Nanoscale characterization of interphase properties in glass fibre composites

Novel experimental techniques, including the nanoindentation and nanoscratch tests and the thermal capacity jump measurement based on differential scanning calorimetry (DSC), have been successfully employed to characterize the properties of the interphase in glass fiber reinforced composites. The nanoindentation test involved a series of indents 400 nm apart and as small as 30 nm in depth to identify the material at the transition region between the fiber and matrix in terms of hardness and elastic modulus. The nanoscratch test involved scratching the specimen along a line across the fiber and matrix, measuring continuously the indenter depth and the tangential force corresponding to a fixed normal force of 2.5 mN. The effective thickness of interphase was also estimated from the consideration of differential heat capacity jump between the fiber-reinforced and unfilled resins in the glass transition region. This method is based on the principle in that as the reinforcement content or fiber volume fraction is increased, the jump in heat capacity in the glass transition region of resin is correspondingly reduced. it is suggested that the macromolecular state of polymer resin in the vicinity of rigid reinforcement surface be modified, which is excluded from the process of glass transition. Parameteric studies are made to evaluate the effects of several important factors, such as matrix material, including vinylester, phenolic and polyester, silane coupling agent concentration and aging in water on the effective thickness of interphase region created between the fiber and matrix.