The invention relates to a curable composition comprising the following components: (A) at least one isocyanate, (B) at least one binder, and (C) at least one polyhedral oligomeric silsesquioxane, with the number of cages z per molecule being 2 or 3 and the number of tin atoms per molecule being at least one. Furthermore, the invention refers to a kit of parts comprising as components A), B), and C); to the use of the polyhedral oligomeric silsesquioxanes in coatings, lacquers, paintings, films and polymer compositions; to coatings obtainable by curing of these compositions; and to the use of polyhedral oligomeric silsesquioxanes containing at least one metal atom, in particular polyhedral oligomeric stannasilsesquioxane to increase the scratch resistance of coatings.