Research On Lanthanum Zirconate Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are one of the most advanced high temperature protective coatings and being wildly used in aeronautic, astronautics, motor industry and heat power station, for their good performance at thermal barrier and oxidation resistance. 8 mol. % yttria partially stabilized zirconia (8YSZ) is currently used as the commercial materials of TBCs. However, the major disadvantage of YSZ is the limited operation temperature of 1200oC for the long-term application. And the search for new candidate materials that can work at even higher temperature has been intensified in future. Among the interesting candidates for TBCs, lanthanum zirconate (La2Zr2O7, LZ) has been proposed as a promising TBCs material for its high melting point, more stable structure and lower thermal conductivity than YSZ. In this work, the preparetion of LZ powder was studied. Different rare earth elements doped LZ powders were researched and LZ-based powders with higher thermal expansion coefficient and better sintering resistance were obtained. Then the powders with good spraying performance were prepared by spray pelletization. And the LZ-based TBCs on Ni super alloy with high bonding strength and thermal shock resistance were fabricated by Air Plasma Spraying (APS). On this basis, the sintering behavior, oxidation resistance and thermal shock resistance of TBCs at 1250 oC were studied in detail for the first time. Besides, the LZ-based/YSZ double ceramic layer (DCL) TBCs deposited on Ni super alloy substrate and LZ-based TBCs deposited on Mo substrate were prepared and their failure mechanism were researched primarily. Details are as follow:The single-phase La2Zr2O7 powders were synthesized by co-precipitation method using ammonia and oxalate ammonium as precipitators, respectively. And the synthesis procedure, calcination temperature, compositon and morphology of products were studied. The results show that, compared with oxalate ammonium, co-precipitation using ammonia is prone to obtain better composition controllable and more homogenous La2Zr2O7 powders with pyrochlore structure at lower temperature (1200oC), despite with lower filtering efficiency.The properties of LZ were modified by rare earth elements adoption. The thermal expansion coefficient (TEC) and sintering behavior of Nd³⁺, Ce⁴⁺ doped LZ powder were investigated in this work. It is observed that the proper addition of Nd and Ce into LZ can largely increase its TEC and improve the sintering-resistance. The TEC of La1.6Nd0.4Ce1.0Zr1.0O7 (LNCZ) is 10.4×10^–6/K at 25¹200oC, which is higher than that of commercial 8YSZ and approaches that of Ni super alloy.The LNCZ powder with good spraying performance was obtained by spray pelletization and being heat treatment at 1200oC. Using this kind of powder, APS LNCZ TBCs were fabricated on the surface of Ni super alloy. The effects of spraying parameters on the microstructure and properties of APS LNCZ TBCs were examined and the optimized technological conditions are given out: power is 40kW, stand off distance is 9cm and powder feeding rate is 12g/min. APS LNCZ TBCs with post argon shield heat treatment were employed to improve their properties. According to the experimental results, post argon shield heat treatment could improve the average bonding strength and thermal shock resistance of coatings. The bonding strength value of LNCZ TBCs increases from 1.326MPa to 7.048MPa, and the thermal shock life increases from 15 to 65 cycles (while 50% of the ceramic coat was delaminated) at the same time, after being heated at 1200℃for 2h.The sintering behavior of LNCZ coat and its effects on the microstructure, thermophysical and mechanical properties of coat were investigated. The porosity of coat increases from 11.35% to 15% after being calcined at 1250℃for 5h. The increasing of intersplat gaps, intrasplat cracks and three-dimensional coarse pore may be the reasons. Thermal conductivity and hardness are more sensitive to the changes in microstructure. After an exposure to a temperature of 1250℃for 5h, the coat's thermal conductive decreases from 0.88W/(m·K) to 0.75W/(m·K), while the hardness increases from 1.97GPa to 2.73GPa.The oxidation and thermal shock behavior of LNCZ TBCs at 1250oC were studied in detail. The failrue mechanisms in LNCZ TBCs were discussed by cracks analysis and the investigation of element's diffusion, combined with the theory of failure on thermal fatigue of PS TBCs as reported. The analysis indicates, during isothermal oxidation and thermal cycling, the breakage all occurs at the LNCZ coat interior primarily with a part of the interface between LNCZ and the thermally grown oxidation (TGO) and the TGO interior. But the failure mode and mechanism during these two processes are different.The failure mode of LNCZ during isothermal oxidation is edge-delamination. Its failure is dominated by one type of crack——parallel mode I crack which is initiated in LNCZ coat near the crests of bond coat undulations by tensile sintering stress and residual stress. The main cause of crack propagation and thickening is the sintering stress and the thermal growth stress caused by TGO product between bond coat and LNCZ coat. The reaction between TGO and LNCZ results in the loose microstructure near the interface of TGO/LNCZ and compressed stress in LNCZ coat. The parallel crack fast propagates along the weak area and induces the ultimate failure of the TBCs.The failure mode of LNCZ TBCs during thermal cycling is cracking and bucking- delamination. Its failure is dominated by two types of cracks——mode I crack and modeâ…¡crack, including the vertical mode I crack initiated in the LNCZ coat by residual stress, the parallel mode I crack between the ceramic splat, the mode I crack along the LNCZ/TGO interface at undulation crests, modeâ…¡crack within the LNCZ coat in the troughs adjacent to TGO and mode I crack within the LNCZ coat in the troughs of undulations which are induced by the thermal mismatch stress dues to the differences in thermal expansion between LNCZ coat and substrate. As the TGO being thicken, the thermal mismatch stress increases. The increased thermal mismatch stress and thermal mismatch stress can accelerate the propagation of all cracks along the weak area, such as the interface of LNCZ/TGO or TGO interior. The coalescence of transverse cracks induces the ultimate failure of the TBCs.The reasons for fast failure of LNCZ TBCs include the lower fracture toughness of LNCZ, rapid formation of brickle and porous oxides, such as NiO and (Cr,Al)2NiO4, and worse thermochemical compatibility between LNCZ and TGO. In order to release the thermal stress between bond coat and LNCZ coat and prevent the reaction between LNCZ and TGO, LNCZ/8YSZ DCL-TBCs were designed and fabricated. After being combined with 8YSZ, the thermal barrier ability of TBCs degrades slightly but its thermal shock resistance has been improved significantly. Just only 5% of the ceramic coat was delaminated after thermal shock for 45 cycles under 1250oC in an air furnace. The failure mode of DCL-TBCs during thermal cycling is bucking-delamination near the interface of LNCZ/8YSZ and 8YSZ/TGO by sequence. The failure mechanisms of LNCZ coat and 8YSZ coat are in the light of thermal mismatch strain and thermal growth stress, respectively.La_1.4Nd_0.6Zr₂O₇ (LNZ) TBCs on Mo substrate with high bonding strength was prepared for the first time. The mixture of Mo and LNZ powder was used as bond coat materials. The behaviors of LNZ TBCs during thermal cycling under 1200oC were examined. The result shows that the thermal shock life is very short. The reaction between LNZ and MoO3 from the oxidation of Mo results in the failure of LNZ TBCs on Mo substrate. Avoiding the oxidation of Mo or the osculation between molybdenum oxide and ceramic coat is favorable for the integrality of LZ-based TBCs on Mo substrate.