Elodie Leveugle

Computer modeling of laser melting and spallation of metal targets

The mechanisms of melting and photomechanical damage/spallation occurring under extreme superheating/deformation rate conditions realized in short pulse laser processing are investigated in a computational study performed with a hybrid atomistic-continuum model. The model combines classical molecular dynamics method for simulation of non-equilibrium processes of lattice superheating and fast phase transformations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons. The kinetics and microscopic mechanisms of melting are investigated in simulations of laser interaction with free-standing Ni films and bulk targets. A significant reduction of the overheating required for the initiation of homogeneous melting is observed and attributed to the relaxation of laser-induced stresses, which leads to the uniaxial expansion and associated anisotropic lattice distortions. The evolution of photomechanical damage is investigated in a large-scale simulation of laser spallation of a 100 nm Ni film. The evolution of photomechanical damage is observed to take place in two stages, the initial stage of void nucleation and growth, when both the number of voids and the range of void sizes are increasing, followed by the void coarsening, coalescence and percolation, when large voids grow at the expense of the decreasing population of small voids. In both regimes the size distributions of voids are found to be well described by the power law with an exponent gradually increasing with time. A good agreement of the results obtained for the evolution of photomechanical damage in a metal film with earlier results reported for laser spallation of molecular systems and shock-induced back spallation in metals suggests that the observed processes of void nucleation, growth and coalescence may reflect general characteristics of the dynamic fracture at high deformation rates.

Laser processing of polymer nanocomposite thin films

Current biotechnology and sensor research has enhanced the drive to establish viable methods for depositing high-quality polymer thin films. In this research, thin films of poly(methyl methacrylate) (PMMA) were prepared by matrix-assisted pulsed-laser evaporation (MAPLE). Up to 2wt% of carbon nanotubes were subsequently added to MAPLE target systems for deposition of polymer nanocomposite films. Targets were ablated using a 248nm (KrF) laser at fluences ranging from 0.045to0.75J∕cm2. In addition, polymer concentration in MAPLE targets was varied between 1 and 5wt% relative to the matrix solvent, in this case toluene. Films were deposited on Si substrates at room temperature in an Ar atmosphere. Molecular-dynamics simulations of MAPLE were utilized for interpretation of experimental observations. Particularly, the ejection of large clusters consisting of both PMMA and toluene molecules was studied and related to the observed morphology of the deposited films.

Atomic/Molecular-Level Simulations of Laser–Materials Interactions

Molecular/atomic-level computer modeling of laser–materials interactions is playing an increasingly important role in the investigation of complex and highly nonequilibrium processes involved in short-pulse laser processing and surface modification. This chapter provides an overview of recent progress in the development of computational methods for simulation of laser interactions with organic materials and metals. The capabilities, advantages, and limitations of the molecular dynamics simulation technique are discussed and illustrated by representative examples. The results obtained in the investigations of the laser-induced generation and accumulation of crystal defects, mechanisms of laser melting, photomechanical effects, and spallation, as well as phase explosion and massive material removal from the target (ablation) are outlined and related to the irradiation conditions and properties of the target material. The implications of the computational predictions for practical applications, as well as for the theoretical description of the laser-induced processes are discussed.

Ejection of matrix-polymer clusters in matrix-assisted laser evaporation: Coarse-grained molecular dynamics simulations

Molecular-level dynamic simulations are performed to investigate the mechanisms of molecular ejection and transport in laser ablation of frozen polymer solutions, as related to the matrix-assisted laser evaporation (MAPLE) technique for polymer film deposition. Coarsegrained description of molecular matrix and polymer molecules is used in the model, allowing for large-scale simulations of the ejection of multiple polymer molecules from MAPLE targets with different polymer concentrations, from 1 to 6 wt.%. The ejection of polymer molecules is observed only above the threshold for the collective material ejection (ablation). Ablation is driven by the phase explosion of the overheated matrix material, which proceeds through the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets and gas-phase matrix molecules. The polymer molecules resist the decomposition of the transient foamy liquid structure and stabilize the matrix droplets. In all simulations the polymer molecules are ejected as parts of large matrixpolymer droplets/clusters that are likely to retain a large fraction of matrix material at the time of the deposition on a substrate. The ejection and transport of large matrix-polymer droplets is related to high-resolution scanning electron microscopy (SEM) images of polymer films deposited in MAPLE, where morphologies of the films are found to be indicative of active processes of matrix vaporization and escape from the deposited matrix-polymer droplets.