Nadia L. Ahlborg and Dongming Zhu

Glenn Research Center, Cleveland, Ohio

Calcium-Magnesium-Aluminosilicate (CMAS)

Reactions and Degradation Mechanisms of

Advanced Environmental Barrier Coatings

NASA/TM—2013-218091

November 2013
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Nadia L. Ahlborg and Dongming Zhu

Glenn Research Center, Cleveland, Ohio

Calcium-Magnesium-Aluminosilicate (CMAS)

Reactions and Degradation Mechanisms of

Advanced Environmental Barrier Coatings

NASA/TM—2013-218091

November 2013

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

40th International Conference on Metallurgical Coatings and Thin Films (ICMCTF 2013)

sponsored by AVS Advanced Surface Engineering Division

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2013-218091 1 

Calcium-Magnesium-Aluminosilicate (CMAS) Reactions and 

Degradation Mechanisms of Advanced Environmental 

Barrier Coatings 

Nadia L. Ahlborg* and Dongming Zhu 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The thermochemical reactions between calcium-magnesium-aluminosilicate- (CMAS-) based road 

sand and several advanced turbine engine environmnetal barrier coating (EBC) materials were studied. 

The phase stability, reaction kinetics and degradation mechanisms of rare earth (RE)-silicates Yb2SiO5, 

Y2Si2O7, and RE-oxide doped HfO2 and ZrO2 under the CMAS infiltration condition at 1500 C were 

investigated, and the microstructure and phase characteristics of CMAS-EBC specimens were examined 

using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Experimental results showed 

that the CMAS dissolved RE-silicates to form crystalline, highly non-stoichiometric apatite phases, and in 

particular attacking the silicate grain boundaries. Cross-section images show that the CMAS reacted with 

specimens and deeply penetrated into the EBC grain boundaries and formed extensive low-melting 

eutectic phases, causing grain boundary recession with increasing testing time in the silicate materials. 

The preliminary results also showed that CMAS reactions also formed low melting grain boundary phases 

in the higher concentration RE-oxide doped HfO2 systems. The effect of the test temperature on CMAS 

reactions of the EBC materials will also be discussed. The faster diffusion exhibited by apatite and 

RE-doped oxide phases and the formation of extensive grain boundary low-melting phases may limit the 

CMAS resistance of some of the environmental barrier coatings at high temperatures. 

Introduction 

The ever-present needs for developing higher efficiency high performance turbine engines have created 

demand for new hot-section component materials that are lighter and more temperature capable. Current 

Ni-based superalloys are reaching the upper limit of their temperature capabilities, and so silicon carbide 

(SiC) fiber-reinforced SiC/SiC ceramic matrix composites (CMCs) have been envisioned as alternative next 

generation turbine engine hot-section materials (Ref. 1). Silicon-based ceramics and composites are 

desirable because they have low density, superior strength and oxidation resistance in high temperature 

oxidizing environments. However, a major issue with Si-based materials is their vulnerability to corrosion 

and performance degradation in combustion water vapor containing environments due to volatilization of 

the protective silica (SiO2) scales, leading to recession of the ceramics and strength degradation under 

combined thermo-mechanical loading conditions. An environmental barrier coating (EBC) can be used to 

prevent the SiC/SiC CMC from reacting with water vapor (Refs. 1 to 3). Advanced HfO2, ZrO2, and rare 

earth silicate (RE silicate) EBC systems have also been developed, achieving significantly improved 

temperature capability and environmental protection of SiC/SiC CMCs (Refs. 4 to 6). 

The significantly higher operating temperatures envisioned above 1500 °C for next generation turbine 

engine hot-section components impose additional material design constraints and raise serious component 

environmental durability issues. Many common silicon-containing sand dust and volcano ash materials 

can melt at approximately 1200 °C. During engine operation, these materials deposit on the EBC and 

form glassy melts of calcium-magnesium aluminosilicate (CMAS), which can significantly reduce the 

 

*NASA Glenn Research Center, summer intern from The Ohio State University. 
NASA/TM—2013-218091 2 

silicon-based EBC temperature capability. This molten CMAS interacts chemically and mechanically 

with the EBC to cause premature coating failure. Thus when designing EBC materials, it is critical to 

understand the high-temperature interactions between the coating materials and CMAS in order to create 

CMAS-resistant coatings. Research work has been done to determine the mechanisms by which CMAS 

can cause failure and performance degradations in yttria stabilized zirconia thermal barrier coatings 

(Refs. 7 and 8) and early generation EBCs (Refs. 9 to 12). 

The objective of the present paper is to investigate the stability of candidate rare earth silicates and 

rare earth oxide-doped HfO2 and ZrO2 environmental barrier coatings in contact with CMAS at high 

temperature. The emphasis is placed on the reactions and degradation mechanisms between CMAS and 

EBC materials particularly at 1500 °C, and compared with the results at 1300 °C. The current work is 

thus aiming at providing information on the CMAS resistance of RE silicates and RE oxide-doped HfO2

and ZrO2 systems. 

Experimental Materials and Methods 

The two primary EBC materials investigated in this study were ytterbium monosilicate (Yb2SiO5) and 

yttrium disilicate (Y2Si2O7). These materials were NASA-designed and processed in the form of plasmahomogenized hollow-sphere (HOSP) powders to NASA powder specifications by Sulzer Metco (US), 

Westbury, New York, USA. In addition to the RE silicates, HOSP NASA four-component HfO2-

23.9mol%(Gd,Yb,Y)2O3 and six-component ZrO2-4.4mol%(Y, Gd, Yb)2O3-2.6mol%(TiO2+Ta2O5) 

compositions (Ref. 6) were also investigated. 

Two types of samples were produced for this study. For the first kind, EBC powders were sintered 

using a hot press technique at 1500 °C and 103 MPa load in a vacuum environment to create dense, 

monolithic slabs ~4 mm thick or discs ~2 mm thick, which were then cut into several specimens. Each 

sample was prepared for CMAS exposure by polishing the surface of interest using 320-grit SiC paper 

and cleaning the surface with ethanol. To provide a baseline understanding for material behavior at 

1500 °C, one specimen of each material was heat treated in air for 50 hr at 1500 °C. A model CMAS frit 

containing 35mol%CaO-10mol%MgO-7mol%Al2O3-48mol%SiO2 and minor amounts of Fe2O3 and NiO 

was received from GE Aviation, Evendale, Ohio, USA. The CMAS frit was mixed with ethanol to form a 

paste and placed on each specimen surface to give a CMAS surface concentration of ~40 mg/cm2

. The 

specimens were placed in an alumina crucible and heat treated in air for 5, 50, 100, 150, or 200 h at 

1500 °C. Every 50 h, the furnace was cooled to room temperature and the samples were removed and 

photographed. Because of observed volatility of CMAS and the reaction products, for samples tested 

longer than 50 h, an additional 40 mg/cm2

 CMAS was applied to the sample surface every 50 h. 

Compared to a single application of CMAS, this testing procedure better simulates engine operation 

where the source of CMAS is continuously available. To provide a better understanding of the CMASEBC reaction, additional tests were conducted for both of the RE-silicate materials at 1300 °C for 50 h. 

The second type of sample was produced by mechanically mixing 25 and 30 wt% CMAS frit with 

Yb2SiO5 and Y2Si2O7 EBC powders, respectively. The CMAS amounts were adjusted to reflect a higher 

concentration that would occur in a thin turbine coating system. The mixtures were sintered via hot press 

at 1050 °C and 103 MPa load in vacuum using a SiC/SiC CMC as a backing to produce two 1 mm thick 

RE-silicate + CMAS blend coating specimens onto CMC substrates. It was thought that these samples 

would more accurately represent the original CMAS composition in the closed, high pressure turbine 

section environment because the CMAS would volatilize less and thus react more completely with the 

EBCs when conducting the tests at such high temperatures. 

 
NASA/TM—2013-218091 3 

Following testing, specimens were examined using x-ray diffraction (XRD) to determine phase 

structure using Cu Kα radiation with a wavelength of 1.541 Å. Each specimen was then sectioned using a 

low-speed diamond saw. One half of the specimen was mounted and polished using standard 

metallographic techniques and examined using a field-emission scanning electron microscope (SEM). 

The other half was reserved for examination of the surface with electron microscopy. Qualitative 

chemical compositions were obtained from surfaces and cross-sections using electron dispersive 

spectroscopy (EDS). 

Experimental Results 

Yb2SiO5-CMAS Surface Reactions 

Figure 1 shows cross-sectional images of the Yb2SiO5 in the as-sintered condition and after 50 h of 

heat treatment in air at 1500 °C. There was little change in microstructure observed over time. 

Figure 2 shows images of Yb2SiO5 before and after CMAS reactions. The dark areas in the 5 h tested 

specimen (Fig. 2(b)) are Yb2SiO5-CMAS reaction products. There is visibly much less of this reacted 

material on the 50 and 200 h tested specimens, suggesting that the CMAS components may be highly 

volatile at 1500 °C and atmospheric pressure in air. 

SEM images of Yb2SiO5 reacted with CMAS for 50 h are shown in Figure 3. EDS was used to 

qualitatively determine the elemental composition in each region of the micrographs. CMAS reacted with 

the Yb2SiO5 to form a Ca-Yb-Si-containing layer with an average thickness of ~60 to 100 μm. The 

surface of this reaction layer (Fig. 3(b)) consisted of roughly hexagonal-shaped grains with an average 

diameter of 17 μm. Figure 4 shows cross-sectional images of Yb2SiO5 reacted with CMAS for 200 h. The 

heavily reacted surface layer had the same approximate composition, crystal structure, and average 

thickness as in Figure 3(a). The enlarged regions in Figures 4(b) and (d) have eutectic microstructures, 

indicating that they solidified from a low-melting liquid due to the higher CMAS content. EDS shows that 

the lighter phase of this eutectic is rich in Al, Mg, Si, and Yb, while the dark phase is rich in Al, Ca, Mg, 

and Si. As apparent from Figure 4(c), these low-melting particles are located between grains, providing 

evidence for preferential CMAS attack at grain boundaries. These particles were found to penetrate the 

entire specimen cross-section, which has a bulk thickness of ~4 mm. 

XRD was used to determine that the reaction layers in Figures 3(a) and 4(a) had a crystal structure 

similar to calcium-ytterbium-silicate-oxyapatite (Ca2Yb8(SiO4)6O2), a variation of the mineral apatite. The 

x-ray scan of the 200 h-reacted Yb2SiO5 is compared to a known apatite structure in Figure 5. The Al, Yb, 

Mg, Ca-containing grain boundary phases may be too small in quantity and/or preferred orientation with 

glass phases to contribute significantly to the diffraction pattern. 

In order to better understand the grain size behavior, volatility, and surface melting of rare earth 

silicates, CMAS surface reaction treatments were carried out for 5 h at 1500 °C and 50 h at 1300 °C in 

air, and compared to 50 and 200 h treatments at 1500 °C. The XRD profile of the 5 h reacted specimen 

was found to be almost identical to the 200 h reacted sample in Figure 5, indicating that these were 

Ca2Yb8(SiO4)6O2 grains. The surface of this specimen (Fig. 6(a)) has roughly hexagonal grains similar to 

those in the 50 h CMAS-reacted specimen (Fig. 3(b)). This suggests that the initially fast CMAS reaction 

kinetics and less further degradation of the specimens due to the CMAS volatility at the high temperatures 

and therefore the loss of available CMAS for reactions. The apatite grains appear to grow from an 

amorphous, CMAS-rich melt matrix, demonstrating that the apatite surface layer forms from a CMASrich melt. In contrast, the surface of the 1300 °C reacted specimen is glassy, with few notable 

microstructural features. This indicates significantly less reactions and slower kinetics at the lower testing 

temperatures. 
NASA/TM—2013-218091 4 

Y2Si2O7-CMAS Surface Reactions 

Figure 7 shows images of Y2Si2O7 specimens before and after reacting with CMAS for various times 

at 1500 °C. The same issue of CMAS volatility at atmospheric pressure observed in Yb2SiO5 was also 

observed for this material at such a high test temperature. Figure 8 shows SEM images of Y2Si2O7 reacted 

with CMAS for 50 h. The region shown in Figure 8(a) represents a portion of the ~1 mm thick wellmelted layer in contact with high CMAS concentration, which has been converted from Y2Si2O7 to a 

Ca-Si-Y-rich reaction product with isolated Si-rich regions. The surface of reaction layer (Fig. 8(b)) 

consists of angular, needle-like grains. SEM micrographs of Y2Si2O7 reacted with CMAS for 200 h are 

shown in Figure 9. The light colored phase shown in Figure 9(a) is a portion of the ~1 mm thick melted 

layer and is rich in Ca, Si, and Y. The darker phases located between the lighter grains are rich in Al, Si, 

and Y and can be found throughout the entire ~4 mm thick cross-section. Unlike the Yb2SiO5-CMAS 

reaction products, these grain boundary phases are not visible in the surface images (Fig. 9(b)), possibly 

due to even higher volatility in the Y, Al, Ca, Mg, Si-containing phases. XRD identifies the Ca-containing 

Y2Si2O7-CMAS reaction product as calcium-yttrium-silicate-oxyapatite (Ca2Y8(SiO4)6O2), as shown in 

Figure 10. This material has a very similar structure to the Yb2SiO5-CMAS reaction layer. 

It can be seen from Figure 11(a) that the Y2Si2O7 reacted with CMAS for 5 h at 1500 °C has a surface 

microstructure nearly identical to the 50 h CMAS reacted specimen in Figure 8(b) with the same amount 

CMAS applied. As with Yb2SiO5, the result suggests that the reaction occurred fast in the initial a few 

hours, and the reaction and CMAS loss due to its volatility prevented further significant more reactions in 

the test condition. The surface of the 1300 °C reacted sample in Figure 11(b) contains apatite grains of 

similar shape to the 1500 °C CMAS-reacted specimens, while un-reacted glass phase CMAS seemed still 

present. The XRD profile of this specimen is nearly identical to Figure 10, showing the crystalline 

calcium-yttrium-silicate-oxyapatite (Ca2Y8(SiO4)6O2) phase. 

RE-Silicate + CMAS Blend 

The hot-pressed rare earth silicate-CMAS blends are shown before and after heat treatment in 

Figure 12. Before heat treatment, both specimens appear dense and have a mottled appearance. Heat 

treatment at 1500 °C results in complete melting of both samples. This melt partially infiltrated the porous 

alumina crucible to penetrate and react with a portion of the ceramic brick beneath it. The experiments 

approximately determined upper temperature limits of the coating materials in contact with CMAS, and 

showing some extensive melting of the Yb2SiO5 and Y2Si2O7 specimens at the high 25 to 30 wt% CMAS 

concentrations at 1500 °C. 

RE-Oxide Doped HfO2- and ZrO2-CMAS Surface Reactions 

Images of the RE-oxide doped HfO2 reacted with CMAS for 100 h are shown in Figure 13. Based on 

initial examinations, the large, light colored grains contain HfO2 with a much lower RE-oxide 

concentration than the original as-processed coating material. The darker grain boundary phases are rich 

in both CMAS elements and RE elements. These RE-rich grain boundary phases penetrated through the 

entire specimen, which had a bulk thickness of ~2 mm. 

Figure 14 shows images of the six-component ZrO2 reacted with CMAS for 100 h. The EDS analysis 

indicates that the large, white grains visible in the cross-section and surface images contain mainly ZrO2. 

The dark grain boundary regions appear to be rich in Si and Zr. No Ca, Mg, or Al was detected in the 

sample cross-section of surface, possibly due to high volatility of the unreacted CMAS components. 
NASA/TM—2013-218091 5 

Discussion 

RE-Silicates 

The main mechanism of CMAS penetration in the low CMAS concentration surface reaction tests 

appears to be grain boundary attack. As mentioned in more detail later, some surface melting may have 

occurred at relatively short CMAS exposure times during which CMAS concentration was high. High 

CMAS volatility likely reduced the CMAS surface concentration at longer exposure times. In the Yb2SiO5

specimens, the widening of the grain boundary regions containing low-melting eutectics with testing time 

implies continuing dissolution of the rare earth (Yb) and recession of the grain surfaces even when the 

overall CMAS surface concentration has decreased due to CMAS volatilization. In both materials, low 

melting grain boundary phases were observed throughout the bulk thickness of the 200 h reacted 

specimens, demonstrating overall poor CMAS resistance, particularly along the grain boundaries, at the 

test temperatures. Because EBC layers generally have a thickness no greater than 200 μm, CMAS would 

penetrate the entire RE silicate EBC top coat quickly, causing the EBC to fail and also exposing the 

intermediate and bond coat layers to CMAS and other environment attack. 

Y2Si2O7 appears to have suffered a more severe reaction, possibly due to its higher SiO2

concentration. The needle-like grains in the surface images (Figs. 8(b), 9(b), and 11(a)) of Y2Si2O7 are 

formed from a liquid phase, indicating at least the surface of the EBC experiences localized melting at 

1500 °C. This also appears to be the case for Yb2SiO5, as the 5 h CMAS experiments confirmed localized 

melting of the Yb2SiO5 and Y2Si2O7 surfaces in contact with high CMAS concentrations. However, more 

work is under way to explain the difference in grain morphology and grain growth kinetics. The Y2Si2O7-

CMAS reaction kinetics appeared faster than the Yb2SiO5-CMAS kinetics, as evidenced by the fully 

formed apatite grains on the 5 h reacted Y2Si2O7 specimen and the partially formed apatite grains in the 

5 h reacted Yb2SiO5 specimen. CMAS attack on Y2Si2O7 appears to be a significant problem even at 

1300 °C, as demonstrated by the extensive localized melting and formation of Y-containing apatite grains 

on the specimen surface (Fig. 11(b)). Conversely, Yb2SiO5 does not display the same aggressive CMAS 

attack at 1300 °C, which can be seen the lack of a crystalline apatite reaction product (Fig. 6(b)) during 

the short CMAS exposure. 

The CMAS volatility observed in the CMAS surface reaction tests may be unrealistic, as the effective 

compositions of a localized CMAS source may change rapidly in the 1500 °C air furnace test conditions 

and no constant CMAS concentration is maintained during the given testing cycle times. The closed 

turbine section environment and continuous deposition of CMAS during the engine operation would 

lessen CMAS volatility, effectively raising the CMAS concentration on the EBC surface and thus likely 

promoting even more severe CMAS infiltration and attack. In this way, the rare earth silicate-CMAS 

blend experiments represent a more realistic scenario and help determine the upper temperature limit of 

the CMAS concentrations. In these tests, the CMAS dissolved the entire EBC specimen, including 

partially the SiC/SiC CMC substrates. This is an extreme demonstration of the possible catastrophic 

effects of CMAS on EBCs and turbine section components. More work must be done to determine the 

upper CMAS concentration limits that RE-silicates and other candidate EBC materials can endure for 

their designed lifetimes without catastrophic failure. 

The RE silicate-CMAS reaction layers observed in the coating experiments suggest very fast transport 

of CMAS into the bulk grains of the specimens via a solid state diffusion process. An effort was made to 

explain this fast diffusion by examining the stoichiometry of the apatite reaction layers. An atomic 

number-absorption-fluorescence (ZAF) technique was used to produce approximate semi-quantitative 

molar percentages of oxides from the EDS data collected from the Yb2SiO5-CMAS and Y2Si2O7-CMAS 

reacted specimens. These molar percentages were then used to determine the ratio of alkaline earth oxides 

(AE oxides) to RE oxides in the apatite reaction products. This ratio is shown plotted as a function of 

CMAS reaction time in Figure 15. Stoichiometric apatite has an AE oxide-RE oxide ratio of 0.5. Form the 

composition standpoint, the non-stoichiometric Ca2Yb8(SiO4)6O2 appears to be Yb2O3-lean (with a more 

oxygen vacant sublattice), while the Ca2Y8(SiO4)6O2 appears Y2O3-rich (with an AE vacant sublattice). 
NASA/TM—2013-218091 6 

The modified phase diagram shown in Figure 16 indicates the shifted compositions of the two reaction 

products. The non-stoichiometric materials contain high concentration anion and cation vacancies and 

defects, which may substantially increase the rate of CMAS element diffusion in the solid material. 

RE-Oxide Doped HfO2 and ZrO2

The doped HfO2 and ZrO2 specimens generally exhibited less melting as compared to RE-silicates 

thus may have showed better CMAS phase stability than the RE-silicates. In the case of high rare earth 

element doped HfO2, the CMAS appears to penetrate the specimen at grain boundaries and maybe 

forming higher melting RE-aluminosilicate phases. The low concentration of RE elements in the HfO2

bulk grains indicates that these elements also substantially dissolve into the CMAS grain boundary melts 

to form the grain boundary reaction phases. Unlike the rare earth silicates, however, the doped HfO2

seems to react with CMAS very little but seemed also recessed due to loss of RE elements. 

The doped ZrO2 had much lower dopant concentrations and exhibited the least reaction and maybe 

the best CMAS resistance of the materials studied. The lack of Ca, Mg, and Al in the bulk specimen 

suggests that the unreacted CMAS volatilized during the course of each 50 h heat treatment. Experiments 

involving higher CMAS concentrations will have to be done to fully understand the interaction between 

CMAS and this six-component ZrO2 composition. 

Summary and Conclusions 

The thermochemical reactions at 1500 °C between calcium-magnesium-aluminosilicate- (CMAS) 

and several EBC materials were studied. After 50 h, Yb2SiO5 and Y2Si2O7 developed a locally melted, 

Ca- and RE-containing reaction layer from which non-stoichiometric apatite-like grains precipitated. 

CMAS penetrated the specimen preferentially at grain boundaries and dissolved the EBC material to form 

low-melting grain boundary pockets. After 200 h, these low melting phases extended through the entire 

specimen, indicating that CMAS penetrated the bulk thickness of ~4 mm. Y2Si2O7 appears to react more 

quickly with CMAS forming even lower melting phases than Yb2SiO5. RE-silicate + 25 to 30 wt% 

CMAS specimens were fully melted after 50 h heat treatment at 1500 °C, indicating that there is an upper 

limit to the allowable CMAS concentration in these materials. RE-oxide doped HfO2 and ZrO2

demonstrated better CMAS reaction and phase stability than the RE-silicates, although appears that high 

RE-oxide dopant concentration still facilitated severe grain boundary CMAS attack in HfO2 and the grain 

boundary surface recession. Low rare earth dopant ZrO2 appeared relatively stable and unreacted under 

the experimental conditions, and the effect of transition metal dopants Ta and Ti on CMAS resistance is a 

future topic to be studied. Higher CMAS exposure and some controlled coating experiments must be 

carried out to fully understand the EBC material’s CMAS-resistance capabilities. 

References 

1. Spitsberg, I. and Steibel, J. “Thermal and Environmental barrier Coatings for SiC/SiC CMCs in 

Aircraft Engine Applications,” International Journal of Applied Ceramic Technology, Vol. 1, No. 4, 

2004, pp. 291-301. 

2. Lee, K.N. “Current Status of Environmental Barrier Coatings for Si-Based Ceramics,” Surface and 

Coatings Technology, Vol. 133-134, Nov. 2000, pp. 1-7. 

3. Lee, K.N., et al. “Upper Temperature limit of Environmental Barrier Coatings Based on Mullite and 

BSAS,” Journal of the American Ceramic Society, Vol. 86, No. 8, 2003, pp. 1290-1306. 

 
NASA/TM—2013-218091 7 

4. D. Zhu, R.A. Miller, and D.S. Fox, “Thermal and Environmental Barrier Coating Development for 

Advanced Propulsion Engine Systems,” AIAA–2007–2130, 2007. 48th AIAA/ASME/ASCE/ 

AHS/ASC Structures, Structural Dynamics and Materials Conference, v 5, p 5121-5135, 2007. 

5. D. Zhu, N.P. Bansal, R.A. Miller, “Thermal conductivity and stability of HfO2-Y2O3 and La2Zr2O7

evaluated for 1650 °C thermal/environmental barrier coating applications,” Advances in Ceramics 

Matrix Composites IX, Ceramic Transactions, Volume 153, p 331-343, 2004. 

6. D. Zhu and R.A. Miller, “Multi-functionally graded environmental barrier coatings for Si-based 

ceramic components,” Provisional patent application No. 60/712,605, August 26, 2005; US Patent 

Application No. 11/510 573, August 28, 2006. U.S. Patent No. 7,740,960 B1, June 2010. 

7. Mercer, C., Faulhaber, S., Evans, A.G., Darolina, R. “A delamination mechanism for thermal barrier 

coatings subject to calcium-magnesium-alumino-silicate (CMAS) infiltration,” Acta Materialia, 

Vol. 53, 2005, pp. 1029-1039. 

8. Kramer, S., Yang, J., Levi, C.G., “Thermochemical Interaction of Thermal Barrier Coatings With 

Molten CaO-MgO-Al2O3-SiO2 (CMAS) Deposits,” Journal of the American Ceramic Society., 

Vol. 89, No. 10, 2006, pp. 3167-3175. 

9. Grant, K.M., Kramer, S., Lofvander, J.P.A., Levi, C.G., “CMAS Degradation of Environmental 

Barrier Coatings,” Surface and Coatings Technology, Vol. 202, July 2007, pp. 653-657. 

10. Harder, B.J. et al., “Chemical and Mechanical Consequences of Environmental Barrier Coating 

Exposure to Calcium-Magnesium-Aluminosilicate,” Journal of the American Ceramic Society., 

Vol. 94, June 2011, pp. 178-185. 

11. C.G. Levi, J.W. Hutchinson, M.H. Vidal-Setif, C.A. Johnson, “Environmental Degradation of TBCs 

by Molten Salt,” MRS Bulletin, Vol. 37, Issue 10, October 2012, pp. 932-941. 

12. D.S. Miladinovich.; D. Zhu, “Mechanical Properties and Durability of Advanced Environmental 

Barrier Coatings in Calcium-Magnesium-Alumino-Silicate Environments,” The 35th International 

Conference on Advanced Ceramics and Composites Daytona Beach, Florida 23-28, Jan. 2011. 

13. G.J. McCarthy, “Crystal chemistry of the rare earths in solidified high level nuclear wastes;” pp. 

665-676 in Proc. Rare Earth Res. Conf., 12th, Vail, Colorado, July 18-22, 1976, Vol. 2. Edited by 

C.E. Lundin, University of Denver, Denver, Colorado, 1976. 

Figure 1.—Sintered and as-processed Yb2SiO5. Cross-section SEM Back-Scattered Electron (BSE) images of 

Yb2SiO5 (a) as-sintered; and (b) after 50 h heat treatment at 1500 C in air. 
NASA/TM—2013-218091 8 

Figure 2.—Yb2SiO5 reacted with CMAS. Macro-photographs of Yb2SiO5 specimens (a) as-sintered; and reacted with 

CMAS for (b) 5 h, (c) 50 h, and (d) 200 h. There is visibly much less of the reacted material on the 50 h and 200 h 

tested specimens, suggesting that the CMAS components may be highly volatile at 1500 C in atmospheric 

pressure in air.

Figure 3.—SEM micrographs of Yb2SiO5 reacted with CMAS for 50 h. (a) cross-section; (b) surface BSE 

images of Yb2SiO5 reacted with CMAS for 50 h at 1500 C; (c) and (d) EDS of regions I and II, respectively. 
NASA/TM—2013-218091 9 

Figure 4.—SEM images of Yb2SiO5 reacted with CMAS for 200 h. (a), (b), (c), and (d) low 

and high magnification surface BSE images of Yb2SiO5 reacted with CMAS for 200 h at 

1500 C; and (e) and (h) EDS of regions I-IV, respectively. 
NASA/TM—2013-218091 10 

Figure 5.—XRD of Yb2SiO5-CMAS reaction layer. The pattern for Ca2Yb8(SiO4)6O2 matches well with the observed 

x-ray scan for the Yb2SiO5-CMAS reaction layer. 

Figure 6.—Yb2SiO5 surface reacted with CMAS for (a) 5 h at 1500 C and (b) 50 h at 1300 C. 

0

5000

10000

15000

20000

25000

30000

35000

10 20 30 40 50 60 70 80

CaYb

4

(SiO4)

3

O (Reference card No. 04-006-0320)

Intensity, Counts

Diffraction angle 2, deg
NASA/TM—2013-218091 11 

Figure 7.—Photographs of Y2Si2O7 specimens reacted with CMAS 

for various time at 1500 C. (a) as-sintered; and (b) reacted with 

CMAS for 5 h, (c) 50 h, and (d) 200 h. Due to the volatility of the 

CMAS and its melts at high temperature, the amount of CMAS for 

reactions was not sustainable for the long-term testing at 1500 C 

in atmospheric pressure in air. 

Figure 8.—Y2Si2O7 reacted with CMAS for 50 h at 1500 C. (a) A cross-section SEM BSE image of the 

Y2Si2O7; (b) A surface SEM BSE image of the Y2Si2O7; (c) and (d) EDS analyses of regions I and II in the 

cross-section surface, respectively. 
NASA/TM—2013-218091 12 

Figure 9.—Y2Si2O7 reacted with CMAS for 200 h at 1500 C. (a) A cross-section SEM BSE image of the 

Y2Si2O7; (b) A surface SEM BSE image of the Y2Si2O7; (c) and (d) EDS analyses of regions I and II in 

the cross-section surface, respectively. 

Figure 10.—XRD of Y2Si2O7-CMAS reaction product. The pattern for Ca2Y8(SiO4)6O2 matches. 

0

1000

2000

3000

4000

5000

6000

7000

10 20 30 40 50 60 70 80

CaY

4

(SiO4)

3

O (Reference card No. 04-006-0319)

Intensity, Counts

Diffraction angle 2, deg
NASA/TM—2013-218091 13 

Figure 11.—Y2Si2O7 reacted with CMAS for (a) 5 h at 1500 C and (b) 50 h at 1300 C. 

Figure 12.—RE-silicate + CMAS mixtures. Digital photographs 

of (a) Yb2SiO5+ 25 wt%CMAS as-sintered; (b)Y2Si2O7 + 

30 wt% CMAS as-sintered; (c) Yb2SiO5 + 25 wt% CMAS 

heat treated for 50 h at 1500 C; and (d) Y2Si2O7 + 30 wt% 

CMAS heat treated for 50 h at 1500 C. 
NASA/TM—2013-218091 14 

Figure 13.—HfO2 reacted with CMAS for 100 h. (a) and (b) Low and high magnification 

cross-sectional images of RE-oxide doped HfO2 reacted with CMAS for 100 h at 1500 C; 

(c) and (d) Low and high magnification surface BSE images of RE-oxide doped HfO2

reacted with CMAS for 100 h at 1500 C. Similar to the rare earth silicate cases, the high 

concentration rare earth doped HfO2 had relatively substantial grain boundary recessions 

and CMAS reaction phases. 
NASA/TM—2013-218091 15 

Figure 14.—ZrO2 reacted with CMAS for 100 h at 1500 C. (a) A cross-sectional image of six-component 

RE oxide-doped ZrO2 reacted with CMAS; (b) A surface BSE image of six-component RE oxide-doped 

ZrO2 reacted with CMAS. 

Figure 15.—Stoichiometry of CMAS reaction layers. The molar ratio of alkaline earthoxides (AEO) CaO+MgO to rare earth-oxides (RE2O3) Yb2O3 and Yb2O3 in the Yb2SiO5-

CMAS and Y2Si2O7-CMAS reaction layers as a function of time. The average and 

stoichiometric molar ratios are indicated as horizontal lines. 
NASA/TM—2013-218091 16 

Figure 16.—AEO-RE2O3-SiO2 phase diagram. Redrawn ternary phase diagram 13 

indicating shifted non-stoichiometric apatite compositions. The letters A and B 

indicate the compositions of CMAS- reacted Yb2SiO5 and Y2Si2O7, respectively. 

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4. TITLE AND SUBTITLE

Calcium-Magnesium-Aluminosilicate (CMAS) Reactions and Degradation Mechanisms of 

Advanced Environmental Barrier Coatings 

5a. CONTRACT NUMBER

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6. AUTHOR(S)

Ahlborg, Nadia, L.; Zhu, Dongming 

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WBS 794072.02.03.03.02 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The thermochemical reactions between calcium-magnesium-aluminosilicate- (CMAS-) based road sand and several advanced turbine engine 

environmnetal barrier coating (EBC) materials were studied. The phase stability, reaction kinetics and degradation mechanisms of rare earth 

(RE)-silicates Yb2SiO5, Y2Si2O7, and RE-oxide doped HfO2 and ZrO2 under the CMAS infiltration condition at 1500 °C were investigated, 

and the microstructure and phase characteristics of CMAS-EBC specimens were examined using Scanning Electron Microscopy (SEM) and 

X-ray Diffraction (XRD). Experimental results showed that the CMAS dissolved RE-silicates to form crystalline, highly non-stoichiometric 

apatite phases, and in particular attacking the silicate grain boundaries. Cross-section images show that the CMAS reacted with specimens 

and deeply penetrated into the EBC grain boundaries and formed extensive low-melting eutectic phases, causing grain boundary recession 

with increasing testing time in the silicate materials. The preliminary results also showed that CMAS reactions also formed low melting 

grain boundary phases in the higher concentration RE-oxide doped HfO2 systems. The effect of the test temperature on CMAS reactions of 

the EBC materials will also be discussed. The faster diffusion exhibited by apatite and RE-doped oxide phases and the formation of 

extensive grain boundary low-melting phases may limit the CMAS resistance of some of the environmental barrier coatings at high

temperatures. 

15. SUBJECT TERMS

Environmental barrier coatings; Calcium-Magnesium Aluminosilicate; Ytterbium silicate; Yttrium silicate; Hafnia; Zirconia; Apatite; 

Glass 

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