engleski

MICRO- AND NANO-POROSITY OF Ta AND Mo SURFACE FORMED BY SOLITONS AND BREATHER MODES BY LASER-MATTER INTERACTIONS

Microscale porosity of Ta and Mo surface has been generated by Nd:YAG laser in the semiconfined configuration in which the plasma expansion was prevented thus causing a high pressure increase. The high pressure pushed the surface layer toward the spinodal, i.e., to the point of absolue instability.Abrupt transition into vapour phase at the spinodal generates a very porous surface layer of ~ 7 – 10 mm of thickness throught bubbling and the cascade of bubble explosions. The bubble explosions generate microscale caverns ; the total area of porous surface was increased ~ 104 times, and this method appears a promissing one for technological applications. Larger magnification reveals porosity of different nature namely, nanoholes of ~ 25 – 30mm in diameter formed on the microscale caverns. The SEM analysis indicates that nanoholes in various domains have either random distribution, 1D arrangement in the regular lines, or regular 2D as the long paralell rows of holes. The holes appear as a fingerprint of thermal/pressure plasma spikes that are in direct contact with metal surface during the laser pulse up to the cooling phase at the end of pulse when such surface morphology stay frozen permanently, becouse of ultrafast cooling at the rate ~ 109 K/s. Characteristics of the nanohole organization indicate that spikes (humps) belong to dispersed solitary plasma waves. Random and (quasi)regular distribution of the stationary and non-stationary solitary waves can be obtained as the solution of equations which includes nonlinerity, dispersion and dissipation, such as the Benney equation. Numerical simulation with selected parameters shows that the soliton amplitude tends to saturation becouse of the mode-mode energy transfer. Collapse of the solitary waves leads to formation of so called breather modes, localized in space which cause formation of 1D or 2D regular spake (hole) arrangement. Microscale porosity (left), random nanoporosity (middle), and regular 2D nanoporosity generated by Nd:YAG laser on tantalum surface in the &laquo ; ; ; ; semiconfined configuration&raquo ; ; ; ; . The breather modes in 1D regular arrangement are obtained from the Boussinesq equation which also gives the imbrication type modes. The breather modes r arrangement are obtained as the solution of the in 2D are obtained from Kadomcev-Petviashvili equation and also give the imbrication type spikes. Thus, the plasma solitary waves, and collapsed solitary waves (breathers) generated in the semiconfined configuration, appear as most probable reason for the generation of nanoporosity.

Laser ablation; Subsurface superheating; Nanoholes; Nonlinear plasma waves; Hump solitons; breather modes

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