In this work, several types of plasma treatments were performed on the surface of polyethylene terephthalate (PET) substrates. Experimental results show that an increase in contact angle is related to an increase in the average roughness. Contact angle values vary from 71 to 84°, with average roughness values ranging between 3.3 and 7.2 nm for different surface treatments. FTIR in mode of attenuated total reflection show that there were no significant chemical changes in the material, while XRD reveals that there are no detectable changes in the structure of the polymers. The stress-strain curves are similar to the material without treatment: linear up to about 8 % of strain, followed by a plastic deformation. The Young modulus, yielding stress and strain of the materials stay, regarding the experimental error, within the values obtained for the untreated polymer: 2.1 GPa, 109.4 MPa and 153.3 MPa, respectively. The adhesion of the titanium nitride (TiN) to the polymer is very strong, without any traces of film removal, despite the test method: ASTM-D3359-97 or peeling test at 90°. The set of results shows that the plasma treatments are effective in promoting adhesion of functional thin films, without any degradation of the polymer's base properties (chemical, structural or even mechanical ones).