The process for preparing a composite film for packaging purposes, that includes coextruding polyethylene terephthalate (PET) and polyethylene-2, 6-naphthalate (PEN) to form a film of polyethylene-2,6-naphthalate. On at least one of the at least one PEN-coated side, a ceramic layer of SiO_x, where x is number between 0.9 and 2, is deposited by means of the thin-film vacuum process through the simultaneous vaporization of silicon dioxide (SiO₂) and metallic silicon, or by the vaporization of silicon monoxide (SiO) in a vacuum. Each of the at least one ceramic layers of SiO_x having a thickness of 10 nm to 200 nm. The resultant composite film retains effective permeability barrier effect for water vapor and gases after sterilization in a water bath or in water vapor at a temperature of more than 90 C.