Abstracts:Yttria stabilized zirconia (YSZ) is an attractive biomaterial with excellent properties, but its deformation mechanism under repetitive loads is still unclear. In this study, the subsurface microstructural changes of YSZ polycrystals under repetitive loading/unloading were investigated by multi-cyclic nanoindentation. Raman spectroscopy analysis revealed that a tetragonal-to-monoclinic (t-m) phase transformation occurred, and multi-cyclic indentation promoted this transformation significantly. Cross-sectional scanning transmission electron microscopy showed that monoclinic variants were extensively generated beneath the free surfaces outside the indent planes, whereas grain refinement was dominant beneath the indenter tip. In addition, load-displacement curve analysis demonstrated that the t-m phase transformation occurred during unloading rather than loading, and that the phase transformation-induced volume expansion caused an incremental change of residual indent depth. The extension of monoclinic variants was promoted by repetitive shear deformation, resulting in significant pileups, as detected by atomic force microscopy. The effect of sample surface preprocessing on indentation behaviors was also investigated. Microcracking and surface spalling were confirmed around the multi-cyclic indents made on a diamond-turned YSZ sample. Based on the experimental results, a model of subsurface microstructure distribution and evolution was established. The findings from this study help clarify the historical effects in subsurface damaging mechanism of brittle polycrystalline materials caused by cyclic tool-workpiece interactions in nanoscale mechanical processing.

Abstracts:Electron beam shrinking is a technique for manufacturing solid-state nanopores with direct visual feedback for better control. The electron beam in scanning electron microscopy (SEM) offers the high-resolution geometry information of nanostructure during the nanopore manufacturing processes. However, the mechanism of electron beam shrinkage is still unclear, which hinders the fabrication of solid-state nanopores array as well as their applications. In this study, Si3N4 nanopores were used as examples to explore the mechanism of electron beam shrinking. The morphology and composition characterization of the Si3N4 nanopores before and after shrinking revealed that the deposition of hydrocarbons dominates the shrinking process, accompanied by some decomposition of Si3N4. Based on this model, the controlled shrinking of nanopores was achieved, which were assembled into solid state nanopores array. The size of the nanopores can be accurately tuned by adjusting the electron beam accelerating voltage, beam current, and magnification. Under the optimal conditions, the smallest diameter of nanopore of 5.3 nm and the fastest shrinkage rate of 2.51 nm/s were obtained. Meanwhile, solid-state nanopores array, including an axisymmetric nanopore array structure, were also created for detecting translocated AuNPs-DNA through the shrunken nanopores. This study demonstrated a facile technique for fine control of the shape and size of nanopores with designed array patterns, which have great applications in producing nanogap electrodes, complex nanostructure, and surface modification or nanostructure repairing on different materials with desired geometries.

Abstracts:Micro-reentrant structures that are wide at the top and narrow at the bottom have important application values. However, the lack of an effective and universal method for fabricating micro-reentrant structures on various metallic materials is a bottleneck that restricts their development and application. In this work, the micro re-entrant structures were proposed to be divided into two parts, viz., the upper roof structures and lower pillar structures. First, superhydrophobic micropillar arrays were fabricated by nanosecond laser ablation followed by a treatment for surface energy reduction. As the electrolyte droplet stands on the top of the micropillars, because of their superhydrophobicity, and the shape of the liquid/gas interface could be regulated, the microroof array could be fabricated on the top of the pillar array by electrochemical deposition, resulting in micro-reentrant structures. The influences of the processing parameters of the laser ablation and electrochemical deposition on the features of the micro-reentrant structures were studied comprehensively. Upon a further reduction in the surface energy with the use of fluoroalkylsilane, superamphiphobicity was achieved with a contact angle larger than 150° for both water and peanut oil. The results of this study enrich the theory and technology of micromachining and promote the practical applications of micro-reentrant structures.

Abstracts:To realize the damage-free, highly efficient, and atomic-level polishing of single-crystal Si, plasma-based atom-selective etching (PASE) is proposed in this study as a generic polishing approach for Si wafers. The polishing effect of PASE is realized through the selective removal of Si atoms with more dangling bonds under high temperature. Plasma diagnostics are carried out to investigate the radical composition, the density of plasma, and the etching temperature. The key parameters of PASE are optimized, and the PASE of Si (100) with a material removal rate greater than 0.7 μm/min was realized. A ground Si (100) surface can be quickly smoothed by PASE, with the Sa roughness being reduced from 195 nm to below 1.0 nm within 5 min, and the polished surface is proven to be crystallographically perfect. The PASE of (110)- and (111)-oriented Si wafers is also proven effective, demonstrating that PASE is a generic polishing approach for Si regardless of orientation. The entire surface flattening of a 2-inch Si wafer was carried out by numerically controlled PASE, and the wafer flatness was reduced from 37.29 μm to 4.92 μm through optimized scanning conditions. Overall, this study has shown that PASE is a promising approach for high-efficiency and high-quality polishing of Si.

Abstracts:Soft-brittle crystals, e.g. KH2PO4 (KDP), are difficult-to-cut due to their high brittleness which can easily generate crack during the machining process. The conventional method to machine this kind of material is by inducing ductile cutting mechanism at room temperature with ultra-precision machining, which can only remove materials at nanoscale level and hence yields very low material removal rate. While some thermal-assisted processes have been recently attempted to improve the machinability of some difficult-to-cut materials, e.g. ceramics, there is no systematic understanding of the temperature effect on material removal mechanism of soft-brittle KDP crystals yet. In this work, the temperature effect on the material removal mechanism has been investigated for the first time using nano-scratch technique. While a decreased hardness and elastic modulus have been observed with the increase of temperature, an increase of fracture toughness has been revealed with a contradictory tendency, indicating a higher capacity of plastic deformation at elevated temperature. In contrast to the almost totally brittle scratch at room temperature (RT) caused by crack propagation and edge chipping, the scratch at 170 °C can achieve more ductile-regime surfaces with a larger critical undeformed cutting depth (3.61 μm), e.g. a significant increase of 8.60 times compared with that at RT (0.42 μm). Moreover, the TEM analysis on the subsurface microstructures shows that a great number of nano grits was generated in the subsurface at RT as the result of crack propagation and interaction, while at elevated temperature some crystallographic lattice misaligned structures (LMS) and nano crystals have been brought about due to the nucleation and evolution of thermal-activated dislocations, which explains the higher plasticity of KDP at elevated temperature. The results present in this paper are of great significance for understanding the specific temperature effect on the brittle-to-ductile transition of the cutting mechanism for future designing thermal-involved processes to machine soft-brittle materials.