Developing a paving system for fabricating ultra-thin layers in ceramic laser rapid prototyping

“Step effect” is one of the major concerns for engineers because it affects the surface quality in layer manufacturing. Instead of constant thickness, adaptive slicing procedures use slices of variable thickness that are governed by the model geometry, manufacturing process, material, and rapid prototyping system. The RP system has the capability of fabricating the layer between a minimum and maximum thickness with certain intermediate thickness during the model making and is indispensable. Due to agglomeration of powder and upward deformation of the layer, the layer’s thickness in powder-based rapid prototyping processes is restricted. To improve surface quality, the current study constructs a paving system to minimize the layer thickness. Based on a wet process-ceramic laser rapid prototyping technique, a paving system, which can fabricate ultra-thin layers to a thickness of 20 μm and can achieve instant layer drying, was successfully constructed. A strategy of compensative layer was induced to improve accuracy of the Z-axis. Due to the ultra-thin layers, a plane surface finishing of R_z = 15∼24 μm was obtained.     

Investigation of glue-bonding process on assembly accuracy

The effect of assembly accuracy due to the displacement of glue-bonding processes using different types of glues and bonding configuration has been investigated systematically using a multi-dimensional capacitance position monitoring system with resolution in around 10 nm. In our study, we observe that an order of magnitude difference in bonding position accuracy may result using different bonding conditions and glue. Our analysis shows while the shrinkage of glue may play an important factor, other factors have to be taken into account in order to get the optimum condition for the glue bonding process. Displacement of less than 0.1 μm level has been demonstrated for a 4-sided side bonding process using UV glue. This level of accuracy is as much as an order of magnitude better than the result obtainable by the commonly used screwing fastening process.     

Multiple fabrications of sacrificial layers to enhance the dimensional accuracy of microstructures in maskless projection microstereolithography

Microstereolithography (MSL) is a promising technology for producing fully three-dimensional microstructures with overhanging features and high-aspect ratios. In conventional stereolithography (SL), a layer thickness of ≈50–100 μm is obtained by using a recoater. However, a recoater cannot be used in MSL, where the layer thickness is typically ≈10 μm or less, since resin flow may break or distort the pre-fabricated layers. In most MSL systems, the resin surface (or layer thickness) is controlled by a free surface technique that employs resin gravity to refresh the resin surface over a given settling time. In addition, a sacrificial layer must be fabricated in MSL to create a flat initial surface and provide support, just as in SL. In this paper, the fabrication methodology and functionality of the sacrificial layer is investigated for microstereolithography microstructures fabricated using the free surface technique. Experimental data are presented that indicate the greater the number of sacrificial layers, the sharper the dimensional accuracy of the microstructures in the building direction. This is because multiple fabrications of the sacrificial layer affect the resin ‘wetting’ status on the substrate or pre-cured surfaces. Several microstructures were fabricated to verify the effect of multiple fabrications of the sacrificial layer on dimensional accuracy in the building direction.     

A space solar cell bonding robot

A space solar cell bonding robot system which consists of a three-axis Cartesian coordinate’s robot, coating device, bonding device, orientation plate, and control subsystem was studied. A method, which can control the thickness of adhesive layer on the solar cell, was put forward and the mechanism was designed. Another method which can achieve the auto-bonding between thin cover-glass and the space solar cell was studied and realized. It produced no air bubble in the adhesives layer under the condition of no vacuum environment, and ensures the assembly dislocation ⩽0. 1 mm. Compared to the conventional method, it has advantages such as no fragment exists, and no adhesives outflow onto the cover-glass and solar cells. __________ Translated from Journal of Shanghai Jiaotong University, 2005, 39(6) (in Chinese)     

Adhesion and friction studies of a nano-textured surface produced by spin coating of colloidal silica nanoparticle solution

This paper presents the results of adhesion and friction studies on a nano-textured surface. The nano-textures were produced by spin coating colloidal silica nanoparticle solution on a flat silicon substrate. Surface morphology was characterized by environmental scanning electron microscopy (ESEM) and scanning probe microscopy (SPM). Adhesion and friction studies were conducted using a TriboIndenter employing diamond tips with 5 μm and 100 μm nominal radii of curvature. The results show that the adhesion forces and coefficients of friction of the nano-textured surface measured by the 100 μm tip were reduced up to 98 and 88%, respectively, compared to those of a baseline silicon oxide film surface.     

Friction Study of a Ni Nanodot-patterned Surface

Nanoscale frictional behavior of a Ni nanodot-patterned surface (NDPS) was studied using a TriboIndenter by employing a diamond tip with a 1 μm nominal radius of curvature. The Ni NDPS was fabricated by thermal evaporation of Ni through a porous anodized aluminum oxide (AAO) template onto a Si substrate. Surface morphology and the deformation of the NDPS were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), before and after friction/scratch testing. SEM images after scratching clearly showed that, similar to what was assumed at the macroscale, the frictional force is proportional to the real area of contact at the nanoscale. It was found that adhesion played a major role in the frictional performance, when the normal load was less than 20 μN and plastic deformation was the dominant contributor to the frictional force, when the normal load was between 60 μN and 125 μN. Surprisingly, a continuum contact mechanics model was found to be applicable to the nanoscale contact between the tip and the inhomogeneous Ni NDPS at low loads. The coefficient of friction (COF) was also found to depend on the size of the tip and was four times the COF between a 100 μm tip and the Ni NDPS. Finally, the critical shear strength of the Ni nanodots/Si substrate interface was estimated to be about 1.24 GPa.     

Micro laser welding of polymer microstructures using low power laser diodes

The use of laser welding for joining micro parts has experienced a substantial increase in popularity during recent years. Specifically translucent microfluidic devices are assembled using laser welding; however, a major issue is the laser beam size of commercially available laser-welding equipment and thus the resulting welding seam size, which may be orders of magnitude larger than microfluidic channels and structures. We have successfully achieved extremely small welding seams using focussed low-power laser diodes. Commercial laser welding stations for polymer assembly will typically operate in the power-region 15–50 Watts. The focussed laser beam will have a size of typically 500 μm × 500 μm and may, depending on optical configuration, be up to several mm². The resulting welding-seam will thus be in the area of 300–600 μm depending on beam energy distribution; additionally the melt will spread to unheated areas due to capillary forces. As microfluidic channels are in 20–100 μm regions, even a very limited amount of stray melt may completely fill a part of a channel and thus render it useless. We have used commercially available “single-die” laser-diodes of optical power 200–500 mW. The beam has been focussed and directed using simple optical installations, resulting in a beam-size in the area of 50 μm × 5 μm full width half maximum (FWHM) We have achieved firm welding seams of width <10 μm, with a welding speed of 15 mm/s and with virtually no noticeable spread of melt.     

Polycrystalline silicon carbide as a substrate material for reducing adhesion in MEMS

The in-use adhesion characteristics of polycrystalline cubic silicon carbide (poly-SiC) films when used as a substrate material in MEMS applications are investigated using micromachined polycrystalline Si (poly-Si) cantilever beam arrays. The detachment lengths greater than 1500 μm are obtained, corresponding to an apparent work of adhesion of less than 0.006 mJ/m². This is to be compared to the detachment lengths of less than 200 μm when poly-Si substrate is used, corresponding to the apparent work of adhesion of greater than 20 mJ/m². To help understand the mechanism leading to the significant reduction in in-use adhesion, the poly-SiC surfaces are characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurement. Based on the data, it is suggested that the topography as well as the slower oxidation rate of poly-SiC films may be responsible for the observed adhesion reduction.     

Effect of boundary layer swirl on supersonic jet instabilities and thrust

This paper reports the effects of nozzle exit boundary layer swirl on the instability modes of underexpanded supersonic jets emerging from plane rectangular nozzles. The effects of boundary layer swirl at the nozzle exit on thrust and mixing of supersonic rectangular jets are also considered. The previous study was performed with a 30° boundary layer swirl (S=0.41) in a plane rectangular nozzle exit. At this study, a 45° boundary layer swirl (S=1.0) is applied in a plane rectangular nozzle exit. A three-dimensional unsteady compressible Reynolds-Averaged Navier-Stokes code with Baldwin-Lomax and Chien’sk-ε two-equation turbulence models was used for numerical simulation. A shock adaptive grid system was applied to enhance shock resolution. The nozzle aspect ratio used in this study was 5.0, and the fully-expanded jet Mach number was 1.526. The “flapping” and “pumping” oscillations were observed in the jet’s small dimension at frequencies of about 3,900Hz and 7,800Hz, respectively. In the jefs large dimension, “spanwise” oscillations at the same frequency as the small dimension’s “flapping“ oscillations were captured. As reported before with a 30° nozzle exit boundary layer swirl, the induction of 45° swirl to the nozzle exit boundary layer also strongly enhances jet mixing with the reduction of thrust by 10%.     

Tribological behavior of thin electroplated and chemically deposited Ni-P coatings on copper substrates

The paper deals with the study of the tribological behavior of thin (1 and 5 μm thick) Ni-P coatings formed on plates made of electrotechnical copper by electroplating and chemical deposition. It has been found that after heat treatment, owing to diffusion processes, the coating components are capable of penetrating into the substrate material—to a depth of 15 μm for electroplated coatings and of 35 μm for chemically deposited coatings)—and influencing the mechanical properties of a specimen such as the hardness and friction coefficient. A correlation between the wear mode of the coating and the method of its deposition has been found. The results show that thin (one μm thick and lesser) Ni-P chemically deposited coatings are promising for being used in friction units of precise mechanical devices.     

Automated surface finishing of plastic injection mold steel with spherical grinding and ball burnishing processes

This study investigates the possibilities of automated spherical grinding and ball burnishing surface finishing processes in a freeform surface plastic injection mold steel PDS5 on a CNC machining center. The design and manufacture of a grinding tool holder has been accomplished in this study. The optimal surface grinding parameters were determined using Taguchi’s orthogonal array method for plastic injection molding steel PDS5 on a machining center. The optimal surface grinding parameters for the plastic injection mold steel PDS5 were the combination of an abrasive material of PA Al₂O₃, a grinding speed of 18000 rpm, a grinding depth of 20 μm, and a feed of 50 mm/min. The surface roughness R_a of the specimen can be improved from about 1.60 μm to 0.35 μm by using the optimal parameters for surface grinding. Surface roughness R_a can be further improved from about 0.343 μm to 0.06 μm by using the ball burnishing process with the optimal burnishing parameters. Applying the optimal surface grinding and burnishing parameters sequentially to a fine-milled freeform surface mold insert, the surface roughness R_a of freeform surface region on the tested part can be improved from about 2.15 μm to 0.07 μm.     

Vacuum suction aid for microlens array formation using LIGA-like process

Microlens array fabrication using a vacuum suction process combed with the LIGA-like process is presented in this paper. The circular patterned array was designed on a photomask and transferred onto a substrate using photoresist patterning. Electroforming technology was used to convert the photoresist patterns into a metallic molds with an array of nozzles. Liquid JSR resist was spun onto the substrate joining the metallic mold to remove microlens array under vacuum conditions. The exposure energy and vacuum pressure were essential parameters in the microlens array manufacturing process. Microlens arrays with 50 μm in diameter at -50 cm-Hg vacuum pressure and 100 μm in diameter at -60 cm-Hg vacuum pressure were successfully formed. The produced microlens arrays presented smooth measured surface profiles coincident with the optical lens geometry.     

A study on the mirror surface machining by using a micro-energy EDM and the electrophoretic deposition polishing

In order to achieve a mirror-like workpiece surface by EDM, a micro-energy EDM or a manual lapping is normally used. However, both methods are time consuming and economically unattractive. To have better performance, a sinking EDM machine was first used in this research on SKD 61 for a micro-energy discharge process followed by the electrophoretic deposition (EPD) process to coat Al₂O₃ particles uniformly on a rotation electrode. The precision polishing process was then applied on the discharged workpiece with suitable parameters including the voltage, electrode rotating speed, pH value of electrolyte, and abrasive concentration. After the EDM process, the surface roughness and the recast layer thickness of a workpiece could also be reduced. The experimental data showed that when 0.3 μm of Al₂O₃ particles was used for the EPD polishing process, the initial roughness of a discharge surface could be improved from 0.52 μm Ra (6.50 μm Rt) to a mirror-like surface of 0.068 μm Ra (0.742 μm Rt). Also, the total working time could be reduced significantly to within the range of 5 to 10 min.     

A multichannel charge-sensitive amplifier for reading out the signals of double-sided silicon detectors of the tracking system for the CBM project

An eight-channel charge-sensitive amplifier chip created by the 0.18 μm CMOS process is intended for reading out the signals of double-sided silicon detectors of the tracking system for the CBM project of the new FAIR accelerator facility (GSI, Darmstadt, Germany). A dynamic range of ≥5 pC is attained for both polarities of the input signal, the power consumption of one channel does not exceed 1.3 mW, and the noise response of the amplifier is characterized as 1024ē + 24ē /pF (the rms deviation). Results of designing the microcircuit and its experimental characteristics are given.     

Phase doppler measurements: system set-up optimization for characterization of a diesel nozzle

A commercial phase Doppler system was set up, optimized and used to measure the time resolved characteristics of the droplets inside a diesel spray. The purpose of this work was to understand exactly the influence of each system parameter, and to find the best setup enabling measurements in the spray zones that are densest and closest to the injector. Parametric studies were performed to gain an understanding of the particle density limits of the system and their dependence on the system parameters. Then the diesel spray produced by a single-hole injector was measured, with the fuel pressure ranging from 300 to 1300 bar and gas density in the test chamber ranging from ambient conditions to 40 kg/m³. The optic parameters (beam waist size, lenses focal length) were chosen to the best expected values allowed by the optical stand-off of the spray enclosure. The receiver slit width, which was found to have a dramatic effect on the detection of droplets during the injection main period, was tested in the range from 100 μm to 25 μm. Tests were carried out with two different slit lengths, namely 1 mm and 50 μm, with results indicating minimal effect on performance. PMT voltage (gain) was held to a moderately low value between 400 and 500 V and the laser power between 400 and 800 mW in the green line. An optimum burst threshold was found to obtain the best quality data regardless of signal background level, which varies greatly in high-density pulsed sprays. In the end, a set of results from the complete nozzle characterization in various conditions is presented in order to show the practical application of the optimization study and to provide some means of appreciating the results accuracy. The results obtained were also used to show that the gas-jet theory can be used to predict if PDPA measurement are possible in a given experimental situation.     

A nanosecond optical parametric oscillator in the mid-IR region with double-pass pump

An optical parametric oscillator (OPO) with double-pass pump based on MgO:PPLN and PPLN periodic structures is described. A compact nanosecond Nd:YLF laser has been used as a pump source at 1.053 μm (the pumping pulse duration is 5–7 ns at a maximum pulse energy of 300 μJ at frequencies of 1–7 kHz). The oscillation threshold of the OPO based on MgO:PPLN was varied in a range of 11–28 μJ at wavelength of 2.1–4.3 μm. The conversion efficiency from the pump wave to an idler wave decreased from 8.6 to 2.5% in the range of 2.0–4.3 μm. For PPLN-OPO the measured threshold was 36 μJ at 4.2 μm and 49 μJ at 4.7 μm. The conversion efficiency of the pump energy into the energy of an idler wave was 3.3μ-0.4% at wavelengths of 4.2–4.7 μm.     

Preparation of mica surfaces for enhanced resolution and cleanliness in the surface forces apparatus

We describe a simple technique for preparing arbitrarily thin mica substrates of unparalleled purity for use in the surface forces apparatus (SFA). Relatively thick mica samples (4–8 μm) are prepared in the conventional way, coated with a sputter deposited layer of silver, and attached (silver side down) to cylindrical lenses with a thick coat of thermosetting epoxy. These surfaces are then placed in a glovebox with the cell of the SFA where they are cleaved once again with a piece of tape, and then mounted and sealed in the SFA for immediate use. With this technique, the surfaces are never exposed to laboratory air, they are insensitive to asperities in the glue/silica support, and may be made arbitrarily thin by removing more mica until the desired thickness is achieved. We find that adhesion between two surfaces cleaved in this manner (∼300 mJ/m²) is significantly larger than for mica prepared in the conventional way (∼200 mJ/m²), and consistent with the adhesion measured in direct cleavage experiments. This suggests a cause for discrepancies in the measured adhesion which have appeared in the literature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure and tribological characteristics of steel under melting by plasma flow and simultaneous Mo and W alloying

New findings of studies of the structural, tribological, and physicomechanical characteristics of structural steel 40Kh treated by plasma flow under melting accompanied by either tungsten or molybdenum alloying are presented. Rutherford back-scattering of ions, scanning electron microscopy (with microanalysis), X-ray fluorescent spectral analysis, X-ray phase analysis, wear-resistance tests, measurement of the coefficient of friction, and transmitting electron microscopy with diffraction are the basic research methods. It is found experimentally that a thin layer 5 μm thick saturated with nitrogen and an alloying element (Mo or W) with regularly arranged crystallites arises on the steel 40Kh surface. The crystallites in this layer have a needle- and ribbon-shaped structure. A deeper layer located about 40 μm thick consists of micro- and nanosized grains. Friction and wear studies of the plasma-flow treated (melt) samples show the steel wear resistance to increase 2–2.5 times and the coefficient of friction to decrease from 0.4–0.5 to 0.10–0.15 compared to the untreated samples.     

A Tribological Study of SU-8 Micro-Dot Patterns Printed on Si Surface in a Flat-on-Flat Reciprocating Sliding Test

Tribological properties of optimized SU-8 micro-dot patterns on Silicon (Si) were evaluated using a flat-on-flat tribometer. Sliding tests on the patterns were conducted against a SU-8 spin-coated 2 × 2 mm² Si substrate at varying normal loads at a fixed rotational speed. It was observed that the pitch of the SU-8 pattern on Si substrate had significant influence on the wear durability. Ultra-thin layer of perfluoropolyether was over-coated onto SU-8 micro-dot patterned specimens for enhanced wear durability, and the specimen of the optimized pitch 450 μm has shown a wear life of more than 100,000 cycles at a normal load of 650 mN.     

Voltage-current characteristics of diodes on MBE-grown Hg<Subscript>0.78</Subscript>Cd<Subscript>0.22</Subscript>Te layers

Results of measuring the voltage-current characteristics (VCCs) of diodes for IR detectors with the cutoff wavelength λ _c = 11μm are presented. The diodes are based on variband Hg_0.78Cd_0.22Te layers grown by molecular beam epitaxy on semi-insulating GaAs substrates. It is found that diffusion current and generation current in the depletion layer of the p-n junction are the main mechanisms of carrier generation at the reverse bias voltage V ₁ < −0.2 V. It is shown that good agreement between calculated and experimental VCCs is achieved if the decrease of the effective depth of recombination levels in the depletion layer as a result of the Poole — Frenkel effect is taken into account, as well as the longitudinal charge spreading in the reverse current diffusion component.