Fiscal Year 2001 Annual Summary Report
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 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: 07/01/99 - 06/30/00
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Project Attributes.
* Number of refereed papers/book chapters published: 1
* Number of refereed papers/book chapters to appear: 1
* Number of books published: 0
* Number of unrefereed reports and other articles: 3
* Number of project presentations: 4
* Number of patents granted and software copyrights: 0
* Number of patents filed but not yet granted: 0
* Number of graduate students supported >= 25% of full time: 2
* Number of post-docs supported >= 25% of full time: 0
* Number of minorities supported: 0
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Summary of Objectives and Approach.
Selective Area Laser
Deposition Vapor Infiltration (SALDVI) is the direct fabrication of ceramics,
metals, and composites from computer representations in which loose powder
layers are densified with solid material deposited from gas precursors into the
pore spaces using chemical vapor deposition and laser heating. SALDVI shapes can be monolithic or multiple
material, and can contain functional embedded devices such as in-situ thermocouples. Our overall objective is to develop a
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) and
Nd:YAG lasers (1.06 mm wavelength), including the effects of gas pressure, the surface
temperature, the laser scanning speed, and a range of starting powders. The time evolution of the SALDVI workpiece
was experimentally investigated for simple single-layer bar geometries by
quantifying the distribution of vapor infiltrated material into the powder bed
as a function of the processing conditions using image analysis. The fabrication of SiC cermets by the vapor
infiltration of SiC into various metal powders was also investigated. The microstructures of SALDVI cermets
fabricated with Cu, Ni, and Mo powders were investigated with x-ray diffraction
(XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy
(EDS), and optical microscopy at a range of processing histories. The formation
of intermetallic silicide phases was observed due to reactions between the Cu
and Ni powders and the vapor deposited SiC matrix. The mechanical properties of three-layer bar geometries were
measured by four-point bend for a range of processing conditions. Recent experimental efforts have focused on
the processing issues related to defect formation in multiple-layer bar and
rectangle geometries. A computer
simulation of the SALDVI process was also developed using finite element
modeling to theoretically predict the infiltration history from the processing
variables and the physical properties of the powder and CVD matrix.
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Summary of Technical Progress.
Experimental results
Silicon carbide has been
deposited SALDVI from the gas precursor tetramethylsilane, Si(CH3)4,
into loosely packed powder layers of SiC, ZrO2, WC or Mo. Layered samples were fabricated for each
powder material using both single line (bar) and multiple line (rectangle)
laser scan patterns and 10 Torr Si(CH3)4, 2.5 mm/s scan speed, 1000°C target temperature, and 120 mm layer
thickness. Samples of SiC and ZrO2
are prone to surface cracking in the bar geometry, and cracking and
delamination of layers in the rectangle geometry. Samples fabricated with Mo powder have no cracks or delamination
defects in either bar or rectangle geometry as well as a better surface
appearance. Figure 1 shows a Mo powder/SiC matrix rectangle with 6 layers (a)
as-fabricated and (b) cross-section across layers.
Thermal stress plays a
major role in cracking in bar and rectangle samples of SiC matrix/SiC powder
and SiC matrix/ ZrO2 powder made by SALDVI. The effective thermal conductivity (keff
)of the SALDVI workpiece correlates well with the severity of thermal
cracking. The lower keff the
more severe the cracking. SiC matrix/Mo
powder samples have the highest keff, and show no cracking. The irregular shape of the SiC and ZrO2
powders, as well as a higher surface roughness, could provide stress risers in
the matrix during processing, increasing their susceptibility to thermal
cracking compared to the spherical Mo powder.
The type of powder was
found to affect the surface appearance and internal structure of SiC matrix
multiple layer SALDVI rectangles.
Samples with SiC and ZrO2 powder show a porous surface
appearance due to displacement of the powder during processing. Mo and WC powder samples have a dense
surface and continuous solid material across adjacent layers. The lower density of the SiC and ZrO2
powders and convective effects in the gas are the likely cause of their poor
structure.
Figure 1. Mo powder/SiC matrix rectangle with 6
layers (a) as-fabricated and (b) cross-section across layers.
Simulation results
A 3D finite element model was developed that simulates
SALDVI of silicon carbide. The model predicts the laser input power and the
distribution of vapor deposited SiC within the powder bed as well as on the
surface of the powder bed (SALD). The model includes closed-loop control of the
laser power to achieve a desired target processing temperature on the top
surface of the power bed. This model considers a moving Gaussian distribution
laser beam, temperature- and porous-dependent thermal conductivity, specific
heat and temperature-dependent deposition rate. A schematic of the model is
shown in Figure 2. The simulation
results compare well with experimental data as shown in Figure 3.
Figure 2. Finite element model of the SALDVI
process.
Figure 3. Comparison between the experimental and
simulated solid fraction distribution in depth direction under the center of
the laser beam after the scanning process at x=7 mm.
1) The simulation
results of the incident laser power history and the distribution of vapor
infiltrated SiC in the powder bed agree fairly well with the experiments.
2) As the laser scanning rate decreases,
the incident laser power increases.
3) As the laser scanning rate decreases, the deposition of
SiC both within the powder bed as well as on the surface of the powder bed
(SALD) increases (Figure 3).
4) Uniform temperature on the top surface
of powder bed during laser scanning process needs non-uniform laser power at
the initial scanning period and nearly uniform laser power elsewhere.
These simulation results offer guidelines for further
experimental studies of the SALDVI process.
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Participation on Other Research Projects.
High Temperature X-ray
Diffraction Studies of Palladium Oxide/Palladium Couples.
Funded by Connecticut Innovztions.
Inc.
SFF of Photonic Crystals,
ONR
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Transitions and DOD Interactions.
<...include contact information too...>
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Software and Hardware Prototypes.
1. Prototype Name: SALD and SALDVI System Prototypes.
+ 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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Current Students and Recent Graduates Supported by ONR.
1. Name: James Crocker
+ US Citizen/Permanent Resident: US Citizen
+ Thesis: Solid Freeform Fabrication Using the SALDVI
Technique
+ Graduated:
+ Home Page:
+ Job: Graduate Assistant
2. Name: Erik Geiss
+ US
Citizen/Permanent Resident: US Citizen
+ Thesis: Laser Processing of "Photonic" Single
Crystals for Micro/Macro System Design
(ONR Photonic Crystal SFF Grant)
+ Graduated
+ Home Page:
+ Job: Graduate Assistant
3. Name: Haoyan Wei
+ US Citizen/Permanent Resident:
+ Thesis: Laser Processing of Materials
+ Graduated:
+ Home Page:
+ Job: Graduate Assistant
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Sabbatical Plans.
none
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Related Research Projects.
Application of laser
processing to create “photonic” single crystals for micro/macro system design.
Multiple material laser
densification applied to dental restorations.
Selective Area Laser Deposition (SALD) joining of silicon nitride silicon carbide in an international collaboration with Dr. Abrahim Ghayad of the Central Metallurgical Research and Development Institute in Cairo, Egypt.
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Miscellaneous.
<...any suggestions/research directions/contacts/sbir
topic/muri topics/etc...>
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