Spongelike metal surface generated by laser in the semiconfined configuration

Abstract

A porous surface of tantalum and molybdenum plates has been formed by a nanosecond laser-matter interaction in the “semiconfined configuration,” in which the laser plasma is trapped between the target and the transparent cover plate. The evolution of the plasma cloud and the pressure above the surface induce a superheated state in the liquid surface layer. Explosion of the plasma disk and formation of a cylindrical blast wave drive the superheated metastable fluid toward the spinodal, i.e., to the point of absolute thermodynamic instability. Phase explosion of a spinodal fluid occurs through the cascade of bubblings and generates a very porous spongelike surface ∼5–7μm thick (in some cases even ∼10μm), which stays frozen permanently because of ultrafast cooling at the end of the laser pulse. The total area of the porous surface is enlarged ∼104 times at the microscale range. At even larger magnification, a different, nanoscale-type porosity has been observed.