Three-dimensional micro-nanoscale deformations of functional thin films by Laser Dynamic Forming

Huang Gao, Purdue University

Abstract

Motivated by the diversified applications of micro-nanoscale systems in our daily life, a multitude of micro-nanoscale manufacturing technologies have been developed to introduce two-dimensional (2D) and three-dimensional (3D) patterned structures to functional devices and materials, which exist especially as thin films to be readily patterned, transferred, handled and integrated. In this study, Laser Dynamic Forming (LDF) demonstrates its capabilities in 2D and 3D micro-nanoscale assembly and patterning of metallic and polymer-laminated materials in a flexible, scalable, fast and consistent way. The formability of functional materials has been improved by utilizing ultrahigh strain rate deformation and through thickness compression during laser-induced shockwave propagation. The forming process completes in tens of nanoseconds, operates at room temperature and atmospheric conditions and requires very limited amount of resource, energy, and equipment to startup. In order to understand the dynamic deformation behaviors during LDF, a multiscale modeling methodology and a constitutive model have been developed, which integrate size effects and ultrahigh strain rate effects by discrete dislocation dynamics (DDD) simulations. The DDD simulations reveal the effects of size (i.e. film thickness) and strain rate on yield strength, strain hardening and strain rate sensitivity, which has been integrated into a conventional semi-physically-based dynamic model, i.e. Mechanical Threshold Stress (MTS). Based on this newly developed model, finite element method (FEM) simulation results agree well with experimental results of deformation depths and thickness variations in thin film deformations. With the in-depth understanding of dynamic deformation mechanisms in LDF, the laser processing conditions have been systematically optimized to achieve meso-micronanoscale patterns consistent over large area in metallic thin films. LDF demonstrates its significant forming capability by high aspect ratio V-grooves and nanobars, sub-50nm nanogratings and multiple-step nanodeformations, which would be costly and difficult to achieve if by conventional lithography and etching. The mechanical strength, indicated by surface hardness in nanoscale, would be improved significantly due to strain hardening and grain size refinement. Since LDF can achieve uniform deformations conformal to the underlying 3D microscale surfaces, it expands its application to directly manufacture and assemble functional laminated films with 3D surfaces for micro-electro-mechanical-systems (MEMS) applications. In this study, functional materials are encapsulated in polymeric layers to mitigate shockwave damage and extend their formability by such compliant substrates. It shows that the functional laminated thin films and sensors can assemble with rigid and flexible substrates and inherit microscale patterns faithfully without failures and fractures by controlled laser intensities. The polymeric layers experience severe thickness reduction over the microscale features and necking at stress and strain concentration. In contrast, the functional layer deforms mildly without significant thinning and fracture, preserving the device function and connectivity. FEM simulations characterize the stress distribution in these heterogeneous materials and stress discontinuity along polymermetal interface in an approximate way.

Degree

Ph.D.

Advisors

Cheng, Purdue University.

Subject Area

Engineering|Mechanical engineering|Nanoscience

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