Silver nanoparticles encapsulated in carbon cages obtained by co-sputtering of the metal and graphite

We have synthesized C-Ag thin films by co-sputtering of a silver-graphite target. The deposition temperature ranged from 77 K to 773 K, the silver concentration varying from 10 to 71 at%. The microstructure of the films has been characterized by transmission electron microscopy (TEM) and small-angle X-ray scattering under grazing incidence (GISAXS) experiments. It is shown that homogeneously distributed silver nanoparticles, having an elongated shape along the direction of the thin film growth, are formed within a more or less graphitized carbon matrix. After liquid nitrogen and room temperature depositions, a preferential crystallographic orientation is observed, dense(111) silver planes being at 90 degrees with respect to the surface layer whereas the carbon matrix is amorphous. A graphitization leading to the encapsulation of the silver nanoparticles in graphite-like carbon has been obtained when the depositions were performed at 773 K for lower silver concentrations without ion-beam assistance and below 573 K for upper silver concentrations with ion-beam assistance. We propose that the demixing of carbon and silver occurs during the co-deposition process by surface diffusion of C and Ag atoms. It is inferred that the presence of silver simply serves as a "catalyst" for the graphitization process at these relatively low temperatures. Furthermore, we have investigated the tribological properties of our C-Ag coatings: a substantial increase in the wear resistance and a significant decrease in friction relative to an austenitic stainless steel substrate is observed. (C) 1998 Elsevier Science B.V. All rights reserved.