Cheng-Da Wu

1. Solder microbumps have been commonly used for vertical interconnects between stacked chips for decades. With increasing demand for high input/output and fine pitch, it is very difficult to reduc...

Selective laser melting (SLM) enables the production of complexly shaped metals with programmable mechanical responses, yet most lattice studies still rely on a single unit cell and a simple columnar build, severely restricting performance improvements. Here, we examine how combining distinct unit cells and rearranging them within the build volume affects the compressive behavior of SLM Corrax maraging stainless steel lattice structures. Three designs are additively manufactured as follows: a columnar cubic-FCCZ lattice, an alternating cubic and FCCZ lattice, and a hybrid lattice (cubic+FCCZ unit cell). In situ 2D digital image correlation (DIC) and finite element analysis (FEA) are used to resolve full-field strain evolution and failure modes under quasi-static compression. The hybrid lattice achieves the highest first maximum compressive strength (418 ± 5.78 MPa) and energy absorption (128.5 ± 6.83 MJ/m³), with its specific energy absorption (44.2 ± 1.48 kJ/kg) outperforming that of the columnar cubic-FCCZ lattice and alternating cubic and FCCZ lattice by 21.76% and 8.07%, respectively. The enhanced performance is attributed to the more uniform stress distribution and delayed shear band localization afforded by the hybrid lattice. These findings show that simultaneously optimizing unit cell geometry and arrangement can significantly expand the design space of metal lattices and provide a practical approach to improving the compressive strength and energy absorption capacity of load-bearing SLM components.

Low-temperature, low-pressure Cu-to-Cu direct bonding technology is a promising solution for next-generation high-density interconnects. Previous studies have shown that many properties of nanomaterials are determined by their structural characteristics. Therefore, the effect of the nanostructure (i.e., twin crystal and twin boundary, TB, sizes) on the bonding mechanism and mechanical response of the direct bonding of (111)-oriented nanotwinned Cu (NT-Cu) is studied using molecular dynamics simulations, where TB size means the TB layer thickness in terms of the number of atoms. The simulation results show that NT-Cu with extremely small twin crystals (e.g., 0.625 nm) have poor diffusivity. The number of dislocations induced by plastic deformation increases with increasing twin crystal size during stretching processes, degrading mechanical strength. The strain hardening of bonded NT-Cu with extremely small twin crystals (e.g., 0.625 nm) is dominated by the strong barrier created by a high density of TBs, whereas that with twin crystal sizes of 2.5–10 nm is dominated by dislocation–TB and dislocation–grain boundary interactions. Bonded NT-Cu with 2–6 atoms per TB layer exhibits softening at initial plastic deformation due to the onset of partial collapse of TBs; however, the strength then significantly increases with a further increase in strain due to strain hardening.

Context Diamond-like carbon (DLC) films are amorphous solids in which carbon atoms are linked by hybridized sp³, sp², and sp bonds. The mechanical and tribological properties of DLC films can be adjusted by tuning the sp²/sp³ bond ratio. These films are typically used as protective coatings for components such as dies, automotive engines, mechanical seals, hard disk drives, biological engineering devices, and micro/nano-electromechanical systems. Further exploration of the mechanical and tribological behavior of DLC films is important for enhancing functional design for the above applications. The simulation results show that single-layer DLC with a higher sp³ ratio has better resistance to indentation. Single-layer DLC with a lower initial sp³ ratio has lower friction and a shorter repeated cycle in the friction force curve due to an increase in the graphitization of the friction interface. Single-layer DLC with a higher sp³ ratio has a higher coefficient of friction because compared with the normal force, the friction force is much more sensitive to an increase in the sp³ ratio. Methods Molecular dynamics simulations based on the Tersoff potential were performed using the open-source code LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator).

Context Nanoporous metals (NPMs) have a three-dimensional bicontinuous porous network structure that consists of interconnected nanoligaments. NPMs have the potential to effectively attenuate and dissipate the effects of impacts and blasts. Previous studies do not provide data on the possible effects of porosity at the nanoscale. Methods Molecular dynamics simulations based on the embedded-atom method potential were performed using the open-source code LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). Results Nanoporous gold (NPG) absorbs shock energy first through the elastic-plastic deformation of ligaments and then through the collapse of ligaments. After the shock loading is relaxed, NPG with lower porosity has a higher amount of springback, and new voids tend to nucleate and grow at collapsed ligaments where the packing is looser. NPG with higher porosity has a better ability to resist the propagation of a shock wave and is subjected to a smaller stress effect.

The effects of the location and size of Kirkendall voids at the nanoscale on the mechanical response of a Cu/Sn solder joint under a tension test are studied using molecular dynamics simulations. The simulation results show that the ultimate stress and ultimate strain of a solder joint with two pre-existing voids decrease with increasing void radius regardless of the distance (L₂) between the centre of void 2 and the interface when the distance (L₁) for void 1 is 4 nm. The elastic deformation period of the solder joint shortens with increasing void radius for void radii ≥3 nm. At the end of the tensile test, a collapse occurs at the layer interface when the two pre-existing voids are very small (e.g. void radius = 1 nm) regardless of the L₂ value. For two pre-existing voids with different radii, cracks at the interface grow toward the larger void and then merge with it when the two voids are close to the interface (e.g. L₁ = L₂ = 4 nm). Fracture occurs at the pre-existing voids when the two voids have large radii (e.g. 4 nm) and are very close to each other.

The effects of heating rate during the thermal coarsening of nanoporous gold (NPG) on its morphological evolution, microstructure, and mechanical properties are studied using molecular dynamics simulations. The simulation results show that lower heating rates promote the coarsening of NPG and decreasing porosity. The coarsening proceeds until the end of the heating process, followed by structural relaxation and recovery of the crystalline structure during the cooling process. NPG coarsened at lower heating rates has better mechanical strength. NPG under compression has a much higher dislocation density than that under tension, which results in a significant tension/compression asymmetry. During compression, NPG necking preferentially occurs at smaller ligament junctions and the orientation is [110].

The effects of the extrusion ratio (diameter of the extrusion mold inlet divided by that of its outlet) and direction on extruded Au nanowires (NWs) are studied using molecular dynamics simulations based on the embedded-atom method. The effects are investigated in terms of atomic trajectories, common neighbor analysis, flow field, system pressure, and the extruded NW length-pressing ram displacement curve. The simulation results show that for the forward extrusion process, a smaller extrusion ratio leads to a larger diameter of the extruded NW and a smoother cross-sectional profile. Extruded NWs obtained at a smaller extrusion ratio have a higher face-centered cubic fraction and a less disordered structure due to lower system pressure. The adhesion effect between extruded NWs and mold outer walls increases with increasing extrusion ratio, resulting in an uneven appearance of extruded NWs and a decrease in their length. The backward extrusion process can produce amorphous NWs from crystal NWs. The forward extrusion process can produce NWs that are longer than those produced with the backward extrusion process.