除了成本高昂外，耐热性差也是导致碳复合材料未能在汽车领域得到广泛应用的一大原因。发动机舱布局紧凑，要在涡轮增压器、尾气后处理元件或高压电池包附近布置碳复合材料，难度很大。

虽然行车时的气流管理可以减轻热效应，但一旦行车和驻车的暖机时间过长，碳材就可能出现分层。

Zicrotec经理、英国热管理专家GraemeBarette表示，铝的熔点高达900℃，铝元件在550-600℃依然可以稳定工作。相比之下，碳复合材料的耐热极限仅为260℃左右，远低于铝。

为推广碳复合材料在车身上的应用，Zicrotec计划和多家OEM合作，共同引进核能行业的热障技术。

“汽车行业人士已经意识到，必须想办法保护排气管附近高温区域的碳复合板，”Barette说，“而且除了排气管附近以外，车内很多地方的温度都超过了碳复合材料可以承受的工作温度。”

Barette表示，在较低的温度范围内，有很多机械性能好的复合材料可以用，“但一旦温度超过260℃，复合材料的机械强度会降低，制造难度也随之增加。”

Barette还告诉《汽车工程》杂志，通过扩大材料和热源之间的气隙或安装传统的隔热板来降低温度，会增加车身质量、侵占空间，而且可能导致不必要甚至是错误的设计变更。

最薄200微米

为了解决以上的问题，Zircotech研发出了陶瓷热障涂层专利技术。据Barette称，有了该新型涂层，复合材料在260℃的临界温度以上仍然能维持安全性和稳定性，而且陶瓷质量轻，不会影响复合材轻量化的优点，也不会侵占发动机舱的空间。

据Barette介绍，Zicrotech自上世纪九十年代就开始广泛研究赛车热管理技术，而这次的全新涂层技术采用了Zicrotec为核电站研发的等离子喷涂工艺。目前已有多家OEM在量产车上使用了这种新型涂层。

该涂层的喷涂工艺采用自动化生产线，其应用范围已从高端车扩展到了量产车。在制备陶瓷隔热层前，必须先确保碳复合材的表面没有其它涂料，接着在经过预处理的金属基底表面涂上粘结层，让陶瓷面层和基底面牢牢粘结。最后涂上陶瓷隔热层，在涂覆过程中，需要控制原料的类型、成分、进料速度、等离子射流的成分、流速、输入的能量、焰炬的几何尺寸、喷嘴的设计、喷嘴的偏移距离和基底的冷却过程等因素。

Barette 补充道，陶瓷层中的空气颗粒能起到很好的隔热作用，涂层最薄仅为200μm（200微米），如另有需求，也可加厚。

由于涂层可以根据不同的应用要求进行调整，因此可以实现种类丰富的表面加工。举例来说，如果复合材料位于电动车电池周围，则可采用导电或不导电的防爆涂层，从而更好地适应电动汽车的特性。而针对外部零件，Zicrotec可提供顶级的表面加工工艺，且有多种颜色可选。

Barette补充道，该陶瓷涂层最高耐受温度为1400℃，具有极佳的抗振动、抗机械损伤性。就算基底发生严重弯曲，也不会影响涂层性能。他强调，陶瓷热障不仅能解决行车过程中的热问题，还有助于推动设计和制造创新，引发新的技术变革。

融入早期设计阶段

该陶瓷涂料的卓越性能已得到了多家OEM的验证。Barette说，“虽然Zircotech可以完成大部分必要的涂料测试，但很多OEM客户倾向于把涂料检测设为公司内部测试的一环。其中陶瓷涂覆复合件的耐用性是重点检测项目。要想取代较重的金属件，复合材必须展示出不逊色于金属的耐用性。为了达到OEM的标准，除热管理测试外，我们还进行了大量测试，如粘结力测试和耐化学性测试等等。”

多年来，Zirotech也在不断丰富碳复合材料以外的热管理解决方案，这也间接反映了汽车研发的变化。

Barette表示，过去，OEM只在研发后期出现热问题时才来咨询Zircotech。到这时才去找解决方案，往往已经来不及或是经费早已用尽，所以现在热障技术已经成为了汽车设计的一部分，这样做也有助于实现车辆配置的优化和车身轻量化。

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.

200-micron minimum

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.

Up-front design

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.

Author: Stuart Birch
Source: SAE Automotive Engineering Magazine