Matching design of thickness ratio to extend the lifespan of double-layered thermal-barrier coatings

The ceramic materials of advanced thermal-barrier coatings (TBCs) must be exposed to high temperatures (over 1200°C) over a long lifespan. To meet this requirement, researchers have recently developed phase-stable ceramic materials. However, most of the newly-developed materials have low fracture toughness and are easily cracked. Double-layered TBCs (DL–TBCs) can potentially provide a fast route to engineering applications of ceramic materials. The present paper attempts to extend the lifespan of DL–TBCs by matching the thickness ratio of the top layer to the bottom layer. First, the lifespans of different DL–TBCs with equivalent thermal insulation were determined in an iso-thermal cyclic test. The lifespan increased and then decreased with increasing thickness ratio. The optimized matching design of the thickness ratio (approximately 2:3) more than doubled the lifespan. Subsequently, the failure mechanism of DL–TBCs with different thickness ratios was analyzed through simulations. Varying the thickness ratio changed the lifespan by influencing the cracking driving force. Finally, to optimize the double-layer structures, the structural evolution was characterized under thermal exposure. Sintering-induced stiffening largely increased the cracking driving force, posing a main failure threat. The simulation suggested that lowering the stiffening degree in the top layer can further extend the TBC lifespan.