Valerie L. Wiesner and Narottam P. Bansal

Glenn Research Center, Cleveland, Ohio

Crystallization Kinetics of Calcium-Magnesium 

Aluminosilicate (CMAS) Glass

NASA/TM—2015-218413

February 2015
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Valerie L. Wiesner and Narottam P. Bansal

Glenn Research Center, Cleveland, Ohio

Crystallization Kinetics of Calcium-Magnesium 

Aluminosilicate (CMAS) Glass

NASA/TM—2015-218413

February 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors thank Dr. Bryan Harder and Dr. Dongming Zhu for helpful discussions, Dr. Richard Rogers for XRD assistance,

Mr. Dereck Johnson for DTA, Dr. Paul Angel for melting the synthetic sand into glass, Mr. Ray Babuder for assistance with 

grinding of the bulk glass and Ms. Joy Buehler for glass specimen preparation. A portion of this work was supported by the 

Pathways Program at NASA Glenn Research Center.

Available from

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This report is available in electronic form at http://www.sti.nasa.gov/ and http://ntrs.nasa.gov/
NASA/TM—2015-218413 1 

Crystallization Kinetics of Calcium-Magnesium Aluminosilicate 

(CMAS) Glass 

Valerie L. Wiesner and Narottam P. Bansal 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The crystallization kinetics of a calcium-magnesium aluminosilicate (CMAS) glass with composition 

relevant for aerospace applications, like air-breathing engines, were evaluated using differential thermal 

analysis (DTA) in powder and bulk forms. Activation energy and frequency factor values for crystallization 

of the glass were evaluated. X-ray diffraction (XRD) was used to investigate the onset of crystallization and 

the phases that developed after heat treating bulk glass at temperatures ranging from 690 to 960 °C for 

various times. Samples annealed at temperatures below 900 °C remained amorphous, while specimens heat 

treated at and above 900 °C exhibited crystallinity originating at the surface. The crystalline phases were 

identified as wollastonite (CaSiO3) and aluminum diopside (Ca(Mg,Al) (Si,Al)2O6). Scanning electron 

microscopy (SEM) and energy dispersive spectroscopy (EDS) were employed to examine the 

microstructure and chemical compositions of crystalline phases formed after heat treatment. 

1.0 Introduction

Particulates, such as sand and volcanic ash, pose a significant threat to the development of nextgeneration, high-efficiency aircraft engines that operate at elevated temperatures (!1200 °C). When an 

engine ingests sand or similar small siliceous debris at temperatures above 1200 °C, the particulates 

transform into molten deposits of calcium-magnesium aluminosilicate (CMAS) (Refs. 1 to 3). Molten 

CMAS infiltrates into the porous thermal and environmental barrier coatings (TBCs, EBCs) that are used 

to protect silicon-based ceramic matrix composite (CMC) turbine engine components from the extreme 

thermal and corrosive environment encountered in flight (Refs. 4 to 6). The penetration of CMAS into the 

coating changes the mechanical properties of the coating, which can result in undesired stresses and can 

prevent the coating from surviving the severe heating and cooling cycles encountered during engine 

operation (Refs. 7 to 11). 

Inducing CMAS crystallization has been shown to be a viable approach to mitigate the deleterious 

effects of CMAS attack on T/EBCs at high temperatures by limiting the depth to which CMAS may 

infiltrate the coating (Ref. 8). Previous T/EBC studies have developed coating materials that promote 

crystallization of CMAS, and these investigations have focused on evaluating the resulting phases that 

develop during CMAS/coating interactions (Refs. 12 to 14). Concurrently, the crystallization behavior of 

CMAS alone has been previously investigated for applications ranging from ceramic glazes (Ref. 15) to 

solid oxide fuel cell sealants (Ref. 16); however, these previous compositions are markedly different from 

those studied in aerospace coatings literature. Because additional compositional constituents, like 

potassium or sodium oxides, have a pronounced effect on CMAS crystallization (Refs. 17 and 18), 

understanding the crystallization behavior of CMAS compositions relevant to aerospace applications is 

vital to develop novel coating material systems that are resistant to CMAS interactions. 

The objective of the current study was to evaluate the crystallization kinetics of a CMAS glass that 

was prepared by melting a synthetic sand composition by differential thermal analysis (DTA) in powder 

and bulk forms. Complementary heat treatments of the bulk glass were performed at various temperatures 

and times. X-ray diffraction was utilized to identify the resulting crystalline phases that developed within 

the glass. Microstructures of the heat treated CMAS specimens and compositions of the crystalline phases 

were analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). 
NASA/TM—2015-218413 2 

2.0 Materials and Methods 

CMAS glass was prepared by melting synthetic sand having a composition of 34 wt% quartz (SiO2), 

30 wt% gypsum (CaSO4·2H2O), 17 wt% aplite (SiO2+KAlSi3O8), 14 wt% dolomite (CaMg(CO3)2) and 

5 wt% salt (NaCl). The synthetic sand was heated to 1550 °C with 30-min isothermal holds at 150, 790, 

and 1275 °C at a rate of 10 °C/min in a platinum crucible in a box furnace. After 1 h isothermal hold at 

1550 °C, the melt was quenched in water. The resulting glass frit was ground to powder using a Fritsch 

Planetary Mono Mill Pulverisette 6 (Idar-Oberstein, Germany) with zirconia milling media in a zirconia 

container. The glass powder was passed through a sieve (d297 Pm mesh) to separate the fine from coarse 

and bulk particles. The composition of the glass powder was analyzed by a Varian Vista Pro Inductively 

Coupled Plasma Atomic Emission Spectrometer (ICP-AES). 

Differential thermal analysis (DTA) was carried out using a Netzsch STA 409 (Burlington, MA) to 

evaluate the crystallization kinetics of the fine CMAS glass powder (particle size d297 Pm) and bulk 

samples having a mass of approximately 100 mg. DTA scans were performed in air from room 

temperature to 1250 °C or 1500 °C at heating rates ranging from 2 to 50 °C/min. Alumina pans were used 

to contain the CMAS glass during testing. The glass transition temperatures (Tg) and crystallization peak 

temperatures (TP) were determined from each DTA scan. 

Bulk CMAS glass specimens weighing ~100 mg were isothermally heat treated on a platinum surface 

in a box furnace in air. Duplicate samples were heated at a ramp rate of 10 °C/min to a temperature 

ranging from 690 to 960 °C for 10 min, 1, 5 or 10 h and then cooled at 10 °C/min to room temperature. 

After heat treatment, one of the two glass specimens was ground using an alumina mortar and pestle. 

Powder X-ray diffraction (XRD) analysis (0.02°/2T step, 2.5 s/step) was performed using a Bruker D8 

Advance diffractometer with Cu KD radiation to evaluate the development of crystalline phases in heat 

treated glass specimens. The second heat treated glass specimen was mounted in epoxy and polished to 

yield a cross-section for microstructural and compositional analysis using scanning electron microscopy 

(SEM) and energy dispersive spectroscopy (EDS). The polished specimen cross-sections were coated 

with a thin layer of carbon for electrical conductivity. A Hitachi 4700II Cold Field Emission Gun SEM 

(Hitachi High Technologies America, Inc., Pleasanton, CA) with EDS analyzer in secondary (SE) and 

back-scattered electron (BSE) imaging modes was used. The average depth of crystalline layers observed 

in heat treated specimens was determined by evaluating five different regions in SEM micrographs of a 

particular specimen and measuring the surface-initiated crystallization depth along 50 lines using ImageJ 

image processing and analysis software. The resulting 250 length measurements were averaged to obtain 

a mean crystal layer thickness for specimen at a particular heat treatment time and temperature. 

3.0 Results and Discussion 

3.1 Composition of Synthetic Sand and CMAS Glass 

XRD spectra of the synthetic sand and CMAS glass powder are presented in Figure 1(a) and (b), 

respectively. XRD results confirmed the presence of quartz (SiO2), gypsum (CaSO4·2H2O), KAlSi3O8, 

dolomite (CaMg(CO3)2) and salt (NaCl) in the synthetic sand. The CMAS glass powder was amorphous 

as expected. The composition of the CMAS glass as analyzed by ICP-AES method was 23.3CaO6.4MgO-3.1Al2O3-62.5SiO2-4.1Na2O-0.5K2O (mol%) and 21.9CaO-4.3MgO-5.4Al2O3-63SiO2-4.3Na2O0.8K2O (wt%). 

3.2 Evaluation of Crystallization Kinetics by DTA 

DTA thermograms of CMAS glass powder and bulk specimens were obtained at various scan rates 

ranging from 5 to 50 °C/min. Representative DTA scans obtained at heating rates of 5 and 50 °C/min are 

shown in Figure 2(a) and (b), respectively. The endothermic peak, which was observed for both powder 

and bulk specimens in DTA thermograms obtained at a scan rate of 10 °C/min, at 704 °C was associated 
NASA/TM—2015-218413 3 

with the glass transition (Tg). The two subsequent exothermic peaks (TP1 and TP2, respectively) seen in 

DTA thermograms of glass powder corresponded to crystallization. Exothermic peak maximum 

temperature values for powder and bulk glass are given in Table 1. The presence of two exothermic peaks 

suggested the crystallization of at least two distinct phases within the glass at different temperatures 

(Ref. 19). The temperature range in which the first exothermic peak was observed in CMAS glass powder 

thermograms was ~842 to 930 °C, and the second peak was around 930 to 1024 °C. A distinct exothermic 

peak corresponding to TP1 could not be identified in DTA thermograms for bulk CMAS glass, but an 

exothermic peak (TB2) likely corresponding to TP2 was apparent in all DTA scans. 

The substantial shift of the second exothermic peak from lower temperatures for the glass powder to 

higher temperatures for the bulk glass suggested that this peak was likely associated with a crystalline 

phase that developed by a surface-nucleated mechanism (Ref. 19). However, further analysis is required 

to fully ascertain the crystallization mechanism(s) at play in the CMAS glass. When a CMAS glass of this 

composition would come into contact with T/EBCs at elevated temperatures, the coating might act as a 

nucleation site, inducing crystallization of CMAS originating at the melt/coating interface. Crystallization 

of CMAS glass could be promoted by introducing dopants into the coating that could initiate 

crystallization by a surface-nucleated mechanism. The endothermic peaks at ~1228 to 1252 °C and 

~1425 to 1470 °C likely involved the melting of the crystalline phases and/or of the residual glass. 

Crystallization kinetics of the CMAS glass powder were evaluated from experimental values of peak 

maximum temperatures (TP1, TP2, and TB2) obtained from DTA scans at various heating rates (refer to 

Table 1) using an approach previously utilized by Bansal et al. (Refs. 20 to 22) The isothermal 

crystallization of a glass can be given by the Johnson-Mehl-Avrami (JMA) equation: (Refs. 23 to 26) 

  n  ln 1 x k t (1) 

where x represents the volume fraction of glass crystallized after time t, k is the reaction-rate constant and 

n is the Avrami exponent, which is a dimensionless quantity that is associated with the morphology of 

crystal growth. The Arrhenius equation can be used to express the temperature dependence of k within 

narrow temperature ranges and can be written as 

k Ȟ exp    E / RT (2) 

where Q is an effective frequency factor, E is the activation energy of the transformation, R represents the 

gas constant and T is the absolute temperature at which the transformation occurs. 

During DTA scans, the temperature of the sample varies at a constant rate, T , which can be defined 

as the change in temperature with respect to time (T = dT/dt), such that T can be related to the 

temperature, T, after a certain time, t, and the initial temperature, Ti, by the following expression: 

T T t i  ș (3) 

Consequently, the reaction rate constant, k, as given in Equation (2), can now be expressed as a 

time-dependent variable for the non-isothermal case using Equation (3): 

    ̧

 ̧

1

·

 ̈

 ̈

©

§

  R T t

E k t

i ș

ȣ exp (4) 

The JMA equation can thus be written in integral form as 

   n t

x k t  ̧

1

·  ̈

©

§   30 ln (5) 1
NASA/TM—2015-218413 4 

By applying the Borchardt assumption (Ref. 27), which implies that the instantaneous reaction rate is 

proportional to the displacement of the DTA trace from its baseline, the maximum rate of transformation 

occurs at the exothermic crystallization peak. Bansal et al. (Refs. 20 to 22) have shown that the 

temperature, TP, of the crystallization peak varies with respect to the heating rate, T, by 

P

P

RT

E

Rv

T E  ̧ 

1

·  ̈

©

§  ̧

 ̧

1

·

 ̈

 ̈

©

§

ln

ș

ln

2

 (6) 

As a result, a plot of ln(TP

2 /T) versus 1 / TP will be linear with slope of E/R and an intercept of 

(ln(E/R) – ln Q. 

Plots of ln(TP

2 /T) versus 1 / TP for each exothermic peak were linear as shown in Figure 3(a) and (b). 

A linear least squares fitting was applied to the experimental data to obtain values for the activation 

energy and frequency factor for both exothermic peaks. The correlation coefficient, R2

, was greater than 

0.99 for both plots shown in Figure 3(a) and (b). Table 2 shows the activation energy and frequency factor 

parameters calculated for CMAS glass in powder and bulk forms. The activation energies for the first and 

second exothermic peaks were 263 and 294 kJ/mol, respectively, for the glass powder. The activation 

energy for the exothermic peak (TB2) for the bulk glass was found to be 286 kJ/mol. This value was 

slightly lower than that for the glass powder, which was expected, because the powder had a higher 

surface area due to the smaller particle size, and the bulk specimens required fewer nucleation sites to 

initiate crystallization. Although not directly comparable due to compositional variance, previous studies 

have reported similar activation energy values for other CMAS glass compositions (Refs. 15, 28, and 29). 

Because asymmetric exothermic peaks were observed in DTA thermograms at varying scan rates, as 

highlighted in Figure 2(a) and (b), n could not be calculated by the Piloyan approach (Ref. 30). 

3.3 Development of Crystalline Phases by X-ray Diffraction 

Figure 4 shows XRD spectra for glass specimens heat treated at various times and temperatures of 

825 to 925 °C. XRD of specimens after heat treatments at 960 °C for various times are presented in 

Figure 5. Although no crystalline phases were detected in glass specimens heat treated at or below 825 °C, 

wollastonite (CaSiO3) (PDF 04-010-0710) was detected as the major phase after annealing at and above 

900 °C. Aluminum diopside (Ca(Mg,Al)(Si,Al)2O6) (PDF 01-076-2502) was determined to be the minor 

phase in specimens heat treated at 925 °C and above. It is unclear if the two phases correspond to a 

particular crystallization peak observed in DTA. The development of crystalline phases in CMAS bulk glass 

heat treated at temperatures ranging from 690 to 960 °C for various times is shown in Table 3. The presence 

of trace amounts of aluminum diopside cannot be ruled out in specimens heat treated at 900 °C for 10 h. It 

was difficult to confirm its presence from XRD data because of the overlap of the primary peaks of 

wollastonite and aluminum diopside phases and also because the crystallinity in the specimen was too small 

when compared with amorphous content. However, the XRD pattern for aluminum diopside provided the 

best fit for the minority phase in spectra of glass specimens heat treated at or above 925 °C. 

At temperatures relevant to aircraft engines (>1200 °C), the viscosity of the CMAS glass will be low, 

causing the CMAS melt to infiltrate into open pores of T/EBC coatings. The rate at which molten CMAS 

infiltrates into the pores of a coating is dependent on its viscosity. Consequently, viscosity values of 

CMAS glass are needed at various temperatures to determine the timescale required for CMAS to 

penetrate a coating. However, as XRD results suggested, crystallization of bulk glass specimens could be 

initiated at temperatures as low as 900 °C when held for 10 h or as quickly as 10 min at 960 °C. The time 

needed to induce crystallization of the CMAS glass could be of the same order as infiltration time into the 

coating at sufficiently high temperatures. Further investigation of the temperature-dependent viscosity of 

this CMAS glass, its thermal properties and its thermochemical interactions with EBC materials is 

currently underway to determine if crystallization will indeed happen within the time frame needed for 

molten CMAS to infiltrate a coating. 
NASA/TM—2015-218413 5 

3.4 Microstructural Evaluation of Heat Treated CMAS Glass 

Figure 6 shows backscatter SEM micrographs of polished cross-sections of the heat treated bulk glass 

specimens exhibiting various extents of crystallization depending on the heat treatment. SEM backscatter 

micrographs revealed that crystallization initiated at the surface of CMAS bulk glass, regardless of whether 

in contact with platinum crucible or air, heat treated at 900 °C and above. No crystalline phases were 

observed in specimens heat treated d825 °C. These results supported the XRD observation described in 

Section 3.3 that the CMAS glass did not crystallize below 900 °C. Surface crystallization appeared to be the 

primary mechanism by which crystal growth occurred in heat treated CMAS specimens. This was consistent 

with DTA scans of glass powder and bulk specimens (shown in Fig. 2) discussed in Section 3.2 as well as 

with previous CMAS investigations of glasses with similar compositions (Refs. 15, 28, 31, and 32). The 

dendritic crystals appeared to develop at the surface and grow radially inward in the specimens examined. 

As shown in Figure 6, the average thickness of the crystalline layer at the surface of heated CMAS 

glass bulk specimens increased with heat treatment temperature above 900 °C as well as with longer 

isothermal hold times at a given temperature. The average thicknesses of the crystalline layer are given in 

Table 4. The thicker surface layer of highly dendritic crystallites observed in specimens heated at 925 °C 

and above suggested that the rate of crystal growth increased with temperature. At heat treatments of 

925 °C for 20 h and 960 °C for 5 h, crystallites fully spanned the thickness of the CMAS specimens, 

which indicated that these samples had the highest degree of crystallinity. The microstructures of CMAS 

glass specimens heat treated at 900 and 960 °C are shown in Figure 7. All specimens heated at and above 

900 °C exhibited a similar microstructure near the sample surface. Well-formed crystal dendrites were 

readily observed in specimens with layers !30 Pm in thickness heat treated above 900 °C, as shown in 

Figure 7. Darker regions that were observed near the surface in all specimens heat treated at 900 °C and 

above did not appear to develop further than ~30 Pm into the sample from the surface. 

Figures 8 and 9 show micrographs and EDS maps of polished cross-sections of specimens heated at 

900 °C for 10 h and at 960 °C for 1 h, respectively, at the crystal growth front. EDS maps corresponding 

to silicon, aluminum and oxygen signals (not shown) exhibited uniform intensity throughout the 

crystalline and amorphous regions of the specimens, suggesting that these elements were present 

throughout the specimen. EDS analysis revealed that the darker areas of the crystal layer near the surface 

of the samples, clearly visible in Figures 7 and 8, contained magnesium, while the lighter areas of the 

crystal layer, seen in Figures 7 to 9, were calcium rich. This suggested that the lighter crystals 

corresponded to a calcium silicate (CaSiO3) phase, since higher calcium signals were observed in 

comparison with the other elements detected. The darker crystalline regions near the surface observed in 

all specimens heat treated at and above 900 °C (shown in backscatter SEM micrographs in Fig. 7) likely 

corresponded to a calcium-magnesium aluminosilicate phase, as EDS maps reflected a higher magnesium 

signal from those areas. 

SEM and EDS analysis suggested that the darker crystalline regions observed in the partially 

crystalline specimens contained magnesium (refer to backscatter SEM micrograph and EDS maps in 

Fig. 8). Although sodium-containing phases were not detected by XRD, EDS mapping of the 

uncrystallized bulk and grain boundaries indicated a higher sodium content than in the crystallized surface 

region, as shown in Figures 8 and 9. If sodium remained within the amorphous regions and along the 

grain boundaries, it likely would not be detected by XRD. 

4.0 Summary and Conclusions 

The crystallization kinetics of CMAS glass powder and bulk specimens was evaluated using DTA. 

Two distinct exothermic peaks, which suggested the likely precipitation of two crystalline phases, were 

observed at ~840 to 930 °C and 901 to 1024 °C in DTA thermograms of the glass powder. Activation 

energies for the first and second peaks were 263 and 294 kJ/mol, and frequency factors were 4.87u108

 s–1

and 1.29u1010 s–1. Only one exothermic peak at ~1054 to 1167 °C was clearly observed in all DTA scans 

of bulk glass specimens. An activation energy value of 286 kJ/mol and a frequency factor of 3.70u108

 s–1
NASA/TM—2015-218413 6 

were determined for this peak, which corresponded to the second exothermic peak in DTA of glass 

powder. The significant shift in the second exothermic peak in DTA scans from ~901 to 1024 °C in glass 

powder to ~1054 to 1167 °C in bulk glass likely suggested a surface-nucleated mechanism for 

crystallization in CMAS glass. 

XRD spectra of glass bulk specimens heat treated at t900 °C showed the development of wollastonite 

(CaSiO3) as a primary crystalline phase. Aluminum diopside (Ca(Mg,Al)(Si,Al)2O6) was also detected as 

a minor phase in specimens heated at !900 °C. Microstructural evaluation by SEM/EDS confirmed the 

presence of compositions corresponding to wollastonite and aluminum diopside crystals that initiated 

from the surface and grew radially inward in glass specimens heated at and above 900 °C. The thickness 

of the crystalline layer increased with temperature as well as hold time. Sodium content was restricted to 

the amorphous region and in between crystallites. Understanding of the crystallization behavior of CMAS 

compositions may help to mitigate damage to protective T/EBC coatings through initiation of CMAS 

crystallization. 

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Diopside System: Structural and Devitrification Study. Part II: Crystallinity Analysis by the 

Rietveld–RIR Method. Journal of the American Ceramic Society 2005;88(11):3131-36. 

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by differential thermal analysis. 1966. 

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Society 2002;85(3):670-74. 

32. Leonelli C, Manfredini T, Paganelli M, Pozzi P, Pellacani GC. Crystallization of some anorthitediopside glass precursors. J Mater Sci 1991;26(18):5041-46. 

 
NASA/TM—2015-218413 8 

TABLE 1.—PEAK MAXIMUM TEMPERATURES FOR 

THE FIRST AND/OR SECOND EXOTHERMIC PEAKS 

OBTAINED BY DTA OF CMAS GLASS POWDER 

(TP1 AND TP2, RESPECTIVELY) AND BULK 

SPECIMENS (TB2) AT VARYING SCAN RATES (T) 

 Powder Bulk 

Scan rate, T [°C/min] TP1 [°C] TP2 [°C] TB2 [°C] 

5 842.1 929.5 1054.2 

10 861.1 954.4 1069.8 

20 893.5 980.5 1108.4 

30 910.7 999.7 1137.4 

40 920 1012.4 1146.5 

50 930 1024.4 1166.9 

TABLE 2.—ACTIVATION ENERGY AND FREQUENCY FACTOR 

VALUES FOR CRYSTALLIZATION OF CMAS GLASS POWDER 

AND BULK SPECIMENS DETERMINED BY DTA 

Parameter Powder Bulk 

Activation Energy for Peak 1, E1 [kJ/mol] 263 ------------- 

Activation Energy for Peak 2, E2 [kJ/mol] 294 286 

Frequency Factor for Peak 1, Q1 [s–1] 4.87×109

 ------------- 

Frequency Factor for Peak 2, Q2 [s–1] 1.29×1010 3.70×108

TABLE 3.—DEVELOPMENT OF CRYSTALLINE PHASES IN CMAS GLASS BULK SPECIMENS 

AFTER DIFFERENT HEAT TREATMENTS 

Heat treatment Crystalline phases detected from XRD 

Temperature, 

°C 

Time Amorphous Wollastonitea

(CaSiO3) 

Aluminum diopsideb

(Ca(Mg,Al)(Si,Al)2O6) 

800 10 h X ͲͲ ͲͲ

825 10 h X ͲͲ ͲͲ

900 10 h X X ? 

925 10 h X X X 

925 20 h X X X 

960 10 min X X X 

960 1 h X X X 

960 5 h ͲͲ X X a

PDF 04-010-0710 wollastonite 

bPDF 01-076-2502 aluminum diopside 

TABLE 4.—THICKNESS OF CRYSTALLINE LAYER THAT DEVELOPED 

IN CMAS GLASS BULK SPECIMENS HEAT TREATED AT VARIOUS 

TEMPERATURES FOR VARYING DURATIONS 

Temperature, 

°C 

Time Crystal layer thickness, 

Pm 

900 10 h 25.6±10.3 

925 10 h 97.5±8.3 

925 20 h Through specimen thickness of ~4 mm 

960 10 min 47.9±6.4 

960 1 h 367.2±13.9 

960 5 h Through specimen thickness of ~4 mm 

 
NASA/TM—2015-218413 9 

Figure 1.—XRD spectra of (a) synthetic sand and (b) the CMAS glass powder. 
NASA/TM—2015-218413 10 

Figure 2.—Differential thermal analysis scans of CMAS glass powder (solid line) and bulk 

specimens (dashed line) obtained at heating rates of (a) 5 qC/min and (b) 50 qC/min. 
NASA/TM—2015-218413 11 

Figure 3.—Plots of ln(TP2 /T) versus reciprocal crystallization peak maximum temperature for the 

(a) first and (b) second crystallization peaks for CMAS glass powder (Ŷ) and bulk (Ƒ). 
NASA/TM—2015-218413 12 

Figure 4.—Powder XRD spectra of CMAS bulk glass heat treated at temperatures ranging from 825 to 

925 qC for various hold times. 
NASA/TM—2015-218413 13 

Figure 5.—Powder XRD spectra of CMAS bulk glass heat treated at 960 qC for various hold times. 

Figure 6.—Backscatter SEM micrographs showing polished cross-sections of CMAS glass bulk specimens heat 

treated at (a) 900 qC for 10 h, (b) 925 qC for 10 h, (c) 925 qC for 20 h, (d) 960 qC for 10 min, (e) 960 qC for 1 h and 

(f) 960 qC for 5 h. 
NASA/TM—2015-218413 14 

Figure 7.—Backscatter SEM micrographs showing polished cross-sections of CMAS glass 

bulk specimens heat treated at (a) 900 qC for 10 h, (b) 960 qC for 10 min, (c) 960 qC for 1 h 

and (d) 960 qC for 5 h. 

Figure 8.—EDS maps for a polished cross-section of CMAS glass bulk specimen heat 

treated at 900 qC for 10 h. (a) Backscatter SEM micrograph and corresponding elemental 

maps of (b) calcium, (c) magnesium and (d) sodium. 
NASA/TM—2015-218413 15 

Figure 9.—EDS maps for a polished cross-section of CMAS glass bulk specimen heat treated at 960 qC for 

1 h. (a) Backscatter SEM micrograph and corresponding elemental maps of (b) calcium, (c) magnesium and 

(d) sodium.