This paper deals with a well known puzzling observation that sometimes
the wetting is improved by a chemical reaction between a liquid and a
solid substrate and sometimes just the opposite effect takes place. H
ere, contact angles of liquid Al on SiO2 and liquid Ti on Al2O3 have b
een measured. The surface and interface structures have been explored
by scanning electron microscopy and energy dispersive X-ray spectromet
ry. According to the experimental observations, it turns out that the
volume change of ceramic substrates during reaction plays a key role i
n the effect of chemical reaction on wetting. If the volume of ceramic
substrate decreases after reaction, the wettability is not improved b
y the chemical reaction. This is the case of liquid Al wetting on SiO2
. If the volume of ceramic substrate increases after reaction, the wet
tability is improved by the chemical reaction. This is the case of liq
uid Ti wetting on Al2O3. Besides our experimental observations, result
s from literature have been reviewed as well, which are in good agreem
ent with the predictions based on the volume change criterion of ceram
ic substrate proposed in this paper.