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J. Micro Nano-Manuf. 2019;7(1):010301-010301-1. doi:10.1115/1.4043692.

This Special Section contains a selection of papers presented at the World Congress of Micro- and Nano-Manufacturing that took place in September 2018 in the charming Slovenian city of Portorož. This annual congress is organized by three societies: the International Institution on Micro-Manufacturing (I2M2), the International Forum on Micro-Manufacturing (IFMM), and the 4 M Association, and it is attended by over a hundred experts on micro- and nano‐manufacturing from all over the globe. Having conducted separate conferences for their respective members for over a decade, from 2015 the three societies started to organize a joint conference that takes place in alternating succession in Americas, Europe, and Asia. WCMNM 2017 took place in Taiwan, WCMNM 2018 was held in Slovenia, and upcoming WCMNM 2019 will be organized in Raleigh, NC.

Commentary by Dr. Valentin Fuster

Special Section Papers

J. Micro Nano-Manuf. 2019;7(1):010901-010901-11. doi:10.1115/1.4042964.

Functionalized metallic nanofeatures can be selectively fabricated via ultrashort laser processing; however, the cost-effective large-area texturing, intrinsically constrained by the diffraction limit of light, remains a challenging issue. A high-intensity near-field phenomenon that takes place when irradiating microsized spheres, referred to as photonic nanojet (PN), was investigated in the transitional state between geometrical optics and dipole regime to fabricate functionalized metallic subwavelength features. Finite element simulations were performed to predict the PN focal length and beam spot size, and nanofeature formation. A systematic approach was employed to functionalize metallic surface by varying the pulse energy, focal offset, and number of pulses to fabricate controlled array of nanoholes and to study the generation of triangular and rhombic laser-induced periodic surface structures (LIPSS). Finally, large-area texturing was investigated to minimize the dry laser cleaning (DLC) effect and improve homogeneity of PN-assisted texturing. Tailored dimensions and densities of achievable surface patterns could provide hexagonal light scattering and selective optical reflectance for a specific light wavelength. Surfaces exhibited controlled wetting properties with either hydrophilicity or hydrophobicity. No correlation was found between wetting and microbacterial colonization properties of textured metallic surfaces after 4 h incubation of Escherichia coli. However, an unexpected bacterial repellency was observed.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):010902-010902-5. doi:10.1115/1.4042965.

The ongoing miniaturization trend in combination with increasing production and functional volume leads to a rising demand for metallic microparts. Bulk forming of microparts from sheet metal provides the potential for mass production of those components by an extensive simplification of the handling. The advantage of a high production rate contrasts with the disadvantage of a low utilization of material. In this context, it is necessary to investigate suitable measures to increase the material utilization. To save cost intensive trial and error tests, numerical analysis could be an appropriate method for a basic process investigation. In this work, a validation with experimental results in the macro- and microscale was used to investigate the eligibility of the finite element method (FEM) for a basic process analysis. For a high transferability, the finite element (FE) models were validated for various tribological conditions and material states. The results reveal that there is a high agreement of the experimental and numerical results in the macroscale. In microscale, conventional FEM shows inaccuracies due to the negligence of size effects in the discretization of the process. This fact limits the application of conventional FE-programs. Furthermore, the results show that lubricated and dry formed blanks lead to the same friction force and process result in the microscale. In addition, the basic formability of the prestrengthened pins in further forming stages was experimentally demonstrated.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):010903-010903-6. doi:10.1115/1.4042966.

The capability to manufacture high-precision components with microscale features is enhanced by the combination of different micromanufacturing processes in a single process chain. This study explores an effective process chain that combines micro-abrasive water jet (μ-AWJ) and microwire electrical discharge machining (μ-WEDM) technologies. An experimental spring component is chosen as a leading test case, since fine geometric features machining and low roughness on the cut walls are required. The advantages deriving from the two technologies combination are discussed in terms of machining time, surface roughness, and feature accuracy. First, the performances of both processes are assessed by experimentation and discussed. Successively, different process chains are conceived for fabricating two test cases with different sizes, displaying some useful indications that can be drawn from this experience.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):010904-010904-7. doi:10.1115/1.4042977.

Additive manufacturing (AM) of metal offers matchless design sovereignty to manufacture metallic microcomponents from a wide range of materials. Green-state micromilling is a promising method that can be integrated into the AM of metallic feedstock microcomponents in typical extrusion-based AM methods for compensating the inability to generate microfeatures. The integration enables the manufacturing of complex geometries, the generation of good surface quality, and can provide exceptional flexibility to new product shapes. This work is a micromachinability study of AISI316 L feedstock components produced by extrusion-based AM where the effects of workpiece temperature and the typical micromilling parameters such as cutting speed, feed per tooth, axial depth of cut, and air supply are studied. Edge integrity and surface roughness of the machined slots, as well as cutting forces, are analyzed using three-dimensional microscopy and piezoelectric force sensor, respectively. Green-state micromilling results were satisfying with good produced quality. The micromilling of heated workpieces (45 °C), with external air supply for debris removal, showed the best surface quality with surface roughness values that reached around Sa = 1.5 μm, much smaller than the average metal particles size. Minimum tendency to borders breakage was showed but in some cases microcutting was responsible of the generation of surface defects imputable to lack of adhesion of deposited layers. Despite this fact, the integrability of micromilling into extrusion-based AM cycles of metallic feedstock is confirmed.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):010905-010905-7. doi:10.1115/1.4043171.

The micro-ultrasonic machining (USM) is suitable for machining hard and brittle materials. When a micro hole is drilled deeply using micro-USM, machining speed slows down and the breakage of micro tool may occur. To solve this problem, this paper proposes the application the planetary movement of micro tool in high-aspect ratio micro holes drilling by micro-USM. The micro holes of about 92 μm in diameter with an aspect ratio larger than ten have been machined. The processing efficiency has been improved. The influence of planetary movement parameters on processing efficiency has been investigated

Topics: Machining
Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):010906-010906-4. doi:10.1115/1.4043236.

During large-area electron beam irradiation, high energy flux pulses of electrons melt a thin layer of material. The objective of this work is to analyze the spatial frequencies of a turned, S7 tool steel surface before and after electron beam melting. It was observed that high frequency features are significantly reduced following melting, but lower frequency features were created and increased the unfiltered areal average roughness. Previous work on laser remelting-based polishing derived a critical frequency that defines the frequency above which higher frequency features are dampened. As the critical frequency depends on the melt duration that the surface experiences, a one-dimensional, transient temperature prediction model was created for this work to estimate the melt time for a single electron beam pulse. This model allowed for the calculation of a critical frequency that showed good ability to predict the frequencies that are dampened.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):010907-010907-8. doi:10.1115/1.4043176.

This paper presents a simulation study toward analyzing the effect of radial throw in micromilling on quality metrics and on the deviation in tool-tip trajectory from its prescribed pattern. Both the surface location error (SLE) and the sidewall (peripheral) surface roughness are analyzed. The deviation in tool-tip trajectory is evaluated considering the flute-to-flute variations in the uncut chip thickness and changes in the tooth spacing angle. Radial throw indicates the instantaneous radial location of the tool axis, thereby capturing all salient features of tool-tip trajectory deviations, such as the general elliptical form of the radial motions. This is in contrast to the concept of run-out, which is a scalar quantity (total indicator reading) indicating the total displacement or change in the radial throw measured from a perfect cylindrical surface for one complete rotation of the axis. As such, measurement and analysis of radial throw is essential to understanding micromachining processes. In our previous work, we described an experimental approach for accurate determination of radial throw when using ultra-high-speed micromachining spindles. In this work, we present a simulation-based study to relate radial throw parameters and form to SLE, sidewall surface roughness, flute-to-flute variations of uncut chip thickness, and changes in tooth spacing angle for a two fluted micro-endmill. As expected, our study concludes that the magnitude, orientation, and form of radial throw all significantly affect the studied quality metrics, tooth spacing angle, and the flute-to-flute chip thickness variations. Specifically, the presence of radial throw with an elliptical form induces up to 50% variation in SLE, up to 20% variation in sidewall surface roughness, up to 60% variation in tooth spacing angle deviations, and up to 50% variation in flute-to-flute chip thickness. As such, the presented simulation approach can be used to assess the direct (kinematic) effects of the radial throw parameters on the quality metrics and chip thickness variations.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):010908-010908-6. doi:10.1115/1.4043275.

In contrast to the well-established stability prediction tools, a robust real-time stability indicator is proposed for micromilling process, and it opens the possibility of online chatter avoidance based on successful detection. In this paper, a robust and easy-to-compute stability indicator is presented. This approach exploits the virtue of a stable milling process—the displacement of the vibrating tool repeats with a period of tooth passing. It has been observed that the standard deviation of the tool displacement sampled at once per tooth passing frequency is indicative of chatter, where a low standard deviation coincides with stable cutting. An increase in standard deviation is the direct consequence of an increase in asynchronous motion of the tool, coinciding with chatter. As it is also well known, this asynchronous vibration of the tool results in distinct marks on the workpiece surface. This paper presents the experimental validation of this real-time stability indicator. The ease of implementation makes the presented stability indicator a strong candidate for applications in chatter avoidance based on detection. The results are also verified against the standard stability prediction method.

Commentary by Dr. Valentin Fuster

Research Papers

J. Micro Nano-Manuf. 2019;7(1):011001-011001-10. doi:10.1115/1.4042383.

As one of the most promising anode materials for high-capacity lithium ion batteries (LIBs), silicon nanowires (SiNWs) have been studied extensively. The metal-assisted chemical etching (MACE) is a low-cost and scalable method for SiNW synthesis. Nanoparticle emissions from the MACE process, however, are of grave concerns due to their hazardous effects on both occupational and public health. In this study, both airborne and aqueous nanoparticle emissions from the MACE process for SiNWs with three sizes of 90 nm, 120 nm, and 140 nm are experimentally investigated. The prepared SiNWs are used as anodes of LIB coin cells, and the experimental results reveal that the initial discharge and charge capacities of LIB electrodes are 3636 and 2721 mAh g−1 with 90 nm SiNWs, 3779 and 2712 mAh g−1 with 120 nm SiNWs, and 3611 and 2539 mAh g−1 with 140 nm SiNWs. It is found that for 1 kW h of LIB electrodes, the MACE process for 140 nm SiNWs produces a high concentration of airborne nanoparticle emissions of 2.48 × 109 particles/cm3; the process for 120 nm SiNWs produces a high mass concentration of aqueous particle emissions, with a value of 9.95 × 105 mg/L. The findings in this study can provide experimental data of nanoparticle emissions from the MACE process for SiNWs for LIB applications and can help the environmental impact assessment and life cycle assessment of the technology in the future.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):011002-011002-7. doi:10.1115/1.4043419.

Various nanocarbons (NCs) were used to study their surface groups under standardized Bohem titration, including: multiwalled carbon nanotube (CNT), graphene (G), Vulcan carbon (VC), and nanodiamond (ND). Endpoint-measured titration using second derivative method to quantify carboxylic, lactonic, and phenolic groups created on treated carbon surfaces shows a high precision comparable to other recent reports and with errors of 1 order of magnitude lower. The results exhibit major concentration of carboxyl group increased after the NCs were oxidized compared to the amount of other functional groups like phenols and lactonic groups. It is important highlight, the concentration ratio of carboxyl group with VC:VC-O was showed at 1:77, exhibited a major result regarding other NCs which exhibited ratios of 1:4.5, 1:1.4, and 1:2.5 for ND:ND-O, CNT:CNT-O, and G:G-O, respectively. It is concluded that VC is a NC that competes and excels in its capacity of oxidation with respect to the popular NCs as CNT, graphene (G), and ND.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):011003-011003-7. doi:10.1115/1.4043420.

This paper presents the development of a prototype exfoliation tool and process for the fabrication of thin-film, single crystal silicon, which is a key material for creating high-performance flexible electronics. The process described in this paper is compatible with traditional wafer-based, complementary metal–oxide–semiconductor (CMOS) fabrication techniques, which enables high-performance devices fabricated using CMOS processes to be easily integrated into flexible electronic products like wearable or internet of things devices. The exfoliation method presented in this paper uses an electroplated nickel tensile layer and tension-controlled handle layer to propagate a crack across a wafer while controlling film thickness and reducing the surface roughness of the exfoliated devices as compared with previously reported exfoliation methods. Using this exfoliation tool, thin-film silicon samples are produced with a typical average surface roughness of 75 nm and a thickness that can be set anywhere between 5 μm and 35 μm by changing the exfoliation parameters.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):011004-011004-7. doi:10.1115/1.4043595.

The unique capabilities of Aerosol Jet® technology for noncontact material deposition with in-flight adjustment of ink rheology in microdroplets are explained based on first principles of physics. The suitable range of ink droplet size is determined from the effectiveness for inertial impaction when depositing onto substrate and convenience for pneumatic manipulation, in-flight solvent evaporation, etc. The existence of a jet Reynolds number window is shown by a fluid dynamics analysis of impinging jets for Aerosol Jet® printing with long standoff between nozzle and substrate, which defines the operation range of gas flow rate according to the nozzle orifice diameter. The time scale for ink droplets to remove volatile solvent is shown to just coincide that for them to travel in the nozzle channel toward substrate after meeting the coflowing sheath gas, enabling the in-flight manipulation of ink properties for high aspect-ratio feature printing. With inks being able to solidify rapidly, 3D structures, such as tall micropillars and thin-wall boxes, can be fabricated with Aerosol Jet® printing. Having mist droplets in the range of 1–5 μm also makes it possible to print lines of width about 10 μm.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2019;7(1):011005-011005-7. doi:10.1115/1.4043502.

This paper reports on a polymer stamp-based mechanical exfoliation method for producing thin (<1 μm) graphite sheets from a highly ordered pyrolytic graphite (HOPG) source by tailoring key exfoliation process parameters, utilizing in-plane shear oscillation during exfoliation, and controlling the thickness of a polydimethylsiloxane (PDMS) stamp. Experiments on the effect of high frequency in-plane shear oscillation and the effect of PDMS stamp thickness are designed to reduce the thickness of exfoliated layers and to minimize surface morphological variations. Results show that the exfoliated sheets consist of a range of layer thicknesses, surface areas, and surface morphological features. The exfoliated HOPG sheets are also found to be thinner, more electrically and thermally conductive, and of higher quality than commercially available pyrolytic graphite sheets.

Commentary by Dr. Valentin Fuster

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