Control of Metal/graphite Interfacial Energy Through the Interfacial Segregation of Alloying Additions.

Equilibrium segregation of Ni to the interface of a solid Pb/graphite and Au/graphite was studied using a solid state wetting approach and the crater edge profiling technique on a scanning Auger microprobe (SAM). All experiments were performed under ultra high vacuum (UHV) to reduce the effects due to surface adsorption of impurities. For the Pb/graphite system, increasing amounts of Ni ranging from 0 to 0.2wt% Ni added to Pb were found to systematically lower the contact angle for samples equilibrated at 285 ^circC. No significant surface segregation of Ni was observed at the Pb surface. The reduction of the contact angle was therefore attributed entirely to the lowering of the interfacial energy by interfacial adsorption of Ni. The interfacial energy and interfacial Ni concentration were obtained as a function of bulk Ni content. The presence of excess Ni at the interface was also determined using the crater edge profiling technique on the SAM for various bulk concentrations of Ni in Pb. The temperature dependence of the segregation process was also studied using the solid state wetting approach. The contact angle of Pb(Ni)/graphite was found to vary as a function of temperature for a given Ni content. No temperature dependence was observed in the case of pure Pb/graphite. The change in interfacial energy and the interfacial Ni concentration were obtained as a function of temperature from thermodynamic considerations, and from that the enthalpy and the entropy of interfacial segregation were determined. For the Au/graphite system at 850^circC, addition of 15at%Ni to Au caused a reduction of contact angle by 7.8^circ with accompanying reduction in interfacial energy. Ni was found to segregate to both the free Au surface as well as to the Au/graphite interface. In addition C was also found to segregate to the Au surface thus lowering the surface energy. The modified surface energy was considered in the determination of the interfacial energy and interfacial Ni concentration which was determined to be 0.45 monolayers. Crater edge profiling studies also revealed Ni at the interface. A theoretical model based on the nearest neighbor bond-regular solution approximation was developed for estimating the surface concentration of the solute. This model was used in the Au-Ni system to estimate the surface Ni concentration and the consequent reduction in surface energy. The predicted values compared favorably with the corresponding experimental values. A model was also developed using a similar approach to predict interfacial Ni concentration in a metal/graphite system. This model was used to estimate the Ni content at a Au/graphite interface for a Au-0. 15Ni sample at 850^circC. The predicted interfacial atom fraction did not agree very well with the experimentally determined value. Nevertheless, it did yield a qualitatively correct trend and the right kind of temperature dependence.