NASA/TMm2001-210695

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

March 2001
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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 tensilc 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 ¢_:curring 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

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

on_'t 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. INTRODUCTION

Preforms of SiC fibers are being used as starting material for the fabrication of SiC- and Al203-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 1
Theobjectivesofthisstudywereseveral:(1),todeterminethebatch-to-batch variations in room temperature

physical and mechanical properties, and microstructure of the SiC preforms typically used for SiC/SiC composites;

(2), to determine the influence of fiber architecture on tensile properties; (3) to determine the damage accumulation

in the preforms; (4), to suggest the influence of preform characteristics on the processing and properties of the SiC/

SiC composites fabricated by the MI approach.

2. EXPERIMENTAL PROCEDURE

The preforms of BN/SiC coated Hi-Nicalon TM and Sylramic TM SiC fibers were purchased from Honeywell

Advanced Composites Inc, Delaware. The Nippon Carbon Company, Japan, and the Dow Coming 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 Syiramic 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 ram thick SiC. The nominal dimensions of the preforms after CVI coatings were 229- (L) × 152- (W)

x 2-mm (T).

The preforms were cut into smaller specimens of dimensions 152- (L) × 12.5- (W) × 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 × 12 × l-ram 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 occuring 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 recorde d 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 mm down to 3 mm diamond particle impregnated metal disks, and polished in a vibratory polisher on a micro

cloth using a 0.3 nun diamond powder paste.

aDigital Wave Corporation, Englewood, Colorada.

NASA/TM_2001-210695 2
For transmission electron microscope (TEM) analysis, the preform specimens were sliced using a diamond saw.

Subsequently, each specimen was sandwiched between two sacrificial silicon wafers, vacuum degassed and impregnated with epoxy. After curing, the sandwich assembly was sectioned into smaller specimens of dimensions

2.5 x 2 × 0.5 mm. After polishing one of the sides of the 2.5 × 2 mm surface for flatness, the opposite side of the

specimen was thinned to electron transparency using tripod-polishing (ref. 4). As polishing progresses, the thickness

of the assembly was periodically monitored using a reflected light microscope. As the thickness of the sacrificial

silicon wafer decreased, its color also changed. When the color of the sacrificial silicon turned from orange to yellow, the polishing was stopped. Tripod polishing makes it possible to mechanically polish a large area of a very thin

specimen with no or minimum mechanical damage. The superficial mechanical damage seen on some specimens

was removed by low angle (<7 °) argon ion-beam milling for about 10 to 20 rain. The present transmission electron

microscopy results include imaging and quantitative analyses of the transverse cross-section of the specimens. In

this mode, the fibers were observed "end-on." Diffraction contrast and phase contrast images, selected-area, and

microdiffraction data were recorded using a Philips CM200 electron microscope operated at 200 kV. The TEM is

coupled to an energy dispersive X-ray (EDX) spectrometer for chemical identification.

3. RESULTS AND DISCUSSION

Because of its low viscosity, epoxy uniformly fills the available open pore space in the preform specimens.

Typical cross-sections of an epoxy infiltrated 5HS woven BN/SiC coated Hi-Nicalon SiC fiber preform specimens

are shown in figure 1. Figure 1(a) indicates that the Hi-Nicalon tows are oblong and show gaps between the fiber

tows. A higher magnification photograph of the cross-section (fig. 1(b)) shows that the diameter of the SiC fibers

within the tow varies from 10 to 16 pain. 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 pro, 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 6 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 6 percent, and CVI SiC coating thickness variations from 2 to 5 lam. 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 3
the other two preform specimens, whereas the average pore fraction of 5 HS preform is lower than those of the other

two preform specimens. The average Archimedes density values for the PW, 5HS, and 8HS woven Hi-Nicalon

preform specimens are generally lower than those for the 5HS woven Sylramic SiC preforms primarily due to the

density difference between the Hi-Nicalon-2.74 gm/cc (ref. 5) and Sylramic SiC fibers-3.0 gm/cc (ref. 6). Other

noticeable points are that the standard deviation and the difference in extreme values remain nearly the same for

both fiber types for the same architecture.

Attempts were made to determine size range of interconnected porosity in the preforms by mercury porosimetry

and nitrogen adsorption techniques. In the case of the porosimeter, mercury easily filled the preform at standard

temperature and pressure conditions, making further infiltration of mercury under pressure impossible. On the other

hand, pore size determination by the adsorption technique could not be used because preform surface repelled nitrogen gas.

The room temperature tensile stress-strain curves for the epoxy infiltrated PW, 5HS, and 8 HS Hi-Nicalon SiC

preform specimens are shown in figure 7. For comparison purposes, included in the figure is the stress-strain curve

for the unreinforced epoxy. Clearly, the modulus and UTS for the unreinforced epoxy is significantly lower than

those for the epoxy infiltrated preform specimens. This suggests that the epoxy matrix does not influence the deformation behavior and tensile properties of the preform specimens. In figure 7, all three weaves displayed similar features on the stress-strain curves: An initial linear region followed by a nonlinear region. The stress corresponding to

deviation from hnearity (DFL) for the 5HS and PW specimens appears to be the same, but the DFL value for the

8 HS preform specimens is slightly lower than that of the other two weaves. Also noticed is that the UTS of 8HS

preform specimens is greater than that of the other weaves, and elastic modulus of the 5HS and PW specimens is

nearly the same.

Although ultimate tensile strength and failure strain for the 5HS Sylramic preform specimens are relatively

lower than those for the 5HS Hi-Nicalon preform specimens, the stress-strain behavior for the 5HS Sylramic preform specimens exhibited features similar to that of the 5HS Hi-Nicalon preform specimens. Therefore, its stressstrain curve is not shown in figure 7.

Variation of the elastic modulus with open porosity is plotted in figure 8 for the epoxy infiltrated Hi-Nicalon

and Sylramic preform specimens. The solid lines in both figures represent the predicted modulus based on rule-ofmixtures with the assumption that transverse tows do not contribute to the modulus. For this calculation the following constituent property values were used: Esic(Hi.Nicalon ) = 270 GPa (ref. 5), Esic(Sylramic ) = 390 GPa (ref. 6),

E(CVD aN_ = 62 GPa (ref. 7), E(CVD SiC_= 425 GPa (ref. 8) Vsi C = 40%, VBN = 6%, and Vcv I sic = 20%. 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 6 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 static and hysteresis stress-strain curves of 5HS Hi-Nicalon specimen. The static and hysteresis stress-strain curves are almost identical. The hysteresis stress-strain curves have slightly larger strains for the

same stress compared to the static stress-strain curve. 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 is shown in figures l0 and 11, respectively, for the PW, 5HS, and 8HS Hi-Nicalon and 5HS Sylramic

SiC preform specimens.

NASA/TM--2001-210695 4
The onset of "0 °'' cracking was determined by extrapolating the hysteresis loop width versus stress or strain to

a value of zero stress or strain, respectively. This occurred for a stress and strain of -90 MPa and -0.075 to

0.08 percent, respectively, for all the preforms tested. In other words, there was no discernable effect of architecture

or fiber type on the hysteresis behavior. This suggests that the "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 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 (refs. 11).

The AE results are shown in figure 12 plotted as normalized cumulative AE energy versus strain. The data pertains to the stress strain data of figure 9 and other hysteresis tensile tests not shown. 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 stress-strain 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 occumng 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 B N/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/tiat 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 fiber and closed porosity varies between 2 to 6 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 5
4. SUMMARY OF RESULTS

The room temperature properties, damage accumulation, and microstructure of BN/SiC coated Hi-Nicalon and

Sylramic SiC fiber preforms were characterized. The influence of fiber architecture on properties was investigated.

Key findings are the following.

1. The variation in Archimedes, and physical densities, and pore fraction is much greater between the batches

(~20 percent) compared to within a batch (-10 percent) for the BN/SiC coated Hi-Nicalon SiC and Sylarmic SiC

fiber preform specimens.

2. Both Hi-Nicalon SiC and Sylarmic SiC fiber preforms show an inverse relationship between the modulus

and pore fraction.

3. Fiber architecture has no effect on the room temperature tensile properties of the Hi-Nicalon preforms.

4. Fiber architecture and fiber modulus have no effect on first matrix cracking, or the onset stress and strain of

cracks which are bridged by the fibers, i.e., "0 °'' bundle cracks.

5. As the fiber modulus increased so did the "0 °'' bundle cracking stress.

6. In Sylramic SiC fiber preforms, there is no chemical reaction between the BN and SiC layers, but in

Hi-Nicalon SiC preforms, a graded region of small grained SiC and BN was observed between the layers. The BN

interface is amorphous and the SiC grains are columnar.

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

SYLRAMIC SIC FIBER PREFORMS

Fiber Wcave # Tested

specimens

Hi-Nicalon PW 19

Hi-Nicalon 5HS 57

Hi-Nicalon 8HS I 1

Sylramic 5HS 79

alndicatc range of values.

Archimcdcs

dcnsib',

[_m/cc

2.71 +0.02

"(2.67 2.73)

2.68±0.12

"(,2.54 2.791

2.91 ±0.04

"(2.77 2.95)

Physical

density,

_m/cc

1.63±0.14

a(1.47- 1.87)

2.08:L0.19

_(1.69 - 2.29,)

1.74+0.20

a( 1.46 - 1.95)

t .86+0.13

"(1.59 2.18)

Porc

fraction

0.40±0.05

_(0.31 (I.46)

0.26+0.06

"{,0.19 0.41)

0.35i0.05

_(0.30 0.43)

0.36+0.05

( ._ 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+O.13 8±0.69 0.27+0.03

Hi-Nicalon PW 63+14 0.06-+4).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 preforms

showing tow distribution and fiber distribution within a tow,

NASA/TM--2001-210695 7
Figure 2.--Optical photograph of the cross-section of

epoxy infiltrated BN/SiC coated Sylramic SiC preforms showing tow distribution and CVD coating

thickness variation within a tow.

Figure 3.toTEM micrographs of the interface regions of BN/SiC coated SiC fiber prefoms: (a) Hi-Nicalon

(b) Sylramic.

NASA/TM--2001-210695 8
.__ 2.75

U)

C

a 2.50 ......

1 2 3 4 5 6 7

Batch #

.__

a

 3oo{N r0)

2.75

2.50

1

.....n

2

N

3 4 5 6

N

7 8 9

Batch #

Figure 4.--Batch to batch variation in Archimedes

density for BN/SiC coated Hi-Nicalon and Sylramic

SiC preforms.

2.50

O

2.00

E

o_ 1.50

_1.00

g o.5o

r't

0.00 w , i i , l

1 2 3 4 5 6 7

Batch #

2.50

0

2.00

E

o_ 1.50

"_ 1.00

0.50

0.00 NNN NN 

1 2 3 4 5 6 7 8 9

Batch #

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

for BN/SiC coated Hi-Nicalon and Sylramic SiC

preforms.

0el; .__ 0.40

0.30

0.20 _

_0.10

n

0.00 ,

1 2 3 4

Batch #

5 6 7

.__ 0.40

0.30

0.20

o 0.10 Q.

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 Hi-Nicalon and Sylramic SiC

preforms.

450

400

,_ 350

- 300

¢0

250

200

150

i._ 100

50

0

8HS --_ _ j

_:/_-- 5HS

_"_j--J-J "- PW

f__(.,-- Epoxy

i i =

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 9
a.

0 _- Rule-of-mixtures

200

E 100 -_ (a) -O

_¢ 0 / , ,

0.0 0.1 0.2

Pore fraction

0.3 0.4

O.

.. _- Rule-of-mixtures

10O 200

E

__ 0

C

0.0 0.1 0.2 0.3 0.4

Pore fraction

Figure 8.mVadation of room temperature tensile

modulus with pore fraction for BN/SiC coated

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

Epoxy preform

400 E = 98 GPa -7

350 HN/BN/CIV SiC _//

300 E = 200 GPa _._ _.__. :j

150 ......

\

CO 100 "_':: •

rm

0 #

0 _ E = 114GPa ,

0,0 0.2 0.4 0.6 0.8 1.0

Strain, %

Figure 9.--Typical unload-reload hysteresis curve for

epoxy infiltrated 5HS Hi-Nicaion SiC preforms and a

CVI SiC/SiC composite.

-.- PW-100 GPa

o_ 0.09] sPW'115GPa J 5HS-114 GPa ? O.08 ] 8HS-94 GPa /

0.07 1 8HS-118 GPa /_ /

iO_0"0510"06 1 5HS Sylramic-124 GPa/_:

0.04 1

oo31

= o.o21

-i°'°'1

_ 0.00/

50 100 150 200 250 300 350 400

Peak stress, MPa

Figure 10.--Maximum hysteresis loop width versus

peak stress for various epoxy infiltrated BN/SiC

coated Hi-Nicalon and Sylmmic SiC preforms.

o_

10

O.

oo

J_

e-

_E

:E

0.06

0.05

0.04

0.03

0.02

0.01

0.00

0.0 0.1

*PW

-PW

o 8HS

• 8HS

. 5HS

° 5HS Sylramic

• O

o

A O

o

O

: I -I I I I I,- t

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Peak strain, %

Figure 11.--Maximum hysteresis loop width versus

peak strain for various epoxy infiltrated BN/SiC

coated Hi-Nicalon and Sylmmic preforrns.

NASA/TM--2001-210695 10
CVl SiC matrix--.

0.9 ..... 2._ _ :_ ..........

N_ 0.60"7 .....f___.._._. _--PW

"_0.5

_ 0.30"4_,_8HS epoxy ,

0.2

0.1

o 0.0 z

0.0 0.2 0.4 0.6 0.8 1.0

Strain, %

Fg3ure12.--Comparison of AE cumulative energy with

loading for epoxy infiltrated 8HS Hi-Nicalon SiC

preforms and 8HS Hi-Nicalon SiC/SiC composites.

_) (a}

1.00 mm

k

150 _m

I I

(b)

Figure 13.--SEM photographs of the tensile fracture

surfaces of epoxy infiltrated 5HS woven Hi-Nicalon

SiC preforms showing (a) failure across the specimen, and (b) fiber pull out within a tow.

10.0 i_m

4

(b)

Figure 14.--SEM photographs of the tensile fracture

surfaces of epoxy infiltrated 5HS woven Sylramic

SiC preforms showing (a) bdttle and pullout regions

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

NASA/TM--2001-210695 11
REPORT DOCUMENTATION PAGE I FormApproved

I

OMB No. 0704-0188

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

I March 2001 Technical Memorandum

4. TITLE AND SUBTITLE

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

Sylramic SiC Fiber Preforms

6. AUTHOR(S)

Ramakrishna I". 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

5. FUNDING NUMBERS

WU-714-04-30---00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12626

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210695

11. SUPPLEMENTARY NOTES

Ramakrishna T. Bhatt, U.S. Army Research Laboratory, Glenn Research Center; Yuan L. Chert, 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 12b. DISTRIBUTION CODE

Unclassified - Unlimited

Subject Category: 24 Distribution: Nonstandard

Available electronically at httD://idtrs._rc.nasa.eov/GLTRS

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

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 l0 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 '1) °" 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

OF REPORT OF THIS PAGE

Unclassified Unclassified

NSN 7540-01-280-5500

19. SECURITY CLASSIRCATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

17

16. PRICE CODE

A03

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribedby ANSI Std. Z39-18

298-102