Hot corrosion behaviors of SrZrO3 ceramic co-doped with Y2O3 and Yb2O3

Available online at https://www.360docs.net/doc/d611447607.html,

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Journal of the European Ceramic Society34(2014)

3917–3924

Hot corrosion behaviors of SrZrO3ceramic co-doped with Y2O3and Yb2O3 in molten Na2SO4,V2O5,and Na2SO4+V2O5salts mixture Hongying Dong a,Yi Ren b,Dongxing Wang b,Xiaoying Li b,Yu Bai b,Jun Wang b,Wen Ma b,?

a School of Chemical Engineering,Inner Mongolia University of Technology,Hohhot010051,China

b School of Materials Science and Engineering,Inner Mongolia University of Technology,Hohhot010051,China

Received13February2014;received in revised form8May2014;accepted15May2014

Available online6June2014

Abstract

The hot corrosion behaviors of Sr(Y0.05Yb0.05Zr0.9)O2.95(SYYZ)ceramic were investigated in Na2SO4,V2O5,and Na2SO4+V2O5salts mixture, respectively.Na2SO4did not react with SYYZ ceramic at900,950and1000?C.m-ZrO2,YVO4and YbVO4were the main corrosion products on the SYYZ ceramic surface in V2O5at800and900?C,whereas Sr3V2O8and t-ZrO2appeared at1000?C.In Na2SO4+V2O5salts mixture, the corrosion products were Sr3V2O8and t-ZrO2at800and900?C on the SYYZ ceramic surface,however,a new phase of SrZrO3developed at 1000?C.The phase transformation and chemical interaction are the primary corrosion mechanisms for degradation of SYYZ ceramic.

Keywords:SrZrO3;Rare-earth oxides;Hot corrosion;Thermal barrier coatings

1.Introduction

Thermal barrier coatings(TBCs)are extensively used to pro-tect hot-section metallic components in gas turbine engines and therefore to achieve increasing turbine inlet temperature, with consequence of improved fuel ef?ciency,higher thrust and power.1Typical TBCs are multilayered systems consisting of a ceramic top coat for thermal insulation,a thermally grown oxide (TGO)scale,a metallic bond coat to against oxidation/corrosion and alleviate stress mismatch between top coat and substrate, and a superalloy substrate.To date,6–8wt.%yttria partially sta-bilized zirconia(YSZ)has been considered as a standard TBC material due to its low thermal conductivity,phase stability at temperatures below1200?C and chemical inertness in combus-tion atmosphere.2However,YSZ can not be long-term operated above1200?C because of phase transformations and accelerated sintering,resulting in a reduction of strain tolerance,an increase in Young’s modulus,and a volume change of the coating.3

?Corresponding author.Tel.:+864716575752;fax:+864716575752.

E-mail address:wma@https://www.360docs.net/doc/d611447607.html,(W.Ma).

In recent years,numerous efforts have been devoted to explore new TBC candidate materials with lower thermal con-ductivity,high phase stability and high sintering resistance,4–14 to meet the demand of the next generation of advanced engines. Among the alternate TBC materials,SrZrO3co-doped with Y2O3and Yb2O3(Sr(Y0.05Yb0.05Zr0.9)O2.95,SYYZ)with promising thermophysical properties than YSZ has been inves-tigated and the results indicate that,SYYZ is of great interest for TBC applications.15

In addition to applications in aero engines,TBCs are applied increasingly in land-based industrial engines and engines for marine applications that usually operated in corrosive envi-ronments or burn low-quality fuels containing impurities such as vanadium,sulfur and sodium.In such cases,hot cor-rosion becomes predominant and crucial to the lifetime of TBCs.During operation of turbine engines,molten vanadate and sulfate salts condense on the TBCs at temperatures of 600–1000?C.16,17Many literatures reported the hot corrosion behavior of YSZ and its hot corrosion mechanism is well understood.18–26The YSZ top coat was found to undergo a dis-ruptive phase transformation from t -ZrO2to m-ZrO2by the reaction of molten deposits with Y2O3stabilizer.The corrosion

3918H.Dong et al./Journal of the European Ceramic Society34(2014)3917–3924

behavior of some new TBC candidate materials was also inves-tigated,such as Gd2Zr2O7ceramic in contact with V2O5 in air at700–850?C,27SmYbZr2O7ceramic in contact with V2O5in air at600–1000?C,28NdMgAl11O19ceramic in con-tact with V2O5in air at650–950?C,29,30Yb2Zr2O7ceramic against V2O5and Na2SO4in air at900–1100?C,31as well as La2Zr2O7coatings,26,32,33LaTi2Al9O19/YSZ coating,34 YSZ/LaMgAl11O19composite coatings,35Gd2Zr2O7and YSZ/Gd2Zr2O7composite coatings36in contact with vanadate and/or sulfate salt in air at different temperatures.

However,no data on the phase evolution and microstruc-ture change of SYYZ ceramic upon high temperature exposure to molten Na2SO4,V2O5and Na2SO4+V2O5salts mixture, respectively,are available in open literatures.

In this contribution,SYYZ ceramic was prepared by pres-sureless sintering and exposed to molten Na2SO4,V2O5and Na2SO4+V2O5salts mixture,respectively,at800–1000?C for a period of10h.The hot corrosion resistance and the hot corrosion mechanisms of SYYZ ceramic were discussed.

2.Experimental procedure

SYYZ power was synthesized by solid-state reaction at 1450?C for up to72h using SrCO3(>98%,Shanghai Reagent Co.Ltd.,China),Y2O3(99.99%,Grirem Advanced Materials Co.Ltd.,China),Yb2O3(99.99%,Grirem Advanced Materials Co.Ltd.,China),ZrO2(99.99%,Grirem Advanced Materials Co.Ltd.,China)as starting materials.The synthesized SYYZ powder was cold pressed to form a pellet with10mm in diam-eter,and then sintered in a muf?e furnace at1700?C for6h to form SYYZ ceramic.

The Na2SO4,V2O5and Na2SO4+V2O5(mole ratio:1:1) salts mixture were spread over the SYYZ ceramic surfaces by using a very?ne glass rod at concentrations of20,10and 10mg/cm2,respectively,then SYYZ ceramic was subjected to isothermal heat treatment at temperatures of900,950,1000?C for Na2SO4coated one,800,900,1000?C for both V2O5and Na2SO4+V2O5coated ones for10h duration in air.Finally,the specimens were removed from the furnace after they reached room temperature and then were subjected to characterization.

The phase analyses of SYYZ ceramic before and after hot corrosion were characterized by X-ray diffraction(XRD,Model D/MAX2200,Rigaku Co.Ltd.,Japan).The surface and cross-section microstructure analyses of SYYZ ceramic were carried out using a scanning electron microscopy(SEM)(Model JXA 840,JEOL,Japan)equipped with an energy dispersive spec-trometer(EDS).

3.Results

3.1.Hot corrosion behavior of SYYZ ceramic in molten

Na2SO4salt

The hot corrosion behavior of SYYZ ceramic in molten Na2SO4salt was examined at900,950and1000?C.The XRD patterns of SYYZ ceramic before and after hot corrosion are pre-sented in Fig.1.Considering the melting point of Na2SO4

being Fig.1.XRD patterns of SYYZ ceramic before(a)and after hot corrosion in molten Na2SO4salt for10h at different temperatures:(b)900?C,(c)950?C and(d)1000?C.

884?C,the corrosion temperatures were set to above900?C to make it possible to melt Na2SO4.The single phase SYYZ ceramic with perovskite structure was prepared by pressureless sintering at1700?C for6h(Fig.1a),no new phase developed during hot corrosion process in molten Na2SO4salt at tempera-tures of900,950and1000?C,indicating no reaction happened between SYYZ ceramic and Na2SO4.

The surface and cross-section microstructures of SYYZ ceramic after hot corrosion in molten Na2SO4salt at900,950 and1000?C for10h are shown in Figs.2and3,respectively. Small particles appeared on the SYYZ ceramic surface after hot corrosion at900?C(Fig.2a),then the porous structure formed progressively with increasing corrosion temperature from950 (Fig.2b)to1000?C(Fig.2c).The phenomena are obvious in the cross-section microstructures of SYYZ ceramic after hot corro-sion,which are shown in Fig.3.The corroded layer thickness of SYYZ ceramic was increased from～5?m to～12?m and ～18?m with increasing corrosion temperature from900to950 and1000?C.

3.2.Hot corrosion behavior of SYYZ ceramic in molten

V2O5salt

Fig.4shows the XRD patterns of SYYZ ceramic before and after hot corrosion in molten V2O5salt at800,900and 1000?C for10h.After hot corrosion at800?C for10h,the SYYZ ceramic surface decomposed entirely and formed m-ZrO2,YVO4and YbVO4as hot corrosion products(Fig.4b). The contents of m-ZrO2,YVO4and YbVO4increased with increasing corrosion temperature from800to900?C(Fig.4c). However,the new phases of Sr3V2O8and t-ZrO2appeared with decreasing contents of m-ZrO2,YVO4and YbVO4at1000?C (Fig.4d).

The surface microstructures and EDS analyses of SYYZ ceramic after hot corrosion in molten V2O5salt at800,900 and1000?C for10h are shown in Fig.5.The SYYZ ceramic surface after hot corrosion at800and900?C became porous and consisted of light grey area A and dark grey area B(Fig.5a and b),which were identi?ed as m-ZrO2in area A,YVO4and

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Fig.2.Surface microstructures of SYYZ ceramic after hot corrosion in molten Na2SO4salt for10h at different temperatures:(a)900?C,(b)950?C and(c)1000?

Fig.3.Cross-section microstructures of SYYZ ceramic after hot corrosion in molten Na2SO4salt for10h at different temperatures:(a)900?C,(b)950?C and(c) 1000?C.

YbVO4in area B by EDS analyses together with XRD results.

After hot corrosion at1000?C for10h,the SYYZ ceramic sur-

face had much different microstructure in contrast to the SYYZ

ceramic surfaces corroded at lower temperatures(Fig.5c).The

ceramic surface consisted of the dense microstructure and the

agglomerated particles indicated by A and B,respectively.The

phase constitutes of the SYYZ ceramic surface were con?rmed

as Sr3V2O8in area A,as well as t-ZrO2,m-ZrO2,YVO4and

YbVO4in area B.

It is worth noting that,no YVO4was identi?ed in microstruc-

ture analyses for SYYZ ceramic after hot corrosion test,even the

YVO4peaks appeared in Fig.4.This might be due to the peak

overlap of Y and Zr element.In addition,YVO4and YbVO4as

hot corrosion products should be appeared simultaneously.

Fig.6shows the cross-section microstructures and EDS anal-

yses of SYYZ ceramic after hot corrosion in molten V2O5

salt at800,900and1000?C for10h.Fig.6(a)and(b)had

almost the same microstructures consisting of light grey area

A and dark grey area B.The light grey area A was rich

Fig.4.XRD patterns of SYYZ ceramic before(a)and after hot corrosion in molten V2O5salt for10h at different temperatures:(b)800?C,(c)900?C and (d)1000?C.ZrO2accompanied by YVO4,YbVO4and Sr3V2O8by XRD and EDS analyses,whereas rich Sr3V2O8included in area B accompanied by YVO4,YbVO4and ZrO2.A very different cross-section microstructure of SYYZ ceramic after hot cor-rosion at1000?C is presented in Fig.6(c).A dense dark grey layer developed on the SYYZ ceramic surface,which has been identi?ed as Sr3V2O8together with small amount of t-ZrO2, YVO4and YbVO4.The composition of the light grey area B in Fig.6(c)was similar to that of the light grey area A in Fig.6(a) and(b),consisting of small amount of ZrO2,YVO4,YbVO4and Sr3V2O8.

3.3.Hot corrosion behavior of SYYZ ceramic in molten

Na2SO4+V2O5salts mixture

Fig.7shows the XRD patterns of SYYZ ceramic before and after hot corrosion in molten Na2SO4+V2O5salts mixture at 800,900and1000?C for10h.As shown in Fig.7(b),Sr3V2O8 and t-ZrO2developed during hot corrosion at800?C,except for some residual Na2SO4.The contents of Sr3V2O8and t-ZrO2 increased progressively with increasing corrosion temperature from800to900?C(Fig.7c),and Na2SO4disappeared simulta-neously.After hot corrosion at1000?C,the content of Sr3V2O8 decreased remarkably,and a new phase of SrZrO3appeared.

The surface microstructures and EDS analyses of SYYZ ceramic after hot corrosion in molten Na2SO4+V2O5salts mix-ture at800,900and1000?C for10h are shown in Fig.8.The SYYZ ceramic surface after hot corrosion at800?C consisted of rod/plate like crystals area A and small particles area B(Fig.8a), which were identi?ed as Sr3V2O8in area A,t-ZrO2with residual Na2SO4in area B by EDS analyses together with XRD results. With increasing corrosion temperature to900?C,the small t-ZrO2particles became sintering(Fig.8b).Two distinct areas A and B appeared in Fig.8(c),which were identi?ed as Sr3V2O8 in area A,t-ZrO2together with SrZrO3in area B.

Fig.9shows the cross-section microstructures of SYYZ ceramic after hot corrosion in molten Na2SO4+V2O5salts

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Fig.5.Surface microstructures and EDS analyses of SYYZ ceramic after hot corrosion in molten V 2O 5salt for 10h at different temperatures:(a)800?C,(b)900?C and (c)1000?C.

mixture at 800,900and 1000?C for 10h.As shown in Fig.9(a),A ～10?m thick porous layer developed on the SYYZ ceramic surface after hot corrosion in molten Na 2SO 4+V 2O 5salts mixture at 800?C.The corroded layer thickness increased to ～13and ～22?m with increasing corrosion temperature to 900(Fig.9b)and 1000?C (Fig.9c),respectively.The corroded layer of SYYZ ceramic after hot corrosion at higher temperatures was denser than that of 800?C corroded one.4.Discussion

4.1.Hot corrosion mechanism of SYYZ ceramic in molten Na 2SO 4salt

SYYZ ceramic did not react with Na 2SO 4at 900,950and 1000?C according to the XRD results in Fig.1.However,

the corroded layer thickness increased with increasing corro-sion temperature,which is obvious in Fig.3.The corrosion mechanism of SYYZ ceramic in molten Na 2SO 4salt seems to be mineralization.Mineralization has been used extensively in obtaining phase equilibrium in silica systems and in certain zirconia systems.37Tani et al.38reported that molten NaVO 3apparently had acted to mineralize CeO 2(20wt%)-ZrO 2.4.2.Hot corrosion mechanism of SYYZ ceramic in molten V 2O 5salt

During hot corrosion test of SYYZ ceramic in contact with molten V 2O 5salt at 800and 900?C,the hot corrosion products of m -ZrO 2,YVO 4,YbVO 4and Sr 3V 2O 8developed simultane-ously.m -ZrO 2was rich on the corroded SYYZ ceramic surface,whereas m -ZrO 2and Sr 3V 2O 8were uniformly distributed under

H.Dong et al./Journal of the European Ceramic Society34(2014)3917–3924

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Fig.6.Cross-section microstructures and EDS analyses of SYYZ ceramic after hot corrosion in molten V2O5salt for10h at different temperatures:(a)800?C,(b) 900?C and(c)1000?C.

the upper corroded layer.The possible reactions are given as follows:

3SrZrO3(s)+V2O5(l)→Sr3V2O8(s)+3m-ZrO2(s)(1)

Y2O3(s)(inSYYZ)+Yb2O3(s)(inSYYZ)

+2V2O5(l)→2YVO4(s)+2YbVO4(s)(2) After hot corrosion at1000?C,a dense layer mainly consist-ing of Sr3V2O8formed on the SYYZ ceramic surface,which might suppress the hot corrosion of SYYZ ceramic by V2O5 effectively.

The reactions between vanadium compounds and ceramic oxides follow a Lewis acid-base mechanism,where the

acid Fig.7.XRD patterns of SYYZ ceramic before(a)and after hot corrosion in molten Na2SO4+V2O5salts mixture for10h at different temperatures:(b) 800?C,(c)900?C and(d)1000?C.

3922H.Dong et al./Journal of the European Ceramic Society34(2014)3917–3924

Fig.8.Surface microstructures and EDS analyses of SYYZ ceramic after hot corrosion in molten Na2SO4+V2O5salts mixture for10h at different temperatures: (a)800?C,(b)900?C and(c)1000?C.

Fig.9.Cross-section microstructures of SYYZ ceramic after hot corrosion in molten Na2SO4+V2O5salts mixture for10h at different temperatures:(a)800?C,(b) 900?C and(c)1000?C.

H.Dong et al./Journal of the European Ceramic Society34(2014)3917–39243923

vanadium compounds react more readily with ceramic oxides that have stronger basicity.As reported in literature,the basicity of strontium oxide,yttrium oxide,ytterbium oxide,and zirco-nium dioxide follows the order:SrO>Y2O3>Yb2O3>ZrO2, indicating that molten vanadium compounds has the tendency to react with SrO more easily.39,40

4.3.Hot corrosion mechanism of SYYZ ceramic in molten

Na2SO4+V2O5salts mixture

During the hot corrosion of SYYZ ceramic in molten Na2SO4+V2O5salts mixture,the uniformly spread Na2SO4+V2O5salts mixture underwent following steps. As the temperature increases from room temperature to designated corrosion temperature,V2O5melts?rstly due to its lower melting point of690?C,followed by Na2SO4with melting point of884?C.Then NaVO3with melting point of 610?C forms above884?C.Therefore,a part of V2O5reacted with SYYZ ceramic?rstly at800?C by Eq.(1),at the same time,Na2SO4did not melt due to its melting point higher than800?C.NaVO3developed by Eq.(3)with increasing temperature to900?C.NaVO3might react with SYYZ ceramic to form Sr3V2O8and t-ZrO2,which can be described by Eq.(4) Na2SO4(l)+V2O5(l)→2NaVO3(l)+SO3(g)(3) 3SrZrO3(s)+2NaVO3(l)→Sr3V2O8(s)+3t-ZrO2(s)

+Na2O(l)(4) As compared to the V2O5corroded SYYZ ceramic,no YVO4and YbVO4developed during hot corrosion in molten Na2SO4+V2O5salts mixture,this might be due to the lower concentration of V2O5.V2O5/NaVO3was consumed to react with SYYZ ceramic to form Sr3V2O8,with no more V2O5/NaVO3enough to react with Y2O3and Yb2O3,resulting in dissolving of Y2O3and Yb2O3into m-ZrO2to form t-ZrO2. The formation of SrZrO3at1000?C might be due to the reversal reaction of Eq.(4),this needs further investigation in future.

The residual Na2SO4on the SYYZ ceramic surface is con-?rmed by XRD analysis,which is shown in Fig.7(b).This can be explained by phase diagram of V2O5-Na2SO4.41The mole ratio of V2O5to Na2SO4in starting mixture is1:1,V2O5melts ?rst due to its lower melting point and reacts with SYYZ ceramic during hot corrosion test,resulting in more Na2SO4left on the SYYZ ceramic surface compared to V2O5.With temperature increase to melting point of Na2SO4,NaVO3develops by reac-tion(3)with mole ratio of V2O5to Na2SO4to be1:1,therefore, some Na2SO4remains on the SYYZ ceramic surface.

4.4.Thermodynamic properties of Y-V-O,Yb-V-O and

Sr-V-O systems

In order to predict the hot-corrosion behavior of SYYZ ceramic in different corrosion environments from the chemi-cal thermodynamic point of view,the Gibbs energy should

be Fig.10.Gibbs energy of Y-V-O,Yb-V-O and Sr-V-O systems as a function of temperature.

known as a function of temperature.The Gibbs energy can be calculated as follows:

f G?(298K)= f H?(298K)+T f S?(298K)(5)

The thermodynamic data are selected from Yokokawa’s work42and Barin’s work.43

Fig.10shows the Gibbs energy of Y-V-O,Yb-V-O and Sr-V-O systems as a function of temperature.It is evident that SrZO3can readily react with V2O5to form Sr3V2O8and ZrO2, followed by Y2O3and Yb2O3interacted with V2O5to form YVO4and YbVO4,respectively.The calculated results are in good agreement with the experiments results.It is more dif-?cult for SrZO3to react with NaVO3than with V2O5from thermodynamic point of view as shown in Fig.10.

5.Conclusions

Single phase SYYZ ceramic was successfully prepared by pressureless sintering at1700?C for6h.The hot corrosion tests of SYYZ ceramic in contact with molten Na2SO4,V2O5,or Na2SO4+V2O5salts mixture were conducted at temperatures of800–1000?C in air.Some conclusions can be drawn based on the experiments results:

(1)SYYZ ceramic did not react with molten Na2SO4

(20mg/cm2)at temperatures of900–1000?C,mineraliza-tion is the main corrosion mechanism in this case.

(2)SYYZ ceramic reacted with molten V2O5salt(10mg/cm2)

at800and900?C to form a porous layer consisting of m-ZrO2,YVO4,YbVO4and Sr3V2O8,whereas a dense layer mainly consisting of Sr3V2O8formed on the SYYZ ceramic surface at1000?C,which might suppress the further hot corrosion of SYYZ ceramic by V2O5effectively.

(3)SYYZ ceramic reacted with molten Na2SO4+V2O5

salts mixture(10mg/cm2)at800and900?C to form Sr3V2O8and t-ZrO2.The lower concentration of V2O5 in Na2SO4+V2O5salts mixture compared to V2O5alone

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corroded case is responsible for the development of t-ZrO2.

The formation of SrZrO3at1000?C might be due to the reversal reaction of Eq.(4),this needs further investigation in future.

Acknowledgements

The authors gratefully acknowledge the?nancial support by the National Natural Science Foundation of China(51062012, 51062013),and the Program for New Century Excellent Talents in University(NCET-11-1017).

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