Y2O3 and Yb2O3 Co-doped Strontium Hafnateas a New Thermal Barrier Coating Material

Wen Ma, Peng Li, Hongying Dong, Yu Bai, Jinlan Zhao, and Xiaoze Fan

(Submitted April 25, 2013; in revised form August 10, 2013)

Y2O3 and Yb2O3 co-doped strontium hafnate powder with chemistry of Sr(Hf0.9Y0.05Yb0.05)O2.95

(SHYY) was synthesized by a solid-state reaction at 1450 �C. The SHYY showed good phase stabilitynot only from 200 to 1400 �C but also at a high temperature of 1450 �C for a long period, analyzed bydifferential scanning calorimetry and x-ray diffraction, respectively. The coefficient of thermal expansionof the sintered bulk SHYY was recorded by a high-temperature dilatometer and revealed a positiveinfluence on phase transitions of SrHfO3 by co-doping with Y2O3 and Yb2O3. The thermal conductivityof the bulk SHYY was approximately 16% lower in contrast to that of SrHfO3 at 1000 �C. Goodchemical compatibility was observed for SHYY with 8YSZ or Al2O3 powders after a 24 h heat treatmentat 1250 �C. The phase stability and the microstructure evolution of the as-sprayed SHYY coating duringannealing at 1400 �C were also investigated.

Keywords perovskites, TBC, thermal conductivity, thermalexpansion

1. Introduction

In recent years, much effort has been devoted to thedevelopment of new ceramics as thermal barrier coat-ing (TBC) materials, in order to increase the efficiencyof gas turbines (Ref 1, 2). The current state-of-the-artTBCs are usually based on 7-8 wt.% Y2O3-stabilizedZrO2 (YSZ) due to its low thermal conductivity, phasestability at temperatures below 1200 �C and chemicalinertness in combustion atmospheres (Ref 3). However,YSZ cannot be operated long-term at ~1200 �Cbecause of phase transitions and accelerated sintering(Ref 4-6).

In order to remedy such phenomenon, a wide searchhas been conducted for new TBC materials which allowoperation temperatures higher than 1200 �C withoutphase transition, aside from thermochemical and ther-mophysical compatibility with the substrate layer to be

coated. Recently, perovskites (ABO3) have been consid-ered for this purpose. Such perovskites are usually char-acterized by high melting point, low thermal conductivityand high coefficient of thermal expansion (CTE). Per-ovskites additionally offer the possibility of extensivesubstitution of ions at the A or/and B site, which enablesthe properties of the materials to be selectively influencedby mixed-crystal formation (Ref 7).

Among the perovskites, SrHfO3 has been proven as one ofthe most refractory oxides with melting point as high as2927 �C (Ref 8). Kennedy and Howard (Ref 9) examined thephase transition of SrHfO3 by a high-resolution neutronpowder diffraction method, and reported the phase transitionsas follows:

orthorhombicPbnm

�!600�Corthorhombic

Cmcm�!750�C

tetragonalI4=mcm

�!1130�CcubicPm�3m

:

The phase transitions of SrHfO3 normally give anegative influence for TBC applications. Fortunately,three phase transitions occur with very small volumechange (Ref 9), which are 0.6, 0.5, and 0.07% fromorthorhombic structure (Pbnm) to cubic structure(Pm�3m) in sequence, respectively. In addition, the valuesof the thermal conductivity of SrHfO3 are ~5.20 W/mK atroom temperature and ~2.90 W/mK at 1000 �C (Ref 10),these values are too high to utilize it for TBC application.The value of better TBC materials should have muchlower thermal conductivity (<2 W/mK). Normally, theimproved phase stability and lowered thermal conduc-tivity for TBC materials could be realized by co-dopingwith some selected oxides, which is evidently confirmedby SrZrO3 (Ref 11-13).

In the present study, the phase stability and thethermophysical properties of Y2O3 and Yb2O3 co-dopedSrHfO3 (SHYY) powder and bulk material, as well asthe phase stability and the microstructure evolution ofthe as-sprayed SHYY coating during annealing arepresented.

This article is an invited paper selected from presentations at the2013 International Thermal Spray Conference, held May 13-15,2013, in Busan, South Korea, and has been expanded from theoriginal presentation.

Wen Ma, Peng Li, Yu Bai, Jinlan Zhao, and Xiaoze Fan, Schoolof Materials Science and Engineering, Inner Mongolia Universityof Technology, Hohhot 010051, Inner Mongolia P. R. China; andHongying Dong, School of Chemical Engineering, InnerMongolia University of Technology, Hohhot 010051, InnerMongolia P. R. China. Contact e-mail: wma66@163.com.

JTTEE5

DOI: 10.1007/s11666-013-0006-9

1059-9630/$19.00 � ASM International

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2. Experimental Procedure

Solid-state synthesis of SHYY was carried out throughball milling using the starting powders of Y2O3 (99.99%,Grirem Advanced Materials Co. Ltd., China), Yb2O3

(99.99%, Grirem Advanced Materials Co. Ltd., China),SrCO3 (>98%, Shanghai Reagent Co. Ltd., China) andHfO2 (99.99%, Aldrich, USA). The powder mixture wasmilled for 24 h using zirconia balls as milling media indeionized water with a solid loading of 50 wt.%. Theweight percentage of both the powder mixture and thezirconia balls was 50%. The milled suspension was thendried and sintered at 1450 �C for 24 h. The powder aftersintering was crushed with a milling machine for 15 minusing zirconia balls as milling media to avoid introducingcontaminants. The synthesis procedure was repeated threetimes to obtain a purer phase material. The bulk SHYYwas cold-pressed under 30 MPa and sintered at 1700 �Cfor 6 h in air. The sample dimensions for CTE and thermaldiffusivity measurements were 3 9 4 9 25 mm andU10 9 1~2 mm, respectively. The CTE, thermal diffusiv-ity, and specific heat capacity of the bulk SHYY wererecorded by a high temperature dilatometer (Model DIL402PC, Netzsch, Germany), laser flash method (ModelLFA 427, Netzsch, Germany), and a simultaneous thermalanalysis apparatus (Model STA 449PC, Netzsch, Ger-many) equipped with a sample holder for thermo-gravimetry-differential scanning calorimetry (TG-DSC)analysis, respectively. The TG-DSC measurement wasconducted in air atmosphere. The density (q) of the sin-tered sample was measured according to Archimedesprinciple. The porosity of the bulk SHYY used for thermaldiffusivity measurement was ~5%. The thermal conduc-tivities of the sintered samples were calculated with theEq 1 and then calibrated with Eq 2:

k ¼ DthðTÞCpðTÞqðTÞ ðEq 1Þ

k=k0 ¼ 1� 4P=3; ðEq 2Þ

where k;DthðTÞ;CpðTÞ; qðTÞ; k0 and P are the thermalconductivity, the thermal diffusivity, the specific heatcapacity, the measured density, the calibrated value of thethermal conductivity, and the porosity, respectively.

The phase analysis of the synthesized powder and thecoating was characterized by x-ray diffraction (XRD)(Model D/MAX 2200, Rigaku Co. Ltd., Japan) at awavelength of 1.5406 A.

The synthesized SHYY powders were milled withethanol and 1.5 wt.% dispersing agent, then subsequentlyspray-dried. Sieved particles with sizes between 48 and100 lm were used for plasma spraying.

The SHYY coating was air plasma sprayed (F4 gun,Sulzer Metco, Switzerland) on steel substrates to a totalcoating thickness of ~1 mm. The spray parameters were asfollows: current = 600 A, voltage = 60 V, plasma gasAr/H2 = 40/10 L/min, spray distance = 120 mm. The free-standing coating, which was produced by removing thesteel substrate from the sprayed coating with hydrochloricacid, was used to investigate the phase stability and

microstructure evolution. The microstructure of the coat-ing was investigated by a scanning electron microscopy(SEM) (Model JXA 840, JEOL, Japan).

The chemical compatibility of SHYY with 8YSZ orAl2O3 was analyzed by XRD after heat treatment at1250 �C for 24 h. The weight fraction of the two compo-nent powders in the mixture was 50%.

3. Results and Discussion

3.1 Synthesis of the SHYY Powder and Its PhaseStability

The XRD patterns of the SHYY powder sintered at1450 �C for different times are presented in Fig. 1. The pow-der has an orthorhombic structure after sintering at 1450 �Cfor 24 h without any second phase (Fig. 1a), indicating SHYYwas successfully synthesized. The synthesized SHYY powderwas heat treated at 1450 �C up to 360 h (Fig. 1b-g), and no newphase developed during the long period heat treatment,implying very high long-term phase stability.

In addition to the long-term phase stability for TBCcandidate materials at high temperatures, the phase sta-bility of those materials in the temperature range betweenroom temperature and high temperature is also important.Figure 2 shows the differential scanning calorimetry(DSC) curve of the SHYY powder, the DSC curve of theSrHfO3 is also presented for comparison. No obvious peakappears in the DSC curve of the SHYY in the testedtemperature range compared to SrHfO3, revealing thatthe phase transition from tetragonal to cubic for SrHfO3

indicated by an arrow at ~1050 �C is suppressed byco-doping with Y2O3 and Yb2O3.

3.2 Thermophysical Properties of the Bulk SHYY

Figure 3 shows the CTEs of the bulk SHYY, as wellas SrHfO3 for comparison. The CTE of SrHfO3 is

Fig. 1 XRD patterns of the SHYY powder sintered at 1450 �Cfor different times: (a) 24 h, (b) 48 h, (c) 72 h, (d) 144 h, (e) 216 h,(f) 288 h, and (g) 360 h

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8.09-10.24 9 10�6 K�1 (200-1300 �C), whereas it is~10.5 9 10�6 K�1 (200-1100 �C) for YSZ (Ref 14). Theslope of the CTE curve of SrHfO3 has three changes at~650, ~760, and ~1050 �C, which are in accordance withthe phase transitions of SrHfO3 between room tempera-ture and high temperature. The CTE of SHYY is8.60-10.39 9 10�6 K�1 (200-1300 �C), which is higher thanthat of SrHfO3 in the whole tested temperature range.Furthermore, the CTE of SHYY has no obvious abnormalchange during measurement, indicating that co-dopingwith Y2O3 and Yb2O3 can improve the phase stability ofSrHfO3.

The thermophysical properties such as thermal diffu-sivity and heat capacity for the bulk SHYY and SrHfO3

were measured. Figure 4 shows the thermal diffusivities ofthe bulk SHYY and SrHfO3. The thermal diffusivity ofSHYY decreases with increasing temperature, it is ~10%lower than that of SrHfO3 in the whole tested temperaturerange. The Cp curves of the bulk SHYY and SrHfO3 areshown in Fig. 5. The Cp values of SHYY are slightly lowercompared to SrHfO3 and, in addition, the obvious threepeaks representing three-phase transitions in SrHfO3 donot appear in the Cp curve of SHYY (Ref 9). This con-firms that the phase stability of SrHfO3 can be improvedby co-doping with Y2O3 and Yb2O3. It is worth to notethat the three phase transition temperatures are ~650,~760, and ~1030 �C respectively, which are different fromthat of reported by Kennedy and Howard (Ref 9). Thismight be due to the different measurement methods. TheCp curve of SrHfO3 is much different from that ofYamanaka�s results, in which no peak can be observed inthe tested temperature range, even both results weremeasured by a DSC (Ref 10). Normally, the heat capacitymeasurement is very sensitive to phase transition of amaterial, it is also observed in SrZrO3 (Ref 15).

The calibrated values of the thermal conductivity forboth bulk SHYY and SrHfO3 as a function of temperatureare shown in Fig. 6. The decrease in the density due tothermal expansion was not taken into account in the cal-culation, so that the real thermal conductivity valuesshould be slightly lower.

The thermal conductivity of the bulk SHYY is~1.92 W/mK at 1000 �C, which is not only ~16% lowerthan that of SrHfO3, but also ~10% lower than thatof 8YSZ (bulk, 2.1-2.2 W/mK, 1000 �C) (Ref 16). The

Fig. 3 CTEs of the bulk SHYY and SrHfO3

Fig. 4 Thermal diffusivities of the SHYY and SrHfO3

Fig. 5 Heat capacities of the SHYY and SrHfO3

Fig. 2 DSC curve of the synthesized SHYY powder

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reasons for the reduction in thermal conductivity ofSHYY are the introduction of vacancies and strain fieldsby Y3+ and Yb3+ cation addition. Y2O3 and Yb2O3 dis-solve into the SrHfO3 matrix and create oxygen vacanciesto maintain the electroneutrality of the lattice. Y3+ andYb3+ cations also introduce strain fields as well as vacan-cies into the lattice, both of which will lower thermalconductivity by reducing the phonon mean free path.Another reason might be the formation of the Y

0

HfV��OYb

0

Hf

defect cluster at nanoscale, which is believed to beresponsible for the significant reduction of thermal con-ductivity. The defect clusters in SHYY have not beenverified independently; however, the defect clusters inNd2O3-Yb2O3 (and/or Sc2O3) co-doped YSZ have beenidentified by TEM analyses (Ref 17).

3.3 Chemical Compatibility of the SHYY with 8YSZor Al2O3

The XRD patterns of SHYY with 8YSZ or Al2O3

mixtures after heat treatment at 1250 �C for 24 h areshown in Fig. 7. There is no new reaction product betweenSHYY and 8YSZ (or Al2O3) appearing either inFig. 7(a) or in Fig. 7(b) with the exception of a minorYb2O3 phase. The addition of 8YSZ or Al2O3 into SHYYmight result in the precipitation of Yb2O3 from SHYY.The XRD results indicate that SHYY has a good chemicalcompatibility with 8YSZ or Al2O3, giving the possibility toapply a one-ceramic-layer (SHYY) coating system or adouble-ceramic-layer (SHYY/8YSZ) coating system. Theprimary thermal cycling results showed that the SHYY/8YSZ coating had much longer thermal cycling lifetimethan that of the SHYY coating.

3.4 Phase Stability and Microstructure Evolutionof the SHYY Coating

The XRD patterns of the SHYY coating in theas-sprayed case and after heat treatment at 1400 �C fordifferent times are shown in Fig. 8. The Yb2O3 phasedeveloped after plasma spraying in the as-sprayed coating.No new phase developed during heat treatment at 1400 �C

from 72 to 288 h, indicating that the SHYY coating hasgood phase stability at 1400 �C. The phase stability of theas-sprayed SHYY coating was also evaluated by TG-DSCanalysis. Figure 9 shows the TG-DSC curves of theas-sprayed SHYY coating, both of the curves are verysmooth and horizontal in the tested temperature range.This indicates that the coating has good phase stabilityfrom room temperature to 1400 �C, even with a minoramount of Yb2O3.

The precipitation of Yb2O3 in the SHYY coating uponspraying can be explained as follows. It is believed to bedue to the different vapor pressures for HfO2, SrO, Y2O3,and Yb2O3, which are 7 9 10�8 atm (2500 �C), 2 910�5 atm (2500 �C), 1 9 10�6 atm (2500 �C), and6 9 10�8 atm (2500 �C), respectively (Ref 18). Duringthermal spraying, SrO volatilizes more than HfO2 due toits higher vapor pressure, resulting in the coating compo-sition deviating from stoichiometric SrHfO3. Hf4+ is par-tially substituted in SrHfO3 by Y3+ and Yb3+, and the

Fig. 7 XRD patterns of SHYY + 8YSZ (a) and SHYY + Al2O3

(b) mixtures after heat treatment at 1250 �C for 24 h

Fig. 8 XRD patterns of the SHYY coating in as-sprayed case(a), after heat treatment at 1400 �C for (b) 72 h, (c) 144 h, (d) 216h, and (e) 288 h

Fig. 6 Thermal conductivities of the bulk SHYY and SrHfO3

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vapor pressure of SrO is higher than that of both Y2O3

and Yb2O3. Therefore, Yb2O3 might exceed its solubilityin the as-sprayed coating compared to the feedstockpowder, which is also observed in Yb2O3-doped SrZrO3

(Ref 11). Furthermore, the amount of Yb2O3 is minor inFig. 8, owing to the similar vapor pressure for both HfO2

and Yb2O3. Thus, the mixture of SHYY and a minoramount of Yb2O3 developed upon spraying.

Figure 10 shows the fracture microstructures of theSHYY coating after heat treatment at 1400 �C for differ-ent times. The as-sprayed SHYY coating consists of aporous structure and a layered structure, which is com-prised of the unmelted particles and the recrystallized

microstructure of the well-melted particles. Obviously, thelayered structure is composed of columnar crystals. Thecolumnar structure within a single splat still remains afterheat treatment for 216 h, indicating that the SHYY coat-ing has good sintering resistance. Considering that a singlemicrograph of a fracture surface is not a good indication ofthe sintering, the thermal conductivity measures will beused to investigate the sintering of the coating in future.

4. Conclusions

SHYY was successfully synthesized at 1450 �C after24 h without any secondary phase. The phase transitionfrom tetragonal to cubic for SrHfO3, investigated by DSCmeasurement, was suppressed by co-doping with Y2O3

and Yb2O3, indicating that SHYY has a good phase sta-bility between room temperature and 1400 �C. The long-term annealing of SHYY at 1450 �C for 360 h revealedvery high phase stability at high temperatures. The CTE ofSHYY was 8.60-10.39 9 10�6 K�1 (200-1300 �C) andexhibited no abnormal change during measurement,indicating that co-doping with Y2O3 and Yb2O3 canimprove the phase stability of SrHfO3. The thermal con-ductivity of SHYY was ~1.92 W/mK at 1000 �C, which is~16% lower than that of SrHfO3, indicating that co-dop-ing with Y2O3 and Yb2O3 is an effective method inreducing the thermal conductivity of SrHfO3. SHYY alsohas a good chemical compatibility with 8YSZ or Al2O3.

The free-standing SHYY coating was prepared by airplasma spraying. The coating consisted of SHYY and a

Fig. 10 Fracture microstructures of the SHYY coating after heat treatment at 1400 �C for different times: (a) as-sprayed, (b) 72 h,(c) 144 h, (d) 216 h

Fig. 9 TG-DSC curves of the as-sprayed SHYY coating

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minor amount of Yb2O3 secondary phase, and had goodphase stability after heat treatment at 1400 �C for 288 h.The columnar structure within a single splat still remainsafter heat treatment for 216 h.

Acknowledgments

The authors gratefully acknowledge the financial sup-port by the National Natural Science Foundation of China(51062012), the Key Project of Chinese Ministry of Edu-cation (210035), and the Program for New CenturyExcellent Talents in University (NCET-11-1017).

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