Structure and Properties of Sputtered Ni-C and Ni-CNx Nanocomposite Thin Films
Zs. Czigány, K. Sedláčková, Gy.J. Kovács, G. Radnóczi
Research Institute for Technical Physics and Materials Science,
H-1525 Budapest, P.O. Box 49, Hungary
Nanocomposite structures composed of a crystalline and an amorphous phase are of rapidly growing interest. With the real-time control of the deposition parameters functionally gradient thin films can be produced with remarkable enhanced mechanical, magnetic and electronic properties compared to conventional single-phase coatings. Nanoscale composite films are usually produced by plasma deposition methods. The nanosize of the crystalline phase is ensured by the formation of a continuous layer of the (amorphous) matrix during growth, blocking the grain growth. Our research is directed to dc magnetron sputtered metal-containing fullerene-like amorphous carbon or carbon-nitride films. We have shown that the metallic component can enhance the formation of fullerene-like structures. We discuss the structure of C-Ni and CNx-Ni nanocomposite films and their mechanical properties.
According to TEM analysis of the films prepared below 400 C the nanocomposite is composed of columnar Ni containing crystalline grains and a partially disordered carbon (or CNx) matrix (width of 1-3 nm) localised between the crystallites. The matrix shows fullerene-like ordering or it is amorphous. In the case of the film deposited at 200˚C the column length is equal to the layer thickness below 18at% Ni. With increasing C content the column length decreases. This decrease is caused by hindering of crystal growth in the direction of the substrate normal due to covering the growth surface by a carbon layer and the resulted repeated nucleation. The new crystallite can grow as long as the formation of the covering layer does not lead to a repeated nucleation again. Below 400oC the growing crystallites have hexagonal lattice symmetry fitting the hexagonal Ni3C (JCPDS 04-0853) unit cell. The existence of the carbide Ni3C phase is also supported by ESCA. The development of the film morphology in thin layers with increasing Ni content is shown in the figure. In all compositions a fullerene-like carbon layer encapsulates the Ni3C crystallites, limiting their lateral growth. A regular nanocomposite of uniform grain size and uniformly thick intergrain phase develops at the selected growth conditions as long as the Ni content remains below 18 at%. At the thickness, chosen for the high resolution imaging (10 nm) the structure is composed of crystallites of 3-5 nm in diameter, and an isolating matrix of about 2-3 nm thickness, correspondingly. As the Ni/C ratio increases the column diameter increases to 5-7 nm. At Ni concentrations above 18 at% enhanced coalescence of the crystalline grains can be observed, the uniformity of the thickness of intergrain carbon phase breaks down.
According to Scanning Tunneling Specrtroscopy the majority of the films (53%) exhibit a semiconductor-like behavior, having a gap of 1-2 eV. A large fraction (43%) is rather similar to a graphitic I-V curve with a zero gap, while metallic behavior was detected only in a minor quantity (4%) of them. These results suggest that the metallic particles are mostly encapsulated into fullerene-like shells.
The nanoindentation hardness of the films is relatively high (10-14 GPa) below 200oC deposition temperature, tending to the value of metallic Ni (2GPa) above 600oC. The elastic modulus have a maximum of 130 GPa at Ts=200oC, and shows the same behavior as the hardness. Elastic recoveries of 50-55% were measured in the nano-indentation tests. At temperatures above 400oC, the films are quite soft, 2-4 GPa, and have only about 30% elastic recovery. The friction coefficient is fairly low for the whole temperature range and it is always lower for the nitrogen containing (CNx) layers than that for C-Ni ones.
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a) 5 W, 8at% of Ni/ b) 10 W, 12at% of Ni
/ c) 15 W, 18 at% of Ni
/ d) 25 W, 50 at% of Ni
Lateral-view HRTEM images of C-Ni nanocomposites, corresponding to different Ni content.
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