CERAMIC FIBERS
AND COATINGS

ADVANCED MATERIALS FOR
THE TWENTY-FIRST CENTURY

Committee on Advanced Fibers for
High-Temperature Ceramic Composites

National Materials Advisory Board

Commission on Engineering and Technical Systems

National Research Council

• Notice
• Committee
• Preface
• Acknowledgments
• Contents
• Executive Summary

Publication NMAB-494
National Academy Press
Washington, D.C. 1998

NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences.

The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering.

The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine.

The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council.

This study by the National Materials Advisory Board was conducted under Contract No. MDA972-92-C-0028 with the Defense Advanced Research Projects Agency. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project.

Additional copies are available for sale from:
National Academy Press
Box 285
2101 Constitution Ave., N.W.
Washington, DC 20055
800-624-6242
202-334-3313 (in the Washington Metropolitan Area)
http://www.nap.edu

Library of Congress Catalog Card Number: 98-84558

International Standard Book Number: 0-309-05996-8

Cover: Eight-harness woven cloth. Each "thread" is a single tow containing as many as 800 individual filaments. Source: Dow Corning Corporation.

Copyright©1998 by the National Academy of Sciences. All rights reserved. Printed in the United States of America.

COMMITTEE ON ADVANCED FIBERS FOR
HIGH-TEMPERATURE CERAMIC COMPOSITES

DAVID W. JOHNSON (chair), Lucent Technologies, Murray Hill, New Jersey

ANTHONY G. EVANS, Harvard University, Cambridge, Massachusetts

RICHARD W. GOETTLER, McDermott Technology-Lynchburg Research Center, Lynchburg, Virginia

MARTIN P. HARMER, Lehigh University, Bethlehem, Pennsylvania

JONATHAN LIPOWITZ, Dow Corning Corporation, Midland, Michigan

KRISHAN L. LUTHRA, General Electric Corporate Research and Development, Schenectady, New York

PAUL D. PALMER, Thermo Fibergen, Bedford, Massachusetts

KARL M. PREWO, United Technologies Research Center, East Hartford, Connecticut

RICHARD E. TRESSLER, Pennsylvania State University, University Park

DAVID WILSON, 3M Corporation, St. Paul, Minnesota

National Materials Advisory Board Staff

SANDRA HYLAND, senior program manager

CHARLES T. HACH, research associate

JANICE M. PRISCO, project assistant

ROBERT E. SCHAFRIK, director (until November 1997)

RICHARD CHAIT, director (after February 1998)

Government Liaison Representatives

ERNEST CHIN, Army Research Laboratory, Aberdeen Proving Ground, Maryland

JAMES A. DICARLO, NASA Lewis Research Center, Cleveland, Ohio

STEVEN FISHMAN, Office of Naval Research, Arlington, Virginia

RONALD KERANS, Wright-Patterson Air Force Base, Ohio

S. CARLOS SANDAY, Naval Research Laboratory, Washington, D.C.

MERRILL SMITH, U.S. Department of Energy, Washington, D.C.

NATIONAL MATERIALS ADVISORY BOARD

ROBERT A. LAUDISE (chair), Lucent Technologies, Murray Hill, New Jersey

G.J. ABBASCHIAN, University of Florida, Gainesville

MICHAEL I. BASKES, Sandia/Livermore National Laboratory, Livermore, California

JESSE (JACK) BEAUCHAMP, California Institute of Technology, Pasadena

FRANCIS DiSALVO, Cornell University, Ithaca, New York

EARL DOWELL, Duke University, Durham, North Carolina

EDWARD C. DOWLING, Cyprus Amax Minerals Company, Englewood, Colorado

THOMAS EAGAR, Massachusetts Institute of Technology, Cambridge

ANTHONY G. EVANS, Harvard University, Cambridge, Massachusetts

JOHN A. GREEN, The Aluminum Association, Washington, D.C.

SIEGFRIED S. HECKER, Los Alamos National Laboratory, Los Alamos, New Mexico

JOHN H. HOPPS, JR., Morehouse College, Atlanta, Georgia

LISA KLEIN, Rutgers, The State University of New Jersey, Piscataway, New Jersey

MICHAEL JAFFE, Hoechst Celanese Corporation, Summit, New Jersey

SYLVIA M. JOHNSON, SRI International, Menlo Park, California

HARRY LIPSITT, Wright State University, Yellow Springs, Ohio

ALAN G. MILLER, Boeing Commercial Airplane Group. Seattle, Washington

RICHARD S. MULLER, University of California, Berkeley

ROBERT C. PFAHL, Motorola, Schaumburg, Illinois

ELSA REICHMANIS, Lucent Technologies, Murray Hill, New Jersey

KENNETH L. REIFSNIDER, Virginia Polytechnic Institute and State University, Blacksburg

JAMES WAGNER, Case Western Reserve University, Cleveland, Ohio

BILL G.W. YEE, Pratt and Whitney, West Palm Beach, Florida

Preface

The U.S. Department of Defense and the National Aeronautics and Space Administration requested that the National Research Council (NRC) conduct a study to recommend future research and development for advanced ceramic fibers and fiber coatings for high-temperature ceramic matrix composites (CMCs). The scope of this study was limited to fibers and their coatings or interfaces, independent of CMC processing and matrix materials, to bring to the forefront the current limitations on the strength and toughness of CMCs, particularly at high temperatures. This report represents the work of the Committee on Advanced Fibers for High-Temperature Ceramic Composites, under the auspices of the National Materials Advisory Board, which was established by the NRC for this purpose.

The properties of the principal classes of high-performance synthetic fibers, as well as several methods of synthesizing and processing them, were discussed in a 1992 NRC report entitled High-Performance Synthetic Fibers for Composites. This report included an excellent assessment of fibers for polymer matrix and metal matrix composites and CMCs, carbon-carbon composites, as well as fibers for nonstructural applications. The 1992 report, however, did not address microstructure/property relationships in ceramic fibers or the relationship between processing and property retention at elevated temperatures. Since the publication of the 1992 report, the need for improved fiber coatings has been more widely identified as critical. In light of the continuing demand for higher temperature performance, this report is focused on the capabilities and requirements of ceramic fibers and ceramic fiber coatings.

The need for improved high-temperature materials is evident in the continuing drive by industry, government, and academia to improve the performance, efficiency, and durability of components used in high-temperature applications. For example, a recent NRC report, Intermetallic Alloy Development: A Program Evaluation, describes the objective of the Oak Ridge National Laboratories intermetallics program as the development of intermetallic alloys for high-temperature structural applications. A 1996 NRC report, Coatings for High-Temperature Structural Materials: Trends and Opportunities, discusses ways to protect the metallic components of turbine engines from their operating environments so they can be used at higher temperatures. Because of the inherent stability of CMCs at high temperatures, they continue to hold great promise for use at high temperatures. Successful implementation of CMCs, however, will require assessing the performance and cost of the constituent fibers and fiber coatings. Therefore, the committee was asked to fulfill the following objectives in this study:

• Characterize the current state of the art in high-temperature fibers and interface materials and identify current domestic and foreign research and development capabilities and production capabilities.

• Assess the capability of current fibers to meet future performance needs.

• Recommend promising directions for research on fibers and coatings to improve performance at high temperatures.

• Identify materials processing technologies that have the potential to produce high-temperature ceramic fibers and coatings cost effectively.

• Identify incentives for and barriers to the development of commercial-scale high-temperature fibers for low-volume applications.

Initially, the committee had intended to address Japanese fiber and coating efforts in a separate section. Given the advances that have been made in the United States and Europe, however, the committee determined that a section dedicated solely to Japanese efforts was not warranted. The state of the art in ceramic fiber and coating technology—in the United States, Europe, and Japan—is discussed in Chapter 3.

To address the study objectives, the committee met four times over a period of 15 months interspersed with several teleconferences. Two of the face-to-face meetings were focused on gathering information, and two were devoted to analyzing information and producing the report. Representatives of the National Aeronautics and Space Administration, the U.S. Department of Defense, and the U.S. Department of Energy materials programs, as well as representatives of nongovernmental entities, were invited to discuss the long-term material performance requirements of high-temperature components and the capability of current materials to fulfill them. Current producers of CMCs were requested to define fiber requirements for composite fabrication, as well as composite performance and supply capabilities.

One of the information gathering meetings was held in parallel with the American Ceramic Society's 21st Annual Conference on Composites, Advanced Ceramics, Materials, and Structures, in Cocoa Beach, Florida. At this meeting, the committee attended 15 presentations given by representatives of industry, government, and academe from the United States, Japan, and Germany. Presentations were, for the most part, focused on the current status and future directions of fiber and coatings technologies. In addition, two of the major manufacturers of jet engines presented the general requirements for CMCs to be used in gas turbine engines. The information collected by the committee was used to assess the current state of the art in ceramic fiber and coating technology and to determine the direction researchers and manufacturers should take to further these technologies. At a second information gathering meeting, held at the National Academy of Sciences in Washington, D.C., several representatives of CMC manufacturers discussed their requirements for ceramic fibers and ceramic fiber coating capabilities.

After reviewing these briefings, the committee considered the following questions:

• What requirements for fibers and interfaces are created by CMC processing?

• What are the major CMC markets/applications?

• What are the requirements for fibers and fiber coatings for these markets?

• Do available fibers meet or come close to meeting CMC requirements now?

• How sensitive are current and potential CMC applications to the costs of fibers?

The committee then considered future needs and opportunities for improving the performance and lowering the cost of ceramic fibers and coatings. The discussion included processing improvements that have the potential for improving fibers and coatings, as well as mechanisms for reducing (to some extent) the cost of developing and manufacturing these mate rials. Finally, the committee developed the conclusions and recommendations presented in this report.

Acknowledgments

The Committee on Advanced Fibers for High-Temperature Ceramic Composites gratefully acknowledges the information provided by the following individuals: Hans-Peter Baldus, Bayer AG; Ted Besmann, Oak Ridge National Laboratory; John Brennan, United Technologies Research Center; Mike Burke, Westinghouse; Phillip Craig, DuPont Lanxide, Inc.; Said El-Rahaiby, Department of Defense-Ceramics Information Analysis Center; Dick Enghalt, Synterials; Doug Freitag, Dupont Lanxide; Hans Friedericy, AlliedSignal; Randy Hay, Wright Patterson Laboratory; Bill Hong, Institute for Defense Analysis; K. William Householder, Dow Corning; Hiroshi Ichikowa, Nippon Carbon Company; Kioyshi Kumagawa, UBE Industries; Withold Kowbel, Materials and Electrochemical Research Group; Gary Linsey, Pratt and Whitney; Richard Lowden, Oak Ridge National Laboratory; Michael Millard, General Electric; Peter Morgan, Rockwell International; Koichi Niihara, Osaka University; Paul Nordine, Containerless Research Corporation; John Porter, Rockwell International; Michael Sacks, University of Florida; Andrew Szweda, Dow Corning; Michio Takeda, Nippon Carbon Company; Tom Tompkins, 3M Company; Richard Wagner, McDermott Technologies, Inc.; and Richard Weber, Containerless Research Corporation.

The committee would like to thank the American Ceramic Society for granting the National Research Council permission to use the following figures in this report: Figures 3-1, 3-2, 3-6, 3-8, 3-12, 3-13, 3-15, 3-16, 3-17, 3-21, 3-23, 3-24, 4-2, 5-2, 5-3, 6-6, 6-8, 6-9, 6-10,6-12, 6-14,6-15, 6-16,6-17, and 6-18. These figures were reprinted with permission of The American Ceramic Society, Post Office Box 6136, Westerville, Ohio 43086-6136. The American Ceramic Society maintains copyright to all of the aforementioned figures. All rights reserved.

This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the NRC's Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The content of the review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their participation in the review of this report: Norbert S. Baer, New York University; John J. Brennan, United Technologies Research Center; I. Wei Chen, University of Pennsylvania; Barry S. Draskovich, Allied Signal; Sylvia M. Johnson, SRI International; Harry A. Lipsitt, Wright State University; David B. Marshall, Rockwell International Science Center; and Dennis C. Nagle, Johns Hopkins University.

While the individuals listed above have provided many constructive comments and suggestions, responsibility for the final content of this report rests solely with the authoring committee and the NRC.

The committee wishes to express its sincere appreciation to the staff of the National Materials Advisory Board for its unswerving support. Sandra Hyland, senior program officer, and Charles Hach, research associate, dedicated much time and energy to bringing the report into being. Janice Prisco very effectively handled many issues as the senior project assistant.

The committee chair especially thanks the committee members for their dedication to producing a high-quality report on a tight schedule. Without their freely given time and efforts, this report would not have been possible.

Contents

EXECUTIVE SUMMARY




1  INTRODUCTION

Approach
Potential Ceramic Matrix Composite Applications
Composite Materials
Ceramic Fibers and Coatings
Engineering Requirements
Report Organization


2  CURRENT AND FUTURE NEEDS
Implementation of New Materials
Ceramic Matrix Composite Design and Life Predictions
Ceramic Matrix Composite Applications and Requirements
Manufacturing Requirements
Implications for Fiber Properties


3  STATE OF THE ART IN CERAMIC FIBER PERFORMANCE
Candidate Fibers
Temperature and Time Dependence of Properties of Non-Oxide Fibers
Temperature and Time Dependence of Properties of Oxide Fibers
Performance Characteristics Compared to Performance Goals
Recommendations and Future Directions


4  CERAMIC FIBER PROCESSING
Non-Oxide Fiber Processing
Oxide Fiber Processing
Recommendations and Future Directions


5  MATERIALS AND MICROSTRUCTURES
Opportunities for Fiber Development
Polycrystalline Oxides
Polycrystalline Silicon Carbide
Amorphous Fibers
Recommendations and Future Directions


6   INTERFACIAL COATINGS
Coatings for Non-Oxide Composites
Oxide Fiber Coatings
Recommendations and Future Directions


7  COST ISSUES
Price vs. Costs
Types of Cost
General Fiber Manufacturing Costs
Manufacturing Ceramic Fibers
Oxides vs. Non-Oxides
Findings
Conclusions
Recommendations and Future Directions


8  RECOMMENDATIONS AND FUTURE DIRECTIONS
Engineering Data
Fiber Coatings
Development of Oxide Fibers
Development of Non-Oxide Fibers
Manufacturing Costs
Priorities


REFERENCES


BIOGRAPHICAL SKETCHES OF COMMITTEE MEMBERS

Tables, Figures, and Boxes



TABLES
ES-1 Typical Property Ranges for Ceramic Fibers

2-1 CMC Performance Attributes and Weaknesses
2-2 Industrial Power Generation Applications
2-3 Aircraft Applications
2-4 Space Applications (United States)
2-5 CMC Processing Environments

3-1 Commercially Available and Developmental Ceramic Fiber for CMCs
3-2 Selected Commercially Available Ceramic Fibers

4-1 Processes for Commercial and Developmental Polymer-Derived Ceramic Fibers
4-2 Composition and Precursors of Commercially Available Oxide Fibers


FIGURES
1-1 Ideal stress-strain behavior of continuous fiber-reinforced CMC compared to the stress-behavior of an unreinforced matrix
1-2 A schematic representation of the constituents of a CMC
1-3 A scanning electron micrograph of the fracture surface of a Hi-Nicalon fiber-reinforced SiC matrix composite made by chemical vapor infiltration (CVI) showing fiber pullout
1-4 A set of "Saturn" turbine engine combustor liners (inner and outer) fabricated by DuPont Lanxide Composites, Inc.
1-5 Eight-harness woven cloth
1-6 Single filament of boron nitride-coated Nippon Carbon Nicalon^TM non-oxide ceramic fiber
1-7 Tow of 3M Nextel 610 polycrystalline ceramic oxide fibers

2-1 Cross section of the hot stage components of a General Electric large utility gas turbine
2-2 Thermal performance requirements for various high-temperature applications
2-3 3M Type 203 Nextel fiber-reinforced SiC matrix ceramic composite candle filter

3-1 Strength vs. temperature of SiC-based fibers and oxide fibers
3-2 (a)Young's moduli vs. temperature for SiC-based fibers. (b) Young's moduli vs. temperature for oxide fibers
3-3 Tensile strength vs. temperature of various SiC fibers
3-4 Fast fracture strength at temperature for commercial fibers based on silicon compounds and alumina
3-5 Room-temperature strength retention after short thermal exposure (one to ten hrs) of commercial silicon-based and alumina-based fibers
3-6 Tensile strength of SiC fibers after heat treatment in argon for one hour
3-7 Room-temperature tensile strength of SiC fibers after 10-hour thermal exposure in argon
3-8 Tensile strength of silicon carbide fibers after aging for 10 hours at 1,550°C (2,822°F) in argon
3-9 Tensile strength of SiC fibers after exposure for 10 hours in dry air at 1,400°C (2,552°F)
3-10 Tensile strength of SiC fibers after heat treatment at 1,000°C (1,832°F) in air
3-11 Structure of the diffusion barrier coating forming on oxidation at the surface of SiBN₃C
3-12 Temperature dependence of electrical conductivity in heat-treated Si-C-O fibers
3-13 Relationship between electrical resistivity and C/Si molar ratio in Si-C fiber
3-14 One-hour stress relaxation ratio of polymer-derived SiC fibers and other polycrystalline SiC and Al₂O₃ fibers
3-15 Rupture strength of SiC fibers in air (open symbols) and argon (closed symbols) at 1,200°C (2,192°F) () and 1,400°C (2,552°F) ()
3-16 Thermal activation plot for Hi-Nicalon fast fracture and rupture strength in air
3-17 Thermal activation plots for average strength of as-produced SiC fibers
3-18 Tensile strength as a function of test temperature for Nextel 610 and Nextel 720 single fibers
3-19 Typical creep curves for polycrystalline alumina-based oxide fibers at 1,090°C (1,994°F) in air
3-20 Creep rate of Nextel 720 compared to other commercial oxide fibers
3-21 One-hour bend stress relaxation ratio for directionally solidified eutectic YAG/alumina fibers grown by edge-defined film growth (EFG), for polycrystalline alumina-based fibers, and for c-axis sapphire fibers
3-22 Stress rupture plot for Nextel 610 fibers
3-23 The 100-hour rupture strength of polycrystalline and monofilament alumina-based fibers
3-24 Thermal activation plots for average strength of as-produced Al₂O₃-based fibers at a gauge length of ~ 25 mm (1 in)
3-25 Comparison of optimum strength map of A1₂O₃ fiber-reinforced CMC with strength behavior of an advanced superalloy

4-1 Typical processing scheme for producing ceramic fibers from organometallic polymer precursors
4-2 Schematic illustration of the pyrolysis process
4-3 Flow chart of chemical processing of ceramic oxide fibers
4-4 Schematic illustration of the dry-spinning process
4-5 Differential thermal analysis (DTA), differential thermogravimetric analysis (DTGA), and thermogravimetric analysis (TGA) of chemically-derived alumina fiber
4-6 æ-Al₂O₃ fiber with large grain size resulting from low nucleation density
4-7 Nextel 610 fiber showing small grain size resulting from the addition of nucleation agents

5-1 Predicted creep rates for alumina fibers as a function of aspect ratio using a two-dimensional model
5-2 Tensile creep rate of A1₂O₃-SiC nanocomposite containing 5 volume percent of 0.15 µm (0.006 mils) SiC particles and undoped Al₂O₃ of the same grain size
5-3 The steady-state tensile creep rate for undoped alumina and alumina doped with 1,000 ppm Y₂O₃
5-4 High resolution secondary ion mass spectroscopy compositional maps showing dopant segregation in yttrium and lanthanum doped alumina

6-1 Tensile test results of Nicalon fiber-reinforced SiC matrix composites prepared by CVI
6-2 Schematic representations of the progression of oxidation along the fiber/coating/matrix interface of uniaxial SiC/C/SiC composites with fiber ends exposed
6-3 Depth of oxidation of fiber coatings for uniaxial SiC/C/SiC composites with fiber ends exposed
6-4 Tensile test results of Hi-Nicalon fiber-reinforced SiC-Si matrix composites prepared by melt infiltration
6-5 Tensile test results for hot pressed Nicalon SiC fiber-reinforced glass-ceramic matrix composites with a BN fiber coating
6-6 Depth of coating oxidation in Hi-Nicalon fiber-reinforced SiC-Si matrix composites prepared by melt infiltration
6-7 Polished section of a hot pressed SiC fiber-reinforced glass-ceramic matrix composite with a BN fiber coating exposed to a tensile stress rupture experiment at 1,200°C (2,192°F), 69 MPa (10 ksi), 11,725 hours in air
6-8 A schematic representation of the matrix cracking on exposure to stresses above the matrix cracking strength in continuous fiber-reinforced ceramic composites
6-9 Schematic representations of the progression of oxidation in a crack/matrix/fiber coating region shown in the elliptical region in Figure 6-8
6-10 Porous alumina fiber coating deposited from a mixture of boehmite and Darvan C
6-11 Effect of thermal exposure on the ambient stress-strain behavior for all-oxide composites in the 0°/90° orientation
6-12 Nextel 720 fiber-reinforced calcium aluminosilicate glass-ceramic matrix composites show significant modulus retention following carbon interface burnout indicating matrix-to-fiber load transfer
6-13 Hexagonal ß-alumina (NaA1₁₁O₁₇) and magnetoplumbite (hibonite [CaA1₁₂O₁₉]) structures
6-14 Transmission electron micrographs of cracks propagating transgranularly along the basal planes of the textured hibonite interphase
6-15 Scanning electron micrograph showing crack deflection at alumina-monazite interfaces
6-16 Scanning electron micrograph and transmission electron micrograph showing debonding along scheelite-Nextel 610 interfaces
6-17 Schematic illustration of immiscible liquid coating technique
6-18 Example of oxide fiber coatings deposited via the immiscible liquid coating technique (ErTaO₄-coated Nextel 610 fibers in an alumina-ErTaO₄ matrix)
6-19 LaPO₄ coatings on Nextel 720 fibers deposited by heterocoagulation techniques

7-1 Elements in manufacturing costs
7-2 Fiber price premiums
7-3 Projected costs of fibers as a function of annual production


BOXES
1-1 Damage Tolerant Ceramic Matrix Composites

2-1 Sample CMC Applications





Acronyms




ACF activated carbon fiber

BSR bend stress relaxation

CFCC continuous fiber-reinforced ceramic composite
CMC ceramic matrix composite
CTE coefficient of thermal expansion
CVD chemical vapor deposition
CVI chemical vapor infiltration

DTA differential thermal analysis
DTGA differential thermal gravimetric analysis

EFCC externally fired combined cycle

IGCC integrated gasification combined cycle
IMC intermetallic matrix composite

MMC metal matrix composite

NMR nuclear magnetic resonance
NRC National Research Council

PAN polyacrylonitrile
PFBC pressurized fluid bed combustion
PMC polymer matrix composite

R&D research and development

TEM transmission electron microscopy
TGA thermal gravimetric analysis
TPV thermophotovoltaic

UCSB University of California-Santa Barbara
UF University of Florida
UHC unburned hydrocarbons
UTS ultimate tensile strength

Executive Summary

High-temperature ceramic fibers are the key components of ceramic matrix composites (CMCs). Ceramic fiber properties (strength, temperature and creep resistance, for example)—along with the debonding characteristics of their coatings—determine the properties of CMCs. This report outlines the state of the art in high-temperature ceramic fibers and coatings, assesses fibers and coatings in terms of future needs, and recommends promising avenues of research. CMCs are also discussed in this report to provide a context for discussing high-temperature ceramic fibers and coatings.

Continuous ceramic fibers in CMCs offer the potential for high performance in high-temperature corrosive environments. CMCs can be fabricated into complex shapes for use in thermostructural environments much the way carbon composites are fabricated for use in less aggressive environments. Unlike monolithic ceramic bodies in which the mechanical strength is determined by the largest flaw in a critical position, CMCs are relatively flaw tolerant because the load is borne by a multiplicity of fibers. Thus, CMCs have toughness and damage tolerance comparable to metals with the added advantages of lower density and greater stability at high temperatures.

For the past 15 years, research and development of CMCs has been sustained because of their potential for military and commercial applications. The applications of interest include (1) aircraft engine components, such as combustors, turbines, compressors and exhaust nozzles; (2) ground-based and automotive gas turbine components, such as combustors, first and second stage turbine vanes and blades, and shrouds; (3) aerospace engines for missiles and reusable space vehicles; and (4) industrial applications, such as heat exchangers, hot gas filters, and radiant burners.

Technical shortcomings must still be overcome, however, before CMCs can be widely used in thermostructural applications. These shortcomings provide research opportunities, particularly for the development of fibers and fiber coatings. The following list describes these opportunities:

• Fiber coatings for non-oxide composites have demonstrated adequate performance in short-life applications (e.g., rocket nozzles). These fiber coatings have also been demonstrated to be adequate in composite samples (e.g., test coupons) during long-time exposures to stress at high temperatures in laboratory tests. However, fiber coating technologies for long-life applications (e.g., turbine engine components) have not been demonstrated in component testing.

• Several coatings for oxide ceramic fibers have enabled model composite systems to demonstrate damage tolerant behavior. However, no fiber coatings have been proven to be effective in actual (as opposed to model) oxide composite systems.

• Non-oxide fibers are generally creep resistant but lack chemical stability; they are prone to oxidation. Consequently, stress oxidation limits the durability of non- oxide composites, especially at intermediate temperatures (i.e., 700 to 900°C [1,292 to 1,652°F]) under cyclic loading conditions. There are no known concepts for producing a coating that can prevent oxidation of the fibers for more than 100 hours, after matrix cracks occur (and remain open). Longer life non-oxide composites will require a combination of oxidation resistant fiber coatings and matrix sealing concepts that protect the fiber from oxidation—particularly when the composite is subject to cyclic thermomechanical loads that can cause sealed cracks to reopen. Such concepts have been developed but have not been tested.

• Oxide fibers are generally environmentally stable but are subject to excessive creep at high temperatures. Preliminary work indicates that microstructural modifications have the potential for enhancing creep resistance.

A significant barrier to progress is the paucity of engineering data¹ on CMCs, which reflects a lack of access to data generated by classified projects, as well as a general lack of engineering data. Advances have been further impeded by the high cost of coated fibers, which are attributable to low production volumes and, in some cases, to high precursor costs. Consequently, the stability and breadth of the vendor base for fibers, coatings, and coated fibers is questionable.

APPROACH

To conduct this study, the National Research Council (NRC) convened a committee with expertise in ceramic fiber processing, nanoparticle reinforced ceramics research, ceramic fiber research, high-temperature ceramic fiber-matrix interfacial coatings, and synthesis of nanomaterials. The committee also has expertise in ceramic fiber economics, including cost analysis and the determination of the commercial potential of advanced materials. To accomplish the overall objective of identifying research directions to meet the mate rial property requirements of advanced fibers and coatings for high-temperature ceramic composites, the committee took the following steps:

• characterized the current state of the art in high-temperature fibers and interface materials and identified current domestic and foreign capabilities (both R&D and production capabilities)

• assessed the capabilities of current fibers to meet future performance needs

• recommended promising research directions for developing fibers and coatings for improved performance in high temperature applications

• identified materials processing technologies that have the potential to produce high-temperature ceramic fibers and coatings cost effectively

• identified incentives for and barriers to the development of commercial-scale high-temperature fibers for low-volume applications

By limiting the scope of this study to fibers and their coatings, independent of CMC processing and matrix materials, the committee was able to focus on issues that limit the strength and toughness of CMCs, particularly at high temperatures. The discussion of composite materials in this report is limited to providing a context for discussions, conclusions, and recommendations regarding ceramic fibers and coatings.

HIGH-TEMPERATURE CERAMIC FIBERS

Non-Oxide Ceramic Fibers

Non-oxide fibers are typically based on silicon carbide (SiC) and are fabricated by several processes: (1) spinning a melt of organometallic precursors (the most favored route); (2) spinning a solution of organometallic precursors (dry spinning); (3) extrusion spinning of a ceramic powder in a polymeric binder; (4) chemical vapor deposition (CVD) of vapor species onto a monofilament core; and (5) conversion of carbon fiber to SiC using Si-containing vapor species.

Spun non-oxide fibers (i.e., those made by the first three processes above) are produced in tows consisting of hundreds of filaments with diameters of 10 to 20 µm. Typical ranges of properties are given in Table ES-1. All of these fibers are based on SiC except for amorphous Si-B-N-C, a promising new fiber.

The best non-oxide fibers have good creep resistance but are susceptible to degradation by formation of an amorphous silica layer upon oxidation. This layer offers some resistance to further oxidation, but prolonged exposure to oxidizing environments results in oxidative embrittlement of the composite.

Oxide Ceramic Fibers

All currently available commercial oxide fibers are based on aluminum oxides. Examples are alumina (A1₂O₃) yttrium aluminum garnet (YAG) and mullite (3A1₂O₃-2SiO₂).Commercial polycrystalline oxide fibers are produced by spinning and hydrolyzing precursors. First a fiber precursor solution is filtered and concentrated to remove excess solvent, forming a viscous spin dope. Then, continuous filaments are extruded by spinning. The filaments are pyrolyzed to remove volatile components and then heat treated above 800°C (1,472°F) to crystallize and sinter the fiber. Polycrystalline oxide fibers are produced in tows of 200 to 1,000 fibers with diameters of 10 to 16 µm (0.39 to 0.63 mils). Typical ranges of properties are listed in Table ES-1.

Oxide fibers are inherently resistant to oxidation but have limited creep resistance because of higher diffusivities compared to SiC. Creep rates decrease with increasing grain size, but this advantage is offset by decreasing strength. However, significant improvements have been achieved in the past decade by reducing the amorphous phase content at the grain boundaries of oxide fibers.

FIBER COATINGS

Damage tolerance in a CMC requires a weak interface between the fibers and the matrix; fiber coatings are engineered to provide this weak interface. Fiber coatings also protect the fiber from environmental attack during composite fabrication and use.

Coatings for Non-Oxide Fibers

Most work on coatings has been concentrated on SiC fibers used in non-oxide CMCs. Tough composite behavior has been reported only when coatings consist predominantly of carbon or boron nitride (BN). Oxidation of the fiber/coating/matrix interface is a major limitation for non-oxide composites. This interface may be exposed to an oxidizing environment when cracks develop in the matrix (which otherwise acts as a barrier to oxygen ingress) under load. Oxidation of the fiber/coating/matrix interface degrades the fiber and its debonding characteristics, reducing both the strength and toughness of the composite.

Carbon coatings can be formed by the in-situ decomposition of Si-C-O fibers or applied by CVD. Carbon coatings are typically 0.1 to 0.3 µm (0.004 to 0.01 mils) thick. During oxidation of the fiber coating, carbon is converted to carbon monoxide (CO) or carbon dioxide (CO₂), leaving a gap. The SiC fiber then oxidizes, forming a silicate glass (SiO₂), which tends to close the gap. However, if the gap is not closed quickly enough, oxidation may proceed along the fiber/matrix interface, bonding the fiber to the matrix and causing embrittlement of the composite. This phenomenon, sometimes called pesting, is most prevalent at intermediate temperatures.

BN is the only other fiber coating that has been demonstrated to enable "tough" composite behavior. BN coatings are typically deposited via CVD and are 0.3 to 0.5 µm (0.01 to 0.02 mils) thick. Because the oxidation product of a BN coating is a borate (B₂O₃) glass, which protects against oxidation at intermediate temperatures, the degradation rate of BN-coated fibers is lower than for carbon-coated fibers. As the temperature is increased, the B₂O₃ reacts with SiO₂ (the oxidation product of either the SiC fiber or the matrix) to form a borosilicate glass. However, in wet atmospheres, the B₂O₃ volatilizes (as boron hydroxides), thus compromising its ability to prevent further oxidation of the fiber, which ultimately leads to embrittlement of the composite.

Coatings for Oxide Fibers

Oxide coatings that are chemically compatible with commercially available oxide fibers have been identified, but adequate debonding and friction have yet to be demonstrated in a composite system. Several coating strategies and materials are being investigated. For example, porous and fugitive coatings have been used, as well as porous matrices (with no coating). These provide toughness as long as sintering between the fibers and particles in the matrix/coatings can be suppressed.

Layered oxides are being studied because of their potential debonding characteristics. A class of sheet silicate minerals known as fluoromicas exhibit easy delamination along crystal planes but are chemically incompatible with current fibers and matrices. Beta alumina (ß-Al₂O₃) and magnetoplumbites are compatible with alumina fibers and have sufficiently low fracture energies to provide the weak fiber/matrix interface needed for damage tolerant composites. The magnetoplumbite mineral hibonite, CaAl₁₂O₁₉, has been studied extensively, but Ca tends to diffuse into matrices during hot pressing, which degrades composite properties. Other layered oxides that have been studied preliminarily include perovskites, such as KCaNb₃O₁₀ and BaNd₂Ti₃O₁₀.

"Nonwetting" oxides have shown particular promise. Nonwetting refers to the tendency of the interface between the coating and the fiber to debond readily. The monazite class of compounds (e.g., lanthanide phosphates) and sheelites fall into this category. These compounds have high melting points and are chemically stable (when stoichiometric).

Several coating technologies are used for oxide fibers. CVD can be used, but maintaining stoichiometry is difficult. Solution-based precursors are better for controlling stoichiometry, but maintaining coating uniformity and preventing bridging between fibers in a tow is difficult. The electrostatic attraction between particles in a slurry and the fiber can also be used to deposit fiber coatings. All currently proposed oxide coatings have promising features but have questionable debonding and frictional characteristics, as well as uncertain processing technologies.

RECOMMENDATIONS AND IMPACTS

The committee formulated many specific recommendations on both fibers and coatings that are presented in the body of the report, the most important of which are listed here. The committee also describes the anticipated impact of these recommendations on the field, which is crucial for moving CMC technology forward. These recommendations are followed by a discussion of the rationale for their prioritization. Although the focus of this report is on high-temperature ceramic fibers and coatings, the committee believes that a database of CMC properties is needed to establish research goals and performance criteria to design better fibers and coatings for future CMCs.

Recommendation 1. Building and disseminating an engineering database for actual (as opposed to model) CMCs is essential. The committee recommends the following:

• Existing nonclassified data that are not broadly accessible because they are associated with classified or restricted studies should be made generally available. Wherever possible, agency-owned engineering data should be made accessible, and new programs should avoid restricting data.

• Data that are currently classified should be reassessed to determine if they can be declassified and, if so, they should be made more generally available.

• Low risk government-sponsored insertion programs for CMCs should be expanded to demonstrate the field performance of CMC components.

• Standardized tests for obtaining engineering data on CMCs should be developed and instituted.

Impact. Researchers would be able to determine when ceramic fibers and coatings limit CMC properties and focus materials research on overcoming these limitations. Design engineers would gain confidence in using CMCs when data is available on CMC component performance in operating environments (i.e., field tests). Furthermore, following the recommendations listed above would elucidate composite failure modes, facilitate vetting less viable CMC systems, and establish a foundation for breaking out of the market size-cost impasse that has stymied investments in facilities for the production of fibers and coatings.

Recommendation 2. Coatings for high-temperature ceramic fibers must be improved.

• Non-oxide fiber coatings research should be focused on:
- concepts that enable high durability with cracked matrices at the "pest" temperature
- a system approach that includes concepts for improving the oxidation resistance of fiber coatings in dry and moist atmospheres and "sealing" matrix cracks as they form
- investigating regenerative in-situ coatings

• Research dedicated to improving the fiber/matrix interface for oxide CMCs should focus on:
- investigations of weakly bonded, thermally stable oxide coatings for oxide-oxide composites (e.g., nonwetting coatings)
- approaches that do not use coatings (e.g., porous matrices)

• emerging amorphous non-oxide fibers, such as Si-B-N-C fibers, to verify the stability, creep resistance, and utility of the in-situ coatings
• microstructural refinement to improve performance in crystalline non-oxide fibers

• capital nonintensive approaches
• processes that leverage past investments
• in-situ and liquid precursor coatings

DISCUSSION OF PRIORITIES

The five recommendations above fall into three categories. Recommendation 1 addresses increasing the knowledge base of, and confidence in, existing CMC technology. The committee anticipates that applications for CMCs will expand if engineering designers have access to the information they need to make materials selection decisions. Thus the committee places a high priority on this recommendation.

Recommendations 2, 3, and 4 (listed in order of decreasing priority) are related to performance, which is also considered to be a high priority. Recommendation 2 is the most important in this category. The oxidation resistance of oxide fibers is attractive, but poor creep resistance is a significant limitation. Thus, Recommendation 3 addresses the need to improve this property. Recommendation 4 (regarding non-oxide fibers) is last in this category because the committee concluded that resources directed toward property improvement in fiber coatings and oxide fibers was more important. The committee is satisfied that the preliminary properties reported for Si-B-N-C amorphous fibers are sufficiently attractive to stimulate the research needed to verify them.

Recommendation 5 is last, not because cost is unimportant but because, at the current stage of the technology, performance rather than cost has limited the use of CMCs. The committee concluded that, at this time, improving properties should be of higher priority than reducing costs.

NOTE

¹Engineering data are defined as coupon tests, subelement tests, and component tests, as well as design and life prediction results.