CERAMIC DEFENSE: Pressing with Controlled Combustion

June 1, 2006
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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 post-machining/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 large-scale 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.1-5 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, multi-layered 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-net-shaped parts for a variety of markets. Anticipated applications of CDC-processed 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 2100ÂșC 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; e-mail karthik@utroninc.com ; or visit http://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.

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