IntroductionWith the development of aero-engine towards large thrust, high thrust-to-weight ratio, high thermal efficiency, low fuel consumption and long life, the turbine inlet temperature is bound to continuously increase. Thermal barrier coating (TBC) with high thermal insulation and long life becomes popular. Multi-rare-earth doped zirconia (especially Gd₂O₃-Yb₂O₃-Y₂O₃ co-doped ZrO₂, GdYb-YSZ) as one of the most potential candidate materials for ultra-high temperature TBCs has a better high-temperature phase stability and a lower thermal conductivity rather than YSZ. It is easy to generate thermal stress during thermal cycling due to GdYb-YSZ with a low fracture toughness, resulting in a poor thermal cycling performance of GdYb-YSZ/NiCrAlY TBC. The design of double ceramic layer (DCL) TBC can alleviate the thermal stress mismatch and effectively solve this problem. In the DLC TBCs system, the thickness ratio between ceramic layers can seriously affect the thermal insulation effect and thermal cycle life of the coating. However, little work on the effect of thickness ratio of GdYb-YSZ/8YSZ ceramic layers on the mechanical properties and thermal cycle life of GdYb-YSZ/8YSZ DLC TBCs system has been reported yet.In this paper, four kinds of GdYb-YSZ/8YSZ DLC TBCs systems with different ceramic layer thickness ratios were prepared. The microstructure, mechanical properties and water quenching-thermal shock cycle properties of GdYb-YSZ/8YSZ DLC TBCs systems with different ceramic layers thickness ratios were investigated. The failure behavior and failure mechanism of GdYb-YSZ/8YSZ DLC TBCs at 1150 ℃were also discussed.MethodsGdYb-YSZ/8YSZ DLC TBCs systems with different ceramic layer thickness ratios (i.e., GdYb-YSZ:8YSZ=5:1, 2:1, 1:1, and 1:2) were prepared with GH4169 nickel-based superalloy as a matrix, NiCoCrAlY as a bond coating, 8YSZ as the first ceramic layer, and GdYb-YSZ as the second ceramic layer., and then four TBC systems on the surface of GH4169 nickel-based superalloy were prepared by a model UniCoatPro^TM atmospheric plasma spraying system (Oerlikon Metco Inc., USA). Subsequently, the microstructure and phase composition of four kinds of GdYb-YSZ/8YSZ DLC TBCs systems were characterized. The bonding strength and water quenching-thermal shock cycle behavior at 1150 ℃ were tested, and the failure mechanism was analyzed. To analyze the water quenching-thermal shock cycle behavior at 1150 ℃, the coated sample was kept in a high-temperature furnace at 1150 ℃ for 5 min and then quickly cooled in distilled water to room temperature. After drying, the sample surface was recorded as a cycle experiment. Repeating the steps above until the cracking or peeling area of the coating surface of greater than 5% was considered as the TBC failure.Results and discussionMost of the GdYb-YSZ and 8YSZ spraying powders were melted in a high-temperature plasma flame flow, and interacted with the matrix to form the smooth structures via high speed jetting. A small number of unmelted and partially melted particles can form a rough microstructure during deposition, accompanied by micro-cracks and pores. The interfaces of each layer are closely bonded and clearly demarcated in GdYb-YSZ/8YSZ/NiCoCrAlY/substrate TBC system. The average bonding strength of the four GdYb-YSZ/8YSZ thermal barrier coating systems all is greater than 40 MPa. The macro-image of fracture after tensile test shows that most of the fracture occurs at the interface between 8YSZ and NiCoCrAlY. The average porosity of GdYb-YSZ/8YSZ TBC systems with different ceramic layer thickness ratios (i.e., 5:1, 2:1, 1:1, and 1:2) calculated by a software named Image J is 3.39%, 2.65%, 4.0%, and 2.64%, respectively. The GdYb-YSZ ceramic layer is composed of 98% (mole fraction) c-ZrO₂ phase and 2% m-ZrO₂ phase, which is basically the same as the phase composition in the spraying powder.Four kinds of TBCs with different thickness ratios are intact after 80 quenching-thermal shock cycles at 1150 °C. GdYb-YSZ/8YSZ TBC systems with four ceramic layer thickness ratios (5:1, 2:1, 1:1, and 1:2) begin to peel off at the edge of the coating after 85, 165, 120, and 195 water quenching-thermal shock cycles, respectively. The water quench-thermal shock cycle resistance of the GdYb-YSZ/8YSZ TBC system becomes better as the thickness of the GdYb-YSZ ceramic layer decreases and the thickness of the 8YSZ ceramic layer increases at a constant total thickness of the ceramic layer. The GdYb-YSZ/8YSZ TBC systems with different ceramic layer thickness ratios (i.e., 5:1, 2:1, 1:1, and 1:2) have a quenching-thermal shock cycle life of 125, 195, 200, and 205 times at 1150 ℃, respectively. The GdYb-YSZ/8YSZ TBC systems with different ceramic layer thickness ratios are mainly characterized via the peeling of ceramic layer during the water-quenching-thermal shock cycle at 1150 ℃. The thermal stress is one of the main factors leading to the failure of TBCs. The residual stress of 8YSZ ceramic layer with different layer thickness ratios of 5:1, 2:1, 1:1 and 1:2 is 0.01, 0.08, 0.12 GPa and 0.15 GPa, respectively, after the water quenching-thermal shock cycle failure at 1150 ℃. The stress increases gradually with the increase of the thickness of the ceramic interlayer 8YSZ. The residual stress (i.e., compressive stress) in the TGO layer in the coating system with different layer thickness ratios is 0.51, 0.29, 0.72 GPa and 0.62 GPa, respectively. The compressive stress of the TGO layer will cause the 8YSZ/TGO interface to buckle, and the tensile stress will be generated at the crest of the wave, thus promoting the generation of cracks. In the process of water quenching-thermal shock cycle of GdYb-YSZ/8YSZ TBCs, some micro-cracks are easy to form on one side of the 8YSZ ceramic layer near the 8YSZ/TGO interface under the combined action of residual stress in the ceramic layer and TGO layer. The microcracks can further spread into TGO layer as the TGO layer increases, seriously weakening the interface bonding force between ceramic layer and adhesive layer, and finally leading to the failure of coating spalling.ConclusionsGdYb-YSZ/8YSZ double ceramic layer thermal barrier coatings (i.e., DCL TBC) with different layer thickness ratios (i.e., GdYb-YSZ/8YSZ = 5:1, 2:1, 1:1, and 1:2) were prepared by an atmospheric plasma spraying technology. GdYb-YSZ ceramic top coated with four coatings were composed of c-ZrO₂ phase and a small amount of m-ZrO₂. The bonding strength of the four coatings was greater than 40 MPa, and the average bonding strength of the coatings was the maximum when the thickness of GdYb-YSZ/8YSZ was 5:1. The thermal shock cycle resistance of GdYb-YSZ/8YSZ TBC system became better as the thickness of the GdYb-YSZ ceramic coat decreased and the thickness of the 8YSZ ceramic coat increased. At the thickness ratio of GdYb-YSZ/8YSZ of 1:2, the water quenching-thermal shock of the TBC was up to 205 cycles at 1150 ℃, having the optimum thermal shock resistance. In addition, GdYb-YSZ/8YSZ TBCs with different ceramic layer thickness ratios did not undergo a phase transformation under the water-quenching-thermal shock cycle at 1150 ℃. The failure mechanism of GdYb-YSZ/8YSZ TBCs with different ceramic layer thickness ratios under the quench-thermal shock cycle at 1150 ℃ was as follows, i.e., in the process of water quenching-thermal shock cycle at 1150 ℃, the residual stress in 8YSZ coating increased with the increase of the thickness of 8YSZ ceramic layer, and micro-cracks were easy to form on one side of 8YSZ ceramic layer at near 8YSZ/TGO interface. In addition, the formation of TGO in the process of thermal cycling was accompanied by the generation of compressive stress. TGO further increased, and the micro-cracks in the coating expanded into TGO with the increase of the number of thermal cycles, reducing the binding force of the coating, and eventually causing the coating to flake and fail.