除了成本高昂外，耐熱性差也是導致碳復合材料未能在汽車領域得到廣泛應用的一大原因。發(fā)動機艙布局緊湊，要在渦輪增壓器、尾氣后處理元件或高壓電池包附近布置碳復合材料，難度很大。
雖然行車時的氣流管理可以減輕熱效應，但一旦行車和駐車的暖機時間過長，碳材就可能出現(xiàn)分層。
Zicrotec經(jīng)理、英國熱管理專家GraemeBarette表示，鋁的熔點高達900℃，鋁元件在550-600℃依然可以穩(wěn)定工作。相比之下，碳復合材料的耐熱極限僅為260℃左右，遠低于鋁。
為推廣碳復合材料在車身上的應用，Zicrotec計劃和多家OEM合作，共同引進核能行業(yè)的熱障技術。
“汽車行業(yè)人士已經(jīng)意識到，必須想辦法保護排氣管附近高溫區(qū)域的碳復合板，”Barette說，“而且除了排氣管附近以外，車內(nèi)很多地方的溫度都超過了碳復合材料可以承受的工作溫度。”
Barette表示，在較低的溫度范圍內(nèi)，有很多機械性能好的復合材料可以用，“但一旦溫度超過260℃，復合材料的機械強度會降低，制造難度也隨之增加。”
Barette還告訴《汽車工程》雜志，通過擴大材料和熱源之間的氣隙或安裝傳統(tǒng)的隔熱板來降低溫度，會增加車身質(zhì)量、侵占空間，而且可能導致不必要甚至是錯誤的設計變更。
最薄200微米
為了解決以上的問題，Zircotech研發(fā)出了陶瓷熱障涂層專利技術。據(jù)Barette稱，有了該新型涂層，復合材料在260℃的臨界溫度以上仍然能維持安全性和穩(wěn)定性，而且陶瓷質(zhì)量輕，不會影響復合材輕量化的優(yōu)點，也不會侵占發(fā)動機艙的空間。
據(jù)Barette介紹，Zicrotech自上世紀九十年代就開始廣泛研究賽車熱管理技術，而這次的全新涂層技術采用了Zicrotec為核電站研發(fā)的等離子噴涂工藝。目前已有多家OEM在量產(chǎn)車上使用了這種新型涂層。
該涂層的噴涂工藝采用自動化生產(chǎn)線，其應用范圍已從高端車擴展到了量產(chǎn)車。在制備陶瓷隔熱層前，必須先確保碳復合材的表面沒有其它涂料，接著在經(jīng)過預處理的金屬基底表面涂上粘結層，讓陶瓷面層和基底面牢牢粘結。最后涂上陶瓷隔熱層，在涂覆過程中，需要控制原料的類型、成分、進料速度、等離子射流的成分、流速、輸入的能量、焰炬的幾何尺寸、噴嘴的設計、噴嘴的偏移距離和基底的冷卻過程等因素。
Barette 補充道，陶瓷層中的空氣顆粒能起到很好的隔熱作用，涂層最薄僅為200μm（200微米），如另有需求，也可加厚。
由于涂層可以根據(jù)不同的應用要求進行調(diào)整，因此可以實現(xiàn)種類豐富的表面加工。舉例來說，如果復合材料位于電動車電池周圍，則可采用導電或不導電的防爆涂層，從而更好地適應電動汽車的特性。而針對外部零件，Zicrotec可提供頂級的表面加工工藝，且有多種顏色可選。
Barette補充道，該陶瓷涂層最高耐受溫度為1400℃，具有極佳的抗振動、抗機械損傷性。就算基底發(fā)生嚴重彎曲，也不會影響涂層性能。他強調(diào)，陶瓷熱障不僅能解決行車過程中的熱問題，還有助于推動設計和制造創(chuàng)新，引發(fā)新的技術變革。
融入早期設計階段
該陶瓷涂料的卓越性能已得到了多家OEM的驗證。Barette說，“雖然Zircotech可以完成大部分必要的涂料測試，但很多OEM客戶傾向于把涂料檢測設為公司內(nèi)部測試的一環(huán)。其中陶瓷涂覆復合件的耐用性是重點檢測項目。要想取代較重的金屬件，復合材必須展示出不遜色于金屬的耐用性。為了達到OEM的標準，除熱管理測試外，我們還進行了大量測試，如粘結力測試和耐化學性測試等等。”
多年來，Zirotech也在不斷豐富碳復合材料以外的熱管理解決方案，這也間接反映了汽車研發(fā)的變化。
Barette表示，過去，OEM只在研發(fā)后期出現(xiàn)熱問題時才來咨詢Zircotech。到這時才去找解決方案，往往已經(jīng)來不及或是經(jīng)費早已用盡，所以現(xiàn)在熱障技術已經(jīng)成為了汽車設計的一部分，這樣做也有助于實現(xiàn)車輛配置的優(yōu)化和車身輕量化。
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
陶瓷涂層可解決輕量化材料的熱效應問題，例如在碳tu復合材質(zhì)的剎車密封蓋上涂覆陶瓷隔熱層。（圖片來源：Zicrotec）
Graeme Barette表示，有了新型陶瓷隔熱涂層，碳復合材料在260℃以上仍能維持安全性和穩(wěn)定性。（圖片來源：Zircotech）
