Amorphous hydrogenated carbon films as barrier for gas permeation through polymer films

The influence of amorphous hydrogenated carbon (a-C:H) coatings on the gas permeation through polymer films was investigated. a-C:I-S films were deposited from a 13.56-MHz RF glow discharge in methane or acetylene atmosphere. Thin poly(ethylene terephthalate) and polyimide foils were used as substrates. The permeation of the gases H-2, N-2, O-2 and CO2 was measured and the reduction of the permeability coefficient was correlated to composition and density of the a-C:H films. The stoichiometry of the layers was analyzed using ion-beam techniques on films deposited onto silicon samples. The a-C:H/PET surfaces were analyzed using optical microscopy and atomic force microscopy (AFM). Multilayer structures comprising different types of a-C:H films were also investigated. A reduction of the permeability coefficient by 80% for hard, dense and 94% for soft, polymer-like layers was found. Surprisingly, the barrier efficacy of the coating decreases with increasing a-C:H film density. This unexpected result is attributed to the appearance of a network of deep cracks spread out over the whole coating. (C) 2000 Elsevier Science B.V. All rights reserved.