Process-Induced Carbon and Sub-Layer

in SiC/BN/SiC Composites:

Characterization and Consequences

NASA/TM—2001-211069

July 2001

L.U.J.T. Ogbuji

QSS Group, Inc., Brook Park, Ohio

D.R. Wheeler

Glenn Research Center, Cleveland, Ohio

T.R. McCue

QSS Group, Inc., Brook Park, Ohio
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NASA/TM—2001-211069

July 2001

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

25th Annual International Conference on Advanced Ceramics and Composites

sponsored by the American Ceramic Society

Cocoa Beach, Florida, January 21–26, 2001

L.U.J.T. Ogbuji

QSS Group, Inc., Brook Park, Ohio

D.R. Wheeler

Glenn Research Center, Cleveland, Ohio

T.R. McCue

QSS Group, Inc., Brook Park, Ohio

Process-Induced Carbon and Sub-Layer

in SiC/BN/SiC Composites:

Characterization and Consequences
Acknowledgments

The assistance of Mike Cuy with burner rig exposures of our samples at NASA Glenn Research Center

is gratefully acknowledged.

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Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NASA/TM—2001-211069 1 

PROCESS-INDUCED CARBON SUB-LAYER IN SiC/BN/SiC 

COMPOSITES: CHARACTERIZATION AND 

CONSEQUENCES 

L.U.J.T. Ogbuji 

QSS Group, Inc. 

Brook Park, Ohio 44142 

D.R. Wheeler 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

T.R. McCue 

QSS Group, Inc. 

Brook Park, Ohio 44142 

ABSTRACT 

Following our detection of films of elemental carbon in the Hi-NicalonTM/BN/SiC 

composite and its deleterious effect on oxidative durability, we have examined 

other SiC/BN/SiC systems. The problem is pervasive, and significant residues of 

free carbon are confirmed in SylramicTM/BN/SiC materials. Effective techniques 

for routine detection and characterization of adventitious carbon in SiC/BN/SiC 

composites are discussed. 

INTRODUCTION 

SiC/BN/SiC composites can be undermined by a layer of free carbon underlying 

the BN interphase, leading to severe impairment of composite oxidative durability 

at intermediate temperatures [1,2]. In Hi-NicalonTM/BN/SiC the deleterious layer 

originates from excess carbon content of the non-stoichiometric fiber [1-4]. Close 

examination of some SylramicTM/BN/SiC composites have revealed similar layers 

of elemental carbon on the fiber, resulting in severe burner rig degradation as was 

exhibited by Hi-NicalonTM composites [1,2]. In SylramicTM/BN/SiC the carbon 

may have come from inadequate removal of the fiber sizing during processing [5]. 

In either case the carbon source is intrinsic to the system itself, whether it comes 

from the fiber or a processing step. Thus, the possibility of SiC/SiC contamination 

by deleterious free carbon is strong, and its consequences must be considered. 

High-resolution TEM offers an excellent means of resolving the thin carbon layer 

in question (≤ 150 nm thick by most estimates [1,5]), but we have achieved better 

results with surface-sensitive techniques [6,7]. Other workers have used TEM to 

examine SiCf/SiCm composites for adventitious carbon: Giannuzzi and Lewinsohn 

[8,9] reported that, in a composite reinforced with NicalonTM fibers coated with a 

150nm layer of carbon, the carbon coating was found to thicken about an order of 

magnitude as a result of infiltrating the SiC matrix. That lends strong support to 
NASA/TM—2001-211069 2 

our suggestion regarding the origin of free carbon layers in NicalonTM/BN/SiC 

systems [1]. Now that interphase integrity is an issue for SiC/BN/SiC composites, 

it seems prudent to examine each variety of SiC/BN/SiC for free carbon that can 

compromise composite durability. Hence, it is useful to establish simple, reliable 

techniques for its detection. This paper addresses that issue and gives a summary 

of our results for SylramicTM/BN/SiC. Our findings for Hi-NicalonTM/BN/SiC 

composites have been described elsewhere [1,2,6,7]. 

PROCEDURE 

The composites in this study were comprised of SylramicTM (stoichiometric SiC) 

fibers coated with ~0.5μm interphase layer of BN, in a dense melt-infiltration 

(MI) SiC matrix. They were in two groups. One group consisted of samples made 

with fibers that had been protected with polyvinyl alcohol (PVA) sizing prior to 

composite processing, the second group with polyethylene oxide (PEO) sizing. 

They are hereafter referred to as “PVA” and “PEO” materials. There were two 

PVA materials and seven PEO materials, in the form of standard tensile bars, 6” x 

0.5” x 0.08” (152 x 12.7 x 2 mm). 

They were exposed in a 0.3 Mach atmospheric-pressure burner rig for 100 hours at 

800o

C and broken in tension, which we have found a reliable screening test for 

the weakening and embrittlement indicative of interphase degradation in SiC//SiC 

composites. Coupons were machined for scanning electron microscopy (SEM) 

from the ends (which were unaffected by the burner rig flame) mounted, polished, 

and studied with a high-resolution Hitachi S4700, operated at 2-6 kV. For Auger 

Electron Spectrometry (AES), thin and flat strips (~25 mm x 2 mm x 0.5 mm) 

were cut from the shoulders of the bars (also far from the flame-affected zone), 

using only water lubrication to avoid contamination by carbon in resin lubricants. 

They were cleaned by sonication in alcohol and examined with Fisons Instrument 

Microlab (Model 310-F) spectrometer, operating at 2.0 kV, the samples having 

been broken in-situ in the AES chamber to minimize adventitious carbon. Other 

tools explored in this study included laser raman spectrometry. 

RESULTS AND DISCUSSION 

Tensile Tests 

All seven PEO materials exhibited drastic drops of strength and strain to failure 

following exposure in the burner rig. The as-received material exhibited a tensile 

strength of ~410 MPa and failure strain of 0.3%; after burner rig exposure these 

values were down to 175-226 MPa and 0.09-0.16%, respectively. The 50-60% 

decrease in strength and strain to fracture were very similar to the severity of 

degradation seen in Hi-NicalonTM/BN/SiC composites following the same burner 

rig exposure [1,2,6,7]. In contrast, none of the PVA materials showed significant 
NASA/TM—2001-211069 3 

weakening or embrittlement following exposure. This was the first indication of 

an important difference between the PVA and PEO sizing in the durability of 

SiC/BN/SiC composites. 

Scanning Electron Microscopy 

Three PEO materials and one PVA material are used to illustrate some features of 

interest in the microstructures. Fig. 1 is an SEM image of the fracture surface of a 

PEO material. The EDS spectra show that the patch labeled A is bare fiber (Si/C 

peak ratio of ~5.0 being characteristic of SiC), and patch B has an overwhelming 

carbon content. Fig. 2 shows a polished surface from another PEO material. In 

the image on the left, two interphase layers are discernible between the fiber and 

CVI-SiC cladding; and the corresponding EDS spectra confirm that feature B is 

boron nitride (with some carbon), while layer C is almost entirely carbon. Where 

BN is absent (see spectrum D and its corresponding image on the right side), the 

gap between fibers is occupied by carbon. Optical fluorescence tests showed the 

carbon patches (B & D) were not from the epoxy mount. In contrast, no carbon 

layer was observed by SEM/EDS in the PVA materials. 

Fig. 1, SEM image and spectra from Sylramic/BN/SiC composite with a high 

excess of carbon in the interphase. 

B

A

A B
NASA/TM—2001-211069 4 

Auger Electron Spectrometry 

Ceramic-matrix composites are designed for preferential fracture through the 

interphase. That behavior was exploited to advantage in this study: any carbon 

layers present were preferentially exposed on a fracture surface and its signal 

thereby enhanced for detection by surface-sensitive techniques. Hence, detection 

of interfacial carbon in SiC/BN/SiC composites by AES is highly reliable even 

when it is too thin to resolve in cross section. 

Fig. 2, SEM images and EDS spectra from polished sections of a PEO composite 

AES offers the advantage of revealing the chemical state of the carbon detected, 

so that free carbon is distinguishable from Si-bonded carbon in SiC [7]. This point 

is illustrated in Fig. 3, which compares AES survey spectra taken from fracture 

surfaces in four SylramicTM/BN/SiC samples: three PEO materials (a-c) and one 

PVA material (d). In sample d the silicon signal is of comparable intensity to that 

of carbon, whereas in samples a-c the silicon seems to be absent. This indicates 

that samples A-C have high amounts of elemental carbon, while sample d is free 

D

B D

C

B

C
NASA/TM—2001-211069 5 

of elemental carbon, being essentially the SiC fiber surface. Further confirmation 

comes from fine-structure details indicated with arrows in the spectra: a shoulder 

or kink in a spectrum at ~260 eV is the hallmark of elemental carbon, and its 

absence a clear indication that the carbon is bonded (in this case to Si) [7,10]. 

Fig. 3, AES survey spectra from four different SylramicTM/BN/SiC composites 

showing that free carbon is associated with those materials made with PEO-sized 

fibers (a-c), but not those with PVA-sized fibers (d). 

This result suggests that PEO sizing is more likely than PVA to leave substantial 

residues of deleterious carbon on fibers. This may reflect an intrinsic difference 

between PEO and PVA (such as different char yields upon de-sizing), or merely 

differing amounts of sizing used: Manufacturers’ data show that PEO-sized fibers 

had ~2 wt. % sizing, whereas PVA-sized fibers had only ~0.2 wt. % of sizing. 

0 200 400 600 -6000

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NASA/TM—2001-211069 6 

The amount of residual free carbon observed in the PEO materials varied greatly: 

from very little in some to substantially thick layers in others. Fig. 4 is an AES 

depth-profile of the fiber coating from the PEO material in Fig 1. The silicon and 

carbon profiles are shown accentuated for comparison. (Profiles of all the other 

constituents lie at the bottom of the chart, close to the detection threshold.) The 

evident 160-nm layer of free carbon is the thickest observed: depth-profiling of 

the other PEO materials indicated the carbon layers to be 15-80 nm deep. 

Fig. 4, AES depth profile from a SylramicTM/BN/SiC composite made with 

PEO-sized fibers, showing a layer of free carbon ~160 nm thick on the fiber. 

It was mentioned in Section 3.1 that all seven PEO materials examined exhibited 

severe degradation in the burner rig, while the two PVA materials did not. It 

should be noted that free carbon was detected in all the PEO samples but not in 

the PVA samples. The correlation seems clear: free carbon at the interphase leads 

to severe degradation in the burner rig. The degradation of SylramicTM/BN/SiC is 

similar to what was observed in Hi-NicalonTM/BN/SiC composites. 

An important difference between the two systems is the origin of the offending 

carbon. In Hi-NicalonTM composites it comes from excess carbon in the fiber, 

while in SylramicTM materials the residue of improperly removed (or unsuitable) 

sizing appears to be the source of carbon. In Hi-NicalonTM systems the carbon 

diffuses from the fiber during matrix infiltration and hence appears uniform and 

0.0%

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CD-00-80699

BN

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BN

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C sublayer

a b

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t

NASA/TM—2001-211069 7 

continuous on the fibers. In contrast, the free carbon on PEO-sized SylramicTM

fibers were quasi-continuous, appearing somewhat like spiral ribbons around the 

fibers. That difference is depicted schematically in Fig. 5 for NicalonTM fibers (a), 

and SylramicTM fibers (b). 

Raman Spectroscopy 

Raman spectrometry was also used on polished cross-sections, without success. 

Results indicated that graphitic carbon was present in the samples but resolution 

was inadequate to distinguish between carbon signals from the matrix, fiber, and 

interphase. Thus Raman spectrometry proved unsuitable for the detection of 

interfacial carbon in these materials. 

Fig. 5, Schematic illustration of carbon layers on SiC fibers: they are thought to 

be uniform on NicalonTM (a), but ribbon-like on PEO-sized SylramicTM fibers (b) 
NASA/TM—2001-211069 8 

SUMMARY AND CONCLUSION 

Elemental carbon was observed in SylramicTM/BN/SiC composites made with 

PEO-fibers, as a (quasi)continuous layer on the fiber analogous to the layer of free 

carbon found on Hi-NicalonTM fibers. As with Hi-NicalonTM/BN/SiC composites, 

the presence of free carbon correlated with degradation of composite behavior in 

these materials. This study has shown that SEM and AES, used in combination, 

are effective techniques for detecting deleterious carbon layers in SiC/BN/SiC 

composites. 

REFERENCES 

1. L.U.J.T. Ogbuji, “Degradation of SiC/BN/SiC Composite in the Burner Rig”, 

p. 257 in Ceram. Eng. & Sci. Proc., vol. 19, issue 4 (1998) 

2. L.U.J.T. Ogbuji, “A Pervasive Mode of Oxidative Degradation in a SiC-SiC 

Composite”, J. Am. Ceram. Soc., 81 [11] 2777 (1998) 

3. E.Y. Sun, S.R. Nutt, and J.J. Brennan, “Interfacial Microstructure and 

Chemistry of SiC/BN Dual-Coated Nicalon-Fiber-Reinforced Glass-CeramicMatrix Composites”, J. Am. Cer. Soc., 77 [5] (1994) 1329-39 

4. G. Chollon, R. Pailler, R. Naslain, F. Laanani, M. Monthioux, and P. Olry, 

“Thermal Stability of a PCS-Derived SiC Fibre with a Low Oxygen Content 

(Hi-Nicalon)”, J. Mater. Sci., 32 [2] (1997) 327-47 

5. L.U.J.T. Ogbuji, Silicon-Based Ceramic-Matrix Composites for Advanced 

Turbine Engines: Some Degradation Issues”, p. 767 in Proc. Parsons 2000: 

Fifth International Charles Parsons Turbine Conf. (A. Strang et al. 

eds.),Cambridge University, 3-7 July 2000; Cambridge University Press, UK, 

2000 

6. L.U.J.T. Ogbuji, “Degradation of SiC/SiC Composite in the Burner Rig: 

Investigation by Fractography”, p. 369 in Materials at High Temperatures, 

Proc. 4th International Conf. on the Microscopy of Oxidation (G. Tatlock, ed.), 

Cambrian Printers, UK, 2000 

7. L.U.J.T. Ogbuji, “Identification of Carbon Sublayer in a Hi-Nicalon/BN/SiC 

Composite”, J. Mat. Sci. Let. 18 (1999) 1825 

8. L.A. Giannuzzi and C.A. Lewinsohn, “Interphase Oxidation in SiC/SiC 

Composites at Varying Partial Pressures of Oxygen”, p. 115 in Ceram. Eng. & 

Sci. Proc., vol. 20, issue 4 (E. Ustundag and G. Fischman eds.), Am. Ceram. 

Soc., Westerville OH, 1999 

9. L.A. Giannuzzi and C.A. Lewinsohn, “Interphase Oxidation in SiC/SiC 

Composites”, p. 469 in Ceram. Eng. & Sci. Proc., vol. 21, issue 3 (T. Jessen 

and E. Ustundag, eds.), Am. Ceram. Soc., Westerville OH, 2000 

, D. Briggs and M.P. Seah (eds), John Wiley & Sons, NY 1990 

 

Spectroscopy

Surface Analysis, 2nd Edition, vol. 1: Auger and X-ray Photoelectron 

10. D. Briggs and J.C. Riviere, Ch. 3 (“Spectra Interpretation”) in Practical 
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Composites; Fiber sizing; Interphase degradation; Carbon

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Prepared for the 25th Annual International Conference on Advanced Ceramics and Composites sponsored by the American Ceramic Society, Cocoa Beach, Florida, January 21–26, 2001. L.U.J.T. Ogbuji and T.R. McCue, QSS Group, Inc.,

2000 Aerospace Parkway, Brook Park, Ohio 44142 (work funded under NASA Contract NAS3–00145); D.R. Wheeler,

NASA Glenn Research Center. Responsible person, D.R. Wheeler, organization code 5140, 216–433–5837.

L.U.J.T. Ogbuji, D.R. Wheeler, and T.R. McCue

Process-Induced Carbon and Sub-Layer in SiC/BN/SiC Composites:

Characterization and Consequences

Following our detection of films of elemental carbon in the Hi-NicalonTM/BN/SiC composite and its deleterious effect

on oxidative durability, we have examined other SiC/BN/SiC systems. The problem is pervasive, and significant

residues of free carbon are confirmed in SylramicTM/BN/SiC materials. Effective techniques for routine detection and

characterization of adventitious carbon in SiC/BN/SiC composites are discussed.