Abstract: The present invention provides method and process for forming a barrier layer on a flexible substrate. The continuous roll-to-roll method includes providing a substrate to a processing chamber using at least one roller configured to guide the substrate through the processing chamber. The process includes depositing a barrier layer adjacent the substrate by exposing at least one portion of the substrate that is within the processing chamber to plasma comprising a silicon-and-carbon containing precursor gas. The present invention is further directed to a coated flexible substrates comprising a barrier layer based on the structural unit SiC:H. The barrier layer possesses high density and low porosity. Still further, the barrier layer exhibits low water vapor transmission rate (WVTR) in the range of 10?2-10?3 g.m?2d?1 and is appropriate for very low permeability applications.