structural stability of thermal barrier coatings...

23. - 25. 5. 2012, Brno, Czech Republic, EU

STRUCTURAL STABILITY OF THERMAL BARRIER COATINGS

PRODUCED BY THERMAL SPRAYING

Ladislav ČELKO a,b, Lenka KLAKURKOVÁ a,b, Karel SLÁMEČKA a,b, Simona POSPÍŠILOVÁ a,b,

Martin JULIŠ a,b, Karel NĚMEC a,b, Tomáš PODRÁBSKÝ a,b, Jiří ŠVEJCAR a,b

a Brno University of Technology (BUT), Central European Institute of Technology (CEITEC), Centre for

Advanced Materials, Research Programm Structure and Phase Analysis, Technická 3058/10, 616 00 Brno,

Czech Republic, [email protected]

b Brno University of Technology (BUT), Faculty of Mechanical Engineering, Institute of Materials Science and

Engineering, Technická 2896/2, 616 69 Brno, Czech Republic, [email protected]

Abstract

One type of functionally graded coating system is thermal barrier coating (TBC), where the combination of

ceramic and metallic coating is used both to reduce the temperature and to increase oxidation and corrosion

resistance of the substrate. TBCs usually consist of the top ceramic coating based on YSZ (ZrO2+Y2O3) and

the metallic bond coating of M-CrAlY type, where M means Ni, Co or their appropriate combination. Electron

beam physical vapour deposition (EB-PVD) or vacuum (VPS), low pressure (LPPS) or atmospheric (APS)

plasma spraying techniques are most frequently used as a deposition method. Despite some requirements

on the product shape simplicity, the air plasma spraying offers high productivity, sufficient quality and much

lower production costs in comparison with the EB-PVD technology. The contribution deals with high

temperature structural stability of TBCs of YSZ + NiCrAlY and YSZ + CoNiCrAlY types that were produced

by air plasma spraying on the INCONEL 713LC polycrystalline nickel based superalloy substrate.

Immediately after deposition, the specimens were exposed to temperature of 1000°C for 5-500 hours in the

common ambient atmosphere. The changes in both microstructure and chemical composition were studied

by means of scanning electron microscopy, energy dispersive microanalysis and image analysis methods.

Keywords: Plasma Spraying, Thermal Barrier Coatings, Nickel-based Superalloys, Thermal exposure,

Microstructure

1. INTRODUCTION

High temperature coatings are important materials with main applications in aircraft and power generation

industries, where they are used primarily for the protection of turbine blades and vanes. These materials

operate in extremely harsh working conditions, including high temperatures and temperature gradients,

abrupt thermal changes (thermal shocks), the presence of oxidizing and corroding atmosphere, high

pressures and multiaxial stresses of broad amplitude and frequency spectra [1,2].

Based on the functional principle, two main groups of coatings exist: (BCs) bond coatings, which could be

further subdivided into diffusion and overlay coatings [3,4], and, (TBCs) thermal barrier coatings. A functional

graded thermal barrier coating consists of two coatings: the thermally insulating ceramic top coat and the

metallic aluminium-containing bond coat (usually of the MCrAlY type), which provides oxidation resistance

and compensates for different thermal expansion coefficients of the substrate and the top coat [2].

The deposition of the ceramic top coat is done either by electron beam physical vapour deposition or by

plasma spraying, although other deposition methods are also being explored, e.g. [5]. In the case of plasma

spraying, a raw coating material is injected in the form of powder into the high-temperature and/or high-

velocity plasma jet. The powder is melted and propelled toward the substrate, where the molten particles


solidify and form “splats”. Plasma spray deposition results in a lamellar structure with 10-15 wt.% ceramic

coating porosity and a certain volume fraction of cracks that are generated in order to relieve stresses

caused by different thermal contraction of the top coat and the bond coat substrate, and by the rapid cooling

of droplets. Moreover, because of the relatively weak adhesion, further delamination cracks are formed at

the splat boundaries, having a significant effect on thermal conductivity and thermal shock resistance. The

main advantage of plasma spraying consists in high deposition rates and low production costs.

The paper focuses on reporting the structural stability of thermal barrier coatings produced by means of

thermal spraying onto the Inconel 713LC substrate surface. Specimens were subjected to high temperature

exposure for different dwell-times in ambient environment. The changes in a coatings microstructure and

chemical composition were investigated in detail.

2. MATERIAL AND METHODS

Functional graded coating systems were prepared from commercially available powders NiCrAlY (GTV

60.46.8), CoNiCrAlY (GTV 60.95.1) and ZrO2/Y2O3 (GTV 40.23.1) that were purchased from company GTV

GmbH. Nominal chemical composition and the average particle size guaranteed by the manufacturer are

collected in Table 1.

Table 1 Nominal chemical composition (wt %) and the average grain size of used powders [m]

Powder Average size Al Y Zr Cr Ni Co

NiCrAlY 15 – 38 10.0 1.0 --- 22.0 bal. ---

CoNiCrAlY 20 – 45 8.0 0.5 --- 21.0 32.0 bal.

ZrO2/Y2O3 20 – 45 --- 8.0 bal. --- --- --

Coatings were deposited on specimens made of a polycrystalline nickel-based superalloy Inconel 713LC

substrate. Substrate’s surface was grounded by the #600 sandpaper and polished by 3 m diamond paste

prior to deposition. Thermal barrier top coat was prepared by plasma spraying in standard ambient

atmosphere in cooperation with the company S.A.M. Two TBCs were prepared: ZrO2/Y2O3 + NiCrAlY a

ZrO2/Y2O3 + CoNiCrAlY. The thicknesses of individual coating layers were 50±5 m and 30±5 m (ceramic

ZrO2/Y2O3 topcoat) and 210±10 m and 450±50 m (NiCrAlY and CoNiCrAlY bond coat), respectively.

Structural stability of all systems was studied under isothermal annealing in the tubular furnace HERAEUS.

Specimens were exposed to temperature 1000°C in the ambient atmosphere with dwell-time in the range of

5-500 hours. Metalographic samples from both the as-sprayed state (VS) and thermally-exposed state (TS)

specimens were obtained by means of the deformation-free dividing equipment AKUTOM by STRUERS. All

samples were grounded by increasingly finer sandpapers (starting at #120 up to #1200) under intensive

water cooling, polished by 3m and 1m diamond pastes and finally chemically polished by means of

OPCHEM. Microstructural observations were carried out by means of the scanning electron microscope

(SEM) XL30 by PHILIPS equipped with the energy dispersive X-ray spectroscopy microanalyser by EDAX

company.

3. RESULTS AND DISCUSSION

Microscopical observations revealed that the ZrO2/Y2O3 topcoats are present in the form of continuous locally

irregular layers that traced the geometry of the underlying MCrAlY bond coat. Considerable number of

microcracks, especially at interfaces of individual splats, and high number of closed pores were observed.

The bond coating NiCrAlY (Fig. 1a,c,e) and CoNiCrAlY (Fig. 1b,d,f) layers deposited on the IN 713 LC

substrate are comparatively more uniform and coherent. In this case, the microstructure and the surface

relief, which is essential for the geometry of the top coat, correspond to manual deposition directly onto the

plane substrate specimens. The microstructure consists primarily of flattened splats, which are generated


when metallic molten particles solidify after impact on the substrate, and by oxides formed at their interfaces.

Occasionally, partially unmelted particles were observed as well [2].

The bond coat / top coat interface is characterized by the presence of very fine local discontinuities and the

continuous thin thermally grown oxide (TGO) layer. The threshold thermal exposure conditions for its

formation are 1000 °C / 5 h in the case of the ZrO2/Y2O3 + NiCrAlY system (Fig. 1c) and 1000 °C / 25 h for

the ZrO2/Y2O3 + CoNiCrAlY system, respectively (Fig. 1d). In both cases, the thickness of TGO layer slightly

increases with increasing dwell-time; this effect, however, is more pronounced for the ZrO2/Y2O3 + NiCrAlY

coating system, Fig 1.

Fig. 1 SEM micrographs of ZrO2/Y2O3 + NiCrAlY (a) VS, (c) TS: 1000 °C/ 5 h, (e) TS: 1000 °C/ 500 h and

ZrO2/Y2O3 + CoNiCrAlY (b) VS, (d) TS: 1000 °C/ 25 h, (f) TS: 1000 °C/ 500 h thermal barrier coatings.

SEM microphotograps of VS specimens with ZrO2/Y2O3 + NiCrAlY coating system are shown as Fig. 2a,b.

Chemical composition was studied at several locations as indicated by numbered white cross marks. Present

phases were determined based on this analysis, known stochiometry of ceramic and intermetallic phases [6]

and previous results of study on short-term structural stability of M-CrAlY coating systems [7].

a) b)

c) d)

e) f)


Fig. 2 SEM microphotographs of VS microstructure of ZrO2/Y2O3 + NiCrAlY thermal barrier: (a) ceramic

ZrO2/Y2O3 top coat, (b) metallic NiCrAlY bond coat.

SEM microphotograps of TS specimens with ZrO2/Y2O3 + NiCrAlY coating system is presented as Fig. 3a-d.

Clearly, the ceramic top coat does not experience any substantial changes of chemical composition during

the thermal exposure. On the contrary, the metallic bond coat undergoes distinct changes during the thermal

exposure. In the VS, NiCrAlY-based bond coating belongs to two-phases coating consisting of solid solution

-NiCr (77-84%Ni, 10-18%Cr, max. 5%Al (wt.%) – Fig. 2b, locations 1,5) and intermetallic phase ‘-Ni(Cr)3Al

(66-68%Ni, 22-23%Cr, 10-12%Al, max. 1%Y – Fig. 2b, locations 2-4). Oxides Al2O3 and Cr2O3 are found at

splats’ interface (38%O, 20%Al, 32%Cr, 2%Y, 8%Ni – Fig. 2b, location 5). After thermal exposure, more

Al2O3, Cr2O3 and NiO oxides (24-45%O, 48-15%Al, 21-4%Cr, 72-3%Ni – Fig. 3c,d), locations 13-15,18,21)

are formed not only at the layers interface but also in the vicinity of flattened particles. This is due to the Al

depletion, which causes complete transformation of initial Ni(Cr)3Al intermetallic phase to NiCr solid solution

(75-77%Ni, 21-24%Cr, 1-3%Al [hm.%] – Fig. 3c,d, locations 16,17,19,20,22) during thermal exposure.

Fig. 3 SEM microphotographs of microstructure of ZrO2/Y2O3 + NiCrAlY thermal barrier after TS: 1000°C /

500 h: (a,b) ceramic ZrO2/Y2O3 top coat, (c) top coat / bond coat interface (d) metallic NiCrAlY bond coat.

a) b)

a) b)

c) d)


SEM microphotograps of VS specimens with ZrO2/Y2O3 + CoNiCrAlY coating system is presented as Fig.

4a,b. Similarly to ZrO2/Y2O3 + NiCrAlY coatings, there are no substantial changes in the chemical

composition of the top coat, while the chemical composition of the CoNiCrAlY bond coat changes quite

substantially.

Fig. 4 SEM microphotographs of VS microstructure of ZrO2/Y2O3 + CoNiCrAlY thermal barrier: (a) ceramic

ZrO2/Y2O3 top coat, (b) metallic CoNiCrAlY bond coat.

As in the previous case, the ceramic top coat does not experience any substantial changes of chemical

composition. In the VS, the CoNiCrAlY bond coat consists of the intermetallic ‘-CoNi(Cr)3Al phase (8-12%Al,

max. 19-22%Cr, 32-37%Co, 31-36%Ni (wt. %) – Fig. 4b, locations 7-11), with Al2O3, Cr2O3, NiO oxides

(24%O, 13%Al, 23%Cr, 27%Co, 13%Ni – Fig. 4b, location 12) at splats’ interface. During the thermal

exposure, an aluminium is going to be depleted in a direction from the ceramic top / metallic bond coat

interface down to the bond coat and the intermetallic phase ‘ is transformed to solid solution (4-5%Al, 22-

23%Cr, 38-40%Co, 34-35%Ni – Fig. 5b, locations 7,8,10) within splats, while Al2O3 – based oxides (34%O,

44%Al, 2%Y, 6%Cr, 7%Fe, 7%Co – Fig. 5b, location 9) form at splats’ interfaces. In the middle and lower

(adjacent the substrate) parts of the bond coat, formation of the solid solution (4-5%Al, 21-23%Cr, 36-

40%Co, 33-38%Ni – Fig. 5c,d, locations 12-14), precipitation and coarsening of intermetallic CoNi(Cr)Al

particles (20%Al, 7%Cr, 20%Co, 53%Ni – Fig. 5c, location 11) were observed.

Fig. 5 SEM microphotographs of microstructure of ZrO2/Y2O3 + CoNiCrAlY thermal barrier after TS: 1000°C /

500 h: (a) ceramic ZrO2/Y2O3 top coat / metallic CoNiCrAlY bond coat interface, (b) upper part of the metallic

CoNiCrAlY bond coat.

a) b)

a) b)


Fig. 5 SEM microphotographs of microstructure of ZrO2/Y2O3 + CoNiCrAlY thermal barrier after TS: 1000°C /

500 h: (c) middle, and (d) lower (adjacent the substrate) part of the metallic CoNiCrAlY bond coat.

4. CONCLUSIONS

Thermal barrier coating of ZrO2/Y2O3 + CoNiCrAlY type shows better structural stability in the thermal

exposure (1000°C, 500 h) when compared with the ZrO2/Y2O3 + NiCrAlY coating system. This corresponds

with delayed formation of the continuous thermally grown oxide layer at the top ceramic / bond metallic

coating interface in the first case (ZrO2/Y2O3 + CoNiCrAlY type). Moreover, the presence of the intermetallic

phases was observed for all 100, 250 and 500 h time dwells. Both studied coating systems undergoes

almost the same degradation mechanism: undesirable oxide based on Al, Ni, Cr and Fe are formed in the

ceramic top coat and the Al is gradually depleted from the metallic bond coat forming Al-based oxides

between individual splats. Due to Al depletion, intermetallic Al-based phases in the metallic bond coat are

transformed into NiCr and CoNiCr solid solutions, respectively. Based on achieved results, the thermal

barrier coating of ZrO2/Y2O3 + CoNiCrAlY type is recommended to be preferred for nickel based superalloy

Inconel 713LC substrate protection.

ACKNOWLEDGEMENTS

The authors want to acknowledge the financial support for this work provided by the projects

(GA 107/12/1922 and GA 107/11/2065) of Czech Science Foundation and “CEITEC - Central European

Institute of Technology” (CZ.1.05/1.1.00/02.0068) from European Regional Development Fund.

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c) d)

structural stability of thermal barrier coatings...

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