A Refractory Coating From Outer Space

A new coating originally developed by NASA for space applications promises to significantly extend the life of refractories while also decreasing the energy use of kilns and other heat-related processes.

A coated ceramic tile undergoes rigorous testing at Wessex’s lab.
“Thermal shock” and “chemical attack” are two phrases certain to strike fear into the hearts of any kiln operator or refractory engineer. No two elements can more quickly degrade refractories and cause them to fail. With most of today’s refractories, a heating and/or cooling rate of greater than 100?F/hour can cause significant refractory damage. Likewise, the use of common fluxing additives (such as fluorspar [CaF2], B2O3, NaOH, etc.) in a process, whether intentional or unintentional, can have a major detrimental effect on refractories, causing accelerated refractory wear/melting and significantly reduced lining life.1

But what if refractories could be treated to make them impervious to thermal shock and chemical attack? How many repairs and rebuilds could be avoided, and how much money could be saved on downtime, if refractories could be made to last twice as long as they do now without erosion or damage?

A new coating developed by the National Aeronautics and Space Administration (NASA) promises to do just that. The coating uses the principle of emissivity (the absorption and release of energy) to resist thermal shock due to thermal cycling, thereby significantly extending refractory life. It is also impervious to chemical and acid attack, enabling it to be used in harsh environments. And it provides a much-needed side benefit—the ability to significantly enhance the efficiency of thermal systems and thereby provide a great deal of energy savings in ceramic manufacturing and other heat-related processes.

Two silicon dioxide-based ceramic tiles were tested using an oxy-acetylene torch (hot enough to cut metals). The untreated tile on the left melted and turned to glass beads within 30 seconds. Under the same conditions, the tile on the right, which was treated with the protective coating, was tested for two minutes and showed little damage. Photos courtesy of NASA.

Bringing the Technology Down to Earth

On March 22, 1994, researchers at Ames Research Center, a division of NASA located in Moffatt Field, Calif., patented a “protective coating for ceramic materials,” or PCCM, that was invented and refined over a period of 10 years to be used as a key element in the thermal control structure of the next generation of space shuttle orbiters (X-33, X-34, etc.).* Designed to protect the orbiters from the extreme temperature of atmospheric re-entry, PCCM could be reused and repeatedly cycled between severe extremes of subzero temperatures (-250?F) and over 2900?F without any damage to the coating or the underlying substrate. In fact, each time the coating was cycled between those extremes, it rearranged its molecular structure to cause stronger bonding to the substrate and an even greater level of thermal protection.

Like many other technologies developed by NASA for use in outer space, PCCM was destined for a range of practical uses within earth’s atmosphere. Through NASA’s technology transfer program, the coating was licensed to Wessex, Inc., of Blacksburg, Va., in 1996 for applications on building materials and structures. According to Dr. John Olver, president of Wessex, the company first focused on applications for steel and wood products, particularly for firewalls in buildings. But as researchers at Wessex further developed and tested the coating,** they began to realize that a number of other materials could also benefit from its thermal protection properties. In February 2001, Wessex expanded its license from NASA to cover all areas except aerospace.

“The obvious application was for ceramics,” Olver says. “We began working with it on certain ceramic refractories, from rigid to non-rigid, and discovered that it worked very well.”

Other potential applications include electronic components, automotive components—“really any application where heat, wear and thermal degradation are factors,” Olver says.

Figure 1. Emissivity data on a silicon carbide fabric. Note how the emissivity of the coated fabric increases with increasing temperature. Data set courtesy of NASA Ames Research Center, U.S. Patent No. 5,296,288.

How the Coating Works

The key to the coating’s thermal performance is that it provides increased hemispherical emissivity—the ability to absorb energy and subsequently release that energy back out from the surface. Emissivity is rated on a scale of 0.0 to 1.0, with 0.0 being the lowest rate of emissivity (least absorption and release), and 1.0 being the highest (complete absorption and release). PCCM provides a known emissivity range from 0.65 to 0.90. In contrast, aluminum has an emissivity ranging from 0.22 to 0.31 at best, and stainless steel has an emissivity of about 0.11.

Figure 2. As radiant heat waves bombard the surface of the coating, the waves bounce around the ceramic molecules and are then absorbed by the emissivity agent. The energy imparted on these molecules causes them to vibrate, creating an effective thermal barrier.
The coating’s emissivity is also a function of temperature—the coating’s performance actually increases in hotter temperatures (see Figure 1). Once the coating has dried, some of the ceramic material migrates to the surface, enclosing the emissivity agent, as shown in Figure 2. As radiant heat waves bombard the surface of the coating, the waves bounce around the ceramic molecules and are then absorbed by the emissivity agent. The energy imparted on these molecules causes them to vibrate. The higher the temperature, the faster the vibration of the molecules, and thus, the more effective the thermal barrier because less heat is transferred to the underlying material. Due to the loss of the energy that is imparted on the molecules, the re-released heat is emitted at an elongated wavelength, resulting in a lower temperature coming back out from the surface.

The coating also decreases the catalytic efficiency of the surface, resulting in a decreased surface temperature and protecting the underlying substrate from heat-related damage. Additionally, because less energy is transferred to the substrate, the coating increases the efficiency of the system by preventing heat transfer and dissipation. And because the coating was originally developed for extreme speeds (up to 14,000 miles per hour) in space applications, it is extremely resistant to wear and abrasion.

The coating is predominantly composed of silicon dioxide, which is widely used in ceramic materials, but the addition of emissivity agents changes the nature of the coating so that it cannot accurately be described as a ceramic coating. Ceramics are known for their tremendous temperature resistance capabilities, but they generally absorb heat energy without releasing energy back out from themselves. This combination of high-temperature materials with emissivity agents yields a superior thermal barrier system capable of withstanding temperatures up to 3000?F, depending on the emissivity agent used in the formulation. Five different emissivity agents are available under Wessex’s license, and the company has modified its formulations to produce a range of 10 different coatings, each formulated for a specific temperature range and type of substrate (rigid, flexible, porous, etc.).

Due to its high silicon dioxide content, the coating has a very low thermal expansion rate. From room temperature to 1090K (~1502?F), the rate is 1.04 x 10-6 cm/ft2. However, the coating can be formulated for use on materials that have much higher thermal expansion rates so that it does not lose adhesion when the underlying material contracts or expands.

The coating can be used on any type of material, whether rigid, flexible or fabric, and can be applied in a number of ways, including brushing, spraying and through the use of doctor blades (blades commonly used to apply an even layer of inks or coatings to a surface). Although the surface of the substrate must be at ambient temperature during the coating process, the coating typically dries within one to two hours, depending on humidity and temperature conditions. According to Olver, a single 0.08-mm-thick coat is all that is required to yield the coating’s thermal and chemical protection benefits.

Practical Applications

Through partnerships with companies such as Danser Inc., Parkersburg, W.Va., and Unifrax Corp., headquartered in Niagara Falls, N.Y., Wessex has been instrumental in the development of new high-temperature, extremely durable ductwork linings, furnace linings and other refractories using the new coating. The coating has also been used to repair refractories in kilns, furnaces and other heat-related applications.

In one application, for instance, a company was experiencing hot spots in one section of its steel ductwork. “The company shut the furnace down, took that section of pipe out and brushed the protective coating on the existing ceramic lining. Then they reinstalled that section of ductwork and started the furnace. Now the ductwork is cool to the touch,” Olver says.

In another application, the material was used to coat a foundry ladle. It improved the heat retention of the ladle and prevented slagging from occurring of the metal to the ceramic interior of the ladle, thus increasing the life of the ladle refractory.

In addition to protecting refractories against hot spots and thermal cycling, the coating has also been used successfully in applications where acids are present in the off-gases. “Acids are deadly on refractories,” Olver says. “The protective coating actually seals the surface of the refractory and makes it impervious to acids.”

According to Olver, one of the greatest potential benefits to the industry is the ability to reduce the amount of raw materials necessary for insulating thermal systems, thus increasing interior volumes while decreasing the size of the system, as well as raw material cost. Additionally, maintenance needs are reduced as the insulation’s lifespan is increased, further reducing the overall cost of the system. And energy costs can be significantly reduced due to the emissivity characteristics of the coating. Wessex is currently quantifying the projected energy savings based on a full-scale test of the coating recently installed in a brick kiln.

While the coating isn’t cheap—it averages from $.50 to $2.50 per sq ft of applied area, depending on the substrate and the type of coating that is used—Olver says that most companies can expect to see a payback within two to three years. The higher the temperatures and the harsher the environment, the faster the payback. “The coating works well at 1600?F, but when you get up around 2000∞F and above, that’s when its benefits really start to emerge,” Olver says.

“What we’ve heard from companies that already have it out in the field is that just from the acid standpoint alone—not even considering the heat savings and the other benefits—they’re going to increase the life of their ductwork anywhere from eight to 10 years with this coating. We’ve done some preliminary calculations on applications such as rebricking electric arc furnaces, and our feeling is that by putting this on the interior of the brick in critical areas—it doesn’t need to be applied everywhere—we can increase the life of those furnaces by at least two or three years. And you can compare that to the cost of taking down the furnace and rebricking it, as well as lost production, and figure out how much you can save,” Olver says.

Olver believes that in most cases, the coating will at least double the life of any refractories on which it is used—but because the coating is so new to this field, no definitive studies have been done to determine at what point the coating will need to be replaced. “We do know from all the work done at NASA that the bond between the coating and the underlying substrate actually improves over time, and that the coating is durable enough to withstand re-entry into earth’s atmosphere at 14,000 miles per hour. What we haven’t been able to determine is what, if anything, would eventually cause the coating to fail in actual field use," Olver says.

Wessex is currently investigating a range of applications for this high-tech coating. According to Olver, the sky—or, in this case, the ionosphere—is the limit.

“Right now, we are primarily working with refractory manufacturers who are willing to work with us, but we’re also working directly with some ceramic manufacturers and other end users to extend the life of their existing refractories,” Olver says. “We’ve also been working with a lot of people from around the world on composites, from aircraft to electronic components, and we’ve recently been working with the military to modify some of our coating formulations for use as protective coating for aluminum and steel in military applications. Additionally, we are working with racing teams to use the material on various components—such as wheels, brakes, and certain engine components—to protect the driver from heat. Most of these applications have been full-scale field-tested with a great deal of success.

“It is a great material with unlimited potential,” Olver adds.

For More Information

For more information about this new coating, contact Dr. John Olver at Wessex Inc., 201 Church St., Suite D, Blacksburg, VA 24060; (540) 961-0999; fax (540) 961-5808; or visit http://www.wessexinc.com.

For more information about NASA’s technology commercialization program, contact Phil Herlth, technology commercialization manager, NASA Ames Research Center - Commercial Technology Office, MS 202A-3, Moffett Field, CA 94035-1000; (650) 604-0625; e-mail pherlth@mail.arc.nasa.gov; or visit http://technology.arc.nasa.gov.

*U.S. patent no. 5,296,288
**The coating is sold commercially as Emisshield Industrial® and Emisshield Spacetec®.

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