The hardnesses of polycarbosilane and polysiloxane coatings subjected to irradiation with increasing doses of He, C and Au ions were measured by means of nanoindentation tests. Diamond-like carbon clusters which are formed during irradiation enhance the hardnesses of the ceramic films which reach in some cases three times that of conventionally annealed specimens. When submitting irradiated films to an additional anneal, the clusters resulting from the segregation of atoms from methyl groups remain more diamond like than those formed directly by radiolysis of phenyl rings, especially when the radiolytic transformation is incomplete (the film having received a low irradiation dose). Moreover, owing to the selective release of hydrogen and the recombination of free radicals at room temperature, no evolution of CHx and COx nor oxidation of unpaired Si atoms occurs during this post-irradiation anneal, contrary to the observations during direct thermal conversion.