Abstract
In this study, the contribution of high thermomechanical fatigue to the gas turbine lifetime during a steady-state operation is evaluated for the first time. An evolution of the roughness on the surface between the thermal barrier coating and bond coating is addressed to elucidate the correlation between operating conditions and the degradation of a gas turbine. Specifically, three factors affecting coating failure are characterized, namely isothermal operation, low-cycle fatigue, and high thermomechanical fatigue, using laboratory experiments and actual service-exposed blades in a power plant. The results indicate that, although isothermal heat exposure during a steady-state operation contributes to creep, it does not contribute to failure caused by coating fatigue. Low-cycle fatigue during a transient operation cannot fully describe the evolution of the roughness between the thermal barrier coating and the bond coating of the gas turbine. High thermomechanical fatigue during a steady-state operation plays a critical role in coating failure because the temperature of hot gas pass components fluctuates up to 140 °C at high operating temperatures. Hence, high thermomechanical fatigue must be accounted for to accurately predict the remaining useful lifetime of a gas turbine because the current method of predicting the remaining useful lifetime only accounts for creep during a steady-state operation and for low-cycle fatigue during a transient operation.
Highlights
Gas turbines form the heart of the electric power and aerospace industries, which has prompted a large amount of research into the use of material, mechanical el, and electrical engineering for increasing their efficiency [1,2,3]
The specimens were annealed for 8000 h in a furnace at a constant temperature of 850, 950, or 1000 ◦ C at a heating rate of 5 ◦ C/min to quantify the effect of isothermal heat exposure on the evolution of coating fatigue
Each image was vertically divided into sections, each of which a to calculate the roughness of the bond coat
Summary
Gas turbines form the heart of the electric power and aerospace industries, which has prompted a large amount of research into the use of material, mechanical el, and electrical engineering for increasing their efficiency [1,2,3]. Coating failure accelerates HGPC degradation because the metal is directly exposed to high operating temperatures, suggesting that HGPC coatings should be carefully examined and promptly repaired To this end, power utility companies schedule periodic maintenance known as overhaul, during which all HGPCs are disassembled and examined by an expert system following standard maintenance guidelines for gas turbines [13,14,15,16,17,18]. The effects of fatigue on gas turbine lifetime have been comprehensively studied to understand the degradation mechanisms and failure modes of HGPCs [28,29,30,31] and to assess their remaining useful lifetime (RUL) [32,33,34] These studies enable one to accurately estimate the operational lifetime of HGPCs and their coating layers in a gas turbine. Factors were considered to accurately predict the RUL of a gas turbine based on the aforementioned aforementioned analyses
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