Advanced Ceramics' It's All in the Polymer

January 1, 2002
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A new liquid, single component, thermosetting SiC precursor polymer is providing the ability to safely and efficiently manufacture a broad range of advanced materials.

The new polymer produces high yield, high purity silicon carbide ceramics.
Can the fabrication processes that yield advanced ceramics actually be made easier? Most of those who have been in the industry for a while might say “not very likely.” The processes have long been costly and unwieldy, with less than desirable yields and purity levels for silicon carbide (SiC) and silicon nitride (Si3N4) ceramic materials. Moreover, product manufacturers have become accustomed to the fact that because these advanced materials are so hard, they are extremely difficult to machine, making them costly and limiting their use to only those high-tech applications, like aerospace, that cannot exist without them.

However, a broad number of industrial markets and numerous emerging high-tech sectors now value materials that resist wear and corrosion, withstand high temperatures, are lightweight yet have high compressive strength, and are extremely hard. This is evidenced by the fact that in the last decade alone, worldwide sales of advanced ceramic materials grew 63% to become a $34 billion market. Projected now to grow 7-20% annually, the advanced ceramics market will probably be one of the fastest growing high-tech sectors of the economy in this century.

Over the past two decades, new processing technologies focused on silicon carbide-based polymers have been developed that can significantly improve the manufacture of higher performing and environmentally friendly materials.* A team of materials scientists and polymer chemists working to solve problems for ceramic manufacturers in processing and product yield learned that if the pre-ceramic polymer could be made to perform in new ways, many of the problems encountered by manufacturers could be eliminated. As those problems were solved, the potential emerged to also break down barriers to even wider use of advanced ceramics.

“In the late 1980s, when Starfire Systems was founded, existing alternative silicon carbide fabrication was not readily scaleable and high production costs inhibited widespread acceptance. Problems in the fabrication, machining and material quality of SiC-based ceramics could only be overcome by marked improvements in the ceramic-forming polymers available at that time. As our product teams designed customized fabrication process solutions, we focused R&D on defining the requirements for a more effective pre-ceramic polymer—one that that would make processing easier, achieve higher ceramic yields, and protect the manufacturer and environment. Then we commercialized the solution,” said Richard Saburro, Starfire’s president and CEO.

The result was a liquid, single component, thermosetting SiC precursor polymer that provides the ability to safely and efficiently manufacture a broad range of advanced materials.

Requirements of an Effective Polymer

To be effective, a pre-ceramic polymer has to do several things well. It needs sufficient fluidity to allow full penetration of the fiber preform, such as a woven or non-woven mat of carbon or ceramic fibers. The precursor should be adjustable in composition to permit functioning as a fiber bond coating, thus avoiding the need for complex and costly chemical vapor deposited fiber interface coatings and treatments. It should also have flexible molecular weights and viscosities to permit functioning as a ceramic binder for sintering, a vehicle for ceramic coatings and as a “glue” for joining ceramics. In order to retain the precursor within the confines of the preform and maintain structural integrity during pyrolysis, it also needs the ability to “turn on” some sort of crosslinking process (curing process) after infiltration.

Additionally, the ceramic yield should be as high as possible, ideally exceeding 75% by mass at all processing temperatures with very few, and preferably environmentally benign, byproducts. Finally, for most applications, where high thermal conductivity, good oxidation stability and resistance to high temperature creep are key issues, the silicon carbide produced should be as close to stoichiometric (one-to-one match of carbon and silicon atoms) as possible, so as to provide a crystalline silicon carbide matrix with minimal amounts of impurity phases, such as unreacted carbon, silicon or silicon oxide.

While a couple of ceramic-forming polymers have become available that produce silicon nitride-based ceramics or silicon oxycarbide ceramics, none can produce high-purity silicon carbide at the required ceramic yield, and all require secondary curing agents—all, that is, except for a new SiC pre-ceramic polymer.

Figure 1. The SiC pre-ceramic polymer provided high ceramic yields at all processing temperatures.

The SiC Pre-Ceramic Polymer

The new polymer,** developed through extensive research, has a nearly one-to-one carbon-to-silicon ratio, minimizing excess carbon after pyrolysis, which can degrade high temperature stability. It is non-toxic and can be chemically altered at the molecular level to tailor viscosity and other properties for use as coatings, composite matrices and high-temperature adhesives. It is fired in low-cost, reusable steel molds under inert gas and is the only pre-ceramic polymer that produces high yield, high purity silicon carbide ceramics (see Figure 1).

Processing requires no solvents, and no chlorides, acids or corrosives are generated during firing, which protects both the manufacturer and the environment. Harmless hydrogen gas is the only byproduct of pyrolysis, allowing a very high 80-85% yield of silicon carbide ceramic. Finally, ceramics can be fabricated to near net shape capacity, saving on machining costs and, with controlled porosity adjustable from 35% to helium-tight, opening up new opportunities in the pollution control industry.

Applying the Polymer in Novel Ways

In addition to using the standard SiC pre-ceramic polymer to safely and efficiently produce high-quality advanced ceramics, small modifications can open up even more opportunities for ceramic manufacturers.

Sintered Materials with Improved Density and Minimal Shrinkage. For sintered ceramics, a binder system and densification aid that is a variant of the SiC pre-ceramic polymer has been developed to enable pressureless sintering after extrusion or cold pressing. The polymer, in combination with a selected powder, such as silicon carbide, is all that is needed to form the desired ceramic. Manufacturers can eliminate the use of sintering aids and solvents since the viscosity of the binder can be tailored to the application. Finally, the binder cures at 200-400?C, resulting in a very strong and machinable green part. Because of these characteristics, more complex shapes are possible.

Figure 2. Key composite properties of a SiC pre-ceramic polymer/Hi-Nicalon fiber composite.
Composites and Coatings with Ultra-High Temperature Stability. For manufacturers trying to achieve composites and coatings with ultra high-temperature stability, the SiC pre-ceramic polymer can be used as a SiC forming binder in combination with selected refractory ceramic and metal powers to create a highly effective thermal and oxygen diffusion barrier. Unlike conventional chemical vapor deposition (CVD) coating techniques, this system† is a “paint on and fire” process using an inert gas furnace at 1100?C for firing. Once densified, composites using this system have demonstrated the ability to withstand up to 10 minutes of exposure to a MAPP gas-oxygen torch at temperatures in excess of 2800?C with minimal effects. In comparison, a 3 mm-thick piece of sintered silicon carbide without the system exhibited burn-through in 30 seconds (see Figure 2 and Table 1).

Table 1.
In these applications and more, the new SiC pre-ceramic polymer is at the core of breaking down barriers that once made the manufacture and application of advanced ceramics difficult. As progress in pre-ceramic polymers continues, the difficulties will disappear, and what was once hard to achieve will become easier. It’s all in the polymer.

For More Information

For more information about the new SiC pre-ceramic polymer and processing applications, contact Starfire Systems, Inc., at 877 25th St., Watervliet, NY 12189; (518) 276-2112; fax (518) 276-3069; e-mail sherwoodw@starfiresystems.com; or visit http://www.starfiresystems.com.

*These technologies were developed by Starfire Systems, Inc.
**The SP-Matrix Polymer, supplied by Starfire Systems.
†The Starfire 2800 System, supplied by Starfire Systems, Inc.


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