Enhanced mechanical properties of polyurethane composite coatings through nanosilica addition

Tin rich surfaces inside Pb-free electronics are more prone to tin whisker growth, thereby resulting in electrical failure of components. Conformal polyurethane (PU) coatings, which can be used as a mitigation strategy for such whisker growth, were investigated in this study. Nanoparticles of silica (∼20 nm in diameter) were added via liquid suspension to the PU resin to improve its mechanical properties. Four compositions of functionalized nanosilica suspensions, 10 wt.%, 20 wt.%, 30 wt.% and 50 wt.% (corresponding to the solid silica weight of 3.50%, 6.74%, 9.75%, and 15.17%, respectively) were evaluated to establish the relationship between microstructure developments and mechanical properties. Fourier-transform infrared spectroscopy (FTIR) and low-kV field emission scanning electron microscopy (FE-SEM) were used to examine the effects of nanosilica suspension addition to the PU resin. Mechanical properties of the coating were not only evaluated by nanoindentation for localized PU matrix properties but also by uniaxial tensile testing for composite properties. Results from both scales show that the addition of the particle suspension up to 20 wt.% (corresponding to 9.75 wt.% nanosilica) makes optimum mechanical performance consisting of good modulus, strength and ductility. This conclusion is also supported from microstructural, as well as chemical aspects of the PU coatings with the addition of nanosilica suspension.