Huang Gao

Large-scale nanoshaping of ultrasmooth 3D crystalline metallic structures

We report a low-cost, high-throughput benchtop method that enables thin layers of metal to be shaped with nanoscale precision by generating ultrahigh-strain-rate deformations. Laser shock imprinting can create three-dimensional crystalline metallic structures as small as 10 nanometers with ultrasmooth surfaces at ambient conditions. This technique enables the successful fabrications of large-area, uniform nanopatterns with aspect ratios as high as 5 for plasmonic and sensing applications, as well as mechanically strengthened nanostructures and metal-graphene hybrid nanodevices. Smooth surface, crystalline 3D metallic nanostructures are fabricated using a laser shock imprinting technique. Laser shock imprinting for patterning metals High-fidelity, small-scale patterning is often a tradeoff between full-pattern methods that may have limited resolution or flexiblity, and serial methods that can create high-resolution patterns but only by slow processes. Furthermore, metals have limited formability at very small scales. Gao et al. developed a method to create very smooth threedimensional crystalline metallic nanoscale structures using a laser to create shockwave impulses. The shockwave creates ultrahigh-strain-rate deformations that overcome the metals normal strength and, thus, resistance to patterning. Science, this issue p. 1352

Deformation Behaviors and Critical Parameters in Microscale Laser Dynamic Forming

Microscale laser dynamic forming (μLDF) is a novel microfabrication technique to introduce complex 3D profiles in thin films. This process utilizes pulse laser to generate plasma to induce shockwave pressure into the thin film, which is placed above a micro-sized mold. The strain rate in μLDF reaches 10 6 ―10 7 S ―1 . Under these ultrahigh strain rates in microscale, deformation behaviors of materials are very complicated and almost impossible to be measured in situ experimentally. In this paper, a finite element method model is built to simulate the μLDF process. An improved Johnson―Cook model was used to calculate the flow stress, and the Johnson―Cook failure criterion was employed to simulate failure during μLDF. The simulation results are validated by experiments, in which the deformation of Cu thin foils after μLDF experiments are characterized by scanning electron microscopy and compared with simulation results. With the verified model, the ultrafast μLDF process is generally discussed first. A series of numerical simulations are conducted to investigate the effects of critical parameters on deformation behaviors. These critical parameters include the ratio of the fillet radius to film thickness, the aspect ratio of mold, as well as laser intensities. The relationship of laser pulse energy and the deformation depth is also verified by a series of μLDF experiments.

Laser-Induced High-Strain-Rate Superplastic 3-D Microforming of Metallic Thin Films

Microforming of metals has always been a challenge because of the limited formability of metals at the microscale. This paper investigates an innovative microforming technique: microscale laser dynamic forming (?LDF), which induces 3-D superplastic forming in metallic thin films. This forming process proceeds in a sequence of laser irradiation and ionization of ablative coating, shockwave generation and propagation in metallic thin films, and conformation of metallic thin films to the shape of microscale molds. Because the deformation proceeds at ultrahigh strain rates, it is found that materials experience superplastic deformation at the microscale. In this paper, experiments are systematically carried out to understand the deformation characteristics of ?LDF. The topologies and dimensions of the deformed samples are characterized by scanning electron microscopy and optical profilometry. The thickness variations are characterized by slicing the cross section of the deformed material using the focused ion beam. The magnitude of deformation depth in ?LDF is determined primarily by three critical factors: film thickness, mold geometry, and laser intensity. The relationships between these factors are explored in process maps to find suitable processing conditions for ?LDF. Nanoindentation tests are conducted to show that the strength of the thin films is increased significantly after ?LDF.

Forming Limit and Fracture Mode of Microscale Laser Dynamic Forming

The microscale laser dynamic forming (LDF) process is a high strain rate microfabrication technique, which uses a pulse laser to generate high pressure by vaporizing and ionizing an ablative coating, and thus produces complex 3D microstructures in thin foils. One of the most important features of this technique is ultrahigh strain rate (typically 10 6―7 s ―1 ), which is theoretically favorable for increasing formability. However, due to the lack of measurement techniques in microscale and submicroscale, the formability of workpieces in LDF is hardly studied. In this article, experiments were carried out on aluminum foils to study the forming limits and fracture of thin films in LDF. The deformation depth was measured by an optical profilometer and the formed feature was observed using a focused ion beam and a scanning electron microscope. Meanwhile, a finite element model based on a modified Johnson―Cook constitutive model and a Johnson― Cook failure model was developed to simulate the mechanical and fracture behaviors of materials in LDF. Experimental results were used to verify the model. The verified model was used to predict the forming limit diagram of aluminum foil in LDF. The forming limit diagrams show a significant increase in formability compared with other metal forming processes.

Laser dynamic forming of functional materials laminated composites on patterned three-dimensional surfaces with applications on flexible microelectromechanical systems

Laser dynamic forming (LDF) is a three-dimensional (3D) forming technique, which utilizes laser to induce shock wave and shape the target thin films onto micro/nanoscale 3D surfaces. This technique has been used to form metals on 3D surfaces. This letter extends LDF to functional and brittle materials sandwiched by elastomeric polymers on patterned 3D surface. The elastomeric polymers absorb the shock energy and minimize the degradation of the functional materials. The patterned 3D surfaces control the plasticity in the structure and therefore retain the function of the structure. The performance was evaluated and mechanisms were studied.

Dislocation Pinning Effects Induced by Nano-precipitates During Warm Laser Shock Peening: Dislocation Dynamic Simulation and Experiments

Warm laser shock peening (WLSP) is a new high strain rate surface strengthening process that has been demonstrated to significantly improve the fatigue performance of metallic components. This improvement is mainly due to the interaction of dislocations with highly dense nanoscale precipitates, which are generated by dynamic precipitation during the WLSP process. In this paper, the dislocation pinning effects induced by the nanoscale precipitates during WLSP are systematically studied. Aluminum alloy 6061 and AISI 4140 steel are selected as the materials with which to conduct WLSP experiments. Multiscale discrete dislocation dynamics (MDDD) simulation is conducted in order to investigate the interaction of dislocations and precipitates during the shock wave propagation. The evolution of dislocation structures during the shock wave propagation is studied. The dislocation structures after WLSP are characterized via transmission electron microscopy and are compared with the results of the MDDD simulation. The results show that nano-precipitates facilitate the generation of highly dense and uniformly distributed dislocation structures. The dislocation pinning effect is strongly affected by the density, size, and space distribution of nano-precipitates.

Direct Integration of Functional Structures on 3-D Microscale Surfaces by Laser Dynamic Forming

This paper demonstrates the scalable and fast-shaping top-down integration capability of laser dynamic forming (LDF), transferring functional structures conformal to three-dimensional (3-D) micro-to-mesoscale curvilinear features on various substrates by the laser-induced shockwave. The functional materials preserve their electrical resistance and temperature coefficient of resistance after the laser shock induced transfer. The ductile interconnections inherit 3-D microscale structures on various substrates without excessive necking and fracture. This process is realized by the lamination of functional materials with cushion layers and the shockwave controlled by laser pulse intensity. The ability of direct transfer is affected by the laser intensity, cushion layer thickness, and geometry of the 3-D substrates. The experiments and numerical study reveal that the cushion layer absorbs most of shockwave energy by large thickness reduction and extends the formability of ductile interconnections. Eventually, the thickness of ductile functional materials is distributed uniformly along the 3-D surfaces. The ranges of the processing conditions for direct integration of functional materials without property degradation are also investigated.

Laser Induced High-Strain-Rate Superplastic 3D Micro-Forming of Metallic Thin Film

Microforming of metals has always been a challenge because of the limited formability of metals at the microscale. This paper investigates an innovative microforming technique: microscale laser dynamic forming (¿LDF), which induces 3-D superplastic forming in metallic thin films. This forming process proceeds in a sequence of laser irradiation and ionization of ablative coating, shockwave generation and propagation in metallic thin films, and conformation of metallic thin films to the shape of microscale molds. Because the deformation proceeds at ultrahigh strain rates, it is found that materials experience superplastic deformation at the microscale. In this paper, experiments are systematically carried out to understand the deformation characteristics of ¿LDF. The topologies and dimensions of the deformed samples are characterized by scanning electron microscopy and optical profilometry. The thickness variations are characterized by slicing the cross section of the deformed material using the focused ion beam. The magnitude of deformation depth in ¿LDF is determined primarily by three critical factors: film thickness, mold geometry, and laser intensity. The relationships between these factors are explored in process maps to find suitable processing conditions for ¿LDF. Nanoindentation tests are conducted to show that the strength of the thin films is increased significantly after ¿LDF.

Laser Shock-Induced Conformal Transferring of Functional Devices on 3-D Stretchable Substrates

This paper discussed a top-down integration method to achieve the three-dimensional (3-D) microscale conformal transferring of functional devices on flexible elastomeric substrates at ambient conditions. By the tunable laser-induced pressure, the functional device inherits the microscale wrinkle-like patterns, without compromising functions. The functional materials are encapsulated in the biocompatible parylene layers to avoid the drastic plastic deformations in functional layers. The electrical resistivity of functional device increases marginally with the applied laser intensity, aspect ratios of microscale features, and overall tensile strain applied to the whole flexible assembly. The stretchability of the transferred functional devices was studied by measuring the electrical property as function of bending and tensile strains. It shows that the device can sustain more than 40% strain in the stretchable substrate. It is demonstrated that the process can achieve the flexible and stretchable functional integration conformal to 3-D micrometer-patterns in a fast and scalable way.