Properties Of Novel Sm₂Zr₂O₇Material And Thermal Barrier Coatings

Materials with higher heat insulation ability and higher operating temperature are knownto have many interesting combinations of physical and mechanical properties. However,forhot components of aircraft engine and gas turbine, conventional ZrO2partically stabilizedwith Y2O3(YSZ) coatings is not meet the demand and new types of ceramic thermal barriercoating materials should be found. Sm2Zr2O7(SZO) is one of the promising candidates dueto their stable phase structure, low thermal conductivity, high melting point, etc. At thesame time, the improvement of thermal shock of SZO thermal barrier coatings (TBCs) isneeded.This paper reports the fabrication of SZO-TBCs with substitution of ions at both A andB sites. The aim is to evaluate and compare the thermal shock performances to providediscrete fundamental information to better understand and, in turn, optimize the coatingstructure and pre-oxidation treatment. The effect of the key factors on thermal shockperformance of SZO-TBCs were determined and the failure mechanisms for the observedthermal shock wasdiscussed.Ceramic materials of SZO and (La0.5Sm0.5)2(Zr1-xCex)2O7(x=0.1,0.2,0.3,0.4,0.5)were synthesized bychemical coprecipitation method. The phase of(La0.5Sm0.5)2(Zr1-xCex)2O7is varied from pyrochlore structure to fluorite structure withincreasing of Ce4+doping amount. According to the impact on the specific surface area of(La0.5Sm0.5)2(Zr0.7Ce0.3)2O7(LSZCO) powder, the factors of the process is ordered indescending sequence as follows: pH value, dispersant content, concentration of reactants,the reaction temperature. LSZCO ceramic powders was fabricated with size distributions of15~40nm in an optimal condition (50oC, pH=11, dispersant content0.4Vol%,concentration of reactants0.6mol/L).The agglomerated SZO powders were prepared by ball-milling, spray drying andinduction plasma spheroidizing technology respectively. The spherical powders fabricatedby the latter two methods are more suitable for the atmospheric plasma spraying (APS).According to the impact on the spheroidization rate of SZO powder, the factors of theinduction plasma spheroidizing process were ordered in descending sequence as follows: hydrogen flow rate, argon flow rate8slpm, pressure of reaction chamber, feeding rate.The spheroidization rate of SZO powder with particle size of20~80μm was98.4%inthe optimal synthesized conditions (hydrogen flow rate60slpm, argon flow rate8slpm,pressure of reaction chamber12psig, feeding rate12g/min). Compared with beforetreatment, the fluidity of SZO powder increased by20.4%.The thermal isulation effect of coatings was evaluated utilizing circular tube methodunder the same boundary conditions.The thermal isulation effect of coatings are ordered indescending sequence as fllows: LSZCO-TBCs, YSZ-TBCs, SZO-TBCs. Compared toSZO-TBCs and YSZ-TBCs, LSZCO-TBCs with coating thickness0.3mm exhibits thethermal isulation from10.3%to28.7%due to the lower thermal conductivity in thecondition of cooling flow3m3/h. The thermal conductivity of SZO materials is reduced by19.1%with doping La3+and Ce4+at A and B sites, respectively.The thermal shock performance of single-ceramic-layer (SCL SZO-TBCs) under1300℃was measured with a burner rig. The thermal stress in the SZO/BC, SZO/TGOs andBC/TGO interfaces are formed due to the different thermal expansion coefficient withdifferent materials. Under the combination of these thermal stresses, SCL SZO-TBCs iscracking with horizontal expansion cracks at the SZO/TGO interface. Such crack should becontributed to the spalling of SZO near the SZO/TGO interface, which leads to failure ofthe coating.Comparison between the thermal expansion coefficient (TEC) of SZO materialspresent that the improvement of LSZCO and YSZ materials is8.3%,14.1%and20%,respectively. The thermal shock lifetime of the SZO-TBCs depend on vacuum per-oxidationtreatment (VHT), materials modification (LSZCO) and YSZ/SZO double ceramic structure(DCL SZO/YSZ), increasing with the extent of33.3%,70.7%,340.0%, respectively. Theresults provide an innovative example of improving the durability of SZO-TBCs byadjusting the TEC of materials. For the DCL SZO/YSZ, the thermal stress at the TGO/BCinterface caused by the increased thickness of TGO is the main factor that lead the failureof coating. The thermal shock lifetime of these coatings is increased up to more than19times compared with SCL SZO-TBCs’by VHT.