Multiscale Modeling of Laser-Induced Surface Nanostructuring of Metals

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NONTECHNICAL ABSTRACTThis award supports computational research and education to develop and use computational modeling tools to understand how very fast laser pulses can modify the structure and surfaces of metals and semiconductors in potentially complex and intricate ways. The ability of very fast laser pulses to deposit energy, for example in the form of heat, in small regions of irradiated material surfaces makes it possible to perform very selective surface modifications and produce unique surface morphologies, metastable phases and unusual arrangements of crystal defects which constitutes microstructure. Fast laser pulses also provide unique opportunities to investigate material behavior under extreme conditions of highly energetic electrons, rapid heating and cooling, and ultrafast mechanical deformation. The objective of this research project is to develop an advanced computational model that contains descriptions of essential physical processes across scales of length and time and is capable of realistic representation of how very fast laser pulses interact with targets made of a wide range of materials from metal alloys to organic compounds. Through the use of very fast computers with many processers that operate in parallel, the PI will apply this model to investigate the main factors that control the surface morphology and microstructure in laser-modified materials. While the laser pulse lasts for only a very short time, the time for the material to respond is much longer. Together with the size of the simulated material sample and complexity of the physical processes stimulated by the response leads to the need for high computer performance. The investigation of how materials respond to short-pulse laser irradiation is aimed to provide insights into the peculiarities of material behavior far from the steady state of equilibrium, under extreme conditions of ultrafast heating, cooling, and deformation rates. Ultrafast laser surface modification has a wide range of potential applications including materials fabrication with enhanced surface properties including hardness, reduced friction, and reduced wear. The involvement of students in all aspects of high-performance parallel computing and close interaction with experimental and computational collaborators worldwide will create a fertile educational environment in the quickly expanding areas of scientific computing and laser-materials interactions. An educational website containing an accessible presentation of basic concepts of laser-materials interactions illustrated by images and animations produced in this project will be developed as a platform for conveying the relevance and excitement of the computational materials research to broader audiences.TECHNICAL ABSTRACTThis award supports theoretical and computational research, and education in short pulse laser modification of material surface morphology. The ability of short pulse laser irradiation to produce unique surface morphologies, metastable phases and unusual microstructure has been demonstrated in experiments and is generally attributed to the conditions of strong electronic, thermal, phase, and mechanical nonequilibrium created in irradiated targets by the laser excitation. Detailed understanding of the relations between the fast nonequilibrium processes caused by the laser energy deposition and the resulting structure and properties of laser-treated regions of the targets, however, is still lacking, thus limiting the expansion of laser technologies into the new domains of nanoscale material processing and fabrication. The main objective of this research project is to provide, through advanced multiscale modeling and theoretical analysis, detailed information on the mechanisms and kinetics of fast nonequilibrium structural and phase transformations triggered by short pulse laser irradiation of metal targets and responsible for the generation of complex hierarchical surface morphology at the nanoscale and microscale, and unusual surface microstructure.A key component of this project is the design and verification of a novel multiscale computational approach combining, in a synergistic manner, large-scale atomistic simulations of the initial material response to short pulse laser excitation, including rapid melting, cavitation, and, at higher laser fluences, explosive boiling and phase decomposition of superheated liquid, with a coarse-grained modeling of the subsequent slower hydrodynamic motion and solidification of the melted surface region. The new computational model will enable a detailed exploration of the complex interplay of the totality of laser-induced processes occurring at different time- and length-scales and will reveal the connections between irradiation conditions, material properties, crystallographic orientation of grains in polycrystalline targets, and the microstructure of surface features generated by laser irradiation.The emergence of new flexible computational methodology for multiscale modeling of material response to a rapid energy deposition is likely to make a broad impact in the general area of computational materials science. The methods developed within this project will be broadly disseminated to the research community and applied, through existing and new collaborations, to different material systems, ranging from metal alloys to organic materials, and various laser processing conditions, for example the presence of a background gas, liquid environment, or a transparent overlayer. Owing to the size of the simulated material, the need for long simulation time, and the complexity of the model, high performance computing simulations are required.The involvement of students in all aspects of high-performance parallel computing and close interaction with experimental and computational collaborators worldwide will create a fertile educational environment in the quickly expanding areas of scientific computing and laser-materials interactions. An educational website containing an accessible presentation of basic concepts of laser-materials interactions illustrated by images and animations produced in this project will be developed as a platform for conveying the relevance and excitement of the computational materials research to broader audiences.