The adhesion between ANFs or ZnO NWs and the UHMWPE fiber surface was initially improved using a surface functionalization treatment that formed a well-adhered nanostructured interphase. Single-fiber pullout was used to investigate and optimize the interfacial reinforcement effect the ANF and ZnO NW interphases have in UHMPWE FRPCs. The ZnO NW interphase on woven UHMWPE fabric was then studied through yarn pullout and impact testing where improvements in the fabric’s inter-yarn friction, impact performance, and energy absorption were demonstrated. Finally, the use of a LIG interphase in aramid FRPCs was shown to suppress delamination and improve both interlaminar fracture toughness and impact resistance. Concurrently, the piezoresistive LIG interphase was shown to be capable of enabling in-situ structural health monitoring via electrical resistance measurements that correlate to impact damage and delamination in aramid FRPCs. The work in this dissertation demonstrates effective methods to modify the chemically inert and smooth surfaces of UHMWPE and aramid fibers through three nanomaterial-based surface modifications that enable improved interfacial and inter-yarn properties and further advances the integration of the FRPCs and woven fabric in high-performance structural and ballistic applications.