Ultra-low thermal conductivity thermal barrier coatings from recycled fly-ash cenospheres

Abstract Thermal conductivity of electrophoretically deposited fly ash (FA) and fly-ash cenosphere (FAC) coatings was measured in the range 100–500 K by the 3ω method. The room temperature thermal conductivity in FAC coatings is found to be as low as 0.08 W m K−1 and slightly higher for FA coatings. The reduction in the thermal conductivity is related mainly to the air-filled core of the cenospheres and the coating’s porosity, which produces a strong heat barrier. The sintering process of the FAC and FA coatings at 1000, 1100 and 1200 °C slightly increases the thermal conductivity as a result of changes in microstructure. The temperature dependence of the effective thermal conductivity of the coatings was modeled within the framework of the self-consistent field concept and a modified Maxwell equation. These results suggest a route for obtaining suitable thermal barrier coatings for high-temperature applications.