STUDY OF THIN ALUMINA COATINGS SPUTTERED ON POLYETHYLENE TEREPHTHALATE FILMS

Thin amorphous alumina coatings have been deposited on polyethylene terephthalate (PET) by r.f. magnetron sputtering in a pure argon plasma. The influence of the deposition parameters (sputtering power and argon pressure) on the assembly properties is discussed. Scanning electron microscopy analysis shows that the microstructure is variable within a wide range, from dense to porous and columnar, in agreement with Thornton's structure zone model. The composition of coatings determined by Rutherford backscattering spectrometry reveals that near-stoichiometric alumina containing a low amount of argon is obtained for an argon pressure of about 1 Pa, Some correlations between the microstructure of deposits and their composition are proposed to explain the excess of oxygen in the coatings elaborated at high pressure. The stresses in the coatings determined by the bending beam method are compressive; the maximum values are obtained in coatings exhibiting the most compact structure. The hardness of the amorphous alumina coatings, determined by a microindentation test, varies from 6.6 to 11.7 GPa depending on the sputtering parameters. The adhesion of alumina on PET is measured by a peel test, with maximum adhesion being obtained at moderate power (1 W cm(-2)) and for about 1 Pa pressure. The adhesion seems to be influenced by the kinetic energy of the sputtered atoms and the total stresses within the coatings.