Published papers

© 2018 Elsevier B.V. Laser processing is a versatile tool for applications in the field of metallic glasses (MGs). Previous studies reported that surface nitridation, i.e. the formation of ZrN phase, occurred when laser irradiation of Zr-based MG was performed in nitrogen gas. Additionally, it was reported that laser thermal shock usually softened the irradiated surface. Therefore, the coupling effects of laser thermal shock and surface nitridation on mechanical characteristics of Zr-based MG should be further clarified. For this purpose, in this study, laser irradiation of Zr-based MG was conducted under various experimental conditions, followed by evaluating the surface characteristics by X-ray diffraction (XRD), nanoindentation, and energy-dispersive X-ray (EDX) spectroscopy. Experimental results indicated that the nanoindentation hardness, serrated flow, and surface shear bands were significantly affected by laser irradiation in nitrogen gas. Both surface softening and hardening were observed which depended on the experimental conditions. A competition mechanism between laser thermal shock and the introduction of ZrN phase was employed to rationalize these results. Furthermore, the affecting layer of ZrN phase along the depth direction was measured by EDX. The obtained results suggest a new method to introduce secondary phase into Zr-based MG surface and further to tune the mechanical characteristics of MGs.

© 2018 Elsevier B.V. Laser irradiation has been recently used to tune the mechanical properties of metallic glasses (MGs). However, the mechanism for plasticity improvement by laser irradiation is still not completely clear and requires further investigation. In this study, nanosecond pulsed laser irradiation is applied on a Zr-based MG surface under various laser powers, scanning speeds, and numbers of irradiation cycles. Then, the mechanical properties of the as-cast and laser-irradiated MG surfaces are characterized by nanoindentation tests. In particular, the indentation hardness, load-depth curve, serrated flow, and residual indent morphology of the surfaces are compared. The results show that the nanosecond pulsed laser irradiation softens the irradiated regions; the effect of the irradiation is slightly dependent on the laser irradiation parameters, and especially the laser scanning speed. Furthermore, laser irradiation considerably affects serrated flows in the load-depth curve and shear bands on the top surface, denoting different plastic deformation characteristics compared with the non-irradiated sample. These differences could be rationalized in terms of laser irradiation-induced formation of pre-existing shear bands in the subsurface layer, as well as thermal effects. The present results are expected to enhance the general understanding of the effects of laser irradiation on the mechanical properties of MGs.

© 2018 A novel method was proposed for generating nanoscale surface patterns on single-crystal diamond by carbon diffusion with a microstructured titanium mold under controlled temperature and pressure. The depth, geometry, and surface integrity of the fabricated patterns were investigated by laser micro-Raman spectroscopy and white-light interferometry, and the titanium molds were analyzed by energy dispersive X-ray spectroscopy. The results showed that at specific temperatures and pressures, three-dimensional patterns with a depth of tens of nanometers and sloped/curved walls could be generated on a diamond surface after a few minutes, without causing any surface graphitization. The intensity profile and penetration depth of carbon atoms into the titanium were experimentally measured.

© 2018 Elsevier Inc. Irradiation of a single-crystal diamond tool was performed with a picosecond pulsed laser to produce a tool with a microgrooved edge. This tool was then used in a metal cutting process to transfer the edge grooves onto a workpiece. Suitable conditions for laser irradiation on the diamond tool were experimentally investigated in terms of groove shape and laser-induced damage to diamond. Two different kinds of cutting experiments were performed; a uniformly grooved surface and a hierarchically structured surface were obtained. The chip formation mechanisms in the metal grooving process were examined. A copper workpiece was rapidly machined the surface of which had microgrooves with a pitch of a few micrometers. An increase of the contact angle was observed on the grooved surface, indicating the improvement of water repellency. This study presents an efficient method to machine microgrooves on metal materials for functional surfaces.

© 2018 Elsevier Inc. A nanosecond pulsed Nd:YAG laser was used to irradiate a sapphire substrate to produce 3-dimensional microstructures with sharp V-shaped grooves. In initial experiments, where single grooves were machined, a maximum taper angle of ∼79° was obtained. At constant laser parameters, the taper angle remained constant. As the taper angle increases from a flat surface, the irradiated area increases while incident fluence decreases; once the incident fluence approaches the ablation threshold, the taper angle becomes constant. The taper angle could be controlled by the laser fluence, scanning speed and incident angle of the laser beam. Using these results, a kind of surface microstructure, comprised of micropyramids with steep walls, was successfully machined for optical measurement applications. The surface roughness, transmittance and crystallinity of the microstructure surface could be controlled by the laser scanning speed. By applying the taper formation mechanism proposed in this study, the micromachining of sharp microstructures with steep walls on various hard brittle materials becomes possible.

© 2018 Elsevier Inc. Porous titanium presents unique material properties with a wide variety of mechanical and biomedical applications. Porous titanium components fabricated by near net-shape technologies require further machining processes to improve the surface quality and form accuracy, and in turn, the added value of the products. In this work, major factors dominating the surface integrity in ultraprecision cutting of porous titanium using single-crystal diamond tools were investigated. The results demonstrated that the presence of pores significantly changed mechanism of cutting. The chip morphology and surface topography depended on pore size and undeformed chip thickness. At an extremely small undeformed chip thickness, a majority pores were closed due to the welding phenomenon, leading to a sharp drop of surface porosity. In contrast, large pores cause craters on the machined surface and segmentation of chips and protruding lamella, especially at a large undeformed chip thickness. A coolant could lubricate the tool-workpiece interface and the shear deformation, which decreased cutting forces. On the other hand, the rapid cooling effect enhanced the work hardening effect and increase the hardness of the machined surface. Tool wear in cutting porous titanium is suppressed compared with that of pure titanium, especially in wet cutting.

© 2018 Society of Photo-Optical Instrumentation Engineers (SPIE). On-machine measurements can improve the form accuracy of optical surfaces in single-point diamond turning applications; however, commercially available linear variable differential transformer sensors are inaccurate and can potentially scratch the surface. We present an on-machine measurement system based on capacitive displacement sensors for high-precision optical surfaces. In the proposed system, a position-Trigger method of measurement was developed to ensure strict correspondence between the measurement points and the measurement data with no intervening time-delay. In addition, a double-sensor measurement was proposed to reduce the electric signal noise during spindle rotation. Using the proposed system, the repeatability of 80-nm peak-To-valley (PV) and 8-nm root-mean-square (RMS) was achieved through analyzing four successive measurement results. The accuracy of 109-nm PV and 14-nm RMS was obtained by comparing with the interferometer measurement result. An aluminum spherical mirror with a diameter of 300 mm was fabricated, and the resulting measured form error after one compensation cut was decreased to 254 nm in PV and 52 nm in RMS. These results confirm that the measurements of the surface form errors were successfully used to modify the cutting tool path during the compensation cut, thereby ensuring that the diamond turning process was more deterministic. In addition, the results show that the noise level was significantly reduced with the reference sensor even under a high rotational speed.

© 2017, Springer-Verlag London Ltd. Titanium is attracting great attentions in aerospace and medical applications where high surface quality plays an important role in improving the product performance. For developing nano-precision machining technology for titanium, clarification of the nanometer-scale chip formation mechanism is essential. In this study, the surface formation mechanism of pure titanium in ultraprecision cutting tests using single-crystal diamond tools was investigated. The results demonstrated that decreasing undeformed chip thickness from the micrometer scale down to the nanometer scale had profound impacts on the shear angle, specific cutting force, and chip morphology. Chip tearing phenomenon occurred when undeformed chip thickness is smaller than a critical value (~ 100 nm), which significantly affected the chip morphology and machined surface integrity. In nanometer-scale cutting, tool feed mark is no longer a major reason of surface roughness; instead, material plucking, debris, scratches, and chip adhesion influenced the surface integrity. The high pressure generated in the nanometer-scale cutting caused a hardness increase in workpiece material and promoted workpiece material adhesion to the tool surface, as well as tool wear.

© 2019, The Author(s). A systematic wear model of the cylindrical grinding process with an alumina abrasive belt from the perspective of single grain sliding wear was established in this study. The model consists of three parts: a single cutting force model derived by applying a stress integration method, a single grain wear height analysis based on the wear rate of alumina, and a grinding mileage prediction of multiple grains with Gaussian distributed protrusion heights. Cutting force, single grain wear height and full-size grinding mileage verification experiments were conducted. The results indicated that the established model was in reasonable agreement with the experimental outcomes, which suggests that this model could be useful in the industry to predict the wear process of abrasive belts.

© 2019, The Minerals, Metals & Materials Society. A cutting process is strongly affected by depth of cut (h0) and tool edge radius (re), the relationship of which is described by relative tool sharpness (RTS = h0/re). In nanometric cutting, the depth of cut is far smaller than the tool edge radius, thus the tool is actually a very blunt one with a highly negative effective tool rake angle (~ − 90°). However, most previous studies have assumed the tool is extremely sharp with a nanometric edge radius, and to date there is very little literature on the cutting mechanism for a blunt tool (RTS < 1). In this study, the material deformation/removal behavior and forces under a small RTS from 1 to 0 were investigated via molecular dynamics. The results show that the surface integrity and chip formation are strongly affected by RTS. As RTS is reduced to 0.25 or smaller, rubbing and ploughing occur with no material removal. In such a circumstance, the force angle drops sharply with remarkable subsurface damage formation in the workpiece. These findings are distinctly different from the common belief, and provide new information for the optimization of the silicon wafer manufacturing process.

© 2019, Fuji Technology Press. All rights reserved. Hybrid structures of single-crystal silicon and high-density polyethylene (HDPE) with high transmittance in the mid-to-far infrared region are used as infrared lens substrates. The hybrids are usually fabricated by high-precision press molding. The Si-HDPE hybrid lens previously fabricated had a low transmittance in the 9–10 μm wavelength region, thereby limiting its application for human body detection. In this study, a Si-polymer hybrid structure was fabricated using a new polymer without any silane coupling agent. Interfacial adhesion between the polymer and the Si substrate was realized with an extremely thin (a few micron thick) layer of an interfacial silane coupling agent. The press molding conditions that led to improved bonding strength and infrared transmittance of the hybrid substrate were investigated. A transmittance similar to that of a single-crystal Si substrate was achieved in the 9–10 μm wavelength range.

© 2020 Elsevier Ltd Silicon nanoparticles can be used for fabricating electrodes in high-performance lithium ion batteries and other high value-added products. Currently, silicon nanoparticles are fabricated by pulsed laser irradiation of single-crystal silicon wafers in water. In this study, we proposed silicon nanoparticle generation by using a nanosecond pulsed laser to irradiate waste silicon powder which is disposed from wire-saw slicing processes of silicon ingots. The laser-induced nanoparticles were backward-transferred and deposited on a glass substrate. It was found that the morphology and amount of the deposited nanoparticles was strongly dependent on the distance between the glass substrate and the waste silicon powder target. Raman spectroscopy showed that the silicon nanoparticles were crystalline, and the average size was ~10 nm. The obtained silicon nanoparticles had high purity, as the diamond abrasives included in the waste silicon powder were removed through oxidation and vaporization during laser irradiation.

© 2020, Fuji Technology Press. All rights reserved. Functional films with multi-directional wavy microgrooves can be applied to reduce fluid drag in turbulent flow applications. For high-efficiency mass production of functional films through polymer imprinting, it is necessary to machine wavy microgrooves on the surfaces of metal roll molds. When wavy grooves are cut, to reduce the follow-up errors of machine tools, a very low cutting speed is normally used, but the mechanism of this cutting is still unclear. In this study, microgrooving experiments were conducted on three different workpiece materials: brass, oxygen free copper, and aluminum alloy, and their cutting mechanisms were investigated. Distinct differences in chip formation behavior and machined surface integrity were identified among these materials. Aluminum alloy was chosen as the most suitable material for roll mold fabrication. Two-directional wavy microgrooves with form accuracy on 1 µm level and surface roughness of less than 10 nm Ra were obtained.

© 2020 The Japan Society of Applied Physics. A huge amount of silicon sludge is disposed of from silicon wafer manufacture, and its reuse is a critical issue. In this study, silicon nanoparticle generation from sludge was explored by nanosecond-pulsed laser irradiation. The nanoparticle production efficiency from silicon sludge was compared with that from silicon wafers. The result showed that using silicon sludge as a laser irradiation target leads to a distinctly higher production efficiency of nanoparticles; the smaller the powder size of the silicon sludge, the higher the efficiency. The generated silicon nanoparticles were crystalline with sizes at the 10 nm level.

© 2020 Elsevier Ltd Zinc selenide is an excellent infrared optical material. In this study, ultraprecision diamond turning experiments were carried out on coarse-grained polycrystalline ZnSe (p-ZnSe) under various conditions and the corresponding surface formation mechanisms were investigated by examining the surface topography, chip morphology, material microstructural change, and cutting forces. Two kinds of surface defects were observed (i.e. plowing-induced micron-scale cleavage craters and tearing-induced submicron-scale tearing pits). It was determined that plowing induced micron-scale cleavage craters could be suppressed by reducing undeformed chip thickness. Furthermore, it was ascertained that tearing-induced submicron-scale tearing pits could be restrained by using a cutting tool with a zero rake angle. The minimal surface roughness was dominated by grain boundary steps formed when the cutting tool crossed twin boundaries at a large angle. A model was proposed for correlating surface defects and boundary steps with crystal orientations and cutting directions. By using a large-nose diamond tool for cutting, a smooth surface of 1.5 nm Sa was obtained. In addition, a zinc blende to cinnabar phase transformation was observed in the cutting chips, and metallization of cutting chips occurred at an extremely small undeformed chip thickness (~20 nm). Moreover, it was discovered that phase transformation inside the workpiece depends on the radius of the tool nose. The findings of this study provide an important reference for ultraprecision machining of brittle polycrystalline materials.

© 2020, Fuji Technology Press. All rights reserved. In ultra-precision diamond turning of freeform optics, it is necessary to obtain submicron-level form accuracy with high efficiency. In this study, we proposed a new method for the quick measurement and compensation of tool contour errors to improve the form accuracy of the workpiece. In this method, the nanometer-scale contour error of a diamond tool is quickly and precisely measured using a white light interferometer and then compensated for, before machining. Results showed that the contour of a diamond tool was measured with an error less than 0.05 μm peak-to-valley (P-V) and the feasibility of error compensation was verified through cutting experiments to create a paraboloid mirror and a microlens array. The form error decreased to 0.2 μm P-V regardless of the contour error of the diamond tools when cutting the paraboloid mirror, and that of the microlens array was reduced to 0.15 μm P-V during a single machining step.

© 2020 Elsevier B.V. Nanoindentation of a Zr-based metallic glass was performed under various loading rates by using a cube-corner indenter. Experimental results showed that increasing the loading rate from 2 to 20 mN/s, the size of serrated flow was greatly weakened, but the number of serrated flow kept very stable, together with very similar shear band features around the residual indents. These results challenge the current understanding derived from the results obtained by using the Berkovich indenter, and can be well explained by the shear-band propagation dynamics model, i.e., loading rate impeded the propagation of shear band rather than its nucleation.

© 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the IMMT Hard coatings are extensively required in industry for protecting mechanical/structural parts that withstand extremely high temperature, stress, chemical corrosion, and other hostile environments. Electrical discharge coating (EDC) is an emerging surface modification technology to produce such hard coatings by using electrical discharges to coat a layer of material on workpiece surface to modify and enhance the surface characteristics or create new surface functions. This paper presents a comprehensive overview of EDC technologies for various materials, and summarises the types and key parameters of EDC processes as well as the characteristics of resulting coatings. It provides a systematic summary of the fundamentals and key features of the EDC processes, as well as its applications and future trends.

© 2020, Springer-Verlag London Ltd., part of Springer Nature. This study explores the feasibility of reusing waste quarry dust as a coating material for surface modification of tungsten carbide cobalt (WC-Co) using electrical discharge coating (EDC). The main objective is to investigate the effect of quarry dust suspension with different concentrations (0–40 g/l) on the characteristics of coated surface such as coating layer thickness, coating element composition and chemical compound, surface morphology, surface roughness and Vickers micro-hardness. Before the EDC process, characterisation of the raw quarry dust was conducted through scanning electron microscopy, X-ray fluorescence and particle size analyser. The results showed that the coating layer thickness was strongly affected by the quarry dust concentration. The formation of hard carbides (SiC, Mg2C3, Fe2C and CaC2) and oxide phases (SiO2, Al2O3, Fe2O3, CaO, CoO and ZnO) in the coated layer due to material migration was confirmed through X-ray diffraction analysis. Vickers micro-hardness of the coated surface was improved significantly with the addition of quarry dust. The best surface finish without micropores and microcracks was obtained using a quarry dust concentration of 10 g/l. These results demonstrated the capability of recycling quarry dust as an environmentally friendly coating material for surface modification.

© 2020 Elsevier B.V. The study reported here confirms that laser surface melting (LSM) can be employed to modify the hardness and the shear banding behaviour of bulk metallic glasses (BMGs). More specifically, by conducting LSM operations on the Zr-based Vitreloy 105 BMG in ambient atmosphere using a nanosecond laser, it was found that surface hardening can be achieved, in addition to the well-known surface softening effect. Besides, it was found that the presence of compressive residual stress and an increased introduction of crystalline precipitates accompanied LSM-induced surface hardening. On the contrary, tensile residual stress and a reduced fraction of crystalline precipitates were observed for a softened surface post-LSM. Finally, differences in shear-banding mechanisms were detected near the surface of the laser irradiated regions. More specifically, overall reduced serrated flow but important surface shear bands events were observed following the LSM-based introduction of compressive residual stress. In contrast, more pronounced serrated flows and the likely distribution of shear banding activity well beneath the irradiated BMG surface was promoted when LSM resulted in the introduction of tensile residual stress.