Crystallization of amorphous Si3N4 and superhardness effect in HfC/Si3N4 nanomultilayers

HfC/Si3N4 nanomultilayers with various thicknesses of Si3N4 layer have been prepared by reactive magnetron sputtering. Microstructure and mechanical properties of the multilayers have been investigated. The results show that amorphous Si3N4 is forced to crystallize and grow coherently with HfC when the Si3N4 layer thickness is less than 0.95 nm, correspondingly the multilayers exhibit strong columnar structure and achieve a significantly enhanced hardness with the maximum of 38.2 GPa. Further increasing Si3N4 layer thickness leads to the formation of amorphous Si3N4, which blocks the coherent growth of multilayer, and thus the hardness of multilayer decreases quickly.