Thermal barrier coatings are susceptible to failures due to spallation and degradation effects caused by sintering. This has led to several studies focused on enhancement of thermal insulation and life span to improve the multiple performance characteristics of coatings.
A typical thermal barrier coating system consists of a ceramic top-coat, metallic bond coat, and a metal substrate. In most land-based gas turbines, the topcoat is normally prepared using a plasma spraying method. However, maintaining high performance during thermal exposure remains a great challenge for the plasma-sprayed thermal barrier coatings. This can be attributed to the sintering effect. Recently, researchers have investigated the effects of retaining the thermally resistant pores during thermal exposure in preventing degradation of thermal insulation. This includes replacing the micropores with newly formed mesopores. Unfortunately, the effects of the new two-dimension mesopores have not been fully evaluated considering the stressed state of the thermal barrier coatings. Additionally, the pores and coating thickness exhibit opposite effects on the thermal insulation and durability. Therefore, there is a great need to simultaneously enhance the thermal insulation and life span performances.
To this note, Li from Xi’an Jiatong University et al. developed a co-design method for the macro- and microstructure to improve multiple performance characteristics of thermal barrier coatings. Specifically, the authors purposed to simultaneously enhance the durability and thermal insulation of the thermal barrier coatings during thermal exposure. Also, they investigated the structural and property changes to determine the contribution of microstructure to the self-enhanced thermal insulation. This work is currently published in the research journal, Applied Surface Science.
Briefly, the authors started their experiments by comparatively cross-examining the effects of newly formed pores on the durability of thermal barrier coatings. Secondly, a co-spraying method was used in the microstructural design to form a hybrid layered coating. To realize the macrostructural design objectives, the thickness was tailored under thermal insulation equivalence. Eventually, the failure mechanism of the thermal barrier coatings was analyzed to evaluate the contribution of the macrostructural design on the durability.
The formation of new two-dimension mesopores was observed during thermal exposure. This was due to the reverse contractions of the dense splats and loose porous nanoheaps that widened the interface between the two zones. The overall thermal insulation performance was self-enhanced by 40%in comparison to monolayered coatings due to the new mesopores. In particular, the larger aspect ratio as compared to the initial two-dimensional micropores was a significant contributor to the enhanced insulation. By tailoring the thickness, the lifetime of the thermal barrier coatings was further extended by 35%. Furthermore, it was worth noting that the decrease in the driving force for spallation was among the responsible mechanism for the improvement on life span.
According to the authors, the co-design approach will significantly contribute to the advancement of thermal barrier coatings with relatively high performance in multiple characteristics including high thermal insulation and long lifetimes. This is due to their durability and degradation resistant properties. The study will also be useful in future tailoring of advanced thermal barrier coatings.
Li, G., & Wang, L. (2019). Durable TBCs with self-enhanced thermal insulation based on co-design on macro- and microstructure. Applied Surface Science, 483, 472-480.Go To Applied Surface Science