Cost is not the only inhibitor to wider use of automotive carbon composites. Their inability to cope with high temperatures is also of serious concern. Locating carbon-composite parts within tightly packed engine bays, in close proximity to turbochargers, exhaust aftertreatment components and even to the high-voltage battery packs of EVs, presents challenges.
Even extended heat soak, both during vehicle travel (with airflow management mitigating thermal effects) and also when stationary, can cause the materials to delaminate.
By comparison, aluminum components typically operate reliably at 550-600°C and steel up to 900°C. But depending on formulation, carbon composites’ thermal limit is way below these figures, at around 260°C, explains Graeme Barette, a Director at Zircotec, a U.K.-based heat management specialist.
The company is now working with OEMs to incorporate thermal barrier technology initially used by the nuclear industry, to support wider carbon-composite applications in vehicles.
“Within the auto industry there is recognition that measures must be taken to protect carbon-composite panels in the region of the hot exhaust exit," Barette noted, "but there are also many other areas of the vehicle that can exceed the comfortable working temperature of the material.”
He explained that while composites with good mechanical properties are widely available to suit the lower range of temperatures, "developing materials suitable for over 260°C leads to compromised mechanical strength and increased manufacturing difficulty."
Reducing the temperatures by increasing the air gap between the composite and the heat source, or introducing conventional heat shielding, increases weight and impinges on available packaging space required. It may also involve unwelcome or unacceptable design changes, he told Automotive Engineering.
To overcome these challenges, Zircotec has developed a patented ceramic thermal coating designed to provide a barrier to carbon composite components. The new coating permits their “safe and reliable” use at temperatures above the crucial 260°C limit, Barette claimed, and preserves the weight saving advantage of a composite without incurring packaging penalties.
He described the coating as a proprietary plasma-spray process, originally developed for the nuclear industry. Zircotec, which has worked extensively in motorsports heat management since the 1990s, is now working with OEMs to provide the coating to production vehicles.
Automation of the process facilitates its use not just for high-performance vehicle applications but if required, also in large volume production. The process of protecting any carbon-composite surfaces requires an initial assurance that they are free of any other coating. The surface of the composite part is then prepared to accept a bond coating to ensure secure adhesion between the ceramic and the substrate.
Barette explained that this is followed by the application of the ceramic barrier while controlling feedstock type and composition, feed rate, plasma gas composition and flow rate, energy input, torch geometry, nozzle design, nozzle offset distance and substrate cooling.
Trapped air particles within the ceramic layer help to contribute to the thermal insulation of the component. Total coating thickness can be as little as 200 μm (200 micron) unless a thicker coating is required, he adds.
Because the coating specification can be tailored to the specific application, a variety of surface finishes are possible. For example, an EV battery surround can receive the application of a flameproof finish with either conducting or non-conducting properties to suit the electrical strategy. For external parts the system has been designed to maintain a class-A surface finish with a wide range of color choice.
The ceramic coating can withstand temperatures up to 1400°C, Barette claimed, is highly resistant to vibration and mechanical damage, and can tolerate significant flexing of the substrate. He stresses that ceramic thermal barrier coatings aren’t just a fix for thermal issues arising during vehicle development; they can enable design and manufacturing goalposts to be moved, bringing a potential step-change in technology.
The ceramic coating has been comprehensively validated across a range of projects. Stated Barette: “Although Zircotec can handle many of the tests required, most OEM customers prefer to incorporate them into their in-house test regime. The durability of coated composite components must often be demonstrably equal to that of the heavier metal parts replaced. This involves an exhaustive range of tests that meet OEM standards in addition to thermal management, from adhesion through to chemical resistance.”
The widening range of more subtle thermal management solutions – not just for carbon composites – indicates a change of engineering emphasis.
In the past, Zircotec would often be consulted by an OEM late in a vehicle program when thermal issues arose during its development. But this is changing, explained Barette. By that phase there may be no time or budget to achieve a solution. Now, automakers are incorporating thermal barrier technology as an integral part of their design philosophy, allowing delivery of a better optimized vehicle, often with a lower mass than would otherwise have been possible.