Self-Enhancing Thermal Insulation Performance of Bimodal-Structured Thermal Barrier Coating

Nanostructured thermal barrier coatings (nano-TBCs) are being extensively studied because of their excellent thermal barrier properties. The occurrence of sintering in TBCs is inevitable in service; however, accelerated sintering of the nano-TBCs may cause premature failure. This study focuses on the changes of microstructure and thermal conductivity of bimodal nano-TBCs during thermal exposure. Results show that there are two stages in the sintering process. It was found that the thermal conductivity increased rapidly in the first stage (from 0 to 20h), with the rate of increase in normalized thermal conductivity equal to 140% of bimodal coating. The continuous healing of the pores was the main structural change. During the following stage (20 to 100h), the thermal conductivity decreased with the rate of increase in normalized thermal conductivity equal to -8% of bimodal coating. The change of structure was the opening of pores. Furthermore, self-enhancing behavior of bimodal composite coatings was discovered. The phenomenon of inevitable sintering in thermodynamics can be used to introduce large-aspect-ratio pores in the in-depth direction, which can greatly slow down the increase in thermal conductivity in service and ultimately increase the lifetime of the thermal insulation. Based on a full study of the sintering mechanism of composite coatings, the present study sheds light on the structural adjustments that lead to a lower thermal conductivity and longer service life in the advanced TBC during high-temperature service.