Dynamics of wetting in reactive metal/ceramic systems: Linear spreading

The kinetics of wetting in the reactive pure aluminium/vitreous carbon (Cv) system was investigated by the sessile drop technique in high vacuum. It was found that the curve showing the radius of the metal drop base R as a function of time t consisted of a central part where the radial spreading of the drop is a linear function of time, and two extremal parts where significant deviations from Linearity are observed. By characterising the Al/Cv interface at different stages of the process, both far from and close to the Al/Cv/vapour triple line, it is shown that (i) in the linear spreading regime wetting kinetics is controlled by a nearly 2D reaction between Al and Cv at the triple line, forming aluminium carbide; and (ii) linearity is closely associated with steady-state growth of the carbide at the triple line. At the beginning of the wetting process, deoxidation of the Al drop and, thereafter, transient-state growth of carbide, cause considerable deviations from linearity in the R(t) curve. Moreover, at the end of spreading, when the contact angle tends towards the steady contact angle of the system, a deviation from linearity is also observed and attributed to the roughness of the reaction product. Finally, examples from the literature are given suggesting that linear spreading may be effective in many reactive metal-on-metal and metal-on-ceramic systems. Copyright (C) 1996 Acta Metallurgica Inc.