NASA/TM--2001-210695/REV1

Microstructure and Tensile Properties

of BN/SiC Coated Hi-Nicalon, and

Sylramic SiC Fiber Preforms

Ramakrishna T. Bhatt

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

Yuan L. Chen

Dynacs Engineering Company, Inc., Brook Park, Ohio

Gregory N. Morscher

Ohio Aerospace Institute, Brook Park, Ohio

August 2002
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

databases, 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 ht@:llwww.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 NASA Access 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--2001-210695/REV1

Microstructure and Tensile Properties

of BN/SiC Coated Hi-Nicalon, and

Sylramic SiC Fiber Preforms

Ramakrishna T. Bhatt

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

Yuan L. Chen

Dynacs Engineering Company, Inc., Brook Park, Ohio

Gregory N. Morscher

Ohio Aerospace Institute, Brook Park, Ohio

National Aeronautics and

Space Administration

Glenn Research Center

August 2002
Document Change History

This printing, numbered as NASA/TM--2001-210695/REV1, August 2002, replaces the previous version

in its entirety, NASAFFM--2001-210695, March 2001.

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

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov
MICROSTRUCTURE AND TENSILE PROPERTIES OF BN/SiC COATED

Hi-NICALON, AND SYLRAMIC SiC FIBER PREFORMS

Ramakrishna T. Bhatt

U.S. Army Research Laboratory

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Yuan L. Chen

Dynacs Engineering Company, Inc.

Brook Park, Ohio 44142

Gregory N. Morscher

Ohio Aerospace Institute

Brook Park, Ohio 44142

SUMMARY

Batch to batch and within batch variations, and the influence of fiber architecture on room temperature physical

and tensile properties of BN/SiC coated Hi-Nicalon and Sylramic SiC fiber preform specimens were determined.

The three fiber architectures studied were plain weave (PW), 5-harness satin (5HS), and 8-harness satin (8HS).

Results indicate that the physical properties vary up to 10 percent within a batch, and up to 20 percent between

batches of preforms. Load-reload (Hysteresis) and acoustic emission methods were used to analyze damage accumulation occurring during tensile loading. Early acoustic emission activity, before observable hysteretic behavior, indicates that the damage starts with the formation of nonbridged tunnel cracks. These cracks then propagate and

intersect the load bearing "0 °'' fibers giving rise to hysteretic behavior. For the Hi-Nicalon preform specimens, the

onset of "0 °'' bundle cracking stress and strain appeared to be independent of the fiber architecture. Also, the "0 °''

fiber bundle cracking strain remained nearly the same for the preform specimens of both fiber types. TEM analysis

indicates that the CVI BN interface coating is mostly amorphous and contains carbon and oxygen impurities, and the

CVI SiC coating is crystalline. No reaction exists between the CVI BN and SiC coating.

1. _TRODUCTION

Preforms of SiC fibers are being used as starting material for the fabrication of SiC- and A1203-based composites. Such composites have potential for high temperature materials applications including for future generations of

aircraft engines because of their high temperature strength (refs. 1 and 2). The preforms are generally fabricated by

cutting the two-dimensional woven fiber cloth to the required dimensions, stacking the woven cloth, and then depositing interface coating(s) into the cloth assembly by chemical vapor infiltration (CVI). The thickness and composition of the interface coating(s) depends upon the subsequent composite processing method. After CVI SiC

infiltration, the preforms are rigid and porous. The interconnected open porosity in the preforms can be filled with

matrix by the melt infiltration (MI), sol gel, or polymer infiltration pyrolysis methods.

For the fabrication of SiC/SiC composites by the nonreactive melt infiltration method, the SiC preforms are

prepared by first depositing a layer of boron nitride (BN) and then an outer layer of SiC. The BN/SiC coated preforms are subsequently infiltrated with a slurry of silicon carbide particles and then with liquid silicon (ref. 3). This

processing method is scalable for the fabrication of complex, near net shape components with minimum damage to

the fibers and interface. However, the state-of-the-art SiC/SiC composites show reproducibility and reliability problems which significantly limit their use temperature to 1200 °C (refs. 2 and 3). The poor reproducibility of mechanical properties in this system is possibly due to batch to batch variations in properties of the starting material, i.e., the

SiC preforms, or due to poor process control during the melt infiltration. To understand the source of reproducibility

and reliability problems, the current study was performed.

NASA/TM--2001-210695/REV 1 1
Theobjectivesofthisstudywereseveral: (1),todeterminethebatch-to-batch variationsinroomtemperature

physical andmechanical properties, andmicrostructure oftheSiCpreformstypicallyusedforSiC/SiCcomposites;

(2),todeterminetheinfluenceoffiberarchitecture ontensileproperties; (3)todeterminethedamageaccumulation

inthepreforms; (4),tosuggest theinfluenceofpreformcharacteristics ontheprocessingandpropertiesoftheSiC/

SiCcomposites fabricatedbytheMIapproach.

2.EXPERIMENTAL PROCEDURE

ThepreformsofBN/SiCcoatedHi-NicalonTM and Sylramic TM SiC fibers were purchased from Honeywell

Advanced Composites, Inc., Delaware. The Nippon Carbon Company, Japan, and the Dow Corning Company,

Midland, fabricated the Hi-Nicalon and Sylramic fibers, respectively. Albany International Techniweave, Inc.,

Rochester, NY, performed the weaving of the fiber tows into mats. In the case of the Hi-Nicalon SiC fiber preforms

three types of weaves namely the plain weave (PW), 5 harness satin (5HS), and 8HS were studied. Whereas in the

case of Sylramic SiC preforms, only 5HS weave was investigated. The preforms contained 8 layers of two-dimensional woven SiC fiber mats and each fiber tow in the mat contained a dual layer of ~0.5 gm thick BN followed by a

layer of 3 to 5 gm thick SiC. The nominal dimensions of the preforms after CVI coatings were 229- (L) x 152- (W)

x 2-mm (T).

The preforms were cut into smaller specimens of dimensions 152- (L) x 12.5- (W) x 2-mm (T) using a diamond

impregnated metal bonded cutoff wheel and water based grinding fluid. The cut specimens were degreased and then

dried in an oven. The Archimedes and bulk densities of specimens were measured. For Archimedes density measurements, methyl ethyl ketone liquid was used.

To avoid delamination and compressive failure within the grips of the tensile testing machine, the preforms

specimens were impregnated with epoxy. Epoxy infiltration was performed in a vacuum chamber at room temperature. The excess epoxy layer present on the surface of the vacuum impregnated specimens was removed by grinding

the specimens on a 200-grit emery paper. A few unreinforced epoxy specimens of dimensions similar to that of

epoxy infiltrated preform specimens were also prepared and tensile tested to determine their base line tensile

properties.

For tensile testing, dog-bone shaped specimens were machined from the epoxy-infiltrated preform specimens by

using an ultrasonic SiC slurry impact machine. At each specimen end, two glass fiber-reinforced epoxy tabs of

dimensions 37 x 12 x 1-mm were bonded, leaving ~78 mm between grips. A spring-loaded clip-on gauge was

attached to the 25-mm length straight section of the dog bone specimen to monitor the displacement during the tensile test. The specimens were tested at room temperature until failure in a servo-controlled tensile testing machine

equipped with self-aligning grips at a crosshead speed of 1.3 mm/min.

In some specimens, the damage occurring during tensile deformation was monitored by two different methods:

the hysteresis loop and acoustic emission. In the first method, the specimen was initially loaded to a predetermined

load level below the stress corresponding to deviation from linearity (DFL) and then fully unloaded. The specimen

was then reloaded to a slightly higher load level and unloaded. This procedure was continued until ultimate failure of

the specimen. After the test, the maximum loop width was determined for the different peak stress hysteresis loops.

Acoustic emission was recorded with two wide-band (50 kHz to 2.0 MHz), high fidelity sensors (Model a1025)

that were placed outside the tapered region of the dog-bone specimen. Vacuum grease was used as a couplant, and

alligator clamps were used to mount sensors to the specimen. Acoustic emission waveforms were recorded using a

2-channel, Fracture Wave Detector (FWD). The FWD consisted of a computer (Pentium, 233 MHz) with a 12-bit,

30 MHz analog to digital acquisition board. Each sensor was connected to a preamplifier and filter trigger module,

which was fed into the computer. The preamplifier was set at 20 dB; the filter signal was amplified

3 dB. The load and strain were also recorded with the FWD computer. The post-test analysis was performed on

Wave Detector TM software provided by the FWD manufacturer.

For optical microscopy, some of the specimens were mounted in a metallographic mold, ground successively on

40 gm down to 3 gm diamond particle impregnated metal disks, and polished in a vibratory polisher on a micro

cloth using a 0.3 gm diamond powder paste.

aDigitalWave Corporation,Englewood,Colorado.

NASA/TM--2001- 210695/REV 1 2
Fortransmission electronmicroscope(TEM)analysis, thepreformspecimens wereslicedusingadiamondsaw.

Subsequently, eachspecimenwassandwiched betweentwosacrificial siliconwafers, vacuumdegassed andimpregnatedwithepoxy.Aftercuring,thesandwichassemblywassectionedintosmallerspecimens ofdimensions

2.5x 2x 0.5mm.Afterpolishingoneofthesidesofthe2.5x 2mmsurfaceforflatness, theoppositesideofthe

specimenwasthinnedtoelectrontransparency usingtripod-polishing (ref.4).Aspolishingprogresses, thethickness

oftheassemblywasperiodicallymonitoredusingareflectedlightmicroscope. Asthethicknessofthesacrificial

siliconwaferdecreased, itscoloralsochanged. Whenthecolorofthesacrificial siliconturnedfromorangetoyellow,thepolishingwasstopped. Tripodpolishingmakesitpossibletomechanically polishalargeareaofaverythin

specimenwithnoorminimummechanical damage. Thesuperficial mechanical damageseenonsomespecimens

wasremovedbylowangle(<7°)argonion-beammillingforabout10to20min.Thepresent transmission electron

microscopy resultsincludeimagingandquantitativeanalysesofthetransverse cross-section ofthespecimens. In

thismode,thefiberswereobserved"end-on."Diffractioncontrast andphasecontrast images,elected-area, and

microdiffraction datawererecordedusingaPhilipsCM200electronmicroscopeoperatedat200kV.TheTEMis

coupledtoanenergydispersiveX-ray(EDX)spectrometer tbrchemical identification.

3.RESULTSANDDISCUSSION

Becauseofitslowviscosity, epoxyuniformlyfillstheavailableopenporespaceinthepreformspecimens.

Typicalcross-sections ofanepoxyinfiltrated5HSwovenBN/SiCcoatedHi-NicalonSiCfiberpreformspecimens

areshowninfigure1.Figurel(a)indicatesthatheHi-Nicalontowsareoblongandshowgapsbetweenthefiber

tows.Ahighermagnification photographofthecross-section (fig.l(b))showsthathediameter oftheSiCfibers

withinthetowvariesfrom10to16bun. The dark ring around the SiC fibers in figure l(b) is the BN coating. The

gray coating on top of the dark ring represents the CVI SiC coating. When fibers are well separated, the BN coating

is seen on the fibers, but when fibers clumped together, the BN coating is not visible. Also noticed are a few isolated

regions of closed porosity within the coated tows. In many regions, the 0° tow of one ply rests on 0° tow of the adjacent ply and the 90 ° tow rests on 90° tow with no CVI SiC coating between the tows. Optical examination of the

cross-section of the epoxy infiltrated 5HS woven BN/SiC Sylramic SiC fiber preforms indicates that the fibers are

tightly packed inside the tows, and have a relatively uniform diameter of -9 bun, discounting a few smaller diameter

fibers. The BN coating is relatively uniform around the fiber, and the thickness of the SiC coating increases from the

center to the outer periphery of the tow. On many tows, a thick SiC layer was noticed on the outer periphery, completely surrounding the tows. The coated tows contain nearly 2 to 8 vol % of closed porosity, which is not even

accessible for the epoxy infiltration.

The TEM micrographs of the cross-section of the BN/SiC coated Hi-Nicalon and Sylramic SiC fiber preforms

are shown in figure 3. In both preforms, the BN coating is mostly amorphous and the SiC coating is crystalline. The

SiC coating is composed of columnar grains that are aligned radially around the SiC fibers. In some Hi-Nicalon

preform specimens, a graded region of fine grained SiC and BN was noticed between the BN and SiC coating. In

contrast, the boundary between the BN and SiC coating is sharp and distinct in the Sylramic SiC preform specimens.

Quantitative EDX analysis indicates that the BN coating contains up to 8 percent carbon and oxygen impurities.

The variations in Archimedes and bulk densities, and volume percent open porosity within and between batches

of the PW, 5HS, and 8HS woven Hi-Nicalon and 5HS Sylramic SiC preforms were measured. The data for 5HS

preforms are shown in figures 4 to 6. In some batches, only one specimen was tested, where as in other batches up to

17 specimens were tested. The scatter band is shown for batches where a statistically significant number of specimens were tested. The scatter band represents two standard deviations. The Archimedes density within a batch is

uniform, but between batches, it varies up to 10 percent. This suggests variation in closed porosity or the SiC coating thickness, or both. In fact, the optical micrographs of the cross-section of different batches of preforms show

closed porosity variations between 2 and 8 percent, and CVI SiC coating thickness variations from 2 to 5 gm. In

contrast, the variations in bulk density and volume percent porosity were significant both within a batch as well as

between the batches (figs. 5 and 6). Some of the variation in physical density can be attributed to error in thickness

measurement of the preform specimens because of the nodular surface. To minimize the error, the thickness measurements were made at fifteen different locations along the length of the specimen to obtain an average value. The

physical property data for the preforms are summarized in table I. The scatter band in the data values represents two

standard deviations. The range of values measured is shown in the brackets. When Hi-Nicalon preform specimens

are compared, the average Archimedes and physical density values for 5HS are generally higher than the those of

NASA/TM--2001-210695/REV 1 3
theothertwopreformspecimens, whereastheaverageporefractionof5HSpreformislowerthanthoseoftheother

twopreformspecimens. TheaverageArchimedes densityvaluesforthePW,5HS,and8HSwovenHi-Nicalon

preformspecimens aregenerallyowerthanthoseforthe5HSwovenSylramicSiCpreformsprimarilyduetothe

densitydifferencebetweentheHi-Nicalon-2.74 gm/cc(ref.5)andSylramicSiCfibers-3.0gm/cc(ref.6).Other

noticeablepointsarethathestandarddeviationandthedifferenceinextremevaluesremainnearlythesamefor

bothfibertypesforthesamearchitecture.

Attemptsweremadetodeterminesizerangeofinterconnected porosityinthepreformsbymercuryporosimetry

andnitrogenadsorptiontechniques. Inthecaseoftheporosimeter, mercuryeasilyfilledthepreformatstandard

temperature andpressureconditions, makingfurtherinfiltrationofmercuryunderpressureimpossible. Ontheother

hand,poresizedetermination bytheadsorptiontechniquecouldnotbeusedbecausepreformsurfacerepellednitrogengas.

Theroomtemperature tensilestress-strain curvesfortheepoxyinfiltratedPW,5HS,and8HSHi-NicalonSiC

preformspecimens areshowninfigure7.Forcomparison purposes, includedinthefigureisthestress-strain curve

fortheunreintbrced epoxy.Clearly,themodulusandUTStbrtheunreinforced epoxyissignificantlylowerthan

thosefortheepoxyinfiltratedpreformspecimens. Thissuggests thatheepoxymatrixdoesnotinfluencethedeformationbehavior andtensilepropertiesofthepreformspecimens. Infigure7,allthreeweavesdisplayedsimilarfeaturesonthestress-strain curves: Aninitialinearregionfollowedbyanonlinear region.Thestresscorresponding to

deviationfromlinearity(DFL)forthe5HSandPWspecimens appearstobethesame, buttheDFLvalueforthe

8HSpreformspecimens islightlylowerthanthatoftheothertwoweaves. AlsonoticedisthatheUTSof8HS

preformspecimens isgreaterthanthatoftheotherweaves, andelasticmodulusofthe5HSandPWspecimens is

nearlythesame.

Althoughultimatetensilestrengthandfailurestrainforthe5HSSylramicpreformspecimens arerelatively

lowerthanthoseforthe5HSHi-Nicalonpreformspecimens, thestress-strain behavior forthe5HSSylramicpreformspecimens exhibitedfeaturessimilartothatofthe5HSHi-Nicalonpreformspecimens. Therefore, itsstressstraincurveisnotshowninfigure7.

Variationoftheelasticmoduluswithopenporosityisplottedinfigure8fortheepoxyinfiltratedHi-Nicalon

andSylramicpreformspecimens. Thesolidlinesinbothfiguresrepresent thepredictedmodulusbasedonrule-ofmixtureswiththeassumption thatransversetowsdonotcontributetothemodulus. Forthiscalculationthefollowingconstituent propertyvalueswereused: Esic(Hi_Nicalon ) = 270 GPa (ref. 5), Esic(Sylramic ) = 390 GPa (ref. 6),

E(CVD BN) = 62 GPa (ref. 7), E(CVD SiC) = 425 GPa (ref. 8) Vsi c = 40%, VBN = 6%, and VCVIsic = 13 to 52%.

For the two fiber types, an inverse relationship between open porosity and elastic modulus was noticed. Also noticed

is that for a fixed pore fraction, the predicted modulus value was slightly higher than the measured value. The discrepancy is possibly due to not accounting for 2 to 8 vol % of closed porosity present in the preform specimens

when the calculations were made. The inverse relationship seen here between porosity and elastic modulus is consistent with CVI literature (refs. 9 and 10). Previous studies indicate that in the early stages of CVI in stacked plies of

fibers, the open porosity decreases, and the tortuosity of interconnected pores changes continuously, but beyond a

critical level of deposition increasing amounts of closed porosity are also formed (refs. 9 and 10).

Table II summarizes the room temperature tensile property data for the four SiC preforms and the unreinforced

epoxy matrix. Each data entry and the corresponding standard deviation in the table represent an average of 7 tests.

The following conclusions can be drawn from the comparison of the data shown in the table. First, fiber architecture

had no effect on the tensile properties of the Hi-Nicalon preforms. Second, the stress or strain corresponding to the

DFL remained nearly the same for all four preforms irrespective of fiber architecture and fiber composition. Third,

primary elastic modulus of Sylramic SiC preforms is higher than that of the Hi-Nicalon SiC preforms. Fourth, the

Hi-Nicalon SiC preforms show significantly higher ultimate strength and strain than the Sylramic SiC preforms.

Figure 9 shows the monotonic (open squares) and hysteresis tensile stress-strain curves of 5HS Hi-Nicalon SiC

fiber preform specimens, and a monotonic stress-strain curve (solid line) for a 5HS Hi-Nicalon SiC/SiC composite

specimen fabricated by CVI. The monotonic and hysteresis stress-strain curves of the 5HS Hi-Nicalon SiC fiber

preforms are nearly identical. Because of its increased CVI SiC content, the 5HS Hi-Nicalon SiC/SiC composite

specimen is much stiffer than the 5HS Hi-Nicalon SiC fiber preform specimens. The onset of 0° fiber bundle cracking is considered to occur when the hysteresis loop width is >0. With increasing stress, the specimen upon unloading

showed increasing amounts of residual strain and increasing loop widths. This indicates progressive damage in the

form of bridged matrix cracks with increasing applied stress in the preform until failure. The variation of maximum

loop width with peak stress and strain are shown in figures 10 and 11, respectively, for the PW, 5HS, and 8HS

Hi-Nicalon and 5HS Sylramic SiC fiber preform specimens.

NASA/TM--2001- 210695/REV 1 4
Theonsetof "0°''crackingwasdetermined byextrapolating thehysteresis loopwidthversusstressorstrain

toavalueofzerostressorstrain,respectively. Thisoccurredforastressandstrainof~90MPaand~0.075to

0.08percent, respectively, forallthepreformstested. Inotherwords,therewasnodiscernibleeffectofarchitecture

orfibertypeonthehysteresis behavior. Thissuggeststhathe"0°'' cracking stress and strain is invariant and is controlled by the CVI SiC coating. It should be noted that for Hi-Nicalon and Sylramic reinforced CVI and/or MI SiC

matrix composites, the onset stress for 0° cracking is higher for the higher elastic modulus composites. However,

the onset strain for all the SiC/SiC composites, ~0.075 percent, was about the same as the epoxy infiltrated composites (ref. 11).

The AE results are shown in figure 12 plotted as normalized cumulative AE energy versus strain. The data pertains to the tensile stress-strain data of figure 9 as well as the other hysteresis tensile tests conducted in this study.

Normalized cumulative AE energy is determined by summing the AE event energies in chronological order of

events and then normalizing that data by the absolute cumulative AE energy of all events. There are two reasons for

analyzing the AE data in this manner. First, AE energy has been shown to be the best parameter relating to matrix

damage (refs. 12 and 13). Second, the absolute AE energy (and number of events) can vary by a factor of two due

primarily to sensor contact. However, the relative (normalized) amounts of AE activity are comparable from specimen to specimen. Also, the onset of AE activity usually occurs at stresses lower than any nonlinearity in the stressstrain curve (refs. 14 and 15), and is a good indicator of first cracking in the material that is usually in the form of

nonbridging tunnel cracks (refs. 11 and 16).

The data plotted in figure 12 are for hysteresis tensile tests. Some events occurred during unloading and subsequent reloading. These correspond to the sharp decreases in strain for AE data in figure 12. This is very common

for hysteresis type testing and is indicative of some damage occurring during unloading and reloading (ref. 17). The

onset of AE activity occurs at ~0.04 percent (~40 to 50 MPa) for all the preforms tested. Interestingly, the strain for

initial AE activity for a CVI SiC matrix composite, also shown in figures 9 and 12, occurs at 0.025 percent, even

less than for the preforms; however, the stress for initial AE activity occurs at ~50 MPa, about the same as for the

preforms. Since initial cracking occurs at stresses well below the formation of hysteresis loops, the first cracks are

presumably nonbridging tunnel cracks (ref. 16). The first cracks in CVI SiC matrix composites have been shown to

emanate from the sharp large pores formed where 90 ° and 0° bundles meet (ref. 11, 18 to 21). Presumably, the same

is true for the preforms, since the stress to initiate first matrix cracking is the same for preforms as for CVI matrix

composites. Figures 13 and 14 show typical tensile fracture surfaces of epoxy infiltrated 5HS BN/SiC coated

Hi-Nicalon and Sylramic SiC preform specimens. Comparison of figures 13 and 14 indicates several differences in

the fracture behavior of two types of preforms. The fracture surface of the Hi-Nicalon preform specimen shows

extensive fiber pullout throughout the cross-section. However, the fracture surface of Sylramic SiC preform specimen shows brittle/flat fracture in some regions and tow pullout in other regions. The fiber tows appeared to be sintered together and show staggered fractures. A higher magnification photograph of the brittle fracture region

indicates fiber to fiber contact with little or no BN coating around the SiC fibers.

In summary, open porosity varies between 20 to 45 vol %, between batches of the preforms of BN/SiC

Hi-Nicalon and Sylramic SiC fibers, and closed porosity varies between 2 to 8 vol %. This large variation in open

porosity is caused by thickness or volume fraction variation of the CVI SiC coating. Variation of porosity and volume fraction of CVI SiC coating will influence both processing parameters and properties of the SiC/SiC composites fabricated by the MI approach. From the processing point of view, it is difficult to formulate a slurry that can

fully infiltrate into a preform containing such a wide variation in porosity, pore size distribution, and tortuosity.

Consequently, some preforms can be fully infiltrated with the slurry and others can contain voids due to choking of

the slurry at the pore neck. These voids cannot be filled with silicon during the final MI step because of capillary

and surface tension forces. From the property point of view, voids and porosity are known to decrease the modulus,

transverse thermal conductivity, and interlaminar tensile strength of composites. In contrast, the variation in volume

fraction of CVI SiC coating may or may not affect properties of a fully dense SiC/SiC composite depending on the

relative differences in the properties of the MI SiC matrix and the CVI SiC coating. If the properties of MI SiC

matrix are similar to that of the CVI SiC coating, then variation in volume fraction of CVI SiC coating in the preform may not affect the properties of the SiC/SiC composites. In other cases, they would. In conclusion, the results

of this study indicate better control in thickness of the CVI SiC coating and the porosity of the BN/SiC coated SiC

preforms is needed to improve reproducibility and reliability of SiC/SiC composites.

NASA/TM--2001-210695/REV 1 5
4.SUMMARYOFRESULTS

Theroomtemperature poperties, damageaccumulation, andmicrostructure ofBN/SiCcoatedHi-Nicalonand

SylramicSiCfiberpreformswerecharacterized. Theinfluenceoffiberarchitecture onpropertieswasinvestigated.

Keyfindingsarethefollowing.

1.ThevariationinArchimedes, andphysical densities, andporefractionismuchgreaterbetweenthebatches

(~20percent) comparedtowithinabatch(~10percent) fortheBN/SiCcoatedHi-NicalonSiCandSylramicSiC

fiberpreformspecimens.

2.BothHi-NicalonSiCandSylramicSiCfiberpreformsshowaninverserelationshipbetweenthemodulus

andporefraction.

3. Fiberarchitecture hasnoeffectontheroomtemperature tensilepropertiesoftheHi-Nicalonpreforms.

4. Fiberarchitectureandfibermodulushavenoeffectonfirstmatrixcracking, ortheonsetstressandstrainof

crackswhicharebridgedbythefibers,i.e.,"0°''bundlecracks.

5.Asthefibermodulusincreasedsodidthe"0°''bundlecrackingstress.

6. InSylramicSiCfiberpreforms, thereisnochemical reactionbetweentheBNandSiClayers,butin

Hi-NicalonSiCpreforms, agradedregionofsmallgrainedSiCandBNwasobservedbetweenthelayers.TheBN

interfaceisamorphous andtheSiCgrainsarecolumnar.

REFERENCES

1. M.Johnson, B.J.Bartlett,andW.A.Troha,Proceedings ofthe13thInternational SymposiumonAirBreathing

Engines, Chattanooga, Tennessee, U.S.A.,1997,editedbyS.Billing,Vol.II,ISABE97-7179, p.1321.

2. D.Brewer, Mat.Sci.andEng.,A261(1999)284.

3. S.K.Lau,S.J.Calandra, andR.W.Ohnsorg, USPat.5,840,221, 1998.

4. J.P.Benedict, S.J.Klepeis,andR.Anderson, Proceedings: MicroscopyandMicroanalysis, Cleveland, Ohio

(August1997),editedby3,(2),(Springer-Verlag, NewYorkInc.,1987)339.

5. M.Takeda, Y.Imai,H.Ichikawa, T.Ishikawa, N.Kasai,T.Seguchi, andK.Okamura, Ceram. Eng.Sci.

Pro,12(1991)1007.

6. J.Lipowitz,J.A.Rabe,A.Zangvil,andY.Xu,Ceram. Eng.Sci.Pro,18(1997)147.

7. "EngineeringPropertyDataonSelectedCeramics, VolI,Nitrides"Report#MCIC-HB-07, (1976)Metalsand

CeramicsInformationCenter,BattelleColumbusLaboratories, Columbus, OH.

8. J.S.Goela,M.A.Pickering, R.L.Taylor,B.W.MurrayandA.Lompado, Appl.Opt.30(1991)3166.

9. W.J.LacyeyandT.L.Starr,"Fiber-Reinforced Ceramic Composites: Materials, Processing, and Technology"

edited by K.S. Mazdiyasni (Noyes Publications, Park Ridge, New Jersey, U.S.A, 1990) p. 347.

10. P.J. Geoghegan, "Flight-Vehicle Materials, Structure, and Dynamics- Assessment and Future Directions"

edited by A.H. Noor, and S.L. Venneri, The American Society of Mechanical Engineers, 345 East 47th Street,

United Engineering Center, New York, NY 10017, 1992) p. 113.

11. G.N. Morscher, J.Z. Gyekenyesi, and R.T. Bhatt, "Mechanical, Thermal, and Environmental Properties and

Performance of Ceramic Composites and Components" ASTM STP 1392 edited by M.G. Jenkins,

E. Lara-Curzio, and S.T. Gonczy, (American Society for Testing and Materials, West Conshohocken, PA,

2000) p. 306.

12. G.N. Morscher, and J.Z. Gyekenesi, Ceram. Eng. Sci. Proc. 1913], (1998) 241.

13. G.N. Morscher, Comp. Sci. Tech. 59[5], (1999) 687.

14. R.Y. Kim, andN.J. Pagano, J. Amer. Ceram. Soc. 74[5], (1991) 1082.

15. Y-J. Luo, S.C. Chang, and I.M. Daniel, J. Comp. Mat. 29, (1995) 1946.

16. B.N. Cox, and D.B. Marshall, J. Amer. Ceram. Soc. 79[5], (1996) 1181.

17. M.G. Jenkins, J.P. Piccola, and E. Lara-Curzio, in "Fracture Mechanics of Ceramics," 12, Edited by R.C. Bradt,

D.P.H. Hasselman, D. Munz, M. Sakal, and V.Y. Shevchenko (Plenum Publishing Corp., USA, 1996) p. 267.

18. G.N. M_rscher_ in ``Review _f Pr_gress in Quantitative N_ndestructive Evaluati_n_,, _9_ Edited by

D.O. Thompson and D.E. Chimenti (Kluwer Academic/Plenum Publishers) in print.

19. L. Guillaumat, and J. Lamon, in "Revue Des Composites et Des Materiaux Advances," 3, (1993) p. 159.

20. L. Guillaumat, and J. Lamon, Comp. Sci. Tech. 56 (1996) 803.

21. P. Pluvinage, A. Parvizi-Majidi, and T.W. Chou, J. Mat. Sci. 31, (1996) 232.

NASA/TM--2001- 210695/REV 1 6
TABLE I. PHYSICAL PROPERTIES OF BN/SIC COATED HI-NICALON AND

SYLRAMIC SIC FIBER PREFORMS

Fiber Weave # Tested

specimens

Hi-Nicalon PW 19

Hi-Nicalon 5HS 57

Hi-Nicalon 8HS 11

Sylramic 5HS

aIndicate range of values.

Archimedes

density,

_m/cc

2.71±0.02

_(2.67 2.73)

2.79±0.03

°(2.67 2.83)

2.68±0.12

_(2.54 2.79)

Physical

density,

_m/cc

1.63±0.14

_(1.47 1.87)

2.08±0.19

_(1.69 2.29)

1.74±0.20

_(1.46 1.95)

Pore

fraction

0.40±0.05

_(0.31 0.46)

0.26±0.06

_(0.19 0.41)

0.35±0.05

_(0.30 0.43)

79 2.91±0.04 1.86±0.13 0.36±0.05

_(2.77 2.95) _(1.59 2.18) _(0.25 0.45)

TABLE II. ROOM TEMPERATURE TENSILE PROPERTY DATA FOR

EPOXY, AND EPOXY INFILTRATED BN/SIC COATED HI-NICALON AND

SYLRAMIC SIC FIBER PREFORMS

Preform Weave DFL DFL strain, E, GPa Ultimate Ultimate

stress, percent tensile tensile

MPa stress, strain,

MPa percent

Epoxy None 3±0.13 8±0.69 0.27±0.03

Hi-Nicalon PW 63±14 0.06±0.005 113±22 352±50 0.86±0.12

Hi-Nicalon 5HS 71±12 0.06±0.02 124±24 370±32 0.74±0.07

Hi-Nicalon 8HS 81±10 0.09±0.02 93±12 405±29 0.83±0.08

Sylramic 5HS 75±14 0.05±0.02 135±30 196±31 0.23±0.03

Figure 1 .--Optical photographs of the cross-section of epoxy infiltrated BN/SiC coated Hi-Nicalon SiC fiber

preforms showing tow and fiber distribution: (a) tows through-the-thickness (b) fibers within a tow.

NASA/TM--2001-2106 95/REV 1 7
Figure 2.--Optical photograph of the cross-section of

epoxy infiltrated BN/SiC coated Sylramic SiC fiber

preforms showing fiber distribution and CVD coating

thickness variation within a tow.

Figure 3.--TEM micrographs of the interface regions of as-fabricated BN/SiC coated SiC fiber preforms:

(a) Hi-Nicalon (b) Sylramic.

NASA/TM--2001- 210695/REV 1 8
3.00

2.75

m

eIll

o 2.50 ......

1 2 3 4 5 6 7

Batch #

3oOtnnnNNl I

(

___2.75

tO 2.50 ........

1 2 3 4 5 6 7 8 9

Batch #

Figure 4.--Batch to batch variation in Archimedes

density for BN/SiC coated SiC fiber preform

specimens: (a) Hi-Nicalon (b) Sylramic.

2501i I nni l

(

2.00

m 1.50

1.00

0.50

a

0.00 ......

1 2 3 4 5 6 7

Batch #

°]innnNnn

(

E2.00

m 1.50

•_ 1.00

t0.50

a

0.00 ........

1 2 3 4 5 6 7 8 9

Batch #

Figure 5.--Batch to batch variation in physical density

for BN/SiC coated SiC fiber preform specimens:

(a) Hi-Nicalon (b) Sylramic.

OSea i

._ 0.40

0.30

• 0.20

n° 0.10

0.00 ....

1 2 3 4

Batch #

fifilI

i i

5 6 7

oso,,bi rrnrF

._ 0.40

0.30

0.20

no 0.10

0.00 ........

1 2 3 4 5 6 7 8 9

Batch #

Figure 6.--Batch to batch variation in open porosity

for BN/SiC coated SiC fiber preform specimens:

(a) Hi-Nicalon (b) Sylramic.

e

45O

400 8HS_ .t

350

j/.y'_-_- 5 HS

300 _ _I_ ....

250 _E¢J" ""_ PW

200 _[_>-_"

150

100

50 //_ /_ Epoxy

0 _ ............. _--

0.0 0.2 0.4 0.6 0.8

Tensile strain, %

Figure 7.--Room temperature tensile stress-strain

curves for epoxy and epoxy infiltrated PW, 5HS,

and 8HS BN/SiC coated Hi-Nicalon SiC fiber preforms.

i

1.0

NASA/TM--2001-210695/REV 1 9
13.

(5 _-- Rule-of-mixtures

/ . .

300

200

E IO0

0

0.0 0.1 0.2 0.3 0.4

t-

_- Pore fraction

13.

(5

_-- Rule-of-mixtures

400 /(b)_ prediction

"-i

2oo

E

0 ....

t0.0 0.1 0.2 0.3 0.4

Pore fraction

Figure &--Variation of room temperature tensile

modulus with pore fraction for BN/SiC coated

SiC fiber preforms: (a) Hi-Nicalon (b) Sylramic.

5HS preform

400 monotonic

350 CVl SiC/SiC t, f/_//

13. monotonic_\ /-_" j/D

300 y __

250 _'

"_ 200

150

"_. 1 O0 / reform

50 hysteresis

0

0.0 0.2 0.4 0.6 0.8

Tensile strain, %

Figure 9.--Typical monotonic and unload-reload

hysteresis tensile stress-strain curves for epoxy

infiltrated 5HS Hi-Nicalon SiC fiber preforms and

monotonic tensile stress-strain curve for a CVl

Hi-Nicalon SiC/SiC composite.

1.0

0.09

t-

-_ 0.08

"'_ 0.07

o_

o° 0.06

•_ 0.05

"_ 0.04

_>, 0.03

t-

,- 0.02

E 0.01

_0.00

50

t /

+ PW

• 5HS Hi-Nicalon

+ 8HS

+ 8HSj / /

+ 5HS Sylramic / /

..A

100 150 200 250 300 350 400

Peak tensile stress, MPa

Figure 1&--Maximum hysteresis loop width versus

peak tensile stress for various epoxy infiltrated

BN/SiC coated Hi-Nicalon and Sylramic SiC fiber

preforms.

0.06

t-

•._ 0.05

Q.

o° 0.04

"_ 0.03

e

0.02

trE

0.01

0.00

0.0

Pw; PW "

8HS Hi-Nicalon

8HS/

5HSJ . o

5HS Sylramic °

A o

t

o

• @

.... ¢_, .... , .... , .... , .... , .... , .... ,.... ,

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Peak tensile strain, %

Figure 11 .--Maximum hysteresis loop width versus

peak tensile strain for various epoxy infiltrated

BN/SiC coated Hi-Nicalon and Sylramic SiC fiber

preforms.

NASA/TM--2001- 210695/REV 1 10
L.

=o

¢)

I.U

<o

E

O

"O

¢)

N

E

o

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.0 0.2 0.4 0.6 0.8 1.0

Tensile strain, %

Figure 12.--Comparison of AE cumulative energy with

loading for epoxy infiltrated PW, 5HS, and 8HS

Hi-Nicalon SiC fiber preforms and an 8HS Hi-Nicalon

SiC/SiC composite.

(a)

1.00 mm

t I

(a)

1.00 mm

I I

150 i_m

t I

(b)

Figure 13.--SEM photographs of the tensile fracture

surfaces of epoxy infiltrated 5HS woven Hi-Nicalon

SiC fiber preforms showing (a) failure across the

specimen, and (b) fiber pull out within a tow.

10.0 i_m

I I

(b)

Figure 14.--SEM photographs of the tensile fracture

surfaces of epoxy infiltrated 5HS woven Sylramic SiC

fiber preforms showing (a) brittle and pullout regions

and (b) fiber to fiber contact in the brittle region.

NASA/TM--2001-210695/REV 1 11
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

August 2002 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Microstructure and Tensile Properties of BN/SiC Coated Hi-Nicalon, and

Sylramic SiC Fiber Preforms

6. AUTHOR(S)

Ramakrishna T. Bhatt, Yuan L. Chen, and Gregory N. Morscher

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

National Aeronautics and Space Administration

John H. Glenn Research Center

Cleveland, Ohio 44135-3191

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

National Aeronautics and Space Administration

Washington, DC 20546-0001

DlJ-714-04-30-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12626-1

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210695-REV1

11. SUPPLEMENTARY NOTES

Ramakrishna T. Bhatt, U.S. Army Research Laboratory, Glenn Research Center; Yuan L. Chen, Dynacs Engineering

Company, Inc., 2001 Aerospace Parkway, Brook Park, Ohio 44142; and Gregory N. Morscher, Ohio Aerospace Institute,

22800 Cedar Point Road, Brook Park, Ohio 44142. Responsible person, Ramakrishna T. Bhatt, organization code 0300,

216-433-5513.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified- Unlimited

Subject Category: 24 Distribution: Nonstandard

Available electronically at httpj_ltl>,grcAmsa_gev/GLTRS

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

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Batch to batch and within batch variations, and the influence of fiber architecture on room temperature physical and tensile

properties of BN/SiC coated Hi-Nicalon and Sylramic SiC fiber preform specimens were determined. The three fiber

architectures studied were plain weave (PW), 5-harness satin (5HS), and 8-harness satin (8HS). Results indicate that the

physical properties vary up to 10 percent within a batch, and up to 20 percent between batches of preforms. Load-reload

(Hysteresis) and acoustic emission methods were used to analyze damage accumulation occurring during tensile loading.

Early acoustic emission activity, before observable hysteretic behavior, indicates that the damage starts with the formation

of nonbridged tunnel cracks. These cracks then propagate and intersect the load bearing "0 °'' fibers giving rise to hysteretic

behavior. For the Hi-Nicalon preform specimens, the onset of "0 °'' bundle cracking stress and strain appeared to be

independent of the fiber architecture. Also, the "0 °'' fiber bundle cracking strain remained nearly the same for the preform

specimens of both fiber types. TEM analysis indicates that the CVI BN interface coating is mostly amorphous and contains

carbon and oxygen impurities, and the CVI SiC coating is crystalline. No reaction exists between the CVI BN and SiC

coating.

14. SUBJECT TERMS

Microstructure; Physical and tensile properties; SiC preforms; TEM; BN/SiC coated SiC

preforms; Mechanical properties; Acoustic emission

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION

OF REPORT OF THIS PAGE OF ABSTRACT

Unclassified Unclassified Unclassified

NSN 7540-01-280-5500

15. NUMBER OF PAGES

17

16. PRICE CODE

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102