NASA/TM-- 1999-208911

U.S. ARMY

ARL-TR-1692

RESEARCH LABORATORY

Oxygen Diffusion and Reaction Kinetics

in Continuous Fiber Ceramic

Matrix Composites

Michael C. Halbig

U.S. Army Research Laboratory, Lewis Research Center, Cleveland, Ohio

Andrew J. Eckel

Lewis Research Center, Cleveland, Ohio

James D. Cawley

Case Western Reserve University, Cleveland, Ohio

February 1999
The NASA STI Program Office... in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA's scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA's institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA's counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA's

mission.

Specialized services that complement the STI

Program Office's diverse offerings include

creating custom thesauri, building customized

data bases, organizing and publishing research

results.., even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

at http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at (301) 621-0134

• Telephone the NASAAccess Help Desk at

(301) 621-0390

Write to:

NASA Access Help Desk

NASA Center for AeroSpace Information

7121 Standard Drive

Hanover, MD 21076
NASA/TM-- 1999-208911

U.S. ARMY

ARL-TR-1692

RESEARCH LABORATORY

Oxygen Diffusion and Reaction

in Continuous Fiber Ceramic

Matrix Composites

Kinetics

Michael C. Halbig

U.S. Army Research Laboratory, Lewis Research Center, Cleveland, Ohio

Andrew J. Eckel

Lewis Research Center, Cleveland, Ohio

James D. Cawley

Case Western Reserve University, Cleveland, Ohio

Prepared for the

22nd Annual Conference on Composites, Advanced Ceramics, Materials, and Structures

sponsored by the American Ceramics Society

Cocoa Beach, Florida, January 17-21, 1998

National Aeronautics and

Space Administration

Lewis Research Center

February 1999
Trade names or manufacturers' names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Price Code: A03
OXYGENDIFFUSIONANDREACTIONKINETICSINCONTINUOUSFIBERCERAMIC

MATRIXCOMPOSITES

MichaelC.Halbig,ArmyResearchLaboratory, VehicleTechnologyCenter, 21000Brookpark

Rd.,Cleveland, OH44135

AndrewJ.Eckel,NASALewisResearchCenter,21000BrookparkRd.,Cleveland, OH44135

JamesD.Cawley,CaseWesternReserveUniversity,10900EuclidAve.,500WhiteBld.,

Cleveland, OH44106

ABSTRACT

PreviousstressedoxidationtestsofC/SiCcompositesatelevatedtemperatures (350°Cto

1500°C)andsustainedstresses(69MPaand172MPa)haveledtothedevelopment ofafinite

differencecrackedmatrixmodel.Thetimestofailureinthesamplessuggest oxidationoccurredin

twokineticregimesdefinedbytheratecontrollingmechanisms (i.e.diffusioncontrolledand

reactioncontrolledkinetics).Microstrnctural analysisrevealedpreferential oxidationalongasfabricated, matrixmicrocracksandalsosuggestedtworegimesofoxidationkineticsdependent on

theoxidationtemperature. Basedonexperimental results,observation, andtheory,afinite

differencemodelwasdeveloped. Themodelsimulatesthediffusionofoxygenintoamatrixcrack

bridgedbycarbonfibers.Themodelfacilitatesthestudyoftherelativeimportanceof

temperature, thereactionrateconstant, andthediffusioncoefficient ontheoveralloxidation

kinetics.

INTRODUCTION

Ceramicmatrixcomposites, specificallycontinuouscarbonfibersinasiliconcarbide

matrix,areproposedforhigh-temperature stuctural applicationsduetotheirhigh-strengthto

densityratiosandabilitytowithstandhightemperatures. Hightemperaturecapabilityallows

enginestooperatemoreefficiently,andwithareductionithecomplexcoolingsystemsoften

requiredformetallicalloys.Performancealsoisincreasedbyweightreductionsfromlighter

materials. Fibereinforcement oftheceramicmatrixallowstheinherentlybrittlematerial tofailin

amoregraceful mannerduetocrackbridgingandfiberpull-out.Thesepropertiesmakeceramic

matrixcompositesidealmaterialsforinertenvironments, however, carbonfiberreinforcedceramic

matrixcomposites arepronetocorrosionioxidizingenvironments.

Manystudiesoftheoxidationofcarbonfibersandpyrocarboninterphases inceramic

matrixcomposites involvethermogravimetric analysis(TGA)inwhichweightlossofthe

compositeismonitoredovertime[1,2,3,4]. Thesetypesofstudiescanprovidevaluable

information. However, theoxidationofthecarbonconstituents inthecompositeforunstressed

andstressedstatesiverydifferent.Inanunstressedstate,silicaformsnear1200°Candcracks

closeneartheprocessingtemperature. Theseffectsprotecttheinteriorofthecompositefromthe

outsideenvironment [2,4,5]. However, inrealapplicationconditions, widetemperature ranges

andstresseswillpreventcracksfromsealing.

TheoxidationofcarbonfibersinaSiCmatrixhasbeenstudiedexperimentally inthe

stressedstate[6]. Temperatures rangedfrom350°Cto1500°Candstressesof69MPa(10ksi)or

172MPa(25ksi)wereapplied.AlthoughweightcouldnotbemonitoredasinTGAexperiments,

thetimesandstrainstofailureofthecomposites providedanindicationoftheextentofoxidation.

NASA/TM--1999-208911 1
Timestofailuresuggestedtworegionsofoxidationkineticswithastrongdependence on

temperature. Inthetemperaturerangeof350°C-750°C, timestofailuredroppedsharplyas

temperaturewasincreased. Sampleswentfromsurvivingbeyond25hr.withahighresidual

strengthofailingin91minutesat69MPaandin22minutesatthehigherstressof172MPafor

temperatures of750°C.Inthehighertemperaturerangeof1000°C-1500°C, temperature

differencesdidnothaveassignificant aneffectonsampleife. Samplelivesremainednearthe

samelevelasthoseat750°Cwithsomedeviationsintimestofailureacrossthehighertemperature

range.

Theeffectsofoxidationwerealsodeterminedfromanalysisofpolishedcross-sections

andfracturesurfacesusingopticalandscanningelectronmicroscopes. Oxidationwasobservedto

occurpreferentiallyalongmicrocracks. Polishedcross-sections ofsamplestestedinthelower

temperature rangeshowedoxidationwasrelativelyminimalandoccurredinarough,uneven

patterneartheedges,butoxidationwasalsoobserveddeepintotheinteriorofthesection,

suggestingslowoxidationkinetics.Sectionsfromsamplestestedinthehighertemperaturerange

showedoxidationoccurringprimarilyalongtheedgesinauniformmannersothatareactionfront

couldbeseen.Thisgaveashrinkingcoretypeofeffectasouterfiberswereconsumedasthe

oxidationprocessmovedinward.Thispatternalongwiththeobservationthatfibersdeepinthe

interiorofthesampleremainedfreeofattacksuggeststheoxidationkineticsaremuchfasterthan

inthelowertemperaturerange.Thischangeintheratecontrollingmechanismfromthelower

temperatureregimetothehighertemperature regimehasbeenattributedtoashiftfromthekinetics

beingreactionratelimitedtobeingdiffusionlimited.Thesexperimental resultswereusedto

provideabasisfordevelopingamodelsothatabetterunderstanding ofdiffusioncontrolledand

reactioncontrolledkineticscouldbegained.

A finite difference model was developed which simulates the diffusion of oxygen into a

matrix crack bridged by an array of carbon fibers. The model provides a visual, qualitative and

quantitative basis for understanding the oxidation kinetics. The influence of important variables:

temperature (T), diffusion coefficient (D), and reaction rate constant (K), are studied.

By obtaining a better understanding of the oxidation kinetics of carbon fibers in a ceramic

matrix, the best approaches for protecting against oxidation can be determined depending on the

application conditions: environment, temperature, stresses and required component lives. One

possible approach for protecting the fibers might involve applying a less reactive interphase to the

fiber. Another may be to include an oxidation inhibitor in the matrix that will react with oxygen to

form oxides or glasses to seal cracks or to consume oxygen to provide more time before the carbon

is attacked. Still another approach could be to identify external seal coatings that can better protect

the interior of the material. One approach may be preferred over another due to exposure

conditions, cost and/or mission requirements.

THEORY

The related theory used in developing the finite difference, cracked matrix model will be

briefly discussed. In a ceramic matrix composite, oxygen diffuses down a pore or crack and reacts

with carbon to form an oxide (either carbon monoxide or carbon dioxide). The oxide then diffuses

out of the pore to the atmosphere. Non-Reactive Matrix

See Figure 1. _

02 _ _

O2+ C --->CO2 91 CO or C02 Jj Carbon

02 + 2C --9 2CO NNNNNNNNNNN_ _

Figure 1. Oxygen diffusing into a pore and reacting with carbon.

NASA/TM--1999-208911 2
Theoxidationkineticsfortherecessionofcarboncanbedescribedbythelinearparabolicratelaw:

x_ + x = t, (1)

kp

where x (m) is the recession distance of the carbon, kp is the parabolic rate constant (m2/sec), kl is

the linear rate constant (m/sec) and t is time (sec).

The parabolic and linear rate constants can be related to more familiar mass transport

constants, the diffusion coefficient and the reaction rate constant respectively. Only the relevant

equations will be discussed. More thorough discussions and derivations can be found in papers by

Eckel et al. [7] and Glime et al. [8].

When the linear term in equation (1) dominates and carbon dioxide is the evolving oxide,

the equation for the recession distance is expressed as [7]:

x = t 1/2 kp 1/2 = t 1/2 (2D Cox / Pc ) such that kp = (2D Cox / Pc ) (2)

where D is the diffusion coefficient of oxygen in the pore (m 2/ sec), Cox is the oxygen

concentration of the atmosphere (tool / ms ) and Pc is the molar density of carbon (tool / mS).

In the case where carbon monoxide is the reaction product, Equation (2) for the recession

distance will have a different form. However, the particular oxide that forms and the exact value

for the diffusion coefficient is not of critical importance in this discussion and in the model. The

main concern at this point in the development of the model is to identify trends. The discussion

does not consider Knudsen effects in which molecular-wall collisions are more frequent than intermolecular collisions. Knudsen effects are not a factor until the pore becomes as small as 0.1 gm

[7]. In the material studied, typical cracks in the matrix and seal coating are on the order of 1 gm.

The second limiting case occurs when kp is much larger than kl. The recession distance

can be expressed as [8]:

x = t k_ = t K Cox / Pc such thatk_ = K Cox / Pc (3).

The above equation takes an alternate form when the oxygen concentration is substituted

by the oxygen partial pressure, Z, times the total concentration of gas molecules in the atmosphere,

CT (mol/mS). The total concentration can be substituted using the ideal gas law so that Equation

(3) becomes [71:

k1---- (1/ 0c )(_ CT ) (K) = (1/Pc) ( Z P/RT) K (4)

where P is pressure (Pa), R is the gas constant (J/mol K), and T is the absolute temperature (K).

Substituting values for kp and kl, Equation (1) becomes

t = x_ + x = x2 + x (5).

kp k_ (2D Cox / Pc ) (1/pc)( zP/RT)K

The diffusion coefficient, D, can be obtained from Chapman-Enskog kinetic theory [9] and the

reaction rate constant, K, can be expressed in an Arrhenius form:

NASA/TM--1999-208911 3
DAB= (5.9543X1024)Ts/2 (1 / M la__MJ__]__MB_)_ + ,

P CY2An-Q-'AB

K = 1%exp(-Q/RT) (6) & (7)

where DAB (m 2/ sec) is the diffusion coefficient of gas A (oxygen) diffusing in the stationary or

stagnant atmosphere of gas B (oxide), T is temperature (K), MA and MB are the molecular weights

of gases A and B (kg/mol), and P is pressure (Pa). The symbols CYAB(m) and _2AB(dimensionless)

are Leonard Jones potentials for the collision diameter and collision integral respectively. Q is the

activation energy (J/tool).

From the above equations for the diffusion coefficient (6) and the reaction rate constant

(7), it is seen that one of the two arguments in Equation (5) will dominate depending on

temperature. The diffusion coefficient depends on Ts/2, with the effect being even less when the

decreasing molar density of the gas is considered with increasing temperature. The reaction rate

constant varies exponentially with temperature when large activation energies are considered. It

can be seen that Equation (5) describes a two step process occurring in series. Each step has the

potential to limit the overall process. When the reaction rate constant is small and the diffusion

coefficient is large, the process will be limited by the reaction rate, i.e. reaction controlled.

Oxygen will diffuse into the pore faster than it can react so that the pore becomes saturated in

oxygen. However, if the diffusion coefficient is small compared to the reaction rate constant,

carbon/oxygen reactions occur as soon as oxygen is supplied. In this case the process is controlled

by the slower diffusion step, i.e. diffusion controlled.

MODELING

In order to gain a better understanding of the diffusion and reaction kinetics observed in

the experiments, a finite difference model was developed. The physical setup of the model is an

array of carbon fibers bridging a matrix crack, as illustrated in Figure 2. Figure 3 shows an

example of an experimental observation of oxidation occurring along fibers bridging a crack,

which is the physical situation on which the model is based.

Figure 2. Fibers bridging a cracked matrix.

Figure 3. Oxidation of carbon fibers that bridge

matrix cracks.

NASA/TM--1999-208911 4
A2-Dimensional gridpatternwaslaidoutasa250x250meshofgrids,whichrepresents

anopenplanewithinthebridgedspaceparalleltothetwocracksurfacesofthematrix.Grids

withinthemesharedesignatedaseitheropenspacecontainingthediffusingases, orascarbon

containinggrids.Carbonfibersarerepresented asa12x12arrayoffiberswhichhavediameterof

10grids.Fibersarespacedadistanceof 10gridsfromoneanotherandfromtheouteredge.One

quarterofthegridpatternisshowninFigure4.

Corner 1

Midline _

Across

Smnple y

Midline t

X

Figure 4. One quarter plot of the matrix pattern (right) orientated as shown on the left.

In the model, the outer boundary has a initial oxygen concentration arbitrarily set at 1.0

mol/cm s, which is assumed to stay constant due to the atmospheric conditions. Oxygen diffuses

into the matrix. Concentrations are calculated throughout the matrix based on oxidation kinetics in

which both reactivity and diffusion are both dependent on temperature and one may be limiting the

overall process. Plots of oxygen concentration as a function of grid position showing gradients

high or low in oxygen concentration will give clues to the limiting step in the two step process.

When mass transport occurs by convection, the flux is given by,

Flux = J = K (CA,1 - C_ )

where CA,1(which has a value of 0 mol/cm s) is the oxygen concentration at the reaction front or of

the carbon edge (carbon containing grid) and C_ is the oxygen concentration in the bulk gas

(oxygen containing grid).

When mass transport occurs by diffusion, the flux is given by,

Flux = J = D (C2 - C1 ) / Ax

where (C2 - C1 ) is the difference in oxygen concentration within the stagnant gas across some

distance, Ax. An important unitless parameter, the Sherwood number, Sh, is obtained from the

ratio of the fluxes per the driving force due to concentration differences,

Sh = F(convection)/AC / F(diffusion)/AC = K Ax / D

The Sherwood number will be used extensively throughout the model. For low Sh values, low

temperatures, the kinetics are reaction controlled, and for high Sh values, high temperatures, the

kinetics are diffusion controlled.

NASA/TM--1999-208911 5
Detailofthegridpatternshowinghalfofacarbonfiberandopenporespaceisillustrated

inFigure5. Theoxygenconcentration ofagridisdeterminedbysamplingfromitsfourneighbors.

Thefluxesbetweenthecentral gridandits4neighborsareadded.Thenetfluxintoandoutofthe

centralgridiszero.Thefluxequationisthen

solvedfortheconcentration ofthecentral

grid.Whenallfourneighborsarewithinthe

stagnant gasasforgrid1,masstransport

occursbydiffusionandtheequationforthe

oxygenconcentration ofthisgridis

C(i,j)=sum of oxygen concentrations

4

When one of the neighboring grids contains

carbon, as for grid 2 in Figure 5, mass

transport occurs through convection between

the central grid and the carbon containing

grid. For the other three grids mass transport

occurs by diffusion. The equation for the

oxygen concentration of the central grid becomes

(i,j-1)

(i-l,j)

@ (i,j+l)

(i+l,j)

i

Figure 5. Grid pattern for model.

C(i,j) = [C(i,j-1)+C(i-l,j)+C(i,j+l)l + Sh*C(i+ld)

(3+Sh)

When two neighbors contain carbon and two contain the stagnant gas, as in grid 3, the equation

becomes

COd) = lcfi,i-1)+cfi-l,i)l + ShlCfi,i+l)+Cfi+l,i)l

(2+2Sh)

The program is run for several different Sh values: 0.0001, 0.001, 0.01, 0.1, 1, 10.

Oxygen concentrations are determined throughout the bridged section based on the boundary

condition and the equations for oxygen concentration. The program runs until a quasi-steady state

is reached. Note the program does not remove carbon by reaction at this point; future development

of the program will remove carbon and continuously recalculate oxygen concentrations so that

oxidation patterns relating to diffusion controlled and reaction controlled kinetics are obtained.

The program may be thought of at this point as having a continuous supply of oxygen and carbon,

where oxygen concentrations throughout the section are calculated until steady state is reached. It

should be pointed out that since oxidation really occurs in 3-dimensions, the 2- dimensional model

does not consider or allow the analysis of certain factors. These include 3rd dimensional effects

such as fiber recession and the supply of oxygen through open channels where the carbon fiber has

been consumed.

The obtained data files for a given Sherwood number were used to prepare plots of the

oxygen concentrations across the interior of the fiber bridged matrix. Gradients in oxygen

concentration across the bridged section show regions where oxygen supply is low or high and

give indications about the reactivity. Figure 6 shows the oxygen concentrations for one-quarter of

the whole section for a relatively high Sherwood number, Sh=0.1. The 3-D effect of the plot is due

to plotting oxygen concentrations across the 2-D surface. It is seen that oxygen concentrations

drop sharply upon diffusion into the matrix. This suggests that the carbon is very reactive and

reacts with oxygen as soon as it is supplied. The overall oxidation process is limited by the slow

supply of oxygen to the reaction process at the carbon/oxygen interface, i.e. diffusion controlled

kinetics. The case for a relatively low Sherwood number, Sh=0.001, is shown in Figure 7. The

plot shows that the reactions are slow so that oxygen is able to diffuse into the interior in high

NASA/TM--1999-208911 6
concentrations and saturate the interior. In this case, the two step process of oxygen diffusion and

carbon/oxygen reactions is limited by the slower reaction process, reaction controlled kinetics.

Distance Into the Matrix Distance Into the Matrix

Figure 6 (left) Sh=0.1. Figure 7 (right) Sh=0.001. One-quarter plots of a 250x250 matrix with a 12x12 fiber

array. Interated to equilibrium.

In the two previous figures, the curves at the edges of the plots represent the oxygen

concentration through one-half of the thickness of the section. Full plots of the oxygen

concentrations across the midline are shown in Figure 8 for several Sherwood numbers. High Sh

values illustrate how the process is diffusion controlled. Oxygen is quickly consumed before it can

enter into the interior of the matrix. Low Sh values illustrate how the kinetics are reaction

controlled. Carbon/oxygen reactions are slow so that there is a high concentration of oxygen

throughout the section.

1

0.9

0.8

_0.7

_0.6

o

_0.5

o

80.4

_0.3

0.2

0.1

nd 10

ower)

i i i i i i

0 50 100 150 200 250

Position Across Matrix (0-250)

Sh = Ax K

D

Reaction Controlled

Diffusion Controlled

Figure 8. Oxygen concentrations across the center of the surface for several Sherwood (Sh) values.

SUMMARY

Experimental studies on C/SiC composite material exposed to stressed oxidation

conditions suggested two types of oxidation kinetics. Lives of the samples were very temperature

NASA/TM--1999-208911 7
dependent in the range of 350°C-750°C, while there was less of a temperature dependence in the

range of 1000°C-1500°C. Microstructural analysis revealed a different type of oxidation pattern

for each temperature range. The related theories for carbon recession, reaction kinetics, diffusion

kinetics, flux due to mass transport, and Sherwood numbers were reviewed. Temperature effects

on the reaction rate constant and the diffusion coefficient were also discussed. Based on

experimental observation, and theory, a finite difference model was presented.

CONCLUSIONS

The oxidation of carbon fibers in a ceramic matrix is a two step process with a varying

temperature dependence. The overall process will be limited by the slower step. The kinetics can

be analyzed by studying the diffusion of oxygen and determining its concentration throughout the

section. The finite difference model is in good agreement with experimental results in suggesting

two types of kinetics. The model supports the conclusion that the kinetics are reaction controlled

at low temperatures, and diffusion controlled at high temperatures.

The next steps in the model development include the removal of carbon as it reacts with

oxygen. In the case of diffusion controlled kinetics, i.e. high Sherwood number, oxygen will

oxidize away the outer fibers first, then continue to move inward so that a shrinking core effect is

seen. For reaction controlled kinetics, i.e. low Sherwood number, there will be a generalized

oxidation of fibers throughout the section. Another developmental step for the model will include

putting an element around the fibers to represent the interphase. Different reactivities of the fiber

and interphase can then be considered.

REFERENCES

1. S. Goujard, L. Vandenbuckle, H. Tawil, "The Oxidation Behavior of Two- and ThreeDimensional C/SiC Thermostructural Materials protected by Chemical Vapor Deposition

Polylayers Coatings," Journal of Materials Science 29, pp. 6212-6220, (1994).

2. Frank Lamouroux, Gerald Camas, and Jacques Thebault, "Kinetics and Mechanisms of

Oxidation of 2D Woven C/SiC Composites: I, Experimental Approach," J. Am. Ceram. Soc., 77

[8] 2049-57 (1994).

3. F. Lamouroux, X. Bourrat, R. Nasalain, J. Sevely, "Structure/Oxidation Behavior Relationship

in the Carbonaceous Constituents of 2-D/PyC/SiC Composites," Carbon, Vol. 31, No. 8, pp. 1273-

1288, 1993.

4. Ludovic Filipuzzi, Gerald Camus, and Roger Naslaim, "Oxidation Mechanisms and Kinetics of

1D-SiC/C/SiC Composite Materials: 1, An Experimental Approach," J. Am. Ceram. Soc., 77 [2],

pp. 459-466, 1994.

5. C. Vix -Guterl, J. Lahaye and P. Ehrburger, "Reactivity of Silicon Carbide and Carbon with

Oxygen in Thermostructural Composites," Carbon, Vol. 31, No. 4, pp. 629-635, 1993.

6. Michael C. Halbig, Andrew J. Eckel, James D. Cawley and David N. Brewer, "Stressed

Oxidation of C/SiC Composites," Proceedings of the 21 StAnnual Conference on Composites,

Advanced Ceramics, Materials and Structures, Cocoa Beach, Florida, January 12-16, 1997.

7. Andrew J. Eckel, James D. Cawley and Triplicane A. Parthasarathy, "Oxidation Kinetics of a

Continuous Carbon Phase in a Nonreactive Matrix," J. Am. Ceram. Soc., 78 [4] 972-80 (1995).

8. William H. Glime and James D. Cawley, "Oxidation of Carbon Fibers and Films in Ceramic

Matrix Composites: A Weak Link Process," Carbon, Vol. 33, No. 8, pp. 1053-1060, 1995.

9. W. Geankoplis, Mass Transport Phenomena, Ohio State U. Bookstore, Columbus, OH, 1978.

NASA/TM--1999-208911 8
Form Approved REPORT DOCUMENTATION PAGE

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

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

February 1999 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Oxygen Diffusion and Reaction Kinetics in Continuous Fiber

Ceramic Matrix Composites

6. AUTHOR(S)

Michael C. Halbig, Andrew J. Eckel, and James D. Cawley

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

NASA Lewis Research Center

Cleveland, Ohio 44135 3191

and

U.S. Amly Resem'ch LaboratoC¢

Cleveland, Ohio 44135 3191

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

National Aeronautics and Space Administration

Washington, DC 20546 0001

and

U.S. Amly Resem'ch LaboratoC¢

Adelphi, Maryland 20783 1145

WU-523-21-13-00

1L161102AH45

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-11540

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1999-208911

ARL-TR-1692

11. SUPPLEMENTARY NOTES

Prepared for the 22nd Annual Conference on Composites, Advanced Ceramics, Materials, and Structures sponsored by

the American Ceramics Society, Cocoa Beach, Florida, January 17-21, 1998. Michael C. Halbig, U.S. Army Research

Laboratory, NASA Lewis Research Center; Andrew J. Eckel, NASA Lewis Research Center; James D. Cawley, Case

Western Reserve University, 10900 Euclid Avenue, 500 White Building, Cleveland, Ohio 44106. Responsible person,

Michael C. Halbig, organization code 5130, (216) 433-2651.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: Categories: 27 and 64 Distribution: Nonstandard

This publication is available from the NASA Center for AeroSpace Information, (301) 6214)390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Previous stressed oxidation tests of C/SiC composites at elevated temperatures (350°C to 1500°C) and sustained stresses

(69 MPa and 172 MPa) have led to the development of a finite difference cracked matrix model. The times to failure in

the samples suggest oxidation occurred in two kinetic regimes defined by the rate controlling mechanisms (i.e. diffusion

controlled and reaction controlled kinetics). Microstructural analysis revealed preferential oxidation along as-fabricated,

matrix microcracks and also suggested two regimes of oxidation kinetics dependent on the oxidation temperature. Based

on experimental results, observation, and theory, a finite difference model was developed. The model simulates the

diffusion of oxygen into a matrix crack bridged by carbon fibers. The model facilitates the study of the relative importance of temperature, the reaction rate constant, and the diffusion coefficient on the overall oxidation kinetics.

14. SUBJECT TERMS

Ceramic matrix composites; Oxidation kinetics

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

15. NUMBER OF PAGES

14

16. PRICE CODE

A03

18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF THIS PAGE OF ABSTRACT

Unclassified Unclassified

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-1B

298-102