NASA Technical Memorandum 107153

Hot Hydrogen Exposure Degradation

of the Strength of Mullite

Thomas P. Herbell, David R. Hull, and Anita Garg

Lewis Research Center

Cleveland, Ohio

June 1996

National Aeronautics and

Space Administration

HOT HYDROGEN EXPOSURE DEGRADATION OF THE STRENGTH OF MULLITE

Thomas E Herbell, David R. Hull,* and Anita Garg

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

ABSTRACT

This study deals with the corrosion of near stoichiometric mullite (3A1203-2SIO2) by pure dry hydrogen

gas. Exposure of the mullite samples was at temperatures of 1050 and 1250 °C for times up to 500 hr. Preferential

attack of the aluminosilicate glass present in the grain boundaries of the mullite occurred after 125 hr at 1250 °C.

Hydrogen scrubbing of the SiO2 from the glassy grain boundaries and the mullite grains yielded a porous aluminarich surface. The room temperature strength increased after short exposure times at 1250 °C (up to 125 hr), then

decreased by 53 percent after exposure for 500 hr. At 1050 °C, all exposure times (25 to 500 hr) decreased the

strength. After 500 hr in hydrogen at 1050 °C, the room temperature strength of mullite decreased 22 percent.

We also observed a rapid 25 percent strength loss after short exposure times at 1050 °C. This is attributed to the

calcium/hydrogen assisted crystallization of the glassy, grain-boundary phase.

INTRODUCTION

Strength retention to high temperature combined with low thermal expansion make the monolithic form of

mullite (3A1203-2SIO2) a potentially useful material for many structural applications. At the time this study was

initiated it was a candidate for the pressure vessel in the hot zone of a ceramic Stirling engine. Current technical

needs emphasize the use of mullite as a matrix and/or as a reinforcing fiber for advanced fiber reinforced ceramic

matrix composites. Muilite coatings are also being developed to enhance the environmental durability of the siliconbased ceramics, SiC and Si3N4. In some applications, the operating conditions will be highly reducing and may

require significant strength retention, even after long time exposure to hydrogen at elevated temperature. Examples

include the ceramic Stirling engine mentioned above and future hydrogen-fueled hypersonic vehicles. Operating

temperatures in a Stirling pressure vessel could approach 1200 °C. Applications for hypersonic vehicles may require

*National Research Council--NASA Research Associate at Lewis Research Center.
evenhigherhydrogenexposuretmperatures. Undertheseconditions, corrosionofthemullitemayoccurbecauseof

uniformmaterial loss,and/orselectivepitting,whichdecreasethefracturestrength.

Theeffectofhightemperaturehydrogenexposureonthemicrostructureofmullite,alongwiththekinetics

ofthereaction,hasbeenthesubjectofseveral investigations. WrightandWolf(Ref.1)foundthatseverechemical

attackofrefractorymulliteoccurswhenexposedtohydrogenabove1370°C. Crowley ( Ref. 2) further studied the

effects of hydrogen on refractory mullite and measured the rate of silica loss at 1425 °C. He concluded that the

weight loss was dependent on the hydrogen flow rate and that the hydrogen-mullite reaction was diffusion controlled. He also observed a decrease in compressive strength with increasing weight loss. Chen (Ref. 3), using

techniques similar to those of Crowley, found the reaction of mullite with hydrogen to be a mixed control process.

He said that the reaction depended on the transportation of the gaseous product species through the porous product

layer and through the gaseous boundary layer. The amount of weight loss was also dependent on the flow rate of

hydrogen over the mullite. The reduction of mullite by hydrogen gas was studied in more detail by Tso and Pask

(Ref. 4). These investigators used a high purity stoichiometric mullite in their tests. Later, Tso (Ref. 5) found that

diffusion through the porous layer was not rate limiting, but the reduction reaction was. All of the above investigators identified the loss of silica from the mullite to be the major reaction. The result of this reaction is a porous

product layer of alpha alumina on the surface.

Al6Si2Ol3(S ) + 2H2 (g)--)3A1203 (s) + 2SiO(g) + 2H20(g ) (1)

A review of the literature reveals that the hydrogen mullite reaction has only been studied at temperatures

above 1350 °C and mainly for short times, i.e., a maximum of 50 hr. In applications such as the Stirling engine

several thousand hr of exposure at somewhat lower temperatures is required.

The objective of this study was to determine the effect of long time hydrogen exposure on the microstructure and retained room temperature strength of near stoichiometric mullite.

2
EXPERIMENTAL

Mullit'testbars6by3by25mmwerepreparedbydiamondgrinding.Thebarswereultrasonicallycleaned

inneatethanol, dried,measuredandweighedbeforeachhydrogenexposure. Approximately20testbarswereused

ineachhydrogentest.

Thehydrogengaswasdriedandpurifiedtoamoisturelevelofabout1ppm.Theflowratewasregulatedat

roomtemperatureandheldconstant a475cc/min.Hydrogenexposuresweredoneinasiliconcarbideresistance

heatedhorizontal tubefurnacewitha99.8percentpureA1203tube.ThespecimenswereplacedonhighpurityAI203

settersinahighpurityA1203boatlocatedinthecentralhot-zoneofthefurnace.Thetemperaturevariationwithin

thefurnacehot-zonewasabout+10 °C.

Heating to the exposure temperature was in high purity helium. Once the desired temperature was achieved,

the atmosphere was switched to hydrogen. At the completion of the exposure cycle, the gas was again switched to

helium for the cool-down cycle. The mullite samples were exposed to hydrogen at 1050 and 1250 °C for times of

25 to 500 hr.

At the completion of the hydrogen exposure, the samples were removed from the furnace, weighed, and

measured. The exposed specimens were subsequently stressed to fracture in a four-point bend fixture. The loading

rate was 0.05 cm/min with an outer span of 1.90 cm and an inner span of 0.95 cm. The following equation:

MOR Strength = 3P(L - a)/2bd 2 (2)

was used to calculate fracture strength.; P is the load at failure, L is the outer span, a is the inner span, b is the

specimen width, and d is the specimen thickness. The original sample dimensions were used to calculate the

fracture stresses. Results showed that the surface recession due to material loss on the surface did not exceed 20 pan

and was considered to have no significant effect (<2 percent) on the strength calculation.

_K-635mullite, Kyocera International, San Diego, CA.
A number of techniques were used to characterize the exposed samples including: X-ray diffraction (XRD),

scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (XEDS), transmission electron microscopy (TEM), selected area diffraction (SAD), electron microprobe, and optical microscopy.

RESULTS AND DISCUSSION

Etching the as-received mullite with a 2:1 mixture of HCI/HF:H20 for five seconds revealed the microstructure shown in Fig. 1. In the as-received sample, the grains were highly acicular with an aspect ratio approaching 8

and an average grain length of 10 pro. A silica-rich glass (marked G in Fig. !) between the mullite grains (marked M

in Fig. 1) makes up about 16 percent of the structure. This is typical of commercial mullites. The slow transport

rates of the constituent species require the use of excess silica to facilitate densification. As shown in Table I, this

mullite also contained calcium and magnesium for enhancement of sintering.

Table I.--Chemical Analysis of Mullite, wt%

Aluminum Silicon Calcium Iron Magnesium

35.4 13.8 0.7 0.3 0.8

Elements not detected (<lppm): B, Ba, Co, Cr, Mn, Mo, Ni, P, Ti, V, Y, Zn, Zr.

Point count approximation on ceramographic sections indicated a porosity of about 3 percent. This value

was confirmed by immersion density measurements. The measured bulk density was 3.05 +0.01 g/cc.

Hydrogen corrosion is seen in the photomicrographs of Fig. 2. This figure shows the as- ground surface of

the mullite in the as-received condition and after 500 hr exposure in hydrogen at 1050 and 1250 °C. Large surface

voids were present in the as received material, but the surface between voids was quite dense (Fig. 2(a)). The initial

effects of surface corrosion are seen as material loss across the surface after 500 hr at 1050 °C (Fig. 2(b)). The

grain-boundary glass phase is the first material removed from the surface, revealing clearly mullite's needle like

structure. Surface corrosion at 1250 °C is more severe. After 500 hr (Fig. 2(c)) the glass is no longer evident and

actual corrosion of the mullite grains is seen.

4
Corrosionof the mullite was so severe at 1250 °C that formation of silica whiskers was frequently observed

near the outlet end of the furnace tube. The formation of gaseous SiO and H20 occurs when SiO2 reacts with

hydrogen according to the following equation:

Si 2(s) + H 2(g)--_ SiO(g) + H20(g) (3)

It is assumed this same reaction is occurring to the aluminosilicate glass located in the grain boundaries.

Corrosion of the mullite grains seen at 1250 °C can also contribute to the formation of SiO by the reaction shown in

Eq. 1. The SiO from these reactions is removed from the reaction layer by the bulk gas and transported to the cooler

outlet end of the furnace tube where conditions are favorable for reoxidation to SiO2. Removal of SiO2 from the

glass phase and the mullite grains leaves regions of porous alpha alumina behind. The resulting alumina-rich surface

was analyzed by XEDS, and Fig. 3 shows the increase in aluminum/silicon XEDS peak height ratio in going from

the as-received material to the exposed material after 125 and 500 hr at 1250 °C.

Removal of the SiO2 also contributes to a drop in density and an increase in porosity near the surface.

Figure 4 shows the effect of exposure time on the density of samples heated in hydrogen at 1250 °C. The density

decrease seems linear with time up to the maximum of 500 hr studied. The porous surface layer formed by hydrogen

corrosion is shown in the back-scattered electron SEM micrographs of Fig. 5.2 Figure 5 (a) is a cross-section of an

as-received mullite specimen with the top surface marked X. Several large voids (10 to 15 Nn in diameter) visible in

this micrograph, were also present in all the exposed samples (Figs. 5 (b to d)). During exposure to H2, escape of

SiO (g) produced by reaction (Ref. 3) leaves porous regions in the material. The extent of the porosity depends on

both the time and temperature of the exposure. At 1050 °C, exposure for 500 hr produces a porous layer only about

4 lain deep marked byY in Fig. 5 (b). At 1250 °C, exposures of 125 and 500 hr produce porous surface layers. The

layer is 150 grn deep after 125 hr and increases to a depth of 375 gm after 500 hr. Chen (Ref. 3) found the reaction

rate to depend on the position of the bars in the furnace and the flow rate of the hydrogen. This implies that removal

2Backscattered electron (BSE) imaging is used to image glass removal. During mounting for ceramographic preparation, epoxy infiltrates the

sample, filling pores where the glass was removed, In BSE imaging, contrast is provided by atomic number differences. The epoxy is composed

of carbon, atomic number 6, and the mullite has an average atomic number of 9.85. Therefore, the epoxy filled voids show up darker than the

mullite.
of reaction products from the boundary layer, not transport through the porous reaction layer, is the rate controlling

step in the corrosion process.

Fracture strength data for mullite bars exposed to hydrogen at 1050 and 1250 °C are shown in Fig. 6. Data

for short time exposures in helium is also presented in the same figure. At room temperature, the fracture strength

of as received muilite bars was approximately 211 MPa. All 1050 °C exposures produced a significant strength loss.

Most of this strength loss occurred in the first 25 to 50 hr, and was independent of exposure atmosphere, H2 or

He. After 500 hr in hydrogen the measured strength loss averaged 22 percent. The slight strength recovery, from

--150 MPa to --167 MPa, observed between 125 and 500 hr exposure time is not believed to be of great significance.

The loss in strength after 500 hr of exposure is attributed primarily to formation of a thin porous reaction layer due

to corrosion of the grain-boundary phase between the mullite grains. Because of the different strength behavior of

bars exposed at 1250 °C, to be described later in this report, an attempt was made to rationalize the sudden loss in

strength for short exposure times at 1050 °C in both hydrogen and helium through microstructural characterization.

Transmission electron microscopy revealed the presence of a faulted crystalline grain-boundary phase in

samples exposed at 1050 °C for 500 hr (Figs. 7 (a) and (b)). This phase is present after both helium and hydrogen

exposures. Strength reductions of 22-28 percent were observed when the crystalline grain-boundary phase was

present regardless of surface corrosion. Dilatometric analyses indicate that the formation of the crystalline phase

does not alter the thermal expansion behavior. Differential thermal analysis traces in air and helium were featureless.

In contrast to 1050 °C, samples heated to 1250 °C, even for 500 hr, contain no crystalline grain-boundary phase

(Figs. 7 (c) and (d)). Figs. 8 (a) and (b) shows the corresponding XEDS for the grain-boundary and the superimposed mullite phase in K-635 mullite for each temperature. Comparison of Figs. 8 (a) and (b) suggests that the

crystalline grain-boundary phase in the 500 hour/1050 °C sample was rich in calcium and also contains some sodium

in addition to aluminum and silicon. To identify this grain-boundary phase, selected area diffraction (SAD) patterns

were recorded from several crystalline grains in different orientations. In each case, the SAD pattern could be

unambiguously indexed to a calcium aluminosilicate hydride called beidellite, [(Ca, Na)2 AI2Si4Ot0(OH)2]. H20_6_.

Beidellite has an orthorhombic crystal structure with unit cell parameters: a=0.518 nm, b=0.897 nm, and c=1.76

rim. The representative SAD patterns along zone axes [121], [421], and [210] are shown in Figs. 9 (a) to (c)
respectively. Beidellite is a clay mineral which belongs to the montmorillonite group and is capable of absorbing

water molecules between its sheet structure (Ref. 7).

Many high density ceramic compositions contain a glassy grain-boundary phase which, when exposed to

high temperatures, can devitrify to a variety of crystalline phases. In sintered alumina materials it has been found

that calcium facilitates the crystallization of the glassy grain-boundary phase (Ref. 8). It was therefore logical to see

if calcium was the cause of the strength reducing recrystallized grain-boundary glass observed in the sample noted

above. A sample of another high density mullite material, designated mullite-I,3 which contained no detectable

amounts of calcium was exposed to dry hydrogen for 50 hr at 1050 °C. TEM of this sample, Figure 10, reveals only

amorphous grain-boundaries. Figure I 1 shows the XEDS of such a grain-boundary. No calcium is detected. This

observation led us to conclude that as with alumina, calcium plays a significant role in the recrystailization of the

grain-boundary glass phase at 1050 °C. The presence of this recrystallized grain-boundary phase seems detrimental

to the high temperature strength of mullite.

A different strength behavior was observed for samples exposed at 1250 °C. Short time exposure (_<50 hr)

to both hydrogen and helium resulted in a sharp strength increase of about 38 percent. For times > 50 hr, strength of

mullite dropped continuously, but bars exposed to hydrogen for 125 hr were still stronger than the baseline material.

The increase in strength at short exposure times is suspected to be related to crack healing promoted by the flow of

silicate glass on the surface and needs further study. Strength degradation after 500 hr at 1250 °C was approximately

53 percent. Crystallization of the glassy, grain-boundary phase that led to the strength degradation at 1050 °C was

not observed. The strength loss after exposure at 1250 °C is attributed to severe corrosion and actual removal of the

glassy grain boundaries as shown in Fig. 5.

SUMMARY

Near stoichiometric mullite was exposed to pure hydrogen at 1050 and 1250 °C for times up to 500 hr.

I. Exposure to hydrogen for 500 hr at 1050 °C reduced room temperature strength by 22 percent. At

1250 °C the corresponding loss was 53 percent.

_IspsenCeramics,Pecatonica, IL.
2. Strengthlossafterlongexposuretimesisrelatedtotheremoval ofSiO2fromthegrainboundaries(and

fromthemullitegrainsat1250°C).

3. Strengthlossaftershortexposuresat1050°Cisduetotheformationofacrystallinegrain-boundary

phase,beidellite.

4. Inhydrogenous atmospheres, thepresenceofcalciumpromotesthecrystallizationofthegrain-boundary

phase,bedielliteat1050°C.

5. Strengthenhancement aftershortexposuresat1250°Cissuspectedtobedueto healingofsurface

flawsduetoflowofthesilicateglassphaseonthesurface.

CONCLUSIONS

Roomtemperaturefacturestrengthofthemulliteusedinthisstudyisseverelydegradedbylongtime

exposuretohydrogenat1050and1250°C.Corrosion of the grain boundaries occurs at both temperatures and at

1250 °C, even the mullite grains are slightly attacked. The presence of calcium promotes the intermediate temperature (1050 °C in this case) recrystallization of the glassy, grain-boundary phase which degrades the strength of

mullite even at short exposure times. These results indicate that the calcium content of mullite intended for use at

temperatures ca 1050 °C should be as low as possible. Pure mullite without the presence of a secondary glass phase

may be suitable for use in hydrogen below 1250 °C. Production of mullite without a glass phase is, however,

extremely difficult.

REFERENCES

1. R. E. Wright and H. I. Wolf, "Refractory Problems in Production of Hydrogen by Pyrolysis of Natural Gas", J.

Am. Cer. Soc., 31 [2] 31-38 (1948)

2. M.S. Crowley,"Hydrogen-Silica Reactions in Refractories", Bull Am. Cer. Soc., 46 [7] 679-682 (1967)

3. C. I. Chen, "The Reduction of Silica and Mullite in Hydrogen", Ph.D. Thesis, Ohio State University, (1979)

4. S. T. Tso and J. A. Pask, "Reaction of Silicate Glasses and Mullite with Hydrogen Gas", J. Am. Cer. Soc.,

68 [8] 383-387 (1982)

5. S. T. Tso, "The Corrosion of Silicate Materials by Hydrogen Gas and Hydrofluoric Acid Solution", Ph.D. Thesis,

University of California, Lawrence Berkley Laboratory, (1979)
6.JCPDSInternational CenterforDiffractionData,Card19-150,(1979).

7.C.Klein,andC.S.Hurlbut,Jr.,Manual of Minerology, Twentieth Edition, 571, (1977).

8. C. A. Powell-Dogan et.al., "Residual Stress-Induced Grain Pullout in a 96% Alumina", J. Am. Cer. Soc., 74 [3]

646-649 (1991)

Figure 1._Cemmograph of as-received mullite

K-635, etched with 2:1 HCI/Hf:H20.
Figure2.---SEMmicrographsofmulliteK-635barsurfaces

afterexposureinhydrogen.(a)As-received. (b)500hr,

1050°C.(c)500hr,1250°C.

l0
10

O

.m

.m_5

O_

7.7

4.1

I

As-received 125 hr

Exposure condition

9.1

500 hr

Figure 3.--Surface AI/Si ratios (XEDS) of mullite

exposed to hydrogen at 1250 °C.

3.1

¢J

3.0

m 2.9

a

2.8

Error bars = One standard deviation

I I I I

0 100 200 300 400

Exposure time, hr

Figure 4.mDensity of mullite K-635 as a function of

hydrogen exposure at 1250 °C.

5OO

1!
4

Figure 5._BSE micrographs reveal porous reaction layer of mullite K-635 after hydrogen exposure. (a) As-received.

(b) 500 hr, 1050 °C; Y = 4 _m. (c) 125 hr, 1250 °C; Y = 150 I_m. (d) 500 hr, 1050 °C; Y = 300 _m.

]2
Temperature,

oC

• 1050

• 1250

Closed symbols denote hydrogen

350 o. i-- Open symbols denote helium

I(10) 25 Error bars - 95% confidence limit

n-"-_ I_ ('=_) ( ) - sample sizes

O

250

 2o)

_ (17)(10)

2

__ I I I E I I

5O

0 100 200 300 400 500 600

Exposure time, hr

Figure 6.mRoom temperature fracture strength of mullite

K-635 as a function of exposure time.

]3
Figure 7.--TEM micrographs of grain-boundary phase in mullite K-635 after hydrogen exposure. M = mullite,

CG = crystallized glass, G = glass. (a) 500 hr, 1050 °C; bright field. (b) 500 hr, 1050 °C; dark field. (c) 500 hr,

1250 °C; bright field. (d) 500 hr, 1250 °C; dark field.

14
3000

2700

2400

2100

1800

lSOO o

O 1200

9O0

600

3O0

0

0

Na

1

Si

AI

Ca

2 3 4

keV

(a) I

5

1800

1600

1400

1200

"E 1000

o 800 O

6O0

400

200

b

k

0

O

AI

1

Si

Ca

2 3 4 5

keV

Figure 8.--XEDS of grain-boundary phase in mullite K-635

after hydrogen exposure. (a) 500 hr, 1050 °C. (b) 500 hr,

1250 °C.

]5
(a)

e

ii '

,ii •

• • •

• • •

(012)

(101)

(711)

(ooo)

(111)

(101)

(Ol2)

(b)

@0@ O@

00@@@ O@

ooo0@@Ooo

o@OOO o@

@@@@O

(1"_0) (112) (104)

(o_}) (ooo) (o12)

(104) (71_)) (720)

• • •

(121) (120) (12.1)

• • •

(001) (000) (001)

• • •

(121) (120) (121)

(c)

Figure 9.mRepresentative selected area diffraction patterns of crystalline phase formed in mullite K-635

at 1050 °C. Simulated zone axis patterns verify match with beidellite [(Ca, Na)2 AI2Si4010(OH)2]. H20.

(a) Zone axis [127]. (b) Zone axis [421]. (c) Zone axis [270].

16
Figure10.BTEMmicrographsofmullite!afterhydrogenexposureat1050°C.(a)Brightfield.(b)Dark

field.

2400--

2000

1600

C

_ 1200

8OO

O

Si

AI

400

0 __ I__

0 2 3 4

keV

1

Figure 11.--XEDS of grain-boundary phase in mullite !

after hydrogen exposure at 1050 °C.

]7
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

June 1996 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Hot Hydrogen Exposure Degradation of the Strength of Mullite

6. AUTHOR(S)

Thomas P. Herbell, David R. Hull, and Anita Garg

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

WU-242-20-06

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-10091

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM- 107153

11. SUPPLEMENTARY NOTES

Thomas R HerbellandAnita Garg, NASA Lewis Research Center; David R. Hull National Research Council-NASA

Research Associate atLewis Research Center. Responsible person, Thomas R Herbell, o_anizafion code 5130,

(216)433-3246.

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Unclassified - Unlimited

Subject Category 27

This publication is available from the NASA Center for Aerospace Information, (301 ) 621-0390.

13. ABSTRACT (Maximum 200 words)

This study deals with the corrosion of near stoichiometric mullite (3AI203-2SIO2) by pure dry hydrogen gas. Exposure

of the mullite samples was at temperatures of 1050 and 1250 °C for times up to 500 hours. Preferential attack of the

aluminosilicate glass present in the grain boundaries of the mullite occurred after 125 hours at 1250 °C. Hydrogen

scrubbing of the SiO 2 from the glassy grain boundaries and the mullite grains yielded a porous alumina-rich surface. The

room temperature strength increased after short exposure times at 1250 °C (up to 125 hours), then decreased by 53 percent

after exposure for 500 hours. At 1050 °C, all exposure times (25 to 500 hours) decreased the strength. After 500 hours in

hydrogen at 1050 °C, the room temperature strength of mullite decreased 22 percent. We also observed a rapid 25 percent

strength loss after short exposure times at 1050 °C. This is attributed to the calcium/hydrogen assisted crystallization of

the glassy, grain-boundary phase.

14. SUBJECT TERMS

Mullite; Hydrogen exposure; Strength loss; Grain-boundary

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