Gary J. Cheng

Microstructure and mechanical property characterizations of metal foil after microscale laser dynamic forming

This article discusses the feasibility of a new microforming technique—laser dynamic forming (LDF). LDF is a new hybrid forming process, combining the advantages of laser shock peening, and metal forming, with an ultra high strain rate forming utilizing laser shock waves. Experiments are conducted on copper foils to demonstrate this forming process. After the forming process, the mechanical and microstructure of the formed work piece will be characterized. Electron backscatter diffraction will be used to investigate the grain microstructure and misorientations quantitatively. The residual stress distributions will be measured using x-ray diffraction. The key factors for the improved formability of this high strain rate microforming process will be discussed in detail. With further development, LDF may become an important microforming technology for various materials.

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

Three-Dimensional Printing of Complex Structures: Man Made or toward Nature?

Current three-dimensional (3D) printing techniques enable the fabrication of complex multifunctional structures that are unimaginable in conventional manufacturing. In this Perspective, we outline recent progress in materials and manufacturing and propose challenges and opportunities for the future development of 3D printing of functional materials. The success of future 3D printing relies not only on multifunctional materials and printing techniques but also on smart design of complex systems. Engineers need to understand advanced materials, additive manufacturing, and, more importantly, creative design. Fortunately, we can learn from many structures that exist in nature and adapt them to engineered structures.

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.

Highly conductive and transparent alumina-doped ZnO films processed by direct pulsed laser recrystallization at room temperature

Highly conductive and transparent alumina-doped ZnO (AZO) thin films (250 nm) are deposited at room temperature using pulsed laser deposition (PLD) and direct pulsed laser recrystallization (DPLR). Morphological characterizations show that the AZO films undergo recrystallization and growth during DPLR, which leads to less internal imperfections in AZO films and hence better film conductance. Electrical-optical characterizations show that DPLR results in significant improvement in conductivity, Hall mobility, and transmission from UV to NIR regions. Decrease in carrier concentration density in AZO film is observed. Compared with PLD, DPLR processed AZO films also possess smaller band gap which leads to broader solar spectrum acceptance.

Deformation-induced martensite and nanotwins by cryogenic laser shock peening of AISI 304 stainless steel and the effects on mechanical properties

Laser shock peening (LSP) of stainless steel 304 was carried out at room and cryogenic temperature (liquid nitrogen temperature). It was found that the deformation-induced martensite was generated by LSP only when the laser-generated plasma pressure is sufficiently high. Compared to room temperature laser shock peening (RT-LSP), cryogenic laser shock peening (CLSP) generates a higher volume fraction of martensite at the same laser intensity. This is due to the increase in the density of potential embryos (deformation bands) for martensite nucleation by deformation at cryogenic temperature. In addition, CLSP generates a high density of deformation twins and stacking faults. After CLSP, an innovative microstructure, characterised by networks of deformation twins, stacking faults and composite structure (martensite and austenite phases), contributes to material strength and microstructure stability improvement. The combined effect of higher surface hardness and a more stabilised microstructure results in greater fatigue performance improvement of the CLSP samples compared to that of the RT-LSP samples.

3D stereolithography printing of graphene oxide reinforced complex architectures.

Properties of polymer based nanocomposites reply on distribution, concentration, geometry and property of nanofillers in polymer matrix. Increasing the concentration of carbon based nanomaterials, such as CNTs, in polymer matrix often results in stronger but more brittle material. Here, we demonstrated the first three-dimensional (3D) printed graphene oxide complex structures by stereolithography with good combination of strength and ductility. With only 0.2% GOs, the tensile strength is increased by 62.2% and elongation increased by 12.8%. Transmission electron microscope results show that the GOs were randomly aligned in the cross section of polymer. We investigated the strengthening mechanism of the 3D printed structure in terms of tensile strength and Youngs modulus. It is found that an increase in ductility of the 3D printed nanocomposites is related to increase in crystallinity of GOs reinforced polymer. Compression test of 3D GOs structure reveals the metal-like failure model of GOs nanocomposites.

Crystalline Nanojoining Silver Nanowire Percolated Networks on Flexible Substrate

Optoelectronic performance of metal nanowire networks are dominated by junction microstructure and network configuration. Although metal nanowire printings, such as silver nanowires (AgNWs) or AgNWs/semiconductor oxide bilayer, have great potential to replace traditional ITO, efficient and selective nanoscale integration of nanowires is still challenging owing to high cross nanowire junction resistance. Herein, pulsed laser irradiation under controlled conditions is used to generate local crystalline nanojoining of AgNWs without affecting other regions of the network, resulting in significantly improved optoelectronic performance. The method, laser-induced plasmonic welding (LPW), can be applied to roll-to-roll printed AgNWs percolating networks on PET substrate. First principle simulations and experimental characterizations reveal the mechanism of crystalline nanojoining originated from thermal activated isolated metal atom flow over nanowire junctions. Molecular dynamic simulation results show an angle-dependent recrystallization process during LPW. The excellent optoelectronic performance of AgNW/PET has achieved Rs ∼ 5 Ω/sq at high transparency (91% @λ = 550 nm).

Stability, Antimicrobial Activity, and Cytotoxicity of Poly(amidoamine) Dendrimers on Titanium Substrates

In this article, we present the first report on the antibacterial activity and cytotoxicity of poly(amidoamine) (PAMAM) dendrimers immobilized on three types of titanium-based substrates with and without calcium phosphate coating. We show that the amino-terminated PAMAM dendrimers modified with various percentages (0-60%) of poly(ethylene glycol) (PEG) strongly adsorbed on the titanium-based substrates. The resultant dendrimer films effectively inhibited the colonization of the Gram-negative bacteria Pseudomonas aeruginosa (strain PAO1) and, to a lesser extent, the Gram-positive bacteria Staphylococcus aureus (SA). The antibacterial activity of the films was maintained even after storage of the samples in PBS for up to 30 days. In addition, the dendrimer films had a low cytotoxicity to human bone mesenchymal stem cells (hMSCs) and did not alter the osteoblast gene expression promoted by the calcium phosphate coating.

Laser Engineered Multilayer Coating of Biphasic Calcium Phosphate/Titanium Nanocomposite on Metal Substrates

In this work, laser coating of biphasic calcium phosphate/titanium (BCP/Ti) nanocomposite on Ti-6Al-4 V substrates was developed. A continuous wave neodymium-doped yttrium aluminium garnet (Nd:YAG) laser was used to form a robust multilayer of BCP/Ti nanocomposite starting from hydroxyapatite and titanium nanoparticles. In this process, low power coating is realized because of the strong laser-nanoparticle interaction and good sinterability of nanosized titanium. To guide the optimization of laser processing conditions for the coating process, a multiphysics model coupling electromagnetic module with heat transfer module was developed. This model was validated by laser coating experiments. Important features of the coated samples, including microstructures, chemical compositions, and interfacial bonding strength, were characterized. We found that a multilayer of BCP, consisting of 72% hydroxyapatite (HA) and 28% beta-tricalcium phosphate (β-TCP), and titanium nanocomposite was formed on Ti-6Al-4 V substrates. Significantly, the coating/substrate interfacial bonding strength was found to be two times higher than that of the commercial plasma sprayed coatings. Preliminary cell culture studies showed that the resultant BCP/Ti nanocomposite coating supported the adhesion and proliferation of osteoblast-like UMR-106 cells.