Maxwell-Garnett theory – a method to investigate the quality of SiO_x coatings

For single-layer barrier coatings, the influence of coating defects of thin SiO_x coatings on the barrier properties was investigated. Coating defects only originate from the coating process; they seem to form independently from the surface roughness of the substrate. The coatings were deposited in a roll-to-roll process (width 1 m) by using the plasma-enhanced chemical vapour deposition technique. The SiO_x coatings, applied in this work, have thicknesses in the range of 30–50 nm with a carbon content of less than 1 at-%. They show an oxygen transmission rate of 1·3 ± 0·4 cm³/(m² day bar) at best. The focus of this research is to investigate the influence of plasma pressure on the coating defects and therefore to the barrier properties. In order to identify nanosized coating defects inside the coating, the Maxwell-Garnett theory was used. The defect volume was indirectly calculated with the help of ellipsometric measurement data. The Maxwell-Garnett theory is a very fast method to investigate the quality of barrier coatings.