Mechanism of Porosity Formation in Transfer Films in Electromagnetic Launchers

Melting occurs at the armature-rail interface during electromagnetic launch due to resistive heating. The liquid metal film at the interface is thought to decrease the friction and contact electrical resistance. The liquid metal is quenched at the rail surface, leaving a solid transfer film with surface properties that may affect the efficiency of subsequent launches. Notably, the transfer film contains pores which increase the overall rail electrical resistance. Reduction of atmospheric water vapor by liquid aluminum to produce hydrogen is thought to be the main source of porosity. To further examine the pore formation mechanism, we performed launch experiments using steel armatures and copper rails lubricated with two silver-bismuth alloys. Spherical pores similar to those observed in aluminum films are also present in silver-bismuth transfer films. Neither liquid silver nor bismuth is capable of reducing water vapor to hydrogen. This strongly suggests that the porosity is not due to hydrogen in these films. Air entrainment occurs in forced wetting when the three-phase contact line velocity exceeds a critical value. The critical wetting velocity is a function of surface tension and viscosity and is estimated to be 4-40 m/s for liquid bismuth at 600 K and a comparable value for liquid aluminum. Armature velocities exceed these values nearly instantaneously during launch. These results indicate that air entrainment is likely the source of porosity in transfer films.