Thermal cycling behaviour of plasma sprayed lanthanum zirconate based coatings under concurrent...

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Ceramics International 39 (2013) 1413–1431

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Thermal cycling behaviour of plasma sprayed lanthanum zirconatebased coatings under concurrent infiltration by a molten

glass concoction

C.S. Ramachandrana,n, V. Balasubramaniana, P.V. Ananthapadmanabhanb

aCentre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608002,

Tamilnadu, IndiabPlasma Spray Technologies Section (PSTS), Laser and Plasma Technology Division (L&PTD), Bhabha Atomic Research Centre (BARC), Anu Shakthi

Nagar, Trombay, Mumbai 400085, Maharashtra, India

Received 7 July 2012; received in revised form 21 July 2012; accepted 24 July 2012

Available online 16 August 2012

Abstract

Thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies

and performance. However, during aero engine operation, environmentally ingested airborne particles, which includes mineral debris,

sand dust and volcanic ashes get ingested by the turbine with the intake air. As engine temperatures increase, the finer debris tends to

adhere to the coating surface and form calcium magnesium alumino-silicate (CMAS) melts that penetrate the open void spaces in the

coating. Upon cooling at the end of an operation cycle, the melt freezes and the infiltrated volume of the coating becomes rigid and

starts to spall by losing its ability to accommodate strains arising from the thermal expansion mismatch with the underlying metal. The

state-of-the-art ZrO2-7-weight% Y2O3 (YSZ) coatings are susceptible to the aforementioned degradation. Rare-earth zirconates have

generated substantial interest as novel thermal barrier coatings (TBC) based primarily on their intrinsically lower thermal conductivity

and higher resistance to sintering than YSZ. In addition, the pyrochlore zirconates are stable as single phases at up to their melting

point. La2Zr2O7 (LZ) is one among such candidates. Hence, the present study focusses on the comparison of cyclic molten CMAS

infiltration behaviour of the base metal Inconel 738 (BM), the bond coat NiCrAlY (BC), the duplex YSZ, the LZ coating and a five

layered coated specimen with LZ as top layer. Among those coatings mentioned above, the five layer coated specimen showed excellent

CMAS infiltration resistance under thermal cycling conditions.

& 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: B. Failure analysis; D. Glass; D. Spinels; D. ZrO2

1. Introduction

The Thermal Barrier Coatings (TBCs) are regarded asone of the most successful innovations and applications ofcoatings in industry. Durability and performance ofadvanced gas turbines for aircraft propulsion, powergeneration, marine propulsion and industrial engine appli-cations have been improved with the application ofthermal barrier coatings (TBCs) to high pressure turbine

e front matter & 2012 Elsevier Ltd and Techna Group S.r.l. A

g/10.1016/j.ceramint.2012.07.084

ing author. Tel.: þ91 4144 230382/þ91 9843892693;

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esses: [email protected].

om (C.S Ramachandran.).

(HPT) aerofoils [1]. TBCs are typically deposited viaelectron beam physical vapour deposition (EB-PVD) oratmospheric plasma spraying (APS). Several million tur-bine blades and vanes are currently flying. CommercialTBCs are comprised of an oxidation resistant bond coating(typically Pt–Al, NiCrAlY or NiCoCrAlY), a thermallygrown oxide (predominately alpha-alumina), and anEB-PVD/APS deposited ceramic coating, which is typi-cally comprised of tetragonal yttria-stabilized zirconia(7–8 wt.% per cent Y2O3 stabilized ZrO2) [2]. With theever increasing demand to increase the turbine inlettemperature (TIT) for improved engine efficiency, a primereliant TBC system to effectively protect the hot sectionturbine components has been a critical requirement for gas

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C.S. Ramachandran et al. / Ceramics International 39 (2013) 1413–14311414

turbine engines. Extensive efforts have been made toidentify the failure mechanisms of the TBCs to increasethe durability and reliability of TBCs [3].

A major disadvantage of the state of the art yttria-stabilized zirconia (YSZ) coating is the limited operationtemperature at and above 1200 1C for long-term applica-tion. At such higher temperatures, phase transformationsfrom the ‘(t0)’-tetragonal prime to tetragonal and cubic‘(tþc)’ and then to monoclinic ‘(m)’ occur, giving rise tothe formation of cracks, leading to the delamination of thecoating [4]. An additional concern regarding the degrada-tion of the TBCs apart from the aforementioned phasetransformation issue is the presence of siliceous minerals(dust, sand, volcanic ash, runway debris) ingested with theintake air. These contaminants deposit onto the coatedsurfaces of the components, yielding glassy melts of lowmelting eutectics in the quaternary CaO–MgO–Al2O3–SiO2 (CMAS) system when the surface temperaturesexceed 1200 1C [5]. The problem is more acute in aircraftengines, but there is anecdotal evidence that it also affectsindustrial and power generation turbines. These aggressiveglassy compounds can deposit, melt, and degrade theTBCs via repeated freeze–thaw action and, to a certainextent, direct chemical reaction with TBC constituents.The thermo-mechanical and thermo-chemical interactionscan accelerate the failure of TBCs and underlying compo-nents by the infiltration of molten deposits into the porousceramic top coat. This has been frequently reported [6],and it warrants an understanding of the interaction of suchmolten deposits with the underlying metallic bond coatand base metal. The search for new materials that canwithstand higher gas-inlet temperatures (above 1200 1C)has been intensified within the last decade. Lanthanumzirconate (La2Zr2O7, LZ) was recently proposed as apromising TBC material. Thermal properties of both bulkLZ material and its coating have been studied [7].

Compared with YSZ, it has a lower thermal conductivity(1.56 W m�1 K�1 for LZ, 2.1–2.2 W m�1 K�1 for YSZ,bulk materials, 1000 1C) and lower sintering ability.Further LZ maintains single cubic pyrochlore phase fromroom temperature up to its melting point. It has a lowYoung’s modulus (175 GPa for LZ and 200–220 GPa forYSZ) and the material is oxygen non-transparent com-pared to YSZ, which makes it an attractive candidate as aTBC material [8]. The present investigation addresses thepotential effect of the infiltration of CMAS under thermalcycling condition on the base metal Inconel 738 (BM), theHigh Velocity Oxy Fuel sprayed NiCrAlY bond coat (BC),the YSZ duplex coating and the LZ duplex coating.

Table 1

Chemical composition of the base metal.

Material Chemical composition

Zr C Mn Si Nb Ta

Inconel 738 0.1 0.17 0.2 0.3 0.9 1.7

In addition, the authors have also attempted to comparethe cyclic CMAS attack behaviour of the aforementionedspecimens with that of a five layer (with LZ coating as thetop layer) coated specimen, since it survived more numbersof thermal cycles at 1280 1C (without CMAS) compared toother coating architectures [9].

2. Experimental work

2.1. Deposition of coatings

An agglomerated and sintered Yttria Stabilized Zirconia(YSZ) spherical powder with the size ranging between10–45 mm (Make: H.C. Stark, AMPERIT 827.054powder (ZrO2, 7 Wt.% Y2O3)) was used. Since theLanthanum Zirconate (LZ) powder is not commerciallyavailable, the same was prepared in our laboratory. Themethod of preparing the plasma spray quality lanthanumzirconate power is available in our previously publishedpaper [10]. The substrate coupons were of nickel basedsuper alloy Inconel 738 material (BM). The chemicalcomposition of the base material (BM) is shown inTable 1.The dimensions of the substrate (BM) couponswere 25.4� 12.2� 3 mm. The corners and the edges of thesubstrate (BM) coupons were chamfered and roundedprior to grit blasting. Grit blasting was carried out usingcorundum grits of 16 mesh size using an automated highpressure suction blasting system (Make: MEC; India.Model: MEC SUBC MK III) to achieve a roughnessaverage (Ra) of 9–11 mm. The surface roughness wasmeasured using a diamond stylus surface roughness tester(Make: Mitutoyo, Japan; Model: SFTT301). The ceramicpowders were plasma sprayed over NiCrAlY (Ni-22Cr-10Al-1Y (wt.%) Make: Praxair NI-343, Size: 10–45 mm) bondcoat (BC), which was previously deposited using HVOF(Make: MEC; India. Model: MEC HIPOJET 7100) processon to the grit blasted Inconel 738 coupons (BM). Thecoatings were made on all the six sides of the grit blasted(BM) coupons. The plasma spray deposition of the YSZ andthe synthesized LZ powders were carried out using a semi-automatic 40 kW IGBT-based Plasmatron (Make: Ion ArcTechnologies; India. Model: APSS-II). Fig. 1 displays thecoatings prepared for CMAS attack test. The various speci-men types considered for the investigation are shown inFig. 1a. The total thickness of all the coatings was keptconstant at 500 mm. Since the intermixed layers wereinvolved, the two powder mixtures namely the LZþYSZand the YSZþNiCrAlY powder mixtures (50þ50 wt%)were mixed, respectively, in a ball mill (Make: VBCC; India,

Mo W Al Ti Co Cr Ni

1.7 2.3 3.6 3.4 3.4 16 Bal.

Fig. 1. Coatings prepared for cyclic molten glass infiltration test. (a) Considered specimen architectures, (b) coated coupons for CMAS test, (c) cross

section of BC specimen, (d) cross section of YSZ duplex coated specimen, (e) cross section of LZ duplex coated specimen, (f) cross section of five layers

coated sample.

C.S. Ramachandran et al. / Ceramics International 39 (2013) 1413–1431 1415

Model: HEBM-11) for four hours in dry condition withzirconia balls as the mixing medium prior to deposition.The porosity levels of the ceramic coatings were main-tained between 13 and 15 vol.% by appropriately modify-ing the APS process parameters. The HVOF sprayed BChad a porosity of around 2.0 vol.%. The process parametersettings to deposit the coatings using APS and HVOFspray processes are shown in Table 2. The anode-nozzlediameter of the APS gun was 6 mm. The injection of thepowder was carried out inside the anode nozzle right infront of the plasma arc root and perpendicular to the

plasma flow direction. The powder carrier gas was Argonhaving a purity of 99.9%. The powder injector diameterwas 2.7 mm and the injector was made out of oxygen freehigh conductivity (OFHC) copper material. The anode, thecathode, the powder injector and the plasma power supplycables of the APS system are intensively cooled withcirculating water having a flow rate of 38 lpm and atemperature of 18 1C. The internal diameter of the HVOFgun’s throat was 12 mm. The powder was injected into theHVOF gun coaxially. The HVOF gun houses a 3.2 mmorifice controlled powder injector made out of 316 LN

Table 2

Process factors and their respective levels.

APS/ HVOF factors YSZ

(APS)

LZ

(APS)

NiCrAlY

(HVOF)

Power (kW) 26 24 –

Stand-off distance (mm) 115 115 210

Primary/secondary gas flow rate (APS:

Ar/N2, HVOF:

Air/Butane/O2) (lpm)

38/5 38/4 950/80/250

Powder feed rate (gpm) 21 21 38

Carrier gas flow rate

(APS: Ar, HVOF:

Moisture free air) (lpm)

11 9 12


stainless steel material. The 250 lpm of moisture freecompressed air supplied into the HVOF gun takes careof the cooling requirements satisfactorily. The depositedcoatings as such are shown in Fig. 1b.

2.2. Characterization and mechanical properties evaluation

of coatings

The coated specimens were sectioned using a slow speedmetallurgical sample saw (Make: Ducom, India; Model: MSS-10) equipped with resin bonded diamond cutting disc. Cus-tomary metallographic procedures were adopted to polish thecross-section of the coatings. The cross-sectional images of thespecimens were captured using optical microscope (Make:Meiji; Japan. Model: MIL-7100). The porosity was analyzedas per ASTM: B276 standard on the polished cross section ofthe coating using the same optical microscope equipped withimage analyzing system. A 100 mm square area was selected onthe polished cross section of the coating and the image wasanalyzed. The same procedure was repeated at five randomlocations to find out the average percentage volume ofporosity. The microhardness measurements were made onthe polished cross-sections of the coatings, using a Vickersmicrohardness tester (Make: Shimadzu; Japan. Model: HMV-2T). A load of 2.94 N and a dwell time of 15 s were used toevaluate the hardness. Hardness values were measured at 10random locations on the polished cross-section of a coating.The tensile bond strength test was carried out as per ASTM:C633 standard using a universal testing machine (Make: FIEBlue Star; India. Model: UNITEK-94100). A commerciallyavailable heat curable epoxy was used as an adhesive, to testthe coated specimens. The method of conducting the bondstrength test and the analysis thereof can be referred in ourpreviously published paper [11]. The coatings were alsosubjected to SEM and EDAX analysis (Make: Quanta;Switzerland. Model: 3D FEG-I). The coatings were alsoanalyzed using X Ray Diffraction (Make: Rigaku, Japan;Model: ULTIMA-III).

2.3. Cyclic CMAS infiltration test method

The cyclic CMAS attack test was carried out, using a1800 1C tubular sintering furnace (Make: VBCC; India,

Model: VBHTSF-1800MF12). For cyclic CMAS infiltrationtests, a composition of 33CaO–9MgO–13Al2O3–45SiO2 wasselected [12]. The compositions are given on weight percentagebasis. The CMAS ingredients were mixed in a ball mill forfour hours with suitable amounts of demineralized water andcorundum balls as mixing medium. The mixed ingredientswere placed in a Pt–Rh crucible and melted in a furnace at1550 1C for four hours. To ensure homogeneity, the resultingglass was crushed and then remelted using the same conditionsas the first melting. The remelted CMAS glass was thencrushed again in a ball mill in dry condition with corundumballs, and the consequential frit was screened using a #500mesh sieve. The CMAS glass thus prepared was used on thespecimens for the cyclic CMAS attack test. Prior to applyingthe CMAS concoction, the specimens were pre-heated at250 1C for one hour, as preheating provided good adhesion ofCMAS to the specimen. The CMAS frit was mixed withethanol using an agate mortar and pestle to form slurry. TheCMAS slurry concoction was spread over the top surfaces ofthe preheated specimens using a glass fibre brush. The CMASconcentration was kept in the range of 34–36 mg/cm2 leavinga 3 mm distance from the edges (without CMAS spreading) toavoid edge effect. These CMAS containing specimens weredried in the furnace at 250 1C for one hour to remove themoisture from the CMAS deposit.Post drying, the CMAS covered specimens were placed

on recrystallized alumina crucibles and inserted directlyinto the hot zone having a temperature of 1280 1C. In thishot zone the specimens were exposed to the aforesaidtemperature for 50 minutes (the CMAS mixture wasmolten on the specimen surface during the exposure). Thiswas followed by cooling the coupons for 10 minutes undera diffused air cooling arrangement leading to roomtemperature. This one hour cycle was repeated for 50times (50 cycles). Further, during cyclic CMAS exposure,the weight differences of the coatings were measured oncein every two cycles. The weight of crucibles and thespecimens were measured together by means of a precisionweighing machine (Make: Shimadzu; Japan. Model: AW320) with 0.01 mg of resolution to determine the specificweight gain of the specimens. The spalled scales of thespecimens were also included at the time of the measure-ments of weight change to determine the specific weightgains of the specimens. The samples were not supplemen-ted with additional CMAS concoction during cycling.

3. Results

The cross-sectional micrographs of the coatings beforecyclic CMAS attack test are presented in Fig. 1c–f. Thecross-sectional micrograph of the BC is shown in Fig. 1cwhich display well packed dense lamellar structure. Theduplex YSZ and LZ coatings (Fig. 1d and e) display asharp and clear interface between the ceramic coat and theBC. The LZ–YSZ coating interface and the YSZ–BCinterface of the five layered coating do not exhibit a clearinterface (Fig. 1f) due to the presence of the 50 mm thick


intermixed interfacial layers. The evaluated mechanicalproperties of the coatings represented in graphical formatwith error bars are shown in Fig. 2. The hardnessdistribution of the LZ, YSZ and the five layered coatingsare shown in Fig. 2a. An average microhardness value of954 HV0.3 was recorded in the case of LZ coating and anaverage hardness value of 907 HV0.3 was recorded inthe case of YSZ coating. The average hardness valuesof the LZ–YSZ interface layer and the YSZ–BC interlayerof the five layered coating were 932 and 625 HV0.3

respectively. The tensile bond strengths of the LZ, YSZand the five layered coatings are shown in Fig. 2b.A tensile bond strength value of 12 MPa was recorded inthe case of the LZ duplex coating; 15 and 20 MPa were thebond strengths of the YSZ duplex and the five layeredcoatings, respectively. The weight gain details of thecoatings after CMAS test are shown in Fig. 3. The specificweight gains of the coatings and the BM, due to cyclicCMAS attack are shown in Fig. 3a. From the figure it canbe said that all the coatings and the BM do not exhibit aparabolic weight gain pattern. The cumulative weight gains

Fig. 2. Mechanical properties of the coatings. (a) Microhardness

Fig. 3. Weight gain of the coatings after the cyclic CMAS test. (a) Specific

(b) Cumulative weight gain of specimens up to 50 CMAS attack cycles

of the coatings and the BM are shown in Fig. 3b. Thehighest weight gain of 26 mg/cm2 was recorded in thecase of the BM. A cumulative weight gain of 19, 9, 14 and6 mg/cm2 were recorded in the case of the BC, YSZ, LZand the five layered coated specimens, respectively.

4. Discussion

4.1. Interaction between CMAS and base metal (BM)

Inconel 738

The cumulative weight gain of the BM is 26 mg/cm2

(Fig. 3b). The measured weight gain is a combination ofinfiltration, oxidation along with the infiltration process whenthe metallic phases transform into the ceramic phases and thetransformation between the ceramic phases formed.

4.1.1. Surface analysis of BM after CMAS attack

The SEM image of the BM exposed to 50 CMAS attackcycles is shown in Fig. 4. The observation of the surface ofthe BM showed that the oxide scales of the BM spalled

distribution in coatings, (b) Tensile bond strength of coatings

weight gain of coatings with respect to number of CMAS attack cycles,

Location B

Location A

CMAS Infiltrated paths

Oxidation Scales

Residual CMAS glass island

Fig. 4. SEM image of top surface of BM exposed to CMAS indicating oxide scale formation, CMAS infiltrated paths and residual CMAS glass island.

Cracked oxide scale containing solidified CMAS

Location C

EDS confirming the spinel ((Ni,Co)(Al,Cr) 2O4)

formation at location C

EDS confirming the CMAS composition at location C

Spinel

CMAS

Fig. 5. Cross-sectional image of BM sample exposed to CMAS showing the cracked oxide scale containing the spinel and the solidified CMAS.


rigorously after the 50-hour cyclic CMAS attack test asseen in Fig. 4. The surface of the BM also exhibits twodifferent morphologies, indicated as location A and loca-tion B in Fig. 4. Location A in Fig. 4 indicates that themolten CMAS has attacked the grain boundaries of theBM and penetrated through it. Since grain boundaries areenergy rich regions, the low viscous glass preferentiallyattacks the grain boundaries, leaving behind lots of entryholes/infiltration paths. The grains were not left unaf-fected. Severe oxidation of the grains has taken place and athick oxide scale was found on the surface. There weresome isolated regions where the un-infiltrated CAMS glasscould be seen on the surface of the BM as seen in LocationB in Fig. 4. It can be observed in Fig. 3a that there is anincrease in the weight with the increase in the number ofthe CMAS attack cycles. The increase in specific weightgain may be attributed to the higher fluidity of the CMASconcoction. The highly fluidic CMAS acts as an oxygencarrier leading to a combined increase in the diffusion and

transport phenomenon. The protective scale is destroyedor eliminated by molten CMAS and consequently themetal surface is exposed to the direct action of thedestructive environment. The molten CMAS also providesrapid diffusion paths for reactant species which lead to thecracking and exfoliation of the protective scale. The cracksmay have allowed the aggressive liquid CMAS glass toreach the virgin metal surface, contributing to the con-tinuation of exfoliation of the oxidized BM scales, till theend of the test.

4.1.2. Cross-sectional analysis of BM after CMAS attack

The cross-sectional morphology of the BM after 50cycles of CMAS attack is shown in Fig. 5. From the figure,one could see that the thickness of the CMAS infiltratedzone is almost 350 mm thick and is characterized by oxidescale with large cracks running parallel to the surface ofthe BM. One can see the presence of solidified CMASinside those cracks. Upon closer inspection of the glass


solidified inside the cracks (Location C in Fig. 5), it wasobserved that the molten CMAS has reacted with the BM,leading to the formation of spinels. Islands of unreactedCMAS were also found along with the spinel. The crystalstructure of the spinel exhibits, almost regular hexagons,and some rectangular crystals. The glass islands wereanalyzed using EDS and the CMAS composition wasconfirmed (Fig. 5). The EDS analysis of the crystallizedspinel is shown in Fig. 5. The EDS analysis matches withthat of the spinel having the composition (Ni,Co) (Al,Cr)2O4. The surface oxides of the BM specimen are moreporous and coarse. A series of chemical reactions that cantake place during the cyclic CMAS attack on the BM areelucidated in the following section.

The chromium and the aluminium have more affinitytowards oxygen; hence they form in the initial stages ofCMAS attack

2Crþ3/2O2-Cr2O3 (1)

2Alþ3/2O2-Al2O3 (2)

The Nickel will also participate in the oxidation process

Niþ1/2O2-NiO (3)

The formations of Al2O3 and Cr2O3 on the surface arebeneficial for oxidation resistance, but these oxide scaleswere not continuous and hence permitted the transporta-tion oxide species. This oxide transport enables theformation of spinels, as mentioned below

NiOþCr2O3-NiCr2O4 (4)

NiOþAl2O3-NiAl2O4 (5)

Cobalt also undergoes the same kinds of reactions toform spinels. The aforementioned reactions, the SEM andthe EDS analysis confirm the formation of complex oxideson the BM surface. The formation of these complex oxides

Location E

CMAS

Location D

TGO

CMAS

Fig. 6. SEM image of top surface of BC exposed to CMAS indicating T

imposes a severe strain on the oxidized scale [13]. Such akind of severe strain may result in the cracking andspallation of the oxide scale, exposing the virgin BM tothe aggressive molten CMAS concoction. This tendency ofthe spalling has also been explained as a result of thepredominance of cationic diffusion in the growth of scaleswhere no oxide is formed at or near the external surface.Voids and porosity gradually develop at or near the metalscale interface due to the reduced volume of metal and lackof sufficient plastic deformation of scale [14]. This leads toa reduction in adherence as well as coherency of the scale,thereby increasing the tendency of spalling.

4.2. Interaction between CMAS and NiCrAlY bond

coat (BC)

The specific weight gain pattern of the NiCrAlY BCexposed to 50 cycles of CMAS attack is shown in Fig. 3a.The cumulative weight gain of the BC is 19 mg/cm2

(Fig. 3b). The specific weight gain pattern and thecumulative weight gain value indicate that the BC hasprovided sufficient protection to the BM from the moltenCMAS attack.

4.2.1. Surface analysis of BC after CMAS attack

For the BC, the surface structure comprised of acombination of crystallized glass, oxidized splats, CMASinfiltration paths, ridges and nodules (Fig. 6). The whiskersand nodules are usually deemed as characteristic featuresfor growing Al2O3 [15], which can be seen as residues ofoutward growth, due to cation lattice and surface diffu-sion, as seen in Location D in Fig. 6. The formation ofAl2O3 was confirmed using the EDS analysis, which isshown in Fig. 6. On the surface of BC, spallation ofalumina scale occurred as well. It is well known thatNiCrAlY coatings protect the substrate superalloy againstoxidation, by forming a protective oxide scale [16]. In this

EDS confirming the confirming the TGO/ alumina

formation at location D formation

Oxidation Scales

CMAS Infiltrated paths

GO formation, CMAS infiltrated paths and oxide scale formation.


study, the protective oxide scale formed on the coating wasmainly Al2O3 scale, which could act as a barrier and shieldthe coating to a certain extent, from the incursion of liquidCMAS concoction. The depletion of Al from the coating isconsidered as a key factor in assessing the service life ofNiCrAlY coating, at high temperature. During the hightemperature cyclic CMAS exposure, the loss of Al byinward and outward diffusion must be considered simul-taneously. Due to the combination of oxidation andinterdiffusion, the Al content decreases and correspond-ingly leads to the alternation of coating structure. Here,the NiCrAlY coating degraded rapidly at high temperatureand showed a limited capability in self-healing. During thehigh temperature cyclic CMAS attack, the protectiveAl2O3 scale would be consumed in two ways. One is thatthe stress caused by the thermal cycling will lead topossible crack or spallation of Al2O3 scale. Another isthe reactions with CMAS concoction might result in thedissolution of the Al2O3. As the CMAS infiltrationproceeds, Al reservoir in the coatings will be consumedto repair the Al2O3 until being exhausted. Once the Al2O3

scale cannot be maintained, the molten CMAS would reactdirectly with the coatings and lead to the ultimate failure.The molten CMAS concoction, which was deposited onthe superalloy, can flux and dissolve the normally protec-tive oxides such as Cr2O3 and Al2O3. The surface alsoexhibits number of CMAS infiltration paths as seen inLocation E in Fig. 6 which is suggestive of the penetrationof molten CMAS along the splat boundaries.

4.2.2. Cross-sectional analysis of BC after CMAS attack

The cross-section of the BC sample is shown in Fig. 7.The CMAS attacked BC surface also exhibits scaleformation to a thickness of around 180 mm as seen inFig. 7. The figure shows that due to the selective oxidationof chromium and aluminium, Cr2O3 and Al2O3 are formedalong the boundaries of nickel-rich splats, and in pores andblocked the passages and thus enabled the BC to develop a

CMAS Spinel

Loc

C

Fig. 7. Cross-sectional image of BC sample exposed to CMAS indica

temporary barrier against the penetration and diffusion ofliquid CMAS. Additionally, very low porosity and the flatsplat structure of the coatings have also contributed indeveloping resistance to CMAS penetration at highertemperatures, since the molten glass mostly propagatealong the splat boundaries and through the pores andvoids. Due to the dense and flat splat structure of thecoatings, the distance from the coating surface to thecoating-substrate interface along splat boundaries, ishighly increased, which enables the coatings to developresistance to CMAS infiltration.The formation of thick oxide layers in the scale, have

contributed to the better CMAS infiltration resistance ofBC, compared to the BM. Though the Cr2O3 and theAl2O3 have acted as barriers, the penetration of theaggressive liquid CMAS concoction into the BC has notbeen completely eliminated. Location F in Fig. 7, on theother hand, displays the evidence of the presence ofcrystallized CMAS and the formation of the spinel. Theformation spinel phase (thin elongated platelets) could bedue to the dissolution of Al and Ni by the molten CMASconcoction, developing into the generation of porousmixed oxides. From the EDS elemental analysis shown inFig. 7, it could be said that this fluxing mechanism isresponsible for disassociating the protective alumina layerand the formation of nickel oxide (NiO) and porous spinel(Ni)(Al,Cr)2O4. In this way, the penetration of molecularoxygen and the molten CMAS along splat boundaries, isallowed in the BC, thereby promoting its oxidation andcrack formation tendency. The molten CMAS consumedthe thermally grown oxide (TGO), alpha-Al2O3 during thetest as shown in Fig. 7.By conducting the EDS analysis (Fig. 7), on the crystal-

lized CMAS, it was found that the composition is rich inAl content, which confirmed the dissolution of the TGO,alpha-Al2O3. Crystallization of CMAS glass throughenrichment of Al content has been reported earlier [17],where anorthite, CaAl2Si2O8 (CMAS glass composition

EDS confirming the Al rich CMAS at location F

EDS confirming the spinel ((Ni)(Al,Cr) 2O4) formation

at location F

ation F

MAS

BC

BM

ting CMAS infiltrated zone containing spinel and Al rich CMAS.


which is easier-to-crystallize) was found to be the pre-dominant crystallite. The spalled scales of the BC werecrushed and were exposed to the XRD analysis. From theXRD pattern (Fig. 8) it could be inferred that thehexagonal anorthite and the cubic spinel were the majorphases and nickel oxide and orthogonal magnesium alu-minate were the minor phases. Based on CaO-SiO2-Al2O3

ternary phase diagram [18], typical CMAS glass composi-tion without considering the Mg content, falls in pseudo-wollastonite field, whose glass composition is difficult tocrystallize. Enrichment of Al content in CMAS shifts this‘‘difficult-to-crystallize’’ pseudo-wollastonite glass compo-sition to a ‘‘crystallizable’’ Al-rich glass composition thatfalls in the anorthite field. Thus the dissolution of alpha-Al2O3 by CMAS resulted in crystallization of CMAS toanorthite (CaAl2Si2O8) phase. Concurrently, the localizedenrichment of Mg content promoted the formation ofMgAl2O4 spinel. The formation of anorthite phaseimproved the CMAS infiltration resistance of the BC toa certain extent, which resulted in the low weight gaincompared to BM. The spallation of BC was also observedduring the test, which may have resulted due to the rapidgrowth of void-like defects lying adjacent to coatingprotuberances, where the tensile radial stress (stress devel-oped during cooling as a result of the thermal contraction)mismatch between the oxide scale and the coating ismaximum. The formation of cracks in the coating usuallyoriginates from stresses developed in the oxide scale-BCinterface.

4.3. Interaction between CMAS and YSZ duplex coated

samples

The cumulative weight gain of YSZ duplex coatedspecimen is 9 mg/cm2 (Fig. 3b). The aforementioned valueand the specific weight gain pattern for 50 CMAS attack

Fig. 8. XRD analysis of crushed BC scale after 50 CMAS attack cycles.

cycles (Fig. 3a) indicate the superior CMAS infiltrationresistance of YSZ coating compared to BM and BC.

4.3.1. Surface analysis of YSZ duplex coated samples after

CMAS attack

The top surface microstructure of the YSZ duplexcoating exposed to CMAS attack is shown in Fig. 9. Thesurface exhibits exfoliated coating lamellae through whichthe CMAS has infiltrated. The CMAS attack on thermallysprayed YSZ coatings is very aggressive due to the splat-layered structure of the coating. The CMAS dissolves thecoating lamellae by attacking the micro-cracks and splatboundaries first. Fig. 9 also displays the presence ofspallation cracks on the coating surface. These cracksmight be associated with the presence of CMAS depositand coating densification. Severe frothing was observedduring the initial stages of the test. The frothing reactiongenerates new kind of porosities. The surface depositsappear to have been in a liquid state with sufficient gascontent to cause frothing. Frothing of molten glass canoccur when calcia and silica are present among thecomponents. But the cause of the frothing phenomenonis still under debate.

4.3.2. Cross-sectional analysis of YSZ duplex coated samples

after CMAS attack

The cross-sectional micrograph of the YSZ duplexcoated sample is shown in Fig. 10. There are lots of pores,voids and cracks present throughout the cross-section ofthe coating and these cavities and cracks have been filledwith crystallized glass. It has been reported that the CMAScomposition starts to melt at approximately 1235 1C whenmade from the constituent oxides. It is completely moltenat 1250 1C and it can infiltrate the pores and cracks presentin the coating. Further, the possible reason behind the

CMAS infiltrated lamella

Interlamellar Cracks

Fig. 9. SEM image of top surface of YSZ duplex coated sample exposed

to CMAS.

Crystallized CMAS

Oxide stringers

TGO

BC

YSZ coating

BM

Location GCMAS

ReprecipitatedZrO2 grains

EDS confirming the reprecipitated ZrO2

formation at location G

Fig. 10. Cross-sectional image of YSZ duplex coating sample exposed to CMAS indicating CMAS infiltration and reprecipitated spherical shaped

ZrO2 grain formation.


formation of voids and pores is that the air within theporous coating structure tries to escape from it and istrapped within the solidified CMAS infiltrated coatinglayer. The large voids could also be produced due to somereactions that produce gases, when the TBC is severelyattacked by CMAS which can be seen in Fig. 10. Thecoating failure observed in this study involves the spalla-tion of the top layer coating which has been infiltrated withthe CMAS. The incipience of the CMAS attack seems toinvolve physical damages.

The CMAS attack on YSZ duplex coated specimens alsoinvolves the following thermo-chemical mechanisms:

1.
The molten CMAS glass wets YSZ and infiltrates intopores and cracks in the TBCs.
2.
YSZ grains dissolve in the molten CMAS glass and thenreprecipitate as globular grains that are depleted in Ysolute. A higher magnification SEM image taken atlocation G displays (Fig. 10) globular ZrO2 grains alongwith crystallized CMAS. Globular nature of the ZrO2
grains in the glassy matrix provides a strong indicationfor diffusion-controlled dissolution–reprecipitation [19];the transformation of t0-ZrO2 to m-ZrO2 appears tohave taken place during that dissolution–reprecipitation.The EDS (Fig. 10) analysis performed on the globulargrains taken at location G confirms that those are ZrO2

grains.
3. The Zr and Y elements incorporated in the CMAS glass
have little or no effect on the glass behaviour.
4. CMAS glass penetrates YSZ grain boundaries, resulting in
the energetically favourable dispersion of exfoliated YSZgrains in glass. i.e., one ZrO2/ZrO2 grain boundary and twoZrO2/glass interfaces. This phenomenon has been observedin dense, polycrystalline Al2O3 and Si3N4 in contact withmolten glasses. However, in those cases the dispersed Al2O3

and Si3N4 grains are generally faceted [20].

5.
Possible diffusion of Y from YSZ grains to the CMASglass occurs, in conjunction with counter-diffusion ofAl, Si, Ca, and Mg from the CMAS glass into YSZgrains [21].
The phases present in the YSZ coatings before and afterCMAS exposure are displayed in Fig. 11. The XRD plot ofthe as sprayed YSZ duplex coating shown in Fig. 11adisplays the presence of t0 (tetragonal prime) phase whichgenerally occurs due to the rapid quenching of the APSsprayed YSZ splats, whereas the XRD pattern of theCMAS infiltrated YSZ coating (Fig. 11b), shows thepresence of t (tetragonal), m (monoclinic) and c (cubic)phases of ZrO2 along with anorthite and orthorhombicmagnesium aluminate/magnesia spinel. The Interactionbetween the YSZ and CMAS was observed to occurstarting at 1250 1C, where CMAS was found to melt andinfiltrate the porous YSZ coating. CMAS melt in the YSZresulted in the dissolution of the YSZ followed by thereprecipitation of ZrO2 grains with different polymorphsand composition based on the local melt chemistry. Thedisruptive phase transformation of t0 to tþm and t tomþc occurred. This phase transformation to Y2O3-poormonoclinic ZrO2 (m) and Y2O3-rich cubic ZrO2 (c) phasesis attributed to the deviation in the content of Y2O3

stabilizer from the original YSZ solid solution duringreprecipitation [22]. The YSZ coating exposed to theCMAS melt was found to undergo the disruptive phasetransformation to m-ZrO2 significantly, with some amountof cubic (c) ZrO2. This could be due to an increase in Ycontent in CMAS during simultaneous dissolution of YSZand ingression of CMAS melt through the thickness ofthe YSZ coating. Thus, reprecipitation of Y2O3-depletedmonoclinic ZrO2 and consequently increased the Y contentin the CMAS melt and forced the reprecipitation of

Fig. 11. XRD patterns of YSZ coating before and after CMAS test. (a) XRD pattern of as deposited YSZ duplex coating. (b) XRD analysis of crushed

YSZ coat (spalled) after 50 cycles of CMAS attack


Y2O3-rich cubic ZrO2 phase. Further, the presence of c-phase of ZrO2 in the CMAS attacked YSZ coating is anindication, that the Ca atoms have diffused into YSZ andincreased the stability of c-YSZ.

The incorporation of calcium into the YSZ layer canlead to an increase of YSZ volume and a change in stressdistribution within the whole TBC system [23]. Uponthermal cycling, this can lead to crack nucleation and theirfurther growth at the YSZ/BC interface. This failuremechanism can be witnessed in Fig. 10, where the innersurface of the YSZ layer was significantly decomposed andinfiltrated by the CMAS. It is a known fact that, too muchphase transformation of (t–m) ZrO2 would cause largevolume expansion and deteriorate the integrity of TBCsystem, and eventually spallation failure of TBC. On theother hand, the CMAS led to an accelerated sintering ofYSZ coating. Sintering usually leads to shrinkage at thesurface of a TBC. The sintering effect decreases graduallyfrom the surface to the bond coat. This leads to an increasein Young’s modulus which decreases strain tolerancecapability of the coating and increases top-coat stresses[24]. The thermal diffusivity of the YSZ coating withCMAS infiltration will be higher than that of the coatingwithout CMAS infiltration. The sintering will also increasethe thermal diffusivity [25]. Therefore, it can be establishedthat the CMAS penetration could compromise the thermalbarrier capability of TBC. As a result, the cooling effec-tiveness could be decreased and cause local over-heating ofthe BM and BC. Simultaneously, due to degradation ofthermal barrier effects, the temperature at the interfacebetween the YSZ top coat and the BC will rise upaccordingly. Accelerated thickening of thermally grownoxide (TGO) could occur, which would lead to prematurespallation of TBC lamellae. The SEM micrograph shownin Fig. 10 also displays phase assemblage of the TGO layerconsisted of alpha-alumina. While most sections of theTGO layer which appeared dense and homogeneous,some regions revealed a high defect density characterized

by clustered porosity and numerous microcracks. Thesemicrocracks in the TGO layer provided a convenient pathfor further diffusion of CMAS constituents through theTGO layer to the TGO/bond coat interface.The variation in the stress distribution also changes the

strain energy release rate and failure mechanisms. Thismay be the mechanism by which the horizontal cracksform and the TBCs spall. The weight gain of these ceramiccoated samples is due to the TGO formation and the highinternal oxidation of the BC and the BM. The EDSelemental mapping results of the YSZ duplex coatingexposed to 50 cyclic CMAS exposures is shown inFig. 12. From Fig. 12 it could be seen that at the verytop, a thin film of unpenetrated CMAS still remains. Thedark areas are pores and voids. It can be seen clearly thatthe YSZ coating is almost destroyed, with the CMAS glassreaching to the bottom of the top coat through the cracksand inter connected porosities. Fig. 12 shows correspond-ing Ca, Mg, Al and Si EDS elemental maps, respectively,and thus bears an evidence of CMAS reaching the bottomof the TBC. The molten glass wets the YSZ coating andinfiltrates into the pores and vertical cracks in the TBC.Simultaneously, the ZrO2 grains dissolve in the moltenCMAS glass and then reprecipitate in globular forms. It ispossible that these, and alternative, mechanisms are activein this investigation. The silica has a significant influenceon the sintering behaviour of YSZ coating, promotingsintering of the coating during high-temperature annealing.The silica glass has a low viscosity at elevated temperaturesand can promote densification by liquid-phase sintering[26]. It has been shown by researchers that SiO2 wasdetrimental to the thermal cyclic life times of YSZcoating. In zirconia ceramics, SiO2 was segregated at grainboundaries and enriched in triple points. Y2O3 was pre-cipitated from YSZ grain boundaries, which led to localinstability. Otherwise, SiO2 caused the ZrO2 to undergosuper-plasticity and accelerated the sintering speed [27].It can be supposed that for the TBC with deep CMAS

50 µm

YSZ

BC

Fig. 12. EDS elemental mapping results of YSZ duplex sample exposed to CMAS attack.



Fig. 13. SEM image of top surface of LZ duplex coated sample exposed

to CMAS.


penetration, the thermal barrier function would be severelycompromised.

4.4. Interaction between CMAS and LZ duplex coated

samples

The specific weight gain pattern of the LZ coating isshown in Fig. 3a. In the figure one can see that the specificweight gain of LZ coating is less compared to YSZ duplexcoated specimen up to 23 cycles, but after 23 cycles theweight gain is high, compared to YSZ duplex coatedsample. The cumulative weight gain (Fig. 3b) of LZ duplexcoated sample is 14 mg/cm2 and this weight gain value ishigher than that of the value recorded for YSZ duplexcoating.

4.4.1. Surface analysis of LZ duplex coated samples after

CMAS attack

The top surface morphology of hot corroded LZ duplexspecimen is shown in Fig. 13. From the figure it can be seenthat the LZ duplex coating has experienced severe crackingand delamination during the CMAS attack test. These crackscan further contribute to the deterioration of the coating. Inthe same figure, one can see that the CMAS has penetratedand exfoliated the LZ coating lamellae (Fig. 13). To establisha self-consistent thermo-mechanical interpretation of thecracking and delamination phenomena evident in the CMASpenetrated zone, three aspects of the mechanics must berationalized [28]: (i) The temperatures and thermally inducedstresses. (ii) Channel cracking at the surface and thepenetration of these cracks into the CMAS-penetratedTBC. (iii) Delamination in the TBC at various levels beneaththe surface. The cooling scenario envisages the TBC being ona thick, actively cooled substrate. In this investigation, the

TBC has been at the high temperature for a sufficiently longperiod so that any stresses fully relax by creep. Moreover,cooling is sufficiently rapid so that the response throughoutthe surface must be thermo-elastic. The basic context of theCMAS penetrated coating under cyclic thermal loading asper the aforementioned phenomenon is as follows: Uponcooling to ambient temperature from 1280 1C the CMASsolidifies, the penetrated layer develops composite thermo-mechanical properties comparable to that of dense LZcoating.Once the CMAS solidifies, the high in-plane stiffness

causes large, near-surface, tensile stresses. The ensuing stressstate is comparable to that associated with a thin film in


residual tension on a thick substrate, with a correspondingpropensity for delamination. Thus the tensile stresses inducea delamination that extends parallel to the surface at acharacteristic depth, subject to a steady-state energy releaserate [29]. Spallation of coating lamellae could be theconsequence of cumulative delamination and material losscaused by sequential cold shocks. The shock requirements forforming surface connected vertical cracks are less stringentthan those for forming delaminations. The expectation,therefore, is that vertical cracks develop first [30]. Thedelamination normally gets extended from the verticalseparations. Upon reheating the CMAS fluid would bedrawn in by capillarity, filling the delamination. The lamellae,where the CMAS has penetrated, the delaminations areevident in the microstructure as seen in Fig. 13.

4.4.2. Cross-sectional analysis of LZ duplex coated samples

after CMAS attack

The cross-sectional micrograph of the CMAS attackedLZ duplex specimen is shown in Fig. 14. After 50 CMASattack cycles, several transverse cracks have propagatedand extended inside the LZ coat and most of the cracks arefilled with CMAS. It would be susceptible to cause thedebonding of the LZ coat from the BC. Further, theCMAS also have penetrated the open porosities andcracks. Open vertical cracks are also in evidence, startingat the surface and extending partially or fully through theCMAS penetrated layer (Fig. 14). These cracks are openand form a characteristic mud-crack pattern at the surface(Fig. 13) typical of thermal stress cracks generated oncooling. Moreover, the cross-sections indicate that thechannel cracks initiate and propagate substantially withinthe CMAS-filled LZ coating. The depth of CMAS pene-tration is location dependant. In the deeply penetratedregions, multiple sub-surface delaminations are evident asseen in Fig. 14: all originate from the extremity of one ofthe channels. They are located at three primary levels [31]:

Crystallized

CMAS

Oxide stringe

TGO

BC

LZ coating

BM

Location

Fig. 14. Cross-sectional image of LZ duplex coating sample exposed

(La8Ca2(SiO4)6O2) grain formation.

(i) just above the bond coat, (ii) adjacent to the bottom ofthe CMAS penetrated layer and (iii) just below the topsurface. Those at level (iii) are filled with CMAS. Level (ii)delaminations often link causing a section of the pene-trated material to spall, resulting in a ‘‘slab’’ of ejectedmaterial.Level (i) delaminations are extremely long and especially

detrimental to the durability of the TBC. The crack anddelamination patterns in Fig. 14 suggest that, where theCMAS has penetrated, tensile stress is developed at thesurface on cooling, causing channel cracks to be formedand extended (fully or partially) through the penetratedlayer. On further cooling, the energy release rates on thelevel (ii) become large enough so that delaminations formfrom the channel cracks and extend adjacent to the CMASpenetrated layer. Thereafter, the energy release rate on thelevel (i) becomes large enough to extend delaminationsfrom deep channel cracks, just above the bond coat/ TGOlayer (in mixed mode). The TGO layer formation is evidentat the LZ–BC interface (Fig. 14), but at many locations ithas been consumed by the CMAS. Though the CMASinfiltration has played a role in partial failure of LZ duplexcoating, the main reasons for the rapid weight gain of LZcoating compared to YSZ coating after twenty three cycles(Fig. 3a) is attributed to the rapid propagation of numer-ous longitudinal and transverse cracks induced by thethermal shock created in the CMAS cycling experiments.The cross-section of the coating indicates (Fig. 14) exten-sive cracking and the total integrity of the coating beinglost albeit filled with CMAS. The extensive cracking isdue to the low fracture toughness (LZ: 1.4 MPa m1/2,YSZ: 1.8 MPa m1/2) and the low coefficient of expansion(LZ: 9.1� 10�6 K�1, YSZ: 10.7� 10�6 K�1) [32].The low fracture toughness leads to the initiation and

growth of micro-cracks even with lower stress levels andthe low coefficient of expansion leads to the mismatchin the coefficient of thermal expansion between the top

rs

H

CMAS

Apatite (La 8Ca 2(SiO4)6O2)

EDS confirming the apatite (La 8Ca 2(SiO4)6O2) formation

at location H

to CMAS indicating CMAS infiltration and needle shaped apatite


ceramic layer and the BC. This thermal expansion mis-match results in an increase of residual stresses that cancause the failure of coatings even though this coating iscomparatively resistant to infiltration by CMAS up to 23cycles compared to YSZ duplex coating. A higher magni-fication image taken at the location H of the CMASinfiltrated LZ duplex coating is shown in Fig. 14. In thefigure one can see the presence of rod shaped crystals alongwith crystallized CMAS glass. The EDS pattern taken atlocation H (Fig. 14) on the rod shaped crystals confirmsthe existence of elements namely the La, Ca, Si and O,which corresponds to the chemical composition of alanthanum based apatite (La8Ca2(SiO4)6O2). The phasespresent in the LZ coatings before and after CMASexposure are displayed in Fig. 15. The XRD analysis ofthe as deposited LZ coating exhibited peaks of a cubicpyrochlore structure as seen in Fig. 15a. Exposure of thecoating to molten CMAS resulted in the appearance ofpeaks (Fig. 15b) attributed to the formation of apatite(La8Ca2(SiO4)6O2), hexagonal anorthite (CaAl2Si2O8),magnesia spinel/orthogonal magnesium aluminate (MgAl2O4)together with the original LZ cubic pyrochlore peaks.Similarly, The EDS elemental mapping result of thecross-section of the LZ duplex coated specimen is shownin Fig. 16. The Infiltration of CMAS through the cracksand interconnected porosities could be witnessed in themapping results corresponding to the elements namely Ca,Mg, Al and Si. The CMAS has penetrated throughthickness of the LZ coating thereby completely jeopardiz-ing its integrity. The liquid CMAS has also consumed theTGO layer and entered into the bond coat lamellae.

4.5. Interaction between CMAS and five layered coating

The specific weight gain pattern of the sample with fivelayered coating for 50 cycles of CMAS attack is shown inFig. 3a. From the figure it could be inferred that thespecific weight gain of sample with five layered coating is

Fig. 15. XRD patterns of LZ coating before and after CMAS test. (a) XRD

LZ coat (spalled) after 50 cycles of CMAS attack.

less compared to all the considered coatings and the BM.The cumulative weight gain (Fig. 3b) of the five layercoated sample comprising of the LZ as top layer is6 mg/cm2. The aforementioned evidences indicate the supe-rior CMAS infiltration resistance of the sample providedwith the five layered coating.

4.5.1. Surface analysis of five layered coating after CMAS

attack

The surface morphology of the five layered coating(Fig. 17) looks similar to that of the LZ duplex coatedspecimen, but the formation of micro-cracks were largelymitigated due to the gradient nature of the coating(Fig. 17a). The molten CMAS reacted with the top LZlayer and the respective products of reaction were found atcertain locations on the surface of the LZ top layer as seenin Fig. 17b. The molten CMAS has infiltrated the topsurface of the five layered coating, but the exfoliation ofthe lamellae has been minimized to a certain extent. TheLZ lamellae have reacted with molten CMAS and thereaction products have undergone delamination and chip-ping, due to the thermal shock, as could be seen inFig. 17a. Furthermore, cyclic attack by CMAS attackchipped certain lamellae of the LZ top layer and pavedway for the intrusion of molten CMAS into the coating.Then the liquid phase CMAS got infiltrated along thegrooves and penetrated into pores and cracks within thetop LZ layer. During the cooling process, the moltenmixture got crystallized from liquid phase and got trappedin the pores and cracks within the top LZ layer of the fivelayered coating.

4.5.2. Cross-sectional analysis of five layered coating after

CMAS attack

The cross-sectional image of the CMAS infiltrated fivelayered coating is shown in Fig. 18. The coating micro-structure in Fig. 18 reveals that CMAS penetrates the splatboundaries of the LZ layer and also has dislodged the

pattern of as deposited LZ duplex coating. (b) XRD analysis of crushed

Fig. 16. EDS elemental mapping results of LZ duplex sample exposed to CMAS attack.



Fig. 17. Images of the top surface of the five layered coating exposed to CMAS taken at different locations. (a) Image of top surface of five layer coated

sample exposed to CMAS showing interlamellar cracks and exfoliated lamellae. (b) Image of top surface of five layer coated sample depicting the

LZ-CMAS reaction front.


reacted layer crystallites, increasing the area of interactionwith the melt. The observation is that the CMAS haspenetrated the LZ top layer through the splat boundaries,but the penetration rate, the propensity of CMAS penetra-tion and subsequent cracking of ceramic lamellae has beenretarded. The most plausible scenario for the infiltrationprogressing through the thickness of the coating involvesthe following order. Stage I: when the surface temperatureof the five layered coating reaches 1280 1C, the moltenfraction of the CMAS deposit penetrates through thecracks of the coating. Stage II: because the temperature

of the melt decreases with depth into the coating, theviscous drag increases and the penetration rate reduces.Nevertheless, for glasses of this chemistry, the viscosity issufficiently low so that the penetration can continue wellbelow the isotherm. Indeed, the glass transition tempera-ture of similar glasses (TgE800 1C) implies that a sig-nificant layer of under-cooled CMAS is likely to developwithin the TBC before the flow becomes sufficientlyviscous to fully stop penetration [33]. Stage III: crystal-lization within this under-cooled layer commences(conceivably modified by the reaction taking place between

Location I

TGO BC

LZ

CMAS

LZ + YSZ

YSZ

YSZ + BC

BM

CMAS

La8Ca2(SiO4)6O2

Fig. 18. Cross-sectional image of surface of five layer coated sample indicating CMAS infiltration and needle shaped apatite (La8Ca2(SiO4)6O2)

grain formation.


the molten CMAS and LZ), constricting the infiltrationpath and eventually blocking the infiltration. The thermo-chemical interaction between LZ top layer and CMAS isarguably linked to the arrest of the CMAS infiltration, andthus deserves further analysis. The morphology and che-mical composition of the crystals found at location I in thefive layered coating (Fig. 18) are the same as the onesfound in the case of the LZ duplex coated sample. Thepoint mentioned above and the XRD result shown inFig. 15b confirm the formation of apatite (La8Ca2-(SiO4)6O2), hexagonal anorthite (CaAl2Si2O8) and magne-sia spinel (MgAl2O4). In striking contrast to the YSZ andLZ duplex coatings, the five layered coating is intact, withvoids and cracks appearing only in the top LZ layer, wherethe reaction has taken place between the LZ and themolten CMAS.

The reaction layer consists primarily of large acicularneedle shaped apatite crystals (La8Ca2(SiO4)6O2) protrud-ing into the residual anorthite CMAS glass (CaAl2Si2O8),interspersed with some globular particles. The crystallinereaction layer thus formed on the top LZ layer isimpervious to molten CMAS. The following explanationmight be the reason behind the aforementioned effect.Since the La content in LZ is significantly higher than theY content in YSZ, the dissolution of LZ grains appears toalter greatly the local CMAS composition (La enrichment)relative to the YSZ case. This appears to result in thecrystallization of the penetrated molten CMAS, withapatite (Location I in Fig. 18). The apparent inability ofapatite to incorporate significant amounts of Mg or Allead to the super-saturation of these oxides in the melt andtheir precipitation as anorthite and spinel, the second andthird crystalline products of the reaction. Thus, theformation of this impervious, stable crystalline reactionlayer prevents the penetration of the molten CMAS to acertain extent. The change in viscosity of the moltenCMAS while infiltrating through the coating will also play

a major role in the mitigation process. A more likely effectof the thawed species could be on the viscosity of the meltand the ensuing inward flow. There are documented effectsof rare-earth additions increasing the viscosity of alumi-nosilicate glasses [34], but the latter contained no othermodifiers like Ca or Mg, and much higher concentrationsof rare earth in the melt. Conversely, the crystallization ofapatite would increase the Ca:Si ratio in the melt, decreas-ing its viscosity. Perhaps, the effects of the dissolution ofLZ into the CMAS in promoting the crystallization of themelt are more important than those on the melt viscosity.The 50-hour cycling test used in the investigation is ofsufficient length to illustrate fully developed molten CMASattack of TBCs as well as the deceleration of this attack.Also, the peak cycling temperature of 1280 1C used here istypical of TBC surface temperature reached in the nextgeneration aero engine gas-turbines.Furthermore, the CMAS concentration (35 mg cm�2)

used here is sufficiently high to distinguish betweencomplete CMAS penetration in YSZ and LZ duplexcoatings and partial penetration in the case of five layeredcoating with LZ as the top layer. The partial penetration islikely to result in some loss of strain tolerance in thecoating relative to the situation without CMAS, but thisloss is expected to be significantly lower compared to thecomplete penetration in the case of YSZ & LZ duplexcoated specimens. The LZ top layer impeded the infiltra-tion of the molten CMAS into the underlying YSZcoating. Consequently, destabilization of YSZ was con-siderably restrained. Additionally, oxidation of the BC wasreduced due to the presence of the LZ top layer. The betterCMAS resistance of the five layered coated specimencan be regarded as the result of the composition andstructure distribution of the gradient region in the coating.It is explained that the thermal stresses arising from thedifferent thermal expansions were greatly relaxed in thiskind of gradient coating [35]. This implied that the top LZ


layer impeded the oxidation of the bond coat during cyclicCMAS exposure due to the oxygen non-transparentnature of the LZ material. Further, the crack formationin the top LZ layer of the five layered coating is alsogreatly mitigated as seen in Fig. 18, which also played amajor role in restraining the oxygen diffusion into thecoating compared to the LZ duplex and YSZ duplexcoatings. The EDS elemental mapping on the cross-section of the five layered coating (Fig. 19) suggests thatthe molten CMAS concoction was found to react with thetop LZ layer and it formed a dense reaction front. Thereare also evidences of the CMAS which got infiltrated intothe five layered coating, but the penetration was largelyrestricted to the top LZ layer and some minimal permea-tion can also be seen in the LZþYSZ intermixed layer ofthe coating. This indicated that the LZ top layer acted as abarrier against the infiltration of molten CMAS into theYSZ coating.

The thermal stresses can also play a major role in thefailure of coatings during cyclic CMAS tests. These stressescan be decreased significantly by employing a gradientcoating or by inducting intermixed interfacial layers in thecoating [36]. The bond strength can be improved signifi-cantly by introducing such intermixed interfacial layers in acoating [37]. The microhardness distributions of the duplexcoatings and the five layered coating are displayed inFig. 2a. It can be observed from the figure that themicrohardness changes gradually through the five layeredcoating, while a significant difference exists for the duplexcoatings. The gradual variation of the microhardness ofthe five layered coatings can reduce the large difference of

LZ

LZ + YSZ

50 µm

Fig. 19. EDS elemental mapping results of top two layers of

the elastic modulus between ceramic and metal layers. Theincrease of bond strength of the five layered coating(Fig. 2b) is also because of the gradual change of themicrostructure without sharp interface between differentlayers. The high bond strength and the gradual variation ofthe microhardness between the top ceramic coating and thesubstrate are beneficial to maintain the coating integrity(alleviation of cracks) under the strain generated by thealternating compressive and tensile stresses generated duringthe cyclic CMAS attack test. Compared with the otherconsidered coatings, the gradient coating improved theCMAS infiltration resistance in the 50-hour cycling test.Therefore, it can be summarized that the resistance of thegradient coating to cyclic CMAS attack is superior to thatof the traditional two-layered YSZ (duplex) coating. Thelast failure step of the five layered coating may be accom-plished firstly by delamination of the CMAS infiltrated layercontaining the crystallized reaction products. Furthermore,if continuously in service, the coating experiences gradualloss of material from the surface and or spallation near theLZ top coat/ LZþYSZ interface. The spallation may beattributed to the thermal expansion mismatch between thesolidified reaction layer and the LZ layer. It was found thatin the case of the five layered coating, in spite of the partialspallation of LZ top layer and the infiltration of CMAS intothe intermixed LZþYSZ layer, the third layer from the topi.e., the YSZ layer remained almost unattacked (Fig. 18).As a result, the five layered coating, with LZ top layer caninteract not only with CMAS deposits, but also other typesof deposits (salts, ash, and contaminants), and lessen theirattack on TBCs.

the five layered coated sample exposed to CMAS attack.


5. Conclusions

The state of the art YSZ duplex, the LZ duplex and thefive layered coatings were fabricated by the APS processand the cyclic CMAS infiltration behaviour of these coat-ings were systematically investigated and compared withthose of the HVOF sprayed NiCrAlY bond coat (BC) andthe bare Inconel 738 substrate (BM) coupons. The con-clusions that were attained are put forth as follows:

�
The bare Inconel 738 substrate (BM) coupons showedconsiderable weight gain during cyclic CMAS attackdue to the infiltration of the molten CMAS though thegrain boundaries. The grains underwent severe oxida-tion and formed complex spinels. These spinels(Ni,Co)(Al,Cr)2O4 imposed severe strain in the oxidizedscale leading to its extensive spallation. � The HVOF sprayed NiCrAlY coating (BC) improved
the infiltration resistance of the BM in the givenconditions. The molten CMAS consumed the thermallygrown oxide (TGO), alpha-Al2O3 during the test. TheCMAS glass got enriched with Al content and formedthe anorthite phase along with complex spinels. Theanorthite CaAl2Si2O8 glass composition which is easier-to-crystallize prevented the molten glass from infiltrat-ing deep into the BC; hence the oxidative weight gain ofBC was comparably lesser than that of the BM.
� Molten CMAS was found to melt and infiltrate the
porous YSZ coating. CMAS infiltration in the YSZcoating resulted in the dissolution of the YSZ followedby the reprecipitation of globular ZrO2 grains withdifferent polymorphs and composition based on thelocal melt chemistry. The disruptive phase transforma-tion of t0 to tþm and t to mþc occurred. This phasetransformation is attributed to the deviation in thecontent of Y2O3 stabilizer from the original YSZ solidsolution during reprecipitation.
� The CMAS infiltration did play a role in the partial
failure of LZ duplex coating. The main reasons for therapid weight gain of LZ duplex coating compared tothat of the YSZ coating after 23 cycles, is attributed tothe rapid propagation of numerous longitudinal andtransverse cracks, induced by the thermal shock createdin the CMAS cycling experiments.
� Exposure of the LZ duplex coating to molten CMAS
resulted in the formation of apatite (La8Ca2(SiO4)6O2),hexagonal anorthite (CaAl2Si2O8), magnesia spinel/ ortho-gonal magnesium aluminate (MgAl2O4) together with theoriginal LZ cubic pyrochlore peaks. Though the forma-tion of the aforementioned phases should have mitigatedthe CMAS infiltration, the intrinsic low coefficient ofexpansion and the low fracture toughness of LZ lead tothe formation of numerous cracks through which theoxidation took place and eventually increased the weightgain of the sample, compared to YSZ.
� In the case of the five layered coating, the presence of a
LZ top layer reduced the infiltration of molten CMAS

into the YSZ coating, which refrained the structuraldestabilization of the inner YSZ. The LZ top layerdeveloped a reaction layer made of needle shapedacicular apatite (La8Ca2(SiO4)6O2), hexagonal anorthite(CaAl2Si2O8) and magnesia spinel (MgAl2O4) duringexposure to the molten CMAS concoction. The lantha-num base apatite got crystallized inside the infiltratedcracks and prevented further infiltration, thereby redu-cing the oxidation rate of the five layered TBC.
� The gradual change in the microhardness distribution
and the high adhesion strength provided a large reduc-tion in stress and stress gradient that formed duringhigh temperature cyclic CMAS attack on the fivelayered coating. The mitigation the crack formationdue to the gradient nature of the layers prolonged thelifetime of the five layered coating.
� The developed five layered coating can be successfully
applied on aero engine turbines to increase theirefficiency and to mitigate the molten CMAS infiltration.

Acknowledgement

The corresponding author wishes to express his sincerethanks to the Department of Science and Technology(DST), Government of India, New Delhi for the fellowshipand the financial support to carry out this investigationthrough sponsored fast track scheme for young scientistsR&D project no. SR/FT/ETA-01/2009.

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