INFLUENCE OF SULFUR ON SURFACE CARBON MONOLAYER FORMATION AND GRAPHITE GROWTH ON NICKEL

Polycrystalline nickel foils (5 mum thick) were carburized and decarburized in CH4-H-2 or H-2 atmospheres, respectively, at 1360 K The formation of a graphitic carbon monolayer was observed for carbon activities a(C) above a critical value a(C,cr) of 0.75. The monolayer exhibits an atomic ratio of 2C/1 Ni as determined by in situ AES analysis. The carbon monolayer formation is accompanied by surface restructuring and proceeds in dependence on the time of sample exposure to the carburizing atmosphere. For carbon activities a(C) > 1, subsequent to the formation of the carbon monolayer, a slow epitaxial growth of graphite occurs. By adding H2S to the carburizing atmosphere, the critical carbon activity a(C,cr) is shifted to higher values and, thus, the carbon monolayer formation is suppressed. At a(C) > 1, graphite is formed even in the presence of H2S, however, at partial pressure ratios p(H2S)/P(H2) of about 3 x 10(-6) a change in the mode of graphite formation from the slow epitaxial growth to a relatively fast growth of irregular globular graphite takes place. This phenomenon can be understood in terms of the effects of sulfur on the surface restructuring of nickel.