• SALDVI
  • Powder Bubbling
    • disruption of the uninfiltrated loose powder particles by a stream of gas was visually observed under certain SALDVI conditions
    • the gas flow was attributed to natural buoyancy convection caused by density gradients in the gas due to the steep temperature gradients induced by localized laser beam heating
    • a simple model showed that the powder levitation depends on the particle diameter, particle density, gas viscosity, temperature gradient, and layer thickness
    • larger and denser powder particles eliminated the observed powder bubbling
  • Powder Preheating
    • a resistance heater was used to preheat a 20 mm-diameter powder cavity
    • temperatures from 300 to 400(C were measured at the powder surface, depending on the gas composition
    • preheating reduces temperature gradients during SALDVI
  • Ramping of laser power
    • the capability to automatically control the rate at which the laser power and temperature increase or decrease at the beginning or end of a scan was instituted
    • ramping of the laser power and temperature minimizes the effects of thermal shock during SALDVI
  • Addition of secondary gases to primary SiC precursor gas
    • for a given laser beam size and SiC source gas partial pressure, the width of a SALDVI line was experimentally observed to vary with the addition of secondary gases (Ar, He, H2, and N2). Reducing the line width would enable fabrication of shapes with finer features
    • the severity of powder bubbling was also experimentally determined to depend on both the type and quantity of the secondary gas
    • a modeling effort is being developed that considers the gas thermal conductivity, viscosity, and diffusivity, as well as the temperature distribution in the SALDVI workpiece/powder bed to better understand these phenomena
• SALD SiC Material Analysis
  • X-ray diffraction: beta silicon carbide with peak broadening indicating a nano-cyrstalline grain strucutre.
  • Nuclear magnetic resonance: spectra points to a mixed morphology of alpha and beta SiC.
  • Transmission electron microscopy: supports nano-crystalline morphology. Shows presence of twins that provides explanation for NMR spectra presence of alpha SiC.

• The goal of this research effort is to join together ceramic parts by using filler material from the SALD gas-phase reaction. This method of fabrication is a patented process. The flexibility of SALD allows for monolithic ceramic joint structures to be formed without any processing additives that can lower the joined sample's use temperature.
  • Research approaches:
    • Use of butt-end and beveled tube section of different compositions: silicon carbide composite fiber tubing, clay-bonded silicon carbide tubing, high purity/high density silicon carbide tubing(i.e. Morton CVD, Hexoloy).
    • Forming laminar-type deposition to improve joint strength and sealing. Achieve by varying rotation speed and transverse velocity of rastered beam, use of a pre-heat laser.
    • Evaluate the mechanical strength of joints by bend testing, the hermetic seal of the joint with a vacuum system and the bonding of the deposit to the tube substrate by microscopy.
• Progress to date: Over 20 tube joints have been fabricated to varying degrees of mechanical strength and seal quality.

• Theoretical deposition maps have been determined using a modified CET 89 free energy minimization computer program. The thermodynamically stable solid products from gas-phase environments are calculated over a specified temperature range at a specified total pressure. The devised maps provide guidance in selecting precursors, precursor ratios and deposition temperatures.

Detailed Summary of Technical Progress

The SALD/SALDVI, 4 inch by 4 1/2 inch PDS SFF system and the SALD Joining system are fully operational and functioning in a continuous mode. Two more systems are sharing a third hood, one examining the SALD deposition of SiC, Si3N4 and C and the other looking at photolytic decomposition of gas precursors for SALD.

The SALDVI work continues to focus on fabricating SiC bulk shapes by infiltration of SiC powder particles with vapor deposited SiC derived from tetramethylsilane (TMS) gas. Progress in SALDVI involved improvements to process control as well as expanding the overall understanding of the process through experiments and theoretical modeling. Process control was enhanced to allow automatic control of the heating and cooling rate whenever the laser beam is turned on or off. The capability to preheat the powder bed and SALDVI workpiece was also added. A theoretical description of buoyancy convection relevant to typical SALDVI geometries and operating conditions was developed. This theoretical description provides insights into the experimentally observed powder bubbling phenomenon. Additional theoretical modeling is being investigated relative to the effect secondary and inert gases have on properties such as effective process zone size, degree of infiltration, and the structure and chemistry of the vapor deposited material. Examples of single and mulitple SALD/SALDVI silicon carbide/carbon thermocouples in a silicon carbide matrix, along with their temperature and emf responses, can be found at the UCONN website .

SALD Joining serves as the other main research focus in the SFF laboratory. SALD Joining attempts to use the SALD deposit to act as a filler material to join ceramic parts together, similar to metal welding. Investigation on this project has yielded several successfully joined silicon carbide tube structures using silicon carbide deposited filler from the gas-phase pyrolosis of tetramethylsilane. Adhesion of the deposit to the tube surface remains excellent. Bend testing of joined samples produced poor strength values, likely due to the butt-end nature of the joint geometry. This leaves an existing 'crack' at the joint seam the length of the tube wall. Beveling of the tube ends has become a central focus to achieve stronger joints, not only to reduce the initial crack length but to also increase the surface area of tube/deposit adhesion interface. The hermetic quality of the several joints was examined, compared to a monolithic standard of the base tube used. The SALD joints held a steady-state vacuum(in Torr) within a factor of 10 of the vacuum held by the standard. Improvements in the deposit morphology, from a billowy, porous nature to a more laminar deposition, are expected to improve these hermeticity values. Several examples of silicon carbide tubes connected by the SALD joining method can be found at the UCONN website .

Transitions and DOD Interactions.

Software and Hardware Prototypes.

Thermocouple and Joining Samples

List of Publications.

Non-refereed Papers

1. "Gas-Phase Selective Area Laser Deposition(SALD) Joining of SiC Tubes with SiC Filler Material" by S. Harrison and H.L. Marcus,Proceedings of the 1998 Solid Freeform Fabrication Symposium, Austin, TX, August, 1998.
2. "Preparation and Properites of In-Situ Devices Using the SALD and SALDVI Techniques" by J.E. Crocker, L. Sun, L.L. Shaw and H.L. Marcus, Proceedings of the 1998 Solid Freeform Fabrication Symposium, Austin, TX, August, 1998.
3. "Gas(SALD) and Gas/Powder(SALDVI) Solid Freeform Fabrication" by J.E. Crocker, S. Harrison, L. Sun, L.L. Shaw and H.L. Marcus, JOM, to be published

Refereed Papers

1. "Gas Phase Solid Free-Form Fabrication and Joining of Ceramics", by K.J. Jakubenas, J.E. Crocker, S. Harrison, L. Sun, L.L. Shaw and H.L. Marcus, Naval Research Reviews, to be published
2. "In-Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques: Parts I. Thermochemical Modeling" by L. Sun, K.J. Jakubenas, J.E. Crocker, S. Harrison, L.L. Shaw and H.L. Marcus, Materials and Manufacturing Processes to be published in issue #1, vol. 14, 1998.
3. "In-Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques: Parts II. Evaluation of Processing Parameters" by L. Sun, K.J. Jakubenas, J.E. Crocker, S. Harrison, L.L. Shaw and H.L. Marcus, Materials and Manufacturing Processes to be published in issue #1, vol. 14, 1998.
4. "In-Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques: Parts III. Fabrication and Properties of the SiC/C Thermocouple Device " by L. Sun, K.J. Jakubenas, J.E. Crocker, S. Harrison, L.L. Shaw and H.L. Marcus, Materials and Manufacturing Processes to be published in issue #1, vol. 14, 1998.

Invited and Contributed Presentations.

1. Workshop on Layered Manufacturing, Oxford, England, June, 1998, Invited Speaker
2. "Gas-Phase Selective Area Laser Deposition(SALD) Joining of SiC Tubes with SiC Filler Material", 1998 Solid Freeform Fabrication Symposium, August 10th 12th, 1998, Austin, TX
3. "Preparation and Properites of In-Situ Devices Using the SALD and SALDVI Techniques", 1998 Solid Freeform Fabrication Symposium, August 10th-12th, 1998, Austin, TX
4. "Properties of SiC/C Thermocouple Device Made with SALD and SALDVI Techniques", 1998 American Ceramic Society Meeting, May 3rd -May 6th, 1998, Cincinnati, OH
5. "Selective Area Laser Deposition of Silicon Nitride", 1998 American Ceramic Society Meeting, May 3rd -May 6th, 1998, Cincinnati, OH

Honors, Prizes or Awards Received.

Project Personnel Promotions.

Project Staff.

• Dr. Kevin Jakubenas, post-doctoral fellow from November, 1996 to February, 1998. Dr. Jakubenas' doctoral research focused on SALD of titanium dioxide and silicon dioxide along with selection methods for SALD gas precursors.

Multimedia URL.

1. EOYL FY98
2. QUAD FY98
3. SALD Joining Video

Keywords.

1. Gas-Phase Solid Freeform Fabrication
2. Selective Area Laser Deposition (SALD)
3. Selective Area Laser Deposition Vapor Infiltration (SALDVI)
4. Embedded Thermocouple Fabricated by SALD/SALDVI
5. Ceramic Joining by SALD
6. Laser-induced Gas Reactions

Business Office

• Business Office Phone Number: (860)486-1729
• Business Office Fax Number: (860)486-1727
• Business Office Email: grtadm18@uconnvm.uconn.edu

Expenditures

1. Est. FY98: 100%
2. FY97: 100%
3. FY96: 100%
4. FY95: 100%
5. FY94: 100%

Current and Former Students

1. Name: Mr. Shay Harrison
   • Position: Graduate Research Assistant
   • Nationality: United States
   • Task: Ceramic joining project, including design of the joining system and performing deposition experiments
   • Thesis: SALD Joining of SiC with SiC Filler Materials
   • Graduated: PhD, 1st Quater, 1999
2. Name: Mr. James E. Crocker
   • Position: Graduate Research Assistant
   • Nationality: United States
   • Task: Embedded sensor project, focusing on the SALDVI deposition of the silicon carbide matrix
   • Thesis: Making Silicon Carbide Shapes by Selective Area Laser Deposition Vapor Infiltration
   • Graduated: Masters, December, 1997
3. : Name: Mr. Lianchao Sun
   • Position: Graduate Research Assistant
   • Nationality: China
   • Task: Embedded sensor project, focusing on the SALD deposition of the thermocouple lines and insulating layers
   • Thesis:
4. : Name: Mr. Erik Geiss
   • Position: Graduate Research Assistant
   • Nationality: United States
   • Task: Design and implementation of a photolytic deposition system
   • Thesis:

Book Plans

1. Title: Proceedings of the 1998 Solid Freeform Fabrication Symposium edited by D.L. Bourell, J.J. Beaman, H.L. Marcus, R.H. Crawford, and J.W. Barlow
   • Publisher: University of Texas at Austin
   • Publication Year: 1998
   • Type of Publication: SFF conference proceeding
   • Additional Support: No

Sabbatical Plans

Related Research

1. The Concurrent Design and Manufacturing Simulation Laboratory (CDMS Lab), Dept. of Mechanical Engineering, University of Connecticut
2. University of Texas at Austin
3. Massachusettes Institute of Technology (MIT)
4. Rutgers University
5. Stanford University

History

• Dr. Marcus has been involved in the solid freeform fabrication research field for over 10 years, beginning with his involvement is selective laser sintering(SLS) investigations at the University of Texas at Austin. Gas-phase SFF is a tangent from the SLS work, using gas precursors as the source of material to form a desired object. The initial gas-phase work dealt with the deposition of carbon and silicon carbide by straight SALD. Iterations of the SALD approach lead to SALDVI, the vapor infiltration of the solid reaction product into consecutive layers of powder, and SALD joining, the use of the reaction product to 'weld' together parts. Dr. Marcus left the University of Texas at Austin in the summer of 1995 to become the director of the Institute of Materials Science(IMS) at the University of Connecticut. At IMS, Dr. Marcus began a new gas-phase program, building up a fully equipped laboratory and launching the SALD/SALDVI thermocouple program and the SALD ceramic joining effort. Future investigations will focus on inhomogenous chemical deposition to form chemically varied, gradient structures.