Solid Freeform Fabrication from Gas Precursors Using Laser Processing
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Principal/co-Principal
Investigator.
* PI/co-PI Name(s): Harris L. Marcus
* PI Institution: University of Connecticut, Storrs
* PI Phone Number: (860) 486-4623
* PI Fax Number: (860) 486-4745
* PI Street Address: 97 North Eagleville Road
* PI City,State,Zip: Storrs, CT 06269-3136
* PI E-mail Address: hmarcus@mail.ims.uconn.edu
* PI URL Home Page: http://www.ims.uconn.edu/metal/faculty/marcus.htm
* Grant/Contract Number: N00014-95-1-0978
* Period of Performance: 07/01/98 - 06/30/99
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Productivity
Measures.
* Number of refereed papers submitted not yet published: 1
* Number of refereed papers published: 3
* Number of unrefereed reports and articles: 6
* Number of books or parts thereof submitted but not published: 0
* Number of books or parts thereof published: 1
* Number of project presentations: 10
* Number of patents filed but not yet granted: ?
* Number of patents granted and software copyrights: ?
* Number of graduate students supported >= 25% of full time: 4
* Number of post-docs supported >= 25% of full time: 0
* Number of minorities supported: 1
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Summary of Objectives
and Approach.
Three research areas are addressed in this report: Selective Area Laser Deposition Vapor Infiltration (SALDVI), Selective Area Laser Deposition (SALD) for in-situ device fabrication, and Selective Area Laser Deposition (SALD) for joining of ceramics with SiC filler material.
A. SALDVI:
Selective Area Laser Deposition Vapor Infiltration (SALDVI) is the direct fabrication of ceramics, metals, and composites from computer representations using gas precursors, powder layers, and laser scanning. SALDVI shapes can be monolithic or multiple material, and can contain functional embedded devices such as in-situ thermocouples. Our objective is to develop a better theoretical understanding of the SALDVI process and to assess the ability of the process to produce functional tailored ceramic and composite structures for various applications including shapes with embedded devices. Previously we have experimentally investigated the laser chemical vapor infiltration (LCVI) of silicon carbide from tetramethylsilane Si(CH3)4 gas using a CO2 laser (10.64 mm wavelength), including the effects of gas pressure, the surface temperature, and the laser scanning speed. Here we examine in more detail the time evolution of the SALDVI workpiece and we quantify the amount of infiltration as a function of the processing conditions using image analysis.
Our research approach is to fabricate test samples using starting powders that vary in particle size, shape, packing fraction, thermal conductivity, and optical reflectivity, absorptivity, and transmissivity at the CO2 and Nd:YAG wavelengths. The amount of infiltration that occurs for each processing condition is quantified using image analysis. That is, the areal density distribution is mapped in the powder layer as revealed by metallographic cross-sectioning and optical microscopy. These results are used to select an optimal system for further investigation and to guide the fabrication of more interesting geometries.
B. SALD of Graphite, SiC and Si3N4 for In-situ Device Fabrication:
The goal of this research is to demonstrate the feasibility of embedding in-situ sensors with a ceramic matrix using the Selective Area Laser Deposition (SALD) technique. SiC/C thermocouple sensor was selected to be embedded within a SiC or SiC/Si3N4 composite matrix and electically insulated with the silicon nitride layers. To achieve the proposed goal, a systematic methodology was applied in this study. First, the various chemical systems related to the thermal vapor deposition were thermodynamically modeled by direct minimization of the total Gibbs free energy method. Second, the detailed experiments were designed to evaluate the validity of the theoretical predictions and to fundamentally understand the SALD process such as effects of processing parameters on morphology, microstructure, and composition of the deposits. Third, based on the understanding achieved during the first and second stage of this study, the in-situ thermocouple sensor device was fabricated and functionally tested.
C. SALD for joining of ceramics:
This research focuses on SALD Joining of silicon carbide structures with silicon carbide filler material. A variety of SiC substrates were examined, as were various conditions of gas precursor pressures and mixtures, laser beam scan speed and joint configuration. The SALD material was characterized for composition and structure by x-ray diffraction, transmission electron microscopy and nuclear magnetic resonance. Successful joints were measured for mechanical bend strength, hermeticity of the joint seal and microhardness of the filler material. The SALD joints were evaluated based on comparisons to monolithic tube properties.
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Summary of Technical
Progress.
A. SALDVI:
SALDVI test samples were fabricated from tetramethylsilane gas and more than 15 starting powders (carbides, nitrides, oxides, metals) varying in their physical properties. Also, powders of different size were mixed to vary the packing fraction and surface area. Laser scanning was performed in the gas atmosphere using different temperature distributions and local heating times. Image analysis was used to measure the variations in the amount of infiltration with position in the samples for each powder and processing condition. A comparison of the measured infiltration profiles shows that the properties of the starting powder influence the vapor infiltration in the powder layer. The variations in the measured infiltration profiles are being correlated with the variations in the physical properties of the starting powders to obtain a general predictive model of the process. The results show that solid densities of more than 90 percent can be obtained in a region within 250 microns of the free surface of the powder layer under certain processing conditions.
B. SALD of Graphite, SiC and Si3N4 for In-situ Device Fabrication:
It was found that the experimental results are in excellent agreement with the theoretical predictions. With the use of acetylene (C2H2), tetramethylsilane (TMS), and a mixture of TMS and ammonia, graphite, SiC, and Si3N4 were successfully deposited, respectively. Strong temperature dependency of the SALD products in morphology, composition, crystal structure and size, growth kinetics and relevant properties were revealed. The predicted carbon co-deposition and the role of hydrogen gas in eliminating this co-deposition in SiC or Si3N4 were experimentally confirmed by a Raman scattering study. The functional test on the fabricated device showed that the embedded SiC/C thermocouples exhibited stable and repeatable response to temperature variations. Thus, the overall results indicated that it is feasible to embed the in-situ sensors within a ceramic matrix using the SALD technique.
C. SALD for joining of ceramics:
SALD Joining produces silicon carbide joints by depositing silicon carbide filler material that adheres to the substrates. Preheating the silicon carbide substrate during the SALD deposition process is a controlling parameter for producing high density, high purity, defect-reduced silicon carbide filler. The preheating reduces the thermal gradient occurring during the joining process. SALD tube joint structures show greatly reduced mechanical bend strengths compared to monolithic tube standards. The origins for the lower strength are the initial crack found at the joint seam of the butt-end tube configuration and the reduction in the cross-sectional area of the SALD filler deposit compared to the full tube cross-sectional area due to the poor ‘throwing power’ of the laser associated with localized CVD. The SALD gas-phase decomposition of tetramethylsilane, with and without hydrogen, deposits nano-crystalline silicon carbide. Characterization of the SALD silicon carbide filler material shows deviations from the expected beta-polytype in the XRD, NMR and TEM patterns that are attributed to twinning faults. This is the first evidence of twins in SALD silicon carbide but is consistent with the very low stacking fault energy for silicon carbide.
The SFF laboratory was designed and built to provide full SALD and SALDVI deposition system capabilities. Three such systems are operational at the present time.
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Transitions and DOD
Interactions.
none
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Software and Hardware
Prototypes.
1. Prototype Name: SALD and SALDVI System Prototypes
+ URL:
+ Availability: Available in SFF laboratory, Institute of Materials Science, UCONN
+ Description: Hardware and software of performing gas phase SFF of structures and infiltration of powder layers.
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Honors, Prizes,
Awards, or Promotions Received.
none
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URLs.
UCONN SFF website: http://www.ims.uconn.edu/~hmarcus/
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Current Students and
Recent Graduates Supported by ONR.
1. Name: Shay Harrison
+ US Citizen/Permanent Resident: US Citizen
+ Thesis: SALD Joining of SiC with SiC Filler Material (adv. Marcus)
+ Graduated: May, 1999
+ Job: Material Scientist, Delphi Automotive Systems, Indianapolis, IN
2. Name: Lianchao Sun
+ US Citizen/Permanent Resident: China
+ Thesis: Fundamental Studies on Selective Area Laser Deposition of Graphite, Silicon Carbide and Silicon Nitride for In-Situ Device Fabrication (adv. Shaw)
+ Graduated: July, 1999
+ Job: Post Doctoral Fellow, Brown University
3. Name: James Crocker
+ US Citizen/Permanent Resident: US Citizen
+ Thesis: Solid Freeform Fabrication Using the SALDVI Technique
+ Graduated:
+ Job: Research Assistant
4. Name: Erik Geiss
+ US Citizen/Permanent Resident: US Citizen
+ Thesis: Laser Processing of “photonic” Single Crystals for Micro/Macro System Design
+ Graduated:
+ Job: Research Assistant
5. Name: Helene Ansquer
+ US Citizen/Permanent Resident: France
+ Thesis:
+ Graduated:
+ Job: Visiting Master of Science Intern investigating SALD of layered structures
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Sabbatical Plans.
none
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Related Research
Projects.
SALD and SALDVI of borides in an international collaboration with Dr. Adel Francis of the Central Metallurgical Research and Development Institute in Cairo, Egypt. This research is being performed under the Joint Fund Program between Egypt and the United States.
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Miscellaneous.
Application of laser processing to create “photonic” single crystals for micro/macro system design.
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