Nano-porosity in silica reinforced methyltrimethoxysilane coatings studied by positron beam analysis

The porosity in particle reinforced sol-gel coatings has been studied. Silica particles (Ludox-TM40) are introduced into methyl silicate coatings to increase the hardness, the elastic modulus and the fracture toughness. The methyl silicate has a relatively low density (about 1.2 g/cm2), while the silica particles are known to be porous. However, the porosity of the silica particles is not accurately known. For model calculations on mechanical properties like the E-modulus this porosity should be known. Positron Beam Analysis (PBA), using the Doppler Broadening (DB) and 2D-Angular Correlation of Annihilation Radiation (2D-ACAR) techniques, was therefore performed for analysis of the porosity. Samples with different weight fractions (0, 20 and 63 wt.%) of silica particles of typically 40 nm in diameter and treated at different curing temperatures (623 and 723 K) were measured. With increasing filler content we observed a decrease in the positron annihilation S-parameter and a broadening of the para-positronium (p-Ps) fraction. By neglecting positron diffusion we can separate porosity in the matrix from that in the particles. This assumption is valid as long as the expected positron diffusion length is short compared to the size of the filler particles, as in the present case. A more detailed description takes into account the local environment of the filler particles affecting their adhesion to the matrix. It is concluded that the density of the silica particles is about 1.4 g/cm2.