Modeling thermal conductivity of thermally sprayed coatings with intrasplat cracks

Thermal spraying is one of the most important approaches for depositing thermally insulating ceramic top coatings for advanced gas turbines due to the low thermal conductivity of the coating resulting from its lamellar structure. The thermal conductivity of the coating has been explained based on the concept of thermal contact resistance and correlated to microstructural aspects such as splat bonding ratio, splat thickness, and the size of the bonded areas. However, the effect of intrasplat cracks on the thermal conductivity was usually neglected, despite the fact that intrasplat cracking is an intrinsic characteristic of thermally sprayed ceramic coatings. In this study, a model for the thermal conductivity of a thermally sprayed coating taking account of the effect of intrasplat cracks besides intersplat thermal contact resistance is proposed for further understanding of the thermal conduction behavior of thermally sprayed coatings. The effect of the intersplat bonding ratio on the thermal conductivity of the coating is examined by using the model. Results show that intrasplat cracks significantly decrease the thermal conductivity by cutting off some heat flux paths within individual splats. This leads to a deviation from the typical ideal thermal contact resistance model which presents cylindrical symmetry. Based on the modified model proposed in this study, the contribution of intrasplat cracks to the thermal resistivity can be estimated to be 42-57 % for a typical thermally sprayed ceramic coating. The results provide an additional approach to tailor the thermal conductivity of thermally sprayed coatings by controlling the coating microstructure.