Microgroove Arrays Research Articles

Effect of surface wettability on tribological properties of Al2O3/TiC ceramic under wet lubrication

The effect of surface wettability on the friction properties of Al2O3/TiC ceramic was investigated to improve the friction and wear of Al2O3/TiC ceramic tool surfaces. The surface wettability of Al2O3/TiC ceramic was changed by preparing different microgroove arrays on its surface. The surface contact angles of the different samples were measured to characterize surface wettability. The reciprocating linear friction tests were implemented under liquid lubrication by a ball-disc type friction machine and the friction coefficient was measured. The worn surface characteristics of Al2O3/TiC ceramic were observed using SEM. Results showed that the surface wettability effectively improved the friction and wear properties of Al2O3/TiC ceramic. The friction coefficient of Al2O3/TiC ceramics decreased as surface wettability was enhanced. The surface wettability enhancement of 86.8% corresponds to a reduction in the friction coefficient of 73.6%. The better the wettability, the smoother the worn surface. The mechanism responsible was explained as micro-textures promoted capillary adsorption of lubricating fluid on the surface of Al2O3/TiC ceramic. The additional capillary adsorption force reduced the solid-liquid interfacial tension, thereby enhancing the surface wettability of the Al2O3/TiC ceramic, which promoted spreading ability of lubricating fluid. Therefore, the lubricating fluid can form a continuous lubricating film at the friction interface to achieve the effect of reducing the friction.

Research on rhenium–iridium alloy coating on microgroove molds in precision glass molding

There is an increasing demand for microgrooves in optical systems with a pitch at wave length level. Precision glass molding (PGM) is one of the most efficient methods for fabricating microgrooves on glass surface. Nickel–phosphorus (Ni–P) exhibits excellent cutting properties and is developed as the mold material for glass microgrooves. However, the phosphorus in Ni–P mold tends to diffuse into the glass during the PGM process at high temperatures, which may reduce the optical performance of the molded glass microgroove component. In addition, the atomic diffusion increases the interface friction coefficient between the glass and the mold, resulting in the decrease of molding accuracy of the microgroove array. To solve these problems, a rhenium–iridium (Re–Ir) alloy coating is deposited on the surface of the Ni–P microgroove mold by ion sputtering. In this paper, the surface roughness of the mold before and after coating with Re–Ir alloy coating is investigated. The mechanical properties of Ni–P and the Re–Ir alloy coating are obtained using a combination of finite element method (FEM) and experimental tests. The mold deformation after heating to the molding temperature is analyzed. The results show that the mechanical properties of the Ni–P mold surface can be strengthened after being plated with Re–Ir alloy coating, the Re–Ir coating isolates the diffusion of phosphorus from the Ni–P mold, and improve the forming accuracy of glass microgrooves.

Key machining characteristics in ultrasonic vibration cutting of single crystal silicon for micro grooves

Structured complex silicon components have been widely used in solar cells, biomedical engineering and other industrial applications. As silicon is a typical brittle material, ultrasonic vibration cutting (UVC) is a promising method to achieve better cutting performance than conventional techniques. High-frequency 1D UVC possesses higher nominal cutting speed and material removal rate than many 2D/3D UVC systems, and thus, it has great development potential in industrial applications of structured silicon components. However, few researchers have applied 1D UVC to the cutting of structured silicon surfaces, since its main drawback is tool marks imprinted by the vibration on machined surface. In this study, to uncover the key machining characteristics under the condition of 1D UVC, a series of tests involving diamond cutting grooves were first performed on the silicon surface. The machined surface and chips were subsequently measured and analyzed to evaluate the critical undeformed chip thickness, surface characteristics, and chip formation. Regarding the main drawback of 1D UVC, a novel theoretical model was developed for predicting the length of tool marks and evaluating the impact of tool marks on the surface finish. The results demonstrated that the critical undeformed chip thickness of silicon reached 1 030 nm under a certain vibration amplitude and that an array of micro grooves was generated at the plastic region with a surface roughness (Ra) as low as 1.11 nm. Moreover, the micro topography of the continuous chips exhibited discontinuous clusters of lines with diameters of dozens of nanometers, only composed of polysilicon. The novel theoretical model was able to predict the length of tool marks with low error. Thus, the impact of tool marks on the surface finish can be reduced and even eliminated with help of the model.

Polarization-controlled microgroove arrays induced by femtosecond laser pulses

Using pulsed femtosecond laser irradiation, we demonstrate the creation of an array of microgrooves within a single laser spot on metals. The orientation of these grooves is not limited to being parallel to the plane of the laser beam's propagation but can orient at any angle up to 30° from parallel. We control the orientation of the microgrooves by proportionally varying the laser's polarization. Polarization, angle of incidence, and structural evolution dynamics have been thoroughly studied to help us understand this phenomenon. Our studies suggest that the formation of angled microgroove arrays is due to a feedback effect occurring between defect-focused ablation and polarization-dependent laser-induced periodic surface structures.

Influence of laser surface texturing on a low‐adhesion and superhydrophobic aluminium alloy surface

An aluminium alloy surface with a microgroove array was fabricated via laser processing, and exhibited superhydrophobicity. The as-prepared surface showed excellent anti-adhesion properties and potential self-cleaning behaviour. The wettability of the fabricated surface was investigated by measurement of the water contact angle. The surface morphology and chemical composition were characterised by scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The wettability of the surface was successfully modified from hydrophilicity to superhydrophobicity by the laser processing. The water contact angle was increased to 155°, and the sliding angle reduced to <;5°, without any chemical modification. Water droplets on the surface existed in an almost perfectly suspended state, resembling a model Cassie state. The superhydrophobic surface showed excellent stability under conditions of external extrusion. This easy and inexpensive modification method is expected to play an important role in extending the applications of aluminium alloys.

Bioinspired Fabrication of Bi/Tridirectionally Anisotropic Sliding Superhydrophobic PDMS Surfaces by Femtosecond Laser

AbstractPrecisely arranged multiscale microstructures in living systems provide inspirations for the fabrication of artificial advanced materials and devices. Recently, uni‐bioinspired surfaces with anisotropic wettability are built for various applications. Here, a kind of bi‐bioinspired (rice leaf and butterfly wing) tridirectionally anisotropic sliding superhydrophobic surface for the first time is reported. First, a microgroove array structured polydimethylsiloxane surface is constructed by selective femtosecond laser ablation. The as‐prepared surface shows bidirectional anisotropy which can be easily tuned by the width (L) and the depth of the microgrooves. A “steps” structure is further introduced into the microgrooves. Interestingly, tridirectionally anisotropic sliding superhydrophobicity that combined the rice‐leaf‐like and butterfly‐wing‐like anisotropy is achieved on such surface. The tridirectional anisotropy is demonstrated to result from the directional “steps” structures and the uneven distribution of rough structures. The bi/tridirectionally anisotropic sliding property will provide a fresh cognition of anisotropic wettability for researchers and have enormous potential applications in smart microfluidic systems.

Ultra-precision cutting of linear micro-groove array for distributed feedback laser devices

The tunable range and beam linewidth of distributed feedback laser were determined by fabrication quality of the periodic linear micro-groove array on substrate, which grooves are high sloped and ranged sub-micron to few microns. Compared with the common micro-groove arrays (size from tens of microns to sub-millimetre), the ultra-precision cutting of these tiny and high-sloped linear micro-groove arrays are more difficult to realise. In this paper, a series of investigations was proposed for ultra-precision cutting of linear micro-groove array on polymethyl methacrylate (PMMA). Firstly, the different cutting processes included planing and fly-cutting were conducted. The workpiece morphology, surface quality and machining efficiency of these processes were investigated. Then, the fly cutting process was optimised by the calculation of non-free chip zone and the analysis of chip interference by cutting experiments. Finally, a novel tool positioning method based on acoustic emission was presented in order to realise high accuracy control of micro-groove arrays dimensions.

Electrochemical micromachining of microgroove arrays on phosphor bronze surface for improving the tribological performance

A high friction coefficient and a low wear rate of contacted surfaces are essential elements to friction pairs between the stator and the rotor in ultrasonic motors. It has been shown that surface textures have a significant effect on improving the tribological performance of friction pairs. In this paper, microgroove arrays are introduced to the stator surface for improving the tribological performance of friction pairs between the stator and the rotor in ultrasonic motors. Microgrooves were fabricated on a phosphor bronze surface by through-mask electrochemical micromachining (TMEMM). Parameters, namely, the electrolyte inlet pressure, applied voltage, pulse duty cycle, and frequency, were varied to investigate their influences on the dimensions and morphology of the microgrooves. Results showed that the width and depth of the microgrooves were strongly affected by the applied voltage and frequency, while the morphology of the microgrooves was dependent on the electrolyte inlet pressure and the pulse duty cycle. Compared with a smooth surface, the friction coefficient increased from 0.245 to 0.334 and less abrasion was obtained when a surface was textured with microgrooves of which the width and depth were 185.6 and 57.6 μm, respectively. Microgroove arrays might play an important role in enhancing the performance of ultrasonic motors.

Fluid jet-array parallel machining of optical microstructure array surfaces.

Optical microstructure array surfaces such as micro-lens array surface, micro-groove array surface etc., are being used in more and more optical products, depending on its ability to produce a unique or particular performance. The geometrical complexity of the optical microstructures array surfaces makes them difficult to be fabricated. In this paper, a novel method named fluid jet-array parallel machining (FJAPM) is proposed to provide a new way to generate the microstructure array surfaces with high productivity. In this process, an array of abrasive water jets is pumped out of a nozzle, and each fluid jet simultaneously impinges the target surface to implement material removal independently. The jet-array nozzle was optimally designed firstly to diminish the effect of jet interference based on the experimental investigation on the 2-Jet nozzles with different jet intervals. The material removal and surface generation models were built and validated through the comparison of simulation and experimental results of the generation of several kinds of microstructure array surfaces. Following that, the effect of some factors in the process was discussed, including the fluid pressure, nozzle geometry, tool path, and dwell time. The experimental results and analysis prove that FJAPM process is an effective way to fabricate the optical microstructure array surface together with high productivity.

Cutting performance of micro-textured polycrystalline diamond tool in dry cutting

This study investigates the performance of micro-grooved polycrystalline diamond (PCD) tool in dry machining of Ti6Al4V titanium alloy. Regular micro-groove array was successfully fabricated on the rake face of PCD tools by a fiber laser. The effect of the micro-grooved PCD tools and the untextured PCD tools on cutting force, average friction coefficient, and anti-adhesion was studied in detail. It is found that better tribological properties of the tool–chip interface can be achieved by the micro-grooved PCD tools compared with that of the untextured PCD tools even under dry cutting conditions. Furthermore, the action mechanism of the micro-grooved PCD tools on the friction behaviors has been discussed and revealed by the tool-chip contact length, direction of micro-grooves, and formation of TiC.

Superhydrophobic surface prepared by micromilling and grinding on aluminium alloy

An efficient method for processing superhydrophobic surface is presented in this paper, which can realise the microstructure machining on a large area. First, the microgroove arrays were processed by high speed precision micromilling machine on the surface of the aluminium alloy. Then, the surface of the microgroove was ground by 1000 # electrostatic sand alumina water resistant abrasive belt. The machined surface topography was observed by SEM, which showed that the microgroove arrays were uniform. The contact angles of the water on the microgroove arrays were measured in different directions. In the parallel direction, the contact angle of microgroove arrays is 142±0·5°, whereas in the vertical direction, it arrives 160±0·6°. The above values of contact angle mean that a stable superhydrophobic metallic surface was prepared by the micromilling and grinding.

Research on the Shape Precison of the Micro-groove in the Silicon under Diamond Fly-cutting

Based on the mechanical physical characteristics of the silicon wafers and the superior processing character of the diamond fly-cutting, the diamond fly-cutting was applied to the machining of the microgroove in the silicon, and the effect of the processing parameters on the machining shape precision of the microgroove arrays in the silicon was mainly studied. Firstly, the cutting model of the microgroove in cross section under the diamond fly-cutting was established and the theoretical curvature radius of the microgroove was obtained. Secondly, the shape precision of the microgroove is deduced. Finally, the processing experiment of the microgroove in the silicon was carried out. The experimental results reveal that the higher spindle speed n and the smaller cutting feed f are beneficial to machine the better microgroove in the silicon, but it is important to choose the appropriate cutting depth to machine the better microgroove in the silicon.

Inside Cover: Spontaneous Formation of Microgroove Arrays on the Surface of p-Type Porous Silicon Induced by a Turing Instability in Electrochemical Dissolution (ChemPhysChem 8/2015)

Inside Cover: Spontaneous Formation of Microgroove Arrays on the Surface of p-Type Porous Silicon Induced by a Turing Instability in Electrochemical Dissolution (ChemPhysChem 8/2015)

Spontaneous Formation of Microgroove Arrays on the Surface of p-Type Porous Silicon Induced by a Turing Instability in Electrochemical Dissolution.

Self-organization plays an imperative role in recent materials science. Highly tunable, periodic structures based on dynamic self-organization at micrometer scales have proven difficult to design, but are desired for the further development of micropatterning. In the present study, we report a microgroove array that spontaneously forms on a p-type silicon surface during its electrodissolution. Our detailed experimental results suggest that the instability can be classified as Turing instability. The characteristic scale of the Turing-type pattern is small compared to self-organized patterns caused by the Turing instabilities reported so far. The mechanism for the miniaturization of self-organized patterns is strongly related to the semiconducting property of silicon electrodes as well as the dynamics of their surface chemistry.

Study on 5-axial milling on microstructured freeform surface using the macro-ball cutter patterned with micro-cutting-edge array

A 5-axial micro-replication milling of microstructured freeform surface is proposed by a novel ball cutter, on which micro-cutting-edge array is patterned by a diamond wheel V-tip in micro-grinding. It can efficiently and precisely machine arbitrary-curved microgroove and micro-pyramid arrays on aluminium alloy and die steel. The form errors reach 6.6μm in 253.6μm in microstructure depth and 1.6μm with 50mm in macro-freeform, respectively. The rake angle, however, is decreased so as to increase cutting temperature. Moreover, increasing wheel speed and decreasing feed speed decrease micro-form errors and surface roughness. The cross-spark-out cutting may deburr.

Numerical and experimental analysis of heat transfer in turbulent flow channels with two-dimensional ribs

The repeated ribs surfaces are known for their effective enhancement of heat transfer, which is widely demanded in many scientific and industrial applications. In this paper, further improvements were made by the introduction of a textured asymmetric arc rib structure, on which arrays of secondary micro grooves are superimposed onto a primary asymmetric surface. Numerical simulation has been carried out on the turbulent force convection flow in a single-phase channel with two-dimension (2D) ribbed internal surface in the range of Reynolds number from 20,000 to 60,000. A uniform heat flux is applied on the external surface of channel. In contrast to the conventional optimized symmetric triangular rib, the advanced compound rib could improve the performance of heat transfer while minimizes the pressure drop. These improvements are closely correlated to the promotion of the separated flow which could reattach the channel surface. Since the geometry of the novel compound rib is difficult to fabricate by conventional means, ultra-precision raster milling (UPRM) is used to generate these micro-structures. According to prediction of numerical simulation, the experimental cooling works have been designed and conducted for the considered shaped ribs with the significant dimensionless pitch of 5 and 6. A good agreement was found between the simulation and experimental results.

Performance of cemented carbide tools with microgrooves in Ti-6Al-4V titanium alloy cutting

Microgroove array with different parameters was fabricated on the rake face of the cemented carbide tools by a fiber laser. The experiments of orthogonal cutting of Ti-6Al-4V titanium alloy were carried out by cutting tools with microgrooves. The effect of microgrooves of different parameters on cutting performance of tool-chip interface was investigated under dry cutting and cryogenic minimum quantity lubrication (CMQL) conditions. Results show that cutting tools with microgrooves combined with CMQL can improve adhesion and reduce the thrust force and average friction coefficient of the rake face of cemented carbide tools.

Controllable underwater anisotropic oil-wetting

This Letter demonstrates a simple method to achieve underwater anisotropic oil-wetting using silicon surfaces with a microgroove array produced by femtosecond laser ablation. The oil contact angles along the direction perpendicular to the grooves are consistently larger than those parallel to the microgroove arrays in water because the oil droplet is restricted by the energy barrier that exists between the non-irradiated domain and the trapped water in the laser-ablated microgrooves. This underwater anisotropic oil-wetting is able to be controlled, and the anisotropy can be tuned from 0° to ∼20° by adjusting the period of the microgroove arrays.

Spatiotemporal Patterns Formed on p-Type Silicon Electrodes during Electrochemical Dissolution

Formation of spatiotemporal patterns has been reported in many electrochemical systems [1]. Applications of spatiotemporal patterns to other fields such as materials science and biological science are important for further progress of nonlinear dynamical systems. Since there are lots of parameters for the emergence of the patterns, it is difficult to understand the mechanisms for pattern formation. Spatiotemporal patterns are often observed in electrochemical systems that show negative differential resistance (NDR) on their current density vs. potential curves. We have found that an S-shaped NDR was observed when a p-type silicon (p-Si) electrode was anodized in an HF solution containing organic solvents. A well-aligned microgroove array was formed after electrochemical dissolution at a potential in the S-shaped NDR. Although the spatial pattern was formed at the S-shaped NDR region, oscillatory instability in current or potential was not observed. In the present study, we report that an oscillatory instability in current is observed at the S-shaped NDR region. For the electrochemical measurements, p-Si (100) with a resistivity of 1-4.5 W cm was used as the working electrode. Platinum (Pt) rods were used not only as the counter electrode, but also as the indicator electrode. Electrolyte solution was a mixture of aqueous HF (47 wt.%), ultrapure water and methanol (MeOH) with a composition of 5:6:29 in volume. Since the resistance of the solution is significant to estimate the potential applied at the electric double layer, the resistance was measured by impedance spectroscopy. Temperature of the solution was controlled by a water bath. Figure 1 depicts current density vs. potential (j-U) curves together with current density vs. iR-corrected potential (j- f) curves measured in the solution at 20°C and 30°C. When using the solution at 20°C, an S-shaped NDR is observed after correcting the iR-drop in the solution. We have confirmed that an array of microgrooves is spontaneously formed at potentials on the NDR branch. No oscillatory instability is observed in the curves. In the solution at 30°C, a fluctuation in current is observed on the j-U curve. After the correction of iR-drop in the solution, a clear oscillatory instability is observed in the region where the S-shaped NDR is detected in the solution at 20°C. Time developments of current measured at a constant potential in the NDR region was plotted in Figure 2. Clear current oscillations are not observed in the solution at 20°C although the current fluctuates with time. In contrast, a current oscillation whose period is roughly 1 min is observed. This difference suggests that a bifurcation from the Turing to Turing-Hopf occurs by changing the temperature of the solution from 20 °C to 30°C.[1] K. Krischer, N. Mazouz, P. Grauel, Angew. Chem. Int. Ed. 40, 850 (2001).

Porosification of Silicon from the Viewpoint of Nonlinear Dynamics

Anodic dissolution of silicon in HF solutions leads to formation of porous silicon. Formation of a wide variety of porous structures has been reported. In the course of the studies, electrochemical oscillations have been found in silicon anodization. Because a symmetry breaking is necessary for the formation of porous silicon, such oscillatory phenomena have attracted keen attention for the understanding of porous silicon formation. Formation of macroporous silicon, whose pore diameter is several micrometers, has been explained based on the current burst model proposed by Föll and his colleagues. In the current burst model, macropore formation has been explained based on desynchronization of local oscillators on a silicon surface.Recent studies on nonlinear dynamics in electrochemistry have shown that spatiotemporal pattern formation is closely related to a negative differential resistance (NDR) observed in the current density vs. potential curve. However, NDRs have not been considered in porous silicon formation so far. We have focused our attention on the emergence of NDR in anodization of silicon, and have discussed the spatial pattern formation on silicon. Recently, we have found that a clear S-shaped NDR is observed after the correction of iR-drop in the electrolyte solution, when using an HF solution containing alcohol such as 2-propanol. In the S-shaped NDR region, an array of microgrooves is spontaneously formed on the silicon surface. Since an S-shaped NDR is induced by a positive feedback which is independent on electrode potential, while electrode potential plays a role of an negative feedback. In the present case, we think that the microgroove array is induced by Turing instability. According to earlier studies, the size of Turing patterns observed in electrochemical systems is on the scale of millimeters. Astonishingly, the size of the present Turing pattern is on the scale of micrometers. In the present paper, the reason why the characteristic size is so small will be discussed in detail. In addition to the Turing pattern formation, formation of other spatiotemporal patterns in silicon electrochemistry, which is difficult to be explained in the classical framework of spatiotemporal pattern formation in electrochemistry, will be discussed.