To anyone who has been in the ceramic industry for any length of time, the idea of “low-cost silicon nitride (Si₃N₄) ceramics” is an oxymoron. The raw materials and manufacturing process required to make Si₃N₄ ceramics are inherently expensive, leading to high costs for the manufacturer and an extremely pricey end product. For this reason Si₃N₄ ceramics have been limited to only the most elite, high-tech applications—applications for which the users are willing and/or able to pay the price for Si₃N₄ strength and performance.

Recently, however, a new technology has been developed that can significantly lower the cost of manufacturing Si₃N₄ ceramics. This technology offers manufacturers of silicon nitride and aluminum oxide (Al₂O₃) ceramics the potential to broaden their market share and explore a variety of applications that were previously unattainable.

The New Si₃N₄ Technology

The new technology was developed over a period of 20 years at Eaton Corp. in Southfield, Mich., for high-volume automotive operations. In 1999, the company granted James Edler, head of the Si₃N₄ R&D operation, an exclusive worldwide license for the technology. Edler immediately created a new company, SiNeramics Inc. based in Ferndale, Mich., to further develop the technology and explore potential new markets.

The biggest difference between the new technology and conventional Si₃N₄ and Al₂O₃ manufacturing is the raw material, says Edler, who serves as president and CEO of the new company. The technology uses low-cost silicon metal as its base material, rather than the conventional silicon nitride or aluminum oxide powders. The cost savings from the raw material alone is as much as 90 to 95% for current manufacturers of Si₃N₄, and as much as 50% for manufacturers of Al₂O₃ (see Figure 1).

According to Edler, there’s nothing notable about the silicon metal material itself—it’s the formulation that makes it successful. “Companies originally made silicon nitride using silicon metal, but they couldn’t get it to sinter properly and they had problems with the nitridation cycle being highly exothermic, which caused problems in their furnaces. For these reasons, the bulk of researchers stopped making silicon nitride from silicon metal and started making it instead from chemically derived silicon nitride powders—a material that was much more expensive but gave them the results they needed,” Edler says. “The problem with that route is that the cost never came down. Silicon nitride powders, and the rest of the processing that goes with them, have always been extremely expensive.”

The new technology is based on water processing and the use of a low-cost cerium oxide additive, which enables manufacture similar to aluminum oxide ceramics. The recipes are provided through a licensing agreement with SiNeramics. Like conventional Si₃N₄ and Al₂O₃, the material is balled milled, and the resulting slurry is spray dried into a powder form. The powder is then pressed into a green shape and nitrided and/or sintered at under 1800°C—a lower temperature than what is required for conventional silicon nitrides—with conventional furnace technology and with little or no over-pressure during sintering. The finished blank can then be ground, if necessary, to complete the final product.

The process was designed to be extremely similar to aluminum oxide processing (see Figure 2). “Any company that makes aluminum oxide already has most of the technology it needs to manufacture silicon nitride ceramics, so it would be really easy to switch over,” Edler says. The only new equipment required would be a furnace to handle the nitridation or sintering operations, depending on the required strength of the final product. (Nitridation yields a Si₃N₄ product with a slightly lower density [75-85%] compared to sintering [98-100%].) Current manufacturers of Si₃N₄ would already have all of the required machinery in place (with the exception of the nitridation furnace). For companies that don’t want to get involved in the material end of the process, the raw powder or billets of material could be provided in a ready-to-manufacture form.

“It’s really a simple process,” says Edler.

Market Opportunities

Extensive testing on components produced with the new technology has shown that the performance of the end product is equal to that of most conventional Si₃N₄ components and, in many cases, far exceeds that of conventional Al₂O₃ components (see Table 1), allowing the technology to be used in a range of applications.

The high durability, thermal shock resistance and creep resistance of Si₃N₄ make it ideally suited for refractory applications in alumina and steel manufacturing plants. For instance, rather than making furnace nozzles and valves from an expensive Si₃N₄ or a cheap refractory with a short lifespan, the new technology can be used to manufacture a relatively inexpensive Si₃N₄ that will increase profits for the Si₃N₄ manufacturer and lower maintenance and replacement costs for the aluminum or metal producer. Tests are also under way to evaluate the use of the Si₃N₄ technology in refractory bricks and other high-temperature applications. In the past, conventional Si₃N₄ has been far too expensive for these types of applications.

Silicon nitride is also impervious to aluminum, making it ideal for aluminum casting applications. And because it can be threaded and machined to intricate shapes and sizes, it can also be used for brazing fixture components and other intricate applications.

The high performance and low cost of the new technology opens a number of market opportunities that were previously reserved for lower-cost metals, such as ball bearings, automotive and engine components, cutting tools, biomedical and electrical products, and fluid handling components. Comparisons between high-performance steel and Si₃N₄ ceramic ball bearings for roller blades, for instance, have shown a significant improvement in spin time with the Si₃N₄ bearings (see Figure 3).

Testing for over 500 hours on natural gas valve seat inserts for engines showed a significant reduction in wear compared to metal inserts (see Figure 4). Additionally, engine valve train components exhibited no wear or surface fatigue after 5000 hours of testing at 2000 cam RPM, and tests are ongoing in a variety of other areas. “The applications of this technology are really only limited by the user’s imagination,” says Edler.

In short, the technology enables Si₃N₄ to become a serious consideration for just about any ordinary, average refractory or ceramic application. “Silicon nitride is no longer for the exclusive world of the turbine engine, fighter jet and others who can afford to pay high prices,” says Edler. “With the new Si₃N₄ technology, it’s now available to the masses.”

For More Information

For more information about the new Si₃N₄ technology and licensing opportunities, contact SiNeramics, 1980 E. Nine Mile Rd., Ferndale, MI 48220; (248) 542-6756; fax (248) 542-6784; or e-mail sineramics@sineramics.com. The company also welcomes visits to its Ferndale manufacturing facility.

SIDEBAR: Si₃N₄ Bearings Lead to Roller Hockey Championships

Real-life tests of Si₃N₄ ceramic ball bearings in roller hockey skates have made several Michigan roller hockey teams firm believers in the technology. During the Winter Indoor Nationals Championships held by NARCh (North American Roller Hockey Championships), four teams—Tour Venom, Easton Detroit, Detroit Iron Horse and Michigan State University—wore skates with the Si₃N₄ ceramic ball bearings, and all four teams won the championship in their division.

James Edler had the opportunity to witness the Michigan State game. “I was sitting in between the Michigan State and Altoona fans, and you could hear them going back and forth trying to figure out what had changed,” Edler says. “It was a 9-1 match, with Michigan State beating Altoona, and this was the score by the second period. Michigan State had played the day before with different skates, and it had been a 3-3 tie. It was such a remarkable difference.”

The skates have also led to victories for numerous speed skaters.