Evolution of the sp2 content and revealed multilayer growth of amorphous hydrogenated carbon (a-C:H) films on selected thermoplastic materials

Amorphous hydrogenated carbon (a-C:H) films were gradually deposited on high-density polyethylene (HDPE), polyethylene terephthalate (PET) and polyoxymethylene (POM) via an indirect (f-type) and a direct (r-type) plasma-enhanced chemical vapor deposition (PECVD) process with acetylene plasma. The surface morphologies of the thicker r-depositions on the three different thermoplastics have been analyzed by atomic force microscopy (AFM) at varying micrometer scales. Absorbance spectroscopy has been used to characterize the optical properties of all coatings. Intrinsic stress release phenomena are revealed on thicker layers through the detection of characteristic surface corrugations. Based on contact angle measurements, the different size of these morphological entities is discussed in terms of different surroundings at the formed sp2 clusters in the films. Superimposed structures developed between coalescence grains have been furthermore observed on top of the thicker layer on PET, demonstrating that the growth of a-C:H material on plastics occurs by a multilayer mode. For all coatings, the total sp2 content increases with increasing f- and r-deposition. Even if thinner layers show dissimilar sp2 contents and even if different specific sp2 organizations exist between the developed coatings on POM compared to HDPE and PET, for thicker r-depositions the total sp2 content assimilates for all films.