Published papers

© 2018, Springer-Verlag London Ltd., part of Springer Nature. We investigated the wear modes and mechanisms of alumina grits during abrasive belt grinding of U71Mn steel under various normal forces and grinding speeds. Our approach consisted of an experimental procedure that produced sufficient wear followed by a resin-embedding method for observing the cross sections and topographies of grits. Statistics of wear modes and protrusion heights were calculated to visualize and quantify abrasive belt wear. The results demonstrated that different subsurface structures emerged inside the worn grits, and these structures were sensitive to process parameters. Transitions between abrasion and fracture wear modes were frequently observed. The effects of abrasive belt wear and process parameters on wear modes were clarified. The critical protrusion height of the abrasive belts for each wear mode of abrasive grits was experimentally determined.

© 2019 IOP Publishing Ltd. The edges and notches of silicon wafers are usually machined by diamond grinding, and the grinding-induced subsurface damage causes wafer breakage and particle contamination problems. However, the edge and notch surfaces have large curvature and sharp corners, thus it is difficult to be finished by chemo-mechanical polishing. In this study, a nanosecond pulsed Nd:YAG laser was used to irradiate the edge and notch of a boron-doped single-crystal silicon wafer to recover the grinding-induced subsurface damage. The reflection loss and the change of laser fluence when irradiating a curved surface were considered, and the damage recovery behavior was investigated. The surface roughness, crystallinity, and hardness of the laser recovered region were measured by using white light interferometry, laser micro-Raman spectroscopy, and nanoindentation, respectively. The results showed that after laser irradiation the damaged region was recovered to a single-crystal structure with nanometric surface roughness, and the surface hardness was also improved. This study demonstrates that laser recovery is a promising post-grinding process for improving the surface integrity of the edge and notch of silicon wafers.

© 2018 The Society of Manufacturing Engineers This paper presents an investigation of the effects of tool rake angle and nose radius on the surface quality of ultraprecision diamond-turned porous silicon. The results showed that as rake angle decreases, the high-stress field induced by the tool edge increases, causing microcracks to propagate extensively near the pore walls. As a result, the ductile-machined areas shrank under a negative tool rake angle. On the other hand, brittle fracture occurred around pores released cutting pressure significantly. These trends of rake angle effects are distinctly different from those in the cutting of non-porous silicon. Finite element simulation of stress in the cutting area agreed with the experimental results. The results also indicated that using a tool with a bigger nose radius suppressed brittle fractures around the pore edge and improved surface quality. Raman spectroscopy of the ductile-machined surfaces revealed that the amorphization of the subsurface layer became more significant when decreasing tool rake angle or increasing tool nose radius. By choosing the optimal tool geometry, a high quality surface can be achieved on porous silicon, which demonstrates the capability of the diamond turning process to fabricate high-precision components.

© 2018, Springer-Verlag London Ltd., part of Springer Nature. The performances of carbon nanofiber (CNF) reinforced copper (Cu) composite electrodes on the reaction-bonded silicon carbide (RB-SiC) ceramic were experimentally investigated by using electrical discharge machining (EDM). The effects of CNF content on material removal rate (MRR), electrode wear ratio (EWR), surface roughness (SR) and surface topography were examined. Results indicated that the addition of CNF to the Cu electrode increased the MRR of the workpiece. In addition, increasing the added CNF content increased the EWR of the Cu–CNF composite electrode and the SR of the machined surface. The results of one-way ANOVA indicated that the Cu–1.0 wt% CNF composite electrode exhibited the optimal MRR and EWR values, although this electrode resulted the maximum SR on the RB-SiC machined surface.

© 2018 Elsevier B.V. CrBC and CrBCN coatings with low and high B contents were deposited on 316L steel and Si wafers using an unbalanced magnetron sputtering system. Mechanical properties including hardness (H), elastic modulus (E) and fracture toughness (K Ic ) as well as residual stresses (σ) were quantified. A clear correlation between structural, mechanical and tribological properties of coatings was found. In particular, structural analyses indicated that N incorporation in CrBC coatings with high B content caused a significant structural evolution of the nanocomposite structure (crystalline grains embedded into an amorphous matrix) from nc-CrB 2 /(a-CrB x , a-BC x ) to nc-CrN/(a-BC x , a-BN). As a result, the hardness of CrBC coating with high B content decreased from 23.4 to 16.3 GPa but the fracture toughness was enhanced. Consequently, less cracks initiated on CrBCN coatings during tribological tests, which combined with the shielding effect of a-BN on wear debris, led to a low friction coefficient and wear rate.

© 2019 IOP Publishing Ltd. Multi-cyclic nanoindentation experiments with various holding loads were carried out on single-crystal Ge to investigate pressure-induced phase transformation behaviors. Cross-sectional transmission electron microscopic observation and laser micro-Raman spectroscopy of the indents revealed various phase transformation behaviors at different holding loads. Distinctive phase transformation behaviors were observed under various loading and unloading conditions. With a low holding load, a broad phase transformed region containing r8-Ge phase and dislocations was generated. A middle holding load promoted twinning but limited phase transformations. On the other hand, a densely transformed region containing dc-Ge, a-Ge, r8-Ge and st12-Ge was widely formed at a high holding load. In addition, the calculation results of the average contact pressure in the last unloading step verified the occurrence of these phase transformations. By integrating these results, a path diagram for phase transformations of single-crystal Ge during multi-cyclic nanoindentation was established, which is useful for understanding the subsurface damage formation in mechanical processing of the material.

© 2019 Elsevier Inc. The formation of laser-induced periodic surface structure (LIPSS) has been observed on various materials. As LIPSS can be used for modifying various surface properties such as wettability, tribology and optical performance, the control of the period and topography of LIPSS is an important issue. In this study, picosecond pulsed laser irradiation was performed on a stainless tool steel in various types of media including the air, water, sugar syrup (water and sucrose (C 12 H 22 O 11 )), decane (C 10 H 22 ), and ethanol (C 2 H 5 OH). Different surface morphologies were observed on the steel surface after laser irradiation, depending on the types of media. A strong relationship was identified between the period of LIPSS and the refractive index of liquid. In addition to the change of LIPSS period, complicated mesh patterns were formed on the surface irradiated in decane and ethanol, indicating the significant effect of thermal conductivity of the liquids on surface formation mechanisms. This study demonstrated the possibility of controlling the period and topography of LIPSS by using different media.

© 2019 by the authors. Silicon micropillars with tunable sizes are successfully fabricated on copper foils by using nanosecond-pulsed laser irradiation and then used as anodes for lithium-ion batteries. The size of the silicon micropillars is manipulated by using different slurry layer thicknesses ranging from a few microns to tens of microns. The effects of the pillar size on electrochemical properties are thoroughly investigated. The smaller the pillars, the better the electrochemical performance. A capacity of 1647 mAh g-1 at 0.1 C current rate is achieved in the anode with the smallest pillars, with 1215, 892, and 582 mAh g-1 at 0.2, 0.5, and 1.0 C, respectively. Although a significant difference in discharge capacity is observed in the early period of cycling among micropillars of different sizes, this discrepancy becomes smaller as a function of the cycle number. Morphological studies reveal that the expansion of micropillars occurred during long-term cycling, which finally led to the formation of island-like structures. Also, the formation of a solid electrolyte interphase film obstructs Li+ diffusion into Si for lithiation, resulting in capacity decay. This study demonstrates the importance of minimizing the pillar size and optimizing the pillar density during anode fabrication.

Copyright © 2019 by ASME. Metal additive manufacturing (AM) has been attracting attention as a new manufacturing method, but a surface finishing process is usually needed to improve the surface quality. As a new surface finishing process, ultrasonic vibration-assisted burnishing (UVAB) is promising. In this study, UVAB was performed on an additive-manufactured AlSi10Mg workpiece to improve its surface/subsurface integrity. The effects of ultrasonic vibration (UV) and lateral tool pass width on the burnishing performance were investigated. It was observed that the surface roughness, filling ratio, and hardness of the surface layer were simultaneously improved after burnishing. This study shows the effectiveness of applying UVAB to improve the surface quality of additivemanufactured products for various industrial uses.

© 2019 Optical Society of America Traditional methods for distortion measurement of large-aperture optical systems are time-consuming and ineffective because they require each field of view to be individually measured using a high-precision rotating platform. In this study, a new method that uses a phase diffractive beam splitter (DBS) is proposed to measure the distortion of optical systems, which has great potential application for the large-aperture optical system. The proposed method has a very high degree of accuracy and is extremely economical. A high-precision calibration method is proposed to measure the angular distribution of the DBS. The uncertainty analysis of the factors involved in the measurement process has been performed to highlight the low level of errors in the measurement methodology. Results show that high-precision measurements of the focal length and distortion were successfully achieved with high efficiency. The proposed method can be used for large-aperture wide-angle optical systems such as those used for aerial mapping applications.

© 2019 Elsevier Inc. A nanosecond pulsed Nd:YVO4 laser was irradiated on a boron-doped single-crystal silicon wafer after rough and fine grinding processes to recover the grinding-induced subsurface damages. The surface topography of samples was investigated by using a white-light interferometer, a scanning electron microscope, and an atomic force microscope; while the crystallinity was analyzed by a laser micro-Raman spectrometer. It was found that surface nanostructures were generated by the Mullins-Sekerka instability, which remained on the surface under recoil pressure and surface tension. The rough grinding-induced deep subsurface damages influenced the interface instability between liquid and solid silicon during recrystallization process. By increasing pulse width and decreasing laser peak irradiance, the subsurface damage was recovered and a flat surface with surface roughness of ~1 nm Sa was obtained. This study reveals important correlations among grinding-induced latent subsurface defects, laser peak irradiance and nanoscale surface topography formation in laser recovery, which contributes to high quality silicon wafer manufacturing.

© 2019, Springer-Verlag London Ltd., part of Springer Nature. Silicon carbide (SiC) is heavily used in the industry due to its resistance to chemical wear and excellent mechanical properties such as high hardness and high stiffness. However, these properties make it difficult to produce micro and nanostructures on the surface of SiC by conventional methods. In this study, high-density microcones that protrude ~ 10 μm above the initial surface have been fabricated by nanosecond pulsed Nd:YAG laser irradiation (λ = 532 nm) on reaction-bonded SiC. Geometrically aligned cones were also fabricated by modifying the laser scanning path, and effect of different parameters such as pulse frequency, laser fluence was studied. It was observed that the surface morphology of microcones was affected by the pulse width and beam overlap. X-ray spectroscopy and Raman spectroscopy showed that the microcones were mainly composed of silicon. Formation of these cone structures made the surface highly superhydrophilic with a contact angle of ~ 0°.

© 2020, Springer-Verlag London Ltd., part of Springer Nature. It has been previously reported that electrical discharge machining (EDM) performance can be improved by tool electrode vibration. However, the mechanism of vibration-assisted EDM has not been clarified and the optimal vibration condition remains unknown due to the difficulty in direct process observation. In this study, direct observation of the EDM gap by using a high-speed video camera through a transparent workpiece electrode made of single-crystal 4H-SiC was attempted. It was found that tool vibration caused discharge dispersion and discharge frequency increase, and this tendency was more significant at higher amplitudes and higher frequencies. In addition, tool vibration-induced periodic discharge was observed, which prevented concentrated discharge and short-circuiting. Accordingly, the machining speed was improved and surface roughness was reduced. This study succeeded for the first time in observing the discharge behaviors in vibration-assisted EDM, thus revealed the effects of key parameters for process optimization.

© 2019 Elsevier Ltd Depth of cut (h0) and tool edge radius (re) are two key parameters in nanometric cutting, investigating both of the two parameters simultaneously can provide comprehensive understanding of the cutting mechanism. In this paper, relative tool sharpness (RTS), which is quantified as h0/re, is employed as a factor to examine the subsurface damage and material recovery in nanometric cutting of mono-crystalline silicon using molecular dynamics (MD) simulation. Various RTS values were generated by changes of cutting depth at different tool edge radius of 1, 3 and 5 nm respectively. Results indicate that there is always a layer of particles which sticks on the tool surface and influences the machined surface, even at RTS = 0. Besides that, the increase of RTS results in subsurface damage layer serration, which is caused by stick-slip phenomenon between the tool and workpiece. A bigger RTS causes a bigger depth of serrations, although the number of serrations remains constant. Increase in RTS also causes the formation of the hexagonal diamond structure. The material recovery drops dramatically by RTS increase. Using a sharper tool edge (RTS<0.25) at a cutting depths below 1 nm does not necessarily decrease subsurface damage due to the drastic stress concentration. It is also demonstrated that silicon amorphisation can occur in the unmachined region in front of the tool due to the hydrostatic pressure wave caused by tool advancement.

© 2020, Korean Society for Precision Engineering. Lubrication can improve the tool-workpiece interface condition in ultraprecision machining, especially in microgrooving processes. The aim of this study was to compare the effects of dry, liquid (cutting oil), solid (wax) and hybrid (cutting oil and wax) lubrication conditions, on the creation of arc-sectioned microgrooves on single-crystal silicon wafers using a single-crystal diamond tool. Micro cutting forces, chip morphology, and surface quality at different locations of the grooves were investigated. Rose-shaped continuous chips were observed in dry cutting, indicating stable ductile mode material removal. The entrance side of the groove was always smoother than the exit side with a higher degree of phase transformation. Chip adhesions on the tool edge were found in dry and oil lubricated cutting, whereas wax coating could prevent the adhesion. Wax coating played the dual roles of a chip breaker as well as a high-performance lubricant, thus improved the surface integrity. This study demonstrated the capability of using wax coating as an environmentally friendly solid lubricant for microgrooving of single-crystal silicon and possibly for other hard brittle materials.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Silicon nanofiber clusters were successfully generated by the irradiation of millisecond pulsed laser light on silicon sludge disposed from wafer back-grinding processes. It was found that the size, intensity, and growing speed of the laser-induced plume varied with the gas pressure, while the size and morphology of the nanofibers were dependent on the laser pulse duration. The generated nanofibers were mainly amorphous with crystalline nanoparticles on their tips. The crystallinity and oxidation degree of the nanofibers depended on the preheating conditions of the silicon sludge. This study demonstrated the possibility of changing silicon waste into functional nanomaterials, which are possibly useful for fabricating high-performance lithium-ion battery electrodes.

© 2020 The Authors The submicron-level structured surface induces viewing-angle-dependent iridescence, and has wide applications in multi-color printing, micro-display projection, invisibility cloak technology. In this study, several types of two-level structures, which consist of the first-order micro geometric features corresponding to the pattern shape and the second-order submicron grooves corresponding to the diffraction grating, are designed to directly induce a variety of iridescent patterns based on their shape regulations. To fabricate these two-level iridescent structures with high accuracy in a single step, vibration-assisted fly cutting (VAFC) is proposed. VAFC involves the low-frequency vibration of the workpiece in the vertical direction during the feed in the horizontal direction and the high-speed rotation of the diamond cutting tool. A 3D theoretical model is established for the numerical simulation of the generating process of the iridescent structures and the regulation of the first-order geometric feature by vibration parameters. As the key parameter to the iridescent color, the spacing of the second-order submicron grooves is flexibly controlled by the feed rate. A VAFC platform is developed to fabricate the two-level structures and an optical detection system is setup to test the surface iridescence. Various vivid colors are regulated by the two-level structures with high saturation and uniformity.

© 2020 Multiscale structured surfaces are a way to provide advanced, otherwise not attainable functionality on a technical part. Applications of such parts can be manifold, and numerous works have already covered the transfer of natural examples into bio-inspired surfaces or the geometrical and functional metrology of such surfaces. After briefly presenting typical functionalities of multiscale structured surfaces, this keynote paper will focus on the available manufacturing processes and review their capabilities to generate multiscale structured surfaces. To compare such processes, the so-called “multiscality” is defined that characterizes the structured surfaces according to the lateral and vertical extent of the individual stacked elements and is used as a first indicator to assess the difficulty of their manufacture. As the boundaries of what is considered a multiscale structure are diffuse, ranges of low, medium and high multiscality are defined instead. After presenting the state of the art of manufacturing processes currently utilized for the manufacture of (not only multiscale) structured surfaces, this keynote paper summarizes the capabilities of single-step and multi-step/multi-physics approaches for their applicability across different scales and gives an outlook on which processes could potentially become relevant in the future.

© 2020 Elsevier Ltd Low power nanosecond laser irradiation has potential applications in surface processing of metallic glasses (MGs). However, the response of MGs to a relatively low power intensity (1011 W/m2) has not been investigated. In this study, surface microstructures and characteristics of Zr-based MG after low power nanosecond laser irradiation were studied. Micro-cracks with depth in nanoscale and width in microscale were observed on the irradiated MG surface. The effects of laser parameters on the formation and evolution of micro-cracks were further investigated, and surface characteristics were characterized by using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The results showed that under the peak laser power intensity of 2.3 × 1011 W/m2, when increasing the scanning speed from 1 to 5 mm/s, the maximum width of micro-cracks reduced from 3 μm to be less than 1 μm; further increasing to 10 mm/s, no micro-cracks appeared. When increasing the laser power intensity, the micro-groove structure would be formed with pileup around it and line cracks, open cracks and laser pulse tracks on it. During multi-line laser scanning, the pulse overlap rate also affected the formation and distribution of micro-cracks. At last, the formation mechanism of micro-cracks was discussed. These results would be meaningful for understanding the laser-MG interaction as well as guiding the selection of laser parameters for various applications.

© 2020, The Author(s). Ultrashort pulsed laser-induced periodic surface structures (LIPSS) can be generated on different kinds of materials, which are widely utilized for modifying surface properties such as wettability, adhesion, and tribological, as well as optical performances. Previous studies have focused mainly on one-dimensional LIPSS (i.e., line structure) generation. In this study, a picosecond pulsed laser was used to irradiate stainless-steel surfaces for generating two-dimensional LIPSS, namely nanodot structures, by cross-scanning the laser beam for a different number of times. The obtained nanodot structures were found to be super hydrophilic just after laser irradiation, but turned to be hydrophobic after exposure in air for a few days. By cross-scanning the laser beam for the same number of times, local LIPSS rewriting was realized. This study showed the possibility of improving the homogeneity of the surface properties of steel materials through laser-induced nanodot structuring.