SURFACE CRACKING AND INTERFACE REACTION ASSOCIATED 

DELAMINATION FAILURE OF THERMAL AND ENVIRONMENTAL BARRIER 

COATINGS 

Dongming Zhu, Sung R. Choi, Jeffrey I. Eldridge, Kang N. Lee and Robert A. Miller 

NASA John H. Glenn Research Center 

21000 Brookpark Road, Cleveland, OH 44135 

ABSTRACT 

In this paper, surface cracking and interface reactions of a BSAS coating and a multi-layer 

ZTO2-8wt%Y2O3 and mullite/BSAS/Si thermal and environmental barrier coating system on 

SiC/SiC ceramic matrix composites were characterized after long-term combined laser thermal 

gradient and furnace cyclic tests in a water vapor containing environment. The surface cracking 

was analyzed based on the coating thermal gradient sintering behavior and thermal expansion 

mismatch stress characteristics under the thermal cyclic conditions. The interface reactions, which 

were largely enhanced by the coating surface cracking in the water vapor environment, were 

investigated in detail, and the reaction phases were identified for the coating system after the longterm exposure. The accelerated coating delamination failure was attributed to the increased 

delamination driving force under the thermal gradient cyclic loading and the reduced interface 

adhesion due to the detrimental interface reactions. 

INTRODUCTION 

Thermal and environmental barrier coatings (TiEBCs) will play a critical role in future 

advanced gas turbine engines to protect Si-based engine components in the harsh combustion 

environments. In order to develop high performance ceramic coating systems, advanced laser 

based high-heat-flux approaches have been established to test coating systems under the simulated 

engine heat flux condition [l, 21. The laser thermal gradient test rigs can establish a required 

thermal gradient across the coating system, so the coating can be evaluated at very high surface 

temperatures while the coating/substrate interface and the substrate can still be maintained below a 

safe temperature limit. The laser thermal gradient approaches have been demonstrated in 

evaluating cyclic and sintering behavior of thermaUenvironmenta1 barrier coatings on SiC/SiC 

substrates [3,4]. 

The purpose of this study is to investigate coating cracking and interface chemical reactions 

induced failure of typical thermal and environmental barrier coating system on SiC/SiC ceramic 

matrix composite (CMC) substrates under laser thermal gradient cyclic test conditions. The coating 

surface and delamination cracking was analyzed based on the coating thermal gradient sintering 

behavior and thermal expansion mismatch stress characteristics under the thermal cyclic 

conditions. The thermal straidstress characteristics under large thermal gradients were also 

described. The accelerated coating failure was discussed based on the delamination driving force 

This is a preprint or reprint of a paper intended for presentation at a 

conference. Because changes may be made before formal 

publication, this is made available with the understanding that it will 

not be cited or reproduced without the permission of the author. 

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https://ntrs.nasa.gov/search.jsp?R=20030032264 2018-05-15T16:00:06+00:00Z
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under the thermal gradient cyclic loading and the interface degradation due to the BSAS phase 

chemical stability and reactions in high temperature water vapor environment. 

EXPERIMENTAL MEHOD AND MATERIALS 

The laser high-heat-flux based technique was employed to test a barium-strontiumaluminosilicate (BSAS) and a multi-layered ZrOz-8wt%Yz03 and BSAS+mullite rnixture/Si 

TBCEBC system on SiC/SiC CMC substrates under thermal gradient cyclic conditions. The 

thermal gradient cyclic testing of the ceramic coatings was carried out using a 3.0 kW C02 laser 

(wavelength 10.6 pm) high-heat flux rig. The general test approaches have been described 

elsewhere [l-41. The surface test temperature was at approximately at "C, and interface 

temperature was at 1300°C. The coatings specimens were also subject o alternating the laser 

environment at 1300°C (every 50 hours). The thermal cyclic tests were conducted using 1 hour hot 

time temperature cycles, with 3 min cooling between each cycle to ensure that the test specimens 

were cooled below 100°C. The coatings were tested up to total 400, 1 hour cycles. The coating 

surface cracking and interface reactions of the coating systems were characterized after the testing 

using a scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy 

(EDS) System. 

thermal gradient cycling test in air and the furnace thermal cycling tests 2? in a 90% water vapor 

/ RESULTS AND DISCUSSION 

Fig. 1 shows SEM microstructures Y t e BSAS/Si environmental barrier coating after laser 

thermal gradient cyclic testing. It can be seen from Fig. 1 (a) that the BSAS coating was first 

decomposed and precipitated a lower silicon (Si) containing phase, with the matrix being a high Si 

containing BSAS. As the coating was further cyclically tested at &-surface temperature of 1482C, 

\p the BSAS matrix was melted because of the formation of the higher Si and lower melting point 

BSAS. The surface cracking was also observed after coating melting and glass phase formation. 

n 

J 

Fig. 1 (a) The BSAS coating was decomposed into low Si (bright second) and high Si (dark 

matrix) phases during initial laser thermal cyclic testing. (b) The higher Si phase became 

the glass phase and surface cracks were formed after 100 hour cyclic testing at the surface 

temperature of 1482°C. 

2 
, .I 

Fig. 2 shows a SEM cross-section micrograph of the BSAS coating after 100 hour laser 

thermal gradient cycling test. The interface reactions between the Si bond coat and BSAS coating 

resulted in a dark reaction layer with the layer thickness of 30-40 microns. The EDS analysis has 

shown that this BSAS region has very high Si concentrations due to the Si diffusion from the Si 

bond coat into the BSAS. The high Si concentration BSAS phase was also decomposed into 

mullite phase (very dark needle-shaped phase) within the high Si BSAS reaction region. Although 

the coating interface melting was not observed under the 1300°C interface temperature of the 

thermal gradient testing for 100 hours, the high Si concentration BSAS reaction layer has been 

reported to facilitate the coating melting and the glass phase formation under the uniform 

temperature furnace testing in a water vapor environment at higher temperatures [5]. 

Fig. 2 SEM cross-section micrograph of the BSAS coating after 100 hour laser thermal gradient 

cycling test. The interface reactions resulted in a dark reaction BSAS layer with high Si 

concentrations. The mullite phase (very dark needle-shaped phase) precipitation was also 

found within the high Si reaction BSAS region. 

Fig. 3 shows cross-section micrographs and compositional maps of the ZrOz8wt%Yz0JBSAS+mullite mixture/Si on a SiC/SiC ceramic matrix composite substrate, after 100 

hours testing in a combined alternating laser thermal gradient cyclic and furnace water vapor 

exposure cyclic testing. It can be seen that substantial thermal gradient induced coating surface 

wedge-shape cracking has occurred and the resulting delamination was observed for the coating 

specimen. As shown in Fig. 3, significant interface reactions within the mullite+BSAS EBC layer 

occurred. The reaction affected zone can be as thick as 30 to 40 microns, especially near the 

interface and the BSAS phase (bright phase) regions. The reactions between mullite and BSAS 

phases were also noticed with a typical reaction layer thickness of 2-5 microns. However, the interdiffusion of Ba and AI between the mullite and BSAS can occur at a much greater distance, as 

indicated by the EDS composition maps in Fig. 3 (d) and (e). The reactions were further enhanced 

by the surface crack penetrations into the EBC layer which allowed the water vapor fast access to 

the coating at the EBC/Si interface. The accelerated coating damage under the thermal gradient 

cyclic testing combined with the furnace water vapor testing was attributed to the increased 

delamination driving force under the thermal gradient cyclic loading and the reduced interface 

adhesion due to the detrimental interface reactions [4,6]. 

3 
The coating surface cracking is largely due to ceramic top coat sintering and thermal 

expansion mismatch, especially under the large thermal gradient conditions. As can be seen in Fig. 

4, with a relatively thick, higher thermal expansion Zr02-Y203 TBC layer on mullite-BSAS EBC 

and CMC substrates, the coating surface cracking and delamination driving force can increase 

significantly, based on a thermal gradient testing delamination model proposed in the literature [7]. 

The significantly larger thermal stresses generated under thermal gradients will also accelerate the 

coating delamination under the thermal cyclic conditions. 

(a) Optical micrograph (b) SEM micrograph 

(0 Si Map 

(e) Ba Map 

(h) Sr Map 

Fig. 3 SEM images and EDS element composition maps of the Zr02-8wt%Y203/BSAS+mullite 

mixture/Si on SiC/SiC ceramic matrix composite substrates after laser testing, showing 

significant interface reactions between BSAS and bond coat Si layer, and the BSAS and 

mullite. (a) and (b) Optical and SEM micrographs of the coating after 100 hours testing, 

showing the surface cracking and interface reactions; and (c) to (h) SEM cross-section 

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micrograph and the corresponding EDS elemental maps for the coating systems near the 

interface. 

Crack length anBC thickness tk 0.016 

6 0.014 

5 

0 I 2 3 4 5 

4 

.c 0.01 2 - - 

I Yi 2 

e4 :4 : 0.010 

03 0 b! u 0,008 

Y 

Y "! - ! 5E 

+- : 2 +_ 

J 0.006 

31 V 0.004 

0 0.002 

0 

r 

- 62 

v 

L 

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 I .o 

Tcuc ""b,,.emcw.mk =.*mi TBC thickness I /total costing thickness t Ih Ud 

Fig. 4 (a) Zr02-Y203 TBC layer on mullite-BSAS EBC and CMC substrates, the coating surface 

cracking and delamination energy release rates can increase significantly with a relatively 

thick, and larger thermal expansion Zr02-Y203 TBC layer on mullite-BSAS EBC on 

CMC substrates. (b) The significantly larger thermal stresses/strains will be generated 

with increasing the thermal gradients (the temperature difference between the ceramic 

coating and the substrates). 

Fig. 5 shows the SEM cross-section micrographs of the ZrO~-8wt%Y2OJBSAS+mullite 

mixture/Si on a SiC/SiC ceramic matrix composite substrate, after 400 hours of testing in the 

combined laser thermal gradient and furnace water vapor cyclic testing. Extensive coating 

delamination and somemspallation were observed during the testing. From Fig 5 (a) and (b), it 

can be seen that, in addition to the more significant reactions between the EBC mullite, BSAS and 

Si bond coat, large interfacial pores developed due to the water vapor attack through the vertical 

cracks in the EBC coatings. From Fig. 5 (c) and (d), it can also be seen that a low melting point 

eutectic phase around the BSAS, Si, and mullite interfaces formed. For the top Zr02-Y203 coating 

side, no obvious reactions were found between mullite and Zr02-8wt%Y203 coatings after the 400 

hours combined laser and furnace water vapor thermal cycling test. 

J 

5 
‘I 

(c> (4 

Fig. 5 SEM cross-section micrographs of the Zr02-8wt%Y203/BSAS+mullite mixture/Si on a 

SiC/SiC ceramic matrix composite substrate, after total 400 hours testing in the combined 

laser thermal gradient and furnace water vapor cyclic testing. (a) Significant interfacial 

pore formation due to the water vapor attack through the vertical cracks under the thermal 

gradient cycling conditions. (b) Low melting eutectic phase was also observed near the 

BSAS-mullite-Si interfaces after the testing. 

CONCLUSIONS 

A laser heat-flux thermal gradient test approach has been used to investigate the interface 

reactions and failure of plasma-sprayed BSAS and ZrOz-8wt%Y20JBSAS+mullite/Si coatings on 

SiC/SiC ceramic matrix composites under thermal gradient cyclic conditions. The failure of the 

coating system can be characterized as wedge-shaped surface coating cracking, surface crackingenhanced coating delamination and interface debonding and spallation under the thermal cyclic 

conditions. Significant BSAS phase related chemical reactions were observed due to its low 

temperature stability and high diffusion activity. The BSAS coatings were found to melt at even 

relatively low temperatures (1300-1482°C) because of the formation of higher Si content, and low 

melting-point phases, either at the surface due to its high temperature decomposition into high Si 

containing matrix phase with low Si BSAS precipitates, or near the interface when reacted with Si 

bond coat. The accelerated coating delamination failure was attributed to the increased 

delamination driving force under the thermal gradient cyclic loading and the reduced interface 

adhesion due to the detrimental interface reactions. 

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ACKNOWLEDGMENTS 

This work was supported by NASA Ultra-Efficient Engine Technology (UEET) Program. 

The authors are grateful to George W. Leissler and Terry R. McCue, QSS group at NASA Glenn 

Research Center for their assistance in the preparation of plasma-sprayed TBC/EBC coatings, and 

the SEM analysis, respectively. 

REFERENCES 

D. Zhu and R. A. Miller, “Thermal Conductivity and Elastic Modulus Evolution of 

Thermal Barrier Coatings Under High Heat Flux Conditions,” NASA Glenn Research 

Center, Cleveland, Ohio, NASA TM-209069, April 1999. Journal of Thermal Spray 

Technology, vol. 9, pp. 175-180,2OOO 

D. Zhu, N. P. Bansal, K. N. Lee, and R. A. Miller, ‘Thermal Conductivity of Ceramic 

Thermal Barrier and Environmental Barrier Coating Materials,” NASA Glenn Research 

Center, Cleveland NASA TM-211122, September 2001. 

D. Zhu, K. N. Lee, and R. A. Miller, ‘Thermal Conductivity and Thermal Gradient Cyclic 

Behavior of Refractory Silicate Coatings on SiC/SiC Ceramic Matrix Composites,” 

Ceram. Eng. Sci. Proc., vol. 22, pp. 443-452, 2001. 

D. Zhu, K. N. Lee, and R. A. Miller, “Sintering and Cyclic Failure Mechanisms of MultiLayered Thermal and Environmental Barrier Coating Systems under High Thermal 

Gradient Test Conditions,” Ceram. Eng. Sci. Proc., vol. 23, pp. 505-516, 2002. 

K.N. Lee, D.S. Fox, J.I. Eldridge, D. Zhu, R.C. Robinson, N.P. Bansal, and R.A. Miller, 

“Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and 

BSAS,” NASA TM-2002-2 1 1372. 

Dongming Zhu, Kang N. Lee and Robert A. Miller, “Thermal Gradient Cyclic Behavior 

of Thermal/Environmental Barrier Coating System on SiC/SiC Ceramic Matrix 

Composites,” in Proceedings of ASME Turbo Expo 2002, June 3-6, 2002, ASME Paper 

GT-2002-30632, Amsterdam, The Netherlands. 

J. H. Hutchinson and A. G. Evens, “On the delamination of thermal barrier coatings in a 

thermal gradient,” Surface and Coatings Technology, vol. 149, 149 (2002), pp. 179-184, 

2002. 

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