The study reported here examines wetting between Al2O3 and a series of Ag–CuO
air braze filler metals. Based on in‐situ contact angle measurements, two transitions in wetting behavior were identified in the composition range of 0–40 mol% CuO. The first transition
occurs directly at the miscibility gap boundary composition (for a given temperature) of the Ag–CuO phase diagram and is attributable to the presence of two liquid
phases, one of which is rich in copper oxide and preferentially wets the alumina substrate via the mechanism predicted by Cahn's critical point wetting theory. The second transition occurs at a filler metal composition that falls well within the miscibility gap for the Ag–CuO
system and may occur due to the formation of a reaction product between the substrate and the molten filler metal, as denoted by small, discontinuous reaction regions.