Research Papers

J. Micro Nano-Manuf. 2013;1(4):041001-041001-9. doi:10.1115/1.4025255.

This paper studies the effects of crystallography on the microscale machining characteristics of polycrystalline brittle materials on a quantitative basis. It is believed that during micromachining of brittle materials, plastic deformation can occur at the tool-workpiece interface due to the presence of high compressive stresses which leads to chip formation as opposed to crack propagation. The process parameters for such a machining process are comparable to the size of the grains, and hence crystallography assumes importance. The crystallographic effects include grain size, grain boundaries (GB), and crystallographic orientation (CO) for polycrystalline materials. The size of grains (crystals), whose distribution is analyzed as a log-normal curve, has an effect on the yield stress of a material as described by the Hall–Petch equation. The effects of grain boundary and orientation have been considered using the principles of dislocation theory. The microstructural anisotropy in a deformed polycrystalline material is influenced by geometrically necessary boundaries (GNB) and incidental dislocation boundaries (IDB). The dislocation theory takes both types of dislocations into account and relates the material flow stress to the dislocation density. The proposed analysis is compared with previously reported experimental data on polycrystalline germanium (p-Ge). This paper aims to provide a deeper physical insight into the microstructural aspects of polycrystalline brittle materials during precision microscale machining.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(4):041002-041002-8. doi:10.1115/1.4025461.

Various ways of fabricating a three-dimensional (3D) component in a single-layer exposure using spatial variation of exposure dose have been presented in the literature. While some of them are based on dynamic mask process, more recently, a process based on varying intensity of a scanning Gaussian laser beam termed as “bulk lithography” has been proposed. In bulk lithography, the entire varying depth 3D microstructure gets fabricated because of spatial variation of intensity of laser imposed at every point in single layer scan. For the bulk lithography process, this paper first presents experimental characterization of unconstrained depth photopolymerization of resin upon exposure to Gaussian laser beam. Experimental characterization carried out for two resins systems: namely 1,6 hexane diol-diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA), over relatively wider range of Ar+ laser exposure dose and time, show behavior well beyond Beer–Lambert law. A unified empirical model is proposed to represent the nondimensional depth variation with respect to the time and energy of exposure for both resins. Finally, using these models, successful fabrication of several microstructures including micro-Fresnel lens, textured curved surface, otherwise difficult or impossible to fabricate, is demonstrated. Several advantages of the bulk lithography as compared to other similar processes in the literature are highlighted.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(4):041003-041003-10. doi:10.1115/1.4025554.

Hot embossing process has emerged as a viable method for producing small, complex, precision parts in low volumes. It provides several advantages such as low-cost for molds, high replication accuracy for microfeatures and simple operation. The adaptation of this process for producing high fidelity hot embossed feedstock based metallic powders without the need for machining of the die mold is outlined. This was achieved through a combination of powder metallurgy and plastic hot embossing technologies to produce net-shape metal or hard materials components. In this paper, the manufacturing of molds that are suitable for the production of microfluidic systems using the replication technique is discussed. Variations of parameters in the replication process were investigated. An experimental rheological study was performed to evaluate the influence of the mixing parameters on the rheological behavior and thermal stability of 316L stainless steel feedstock. The effects of the solid loading on the feedstock rheological properties and tolerance control as well as mechanical properties and microstructures were investigated.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(4):041004-041004-9. doi:10.1115/1.4025538.

The microscale drilling performance of a Zr-based bulk metallic glass (BMG) is investigated in this paper. Crystallization, drill temperature, axial force, spindle load (SL), acoustic emissions (AE), chip morphology, hole diameter, and entry burr height are measured and analyzed with varying cutting speed and chip load. The progression of tool wear is assessed using stereo-microscopy techniques. At small chip loads, minimum chip thickness (MCT) is observed to shift cutting mechanics from a shear-dominated to a ploughing-dominated regime. Consequently, evidence of drill instability and larger burr height are observed. As drilling temperatures rise above the glass transition temperature, the BMG thermally softens due to the transition to a super-cooled liquid state and begins to exhibit viscous characteristics. In the tool wear study using tungsten carbide microdrills, rake wear is found to dominate compared to flank wear. This is attributed to a combination of a high rate of diffusion wear on the rake face as well as lower abrasion on the flank due to the decreased hardness from thermal softening-induced viscous flow of BMG.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(4):041005-041005-11. doi:10.1115/1.4025977.

In electrochemical micromachining (EMM) of microfeatures using straight cylindrical microtools, sidewalls of the structure tapers as depth increases. Disk microtool electrodes are used to minimize the taper formation during the machining of microfeatures. At present disk microtool electrodes are fabricated by wire electrical discharge grinding, reverse electro discharge machining (EDM), and microwire electro discharge machining method, which needs separate EDM machine as well as fabricated microtools suffer from thermal defects like microcracks on surface, residual stress, deformation, and needs careful handling. To overcome these limitations, new method is proposed to fabricate disk microtool electrode by EMM. Also the influences of EMM process parameters like applied voltage, pulse frequency, duty ratio, electrolyte concentration on shank diameter, material removal rate, and surface quality are investigated. Disk microtool electrode of disk height 70 μm, disk diameter 175 μm, shank diameter 93 μm, and shank height 815 μm have been fabricated from tungsten microrod of 300 μm diameter by proposed method and used to machine microfeatures like cylindrical hole with reduced taper angle, reverse taper hole, taper free microgroove, and 3D microstructure with plane surfaces on stainless steel by EMM. Effects of disk height on machining accuracy during generation of microhole, in the form of taper angle are also presented in the paper. Proposed method of developing disk electrode by EMM will be very useful for fabricating disk microtool electrodes with different disk diameters, disk heights, shank diameter, and shank height with desired surface quality by controlling various process parameters. Disk microtools with lower disk heights are more effective to generate microfeatures with minimum taper.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(4):041006-041006-6. doi:10.1115/1.4025979.

Elastic deformation molding method is a newly developed aspheric machining technology which can convert complex aspherical surface machining to simple flat surface machining. The residual stress induced in lapping process of elastic deformation molding method has significant influence on machining precision. In this paper, the influence of residual stress induced lapping process on machining accuracy is analyzed and discussed through experiment. An experiment with elimination of residual stress is carried out. The machining result shows that distortion amount has reduced form 2 μm to 0.8 μm, which means the residual stress can be effectively eliminated. A machining form accuracy of P-V 0.56 μm is obtained.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(4):041007-041007-6. doi:10.1115/1.4025991.

The precision ball is the most key component of ball bearing, which is widely used in many precision mechanical fields. This paper presents a variable-radius V-groove lapping method since this method can make the spin angle vary between 0 deg and 90 deg, and the lapping trajectory can cover the ball's surface. Based on this lapping method, an observation of lapping surface of balls experiment is set up in which blackened balls is brightened effectively only after 10 circulations of lower plate in 2 min. Next, another experiment is also done: A batch of G16 level steel balls has been lapped to G5 level in the machining experiment. All results illustrate that the novel method has properties of high efficiency, high precision, and high consistency, which needs further study and then it is likely to replace the traditional one in the future.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Micro Nano-Manuf. 2013;1(4):044501-044501-5. doi:10.1115/1.4025462.

In this study, efficient, high-precision grinding of cemented carbide alloys using a specific grinding wheel was performed, and the ground surface characteristics were investigated in detail. The results showed that final finishing using a chromium-bonded wheel produced an extremely smooth surface with an average roughness Ra of 4 nm. The grinding process produced a chromium- and copper-rich surface layer, as well as a large amount of diffusion of oxygen. Adhesive strength tests using a microscratching method were also carried out on ground substrates coated with diamond-like carbon (DLC) films. The surface ground by the chromium-bonded wheel exhibited superior adhesive strength due to its strong chemical affinity with the DLC film.

Commentary by Dr. Valentin Fuster

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