Evaluation of internal thermal barrier coatings for exhaust manifolds

Seven different thermal barrier coatings (TBC) intended for coating the inside of an exhaust manifold to reduce its material temperature were studied. They comprised five plasma-sprayed (mullite, forsterite, La2Zr2O7, 8YSZ, and nanostructured 8YSZ) and two sol-gel composite (one sprayed and one dipped) coatings, which were examined for their thermal insulation properties and oxidation and spallation resistance. Thermal cyclic tests in air and in exhaust gas in a diesel test engine showed that thermal expansion mismatch between substrate and TBC was most crucial for TBC lifetime. Moreover, thermal modeling indicated that it is possible to reduce the material temperature by 50 °C, which is important for improving the fatigue life of exhaust manifolds. This reduction can be obtained with a 0.2 mm thick TBC with thermal conductivity close to 0.1 W/m K, or a 3-6 mm thick TBC with thermal conductivity 1.5-3 W/m K.