Burner Rig Testing of Thermal Barrier Coatings for Lifetime Prediction

Abstract

Thermal barrier coating lifetime prediction has been commonly performed using furnace cyclic test results. This testing method causes coating failures driven by the bondcoat oxidation. This allows definition of lifetime prediction models representative of the field experience for thin thermal barrier coating systems where the difference between the bondcoat temperature and the coating surface are limited to 100–200 °C. Thick thermal barrier coating systems can experience coating surface temperatures 500 °C higher than the bondcoat temperature. In such cases sintering and phase transformations in the ceramic layers can also affect the coating lifetime. For this reason cyclic test methods like thermal gradient burner rig and laser heat-flux tests have been developed. They allow to test a coating system with surface temperatures >1400 °C while keeping bondcoat temperature <900 °C. The main issue of such tests is the often limited samples statistic, the reproducibility of the test conditions, and the coating failure mode that is not representative of the field experience. In Alstom, a burner rig test has been developed to solve these issues. It allows to test in parallel 10 samples, with a closed loop control system allowing live adjustment of the heat and cooling air input to keep an individually controlled constant thermal gradient with a homogeneous temperature distribution on the sample surface. Modeling of the test has been performed to understand the coating failure mechanism and to adapt the testing conditions such to get a failure mechanism closer to the relevant degradation mechanisms experienced in the field. Testing of coatings coming from the same production batch in various test campaign shows a low scatter in test results confirming that the burner rig test design allowed solving the test reproducibility and samples statistics issues. Examples will be shown how this burner rig test can be used for the development of lifetime prediction rules for thermal barrier coating systems.