Microstructure And Properties Of CN-based Nanocomposite Films

Amorphous CNx films have been extensively investigated due to their excellent mechanical, optical and electrical properties and practical applications in magnetic disk coatings, cast machining, cold-cathode-emission and space protection etc. Recently, ferromagnetic metal composite materials and superhard nanomultilayers become the focuses in the field of condensed matter physics and materials science. Carbon nitride films have controllable conductivity and high hardness, and can be used as one of the component in above systems.Carbon nitride films, transition metal (Ni, Fe, Co, Cu, Ti)-CN nanocomposite films and TiN/CN multilayers were fabricated using facing-target reactive sputtering. Their chemical composition, microstructure, magnetic, electrical transport and mechanical properties were studied systematically.It was found that the N concentration in the carbon nitride films does not change with the nitrogen partial pressure PN linearly, which rises quickly to a saturate value of～33% at a PN of 20%, and does not change with further increasing PN. The pure C films are mainly composed of sp3 C atoms. Increasing PN makes both the number of sp2 C atoms and the size of aromatic sp2-hybridized C clusters increase, while the sp2 C cluster remains a rather small size. The optical and electrical properties of the films are mainly determined by the defects existing in the films and the mechanical properties correlate closely with the sp3 C content.An enhanced spin-dependent magnetoresistance (MR) was observed at low temperatures in the (Fe, Co, Ni)-CNx nanocomposite films fabricated at room temperature. The maximum MR reaches–59% at 3 K and 90 kOe. The MR has a weak saturation trend with increasing field to 90 kOe. Above 20 K, the MR is very small (<1%), but as temperature is below 20 K, it follows the relation of log MR∝?T. The maximum MR, MR enhancement and carrier transport mechanism can be tailored by changing the insulation of the CNx matrix through tuning PN or altering the Ni content in the films. The enhancement of the low-temperature negative MR was clarified by introducing a new relation of spin polarization P with T, P = P0 exp(?βTα), into the high-order tunneling model. TiN/CN multilayers were fabricated and superhardness phenomenon was observed. When the two components (TiN and CN) in the multilayer are amorphous, no enhancement in hardness can be observed. While with the crystallization of the TiN layer, the hardness of the multilayers is enhanced greatly and becomes higher than that calculated using the rule of mixtures. The variation of hardness with the crystallinity of TiN layers suggests that the mechanism of the deformation in amorphous TiN/CN multilayers is different from that in the crystalline ones and the dislocation related mechanism in the crystalline multilayers is very important for the appearance of the superhardness effect. The real hardness of the multilayers free from the substrate effect was extracted using Bhattacharya-Nix empirical equation based on the data measured from nanoindentation with continuous stiffness measurement technique. Large deviation between the film's hardness and measured hardness was found for the films with large hardness.