Using Thermal Conductivity in Thermal Barrier Coating Design
Thermal barrier coatings (TBCs) are used widely in the aviation, aerospace, and automotive sectors to manage the hazards posed by exhaust heat. These materials are used to provide thermal insulation to and prolong the service of hot section metal engine components – particularly in aviation, where fuel burns at temperatures of up to 2000 °C and the metal engine components begin to melt around 1200 °C, these components are necessary to the safe operation of the engine.
An ideal TBC is a material with a relatively low thermal conductivity, high toughness even at elevated temperatures, without much sintering behavior at operational temperatures after application and without any phase changes in its operational temperature range. Many TBC applications also require the material to be refractory. The existing state-of-the-art material is a form of yttria-stabilized zirconia (YSZ). YSZ is a refractory material with a thermal conductivity which is highly dependent upon yttria loading, sintering conditions, and morphology, which makes it tunable to the application. However, YSZ has some critical flaws as a TBC material at very high temperature applications: It has well-characterized sintering and phase-change behavior above 1200°C, which renders it unsuitable above that temperature. In aviation, this poses a challenge as it is well known that engine efficiency is optimal at firing temperatures on the order of 1350 °C – 1425 °C. Improving jet engine efficiency thus requires a TBC material capable of withstanding temperatures in excess of 1400 °C. There is considerable research interest in developing ceramics appropriate for use as TBCs in high-temperature advanced jet engine operations.
A newer material under study by the TBC field is LaTi2Al9O19, commonly called LTA. It is widely known to have excellent phase stability up to 1600 °C and thermochemical stability. However, it possesses low fracture toughness, which makes it inappropriate for high-cycling applications such as aviation.
Figure 1. C-Therm TCi Thermal Conductivity Analyzer.
Researchers from the UCL Institute for Materials Discovery at the University College London recently published work in Scripta Materialia concerning the development of a TBC material at temperatures up to 1500°C. Using a combination of LTA and 4% YSZ (4YSZ), they were able to develop a new composite ceramic. In comparison with LTA, their composite has a lower coefficient of thermal expansion, superior harness, and comparable Young’s modulus. They used the C-Therm TCi Thermal Conductivity Analyzer, based upon the patented Modified Transient Plane Source (MTPS) thermal conductivity analysis technique, to measure how their novel composite ceramic compared with two control samples.
Figure 2. Thermal conductivity comparison of LTA, 4YSZ, and LTA-4YSZ composite.
As seen in Figure 2, the thermal conductivity of the LTA-4YSZ composite is less than that of either of the component materials the composite was formed from – meaning the composite is likely to have superior TBC performance. The authors suggest phonon scattering may be responsible for the reduction in heat transfer efficiency.
To read more about this work, including details on composite synthesis and the exact results from the other analysis methodologies applied to these materials, please purchase the paper here.