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Although the UK’s enthusiasm for nuclear power has waned, the spark of creativity that its development fostered still glows. Originally developed for nuclear applications, a durable, heat-resistant ceramic coating* has found increasing uses in the automotive industry, the medical sector and beyond.
Since the internal combustion engine is a thermodynamic system rendered in metal, thermal management is fundamental to automotive engine design. Ceramic coatings have been used in the automotive field before, to retain heat within specific sub-systems or to protect components and operators from excessive temperatures. More durable and simpler to package than wrap, today’s road vehicles require more efficient methods of heat management-and ceramic coatings are the answer.
*Thermohold, available from Zircotec, Oxfordshire, UK.
Cabin FeverMany of the original customers for these coatings were in the motorsport sector. “We were approached by Prodrive [which runs the Subaru World Rally Team] back in 1994,” says Andy McCabe, technical director of Zircotec, a ceramic coating manufacturer headquartered in Oxfordshire. “Their drivers were getting hot. We found that putting our coating on the exhaust pipe-not only under the bonnet, but running under the entire length of the car-helped lower cabin temperature by between 6 and 8ºC.”
From this initial application, word spread about the benefits of using ceramic coatings, and positive word-of-mouth led to the use of the system in touring cars and, subsequently, Formula One. In addition to driver comfort, coating hot components offers other advantages. For Formula One teams, one of the major benefits comes about because of the formula’s extensive use of carbon fiber composites. These composite components are very light but can be susceptible to delamination when exposed to high temperatures.
To solve this problem, a new technique has been developed to plasma spray coat composites and laser sintered nylon. Formula One teams have found that by coating the exhaust components of their cars, carbon fiber composites can be reliably 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. This also gives designers greater flexibility to design vehicles that are as aerodynamic as possible, with the bodywork following the underlying structures much more closely.
Thermal ManagementManaging heat can also improve reliability. Reducing the amount of heat escaping around the powertrain can increase the durability and effectiveness of engine and transmission oils, improve engine cooling, and help to protect ancillaries such as regulators, wiring and ignition systems from the degrading affects of excessive heat. The effect of coating the exhausts extends beyond simply protecting heat-sensitive components-ensuring that thermal energy stays where it is supposed to can help engines perform as they were designed. Retaining heat energy in the exhaust system means that the gases within the exhaust move faster, reducing backpressure on cylinders and helping to increase engine efficiency.
Protecting air intakes from locally generated heat also means the engine receives denser, more oxygen-rich air, which helps boost performance further. A 5ºC drop in air intake temperature tends to boost power output by around 1%. The 30ºC reductions seen by customers mean that 6% rises in horsepower are possible.
When Litchfield, an importer and tuner of Japanese performance cars, decided to create its own version of the Subaru Impreza, performance and an OEM appearance were its two main goals. Litchfield found that coating the up- and down-pipes to and from the turbocharger helped keep the entire engine bay cool. An added benefit was that more of the energy in the hot exhaust gases leaving the engine reaches the turbo rather than being radiated away from the up-pipe.
“The more heat you can keep in the system, the faster the turbo will spool up, making the car more responsive,” says Managing Director Iain Litchfield. “We’re finding that it’s bringing the turbo up to speed 300 or 400 rpm sooner, which you can really feel in the crispness of the throttle response.”
Perfect FinishThe use of the coating has not been restricted to the futuristic technology of cutting-edge motorsports, however. The cream finish that the coating exhibits is becoming popular with restorers of the classic racing cars of the 1950s and 60s, which would have originally featured asbestos-based paints as a safety feature on their exposed exhaust pipes. Users have found that the coating offers a superior level of heat shielding and greatly improved durability without impacting the original appearance of the vehicle.
A variety of finishes have been developed for the coating, including black, dark grey and silver. The silver, for instance, has been used to coat the manifolds of a classic Aston Martin DB4 to protect the rest of the engine bay. Modern fuels tend to generate much higher temperatures than the blends used in the 1960s, meaning that classic components are being subjected to much greater thermal loads than would have been anticipated at the time of their design. The coating helps ensure that the finish of a carefully restored engine bay is retained for many years to come.
Cut the WrapMany of the benefits derive not just from the thermal insulation properties of the zirconia-based coating, but also from the application method. Rather than being painted on, the ceramic is ionized in a high-powered electric arc so that fine, molten particles can be sprayed onto a surface. The result is a coating that adheres much more effectively to the parts being sprayed than the wraps and paints used in the past.
In order to create a highly resilient coating, all parts are first degreased and shot-blasted to provide a clean and predictable surface to spray on. A nickel-based coat is plasma-sprayed to give a rough surface so that the ceramic coating can adhere as firmly as possible. In addition, this bond coat also minimizes the thermal mismatch between the substrate and the ceramic coating. The final result is a highly uniform coating of between 300 and 350 microns in thickness, with around a third of that made up by the undercoat. The effect on the component’s weight is an additional 1.6 kg per square meter.
The current spraying process is labor intensive and requires complex extraction, as well as strict health and safety controls for the operators. Huge growth in the medical market (a similar coating is used for coating medical implants like replacement hip and knee joints), has led to robotized spraying. With the expansion into OEM road car and commercial vehicle markets, a new robotized spray booth has been commissioned that will come on stream later this year.
Not Just AutomotiveA spraying method was developed that enables the production of free-standing ceramic components in complex shapes, including components used in the manufacture of optical fibers. The fibers are created by drawing glass through an electromagnetic induction furnace at high temperatures. The ceramics are used to create the susceptors that conduct the furnace’s heat through to the glass. This requires the ability to withstand thermal shock, since the components are heated to around 2000ºC and allowed to cool again.
The technology’s first use outside the nuclear industry was for medical implants. Starting with the same coating technology, it was discovered that orthopedic implants formed a more reliable join to re-growing bone if they were given a rough, textured coating first. The durability of the biocompatible titanium coating gives reassurance when using it over such long periods in such a sensitive environment.
Another coating was developed that incorporates hydroxyapatite, a substance similar to human bone. A textured layer of hydroxyapatite creates a structure that bone can grow into, creating an even stronger bond between the implant and the damaged bone.
Road Car ApplicationsApplications in the vehicle market have been varied. Several high-performance car manufacturers are adopting the technology to prevent heat from the exhaust pipe from damaging surrounding materials. The high-performance Koenigsegg from Sweden is the latest example (see sidebar).
For the diesel engine market (both road car and commercial vehicles), the ceramic coating can offer more precise temperature control, helping particulate filters reach optimum operating temperature more quickly and improving system performance. By significantly reducing heat loss from the exhaust system, the coating retains high exhaust gas temperatures, reducing warm-up times for after-treatment systems and the need for close coupling. This allows for more consistent control of exhaust gas temperatures. The technology is expected to be particularly useful for keeping particulate filters at their operating temperature.
“Maintaining sufficient heat in the after-treatment system is an increasing challenge, especially as catalysts and filtration stack up, allowing efficient working temperatures to be maintained,” says McCabe.
Keeping the thermal energy in the exhaust stream can also significantly improve turbo response, which makes the coating ideal for selective catalytic reduction (SCR) diesel exhaust systems. The coating protects surrounding components from the high temperatures reached in the catalyst converter and removes the need for separate heat shields.
New derivatives (including different surface finishes and colors) are under development with auto manufacturers clearly in mind. With several concept cars using the ceramic coating as an alternative to some traditional brightwork, the technology may well find additional uses in the near future.
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.org.
SIDEBAR: Keeping the Swedes CoolKoenigsegg’s £540,000 (~ $1.1 million) CCX (Competition Coupe X) supercar is the latest vehicle to benefit from using the ceramic coating for its exhaust manifold. The CCX delivers incredible performance, dispatching 0-60mph in 3.2 seconds with an 806 bhp engine helping the car achieve over 395 km/h.
Installing the 4.7 liter engine in a sleek, aerodynamic carbon fiber body means package space is at a premium. The result is a very restricted engine bay with sensitive components and the painted composite parts close to the exhaust. The CCX also incorporates a new exhaust system with catalysts moving nearer to the engine (to improve light-off time), further increasing temperatures under the hood.
Koenigsegg needed a solution that would dramatically reduce temperatures to safeguard electrical components and the composite bodywork. Limited space meant exhaust wrap was not feasible, and it would be unsightly for such a highly aesthetic engine bay. The engineers turned to spray coatings in order to achieve a robust and effective solution that would satisfy rigorous OEM quality standards.
“The Zircotec coating offered us an immediate solution and a substantial improvement over the coating we were already using,” says Koenigsegg Chief Operating Officer Jeff Stokes. “Plus, it has more significantly reduced under-bonnet temperatures.” As a niche performance car manufacturer, the adoption of the ceramic coating provided other benefits to Koenigsegg. “There was no tooling investment for ordinary heat shields, it is low weight and it needs minimal package space,” adds Stokes.