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Thin plasma polymerized gas barrier coatings were applied on PET films using low-pressure microwave excited hexamethyldisiloxane plasma. Oxygen and water vapor transmission rates were determined for the same barrier coating of varying thickness and correlated with the coating porosity. The porosity was quantified on different scales with two complementary methods. A plasma etching process with subsequent high-resolution SEM imaging and an automated defect detection as well as an overall porosity measurement by means of cyclic voltammetry. This method combination can be used to assign and classify Knudsen diffusion and Knudsen diffusion for different coatings and defect distributions applied. Film growing process could be observed with correlating defect distributions and it could be determined that almost no macro defects with a radius rd > 150 nm are present while growing process. With visually closed coatings, the entire oxygen and water vapor permeation could be attributed to Knudsen and solid body diffusion, although it is difficult to divide the proportions of these two processes.

Thin SiNwOxCyHz coatings were deposited from hexamethyldisilazane as a precursor in a microwave driven low pressure plasma enhanced chemical vapor deposition process, in order to investigate their suitability as silicon based separating layers in membranes for gas separation. Polydimethylsiloxane composite membranes were used as substrate, as they have a dense and defect free surface and by this provide a smooth surface to ensure a homogenous and defect free coating. To evaluate correlations between process parameters, coating properties and permeation/selectivity performance, the influence of different compositions of the feed gas (auxiliary gas and monomer) on the chemistry and structure of the coatings and subsequently on permeability were investigated. For this, auxiliary gas was varied (N2, none and Ar) and coatings/membranes were analysed regarding their structural properties with atomic force microscopy and cyclic voltammetry as well as their chemical properties with x-ray photoelectron spectroscopy. Correlations between those properties and the permeation properties were examined. The investigations reveal that coating and gas transport properties can be adjusted by changing the auxiliary gas type. Membrane selectivities could be produced that are above Knudsen selectivity, especially for the gas pairings CO2/N2 (up to 15), He/N2 (up to 9) and CO2/CH4 (up to 8) at 30 °C.

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Hexamethyldisiloxane (HMDSO)‐ and hexamethyldisilazane (HMDSN)‐based plasma polymeric coatings are compared as elements of multilayer barrier coatings, consisting of an alternating structure of organosilicon and silicon oxide layers. Furthermore, these coatings are examined with regard to their deposition rate, critical layer thickness, surface roughness, atomic composition, and gas permeability. It is investigated how these attributes correlate in the overall barrier performance of a multilayer coating system. The concluded examinations show that HMDSN delivers best overall barrier performances as a precursor at high energy densities. For lower energy densities, the use of HMDSO is preferable. Intermediate layer thickness should not exceed six nanometers for best barrier performance in the given experiment configuration.