The main objective of this study was to prepare bead milled-silica nanoparticles (SiO2) as reinforcing materials for transparent hard coating films. SiO2 dispersed in Dowanol PM without any stabilizer was used as a main component in the nanocomposite hard coating films to improve hardness of Poly methyl methacrylate (PMMA) sheets. However, the major challenge in hard coating formulation is the dispersion of nanoscale SiO2 particles. Bead milling machine (MiniCer, NETZSCH, Germany) equipped with different sizes of zirconia (ZrO2) beads (0.1, 0.5, and 1.0 mm) was used for dispersing 40wt% SiO2 in Dowanol PM to achieve target sizes of 200, 500, and 800 nm. The dispersed nanoparticles were characterized by UV-visible spectroscopy for their optical transmission, transmission electron microscopy (TEM) for particle morphologies, and dynamic light scattering technique (DLS) for the particles sizes. The milled-SiO2 nanoparticles were stable in Dowanol PM as suspensions with their particles sizes closed to the target sizes. The 200-nm suspension showed the longest storage time without any aggregate formation. Whereas, the dispersed nanoparticles suspensions with the particles size >500 nm formed agglomerates during storage. The SiO2/MTMS nanocomposite coating film was then prepared coated milled-SiO2 suspension in Dowanol PM on PMMA sheets. The film with 200 nm SiO2 showed the highest transparency (92% at 550 nm) which was like the uncoated PMMA sheets. At thickness of 3-microns, SiO2/MTMS nanocomposite films could improve pencil hardness of PMMA sheets from <H to 3H.
Friction and Wear Properties of Hard Materials Against Metals. Comparison of Hard Coating Films and Sintered Ceramics.