Published papers

Laser nitriding is a useful method to improve the surface hardness of materials, and it has been newly introduced to harden the surface of Zr-based metallic glass (MG). However, the currently achieved improvement in surface hardness is only 15.7%. To explore the potential of laser nitriding of Zr-based MG, orthogonal experiments were designed and performed in this study. By nanosecond pulsed laser irradiation in nitrogen atmosphere, the effects of laser parameters (average laser power, scanning speed, overlap rate and number of irradiation cycle) on the surface hardness of Zr-based MG were systematically investigated. The experimental results showed the highest surface hardness reached 18.21 GPa (Meyer's hardness) by orthogonal design, which is about 2 times higher than that of the as-cast MG. Furthermore, the laser affecting layer consisting of nitride phase reached tens of microns. The obtained results demonstrated that significant improvement in surface hardness could be achieved by selecting appropriate laser irradiation parameters, which would be beneficial to the engineering applications of MGs as surface contact materials.

The surface with micro-convex prepared by laser ablation is beneficial to improving the functional properties of metallic glasses (MGs). However, accompanied by the formation of micro-convex, surface hardness of the ablated area generally decreases due to the softening effect caused by laser thermal shock. Previous studies have shown that the mechanical properties of laser-ablated MG surface are significantly different when using different gas atmospheres. In this study, a comparative investigation was performed to analyze the influence of gas atmosphere (i.e. argon, nitrogen, and air) on the formation of micro-convex as well as its surface characteristics and mechanical properties. Experimental results showed that the atmosphere type did not affect the formation of micro-convex, but significantly affected the surface morphology and element composition of the laser-ablated area. The element analysis and Raman spectral measurements indicated that laser ablation in nitrogen or air atmosphere resulted in local nitridation or oxidation. The results of nanoindentation tests demonstrated that laser ablation in nitrogen or air atmosphere also resulted in surface hardness inhomogeneity, i.e., softening and hardening effects coexist in the ablated area, which could be attributed to the combined influence of laser thermal shock, the introduction of secondary phase as well as laser ablation induced loose structure.

Gear hobbing is widely employed to manufacture automotive gears, where the productivity depends on the cutting speed. Currently, gear hobbing is performed at ∼300 m/min using high-speed steel hob cutters. In this study, ultra-high-speed gear hobbing was attempted using a large-diameter cemented carbide hob cutter on a gear grinder. This enabled a cutting speed up to 2450 m/min. Many interesting phenomena were acquired in this speed range. Chips were severely oxidized, whereas the gear surface was not affected. Compressive residual stress was generated at the gear surfaces with low surface roughness and high hardness, while the wear of hob cutter was insignificant.

Nanometric cutting removes material at nanoscale and generates high-quality surfaces with a nanometric finish. In past decades, it has thrived as a mainstream manufacturing technology to produce critical components to advance scientific discovery and promote innovation in various fields, such as astronomy, aerospace, microelectronics, optics and photonics, biology and quantum technology. Therefore, it is timely to develop a review article capturing such developments and establishing directions for future advancement. This article systematically reviews the fundamental issues such as cutting model, material deformation mechanism and tool wear in nanometric cutting. It also presents the working principles of innovative ion implantation-assisted, laser-assisted and ultrasonic vibration-assisted nanometric cutting methods to overcome the challenges of machining difficult-to-cut materials. Practical techniques for the generation of high-quality complex or structured surfaces are also discussed. Finally, challenges and future perspectives of nanometric cutting, as well as the evolution towards atomic and close-to-atomic scale manufacturing, are outlined.

Metallic glasses (MGs) are a promising candidate for advanced structural applications due to their superior mechanical properties. Improving their surface mechanical properties would be of great importance for promoting their structural and functional applications. In this study, SiC particles were used as the reinforcement to improve the mechanical properties of Zr-based MG via laser surface alloying. The influences of the average laser power and overlap ratio between neighboring laser processing lines on the microstructure and mechanical properties of the laser-alloyed surface layer were investigated. The experimental results indicated that ZrC and SiC phases were successfully introduced into the MG matrix by laser surface alloying, and the content of these two hard ceramic phases was dependent on the laser processing parameters. The formed laser-alloyed surface layers exhibited a significant improvement in overall hardness compared with the as-cast specimen. At a relatively high overlap ratio of 70%, the average hardness of the MG matrix within the laser-alloyed surface layer reached 28.91 GPa, which was three times higher than that of the as-cast specimen (6.46 GPa). Furthermore, the microstructural characteristics and mechanical properties of the cross-sections of the laser-alloyed samples were characterized. The thickness of the laser-alloyed surface layer reached several tens of microns, and the SiC particles were uniformly dispersed in the whole laser-alloyed surface layer. This study confirms the feasibility for improving the mechanical properties of MGs by laser surface alloying, which is expected to broaden the application of MGs as structural and functional components under harsh severe conditions.

Manipulating materials at nanoscale is challenging but rewarding. Nanostructures exhibit some unique properties distinguished from macroscopic bulk materials, so fabrication of nanostructures on the material surface is beneficial to broaden their functional applications. In this study, by nanosecond pulse laser irradiation in nitrogen atmosphere, it was found that unique nanoporous structures could be formed on the Ti6Al4V surface. The effects of laser parameters including the laser power, overlap rate, scanning speed, and repetition frequency on the formation and evolution of the nanoporous structures were systematically investigated. By changing the laser parameters, the nanoporous structures with various porosities could be easily achieved on the Ti6Al4V surface. According to the structural evolution and chemical composition, the formation mechanism of the nanoporous structures was discussed. Furthermore, by characterizing the micro-mechanical properties and wettability, it was found that the surface with nanoporous structures exhibited similar surface hardness but improved tribological performances compared to the original Ti6Al4V surface, and the surface wettability could be tuned. This study provides a simple method for fabricating nanoporous structures on the Ti6Al4V surface, which would broaden its functional applications.

The rolling element bearing is a fundamental component of any rotating machinery. During operation, wear debris and lubricant impurities create dents and bumps on the bearing raceway surfaces. Such localized defects produce transient vibration impulses at one of the bearing characteristic frequencies. Having a combination of multiple types of point defects on the raceway results in superimposed vibration patterns, which reduce the ability to recognize these defects’ effects. In this paper, a 6-DOF dynamic model is developed to accurately investigate the vibration characteristic of a ball bearing with a multipoint defect comprising a dent and bump on its raceway surface. The model considers the effects of time-varying contact force produced due to defects, lubricant film damping, bearing preload, and the inertia effect of rolling elements. The simulation results reveal the vibration behavior of multipoint defect bearings. In addition, bearing vibration response is affected by the number of defects, the angle between them, and the type and size of each defect. Furthermore, it is challenging to predict bearing defects parameters such as the numbers, types, sizes, and angles between adjacent defects from acceleration signal analysis without jerk signal analysis. The validation of the model is proved using signals from the Case Western University test setup.

Femtosecond pulsed laser irradiation of yttria-stabilized zirconia (YSZ) was performed to investigate the feasibility and fundamental characteristics of nanopore structure fabrication. Numerous nanopores were successfully generated on the YSZ surface with only a single laser pulse shot by controlling laser power near the ablation threshold. The nanopore generation mechanism involves light focusing by the convex shape of a crystal grain and ablation inside the grain. By increasing laser power, the number of nanopores increased. Cross-sectional observation revealed that the depth of nanopores was about 500 nm, and nanopores were generated inside the YSZ grains of the top layer while keeping the grains' shapes unchanged. It was found that higher scanning speed and a small number of scans enabled nanopores fabrication without changing the original surface shape, avoiding the removal of the surface layer. Laser-irradiated surfaces of YSZ were characterized by Raman spectroscopy, and it was found that there was no significant phase change in the specimen after nanopore generation compared with the original YSZ surface, confirming that there was no thermal damage to the bulk material. The surface with nanopores showed an enhancement in surface hydrophilicity. These findings demonstrated the possibility of selectively generating nanopores in the top surface of YSZ for use in functional surfaces without serious thermal damage to the workpiece. It is expected that the nanopore generation will lead to new applications of YSZ.

High-entropy alloys (HEAs) are promising structural and functional materials owing to their superior mechanical and chemical properties. High surface quality and good machinability are essential for micro-machining high-end precision equipment. Here, laser repairing (LR) was firstly employed to eliminate surface defects and improve the machinability of FeCoCrNiV. After LR, the microstructures and mechanical properties of FeCoCrNiV were investigated. The results show that the surface morphology and chemical composition uniformity of FeCoCrNiV were significantly improved after LR, in which the high melting-point V-rich precipitates concentrated on the original surface were homogeneously distributed throughout the substrate. The surface hardness decreased from 10.5 to 4.5 GPa due to the growth of grains, and the hardness distribution became more homogeneous after LR, providing the opportunity to improve its machinability. In ultrasonic vibration diamond turning tests, the machined surface quality, tool wear, and cutting chips indicated that the laser-repaired material exhibited superior machinability. A sinusoidal free-form surface with high surface quality was then constructed by ultrasonic vibration-assisted diamond turning on the laser-repaired surface. This study provides a method to repair surface defects of HEAs and improve their machinability, which is beneficial to their practical applications.

Single-crystal silicon carbide (SiC) is one of the most attractive materials for electronics and optics but extremely difficult to cut owing to its hard and brittle properties. While in previous studies, the focus has been placed on machining flat surfaces, in this study, the mechanisms of tool-workpiece interaction in cutting curved microstructures on 4H–SiC (0001) were explored through the ultraprecision diamond turning of micro-dimples. The surface/subsurface of both machined workpieces and used diamond tools were characterized, and the machining characteristics, such as chip formation and cutting forces, were also investigated. It was found that microcracks occurred easily in the feed-in/cut-in area of the dimples, which is caused by a large friction-induced tensile stress due to a large thrust force. The dimples located on the secondary cleavage directions <10−10> (S-dimples) were easy to produce crack-free surfaces, while the dimples located on the primary cleavage directions <−12−10> (P-dimples) were very prone to cause cracks on surfaces, even though the chips were formed in a ductile mode. The dimples located on the in-between direction (I-dimples) were moderately prone to surface cracking. It was also found that although the S-dimple has a crack-free surface, it has the thickest subsurface damage (SSD) layer containing a disordered layer, dislocations, and stacking faults; the SSD layer of the P- and I-dimples do not contain stacking faults; and the SSD layer of the I-dimple is the thinnest. Flank wear with nanoscale grooves on the diamond tool was significant without edge chipping and diamond graphitization detected. By optimizing the cutting conditions, a crack-free micro-dimple array was fabricated with nanometric surface roughness. The findings from this study provide guidance for the manufacture of curved SiC parts with high surface integrity, such as molds for replicating microlens arrays and other freeform surfaces on glass.

The evolution of deformation mechanisms with increasing depth of cut (DOC) in micro-grooving of Zr-based metallic glass was deeply investigated in this study for the first time. The micro-grooving characteristics of metallic glass were initially studied by nanoscratch test, which verified that the material removal mode abruptly changes from extrusion mode to cutting mode with increase of DOC. Based on the free-volume theory, a dynamic model coupling stress, temperature and free volume concentration was used to simulate the change of deformation characteristics with DOC. The results showed that the deformation mechanism of metallic glass is closely related to free volume flow, which is significantly affected by average strain rate and free volume flow coefficient in deformation zone and can be tuned with different DOC. Considering that free volume flow is determined by its flow coefficient and concentration, we proposed to control the deformation characteristics of metallic glass in micro-grooving by heat treatment and verified its effectiveness experimentally.

Relative tool sharpness (RTS) is identified as the ratio of undeformed chip thickness to tool cutting edge radius. This paper studies the effects of RTS on the surface generation mechanism of precision turning of electroless nickel-phosphorus (Ni-P) coating. An R-shaped tungsten carbide (WC) tool was adopted for the face turning experiment. The cutting edge radius was 1.84 µm measured by a laser scanning confocal microscope (LSCM). The chip formation behavior, cutting forces and surface morphology were investigated under different RTS values. Results showed that the chip changes from continuous to discontinuous as RTS decreases from 0.54 to 0.27, indicating the transition of the material removal mechanism. The periodical fluctuations with small amplitudes on the machined surface are associated with the high-frequency tool-tip vibration. The low-frequency fluctuations of the cutting forces are related to the material swelling and recovery. The optimal machined surface roughness was obtained at the RTS of 0.38.

Surface micro-patterning is an emerging technique to endow a wide range of materials with specific functions, such as superhydrophobicity and anti-reflection, but it is still technically challenging and inordinately expensive via existing manufacturing approaches. Compared with crystalline metals, metallic glasses (MGs) are talented with some unique mechanical and chemical properties due to their isotropic internal structure. Here, a novel two-step method, involving nanosecond pulsed laser irradiation and mechanical polishing, was proposed to fabricate micro-dimple arrays with good surface quality on the MG substrate. In the first step, multi-pulse laser irradiation experiments were performed to study the correlation between the peak laser power intensity and the feature sizes of the micro-dimple (i.e. the depth and diameter of the micro-dimple as well as the height of the pile-up), and then various micro-dimple arrays were fabricated on the MG substrate by adjusting the peak laser power intensity and the interval between adjacent laser shots. Subsequently, aiming at improving the surface quality, the micro-dimple arrays were mechanically polished under wax sealing. The experimental results indicated that after polishing, the pile-ups around the micro-dimples were completely removed, and at the same time, the surface quality of their inner walls was improved. Furthermore, based on experimental results, the possible material removal mechanism during polishing was discussed. This study provides a convenient method to fabricate micro-dimple arrays with good surface quality on the MG substrate, which would be one of the potential alternatives for large-area, low-cost micro/nano-manufacturing.

A comprehensive dynamic model is introduced try to further reduce dimensional errors of micromilled micro straight thin walls based on an in-process cutting parameter compensation device. First, a mathematical model is established to predict the dynamic elastic deformation of the thin walls in the micromilling process. Next, the radial deformation and the runout of the cutter are modeled. Then, the comprehensive in-process parametric compensation model and strategy are established. Finally, a real-time in-process cutting force measurement and cutting parameter compensation device is applied to the micromilling system and thin wall fabrication experiments have been conducted. It shows that the comprehensive dimensional error model and iterative compensation method are effective, and the dimension and shape precision of the micro straight thin wall has been obviously improved after the compensation of the radial cutting depth. The average relative errors have been reduced from 6.86% to 1.10%∼1.70%. The in-process compensation method introduced in this study is efficient and convenient as long as compensation strategies are integrated in the controlling unit for a specific micromilling process.

As a promising surface modification method, nanosecond pulsed laser nitriding has been widely employed in enhancing the surface hardness and chemical corrosion of Zr-based metallic glass (MG). However, laser nitriding generally leads to poor finished surface quality, limiting its potential applications. For flattening the laser-nitrided surface, the most common methods, i.e., mechanical polishing (MP) and a novel proposed flattening approach, laser polishing (LP), were comparatively studied. A systematical comparison between MP and LP on three types of laser-nitrided surfaces was performed to evaluate the surface roughness, surface hardness, and plastic deformation. The experimental results indicate that the surface roughness of these three laser-nitrided surfaces is reduced by 56.4%, 58.1%, and 44.6% by MP. In contrast, a remarkable improvement in the surface quality is achieved with a maximum reduction in surface roughness by 80.4%, 81.5%, and 74.2% by LP, respectively. While in terms of the surface mechanical properties, the LP surfaces exhibit slightly lower hardness than the corresponding MP surfaces, which are both enhanced in hardness compared with as-cast MG. Additionally, no matter on the MP or LP surfaces, the shear bands and serrated flows have been significantly suppressed compared with as-cast MG, demonstrating that the micro-scale plastic deformation of as-cast MG has been modified. Although the obtained results indicate that the surface roughness of the laser-nitrided surface could be improved by both MP and LP, in terms of environmental friendliness, flexibility, and controllability, the LP shows dominant advantages and enormous potential in industry and consumer products.

It is indispensable to explore the appropriate level of surface residual stress to enhance the wear resistance of coated tools. TiAlSiN-coated carbide tools with dissimilar surface residual compressive stresses (− 300 ~ − 600 MPa) are acquired via micro-sandblasting, which are used in the high-speed sliding friction experiments and turning experiments. The influence of residual stress on the sliding wear and cutting wear performance of TiAlSiN coated tools is investigated. The results show that with the improvement of surface residual compressive stress (absolute value), the friction coefficient and wear rate are reduced. The sliding wear resistance of the coated tool is enhanced. In addition, the cutting performance of the tool is also improved with the increase of residual compressive stress (absolute value). However, the excessive surface residual compressive stresses (absolute value) can lead to severe coating peeling, which in turn impairs the capability of wear resistance. The TiAlSiN-coated carbide tool with the stress of − 500 MPa presents the best wear resistance in the high-speed sliding wear and cutting wear process.

Highlights Effects of tool geometry on machining characteristics of soft-brittle materials are reviewed. The differences in material behaviors between soft- and hard-brittle materials are identified. The surface formation mechanisms for various processing methods are compared. The optimal tool geometries and parameters for different machining methods are analyzed. Challenges, feasibility, and future perspectives in this area are outlined.

Highlight A hybrid surface roughness model combining the theoretical model and deep learning method are suggested. SSGB and GB roughness component reduce due to a decrease of misorientation angle. Transcrystalline/intercrystalline fracture mode of work material is analyzed at different tool sharpness. A flat copper surface finish of Sa 1.314 nm without SSGB is attained.

Chalcogenide (ChG) glasses with micro-nano structure arrays are increasingly demanded in infrared optical systems owing to their excellent antireflective properties, and can be mass-produced by precision glass molding (PGM). Nickel-phosphorus (Ni–P) plated molds are commonly used in the PGM process, but the Ni atoms tend to react with the ChG glass at high temperatures. In this study, we propose a novel two-step PGM process. A Ni–P mold is used to transfer antireflective micro-nano structure array to silicate glass. Then the silicate glass, which has a higher glass transition temperature (Tg) than the ChG glass, is used as the mold to replicate the antireflective structured surface to the ChG glass. The results show that the ChG glass micro-nano structure array has high forming accuracy and favorable infrared optical transmission characteristics. The silicate glass is proved to be an appropriate mold material for the ChG glass molding process. This work provides a novel type of the PGM techniques for ChG glass materials.

With significant technological progress in optics fields, such as VR and 3D sensing technologies, the demand for optical components with complex shapes (e.g., freeform surfaces) has expanded significantly in recent years. Ultra-precision diamond turning is one of the common manufacturing methods used for freeform optics. There are two tool-motion mechanisms for freeform diamond turning: slow tool servo (STS), which moves the entire tool rest, and fast tool servo (FTS), which only drives the diamond tool. In particular, high-frequency and long-stroke FTS units driven by a separate control system from the machine tool controller have recently been developed as a new trend in FTS-based diamond turning. However, this type of independent FTS control system causes a time delay between the machine tool instruction and the FTS tool motion. Even a millisecond-scale time delay can cause a considerable angular misalignment owing to the high rotation rate of the spindle, resulting in a micron-scale form error on the workpiece surface. In this paper, a novel method for precisely measuring the time delay from the machined workpiece surface and compensating for the misalignment based on the measured time delay was proposed. To measure the time delay, spherical dimples were machined on the workpiece surface by using STS and FTS. Subsequently, the angular misalignment and time delay were obtained by comparing the measured surface profiles at specific positions of the spherical dimples. Thus, the time delay can be measured with microsecond-scale accuracy. Based on the measured time delay, the shifted tool position along the spindle axis by the time delay was compensated to the ideal tool position by considering the cutting velocity. Consequently, the angular misalignment was completely eliminated, and submicron-level form accuracy was achieved. This calibration method of angular misalignment for an independent FTS unit is expected to significantly improve the accuracy of ultra-precision machining of freeform optics.