Functional interphases for structural composites

Safety of buildings and other structures is of great importance in engineering, since the structural integrity of a component may be compromised over time due to a variety of internal and external loads. The use of advanced materials, such as fiber-reinforced composites (FRCs), has become essential in construction due to their superior properties. However, FRCs suffer from interfacial debonding, i.e. the nucleation and the propagation of cracks at the contact region between matrix and fibers. Robust interfacial adhesion and self-healing capability are crucial to reduce defects and extend the material service life. The intent of this Doctoral Thesis was to provide a contribution in this field, by focusing on two primary research areas: the enhancement of the interfacial adhesion and the development of self-healing interfacial capability. Both research areas involved the investigation of microscale interfacial properties through the fabrication of microcomposites subjected to microdebonding tests, in order to allow highly accurate estimations of the interfacial properties. In the first research area, two innovative methods such as triboelectrification and laser treatment were explored. Triboelectrification involved the rubbing of glass fibers (GFs) against a polytetrafluoroethylene (PTFE) surface to generate surface charge, therefore to attract graphene oxide (GO) nanosheets on the fibers and to create a compact and homogeneous coating. The resulting interfacial adhesion with epoxy resin (EP), which was expressed as interfacial shear strength (IFSS), was enhanced up to 45% compared to neat fibers. On the other hand, basalt fibers were treated with CO2 laser to increase their mechanical interlocking with the EP, and in this case a 8% improvement in IFSS was obtained. In the second research area, a continuous film and a nanostructured coating made of poly(ϵ-caprolactone) (PCL) were deposited on GFs and carbon fibers (CFs) to impart interfacial self-healing properties. Scanning electron microscopy and optical microscopy analyses confirmed the formation of well-applied and homogeneous coatings. In the case of the PCL film, an increase of 16% in IFSS with EP was obtained, while in the case of the PCL nanostructured coating different effects on IFSS were obtained, ranging from +26% for glass fibers to -60% for carbon fibers. Self-healing efficiency (H.E.), calculated as the ratio of IFSS before and after thermal mending, reached an impressive 100% for the PCL-film coated GFs, with a gradual decrease over multiple debonding/healing cycles. An alteration of the matrix droplet's meniscus was observed, which was hypothesized being the cause of the decrease over multiple H.E., due to the modification of the stress state near the fiber/droplet contact point. Moreover, also in the case of PCL-nanocoated CFs, the interfacial debonding was completely restored. Future steps of this research will include the development of a finite element model (FEM) of the interfacial region to simulate the microdebonding test and to provide a comprehensive understanding of the interfacial debonding and healing mechanisms over multiple healing cycles.