(205b) Mechanical Properties and Microstructure of the Nanoparticle Silica/Polymer Composite Coatings

Nanocomposite coatings containing ceramic nanoparticles and polymers are used in variety of applications such as paints, ink-jet print media, abrasion resistance coatings and optical coatings. Cracking from drying-induced stresses is frequently a problem in these coatings, which are prepared from dispersions of ceramic nanoparticles. The polymer is added to prevent cracking. The polymer provides strength to the porous coating, but changes the microstructure by filling in pore space. The polymer also affects the stress and the mechanical properties. Therefore, understanding the effect of polymer content on stress, microstructure and mechanical properties is necessary to create functional nanocomposite coatings. The goal of this research is to characterize microstructure, stress, and elastic modulus of nanocomposite coatings composed of silica nanoparticles and polyvinyl alcohol (PVA) polymer over the entire range.

Nanocomposite coatings were fabricated from suspension of nanosized silicon oxide particles (20 nm primary particle diameters within branched aggregates of 150 nm diameters) and polyvinyl alcohol (PVA) polymer (molecular weight range of 85,000-140,000 & degree of hydrolysis of 87.8%). The suspensions were deposited as coatings onto silicon substrates and dried at room temperature and nitrogen gas flow of ~ 3L/min. The final dried coating stress as measured by a cantilever deflection device indicated that the stress goes through maximum as the amount of polymer in the coating increases. The highest final stress was found to be ~ 110MPa at a PVA content of 60 wt%. The nanoparticle coating microstructure was characterized by imaging the dried samples using scanning electron microscopy (SEM). Porosity measurement using the nitrogen gas adsorption (micrometrics ASAP 2000) reveals that all the pores in the coatings are filled at 50wt% PVA content. The mechanical properties of silica nanocomposite coatings of varying composition were studied by the nanoindentation. The modulus of the composite coating increased as the polymer was added, reached maximum and decreased with further addition of polymer to the coating. The research demonstrates that the mechanical properties and the microstructure can be tuned be varying the relative amount of polymer in the silica nanoparticles coating. For example porous but strong and crack free nanocomposite coatings can be prepared with 30 wt% polymer. Furthermore, nonporous coatings with high modulus can be prepared with 50 wt% polymer content.