A Test Rig for Evaluating Thermal Cyclic Life and Effectiveness of Thermal Barrier Coatings inside Exhaust Manifolds

Thermal Barrier Coatings (TBCs) may be used on the inner surfaces of exhaust manifolds in heavy-duty diesel engines to improve the fuel efficiency and prolong the life of the component. The coatings need to have a long thermal cyclic life and also be able to reduce the temperature in the substrate material. A lower temperature of the substrate material reduces the oxidation rate and has a positive influence on the thermo-mechanical fatigue life. A test rig for evaluating these properties for several different coatings simultaneously in the correct environment was developed and tested for two different TBCs and one oxidation-resistant coating. Exhausts were redirected from a diesel engine and led through a series of coated pipes. These pipes were thermally cycled by alternating the temperature of the exhausts. Initial damage in the form of cracks within the top coats of the TBCs was found after cycling 150 times between 50°C and 530°C. Temperature calculations showed that, besides evaluating the thermal cyclic life, the test method has the potential to provide a quick ranking of coating materials with respect to thermal insulation by measuring the temperature on the outer surface of the coated pipes. One of the major advantages of the presented test method compared to other methods described in the literature is that it ranks the thermal cyclic life and thermal properties of different coatings under realistic conditions in the correct environment. More cycles and higher temperatures are recommended for future tests, to accelerate the test, as well as evaluate whether the initial cracks in the TBCs will lead to spallation.