Polymers in the form of films are nowadays used in a wide range of applications, from the packaging of several products in food industry to the cutting edge organic optoelectronic devices. Despite their remarkable advantages, the polymer films suffer from poor mechanical and barrier properties. Overcoming of these drawbacks can be achieved by using inorganic, hybrid polymers and/or nanocomposite thin films to coat the flexible polymer substrate. In this way the permeation of the harmful atmospheric gases (H2O and O-2) through the polymer can be significantly reduced and, as a result, the product service life is prolonged. In this work, the nanomechanical behavior of 50 mu m thick flexible poly(ethylene terephthalate) (PET) substrate coated with AlOx barrier thin film (40 nm and 70 nm thick) is studied. A synergy of state-of-the-art experimental techniques, such as depth-sensing Nanoindentation, Scratch Test and Atomic Force Microscopy (AFM), was used to shed some light on the deformation mechanisms and the adhesion failures of the inorganic thin film on flexible PET substrate. Several nanoindents were made on each sample with a sharp Berkovich-type indenter, in various maximum normal applied load (P)/indentation depths. Analysis of the measured load-displacement (P-h) curves showed that for low loads (P <= 35 mu N) the contact between the indenter and the sample was almost elastic. The AFM images provided valuable information about deformation during higher normal applied load (P > 500 mu N) nanoindentation, which induces elastic/plastic deformation to the samples. Delamination/cracking of the barrier thin film from the PET flexible substrate was not observed even though the indenter penetrated almost 7 times the thickness of the thin film. On the other hand, cracking, fragmentation and finally delamination of the barrier thin film occurred during the ramping load Scratch Test. (C) Koninklijke Brill NV, Leiden, 2011