Properties of nano-hybrid sol-gel coating films synthesized with colloidal silica and organoalkoxy silanes

Colloidal silica (CS)-silane sols were synthesized as a dispersion medium of dispersoid, ceramic powders. Functionalizing silanes for nano-hybrid sols were methyltrimethoxysilane (MTMS) and gamma-glycidoxypropyltrimethoxysilane (GTMS). GTMS was added to sols in different amounts. The stability of the sol was evaluated as a function of reaction time up to 10 days through the variation of the viscosity and the transmittance. The viscosity of the sols increased only slightly with reaction time before 10 days. When the sols were reacted for 10 days, the viscosity increased and the transmittance decreased markedly, as well. The coating films were formed on a glass substrate using a spin coating procedure. The coating films were flat and transparent without cracks. Also, the properties of the coating films such as contact angle, roughness, and thermal property were characterized. The CS/MTMS coating films showed a large contact angle. When a large amount of GTMS was added the contact angle decreased due to the epoxy group. The roughness of the coating films was found to be very low, less than 20 nm. However the roughness of the coating films reacted for 10 days increased due to degradation of the stability of the sol. No thermal degradation of the CS/MTMS coating film occurred up to 600 degrees C. The CS/MTMS/GTMS coating films degraded at 400 degrees C due to decomposition of the epoxy group which has a long organic chain easily degraded by the thermal shock.