Thermal conductivity in mullite/ZrO2 composite coatings

Mullite-based multilayered structures have been suggested as promising environmental barrier coatings for Si3N4 and SiC ceramics. Mullite has been used as bottom layer because its thermal expansion coefficient closely matches those of the Si-based substrates, whereas Y–ZrO2 has been tried as top layer due to its stability in combustion environments. In addition, mullite/ZrO2 compositions may work as middle layers to reduce the thermal expansion coefficient mismatch between the ZrO2 and mullite layers. Present work studies the thermal behaviour of a flame sprayed mullite/ZrO2 (75/25, v/v) composite coating. The changes in crystallinity, microstructure and thermal conductivity of free-standing coatings heat treated at two different temperatures (1000 and 1300 °C) are comparatively discussed. The as-sprayed and 1000 °C treated coatings showed an almost constant thermal conductivity (K) of 1.5 W m−1 K−1. The K of the 1300 °C treated specimen increased up to twice due to the extensive mullite crystallization without any cracking.