SILVER NANOPARTICLES ENCAPSULATED IN CARBON CAGES OBTAINED BY COSPUTTERING OF THE METAL AND GRAPHITE

We have synthesized C-Ag thin films by co-sputtering of a silver-graph ite 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 nanopa rticles, having an elongated shape along the direction of the thin fil m 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 be ing 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 wh en the depositions were performed at 773 K for lower silver concentrat ions without ion-beam assistance and below 573 K for upper silver conc entrations with ion-beam assistance. We propose that the demixing of c arbon and silver occurs during the co-deposition process by surface di ffusion of C and Ag atoms. It is inferred that the presence of silver simply serves as a ''catalyst'' for the graphitization process at thes e relatively low temperatures. Furthermore, we have investigated the t ribological properties of our C-Ag coatings: a substantial increase in the wear resistance and a significant decrease in friction relative t o an austenitic stainless steel substrate is observed. (C) 1998 Elsevi er Science B.V. All rights reserved.