Exploring the Interface: Improving the Mechanical Properties of Silicon Nitride Fibre - Polymer Composites

M. Hayward, J. Johnston, T. Dougherty, K. deSilva
Victoria University of Wellington, Nuenz Limited, NZ and NZ Product Accelerator,
New Zealand

Keywords: reinforced polymers, interfacial adhesion, silicon nitride nanowires, polymer composites


Interfacial interactions between a fibre reinforcement and a polymer matrix are a fundamental contributor towards the mechanical, thermal and electrical properties of fibre-polymer composites. Strong adhesion at the phase boundary facilitates efficient property transfer between the fibre additive and the polymer resin creating advanced, high performance composite materials. Silicon nitride (Si3N4) fibres are an attractive reinforcement material for polymer composites due to their chemical inertness, rigidity, high aspect ratio, non-flammability and high tensile strength. Effective transfer of these properties to the polymer matrix can produce a polymer composite material with enhanced mechanical properties, such as hardness and tensile strength. This requires excellent dispersion of the fibres within the matrix and a strong chemical bond linking the phase boundary between the Si3N4 fibre surface and the polymer resin. Bifunctional coupling agents containing both inorganic and organic functionalities can be used to modify the surface of silicon nitride fibres to facilitate a chemical link between the fibre surface and polymer resin. This work examines and compares the effects of various coupling agents, including silane, titanate and zirconate based coupling agents, on the interfacial adhesion within the composite structure. This study focuses on exploring various surface functionalisation techniques to alter the surface chemistry of silicon nitride fibres. Our work is devoted to maximising the interfacial attraction and chemical binding to a polymer matrix, optimising the dispersion of the fibres, and assessing how this effects the physical and mechanical properties of the composite material. We report the dispersion and adhesion of silicon nitride fibres in commercial polymer resins with the aim of improving the processability, durability, hardness and tensile strength of the composite. Si3N4 fibres were functionalised with a monolayer coverage of coupling agent and incorporated into a commercial resin. The fracture surfaces of the composites were analysed via scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) which show enhanced adhesion and interfacial binding of the functionalised fibre surface to the polymer resin compared to the unfunctionalised fibre surface. FTIR analysis of the composite materials illustrate a downfield shift in the resin peaks after functionalisation. This is due to a chemical interaction between the coupling agent and the polymer matrix causing a change in the polymer structure network. The formation of a chemical bridge between the fibre surface and the polymer matrix facilitates effective transfer of properties and superior composite performance. The physical and mechanical properties of the composites materials were analysed and compared. The viscosities of the filled pre-polymer resins were measured to assess the processability of the composite resin. It was found that addition of functionalised fibres lowered the viscosity of the pre-polymer and hence improves the ease of manufacture and processability. The functionalised Si3N4 composites demonstrated increased hardness and tensile strengths when compared to the neat polymer and unfunctionalised Si3N4 composites.