The reduction of helium gas permeability through polyetheretherketone (PEEK) microfilm (250 mu m) coated with titanium (Ti) or tetrahedral amorphous carbon (ta-C) nanofilm was investigated. Two morphologies of PEEK film were used (amorphous and semi-crystalline), and four thicknesses of nanofilm were deposited (5, 20, 50, or 100 nm). Both types of nanofilm coatings reduced the overall permeability of the PEEK film, with the greatest permeability reduction of similar to 83% seen for the 100 nm Ti coating on semi-crystalline PEEK. Defects were identified in both types of coating; however the nature of the defects was different. The ta-C coatings exhibited a network of microcracks, while pinholes were present in the Ti coatings. The defects in both coatings were considered the dominant path for gas flow. Overall, Ti performed better than ta-C with regard to permeability reduction, and Ti also demonstrated a correlation between coating thickness and permeability reduction. (C) 2011 Elsevier B.V. All rights reserved.