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Surface & Coatings Technolo

Characterization of thermal barrier coatings with a gradient in porosity

A. Portinhaa, V. Teixeiraa,*, J. Carneiroa, J. Martinsb, M.F. Costac, R. Vassend, D. Stoeverd

aGRF-Functional Coatings Group, Physics Department, University of Minho, Campus de Azurem, 4800 Guimaraes, PortugalbMechanical Engineering Department, University of Minho, Campus de Azurem, 4800 Guimaraes, Portugal

cPhysics Department, University of Minho, Campus de Gualtar, 4700 Braga, PortugaldInstitute for Materials and Processes in Energy Systems 1, Forschungszentrum Julich GmbH, D-52425 Julich, Germany

Received 11 February 2004; accepted in revised form 8 July 2004

Available online 11 September 2004

Abstract

A major problem in thermal barrier coatings (TBC) applied to gas turbine components is the spallation of ceramic coating under thermal

cycling processes. In order to prevent spallation and improve the thermomechanical behaviour of the TBC, graded ceramic coatings can be

produced. For this purpose we are developing a new concept of Thermal Barrier Coating (TBC) that consist of a conventional NiCoCrAlY

bond coat and an atmospheric plasma sprayed ZrO2–8 wt.%Y2O3 top coat graded in porosity on an Inconel 738 LC substrates. The aim of

this work is to produce coatings with low thermal conductivity and better thermomechanical behaviour due to the gradient in porosity which

reflects a gradient in the elastic properties. Absolute porosity was measured with a mercury porosimetry and by image analysis. The second

technique was also used to estimate the porosity variation along the cross-section. Optical Microscopy (OM) and Scanning Electron

Microscopy (SEM) were used to observe the morphology and coating microstructure. The microhardness was measured with a Vickers

indenter and 0.981 N load. The microhardness has been evaluated for coatings in as-sprayed condition and after annealing at 1100 8C during

100 h. The results show a fast increase of the hardness after annealing. After thermal shock heating at 1000 8C, 1 h and quickly cooling in

water no spallation was observed for 100 cycles.

D 2004 Published by Elsevier B.V.

Keywords: Stabilised zirconia coatings; Thermal barrier coatings; Graded in porosity; Microhardness

1. Introduction

Ceramic coatings of engineering materials such as

zirconia partially or totally stabilized are used for a variety

of technological applications requiring thermal insulation,

wear and erosion resistance or protection from oxidation,

sulfidation and hot corrosion. These kinds of coatings have

been applied as Thermal Barrier Coatings (TBCs) for

protection of metallic components in gas turbines (vanes,

blades, shrouds, etc.) and diesel engines, and improve

performance at high temperatures [1–3]. The TBC concept

allows increase in operating temperature and/or reducing the

cooling systems due to the temperature gradient across the

thick ceramic coating, that permit better thermodynamic

0257-8972/$ - see front matter D 2004 Published by Elsevier B.V.

doi:10.1016/j.surfcoat.2004.07.094

* Corresponding author. Tel.: +351 253510465/400; fax: +351

253510401.

E-mail address: vasco@fisica.uminho.pt (V. Teixeira).

performance, lower emissions without requiring major alloy

development. Zirconia coatings are very interesting materi-

als because of their outstanding mechanical, thermal, optical

and electrical properties. Zirconia has a high melting point,

high resistance to oxidation, low thermal conductivity, high

hardness, and high coefficient of thermal expansion. These

ceramic coatings are widely use in many technological

applications such in components at high temperature and

adverse corrosive environments, oxygen sensors [4], optical

coatings [5], etc.

TBCs, traditionally, consist in a thick partially stabilized

ZrO2 top coating commonly deposited by atmospheric

plasma spraying (APS) on superalloys precoated with a

metallic bond coat (NiCoCrAlY) produced by vacuum

plasma spraying (VPS) [6]. The partially stabilized ZrO2

top coat has a porous and laminar structure and consists of

splats with cracks perpendicular to the surface, this porous

structure allows the increase in the thermal isolation and the

gy 195 (2005) 245–251

Fig. 1. Model of TBC graded in porosity along cross section. The graded

coating was divided in four layers. The porosity was increasing from layer

1 to 4.

A. Portinha et al. / Surface & Coatings Technology 195 (2005) 245–251246

cracks permit better stresses accommodation [1,7–9]. The

metallic bond coat reduces the diffusion of contaminants

and the mismatches of the thermal expansion between top

coat and the substrate and this leads to an increase of the

lifetime of operation for the components.

Zirconia (ZrO2) crystallizes in three distinct polymorphs

according to the temperature and pressure conditions. The

main phases of ZrO2 are the monoclinic phase (m) stable at

room temperature, the tetragonal phase (t) stable between

1170 and 2370 8C and cubic phase (c) stable from 2370 to

2680 8C [10–14]. When the applications for these coatings

involve operation in range of temperatures that involve

phases transformations, it is necessary to stabilize the high

temperature phases at room temperature because the tZm

phase transformation is accompanied by a 3–5% volume

expansion, and this volume expansion can cause high

residual stresses and microcracks leading to delamination

and spallation of coatings.

To prevent this transformation, it is necessary to stabilize

the high temperature phases at room temperature. For this

purpose, these phases can be stabilized by doping ZrO2 with

other oxides such as Y2O3, CeO2, MgO, CaO, Gd2O3 or

Al2O3 [10–17].

The tetragonal phase of zirconia also can be stabilized at

room temperature by decreasing the grain size for a few

nanometers (about 6 nm) and it can be obtained producing

nanolayered coatings of ZrO2/Al2O3 and ZrO2/TiO2 [13,16].

These coatings with nanometer grain size and nanolayered

structures also lead the decrease in thermal conductivity.

Nowadays, higher operation temperatures are required

and in order to obtain systems of coatings that allow its

range of temperatures we need to develop new materials for

coatings or new architectures for the existing materials.

These new concepts of TBCs should have lower thermal

conductivity, and be more stable at higher temperatures than

the 7–8 wt.% yttria-stabilized zirconia (YSZ) [6,18]. In

addition, these new materials should have other properties

comparable to the YSZ like thermal expansion coefficient,

corrosion resistance.

It is known that increasing the porosity, it will reduce the

thermal conductivity; however, above certain values, this

increase can degrade the mechanical integrity due to the

decrease in cohesion between lamellas. Producing coatings

with porosity variable, increasing to the surface we probably

can have coatings with higher porosity content with the

same or better thermomechanical behavior because the

adherence and residual stresses are maintained at the

interface but increasing the porosity towards the surface a

reduction both in elastic modulus is expected and the level

of residual stresses. These properties can be also controlled

with better rigor if we will control the pore geometry [19].

New materials, stable at high temperatures and with lower

thermal conductivity are under development in order to

allow the use of high inlet temperatures [6,17,20]. In

addition, multilayer systems with different functions are in

study: layers for chemical insulation, with an intermediate

conventional zirconia partially stabilized layer and a new

material top layer or a graded structures changing the

chemical composition from interface with bond coat to the

surface using 100% YSZ at interface and then reducing it

contents in substitution with the new materials like lantha-

nides [6]. New dopants have also presented good results,

applied alone or joint with the yttria (Y2O3) that have

substantial reductions in thermal conductivity reaching 40%

for coatings codoped with Y2O3 and Gd2O3 [17,20,21].

In order to obtain better thermal insulation, in this

contribution we present a new concept of TBC. It consists in

a conventional bond coat and a graded ZrO2–8 wt.%Y2O3

top coat that is graded in the porosity (see Fig. 1).

In this paper, we report on the study of structural properties

of ZrO2Y2O3 multilayered coatings focusing on the porosity

of the microlayers. In order to increase the efficiency of the

thermal barrier, different layers with different porosities

increasing towards the surface were studied.

The SEM images are processed using dedicated routines,

in order to measure the porosity of the coatings [22,23].

Not only the porosity values for each layer were obtained

but also it was evaluated the way the porosity changes along

the coatings cross-section. To obtain the total porosity, we

also determine the porosity by mercury intrusion [23].

The microhardness was measured with a Vickers indenter

in as-sprayed condition and after annealing. The values of

microhardness were thus related with the deposition

conditions, heat treatment and porosity variation [24,25].

2. Experimental setup

Thermal barrier coatings (TBCs) investigated were

produced by plasma spraying. A Sulzer Metco AG vacuum

plasma spraying (VPS) system was used to deposit a

NiCoCrAlY bond coat (Ni 192-8 powder by Praxair Surface

Technologies, Indianapolis, IN) on square plates of a nickel

superalloy Inconel 738 LC previously sand-blasted with

alumina particles which are 0.71-Am average size and

ultrasonically cleaned in acetone. The substrate dimensions

were 40�40�3 mm.

The ceramic powder used for top coats was ZrO2–8

wt.%Y2O3 (Metco 204 NS Sulzer Metco GmbH, Germany)

and was sprayed by atmospheric plasma spraying (APS)

Table 1

Deposition parameters

Deposition parameters APS All samples

HP GPI GPII GPIII VPS

Power (kW) 14.5 20.5 to 14.5 20 to 15 20.5 to 14.5 49.4

Int. of current (A) 240 300 to 240 300 to 240 300 to 240 733

Gas plasma (slpm) 20.1/13.1 (Ar/He) 20.1/13.1 (Ar/He) 20.1/13.1 (Ar/He) 20.1/13.1 (Ar/He) 50.6/9.1 ( Ar/H2 )

Gas transport (Ar) (slpm) 1.5 1.5 1.5 1.5 1.7

Diameter of nozzle (mm) 10 10 10 10 7

Flux of powder (g/min) 8.5 8.5 8.5 8.5 40

Distance of gun to substrate (mm) 90 90 120 90 to 120 275

Pressure in the chamber (mbar) atm atm atm atm 60

Substrate temperature (K) 423 473–423 453 423 1073–1093

Thickness of the total layer (Am) 300 265 305 200 140

Gun speed (mm/s) 500 500 500 500 440

A. Portinha et al. / Surface & Coatings Technology 195 (2005) 245–251 247

using a Sulzer Metco Triplex gun. Deposition conditions are

presented in Table 1. For characterization of the as-sprayed

condition of the top coat, steel substrates were coated at the

same time.

The samples were annealed at 1100 8C for 100 h in air,

and were made a thermal shock heating at 1000 8C during 1

h and after cooling in water for 100 cycles.

The microhardness was measured with a Vickers

indenter with 0.981 N load and was measured along the

whole cross-section.

Total porosity levels were evaluated in as-sprayed

condition by mercury intrusion after removal of the steel

substrate from the coating with hydrochloric acid.

The morphology of the coatings was analyzed by optical

and scanning electron microscope (SEM). Micrographs with

two magnifications (400� and 500�) from polished cross-

Fig. 2. SEM micrographs showing microstructure and porosity on ZrO2–8 wt.%

columnar structure in each lamella, (b) cross-section after annealing in which the Th

top coat where the increase in porosity towards the surface is visible.

sections were used for image analysis for the determination

of the total porosity and the porosity profile through the

cross-section.

3. Results and discussion

3.1. Microstructural characterization

In Fig. 2 is presented SEM cross-sectional micrographs

for thermal barrier coatings showing the microstructure

before and after annealing. The thickness was determined by

SEM analysis (see Table 1) of all coatings, and combining

this analysis with deposition parameters was estimated the

thickness of each microlayer for the different porosities

along the cross-section. The coatings present a porous and

Y2O3 coatings produced by APS. (a) Typical lamellar microstructure with

ermal Grown Oxide (TGO) is clearly visible, (c) cross-section of as-sprayed

Table 2

Coating porosity measured by image analysis and Hg porosimetry

Samples HP GPI GPII GPIII

Hg porosity (%) 14.75 15.31 15.29 13.38

Image analysis (%)a 11.79 13.08 15.48 13.34

Image analysis (%)b 11.15 10.76 12.73 9.34

Image analysis after annealing (%)a 8.44 8.33 9.90 10.57

Image analysis after annealing (%)b 6.94 7.99 7.40 8.27

a Porosity with small cracks and ribbons.b Porosity without ribbons.

A. Portinha et al. / Surface & Coatings Technology 195 (2005) 245–251248

lamellar structure (see Fig. 2a) which is characteristic for this

kind of coatings [26]. In plasma spraying, the microstructure

of the coatings is strongly dependent on processing

conditions. The splats are separated by interlamellar pores

resulting from rapid solidification of the lamellae, very fine

voids formed by incomplete intersplat contact or around

unmelted particles, and cracks due to thermal stresses and

tensile quenching relaxation stresses. The presence of cracks

also increases the strain tolerance and enhances the thermal

shock resistance for TBCs in service. From the SEM

micrographs it is possible to observe the variation of the

porosity for different coatings and also its variation from the

interface with bond coat to the surface as is demonstrated in

Fig. 2c, the quantification of this variation along cross-

section has been measured by image analysis. Additionally,

small microcracks with diameters of about 200 nm are

observed. These cracks are originated from the thermal

stresses which arise from the rapid cooling during the spray

process (quenching stresses).

After annealing in air, all coatings present a sintered

structure and consequently a reduction in the porosity levels

which is in agreement with other studies [27]; these

sintering effects will promote the increase in thermal

conductivity, in elastic modulus and a loss of the strain

tolerant behavior. A thermally grown oxide (TGO) observed

between bond coat and top coat for the annealed samples is

shown in (Fig. 2b) which have 5 Am after 100 h at1100 8Cin air. Energy dispersion X-ray spectrometry (EDX) analysis

reveals that the TGO is predominantly aluminum oxide. The

aluminum growth oxide at the interface between bond coat

and top coat results from the diffusion of the aluminum

present in the bond coat to it surface, this oxide protects the

alloy from oxidation. However, when it reaches certain

thickness and due the interface irregularities develops high

residual stresses that promote microcracking which are

responsible for the spallation of top coat and system failures.

After thermal shock in all samples, no spallation was

observed and only it has seen some densification at the

surface and the appearance of small cracks perpendicular to

the coating plane that improve the strain accommodation.

This behavior is important and means that our coatings

show an excellent thermal shock resistance. In our opinion,

the gradation in porosity improves this resistance because

the coatings have better accommodation of thermal stresses

during the quenching period.

3.2. Porosity measurements

Porosity of TBCs can be characterized qualitatively by

microstructure observation and quantitatively by mercury

intrusion porosimetry (MIP) technique besides coating

density measurement. The direct examination of coatings

microstructure from cross-section of coatings using a (SEM)

gives comparative information about porosity for the

different coatings. In conjunction with a backscattered

electron (BSE) detector, the chemical composition of the

microstructure is represented in the images by gray level

variation. Pores appear very dark, which permit them to be

distinguished and quantified by image analysis. By this

method, we can’t obtain information about the 3-D pore

network or connectivity between them [22,23].

For the analyzed coatings, two series of images were

acquired, one with 400� magnification and the other with

500� magnification.

Using the MIP, it is possible to obtain measurements of

total porosity for open pores and the evaluation of pore size

distribution. MIP does provide information about the

connectivity of the pores and microscopy reveals informa-

tion about pore geometry, so there is interest in combining

these two techniques for a more complete analysis.

Mercury intrusion porosimetry is based on the premise

that a nonwetting liquid (one having a contact angle greater

than 908) will only intrude capillaries under pressure.

Mercury must be forced using pressure into the pores of a

material. The pore size distribution is determined from the

volume intruded at each pressure increment. Total porosity

is determined from the total volume intruded.

The digital micrographs were evaluated on a Matrox II

program for image analysis.

The pores were identified by thresholding the brightness

of the pores to produce a binary image, after the dark area

fraction in the binary image was evaluated and the

percentage determined.

The corresponding porosity values for the different

coatings are presented in Table 2.

We can see in Table 2 a considerable difference between

the measured Hg porosities and the porosities evaluated by

image analysis. In addition, a reduction was found in

porosity values after annealing for all samples, its reduction

is mainly due to the sintering effects. While the Hg

porosimetry gave reliable results for small pores and

microcracks, it failed for pores with radius larger than 80

Am. For large pores the mercury fills it without any external

applied pressure (because of the weight of the mercury and

the size of the pores) and was therefore not measured.

Contrarily, the image analysis is a technique to analyse

porosity in which the contribution of small pores and small

microcracks between and through the lamellas within the

plasma-sprayed coatings is difficult to measure in contrast to

the large pores. This microcracked microstructure leads to

relatively low thermal conductivity values in APS TBCs and

Fig. 3. Porosity distribution along the cross section determined by image analysis: (b, d, f) porosity with small cracks and ribbons; (a, c, e) porosity without

ribbons.

A. Portinha et al. / Surface & Coatings Technology 195 (2005) 245–251 249

enhances the strain tolerance in service. In agreement with

SEM analysis and deposition parameters the thickness of

each microlayer was estimated in the graded coatings. Using

image analysis the porosity values of each layer was

determined. In Fig. 3, the porosity variation along the

cross-section for these three kinds of coatings is presented.

The porosity increases from the bond coat interface

towards the surface of the top coat. In as-sprayed condition

the absolute porosity variation ranges from 3% to 5% from

interface to top coat surface and in annealed condition we

observe a significant reduction in its variation (from 1% to

2%). This reduction is due the sintering effects at high

temperature [27,28].

The pore size distribution determined by Hg porosimetry

can be seen in the Fig. 4 for all coatings. In this graph is

represented the cumulative porosity function of the pore

sizes and a typical bimodal distribution for the pore radii is

found. All coatings have almost the same behavior in terms

of the pore size distribution, although being the profile of

porosity gradation through the coatings thickness different

for each. The nongraded coatings (HP) present more pores

in the 0.04–0.4 Am radius range than the graded samples.

The GPII samples have a further contribution of pores with

Fig. 4. Pore size distribution of as-sprayed coatings obtained by Hg

intrusion.

radii lower than 0.008 Am for total porosity that implies

more small cracks or interlamellar pores. One fraction of

porosity represents microcracks and the other, larger one

opens pores. The pores with radii lower than 0.2 Am have

lower contribution to the total porosity and its amount is

about 1% to 2%. Pores with radii bigger than 1 Am have also

for all coatings one contribution of 2.5% for the total

porosity. The porosity with radii in the range 0.1–1 Am has

the most influence in the total porosity which is responsible

for about 8.5%. The fraction under 0.2 Am represents the

microcracks through the lamellas and between them that are

very important for the strain accommodation.

Fig. 5 shows the relation between the porosity and the

deposition parameters. We observe that porosity increase

when increase the working distance and decrease signifi-

cantly for higher power for the plasma gun. These results

are due the less velocity of the melted particles in the first

case when they reach to the coating in growth and in the

second case the particles have more velocity and higher

temperature.

3.3. Microhardness measurements

Fig. 6(a1) and (a2) shows the microhardness distribution

of the ceramic top coatings for as-sprayed condition. It can

Fig. 5. Porosity evolution: with power of the plasma gun and with working

distance.

Fig. 6. Microhardness measurements in atmospheric plasma sprayed coatings, along the cross section; (a1) and (a2) in as sprayed condition; (b1) and (b2) after

annealing at 1100 8C during 100 h.

A. Portinha et al. / Surface & Coatings Technology 195 (2005) 245–251250

be observed that the microhardness decrease towards the

surface in graded samples (GPI, GPII and GPIII) and

slightly increased in case of the constant porosity. For the

sample with constant deposition parameters, we observe a

little decrease in the porosity values towards the surface that

might be explained by the increase of the surface temper-

ature during deposition, and justifies the small microhard-

ness increase on sample HP. Not only can the reduction in

the porosity contribute to an increase of the hardness but

also the thermal residual stresses within coatings are

important. The successive material arriving to the substrate

solidify on a surface at lower temperature that rises with the

coating growth which slight decrease the cooling velocity of

splats. With this effect, the layers more close to the final

coating top surface should present a more dense structure

that has higher elastic properties, which is favorable to the

formation of higher residual stresses and also present more

hardness. The decrease in microhardness for the graded

samples is due the increase in porosity along the cross-

section, this variation can be observed in the Fig. 7 and it is

clear a reduction on the harness values for the higher

porosities. Sample GPII has less decrease because the

variation in porosity is minor too. In addition, it is observed

that the microhardness values have a considerable standard

deviation, corresponding to the inhomogeneous, porous

Fig. 7. Relation between hardness and porosity (as-sprayed coatings).

microstructure present in thermal barrier coatings obtained

by plasma spraying.

After annealing, all coatings show higher values for the

microhardness (Fig. 6 (b1) and (b2)) and maintain gradients

from the bond coat interface to the surface. In addition, the

microhardness of the bond coat was measured and gave

constant values along the cross-section (about 450 HV).

The higher values for the annealed coatings are due the

sintering effects at high temperature [26,27].

3.4. Structural analysis and phase transformation

The structural analysis was performed to determine the

structure for all coatings and to determine the volume of

phase modification. For the as-sprayed coatings, all present

a polycrystalline structure in the tetragonal phase and the

main peak diffraction is for (111) planes. After annealing

and thermal shock a very small amount of monoclinic phase

was detected for the main diffraction peaks m(111) and

m(�111). It is known that the presence of monoclinic phase

in TBC’s is not wanted because reduce the TBC’s lifetime

due the volume increase during the tetragonal to monoclinic

transformation. It is well observed that a small shift in the

peak positions that can be explained by the increase in

thermal stresses after annealing and after thermal shock.

4. Conclusions

With the modification of deposition parameters, we can

get a thermal barrier coatings graded in porosity along the

cross-section, and this way we improve the thermal shock

resistance and can be an important factor for decrease the

thermal conductivity. Improving the thermal conductivity

and thermal shock resistance, it is possible to increase the

inlet temperatures in gas turbines and their performance.

In the as-deposited condition, all coatings have a lamellar

structure which is a characteristic for this type of deposition

technique. After annealing and thermal shock a thermal

grown oxide with about 5 Am was observed as coatings

sintering effects.

A. Portinha et al. / Surface & Coatings Technology 195 (2005) 245–251 251

Small cracks and interlamellar defects gave a rather small

contribution to the total porosity in image analysis measure-

ments, in contrast to Hg-intrusion investigations.

All coatings present a bimodal pore size distribution. The

porosity increases towards the surface in graded coatings,

decreases with increasing plasma gun power and increases

with increasing working distance.

The microhardness decreases from the interface to the

surface and increase after annealing. As it was expected, the

microhardness decreases when the porosity increases. Non-

graded, as-sprayed HP sample showed a slightly increase in

the hardness from the interface to the surface which was

explained by the increase of surface temperature during

deposition that implies a densification of the structure with

lower porosity and higher thermal residual stress within the

coating.

The annealed coatings showed reduced porosity levels

due the sintering effects.

After heat treatments, it was observed a very small

amount of monoclinic phase.

Acknowledgments

This work was financially supported by FCT-Portu-

guese Foundation for Science and Technology under the

project POCTI/EME/39316/2001: bPVDCOAT-Composite

and multilayered protective coatings for efficient energy

systemsQ. The cooperative work is also supported by

German–Portuguese Cooperative Programme ICCTI-

DAAD and European Commission-DG-XII under con-

tracts: ICCTI-DAAD/ 423/2000, bComposite Coatings for

high temperature applicationsQ and COST 522, WP2/SP2-

1999/01: bResidual stresses and failure in multilayered and

functionally graded coatings for advanced energy systemsQ(projects leader: V. Teixeira). A. Portinha is grateful for

the Research Grants supported by F.C.T.-Portuguese

Foundation for Science and Technology. The authors

acknowledge to Alcino Monteiro for some hardness

measurements.

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