Cite this: RSC Advances, 2013, 3,17034

Graded YSZ/Al2O3 hot corrosion resistant coating withenhanced thermal shock resistance

Received 11th May 2013,Accepted 15th July 2013

DOI: 10.1039/c3ra42326c

www.rsc.org/advances

Ruili Liu, Shuai Yuan,* Zhuyi Wang, Yin Zhao, Meihong Zhang and Liyi Shi*

In order to reduce the interfacial thermal stress, a novel functionally graded YSZ/Al2O3 nanocoating (YSZ/

Al2O3 FGC) which is hot corrosion resistant was developed with the sols of ZrO2, Y2O3 and AlOOH in this

paper. SEM and XRD analyses indicated that the YSZ/Al2O3 FGC was about 4.2 mm in thickness and

composed of t9-YSZ and c-Al2O3 nanoparticles. Hot corrosion tests showed that the YSZ/Al2O3 FGC

remarkably restrained the infiltration of the molten NaVO3 salt into the YSZ substrate. A thermal shock test

at 700 uC showed that the thermal shock resistance of the YSZ/Al2O3 FGC was enhanced greatly compared

with that of the Al2O3 coating. The improvement of the YSZ/Al2O3 FGC in the thermal cycle lifetime could

be attributed to its compositionally graded structure, which decreased the thermal stress due to the

thermal expansion coefficient mismatch between the YSZ substrate and the Al2O3 coating.

Introduction

Thermal barrier coatings (TBCs) are widely used as protectivecoatings on hot section components in industrial gas turbineengines and diesel engines to increase the operating tempera-ture and thus improve the efficiency, performance anddurability of the engine.1 A typical TBC consists of a bondcoat applied onto a substrate followed by an application of aceramic YSZ topcoat (6–12 wt% Y2O3 stabilized ZrO2).2

Industrial gas turbines and diesel engines are usually operatedwith fuels containing vanadium. In this case, a molten saltconsisting of vanadate is formed on the surface of the turbinecomponents during long term operation at elevated tempera-ture.3–8 The molten vanadate reacts with yttria to form YVO4,inducing the depletion of the Y2O3 stabilizer from the ZrO2

matrix and thus the structural destabilization of ZrO2,3–5 andeventually delamination and spalling of the ceramic coatingoccur. Because of the high amount of impurities and quite lowmean temperature in the combustion chamber of industrialgas turbines and diesel engines, the conditions are favorablefor hot corrosion. For these reasons, the major failuremechanisms that cause TBC spallation in industrial gasturbines and diesel engines are hot corrosion, thermal cyclingand mechanical loading.6

In order to improve the hot-corrosion resistance of TBCs,the outer surface of the YSZ coating may be isolated from themolten salt by depositing corrosion resistant materials. Thewell-known engineering ceramic material alumina (Al2O3) hasa very low solubility, particularly in molten salts, and is

expected to show an excellent corrosion resistance.9 It wasshown that Al2O3 dispersion in a NiCrAlY coating enhancedthe hot corrosion resistance in Na2SO4–V2O5 molten salts.3 AnAl2O3 overlay has been deposited on the outer surface of theYSZ coating.4,5 It remarkably restrained the infiltration of themolten sodium and vanadium into the YSZ coating, whichhindered the structural destabilization in the inner YSZ. Itindicated that an Al2O3 coating is very useful in applications asa hot corrosion resistant material.

However, there are also some problems with Al2O3 coatings,restricting its application as a hot corrosion resistant material.The most important problem is the misfit of the thermalexpansion coefficient between the Al2O3 coating and thesubstrate, which makes for the spallation failure during thecycling process. It has been reported that a Al2O3/13wt%TiO2

coating was delaminated from the metal substrate because ofthe thermal expansion mismatch between the ceramic coatingand the metal substrate.10 During thermal cycling, the ceramiccoating would experience tensile stress due to the thermalexpansion coefficient of Al2O3 being lower than the metalsubstrate, which finally induces a fracture between the topcoating and the substrate. The thermal expansion coefficientof YSZ is close to the metal, with a higher thermal expansioncoefficient than Al2O3.11 This indicates that when Al2O3 is usedas a hot corrosion material on the outer surface of a YSZcoating, a large thermal stress exists between the YSZ andAl2O3 layers due to the mismatching of their thermalexpansion coefficients, which will limit the application ofalumina as a hot corrosion resistant material on YSZ TBCs.

One approach to solve the above limitation of the aluminamaterial is the development of a functionally graded coatingwith a gradual composition variation between two pure layers.Temperature-dependent material properties of functionally

Research Centre of Nanoscience & Nanotechnology, Shanghai University, Shanghai

200444, China. E-mail: s.yuan@shu.edu.cn; shiliyi@shu.edu.cn; Tel: +86 21

66136082

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17034 | RSC Adv., 2013, 3, 17034–17038 This journal is � The Royal Society of Chemistry 2013

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graded materials made of two or more constituent phases withcontinuous and smoothly varying compositions vary continu-ously in the thickness direction according to a simple powerlaw distribution in terms of the volume fraction of thecompositions.12–14 Accordingly, the thermal stress of afunctionally graded coating system in a high temperatureenvironment is expected to be reduced.

In this paper, a new functionally graded YSZ/Al2O3

nanocoating (YSZ/Al2O3 FGC) was deposited onto the surfaceof a YSZ substrate by a dip coating method with pure andcomposite sols of ZrO2, Y2O3 and AlOOH in order to reduce theinterfacial thermal stress and subsequently elevate the coatinglifetime. The hot-corrosion property of the YSZ/Al2O3 FGC wasresearched. The thermal shock behavior of the YSZ/Al2O3 FGCcompared with that of an Al2O3 coating was investigated.

Experimental

Materials

YSZ substrates were purchased from Shanghai Daheng Opticsand Fine Mechanics Co., Ltd. The monoclinic ZrO2 sol (15.3wt%) was made in our lab. The Y2O3 sol (16.6 wt%) and AlOOHsol (17.4 wt%) were purchased from Dalian Snowchemical Co.,Ltd. The YSZ sol was prepared by mixing the Y2O3 sol and theZrO2 sol and diluting until the final solid contents of Y2O3 andZrO2 were 0.1 wt% and 1.8 wt%, respectively. The AlOOH solwas diluted until the final solid contents were 8.7 wt% and 1.8wt%. The YSZ sol was mixed with the AlOOH sol (1.8 wt%) toform three sol blends, i.e. 75%YSZ/25% AlOOH, 50%YSZ/50%AlOOH, 25%YSZ/75% AlOOH.

Preparation and characterization of the YSZ/Al2O3 FGC andthe Al2O3 coating

To prepare the YSZ/Al2O3 FGC, the YSZ substrate wascontinuously dipped into the sols of 1.8 wt% YSZ, 75%YSZ/25%AlOOH, 50%YSZ/50%AlOOH, 25%YSZ/75%AlOOH and 8.7wt% AlOOH. The (advance and withdraw) dipping speeds werecontrolled at 20 mm per minute and the specimens were heldin the solutions for 120 s. The deposition cycles were repeated5 times in the YSZ and YSZ/AlOOH sols and 25 times in theAlOOH sol. To prepare the Al2O3 coating, the YSZ substratewas dipped into the sol of 8.7 wt% AlOOH with 25 depositioncycles. The as-deposited films were further annealed in airisothermally at 700 uC for 2 h in a muffle furnace (CarboliteRWF). The annealed specimen was sectioned with a slowspeed abrasive diamond cutter. The microstructure andcomposition of the coating surface and the cross section werecharacterized by a scanning electron microscope (SEM,Hitachi S-4800) equipped with an energy-dispersive spectro-meter (EDS).

The phases of the coatings were represented by those of thepowders of the pure and composite sols above in order toavoid the disturbance of the YSZ substrate. The pure andcomposite sols above were dried at 100 uC and annealed at 700uC for 2 h. The powders were characterized by X-ray diffractionmeasurements. The X-ray diffraction patterns were collected atroom temperature by scanning steps of 0.02u (2h) over a 10u ,

2h , 90u angular range using a diffractionmeter (Rigaku D/MAX 2550 V PC) operated at 40 keV and 200 mA (Cu–Karadiation, l = 1.5418 Å).

Hot corrosion tests

To evaluate the effectiveness of the YSZ/Al2O3 FGC on the hotcorrosion resistance of the YSZ substrate, the YSZ substrateswith and without the YSZ/Al2O3 FGC and Al2O3 coating werecoated with 0.5 mg cm22 of salt by spreading an aqueoussolution of NaVO3 (20 mg ml21) then placing it into a mufflefurnace which was isothermally held at 700 uC for 60 min.After exposure, the samples were cooled down to roomtemperature in the furnace. After hot corrosion testing, theexposed samples were cleaned in de-ionized water. The cross-section was obtained by sectioning the substrate with a slowspeed abrasive diamond cutter. The microstructure andcomposition of the coating surface and the cross section ofthe specimens were characterized by SEM, EDS and XRD.

Thermal shock tests

Thermal shock tests were performed in a muffle furnace wherethe temperature had settled to 700 uC by a method similar toref. 15. Each cycle involves inserting samples into the furnace,followed by 30 min of holding them in the furnace and thenputting them in air for 10 min until the samples were cooled toambient temperature. The above steps were repeated. Eachsample was observed visually during the thermal cycle using amagnifier (64). When the damage area reached approximately20% of the total coating area, the tests were terminated andthe thermal cycle numbers were recorded as the lifetime of thecoatings.16 The thermal shock life was used to characterize thethermal shock resistance of the coating system and it wasobtained by averaging three measurements. The surface andcross sectional morphologies of the ceramic coatings wereexamined by SEM after the thermal shock experiment.

Results and discussion

Characteristics of the YSZ/Al2O3 FGC and Al2O3 coating

Fig. 1 shows the surface and cross sections of the YSZ/Al2O3

FGC and Al2O3 coating. The surfaces of the as-prepared YSZ/Al2O3 FGC (Fig. 1a) and Al2O3 coating (Fig. 1b) revealed auniform coating structure. From the magnified surfacephotographs (Fig. 1c, d) it can be seen that the coatings werecomposed of nanoparticles with diameters of about 30 nm. Itcan be seen from the SEM images of the cross sections (Fig. 1e,f) that the YSZ/Al2O3 FGC and Al2O3 coating were dense andadhered to the YSZ substrates. The total thickness of thedeposited layers in the YSZ/Al2O3 FGC and the Al2O3 coatingwere measured to be 4.2 mm and 1.4 mm, respectively. Fig. 2 isthe cross-sectional SEM micrographs of the YSZ/Al2O3 FGCand the corresponding elemental maps: Zr, Y and Al.Elemental-distribution map analysis revealed the gradualdistribution of the three elements along the thickness of thecoating. The interfaces between the different layers in the YSZ/Al2O3 FGC (Fig. 1e) were distinctive, implying that thecomposition had been graded as designed. It is expected that

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the properties of the YSZ/Al2O3 FGC system will exhibit agradual change from one layer to another.

The XRD patterns of the dried and calcinated powders of theabove sols were detected to represent the phases of thecoatings in order to avoid the disturbance of the YSZ substrate.Fig. 3 shows the XRD patterns of the pure and compositepowders of YSZ and Al2O3 dried at 100 uC and calcinated at 700uC for 2 h. The dried YSZ and Al2O3 powders were composed ofa monoclinic zirconia phase and AlOOH, according to Joint

Committee on Powder Diffraction Standards (JCPDS) No. 24-1165 and 49-0133, available at the International Centre forDiffraction Data (ICDD). After calcination at 700 uC for 2 h, thepure and composite YSZ and Al2O3 powders were transformedto non-equilibrium tetragonal (t9) YSZ (JCPDS 24-1164) andc-Al2O3 (JCPDS 46-1131), respectively.17 This indicated thatafter annealing at 700 uC, Y2O3 was doped into the crystallattice of ZrO2 and the t9-YSZ phase was formed. It could beinferred that the as-prepared YSZ/Al2O3 FGC was composed oft9-YSZ and c-Al2O3.

Hot corrosion resistance

The hot corrosion resistance of the YSZ/Al2O3 FGC and Al2O3

coating was tested and compared with that of the YSZsubstrate without the overlay. The color of the YSZ substratewithout the overlay changed from white to yellow afterexposure to the molten salt. Further SEM and EDS examina-tions are shown in Fig. 4a. It shows that the surface of the YSZ

Fig. 1 SEM pictures of the YSZ plates with YSZ/Al2O3 FGC and Al2O3 coating. (a,c) coating surface of the YSZ/Al2O3 FGC; (b, d) coating surface of the Al2O3

coating; (e) cross section of the YSZ/Al2O3 FGC; (f) cross section of the Al2O3

coating.

Fig. 2 Cross section SEM micrograph and corresponding element mapping ofthe YSZ/Al2O3 FGC on the YSZ substrate.

Fig. 3 XRD patterns of the pure and composite powders of YSZ and AlOOH(Al2O3) dried at 100 uC (A) and calcined at 700 uC (B). (a) powders of YSZ; (b)powders of 75%YSZ/25%AlOOH(Al2O3); (c) powders of 50%YSZ/50%AlOOH(Al2O3); (d) powders of 25%YSZ/75%AlOOH(Al2O3); (e) powders ofAlOOH(Al2O3).

17036 | RSC Adv., 2013, 3, 17034–17038 This journal is � The Royal Society of Chemistry 2013

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plate became coarse with many columnar particles on thesurface and inside the coating.

EDS analysis showed that the particles were composed ofyttrium, vanadium and oxygen. Zirconium was also detected inthe EDS spectrum, which may be from the YSZ substrate. Thisindicates that the particles should be YVO4.8 Further X-raydiffraction analysis was performed on the YSZ substrate beforeand after the hot corrosion test, as shown in Fig. 4b. The X-raydiffraction pattern of the YSZ substrate before the hotcorrosion test indicated that it contained predominantlyt9-YSZ. After exposure to the molten salt at 700 uC for 1 h,the YVO4 phase was formed, implying the leaching of Y2O3

from YSZ by the reaction of Y2O3 with NaVO3. As a result, theintensity of t9-phase remarkably decreased and a substantialamount of the m-phase was formed due to the leaching ofY2O3 from YSZ. The above results are consistent with those inref. 5.

The color of the YSZ plates with the YSZ/Al2O3 FGC andAl2O3 overlay remained white after the hot corrosion test. SEMexamination (Fig. 4c, d) showed that no YVO4 crystal wasformed on the surfaces of the YSZ plates with the YSZ/Al2O3

FGC and Al2O3 overlay and the surface morphology of theplates was the same as the specimens before corrosion. Thisindicates that the YSZ substrates were effectively protectedfrom the molten salt by the YSZ/Al2O3 FGC and Al2O3 coating.This favorable result may be due to the anticorrosion effect ofthe Al2O3 component.9

Thermal cycling behaviors

The thermal cycling lives of the YSZ/Al2O3 FGC and the Al2O3

coating at 700 uC were 43 and 25 times, as shown in Fig. 5.This indicates that the thermal shock property of the YSZ/Al2O3 FGC gets ahead of that of the conventional Al2O3

coating. In order to study the failure mechanisms of thecoating systems, the surface and cross sections of the failed

specimens were examined, as shown in Fig. 6. The spallationfailure in the YSZ/Al2O3 FGC (Fig. 6a, c) and the Al2O3 coating(Fig. 6b, d) took place by typical delamination cracking at theinterface between the coating and the YSZ substrate. Thestress produced during the thermal cyclic process due to thethermal expansion mismatch between the Al2O3 coating andthe YSZ substrate results in the spallation of the top coatingwhen it is large enough accompanied by relaxation of theinterfacial residential stress.9

The improvement of the YSZ/Al2O3 FGC in the thermal cyclelifetime compared to the Al2O3 coating could be attributed toits compositionally graded structure as the graded structureprovides a more gradual transition in properties through thecoating thickness, according to the power law, compared withthe sudden change of properties in a monolithic Al2O3 coatingsystem.12 In the YSZ/Al2O3 FGC system, the thermal stressoriginating from the thermal expansion coefficient mismatchbetween the YSZ substrate and the Al2O3 coating wasdecreased, which is beneficial to improve the binding strength

Fig. 4 SEM photograph and EDS spectrum (inserted picture) of the YSZsubstrate after the hot corrosion test (a). XRD patterns of the YSZ substratebefore and after the hot corrosion test (b). SEM photographs of the YSZ/Al2O3

FGC (c) and the Al2O3 coating (d) after the hot corrosion test.

Fig. 5 Thermal shock lives of the YSZ/Al2O3 FGC and Al2O3 coating.

Fig. 6 SEM photographs of the YSZ/Al2O3 FGC coating and the Al2O3 coatingafter the thermal shock test.

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of the coating system. The probability of delamination can bereduced due to the lower residual stress.

Conclusions

A novel functionally graded YSZ/Al2O3 hot corrosion resistantnanocoating was developed by the sol–gel method with ZrO2,Y2O3 and AlOOH sols. It was found that the YSZ/Al2O3 FGC wascomposed of t9-YSZ and c-Al2O3 nanoparticles. The hotcorrosion tests showed that the YSZ/Al2O3 FGC acted as abarrier against the infiltration of vanadium into the YSZsubstrate, which may hinder the structural destabilization ofthe inner YSZ. The thermal shock test result showed that thethermal shock property of the YSZ/Al2O3 FGC coating systemovermatches that of the Al2O3 coating. The improvement of theYSZ/Al2O3 FGC in the thermal cycle lifetime compared to theAl2O3 coating could be attributed to its compositionally gradedstructure, which decreased the thermal stress due to thethermal expansion coefficient mismatch between the YSZsubstrate and the Al2O3 coating.

Acknowledgements

The authors acknowledge the support of the Shanghai LeadingAcademic Discipline Project (S30107) and the DongguanScience and Technology Bureau (2012108102029).

Notes and references

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17038 | RSC Adv., 2013, 3, 17034–17038 This journal is � The Royal Society of Chemistry 2013

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