High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—Relevance of Variation in Materials Systems of TBC Components

doi:10.4236/jsemat.2013.31A016

Paper Menu >>

Journal Menu >>

Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 116-132

http://dx.doi.org/10.4236/jsemat.2013.31A016 Published Online February 2013 (http://www.scirp.org/journal/jsemat)

High Temperature Sintering and Oxidation Behavior in

Plasma Sprayed TBCs [Single Splat Studies] Paper

2—Relevance of Variation in Materials Systems of TBC

Components

Swarnima Deshpande

Center for Thermal Spray Research, Department of Materials Science and Engineering, State University of New York at Stony Brook,

New York, USA.

Email: [email protected]

Received December 1st, 2012; revised January 3rd, 2013; accepted January 11th, 2013

ABSTRACT

The TBC system’s response to thermal exposure at high temperature is discussed here. The relevance of the micro-

structural aspects of each component of the TBC system is emphasized. The top coat is a YSZ ceramic coating consist-

ing of a collection of splats on top of one another. The most important aspect of this layer is the inherent inter-splat and

intra-splat porosity which undergoes sintering during thermal exposure. This study investigates the effect of thermal

exposure on the microstructure and sintering behavior in single splats produced using different starting powders since

this has been shown to influence the basic microstructure of YSZ topcoat. The bond coat is an MCrAlY metallic coating

which serves as an Al reservoir and allows the formation of a protective alumina, Thermally Grown Oxide (TGO) layer

between the bond coat (BC) and the top coat (TC) layers. This oxide scale formed upon thermal exposure prevents fur-

ther oxidation of the underlying component (substrate) and thus provides protection. As such, the content of free Al in

the bond coat layer is of significance and makes it crucial to understand the influence of bond coat microstructure evo-

lution and oxidation involved during its formation. The interaction between the bond coat, the TGO and the top coat

layers is examined in this study to understand the high temperature behavior of the TBC system with regards to varia-

tions in the top coat and bond coat material systems used.

Keywords: TBC; HVOF Bond Coat; Top Coat; Thermal Exposure; Vacuum Environment; Oxidation;

TGO Imperfections; Sintering; Microcracks

1. Introduction

This study seeks to look into the influence of different

material systems used for TBC system components. Dif-

ferent YSZ powders can be used to spray the topcoat and

a variety of spray methods can be used to generate the

bond coats. Previous study involving thermal exposure

using single splats has indicated that the microcracks in

these splats provide a path for the oxygen to reach the

aluminum depleted areas in the underlying bond coat [1st

paper, JSEMAT 2013]. Hence it was thought necessary

to examine the TGO formation and interactions with YSZ

during thermal exposure as a function of the initial YSZ

splat layer and the as-sprayed bond coat microstructures.

High temperature and thermal cycling behavior of

TBC systems has been a subject of great interest. When a

TBC system is thermally exposed, it is transparent to

oxygen because of the abundance of oxygen ion vacan-

cies in ZrO2 and hence oxygen ingresses through the

YSZ top coat. The bond coat undergoes oxidation and

leads to the formation of Thermally Grown Oxide (TGO).

This bond coat is typically an MCrAlY alloy, designed as

a local Al reservoir, enabling α-alumina to form in pref-

erence to other oxides. The TBC Alumina is preferably

formed because of its low oxygen diffusivity and supe-

rior adherence [1]. The thickness of TGO increases with

thermal exposure and induces the strain energy for the

crack propagation during spallation [2].

After exposure the TGO has a convoluted morphology

with major imperfections, the most prominent being the

undulations of the original TBC/bond-coat interface. This

is outlined by the dark gray TGO layer. The thicker areas

of TGO are predominantly α-alumina containing veins of

yttrium aluminates. Other oxides occur in isolated do-

mains within the TBC next to the TGO and have a lighter

gray contrast. These are typically spinels comprising oxi-

des of Cr/Ni/Co often with associated internal porosity

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

117

and these spinels when formed act as preferential sites

for failure [3].

Shillington and Clarke studied the Changes in TGO

during Oxidation [3]. Pieces of the same bond-coat al-

loy were oxidized for different times (64, 128, 256, 384,

512 and 640 h) at 1121˚C. It was seen that after 128 h,

the only phase detectable was α-alumina. After 256 h, the

first indications of the formation of spinel and α-chromia

were detected. Alumina was no longer continuous after

384 h and the oxide was principally a mixture of spinel

and α-chromia. Alumina was no longer detectable after

512 h.

Other studies have indicated that differences in the

YSZ powder used generate differences in splat mor-

phologies and porosity content of the TBC topcoat [4-6].

Also, bond coats (NiAl) sprayed by different processes

have been extensively examined to understand the oxida-

tion involved during spraying and its influence on micro-

structural evolution. It has been shown that Air plasma

spraying and Wire arc spraying involve different mecha-

nisms of oxidation during the entire spray process and

generate coatings with a much higher oxide content com-

pared to HVOF spray process [7]. This study aims to

examine the effect of these variations in the component

material systems on the behavior of the TBC system

upon subsequent thermal exposure.

2. Experimental Approach

Single splats of YSZ were collected on NiCrAlY/

CoNiCrAlY bondcoat surfaces in order to conduct ther-

mal exposure studies on the same. For the first set of coat-

ings, NiCrAlY coatings were air plasma sprayed onto In-

conel 718 superalloy substrates. Processing conditions for

the same are tabulated below in Table 1.

Zirconia powders with different powder morphologies

were used to spray splats onto the polished surfaces of

these MCrAlY bond coats using APS process with spray

parameters as indicated in Table 2. The PSZ powders

Table 1. Deposition parameters for APS sprayed NiCrAlY

Bond coat.

Gun Sulzer F4MB

Gun voltage 68 V

Gun current 500 A

Primary gas (Ar) 50 SLPM

Secondary gas (H2) 10 SLPM

Carrier gas (Ar) 3 SLPM

Spray distance 120 mm

Powder Feed rate 40 g/min

Table 2. Deposition parameters for APS sprayed YSZ splats.

Gun PT-F4MB

Gun voltage 65 V

Gun current 650 A

Primary gas (Ar) 40 SLPM

Secondary gas (H2) 8 SLPM

Carrier gas (Ar) 3 SLPM

Spray distance 100 mm

Powder Feed rate 10 g/min

Gun traverse speed 10 mm/sec

Substrate rotational speed 160 rpm

used were:

1. Fused and Crushed Zirconia & 2. HOSP Zirconia

(Plasma-Densified Hollow Spheres).

For the second set of coatings, splats were collected on

polished surfaces of HVOF and VPS sprayed CoNiCrAlY

bond coats obtained from Engelhard Surface Technolo-

gies to analyze the effect of the bond coat system.

Thermal exposure behavior of these splats in air was

studied using a Thermolyne 47,900 box furnace. The

splats were isothermally exposed in Air at 1100˚C for 8

hr and 24 hrs to see the effect of duration. Also, thermal

exposure was studied in vacuum to eliminate effect of

bond coat oxidation. For this purpose, the samples were

sealed in quartz tubes prior to treatment, up to a vacuum

of 10−5 torr. This set of samples was subjected to an in-

termediate vacuum heat treatment at 1100˚C for 2 hr fol-

lowed by a subsequent isothermal exposure in Air at

1100˚C for 24 hr.

Particular splats were identified and the microcrack

network, surface roughness, and splat lifting/spalling

before and after HT were compared. SEM Back-scattered

imaging (Leo 1550, FEG) was employed on splat cross-

sections to observe the splat dimensions, microcrack sin-

tering and effect of TGO growth after HT. Energy Dis-

persive Spectrometry (EDS) gave elemental composition.

Atomic Force Microscopy (AFM) was employed to

quantify surface roughness of splats.

3. Results and Discussion

The thermal exposure studies were conducted using sin-

gle splats on bond coat. Single splats as opposed to free-

standing coatings include the splat/substrate interfacial

interaction. Absence of thick top coat minimizes the fac-

tors introduced by TBC coating and behavior can be more

related to the bond-coat chemistry and microstructure.

Only isothermal heat treatments were carried out in order

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

118

3.1.1. Splat Interfaces in Coating to eliminate cyclic TGO elongation effects.

The number of interfaces in a coating is directly related

to the thicknesses of individual splats which in turn are

affected by flattening and spreading. Composite SEM

cross-sectional images of coatings for both powder mor-

phologies were used to observe the splat interfaces as

shown in Figures 2(a) and (b). Several visible splat

boundaries in these cross-sections are marked and splat

thicknesses are noted for each of these splats. Then all

these splat thickness values are used to compute the av-

erage splat thickness for each case.

3.1. Effect of Variation in YSZ Top Coat

Microstructure

Changing the feedstock translates into changes in the as-

sprayed splat dimensions which in turn have a distinct

effect on the top coat structure. Different morphology

YSZ powders, Fused and Crushed (FC) and Plasma Den-

sified Hollow Spheres (HOSP) when sprayed through the

plasma exhibit different flow behavior and different mel-

ting efficiencies and form splats with different dimen-

sions.

The average splat thickness for FC YSZ coating aver-

aged over 13 values was 4.14 μm and that calculated for

HOSP YSZ coating using 18 different values was 2.46

μm. Thus it is once again seen that fused and crushed

YSZ powders produced coatings with splats 1.7 times

thicker than in the case of HOSP YSZ. Hence it was in-

ferred that within a given thickness of say 250 μm, the

average number of interfaces created by HOSP coating

Cross-section images of splats in Figure 1, show the

diameters and thicknesses for FC and HOSP splats. Mi-

crocrack widths in each case are also measured. It can be

noted that the FC zirconia splats are approximately 1.75

times thicker compared to HOSP splats. This is further

addressed in the following section.

Fused and Crushe

HOS

2.61



177.9



m 110.0



0.49



(a)

Fused and Crushe

HOS

0.48 m

0.52



m 0.45 m

0.48



0.45



0.52 m

0.41 m

(b)

Figure 1. (a) Cross-sections showing splat dimensions; (b) Cross-sections showing microcrack widths.

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

119

(b1)

(b2)

(b3)

(a) (b)

Figure 2. (a) Composite image of FC YSZ coating at 5000×; (b) Composite image of HOSP YSZ coating (b1) and (b2) at

5000×, (b3) at 10,000×.

will be 103 - 104 whereas the number of interfaces in

case of a FC coating will be 61 - 62. Thus larger splat

thickness translates into lower number of interfaces per

given thickness of coating. This difference in the splat

structure of the two coatings is not insignificant when it

comes to coating properties such as thermal conductivity

and elastic modulus. The implication of these results has

also been discussed in more detail in another study [8].

Hence, in order to examine the influence of top coat

microstructure on the TBC system’s behavior under ther-

mal exposure, different feedstock of YSZ were used to

collect the splats and then these were subjected to similar

heat treatments.

The two TBC system samples consisted of superalloy

substrates, MCrAlY bond coats (BC) and top coats (TC)

made of single splats collected using FC or HOSP YSZ

powders. These were subjected to the various heat treat-

ments as mentioned in Section 2 and are categorized as

given below in Table 3. Henceforth the samples will be

referred to by the YSZ powder type and the Set number

indicating the type of thermal exposure.

Specific splats were identified for each sample (pow-

der type) and set (thermal exposure type) and the top

surface microstructures as well as cross-sections of these

were examined before and after thermal exposure. SEM,

AFM and EDS analysis were employed for observation

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

120

Table 3. Different types of thermal exposure treatments on

FC and HOSP splats.

Samples

1. (Substrate/BC/FC TC)

2. (Substrate/BC/HOSP TC) Thermal exposure

Set 1 As-sprayed splats

Set 2 Isothermally exposed in Air for 8 hr

at 1100˚C

Set 3 Isothermally exposed in Air for 24 hr

at 1100˚C

Set 4

Isothermally exposed in Vacuum for

2 hr at 1100˚C and then Isothermally

exposed in Air for 24 hr at 1100˚C

and comparison of several effects of thermal exposure.

3.1.2. Splat Dimensions

The top surface microstructures of the single splats of FC

and HOSP YSZ taken at the same magnification before

and after different exposure times in air show that the

splat dimensions have not changed as an effect of ther-

mal exposure. The cross-section images of splats are

compared to show that the splat thicknesses also remain

the same after thermal exposure; 2.6 μm for FC and 1.49

μm for HOSP splats after 24hr HT and 8hr HT respect-

tively (Figure 3).

3.1.3. Microcrack Sintering

The microcrack network is observed in the top surface

SEM micrographs of individual splats. It is seen that this

network is not altered after thermal exposure at 1100˚C.

Most of the fine microcracks are retained when the splats

are thermally exposed in air. This is also confirmed by

measuring microcrack widths in the cross-section images

of splats before and after different thermal exposure times

(Figure 4).

However in case of samples of Set 4 that have under-

gone the intermediate vacuum thermal exposure, the mi-

crocrack network is modified. Most fine microcracks in

this case have started to sinter and are not as noticeable

in the top surface microstructures of splats. The cross-

section images also show that microcracks widths have

reduced significantly after the subsequent 24 hr Air HT

(Figure 5). This indicates that the intermediate vacuum

HT is responsible for instigating the sintering of these

microcracks. This may be related to the increase in lattice

spacing that was observed by Thornton et al. [9] when a

TBC coating was heat treated in vacuum. In vacuum,

there is no oxygen to replace that lost from zirconia in

the formation of bond coat oxide and less oxygen appears

to cause larger lattice spacing [9]. Increased lattice spac-

ing in turn could be initiating the sintering of micro-

cracks.

3.1.4. TGO Growth

Bond coat oxidation at high temperature, leads to the for-

mation of TGO and thereby creates another layer at the

interface between BC and TC. The TGO layer thickness

gradually increases with HT duration.

Figure 6 shows the development and growth of the

TGO layer between the bond coat and the top coat upon

thermal exposure in Air at 1100˚C. It is seen that for both

the powders, the samples show increased TGO thickness

with increasing duration of thermal exposure. For FC

YSZ samples, the TGO thickness increases from ap-

proximately 1.5 μm after 8 hr HT to 2.7 μm after 24 hr

HT. For the HOSP YSZ samples, the TGO thickness is

seen to increase from approximately 1.3 μm after 8 hr

HT to 2.0 μm after 24 hr HT. This TGO layer is mainly

constituted by Alumina since Al in the bond coat is pref-

erably oxidized but at some places Chromia is also for-

med as disclosed by EDS. Previous studies also show

that other oxides occur in isolated domains within the

TBC next to the TGO and have a lighter gray contrast.

These are typically spinels comprising oxides of Cr/Ni/Co

often with associated internal porosity [10].

3.1.5. Splat Surface Roughening

Figure 7 shows the splats and large magnification im-

ages (taken at 50,000×) of their top surfaces before and

after different thermal exposures. The grain structure on

the splat surfaces is thereby visible. In case of both FC

and HOSP splats it is observed that at the areas marked

by purple arrows, the columnar grains seem to have risen

upwards. A comparison of the splat surfaces between as-

sprayed and thermally exposed splats indicates that the

splat surface has roughened overall. This splat surface

roughness was quantified using AFM in case of HOSP

powder samples for as-sprayed splats and those exposed

for 8hr in Air at 1100˚C. The results of this measurement

are shown in Figure 8 below. The mean surface rough-

ness of the as-sprayed splat was approximately Ra = 11.9

nm and that of the thermally exposed splat was approxi-

mately Ra = 153 nm.

In order to examine the cause behind the increased

surface roughness, cross-sections of the splat were ob-

served and the TGO layer generated after 24 hr HT in Air

at 1100˚C was characterized. In most areas the TGO

consists primarily of Alumina, as in Figure 9(a), but in

some areas oxides like Chromia also form and exhibit a

lighter grey contrast as discussed in Section 3.1.4 above.

Studies have shown that such other oxides like spinels

when formed also act as preferential sites for failure [10].

Reason might be that the interfacial fracture resistances

of the TBC/α-chromia and the TBC/spinel interfaces are

lower than that of the TBC/α-alumina interface originally

present [11]. Figure 9(b) indicates the splat being lifted

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

121

FC YSZ splat HOSP YSZ splat

2.61 m2.61 m2.61 m

Set 3 (splat thickness)

Set 1 (splat thickness)

2.6 m2.6 m2.6 m

Set 3

Set 1

Set 2

Set 1 Set 2 Set 1

Set 3

Set 1

1.49 m1.49 m1.49 m1.49 m

Set 2 (splat thickness)

Set 1 (splat thickness)

Figure 3. Splat dimensions before and after HT in Air@1100˚C (for FC and HOSP splats).

FC YSZ spla

HOSP YSZ splat

Set 2 (microcrack network)

Set 1 (microcrack network)

0.52 m

0.48 m

0.45 m0.52 m

0.48 m

0.45 m

Set 2 (microcrack width)

Set 1 (microcrack width)

0. 48 m

0. 48 m0. 52 m

0. 48 m

0. 48 m0. 52 m

Set 2 (microcrack network)

Set 1 (microcrack network)

Set 3(microcrack network)

Set 1 (microcrack network) Set 3(microcrack network)

Set 1 (microcrack network)

Set 2 (microcrack width)

Set 1 (microcrack width)

0.48 m0.52 m

0.45 m0.41 m

0.48 m0.52 m

0.45 m0.41 m

0.30 m

0.39 m

0.30 m

0.39 m

Figure 4. Microcracks in splats before and after HT in Air@1100˚C (for FC and HOSP splats).

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

122

Set 4(microcrack network)

Set 1 (microcrack network) Set 4(microcrack network)

Set 1 (microcrack network)

FC YSZ splat HOSP YSZ splat

0. 09 m0. 09 m0. 09 m

Set 4 (microcrack width)Set 4 (microcrack width)

0.17 m0.17 m

Figure 5. Microcracks in splats before and after HT in vacuum + HT in Air@1100˚C (for FC and HOSP splats).

FC YSZ splat HOSP YSZ splat

Set 1 (interface between BC and TC)

1.3 m1.1 m

Chromium oxide

Aluminum oxide

1.3 m1.1 m

Chromium oxide

Aluminum oxide

2.7 m

Aluminum oxide

2.7 m

Aluminum oxide

Set 2 (TGO layer between BC and TC)

Set 3 (TGO layer between BC and TC)

Set 1 (interface between BC and TC)

Set 2 (TGO layer between BC and TC)

Set 3 (TGO layer between BC and TC)

Chromium oxide

1.27 m

0.74 m

Aluminum oxide

Chromium oxide

1.27 m

0.74 m

Aluminum oxide

1.99 m

Aluminum oxide

1.99 m

Figure 6. TGO layer thickening with increasing duration of HT in Air@1100˚C (for FC and HOSP splats).

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

123

FC YSZ splat HOSP YSZ splat

Set 3

Set 1

Set 2

Set 1

Set 3

Set 1

Set 2

Set 1

Figure 7. Roughening of splat surface due to TGO layer thickening w i th ther mal exposure (for FC and HOSP splats).

HOSP YSZ splat

Section AnalysisRoughness Analys is

Mean Roughness (Ra) 11.852 nm

Section AnalysisRoughness Analys is

Mean Roughness (Ra) 11.852 nm

Secti on A nalysisRoughnes s Analys is

Mean Roughness (Ra) 153.08 nm

Secti on A nalysisRoughnes s Analys is

Mean Roughness (Ra) 153.08 nm

Set 2

Set 1

Figure 8. Roughening of splat surface due to TGO layer thickening with thermal exposure.

upwards at such a location.

Thickness imperfections in TGO enlarge in regions

where O2-diffusivity through TGO is exceptionally large

i.e. at locations where TGO contains oxides other than

alumina [1]. These TGO undulations must then push the

grains in the splats upward and cause splat lifting or may

be spalling. This is visible in Figures 9(c) and (d) and

explains the increase in surface roughness of splats as

measured in Figure 8.

3.1.6. NiO O utgrowth

nother effect of thermal exposure is the appearance of

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

124

(b) Non-Uniform TGO layer (Alumina + Chromia)  splat being lifted upwards

(a) Uniform TGO layer (Alumina)

FC YSZ splat

OAl

EDS analysis of Alumina and Chromia regions

of TGO

(for FC and HOSP splats)

(d) Thickness imperfection in TGO layer at the location of chromia formation

HOSP YSZ splat

Figure 9. TGO layer between bond coat and YSZ top coat  generated after 24 hr HT in Air at 1100˚C Effect of Chromia

ormation at TGO/YSZ splat interface. f

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

125

some outward oxide growth from the bond coat, visible

at larger microcracks on the top coat surface, as marked

by green arrows in Figure 10. This outgrowth was as-

sumed to originate from the bond coat oxide below the

splat. It was also observed when cross-section images

were examined. EDS analysis was conducted and these

outgrowing oxide grains gave an EDS pattern showing

Ni and O peaks, also shown in Figure 10.

APS NiCrAlY bond coat microstructure (Figure 11)

contains internal oxide chunks of alumina. These regions

may be locally depleted of free aluminum. Microcracks

in splats are seen to coincide with oxide points in the

bond-coat, Figure 11.

Several studies have shown that oxygen diffusion

through TGO along grain boundaries causes more TGO

growth at these boundaries [1,12,13]. Similarly at these

microcrack positions, there is a path for oxygen to reach

a BC area that is locally depleted of aluminum [7]. As

such, other oxides could form at these microcrack posi-

tions.

However a similar investigation of the Set 4 samples

that underwent the intermediate vacuum HT (Figure 12)

revealed no such oxide outgrowth. The surfaces of splats

indicated almost sintered microcracks and no outgrowth

in the top surface as well as the cross-section images.

The outward oxide growth is curtailed probably because

the intermediate vacuum treatment started the sintering

or sealing of most microcracks from their bottom end.

One possible explanation for this microcrack sintering

under vacuum has been discussed above in Section 3.1.3.

The oxide outgrowth in case of thermal exposure in

Air is also thought to be related to the bond coat mic-

rostructure and will be addressed in the following Sec-

tion 3.2.1.

3.2. Effect of Variation in MCrAlY Bond Coat

Microstructure

The bond coat microstructure is greatly influenced by the

spraying technique adopted for the coating formation.

Ni-5 wt% Al bond coatings obtained using four different

spraying techniques are compared in Figure 13. Wire arc

spraying, Air plasma spraying, HVOF spraying and Cold

spraying, each differ w.r.t their feedstock injection, mel-

ting methods, spraying parameters as well as oxidation

involved during spraying. As such they result in distinct

particle conditions during flight. Wire arc and APS gen-

erate lower particle velocities compared to HVOF and

Cold spray. HVOF sprayed droplets have lower tempera-

tures and Cold spray is a unique process in which entire

deposition takes place in the solid state [14].

As such these processes lead to distinct differences in

the microstructures of coatings produced. The porosity

and oxide content in the micrographs are estimated using

Image analysis, shown in Figure 14.

Both wire arc and APS coatings show significant oxi-

dation and porosity. HVOF coating depicts a much lower

“dark area” but a significant portion of this is oxide.

Cold-spray coatings indicate low porosity and almost no

oxide formation, probably due to high impact velocities

and low process temperatures.

Figure 15 summarizes the process of bond coat mi-

crostructure evolution by describing the key mechanisms

involved during spraying such as in-flight oxidation, sin-

gle splat formation, post-impact oxidation and splat-splat

linkage leading to coating buildup. Schematics for me-

chanisms and corresponding microstructural observations

are shown.

Oxide segregation during in-flight oxidation in case of

APS process causes splashing of oxide beneath the splat,

whereas in case of HVOF spraying, complete spreading

of splat before solidification generates very flat, disk sha-

ped splats having better contact with substrate. Less in-

flight oxidation and slow solidification in the HVOF

process generates an Al depletion region below the post

impact oxidation on splat surface. Post impact oxidation

in case of APS splats however, generates poorer wetting

and gives rise to lamellar porosity between the new splat

and oxide of the previous layer. Metal-metal contact in

both processes generates inter-splat coalescence.

Nano scale observations revealed oxide bands after

every splat in case of APS coating and more splat coales-

cence in HVOF coating. Phases in each coating were

analyzed showing fcc Ni as the primary phase in APS

and gamma-Ni (richer in Al) in case of HVOF. The

above observations are investigated and discussed in de-

tail elsewhere [7].

These observations make it very apparent that the

choice of bond coat will also play a role in the high tem-

perature behavior of the TBC system as the bond coat is

an integral component of such a system. Hence, in order

to examine the influence of bond coat microstructure on

the TBC system’s behavior under thermal exposure,

bond coats sprayed by different techniques were used.

Top coat splats were collected on these bond coat sur-

faces and the obtained TBC systems as a whole were

then subjected to similar heat treatments. Table 4 shows

a list of the samples used.

The top surfaces of splats and cross-sections of coat-

gs were compared before and after thermal exposure.

Once again different effects of thermal exposure were

observed upon examination.

3.2.1. Ni O Outgrowth—HT in Air

Figure 16 shows the as-sprayed splats and the splats af-

ter 8 hr HT in Air at 1100˚C on APS, HVOF and VPS

bond coat surfaces.

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

126

FC YSZ spla

HOSP YSZ spla

Set 1 Set 2 (outward oxide growth)

Set 3(outward oxide growth) Set 1

Set 2 (outward oxide growth)

Set 1

Set 3(outward oxide growth)

Set 1

EDS analysis of Oxide outgrowth

Ni and O peaks

FC Set 2 (outward oxide growth) HOSP Set 2 (outward oxide growth)

ONi

Figure 10. NiO outgrowth through microcracks in splats after HT in Air @ 1100˚C (for FC and HOSPsplats).

O K

Al K

Ni L

O K

Al K

Ni L

Figure 11. Microcracks coinciding with oxide points.

Table 4. Different types of thermal exposure treatments on

FC and HOSP splats.

No. TBC system components Thermal exposure

Single splats of YSZ on

APS, HVOF and VPS bond

coat

Isothermally exposed in Air

for 8 hr at 1100˚C

2. Thin YSZ coating on APS

bond coat

Isothermally exposed in Air

for 24 hr at 1100˚C

3. Thin YSZ coating on HVOF

bond coat

Isothermally exposed in Vacuum for

2 hr at 1100˚C & then Isothermally

exposed in Air for 24 hr at 1100˚C

In case of APS bond coat, outward oxide growth and

splat surface roughening is observed as discussed before

in Sections 3.1.6 and 3.1.5 respectively. But in case of

HVOF and VPS bond coats, no such oxide outgrowth is

seen and splat surface roughening is also minimal.

The HVOF and VPS bond coats have much lesser in-

herent oxidation as seen in Section 3.2. Lesser in-flight

oxidation was reported for HVOF compared to APS and

of course VPS process pretty much eliminates oxidation

during spraying. As such, the Al concentration in the BC

is uniform, there are no alumina chunks, and microcracks

cannot coincide with any Al depleted areas. This natu-

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

127

Set 4  Sealed microcracks

No oxide outgrowth

FC YSZ splat HOSP YSZ splat

Set 1

Set 4 (no oxide outgrowth)

Set 1

Set 4 (no oxide outgrowth)

Figure 12. Microcracks in splats before and after HT in Vacuum + HT in Air@1100˚C (for FC and HOSP splats).

APS coating showing

thinner splats

Cold spray coating showing

very dense cross-section

Wire Arc coating showing thicker splats

HVOF coating showing very

few visible splat boundaries

Figure 13. Bond coat microstructures using different spray-

ing techniques.

rally precludes the abovementioned method of formation

of NiO.

3.2.2. Sintering Under High Temperature

Figure 17 shows TBC samples of thin YSZ coating on

APS BC and thin YSZ coating on HVOF bond coat sub-

jected to the 24 hr HT in Air at 1100 degrees.

Careful examination shows visible sintering of inter-

Wire-arc APS HVOF Cold

16.5

20.5

3.1 1.4

Wire-arc APS HVOF Cold

16.5

20.5

3.1 1.4

Wire-arc APS HVOF Cold

16.5

20.5

3.1 1.4

Figure 14. Porosity and oxide content in bond coat micro-

structures obtained using image analysis.

lamellar pores and intersplat boundaries in the APS BC

sample, as marked by yellow arrows. Most microcracks

are however still retained as indicated by red star sym-

bols.Whereas, upon inspection of the HVOF BC sample,

interlamellar pores and intersplat boundaries are seen to

be still present as shown by pink arrows and red stars

indicate that microcracks have started sintering.

Although this is not completely understood, one possi-

ble theory is that as observed before, during HT in Air,

NiO outgrowth occurs through microcracks of YSZ

splats on APS BC whereas; this is not the case with

HVOF bond coats. Since this basic hindrance to sintering

(blocking of microcracks) is now missing, the micro-

cracks can sinter.

If these same samples are subjected to intermediate

vacuum HT and then the 24 hr Air HT (Figure 18), then

however, most microcracks are seen to have sintered in

ase of both bond coat samples. This is in agreement c

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

128

APS bond coat HVOF bond coat

Internal

oxide,

Al2O3

Internal

oxide,

Al2O3

Oxide

segregation

Al2O3

Oxide

segregation

Al2O3

SubstrateSubstrate SubstrateSubstrateSubstrateSubstrate

SubstrateSubstrateSubstrateSubstrate

SubstrateSubstrateSubstrateSubstrate SubstrateSubstrateSubstrateSubstrate

Oxide splashing

beneath splat

[1]

Oxide splashing

beneath splat

Oxide splashing

beneath splat

[1]

Complete spreading of splat before solidificationComplete spreading of splat before solidification

5 m5 m

[2]

Al depletion below oxide

[2]

Al depletion below oxide

Coalescence between two

metal splats with no oxide

in between

Coalescence between two

metal splats with no oxide

in between

Lamellar pore

between new

splat and oxide

of previous layer

[3]

Lamellar pore

between new

splat and oxide

of previous layer

[3]

2 m2 m2 m2 m

Substrate

Oxide growing on top of splat

SubstrateSubstrateSubstrateSubstrate

Oxide growing on top of splat

Oxide bands

after every splat

[4]

Oxide bands

after every splat

[4] More splat

coalescence

[5]

More splat

coalescence

[5]

[6]

Ni (200)

Ni (020)Ni (2 20)

fcc Ni

[6]

Ni (200)

Ni (020)Ni (2 20)

fcc Ni

Ni (200)

Ni (020)Ni (2 20)

Ni

(200)

Ni

(022)

Ni

(311)

Ni (200)

Ni (020)Ni (2 20)

Ni (200)

Ni (020)Ni (2 20)Ni

(111)

fcc NiNi

Ni

(200)

Ni

(022)

Ni

(311)

Ni

(111)

Ni

(200)

Ni

(022)

Ni

(311)

Ni

(111)

Ni

(200)

Ni

(022)

Ni

(311)

Ni

(111)

Ni

(111)

Ni

(311) Ni

Figure 15. Bond coat microstructure evolution—influence of spray process.

APS bond coat HVOF bond coat VPS bond coat

Set 1

Set 2 (splat surface roughening and

oxide outgrowth)

Set 1

Set 2 (no splat surface roughening and

no oxide outgrowth)

Set 1

Set 2 (less splat surface roughening,

no oxide outgrowth)

Figure 16. Effects on top coat single splats after 8 hr HT in Air@1100˚C (for APS, HVOF and VPOS bond coats).

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

129

APS bond coat HVOF bond coat

Set 3 (interlamellar boundaries and microcracks)

Figure 17. Sintering in top coat after 24 hr HT in Air@1100˚C (for APS and HVOF bond coats).

Set 4 (interlamellar boundaries and microcracks)

APS bond coat HVOF bond coat

Set 4 (interlamellar boundaries and microcracks)

Figure 18. Sintering in top coat after 2 hr HT in Vacuum + 24 hr HT in Air@1100˚C (for APS and HVOF bond coats).

with the previous observations made in case of single

splats.

3.2.3. TGO Growth

The TGO formation in these samples with different bond

coats was also considered and is illustrated in Figure 19.

For the same duration of HT i.e. 24 hr in Air at 1100 de-

grees, APS bond coat shows faster TGO growth than

HVOF bond coat. The TGO thickness is seen to be 2.7

μm for APS bond coat sample and 1.7 μm for the HVOF

bond coat sample.

3.2.4. TGO Imperfections

When the elemental composition along the TGO in both

samples is compared, then for the APS bond coat sample,

many locations along the TGO length show formation of

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

130

other oxides like Chromia.However, for the HVOF bond

coat sample, although some locations show TGO thick-

ening, chromia formation is not visible. This is clearly

shown in Figure 20.

Once again this is related to inherent oxidation levels

in both bond coats. APS bond coats have undergone

more oxidation during their formation and hence have

lesser free Al available for TGO formation. Al depletion

2.7 m2.7 m

1.7 m1.7 m

APS bond coat HVOF bond coa

Figure 19. TGO growth after 24 hr HT in Air@1100˚C (for APS and HVOF bond coats).

Set 4 (TGO layer at BC/TC interface)

APS bond coat HVOF bond coat

Set 4 (TGO layer at BC/TC interface)

Locations of Chromia formation

GO thickening but no Chromia formation

Figure 20. Thickness imperfec tions in TGO—thicke ning and for m ation of othe r oxide s (for APS and HVOF bond coats).

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

131

in BC causes other oxides to form, as also shown by

Tolpygo and Clarke [15,16]. HVOF bond coats have

more free Al retained in the coating and hence they have

a longer way to go before Al depletion occurs.

Fewer instances of chromia formation means fewer

occurrences of thickness imperfections in the TGO layer

and hence reduced splat surface roughening in case of

HVOF and VPS bond coats. This further explains the

observations in Figure 16, Section 3.2.1.

4. Conclusions

In this study, the following effects of thermal exposure

were considered and compared as an outcome of varia-

tions in top coat splat dimensions and bond coat micro-

structures.

 Microcracks sintering in splats.

 TGO growth at interface between top coat and bond

coat.

 Splat surface roughening.

 NiO outgrowth occurring through microcracks in

splats.

 Effect of intermediate Vacuum HT during thermal

exposure.

It was seen that in case of top coat, different YSZ

feedstock give different initial splat dimensions in the

as-sprayed splats but the splats show the same high tem-

perature behavior when subjected to similar heat treat-

ments. Intermediate vacuum heat treatment alters the

microcrack sintering behavior observed for Air heat

treatment and also prevents the NiO outgrowth upon

thermal exposure. But these effects are also similar for

both YSZ powders. So, the top coat microstructure with

respect to single splats does not have an influence on

high temperature behavior of the system.

In case of bond coats, however, when different spray-

ing techniques are used, the inherent oxidation levels in

the bond coat microstructures are different and this does

influence the behavior of the system upon high tempera-

ture exposure.

The free Al available in the BC for TGO formation

dictates the occurrence of chromia formation and hence

determines the extent of TGO thickness imperfections

leading to splat surface roughening or spallation.

Absence of Alumina chunks in the HVOF and VPS

bond coat microstructures prevents the described method

of formation of NiO outgrowth through microcracks and

stimulates microcracks sintering which is not observed in

APS bond coats under similar thermal exposure.

5. Acknowledgements

We would like to thank Glenn Bancke, AnirudhaVaidya,

John Gutleber and Li Li (CTSR) for preparation of the

specimens and spraying diagnostics.

REFERENCES

[1] A. G. Evans, D. R. Mumm, J. W. Hutchinson, G. H.

Meier and F. S. Pettit, “Mechanisms Controlling the Du-

rability of Thermal Barrier Coatings,” Progress in Mate-

rials Science, Vol. 46, No. 5, 2001, pp. 505-553.

doi:10.1016/S0079-6425(00)00020-7

[2] A. Nusair Khan and J. Lu, “Behavior of Air Plasma

Sprayed Thermal Barrier Coatings, Subject to Intense

Thermal Cycling,” Surface and Coatings Technology,

Vol. 166, No. 1, 2003, pp. 37-43.

doi:10.1016/S0257-8972(02)00740-5

[3] A. Rabiei and A. G. Evans, “Failure Mechanisms Associ-

ated with the Thermally Grown Oxide in Plasma Sprayed

Thermal Barrier Coatings,” Acta Materialia, Vol. 48, No.

15, 2000, pp. 3963-3976.

doi:10.1016/S1359-6454(00)00171-3

[4] A. Kulkarni, S. Sampath, A. Goland and H. Herman,

“Porosity-Thermal Conductivity Relationships in Plasma

Sprayed Zirconia Coatings,” In: C. C. Berndt, Ed., Ther-

mal Spray: Surface Engineering via Applied Research

(ITSC 2000), ASM International, Materials Park, Mont-

real, 8-11 May 2000, pp. 1061-1066.

[5] A. Kulkarni, Z. Wang, T. Nakamura, S. Sampath, A. Go-

land, H. Herman, A. J. Allen, J. Ilavsky, G. Long, J.

Frahm and R. W. Steinbrech,“Comprehensive Micro-

structural Characterization and Predictive Property Mod-

eling of Plasma-Sprayed Zirconia Coatings,” Acta Mate-

rialia, Vol. 51, No. 9, 2003, pp. 2457-2475.

doi:10.1016/S1359-6454(03)00030-2

[6] S. Deshpande, A. Kulkarni, S. Sampath and H. Herman,

“Application of Image Analysis for characterization of

Porosity in Thermal Spray Coatings and Correlation with

Small Angle Neutron Scattering,” Surface and Coatings

Technology, Vol. 187, No. 1, 2004, pp. 6-16.

doi:10.1016/j.surfcoat.2004.01.032

[7] S. Deshpande, S. Sampath and H. Zhang, “Mechanisms

of Oxidation and Its Role in Microstructural Evolution of

Metallic Thermal Spray Coatings—Case study for NiAl,”

Surface and Coatings Technology, Vol. 200, No. 18-19,

2006, pp. 5395-5406. doi:10.1016/j.surfcoat.2005.07.072

[8] Z. Wang, A. Kulkarni, S. Deshpande, T. Nakamura and H.

Herman, “Computational Approaches to Estimate Proper-

ties of Thermally Sprayed Porous Coatings,” Acta Mate-

rialia, Vol. 51, No. 18, 2003, pp. 5319-5334.

doi:10.1016/S1359-6454(03)00390-2

[9] J. Thornton, D. Cookson and E. Pescott, “The Measure-

ment of Strains within the bulk of Aged and As-Sprayed

Thermal Barrier Coatings Using Synchrotron Radiation,”

Surface and Coatings Technology, Vol. 120-121, 1999,

pp. 96-102. doi:10.1016/S0257-8972(99)00340-0

[10] A. Rabiei and A. G. Evans, “Failure Mechanisms Associ-

ated with the Thermally Grown Oxide in Plasma Sprayed

Thermal Barrier Coatings,” Acta Materialia, Vol. 48, No.

15, 2000, pp. 3963-3976.

doi:10.1016/S1359-6454(00)00171-3

High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies] Paper 2—

Relevance of Variation in Materials Systems of TBC Components

132

[11] E. A. G. Shillington and D. R. Clarke, “Spalling Failure

of a Thermal Barrier Coating Associated with Aluminum

Depletion in the Bond-Coat,” Acta Materialia, Vol. 47,

No. 4, 1999, pp. 1297-1305.

doi:10.1016/S1359-6454(98)00407-8

[12] T. Xu, M. Y. He and A. G. Evans, “A Numerical As-

sessment of the Durability of Thermal Barrier Systems

That Fail by Ratcheting of the Thermally Grown Oxide,”

Acta Materialia, Vol. 51, No. 13, 2003, pp. 3807-3820.

doi:10.1016/S1359-6454(03)00194-0

[13] A. M. Karlsson, J. W. Hutchinson and A. G. Evans, “The

Displacement of the Thermally Grown Oxide in Thermal

Barrier Systems upon Thermal Cycling,” Materials Sci-

ence and Engineering, Vol. 35, No. 1-2, 2003, pp. 244-

257. doi:10.1016/S0921-5093(02)00843-2

[14] S. Deshpande, S. Sampath, P. I. Gouma and H. Herman,

“Microstructural Characterization across Length Scales of

Thermal Sprayed Ni5wt%Al Coatings,” In: B. R. Marple,

Ed., Thermal Spray: Advancing the Science and Applying

the Technology (ITSC 2003), ASM International, Material

Park, Orlando, 5-8 May 2003, pp. 1419-1428.

[15] R. Panat, S. Zhang and K. J. Hsia, “Bond Coat Surface

Rumpling in Thermal Barrier Coatings,” Acta Materialia,

Vol. 51, No. 1, 2003, pp. 239-249.

[16] V. K. Tolpygo and D. R. Clarke, “Surface Rumpling of a

(Ni,Pt)Al Bond Coatinduced by Cyclic Oxidation,” Acta

Materialia, Vol. 48, No. 13, 2000, pp. 3283-3293.

doi:10.1016/S1359-6454(00)00156-7