Specifying the interlayer turning point and dehydrogenation in a-C:H layers plasma deposited on high-density polyethylene with X-ray synchrotron techniques

Highlights: • Polymer to amorphous-hydrogenated (a-C:H) carbon transition point. • Interlayer between polymer and a-C:H specified to max. 40 nm. • Confirmation of a dehydrogenization process in a-C:H layers. • Analysis of thickness dependent sp³/sp² contents. -- Abstract: High-density polyethylene (HDPE) samples were coated with amorphous carbon layers (a-C:H) increasing in thickness from nm to µm using an radio frequency plasma source. The deposited layers were examined in detail by synchrotron based X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) to determine the binding states of the carbon atoms to each other and their status in the network. Particular attention was paid to coatings between 500 and 1500 nm, where dehydrogenation of the a-C:H layer occurs, and to thin depositions from 0 to 50 nm to specify the interlayer turning point from the polymer to the pure a-C:H network equivalent to the end point of a mixed phase. Since this is decisive for the adhesion of the carbon layer to the polymer, this area was additionally investigated by means of diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Herein the dimension of the interlayer between a-C:H layers on HDPE could be consistently narrowed to a thickness of more than 30 nm and less than 40 nm in all methods, as later the a-C:H network is appearing. The dehydrogenation process in the growing a-C:H layer was also confirmed, leading to a decrease in the sp³ respectively increase in sp² bonds.