The wetting of graphite by molten silver was investigated at the temperature range from 1000°C to 1200°C using the sessile drop method coupled with a video enhanced image digitization technology and a best-fitting algorithm. At the temperature range from 1150°C to 1175°C the contact angle θ was observed suddenly increasing and then decreasing, repeatedly. A similar behavior was not found in the control test performed on wetting of refractory (ZrO2) by molten silver at the same range of temperature.
This trend has never been reported in literature for the wetting of graphite by molten silver, probably since most of the studies investigating this system have been conducted at temperatures ~1000°C. However other research groups have found an analogous behavior for different systems (wetting of sapphire by Al and Cu, wetting of SiO2 by Ag and Cu, and wetting of SiC by Ag and Cu), observing the drop repeatedly spreading and rapidly contracting at temperatures higher than 1130°C (T>950°C for micro-sized droplets). In all these cases a morphological surface analysis of the substrate showed several “rings”.
Therefore a surface analysis of the graphite used in our measurements was performed (AEP 3D surface profilometer), showing a similar rings-shaped surface morphology. The correlation between the wetting diameters, the ring diameters and the observed drop contractions was then brought out. The measured contact angle decreases while a “trough” forms at the three phases contact circle. At the observed θ sudden increase, the drop is actually “jumping out” from the trough, since the rate of the formation of the trough is lower than the rate at which the drop changes its shape. The process starts then again, with the formation of a new trough. The Young's equation could still describe the real angle formed between the substrate and the drop, but the measured θ is obviously affected by the trough geometry.
After an analysis (SEM and ESCA) of the silver used in our measurements, a small percentage of carbon was found, while no carbon was present in the silver used for wetting the refractory in the control test. A hypothesis could be that the carbon removed from the substrate diffuses at these specific temperatures along the interface and then leaves it as the gaseous product of a reaction with the oxygen present as impurity in the working gas (Ar). The specific rings-shaped morphology of the graphite could be thus related to the more intense evaporation of these gases at the three phases contact circle than at the center of the drop due to the static pressure exerted by the drop itself.