Inorganic-organic hybrid nanocomposite sols were prepared using tetraethyl orthosilicate (TEOS), 3-(glycidoxypropyl)trimethoxysilane (GLYMO), n-butanol, water, methanol, and catalytic amounts of HCl and Al(acac)(3). Hydrolysis-condensation reactions of TEOS at pH approximate to 1.3 generates silica nanoparticles in the sol that remain bonded with the -Si-O-Si- network and protected by the organic functionality of GLYMO. The pH of the final sol was adjusted to close to the isoelectric point of silica (pH similar to 2) to increase the shelf life of the sol. The resulting sol (obtained from the best optimized composition TEOS:GLYMO = 2.33:1) when deposited on CR-39 or related plastics yielded optically transparent and spot-free hard coatings after thermal curing at 95 degrees C. About 1.5-2 mu m thick coatings serve all international specifications required for hard coatings. Thermal curing in the presence of Al(acac)(3) ensured polymerization of GLYMO originated epoxy groups to polyethylene oxide (PEO). BET surface area measurement confirms that the cured coatings are nonporous (surface area 0.6-0.8 m(2) g(-1)) in nature. The density of the coating was measured by the X-ray reflectivity technique (XRR) and found to be 1.70 g cm(-3). TEM shows flaky plastic-like characteristics of the coatings, and small-angle X-ray scattering (SAXS) study reveals the presence SiO2 nanoparticles of average size 5.4 nm inside the coatings. The pencil hardness value of the coatings (thickness 1.5-2 mu m) was >6H. The high hardness of these nanocomposite coatings is mainly due to the in situ generated silica nanoparticles chemically bonded with the highly cross-linked silica-PEO network.