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One factor that has hindered the growth of composites, however, is high temperatures. The thermal environment in space and aerospace applications is not always conducive to the use of composites, due to the composite resins melting at very high temperatures. Furthermore, shock, impact or repeated cyclic stresses can cause the laminate to separate at the interface between two layers, a condition known as delamination.
Seizing the Zirconia OpportunityWith opportunities arising from the application of composite materials in harsh environments, what can be done? One solution lies with zirconia, according to Zircotec, a UK-based ceramic coating manufacturer. “The latest zirconia-based ceramic coatings help protect surfaces from failing, allowing composites to be used instead of heavier materials such as stainless steel or bulky heat shields,” explains Andy McCabe, technical director of Zircotec. “Shields and wraps can, after all, reduce overall performance and cancel out the composite’s weight-reducing properties.”
High-temperature plasma-sprayed ceramic coatings provide lightweight, easily packaged and highly durable thermal barriers suitable for a wide range of highly aggressive environments. Zirconia has a thermal efficiency of less than 1.7 W/m K (compared with 4 W/m K for alumina), creating a coating that is very effective at inhibiting the radiation of heat from a surface. Tests for a typical application have shown a reduction in composite surface temperature of more than 125ºC (see Figure 1).
Zirconia is very useful in its “stabilized” state and is an effective thermal barrier coating in jet turbine and other high-performance applications. Elsewhere, zirconia is used as a refractory material in insulation, abrasives, enamels and ceramic glazes.
Localizing the Ceramic CoatOne new engineered coating* enables thermodynamic management parameters to be varied and controlled in all three dimensions. This allows significant thermal protection with only minimal weight gain, as low as 0.03 g/cm2 for some applications. The process, for which a patent application has been filed, offers the opportunity to “engineer” the coating to suit specific customer requirements, adjusting the coating properties both through thickness and across the surface of a component to cope with “hot spots” and differing forms of heat transfer such as radiant, conductive or convective heating.
“A reflective surface layer coating can be applied to help protect against radiant heat, or the thickness of the ceramic can be increased in certain areas where a hot spot can occur,” says McCabe. “We also have the ability to build in a conductive sub-layer that will help to dissipate heat away from any hot spots and help deal with transient heating situations. This means that just the right amount of coating can be applied to deliver the necessary protection while minimizing its weight impact.”
*Developed by Zircotec, Oxfordshire, UK.
Applying the CoatTo ensure optimum results, a technical team works alongside clients to specify the most appropriate coating arrangement for a given application. “We ensure that by combining computer optimization with a range of coating parameters we can offer a solution to any thermal issue,” claims McCabe.
The “engineered” multi-layer solution can be adjusted to suit the operating environment, preventing delamination or structural failure. Based on proprietary technology,** the coating is designed to be robust and is highly resistant to vibration and mechanical damage.
The deposition technique uses an optimized high-temperature plasma spraying process. The surface is first prepared to clean the substrate and give it a key to enhance adherence of the coating. Then a special bond coat is applied, also using the plasma spray. This is a key stage, as the bond coat accommodates the high thermal expansion and contraction that the component will experience in its working cycle, contributing to its high durability.
**ThermoHoldTM, developed by Zircotec, Oxfordshire, UK.
Durable Good ValueMany industries have found thermal barrier coatings to be of excellent value compared to alternative solutions. “There is no additional fitting time required, while wraps and heatshields can take several hours,” explains McCabe. “Physical barrier solutions can, in some cases, require frequent realignment, unlike the completely maintenance-free coatings.” Because of the ceramic coating’s inherent qualities, life expectancy is extended and resources and finances can be released for other research and development activities.
Even coating metallic surfaces has allowed more composites to be used safely. “Formula One teams discovered that by coating the exhaust components of their cars, carbon fiber composites can be used much closer to the exhaust than would otherwise be possible, allowing a greater number of components to be made from carbon fiber and reducing the need for heat shielding,” says McCabe.
What's Next?With applications in many of the world’s most advanced jet engines increasing, further demand for ceramic coatings is expected from aerospace airframe builders and engine makers who are forever in the pursuit of weight reduction and packaging optimization. “We believe that this is the only process of its type available commercially, and with extremely positive test results, the opportunities for composite applications of ceramic coatings are endless,” concludes McCabe.
For additional information regarding high-temperature ceramic coatings, contact Zircotec at 528 10 Unit 2, Rutherford Ave., Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0QJ, UK; (44) 01235-434326; fax (44) 01235-434329; e-mail firstname.lastname@example.org; or visit www.zircotec.com.