Strengthening mechanism of porous silica films derived by two-step catalysis

A scratch-resistant improvement of sol-gel derived nano-porous silica films by a base/acid two-step catalysis is reported. The silica films were prepa red with the two-step catalytic sol-gel process and a dip-coating method, a nd then post annealed in air and a mixed gas atmosphere. The films were cha racterized with FE-SEM, AFM, TEM, XPS, FTIR, an ellipsometer, and an abrasi on test. With the post annealing, the frequency shift of the FTIR absorptio n omega (4)(TO3) peak of the films to a lower wavenumber results from a dec rease in the average Si-O-Si bridging angle. An increase in the full-width at half-maximum of the omega (4) peak is ascribed to a wide distribution of the bridging angle because of formation of strained Si-O-Si bonds caused b y the post annealing. XPS analysis shows that the effect of the mixed gas a nnealing on the binding energy and full-width at half-maximum (FWHM) for Si (2p) and O (Is) peaks is attributed to the further condensation reaction o f the films resulting from the disappearance of the OH groups. The abrasion and adhesion tests indicate that the two-step catalysis obviously improves the scratch resistance and adhesion of the films, and that the mixed gas t reatment further strengthens the silica network. The increase in strength i s attributed to more Si-O-Si bond cross-linkages between silica particles f ormed by the two-step catalysis and the mixed gas treatment.