Ablation laser femtoseconde de verres métalliques de Cu_x Zr_(1−x) : une étude par
dynamique moléculaire
Laser ablation of CuxZr1−x (x = 0.33, 0.50 and 0.67) metallic glasses along with crystalline CuZr2 in the C11b structure was investigated using a molecular-dynamics scheme combined with a two-temperature model (MD-TTM). The ablation threshold (Fth) was determined for each of the 4 samples leading to the facts that Fth is lower for samples containing a larger amount of Cu atoms since their cohesion is weaker than Zr atoms’ in all the targets. For x=0.33, Fth is lower in the crystal than in the glass because the electron-phonon coupling is weaker in the latter, which implies that the energy is transferred more slowly from the electronic system towards the ionic system for a-CuZr2 than for c-CuZr2 . The velocity of the pressure wave generated by the laser pulse grows with increasing fluence for the crystalline sample as opposed to amorphous samples in which the pressure wave velocity is essentially constant with increasing fluence. This is explained by the fact that the shear modulus grows with increasing pressure in the c-CuZr2 and is constant in the glassy samples. Finally, the heat-affected zone (HAZ) was caracterized by the melting depth and the plastic deformations after the laser pulse. The lower melting temperature for amorphous samples causes the melting depth to be more important in them than in the crystal. In the metallic glasses, plastic deformations were identified as shear transformation zones (STZ) which merge and coalesce at higher fluence. No plastic deformation has been observed in c-CuZr2 other than some light deformations near the melting front, essentially caused by residual strains. In summary, this study allows a better understanding of the laser ablation of metallic glasses and the collateral damages associated with it.
Two-temperature model