abstract - wit press...structural materials modificated by ceramic coating were exposed at...

Correlation between microstructure and

degradation characteristics of plasma-spray

ceramics coating under cyclic heating condition

K. Sanada* & Y.

Abstract

Since plasma-spray ceramic coating is comparatively easy to form thick coating,this coating method is already applied as thermal barrier coating. When thesestructural materials modificated by ceramic coating were exposed at high-temperature environment, various problems such as corrosion, erosion andoxidation and so on. have been experienced. And in case when applyingceramics coating to actual components, it is very important to understand theirdegradation characteristics. Therefore, in this study, oxidation degradationcharacteristics of plasma-spray ceramic coating was evaluated under cyclicheating condition. And then, by means of observing their micro structure, theinterrelation between the above result and their degradation characteristics wasexamined. And, for the purpose of improving oxidation-resistant property, theeffect of sealing treatment by ethylsilicate and nickel plating was examined.The result of cyclic heating test showed that in the early stages of cyclic heating,LPPS coating have superior oxidation-resistant property than APS coating.This result is caused by dense microstructure of LPPS coating. However, withthe increase in cycles of heating, this relation is reversed due to the macrocrackgeneration in LPPS coating. And it is recognized that many pores in APScoating contributed to relieve the damage in cyclic heating. And it wasrecognized that sealing treatment is effective for improving oxidation-resistantand relieving the damage.

Transactions on Engineering Sciences vol 17, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533

116 Surface Treatment, Computer Methods and Experimental Measurements

1 Introduction

As for gas turbine components, jet engine and so on, the requirement forimprovement of their efficiency and output, caused the employment of higher-temperature environment.^ For using these structural material in such a severeenvironment, the ceramic coating technologies have been developed. Sinceplasma-spray ceramic coating is comparatively easy to form thick coating, thiscoating method is already applied as thermal barrier coating.̂ ^ When thesestructural materials modificated by ceramic coating were exposed at high-temperature environment, various problems such as corrosion, erosion andoxidation and so on, have been experienced. And in case when applyingceramics coating to actual components, it is very important to understand theirdegradation characteristics. Therefore, in this study, through measuring theweight gain of specimens by oxidation reaction with the number of heat cycles,oxidation degradation characteristics of plasma-spray ceramic coating wasevaluated under cyclic heating condition which is a severe environment forstructural materials. And then, by means of observing their microstructure, theinterrelation between the above result and their degradation characteristics wasexamined. And for the purpose of improving oxidation-resistant property, theeffect of sealing treatment by ethylsilicate and nickel plating was examined.

2 Experimental procedures

2.1 Specimens

For experimental specimens, both APS coating and LPPS coating wereprepared. APS coating is fabricated under atmospheric pressure, on the otherhand, LPPS coating is fabricated in the vacuum chamber of 20kPa. Thesesubstrate was austenitic stainless steel (AISI304) performed sandblast treatment.And as top coating layer, ZrCh containing 8wt.% YzOs was applied to bothAPS coating and LPPS coating. Top layer was 240 JJL m in thickness. On theother hand, as bonding layer with the intention of improving adhesion andreliefmg thermal stress, NiCrAlY was applied for APS coating, and Ni-20Crwas applied for LPPS coating. Both of bonding layers was 120 \i m in thickness.The chemical compositions of each layer were shown in Tables 1, 2 and 3. Andspecimen geometry was shown in Figure 1. Total thickness of specimen withcoated layers were shaped into 1.5mm thick to eliminate the effect of oxidationon the surface of substrate exposed to environment.

It has been known that top layer ZrOz is transformed by high temperatureexposure. Therefore, the structural analysis was conducted by X-ray diffractionfor confirming its crystallographic phase. The results were shown in Figure 2and Figure 3. Since these results showed that the top coated layer of both APS


Surface Treatment, Computer Methods and Experimental Measurements 117

Table 1 Chemical compositions ofZrOz

Ingredients

wt.%

ZlO2

91.63

Y2O3

7.87

AbO3

0.02

SiCfc

0.23

FeaOs

0.06

Table 2 Chemical compositions of NiCrAlY

Ingredients

wt.%

Ni

Bal.

Cr

224

Al

9.31

Y

0.86

Fe

0.13

Si

0.10

Table 3 Chemical compositions of Ni-20Cr

Ingredients

wt.%

Ni

Bal.

Cr

19.18

C

0.11

Si

1.50

S

0.008

Mn

0.85

Top coating layer |

Bond coating layer(NiCrAlY for APS coating, SubstrateNi-20Cr for LPPS coating) (AISI 304)

Figure 1 Specimen geometry

for cyclic heating test

1000

CU£

Time (hr.)

Figure 4 A heat cycletemperature pattern



(kcps)Q l l l l l l l l l l l l l l l l l l l l l . i l . . l l l l l l l l l l l l l l l l l l i l l C l l l l f l l l l f l l l l . - l A - L t . l_l_J_l—l ,

3̂2-]

t-ZrOz

1 <^ t-ZKh

1 4 A A . _ ._ ,20 30 5O4O 5O BO 7O GO

Figure 2 XRD profile of APS coating before testing

9O (deg)

4:

2-

1-I t - Z r O i

/ A2O 3O 4O 5O

t-ZrOz

60 70 80 90Figure 3 XRD profile of LPPS coating before testing

: Top coating layer8wt.%Y203-Zr02)

Bond coating layer

,0.5mm

0.24mm0.12mm11.28mm

Substrate/(A1SI304)

(a) Surface morphology

FigureS Specimen geometryfor sealing treatment

(b) Cross sectional morphology

Figure 6 SEM micrograph ofAPS ceramic coating

before testing



coating and LPPS coating have a crystal structure mainly consisted oftetragonal phase, it was recognized that these top coating layers have superiormechanical property such as high-toughness.

2.2 Cyclic heating test

The electric furnace was applied for cyclic heating test. Experimentalenvironment was laboratory air, and a heat cycle temperature pattern wasshown in Figure 4. And the weight gain of specimens by oxidation reactionwith the number of heat cycles was measured by electric chemical balance.

2.3 Sealing treatment

For the purpose of improving oxidation-resistant property of APS coatedspecimen, sealing treatment by ethylsilicate and nickel was conducted. Thespecimen geometry to which sealing treatment was conducted was shown inFigure 5. For sealing treatment by ethylsilicate, ethylsilicate containing ahardening agent was applied, this sealing treatment was done by brushing. Andfor sealing treatment by nickel, the electroplating was done in nickel sulfamatesolution containing boric acid at 5CTC, 0.5mA/dirT for Ihour.

3 Results and discussions

3.1 Microstructure of coating layers

SEM micrographs of APS coating before testing were shown in Figure 6. Itwas recognized that there were a lot of cracks on surface of APS coating(Figure 6-(a)) . It is considered that these cracks generation was caused by therapid cooling of coated material to room temperature. And in the cross sectionof APS coating, it was recognized that there were many pores in the top coatinglayer.(Figure 6-(b)) . This feature is caused by fabricating process of sprayingwhile catching the air. Figure 7 showed the microstructure of surface and crosssection of LPPS coating before testing. It was recognized that there weremicrocrack on the surface of LPPS coating (Figure 7-(a)). However, formationof microcrack had been suppressed than APS coating, because the row particleswere easily kept at high-temperature and melted well due to the decompressedenvironment. Microstructure of cross section of LPPS coating before testingwas shown in Figure 7-(b). It was recognized that LPPS coating havecomparatively few pores and dense micro structure than APS coating. Thisfeature of coated layers were brought about by catching few air in fabricatingprocess.



3.2 Degradation behavior of plasma-spray ceramic coatingunder cyclic heating condition and its governing factor

The results of cyclic heating test under atmospheric environment were shownin Figure 8. This figure indicated the correlation between the weight gain ofspecimens by oxidation reaction and the number of heat cycles. Since theweight gain of LPPS coating in the early stages of heating cycles had beensuppressed than APS coating, it was recognized that LPPS coating havesuperior oxidation-resistant property than APS coating. However, with theincrease in cycles of heating, oxidation rate of LPPS coating was extremelyaccelerated. Therefore, for investigating the governing factor of this behaviorobservations of the microstructure of APS coating and LPPS coating after 10cycles was conducted and shown in Figure 9. As mentioned before, APScoating is fabricated under atmospheric pressure, and its microstructure isfeatured by many pores. This characteristic feature is caused by fabricatingprocess of being sprayed with catching the air in itself. In case when thesepores are closed, this closed pore contribute to suppress thermal stressgeneration, and to improve the thermal barrier property. However, in case whenthis pore is opened, this opened pore causes the oxidation reaction of thesubstrate and bonding layer. And it was considered that the oxidationcharacteristics in early stages of cyclic heating was closely related with thedegree of the existence of opened pores.̂ With the number of heat cyclesincreased, it was recognized that the macrocracks formation on the surface ofAPS coating (Figure 9-(a),(b)) . However, when the oxidation reaction hadbeen advanced to some degree, the oxidation reaction was restrained by itsoxidation products because of these cracks were not reaching to the bondcoating layer. And it is considered that oxidation rate is governed by theparabolic law.

On the other hand, it was recognized the macrocrack formation on LPPScoating too, however, this morphology was a little different from that of APScoating. These cracks in the top coat of LPPS coating had reached to the bondcoating layer. These become the route of penetrating oxygen, and the boundarybetween the top coating layer and the bond coating layer was oxidizedpreferentially.̂ Since this phenomenon caused the continuous increase ofweight gain by oxidation reaction, it is considered that the weight gain of LPPScoating with the number of cycles was extremely accelerated comparing withAPS coating.

The macrocrack generation in LPPS coating was caused by the mismatch ofcoefficient of thermal expansion between ceramic and metal. In case of APScoating, since many pores is included in coating layer, APS coating iscompatible with expansion of the substrate under high-temperatureenvironment. In case of LPPS coating, since this coating have few opened



(a) Surface morphology

(b) Cross sectional morphology

Figure 7 SEM micrograph ofLPPS ceramic coatingbefore testing

4 8 12 16

Number of cycles

Figure 8 Relationship betweennumber of heat cyclesand weight gain

(a) Surface morphology of APScoating after 10 heat cycles

(c) Surface morphology of LPPScoating after 10 heat cycles

(b) Cross sectional morphology ofAPS coating after 10 heat cycles

(d) Cross sectional morphology ofLPPS coating after 10 heat cycles

20

Figure 9 SEM micrograph of ceramic coating after 10 heat cycles



(a) Top view of APS coatingwithout sealing treatment

(c) Top view of APS coatingwith sealing treatment

(b) Side view of APS coatingwithout sealing treatment

(d) Side view of APS coatingwith sealing treatment

Figure 10 Out looks of specimens sealing treated by ethylsilicateafter 10 heat cycles at 1000'C

0.001 -

(a) BSE image

10 20Cyclic heating number

Figure 12 Relationship betweennumber of cycles and

weight gain of APS coatingFigure 11 Cross sectional morphology of ard sealing treated

Nickel sealing treated APS coating y^pg coatingafter 10 heat cycles

(b) X-ray image (Nickel)



pores, it have superior oxidation-resistant property than APS coating in earlystages of cyclic heating. However, because LPPS coating have few closedpores, and it has dense microstructure, it has less compatible properties with theexpansion of the substrate comparing with that of APS coating. Since thischaracter caused the macrocrack generation, instead of superior oxidationresistance in early stages of cyclic heating, it is considered that the oxidation-resistant property of LPPS coating was degraded, as shown in Figure 8.

From the above mentioned results, LPPS coating attempted to form densemicrostructure by decreasing pores with the intension of improvement ofoxidation-resistance, have superior oxidation-resistant property in the earlystages of cyclic heating. However, with increase in number of cyclic heating,the damage to coating layer increase than APS coating. And in case ofconsidering about the reliability in long use, it was recognized that APS coatingwith some pores has superior characteristics comparing to LPPS coatingespecially under cyclic heating condition.

3.3 The effect of sealing treatment

For the purpose of improving oxidation-resistant property, the effects of sealingtreatment by ethylsilicate and nickel plating were investigated. Out looks ofspecimen sealing treated by ethylsilicate after 10 cycles of heating were shownin Figure 10. In specimen with no sealing treatment, formation of macrocrackswas recognized. On the other hand, in specimen with sealing treatment,macrocrack generation was not recognized. Figure 11 showed cross sectionalmorphology of specimen sealing treated by nickel plating after 10 cycles ofheating. It was recognized that the generated pores in coating layer were sealedby nickel. And then, in case when the cracks were formed, it was recognizedthat the crack propagating was suppressed by nickel phase. In short, thisbehavior was caused by propagating the crack into nickel ductile metallic phase,the crack tip becomes blunted, and stress concentration is relieved.^

And in Figure 12, sealing treatment is effective in improving oxidation-resistant property. From these results, it was recognized that sealing treatmentby nickel plating contributes for the improvement not only the oxidationresisting property but also that of suppressing crack propagation and exfoliation.

4 Conclusions

Correlation between microstructure and degradation characteristics of plasma-

spray ceramics coating under cyclic heating condition were investigated.

The results obtained were summarized as follows:(1) Microstructure of APS coating before testing is featured by its a lot of



micro pores. It is considered that this feature is caused by fabricating processof spraying while catching the air.

(2) LPPS coating have comparatively few pores and dense microstructure thanAPS coating, because LPPS coating is fabricated in the decompressionchamber of 20kPa.

(3) From the result of cyclic heating test, in the early stages of cyclic heating,LPPS coating have superior oxidation-resistant property than APS coating.This result is caused by dense microstructure of LPPS coating.

(4) With the increase in cycles of heating, the weight gain of LPPS coatingwith the number of cycles was extremely accelerated comparing with APScoating due to the macrocrack generation in LPPS coating.

(5) From the above results, it was recognized that many pores in APS coatingcontributed to relief the damage in cyclic heating.And in case of considering about the reliability in long use, it is consideredthat APS coating with some pores is superior characteristics to LPPScoating under cyclic heating condition.

(6) Sealing treatment by ethylsilicate and nickel plating is effective inimproving oxidation-resistant property, moreover, it was recognized thatnickel plating is effective in suppressing crack propagation.

References

[1] Y.Ito & M.Saito, Zazryo ATagakw, Vol.30, No.2, pp.86-93, 1993[2] N.Birks, High Temperature Materials for Gas Turbines, Int. Symp. on High

Temp. Materials to Solve Global Environmental Problems, Tokyo, 1993[3] Y.Mutoh, M.Ohki, M.Takahashi, Effect of Thermal Barrier Properties of

Thermal Barrier Coatings, Asian Pacific Conf. for Fracture and Strength,Korea, 1996

[4] Y.Ito, Kikai no kenkyu, Vol.47, No. 10, pp. 1040-1048, 1995[5] K.Sanada, Y.Kimura and K.Ishikawa, Proc. of JSME No.96-1,

pp.659~660, 1996


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