Method Evaluates Thermal Stress in Barrier Coatings

MONDAY, APRIL 29, 2024


A research team at the Kunming University of Science and Technology in Kunming, China, recently created a new high-throughput multiscale evaluation method for thermal stress in thermal barrier coatings.

The team explained that the new testing method will account for the phase transition of top ceramic materials by grouping first-principles calculations with finite element simulations.

About the Method

According to a press release provided to phys.org, TBCs are widely used in gas turbine engines to reach higher working temperatures and improve engine efficiency. The phase transition of the ceramic layer goes along with a large volume difference, causing the concentration of thermal stress and leading the TBCs to fall off and fail.

Because of this, the team believes that it is necessary to quantitatively evaluate the magnitude and distribution of thermal stress caused by the phase transition in the ceramic layer.

With this new method, the researchers hope to evaluate and visualize thermal stress of the real TBCs' structure under thermal cycling by multifield coupling, potentially providing an important theoretical basis for the life prediction and reverse design of coating materials.

"In this report, we develop a high-throughput multiscale evaluation method for thermal stress in multilayered systems, which considers the phase transition of the top ceramic materials by coupling first-principles calculations with finite element simulations,” said Xiaoyu Chong, a professor at the Faculty of Materials Science and Engineering at Kunming University of Science and Technology.

“This approach can quantitatively evaluate and visualize the thermal stress in TBCs based on real structures, considering the actual service environment subjected to thermal cycling."

The team added that previous it was hard to observe the phase transformation process of ceramic coatings. As one of the main reasons for coating failure, thermal stress is reportedly subject to a lack of quantitative testing and characterization methods, and the high temperature service environment can increase the difficulty of phase transformation thermal stress testing.

"The thermophysical properties' input in finite element simulations are calculated by first-principles calculations, in which the multiscale method can consider the influence of phase transition and temperature and simultaneously reduce the cost and time of obtaining thermophysical properties by experiments," Chong said.

For their research, the team built a high-throughput multiscale evaluation method for thermal stress in multilayered systems, accounting for the phase transition of the top ceramic materials by grouping first-principles calculations with finite element simulations.

"The thermophysical properties input in finite element simulations are reportedly found through first-principles calculations, where the multiscale method can consider the influence of phase transition and temperature, also reducing the cost and time of obtaining thermophysical properties by experiments," explained Chong.

"The finite element simulations coupled with multiple physical fields can visualize and quantitatively evaluate thermal stress of TBCs. However, the thermophysical properties required for finite element simulations are derived from experimental measurements, which ignores the effects of phase transition and temperature," said Mengdi Gan, the first author of the paper and a Ph.D. student supervised by Chong.

This new approach is expected to help accurately observe and visualize the thermal stress in TBCs based on real structures, using the actual service environment subjected to thermal cycling.

For the project, rare earth tantalites (RETaO4) were introduced as ceramic layers, with results reportedly showing that thermal stress undergoes a speedy escalation near the phase transition temperature, specifically in the TBCs_GdTaO4 system.

This discontinuity in thermal stress, according to the team, could potentially come from the large alterations in Young's modulus and thermal conductivity near the phase transition temperature. The TBCs_NdTaO4 and TBCs_SmTaO4 systems have reportedly shown notable temperature drop gradients and low thermal stress fluctuations, which is reportedly beneficial for extending the service lifetime of the TBCs.

This approach has reportedly helped with the prediction of failure mechanisms and has provided theoretical guidance for the reverse design of TBCs materials to obtain low thermal stress systems.

Other contributors include Mengdi Gan, Tianlong Lu, Wei Yu, Jing Feng from the Faculty of Materials Science and Engineering at Kunming University of Science and Technology in Kunming, China.

The team’s research was published in the Journal of Advanced Ceramics.

Other Barrier Coatings

Last month, researchers at the Skolkovo Institute of Science and Technology (Skoltech) reported that they had identified promising ceramic materials for metal coatings that allow gas turbines to get hotter and produce more power.

According to Skoltech, if further tests proved successful, the coatings would enable power plants to produce more electricity and jet planes to consume less fuel. Thermal barrier coatings are used to protect turbine blades at power plants and in jet engines. The blades themselves are reportedly made of nickel-based superalloys.

While these materials offer a combination of high-temperature strength, toughness and resistance to degradation, as things get really hot, the superalloy softens and may even melt. Protective coatings reportedly make it possible to operate turbines at higher temperatures without compromising their integrity.

According to the scientists, a material for thermal barrier coatings has to meet several requirements, such as a very high melting point and a very low thermal conductivity. The latter property is particularly hard to compute because it depends on the intricate “anharmonic” effects in crystals.

Additionally, when heated, the material should expand at about the same rate as the superalloy, or it will flake off the surface. The material should not experience any phase transitions between room temperature and the operating temperature of the turbine, which would cause the coating to crack.

It should also be able to withstand the effects of dust particles and oxygen at high temperatures, as well as prevent oxygen ions from reaching the underlying metal and oxidizing it.

The Skoltech study also highlighted several materials that can reportedly surpass yttria-stabilized zirconia, including yttrium niobate (Y3NbO7), the perovskite structures BaLaMgTaO6 and BaLaMgNbO6, and seven more materials.

   

Tagged categories: Asia Pacific; Barrier coatings; Ceramic coatings; Coating Materials; Coating Materials; Coatings; Coatings Technology; Colleges and Universities; EMEA (Europe, Middle East and Africa); Energy efficiency; Latin America; North America; Program/Project Management; Quality Control; Research; Research and development; Temperature; Testing + Evaluation; Thermal-barrier coatings; Z-Continents

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