Micro-dimple Arrays Research Articles

Electrochemical Dissolution Behavior of ZCuPb10Sn10 Alloy in NaNO3 Solution

Copper alloys, such as ZCuPb10Sn10, have been widely applied to friction pairs in various products. Surface texture, such as micro-dimple array has attracted significant attention from researchers worldwide to improve tribological performance. To generate micro-dimple array on ZCuPb10Sn10 alloy by electrochemical machining, it is essential to investigate the electrochemical dissolution behavior of ZCuPb10Sn10 in NaNO3 solution. In this paper, the electrochemical dissolution behavior of ZCuPb10Sn10 alloy in NaNO3 solution is investigated through experimental tests. Anodic polarization, Tafel polarization, and electrochemical impedance spectroscopy were conducted to investigate its passive and corrosion behavior. The microstructure and composition of the dissolved surfaces were analyzed under various conditions. Additionally, a model was proposed to explain the electrochemical dissolution process of ZCuPb10Sn10 alloy in NaNO3 solution under high pressure hydrostatic conditions. Ultimately, a NaNO3 solution with 10% in concentration and 20 °C in temperature was selected as the electrolyte and a micro-dimple array with an average diameter of 119.7 μm and a depth of 7.4 μm was successfully generated with through-mask electrochemical micromachining on the surface of ZCuPb10Sn10 alloy.

Numerical simulation and experimental research on through-mask electrochemical machining of micro-dimple arrays using inner-jet cathodes

The surface micro-dimple arrays can effectively improve the friction and lubrication performance of moving parts and are widely used in cutting tools and mechanical transmissions. Through-mask electrochemical machining (TMECM) is a process that uses the anodic dissolution principle to remove materials at low temperatures. It is suitable for efficiently processing large-area micro-dimple arrays on metal surfaces. However, the previous TMECM method still suffers from the problem of low processing adaptability. In this paper, a new method for scanning-cathode TMECM by utilizing the fluid dynamic pressure effect was proposed, and the flow field and electric field models of various inner-jet cathodes were developed as well. Through numerical simulations of the flow field and electric field, the flow velocity, electrolyte pressure, and current density distributions for different inner-jet cathodes were investigated to determine an appropriate cathode. Contrasting experiments were conducted to investigate the variations in electrolyte flow states and machining currents for different inner-jet cathodes. Additionally, the changes in micro-dimple dimensions under different machining parameters were investigated. Moreover, the micro-dimple arrays of 385.7 μm in diameter and 111.8 μm in depth, 288.8 μm in diameter and 40.3 μm in depth, and micro-dimple arrays with an etching factor of 1.69 were fabricated, and the results demonstrated the good processing adaptability of the scanning-cathode TMECM.

Laser-Fabricated Micro-Dimples for Improving Frictional Property of SKH51 Tool Steel Surfaces

Friction involved in metal-forming processes typically leads to the wear of tool and die surfaces, and in turn shortens the tool’s service life. A thriving need for reducing surface friction requires the tool surface to be modified. This paper presents the surface modification of SKH51 tool steel, on which the hexagonal array of micro-dimples is fabricated by a nanosecond pulse laser. Using the average laser power of 25 W can create decent dimples for trapping lubricant and enabling hydraulic pressure at the surfaces in contact. The effect of dimple density and sliding speed on the coefficient of friction was examined in this study through the pin-on-disc test, in which a stainless steel pin was applied against the tool steel disc with a constant load. The laser-textured tool steel surface with a dimple density of 35% had a friction coefficient of 0.087, which was lower than that of the untextured surface by 12.6% when using a sliding speed of 15 cm/s. In addition to friction reduction, there was no substantial wear found on the laser-textured surface compared to the untextured sample. The findings of this study can be a processing guideline and benefit the treatment of tool and die surfaces for friction and wear reduction in metal-forming and related processes.

Mechanisms of tool-workpiece interaction in ultraprecision diamond turning of single-crystal SiC for curved microstructures

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.

Fabrication of Micro-Dimple Arrays by EMM and RUREMM on Cylindrical Surface

To achieve high precision, stability, and good surface quality when producing micro-dimple arrays on cylindrical surfaces, we propose a new processing method known as radial ultrasonic rolling electrochemical micromachining (RUREMM) in this study. This method is based on the electrochemical micromachining (EMM) and ultrasonic machining principle. The relevant simulation model was created, and ANSYS researched the flow field characteristics of the electrolyte between the array electrodes and the workpiece. Micro-dimple arrays were created on a SS304 cylindrical surface with the consideration of the effects of the machining parameters, including ultrasonic amplitude and applied pulse voltage. Compared with the EMM, the average width of the micro-dimples is reduced by 24.5%, the aspect ratio of the dimple is increased by 108.0%, and the surface roughness of micro-dimples is decreased by 59.7%. In addition, the localization and the surface quality of micro-dimples by RUREMM can be improved when using appropriate machining parameters.

Experimental study on the evolution process of Zr702 microdimple array generated with masked jet electrochemical machining

Experimental study on the evolution process of Zr702 microdimple array generated with masked jet electrochemical machining

A novel method for fabricating micro-dimple arrays with good surface quality on metallic glass substrate by combining laser irradiation and mechanical polishing under wax sealing

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.

On-machine chromatic confocal measurement for micro-EDM drilling and milling

On-machine surface measurement of micro-scale components after micromachining becomes crucial to satisfy the increasing quality requirements. It implements a seamless measurement activity without interrupting manufacturing actions and hence facilitates the closed-loop quality control inline. Chromatic confocal probe sensing is a promising technique for on-machine integration thanks to its capability of assessing the surface with high accuracy and high speed. This paper presents the development of an on-machine measuring system using a chromatic confocal sensor, integrated in a micro-electrical discharge machining (micro-EDM) machine. The effects of scanning speed are qualitatively investigated in relation to outliers and quantitatively reflected through surface height parameters. In addition, the effects of surface gradient and surface contamination upon the measurement quality are studied. Two study cases, i.e. an array of micro-dimples and a micro-gear which are drilled and milled by micro-EDM, respectively, have been implemented to evaluate the developed system and to compare with off-line commercial confocal microscope. This comparative study demonstrates the system capability for on-machine areal measurement with sub-μm accuracy and reduced inspection time of 64%.

Ultraprecision grinding of microdimple array mold made of cemented carbide using nano-polycrystalline diamond tool

Ultraprecision grinding of microdimple array mold made of cemented carbide using nano-polycrystalline diamond tool

A high-frequency non-resonant elliptical vibration-assisted cutting device for diamond turning microstructured surfaces

In recent years, research has begun to focus on the development of non-resonant elliptical vibration-assisted cutting (EVC) devices, because this technique offers good flexibility in manufacturing a wide range of periodic microstructures with different wavelengths and heights. However, existing non-resonant EVC devices for diamond turning can only operate at relatively low frequencies, which limits their machining efficiencies and attainable microstructures. This paper concerns the design and performance analysis of a non-resonant EVC device to overcome the challenge of low operational frequency. The structural design of the non-resonant EVC device was proposed, adopting the leaf spring flexure hinge (LSFH) and notch hinge prismatic joint (NHPJ) to mitigate the cross-axis coupling of the reciprocating displacements of the diamond tool and to combine them into an elliptical trajectory. Finite element analysis (FEA) using the mapped meshing method was performed to assist the determination of the key dimensional parameters of the flexure hinges in achieving high operational frequency while considering the cross-axis coupling and modal characteristics. The impact of the thickness of the LSFH on the sequence of the vibrational mode shape for the non-resonant EVC device was also quantitatively revealed in this study. Moreover, a reduction in the thickness of the LSFH can reduce the natural frequency of the non-resonant EVC device, thereby influencing the upper limit of its operational frequency. It was also found that a decrease in the neck thickness of the NHPJ can reduce the coupling ratio. Experimental tests were conducted to systematically evaluate the heat generation, cross-axis coupling, modal characteristics and diamond tool’s elliptical trajectory of a prototype of the designed device. The test results showed that it could operate at a high frequency of up to 5 kHz. The cross-axis coupling ratio and heat generation of the prototype are both at an acceptable level. The machining flexibility and accuracy of the device in generating microstructures of different wavelengths and heights through tuning operational frequency and input voltage have also been demonstrated via manufacturing the micro-dimple arrays and two-tier microstructured surfaces. High-precision microstructures were obtained with 1.26% and 10.67% machining errors in wavelength and height, respectively.

Numerical analyses of rectangular micro-textures in hydrodynamic lubrication regime for sliding contacts

Hydrodynamic lubrication of rectangular micro-textures on sliding contact surfaces is investigated using numerical calculation methods. The theoretical models for the slider surface are developed and the film pressure is used to evaluate the hydrodynamic lubrication based on the Reynolds equation. Meanwhile, the geometry and distribution of the rectangular dimples are optimized for maximizing the average film pressure. Results show that the film pressure is dependent on the geometry and distribution of the rectangular micro-dimples. The optimal geometry of the single rectangular dimple is obtained, and the spacing has an important influence on the film pressure. The distribution types of rectangular dimples affect the hydrodynamic lubrication significantly and the interlaced array of the rectangular micro-dimples is beneficial to enhancing the hydrodynamic lubrication. Meanwhile, the rectangular dimples with 72° interlaced angle exhibits the best effectivity.

One-step fabrication of regular hierarchical micro/nano-structures on glassy carbon by nanosecond pulsed laser irradiation

Fabrication of hierarchical micro/nano-structures on glassy carbon (GC) surface is important for the application of GC as the mold material for glass molding, but it is still challenging due to the high brittleness and hardness of the material. In this study, by using a nanosecond pulsed laser, a hierarchical micro/nano-structure, i.e., micro-dimple with ring-like nanostructure on its inner wall, was generated on the GC surface. The effects of laser parameters, such as laser irradiation time and fluence, on the formation and evolution of the hierarchical micro/nano-structure were experimentally studied. By comparative experiments with graphite, the role of amorphous nature of GC on the formation of very regular hierarchical micro/nano-structure was confirmed. The results obtained by irradiating GC in argon and nitrogen gas further indicated that the formation of hierarchical micro/nano-structure was independent of the gas atmosphere. By controlling the relative positions of point shots, various micro-dimple arrays were successfully fabricated on the GC surface which significantly changed its wetting behavior. Finally, formation mechanism of the hierarchical micro/nano-structure was discussed according to the experimental results as well as some previous literatures. This study provides a one-step method for fabricating hierarchical micro/nano-structures on the GC surface by nanosecond pulsed laser irradiation, which would be meaningful for the functional applications of GC.

Micro-texturing on flat and cylindrical surfaces using electric discharge micromachining

The present work discusses micro-texturing on flat and cylindrical surfaces using the electric-discharge micromachining (EDMM) process. The arrays of micro-dimples are generated on flat Ti-6Al-4V surfaces using a block–electric discharge grinding (block-EDG)–fabricated microtools of an average diameter of 148 µm and 105 µm. Large-area surface texturing on flat Ti-6Al-4V and aluminium surfaces are performed to analyse the variation in water contact angle with varying depths of dimples. Adopting the electric discharge–milling (ED-milling) strategy, micro-pillars of dimensions 242 µm × 166 µm × 50 µm are machined on flat Ti-6Al-4V surfaces. The EDMM process for non-flat surfaces, such as curved (internal and external), spherical and freeform surfaces, is receiving attention in various applications. Machining of the aforementioned surfaces using the EDMM process appears to be problematic, due to the continuous change in curvature, which results in the subsequent spark gap variation. In the present work, processing of cylindrical surfaces for micro-features generation, such as micro-dimple arrays, has been attempted. Arrays of micro-dimples are machined on copper and Ti-6Al-4V cylindrical surfaces. A precise indexing setup is fabricated to hold and index the workpiece at the desired angular positions. Unlike machining on flat surfaces, the relative dimensions of the tool and the workpiece’s curvature result in non-uniform wear at the tool’s end cross-section. Owing to this non-uniform wear of tool electrode caused by the curvature effect of the workpiece, the formation of a microscopic bump/spike is observed on the dimple’s bottom. The depth of the dimple up to which the entire bottom surface of the tool is not exposed to the sparks is defined as its critical depth. For a combination of a tool and a workpiece of diameters 500 µm and 5 mm, respectively, the critical depth of the dimple is found to be 12.53 µm. However, the critical depth increases with a decrease in workpiece diameter, provided the diameter of the tool is constant.

Enhancement of friction performance of fluorinated graphene and molybdenum disulfide coating by microdimple arrays

Two-dimensional (2D) materials, including graphene-family materials and molybdenum disulfide (MoS2), have been used as solid lubricants because of their low interlayer shear strength. In this study, a fluorinated graphene (FG)–MoS2 nanocomposite coating was deposited on a stainless-steel substrate with a laser surface textured dimple array. The FG–MoS2 nanocomposite coating deposited on a textured surface led to the reduction in the coefficient of friction to 0.036, and it significantly reduced wear and extended the lifetime compared with those obtained using the MoS2 coating. The excellent tribological performance is attributed to the synergetic effect between FG and MoS2 nanoflakes and the function of texture, where the MoS2 nanoflakes dominate the lubrication performance because of interlayer shearing, and the FG nanoflakes suppress the influence of external humidity on MoS2 nanoflakes because of their hydrophobic nature. Meanwhile, the textured surface can promote the formation of a tribofilm through changing the stress distribution at the sliding interface and act as a reservoir for nanoflakes, leading to a much lower friction and longer lifetime. This has implications for the development of functional surfaces with excellent lubrication and wear-resistance performances with 2D materials.

Development of superhydrophilic Al foil with micropore arrays via mask electrochemical machining and chemical immersion for efficient oil/water separation

Extreme wettability materials, which can enable high-efficiency oil/water separation, has become a widely discussed topic due to increasingly severe oil pollutions including frequent offshore oil-spills accidents and industrial oily wastewater emission. However, most of existing methods to prepare these materials involved complicated machining processes or dangerous operations, especially the use of corrosive or toxic chemicals. Herein, a novel and environment friendly method including mark electrochemical machining (MECM) and subsequent chemical immersion, was proposed to acquire micropore arrays on Al foil for efficient oil collection from oil/water mixtures. Micro-dimple arrays were generally fabricated by MECM process followed by CuCl2 immersion to make it through. Both simulation and experimental verification were conducted to elucidate how dimple structures varied with machining parameters. The micro-scale rectangular-shaped step-like structures on micropore arrays endowed as-prepared samples with excellent superhydrophibic and underwater superoleophobic properties. Moreover, oils with kinematic viscosities ranging from 0.41 to 74.4 cSt could be separated from water with separation efficiencies above 95%. The superhydrophilic Al foil still showed good durability through repeated oil/water separation tests, and retained its superhydrophilicity even after the storage in water for 120 hours. Therefore, this novel and green method gives researchers new insights into functional materials preparation for practical oil/water separation.

Role of micro-dimple array geometry on the biological and tribological performance of Ti6Al4V for biomedical applications

The purpose of this study was to employ surface texturing, using CO2 pulsing laser, to create micro-dimple arrays and study the role of diverse geometries of micro-dimples on the physical, biological and tribological responses of Ti6Al4V. Scanning electron microscopy and X-ray diffraction were applied to study the structural and chemical composition of surface treated samples, respectively. Furthermore, the effects of laser treatment on the surface hydrophilicity, topography and hardness of samples were investigated using water contact angle measurement, surface roughness tester and micro-hardness tester. Results confirmed the noticeable role of scan rate (0.5, 1, 5 and 10 mm/s) on the morphology of micro-dimples, the chemical structure and hardness of the samples. In this respect, increasing the scan rate of laser irradiation from 0.5 mm/s to 10 mm/s resulted in the formation of acicular α-Ti around micro-dimples with various thicknesses. Moreover, micro-hardness of Ti6Al4V greatly improved to 635 ± 21 HV, when the scan rate enhanced to 5 mm/s. In addition, the effect of surface texturing on the attachment, proliferation and spreading of MG63 cells were investigated. Results confirmed that cell proliferation was significantly improved on the textured Ti6Al4V. Tribological characterization revealed that poor tribological properties of TiO2 layers on Ti6Al4V could be meaningfully modified using laser texturing, depending on the geometries of micro-dimples. Additionally, noticeably reduction of friction was obtained on the textured samples. In summary, laser texturing could be effectively applied to simultaneously enhance the tribological and biological performances of Ti6Al4V alloy for biomedical applications.

Reciprocating electrolyte jet with prefabricated-mask machining micro-dimple arrays on cast iron cylinder liner

Cast iron cylinder liner in the diesel engine is putting forward higher requirements on the tribological performance. The micro-texture machined on the cast iron cylinder liner surface has demonstrated the brilliant potential of friction reduction. However, the existing micro-texturing technologies of the cylinder liner have some limitations, such as the workpiece deformation, the low processing efficiency and the high cost of the machining equipment. In the paper, the reciprocating electrolyte jet with prefabricated-mask (REJP) machining technology was used to generate circular dimple arrays on the cast iron cylinder liner. The micro-machining equipment adopts the direct current power instead of the pulse power. The electrolyte jet as the cathode driven by the step-wise motor realizes the reciprocating movement. In addition, the arc shape of the cathode opening is identical with the cylinder liner to guarantee the same inter-electrode gap along the curved liner. After the electrolyte flows out of the cathode opening, three machining areas including the injected area by the vertical flow, the activated area and the wetted area by the horizontal flow, are formed on the cylinder liner surface by the prefabricated-mask. The circular dimple can be uniformly fabricated in arrays with the following optimized machining parameters: cathode velocity = 10 mm/s, electrolyte flow = 45 ml/s, applied voltage = 10 V, electrolysis time = 60 s. By using the prefabricated-mask with the array hole of 800 μm in diameter, the best dimension of micro-dimple is about 43 μm in depth and 822 μm in diameter. Compared with the non-dimpled cylinder liner, the friction coefficient of the micro-dimpled cylinder liner decreases as much as 12.0% under the heavy load of 20 MPa.

Improvement in accuracy of micro-dimple arrays prepared by micro-electrochemical machining with high-pressure hydrostatic electrolyte

Micro-dimple arrays, as a common surface texture, play an important role in improving the tribological properties and lifetimes of mechanical parts. Through-mask electrochemical micromachining (TMEMM) is an important and popular approach for fabricating micro-dimple arrays. In traditional TMEMM that uses polydimethylsiloxane (PDMS) as a mask, the accumulation and escape of the oxygen bubbles generated on each micro-dimple vary considerably, which decreases the uniformity of the electrical field and leads to poor dimensional uniformity (including diameter and depth) of the micro-dimple arrays. In this paper, high-pressure hydrostatic electrolyte was proposed in order to enhance the uniformity of the electrical field distribution on the workpiece surface. Both the dimensions and machining accuracy of micro-dimple arrays under different applied direct current (DC) voltages during high-pressure hydrostatic TMEMM were investigated experimentally. Two novel phenomena were observed: (i) the dimensions of the micro-dimple arrays were greater at low voltages than at high voltages, and (ii) the machining accuracy was high at both low and high applied voltages but was low at intermediate voltages. At 32 V DC, an array of 14,000 micro-dimples was successfully fabricated with a diameter and a depth of 105.95 and 9.79 μm, respectively, and with diameter and depth deviations of 0.59 μm and 0.21 μm, respectively.

The Effects of Microdimple Texture on the Friction and Thermal Behavior of a Point Contact

This study investigates the effects of the microdimple texture on the friction and surface temperature performances of a ball-on-disk contact, operating under the speed and load ranges that cover typical gearing applications. Circular-shaped microdimple arrays with different dimple center distances and dimple depths are implemented on the ball surface to quantify the impacts of these two parameters on the friction coefficient and the maximum ball surface temperature. In addition, the contacts of three surface texture combinations, namely microdimpled and polished ball surface versus polished disk surface, polished ball surface versus polished disk surface, and ground ball surface versus ground disk surface, are compared to demonstrate any beneficial or detrimental effect of microdimples in heavily loaded high-speed applications. This study adopts a thermal mixed EHL point contact model, whose capability and accuracy have been well demonstrated by comparing to the experimental measurements, to quantify the deterministic tribological behavior within the contact, allowing the exploration of the underlying mechanism that governs the role of microdimples in the elastohydrodynamic lubrication (EHL).

Investigation of foamed cathode through-mask electrochemical micromachining developed for uniform texturing on metallic cylindrical surface

Surface texturing with micro-dimples has been proved to be an effective approach in many fields to improve the functionalities and performances of some mechanical components. Through-mask electrochemical micromachining (TMEMM) is a popular technique to create textures on metallic surface, but it is still very challenging to be used to texture curved surface. In this paper, a novel TMEMM process was proposed to generate micro-dimple array on the cylindrical surface. In this TMEMM process, a foamed metal film with a good pliancy performance and a huge number of interconnected micropores is introduced to serve as the cathode, which is further integrated with the reusable through-mask and workpiece to form a sandwich-like assembly. The current distribution within each region being machined and its correlation with the geometric dimensions of through-hole, as well as with the relative location of the through-hole in the mask, are analyzed numerically. With this TMEMM process, a great number of micro-dimples with a good uniformity in their geometric dimensions can be produced both on the planar and the entire 360° cylindrical workpiece surfaces in only one operation, showing the minimum coefficient of variation of the dimple’s depth and diameter being 5.6 and 2.4%, respectively. The reasons for such a favorable result are interpreted theoretically. The proposed TMEMM process here could have a great potential in generating geometrically consistent surface textures on the curved surface efficiently and low-costly.