Fabrication and characterization of adherent diamond-like carbon based thin films on polyethylene terephthalate by end hall ion beam deposition

This work aims to develop novel techniques, including low energy end-hall ion (EDH) source, nitrogen and silicon doping, to enhance the adhesion strength of Diamond-like carbon (DLC) on Polyethylene Terephthalate (PET) to address their major application issues. For this purpose, Nitrogen doped DLC (N-DLC) and Silicon doped DLC (Si-DLC) thin films with different compositions were prepared on PET substrates using EDHI beam deposition. The composition and bonding states of the films were characterized by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS). The results show that sp3 carbon bonding concentration in Si-DLC is higher than in N-DLC and the doped Si forms sp3 SiC bonds and N forms sp3 CN and sp2 CN bonds in films. Nano-indentation tests were used to measure the hardness and Young's modulus while pin-on-disk sliding tests and scratching tests were used to evaluate the tribological and adhesion behavior of the samples, respectively. The results illustrate hardness reduction and adhesion enhancement in N-DLC and hardness enhancement of Si-DLC. In addition, the coefficient of friction (COF) of N-DLC is lower than pure DLC and Si-DLC, while it has been observed that formation of silicon oxide can reduce COF of Si-DLC. The coating roughness measured by an optical profiler decreases by increasing nitrogen concentration in N-DLC. This is probably because N doping reduces residual stress and thus makes film dense and smooth. By optimizing deposition condition, highly adherent DLC based thin films on PET have been achieved.