Factors affecting TBC furnace cycle lifetime: Temperature, environment, structure and composition

Higher efficiency power generation turbines will benefit both natural gas-fired combined cycle units as well as coal gasification plants burning synthesis gas. Increasing efficiency is primarily linked to the turbine inlet temperature, where increases require improved materials such as a higher performance thermal barrier coating (TBC) with the durability needed by utility customers. An ongoing collaboration between Oak Ridge National Laboratory (ORNL) and the Center for Thermal Spray Research (CTSR) is combining expertise in processing and high temperature oxidation to understand relevant degradation mechanisms and identify promising concepts for further development. Screening of new TBC concepts in the laboratory typically involves furnace cycle testing (FCT) and this technique has been used to explore variables such as temperature, environment, roughness, structure and composition. For base load power generation duty, the traditional 1-h thermal cycle has been replaced in some experiments by 24- or 100-h cycles, which has focused attention on the limitations of FCT. In order to link FCT results with observed long field TBC lifetimes in land-based turbines operating with ∼900 °C metal temperatures, specimens were exposed in FCT at 900 °C and Al loss after 5-20 kh exposures has been used to make initial lifetime predictions. Based on the observed FCT performance of flat coupons, the most recent work has explored cylindrical specimens.