Initial stage of isothermal wetting of bulk Cu₆Sn₅, Cu₃Sn and Cu substrates by liquid Sn

For the first time the initial stage (10⁻³ to 10⁻² s) of isothermal wetting of bulk Cu₆Sn₅, Cu₃Sn and Cu substrates by liquid Sn are studied using a dispensed drop set-up under high vacuum at 300 and 390 °C combined with high-speed video recording. Specific non-reactive wetting experiments of Cu₆Sn₅ by saturated Sn-7.8 wt%Cu liquid alloy are also performed in order to distinguish the reactive and non-reactive spreading stages. Selected samples are characterized by scanning electron microscopy in order to observe the reactive (or non-reactive) interfaces as well as the drop/substrate configuration at the zone close to the triple line. The physical meaning of observed contact angles is discussed based on these experimental results. The equilibrium contact angle of liquid Sn on the unreacted Cu substrate is found to be significantly higher than those on Cu₆Sn₅ and Cu₃Sn substrates. A mechanism of reactive spreading in the liquid Sn/Cu system is proposed.