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CERAMIC DEFENSE: Pressing with
Controlled Combustion
By Karthik Nagarathnam Ph.D.
June 1, 2006
A new high-pressure, combustion-driven compaction process is creating
high-density ceramic parts with improved properties for applications in
defense, energy and other commercial markets.
Above: A close-up of a 300-ton CDC press.
Forming high-density ceramic parts has traditionally presented a number
of challenges. Conventional powder pressing methods often include
quite large pieces of equipment and have many moving mechanical
parts and/or complex hydraulics that make the systems difficult to
maintain. Compaction densities are often lower than desired, and
additional multi-step post-processing, machining or finishing are usually
needed to achieve the final part specifications.1-5
Recently, a new process has been developed to overcome these
challenges. Called combustion-driven compaction (CDC), the technology
uses high pressures (>50 to 150 tons per square inch [tsi]) combined
with natural gas combustion to provide higher pressed green and
sintered part densities, a gentler dynamic loading cycle on the pressed
parts, net-shaped or near-net-shaped forming, faster processing times,
improved density and manufacturing flexibility, reduced part shrinkage,
the potential for nano/composite/multi-layered/functional gradient
materials (FGM) fabrication, improved performance, few or no postmachining/grinding requirements, and scalability to higher-capacity CDC
press sizes with the potential for automation/rapid fabrication, all in a
compact piece of equipment. 6-8
With these benefits, the CDC technology presents a new way to achieve
high-density parts with improved properties for applications in defense,
energy and other commercial markets.
Figure 1. A schematic of the CDC process. 6,7,8
Background
The CDC process uses a controlled release of energy from the
combustion of natural gas and air to compact a variety of ceramic,
metallic and composite powders (see Figure 1). The chamber is first
filled to a high pressure with a mixture of natural gas and air. As the
chamber is being filled, the piston or ram is allowed to move down to
pre-compress and remove entrapped air from the powder. The gas
supply is then closed, and an ignition stimulus is applied. The ignition
causes the pressure in the chamber to rise dramatically, further
compressing the powder into its final net shape.
Figure 2. A typical CDC compaction load. 6,7,8
The process can provide standard or very high compaction tonnages,
resulting in high-density parts with improved properties, in just a few
hundred milliseconds, as shown in Figure 2.
Figure 3. A 300-ton CDC press. 1-3
Unlike mechanical or hydraulic presses, which are typically quite large
and have many moving parts and/or complex hydraulics, the CDC press
is not much larger than a phone booth (see Figure 3), and it has just one
moving part. As a result, it is relatively easy to incorporate into an
existing production operation, and it is also easy to maintain.
Compaction Loading Cycle
As the compaction load to a powdered ceramic or metal is raised, the
part density and properties improve. If the powder is compressed too
rapidly, shock propagation in some materials can cause internal cracks
and separations (over-pressing). Unlike other forging or explosive
forming techniques, the CDC loading rate is much gentler and is highly
controlled, which provides advantages when compacting crack-sensitive
materials, such as brittle ceramic powders, difficult-to-press alloy
powders or composites.
The CDC press uses a two-stage fast (but gentle) loading cycle to
achieve a high density while avoiding shock propagation and defects in
the compacted part. In addition, the load sequence of the CDC
technology allows large tonnage loads to be applied without damage to
the press or die components. The initial gas-fill sequence aligns the ram
and die components while applying a sufficient load to pre-compress the
powder and remove entrapped air. When the fill gas is ignited, the ram
rapidly presses down without slamming into the tooling or powder. In
other words, although the process is fast and powerful, it is smooth and
continuous.
The CDC process routinely operates at compaction loads of 2069 MPa
(150 tsi). This is in sharp contrast to conventional compaction processes,
which generally are limited to 690 MPa (50 tsi). 1-4 One 300-ton CDC
press has operated for six years, indicating that the press has a
proprietary tooling/die life of more than a few thousand cycles.
Press Scaling
Since the CDC press directly converts chemical energy into compaction
energy, it is highly energetic and capable of producing enormous
compaction pressures in a moderate-sized piece of equipment. To date,
10-, 30- and 300-ton presses have been constructed and operated, and
a scaled-up 1000-ton CDC press is in the final stages of
design/assembly. A compact version of a 1000-ton CDC press for largescale part manufacturing (>1 in. diameter with loading pressures up to
150 tsi) can be further scaled up to 3000 tons if needed. Scaling from
one size to the next has been relatively straightforward. Since the
process works more or less like a piston in an automobile (albeit at much
higher pressures), the loads that can be produced are a direct function of
the combustion pressure and the area of the ram (piston) rather than the
overall size of the press. It is therefore possible to scale a CDC press to
very high tonnages without dramatically increasing the size of the press.
The relatively small size of the CDC press compared to traditional
hydraulic or mechanical presses can allow ceramic parts to be made in
almost any industrial or commercial building that has access to bottled or
piped natural gas, including "machining centers." Additionally, the ability
to upgrade cost-effectively to higher CDC tonnages (e.g., 1000 tons or
higher) provides the potential to rapidly scale up a manufacturing
process to meet increasing demand for a new product.
Figure 4. Net shape fabrication potential for aluminum nitride and Cu/AlN ceramic composite cylinders of various functional gradient
compositions (0.5 in. diameter). 2
Processed Geometries and Materials Behavior
In general, CDC compacted parts possess significantly higher green and
sintered densities at CDC pressures of above 50 tsi, which is usually the
threshold limit offered by traditional powder pressing methods.15
Materials that have been processed successfully using the new
compaction technology include pure metals (Cu, Al, W, Mo, Re, Ta and
Fe), alloys (316 SS, 410SS, FL-4400, FLN2-4405, 737SH and Re/Mo
alloys), layered and mixed materials (Al/Ti, SS/Mo, Cu/Ta, Al/Alumina,
SS/Ta, FL-4400/Cu, FL-4400/Al, Fl-4400/Ti and FL-4400/Ta), aluminum
nitride, nano-silicon carbide, nano-boron carbide, a ferrous alloy matrix
with ceramic nanocomposites in various shapes and geometries, multilayered parts made of steel/copper, copper/stainless steel/niobium, AlN,
and nanocomposite magnetic materials such as FeNi-NanoSiO2 (see
figures 4-6).
The CDC process routinely operates at compaction loads in the 150-tsi
range and above, which significantly improves the final quality of the
compacted part both in the green and sintered states. For example, the
FeNi-nanoSiO2 composite material compacted with the CDC technology
had a relatively higher magnetic permeability and lower eddy current
losses compared to a traditionally processed magnet at the tested
frequencies of MHz levels. Most materials processed with the CDC
technology have exhibited a superior surface finish, reduced shrinkage,
and improved mechanical and high-temperature durability properties
compared to parts pressed through conventional means.
Figure 4 illustrates the fabrication potential and technical feasibility for
net-shaped, well-bonded and crack-free AIN ceramic and copper/AIN
composites by high-pressure CDC compaction up to 100 tsi. The
powders used for fabricating these composite materials were relatively
fine (-625 mesh). The typical part size was about 0.5 in. diameter using
an existing die/punch assembly. These results highlight the technology's
potential for developing functional gradient composite materials (FGM).
Figure 5. CDC compacted ceramic discs and rings, which exhibited excellent bonding and high part densities. 3
Nanoceramics (Figure 5) and metal matrix nanocomposites consolidated
by CDC displayed much higher densities (see Table 1) compared to
traditional pressing methods, indicating relatively easier part handling
attributes. Silicon carbide discs made using submicron-sized powders
also indicated excellent densification responses, minimal grain growth
and reduced shrinkage (<50% lower than possible by traditional ceramic
processing methods) (see Table 2 and Figure 6).
Applications and Further Advances
CDC processing is an emerging manufacturing technology that can be
used to develop high-density and high-performance net- or near-netshaped parts for a variety of markets. Anticipated applications of CDCprocessed components include X-ray targets, laser optical mirrors,
projectiles, lighter and stronger armor tiles, cryogenic parts,
accelerator/RF microwave components, fuel cell/battery electrodes,
computer hard disk drive accessories, high-performance engine parts,
high-temperature nozzle liner parts, heat sinks, tooling inserts,
nanostructured composite magnets, gears, bearings, biomedical
components, welding/water jet machining nozzles/electrodes, and
wear/corrosion resistant tribological components.
Figure 6. Crack-free CDC SiC discs pressed at 150 tsi and sintered at 2100ºC for ~30 min. under flowing argon.
Additionally, lightweight and durable ceramics and composites made
from boron carbide, silicon carbide, silicon nitrides or borides or
combinations of composite alloys are highly desirable for increasing
needs in defense applications, as well as in a number of energy and
other commercial applications.
The preliminary results are encouraging, and significant potential exists
for further evaluating and exploring this new fabrication method.
For more information about the CDC process, contact Karthik
Nagarathnam at UTRON Inc., 8506 Wellington Rd., Suite 200,
Manassas, VA 20109; (703) 369-5552, ext. 111; fax (703) 369-5298; email karthik@utroninc.com ; or visithttp://www.utroninc.com .
Author's Acknowledgements
Project funding for the CDC Manufacturing and Materials
Development Program at UTRON Inc. is mostly from the Small
Business Innovative Research (SBIR/STTR) project awards funded
by U.S. Department of Energy-SBIR and DOD SBIR/STTR Programs,
such as the Innovative Science and Technology Office of the
Ballistic Missile Defense Organization (presently MDA) and the U.S.
Navy/ONR. Technical assistance and continued support from
UTRON's CDC manufacturing team is also acknowledged, including
Don Trostle, Jason Zielsdorf, David Kruczynski, Lester Via, Chuck
Martin, Aaron Renner, David Heldt and Dennis Massey.
Links
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Karthik Nagarathnam's e-mail
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UTRON Inc.
Karthik Nagarathnam is senior materials scientist at UTRON Inc., Manassas, Va.