Date of Award

Summer 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Computational Analysis and Modeling

First Advisor

Weizhong Dai

Abstract

Micro heat transfer induced by Ultrashort-pulsed lasers is an important research topic in mechanical engineering and material science. In order to apply ultrashort-pulsed lasers successfully, studying the thermal deformation in double-layered thin films with imperfect thermal interfacial contact induced by ultrashort-pulsed lasers is important for preventing thermal damage. For the ultrashort-pulsed laser, the thermal damage is different from that caused by the long-pulsed lasers, and ultrafast cracks occur after heating.

This dissertation presents a new finite difference method for investigating the thermal deformation in a 3D gold-chromium thin film with imperfect interfacial thermal contact exposed to ultrashort-pulsed lasers. The method is obtained based on the parabolic two-step model and implicit finite difference schemes on a staggered grid. The method accounts for the coupling effect between lattice temperature and strain rate, as well as for the hot electron-blast effect in momentum transfer. In the calculations, a fourth-order compact scheme is employed for evaluating the stress derivatives in the dynamic equations of motion. The method allows us to avoid non-physical oscillation in the solution. In particular, the temperature change across an imperfect thermal interfacial contact can be expressed by the fourth-power law for radiation, which gives nonlinear temperature distribution around the interface, and we obtain successfully the stress change across the interface based on the fourth-power law for radiation by an iterative numerical method.

Numerical results show that when the center part of a top surface was irradiated by ultrashort-pulsed lasers, there are no non-physical oscillations in the solution, and the solution is grid independent; hence, the scheme is considered to be stable. The results also show that the temperature distribution from the top surface discontinuously across the imperfect thermal interface to the bottom, and the displacement and stress alterates from a negative value to a positive value at the center along the z direction, and along x and ydirections, indicating that the central part of the upper layer of the thin film expands during heating. The obtained model and numerical scheme in this dissertation will provide a theoretical tool for studying thermal deformation in multi-layered metal thin film exposed to ultrashort-pulsed lasers which have been used in laser process.

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