The growth and structure of fullerene-like CN x films produced by laser ablation of graphite in low pressure nitrogen were investigated. Deposition conditions were selected based on investigations of CN and C 2 concentration at the condensation surface, vibrational temperature of CN radicals, and kinetic energies of atomic and molecular species. Films were characterized with x-ray photoelectron spectroscopy, Raman spectroscopy, high-resolution transmission electron microscopy, nanoindentation, and stress analyses. The nitrogen content in CN x films directly depended on the concentration of CN radicals at the condensation surface. Formation of fullerene-like structures required a high vibrational temperature of these radicals, which was maximized at about 4 eV for depositions at 10 mTorr N 2 and laser fluences of ∼7J/cm 2. The presence of C 2 had only a minor effect on film composition and structure. Optimization of plasma characteristics and a substrate temperature of 300°C helped to produce about 1-μm-thick solid films of CN x (N/Cratio≈0.2-0.3) and pure carbon consisting of fullerene-like fragments and packages. In contrast to carbon films, fullerene-like CN x films exhibited a high elastic recovery of about 80% in using a Berkovich tip at 5 mN load and indentation depths up to 150 nm. Their elastic modulus was about 160 GPa measured from the unloading portion of an indentation curve, and about 250 GPa measured with a 40 Hz tip oscillation during nanoindentation tests. The difference was related to time dependent processes of shape restoration of fullerene-like fragments, and an analogy was made to the behavior of elastomer polymers. However, unlike elastomers, CN x film hardness was as high as 30 GPa, which was twice that of fullerene-like carbon films. The unusual combination of high elasticity and hardness of CN x films was explained by crosslinking of fullerene fragments induced by the incorporated nitrogen and stored compressive stress. The study demonstrated laser ablation as a viable technique for the growth of fullerene-like CN x films, which may be used as hard protective coatings resisting brittle fracture at high loads and extensive substrate deformations.
ASJC Scopus subject areas
- Physics and Astronomy(all)