Effects of sulfur on interfacial energy between Fe-C melt and graphite

We used an improved sessile drop method, which can measure the equilibrium contact angle of molten Fe-C-S alloys on graphite basal planes using an equilibrated method. We adopted this method for the measurement of the interfacial energy, gamma(s/l), between the basal plane of graphite and the Fe-C melts at 1573 K in a purified He-3%H-2 atmosphere. For the calculation of gamma(s/l), we have to use the true graphite surface energy, gamma(s/v) and the surface tension of the iron melt, gamma(l/v), as the reference. However, we could not get a reliable gamma(s/l) value from the literature, because the gamma(s/v), value could be changed by the contamination of the vapor from the melt. For this reason, we measured the gamma(s/v) value using a nonequilibrated method based on the interfacial energy balance at the trijunction using gamma(l/v) and the contact angle theta and phi, where theta is the contact angle between the liquid metal and graphite substrate observed during the measurement by an optical microscope, and phi is the hidden angle observed at cross section of the sample. We found that the gamma(s/v), and gamma(s/l) values decrease when the sulfur contents are greater than 10 mass ppm sulfur. From 10 to 120 ppm sulfur, the decreasing rate has a constant value on a logarithmic scale of sulfur activities. The morphology of the graphite changes from spheroidal to flake as gamma(s/l) decreases due to increase in the sulfur content. (C) 2005 Springer Science + Business Media, Inc.