Wetting and spreading of Pb, Pb-Cu, and Ag-Cu melts on a polycrystalline copper surface with cobalt particles

Abstract

The specific feature of Cu(Co) solid solutions with a sub-solidus composition involves the formation of submicron-sized cobalt particles on the free surface. In this work, Cu(Co) substrates with cobalt particles on the surface were produced through the annealing of a series of copper-based cobalt solid solutions containing 1–3 at% Co in a hydrogen atmosphere at 1050 °C, leading to the formation of a family of cobalt-based submicron-sized particles with a similar orientation within one grain on the surface. The wetting and spreading of lead and copper-based liquid alloys on the surfaces of these substrates were compared to the wetting of pure copper and pure cobalt. Experiments were conducted via the high-speed video recording of droplet transfer onto the Cu, Co, and Cu(Co) substrates at 400 °C (melt: pure lead), at 850 °C (Pb + 10.3 at% Cu), and at 1070 °C (Cu + 2.5 at% Ag) in a vacuum. The contact angle of the Pb melt on cobalt (46°) differed significantly from the contact angle on copper at 400 °C (32°), while at 850 °C, the wetting for both metals was almost complete (the wetting angles for Cu and Co were 5° and 7°, respectively). The wetting angle for the Cu(Co) solid solution surface with cobalt particles was almost equal to that for pure copper, and the spreading rates were comparable for the two cases. The wetting of the Cu, Co, and Cu(Co) surfaces by the Cu(Ag) melt did not demonstrate any significant differences between the equilibrium contact angles (tending to zero), while the spreading rate differed only slightly. The surface area covered by the particles reached 20%, despite the presence of the particles, which exerted no impact on the contact angle, due to two possible effects: (i) the formation of an adsorbed copper layer on the surface of the cobalt phase confirmed by room-temperature Auger electron spectroscopy, and (ii) the compensation of wetting deterioration by an increase in roughness.