<p>The mechanical performance of a fibre-polymer composite is largely determined by the strength of interfacial adhesion across the fibre-polymer phase boundary. Therefore, a critical step in the design  of novel composite materials is selecting a coupling agent which will maximise the interfacial adhesion and optimise the mechanical properties of the composite. Typically, trial-and-error experimentation is required to find which coupling agent maximises interfacial adhesion and produces the composite with the most desirable mechanical properties. The aim of this thesis is to enhance our current understanding of the key processes involved in composite interfacial adhesion and to use this knowledge to design a novel test method which can predict and quantify which coupling agent will result in the best interfacial adhesion and  maximise a composite’s mechanical performance.</p> <p><br></p> <p>Si3N4 fibres were functionalised with a variety of silane coupling agents (SCAs) and dispersed into four typical thermoset resins. The mechanical performance of each composite was tested by measuring the tensile strength, Young’s modulus, toughness, Shore D hardness and  flexural strength. Testing showed that changing the SCA at the interface had a significant effect on the mechanical performance of the composite. Microstructural analysis of the fractured surfaces of the composites using scanning electron microscopy confirmed that the differences in the mechanical properties were largely determined by the nature of the fibre-polymer interface.</p> <p><br></p> <p>This work focused on developing industrially-suitable techniques for measuring and quantifying interfacial wettability and interfacial chemical bonding as a measure of composite adhesion. Two methods for measuring interfacial wettability were proposed and compared, with consistent results. The results from the wettability tests were compared  to the mechanical performance of the composites. It was concluded that wettability did not have a strong direct influence on the mechanical properties of the composite and that wettability cannot be used, in isolation, as a reliable predictive model for interfacial adhesion and composite performance. FT-IR and Raman spectroscopy were used to  quantify the chemical bonding at the composite interface. SCAs were added directly to the polymer resins and the changes to the peak intensities of the interfacial functional groups were monitored over time. The SCAs that had vibrational modes which demonstrated a significant reduction in peak area were likely to demonstrate the highest amount of interfacial chemical bonding. Analysis via FT-IR  spectroscopy generally proved to be less effective than Raman, as the IR functional group peak intensities were too weak for quantitative conclusions. The SCAs which had the greatest amount of observed interfacial chemical bonding corresponded to the composites with the best mechanical properties indicating that this was a suitable predictive model for interfacial adhesion and composite performance.</p> <p><br></p> <p>The combined effects of wettability and chemical bonding at the interface on the mechanical performance of the composite were analysed. It was concluded that, although the presence of interfacial chemical bonding had a more significant contribution towards improving composite  properties, both factors had some level of influence. The best composite properties were achieved with strong interfacial bonding and a high wettability. Using the various test methods to quantify interfacial wettability and chemical bonding, it was possible to predict which SCA produced the composite with the best mechanical performance. </p>