The application of modified Al2O3 nanoparticles for thin coatings is investigated to improve especially the mechanical characteristics by cross-linking the particles using an additive, together with the formulation parameters as well as the cross-linking kinetics. In general, the experiments demonstrate that the mechanical properties, like hardness and scratch resistance, can be enhanced via particle surface modification in combination with a cross-linking additive. Specifically, the optimal additive concentration was determined in order to obtain coatings with improved mechanical and constant optical properties. These results were related to the structure formation and cross-linking behavior. Mechanisms were identified for this series of experiments to explain how the particles bond with each other and how the coating structure is formed by using different additive ratios.