FABRICATION AND CHARACTERIZATION OF ADHERENT DIAMOND-LIKE CARBON BASED THIN FILMS ON POLYETHYLENE TEREPHTHALATE BY END HALL ION BEAM DEPOSITION

The application of Polyethylene terephthalate (PET) has been steadily increasing during recent years. However, its low wear resistance, high gas permeation rate and poor biocompatibility have significantly limited its applications in some specific areas. On the contrary, diamond-like carbon (DLC) possesses many unique properties such as the high hardness, low friction coefficient, high transparency, good gas barrier, excellent biocompatibility, and low synthesis temperature, which makes it an ideal coating material for modifying the surfaces of PET for applications as food and beverage containers and biomedical implants. Nevertheless, the adhesion of DLC on PET is poor due to the high stress induced by ion bombardment during DLC deposition and the large mechanical property mismatch between the coating and the substrate. Therefore, it is very important to develop techniques to lower the stress of DLC and to enhance the adhesion strength of DLC to PET. In the present thesis work, low energy end-hall (EH) ion source and element (N and Si) doping were used to deposit DLC to lower its internal stress and ion beam treatment was performed to modify the surface of PET to enhance the adhesion between DLC and PET. In addition, two different hydrocarbon gases (CH4 and C2H2) were used as precursors for DLC deposition in order to understand the effect of precursor gas. The structure and properties of the treated PET and deposited DLC films were characterized by various advanced techniques: The adhesion of DLC to PET was assessed using scratch testing; the composition and bonding states of DLC and treated PET were analyzed by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS); the hardness and Young’s modulus were measured by nano-indentation; the tribological properties of the coated materials were evaluated by pin-on-disk sliding testing; and the surface tomography and surface roughness were investigated by atomic force microscopy (AFM), optical profiler, and water contact angle measurements. The main results are summarized as follows. Ion treatment can effectively change PET microstructure and chemical states and thus improve the adhesion of subsequent deposited DLC. The kind of ions and the treatment time play an important role in PET structure changes. Oxygen ion treatment can effectively change the chemical states of PET and improve the adhesion of DLC and can further improve DLC adhesion. Element doping can change the structure of DLC and thus the mechanical properties. Si doping increases sp3 carbon bonding content in the films while N doping decreases sp3 carbon bonding content in the films. The doped Si forms sp3 Si-C bonds and N forms sp3 C-N and sp2C=N bonds in the films. Consequently, the hardness values decrease from 11.5 GPa for pure DLC to 7.0 GPa for N-DLC samples whereas the hardness of Si-DLC increases with increasing the silicon content. In addition, N doping can reduce residual stress of DLC and thus increases adhesion of DLC. The DLC films deposited using methane show better properties than using acetylene. By optimizing the conditions, dense, smooth, and adhesive DLC films have been deposited onto PET using EH ion source by combining N doping and O ion treatment with methane as precursor gas