Preparation And Properties Of Sulotion Percursor Plasma Sprayed ZrO₂-based Thermal Barrier Coatings

The advanced aero engines require high by-pass ratio,high thrust-weight ratio and high turbine inlet temperature due to the rapid development of the aviation industry,making thermal barrier coating?TBC?materials will be operated in much harsh environment.The state-of-the-art 8YSZ TBC cannot meet the demands that the new generation aero engines required,due to the phase transformation and sintering effect at higher temperatures.Therefore,new TBC materials and nano-structured TBC have attracted much attention.In this paper,the preparation and deposition mechanism of the 8YSZ TBC,as well as the phase stability and thermophysical properties of the double-rear-earth co-doped zirconia TBCs were investigated.The physical properties of the 8YSZ solution precursor were characterized by surface tension meter,Ubbelohde viscometer and TG-DSC.The phase compositions,microstructure and microhardness of the SPPS 8YSZ coatings were analyzed by XRD,SEM and microhardness tester,respectively.The SPPS spray parameters of the 8YSZ coating were optimized by Taguchi design of experiments?DOE?.The 8YSZ,Zr_0.92(Yb_0.04Gd_0.04)O_1.96?YGSZ?and Zr_0.92(Yb_0.04Y_0.04)O_1.96?YYSZ?coatings were prepared using the optimized spray parameters.The phase compositions and microstructures of the three coatings were characterized by XRD and SEM after different heat-treatment times,respectively,while the coatings' thermal expansion coefficients?TECs?and thermal diffusivities were evaluated by high-temperature thermal expansion dilatometer and laser-flash thermal analyzer,respectively.The optimal SPPS spray parameters were identified by Taguchi design of experiments?DOE?as follows: solution precursor concentration 2.35 mol/L,hydrogen flow rate of 12 slpm,and feedstock flow rate 28 ml/min.The deposition mechanism of the 8YSZ coating was investigated by changing atomizing pressure of the precursor solution,plasma torch traverse step size,spray distance and substrate preheat temperature.The 8YSZ coating exhibited relatively uniform and dense microstructure,less porosity and unmelted particles,higher deposition efficiency,as well as less unpyrolyzed solution precursor under atomizing pressure 0.13 MPa,plasma torch traverse step size 1-2 mm,spray distance 40 mm,and substrate preheating temperature above 300 oC.The investigation of the 8YSZ,YGSZ and YYSZ coatings heat-treated at 1400 oC for different times showed that,YYSZ had superior phase stability compared both to 8YSZ and YGSZ coatings.The coatings became denser with increasing heat-treatment time,accompanied by decreasing coating porosity.After 200 h heat-treatment,the porosities of the as-sprayed 8YSZ,YGSZ and YYSZ coatings generally decreased from 23% to 13%,21% to 12%,19% to 13%,respectively.However,the porosities of 8YSZ,YGSZ and YYSZ coatings slightly increased after 100 h,50 h,and 200 h,respectively,due to the phase transformation of zirconia.The TEC curves of the three coatings showed no abnormal change after 20 h heat-treatment,whereas the inflection point was appeared obviously with increasing heat-treatment time.The YYSZ coating had a low inflection point temperature of 580 oC compared with the YGSZ and 8YSZ coatings.The anti-sintering curve of the coating showed that the sintering resistance of the YYSZ coating and the YGSZ coating was significantly better than that of the 8YSZ coating.The thermal conductivities of the YYSZ and YGSZ coatings were lower than that of the 8YSZ coating in as-sprayed state.The thermal conductivities of the YGSZ and YYSZ coatings decreased by 60% compared to the 8YSZ coating after 200 h heat-treatment,with the lowest thermal conductivities of 1.38-1.61 W·m^-1K^-1 for the YYSZ coating.