Ceramic Materials for Thermal Barrier Coatings

Ceramic materials for thermal barrier coatings

X.Q. Cao*,a, R. Vassenb, D. Stoeverb

aKey Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, Jilin, ChinabIWV-1, Forschungszentrum Juelich GmbH, D-52425, Germany

Received 30 June 2002; received in revised form 20 February 2003; accepted 7 March 2003

Abstract

This paper summarizes the basic properties of ceramic materials for thermal barrier coatings. Ceramics, in contrast to metals, areoften more resistant to oxidation, corrosion and wear, as well as being better thermal insulators. Except yttria stabilized zirconia,other materials such as lanthanum zirconate and rare earth oxides are also promising materials for thermal barrier coatings.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: Coatings; Mechanical properties; Thermal barrier coatings; Thermal properties

1. Introduction

During the past decade, research eorts were devotedto the development and manufacturing of ceramic ther-mal barrier coatings (TBCs) on turbine parts becausethe traditional turbine material have reached the limitsof their temperature capabilities. TBCs are deposited ontransition pieces, combustion lines, rst-stage bladesand vanes and other hot-path components of gas tur-bines either to increase the inlet temperature with aconsequent improvement of the eciency or to reducethe requirements for the cooling system.13

The earliest ceramic coatings for aerospace applica-tions were frit enamels developed by the NationalAdvisory Committee for Aeronautics (NACA) and thecoating of calcia stabilized zirconia on the exhaust noz-zle of the X-15 manned rocket plane in 1960s is believedto be the rst use of TBCs in manned ight.4 Theworking parts of aircraft jet engines are subjected toserve mechanical, chemical and thermal stresses. Severalceramic coatings such as Al2O3, TiO2, mullite, CaO/MgO+ZrO2, YSZ, CeO2+YSZ, zircon and La2Zr2O7,etc. have been evaluated as TBC materials.The selection of TBC materials is restricted by some

basic requirements: (1) high melting point, (2) no phasetransformation between room temperature and operationtemperature, (3) low thermal conductivity, (4) chemical

inertness, (5) thermal expansion match with the metallicsubstrate, (6) good adherence to the metallic substrateand (7) low sintering rate of the porous microstructure.1,5

The number of materials that can be used as TBCs is verylimited. So far, only a few materials have been found tobasically satisfy these requirements. This paper is believedto be the rst review about the ceramic TBC materialsand is helpful to the selection of TBC materials. In Ref. 6the development of new TBC systems is described. Thefollowing are TBC materials under investigation.Properties of some ceramics that can be used in TBC

system are summarized in Table 1. Among those prop-erties, thermal expansion coecient and thermal con-ductivity seem to be the most important. These data arecollected from dierent references and hence may not becomplete. Metal substrate and bond coats are alsoincluded for comparison. The number before yttria sta-bilized zirconia (YSZ) represents the weight percentageof Y2O3 in ZrO2. The advantages and disadvantages ofother TBC materials are compared with YSZ and listedin Table 2. The improvement techniques of YSZ coat-ings are also summarized in this table.

2. Materials for TBCs

2.1. YSZ

7-8YSZ is the most widely studied and used TBCmaterial because it provides the best performance in

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Journal of the European Ceramic Society 24 (2004) 110

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* Corresponding author.

E-mail address: [email protected] (X.Q. Cao).

high-temperature applications such as diesel enginesand gas turbines,27,29,33,3540 and reports about thismaterial are numerous. YSZ coating has been proved tobe more resistant against the corrosion of Na2SO4 andV2O5 than the ZrO2 coating stabilized by CaO orMgO.36 18-20YSZ coatings has also been studied.24,41 A

major disadvantage of YSZ is the limited operationtemperature (

perature of 1223 K in gas turbine for the ZrO2 coatingstabilized by CaO and MgO was reported.4 On the otherhand, these coatings, possess a high concentration ofoxygen ion vacancies, which at high temperature assistoxygen transport and the oxidation of the bond coat atthe ceramicbond coat interface, namely the formationof thermally grown oxide (TGO) on the bondcoat sur-face. This leads to spallation of the ceramic and sucha mode of failure of the TBC is predominant whenthe coatings are thin as in gas turbines. This problemhas been overcome to a large extent by providingoxidation resistant bond coats such as alumina andmullite.43 A model of life prediction of TBCs hasbeen developed, in which the coating failure wasattributed to stresses arising from the formation ofTGO,13 and in Fig. 1 is shown the relationshipbetween the thermal cycling life of TBCs and thesubstrate temperature.6

The silica impurity (even as low as 1 wt.%) in YSZcoating has a strong detrimental eect on the thermalcycling life.44,45 In bulk zirconia-based ceramics, silicasegregates to grain boundaries with excessive amountscollecting at triple points. Silica at the grain bound-aries leads to changes in the size and shape of grainsand it may dissolve Y2O3 from the YSZ grainboundary regions leading to localized destabiliza-

tion.45 Silica can also cause ZrO2 polycrystal super-plasticity, dramatic increases in sintering rates, anddecreases in electrical conductivity. It may also lead toincreased creep rates, as has been observed with silicon-base ceramics.45 However, silicates have much lower

Table 2

TBC materials and their characteristics

Materials Advantages Disadvantages

7-8 YSZ (1) high thermal expansion coecient (1) sintering above 1473 K

(2) low thermal conductivity (2) phase transformation (1443 K)

(3) high thermal shock resistance (3) corrosion

(4) oxygen-transparent

Mullite (1) high corrosion-resistance (1) crystallization (1023-1273 K)

(2) low thermal conductivity (2) very low thermal expansion coecient

(3) good thermal-shock resistance below 1273 K

(4) not oxygen-transparent

Alumina (1) high corrosion-resistance (1) phase transformation (1273 K)

(2) high hardness (2) high thermal conductivity

(3) not oxygen-transparent (3) very low thermal expansion coecient

YSZ+CeO2 (1) high thermal expansion coecient (1) increased sintering rate

(2) low thermal conductivity (2) CeO2 precipitation (> 1373 K)

(3) high corrosion-resistance (3) CeO2-loss during spraying

(4) less phase transformation between m and t than YSZ

(5) high thermal-shock resistance

La2Zr2O7 (1) very high thermal stability (1) relatively low thermal expansion coecient

(2) low thermal conductivity

(3) low sintering

(4) not oxygen-transparent

Silicates (1) Cheap, readily available (1) decomposition into ZrO2 and SiO2 during thermal spraying

(2) high corrosion-resistance (2) very low thermal expansion coecient

Improvement of YSZ TBCs: (1) post-deposition of the coating with sealants24,25 or laser irradiation of the coating surface26 for better corrosion

resistance; (2) gradient,6,27,28 or multilayered,6,2932 coatings with other materials; (3) thick TBC for better thermal insulation.33,34

Fig. 1. Thermal cycling lives of TBCs as a function of the substrate

temperature. (After Ref. 6. Reprinted with permission of the American

Ceramic Society, copyright 2002. All rights reserved.).

X.Q. Cao et al. / Journal of the European Ceramic Society 24 (2004) 110 3

oxygen conductivity than YSZ, and a thin layer of sili-cates on the top of the bond coat as oxygen barriermight improve the oxidation resistance of the bondcoat.46

2.2. Mullite

Mullite is an important ceramic material because ofits low density, high thermal stability, stability in severechemical environments, low thermal conductivity andfavorable strength and creep behavior.47 It is a com-pound of SiO2 and Al2O3 with composition 3Al2O3.

2SiO2. Compared with YSZ, mullite has a much lowerthermal expansion coecient and higher thermal con-ductivity, and is much more oxygen-resistant than YSZ.For the applications such as diesel engines where thesurface temperatures are lower than those encounteredin gas turbines and where the temperature variationsacross the coating are large, mullite is an excellentalternative to zirconia as a TBC material. Engine testsperformed with both materials show that the life of themullite coating in the engine is signicantly longer thanthat of zirconia.15,48 Above 1273 K, the thermal cyclinglife of mullite coating is much shorter than that ofYSZ.43 mullite coating crystallizes at 10231273 K,accompanied by a volume contraction, causing crackingand de-bonding.43 Mullite is the most promising coatingmaterial for the SiC substrate because their thermalexpansion coecients are similar.16

2.3. Al2O3

a-Al2O3 is the only stable phase among all aluminumoxides. It has very high hardness and chemical inert-ness. The erosion behavior of vacuum plasma sprayedand air plasma sprayed alumina coatings has beenstudied and compared with that of bulk material.49 Theaddition of a certain amount of alumina into YSZcoatings can improve the hardness and bond strengthwithout substantial modication of Youngs modulusand toughness.5052 The hardness of the coating canalso be achieved by spraying an outer coat of aluminaonto YSZ coatings.52 However, the plasma sprayedcoating of alumina contains mainly unstable phasessuch as g and d-Al2O3. These unstable phases willtransform into a-Al2O3 during thermal cycling, accom-panied by a signicant volume change (g!a, 15%)which results in microcrack formation in the coat-ing.53,54 The doping of transition metal oxides such asCr2O3, Fe2O3 and TiO2 into alumina can only partiallystabilize the a phase. On the other hand, alumina hasrelatively high thermal conductivity and low thermalexpansion coecient compared with YSZ.5 Themechanical properties of alumina coating can be sig-nicantly improved by the incorporation of silicon car-bide bers.55 Even though alumina alone is not a good

TBC candidate, its addition to YSZ can increase thehardness of the coating and improve the oxidationresistance of the substrate. A gradient coating of 8YSZAl2O3 has a much longer thermal cycling life than thatof 8YSZ coating.28

2.4. CeO2+YSZ

CeO2 has higher thermal expansion coecient andlower thermal conductivity than YSZ, and the additionof CeO2 into YSZ coating is supposed to be eectivefor the improvement of thermal cycling life. Remark-able improvement of thermal shock tolerability wasattained by the addition of CeO2 into YSZ.

5658 Theceria-doped coating has a better thermal shock resis-tance because mainly:59 (1) there is little phase trans-formation between the monoclinic and tetragonalphase in the CeO2+YSZ coating; (2) stress generatedby bond coat oxidation is smaller in the CeO2+YSZcoating due to better thermal insulation; (3) the thermalexpansion coecient is larger in the CeO2+YSZ coat-ing. However, the addition of CeO2 has some negativeeects, such as the decrease of hardness and stoichio-metry change of the coating due to the vaporization ofCeO2,

56 reduction of CeO2 into Ce2O3 and acceleratedsintering rate of the coating.60,61

2.5. La2Zr2O7

La2Zr2O7 (LZ) was recently proposed as a promisingTBC material.5,6,12,17,30,62,63 It has a cubic pyrochlorestructure which has been discussed in detail by Sub-ramanian et al.64 The crystal structure consists of thecorner-shared ZrO6 octahedra forming the back bone ofthe network and La3+ ions ll the holes which areformed by 6 ZrO6 octahedra. It can largely toleratevacancies at the La3+, Zr4+ and O2-sites withoutphase transformation. Both La3+ and Zr4+-sites can besubstituted by a lot of other elements with similar ionicradii in case the electrical neutrality is satised, givingrise to the possibility of its thermal properties to be tai-lored. It is one of the few oxides with pyrochlore struc-ture (such as La2Zr2O7, La2Hf2O7, Pr2Hf2O7, Ce2Zr2O7and Sm2Ti2O7) that are phase-stable up to their meltingpoints (Fig. 2) and this is a major reason that it isbelieved to have potential as TBC material. On theother hand, La2Zr2O7 has even lower thermal con-ductivity than YSZ. However, the coating of this mate-rial did not give a longer thermal cycling life than YSZcoating which might be explained by its relatively lowthermal expansion coecient and poor toughness.63 Bythe substitution of CeO2 for ZrO2 (LC), the thermalexpansion coecient of the coating can be increasedand its thermal cycling life is largely improved (Fig. 3),comparable with or even better than the standard coat-ing material 8YSZ.6

4 X.Q. Cao et al. / Journal of the European Ceramic Society 24 (2004) 110

2.6. Silicates

From the whole wide group of silicates, only thespraying of zircon (ZrSiO4)

66 or natural zircon sand67 isoccasionally reported. Zircon has a low thermal expan-sion coecient (4.99106 K1, 3001700 K) and highthermal conductivity (3.46 W m1 K1, 3651810 K).7

It is chemically very stable, particularly at lower tempera-tures. With the exception of concentrated hydrouoricacid, zircon is not generally attacked by acids but by basicmaterials at elevated temperatures. Zircon has no truemelting point as it dissociates before melting. The thermaldecomposition of zircon occurs at 1949 K or even as lowas 1558 K depending on the purity.68 During plasma

spraying, zircon dissociates and consequently coatings arecomposed of a mixture of crystalline ZrO2 and amor-phous SiO2. When using zircon as TBC for diesel engines,the decomposed SiO2 in the coating may cause problemsdue to the evaporation of SiO and Si(OH)2.

69 The bar-rier eect is supposed to be due to the ZrO2 phase in thecoating.67 Quite a few other silicates are relatively cheapand readily available and have potential as TBC mate-rials, such as garnet almandine Fe3Al2(SiO4)3, garnetpyrope Mg3Al2(SiO4)3, garnet andradite-grossularCa3Al2(SiO4)3, basalt (glass).

66 The coating of compo-site oxides 2CaO.SiO2-10 to 30 wt.% CaO.ZrO2 showsexcellent thermal shock resistance and hot corrosion.26

2.7. Rare earth oxides

The mixture of rare earth oxides is readily availableand very cheap. The coatings of rare earth oxides(La2O3, CeO2, Pr2O3 and Nb2O5 as main phases) havelower thermal diusivity and higher thermal expansioncoecient than ZrO2, and they have the potential asthermal barrier coatings.70 No further reports about thismaterial are available. Most of the rare earth oxides arepolymorphic at elevated temperatures,71 and the phaseinstability will certainly to some extent aect the ther-mal shock resistance of their coatings.

2.8. Metalglass composite

This is totally a new TBC system.72,73 The powdermixture of metal and normal glass can be plasma-sprayed in vacuum. For a suitable composition thisTBC material has similar thermal expansion coecientwith metal substrate and its thermal conductivity isabout a factor of 2 larger than that of YSZ. There arebasically three reasons for the long thermal cycling life ofmetal-glass TBC, namely high thermal expansion coe-cient (12.3106 K1), good adherence to the bond-coatand the absence of open porosity. Such a coating does nothave open pores, preventing the oxidation of bond-coat

Fig. 3. Coatings after thermal cycling: (3a) LZ, Tsurf.1200C/Tsub.960 C, 775 cycles; (3b) LC, Tsurf.1225 C/Tsub.965 C, 3238 cycles.

Fig. 2. Phase diagram of La2O3ZrO2 system. (After Ref. 65. Rep-

rinted with permission of the American Ceramic Society, copyright

2002. All rights reserved.)


from corrosive gases. Fig. 4 shows the microstructure ofsuch a coating after thermal treatment.

2.9. Y3AlxFe5xO12

Garnet ceramics with compositions of Y3AlxFe5xO12(x=0, 0.7, 1.4 and 5) have been proposed as TBCmaterials.19 YAG (Y3Al5O12), in particular, has super-ior high-temperature mechanical properties, excellentphase/thermal stability up to the melting point (2243 K)and low thermal conductivity. The oxygen diusivity inYAG is about 10 orders of magnitude lower than that inzirconia, implying that it is more oxygen-resistant thanthe latter and can give a better protection to the bondcoat. However, the relatively low thermal expansioncoecient (9.1106/K) and low melting point of thismaterial seem to be the major problem.

2.10. SrZrO3 and BaZrO3

So far, only two materials with perovskite structurehave been studied, i.e. SrZrO3 and BaZrO3.

17 They havevery high melting points (3073 and 2963 K, respec-tively), their thermal expansion coecients (10.9106K1 and 7.9106 K1, respectively, 3031273 K) arecomparatively lower than that of YSZ. A former workabout the TBC of BaZrO3 proved that this coating didnot show better thermal-shock resistance than YSZ.SrZrO3 shows a phase instability, which is expected tobe detrimental to its thermal shock resistance.

2.11. Lanthanum aluminates

A newly developed alumina-based ceramic coatingconsisting of La2O3, Al2O3 and MgO (MMeAl11O19,

M=La, Nd; Me=alkaline earth elements, magneto-plumbite structure) that possesses long term structuraland thermochemical stability up to 1673 K is presentedin Refs. 6,20,74. This coating has signicantly lowersintering rate than zirconia-based TBCs. The low ther-mal conductivity of LHA is caused by its micro-structure, i.e. a random arrangement of LHA plateletsthat build up a microporous coating and the insulatingproperties of the material with its crystallographic fea-ture itself. A study describing the development of anoptimized procedure for the processing, manufacturingand application of LHA as TBCs is reported in Ref. 20.

2.12. (Ca1xMgx)Zr4(PO4)6

It is reported that this material has lower thermalconductivity than zirconia and the thermal expansioncoecient is near zero.75 On the other hand, this mate-rial is less dense than ZrO2 with theoretical density 3.2g/cm3 vs. 5.8 g/cm3. Furthermore, no strength-loss afterair quenching from temperatures up to 1773 K wasobserved, showing a good thermal shock resistance. It isnot clear if this material can really be used for thermalbarrier coatings because of its low thermal expansioncoecient. No further results about this coating arereported.

2.13. LaPO4

Lathanum phosphate is monoclinic with four formulaunits of LaPO4 in the P21/n unit cell.

23 It is consideredas a potential material for thermal insulation coating onNi-based superalloys owing to its high-temperature sta-bility (melting point 234520 K), its high thermalexpansion and low thermal conductivity (comparablewith properties of dense zirconia).22 In addition, lan-thanum phosphate is expected to have good corrosionresistance in environments containing sulfur and vana-dium salts. It does not react with alumina, which is apositive attribute, but bonds poorly to it, which is alimitation for the application. On the other hand,LaPO4 is a line compound that melts congruently butsmall deviations from stoichiometry change its solidustemperature from 2343 to 1853 K on the La-rich side orto 1323 K on the P-rich side. From this point of view, itis very hard to make coating of this material by plasmaspraying, and it is expected that the coating made fromthis material can hardly be used for high temperatureapplications. No thermal cycling tests about this coatingare reported.

3. Summary

In the periodical table of elements, the elementswhose oxides nd applications in TBCs are mainly dis-

Fig. 4. Plasma-sprayed metalglass thermal barrier coating after

annealing at 1000 C for 300 h without cracks and spalling. White andgray strips are metal and glass, and the black spots are pores. (After

Ref. 73. Reprinted with permission of Wiley-VCH, STM-Copyright &

Licenses.)


tributed in IIIB (rare earth elements), IVB (Ti, Zr andHf), IIIA (Al) and IVA (Si). The IIA elements (Mg andCa) can only be used as stabilizers of zirconia. Rareearth oxides are promising materials for TBCs becauseof their low thermal conductivity, high thermal expan-sion coecients chemical inertness. By the doping ofcertain elements, the thermal expansion coecient ofLa2Zr2O7 can be increased. When its thermal expansioncoecient is increased to a value similar to that of YSZ,the La2Zr2O7 coating can be hopefully used in hightemperature applications. On the other hand, as thedual layer MCrAlY/YSZ system seems to reach theiroptimum performance, the development of TBCs mustshift towards dierent materials combinations anddeposition processes. It seems that the concept of mul-tilayer is eective for the improvement of the thermalshock life of TBCs29,76 because no single material satis-es all requirements for TBCs. The multilayer includesan erosion resistant layer as the outer layer, a thermalbarrier layer, a corrosion-oxidation resistant layer, athermal stress control layer and a diusion resistantlayer. Based on the multiplayer system, a double-cera-mic-coating system is newly developed as shown inFig. 5. On the top of 8YSZ coating, another coatingwhose thermal conductivity is lower than that of 8YSZis formed. The top ceramic layer acts as thermal insu-lator to protect the 8YSZ layer. As reported by theauthors, the thermal cycling performance of La2Zr2O7/8YSZ coating is excellent,77 and the CeO2/8YSZ coatingshowed a good thermal shock resistance.78 Taken intotal, several decades of TBC development indicate thatwhen all of the necessary properties for successful TBCsconsidered, YSZ remains dicult to be replaced.

Acknowledgements

This work was nancially supported by Bai Renand 863 projects (2002AA332010). Experiments

about LZ and LC coatings were nished in and nan-cially supported by Forschungszentrum Juelich GmbHwith the help of Dr. R. Vassen and Professor D. Stoever.

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Ceramic Materials for Thermal Barrier Coatings