Variation reduction of a closed-loop precision ceramic micromachining process

Thesis (S.M.)--Massachusetts Institute of Technology, Sloan School of Management; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996.Includes bibliographical references (p. 125-132).This report details the investigation of the micromachining of a TiC.A1203 ceramic using a closed-loop lapping process. Currently the micromachining process laps a ceramic bar with only a priori flatness adjustment. Bar flatness is adjusted prior to the lap using optical measurement of lithography targets. The average value of a critical dimension determines lap completion. The critical dimension is determined with an embedded electronic lapping guide (ELG). The problem with this technique, as it is currently employed, is high product loss due to large variance of the critical dimension across the bar. A six microinch standard deviation is desired. Any product above or below specified critical dimension limits are scrapped, so the variance reduction directly impacts immediate and downstream process yields. An alternative approach is proposed using electrostrictive actuators in a closed control loop to deform the bar during lap processing. The closed-loop lapping (CLL) process significantly decreases critical dimension process variance.by David J. Fanger.S.M

The 3D Characterisation Of Microscale Indentations On Silicon Nitride

Silicon nitride is a high hardness structural ceramic that is often used for the production of ball-bearing components due to its good mechanical properties. However, during manufacturing, microscale pits (called “star features”) can form on the surface of ball components. In this project, microscale pits, on silicon nitride balls are characterised in three-dimensions using a combination of surface and sub-surface characterisation techniques. Investigations into low load (0.5 kg and 1 kg) Vickers micro-indentations on silicon nitride are first carried out to ascertain which techniques are most applicable to evaluation of star features. This includes the determination of key microstructural parameters including microstructure, cracking distribution and residual stress. High resolution SEM imaging and Raman spectroscopy of indentations reveal a primarily intergranular fracture mode for cracking, with surface tensile stresses of ~ 1 GPa at radial crack tips for 1 kg indentations. Micro-XCT resolves sub-surface half-penny, lateral, and radial cracking present after high load 5 kg Vickers indentation into silicon nitride. Focused ion beam (FIB) tomography revealed the presence of half-penny and lateral cracking in low load (0.5 kg and 1 kg) indentations with c/a ratios < 2. The initiation depth of half-penny and lateral cracking has been used to approximate the volume of the compressive stress region that exists beneath the indentation impression. FIB tomography of star features on silicon nitride balls reveals half-penny cracking that has not observed using UV fluorescence microscopy. The lateral extension of lateral cracking from the centre of star features observed via FIB tomography has been found to correlate well with UV fluorescence imaging. In addition, lateral cracking is found to be located at the same depth as adjacent regions of missing material, implying that material removal initiates from lateral cracking. Uplifted zones of up to ~ 0.8 μm vertical displacement were also found to exist surrounding star features. Overall it is found that 3D characterisation methodologies, including FIB tomography, optical profilometry and micro-XCT, are highly effective in the quantification of tribological surface damage in silicon nitride, giving new insights into the distribution and role of surface deformation and fracture

SURFACE CHARACTERIZATION AND TRIBOLOGY IN FLAT LAPPING OF METALS

Lapping is a loose abrasive process employed to remove very small quantities of materials leading to a good surface finish. This research makes several investigations on the lapping process, both qualitative and quantitative. Lapping has been in existence for several decades and yet remains more of an art rather than a science. The principal objective is to create a scientific basis to the study of lapping common metals with common abrasives. The important goals are to study friction, material removal rate, roughness, surface characterization, redox chemistry, burn, and microvoids during flat lapping of aluminum 2024, 304 stainless steel, and 1018 steel. The effects of different abrasives: garnet, silicon carbide, and white aluminum oxide were studied experimentally while lapping aluminum 2024, 304 stainless steel, and 1018 steel.In addition, the area of lapped parts, unfinished zones, and scratched zones were determined using image analysis. Although the aim of lapping is to improve surface finish, sometimes parts are rejected after lapping because of burn, friction, incomplete lapping, scratches, microvoids, and wear. Scratches may be caused by excessive load, low supply of abrasive slurry, or high friction and burn may be caused by excessive load. Uneven distribution of load occurs when the lapping table is not flat, but rather concave or convex in shape. The factors that cause burn, scratches, and incomplete lapping should be minimized.A new method is proposed for calculation of frictional force during lapping using the current consumed in the process. The effects of different abrasives on material removal rate and surface finish on three different types of work materials were evaluated quantitatively. It was found that silicon carbide and white aluminum oxide abrasives removed more material per minute than garnet. Furthermore, from geometric and Energy Dispersive Spectroscopy (EDS) analysis obtained using a Scanning Electron Microscope (SEM), it was confirmed that some abrasives became embedded into the lapped metal substrates. No burn was observed on the lapped samples. Scratches and unfinished lapped parts were observed primarily in 304 stainless steel. There were little or no scratches found on lapped Al 2024 and 1018 steel.Based on the net cell reaction potentials using the Nernst equation, the possible reactions during the lapping process are reactions between magnesium and its hydroxides and white aluminum oxide abrasive. Also, SiO2 from SiC abrasives oxidized Al, Mn, Mg, and Ti in Al 2024 as well as Mn in 304 stainless steel, and Al and Mn in 1018 steel. Analysis of Variance (ANOVA) was performed using Statistical Analysis Software (SASTM 9.1) in order to determine the effects of each variable. ANOVA results revealed that the main effects of abrasive types, size of abrasives, and type of work material had statistically significant influence on material removal rate and surface finish

High-power deep-UV laser for improved and novel experiments on hydrogen

2019 Spring.Includes bibliographical references.This dissertation details the design, performance, and cavity enhancement of a novel, high-power coherent 243.1 nm laser system, and through simulations, its ability to trap hydrogen in a magic wavelength optical trap. This wavelength of light is necessary to address the 1S–2S two-photon transition in hydrogen, and the primary motivation behind development of this laser system is obtaining high enough 243.1 nm powers for two-photon cooling of hydrogen. Due to the light mass of hydrogen, high precision spectroscopy of hydrogen is limited by unwanted motional effects, which could be mitigated with laser cooling and confinement in an optical trap. Besides laser cooling, a high power deep-UV laser system at this wavelength has great utility for improving spectroscopy of hydrogen and other exotic simple systems. High-power fiber lasers from 1-1.2 µm have flourished as a result of advances in ytterbium(Yb)-doped fiber amplifiers. In addition, high-power Yb-fiber lasers between 975-980 nm have also been developed—a notable accomplishment due to gain competition in the > 1 µm spectral region. These systems initially lacked sufficiently narrow spectral bandwidth for efficient harmonic generation, motivating further development since there is significant interest in frequency doubling and quadrupling these sources to produce coherent blue radiation and deep-UV radiation. Here, we generate coherent, high-power deep-UV radiation through frequency quadrupling of a high-power, highly coherent Yb-fiber amplifier at 972.5 nm. The Yb-fiber amplifier system consists of a frequency stabilized master oscillator power amplifier (MOPA) that can be referenced to a coherent frequency comb. This MOPA can be amplified to > 10 W of narrow linewidth power at 972.5 nm in the Yb-fiber amplifier. This is a technically challenging and notable result for this wavelength as gain is much more readily obtained in Yb-doped fibers at the absorption/emission cross-section peak near 975 nm and in the > 1 µm spectral region where the emission cross-section is much larger than the absorption cross-section. This system successfully combated unwanted gain at these wavelengths by using a relatively short (≈ 10 cm), angle-polished Yb-fiber with a large core-cladding ratio, along with aggressive spectral filtering and large amounts of seed power at 972.5 nm. With this narrow linewidth Yb-fiber amplifier, efficient frequency conversion of high power 972-976 nm radiation to 243-244 nm radiation is possible through intracavity doubling. Through successive resonant doubling stages, this system demonstrates > 1 W of highly stable, continuous-wave (CW) 243.1 nm power. To the author's knowledge, this is a record amount of CW deep-UV power below 266 nm, and is made possible thanks to advances in the production of a relatively new non-linear crystal for robust deep-UV generation, cesium lithium borate (CLBO). The precise frequency control of this radiation is established via excitation of the 1S–2S transition in hydrogen, and the viability for two-photon laser cooling on this transition is shown through enhancement of this power to > 30 W of intracavity power in a deep-UV enhancement cavity. At these powers, UV-induced mirror degradation was observed and mitigated by flushing the enhancement cavity mirrors with ultra-pure oxygen. With these powers, rapid two-photon laser cooling of a hydrogen atomic beam approaches reality. The 243.1 nm powers offered by this laser system also offer unique methods for capturing hydrogen in an optical trap. Explored via simulations, single optical scatter capture of hydrogen in a magic wavelength dipole trap is demonstrated, promising exciting new avenues for high precision spectroscopy of hydrogen

Investigating the Effects of SiC Abrasive Particles on Friction Element Welding

The growing demands on reducing the harmful emissions from automobiles have forced automakers to reduce the weight of the vehicle. The increasing demands on improving the fuel economy also has challenged automotive manufacturers to make the vehicle as lightweight as possible. However, the challenge is also to ensure that the vehicle meets safety standards. For the vehicle to meet these standards, it needs to be of adequate strength as well. Automotive manufacturers have adopted a strategy of using multi-material construction to achieve the target. But with multi-material construction comes the requirement of advanced joining techniques that are capable of joining dissimilar materials. The requirement of the advanced techniques is due to the difference in physical and chemical properties of the dissimilar materials to be joined. The conventional methods are either unable to join the dissimilar material or form a joint with defects and of poor quality. Friction Element Welding (FEW) is one of the advanced joining techniques capable of joining dissimilar materials effectively. The process is based on the concepts of friction welding technique where the materials to be joined are heated to the temperature below their melting temperatures. In FEW, a friction element is used to form a friction weld. It has been found that the FEW process although has a low processing time, it is still higher than a few of its competitors. Most of the processing time of the FEW process is taken by the second step of the process, i.e., the cleaning step. Cleaning step parameters are the dominating factors that affect the processing time of the process. The cleaning step involves removing the coatings/impurities present on the bottom sheet of the materials to be joined while also pre- iii heating the friction element. The removal of coatings/impurities, however, can be accelerated with the use of abrasive particles. This study focuses on the effect of abrasive particles on the cleaning time and processing time of FEW. Silicon carbide abrasive particles have a high hardness and provide higher wear rates. The higher wear rates promote the wearing off of coatings from the surface of the materials. Silicon carbide abrasive particles were placed in a pre-drilled pocket in an aluminum top sheet. Design of Experiments (DOE) involved two levels of pocket size, pocket depth, abrasive particle size, and volume fraction of abrasives. The results show that abrasive particle size and volume fraction of abrasive particles were the dominating factors in determining the cleaning step time and overall processing time. Lower particle size and volume fraction of abrasives resulted in a reduction of cleaning time and processing time. Cross-tension strength (CTS) tests were performed, followed by microscopy analysis and hardness testing to study the effect of abrasives on the joint quality. The best case was observed for 6 mm pocket size, 0.2 mm pocket depth, 5 μm abrasive particle size, and 50% volume fraction of abrasives. The best case with abrasives was compared with the FEW sample which does not involve pocket and abrasives. The comparison showed that the inclusion of abrasives results in a reduction in cleaning time by 39.93% and processing time by 14.28%. The CTS of the joints formed with abrasives was slightly higher than the case without abrasives. Both the cases showed a button pull-out failure when subjected to CTS loading conditions. Microstructural analysis showed a presence of hard SiC and wider martensite phase, which is a probable reason for an increase in the joint strength for the joints that involved iv abrasives. The Microhardness tests further supported the CTS results. For the joints involving abrasives, a marginally higher hardness was observed along the cross-section. The significance of this study lies in the opportunities to reduce the processing time of the joining process using abrasive particles

High-power, highly-efficient thulium-doped potassium double tungstate channel waveguide lasers

The subject of this thesis is the development of 2-μm rare-earth lasers in thuliumdoped yttrium-gadolinium-lutetium-co-doped potassium double tungstate film layers. These thulium-doped layers were grown onto undoped potassium yttrium double tungstates by liquid-phase epitaxy and were lapped and polished afterwards, prior to a photo-lithographic process to define channel waveguides. Channels were subsequently obtained by argon-beam milling of the samples, resulting in ridge-type channel waveguides. During another liquid-phase epitaxy growth these channels were overgrown with a double tungstate cladding to obtain buried channel waveguides. The concentration of the co-dopants and the dimensions of the buried channel waveguide are chosen such that the overlap between pump and laser optical modes is maximised, whilst preventing lattice stress and cracking of the layers and ensuring single-transverse-mode operation at both the pump and laser frequency. The fabricated channels on multiple samples have a width of 7.5 − 25 μm and a height of 6.6 − 14.3 μm, and have thulium dopant concentrations of 1.5 − 20at.%. Laser experiments on the channel waveguides were performed by using a Ti:-sapphire laser near 800 nm as the pumping source. The channel waveguides were tested with different out-coupling transmission of up to 89%, provided by various combinations of butt-coupled dielectric mirrors, or an out-coupling transmission of up to 99% in case no mirrors were used. For a 1.5% thulium-doped channel waveguide, a threshold of 7 mW, a slope efficiency of 31.5%, and an output power of 149 mW were measured and a value for the propagation loss of 0.1 ± 0.03 dB/cm at the lasing wavelength of 2 μm were derived from relaxation-oscillation measurements. Laser experiments on channel waveguides with a higher thulium dopant concentration of 5at.% yielded a maximum slope efficiency of 53%. The optimum thulium dopant concentration was 8at.% which yielded a maximum slope efficiency of 81 ± 3%, which is close to the theoretical maximum for this laser of 83%. An output power of 1.6 W was obtained from this laser for 2.3 W of absorbed pump power. The high efficiency is a result of cross-relaxation which increases the maximum quantum efficiency for this laser to ⌘q = 1.94. For higher thulium concentrations of 12at.% and 20at.%, the maximum obtained slope efficiency was 60%. Depending on the out-coupling transmission selectable by the dielectric mirrors, the laser output wavelength was found to shift between 1840 nm and 2037 nm, as a result of the varied threshold inversion. By using a blazed diffraction grating in Littrow configuration, tuning of the laser output wavelength between 1810 – 1950 nm has been achieved

Cumulative Index to NASA Tech Briefs, 1963 - 1966

Cumulative index of NASA Tech Briefs dealing with electrical and electronic, physical science and energy sources, materials and chemistry, life science, and mechanical innovation

To perform a gyro test of general relativity in a satellite and develop associated control technology

A satellite configuration having two gyroscopes with axes parallel to the boresight of a telescope and two at right angles to the telescope and approximately parallel and perpendicular to the earth's axis is proposed for measuring geodetic precessions due to the earth's motion about the sun, higher order geodetic terms calculated from the earth's quadrapole mass moment (0.010 arc-sec/year in a 400 nautical mile polar orbit), and deflection by the sun of the starlight signal for the reference telescope. Data from the experiment also contain large periodic signals due to the annual and orbital aberrations of starlight which are useful in providing a built in reference signal of known amplitude for scaling the relativity signals, and should yield a singularly precise measurement of the parallax of the reference star. The development of the gyroscope and its readout system are discussed, as well as signal integration, drag-free control, and attitude control

Stabilization and Imaging of Cohesionless Soil Specimens

abstract: This dissertation describes development of a procedure for obtaining high quality, optical grade sand coupons from frozen sand specimens of Ottawa 20/30 sand for image processing and analysis to quantify soil structure along with a methodology for quantifying the microstructure from the images. A technique for thawing and stabilizing frozen core samples was developed using optical grade Buehler® Epo-Tek® epoxy resin, a modified triaxial cell, a vacuum/reservoir chamber, a desiccator, and a moisture gauge. The uniform epoxy resin impregnation required proper drying of the soil specimen, application of appropriate confining pressure and vacuum levels, and epoxy mixing, de-airing and curing. The resulting stabilized sand specimen was sectioned into 10 mm thick coupons that were planed, ground, and polished with progressively finer diamond abrasive grit levels using the modified Allied HTP Inc. polishing method so that the soil structure could be accurately quantified using images obtained with the use of an optical microscopy technique. Illumination via Bright Field Microscopy was used to capture the images for subsequent image processing and sand microstructure analysis. The quality of resulting images and the validity of the subsequent image morphology analysis hinged largely on employment of a polishing and grinding technique that resulted in a flat, scratch free, reflective coupon surface characterized by minimal microstructure relief and good contrast between the sand particles and the surrounding epoxy resin. Subsequent image processing involved conversion of the color images first to gray scale images and then to binary images with the use of contrast and image adjustments, removal of noise and image artifacts, image filtering, and image segmentation. Mathematical morphology algorithms were used on the resulting binary images to further enhance image quality. The binary images were then used to calculate soil structure parameters that included particle roundness and sphericity, particle orientation variability represented by rose diagrams, statistics on the local void ratio variability as a function of the sample size, and the local void ratio distribution histograms using Oda's method and Voronoi tessellation method, including the skewness, kurtosis, and entropy of a gamma cumulative probability distribution fit to the local void ratio distribution.Dissertation/ThesisM.S. Civil Engineering 201