Investigation and experimental investigation of characteristic behavior of thermal barrier coated surfaces in dynamic conditions

Gas turbine engines represent a cornerstone of modern technology, serving as the primary power source in the aviation sector and playing a critical role in global energy production. The fundamental principle of their operation involves the combustion of a pressurized air-fuel mixture, which generates high-energy exhaust gases. Unlike reciprocating piston engines, gas turbines utilize these gases to rotate a series of fan blades, known as a turbine, at exceptionally high speeds. The pursuit of greater thermodynamic efficiency and higher power output in advanced gas turbine designs has inexorably led to an increase in turbine inlet temperatures. Contemporary engines operate with exhaust gas temperatures routinely exceeding 1500 °C. These extreme thermal conditions impose severe thermomechanical stresses on the turbine blades, which are typically fabricated from nickel-based superalloys. While these alloys are engineered for high-strength performance at elevated temperatures, they are susceptible to creep, fatigue, and oxidation, which can lead to catastrophic blade failure. To mitigate these risks and enable operation at temperatures beyond the intrinsic melting point of the metallic alloys, Thermal Barrier Coatings (TBCs) are ubiquitously applied. TBCs are advanced multi-layered material systems comprising a metallic bond coat for adhesion and oxidation resistance, and a ceramic top coat—typically yttria-stabilized zirconia (YSZ)—renowned for its low thermal conductivity, high-temperature stability, and wear resistance. This ceramic layer acts as a thermal insulator, substantially lowering the temperature experienced by the underlying metallic substrate. The rationale for prioritizing dynamic testing, as undertaken in this study, stems directly from the inherent and critical limitations of conventional static evaluation methods. For decades, the industry has relied on static tests, such as isothermal furnace cycling and atmospheric burner rig tests, to assess TBC durability. While these methods are valuable for isolating specific failure mechanisms like thermal expansion mismatch between the ceramic and metallic layers and the oxidation kinetics of the bond coat, they provide a dangerously incomplete picture of the service environment. A static test, by its very nature, fails to incorporate the significant mechanical loads that are superimposed upon the thermal loads in an operational engine. The most prominent of these is the immense centrifugal force generated by rotation at tens of thousands of RPM, which induces a constant tensile stress throughout the coating system. Furthermore, static tests cannot replicate the complex aerodynamic forces, including high-frequency vibrations and flutter, which introduce additional mechanical fatigue pathways. They also neglect the severe erosive effects of high-velocity particulate matter within the gas stream, which physically wears away the protective ceramic layer, particularly at the leading edge of the blade. Consequently, life predictions based solely on static data are often non-conservative, meaning they overestimate the coating's true operational lifespan. This discrepancy poses a significant risk to engine integrity and safety. Therefore, the transition to dynamic testing is not merely an academic improvement but an engineering imperative for the accurate prediction of TBC performance and the prevention of premature, in-service failures. To achieve a more realistic simulation of the operational environment, a bespoke dynamic test rig was designed and manufactured specifically for this investigation. This apparatus facilitated the rotation of TBC-coated specimens at high speeds within a high-temperature gas stream, thereby introducing the crucial centrifugal and radial gas flow effects that are absent in static tests. The substrates, representative of turbine blade materials, were coated with TBCs of varying thicknesses. The primary objective was to systematically evaluate the influence of coating thickness on performance and longevity under these dynamic thermomechanical loads. The durability lifespan of each specimen was determined by subjecting it to cyclic heating and cooling while under rotation until coating failure, typically defined by a predetermined level of spallation or delamination. The data acquired from these dynamic experiments were then systematically compared against established data from static tests reported in the literature to quantify the impact of the dynamic environment. The experimental findings revealed several crucial insights into TBC performance that deviate significantly from static test predictions. It was determined that the radial flow of hot gases across the coating surface, a direct consequence of rotation, significantly reduces the coating's operational lifespan. This accelerated degradation is attributed to the combined effects of increased erosive wear and the complex stress fields induced by aerodynamic loading, which act synergistically with the baseline thermal mismatch stresses. Conversely, the study affirmed that the implementation of internal cooling channels, a standard design feature in modern turbine blades, has a demonstrably positive effect on the longevity of the TBC. By creating a steeper thermal gradient across the coating, the cooling channels effectively lower the temperature at the critical bond coat/top coat interface. This reduction in interface temperature slows the growth of the thermally grown oxide (TGO) layer—a primary driver of TBC failure—thereby extending the coating's useful life. Furthermore, a clear correlation between coating thickness and durability was established. The results showed that the lifespan of the TBC increases with increasing coating thickness. However, this relationship is not without limits. While a thicker coating provides superior thermal insulation, it also increases internal stresses and stored strain energy, potentially making it more prone to mechanical failure. Through systematic testing at different thicknesses, an ideal coating thickness of approximately 300 µm was identified. This thickness provides the optimal balance between maximizing thermal protection and maintaining mechanical integrity. Notably, this finding is in strong agreement with optimal thickness values reported in the existing body of literature, which serves to validate the experimental methodology employed in this dynamic study. This research underscores the inadequacy of static testing alone for the accurate life prediction of Thermal Barrier Coatings and highlights a critical void in the materials testing landscape: the lack of a standardized international protocol for dynamic TBC evaluation. Currently, no widely accepted ASTM, ISO, or other regulatory standard exists for conducting durability tests under combined high-temperature and rotational conditions. This absence forces different research institutions, coating developers, and engine manufacturers to rely on their own proprietary, non-standardized test rigs. The consequence is a fragmented and often contradictory body of data, making direct, "apples-to-apples" comparisons of different TBC systems nearly impossible and hindering collaborative progress across the industry. The unique test rig and methodology developed and validated in this study present a significant step toward rectifying this issue. By providing a controlled, repeatable, and realistic simulation of the dynamic engine environment, the experimental setup described herein has the clear potential to serve as a foundational blueprint for the development of a much-needed universal standard. Establishing such a standard would unify testing procedures, enable reliable data comparison, accelerate the development and qualification of more robust coating materials, and ultimately enhance the operational safety and efficiency of future gas turbine engines worldwide.