Published papers

Gallium nitride (GaN) is an important semiconductor material that is widely used in advanced microelectronic and photonic devices. The machining of GaN is challenging because of its hard and brittle nature. Because surface integrity is strongly affected by strain rate during processing, this study conducted nanoindentation tests on GaN at various strain rates using a Berkovich indenter configured at two different orientation angles, to investigate how different strain rates and indenter geometry affect the deformation behavior of GaN. Results showed that, regardless of indenter orientation, the 1/3'11−2−3'{11−22} slip system was activated within the residual indents. Both the hardness and elastic modulus of GaN increased with strain rate. GaN exhibited limited plasticity, leading to sink-in of residual indents, however, the plastic deformation was enhanced at higher strain rates. At a given strain rate, indenter orientation had little influence on hardness but noticeably affected the elastic modulus. The maximum creep depth increased with strain rate, but unlike alloys and wurtzite-structured ZnO crystals, it rapidly approached saturation with little further increase over time. This study is expected to provide guidance for optimizing the surface and subsurface properties of GaN in machining processes and to offer insights into advancing its ultraprecision machining technology.

The dynamic mechanical response of FeCoCrMnNi high-entropy alloys (HEAs) is strongly governed by microstructural characteristics, whereas most existing understanding has been established under quasi-static or low to intermediate strain-rate conditions. Experimental insight into how microstructural features govern the dynamic mechanical behavior and plastic deformation mechanisms under high-strain-rate loading conditions remains limited. In this study, a laser-driven high-strain-rate impact indentation technique was employed to impose controllable localized loading at strain rates up to 105 s−1 while effectively avoiding macroscopic failure. The dynamic mechanical behavior and plastic deformation mechanisms of FeCoCrMnNi HEAs with different grain sizes were systematically investigated. The results show that, during high-strain-rate impact, both the dynamic mechanical response and plastic deformation behaviors of FeCoCrMnNi HEAs exhibit evident grain-size sensitivity. Microstructural analyses demonstrate that the plastic deformation of the coarse-grained HEA is dominated by long-range stacking-fault networks and high-density dislocation accumulation within large grains. In contrast, with decreasing grain size, the fine-grained HEA develops a more complex deformation microstructure, including short-range stacking faults, deformation twins, nanoscale grains with dense boundary networks, and local lattice disorder. Molecular dynamics simulations further confirm the difference in deformation mechanism through quantitative analyses of cluster size of hexagonal-close-packed (HCP) atoms, the fraction of newly generated HCP atoms near grain boundaries, and dislocation evolution. This study clarifies how grain size regulates dynamic mechanical characteristics of FeCoCrMnNi HEAs, providing insights into their scale dependent deformation under extreme impact loading.

Compared with conventional axisymmetric lenses, freeform surfaces provide more flexibility in designing optical lenses and are expected to expand their demand in a wide range of fields, such as virtual and augmented reality and space telescopes. Fast tool servo-based diamond turning is a promising method for machining ultra-precision freeform surfaces. However, interpolation errors occur in machining programs generated with point cloud data, resulting in large form errors, especially for large-size freeform surfaces. To solve this problem, this study proposes a novel machining program that directly evaluates the optical design equation during machining, instead of using conventional point cloud-based machining programs. This method continuously controls the tool position to its ideal position at each control sampling point using two-variable polynomial calculations, thereby reducing interpolation errors. The polynomial coefficients are corrected based on those of the original design equation by considering the tool radius. The freeform surface machining results demonstrate that the proposed method achieves a 75.7% reduction in RMS form error while replacing a point cloud consisting of 993,600 points with only 20 polynomial coefficients. The proposed method also enables machining of multiple surfaces within a single setup, further demonstrating the effectiveness of the proposed machining program. While the form accuracy of the conventional method deteriorates in proportion to the surface curvature and distance from the spindle rotation center, the proposed approach achieves high-precision machining regardless of variations in such factors, distinctly outperforming the conventional bilinear interpolation method.

Carbon fiber reinforced plastic (CFRP) is essential in lightweight structural design due to its exceptional mechanical and physical properties. Although laser processing offers an efficient method for machining CFRP, the substantial thermal damage inherent to the ablation process severely compromises structural integrity. To overcome these limitations, this study introduces an argon-assisted laser processing technique that effectively mitigates thermal damage, enabling fabrication of high-performance CFRP components. The laser ablation behavior of CFRP under air and argon atmospheres is systematically compared. Experimental and simulation results reveal that surface roughness and the width of heat-affected zone (HAZ) decrease with increasing scanning speed or scanning pitch. Crucially, argon-assisted laser processing significantly improves surface quality. Compared to processing in air, this approach yields reductions in surface roughness of up to 23.4%. Furthermore, under a laser power of 3.5 W, a scanning speed of 400 mm/s, and a scanning pitch of 30 μm, the HAZ on the right side of the laser-ablated region is reduced by a maximum of 74.3%. The underlying damage suppression mechanism involves laser energy attenuation, enhanced heat dissipation, and suppression of exothermic oxidation reactions. These findings establish argon-assisted laser processing as a promising strategy for high-fidelity manufacturing of advanced composite components.

Waterjet-driven single-particle impact is a fundamental mechanism governing material removal in abrasive waterjet machining (AWJM). To thoroughly investigate the complex fluid–solid interactions underlying this mechanism, a reliable framework that can precisely capture the physical behavior of the waterjet, abrasive particles, and workpiece erosion is essential. Therefore, a coupled numerical model based on smoothed particle hydrodynamics (SPH) and finite element method (FEM) is proposed to provide a physically consistent representation of all relevant impact phenomena. The model incorporated waterjet pressure, impact angle, abrasive mesh size, and particle radial position as the key process parameters. The results of waterjet-driven particle impact demonstrate that induced impact phenomena, particularly the stagnation zone, critically alter particle impact velocity, trajectory, kinetic energy damping across the jet radius. Moreover, the stagnation zone facilitates abrasive particle embedment by acting as a hydrodynamic confinement region that restricts particle escape. Such influences become even more pronounced with larger eroded craters or cavities. This is attributed to the expansion of stagnation region which amplifies hydrodynamic dissipation, deviates particle trajectory, and increases the probability of particle embedment. Notably, the developed SPH-FEM model successfully captured critical physical phenomena during impact that remain beyond the reach of the traditional computational fluid dynamics (CFD) approach. Overall, the revealed phenomena provide in-depth insight into impact behavior and offer clear guidance for devising a mechanistic framework to enhance material removal efficiency.

Single-crystal diamond (SCD) has excellent mechanical, thermal and physical properties that rank among the highest of all substances. It has widespread applications in diverse fields such as cutting tools, optical windows, spintronics and power electronics. However, SCD is difficult to machine via mechanical processes due to its extremely high hardness. In recent years, laser processing has attracted attention since it enables non-contact efficient machining regardless of material hardness. In this study, femtosecond pulsed laser, which is considered to be capable of high-precision machining with low thermal effect, was used for micromachining of SCD, and the effects of laser irradiation parameters on the surface texture, roughness and profile geometry, as well as subsurface material structural changes, were investigated. It was found that even under femtosecond pulses thermal effect existed and that using a higher laser scanning speed was helpful to eliminate thermal cracks formation. The laser machined surfaces were covered with nanoscale periodical surface ripples with a surface roughness in the level of 0.10 μm Ra. Raman analysis of the SCD surface after laser machining showed that amorphous carbon and nanocrystalline graphite existed as a composite material. The graphitization depended on the laser fluence and hatching width and was particularly pronounced at low fluence and small hatching width. It was also demonstrated that acid cleaning could remove most of the graphite layer and reduce surface roughness. As samples, a few three-dimensional micro cavities with flat bottoms and sharp corners were created. This study demonstrates that femtosecond pulsed laser is a suitable method for machining micro three-dimensional shapes in SCD while it is important to optimize the laser machining conditions to avoid thermal damage and improve the surface quality.

Carbon fiber reinforced plastic (CFRP), as a critical lightweight structural material, has been widely used in automotive and aerospace industries due to its exceptional mechanical properties. However, laser processing of CFRP generally induces severe thermal damage and compromises surface integrity, hindering the fabrication of high-quality components. To overcome this limitation, this study introduces a synergistic cryogenic cooling and lateral gas blowing (SCCLGB) strategy to effectively mitigate thermal damage during laser processing of CFRP. The surface characteristics of CFRP processed using SCCLGB are systematically investigated. Compared with conventional laser processing, SCCLGB reduces the width of the heat-affected zone (HAZ) by up to 93.7%. The underlying mechanisms of thermal damage suppression involve inhibited heat transfer, enhanced convective cooling, and reduced oxidative degradation. Regression models for predicting HAZ width and material removal rate (MRR) are developed and validated experimentally. The individual and interactive effects of processing conditions on both HAZ and MRR are thoroughly analyzed. Practical guidelines for selecting optimal processing conditions based on specific manufacturing requirements are also established. This work establishes a novel strategy for high-precision manufacturing of CFRP, offering a promising pathway toward its potential application in relevant industrial fields.

Microscale grooves and holes were fabricated in carbon fiber reinforced plastics (CFRPs) using femtosecond pulsed laser irradiation, and the characteristics and mechanisms of laser processing were investigated. The relationship between laser fluence and ablation depth was established by measuring the cross-sectional profiles of the microgrooves produced at different laser fluences. Furthermore, the effect of the angle between the edge of the micro holes and the carbon fibers was examined by analyzing the edges and wall surfaces of circular and rectangular holes. Experimental results revealed that the laser processing mechanism was strongly dependent on fiber orientation due to significant heat conduction along carbon fibers. The findings of this study contribute to a deeper understanding of the ultrashort pulsed laser processing characteristics and mechanisms for creating small holes in CFRPs and emphasize the possibility of drilling high-precision holes that can be used in the direct bonding of sensors/IC chips to CFRPs.

Due to its low thermal neutron absorption cross-section and adequate irradiation stability, Zr-4 alloys are widely used in nuclear reactors for critical components such as fuel rod cladding and cooling tubes. These components operate under diverse conditions, including high and ambient temperatures, as well as aqueous environments, where they are exposed to mechanical wear and corrosive media. However, the inherently limited surface hardness of Zr-4 alloy leads to poor wear resistance, potentially compromising component integrity. In this study, laser nitriding is employed to enhance the surface properties of Zr-4 alloy across multiple operating environments. Laser irradiation with a power level of 23 W in a nitrogen atmosphere increases the surface hardness to 8.75 GPa, representing a 495 % improvement over the as-received material. The laser-nitrided surfaces exhibit exceptional wear resistance, achieving maximum reductions in cross-sectional wear area of 90.8 % in room temperature air, 98.6 % in room temperature ultrapure water, and 52.1 % in air at 300 °C, compared to the as-received surface. This enhancement arises from the formation of hard nitride phases and a transformation in the dominant wear regime. Electrochemical tests reveal that the polarization resistance of the laser-nitrided surface processed at a relatively high laser power of 19 W is approximately 34 times higher than that of the as-received surface, demonstrating significantly improved corrosion resistance. These findings highlight laser nitriding as a highly effective surface engineering approach for improving the durability of Zr-4 alloy components in nuclear reactors, with important implications for enhancing the reliability of nuclear energy systems.

The fabrication of freeform optical components is increasingly demanded in emerging technologies such as augmented reality, imaging systems, and compact spectrometers. Fast-tool-servo (FTS) diamond turning has become a promising solution for high-speed, high-precision machining of such surfaces. However, achieving nanometric form accuracy remains challenging due to servo-induced dynamic errors, particularly overshoot and control delay. This study proposes a feedforward methodology to predict and compensate for dynamics-induced form errors in long-stroke FTS systems. By capturing FTS position signals and applying normalized cross-correlation (NCC) analysis, a repeatable motion error referred to as the time-shifted following error (TSFE) was identified through time delay compensation of the servo. The three-dimensional map of TSFE was constructed and integrated into the tool path to compensate for the error before machining. Experimental machining test of a dual-sinewave freeform shape achieved peak-to-valley (PV) form error of 0.26 µm, which was a 56% reduction compared to without measurement compensation. The repeatability of the three-dimensional map of TSFE showed a standard deviation of 0.007 µm on the PV, which confirmed that the TSFE map has enough repeatability to be used as a pre-compensation dataset. Additionally, the FTS system had latent TSFE error regardless the spindle rotation rates. In this study the TSFE map derivation of system indicated approximately 0.5 µm PV. These findings establish a fundamental and broadly applicable framework for improving form accuracy in high-speed freeform optics manufacturing using FTS, contributing to the advancement of ultra-precision machining technologies.

Waste Silicon (Si) powder generated during semiconductor manufacturing has attracted attention as a potential anode material for lithium-ion batteries. This study proposes a novel method to fabricate high-aspect-ratio cubic silicon carbide (3C-SiC) nanowires using a mixture of waste Si and graphite powders through laser processing. The mixture is placed between two plates to sandwich the laser-induced plume. This confinement promotes nanowire growth, increases the aspect ratio, shortens the lithium-ion diffusion length, and enables the fabrication of flexible electrodes. High-speed camera observations confirmed that the sandwiched plume was compressed and remained in air for nearly 10 s. Consequently, nanowires approximately 100–1200 nm in length and 15–60 nm in diameter (aspect ratio greater than 25) were generated. Electron diffraction and XRD analyses revealed that the nanostructures mainly comprised 3C-SiC. Battery performance evaluation tests of the generated nanowires showed that after 10 charge–discharge cycles at a current density of 50 mA/g, the SiC nanowires exhibited a specific capacity of 455 mAh/g, significantly higher than that of Si nanoparticles (291 mAh/g) and comparable to that of C-coated SiC nanoparticles (477 mAh/g). The SiC nanowires outperformed both Si nanoparticles and graphite anodes, demonstrating the potential of the proposed method in next-generation lithium-ion battery (LIB) anode fabrication.

Latent fingerprints (LFPs), though critical in forensic investigations, are often invisible and difficult to detect, especially on transparent material surfaces. Here, we present a novel nanosecond pulsed laser printing technique to replicate LFPs from transparent substrates onto metal surfaces. By placing a metal plate beneath the transparent material with the fingerprint side up, the components of the fingerprint selectively block the laser, enabling the transparent substrate to act as a photomask. This results in controlled ablation of the metal surface, replicating the fingerprint with high fidelity. Additionally, laser-induced backside etching produces a mirrored copy of the fingerprint pattern on the underside of the transparent material. Grayscale analysis confirms enhanced clarity of the printed fingerprints compared to the original LFPs. Importantly, the method is effective on curved, colored, and flexible transparent substrates and allows multiple high-quality replications from a single sample, offering a promising tool for forensic applications.

Single-crystal zinc oxide (ZnO) is an important optoelectronic material, but its micro/nanoscale mechanical behavior is poorly understood. In this study, nanoindentation tests were performed on various crystalline planes of ZnO crystals using a Berkovich indenter at different indenter orientations. The surface and subsurface characteristics of the different crystal planes were analyzed and compared. It was found that indentation on the (0001) plane activated both basal and pyramidal slip systems, which suppressed pile-up. Dislocation pinning caused pop-in events during loading and promoted edge crack formation during unloading due to high elastic recovery. Indentation on the (1−100) and (11−20) planes activated basal and prismatic slip systems. As these systems cannot accommodate dislocation glide along the [0001] direction, corner cracks easily initiated. Meanwhile, dislocations glided toward the surface with minimal resistance, thereby facilitating outward material flow and pile-up formation along the [11−20] and [1−100] directions. Indenter orientation variation did not alter slip system activation for any plane and had minimal impact on plastic flow trends. No phase transformation was detected on the indented surfaces, while dislocation-induced luminescence quenching was observed. This study reveals how crystal orientation governs surface evolution in ZnO, offering guidance for machining method development and processing parameter optimization.

The service reliability and lifespan of Ti6Al4V in high-end equipment application are limited by its poor wear resistance and susceptibility to pitting corrosion. Pulsed laser nitriding and simultaneous microtexturing can improve surface hardness and wear resistance. However, this method faces a mutual constraint between modified layer thickness and structural regularity. In this study, composite microtextures comprising periodic ripples and uniform vein-like structures are fabricated on Ti6Al4V by CW laser nitriding. Inspired by biological surfaces, various biomimetic textures are created, among which the biomimetic fish scale surface (BFS) exhibits remarkable performance enhancement. Its cross-section consists of a dendritic TiN layer on the surface and a dense acicular α'-Ti phase within the molten pool, with the modified layer thickness increasing to nearly 20 μm. Compared with the substrate, the cross-sectional hardness increases by 117 %, while the wear rate decreases by 10 % and 96 % under dry friction and starved lubrication, respectively. Moreover, the corrosion potential shifts positively from −579.84 mV to −299.47 mV, and the corrosion current density decreases by three orders of magnitude. The formation mechanisms of the microtextures are elucidated based on the evolution of microscopic morphologies and finite element simulations. The hardness enhancement is interpreted through analysis of the cross-sectional microstructure, whereas the improvement in corrosion resistance is revealed through impedance spectroscopy and passive film composition. This work provides a new strategy for the integrated construction of nitrided layers and functional microtextures on titanium alloy surfaces.

High-entropy alloy (HEA) is an emerging alloy which consists of five or more metallic elements with equimolar concentrations and exhibits excellent mechanical properties at cryogenic temperature. However, its machinability is almost unknown. In this study, high frequency one-dimensional ultrasonic vibration-assisted diamond turning (UVDT) experiments were conducted on an FeCrCoMnNi-based HEA to investigate the micro-nanoscale material removal mechanisms. Compared with conventional diamond turning, UVDT produced thinner chips, lower cutting forces, less tool wear and better surface integrity. Due to the ultrasonic vibration-assisted burnishing effect, surface scratches were significantly eliminated. A freeform surface was test-fabricated with optical-level finish.

Alumina is an advanced ceramic material that has numerous applications in dentistry and medicine. Ultrasonic-assisted end grinding proved to be one of the most successful and efficient ceramic processing techniques. However, the current state of the art in UAEG of alumina is mostly limited to finishing operations with low material removal rates. In this paper, UAEG is investigated to find the optimum grinding conditions of roughing operations of alumina ceramics. Response Surface Methodology was employed to study all combined effects of the spindle speed, feed rate, depth of cut, and vibration amplitude on the MRR, grinding forces and surface roughness. The surface morphology of the ground surface and the magnitude of the resultant grinding forces were recorded and analyzed for each test. The results showed that the vibration amplitude has the most influential effect on surface roughness with a contribution of 32.08%, while the grinding force is most affected by depth of cut (32.02%). The spindle speed of 9500 rpm, feed rate of 900 mm/min, depth of cut of 47 µm, and vibration amplitude of 2.6 µm are found to be the optimum machining parameters that maximize MRR and minimizing the grinding force and surface roughness.

Complex surfaces are increasingly used in optics, biomedical devices, and aerospace industries. Additive manufacturing and five-axis milling can produce complex shapes, but additional finishing processes are needed to meet the surface quality demands. This keynote paper reviews shape-adaptive finishing processes that enable uniform surface quality improvement without altering the shape. Key factors affecting shape adaptability, processing efficiency, and material applicability are analyzed and optimal process selection strategies for finishing complex surfaces are discussed. Future possibilities and research topics in this area, including process modeling/simulation, digital twin, surface/subsurface metrology, functionality evaluation, and applications of advanced robotics and artificial intelligence, are outlined.

Zinc oxide (ZnO) is a promising material for optics and optoelectronics. However, its machining methods are limited, with polishing being virtually the only available technique, which restricts the fabrication of complex shapes. In this study, for the first time, ultraprecision diamond cutting of single-crystal ZnO was explored. Conventional cutting (CC) and ultrasonic vibration-assisted cutting (UVC) experiments were performed in dry and oil mist lubrication environments. It was found that under dry conditions, both CC and UVC resulted in surface fractures owing to the oxidation-induced chemical tool wear. Using oil mist lubrication significantly reduced tool wear, particularly for UVC. As a result, surface roughness was reduced to 1.9 nm Sa. Cutting in the [11−20] direction was more likely to achieve crack-free surfaces on ZnO than cutting in the [1−100] direction under CC; however, the machinability in cutting in the [1−100] direction was improved by applying UVC. Both CC and UVC processes activated basal and pyramidal slip systems in the subsurface region, accompanied by a high density of dislocations located immediately beneath the surface. UVC further promotes polycrystallization and pyramidal slip activity in the subsurface. A microlens array was fabricated with a surface form error of less than 49 nm peak-to-valley and a surface roughness of less than 2.2 nm Sq by integrating slow tool servo diamond turning with ultrasonic vibration assistance under oil mist lubrication. This study reveals the fundamental cutting characteristics of ZnO, provides guidance for ultraprecision cutting of brittle oxide crystals, and validates a potential solution for fabricating micro-structures on ZnO surfaces.

Creating nanoscale grooves along sloping and curved path is significant for achieving multi-scale structured surfaces with superior surface properties. Attributed to the excellent ability to fabricate micro/nanoscale surface structures, ultra-precision diamond cutting technology is an effective method to engrave sloping-arranged nano-grooves. However, due to the tool-edge extrusion effect, the material removal behavior becomes very complicated when the tool is fed along the sloping path, consequently nano-grooves with weak stiffness are extremely prone to irregular deformation. This paper investigates the formation and cutting-caused deformation of sloping-arranged nano-grooves on amorphous nickel‑phosphorus (a-Ni-P) through mechanism analyzation, simulation, and experiments. The mechanism of nano-grooves deformation and materials residue under the non-free cutting condition is revealed. The influence of the size of the stress zone by the extrusion of the tool edge on the deformation is analyzed. What's more, the influence mechanism of direction of the sloping feed path and its slope on the deformation are investigated. Compared to extruding the long bevels of nano-grooves in the downward feed path, the tool edge extrudes the short bevels of nano-grooves in the upward feed path, making the stress zone smaller and deformation suppressed. To suppress the deformation of undulating sloping-arranged nano-grooves in the full feed path, a novel intermittent-upward feed path is proposed for fly cutting of sloping-arranged nano-grooves on a side-mounted workpiece. The proposed tool feed strategy was demonstrated to be able to improve the machining quality of the undulating sloping-arranged nano-grooves.

Microtextured surfaces, such as those with groove, hole, or protrusion structures, are widely used in various industries for bonding dissimilar materials because of their anchor effect, which enhances adhesion and penetration with other materials. Although surfaces with grooves and protrusions are expected to show higher bonding strength, it is difficult to fabricate such hybrid structures using a material removal process or additive manufacturing. This study established a new burnishing method using a tool with preformed microgrooves at the tip to fabricate groove-with-protrusion structures on metal surfaces by plastic deformation. The tip of a ball-shaped polycrystalline diamond (PCD) tool was irradiated with a femtosecond pulsed laser to generate multiple trapezoid ten-micron scale grooves. Burnishing was then conducted on an oxygen-free copper surface to investigate the characteristics and mechanism of the burnishing-induced plastic deformation behaviors. The results showed that protrusions and grooves, which were several tens of micrometers in height and depth, were simultaneously produced along the tool path on the material surface. Tool sliding on the workpiece induced shear stress, which mainly triggered dislocation movement and plastic deformation enhancement. Cross-sectional observation revealed that grain elongation and refinement remarkably increased with the burnishing depth, indicating that the shear stress from burnishing produced refined grains that flowed and formed the protrusions. This research demonstrates the feasibility of a highly efficient method for simultaneously generating groove-with-protrusion structures with enhanced surface properties.