23rd annual composites, advanced...

23rd Annual Lonterence on

Composites, Advanced Ceramics,

Materials, and Structures: B

Ersan Ustundag Gary Fischman Editors

January 25-29, I999 Cocoa Beach, Florida

Published by The American Ceramic Society 735 Ceramic Place Westerville, OH 4308 I

01 999 The American Ceramic Society ISSN 0 196-62 I9

23rd Annual Conference on

Corn posites, Advanced Ceramics,


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Each issue of Ceramic Engineering and Science Proceedings, ISSN 0 196-62 19, includes a collection of technical articles in a general area of interest These articles are of practical value for the ceramic industries and the general public.The issues are based on the proceedings of a conference. Both American Ceramic Society and non-Society conferences provide these technical articles. Each issue is organized by an editor who selects and edits material from the conference proceedings.The opinions expressed are entirely those of the present0rs.Thet-e is no other review prior t o publication.

23rd Annual Lonterence on

Composites, Advanced Ceramics,


Ersan Ustundag Gary Fischman Editors

January 25-29, I999 Cocoa Beach, Florida

Published by The American Ceramic Society 735 Ceramic Place Westerville, OH 4308 I

01 999 The American Ceramic Society ISSN 0 196-62 I9

Copyright I999 by The American Ceramic Society. All rights reserved.

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Cover photo: “Typical fractographs of ceramic composite joint,” is courtesy of S. Aravindan and R. Krishnamurthy, and appears as figure 7(a) in their paper “Microwave Joining of Al,O,-ZrO,,” which begins on page 7 I .

Contents 23rd Annual Conference on Composites,Advanced Ceramics, Materials, and Structures: B

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

DARPA’s Low Cost Ceramic Composite (LC3) Program Characterization of in situ BN Interface Formed by Nitridation of Nextel 3 I 2 .......................... .3 F.I. Hurwitz, J. Riehl, J . Madsen,TR. McCue, and D.L. Boyd

Simulation of Progressive Damage for High-Temperature Woven Composites .............................. .I I J. Fish, Q.Yu, and D. Wildman

Device Design Surface Finish and Composition Dependence of Valvetrain Friction with Silicon Nitride Tappet Inserts . . . . . . . . . . . . . .25 G.M. Crosbie, R.L. Allor, A. Gangopadhyay, D. McWatt, and PWillermet

Functionally Graded Materials for Gun Barrels . . . . . . . . . . . .33 PJ. Huang, C. Hubbard, J.J. Swab, M.W. Cole, S. Sampath, S. Depalo, J. Gutleber; A. Kulkarni, and J. Margolies

Development of a CMC Thrust Chamber ............... .39 T Nakamura, H. Murata, N. Matuda, F.Tomioka, S. Nishide, and S. Masaki

Design and Characterization of a Co-Extruder . .......... .47 Z. Liang and S. Blackburn

Scale-Up of APCVD Uniform Ceramic Coating of

TJ. Clark

Joining Techniques for Fiber-Reinforced Ceramic

AS. Fareed, C.C. Cropper, and B.R. Rossing

Microwave Joining of Al,O,-ZrO, Composites . .......... .7 I S. Aravindan and R. Krishnamurthy

Transparent Armor Ceramics: AlON and Spinel ........... .79 J.J. Swab, J.C. LaSalvla, G.A. Gilde, PJ. Patel, and M.J. Motyka

Tubular Inner Surfaces via Process Modeling . ........... .55

Matrix Composites .............................. .6 I

V

Service Environment Effects Modeling the Oxidation Kinetics of Continuous Carbon Fibers in a Ceramic Matrix ................... .87 M.C. Halbig and J.D. Cawley

Mechanisms of low-Temperature Environmental Effects

Z. Zhao and D.O. Northwood

Oxidative Pest Degradation of Hi-NicalonIBN/SiC Composite as a Function of Temperature and Time

on Transformation-Toughened Zirconia Ceramics . . . . . . . . . .95

in the Burner Rig .............................. .I05

Partial Pressures or Oxygen

L.U.J.T. Ogbuji

lnterphase Oxidation in SiCISiC Composites at Varying ....................... I I5

L.A. Giannuzzi and C.A. Lewinsohn

Effects of Particulate Debris Morphology on the Rolling Wear Behavior of All-Steel and Si,N,-Steel

D.J. Mitchell, J.J. Mecholsky jc, and J.H. Adair

A Comparative Study of the Tensile, Fatigue and Creep Properties of Sintered (SNW- I000 and GS-44) and HlPed (PY-6) Silicon Nitride Ceramics .............. I33 J. Sankar, G. Choudhury, Q.Wei,V.Vijayrao, and A.D. Kelkar

Modeling Ceramic Composite Hot Gas Candle Fi l ter Material Using Energy Method ...................... I45 X. Huang, R.H. Carter; and KL. Reifsnider

Experimental Study of a Ceramic Hot Gas Candle Fi l ter Material ................................. I53 RH. Carter, X. Huang, and KL. Reifsnider

Environmental Effects of Microstructural Stability in

T. Shibayama, G.W. He, H.Takahashi,Y Katoh, and A. Kohyama

Bearing Element Couples ......................... I23

SiCISiC Composites .............................. I 6 I

Strength of Mansonry Mortars under Field Exposure Conditions ................................... I69 O.Z. Cebeci

vi

Advanced Synthesis and Processing: Materials Behavior Under Extreme Conditions

Fields: ElectriclM agne ticlRF Research Programs on Material Processing in High Magnetic Fields at Tsukuba Magnet Laboratory (invited) ... I79 H.Wada, A. Fukuzawa, H. Ohtsuka,T Ohara, H.Abe, andT Kiyoshi

Powder Consolidation Using Dynamic Magnetic Compaction (DMC) Process ........................ I 9 I

Microwave Induced Combustion Synthesis of Ultrafine Barium Hexaferrite Powders ....................... I99 S. Bhaduri, S.B. Bhaduri, and J.B. Zanotti

A Comparison of Annealing Treatments of an Oxide Ceramic ................................ .207

B. Chelluri

M.S. Morrow, D.E. Schechter; R. Simandi, H.E. Huey and Q.S.Wang

Fields: R FlGravi t at io n a1 Synthesis of Crystalline Materials with High Quality Under Short-time Microgravity (invited) .............. .2 I5 T. Okutani, H. Minagawa, H. Nagai,Y Nakata, M. Suzuki,Y Ito,TTsurue, and K. lkezawa

Auto Ignition Synthesis and Microwave Sintering of Zr0,-CaO .................................... .227 S.B. Bhaduri, S. Bhaduri, and J-G. Huang

Reactive Synthesis of Dense FGMS in the Ti-B Binary System ................................ .235 S. Bhaduri and S.B. Bhaduri

Flight-and Ground-Based Materials Science Programs at NASA ..................................... 24

Zero Gravity Sol-Gel Glass-Metal Composite Production . . . .25

D.C. Gillies

M. Kotwin, K. Lynch, and W. LaCourse

Temperature: High Flux, High Rates Thermal Shock Behavior of Single Crystal Oxide Refractive Concentrators for High-Temperature Solar Thermal Propulsion ........................ .259 D. Zhu, S.R. Choi, N.S. Jacobson, and R.A. Miller

vii

I

lnviscid Melt Spinning of Mullite Fibers ............... .267 B.S. Mitchell

Growth and Diameter Control of A1,03/Y3A1,0,2 Eutectic Fiber by Micro-Pulling-Down Method and I t s High-Temperature Strength and Thermal Stability . . . . . . . ,275 A.Yoshikawa, K. Hasegawa,T Fukuda, K. Suzuki, andY.Waku

Development of Sic-Based Layered Composites with Rare-Earth Silicate Layers ........................ .283 T Fukasawa, M. Kato,Y Goto, S. Suyama, andT. Kameda

Lanthane Aluminate Thermal Barrier Coating . ......... .29 I G.W. Schafer and R. Gadow

Polymer Precursors Recent Advancement of TyrannolSiC Composites R&D ..... .301 T Nakayasu, M. Sato,TYamamura, K. Okamura,Y. Katoh, and A. Kohyama

Fabrication of High Performance SiClSiC Composite by Polymer Impregnation and Pyrolysis Method ........... .309 M. Kotani,A. Kohyama, K. Okamura, andT lnoue

Highly Cross-Linked Precursors to Silicon Carbide . . . . . . . .3 I 7 T Iseki, M. Narisawa, K. Okamura, K. Oka, andT Dohmaru

High-Energy Particle Bombardment Microstructural Stability of SiClSiC Composites under Dual-Beam Ion Irradiation ........................ .325 Y Katoh,T Hinoki,A. Kohyama,T Shibayama, and H.Takahashi

PlasmalCVDICVI High Energy Plasma Ceramic Coating Optimization (invited) .................................... .335 M. Quint and H. Kopech

Design of a Laser CVD Rapid Prototyping System . . . . . . . . .347 C.E. Duty, D.L. Jean, and W.J. Lackey

Corrosion-Resistant CVD Mullite Coatings for Si,N, . . ..... .355 ].A. Haynes, K.M. Cooley, D.P Stinton, R.A. Lowden, and W.Y. Lee

High-Temperature Corrosion of Oxide-Coated Sic in

N.Yunoki, S. Kitaoka, and H. Kawamoto Water Vapor Atmosphere ........................ .363

... V l l l

Bending Properties of CVI SiCdSiC Composites at

H. Araki, W.Yang,Y Shi, S. Sato,T. Noda, and A. Kohyama

Stress-Rupture of New Tyranno Si-C-0-Zr Fiber Reinforced Minicomposites ....................... .379 G.N. Morscher

Thermal Stabilities of CVI SiCdSiC Composites . ......... .387 T. Noda, H. Araki, W.Yang, and A. Kohyama

Thermal Expansion Characteristics of Coated

R. Krishnan,A.D. Kelkar; and J, Sankar

The Effect of Heat Treatment on Thermal and Strength Properties of Sic Fiber Prepared from Activated Carbon Fiber .......................... .403 J. Shirnizu, K. Okada,T.Watabe, and K. Nakajima

Optimization and Scale-Up of Si-N-0 Fiber Synthesis ..... .41 I A.Vital, H.C. Ewing, U.Vogt, and L. Reh

Elevated Temperatures .......................... .37 I

Fiber Composites .............................. .395

iExtreme Conditions~ThermullMechunical The Properties of Ceramic Laminates (invited) . ........ .42 I W.J. Clegg, G. Andrees, E. Carlstrorn, R. Lundberg, A. Kristofferson, R. Meistring, E. Menessier, and A. Schoberth

Shock Compaction of the Titanium-Silicon Ternary

J.L. Jordan and N.N.Thadhani Carbide (Ti,SiC,) Powders ........................ .427

Submicrometer Cutting Tools on the Basis of AI,O, for Machining Alloyed Hard Cast Iron and Hardened Steel .... .435 A. Krell, F! Blank, L. Berger; andV. Richter

High-Temperature Properties and Creep Resistance of Near-Stoichiometric Sic Fibers ..................... .443 H. Serizawa, C.A. Lewinsohn, G.E.Youngblood, R.H. Jones, D.E. Johnston, and A. Kohyama

Creep Response of Fiber Reinforced Ceramic Composites . . .451 UAnandakurnar and R.N. Singh

The Multiaxial Strength of Tungsten Carbide . .......... .459 J. Salem and M.Adarns

Hot Pressing of Mixtures of AlumindZirconia Powders . . . . .467 K. Jakus, K. Bhatia, and J.E. Ritter

ix

Gas Pressure Sintering Silicon Nitride Ceramics Using Rare Earth and Transition Metal as Sintering Aids . ...... .475 A.J. N. Dias, J. Duailibi Fh, C.A.Vilardo,TC. Silva, and M.C.S. N6brega

Commercialization and Use of Engineering Ceramic

Reduced Cost, Improved F.O.D. Resistance and Improved Attachments for Insertion of Silicon Nitride Turbomachinery Components Program Overview: Reduced Cost, Improved F.O.D. Resistance and Improved Attachments for Insertion of Silicon Nitride Turbomachinery Components ........... .485 A.C. Jones

Advanced Silicon Nitride Components: A Cost Analysis .... .497 J.M. Schoenung, E.H. Kraft, and D.Ashkin

Surface Compressive Coatings for Improved F.O.D.

D.Ashkin, J. Rubin, D.K. Shetty, and J. Xi Resistant Silicon Nitride ......................... .505

CopperlGallium Amalgam: A Material for Joining Ceramics and Metals ........................... .5 I3 M. Heim, J.E. Holowczak, L.A.B.Tessarotto, and V.A. Greenhut

Design, Analysis and Testing of Ceramic Components for An Advanced Military Expendable Turbojet Engine ....... .523 A.C. Jones

Oil-Less Silicon Nitride Bearings for An Advanced Military

G.W. Hosang, D.Ashkin, and D.K. Shetty Expendable Turbojet Engine ...................... .533

Commercialization of Ceramics CMC Combustor with a Thick Thermal Protection System-

J. Shi and F!D.Andrew

Manufacturing and CMC-Component Development for

R. Gadow and M. Speicher

A Model of Toughening in Metal Melt-Infiltrated

William B. Hillig

A Detailed Stress Analysis ........................ .543

Brake Disks in Automotive Applications .............. .55 I

Ceramic Composites ............................ .559

X

Cost Effective Processing Fluid Coating Process for Protective Coatings of Carbon Fibers ................................ .57 I R. Gadow, S. Kneip, and G.W. Schafer

Diamond Films on the Leading Edge ................. .579

Co-Extrusion of Solid Oxide Fuel Cell Functional Elements .. .587 Z. Liang and S. Blackburn

Manufacturing of Aluminum Nitride Heat Exchangers by Ceramic Injection Molding ........................ .595 R. Fischec R. Gadow, and G.W. Schafer

M.I. Mendelson

xi

_ ~ _ _

Preface

The International Conference on Engineering Ceramics and Structures-the 23rd Annual Cocoa Beach Conference and Exposition drew a diverse group of people interested in ceramics for various reasons. Scientists and engineers from government, industry, and academia joined to discuss advanced ceramics from materials preparation to commercialization. Three successful symposia focused the program into their subject areas, and the general conference topics assured that a broad perspective of the field was discussed.

The three symposia gave a unique character to the program.They were:

Advanced Synthesis and Processing Materials Behavior under Extreme Conditions, which focused on innovative approaches in materials synthesis and processing in which extreme conditions are present or imposed. Most of these papers are included in these volumes.

Commercialization and Use of Engineering Ceramics, which focused on ways to increase or improve the interactions necessary to bring engineering ceramics to market in a timely fashion.

The Larry Hench Sympsium on Surface-Active Processes in Materials (papers published in Ceramic Transactions Volume I 0 I ) brought a diverse group of engineers, medical practi- tioners and scientists together to discuss surface active materials in honor of a major pioneer in these areas.

These three symposia and the general topics offered perspectives of ceramics that ranged from materials modeling to technology transfer and market development. Such a range of topics created a dynamic program that attendees enjoyed greatly. These volumes are a tribute to that meeting.

Finally, we would like to thank those that helped make this conference possible: the sym- posium chairs who worked to organize their programs, session chairs who kept things running smoothly on stage, and the staff who kept things running smoothly. We would also like to thank the efforts of the Advanced Composite Working Group who ran a conference concurrently and worked hard so we could create the highest value program to participants of both conferences.

Gary Fischman Ersan Ustundag

... X l l l

DARPA’s Low Cost Ceramic Composite (LC3) Program

CHARACTERIZATION OF IN-SITU BN INTERFACE FORMED BY NITRIDATION OF NEXTEL 312 Frances I. Hunvitz, NASA Lewis Research Center, Cleveland, OH 44 135 John Riehl and John Madsen, Northrop Grumman Corp., Bethpage, NY 11714 Terry R. McCue, Dynacs Engineering Co., Inc., Brookpark, OH 44142 Darwin L. Boyd, Kent State University, Kent, OH 44242

ABSTRACT

cost approach to the formation of BN on the fiber surface. Although the BN enriched surface region is extremely thin (< 40 nm), it is effective in providing debonding and composite behavior. Variation in mechanical behavior of Nextel'M 3 12/BN/131ackglasTM composites has been noted among panels fabricated with AF- 10 cloth treated in different nitridation runs. Understanding this variation became the rationale for undertaking a detailed characterization of the nitrided fiber using field emission scanning electron microscopy, low voltage energy dispersive x-ray and Auger electron spectroscopy. Chemical composition and surface morphology are discussed in relation to observed mechanical behavior of NextelTM 3 1 2/BlackglasTM composites tested in bending.

Treatment of NextelTM 3 12 in a reactive environment has been used as a low

INTRODUCTION

framework for fabricating low-cost, structural ceramic matrix composite (CMC) components having potential for high temperature operation and high specific properties in oxidizing environments. The approach is to introduce low cost materials and process technologies to reduce CMC manufacturing cost and at the same time enhance component lifetimes.

The program has selected NextelTM 3 12 fiber for its relatively low cost and ability to form an in-situ BN interface, thus eliminating the need for CVI deposition of an interface coating. The BlackglasTM polymer system was chosen for its relatively low cost and amenability to resin transfer molding (RTM) and resin vacuum infiltration (RVI) processing, and needed temperature capabilities.

3 The focus of the Low Cost Ceramic Composites (LC ) program is to develop a

To the extent authorized under the laws of the United States of America, all copyright interests in this publication are the property of The American Ceramic Society. Any duplication, reproduction, or re ublication of this ublication or any part thereof, without the express written consent of The American Ceramic Society or fee paiBto the Copyright Jkarance Center, is prohibited.

3

BlackglasTM undergoes significant shrinkage on pyrolysis, as do all preceramic polymers, resulting in the need for a number of reinfiltration cycles to process the composite to a desired matrix density and optimize mechanical properties. The resulting matrix is highly microcracked.1

B203, 24% SO2. Processes for forming a BN enriched surface on this fiber by heat treatment in an ammonia containing atmosphere have been reported in the literature, 2-4 and the in-situ BN has been characterized by transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS)5,6, as well as by secondary ion mass spectrometry (SIMS).7>*

The depth of boron enrichment and nitrogen incorporation near the fiber surface is quite small, on the order of 20-30 nm, with maximum boron levels observed at about 5 nm from the surface. Reaction is presumed to occur by diffusion of ammonia into the fiber, to react with B2O3, forming BN, with loss of H20. Loss of SiO from the fiber might be taking place as well.7 Although the surface enrichment is extremely thin, desirable interface properties are shown to be produced when treated fiber is incorporated into BlackglasTM matrix composite^.^ It also has been shown that the interphase does not react with the matrix on oxidative exposure of the composite at 600°C for up to 1000 hours.6

We have noted a larger variation in mechanical properties of composites manufactured from fabric nitrided in different runs than among panels produced from cloth from the same nitridation run; and therefore selected panels and samples of fabric from three different runs were chosen for comparison.

NextelTM 312 ceramic fiber (3M) has the nominal composition 62% A1203, 14%

EXPERIMENTAL

sizing, then treated in an NH3/H2 atmosphere by Allied Signal to produce a BN enriched fiber surface. Panels were fabricated using fabric produced in three different nitridation runs.

from the nitrided AF- 10 five harness satin weave fabric and Blackglas 493 resin and catalyst (Allied Signal) using resin transfer molding, curing, pyrolysis and reinfiltration cycles. The resin was degassed and injected into the tooled, stacked plies along their length using a constant volume injection system (Liquid Control). The long edges of the panel parallel to the flow direction are sealed to prevent race tracking of the resin along the panel edges during the fill. The panel was compressed to a thickness of 1.9 mm to maintain fiber volume fraction. Pyrolysis was carried out in an Inconel 718 retort at a heating rate of 2"C/min to a final

NextelTM 3 12 AF-10 fabric (3M) was cleaned by heating in air to remove fabric

Ten ply composite panels having a fiber volume fraction of 0.5 were prepared

4

temperature of lOOO"C, with a 1 hour hold at that temperature. Pyrolysis runs were performed in high purity argon (99.995%) at approximately 1 atm with a flow rate of 0.056-0.1 12 cm3/h. This procedure permitted a complete exchange of gas in the retort every 15 min. A total of 5 reinfiltraton cycles were used.

Composite mechanical behavior was characterized by three point flexural testing for comparison with prior data, utilizing a sample geometry of 7.62 x 0.635 cm, using a supporting span of 5.08 cm. Testing was at a constant displacement loading rate of 0.127 cdmin. Load versus displacement was measured and recorded and from it values of peak stress, modulus and percent strain (at peak load) were calculated.

Pieces of nitrided Nex teP 3 12 cloth were characterized using field emission scanning electron microscopy (Hitachi 4700), low voltage energy dispersive spectroscopy (1 -3 kV), Auger spectroscopy (AES) and x-ray diffraction analysis.

RESULTS AND DISCUSSION

designated as Runs 50, 62 and 67. Flexural properties of composite samples using fabric from each of these runs is summarized in Figure 1. Significant differences in flexural strength were observed from among the panels, with Panel

Three nitridation runs of AF-10 were performed by Allied Signal, Inc., and are

40 1

r Peak Stress, Ksi 09

T 0 Modulus. Msi lo.* 35

0 7

0 6

0 5

- 30 I - $ 25 v H

e I 20 z 2. 15

e 0 3

0 2

0 1

>

Y 0 4 A

w 10

5

0 0 Run 50 Run 62 Run 67

Nitridation Run

Figure 1: Flexural properties of NextelTMBlackglasTM composites

5

50 having the highest strength, followed by Panels 62 and 67. Strain to failure showed the same decreasing trend across the group of three, while modulus was slightly lower for Panel 50 compared with 62 and 67.

SEM analysis of small pieces of fabric sampled from the three nitridation runs revealed differences in surface texture of the fibers (Figure 2). A piece of fabric which had been heat cleaned but not nitrided was examined for comparison. The

Figure 2: Surface texture of heat cleaned and nitrided fiber

as heat cleaned fiber exhibited a rough, granular texture, which varied in degree of surface roughness among the nitrided samples, with runs 62 and 67 being slightly smoother than the control sample, and run 50 showing the least surface roughness.

Energy dispersive spectroscopy (EDS) at 1 kV minimizes beam penetration and fails to excite heavier elements, increasing the relative signal from light elements and surface sensitivity. This revealed variation in relative peak heights of B, C, N and 0 among the cloth samples, with Sample 50 showing the largest relative concentrations of B and N. At 3 kV there is both increased beam

6

penetration and excitation of heavier elements, so that and A1 and Si peaks are observed as well. Comparison of B and N intensities with the A1 peak again shows the highest BN concentration in Run 50, with decreasing levels in 62 and 67, respectively (Figure 3).

N

P (Run501

0.30 0.60 0.90 1.20 1.50 1.80

I Run 67 1

c - ' .1..,. .KO 0.90 1.20 1.50 1.10

& 0.30

~~

.60 0.90 1.20 1.50 1.80

Figure 3: Energy dispersive spectra of nitrided fabric, 3 kV.

Auger spectroscopy shows incorporation of N to a depth of 30-35 nm in Run 50,25 nm in Run 62, and 15-20 nm in Run 67 (Figure 4). B enrichment at the surface is nominally 50 atomic percent in Run 50,45 and 30 percent, respectively, in Runs 62 and 67. In Run 67, bulk fiber composition is reached at only 15 nm. The Auger results corroborate the EDS finding of greatest N incorporation and highest surfaceBN in Run 50.

7

60.0

50.0

n 40.0 .-

5 - C

2 30.0 0 0 V

0 'E 20.0 2

10.0

0.0 0.0 10.0 20.0 30.0 40.0 50.0

Depth, nm

60.0

50.0

w 40.0 .-

5 c c 8 30.0 c 0 0 u

20.0 .- E 2

10.0

0.0 0.0 10.0 20.0 30.0 40.0 50.0

Depth, nm

Figure 4: AES depth profile of nitrided fabric.

8

Based on the EDS and Auger data, a coating “thickness” is difficult to define, because the nitridation treatment provides a change in surface chemistry of the fiber, rather than an actual “BN” coating layer. This interpretation is supported by the published TEM data.6

X-ray diffraction analysis was performed on ground fiber samples to identify crystalline phases. The as heat cleaned AF- 10 fiber was totally amorphous, whereas all nitrided samples showed the presence of AlllB4033. Relative concentrations of aluminum borate and crystallite size were indistinguishable among the three samples. Crystallization is attributed to temperature exposure during the nitridation treatment.

Improved composite performance, expressed as maximum flexural strength, is seen to coincide with a smoothing, or erosion, of the fiber surface roughness, which would enhance fiber sliding, as well as a change in fiber surface chemistry. The study does not permit separation of the two effects.

SUMMARY AND CONCLUSIONS Nitridation of NextelTM 3 12 by heat treatment in ammonia alters the fiber

surface chemistry and morphology, decreasing surface roughness as compared with the as heat cleaned fiber. Changes in surface chemistry are very localized, and alter the fiber to a depth of not more than 35-40 nm below its surface. Improvement in composite properties may be attributed to a decrease in surface roughness and/or altered bonding between fiber and matrix resulting from a change in surface chemistry. The large effect on flexural strength with fairly subtle variation in extent of nitridation suggests the need for close control of nitridation parameters. It is not possible to ascertain from this study if there is an effect of variation in extent of nitridation with position within a fabric roll, as the nitridation is carried out with loosely rolled fabric placed within a furnace.

ACKNOWLEDGMENTS The authors wish to thank Ralph Garlick of NASA Lewis Research Center for

x-ray diffraction analysis, and Dr. Donald Wheeler for Auger spectroscopy. This work was partially supported by DAWA Technology Development Agreement No. MDA972-93-2-0007 that is being administered by the Materials and Manufacturing Directorate of the Air Force Research Laboratory.

9

REFERENCES 1. R. H. Stawovy, S. L. Kampe, W. A. Curtin, "Mechanical behavior of glass and BlackglasTM ceramic matrix composites," Acta Muter. 45, 53 17-5325 (1997). 2. F. Simpson, J. Verzemnieks, Patent US 4,605,588, August 12, 1986. 3. F. Simpson, J. Verzemnieks, Patent US 4,948,662, August 14, 1990. 4. S. S. Campbell, S. T. Gonczy, "In-situ formation of boron nitride interfaces on NextelTM 3 12 continuous ceramic fiber. I: Nitriding process and BlackglasTM ceramic matrix composite properties," Ceram. Eng. Sci. Proc. 15,327-336 (1 994). 5. N. R. Kasgiwale, et al., "Characterization of BN rich layer on ammonia treated NextelTM 312 fibers," Mat. Res. Soc. Symp. Proc. 365, 389-394 (1995). 6. N. R. Kasgiwale, et al., "Evaluation of ammonia treated NextelTM 3 12 fiber- BlackglasTM matrix composites," Ceram. Eng. Sci. Proc. 16,621-63 1 (1995). 7. E. A. Leone, S. S. Campbell, "The use of secondary ion mass spectroscopy (SIMS) in the analysis of alumina-boria-silica ceramic," Ceram. Eng. Sci. Proc.

8. S. Campbell, E. Leone, M. McNallan, "The effect of processing parameters on the surface nitridation of NextelTM 3 12 ceramic," Ceram. Eng. Sci. Proc. 18, 391- 398 (1 997).

18,381-389 (1997).

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SJMULATION OF PROGRESSIVE DAMAGE FOR HIGH mMPERATURE WOVEN COMPOSITES Jacob Fish and Qing Yu, Rensselaer Polytechnic Institute, Troy, NY 121 80

Durell Wildman, Allison Advanced Development Co., Indianapolis, IN 46241

ABSTRACT

This paper is aimed at developing a nonlocal theory for obtaining numerical approximation to a boundary value problem describmg damage phenomena in a ceramic composite material. The mathematical homogenization method based on double scale asymptotic expansion is generalized to account for damage effects in heterogeneous media. A closed form expression relating local fields to the overall strain and damage is derived. Nonlocal damage theory is developed by intro- ducing the concept of nonlocal phase fields (stress, strain, free energy density, damage release rate, etc.). Numerical results of our model were found to be in good agreement with experimental data of 4-point bend test conducted on composite beam made of Blackglasm/Nextel 5-harness satin weave.

INTRODUCTION Damage in composite materials occurs through Merent mechanisms that are complex and usually involve interaction between microconstituents. During the past two decades, a number of models have been developed to simulate damage and failure process in ceramic composites, among which the damage mechanics approach is particularly attractive in the sense that it provides a viable framework for the description of distributed damage including material stiffness degradation, initia- tion, growth and coalescence of microcracks and voids. Various damage models for brittle composites can be classified into micromechanical and macromechanical approaches. In the macromechanical damage approach, composite material is idealized (or homogenized) as an anisotropic homogeneous medium and damage is introduced via internal variable whose tensorial nature depends on assumptions about crack orientation 191, [151, [161, [191.[211,[181. The micromechanical damage approach, on the other hand, treats each microphase as a statistically homogeneous medium. Local damage variables are defined to represent the state of damage in each phase and phase effective material properties are defined thedter. The overall response is subsequently obtained by homogenization [ll, [171,[221.

From the mathematical formulation stand point, both approaches can be viewed as a two-step procedure. The main difference between the two approaches is in the chronological order in which the homogenization and evolution of damage are carried out. In the macromechanical approach, homogenization is performed first followed by application of damage mechanics principles to

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homogenized anisotropic medium, while in the micromechanical approach, damage mechanics is applied to each phase followed by homogenization.

The primary objective of the present manuscript is to simultaneously carry out the two steps (homogenization and evolution of damage) by extending the framework of the classical mathematical homogenization theory [2][3][14] to account for damage effects. This is accomplished by intro- ducing a double scale asymptotic expansion of damage parameter (or damage tensor in geneml). This leads to the derivation of the closed form expression relating local fields to overall strains and damage. The second salient feature of our approach is in developing a nonlocal theory by introduc- ing the concept of nonlocal phase fields (stress, strain, free energy density, damage release rate, etc.). Nonlocal phase fields are defined as weighted averages over each phase in the chcteristic volume in a manner analogous to that currently practiced in concrete [6], [7] with the only excep- tion beiig that the weight functions are taken to be C? continuous over a single phase and zero else- where. On the global (macro) level we limit the finite element size to ensure a valid use of the mathematical homogenization theory and to limit localization. We consider a 4-point bend test conducted on the ceramic composite beam made of BlackglasTM/Nextel 5-harness satin weave and compare our numerical simulations to experiments [8].

MATHEMATICAL HOMOGENIZATION FOR DAMAGED COMPOSITES In this section we extend the classical mathematical homogenization theory [2] for statistically homogeneous composite media to account for damage effects. The strain-based continuum damage theory is adopted for constructing constitutive relations at the level of microconstituents. Closed form expressions of local strain and stress fields in a multi-phase composite medium are derived. Attention is restricted to small deformations.

The microstructure of a composite material is assumed to be locally periodic (Y-periodic) with a

period defined by a Statistically Homogeneous Volume Element (SHVE), denoted by 0. Let x be

a macroscopic coordinate vector in macro domain and y = x/5 be a microscopic position

vector in 0. Here, < denotes a very small positive number compared with the dimension of Q, and y I x/< is regarded as a stretched coordinate vector in the microscopic domain. When a solid is subjected to some load and boundary conditions, the resulting deformation, stresses, and internal variables may vary from point to point within the SHVE due to the high level of heteroge- neity. We assume that all quantities have two explicit dependencies: one on the macroscopic level x , and the other one on the level of microconstituents y I x/< . For any Y-periodic response

function f , we have f(X, y ) = f(X, y + ky) is the basic period of the

microstructure and k is a 3 by 3 diagonal matrix with integer components. Adopting the classical

nomenclature, any Y-periodic function f can be represented as fC(x) ~ f ( x , y ( x ) ) with super-

script 5 denoting a Y-periodic function f . The indirect macroscopic spatial derivatives of f C can

be calculated by the chain rule as with the comma followed by a subscript variable xi denoting a

partial derivative with respect to the subscript variable (i.e. f,xi = @/ax i ). Summation convention

for repeated subscripts is employed, except for subscripts x and y . The constitutive equation on the microscale is derived from continuum damage theory based on the thermodynamics of irreversible processes and internal state variable theory. To model the isotropic

in which vector

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damage process, we define a scalar damage parameter o' as a function of microscopic and macro-

scopic position vectors, i.e., 0' = O(X, y) . Based on the strain-based continuum damage theory, the free energy density has the form of

where 0' E [ 0, 1 ) is the damage parameter. For small deformations, elastic free energy density

is given as we(&b) = O . % , i j t l & $ E ~ l . The constitutive equation, thermodynamic force (also

known as a damage energy release rate) and dissipative inequality follow from (1)

With this brief glimpse into the constitutive theory, we proceed to outlining the strong form of the governing differential equations on the fine scale - the scale of microconstituents. We assume that microconstituents possess homogeneous properties and satisfy equilibrium, constitutive, kinemat- ics and compatibility equations. The corresponding boundary value problem is governed by the following set of equations:

where o' is a scalar damage parameteq 05 and &a are components of stress and strain tensors;

Lijkl represents components of elastic stiffness; bi is a body force assumed to be independent of

y ; uf denotes the components of the displacement vector. the subscript pairs with parentheses

denote the symmetric gradients defined as "ti, I -(u: xj + u j x i ) ; Q denotes the macro-

scopic domain of interest with boundary r; ru and Tr are boundary portions where displace-

ments lii and tractions ti are prescribed, respectivel; ni denotes the normal vector on r Clearly, a brute force approach attempting discretization of the entire macro domain with a grid spacing comparable to that of the microscale features is not computationally feasible. Thus, a mathematical homogenization method based on the double-scale asymptotic expansion is employed to account for microstructural effects on the macroscopic response without explicitly representing the details of the microstructure in the global analysis. As a starting point, we approximate the dis-

placement field, u,S(x) = ui(x, y) , and the damage parameter, d ( x ) = o(x, y) , in

terms of double-scale asymptotic expansions on !,2 X @ :

1 2

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Strain expansions on the composite. domain with consideration of the indirect differentiation rule

X 0 can be obtained by substituting (5) into (3)

Stresses and strains for different orders of 5 are related by the constitutive equation (3)

S

r = O

The resulting asymptotic expansion of stress is given as

(9) 1 s

O i j ( X , y ) = +x, y ) + o p , y ) + so& y ) + ...

= 0, O(q1): 0-1 I J . 1 , + O$,Yj = ‘ 9 ‘(so): “ 8 , X j + “ h , Y j + bi = O. (lo)

Inserting the stress expansion (9) into equilibrium equation (3) and making use of equation yield the following equilibrium equations for various orders:

O ( p ) : 0-1 iJ.Y,

From the o( 5-2) equilibrium equation (10) we arrive at the classical result up = up(x) .

We proceed to the o(5-l) equilibrium equation (10). From (7) and (8) follow

where Hikl is a Y-periodic function. We assume that dko,(x) is macroscopic damage-induced

strain driven by the macroscopic strain Ekr = Exkl(U0) . More specifically we can state that if

Ekl = 0 , then d;(x) = 0 and oO(X, y ) = 0.

Based on the decomposition given in (12). the o( 5 - l ) equilibrium equation takes the following form:

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