Mechanical and optical properties of plasma deposited superhard nanocomposite coatings

In this work, we investigate the growth and the characteristics of nc-TiN/SiN _1.3 and nc-TiC_xN_y/SiCN superhard coatings. These materials were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a radio frequency discharge using intense ion bombardment. The coatings were deposited from TICl₄/SiH₄/CH₄/N ₂ gas mixtures at a pressure of 200 mTorr (26.66 Pa) and with substrate temperatures ranging from 300 to 500°C. We systematically evaluate the films' mechanical and optical properties, and we interpret them in terms of their microstructure.

The first part of the work describes the nc-TiN/SiN_1.3 system. The mechanical properties and microstructure of coatings deposited at a low temperature of 300°C are systematically compared with those deposited at 500°C.

After establishing the methodology of mechanical measurements, we optimized the nc-TiN/SiN_1.3 from the point of view of hardness and Young's modulus. By changing the amount of SiN_1.3 we identified optimum deposition conditions resulting in the highest hardness and reduced Young's modulus, up to 45 and 350 GPa, respectively.

In the second part of this work, we studied the properties of SiCN films, which were then used as a matrix in the nanocomposite. By adding an optimized amount of carbon to SiN_1.3 (∼28 at.%), a hardness of 33 GPa, a reduced Young's modulus of 200 GPa and an elastic rebound of 85% were obtained. Such high hardness compared to 18 GPa for SiN_1.3 and 26 GPa for SiC, is attributed to the formation of C-N bonds. The new SiCN material was incorporated in the novel, quaternary nc-TiC_xN_y/SiCN system.

The final part of this work describes quaternary nc-TiC_xN _y/SiCN coatings. At an optimum C concentration (∼10 at.%), the nc structure is preserved and it is characterized by very high hardness (55 GPa), high reduced Young's modulus (306 GPa), and an elastic rebound of more than 80%. High resistance to plastic deformation, expressed by a high H³/E_r² ratio of up to 1.8 GPa was obtained; this is higher than reported by any other group. The mechanical stress was between 2.0 and 2.5 GPa in compression. The friction coefficient and the wear rate, measured against diamond, were found to be 0.13 and 12*10⁻⁶ mm³/Nm, respectively. Such superior mechanical performance is attributed to a combined effect of nc structure together with enhanced properties of the SiCN matrix and strong Ti-C bonds in the nanocrystals. (Abstract shortened by UMI.)