New Structural Materials Technologies:

Opportunities for the Use of Advanced

Ceramics and Composites

September 1986

NTIS order #PB87-118523
Recommended Citation:

U.S. Congress, Office of Technology Assessment, New Structural Materials Technologies:

Opportunities for the Use of Advanced Ceramics and Composites–A Technical IMemorandum, OTA-TM-E-32 (Washington, DC: U.S. Government Printing Office, September 1986).

Library of Congress Catalog Card Number 86-600551

For sale by the Superintendent of Documents

U.S. Government Printing Office, Washington, DC 20402
Foreword

This technical memorandum responds to a joint request from the House Committee on Science and Technology and the Senate Committee on Commerce, Science, and

Transportation to analyze the military and commercial opportunities presented by new

structural materials technologies and outline the Federal research and development priorities which are consistent with those opportunities. This memorandum is part of a

larger assessment which will address the impact of advanced structural materials on

the competitiveness of the U.S. manufacturing sector, and offer policy options for accelerating the commercial utilization of these materials.

New structural materials—ceramics, polymers, metals, or hybrid materials derived

from these, called composites—open a promising avenue to renewed international competitiveness of U.S. manufacturing industries. There will be many opportunities for use

of the materials in aerospace, automotive, industrial, medical, and construction applications in the next 25 years.

In recent years, several excellent studies have been carried out on both ceramics

and polymer matrix composites. This memorandum draws on this body of work and

presents a broad picture of where these technologies stand today and where they are

likely to go in the future.

OTA appreciates the assistance provided by the contractors, advisory panel, and

workshop participants, as well as the many reviewers whose comments helped to ensure the accuracy of the report.

#Ak4 + c d

.

JOHN H. GIBBONS

Director

.,, ///
New Structural Materials Technologies Advisory Panel

Rodney W. Nichols, Chairman

Executive Vice President, The Rockefeller University

J. Michael Bowman Arthur F. McLean

Director, Composites Venture Manager, Ceramics Research

E.I. du Pent de Nemours & Co. Ford Motor Co.

Robert Buffenbarger Joseph Panzarino

Chairman, Bargaining Committee Director, R&D of High Performance Ceramics

International Association of Machinists Norton Co.

Joel Clark Dennis W. Readey

Associate Professor of Materials Systems Chairman, Ceramics Engineering Department

Massachusetts Institute of Technology Ohio State University

Laimonis Embrekts B. Walter Rosen

Consultant President

Dix Hills, NY Materials Sciences Corp.

Samuel Goldberg Amy L. Walton

President, INCO-US Inc. Member, Technical Staff

New York, NY Jet Propulsion Laboratory

Sheldon Lambert Alvin S. Weinstein

Consultant Consultant

Piano, TX Concord, NH

James W. Mar Dick Wilkins

Professor, Department of Aeronautics and Director, Center for Composite Materials

Astronautics University of Delaware

Massachusetts Institute of Technology
OTA Project Staff

Lionel S. Johns, Assistant Director

OTA Energy, Materials, and International Security Division

Peter D. Blair, Energy and Materials Program Manager

Richard E. Rowberg, Energy and Materials Program Manager, June 1985—December 1985

Gregory Eyring, Project Director, November 1985—present

Thomas E. Bull, Project Director, June 1985—November 1985

Joan Adams, Analyst, June 1985–January 1986

Laurie Evans, Analyst, June 1986—present

Administrative Staff

Lillian Chapman Linda Long
Contractors

Elaine P. Rothman

Massachusetts Institute of Technology

Cambridge, MA

D.B. Marshall

Rockwell International Science Center

Thousand Oaks, CA

J.E. Ritter

University of Massachusetts

Amherst, MA

J.J. Mecholsky

The Pennsylvania State University

State College, PA

Julie M. Shoenung

Joel P. Clark

Massachusetts Institute of Technology

Cambridge, MA

David W. Richerson

Ceramatec, Inc.

Salt Lake City, UT

R. Nathan Katz

Alfred L. Broz

Army Materials Technology Laboratory

Watertown, MA

Reginald B. Stoops

R.B. Stoops & Associates

Newport, RI

Carl Zweben

General Electric Co.

Philadelphia, PA

Kenneth L. Reifsnider

Virginia Polytechnic Institute and

State University

Blacksburg, VA

John Busch

Frank Field

Materials Modeling Associates

Massachusetts Institute of Technology

Cambridge, MA

Dale E. Chimenti

Thomas J. Moran

Joseph A. Moyzis

Air Force Materials Lab

Wright Patterson AFB

Dayton, OH
Ceramics Workshop Participants: Nov. 14-15, 1985

Alfred Broz

Army Materials Technology Laboratory

Watertown, MA

Joel Clark

Massachusetts Institute of Technology

Cambridge, MA

Tom Henson

GTE Products

Towanda, PA

R. Nathan Katz

Army Materials Technology Laboratory

Watertown, MA

Jack Mecholsky

Pennsylvania State University

State College, PA

David Richerson

Ceramatec, Inc.

Salt Lake City, UT

John Ritter

University of Massachusetts

Amherst, MA

Elaine Rothman

Massachusetts Institute of Technology

Cambridge, MA

Composites Workshop Participants: Nov. 21-22, 1985

Joel Clark

Massachusetts Institute of Technology

Cambridge, MA

John DeVault

Hercules Aerospace Co.

Salt Lake City, UT

Joseph Moyzis

Wright-Patterson AFB

Dayton, OH

Thaddeus Helminiak

Wright-Patterson AFB

Dayton, OH

Kenneth Reifsnider

Virginia Polytechnic Institute & State University

Blacksburg, VA

Reginald B. Stoops

R.B. Stoops & Associates

Newport, RI

Charles L. Tucker

University of Illinois

Urbana, IL

Carl Zweben

General Electric Co.

Philadelphia, PA

vii
Future Applications of Ceramics Workshop Participants:

Dec. 9, 1985

Charles Berg

Northeastern University

Boston, MA

Bill Kuhn

Zesto-Therm

Cincinnati, OH

Rustum Roy

Pennsylvania State University

State College, PA

Jim Wimmer

Garrett Corp.

Phoenix, AZ

Albert Paladino

Advanced Technology Ventures

Boston, MA

Future Applications of Composites Workshop Participants:

Dec. 10, 1985

Charles Berg

Northeastern University

Boston, MA

Bob Hammer

Boeing Commercial Aircraft Co.

Kirkland, WA

D. William Lee

Arthur D. Little, Inc.

Cambridge, MA

Joe Lees

E.I. du Pent de Nemours & Co.

Wilmington, DE

Paul McMahan

Celanese Research Corp.

Summit, NJ

Darrell H. Reneker

National Bureau of Standards

Gaithersburg, MD

John Riggs

Celanese Research Corp.

Summit, NJ

Charles Segal

Omnia

Raleigh, NC

. . . Vlll
Acknowledgments

In addition to those cited above, the following are thanked for useful review comments:

Seymour Newman

Ford Motor Co.

Jerome Persh

U.S. Department of Defense

Richard Helmuth

Portland Cement Association

Charles West

Resin Research Laboratories

Henry Brown

Society for the Advancement of Material and

Process Engineering

Joseph Kirkland

E.I. du Pent de Nemours & Co.

Craig Ballinger

Federal Highway Administration

Michael Greenfield

National Aeronautics and Space Administration

Manfred Kaminsky

Surface Treatment Science International

Harris Burte

Air Force Materials Lab

Jim Stephan

Coors Biomedical

Maxine Savitz

Garrett Corp.

Robert Gottschall

U.S. Department of Energy

Geoffrey Frohnsdorff

National Bureau of Standards

Bernard Maggin

National Research Council

Robert Murphy

BASF Structural Materials, Inc.

Glenn Kuebeler

Hercules Aerospace Co.

Dick Dauksys

Hexcel

E.R. Bell

Shell Chemical

Clyde Yates

U.S. Polymeric

Gail DiSalvo

Ciba-Geigy

David Forest

Ferro Corp.

Nick Spencer

Ciba-Geigy

ix
Contents

Page

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Market Opportunities for Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Research and Development Priorities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Market Opportunities for Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Research and Development Priorities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

General Prerequisites for the Use of Advanced Structural Materials . . . . . . . . . . . . . . . . . 7

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . . 11

The Advanced Structural Ceramics Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Properties of Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Ceramic Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Ceramic Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Design, Processing, and Testing of Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Health Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Applications of Structural Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Future Trends in Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Research and Development Priorities for Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

The Polymer Composites Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Composite Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Properties of Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Design, Processing, and Testing of Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Health and Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Applications of Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Future Trendsin Polymer Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Research and Development Priorities for Polymer Matrix Composites . . . . . . . . . . . . . . . 62

General Prerequisites of the Use of Advanced Structural Materials . . . . . . . . . . . . . . . . . . . 66

Education and Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Multidisciplinary Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Integrated Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Systems Approach to Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

List of Tables

Table No. Page

1. Some Future Applications of Structural Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2. Comparison of Physical and Mechanical Properties of Common

Structural Ceramics With Steel and Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3. Fracture Toughness and Critical Flaw Sizes of Monolithic and

Composite Ceramic Materials Compared With Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4. Common Processing Operations for Advanced Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . 16

5. Selected Processes for the Production of Ceramic Coatings and for the

Modification of Ceramic Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

6. Characteristics and Properties of Ceramic Coatings Often Considered Desirable . . . . 20

7. Comparison of the Mechanical Properties of Various Cements and Aluminum. . . . . . 21

8. Comparison of Some Possible Production-Level NDE Techniques

for Structural Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Contents—continued

Table No. Page

9. Future Diesel Engine Technology Development Scenario . . . . . . . . . . . . . . . . . . . . . . . . . 32

10. Structural Ceramic Technology: Federal Government Funded R&D. . . . . . . . . . . . . . . . 38

11. Breakout of the Fiscal Year 1985 Structural Ceramic Budget According to

the R&D Priorities Cited in the Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

12. Production Techniques for Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

13. NDE Techniques Appropriate for Production, Finished Product, Depot,

and Field-Level Inspections of Polymer Matrix Composite Structures . . . . . . . . . . . . . 49

14. Budgets for Polymer Matrix Composite R&D in Fiscal Years 1985 and 1986 . . . . . . . . 62

15. Hypothetical Multidisciplinary Design Team for a Ceramic Component. . . . . . . . . . . . 66

List of Figures

Figure No. Page

l. The Family of Structural Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2. Probability of Failure of a Ceramic Component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3. Estimated Scenario for Implementation of Ceramic Components

in Structural Application Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4, Projected U.S. Markets for Structural Ceramics in the Year 2000 . . . . . . . . . . . . . . . . . . 25

5. Load-Deflection Curve for a SiC/Glass-Ceramic Composite . . . . . . . . . . . . . . . . . . . . . . . 36

6. Composite Reinforcement Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7. Comparison of the Specific Strength and Stiffness of Various Composites and Metals 44

8. Comparison of General Characteristics of Thermoses and Thermoplastic Matrices. . . . 45

9. Composite Aircraft Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

xi
OVERVIEW

The past 25 years have witnessed remarkable

developments in structural materials technologies;

new nonmetallic materials, such as ceramics and

polymer matrix composites, are replacing metals

in a wide variety of applications, ranging from

cutting tools to tennis rackets. The new materials represent both an opportunity and a challenge

for U.S. manufacturing industries. In an era of

concern about the long-term economic viability

of the manufacturing sector, they open a promising avenue to renewed international competitiveness, new industries, and employment opportunities,

By the year 2000, large markets for ceramic and

composite products will be found in military,

aerospace, automotive, medical, and construction

applications, However, these opportunities cannot be taken for granted. The United States does

not enjoy a comfortable lead (indeed it often lags)

in new structural materials technologies; key advances continue to come from abroad. Moreover,

through well-coordinated government-industry efforts, several countries have succeeded in bringing ceramic and composite products to the market years in advance of comparable U.S. products.

Japan in particular has initiated aggressive programs to commercialize its evolving materials

technologies, In order to realize the promise represented by the new materials, a consensus is developing in the materials community that U.S.

policymakers in government, universities, and industries will have to come together to define feasible goals and materials applications, coordinate

research and development efforts toward these

goals, and, especially, focus on reducing the barriers to commercial introduction of advanced ceramic and composite products.

This technical memorandum assesses the opportunities for the use of structural ceramics and polymer matrix composites in the next 25 years, outlines the research and development priorities

implied by those opportunities, and concludes

with a discussion of some key prerequisites for

their realization. A parallel treatment of metal matrix composites, as well as a discussion of public

policy options for accelerating the development

and commercialization of advanced structural materials, is deferred until the final report. ]

‘Projected publication date September 1987.

1
SUMMARY

In the past 25 years, new structural materials,

such as ceramics, polymers, and composites, have

brought a revolutionary change to the field of

engineering. Never before have engineers had a

comparable opportunity to design the materials

they use as well as the structures they build. In

fact, with advanced ceramics and composites, the

very concepts of materials and structures merge

together, joined by the new concept of integrated

design. Such designs consolidate discrete parts and

functions into a single, multifunctional structure,

leading to highly efficient use of materials and

lower overall costs.

In the future, new structural materials will provide a powerful leverage point for the manufacturing sector of the economy: not only can ceramic and composite components deliver superior

performance, they also enhance the performance

and value of the larger system in which they are

incorporated. When this multiplier effect is taken

into account, the impact of advanced structural

materials on the gross national product (GNP),

balance of trade, and employment could be dramatic. All of the industrialized countries have recognized the opportunities and are competing actively for shares of the large commercial and

military markets at stake.

The Federal Government plays two important

roles in the research and development of advanced

ceramics and composites: one military, and the

other nonmilitary. The Department of Defense

funds some 35 percent of Federal R&D on structural ceramics and about 70 percent of that on

polymer matrix composites. The military and

nonmilitary agencies have very different funding

goals and materials requirements.

In the case of the military, the government is

the customer for materials technology and hardware. It has an interest in securing stable, domestic

sources of material supply. Although materials

cost is an important consideration, typically the

performance advantages obtained are more important. Military applications often provide a first

“window” for the use of advanced materials; however, the technologies may or may not be “spun

off” into commercial applications. Because of the

emphasis on high performance and the fact that

production volumes are typically low, often neither the materials nor the production methods are

appropriate for commercial applications. For national security reasons, restrictions are placed on

the dissemination of DOD-funded research results

in scientific publications and at meetings; export

controls may also be placed on shipments of the

materials abroad. These restrictions tend to further discourage the commercial utilization of materials technologies developed by the military. In

the future, demand for high performance materials in such applications as aircraft, missiles, and

space-based weapons systems will increase, making these among the fastest growing applications

of structural ceramics and composites.

About half of total Federal spending for advanced ceramics and polymer matrix composites

R&D is nonmilitary in nature, including most of

that funded by the Department of Energy, the

National Aeronautics and Space Administration

(NASA), the National Science Foundation, the

National Bureau of Standards, and the Bureau of

Mines. These agencies generally do not act as the

procurers of hardware, but instead fund materials research ranging from basic science to technology demonstration programs, according to

their mission objectives. Where appropriate, they

openly seek to transfer materials technology to

the private sector. For a variety of reasons, the

success rate of these efforts has been low: the technology may require significant further development prior to utilization; the market opportunities for the technology may be too long-term or

poorly defined; or key decisionmakers within the

receiving company may not be committed to commercializing the technology. Federal nonmilitary

materials R&D clearly represents a resource that

can be exploited more effectively by U.S. companies in the future.

An important policy objective for the future

will be to encourage industrial investment in research, development, and commercialization of

ceramic and composite materials. Factors discouraging these investments include: fragmentation of industries and markets, which precludes

2
3

any one segment from undertaking the necessary

research; concerns that R&D investments cannot

be protected because of the high transferability

of the technologies; and the relatively high cost

of capital in the United States compared with competitor nations such as Japan. Federal efforts to

facilitate collaborative R&D arrangements between industries, universities, and government

laboratories, to tighten patent protection for industrial products and processes, and to articulate

policy clearly on export controls and on product

liability issues, are likely to be very important.

Ultimately, however, a strong U.S. competitive

position depends on a judgment on the part of

manufacturers that the new materials offer cost

and performance advantages in the marketplace.

Introduction

Structural materials may be classified as ceramics, polymers, metals, or hybrid materials derived from these, called composites, Wood and

oyster shells are examples of composites that have

been exquisitely engineered by nature to perform

their unique functions. Similarly, manmade composites are engineered materials whose properties

can be tailored to match the design requirements

of a particular structure. The most common composites consist of a host (matrix) material which

is reinforced with particles, whiskers (short single crystals), or fibers of a second material; thus,

there are ceramic matrix composites, polymer matrix composites, and metal matrix composites.

Although ceramics, polymers, and metals generally have complementary combinations of properties, there are many applications in which

designers can choose from several alternative

materials. A new structural material must therefore demonstrate cost or performance advantages

(often both) if it is to be chosen over a more familiar material. Very often, ceramics and composites are competing with well-characterized

alloys of aluminum, titanium, steel, and hightemperature superalloy, Furthermore, new alloys

are being developed and metal processing methods are continually improving. Thus, the minimum cost and performance requirements which

must be satisfied by a ceramic or composite are

becoming more stringent. This competition

among different materials has two important

corollaries. First, the characteristics of structural

ceramics and composites make them more suitable for some applications than others; it is important for government and industry policymakers to identify the opportunities correctly.

Second, the many technical and economic factors

which could tip the balance in favor of one material or another make the market penetration of

ceramics and composites difficult to predict.

Ceramics

Ceramics encompass such a broad class of materials that they are more conveniently defined in

terms of what they are not, rather than what they

are. Accordingly, they may be defined as all solids

which are neither metallic nor organic. Compared

with metals, ceramics have superior wear resistance, high-temperature strength, and chemical

stability; they also generally have lower electrical and thermal conductivity, and lower toughness. The low toughness of ceramics (brittleness)

causes them to fail suddenly when applied stress

is sufficient to propagate cracks which originate

at microscopic flaws (e.g., cracks, voids, or inclusions as small as 20 micrometers) in the material.

The actual stress level at which this occurs can

be very high (e.g., 300,000 psi, or 2,069 MPa) and

theoretically could be much higher, if flaw sizes

could be reduced. Unpredictable failure caused by

poor control over flaw populations is the most

serious handicap to the use of structural ceramics

in load-bearing structures.

Several approaches have been taken to improve

the toughness of ceramics. The most satisfactory

is to design the microstructure of the material to

resist the propagation of cracks. Ceramic matrix

composites, which contain dispersed ceramic particulate, whiskers or continuous fibers, are an

especially promising technology for toughening

ceramics. A different approach is the application

of a thin ceramic coating to a metal substrate. This

yields a component with the surface properties of

a ceramic combined with the high toughness of

metal in the bulk.

Most advanced structural ceramics are fabricated by consolidation of a compact of pure,

finely divided oxide or nonoxide powders in a furnace at high temperatures. However, another class
4 —

of ceramics, called chemically bonded ceramics,

develops its strength at room temperature, typically through chemical reactions involving the uptake of water. Traditional cement pastes and concretes fall in this category. Because these materials

normally contain large flaws which limit their

strength, they are not ordinarily considered “advanced. ” However, in recent years the properties

of chemically bonded ceramics have improved

dramatically as new processing methods have reduced the strength-limiting flaws. The low cost

and flexible fabrication methods of chemically

bonded ceramics could permit them to displace

plastics and metals in a wide variety of applications.

Market Opportunities for Ceramics

Market demand for structural ceramics is not

driving their development in most applications at

the present time. In 1983, the world market for

advanced structural ceramics was estimated as

$250 million, primarily in heat- and wear-resistant

applications. Projections to the year 2000 place

the U.S. market variously between $1 and $5 billion, spread among many new applications discussed below. The larger estimates assume substantial utilization of ceramics in automotive heat

engines, long predicted to be the most important

application of structural ceramics. The analysis

below indicates that significant use of ceramics

in automotive heat engines is not likely by the

year 2000. However, large near-term opportunities are available in other products, such as medical devices.

Japan is the principal competitor to the United

States in advanced ceramics technology. Japanese

companies have made a long-term commitment

to the development of advanced ceramics which

looks beyond the current weak market demand.

They have mounted a well-coordinated effort

which includes: increasing the quality and availability of raw materials; optimizing fabrication

processes; installing the most advanced processing and quality control equipment; initiating manufacturing development; and aggressive marketing of evolving materials. This commitment to the

future of ceramics is likely to give Japanese companies the early lead in capturing the large nearterm markets.

Current Production

Ceramics such as alumina, silicon nitride, and

silicon carbide are in production for wear parts,

cutting tools, bearings, and coatings. The market share for ceramics in these applications is generally less than 5 percent, but substantial growth

is expected, and the U.S. markets for the ceramic

components alone could be over $2 billion by the

year 2000. Research and development funding is

currently being provided by industry and is driven

by competition in a known market.

Ceramics are also in limited production (in

Japan) in discrete engine components such as

turbochargers, glow plugs, and precombustion

chambers. Current military applications include

radomes, armor, infrared windows, and heat

sources.

Near-Term Production

Near-term production (next 10 to 15 years) is

expected in advanced bearings, bioceramics, construction applications, heat exchangers, electrochemical devices, discrete components in automobile engines, and military applications. The

technology feasibility has generally been demonstrated, but scale-up, cost reduction, and design

optimization are required. Large markets are at

stake: especially promising are bioceramics for

dental and orthopedic implants, and chemically

bonded ceramics for construction applications.

Wisely directed government funding will be

required to solve the remaining problems and

achieve a production capability competitive with

foreign sources.

Long-Term Applications

Long-term applications (beyond 15 years) are

those which require solution of major technical

and economic problems. These include an advanced automotive turbine engine, the advanced

ceramic diesel (although ceramics could be used

in military versions of these engines at an earlier

date), some electrochemical devices, military components, and heat exchangers. A variety of other

turbine engines, especially turbines for aircraft

propulsion and for utility-scale power generation,

should also be categorized as long term. In general, the risks are perceived by U.S. industry to
5

be too high to justify funding the needed research.

Advances in these applications are likely to be

driven by government funding.

Research and Development Priorities

The following hierarchy of R&D priorities is

based on the technical barriers that must be overcome before ceramics can be used in the applications discussed above.

Very Important

Processing Science .—This is the key to understanding how processing variables such as temperature, composition, and particle size distribution are connected to the desired final properties

of the ceramic.

Environmental Behavior.—In many applications, ceramics are required to withstand high

temperature and corrosive or erosive environments. Information on the long-term behavior of

ceramics in these environments is essential to predict the service life of ceramics in those applications.

Reliability .—The reliability of advanced ceramics and ceramic composites is the single most

important determinant of success in any application. Progress requires advances in brittle materials design, process control, nondestructive evaluation, understanding crack growth processes,

and life prediction.

Ceramic Composites.—These novel materials

offer an exciting opportunity to increase the

strength and toughness of ceramics.

Important

Joining. -Joining of ceramics to metals, glasses,

and other ceramics is necessary when ceramic

components are incorporated into larger systems.

The principles developed in joining research can

also be applied to coating technology.

Tribology.—This is the study of friction, wear,

and lubrication of surfaces in relative motion. It

is especially relevant to understanding the degradation of ceramic wear parts, bearings, and the

lubrication requirements of high-temperature ceramic engines.

Standardization and Testing.—Although standards exist for ceramic refractories and brick, at

present there is a lack of standards for advanced

ceramics, whether for testing procedures, compositions, or processing history. Such standards

are a prerequisite for confident design and industry acceptance of ceramics.

Desirable

Chemically Bonded Ceramics.—These materials, which include advanced cement pastes and

concretes, represent an outstanding potential for

low-cost, net shape fabrication of ceramic structures. The cost savings associated with the use of

improved concretes in highways and bridges are

likely to be very large compared with the research

investment required.

Polymer Matrix Composites

Polymer matrix composites are organic polymers which have been reinforced with a variety

of short or continuous fibers to provide added

strength and stiffness. The composite is designed

so that the mechanical loads to which the structure is subjected in service are supported by the

reinforcement. The amount of reinforcement and

its geometry can be varied across the structure,

resulting in highly efficient use of material, consolidation of parts, and weight savings.

Polymer matrix composites are often divided

into two categories: reinforced plastics and “advanced composites. ” The distinction is based

on the level of mechanical properties (usually

strength and stiffness); however, there is no unambiguous line separating the two. Reinforced

plastics, which are relatively inexpensive, typically consist of polyester resins reinforced with

low-strength glass fibers; they have been in use

for 30 or 40 years in applications such as boat

hulls, corrugated sheet, pipe, automotive panels,

and sporting goods. Advanced composites, which

are used primarily in the aerospace industry, consist of fiber and matrix combinations which yield

superior strength and stiffness, They are relatively

expensive and typically contain a large percentage of high-performance continuous fibers (such

as high-strength glass, graphite, aramid, or other

organic fibers). In this technical memorandum,
6

market opportunities for both reinforced plastics

and advanced composites are considered.

Chief among the advantages of polymer composites is their light weight coupled with high stiffness and strength along the direction of the reinforcement. This combination is the basis of their

usefulness in aircraft, automobiles, and other

moving structures. Other desirable properties

include corrosion and fatigue resistance. One

generic limitation of polymer matrix composites

is temperature; an upper limit for service temperatures with present composites is about 600° F

(316° C). With additional development, temperatures above 8000 F (4270 C) might be achieved.

In addition to their superior physical and mechanical properties, polymer composites offer

many design and manufacturing advantages. Perhaps the most important of these is the opportunity to consolidate a large number of parts into

one, thus reducing assembly costs. This capability has already been demonstrated in aircraft and

automotive applications. For example, it has been

estimated that the 1,500 structural parts in an automobile could be reduced to a few hundred using

composites, resulting in major savings in tooling

and manufacturing costs.

Most advanced composites today are used in

the aerospace industry. They are fabricated by a

laborious process called “lay -up.” This typically

involves placement of sequential layers of polymer-impregnated fiber tapes on a mold surface,

followed by heating under pressure to cure the

lay-up into an integrated structure. Although

automation is beginning to speed up this process,

production rates are still too slow to be suitable

to high-volume, low-cost applications such as

automotive production lines. New composite

fabrication methods which are much faster and

cheaper will be required before composites can

successfully compete with metals in these applications.

Market Opportunities for

Polymer Matrix Composites

Fiberglass composites have been in use since the

1940s, and about 85 percent of the materials used

in the reinforced plastics/composites industry

today are glass fiber reinforced polyester resins.

Less than 2 percent of the materials are “advanced” composites such as those used in aircraft

and aerospace. Worldwide sales of advanced composites have been projected to grow by 15 percent annually for the remainder of the century,

increasing from a 1985 value of $1.4 billion to

nearly $12 billion by the year 2000. The industry

continues to be driven by aerospace markets, with

defense applications likely to grow by as much

as 22 percent per year in the next few years.

Current Production

Aerospace applications of advanced polymer

composites account for about 60 percent of current sales. Sporting goods, such as golf clubs and

tennis rackets, account for another 20 percent.

The sporting goods market is considered mature,

with projected annual growth rates of 3 percent.

Automobiles and industrial equipment round out

the current list of major users of advanced composites, with a 15 percent share.

Near-Term Production

Composites were introduced into the horizontal stabilizer of the F-14 fighter in 1970, and have

now become the baseline materials in high performance fighter and attack aircraft. The next major challenge for composites will be use in large

military and commercial transport aircraft. Composites currently comprise about 3 percent of the

structural weight of commercial aircraft such as

the Boeing 757, but could eventually account for

more than 65 percent. No technical barriers are

foreseen which might prevent this scenario, although design and certification databases for civil

applications need to be established. Since fuel savings are a major reason for the use of composites

in commercial aircraft, continued low fuel prices

could delay their exploitation.

The largest volume opportunity for polymer

composites is in the automobile. Composites currently are in limited production in body panels,

drive shafts, and leaf springs. By 1995, composite unibody frames could be introduced in limited

production. The principle advantage of a composite unibody would be the potential for parts

consolidation resulting in lower assembly costs.

If composites were to be used extensively in unibodies, the effects on the way automobiles are

designed, built, and serviced would be dramatic.
7

However, production methods some 10 times

faster than current methods will have to be developed in order to take advantage of this opportunist y.

Additional near-term markets for polymer composites include medical implants, reciprocating industrial machinery, storage and transportation of

corrosive chemicals, and military vehicles and

weapons.

Long-Term Applications

Beyond the turn of the century, composites

could be used extensively in construction applications such as bridges, buildings, and manufactured housing. Realization of this opportunity will

depend on development of cheaper materials and

on designs which take advantage of compounding benefits of composites, such as reduced weight

and increased durability. In space, a variety of

composites will be used in the proposed aerospace

plane, and composites are being considered for

the tubular frame of the space station. Composites of all kinds, including metal matrix, ceramic

matrix, and polymer matrix, will be a central feature of space-based weapons systems such as those

under consideration for ballistic missile defense.

Research and Development Priorities

Polymer matrix composites have achieved an

excellent service record and experience indicates

that reliable composite structures can be fabricated. However, in many cases the technology has

outrun the basic understanding of these materials. In order to generate improved materials and

to design and manufacture composites more costeffectively, the following needs should be addressed.

Very Important

Processing Science.—Development of new,

low-cost fabrication methods will be critical for

composites. An essential prerequisite to this is a

sound scientific basis for understanding how process variables affect final properties.

Impact Resistance.—This property is crucial to

the reliability and durability of composite structures.

Delamination.—A growing body of evidence

suggests that this is the single most important

mode of damage propagation.

Interphase.—The poorly understood interracial

region between the fiber and matrix has a critical

influence on composite behavior.

Important

Mechanical Properties.—Better scientific understanding and modeling in the following areas

would permit development of better materials and

improved designs: strength, fatigue, and fracture.

Environmental Effects.—The environments to

which composites are subjected can have a deleterious effect on their long-term reliability. Investigation of the effects of moisture and temperature are particularly important.

Reinforcement Forms and Hybrid Composites.—Innovations in both of these areas would

provide opportunities to improve properties and

reduce costs. The large number of variables makes

a purely empirical approach very inefficient.

Test Methods.—Lack of standardized test methods contributes to the considerable variability in

reported property values.

Desirable

Viscoelastic and Creep Properties.—Deformation resulting from sustained loading can have an

adverse effect on structural performance. A

greater database is required, especially for compressive loading.

General Prerequisites for the Use of

Advanced Structural Materials

Education and Training

At present, the demand for scientists and engineers trained in ceramics and composites technologies outstrips the supply. Education may be the

most effective tool available to government for

accelerating the development of advanced materials in the long run. Several aspects should be

considered. Undergraduate introductory courses

in ceramics and composites should be offered to
8

engineering majors. Undergraduate courses specializing in design with these materials should also

be available to all students in applied mechanics,

including mechanical and civil engineers. Provision should be made through seminars and short

courses for continuing education of engineers in

other areas who want to learn more about ceramics and composites. Also, education of designers, planners, managers, and the general public

through newspapers, magazines, and television

will be especially important in creating demand

for products made from these high-technology

materials.

Multidisciplinary Approach

In recent years there has been an increasing appreciation of the fact that many problems in science and technology are most effectively solved

through the combined efforts of specialists from

a variety of disciplines. In no case is this more

apparent than with advanced materials. The nature of ceramics and composites is such that the

process which transforms the raw materials into

the finished product integrates the discrete steps

of design, manufacturing, and testing into a coherent whole. The familiar distinctions between

the product and the material from which it is made

become blurred, in a way which is qualitatively

different from our experience with traditional materials. This integrative property implies that a

multidisciplinary team of experts is more likely

to solve the problems associated with the development and application of advanced materials

than an alternative approach involving individual researchers.

Integrated Design

The outstanding opportunities presented by ceramics and composites do not lie in part-for-part

replacement of metal in current designs; rather,

new designs which incorporate many parts and

functions into one are made possible. These new

structural shapes and systems for carrying loads

may not look at all like metal designs. Such a design capability will require the development of sophisticated software for computer modeling and

analysis, as well as an extensive database on materials properties. Perhaps the most important requirement is for a better scientific understanding

of how the properties of the microscopic constituents determine the overall behavior of the

structure. Many of the research and development

priorities above are aimed at providing this information.

Systems Approach to Costs

High cost is often cited as the largest barrier

to the use of advanced ceramics and composites.

Indeed, on a dollar-per-pound basis, today’s ceramics and composites often cost several times as

much as the materials they would replace. Although costs can be expected to decline as production volumes increase and manufacturing technologies mature, the baseline costs per pound of

many ceramics and composites may always be

higher than those of steel and aluminum. However, rather than a barrier, cost may in fact be

a potential advantage of the new materials, if the

“system costs, “ including materials, manufacturing, and life cycle of a structure, are all taken into

account. Like multidisciplinary research and integrated design, a systems approach to costs is implied by the holistic nature of advanced materials.

In conclusion, advanced structural materials

represent evolving technologies which will dramatically affect the U.S. economy. The near-term

markets for these materials are large, and the longterm opportunities are just beginning to be recognized. Although the United States faces stiff international competition for the developing world

markets, the United States does possess several

natural strengths which should enable it to excel

in this competition. These include a strong university system, a well-developed computer and software industry, and the largest domestic market

for materials in the world. The challenge to national policy makers is to define a Federal role

which facilitates the maximum exploitation of

these natural strengths. The most important policy issues and options for addressing them will

be explored in the final report.
INTRODUCTION

The past 25 years have witnessed an unprecedented explosion of new structural materials available to the designer and engineer. Polymers,

ceramics, and composites promise to bring a revolution in the performance of a wide variety of

engineering components and structures, and in the

way those structures are designed and fabricated.

Today the pace of technological change in this

field is rapid, and the scope of the opportunity

presented by the new materials is only beginning

to be recognized.

In addition to changing the engineering landscape, advanced structural materials have also

altered the boundaries of traditional policy concerns relating to materials. Historically, the Federal interest in materials has centered around the

problem of shortages of supply of certain “critical” minerals. For example, the United States is

dependent on foreign sources for virtually all of

its chromium, manganese, cobalt, and platinum

group metals. These metals are important ingredients in a variety of steel alloys and superalloy.

To a limited extent, ceramics and composites,

which are made from plentiful raw materials, can

alleviate this problem through substitutional However, the potential of the new materials goes far

beyond simple substitution; they provide performance advantages in both military and commercial applications which cannot be achieved

with metals. Ceramics and composites are now

considered to be “critical” in their own right, both

for national defense and future economic competitiveness. Therefore, the appropriate Federal role

in accelerating the development and commercialization of these materials is likely to be a subject

of active debate in the years ahead.

Advanced materials are often ranked with

microelectronics and biotechnology as the three

most promising “high-tech” industries of the future. Many countries around the world have recognized the exceptional promise of these materials

and have targeted them for priority development,

with the result that the United States, having pi-

‘U. S. Congress, Office of Technology Assessment, Strategic Afaterials: Technologies To Reduce LI. S. Import L’ulnerability, OTAITE-248 (Washington, DC: U.S. Government Printing Office, May

1985],

oneered many of the basic technologies, now finds

itself in a heated race with Japan and several countries in Europe to supply the growing world markets. Fundamental advances in these technologies

are now being made overseas, so that the United

States cannot afford to be complacent if it intends

to be competitive. It is important to note that in

most cases ceramic and composite structures do

not stand alone, but are instead components that

enhance the performance of much larger systems,

such as automobiles or aircraft. Thus, the materials have a significance to the economy extending far beyond the value of the materials themselves.

Structural materials may be classified as ceramics, polymers, or metals, as shown in figure

1. Two or more of these materials can be combined together to forma composite having properties superior to those of the constituents alone.

Figure 1.— The Family of Structural Materials

The family of structural materials includes ceramics, polymers, and metals. Reinforcements added to these materials

produce ceramic matrix composites (CMCs), polymer matrix

composites (PMCs), and metal matrix composites (MMCs).

Materials in the shaded regions are discussed in this report.

SOURCE: Off Ice of Technology Assessment

9
10

The most common form of composite is a host

material (matrix) which is reinforced with particles

or fibers of a second material. Composites are

classified according to their matrix phase; thus,

there are ceramic matrix composites (CMCs),

polymer matrix composites (PMCs), and metal

matrix composites (MMCs). Wood, jade, and oyster shells are natural composites which display

remarkable strength and durability. A common

manmade composite is glass fiber reinforced plastic (FRP), which combines the strength of the glass

with the toughness and corrosion resistance of

plastic,

Each type of structural material has its own advantages and disadvantages. For example, metals

are strong, tough, inexpensive, and readily formable, but are also heavy, chemically reactive, and

limited to service temperatures below about

1,900° F (1,038° C). Ceramics are hard, chemically stable, and useful at high temperatures, but

are also brittle and difficult to fabricate. Polymers

are light and easy to fabricate, but are relatively

weak and restricted to service temperatures below about 600° F (316° C).

Frequently, composites offer an excellent opportunity to eliminate some of the undesirable

properties while retaining the desirable ones. Metals can be reinforced with ceramic fibers to extend their useful strength to higher service temperatures; ceramics can be reinforced with ceramic

fibers to reduce their brittleness; and polymers can

be reinforced with ceramic or organic fibers to

make them, pound for pound, the strongest materials known. The great value of composites is

that the reinforcement type and geometry can be

varied to optimize the performance and minimize

the cost of the overall structure.

In the face of the challenge from ceramics and

composites, metals technologies have not remained static. New steel and aluminum alloys and

new processes such as rapid solidification and

powder metallurgy techniques have been developed. Metal matrix composites promise to increase the hardness and service temperature of metals without requiring a large investment in new

manufacturing plant and equipment. Also, various coatings and surface modification techniques

can be used to protect the metal so that it will not

deteriorate in hostile environments.

The diversity of structural materials available

today offers many options to the designer. Typically, a given structure can be fabricated from any

of a variety of candidate materials. Only rarely

will the performance requirements dictate a single choice. Nor does the challenge to the primacy

of metals come only from the most “advanced,”

most expensive composites or ceramics: unreinforced engineering polymers such as nylons or

polyamides, and chemically bonded ceramics (high

performance cements) offer an inexpensive and

effective alternative to metals in many applications. This capacity for materials substitution also

complicates the task of projecting the future shares

of materials in various markets. However, one can

predict with confidence that the properties and

manufacturing advantages of ceramics and composites will make them as familiar in the 21st century as metals are today.
CERAMICS

Ceramics may be defined as nonmetallic, inorganic solids. By far the most common of terrestrial materials, ceramics in the form of sand

and clay have been used for many thousands of

years to make brick, pottery, and artware. Modern structural ceramics bear little resemblance to

these “traditional” materials; they are made from

extremely pure, microscopic powders which are

consolidated at high temperatures to yield a dense,

durable structure.

The Advanced Structural

Ceramics Industry

The world market for advanced structural ceramics in 1983 was only $250 million. z Of this,

the Japanese market made up nearly half. These

ceramics were primarily used in heat- and wearresistant applications. The limited current markets for ceramics do not exert a strong “market

pull” on the technology; at present, the situation

is characterized by “technology push. ” In the next

10 to 15 years, however, the market opportunities for structural ceramics are expected to expand

rapidly (table l), such that by the year 2000 U.S.

markets are projected variously between $1 bil2G .B. Kenney and H .K, Bowen, “High Tech Ceramics in Japan:

Current and Future Markets, ” American Ceramic Society Bulletin

62(5):590, 1983.

Table 1 .—Some Future Applications of

Structural Ceramics

Application Performance advantages Examples

Wear parts High hardness. low friction

seals

bearings

valves

nozzles

Cutting tools High strength, hot hardness

Heat engines Thermal insulation, high

diesel components temperature strength, fuel

gas turbines economy

Medical Implants Biocompatibility, surface

hips bond to tissue, corrosion

teeth resistance

joints

ConstructIon Improved durability, lower

highways overall cost

bridges

buildings

SOURCE Off Ice of Technology Assessment

Sillcon carbide,

alumina

SIllcon nitride

Zlrconia, silicon

carbide, silicon nitride

Hydroxylapatite,

bioglass,

alumina,

zirconia

Advanced cements and

concretes

lion and $5 billion.3 This estimate reflects only

the value of the ceramic materials and components, and does not include the value of the overall systems in which they are incorporated. When

this multiplier effect is taken into account, a recent study has indicated that the country which

leads in the development of advanced ceramics

will reap large benefits in the form of jobs and

economic expansion.4

Properties of Ceramics

The properties of some common structural ceramics are compared with those of metals in table 2. In general, ceramics have superior hightemperature strength, higher hardness, lower density, and lower thermal conductivity than metals.

The principal disadvantage of ceramics as structural materials is the sensitivity of their strength

to extremely small flaws, such as cracks, voids,

and inclusions. Flaws as small as 10 to 50 micrometers can reduce the strength of a ceramic structure to a few percent of its theoretical strength.

Because of their small sizes, the strength-controlling flaws are usually very difficult to detect and

eliminate.

The flaw sensitivity of ceramics illustrates the

importance of carefully controlled processing and

finishing operations for ceramic components.

However, even with the most painstaking efforts,

a statistical distribution of flaws of various sizes

and locations will always exist in any ceramic

structure. Even “identically prepared” ceramic

specimens will display a distribution of strengths,

rather than a single value. Design with ceramics

is therefore a statistical process, rather than a deterministic process, as in the case of metals. The

situation is illustrated in figure 2.

The curve on the right of figure 2a represents

the distribution of strengths in a batch of identically prepared ceramic components. The curve on

the left is the distribution of stresses to which these

3Gr~~ Fischer, “Strategies Emerge for Advanced Ceramic Business, ” American Ceramic Society Buffetin 65(1):39, 1986.

4

Larry R. Johnson, Arvind P.S. Teotia, and Lawrence G. Hill,

“A Structural Ceramic Research Program: A Preliminary Economic

Analysis, ” Argonne National Laboratory, ANL/CNSV-38, 1983.

11
12

Table 2.—Comparison of Physical and Mechanical Properties of Common Structural Ceramics With

Steel and Aluminum Alloys. SiC: silicon carbide; Si3N4: silicon nitride; ZrO2

: zirconia

Strengtha

Thermal

Density a Room temperature at 1,095” C Hard nessb

conductivity

Material (g/cm3

) strength (M Pa) (M Pa) (kg/mmz

) 250/1 ,100° (W/mC)

Various sintered SiC materials . . . . 3.2 340-550 (flexure) 340-550 (flexure) 2,500-2,790 85/1 75

Various sintered Si3N4 materials . . 2.7-3.2 205-690 (flexure) 205-690 (flexure) 1,366 17/60

Transformation toughened ZrO, . . . 5.8 345-620 (flexure) — 625-1,125 1.713.5

Steels (4100, 4300, 8600, and

5600 series) . . . . . . . . . . . . . . . . . . 7-8 1,035-1,380 (tensile yield) useless 450-650 43

Aluminum alloy . . . . . . . . . . . . . . . . . 2.5 415-895 (tensile yield) useless 100-500 140-225

NOTE. 1 MPa = 145 psi = O 102 Kg/mm Y,

SOURCES: aR. Nathan Katz, “Applications of High Performance Ceramics in Heat Engine Design, ” Materials Science and Engineering 71 ‘227-249, 1985.

bElaine P Rothman, “Ultlmate Properties of Ceramics and Ceramic Matrix Composites, ” contractor report for OTA, December 1985

components are subjected in service, The overlap between the two curves, in which the stress

in service exceeds the strength of the ceramic, determines the probability that the part will fail.

There are several ways to reduce the probability of failure of the ceramic. One is to shift the

strength distribution curve to the right by elimination of the larger flaws (figure 2b). A second

approach is to use nondestructive testing or proof

testing to weed out those components that have

major flaws. This leads to a truncation of the

strength distribution, as shown in figure 2c. Although proof testing of each individual component is expensive and can introduce flaws in the

material which were not there originally, it is

widely used in the industry today.

A third approach is to design the microstructure of the ceramic to have some resistance to fracture (increased “toughness”), and hence, some tolerance to defects. Toughness is a measure of the

energy required to fracture a material in the presence of flaws. For a ceramic component under

stress, the toughness determines the critical flaw

size which will lead to catastrophic failure at that

stress. In fact, the critical flaw size increases with

the square of the toughness parameter; thus, an

increase in the material toughness of a factor of

3 leads to a ninefold increase in the flaw size

tolerance. Reduction in the flaw sensitivity of ceramics is especially important for applications involving a hostile environment which can introduce strength-degrading defects, and thus negate

all efforts to ensure reliability by identifying or

eliminating the largest preexisting flaws. Three recent developments have been shown to improve

the toughness of ceramics: microstructure design,

transformation toughening, and composite reinforcement.

Microstructure Design

The toughness of monolithic ceramics can be

improved considerably by refinement of the polycrystalline grain size and shape. The presence of

elongated fibrous grains, especially in ceramics

based on silicon nitride, has been shown to increase toughness by as much as a factor of 2 over

other monolithic ceramics, such as silicon carbide

and aluminum oxide.5 Numerous mechanisms

have been proposed to account for the observed

toughening: crack deflection, microcracking, residual stresses, crack pinning, and crack bridging. It is likely that several of these mechanisms

operate simultaneously in these materials. The

high toughness is accompanied by high strength,

both of which result from the modified microstructure. Commercial activity in the silicon

nitride-based materials is projected to expand rapidly by 1990 and many new applications should

be established by 1995.

Transformation Toughening

Transformation toughening is a relatively new

approach to achieving high toughness and

strength in ceramics and has great potential for

increasing the use of ceramics in applications requiring wear resistance. The key ceramic material is zirconium oxide (zirconia). Zirconia goes

through a phase transformation from the

‘Nitrogen Ceramics, F.L. Riley (ed, ) (Boston and The Hague: Martinus Nijhoff Publishers, 1983).
13

a)

b)

c)

Figure 2.— Probability of Failure of

a Ceramic Component

d Strength

The probability of failure of a ceramic component is the overlap between the applied stress distribution and the material

strength distribution, as shown in (a). This probability can be

reduced by reducing the flaw size (b), or truncation of the

strength distribution through proof testing (c).

SOURCE R Nathan Katz, “Applications of High Performance Ceramics in Heat

Engine Design,” Materials Science and Engineerfrig 71:227.249, 1985

tetragonal to the monoclinic crystal form while

cooling through a temperature of about 1,1500

C (2,102o F). This phase transformation is accompanied by an increase in volume of 3 percent, similar to the volume increase that occurs when water

freezes. By control of composition, particle size,

and heat treatment cycle, zirconia can be densified at high temperature and cooled such that the

tetragonal phase is maintained down to room temperature. When a load is applied to the zirconia

and a crack starts to propagate, the high stresses

in the vicinity of the crack tip catalyze the transformation of adjacent tetragonal zirconia grains

to the monoclinic form, causing them to expand

by 3 percent. This expansion of the grains around

the crack tip compresses the crack opening, preventing the crack from propagating. The result

is a ceramic that is both tough and strong.

Ceramic Matrix Composites

A variety of ceramic particulate, whiskers

(high-strength single crystals with length-to-diameter ratios of 10 or more), and fibers maybe added

to the host matrix material to generate a composite with improved fracture toughness. The presence of these additives appears to frustrate the

propagation of cracks by at least three mechanisms. First, when the crack tip encounters a particle or fiber which it cannot easily break or get

around, it is deflected off in another direction.

Thus, the crack is prevented from propagating

cleanly through the structure. Second, if the bond

between the reinforcement and the matrix is not

too strong, crack propagation energy can be absorbed by “pullout” of the fiber from its original

location. Finally, fibers can bridge a crack, holding the two faces together, and thus preventing

further propagation.

Table 3 presents the fracture toughness and critical flaw sizes (assuming a typical stress of 700

MPa, or about 100,000 psi) of a variety of ceramics and compares them with some common

metals. The toughness of monolithic ceramics generally falls in the range 3 to 6 MPa-m’ 1

, corresponding to a critical flaw size of 18 to 74

micrometers. With transformation toughening or

whisker dispersion, the toughness can be increased

to 8 to 12 MPa-m’ 2 (critical flaw size 131 to 294

micrometers); the toughest ceramic matrix composites are continuous fiber-reinforced glasses, at

15 to 25 MPa-m’ 1

. It is important to note that in

the latter composites the strength appears to be

independent of preexisting flaw size, and is thus

an intrinsic material property. Metals such as steel

have toughnesses above 40 MPa-m1 (some al-
14

Table 3.—Fracture Toughness and Critical Flaw

Sizes of Monolithic and Composite Ceramic

Materials Compared With Metals.a

A12O3: alumina; LAS: lithium aluminosilicate;

CVD: chemical vapor deposition

Fracture Critical

toughness flaw size

Material (MPa. m’/2

) (micrometers)

Conventional microstructure:

Al 203. . . . . . . . . . . . . . . . . .

Sintered SiC. . . . . . . . . . . .

Fibrous or interlocked

microstructure:

Hot pressed Si3N4 . . . . . . .

Sintered Si3N4 . . . . . . . . . .

SiA10N . . . . . . . . . . . . . . . .

Particulate dispersions:

Al2 05

-TiC . . . . . . . . . . . . . .

SiC-Ti B2 . . . . . . . . . . . . . . .

Si3N4

-TiC. . . . . . . . . . . . . . . .

Transformation toughening:

ZrO2

-MgO . . . . . . . . . . . . . .

ZrO2

-Y2O3 . . . . . . . . . . . . . .

A1203-zro2 . . . . . . . . . . . . .

Whisker dispersions:

Al2O3,-SiC . . . . . . . . . . . . . .

Fiber reinforcement:b

SiC in borosilicate glass .

SiC in LAS . . . . . . . . . . . . .

SiC in CVD SiC . . . . . . . . .

Aluminum c 

. . . . . . . . . . . . . . . . .

Steel c 

. . . . . . . . . . . . . . . . . . . . .

3.5-4.0 25-33

3.0-3.5 18-25

4.0-6.0

4.0-6.0

4.0-6.0

4.2-4.5

4.5

9-12

6-9

6.5-15

8-10

15-25

15-25

8-15

33-44

44-66

33-74

33-74

33-74

36-41

41

165-294

74-165

86-459

131-204

aAssumes a stress of 700 MPa ( - l@,000 Psi).

bThe strength of these composites is independent of preexisting flaw size.

cThe toughness of some alloys can be much higher.

SOURCES David W. Richerson, “Design, Processing Dewdopment, and Manufacturing Requirements of Ceramics and Ceramic Matrix Composites, ”

contractor report for OTA, December 1985, and Elaine P. Rothman,

“Ultimate Properties of Ceramics and Ceramic Matrix Composites,”

contractor report for OTA, December 1985

loys may be much higher), more than 10 times

the values of monolithic ceramics.

The critical flaw size gives an indication of the

minimum flaw size which must be reliably detected by any nondestructive test in order to ensure reliability of the component. Most NDE

methods cannot reliably detect flaws smaller than

about 100 micrometers, corresponding to a toughness of about 7’ MPa-m’ I. Toughnesses of 10 to

12 MPa-m” would be desirable for most components.

Ceramic Coatings

The operation of machinery in hostile environments (e. g., high temperatures, high mechanical

loads, or corrosive chemicals) often results in performance degradation due to excessive wear and

friction, and productivity losses due to shutdowns

caused by component failure. Frequently, the

component deterioration can be traced to deleterious processes occurring in the surface region

of the material. To reduce or eliminate such effects, ceramic coatings have been developed to

protect or lubricate a variety of substrate materials, including metals, ceramics, and cermets (ceramic-metal composites).

The coating approach offers several advantages. One is the ability to optimize independently

the properties of the surface region and those of

the base material for a given application. A second advantage is the ability to maintain close

dimensional tolerances of the coated workpiece,

since very thin coatings (of the order of a few

micrometers) are often sufficient for a given application. Further, cost savings are obtained by

using expensive, exotic materials only for thin

coatings and not for bulk components. This can

contribute to the conservation of strategically critical materials. Finally, it is often cheaper to recoat

a worn part than to replace it.

In view of these advantages, it is not surprising that ceramic coatings have found wide industry acceptance. For example, coatings of titanium

nitride, titanium carbide, and alumina can enhance the useful life of tungsten carbide or high

speed steel cutting tools by a factor of 2 to 5.6 In

1983, annual sales of coated cutting tools reached

about $1 billion.7 Ceramic coatings are also finding wide applications in heat engines. Low thermal conductivity zirconia coatings are now being tested as a thermal barrier to protect the metal

pistons and cylinders of advanced diesel engines.

In turbine engines, insulative zirconia coatings improve performance by permitting combustion gas

temperatures to be increased by several hundred

degrees (F) without increasing air-cooled component metal temperatures or engine complexity.8

Ceramic coatings have also been used to provide

an oxidation barrier on turbine blades and rings.

‘David W. Richerson, “Design, Processing Development, and

Manufacturing Requirements of Ceramics and Ceramic Matrix Composites, ” contractor report prepared for the Office of Technology

Assessment, December 1985.

7U. S. Department of Commerce, Bureau of the Census, Census

of Manufacturing, Fuels, and Electric Energy Consumed, 1984.

‘Tom Strangman, Garrett Corp., personal communication, August 1986.
15

Progress in the use of ceramic coatings in these

and other applications suggest that further research on new coatings and deposition processes

is likely to yield a high payoff in the future.

Design, Processing, and Testing

of Ceramics

It is in the nature of advanced structural materials that their manufacturing processes are additive rather than subtractive. They are not

produced in billets or sheets which are later rolled,

cut, or machined to their final shape. Rather, the

goal is always to form the material to its final

shape in the same step in which the microstructure of the material itself is formed. Because of

the severity of joining problems, the designer is

always conscious of the need to consolidate as

many components as possible together in a single structure. To be sure, these goals are not always realized, and expensive grinding or drilling

is often required. However, to a great extent, the

promise of the advanced materials lies in the possibility of net shape processing, thereby eliminating expensive finishing and fastening operations.

Ceramics Design

Designing with ceramics and other brittle materials is much different from designing with metals, which are much more tolerant of flaws. In

practice, ceramic structures always contain a distribution of flaws, both on the surface and in the

bulk. Ceramic designs must avoid local stress concentrations under loading which may propagate

cracks originating at the flaws.

Two serious barriers to the use of ceramics are

the lack of knowledge among designers of the

principles of brittle material design, and the poor

characterization of ceramic materials of interest.

Greater emphasis needs to be placed on brittle materials in college curricula. Courses at the college

level and minicourses for continuing education on

design for brittle materials should be offered and

publicized. ’

The characterization of commercially available

ceramics for design purposes is generally poor.

The poor quality of the data is due to the fact that

mechanical, thermal, and chemical properties of

ceramic materials vary with the method of manufacture as well as test method. Both carefully

controlled and documented processing procedures

and standard test methods will be required to give

designers the confidence that consistent properties at a useful level can be obtained at a predictable cost.

Computer-aided design (CAD) techniques have

become essential to the design process, particularly in the analysis of mechanical stresses, thermal stresses, and life prediction of the part in service. Sophisticated software to calculate mechanical

and thermal stresses is available; however, these

programs require detailed data on properties such

as thermal expansion coefficient, heat capacity,

and elastic modulus, which often are not available. Current techniques for predicting the service life of ceramics are limited by lack of data on

the behavior of these materials in various environments of interest. These techniques are especially unreliable in high temperature, high stress

regimes where several failure mechanisms are

operating simultaneously.l”

It is not particularly unusual that there should

be a lack of reliable design data in a field like structural ceramics, which is very new and which is

constantly producing new materials. It can even

be argued that the imposition of codes and specifications would be premature, given that there is

no consensus on the best materials or processes.

Although there is a danger in narrowing the field

too soon, there is also a danger in keeping it general for too long. It may be appropriate to choose

one or two materials which seem most promising and concentrate on producing uniform, highquality components from these. Silicon carbide

and silicon nitride would be two possible candidates, because they have already received a large

amount of research funding over the years for heat

engine applications. These materials have a broad

range of potential uses, but designers cannot compare them or use them without a reliable data9Specific proposals to improve ceramics education are given in

the Report of the Research Briefing Panel on Ceramics and Ceramic

Composites (Washington, DC: National Academy Press, 1985).

IOR, Nathan Katz, “Applications of High Performance Ceramics

in Heat Engine Design, ” Materials Science & Engineering 71 ;227-

249, 1985.
16

base on standard compositions having specified

properties.

Processing of Ceramics

The production of most ceramics, including

traditional and advanced ceramics, consists of the

following four basic process steps: powder preparation, forming, densification, and finishing. The

most important variations on these steps are given

in table 4.

Powder Preparation.—Although the basic ceramic raw materials occur abundantly in nature,

they must be extensively refined or processed before they can be used to fabricate structures. The

entire group of silicon-based ceramics (other than

silica) does not occur naturally. Silicon carbide,

silicon nitride, and sialon (an “alloy” of silicon

nitride with aluminum oxide in which aluminum

and oxygen atoms substitute into silicon and nitrogen lattice positions, respectively) compositions

must all be fabricated from gases or other ingredients. Even minerals which occur naturally, such

as bauxite, from which alumina is made, and zircon sands, from which zirconia is derived, must

be processed before use to control purity, particle size and distribution, and homogeneity.

In recent years, the crucial importance of powder preparation has been recognized. Particle sizes

Table 4.—Common Processing Operations for

Advanced Ceramics

Operation Method

Powder preparation Synthesis

Sizing

Granulating

Blending

Solution chemistry

Forming Slip casting

Dry pressing

Extrusion

Injection molding

Tape casting

Melting/casting

Densification Sintering

Reaction bonding

Hot pressing

Hot isostatic pressing

Finishing Mechanical

Chemical

Radiation

Electric

Examples

SiC

Si3,N4

Zr02

Glasses

Combustors, stators

Cutting tools

Tubing, honeycomb

Turbocharger rotors

Capacitors

Glass ceramics

Al ,03

S13N4

Si3N4, SiC, BN

Si3N,, SiC

Diamond grinding

Etching

Laser, electron beam

Electric discharge

SOURCE Off Ice of Technology Assessment

and size distributions are critical in advanced ceramics to produce uniform green (unfired) densities, so that rearrangement can occur to produce

a fully dense, sintered, ceramic part.

Various dopants or sintering aids are added to

ceramic powders during processing. Sinterability

can be enhanced with dopants, which can control particle rearrangement and diffusivities. These

dopants allow sintering at lower temperatures

and/or faster rates. Dopants are also used to control grain growth or achieve higher final densities. The use of dopants, while providing many

beneficial results, can also influence the material’s

properties. Segregation of dopants at the crystalline grain boundaries can weaken the final part,

and final properties such as conductivity, strength,

etc., may differ significantly from those of the

“pure” material.

Forming.—Ceramic raw materials must be

formed and shaped before firing. The forming

process often determines the final ceramic properties. The processing variables in the forming step

affect particle shape, particle packing and distribution, phase distribution, location of pores,

shrinkage, and dry and fired strength.

Forming processes for ceramics are generally

classified as either cold or hot forming. The major cold forming methods include slip casting, extrusion, dry pressing, injection molding, tape casting, and variations. The product of cold forming

techniques is called a “green body, ” which may

be machined before firing. The homogeneity of

the cold formed part determines the uniformity

of shrinkage during firing.

Hot forming methods combine into one step the

forming and sintering operation to produce simple geometric shapes. These techniques include hot

pressing and hot isostatic pressing.

Densification.— Sistering is the primar y

method for converting loosely bonded powder

into a dense ceramic body. Sintering involves consolidation of the powder compact by diffusion on

an atomic scale. Moisture and organics are first

burned out from the green body and then, at the

temperature range where the diffusion process occurs, matter is moved from the particles into the

void spaces between the particles, causing densification and resulting in shrinkage of the part.
.-

Combined with forming techniques such as slip

casting, sintering is an economic method for producing intricate ceramic components. Its drawback lies in the need to use additives and long sintering times to achieve high densities. The

complications introduced by the additives have

been noted above in the discussion of powders.

Finishing.—These techniques include grinding

and machining with diamond and boron nitride

tools, chemical etching, and laser and electric discharge machining. The high hardness and chemical inertness of densified ceramics make the fin-

 ishing operations some of the most difficult and

expensive in the entire process. Grinding alone can

account for a large fraction of the cost of the component, In addition, surface cracks are often introduced during machining, which reduce the

strength of the part and the yields of the fabrication process.

Near Net Shape Processing (NNSP).—This describes any forming process which gives a final

product that requires little or no machining. Typically, ceramics shrink to about two-thirds of their

green volume upon sintering. This shrinkage

makes it extremely difficult to fabricate ceramics

to final net shape. However, if the green ceramic

is machined prior to densification, a near net

shape part can be obtained. Hot isostatic pressing and ceramic coatings also yield parts which

do not require subsequent machining.

Near net shape processes which are currently

used for metals include powder metallurgy and

advanced casting techniques. Since metals are in

direct competition with ceramics in many applications, near net shape processing of ceramics will

continue to be a high priority research area which

will be critical to their cost competitiveness,

Ceramic Matrix Composites

Ceramic matrix composites may consist of continuous fibers oriented within a ceramic matrix,

ceramic whiskers randomly oriented within a

ceramic matrix, or dissimilar particles dispersed

in a matrix with a controlled microstructure. The

potential benefits of ceramic composites include

increased fracture toughness, hardness, and improved thermal shock resistance. Processing methods for particulate-reinforced composites are similar to those for monolithic ceramics.

50μM

Photo credit: Los Alarnos Nafional Laboratory

Fracture surface of a composite formed by hotpressing silicon nitride powder with 30 percent by

volume silicon carbide whiskers.

Whisker Reinforcement.—Ceramic whiskers

are typically high-strength single crystals with a

length at least 10 times the diameter. Silicon carbide is the most common whisker material. Current dispersed whisker composites are fabricated

by uniaxial hot pressing, which substantially

limits size and shape capabilities and requires expensive diamond grinding to produce the final
18

part. Hot isostatic pressing (HIP) has the potential to permit fabrication of complex shapes at

moderate cost, but procurement of expensive capital equipment and extensive process development

are required.

Continuous Fiber Reinforcement.—The primary fibers which are available for incorporation

into a ceramic or glass matrix are carbon, silicon

carbide, aluminum borosilicate, and mullite. Currently, glass matrix composites are more developed than their ceramic analogs. These composites are far tougher than unreinforced glasses

(tabl e

00 to 1,3000 F (59 3

o to 7 0 4o C). Servic e

temperatures up to 2,0000 to 2,2000 F (1,093 o to

1,2040 C) may be obtained with “glass-ceramic”

matrices which crystallize upon cooling from the

process temperature.11 Carbon matrix composites

have the highest potential use temperature of any

ceramic, exceeding 3,5000 F (1,927° C). However,

these composites oxidize readily in air at temperatures above about 1,100° F (s930 C), and require

protective ceramic coatings if they are to be used

continuously at high temperature.12

ilK~r] M, Prewo, J .J. Brennan, and G.K. Layden, “Fiber -

Reinforced Glasses and Glass-Ceramics for High Performance Applications,” American Ceramic Society Bulletin 65(2):305, 1986.

‘zJoel Clark, et al., “Potential of Composite Materials To Replace

Chromium, Cobalt, and Manganese in Critical Applications, ” contractor report prepared for the Office of Technology Assessment,

1984.

Photo credit’ United Technologies Research Center

Tensile fracture surface for

Nicalon SiC fiber-reinforced glass-ceramic

Fabrication of continuous fiber ceramic composites is currently of a prototype nature and very

expensive. Several approaches are under development:

● The fibers are coated with ceramic or glass

powder, laid up in the desired orientation,

and hot pressed,

 Fibers or woven cloth are laid up, then are

infiltrated by chemical vapor deposition

(CVD) to bond the fibers together and fill in

a portion of the pores,

● Fibers are woven into a three-dimensional

preform, then infiltrated by CVD.

● A fiber preform is infiltrated with a ceramicyielding organic precursor, then heat treated

to yield a ceramic layer on the fibers. This

process is repeated until the pores are

minimized.

Considerable research and development will be

necessary to optimize fabrication and to decrease

the cost to levels acceptable for most commercial

applications.

Ceramic Coatings

Many different processes are in use for the fabrication of ceramic coatings and for the modification of surfaces of ceramic coatings and of

monolithic ceramics. Table 5 lists some of the

more important techniques. The choice of a particular deposition process or surface modification

process depends on the desired surface properties.

Table 6 lists some of the coating characteristics

and properties which are often considered desirable. Additional considerations which can influence the choice of coating process include: the purity, physical state, and toxicity of the material

to be deposited; the deposition rate; the maximum

temperature which the substrate can reach; the

substrate treatment needed to obtain good coating adhesion; and the overall cost.

For most coating processes, the relationships

between process parameters and coating properties and performance in various environments are

poorly understood. Coating providers tend to rely

on experience gained empirically. Work is in

progress to establish these relationships for certain processes, e.g. ion beam- or plasma assistedphysical vapor deposition. However, in view of
19

Table S.—Selected Processes for the Production of Ceramic Coatings and for the Modification of Ceramic Surfaces

Process category Process class Process

Ceramic coating processes:

Low gas pressure (’(vacuum”) Chemical vapor deposition (CVD) Pyrolysis

processes Reduction (plasma assisted)

Decomposition (plasma assisted)

Polymerization (plasma induced)

Physical vapor deposition (PVD) Evaporation (reactive, plasma assisted)

Sputtering (reactive, plasma assisted)

Plasma-arc (random, steered)

Ion beam assisted co-deposition

Plasma discharge spraying

Processes at elevated gas pressures

Liquid phase epitaxy processes

Electrochemical processes

Low pressure plasma spraying

Plasma spraying

Flame spraying

Wetting process

Spin-on coatings

Electrolytic deposition

Electrophoretic deposition

Anodization

Electrostatic deposition

Processes for the modification of ceramic surfaces:

Particle implantation processes

Densification and glazing processes

Chemical reaction processes

Conversion processes

Etching processes

Mechanical processes

Direct particle implantation

Recoil particle implantation

Laser beam densification and

glazing

Electron beam densification and

glazing

Gaseous anodization processes

Plasma arc spraying

Combustion flame spraying

Dip coating (e.g., Sol-Gel)

Brush coating

Reverse-roller coating

Cation deposition

Charged colloidal particle deposition

Anion oxidation in electrolytes

Charged liquid droplet deposition

Energetic ion or atom implantation in

solids

Recoil atom (ion) implantation in solids

CW-laser power deposition

Pulsed-laser power deposition

Energetic electron beam power

deposition

Ion nitriding

Ion carburizing

Plasma oxidation

Deposition of molecular species

Formed in gas phase

Diffusion of material from surface into

bulk of substrate

Acidic solutions; lye etching

Disproportionation processes

Thermal diffusion

Chemical etching

Ion etching

Grinding

Peening

Polishing

Sputter process

SOURCE: Manfred Kaminsky Surface Treatment Science International, Hinsdale. IL

the current widespread use of coated machinery Chemically Bonded Ceramics

components, and projected future requirements

for components with advanced coatings, research Hardened cement pastes and concretes fall in

in processing science for ceramic coatings remains the category of chemically bonded ceramics

an important priority. In addition, improved (CBCs), because they are consolidated through

deposition processes are required, particularly for chemical reactions at ambient temperatures rather

the coating of large components or those having than through densification at high temperature.

a complex shape. Due to their low strength compared with dense
20

Table 6.—Characteristics and Properties of

Ceramic Coatings Often Considered Desirable

Good adhesion

Precise stoichiometry (negligible contamination)

Very dense (or very porous) structural morphology

Thickness uniformity

High dimensional stability

High strength

High fracture toughness

Internal stresses at acceptable levels

Controlled density of structural defects

Low specific density

High thermal shock resistance

High thermal insulating properties

High thermal stability

Low (or high) coefficient of friction

High resistance to wear and creep

High resistance to oxidation and corrosion

Adeauate surface topography

SOURCE Manfred Kaminsky, Surface Treatment Science International, Hinsdale,

IL.

ceramics, concrete and other cementitious materials are not normally considered “advanced. ”

However, in recent years, new processing methods have led to significant improvements in the

strength of chemically bonded ceramics, and further development will provide additional improvements. In fact, the ultimate limits on tensile and compressive strength have not yet been

approached.

Cements.—Cements are chemically active

binders which may be mixed with inert fillers such

as sand or gravel to form concrete. Cement pastes

containing minor additives such as organic polymers can also be used as structural materials. By

far the most common cement used in making

CBCs is portland cement. Portland and related

cements are hydraulic; i.e., they react with water

to form an relatively insoluble aggregate. In hydraulic cements, excess water is usually added to

improve the working characteristics, but this

causes the hardened structures to be porous (the

minimum porosity of fully hydrated cements is

about 28 percent) 13 and of low strength. In a recent advance, a high-shear processing technique

together with pressing or rolling are used to remove pores from a low-water calcium aluminate

cement paste containing 5 to 7 percent by weight

organic polymers to improve workability. The

dense paste, which is sometimes called macrodefect-free or MDF cement, has the consistency

Richard A. He]muth, portland Cement Association, perSOIM]

communication, August 1986.

—— -—..-.-——— ——.——. -— —-—

I

I

I

I

—.. .- . . -——.— -J

Photo credits” /mperia/ Chemica/ /ndustries PLC

Top photo: Conventional Portland cement paste

microstructure, showing large flaws (pores).

Bottom photo: Advanced cement paste microstructure,

illustrating the absence of large pores

(magnification x1OO).

Photo credit” CEMCOM Research Associates, Inc

This tool, made from a cement-based composite,

is used for autoclave forming of a fiber-epoxy

jet engine component.

of cold modeling clay, and can be molded or extruded by techniques similar to those used for

plastics. The hardened cement paste has a strength

approaching that of aluminum (table 7) and much

lower permeability than ordinary portland cement

paste. 14 Although MDF cement pastes cost 20 to

I iAccording t. product literature supplied by ImPerial Chemical

Industries, “New Inorganic Materials. ”
21

Table 7.—Comparison of the Mechanical Properties of Various Cements and Aluminum

Material Density (g/cm3

) Flexural strength (psi)a Compressive strength (psi) Fracture energy (J/m2)

Portland cement paste . . . . 1.6-2.0b 725-1,450 4,000-5,000 b

Cement/asbestos . . . . . . . . . . 2.3 5,075

Advanced cementsc . . . . . . . . 2.3-2.5 14,500-21,750 22,000-36,000

Aluminum . . . . . . . . . . . . . . . . 2.7 21,750-58,000 42,000

20

300

300-1,000

10,000

a

1 MPa = 145 psi.

bAccording to information supplied by the Portland Cement Association

c

The advanced cement has the following composition. 100 parts high alumina cement; 7 parts hydrolyzed Poly (vinylacetate),

SOURCE Imperial Chemical Industries.

30 cents per pound compared with portland cement paste at 3 cents per pound, they are still significantly cheaper than metals and plastics.

Processing of hydraulic CBCs is very cheap,

since it involves only adding water, mixing, casting or molding, and allowing the material to set

at room temperature or slightly elevated temperature. Very little dimensional change occurs during the set and cure, so that parts can be made

to net shape. Due to the low processing temperature, a wide variety of reinforcing fibers can

be considered, including metal fibers. Further

work is needed to improve the long term stability of these materials, and the presence of organics

makes them unsuitable for use above about 2000 F

(93° c).

Concrete.—As chemical additives, such as organic polymers, have improved the properties of

cement pastes, chemical and mineral additives

have had a similar effect on concrete. Minerals

such as fly ash and microscopic silica particles help

to fill in the pores in the concrete and actually improve the bonding in the cementitious portion.

This results in greater strength and reduced permeability. In a recently developed concrete, molten sulfur is used as a binder in place of cement.

Sulfur concrete has superior corrosion resistance

in acidic environments and can be recycled by

remelting and recasting without loss of the mechanical properties.

The compressive strength of typical concretes

today is around 5,OOO psi (34 MPa), although concretes with strengths of 10,000 to 15,000 psi (69

to 103 MPa) are becoming common. Under laboratory conditions, compressive strengths of at

least 45,000 psi (310 MPa) have been achieved,

“W. C. McBee, T.A. Sullivan, and H.L. Fike, “Sulfur Construction Materials, ” Bulletin 678, U.S. Department of the Interior, Bureau of Mines, 1985.

10-12 parts water

and there is no indication that the ultimate

strength is being approached. 16 In concrete highrise buildings, the higher compressive strengths

permit use of smaller columns, with consequent

savings in space and materials.

Two deficiencies in concrete as a structural material are its low tensile strength and low toughness. A typical concrete has a tensile strength below 1,000 psi (7 MPa). Steel reinforcement bars

are added to the concrete to provide tensile

strength. In prestressed concrete, high strength

steel wires under tension are used to keep the concrete in a state of compression. To improve

strength and toughness, a variety of reinforcing

fibers, including steel, glass, and polymers, have

been tried, with varying degrees of success. Fiber reinforcement can increase the flexural

strength by a factor of 2.5 and the toughness by

a factor of 5 to 10 above unreinforced materials. 18

This technology, which dates back to the strawreinforced brick of the ancient Egyptians, requires

fiber concentrations which are sufficiently low

(usually 2 to 5 percent by volume) to preserve the

flow characteristics of the concrete, and a chemically stable interface between the fiber and the

concrete over time. Asbestos fibers served this

function for many years; however, because of the

health hazards, new fibers are being sought. A

fully satisfactory reinforcing fiber for concrete has

yet to be discovered.

Nondestructive Evaluation (NDE) of Ceramics

Nondestructive evaluation refers to techniques

for determining properties of interest of a struc-

‘

6 Sidney Mindess, “Relationships Between Strength and Nlicrm

structure for Cement-Based Materials: An Overview’, ” Alateriak Zi’esearch Society S~’mposium Proceedings, 42:53, 1985.

“Ibid.

18American Concrete Institute, “State-o-the-Art Report on FiberReinforced Concrete, ” Report No. ACI 544.11 <-82, 1982.
22

ture without altering it in any way. NDE has historically been utilized for flaw detection in ceramic

materials to improve the reliability of the final

product. In the future, NDE will be utilized for

defect screening, material characterization, inprocess control, and lifecycle monitoring. These

techniques will be applied to the starting materials, during the process, and to the final product.

A key goal for the future will be the evolution

of NDE techniques amenable to automation and

computerization for feedback control. Powder

and green body characterization will be critical

for materials processed from powders. For inprocess characterization, the relation between

measurable quantities, obtained through the use

of contact or noncontact sensors, and desired

properties is crucial. This will require developments in sensor technology as well as theories

which can quantitatively relate the measured NDE

signal to the properties of interest.

In the past, a great deal of emphasis has been

placed on the sensitivity of an NDE technique,

i.e., the size of the smallest detectable flaw. A

more relevant criterion for reliability purposes is

perhaps the size of the largest flaw that can go

undetected. There has been very little emphasis

on the reliability of NDE techniques, i.e. the probability of detecting flaws of various sizes. Experience has shown that most quality problems result not from minute flaws, but from relatively

gross undetected flaws introduced during the

fabrication process.19

Cost of production estimates for high performance ceramic components typically cite the inspection costs as approximately 50 percent of the manufacturing cost .20 Successful NDE techniques for

ceramic components should meet two major criteria: they should reliably detect gross fabrication

flaws to ensure that the material quality of the

component is equal to that of test specimens, and

they should be able to evaluate the quality of a

complex-shaped component in a practical manner. No single NDE technique for ceramics completely satisfies these criteria. However, those

which could be cost-effective for production level

inspections are described in table 8.

Health Hazards

The most serious health hazard associated with

ceramics appears to be associated with ceramic

fibers. Studies carried out at the National Cancer Institute have indicated that virtually all durable, mineral fibers having a diameter of less than

1 micrometer are carcinogenic when introduced

into the lining of the lung of laboratory rats.2l The

carcinogenicity drops with increasing diameter,

such that fibers having diameters greater than 3

micrometers do not produce tumors. Recent studies

on commercially available aluminosilicate fibers

suggest that animals exposed to the fibers develop

an increased number of lung cancers over time

compared with a control group .22 No data on the

effects of ceramic fibers on humans are available,

and no industry standards for allowable fiber

concentrations in the workplace have been estabIQR Nathan Katz and Alfred L. Broz, “Nondestructive Evaluation Considerations for Ceramics and Ceramic Matrix Composites, ”

contractor report prepared for the Office of Technology Assessment,

November 1985.

‘“Ibid.

z IMear] F. Stanton, et al., Journal of the National Cancer hstitute 67:965-975, 1981.

Zzphi]ip J. Lan&-igan, M. I)., The Mount Sinai Medical Center,

personal communication, August 1986.

Table 8.—Comparison of Some Possible Production-Level NDE Techniques for Structural Ceramics

Adaptability to Extent of development

NDE technique Detected flaw type Sensitivity complex shapes required for commercialization

Visual (remote) . . . . . . . . . . surface fair good none

Dye penetrant . . . . . . . . . . . surface good good none

Radiographic . . . . . . . . . . . . bulk 1-20/0 of specimen excellent none

thickness

Ultrasonic . . . . . . . . . . . . . . . bulk and surface good poor some

Holographic . . . . . . . . . . . . . surface good fair large

Thermographic. . . . . . . . . . . surface poor excellent some

Proof test . . . . . . . . . . . . . . . any good, but may excellent none

introduce flaws

SOURCE: Office of Technology Assessment
23

lished. Until such data become available, the animal studies suggest that these fibers should be considered carcinogenic, and treated in a manner

similar to asbestos fibers.

Applications of Structural Ceramics

Figure 3 shows an estimated timetable for the

introduction of ceramic products in various categories. It shows that some advanced structural

ceramics are in production, others have near-term

potential for production, and some are far away

from production.

Current Production

Ceramics such as alumina, silicon nitride, and

silicon carbide are already well established in production for many structural applications in the

categories of wear parts, cutting tools, bearings,

and coatings. The ceramics portion of the market is currently small (generally less than 5 percent) .23 Substantial growth in ceramics production is expected to occur over the next 25 years

by growth of the overall markets, by achieving

an increase in the market share, and by spin-off

applications.

Ceramics are also in limited production (in

Japan) in discrete engine components such as

turbochargers, glow plugs, and precombustion

chambers. Current military applications include

radomes, armor, infrared windows, and heat

sources.

Funding for known near-term markets is currently being provided by industry. Much of the

funding is directed toward development of new

or improved ceramic or ceramic matrix composite materials. Key objectives are to achieve improved toughness, higher reliability, and decreased cost. Development of silicon nitride,

transformation toughened ceramics, and composites has yielded materials with enhanced toughness and reliability, but costs are still high and

reliability remains a problem. Progress in resolving these limitations is occurring, and large increases in the market share for ceramics are projected.

——. .

‘‘U.S, Department of Commerce, A Corrtpetiti;re Assessment of

the U.S. Advanced Ceramics Industry’ (Washington, DC: U. S, C,o\rernment Printing Oftice, March 1984), pp. 38-39.

The United States is competing with other countries, especially Japan, for the growing markets.

Japan has a well-coordinated effort which includes: increasing the quality and availability of

the raw materials; conducting material fabrication process optimization; installing the most advanced processing and quality control equipment

for prototype and production facilities; and aggressively marketing their evolving materials.

Near-Term Production

Near-term production is projected in advanced

construction products, bearings, bioceramics, heat

exchangers, electrochemical devices, isolated components for internal combustion engines, and military applications. The technology feasibility has

generally been demonstrated, but scale-up, cost

reduction, or design optimization are required.

Although much of the feasibility demonstration

has occurred in the United States, foreign industry and government-industry teams, particularly

in Japan, have more aggressive programs to commercialize the near term applications. Large markets are at stake; for example, the world market

for all biocompatible materials has been projected

to be as much as $6 billion by the year 199524;

ceramics could capture 25 to 30 percent of this. 25

Foreign dominance of these markets would adversely affect the U.S. balance of trade. Government, industry, and universities need to team together in focused programs to solve the remaining

problems and achieve production capability competitive with that of foreign countries.

Long-Term Applications

Some potential applications of ceramics require

solution of major technical and economic problems. These high risk applications are categorized

as long term (greater than 15 years away). The

ultimate payoff may be large, but it is not possible to predict confidently that the problems will

be overcome to achieve the benefits.

Long-term applications include the automotive

gas turbine engine, the advanced diesel, some elecj~Larry L. Hench and June Wilson, “Biocompatibility of Silicates

for Medical Use, ” Silicon Biochemistry, CIBA Foundation Symposium No. 121 (Chichester: John Wiley & Sons, 1986), pp. 231-246,

JsLarwL, Hench,universityof Florida, persona] communication,

August 1986.
Figure 3i.—Estimated Scenario for Implementation of Ceramic Components in Structural Application Categories

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Ai,o, SiC PSZ Si3N4

. Composites

TTA TZP Si3N4-BN Wear parts

Advance d construction products I Chemically Composites

bonded

Military applications

Bearings

Bioceramics

Heat exchangers

Electrochemical devices

Heat engines:

Gasoline automotive

Diesel automotive

Automotive turbine

Other turbines

Coatings

BC Armor Coatings Bearings Diesels Turbines

Isolated components

Ai203 Military Commercial

AI203, FDA hip Orthopedic Advance d

clinical approval and dental materials

Recuperated Rotary Tubular Cogeneration Fixed

furnaces military industrial boundary

O 2 sensors Elect rochlorination Na-S battery Fuel cell

02 pump

Exhaust port liner Piston pin

Turbocharger

Cam follower

Pre-chamber, coatings Isolated parts

glow plug Uncooled engine

Isolated

components

Wear and Cutting Turbine Minimum-cooled

corrosion tools components diesel

resistance

SOURCE: David W. Richerson, ""Design, Processing Development, and Manufacturing Requirements of Ceramics and Ceramic Matrix Composites, ” contractor report prepared for the Office

of Technology Assessment, December 1985.
25

trochemical devices such as fuel cells, some heat

exchangers, and some bearings. A variety of other

turbines, especially those for aircraft propulsion

and utility scale power generation, should also be

categorized as long term.

Substantial design, material property, and manufacturing advances are necessary to achieve production of applications in the long-term category.

In general, risk is perceived by industry to be too

high and too long range to justify funding the

needed developments. Advancement will likely

be driven by government funding. In many of

these categories, military use will predate commercial use.

Markets for Advanced Structural Ceramics

Estimated U.S. markets in the year 2000 for several of the categories listed above are shown in

figure 4.

Wear Parts.—Wear parts include such applications as seals, valves, nozzles, wear pads, grinding wheels, and liners. The Department of Commerce has estimated that by the year 2000 ceramics

could capture roughly 6 percent of the wear parts

market, currently dominated by tungsten carbide

cermets, and specialty steels. With the total market estimated at $9 billion, the ceramic portion

would be $540 million. 26

Cutting Tools .—Ceramics have demonstrated

a capability as a cutting tool, especially in competition with tungsten carbide-cobalt cermets (ceramic-metal composites) as inserts for metal turning and milling operations. The advantage of

ceramics compared with carbides is retention of

high hardness, strength, and chemical inertness

to temperatures in excess of 1,000° C (1,8320 F).

This allows use of the ceramics at much higher

machining speeds than can be tolerated by carbides. However, the ceramics have lower toughness than the carbide materials, and have only

been used successfully in the limited operations

of turning and milling. A further impediment to

the use of ceramics, especially in the United States,

Figure 4.— Projected U.S. Markets for Structural

Ceramics in-the Year 2000 (billions of dollars)

1,0

0.5

— m

(,-l

—

—n— l-l

—

—m

co

a—

u

SOURCES aU.S. Department of Commerce, “A Competitive Assessment of the

U.S. Advanced Ceramics Industry” (Washington, DC, U S Govern

ment Printing Office, March 1984)

bHigh Technology, March 1986, P. 14.

c

Larry L. Hench and June wilson, “Blocompatablility of Silicates for

Medical Use, ” CIBA Foundation Symposium 121, pp. 231-246

(Chichester, John Wiley & Sons, Publishers, 1986) Ceramics are as.

sumed to capture 30°/0 of the estimated $3 billion U S blomaterials

market

‘David W. Richerson, “Design, Processing Development, and

Manufacturing Requirements of Ceramics and Ceramic Matrix Com.

posites, ” contractor report prepared for the Off Ice of Technology

Assessment, December 1985

has been equipment limitations. Much of the production metal machining equipment does not have

the rigidity or speed capability to utilize ceramics.

In spite of the current limitations of ceramics

and equipment, an assessment by the Department

of Commerce estimates steady growth of ceramics

for cutting tool applications. It projects a growth

of the total U.S. shipments from $2.2 billion in

1980 to $8 billion in the year 2000, with the

growth of the ceramics portion from $45 million

(2 percent) to $960 million (12 percent) .2’

16U s, Department of Commerce, Op. cit. , March 1984, P. 35. “Ibid.
26

Photo credit: Norton Co

Silicon nitride anti-friction bearing components offer

improved wear resistance and fatigue life compared

to most bearing steel.

Bearings

High-performance ceramic bearings have been

developed for military applications such as missiles. The primary candidate material is hot

pressed silicon nitride. Ceramics offer resistance

to low temperature corrosion, high temperature

stability, low density, and the ability to operate

for a moderate length of time with little or no lubrication. Hot isostatic pressing (HIP) is being developed to improve properties and to decrease

cost by allowing near-net-shape fabrication.

The military developments will yield a technology base that can be applied to commercial products such as instrumentation bearings, hydraulic

and pneumatic activator systems, and ceramic

coatings on the foils in gas bearings. Potential ceramic bearing markets have been estimated to be

$300 million per year. 28

Coatings

Ceramic coatings provide a variety of benefits,

including abrasion resistance, thermal protection,

corrosion resistance, and high-temperature lubrication. Applications include ultrahard coatings for

cutting tools, thermal insulation and lubricating

coatings for adiabatic diesel engines and cooled

‘“High Technology, March 1986, p. 14.

gas turbines, and bioactive glass coatings for metal

orthopedic implants. The list could be expanded

to include other sectors such as mining (e.g.,

drills), utilities (e.g., turbine-generator sets, heat

exchangers), agriculture (e. g., plows and tillers),

and aerospace (e. g., bearings, power transfer

assemblies, and actuator drive systems).

The availability of advanced ceramic coatings

is expected to be a significant benefit to the U.S.

economy. The value of the market for ceramic

coatings is not easily assessed, because the range

of applications is so wide. One estimate is for a

$1 billion market worldwide for all coating materials, about 60 to 70 percent of which is domestic. 29 This estimated market includes jet engine,

printing, chemical, textile, and tool and die applications. As we have seen, this list could be

greatly expanded to include wear parts, bearings,

biomaterials, heat exchangers, and automotive

components in the future. Ceramic coatings

should be considered an extremely important technology for extending the performance of metal

components, and, in some cases, coated metal

structures may be an excellent alternative to

monolithic ceramics.

Advanced Construction Products

Potential applications of advanced cementbased materials include floors, wall panels, and

roof tiles, in addition to pipes, electrical fittings,

and cabinets. The cements can be laminated with

wood or foam to form hard, decorative, and protective surfaces. so Advanced cement pastes cost

20 to 30 cents per pound compared with 75 cents

to 2 dollars for metals and plastics, and could displace them in the future in many common uses.

The development of a cost-effective, durable,

high tensile and compressive strength concrete

would have dramatic implications for the infrastructure of the United States. It has been estimated that between 1981 and the end of the century the nation will spend about $400 billion for

replacement and repair of pavements, and about

$103 billion to correct bridge deficiencies .3’ Cost

29Richerson, op.cit., December 1985.

“Imperial Chemical Industries, op. cit.

31National Research Council, Transportation Research Board,

“America’s Highways: Accelerating the Search for Innovation, ” Special Report 202, 1984.
27

A spring made from a high-strength cement, formed by extrui

Photo credits” CEMCOM Research Associates, Inc.

sion processing. Left: natural length. Right: compressed.

The spring is not intended for any practical use, but demonstrates the versatility and resilience of the material.
28

savings of about $600 million per year could result from implementing new technologies .32 In

addition to reducing repair and maintenance costs,

new materials would provide other benefits. For

example, high compressive strength concrete can

be used to reduce the number and thickness of

concrete bridge girders, significantly reducing the

structural weight;33 in concrete high-rise buildings,

its use permits a reduction in the diameter of the

columns, thus freeing up additional floor space.

The barriers to the development and implementation of new technologies in the construction industry are high. Some of those most often cited

are industry fragmentation (e.g. some 23,000 Federal, State, and local agencies operate the Nation’s

highway industry), a system which awards contracts to the lowest bidder, and low industry investment in research as a percentage of sales (the

steel industry invests 8 times more) .34 The low investment is due in part to the fact that the principal benefits of the use of better materials accrue

to the owner of the highway or bridge (the taxpayer) rather than to the cement producer.35

Legislation is currently before Congress (the

Federal-Aid Highway Act of 1986, S.2405) which

would set aside 0.25 percent of Federal-aid highway funds for a 5-year, $150 million Strategic

Highway Research Program. The program, which

would be administered by the National Research

Council, has targeted six priority research areas

for support: asphalt characteristics, $50 million;

long-term pavement performance, $50 million;

maintenance costs, $20 million; concrete bridge

components, $10 million; cement and concrete,

$12 million; and snow and ice control, $8 million.

Bioceramics

Bioceramics, or ceramics for medical applications such as dental or orthopedic implants, represent a major market opportunity for ceramics

in the future. The overall worldwide market for

biocompatible materials is currently about $3 billion, and this is expected to double or triple in

321bid.

33J.E. Carpenter, “Applications of High Strength Concrete for

Highway Bridges, ” Public Roads 44:76, 1980.

“National Research Council, op. cit., 1984.

‘51bid.

the next decade .3’ Ceramics could account for 25

to 30 percent of this market .37

Bioceramics may be grouped into three categories: nearly inert, surface active, and resorbable. 38 Nearly inert ceramics can be implanted in

the body without toxic reactions. These materials include silicon nitride-based ceramics,

transformation-toughened zirconia and transformation-toughened alumina.

Surface-active ceramics form a chemical bond

with surrounding tissue and encourage ingrowth.

They allow the implant to be held firmly in place

and help prevent rejection due to dislocation or

to influx of bacteria. Surface-active ceramics

which will bond to bone include dense hydroxy -

apatite, surface-active glass, glass-ceramic, and

surface-active composites.

The function of resorbable bioceramics is to

provide a temporary space filler or scaffold which

serves until the body can gradually replace it.

Resorbable ceramics have been used to treat maxillofacial defects, for filling periodontal pockets,

as artificial tendons, as composite bone plates, and

for filling spaces between vertebrae, in bone,

above alveolar ridges, or between missing teeth.

An early resorbable ceramic was plaster of paris

(calcium sulfate), but it has been replaced by trisodium phosphate, calcium phosphate salts, and

polylactic acid/carbon composites.39

Any new material intended for use in the body

must undergo extensive testing before it is approved. Preclinical testing, clinical studies, and

followup may take as long as 5 years to complete. 40 However, ceramics have been in clinical use

for some 15 years, and are gaining acceptance.

Industry interest in bioceramics has increased

since 1980; however, no data were available on

aggregate R&D spending either in industry or the

Federal Government. In contrast, France, Germany,

and especially Japan have aggressive programs of

government support for the commercialization of

36 Hench and Wilson, op. cit., 1986.

JT~rV L. Hench, University of Florida, persona] communication,

August 1986.

3“J.W. Boretos, “Ceramics in Clinical Care, ’’American Ceramics

Society 13ulfetin 64(8):630-636, 1985.

391bid.

dOEduardo March, Food and Drug Administration, persona] communication, August 1986.
29

Photo credit’ R/chards Medical Co

Total hip system including ceramic femoral head and

acetabular cup, with metal femoral stem, The system

is manufactured in West Germany and marketed by

Richards Medical Co.

bioceramics.” All researchers interviewed agreed

that the Japanese effort in bioceramics is considerably larger than the U.S. effort.

Heat Exchangers

Ceramic heat exchangers are of great interest

because they can utilize waste heat to reduce fuel

consumption. Heat recovered from the exhaust

of a furnace is used to preheat the inlet combustion air, so that additional fuel is not required for

this purpose. The higher the operating tempera-

~ I I{lchers(ln, Op, c1 t., December 1985.

ture, the greater the benefit. Ceramic systems have

potential for greater than 60 percent fuel savings.’2

Ceramic heat exchangers may be used in a variety of settings, including industrial furnaces, industrial cogeneration, gas turbine engines, and

fluidized bed combustion. The size of the unit,

manufacturing technique, and material all vary

depending upon the specific application. Sintered

silicon carbide and various aluminosilicates have

been used in low pressure heat exchangers because

of their thermal shock resistance; however, the

service temperature of these materials is currently

limited to under 2,200° F (1,2040 C), Silicon carbide is being evaluated for higher temperatures,

but considerable design modifications will be necessary. 43

Government support has been necessary to

accelerate development of the ceramic materials

and system technology for heat exchangers, in

spite of the design projections of significant fuel

savings and short payback time. The material

manufacturers, system designers, and end users

have all considered the risks too high to invest

their own funds in the development and implementation of a system. Specific concerns include:

the high installed cost (up to $500,000 for a 20

MBTU per hour unit), which represents a significant financial risk to the user for a technology that

is not well proven; the fact that many potential

end users are in segments of industry that presently are depressed; and the fact that designs vary

according to each installation, leading the user to

want a demonstration relevant to his particular

situation.

Many of the ceramic heat exchanger programs

were initiated in the 1970s when there was a keen

sense of urgency concerning the “energy crisis. ”

In recent years, declining fuel prices have generally reduced this sense of urgency. If the current

low fuel prices persist, this could delay the widespread implementation of ceramic heat exchangers

for waste heat recovery.

42s ,M. Johnson and D.J. Rowcliffe, SRI International Report to

EPRI, “Ceramics for Electric Power-Generating Systems, ” January

1986.

tlRicherson, Op. cit,, December 1985.
30

Electrochemical Devices

Though not strictly structural applications of

ceramics, devices in this category utilize ceramics

for both their electrical and structural properties.

Typically, the ceramic, such as zirconia or beta

alumina, serves as a solid phase conductor for ions

such as oxygen or sodium. Examples include oxygen sensors, oxygen concentration cells, solid oxide fuel cells, the sodium sulfur battery, sodium

heat engine, and electrodes for metal winning and

electrochlorination cells. As a group, these applications could comprise a market of over $250 million for ceramics by the year 2000.

44

Heat Engines

The advantages of using ceramics in advanced

heat engines have been widely publicized. These

include increased fuel efficiency due to higher engine operating temperatures, more compact designs, and reduction or elimination of the cooling system .45 Ceramics are being considered in

three general categories: discrete components such

as turbochargers in metal reciprocating engines;

coatings and monolithic hot-section components

in advanced diesel designs; and all-ceramic gas

turbine engines.

A number of sources have predicted that components for heat engines will be the largest area

of growth for structural ceramics over the next

25 years. Market estimates have varied widely.

Kenney and Bowen stated:

The potential demand for ceramic engines

could reach $9 billion in Japan and $30 billion

world wide. If ceramics are used only to partially

replace metals as hot parts in the engine, the demand would be from $1 to $5 billion.46

The Department of Commerce has estimated more

conservatively a U.S. market of $56 million by

1990 and $840 million by 2000. 47 A study by

Charles River Associates estimates U.S. consumption of ceramic heat engine parts at $25 to 45 million in 1990 and $920 million to $1.3 billion by

2000. 48 Some structural ceramic components are

already in limited production for heat engines. Ceramic precombustion chambers and glow plugs

for diesels, and ceramic turbochargers, are now

in production in current model Japanese cars.

These markets will grow, but not to a level that

will account for the projected $1 billion sales for

heat engine components in the year 2000. Growth

to this level would require material and design

technology breakthroughs, as well as manufacturing scale-up and cost reduction. In view of

these technical and economic barriers, the more

conservative estimates are likely to be be the more

accurate.

Gasoline Engines.—The automotive internal

combustion engine offers a vast market for materials. Total sales for 1985 of cars and trucks in

the free world have been estimated at 38.7 million units.49 Any part replacement or new part

would represent a volume market with substantial sales, even if the unit price were small. However, the current engine designs are considered by

automotive companies to be mature, reliable, and

cost-effective. Very few incentives for change exist. Cost reduction remains a significant incentive,

but this is extremely difficult to satisfy for a new

material, whose introduction may require redesign

of adjacent parts, retooling, and modification of

the production line.

Another incentive is to develop new technology which may be applicable to advanced designs.

This consists of both generic and directed R&D

with the primary objective of maintaining a competitive position. Ceramics within this category

which have potential for production include exhaust port liners, cam followers, and turbocharger

components. To date, U.S. firms have not introduced these products, although R&D programs

continue. Ceramic turbocharger rotors, glow

plugs, precombustion chambers, and rocker arm

wear pads are currently in limited production in

Japan.

The United States is well behind Japan in

procuring the advanced production equipment to

“Ibid.

45 Katz, op. cit., 1985.

46 Kenney and Bowen, Op. Cit., 1983 .

“U.S. Department of Commerce, op. cit., March 1984.

“Charles River Associates report prepared for the National Bureau of Standards, ‘Technological and Economic Assessment of Advanced Ceramic Materials, Vol. 2: A Case Study of Ceramics in

Heat Engine Applications, ” NBS-GCR 84-4760-2, August 1984,

4’Richerson, op. cit., December 1985.
31

Phofo credit The Garrett Corp

Ceramic turbocharger turbine rotors

produce ceramic turbocharger rotors. U.S. automotive companies do not appear to have the

same level of confidence as the Japanese that a

ceramic turbocharger market will develop. This

is an institutional barrier based on perceived risk.

In spite of this, U.S. industry is still funding considerable R&D on ceramic turbocharger rotors.

One study has estimated that if a ceramic rotor

price of $15 can be reached, and if performance

and reliability are acceptable, a worldwide market of $60 million could be generated for ceramic

turbocharger rotors by the year 2000.50

50 Elaine P. Rothman, “Advanced Structural Ceramics: Technical Economic Process Modeling of Production and a Demand Analysis for Cutting Tools and Turbochargers, ” Materials Systems Laboratory, Massachusetts Institute of Technology, August 1985.

The ceramic turbocharger has a significance far

beyond its contribution to the performance of the

engine. It is regarded as a forerunner technology

to far more ambitious ceramic engines, such as

the advanced gas turbine. Design, fabrication, and

testing methods developed for the turbocharger

are expected to serve as a pattern for subsequent

ceramic engine technology efforts.

Diesel Engines.—There are several levels at

which ceramics could be incorporated in diesel engines, as shown in table 9. The first level involves

a baseline diesel containing a ceramic turbocharger and discrete ceramic components. The

second level adds a ceramic cylinder and piston,

and eliminates the cooling system. The ceramic

used at this level would provide high-temperature
32

Table 9.—Future Diesel Engine Technology Development Scenario

Technology level Engine configuration Potential ceramic components Potential payoffs

1 State-of-the-art engine, Turbocharger Improved performance

turbocharged Valve train components Reduced cost?

Prechamber, glow plugs Early manufacturing experience

3

2 Uncooled, non-adiabatic Turbocharger Reduced weight - efficiency gain

(no water or air cooling) Valve train components Gives option to improve aero-

(no turbo-compounding) Piston, cap dynamics - efficiency gain

Cylinders, liners Reduced maintenance

Reduced engine systems cost?

Flexibility of engine placement

Adiabatic turbo-compound Turbocharger Very significant reduction in

Turbo-compound wheel specific fuel consumption

Valve train components Improved aerodynamics

Piston, cap Reduced maintenance

Cylinders, liners

Exhaust train insulation

Minimum friction technology Air bearings Lower specific fuel consumption

(could be combined with 1, High-temperature rings

2 or 3) High-temperature bearings

Nongalling wear surfaces

Low friction liquid, lubricant-free

bearings

SOURCE R Nathan Katz," Applications of High Performance Ceramics in Heat Engine Design, ” Material Science and Engineering 71227.249, 1985

strength, rather than thermal insulation. The third

level would utilize ceramics for thermal insulation in the hot section as well as in the exhaust

train. Turbocompounding would be used to recycle energy from the hot exhaust gases to the drive

train. The fourth level would use advanced minimum friction technology to improve the performance of the engine. Appropriate aspects of this

could be utilized at levels one, two, or three,

The four levels listed above place different demands on the ceramic materials. Levels one and

two require a low-cost, high-strength material, but

without insulating properties; sintered silicon nitride or silicon carbide would be possibilities here.

It has been suggested that level two represents the

best compromise for light duty ceramic diesels

such as those in automobiles. 51 Level three would

require an insulating ceramic, probably zirconia

or a zirconia-based composite. This is the level

where the most significant improvements in fuel

efficiency would be realized. The current zirconia and alumina-zirconia transformation toughened ceramics are not reliable at the high stress

of the piston cap and do not have a low enough

coefficient of friction to withstand the sliding contact stress of the rings against the cylinder liner.

51 Katz, Op. cit., 1985.

These materials do seem to have adequate properties, however, for other components such as the

head plate, valve seats, and valve guides.52

Emission requirements will likely affect the size

of the diesel market for passenger cars and trucks.

Diesel engines generally produce a high level of

particulate emissions. The higher operating temperatures of the adiabatic diesel could reduce emissions or allow emission control devices to operate more efficiently. The market for ceramics

could also be affected in another way: a major

candidate for diesel emission control is a ceramic

particle trap. However, such a trap is likely to

be expensive, and could raise the price of the automobile to an unacceptable level.

Ceramic coatings may be an alternative to

monolithic ceramics in diesel applications. Zirconia coatings can be plasma-sprayed onto metal

cylinders to provide thermal insulation. In a joint

program between the United States Army Tank

Automotive Command and Cummins Engine

Co., the combustion zone of a commercial Cummins NH diesel engine was coated with a zirconiabased ceramic and installed in a 5 ton Army truck,

minus the cooling system. The engine accumu52 Richerson, op. cit., December 1985.
33

lated over 15,000 km of successful road testing.53

The current state-of-the-art thickness of zirconia

coatings is 30 to 50 thousandths of an inch, It is

estimated that thicknesses of 125 thousandths will

be required to provide thermal insulation comparable to monolithic zirconia,54 The coating is

not as impermeable nor as resistant to thermal

shock as the monolithic zirconia. However, the

coated metal part has higher strength and toughness than the all-ceramic part.

In the past, confusion has arisen because identical configurations of ceramic and metal engines

have not been compared. It is important to separate out the configuration options, such as turbocharging, turbocompounding, heat recovery,

cooling, etc., from the material options in order

to isolate the benefits of the use of ceramics. Failure to do this has led to overestimation of the benefits of ceramics. For example, a recent study of

a ceramic diesel design funded by DOE indicates

that “a practical zirconia-coated configuration

with a cooled metal liner, intercooled, with combined turbocompounding and Rankine cycle exhaust heat recovery, provides a 26 percent increase in thermal efficiency over a metallic,

cooled, turbocharged, intercooled, baseline engine. ”55 The bulk of the performance improvement was attributed to the turbocompounding

and the Rankine cycle exhaust heat recovery,

Only 5.1 percent was attributed to the improved

thermal insulation.

A recent study by Charles River Associates predicts that the uncooled ceramic diesel engine system will be the first to be commercialized.56 It

projects that the initial introduction will be in the

late 1980s to early 1990s, and could account for

5 percent of new engines manufactured in 1995,

This projection is more optimistic than the above

discussion would imply. Zirconia materials do not

yet exist which can be used for level three technology, where the greatest fuel efficiencies are expected. It remains to be seen whether the elimination of the cooling system will provide sufficient

incentives to U.S. automakers to commercialize

level two ceramic technology. However, Japan in

particular has very active programs both in material and diesel engine development. The Japanese company Isuzu has reported greater than 300

miles of road testing and is projecting 1990 production. ”

Automotive Gas Turbines.—The major incentive for the use of ceramics in turbines is the possibility of operating the engine at turbine inlet temperatures up to about 2,500° F (1,3710 C),

compared with superalloy designs, which are

limited to about 1,900° F (1,038° C) without cooling. This temperature difference translates into an

increased thermal efficiency from around 40 percent to nearly 50 percent. ’g Power increases of 40

percent and fuel savings of around 10 percent have

been demonstrated in research engines containing ceramic components .59 Other potential advantages include reduced engine size and weight, reduced exhaust emissions, and the capability to

burn alternative fuels, such as powdered coal.

Structural ceramics are an “enabling technology” for the automotive gas turbine; i.e., without the use of ceramics, an automotive turbine

cannot be designed and manufactured that can

compete in cost or performance with current gasoline and diesel engines. Extensive design, materials, and engine efforts have been made over the

past 15 years in the United States, Europe, and

Japan. These efforts have resulted in significant

progress in design methods for brittle materials,

the properties of silicon nitride and silicon carbide materials, fabrication technology for larger

and more complex ceramic components, NDE and

proof testing, and engine assembly and testing.

Ceramic components have been operated in

prototype turbine engines in Germany, Sweden,

Japan, and the United States. The tests in the

United States have been highly instrumented development engines in test cells. 60 Current pro-

‘

]Katz, op. cit., 1985.

s4Bi]] Mand]er Cummins Engine Co. , personal communication,

August 1986. ‘

SST More] et a],, ‘‘Ana]ysis of LOW Heat Rejection Engine COncepts, ” Proceedings of the 23rd Automotive Technology Development Contractors Coordination Meeting, to be published by the Society of Automotive Engineers in 1986.

“Charles Rivers Associates, op. cit., August 1984.

sTRicherson, op. cit., December 1985.

‘“John Mason, Garrett Corp., presentation at the Society of Automotive Engineers International Congress and Exposition, February 1986.

“DavidW. Richerson and KM. Johansen, “Ceramic Gas Turbine Engine Demonstration Program, ” Final Report, DARPA Nav}r

contract NOO024-76-C-5352, May 1Q82.

fi”D.~’. Richerson, “Evolution in the U.S. of Ceramic Technology for Turbine Engines, ” American Ceramics Societ}’ Bulletin

64(2 1:282-286, 1985.
34

1. Flow Separator Housing 2. Turbine Shroud 3. Turbine Rotor

4. Inner Diffuser HSG 5. Outer Diffuser HSG 6. Turbine Backshroud

8. Turbine Transition Liner 9. Combustor Baffle

10. Bolts 11. Regenerator Shield 12. Alumina-Silica Insulation

Photo credit The Garrett Corp

Prototype ceramic gas turbine engine components
35

grams have achieved over 100 hours of operation

at temperatures above 2,0000 F (1,0930 C). 61

These achievements, while impressive, are still far

from the performance required of a practical gas

turbine engine, which will involve continuous

operation above 2,5000 F (1,3710 C). The limiting component in these engines appears to be the

rotor, which must spin at about 100,000 rpm at

these temperatures. The best rotors available

today are Japanese. ’z The target automotive gas

turbine engine is designed to provide about 100

horsepower with fuel efficiencies of about 43 mpg

for a 3,000 lb. automobile. 63

The automotive gas turbine would be a more

revolutionar y application of ceramics than the

diesel. The diesel engine is a familiar technology

and incorporation of ceramics can occur in stages,

consistent with an evolutionary design. The gas

turbine, on the other hand, represents a completely new design requiring completely new tooling and equipment for manufacture. Because of

the remaining technical barriers to ceramic gas turbines and the fact that they represent a complete

departure from current designs, it is unlikely that

a ceramic gas turbine passenger car could be

produced commercially before 2010. 64 In view of

this long development time, it appears possible

that this propulsion system could be overtaken

by other technologies, including the ceramic

diesel. One factor which would favor the turbine

engine would be dramatically increased fuel costs.

In the case that traditional fuels became scarce or

expensive, the turbine’s capability to burn alternative fuels could make it the power plant of

choice in the future.

In summary, the outlook for ceramic heat engines for automobiles appears to be highly uncertain. The performance advantages of ceramic engines are more apparent in the larger, more

heavily loaded engines in trucks or tanks than

they are in automobiles. Ceramic gas turbines and

adiabatic diesel designs do not scale down in size

as efficiently as reciprocating gasoline engines. 65

Thus, if the trend toward smaller automobiles

continues, reciprocating gasoline engines are likely

to be favored over advanced ceramic designs.

The prevailing approach of U.S. automakers

is to wait and see if an clear market niche for ceramics develops before investing heavily in the

technology. This is likely to mean that previous

forecasts of the U.S. ceramics heat engine market, which cluster in the $1 to $5 billion range by

the year 2000, are too optimistic. On the other

hand, the Japanese approach, in which ceramics

are steadily being incorporated in engines on a

more experimental basis, reflects greater faith in

the future of the technology. If a substantial automotive market for ceramics does develop, heat

engine applications for ceramics would be one of

the most highly leveraged in terms of economic

benefits and jobs.66 67

Military

Production of ceramics for military applications

is projected to expand substantially during the

next 25 years. 68 Near-term growth is expected for

armor, radomes and infrared windows, bearings

for missiles, and rocket nozzles (carbon-carbon

composites and ceramic-coated carbon-carbon

composites). New applications are likely to be laser mirrors, gun barrel liners, rail gun components, and turbine and diesel engine components.

Ceramics and ceramic composites in many cases

offer an “enabling” capacity which will allow applications or performance that could not otherwise be achieved. Some of the resulting technology will be suitable for commercial spinoff if

acceptable levels of fabrication cost and quality

control cost can be attained.

Diesels.—In military diesels, ceramics provide

much the same benefits as in commercial diesels.

Of particular interest to the military is the elimination of the cooling system to achieve smaller

packaging volume and greater reliability, Con-

“Mason, op. cit., January 1986.

“Richard Helms, General Motors Corp., presentation at the Society of Automotive Engineers International Congress and Exposition, Detroitr MI, February 1986.

“Katz, op. cit., 1985.

“Richerson, op. cit., December 1985,

65UsConge=, Office of TechnoloW Assessment, Increased A u- . .

tomobile Fuel Efficiency and Synthetic Fuels: Alternatives for Reducing Oil Zmports, OTA-E-185 (Washington, DC: U.S. Government Printing Office, September 1982), p, 145.

“Johnson, Teotia, and Hill, op. cit., 1983.

bTChar]es River Associates, Op. cit., August 1984.

~gRicherson, op. cit., December 1985.
36

siderable progress has been made through the use

of ceramic coatings, Monolithic ceramics have

also been tried, but have only been successful in

a few components, and require further development. A military diesel with minimum cooling

achieved primarily with ceramic coatings could

be produced within 5 years. Engines containing

more extensive ceramic components are not likely

to appear before 1995 to 2000. 69

Turbines.—Turbine engines are in widespread

use in the military for aircraft propulsion, auxiliary power units, and other applications. They

are being considered for propulsion of tanks,

transports, and other military vehicles. Ceramics

have the potential to enable advanced turbines to

achieve a large increase in performance: as much

as 40 percent more power, and 30 to 60 percent

fuel savings.70 In addition, they offer lower

weight, longer range, decreased critical cross section, and decreased detectability.

Design and material technologies are available

in the United States to produce high-performance,

ceramic-based turbine engines for short life applications such as missiles and drones. Furthermore, it appears that these engines have potential for lower cost than current superalloy-based

short life engines.71 Longer life engines will require

considerable development to demonstrate adequate reliability. This development must address

both design and materials in an iterative fashion.

While the use of ceramic thermal barrier coatings

in metal turbines is well underway, new turbines

designed specifically for ceramics are not likely

to be available before the year 2000.

Future Trends in Ceramics

Ceramic Matrix Composites

Dramatic improvements in the fracture properties of ceramics have been obtained by reinforcing with continuous, high-strength fibers. Optimum microstructure result in composites that do

not fail catastrophically, and therefore have mechanical properties that are very different from

b91bid.

701bid.

“Ibid.

those of monolithic ceramics, as shown in figure

5. The most developed composites to date are silicon carbide fiber-reinforced glass/ceramic matrices.

Recent analysis suggests that fibers resist the

opening of a matrix crack by frictional forces at

the matrix-fiber interface .72 One of the important

results is that the strength of the composite becomes independent of preexisting flaw size. This

means that strength becomes a well-defined property of the material, rather than a statistical distribution of values based on the flaw populations,

Ceramic matrix composites present an opportunity to design composites for specific engineering applications. This will require a detailed understanding of the micromechanics of failure and

explicit quantitative relations between mechanical properties and microstructural characteristics.

The most important breakthrough in ceramic

composites will come with the development of

new high-temperature fibers that can be processed

with a wider range of matrix materials. New

chemical methods to form fibers and matrices together could yield even more revolutionary advances.

72D. B. Marshall and A.G. Evans, “The Mechanics of

Cracking in Brittle-Matrix Fiber Composites, ” Acta Metal].

2013-2014, 1985.

Figure 5.— Load-Deflection Curve for

a SiC/Glass-Ceramic Composite

400 r

300

-u

(u

3

100

0

0 0.25 0,5

Deflection (millimeters)

Matrix

33(11):

7

600

500

400,-

~

300 g

T

200 x

100

0

Dotted line indicates deflection at which a single crack has

passed completely through the matrix, but remains bridged

by intact fibers,

SOURCE: D.B. Marshall and A,G, Evans, “The Mechanics of Matrix Cracking in

Brittle-Matrix Fiber Composites,” Acta Mefa//. 33(1 1):2013-2021, 1985
37

Chemical Processing of Ceramics

In the future, chemical approaches to the fabrication of ceramics will probably be preferred to

the traditional methods of grinding and pressing

of powders. Chemical methods, such as sol-gel

processing, 73 are already used extensively in the

production of electronic ceramics, and will become increasingly important in structural ceramics

as well. These techniques afford greater control

over the purity of the ceramic and over its microstructure. It has been estimated that by the year

2010, 50 percent of structural ceramics will be

processed chemically .74

Ceramics Factory of the Future

Because of the extreme reliability and reproducibility problems of ceramics, production processes in the future must be highly automated,

Processing equipment will be microprocessor controlled, including automatic load/unload devices

and numerically controlled grinding and finishing machines.

Computer-process control of critical steps combined with nondestructive, in-line test methods

such as acoustic flaw detection and radiography

will form the heart of the manufacturing operation. In the near term, off-line proof testing of critical components will be required.

Quality assurance will be integrated into the

process and will range from incoming raw materials inspection to automated dimensional inspection techniques such as computer controlled coordinate measuring machines of either a contact

or noncontact (optical) configuration. Great emphasis will be placed on the characteristics of the

starting powders, and use of clean rooms will be

the rule in the initial forming steps.

Perhaps the most important processing goal in

the future will be forming to net shape. This will

reduce the costs of ceramics by eliminating costly

grinding steps, and by making more efficient use

of the material. Moreover, it will increase the

“See for example: David W. Johnsonr Jr., “Sol-Gel Processing

of Ceramics and Glass, ” American Ceramic Society Bulletin

64(12):1597-1602, 1985.

‘*R. Nathan Katz, in presentation at the Society of Automotive

Engineers International Congress and Exposition, Detroit, MIr February 1986.

reliability and yields, by eliminating flaws which

are introduced in the finishing operations.

Biotechnology

Structural ceramics could have a significant interaction with the developing field of biotechnology in the future. Ceramics could be used extensively in fermenters, and are likely to be important

in a broad range of product separation technologies.

Fermenters .—Most current fermenters are made

of 316 grade stainless steel. 75 Steel fermenters suffer from several disadvantages, including contamination of the cultures with metal ions, corrosion

caused by cell metabolizes or reagents, and leaks

around gaskets and seals during sterilization and

temperature cycling. Glass-lined steel bowls are

sometimes used, especially for cell cultures, which

are more sensitive to contamination than bacterial

cultures. However, the thermal expansion mismatch between the glass lining and the metal

causes problems during steam sterilization. Ceramics offer a solution to these problems, because

of their chemical stability and low thermal expansion. Ceramics could be used in the bowl and agitator as well as in the peripheral plumbing joints

and agitator shaft seal to prevent leaking.

Separation Technology .—Separation and

purification of the products of cell and bacterial

cultures is a key aspect of biotechnology. In general, the separation techniques are based less on

filtration than on active interactions between a

solid phase and the liquid mixture, as in chromatography. For example, biologically produced

insulin is now being purified with a chiromatographic process based on a modified silica material, 76 Silica or alumina particles can also be used

as a solid support for attaching monoclinal antibodies, which bind to specific proteins and effect

a separation by affinity chromatography .77 The

strength and hardness of the ceramics are key to

the avoidance of deformation of the ceramic particles under conditions of high throughput .78 In

“Richard F. Geoghegan, E.I. du Pent de Nemours & Co., Inc.,

personal communication, August 1986.

“Joseph J. Kirkland, E,I. du Pent de Nemours & Co., Inc., personal communication, August 1986.

“George Whitesides, Department of Chemistry, Harvard University, personal communication, August 1986.

“Ibid.
38

the future, the most efficient processes could be

hybrids based on both filtration and chromatography. 79 As these processes are scaled up to production units, a large increase in the demand for

specially modified silica and alumina column

packing materials can be anticipated.

Energy Production

Power Turbines.—The performance requirements for power turbines are much greater than

those for automotive gas turbines. 80 Power turbines must have a lifetime of 100,000 hours, compared with about 3,000 hours for the auto turbine. In addition, the consequences of power

turbine failure are much greater. In light of these

facts, a recent report has concluded that power

turbines will be commercialized after automotive

gas turbines. 81 While some of the processing technology developed for the auto turbine may be

applicable to power turbines, the scale-up from

a rotor having a 6 inch diameter to one having

a much larger diameter may require completely

new fabrication techniques. The larger ceramic

structures may also require different NDE techniques to ensure reliability. Thus, use of mono-

“Michelle Betrido, Celanese Research Corp., personal communication, August 1986.

80 John50n and Rowcliffe, op. cit., January 1986.

‘*Ibid.

lithic ceramics in the critical hot section components of power turbines is not anticipated in the

next 25 years. However, ceramics may find applications in less critical structures, such as combustor linings, and ceramic coatings could be used

to augment the high-temperature resistance of

cooled superalloy rotor blades .82

Research and Development Priorities

for Ceramics

In 1986, the U.S. Government spent about $60

million on structural ceramics research and development (table 10). R&D expenditures in private industry may be roughly comparable .83 The

Departments of Energy and Defense spent the

largest fractions, at 47 percent and 36 percent, respectively. Funding for fiscal year 1987 is expected

to decrease by over 20 percent, with 50 percent

reductions in the DARPA program and in the

DOE vehicle propulsion program. The following

hierarchy of R&D priorities is based on the opportunities identified above. These are then correlated with the actual spending on structural ceramics R&D in fiscal year 1985.

azlbid.

“Charles River Associates, op. cit., August 1984, p. 38.

Table 10.—Structural Ceramic Technology: Federal Government Funded R&D (in millions of dollars)

FY 1983 FY 1984 FY 1985 FY 1986 Estimated FY 1987

Department of Energy:

Conservation and renewable energy:

Vehicle propulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Advanced materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Industrial programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Energy utilization research . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fossil energy:

Advanced research and technology development . . . . . . .

Energy research:

Basic energy science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

NASA:

Lewis Research Center . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

National Science Foundation. . . . . . . . . . . . . . . . . . . . . . . . .

National Bureau of Standards . . . . . . . . . . . . . . . . . . . . . . . . . .

Department of Defense:

Defense ARPA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

U.S. Air Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

U.S. Army . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

U.S. Navy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Total . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

alnCIUdeS $1.6 for Manpower Salaries.

blncludes $4.0 for TA C OM Diesel.

SOURCE: Robert B. Schulz, Department of Energy

11.4

3.2

1.0

0.5

1,0

3.0

3.0

2.9

7.7

3.0

4.7

1.2

42.6

12.7 11.9 10.0

5.1 6.0 8.7

2.3 1.7 1.5

1.5 1.8 1.8

1,1 1.5 1.2

4.4 4.6 4.5

2.5 5.4 4.5a

3.3 3.6 3.6

2.2

8,2 9.4 10.0

3.4 4.7 4.7

6.0 2.5 4.4b

1.3 1.4 2.3

51.8 54.5 59.4

5.2

6.5

0.8

1.0

0.9

4.5

4.6a

3.7

3.0

5.0

5.0

4.7b

2.3

47.2
39

Very Important

Processing Science.—There is a great need for

generic research to support the development of

practical manufacturing technologies within

industry. The agenda for such research is long,

but

●

●

●

●

●

includes such topics as:

development of near-net-shape processes;

development of pure, reproducible powders,

whiskers, and fibers which can be formed

and densified with a minimum of intermediate steps; role of solution chemistry in

powder preparation and control of interface

properties in composites;

development of practical in-process inspection devices to identify problems at the

earliest possible stage in the process;

iterative development of new equipment

such as hot isostatic presses (HIP) and multistage processes such as sinter-HIP, with emphasis on scaling up to commercially viable

size; and

understanding the relationships between

coating process variables and final properties and performance of ceramic coatings.

Environmental Behavior.—Many of the applications for ceramics mentioned above require

long-term performance in severe environments.

In order to develop higher temperature, corrosion

resistant materials, it is necessary to understand

the long-term behavior of candidate ceramic materials in the anticipated environments.

For heat engine applications, the general

requirement is to understand the mechanical and chemical behavior of advanced

ceramics such as silicon nitride, silicon

carbide, zirconia, and composites based on

these materials in environments of 1,0000 to

1,4000 C (1,8320 to 2,5520 F) in air, carbon

monoxide, or carbon dioxide.

In ceramic wear parts, it will be necessary

to understand the relationship between wear,

erosion, and toughness in the presence of lubricating fluids and gases. Generally, wear

parts include hard materials like tungsten

carbide, titanium boride, and materials

which have good lubrication characteristics,

like silicon nitride.

●

●

●

Since heat exchangers generally fail as a result of long-term corrosion at high temperatures, it is important to understand the

chemical processes of corrosion in environments like salts of sodium, potassium, magnesium, calcium, vanadium, and mixed

metals. In addition to existing materials (e.g.

silicon carbide, cordierite, and zirconium

silicate), newer materials, including silicon

nitride and composites, should be investigated. Corrosion-resistant ceramic coatings

may become important here.

Considering the large size of the potential

markets in bioceramics, it will be critical to

understand the long-term effects of body

fluids on chemical structure and mechanical

properties. In view of the many years which

ceramic implants must serve without failure,

the interaction between slow crack growth

and the body environment should be investigated.

The long-term environmental stability of advanced chemically bonded ceramics will be

crucial to their effectiveness in construction

and other applications. The deterioration of

the properties of some advanced cements in

the presence of moisture remains a problem,

and the chemical degradation of the fiber interface in reinforced cements and concretes

have limited the structural uses of these materials.

Reliability. -No factor is more important to the

success of ceramics in all of the applications discussed than reliability. Since the performance

specifications and environment of each application are different, it will be necessary to establish the most appropriate and cost-effective nondestructive testing methods for each one.

In order to facilitate design, models need to be

developed for predicting the service lifetime of ceramic parts containing various kinds of flaws.

Such models must depend heavily on information

derived from the categories of environmental behavior of ceramics and composite failure mechanisms discussed above. Beyond a dependence on

intrinsic flaws in the material, however, lifetime

also depends on the location and nature of the

flaw relative to the structure itself. It is not suffi-
40

cient to characterize the behavior of a coupon of

the material from which a structure is made; either the structure itself must be tested, or additional models must be available to predict the effects of a particular flaw on a particular structure.

Interphase in Composites.—The interphase between ceramic fiber and matrix is crucial to the

static strength, toughness, and long-term stability of the composite. Very little is known about

the relationship between the properties of the interphase and these overall composite properties.

The capability to modify the surface chemistry

of ceramic fibers and whiskers to provide optimum compatibility between reinforcement and

matrix could yield remarkable improvements in

ceramic performance and reliability,

Important

Joining of Ceramics. -In most applications, ceramics are not used alone; rather, ceramic components are part of larger assemblies. Therefore,

the ceramic must be joined to more conventional

materials in the assembly to function properly.

Broad research in joining of ceramics to metals,

glasses, and other ceramics could have a decisive

impact on a future use of monolithic ceramics,

coatings, and composites. The key to joining is

an understanding of the surface properties of the

two materials and of the interface between them.

In general, the interface is a critical point of weakness in discrete ceramic components such as those

in heat engines, in ceramic coatings on metal substrates, and in ceramic fibers in composites. Principal needs in this area are in the strengthening

and toughening of joints, an understanding of

their high-temperature chemistry, and improved

resistance to corrosion in the environments of interest. As with solution methods in powder preparation, chemistry will make a crucial contribution to this field.

Tribology of Ceramics.—Tribology, the study

of friction, wear, and lubrication of contiguous

surfaces in relative motion, is a field of key importance to ceramic wear parts and heat engine

components. Lubrication is a particularly serious

problem in ceramic engines because of their high

operating temperatures. Ordinary engine oils cannot be used above about 3500 F (177° C). For

operating temperatures of 500° to 7000 F (2600

to 371 o C), synthetic liquids such as polyol esters are available, but are extremely expensive and

require further development. In a low-heat rejection (“adiabatic”) ceramic engine, cylinder liner

temperatures may reach 1,000° to 1,700° F (5380

to 927° C), depending on the insulating effectiveness of major engine components. For this elevated temperature regime, synthetic lubricants

cannot be used effectively. One approach is to use

solid lubricants which would become liquid at elevated temperatures; however, no such lubricants

exist for use in the environments envisioned (high

temperature, corrosive gases). Moreover, the distribution of solid lubricant around the engine is

a persistent problem .84

A second approach involves modifications of

the surface of the component to produce “selfLubrication. ” The lubricant can be introduced

through ion implantation directly into the surface

(to a depth of several micrometers) of the component; it then diffuses to the surface to reduce

friction. Some metal or boron oxides show promise as lubricants. Another alternative is to use surface coatings of extremely hard ceramics such as

the carbides and nitrides of zirconium, titanium,

or hafnium, without any lubrication. At present,

these techniques all lead to sliding friction coefficients which are roughly four times higher than

those achieved at low temperatures with engine

oils. 85 Further research is needed to improve this

situation.

Failure Mechanisms in Composites.—Ceramic

matrix composites offer the best solution to the

problem of the brittleness of ceramics. However,

this field is still in its infancy, and research is characterized by a very empirical approach to mixing, forming, densification, and characterization

of fiber-powder combinations. Fundamental understanding of the failure mechanisms in composites would provide guidance for development of

new, tougher ceramics. This would include an investigation of: multiple toughening techniques,

such as transformation toughening and whisker

reinforcement; the role of interphase properties

giManfred Kaminsky, Surface Treatment Science International,

personal communication, August 1986.

‘

s

Ibid.
41

in fracture; and failure mechanisms in continuous fiber composites.

Standardization and Testing.—Standardized

tests and methods of reporting test data are fundamental to the use of any engineering material;

however, a consensus on even these basic principles has eluded the ceramics community. It is not

surprising that in a new field like structural ceramics, which is constantly inventing new materials and processes, there should be a lag in the

development of standards and test methods.

Standardization of materials must inevitably follow the formation of a consensus that certain materials are worth pursuing. While there is a danger in prematurely narrowing the possibilities,

there may also be a danger in not developing the

materials we already have. This is particularly

true of the relatively well-studied materials: silicon nitride, silicon carbide, and zirconia.

In the case of ceramic composites, the specification of standard tests is more difficult, because

the relationship between the micromechanics of

the fibers and matrix and the macroscopic composite behavior is not well understood. In continuously reinforced composites, for example,

which display progressive rather than catastrophic

failure behavior, the stress at which the composite “fails” is ambiguous.

There is a growing interest, both nationally and

internationally, in the development of standards

for ceramics .8’ Areas which could benefit from

standards include: statistical process control, nondestructive testing, analytical procedures, mechanical properties, performance characteristics

in various environments, design procedures and

terminology .87 Standards are essential for the generation of design data, and for reliability specifications for ceramic materials sold domestically

or abroad,

““Concerned organizations include the American Ceramic Society, the U. S. Advanced Ceramic Association (USACA), the Amer.

ican Society for Testing and Materials (ASTM), the Versailles Project

on Advanced Materials and Standards (VAMAS ), and the National

Bureau of Standards,

‘-Proposal developed by S. J. Schneider, as described in the Ceramic Technology Newsletter, Oak Ridge National Laboratory}’, No,

10, Februrary-April 1986.

Desirable

Chemically Bonded Ceramics.–Chemically

bonded ceramics offer great promise for low-cost,

net shape fabrication of structures in such applications as wear parts and construction. Recent improvements in the tensile strength of CBCs suggest that the limits of this key engineering property

are far from being realized, Further research is required in flaw size reduction, long-term stability

in various environments, and the properties of the

interphase in fiber-reinforced cements and concretes.

Table 11 shows that the actual structural ceramics R&D spending in fiscal 1985 for all government agencies corresponds roughly with the

priority categories recommended above, although

specific projects differ. Processing research accounted for the lion’s share, with 76 percent. No

separate estimate was available of research on the

interphase in composites; no doubt a portion of

this work was included under composite fabrication, listed in table 11 as a subcategory of processing. Also, no separate figure was obtained for

Table 11 .—Breakout of the Fiscal Year 1985 Structural

Ceramic Budget According to the R&D Priorities

Cited in the Text

FY 1985 –

Research area budget percentage

Processing:

Powder synthesis . . . . . . . . ... ... 4

Monolithic fabrication . . . . . . . . . . 32

Composite fabrication . . . . . . . . . . . 32

Component design and testing . . . . 4

Coatings . . . . . . . . . . ... ... ... 4

Machining ... . . . . . . . . . . . . . . . . . <1

Subtotal . . . . . . . . . . . . . . . . . . . . . 76

Environmental behavior ., . . . . ... . 4

Reliability:

Modeling ., . . . . . . . . . . . . . . . . . . ... 2

Time dependent behavior . . . . . . . . 1

Nondestructive evaluation . . . . . . . . 3

Microstructure evaluation . . . 4

Subtotal , . . . . . . . . . . . . . . ... 10

Interphase in composites ... ... . no separate figure

Tribology.

Joining . .

Fracture .

Standards

Total

SOURCE S J

.. . . . . . . . . . . . . . . . . . . . . . . . . 2

. . . . . . . . . . . . . . . . . . . . . . . . . 3

. . . . . . . . . . . . . . . . . . . . . . . . . 5

. . . . . . . . . . . . . . . . . . . . . . . . . <1

. . . . . . . . . . . . . . . . . . . . . . . . . 100 ”/0

Dapkunas, Department of Energy
Federal expenditures on new cements and con- suming that the current breakdown of Federal

cretes. In 1983, however, total U.S. Government ceramics research is similar to that in fiscal 1985,

and industry funding of cement research was esti- a comparison of table 11 with the priorities above

mated at only $1 million, compared with a port- suggest that greater emphasis should be placed on

land cement sales volume over $1 billion.88 As- standards development, joining, tribology, and

88 cement-based materials. National Research council, Transportation Research Board, op.

cit., 1984.
POLYMER MATRIX COMPOSITES

Unlike a ceramic matrix composite, in which

the reinforcement is used primarily to improve the

fracture toughness, the reinforcement in a polymer matrix composite (PMC) lends strength and

stiffness to the relatively weak matrix. The composite is designed so that the mechanical loads to

which the structure is subjected in service are supported by the reinforcement. The function of the

matrix is to bond the fibers together and to transfer loads between them. As with ceramic matrix

composites, the reinforcement may consist of particles, whiskers, fibers, or fabrics, as shown in figure 6.

Polymer matrix composites are often divided

into two categories: reinforced plastics, and “advanced composites. ” The distinction is based on

the level of mechanical properties (usually strength

and stiffness); however, there is no unambiguous

line separating the two. Reinforced plastics, which

are relatively inexpensive, typically consist of

polyester resins reinforced with low-strength glass

fibers (E-glass); they have been in use for 30 to

40 years in applications such as boat hulls, corrugated sheet, pipe, automotive panels, and sporting goods. Advanced composites, which have been

in use for only about 15 years, primarily in the

aerospace industry, consist of fiber and matrix

combinations which yield superior strength and

stiffness. They are relatively expensive and typically contain a large percentage of high-performance continuous fibers, such as high-strength glass

(S-glass), graphite, aramid, or other organic fibers.

In this report, market opportunities for both reinforced plastics and advanced composites are considered.

The properties of the composite depend on the

matrix, the reinforcement, and the boundary layer

between the two, called the “interphase.” Consequently, there are many variables to consider when

designing a composite. These include the type of

matrix, the type of reinforcement, their relative

proportions, the geometry of the reinforcement,

and the nature of the interphase. Each of these

variables must be carefully controlled in order to

produce a structural material optimized for the

conditions under which it is to be used.

Figure 6.—Composite Reinforcement Types

Particulate

SOURCE: Carl Zweben, General Electric Co

43
44

The use of continuous fiber reinforcement confers a directional character (“anisotropy”) to the

properties of composites. PMCs are strongest

when stressed parallel (O O) to the direction of the

fibers (longitudinal direction), and weakest when

stressed perpendicular (90o

) to the fibers (transverse direction). In practice, most structures are

subjected to complex loads, necessitating the use

of fibers oriented in several directions (e. g., 0°,

+ / –45 o

, 900). However, PMCs are most efficiently used in applications which can take advantage of the inherent anisotropy of the materials,

as shown in figure 7, which compares the strength

and stiffness of composites with various metals.

When discontinuous fibers or particles are used

for reinforcement, the properties tend to be more

isotropic, since the reinforcement direction is not

as well defined. Such composites lack the outstanding strength of continuous fiber PMCs, but they

can be produced more cheaply, using the technologies developed for unreinforced plastics, such as

extrusion, injection molding, and compression

molding. Sheet molding compound (SMC, figure

7) is such a material, which is widely used in the

automotive industry.

The Polymer Composites Industry

The reinforced plastics/composites industry

today encompasses a very broad range of materials, products, and manufacturing processes. About

85 percent of the material used is glass fiber-reinforced polyester resin (often called fiber-reinforced

plastic, or FRP). 1 FRP is fabricated by a number

of processes to produce a great variety of products, some of which are noted above.

Less than 2 percent of the material used in the

reinforced plastics/composites industry goes into

‘Reginald B. Stoops, R.B. Stoops & Associates, Newport, RI,

“Manufacturing Requirements of Polymer Matrix Composites, ” contractor report prepared for the Office of Technology Assessment,

December 1985.

Figure 7.–Comparison of the Specific Strength and Stiffness of Various Composites and Metals (Steel: AISI 4340;

Aluminum: 7075”T6; Titanium: Ti-6Al-4V) Specific Properties are Ordinary Properties Divided by Density.

Angles Refer to the Directions of Fiber Reinforcement

4

3

2

1

(0°) Graphite/epoxy

/+

/’

(0°) Kevlarlepoxy

i

+ (O”,90c) Graphitelepoxy

/

+/’

Steel

Aluminum ~ +

Titanium

(0° t 45° 90°) (0”) S-glass/epoxy

I

Graphite/epoxy

t

Sheet molding compound (SMC)

1 2 3 4

Specific tensile strength (relative units)

SOURCE Carl Zweben, General Electr!c Co
“advanced composites” for use in “high-tech” applications such as aircraft and aerospace.2 In 1984,

the world produced some 22 million pounds of

advanced composite materials, mostly in the

United States. The total value of fabricated parts

was about $1.3 billion split among four major consuming industries: aerospace (60 percent), sports

equipment (20o percent), and industrial and automotive (15 percent ).’

It has been estimated that advanced polymer

composites will grow at the relatively high rate

of about 15 percent per year in the next few years,

with the fastest growing sector being the aerospace

industry at 22 percent. By 1995, consumption is

forecast to be 110 million pounds with a value

in 1985 dollars of about $6.5 billion. By the year

2000, consumption is estimated to be 200 million

pounds valued at about $12 billion.4

If we divide these numbers to get a rough idea

of the cost per pound of advanced composite material, we find a value of about $60/pound. This

compares with a value of about $1/pound for steel

or $1 .50/pound for FRP. If these forecasts are correct, it is clear that over this period advanced composites will be used primarily in high value-added

applications which can support this level of material costs. However, we shall see that use of composites can lead to cost savings in manufacturing

and service. Thus, the per-pound cost is rarely

a useful standard for comparing advanced composites with traditional materials.

‘Ibid, These are primarily epoxy matrices reinforced with carbon

fibers.

3According to the market resdarch firm, Strategic Analysis (Reading, PA) as reported in High Technology, May 1986, p. 72. Advanced composites were defined as those reinforced with S-glass or

superior fibers.

“’Industry News, ” SAMPE Journal, July August 1985, p. 89.

Composite

Matrix

45

Constituents

The matrix determines the resistance of the composite to most of the degradative processes which

eventually cause failure of the structure, including impact damage, delamination, water absorption, chemical attack, and high-temperature creep.

Thus, the matrix is typically the “weak link” in

the composite structure.

The matrix phase of commercial composites can

be classified as either “thermoset” or “thermoplastic. ” The general characteristics of each matrix type

are shown in figure 8; however, recently developed matrix resins have begun to change this picture, as noted below.

Thermosets. -Thermosetting resins include polyesters, vinylesters, epoxies, bismaleimides, and

polyamides. Thermosetting polyesters are commonly used in FRPs, and epoxies make up most

of the current market for advanced composite

resins. Initially, the viscosity of these resins is low;

however, these matrix materials undergo chemical reactions which crosslink the polymer chains,

and thus connect the entire matrix together in a

three-dimensional network. This process is called

“curing. ” Thermoses, because of their threedimensional crosslinked structure, tend to have

high dimensional stability, high temperature resistance, and good resistance to solvents. Recently,

considerable progress has been made in improving

the toughness of thermosets.5

‘Norman J. Johnston, “Synthesis and Toughness Properties (>f Re\-

ins and Composites, ” CP 2321, National Aeronautics and Space

Administration, 1984.

Figure 8.—Comparison of General Characteristics of Thermoset and Thermoplastic Matrices

Process Process Use Solvent

Resin type temperature time temperature resistance Toughness

Thermoset . . . . . . . . . . . . . . . . . . . . . . . . . Low 1 High I High II High I

Low

Toughened thermoset . . . . . . . . . . . .

Lightly crosslinked thermoplastic . . . . . t 1 t t 1

Thermoplastic. . . . . . . . . . . . . . . . . . . . . . High I

Low I Low Low I High I

SOURCE Darrel R Tenney NASA Langley Research Center
46

Thermoplastics.—Thermoplastic resins, sometimes called engineering plastics, include some

polyesters, polyetherimide, polyamide imide, polyphenylene sulfide, polyether ether ketone (PEEK),

and liquid crystal polymers. They consist of long,

discrete molecules which melt to a viscous liquid

at the processing temperature, typically 5000 to

700° F (260° to 3710 C), and, after forming, are

cooled to an amorphous, semicrystalline, or crystalline solid. The degree of crystallinity has a

strong effect on the final matrix properties. Unlike the curing process of therrnosetting resins, the

processing of thermoplastics is reversible, and, by

simply reheating to the process temperature, the

resin can be formed into another shape if desired.

Thermoplastics, while generally inferior to thermosets in high-temperature strength and chemical stability, are more resistant to cracking and

impact damage. However, it should be noted that

recently developed high-performance thermoplastics such as PEEK, which have a semicrystalline

microstructure, exhibit excellent high-temperature

strength and solvent resistance.

Thermoplastics offer great promise for the future from a manufacturing point of view, since

it is easier and faster to heat and cool a material

than it is to cure it. This makes thermoplastic matrices attractive to high-volume industries such as

the automotive industry. Currently, thermoplastics are used primarily with discontinuous fiber

reinforcements such as chopped glass or carbon/

graphite. However, there is great potential for

high-performance thermoplastics reinforced with

continuous fibers. For example, thermoplastics

could be used in place of epoxies in the composite

structure of the next generation of fighter aircraft.

Reinforcement

The continuous reinforcing fibers of advanced

composites are responsible for their high strength

and stiffness. The most important fibers in current use are glass, graphite, and aramid (other organic fibers, such as oriented polyethylene, are

also becoming important). Advanced composites

contain about 60 percent of reinforcing fiber by

volume. The strength and stiffness of some continuous fiber composites are compared with those

of sheet molding compound and various metals

in figure 7. For example, unidirectional, high

strength graphite/epoxy has over three times the

specific strength and stiffness of common metal

alloys.

Of the continuous fibers, glass has a relatively

low stiffness; however, its tensile strength is competitive with the other fibers and its cost is dramatically lower. This combination of properties

is likely to keep glass fibers the most widely used

reinforcement for high-volume commercial composite applications. Only when stiffness or weight

are at a premium will aramid and graphite fibers

be used.

Interphase

The interphase of composites is the region where

loads are transmitted between the reinforcement

and the matrix. The extent of interaction between

the reinforcement and the matrix is a design variable, and may vary from strong chemical bonding to weak frictional forces. This can often be

controlled by using an appropriate coating on the

reinforcing fibers. Generally, a strong interracial

bond makes the composite more rigid, but brittle; a weak bond decreases stiffness, but enhances

toughness. If the interracial bond is not at least

as strong as the matrix, fracture and delamination will occur at the interphase under certain loading conditions. Frequently, the most desirable

coupling is intermediate between the strong and

weak limits. The bond is also critical to the longterm stability of the composite, playing a key role

in fatigue properties, environmental behavior, and

resistance to hot-wet conditions.

Properties of Polymer Composites

Composites are designed materials. This is really

the fact that underlies their usefulness. Given the

spectrum of matrix and reinforcement materials

available, the composite properties can be optimized for a specific application. A composite can

be designed to have zero coefficient of thermal

expansion. It can be reinforced with combinations

of fiber materials (hybrid composites) and geometries in order to optimize performance and minimize cost. The inherent anisotropy of the mate-
47

rial means that the composite can have different

properties in different directions. The design opportunities of PMC materials are just beginning

to be realized.

Design, Processing, and Testing of

Polymer Matrix Composites

Composites Design

Because of their larger number of components

and anisotropic properties, polymer composites

are inherently more complex than metals. In fact,

composites are more accurately characterized as

customized structures rather than materials. While

the engineering properties of the homogeneous

resins and fibers can be determined, the properties of the composite depend on the composition,

fiber geometry, and the nature of the interphase.

The categories of mechanical and physical properties used to characterize composites are carried

over from the long engineering experience with

metals. In many cases, however, properties which

are meaningful in metals are not meaningful in

composites. “Toughness” is such a property. In

metals, where the dynamics of crack propagation

and failure are relatively well understood, toughness can be defined relatively easily. In a composite, however, toughness is a complicated function of the matrix, fiber, and interphase, as well

as the reinforcement geometry. The shear and

compression properties of composites are also

poorly defined.

In spite of many years of discussion about standardized test methods for measuring engineering

properties, the composites community has not

been able to reach a consensus on what it is that

the existing tests actually measure. The Army’s

Materials Technology Laboratory (Watertown,

Massachusetts) is currently developing MIL-17,

a handbook on advanced composites which will

begin to address the questions of how data on composites can be obtained and reported. At present,

each company qualifies its material for each separate aerospace or defense application according

to its own individual tests and procedures. Data

on material properties are often developed under

government contract (costing $100,000 to $200,000

and taking about 1 year), and companies are reluctant to share the results. Even when data are reported in the literature, the type of test used and

the statistical reliability of the results are not reported with the data. While the lack of standards

probably does not inhibit the expert designer of

composite aerospace structures, standards could

encourage the use of composites in industries such

as construction, where designers have no familiarity with the materials.

Computer-Aided Design.—Inasmuch as composites are tailored to meet the design requirements

of a particular structure, designers must rely on

computer models to analyze and predict the behavior of composite structures using a database

containing the properties of the resins, fibers, and

unidirectional composites. Such models presuppose a scientific understanding of the underlying

physical, chemical, and mechanical processes

within the composite at a microscopic level, and

the relations between these and the macroscopic

properties. In general, this understanding does not

exist; the most critical needs in this area are discussed below in the section on research and development priorities.

A number of models exist today to address the

needs of composites designers. Elastic behavior

modeling is fairly advanced, with programs available to analyze stress distributions and elastic

deformation in complex three-dimensional shapes.

Several models exist to predict failure in composite structures where the characteristics of the critical flaws are known. These are based on Linear

Elastic Fracture Mechanics (LEFM), a body of theory developed to predict fracture in metals. Extension of LEFM to composites is controversial

today, because the failure modes of composites

are more complex than metals. Very little work

has been done in modeling the long-term behavior

of composites; for example, in the areas of fatigue,

creep, and environmental degradation. Part of the

reason for this is the lack of data on which to test

and validate models.

In spite of the lack of modeling capability, experience to date has shown that designers and manufacturers can produce reliable composite structures. This is no doubt due to two factors. First,

in the face of uncertainty, designers tend to “overdesign”; that is, to be too conservative in their

use of material, to avoid any possibility of mate-
48

rial failure. Second, composite structures have

been extensively tested before use, ensuring that

any potential problems show up during the tests.

Thus, the composite materials themselves have

been proven, in the sense that structures can be

fabricated which are reliable and meet all design

criteria. However, both overdesign and empirical testing are costly and drive up the prices of

composites. The principal benefit of enhanced

modeling capability will be to help to make composites more cost competitive.

Manufacturing of Polymer

Composite Structures

Given the many different fibers and matrices

from which composites can be made, the subject

of PMC manufacturing is an extremely broad one.

However, more than any other single area, low

cost manufacturing technologies are required before composites can be utilized more widely. The

basic steps include: impregnation of the fiber with

the resin; forming of the structure; curing (thermoset matrices) or thermal processing (thermoplastic matrices); and finishing. Depending on the

process, these steps may occur separately or simultaneously. For example, the starting material for

many composites is a “prepreg”; i.e., a fiber tape

or cloth which has been preimpregnated with resin

and partially cured. Therrnoset prepregs may be

stored in the freezer for up to 12 months prior to

use. Some of the more important fabrication processes for composites are listed in table 12.

Nondestructive Evaluation (NDE)

of Polymer Matrix Composites

In general, polymer composites do not have as

great a tendency to brittle fracture as do ceramics.

This means that the critical flaw size in large PMC

structures may be of the order of centimeters, while

in ceramics it is some tens of micrometers. PMC

structures are increasingly used in life-critical

structures such as aircraft wings and fuselages.

Photo credit: Hercules, Inc.

Filament winding of a rocket motor case.
49

Table 12.— Production Techniques for

Polymer Composites

Technique Characteristics Examples

Sheet molding Fast, flexible, 1 -2” SMC automotive body

fiber panels

Injection molding Fast, high volume Gears, fan blades

very short fibers,

thermoplastics

Resin transfer Fast, complex parts, Automotive structural

molding good control of fiber panels

orientation

Prepreg tape Iayup Slow, Iaborious, Aerospace structures

reliable, expensive

(speed improved by

automation)

Pultrusion Continuous, constant l-beams, columns

cross-section parts

Filament windlng Moderate speed, Aircraft fuselage,

complex geometries, pipes, drive shafts

hollow parts

Thermal forming Reinforced thermoplastic All of above

(future) matrices, fast, easy

repair, joining

SOURCE Off Ice of Technology Assessment

This places a special burden on NDE, both in the

factory and in the field.

Although NDE is now used primarily for the

detection of defects in finished structures, in the

future it will be used increasingly for monitoring

the status of composites at intermediate steps in

the production process. Progress in this field will

require development of sophisticated sensors and

feedback control systems.

Requirements for NDE of composites differ

somewhat from those of ceramics. While the flaw

sizes to be detected are not as small, the area of

structure to be investigated is frequently much

larger, up to hundreds of square feet. Thus, techniques are required which can rapidly scan large

areas for flaws or damage. While there are numerous NDE techniques which may be useful in

the laboratory for testing of small specimens for

research purposes, relatively few are appropriate

to production or field-level inspection. Several of

the more important NDE techniques which are

relevant to production, end product, and field level

inspection are listed in table 13. Excellent progress

has been made in production-level techniques such

as ultrasonics, and manufacturers are confident

that large composite surface areas can be inspected

reliably and economically for such flaws as bulk

delaminations.

The inspection and repair of composite structures (e.g., aircraft components) at the depot and

field levels will require a substantial training program for inspectors unfamiliar with composites.

All procedures must be standardized and straightforward, since in general composite experts will

not be available. In the future, as inspection processes become fully computerized, this will be an

excellent application for expert systems which can

guide the operator through the process and alert

him to any detected anomalies.

Table 13.—NDE Techniques Appropriate for Production, Finished Product, Depot,

and Field-Level Inspections of Polymer Matrix Composite Structures

N DE technique Flaw type Sensitivity

Production:

Visual (remote) ... ... , . . Fiber orientation good

Foreign material

Ultrasonic ., . . . . . . . . . . . . . . Porosity, viscosity good

during cure

Dielectrometry . . . . . . . . . . Degree of cure good

End product:

Visual . . . . . . . . . . . . . . . . . . surface good

Ultrasonic . . . . . . . . . . . . . . . bulk good

Radiographic . . . . . . . . . . . . . bulk fair

Acoustic emission ., ... . bulk fair

Depot level:

Ultrasonic ., . . . . . . . . . ... bulk delamination good

Field level:

Ultrasonic . . . . . . . . . . . . . . . . bulk delamination fair

Complex Development for

shapes commercialization

good

poor

good

good

poor

excel lent

good

poor

Door

none

extensive

some

none

some

none

extensive

some

extensive

SOURCE Joseph A. MoyzIs, et al , “Nondestructive Testing of Polymer Matrix Composites,” a contractor report prepared for

OTA, December 1985
50

Health and Safety

There are a number of unique health and safety

issues associated with the manufacture of composite materials. The health hazards stem from

the fact that chemically active materials are used

and workers handling them may breathe harmful

fumes or come in contact with irritating chemicals. The chemical of greatest concern is the styrene monomer used in polyester resins. The problem is most severe when the resin is sprayed, and

the monomer evaporates into the air. Inhalation

of styrene monomer can cause headaches, dizziness, or sore throat; some people become sensitized to the vapors and can no longer work in a

reinforced plastics plant.

The Occupational Safety and Health Administration has specified that styrene monomer concentrations in a plant should not exceed 100 parts

per million .

6 In a plant where spray systems are

used, extensive air handling equipment, spray

booths, and air masks are required to maintain

these standards. Where polyester resins are used

for compression molding, resin transfer molding,

or other enclosed mold systems, the problem can

be dealt with by simple exhaust systems.

A new safety hazard was introduced with the

advent of carbon fibers, which tend to float around

a plant where they are used and, because they are

electrical conductors, can get into unprotected electrical devices to cause short circuits. The fiber concentration in the air can be controlled by a negative pressure exhaust system in the area where they

are used, but all electrical devices in the area

should be sealed (explosion proof).

Recycling and Disposal of Composites

Most polymer composite materials in use today

have thermosetting matrices, and after they have

been cured, have no apparent scrap value. Although attempts have been made to grind them

up and use them as fillers, this has not proven to

be economically practical. The reuse of uncured

FRP composites offers little economic incentive.

Most scrap is simply discarded. One of the potential advantages of composites with thermoplastic matrices is that the scrap can be recycled.

‘World of Composites, quarterly publication of the SPI Reinforced

P]astics/Composites Institute, winter 1986.

Cured composites present no particular problem with disposal; they are chemically inert and

can be used for land fill. Burning them can generate toxic smoke, and so is generally avoided.

The principal problem of disposal arises with

uncured composites. Wet lay-ups, prepregs, SMC,

etc., are still chemically active and pose both health

and safety problems. If used in landfill, the active

chemicals can leach out and cause contamination

of the soil or water. A more serious problem is

that the catalyzed resins may go on to cure and

generate an exotherm that causes spontaneous

combustion or self-ignition. The safe way to dispose of uncured composite material is to first bake

it until it is cured and then dispose of it.

Applications of Polymer

Matrix Composites

Polymer matrix composites are a more mature

technology than structural ceramics. With the experience gained in military applications such as

fighter aircraft and rocket motor casings beginning in the 1970s, composites now have a solid

record of exceptional performance and reliability, They are rapidly becoming the baseline structural material of the defense aerospace industry.

Becaues of their high cost, diffusion of advanced composites into the civilian economy is

likely to be a “top-down” process, progressing

from relatively high value-added applications such

as aircraft to automobiles and then to the relatively “low-tech” applications such as construction. On the other hand, there is also a “bottomup” process at work in which savings in manufacturing costs permit unreinforced engineering

plastics and short fiber-reinforced composites to

replace metals in applications where high strength

and stiffness are not required. Use of sheet molding compound for automobile body panels is one

example of this phenomenon.

Markets for Advanced Polymer

Matrix Composites

Aerospace.—This sector has been estimated to

consume as much as 80 percent of all high-performance PMCS.

7 Growth projections for aero7According to the market study conducted by Business Communications Co., “Advanced Composites: An Evaluation of Commercial Prospects, ” Stamford, CT, as reported in World of Composites,

a publication of the Society of Plastics Industries, winter 1986, p, 4.
51

space usage of composites have ranged from 8.5

percent per year8 to 22 percent per year.9 The primary matrix materials in aerospace applications

are epoxies, and the most common reinforcements

are carbon/graphite, aramid (e. g., Du Pent’s

“Kevlar 49”), and high-strength glass fibers. However, high-temperature thermoplastics such as

PEEK are considered by many to be the matrices

of choice for future aerospace applications. Composites are used extensively today in small military aircraft, military and commercial rotorcraft,

and prototype business aircraft. The next major

aircraft market opportunity for composites is in

large military and commercial transport aircraft.

The principal advantages of PMCs in aerospace

applications are their superior specific strength

and stiffness compared with metals, resulting in

weight savings of 10 to 60 percent over metal designs, with 20 to 30 percent being typical.l0 This

weight reduction can be used to increase range,

payload, maneuverability and speed, or reduce

fuel consumption. It has been estimated that a

pound of weight saved on a commercial transport

aircraft is worth $100 to $300 over its service life,

depending on the price of fuel, among other factors.11 This high premium for weight saved is

unique to this sector, and explains why it leads

all others in growth rate. Additional advantages

of PMCs are their superior fatigue and corrosion

resistance, and vibration damping properties.

Military Aircraft.—Advanced composites have

become essential to the superior performance of

a large number of fighter and attack aircraft (figure 9). Indications are that composites may account for up to 40 percent of the structural weight

of the Advanced Technology Fighter (ATF),

which is still in the design phase. Because the performance advantages of composites in military

aircraft more than compensate for their high cost,

Figure 9.—Composite Aircraft Structure (by percent)

V-22 Rotor Craft

T

AVTEK 400 Corp A/ C

Beech ‘H x

Starship

Com Dosltes

E;

o.- (JU

~ g 40

“In

c . - S-76 (UC

2 ~ 20 -

n F-15

F-14

n B-767~

:ntatlon

Adv

2

Fighter &

Z Fighter Attack A/C

/.

d

A320 Large Commercial &

fra Mllltary A/C

1970 1980 1990

Year of first flight

Business aircraft and rotorcraft

M)l,ta~ rotorcraft

F!ghler and attack aircraft

Large commercial and mllltary a!rcraft

Bomber

SOURCE. Richard N Hadcock, Grumman Aircraft Systems Dlvlslon, “Status and

Viabillty of Composite Materials in Structure of High Performance Aircraft, ” a presentation to the National Research Councfl, Aeronautics

and Space Engineering Board Naval Postgraduate School, Monterey,

CA, Feb 10, 1986

this is likely to be the fastest growing market for

advanced composites over the next decade. One

estimate, which assumes only existing production

plus the ATF, projects a growth from about 0.3

million pounds per year in 1985 to 2 million

pounds per year in 1995. ’2

Commercial Aircraft. —If glass fiber reinforced

composites are included, the volume of composites used in commercial and business aircraft is

about twice that used in military aircraft. 13 In current commercial transport aircraft, such as the

Boeing 767, composites make up about 3 percent

of the structural weight, and are used exclusively

in the secondary (not flight-critical) structure. 14

However, several companies, including Beech and

Avtek, are awaiting FAA approval of “all composite” business aircraft prototypes, which utilize

composites in the wings, empennage, and fuselage. 15

‘According to the market study conducted by Frost & Sullivan,

‘Worldwide High Performance Composites, ” New York, as reported

in World of Composites, a publication of the Society of Plastics Industries, winter 1986, p. 4.

‘According to a market study by Charles H. Kline & Co., “Advanced Polymer Composites, ” Fairfield, NJ, as reported in Plastics

Engineering, June 1985, p. 62.

IOcar] Zweben, “polymer Matrix Composites, ” hontiers h ~aterials Technologies, M.A, Meyers and O.T. Inal (eds. ) (The Netherlands: Elsevier Science Publishers, 1985), p. 365.

llBob Hammer, Boeing Cornrnercid Aircraft CO., personal communication, August 1986.

IZRichard N, HadcWk, Gmmman Aircraft Systems Division, “Status and Viability of Composite Materials in Structures of High Performance Aircraft, ” a presentation to the National Research Council, Aeronautics and Space Engineering Board, N’aval Postgraduate

School, Monterey, CA, Feb. 10, 1986.

“Ibid.

I ~Darre] R, Tenney, NASA Langley Research Center, “Advanced

Composite Materials: Applications and Technology Needs, ” presentation to the Metal Properties Council, Inc., Miami, FL, Dec. 5,

1985.

lsHadcock, op. cit. , Feb. 10, 1986
5 2

built by 1995, has placed the U.S. composite com-

\

l\

‘*

Photo credit” Hercules, Inc.

A modern lightweight fighter incorporating 64 different

components—more than 900 pounds of composite

structure per airplane.

Although growth of the business aircraft fleet

over the next 5 years is expected to be only around

10 percent, two categories (turboprops and turbojets) are expected to grow significantly. ” These

aircraft are also the best candidates for composite fuselages. The estimated value (derived from

cost and volume estimates) of composite fuselages,

assuming all business aircraft manufacturers adopt

this technology, is about $100 million a year.17

These fuselages could account for 1.2 million

pounds of graphite/epoxy consumption annually.

Large transport or commercial aircraft fuselages

will probably not be made from advanced composites until the technology is demonstrated in

business aircraft.

By the year 2000, composites could make up

65 percent of the structural weight of commercial transport aircraft.18 Estimating a structural

weight of 75,000 pounds per aircraft and production of 500 aircraft per year, this application alone

should account for 24 million pounds of advanced

composite per year. Assuming a starting material

value of $60/lb, the market in the year 2000 is

valued at about $1.5 billion for the composite material alone. A more conservative estimate, which

assumes that no new commercial aircraft will be

“Materials Modeling Associates, “Properties, Costs, and Applications of Polymeric Composites, ” Massachusetts Institute of Technology, a contractor report prepared for the Oftice of Technology

Assessment, December 1985.

‘“Ibid.

IRTenney, op. cit. , Dec. 5, 1985.

mercial airframe production at between 1 and 2

million pounds in that year.19

Helicopters.–With the exception of the “allcomposite” business aircraft prototypes, which are

still awaiting certification, composites have been

used more extensively in helicopters than in aircraft. Military applications have led the way, and

the advantages of composites are much the same

as in aircraft: weight reduction, parts consolidation, and fatigue and corrosion resistance. Over

the past 15 years, composites have become the

baseline materials for rotors, blades, and tail

assemblies. Sikorsky’s S-76 commercial model,

which is about 25 percent composite by weight

(figure 9), was certified in the late 1970s. Future

military helicopters, such as the Army’s LHX

(with major airframe design teams at Bell/McDonnell Douglas and Boeing/Sikorsky), or the

Navy’s tilt-rotor V-22 “Osprey” (designed by

Bell/Boeing), have specifications which force

designers to consider composites, which are likely

to comprise up to 80 percent of the structural

weight (figure 9). Materials such as graphite/epoxy are likely to be used in the airframe, bulkheads, tail booms, and vertical fins, while the less

stiff glass/epoxy composites will be used in the

rotor systems. As with aircraft, there could be a

long-term trend away from epoxy resins and

toward thermoplastic resins.

Automotive Industry .—The automotive industry is widely viewed as being the industry in which

the greatest volume of advanced PMC materials

will be used in the future. Because the industry

is mature and highly competitive, the principal

motivation for introducing composites is cost savings. In contrast to the aircraft industry, there is

no clear-cut premium associated with a pound of

weight saved. Nevertheless, Detroit continues to

be interested in saving weight as it pursues the

conflicting goals of larger automobiles and higher

fuel efficiency. Automakers are looking to the vehicle skin/frame systems to provide the next big

leap in weight reduction. Other potential advantages of composites, such as corrosion resistance,

appear to be secondary to the cost issue.

1~Hadcock, op. cit., Feb. 10, 1986.
By far the greatest volume of composite material is sheet molding compound (SMC) used in exterior panels .20 The most visible automotive use

of SMC in recent years is the Pontiac Fiero, which

has an all composite exterior. The Fiero is constructed with a steel “space frame” superstructure

to which the composite body panels are attached.

By using different composite exterior panels, automakers can achieve model differentiation for

limited production runs (100,000 to 200,000 units)

while avoiding the prohibitive tooling costs which

would be involved with use of steel. Automobile

companies have adopted the space frame concept

and will be using PMC materials for exterior

panels in the future. In the short run, these will

be mostly SMC.

The next major opportunity for composites in

automobiles is in structural components,21 Two

structural components currently in service are the

composite drive shaft and leaf spring. Some 3,000

‘“Materials Nlodeling Associates, op. cit., December IQ85.

21P. Beardmore, “Composite Structures for Automobiles, ” Composite Structures 5:163-176, 1986.

53

drive shafts manufactured by filament winding

graphite and E-glass fibers in a polyester resin are

used annually in the Ford Econoline van. 22 Also,

glass fiber reinforced plastic leaf springs in the

Corvette and several other models are in production at the rate of approximately 600,000 per year.

Leaf springs are regarded as a very promising application of composites, and are expected to show

strong growth, especially in light trucks. Prototype primary body structures have been constructed with weight savings of 20 percent or

more.

Advance engineering groups within the Big

Three automobile producers are targeting the mid1990s to launch composite unibody vehicles .23 Initially, composite unibodies will appear in limited

production, in models like the Corvette or Fiero.

Annual production should be in the range of

25,000 to 100,000 vehicles. It is likely that a composite unibody would not look like a conventional

metal unibody, but would have a shape more consistent with the requirements of the manufacturing process used; resin transfer molding or filament winding would be two candidate Drocesses.

However, an estimate of the amount of composite material required can be obtained from the following. A typical mid-size, stamped steel unibody

weighs between 500 and 600 pounds. It is expected

that the equivalent composite system will weigh

25 to 45 percent less. Based on these assumptions,

consumption of composites will amount to about

12,000 tons in the first full year, which represents

about 10 percent of current reinforced plastic consumption in the domestic automotive sector. Estimating the value of composite unibodies at $6/lb,

the estimated market would be about $150 million for these components alone.24

The automakers are exploring composite unibodies for a variety of reasons. Composite vehicles would enable designers to reduce the number

of parts required in assembly; some manufacturers

are looking into a one-piece composite structure.

By reducing the number of parts, better consistency

of parts can be achieved at considerably reduced

Photo credit Ford Motor Co

Compression molded composite rear floor pan

prototype for the Ford Escort. Ten steel components

were consolidated into a single molding, with

15 percent weight savings.

zz~though composite drive shafts are technically successful, Ford

will take them out of production in 1987 in favor of a new aluminum design

“Materials Modeling Associates, op. cit., December 1985.

241bid.
54

assembly costs. Composites also offer substantial

improvements in specific mechanical properties,

with the possibility of reducing weight while increasing strength and stiffness. Finally, PMCs offer greatly improved corrosion resistance over

steel or galvanized steel, since they do not rust.

Observers have estimated that composite automobiles could last 20 or more years, compared to

the current average vehicle lifetime of 10 years .25

Without question, the major technical barrier

to use of PMCs in the automotive industry is the

lack of manufacturing technologies capable of

matching the high production rates of metal

stamping technology. The fact that steel parts can

be stamped in seconds while plastics mold in minutes has led to lower production costs for steel.

A major reason behind the increased use of plastics in automobiles is that the time of manufacture has fallen, not the cost of the equipment,

tools, or materials. The fastest current technologies can process material at the rate of tens of

pounds per minute, while true economy will require rates of 100 pounds per minute or more.26

Thus, there is a gap of roughly an order of magnitude between current and economical rates.

If composites are used extensively in automobile unibodies, there could be large, secondary

economic effects. These would involve the automobile producers and suppliers, the service network, and those involved in disposal of automobiles.

Auto producers will have to build new facilities or modify existing physical plant in order to

fabricate composite unibodies in-house. Alternatively, they may elect to outsource the molding

operations. This would lead to the creation of new

specialized composites molding businesses, similar to the custom molders of plastics which now

serve Detroit. Production lines would have to be

altered to accommodate different processes for joining components together. Segments of the automobile service network would be directly affected

by a change to composite unibodies. Body shops

would have to change their repair procedures, or

switch from repairing to replacing damaged parts.

‘

s Ibid.

z~char]es Sega], President, Omnia, Raleigh, NC, in OTA workshop on “Future Applications of Advanced Composites, ” Dec. 10,

1985.

Rust-proofing services might be eliminated altogether.

Finally, an all-composite unibody will greatly

reduce the supply of steel scrap and may render

the recycling of automobiles unprofitable. Should

this occur, disposing of millions of auto hulks per

year could become a national problem. In addition,

since steel minimills use steel scrap as their starting material, they might be placed in jeopardy.

Reciprocating Equipment.—PMC materials

have considerable potential for use in many different kinds of high-speed industrial machinery. Current applications include such components as centrifuge rotors, weaving machinery, hand-held

tools, and robot arms. All of these applications

take advantage of the low inertial mass, but they

also benefit to various extents from the tailorable

anisotropic stiffness, superior strength, low thermal expansion, fatigue life, and vibration damping characteristics of composites.

The productivity of robotic work stations and

flexible manufacturing cells could be improved if

robots could operate at higher speeds with more

accurate endpoint positioning, Stiffness is the key

mechanical property, since the endpoint accuracy

is limited by bending deflections in the beamshaped robot members. With metal designs, stiffness is obtained at the cost of higher mass, which

limits the robot response time. Consequently, the

ratio of the weight of the manipulator arm to that

of the payload is rarely lower than 10:1.27 Because

of their superior stiffness per unit weight, composites are a promising solution to this problem.

At present, only one U.S. company28 has marketed a robot incorporating composites, although

there are several Japanese models and a number

of other countries are funding research.

Although the benefits of using composites in

reciprocating equipment are clear, initial attempts

to penetrate this market have been disappointing.

The market is a highly fragmented one, and equipment manufacturers, who tend to be oriented

Z7B sThompson and C .K. Sung, “The Design of Robots and In- . .

telligent Manipulators Using Modern Composite Materials, ” A4echanism & Machine Theory 20: 471-482, 1985.

2’Graco Robotics of Lavonia, Michigan manufactures a spray

painting robot with a hollow graphite/epoxy arm. However, this

arm is being phased out in favor of a new aluminum design.
5 5

toward metals, have shown a reluctance to consider the use of a higher cost material (particularly

when its use requires new processes and tooling)

even when performance advantages are demonstrated. No attempt has been made to quantitatively estimate future markets. Composite

penetration is likely to be slow but steady.

Shipping and Storage.—Plastics have long been

known to be inert to chemical attack from a wide

variety of substances. This has made them useful as containers for many corrosive chemicals.

With the addition of fiber reinforcement, the increased strength makes them suitable for large

structures such as large diameter pipe, holding

tanks, and pressure vessels. One generic limitation of composites for these applications is temperature. As the structures are generally made out

of glass fiber reinforced polyester or epoxy, the

maximum service temperature is about 3000 F

(1490 C). A further potential problem is that the

glass fibers are subject to corrosive attack if the

matrix cracks and exposes them to water or other

chemicals. Assuming fiber cost reductions can be

achieved, use of corrosion-resistant graphite or

ceramic fiber reinforcement could be a promising solution to this problem in the future.

Pipe. -Glass fiber reinforced plastic (FRP) pipes

have major applications in the oil and the chemical process industry. The demand for composite

pipe is directly affected by the level of economic

activity within these industries. Chemical manufacturing expenditures have been decreasing over

the past few years, and low oil prices have

brought acquisition of new oil field equipment to

a halt. Consequently, the demand for fiberglassreinforced pipe in these sectors has been declining. 29

Industry sources predict that any growth in the

demand for composite pipe will come from the

petroleum industry. One high-volume application

which has shown strong growth is pipe used in

pumping of gasoline. It is expected that fiberglassreinforced pipe will penetrate 15 to 30 percent of

the oil field market in such applications as sucker

rods, downhole tubing, and casing, However,

growth in these areas is predicated on rising oil

prices. The FRP pipe industry is approximately

Material Modeling Associates, op. cit., December 1985.

30 years old. It saw tremendous growth until

1980, but is now leveling off. A maximum of 5

percent annual growth is expected .30

Storage Tanks.—FRP has captured about 40

percent of the market for underground tanks for

storing fuel and corrosive chemicals and could increase its market share to about 80 percent over

the next 5 to 10 years. 31 FRP underground tanks

are expected to last at least 30 years, compared

with 13 to 15 years for bare steel.

Above-ground tanks for storage of oil, chemicals, and food represent a large potential market for FRP. Above-ground tanks smaller than

about 12 feet in diameter or less than 30 feet high

are predominantly FRP, but for larger tanks, the

difficulty of manufacturing, handling, and shipping with FRP makes steel the more economical

material.

Composites could be used widely in the future

for portable storage tanks and pressure vessels.

The two properties of greatest importance are corrosion resistance and light weight. In Europe

today there are currently some 20,000 tanker

trucks which use an FRP shell reinforced by a steel

frame. In the United States there is one prototype

tanker which utilizes a reinforcement combination of chopped glass fiber and glass filament

wound layers.32

T h ee standaard tanker design fo r

hauling corrosive chemicals (a business which accounts for about 5 percent of the total shipping

market), is stainless steel lined with rubber, epoxy, or glass. These liners must be replaced every

1 to 5 years, at considerable expense. The composite tanker can be produced for approximately the

same price as the steel design, and, based on the

European experience and the performance record

of underground composite tanks, it is expected

to last longer, reduce costs, and increase safety.

As of September 1984, composite tanker trucks

were approved by the Department of Transportation for travel on U.S. roads; however, the difficulty of obtaining insurance continues to be a

serious barrier to use of composite tankers .33

301bid.

s] ~orj~ of Composites, Op. cit,, winter 1986, P. 8.

JzJowph M. Plecnik, et al., “Composite Tanker Trucks Have Made

the Grade, ” Plastics Engineering 41(3):63-65, March 1985.

JJJowph M. pl=ik, Department of Civil Engineering, Long ~ach

State University, personal communication, August 1986.
5 6

Photo credit: The CUMAGNA Corp.

Computer trace of fiber paths in a three-dimensional braided l-beam. This process yields a composite l-beam which is

strong in all directions, but which weighs much less than steel.

Construction.—Construction applications are

potentially a very significant opportunity for composites. For example, most highway bridges in the

United States are over 35 years old, and most railroad bridges are over 70 years old.34 Replacing

or refurbishing even a small fraction of these with

composite materials would involve a substantial

volume of fiber and resin. However, significant

technical, economic, and institutional barriers exist to the implementation of this technology, such

that construction opportunities should be viewed

as long term.

j*John Sca]zi, National Science Foundation, persona] communication, August 1986.

A potentially high-volume market for composites is in construction of buildings, bridges, and

housing. Additional applications include lamp

posts, smokestacks, and highway culverts. Construction equipment, including cranes, booms,

and outdoor drive systems, could also benefit

from use of composites. Because of the many inexpensive alternative building materials currently

being used, cost of the PMC materials will be the

key to their use in this sector. The chief advantage of composites would be reduced overall systems costs for erecting the structure, including

consolidation of fabrication operations, reduced

transportation and construction costs due to
57

—

lighter weight structures, and reduced mainte -

nance and lifetime costs due to improved corrosion resistance. 3 5

Bridges are likely to be the first large scale construction application for polymer composites. The

U.S. Department of Transportation is currently

evaluating composites for use in bridge decking

and stay cables.36 Fiberglass tendons are also being used in place of steel in prestressed concrete

bridge structures. ” Other countries which have

active programs in this area include the Peoples’

Republic of China, England, West Germany, Israel, and Switzerland. Because the largest load

which must be supported by the bridge is its own

dead weight, use of light weight composites would

allow the bridge to accommodate increased traffic

or heavier trucks. Decking materials are likely to

be relatively inexpensive vinylester or epoxy resins reinforced with continuous glass fibers. Cables would probably be reinforced with graphite

or aramid fibers, because of the high modulus and

low creep requirements.

The manufactured housing industry is an especially intriguing potential opportunity for composite materials. In 1984, almost half of all new

housing units were partially manufactured; i.e.,

large components were built in factories, rather

than assembled onsite.38 In the future, factory

manufacture of housing promises to reduce housing costs while still maintaining options for distinctive designs. Composite manufacturing techniques such as pultrusion and transfer molding

could be used to fabricate wall structures containing structural members and panels in a single step,

Components such as I-beams, angles, and channels can also be economically produced by these

techniques. In spite of the opportunities, however,

Japan and several countries in Europe are far

ahead of the United States in composite housing

construction technologies.

An important research need affecting the use

of composites in construction applications has to

“Howard Smallowitz, “Reshaping the Future of Plastic Buildings, ”

Civil Engineering, May 1985, pp. 38-41.

‘

eAccording to information provided by Craig A. Ballinger, Federal Highway Administration, August 1986.

“Engineering News Record, Aug. 29, 1985, p. 11.

“Thomas E. Nutt-Powell, “The House That Machines Built, ”

Technology Review, Nov. 1985, p. 31.

Photo credit Cofnpos/te Technology, /nc

Interior of a nonmetallic, nonmagnetic structure built

for Apple Computer, Inc., to house a microwave device

testing facility. The building components, including

fasteners, are entirely made of fiber-reinforced

composites.

do with adhesion and joining. The joining of composite materials to other materials for the purpose

of load transfer, or to themselves for the purpose

of manufacturing components, requires advances

in technology beyond present levels. This is a particular obstacle when joining may be done by unskilled labor.

An additional technical barrier is need for development of design techniques for integrated,

multifunctional structures. Window frames, which

are made by joining together several pieces of

wood, can be replaced by molded plastic and

composite structures having many fewer pieces,

lower assembly costs, and better service performance. The flexibility of the design and manufacture

of composite materials could also be used to integrate a window frame into a larger wall section,

again reducing the number of parts and the cost

of manufacture, This has been done for certain

experimental bathroom structures where 1avato-
5 8

ries, shower rooms, and other structural components have been integrated in a single molding.39

The principal barriers to the adoption of new

materials technologies in the construction industry in the United States are not so much technological as institutional and economic. Like the

highway construction industry discussed above,

the housing construction industry is highly fragmented. This makes the rate of R&D investment

and adoption of new technology very low. The

performance of housing materials is regulated by

thousands of different State and local building and

fire safety codes, all written with conventional

materials in mind. Further, engineers and contractors lack familiarity with the composite materials and processes. Finally, composites must compete with a variety of low-cost housing materials

in current use. As a result, composites used in

manufactured housing are not likely to be “advanced”; rather, they might consist of wood fibers

pressed with inexpensive resins or laminated structures involving FRP skin panels glued to a foam

or honeycomb core.

Medical Devices.—PMC materials are currently

being developed for medical prostheses and implants. The impact of composites on orthopedic

devices is expected to be especially significant.

While medical devices are not likely to provide

a large volume market for composites, their social and economic value are likely to be high. The

total estimated world market for orthopedic devices such as hips, knees, bone plates, and intramedullary nails is currently about 6 million

units with a total value of just over $500 million.40

Estimates of the U.S. market for all biocompatible materials by the year 2000 have been quoted

above as up to $3 billion per year.41 Polymer composites could capture a substantial portion of that,

sharing the market with ceramics and metals.

Metallic implant devices of this type, such as

the total hip unit which has been used since the

early 1960s, suffer a variety of disadvantages: difficulty in fixation, allergic reactions to various

metal ions, poor matching of elastic stiffness, and

jqKenneth L. Reifsnider, Materials Response Group, Virginia polytechnic Institute, “Engineering Research Needs of Polymer Composites, ” a contractor report prepared for the Office of Technology

Assessment, December 1985.

‘“Ibid. tlHench and Wilson, oP. cit.

mechanical (fatigue) failure. Composite materials have the potential to overcome many of these

difficulties. Not only can the problem of metal

ion release be eliminated, but composite materials can be fabricated with stiffness which is tailored to the stiffness of the bone to which they

are attached, so that the bone continues to bear

load, and does not resorb (degenerate) due to absence of mechanical loading. This is a persistent

problem with metal implants. It is also possible

to create implants from biodegradable composite systems which would provide initial stability

to a fracture but would gradually resorb over time

as the natural tissue repairs itself. Finally, composites can be designed to serve as a scaffold for the

invasive growth of bone tissue as an alternative

to cement fixation. This leads to a stronger and

more durable joint.

Research in composite orthopedic devices is currently being carried out on a relatively small scale

in the labs of orthopedic device manufacturers.

Further research is required to improve in situ

strength and service life, stress analysis, and fabrication and quality control technologies.

In order to overcome the remaining technical

barriers, a cooperative effort of an interdisciplinary team is required. At a minimum, the team

must include expertise in design, engineering,

manufacturing, and orthopedic surgery. Significant strides in this field are being made in Japan,

the United Kingdom, France, Germany, Italy,

Canada, and Australia, as well as in the United

States .42

Future Trends in Polymer

Matrix Composites

Novel Reinforcement Types

Rigid Rod Molecules. —Composites can be reinforced with individual rigid rod-like molecules or

fibers generated from these molecules. One example is poly (phenylbenzobisthiazole), or PBT. Experimental fibers made from this material have

specific strength and modulus on a par with the

most advanced fibrous reinforcement, exceeding

the properties of commercially available metals,

tzReif5nider, Op. Cit” ~ December 1985.
5 9

including titanium, by over a factor of 10. 43 A

particularly exciting possibility is dissolving the

molecular rods in a flexible polymer, and thus creating a composite reinforced by individual molecules. Such a homogeneous composition would

mitigate the problem of matching the thermal expansion coefficient between the reinforcement and

the matrix, and would virtually eliminate the

troublesome interface between them. The future

of this technology will depend on solving the

problems of effectively dissolving the rods in the

“matrix” and on orienting them once dissolved.

Novel Matrices

Because the matrix largely determines the environmental durability and toughness, the greatest

improvements in the performance of polymer

composites in the future will come from new matrices, rather than new fibers. Perhaps the most

significant opportunities lie in the area of molecular design; chemists will be able to design polymer molecules to have the desired flexibility,

strength, high-temperature resistance, and adhesive properties .44 Some of the more promising

directions are discussed below.

Oriented Molecular Structures. -At present the

anisotropic properties of most composites are determined by the directions of fiber orientation. In

the future, it may be possible to orient the individual polymer molecules during or after polymerization to produce a “self-reinforced” structure. The oriented polymers will serve the same

reinforcing function as fibers do in today’s composites. In effect, today’s organic fibers (e.g., Du

Pent’s Kevlar or Allied’s Spectra 900), which consist of oriented polymers and which have among

the highest specific stiffness and strength of all

fibers, provide a glimpse of the properties which

tomorrow’s matrices might have.

Recently developed examples of oriented polymer structures are the liquid crystal polymers

(LCPs). They consist of rigid aromatic chains

modified by thermoplastic polyesters (e.g., poly -

ethyleneterephthalate, or PET), or polyaramids,

tJThaddeus Helminiak, “Hi-Tech Polymers From Ordered

Molecules, ” Chemical Week, Apr. 11, 1984.

~tcharl= p, Wet, R~in Research Laboratories, Inc., Newark,

NJ, personal communication, August 1986.

and have a self-reinforcing fibrous character which

imparts strength and modulus comparable to

those of reinforced thermoplastic molding compounds, such as 30 percent glass reinforced nylons.” The fiber orientations of current LCPS are

hard to control (current applications include

microwave cookware and ovenware, which require high-temperature resistance but not high

strength) and this represents a challenge for the

future.

High-Temperature Matrices.—The maximum

continuous service temperature of organic polymers in an oxidizing atmosphere is probably

around 7000 F,

46 although brief exposures to

higher temperatures can be tolerated. Currently,

the most “refractory” matrices are polyamides,

which can be used at temperatures of 600° F

(316° C), although slow degradation occurs.” If

stable, high-temperature matrices could be developed, they would find application in a variety of engine components and advanced aircraft

structures.

Thermotropic Thermosets. -These hybrid matrices are designed to exploit the processing advantages of thermoplastics and the dimensional

stability and corrosion resistance of thermoses.

The molecules are long, discrete chains which

have the latent capacity to form crosslinks. Processing is identical to thermoplastics in that the discrete polymers are formed at high temperatures

to the desired shape. Then, however, instead of

cooling to produce a solid, the structure is given

an extra “kick,” with additional heat or ultraviolet light, which initiates crosslinking between the

polymer chains. Thus, the finished structure has

the dimensional stability characteristic of a thermoset .48

isRegina]d B, Stoops, R.B. Stoops & Associates, Newport, RI,

“Ultimate Properties of Polymer Matrix Materials, ” a contractor report prepared for the Office of Technology Assessment, December

1985.

qbpaul McMahan, Ce]anese Research Corp., in an OTA workshop, “Future Opportunities for the Use of Composite Materials, ”

Dec. 10, 1985.

d7Aerospace America, May 1986, p. 22.

4aJohn Riggs, Celanese Research Corp. in OTA workshop cited

in footnote 46.
60

The Composites Factory of the Future

It is in the nature of advanced composites that

they do not lend themselves to standard processes

or manufacturing procedures. The very concept

of tailoring a material to each specific application

militates against standardization. It is therefore

unlikely that we will see completely automated

factories in the near future in which manufacturing instructions are fed in at one end and a composite part pops out the other, untouched by human hand.

Computer-Aided Design.—Computer-aided

design (CAD) systems currently focus on threedimensional graphics, and are often coupled with

the capability for stress analysis of a structure.

However, a comprehensive CAD system which

would facilitate the process of choosing suitable

materials, reinforcement geometry, and method

of fabrication is still far in the future. Such a system would require an extensive database on fiber and resin properties, as well as a processing

database which would permit modeling of the

manufacturing steps necessary to fabricate the

part, including the costs of those operations. The

principal advantage of an integrated CAD system

would be to clearly define the options and tradeoffs associated with various production strategies.

Computer-Aided Manufacturing.—Computeraided manufacturing (CAM) would alter the processing of composites in several ways. Based on

the instructions generated by the CAD system,

numerically controlled machine tools would automatically machine molds contoured to the precise outline of the part. CAD instructions would

guide automated lay-up of the part, and sensors

embedded in the molds or in the materials themselves would make the entire manufacturing process an interactive one in which the computer controller would automatically adjust the processing

variables based on real-time information about

the state of the structure being fabricated. With

the advent of robot-assisted manufacturing, NDE

during the production cycle, performed with

prcbes manipulated by the robot and analyzed

by artificial intelligence, could become the way

all high-value composite components are produced.

Integration and miniaturization of sensors will

lead to improvements in the quality and reliability of the information obtained from a nondestructive test.

Nondestructive Evaluation.—Experience to

date gives no indication that composite structures

in service will require more frequent inspection

than metal structures. In the future, it will be

judged inconvenient to take composite systems

out of service merely to inspect them for structural integrity. For such cases it will be necessary

to have NDE performed continuously by devices

that can sense changes in the mechanical properties which would precede a structural failure.

These sensors would be included in the critical

components during manufacture, and their signals would be examined regularly by a computer

which would integrate the data, using AI, and decide whether to alert a human operator. A candidate NDE test for such a system is acoustic emission, in which the sound emitted by a composite

structure under stress can be used to characterize

the flaw population. Another possibility would

be to include optically or electrically conducting

fibers with the structural reinforcement; the degree of transmission of light or electric current by

these fibers could be related to fiber breakage in

the overall structure.

Thermal Forming.—With the increased use of

advanced thermoplastic composites in the future,

new thermal forming technologies will become important. The thermoplastic could be applied either as a hot melt or as a prepreg, which is relatively fast and easy to make with thermoplastics.

Either method will require the development of

new equipment capable of placing the fibers and

compacting the structure while maintaining the

process temperature.

Space

Space Transportation Systems.—over 10,000

pounds of advanced composites are used on the

NASA space shuttle. ” Polymer composites are

also being considered in designs for the proposed

aerospace plane, although such an aircraft probably will not be available until after the year 2000.

The primary limitation on the use of polymer

composites in this application is high tempera-

‘* Tenney, op. cit. , Dec. 5, 1985.
61

— —

to the sun. This thermal cycling produces radial

cracks in graphite/epoxy tubes which can reduce

the torsional stiffness by as much as 30 percent

after only 500 cycles. 52 To reduce the effects of

thermal cycling, the composite tubes would be

coated with a reflecting, thermally conducting

layer to equalize the temperature throughout the

tube. The layer would also protect the composite from atomic oxygen, which is a major cause

of material degradation in low earth orbit, as well

as from solar ultraviolet radiation.

Military .—Composites of all types, including

ceramic, polymer, and metal matrix composites,

are ideal materials for use in space-based military

systems, such as those associated with the Strategic Defense Initiative (SDI). ” Properties such

as low density, high specific stiffness, low coefficient of thermal expansion, and high temperature

resistance are all necessary for structures which

must maneuver rapidly in space, maintain high

dimensional stability, and withstand hostile attack. A program devoted to the development of

new materials and structures has recently been

established within the SDIO. 54

Reference configuration for the NASA manned space Bioproduction station. Composites could be used in the tubular struts

which make up the frame. Living cells can synthesize polymeric molecules

with long chains and complex chemistries which

ture. ’” Flying at speeds exceeding Mach 7, the cannot be economically reproduced in the laboralower surfaces and leading edges would experi- tory. For example, crops and forests are an imence temperatures of 2,0000 to 3,000 F (1,093 portant source of structural materials and chernto 1,6490 C) .51 If PMC materials were available ical feedstocks. A number of crops such as

which could retain high strength and stiffness up crambe, rapeseed, and various hardwood trees are

to 800° F (427° C), they could be used extensively now being evaluated for commercial production

for the cooler skin structure and most of the sub- of lubricants, engineering nylons, and composstructure. ites. 55 For some materials, such as natural rubSpace Station.—Graphite /epoxy composites

and aluminum are both being considered for the

tubular struts in the space station reference design. The goal of reducing launch weight favors

the use of composites; however, their lower thermal conductivity compared with aluminum could

create problems in service. The most serious environmental problem faced by the composite is temperature swings between –250’ F ( –121” C) and

+200” F ( +93 o C) caused by periodic exposure

ber, the United States is totally dependent on foreign sources. Congress has mandated (“Critical

Agricultural Materials Act, ” Public Law 98-284,

1984) that the Department of Agriculture estab-

—.—

‘

2 Tenney, op cit., Dec. 5, 1Q85

1‘Jerome Persh, ‘ \later]als and Structure,\ S[ ](,nc ( ,ln~j T(’c hn~~logy Requirement\ k(lr the DOD St ratcg]c I )ei en~c’ 1 n I t la t 1 i(’ .’l J?lt’l -

ican Ceramic Societ~’ Bullet~r7 04 t 4 }, 555-55Q 1 Q85

‘dThe effort Inclucie\: IIghtwe]ght \tructure\, thermal and CIL,C tr)

ca 1 ma terlals, opt ] ca 1 rnater]a 1> d nd proc e~se~, t rlhologlca” I ma t(, r 1-

a]s, and materials durab] ] ] tv. C u rrent budxets are about S2 ml I I I on
62

lish an Office of Critical Materials to evaluate the

potential of industrial crops to replace key imported materials. Several demonstration projects

of 2,000 acres or more are planned for 1987.

With the possible exception of wood, it is unlikely that biologically produced materials will

compete seriously with polymer matrix composites in the structural applications discussed in this

report. Although natural polymers such as cellulose, collagen, or silk can have remarkably high

strength, their low stiffness is likely to limit their

use in many structures. Nevertheless, their unique

chemical and physical properties make them appropriate for certain specialty applications. For

example, collagen is a biologically compatible material which is being used to generate artificial

skin. 56

Biotechnology may offer a novel approach to

the synthesis of biological polymers in the future.

Genetically engineered bacteria and cells have

been used to produce proteins related to silk.57

In the future, production rates could be accelerated by extracting the protein synthetic machinery from the cells and driving the process with

an external energy source, such as a laser or electric current.58 The flexibility inherent in such a

scheme would be enormous; by simply altering

the genetic instructions, new polymers could be

produced.

Research and Development Priorities

for Polymer Matrix Composites

Federal R&D spending for polymer matrix composites in fiscal years 1985 and 1986 is shown in

table 14; roughly 65 to 70 percent in each year

was spent by the Department of Defense. The

large drop in Federal expenditures from 1985 to

1986 does not reflect a sharp cut in composites

R&D; rather, it can be attributed to the completion of large research programs in 1985 and the

transitioning of the technology (particularly for

“J. Burke, et al., “The Successful Use of Physiologically Acceptable Artificial Skin in the Treatment of Extensive Bum In jury,” Annals of Surgery 194:413-428, 1982.

57 Dennis ~ng, Syntro Corp., La Jolla, CA, pmorkd communication, August 1986.

SsTerence Barrett, FJationa] Aeronautics and Space Administration, persona] communication, August 1986.

Table 14.—Budgets for Polymer Matrix Composite

R&D in Fiscal Years 1985 and 1986 (millions)

Agency FY 1985 FY 1986

Department of Defensea (6.1, 6.2, 6.3A), 55.9 29.2

National Aeronautics Space Agencyb 23 8.7

National Science Foundatio n

c 

., ... ., 1-2 1-2

National Bureau of Standardsd . . ., ., 0.4 0.4

Department of Transportatione ., ., — 0.4

Total . . ., ., 80-81 40-41

SOURCES aJerome Persh, DOO

bMlchael Greenfleld, NASA

cRanga Komandurl, NSF

‘Leslie SmKh, NBS

‘Cratg Ballmger, FHWA

carbon fiber composites) to private industry. Defense applications continue to drive the development of advanced composites, which are used in

an estimated $80 billion of weapons systems .59

Reliable composite structures can now be designed which satisfy all of the engineering requirements of a given application. However, scientific

understanding of composites has lagged behind

engineering practice; in order to design more efficiently and cost effectively, and to develop improved materials, it will be necessary to understand and model several important aspects of

composites. Based on the opportunities outlined

above, some research and development priorities

for composites are suggested below.

Very Important

Processing Science. —The primary goal of processing science is to be able to control the fabrication process to assure complete and uniform cure,

minimize thermal stresses, control resin content,

and assure accurate fiber placement. This requires

a model which can predict the influences of key

process variables and techniques for monitoring

these variables so that pressure and temperature

can be adjusted accordingly. Such models would

also provide useful guidelines for tool design. At

present, modeling is in a very early stage of development.

The existence of low-cost fabrication processes

will be critical to the use of new composite systems, such as low-cost, high-performance thermoplastics reinforced with continuous fibers. For

SeKenneth Foster, Assistant for Materials po]icy, Department of

Defense, personal communication, August 1986.
6 3

example, methods for fabricating shapes with

double curvature are needed. Another important

problem is the impregnation and wetting of fiber

bundles by these relatively viscous plastics. The

effect of processing on microstructure requires further study, as does the influence of residual thermal stresses. The latter is a particular concern for

resins processed at high temperatures. Finally, as

for thermoses, process models are required.

Impact Damage.—The impact damage resistance of a composite structure has a critical effect on its reliability in service. It has been shown

that impact damage that is barely visible to the

naked eye can cause a reduction in strength of as

60 Impact resistance is esp e

-

much as 40 percent.

cially important in primary aircraft structures and

other safety-critical components. Tougher thermoplastic matrix materials promise to improve

the impact resistance of aircraft structures now

made with epoxy matrices.

The complexity of the impact damage process

makes modeling very difficult. However, it would

be very desirable to be able to relate the extent

of damage to the properties of the matrix, fiber,

and interphase, along with factors like reinforcement form. This would facilitate the development

of more reliable materials and structures. In addition, an understanding of impact damage mechanisms would aid in developing protocols for repair of composite structures, a field which largely

relies on empirical methods.

Delamination. -There is a strong body of opinion that delamination is the most critical form of

damage in composite structures (particularly those

produced from prepregs). As noted above, impact

damage barely visible on the surface can cause

dramatic reductions in strength through local

delamination. Delamination may prove to be a

problem of increasing severity in the future, because of the trend toward higher working strains

that tend to accentuate this mode of failure.

Analyses to date have concentrated on growth

and failure associated with compressive loading.

These need to be verified and refined. In addition,

the effects of combined loading, resin fracture

““Carl Zweben, General Electric Co., “Assessment of the Science

Base for Composite Materials, ” a contractor report prepared for the

Otfice of Technology Assessment, December 1985.

toughness, reinforcement form, and environment

need to be investigated.

Interphase.—The interphase has a critical influence on the composite, since it determines how

the reinforcement properties are translated into

the composite properties. The characteristics of

this little-studied region merit thorough investigation. The objective would be to develop a body

of knowledge that would guide the development

of fiber surface treatments, matrices, and fiber

coatings that will optimize composite mechanical properties and provide resistance to environmental degradation.

Important

Strength.—The excellent strength properties of

composites are one of the major reasons for their

use. However, as with many properties of composites, their strong heterogeneity makes strength

characteristics very complex. This heterogeneity

gives rise to failure modes that frequently have

no counterpart in homogeneous materials. Even

in the simplest composites, unidirectional laminates, the relationships between axial and transverse loading (parallel and perpendicular to the

fiber direction) and failure are not well understood. In more complex composites, containing

several fiber orientations and various flaw populations, efforts to model strength have been

largely empirical. It will be important in the future to have analytical models for the various failure modes of unidirectional composites and

laminates that relate strength properties to basic

constituent properties.

Fatigue.—Fatigue of composites is an important design consideration. Fatigue resistance is a

major advantage which composites enjoy over

metals; however, the traditional models for analyzing fatigue in metals do not apply to composites. The risks associated with fatigue failure are

likely to increase because of the trend toward use

of fibers with higher failure strains, and the desire to use higher design allowable for existing

materials. Ideally, it would be desirable to be able

to predict composite fatigue behavior based on

constituent properties. A more realistic near-term

objective is to understand fatigue mechanisms,

and how they are related to the properties of

fibers, matrix, interphase, loading, and environ-
64

ment of the composite. Important topics which

have not received adequate attention are the fatigue properties of the reinforcements and matrix

resins, and compression fatigue of unidirectional

composites. In view of the increasing interest in

thermoplastics, fatigue of these materials also

deserves study.

Fracture.—One of the most important modes

of failure in metals is crack propagation. Arising

at regions of high stress, such as holes, defects,

or other discontinuities, cracks tend to grow under cyclic tensile load. When cracks reach a critical size, they propagate in an unstable manner,

causing failure of the part in which they are located. This, in turn, may result in failure of the

entire structure. In contrast, failure of composites

often results from gradual weakening caused by

the accumulation of dispersed damages, rather

than by propagation of a single crack.

In view of the significant differences in failure

modes between metals and composites, use of linear elastic fracture mechanics (LEFM) to describe

fracture in these complex materials is controversial. It is open to question whether there are

unique values of fracture toughness or critical

stress intensity that describe the fracture characteristics of composites. In order to develop reliable design methods and improved materials in the

future, it will be necessary to develop a body of

fracture analysis which is capable of accounting

for the more complex failure mechanisms.

Environmental Effects.—The environments to

which composites are subjected can have a significant effect on their properties. Environments

that are known to be especially damaging are

those of high temperature and moisture under

load, ultraviolet radiation, and some corrosive

chemicals. The key need in the environmental area

is to develop a thorough understanding of degradation mechanisms for fibers, resins, and interphases in the environments of greatest concern.

This knowledge will lead to more reliable use of

existing materials and provide the information required to develop new, more degradation-resistant

ones.

Reinforcement Forms and Hybrid Composites.

—There are two main reasons for the interest in

new reinforcement forms: improved through-thethickness properties, and lower cost. A pervasive

weakness of composite laminates of all kinds is

that the out-of-plane strength and stiffness, being dependent primarily on the matrix, are much

inferior to the in-plane properties. This is because

in convertional laminates there is no fiber reinforcement in the thickness direction.

New reinforcement forms under development

include triaxial fabrics, multilayer fabrics, twodimensional braids, three-dimensional braids, and

various kinds of knits. In addition, laminates have

been reinforced in the thickness direction by

stitching. From an analytical standpoint, the major drawback to fabrics, braids, and knits is that

they introduce fiber curvature, which can cause

significant loss of strength, compared to a unidirectional laminate. However, multidirectional

reinforcement appears to confer increased fracture toughness on the composite.

Use of several types of fibers to reinforce composites will be driven by the desire to obtain properties that cannot be achieved with a single fiber,

and to reduce cost. For example, glass fibers,

which are cheap but have a relatively low tensile

modulus, can be mixed with more costly, high

modulus graphite fibers to achieve a composite

which is both stiff and relatively cheap.

With both new reinforcement forms and hybrid

reinforcement, there is a great need for analytical methods to identify the configurations required

to produce desired properties. Without such tools,

it will be necessary to rely on intuition and timeconsuming, costly empirical approaches.

Test Methods.—Reliable and standardized test

methods are critical to the evaluation of all materials. For homogeneous materials such as metals, such methods are fairly straightforward. In

composites, however, the macroscopic mechanical behavior is a complex summation of the behavior of the microconstituents. Consequently,

there has been great difficulty in achieving a consensus on what properties are actually being measured in a given test, let alone what test is most

appropriate for a given property. There are currently numerous test methods in use throughout

the industry, and numerous private databases.

This has resulted in considerable property variability appearing in papers and reports. The prob-
6 5

lem is particularly severe for toughness, bending,

shear, and compression properties.

The movement to standardize composites testing began in 1969, but has stalled in recent years. 61

Standardized methods of testing and reporting results are a prerequisite to the establishment of

composite properties databases for design and

process modeling. The lack of standards is also

a significant barrier to the transfer of test results

from one structural application to another.

Desirable

Creep Fracture.—Materials subjected to sustained loading fail at stress levels lower than their

static strengths. This is called creep fracture.

Topics which require study include tensile and

“Kenneth L. Reifsnider, Virginia Polytechnic Institute, personal

communication, August 1986.

compressive loading of both unidirectional composites and laminates, and the influence of temperature and environment. Since the time-dependent degradation of the matrix and interphase

properties are typically greater than those of the

fibrous reinforcements, particular attention should

be paid to transverse matrix cracking and delamination.

Viscoelastic and Creep Properties.—The occurrence of significant deformation resulting from

sustained loading can have an adverse effect on

structural performance in some applications, such

as reciprocating equipment, bridges, and buildings. Consequently, creep behavior is an important material characteristic. This subject could

benefit from development of a database for creep

properties of various fibers, matrices, and composites, especially for compressive loading, which

has received relatively little attention.
GENERAL PREREQUISITES OF THE USE

OF ADVANCED STRUCTURAL MATERIALS

Education and Training

The expanding opportunities for ceramics and

composites will require more scientists and engineers with broad backgrounds in these fields. At

present, only a handful of universities offer comprehensive curricula in ceramic or composite materials. There is also a shortage of properly trained

faculty members to teach the courses. The job

market for graduates with advanced degrees in

ceramic or composite engineering is good, and can

be expected to expand in the future. Stronger relationships between industry and university laboratories are providing greater educational and job

opportunities for students.

There is a great need for continuing education

and training opportunities for designers and engineers in industry who are unfamiliar with the new

materials. In the field of polymer composites, for

example, most of the design expertise is concentrated in the aerospace industry. Continuing education is especially important in “unsophisticated”

industries such as construction, which must purchase, rather than produce, the products they use.

Universities and professional societies are offering seminars and short courses to fill this gap;

these opportunities should be publicized and made

more widely available.

Beyond the training of professionals, there is

a need for the creation of awareness of new materials technologies among technical editors, managers, planners, vice presidents, and the general public. In recent years, the number of newspaper and

magazine articles about the remarkable properties of ceramics and composites has increased, as

has the number of technical journals associated

with these materials. The success of composite

sports equipment, including skis and tennis

rackets, shows that new materials can have a

“high-tech” appeal to the public, even if they are

relatively expensive.

Multidisciplinary Approach

Progress in materials development often requires

a team effort. For example, for a typical ceramic

component, the team might consist of one or more

specialists from each of the disciplines in table 15.

Materials research lends itself naturally to collaborative institutional arrangements in which the

rigid disciplinary boundaries between different

fields are relaxed. The successes of numerous

multidisciplinary materials centers based at universities and national laboratories across the country reflect this fact. A consensus is developing

within the materials community that the exploration of new mechanisms for collaborative work

between university, industry, and government

laboratory scientists and engineers would have a

salutary effect on the pace of advanced materials

development and utilization. This topic will be

examined in greater detail in the final report.

Table 15.—Hypothetical Multidisciplinary Design

Team for a Ceramic Component

Specialist Contribution

Systems engineer . . . . . . . Defines performance

Designer . . . . . . . . . . . . . . . Develops structural concepts

Stress analyst . . . . . . . . . . Determines stress for local

environments and difficult

shapes

Metallurgist . . . . . . . . . . . . Correlates design with metallic

properties and environments

Ceramist . . . . . . . . . . . . . . . Identifies proper composition,

reactions, and behavior for

design

Characterization analyst. Utilizes electron microscopy,

X-ray, fracture analysis, etc.

to characterize material

Ceramic manufacturer . . . Defines production feasibility

SOURCE J.J Mecholsky, “Engineering Research Needs of Advanced Ceramics

and Ceramic Matrix Composites, ” contractor report for OTA, December 1985.

6 6
6 7

Integrated Design

Advanced ceramics and composites should

really be considered structures rather than materials. Viewed in this light, the importance of a design process which is capable of producing highly

integrated and multifunctional structures becomes

clear. Polymer matrix composites provide a good

example. Perhaps the greatest single economic

advantage of composites, beyond their superior

performance, is the potential for reduction in the

cost of manufacture achieved by reducing the

number of parts and operations required in fabrication. For example, a typical automobile has

about 1500 structural parts. It has been estimated

that, with an integrated composite design, this

total could be reduced to a few hundred parts,

with major savings in tooling and manufacturing

costs. 62 Because composites can be tailored in so

many ways to the various requirements of a particular engineering component, the key to op -

timizing cost and performance is a fully integrated

design process which is capable of balancing all

of the relevant design and manufacturing variables. Such a design process will require an extensive database on matrix and fiber properties, as

well as sophisticated software capable of modeling fabrication processes and three-dimensional

analysis of the properties and behavior of the resulting structure. Development of expert systems

software for the design of both materials and

structures will also be important, particularly for

engineers who are new to the field. Perhaps the

most important element in the development of integrated design algorithms will be an understanding of the relationships between the constituent

properties, microstructure, and the macroscopic

properties of the structure. Many of the research

and development priorities listed above are intended to provide this information.

The view of ceramics and composites as structures also sheds light on the often-stated need for

a “database” for design purposes. The discussion

above suggests that the “customized” nature of

advanced structural ceramics and composites militates against the very idea of standardized structures with specified properties which could be cataloged in a database. However, depending on the

blReifsnider, op. cit., December 1985.

application, a two-tracked approach to standardization and data-collection may be the best overall solution. For design of integrated, multifunctional structures, the best approach may be a

database containing the properties of constituents

and unidirectional composites, coupled with

standardized algorithms for modeling and analysis. Destructive testing of the finished structure

would provide data to validate the models. For

applications in which standard ceramic or composite components are desired, such as brackets,

elbows, or beams, the properties and composition of the structures would be specified and

standardized,

Systems Approach to Costs

It is often stated that the three biggest barriers

to the increased use of advanced materials are

cost, cost, and cost. In a narrow sense, this observation is correct. If advanced materials are considered on a dollar-per-pound basis as replacements for steel or aluminum in existing designs,

they cannot compete. This is very often the perspective of potential user industries, who are oriented toward metals processing, and has been the

source of great frustration among ceramics and

composites suppliers. However, the per-pound

cost and part-for-part replacement are rarely valid

bases for comparison. This is the reason that the

costs of ceramics and composites have not been

stressed in this report. A more fruitful approach

is an analysis of the overall systems costs of a shift

to advanced materials, including integrated design, fabrication, installation, and lifecycle costs.

The high per-pound cost of advanced materials is largely a result of the immaturity of the fabrication technology and the low production

volumes. Large decreases in materials costs can

be expected as the technologies mature. For example, a pound of standard high strength carbon

fiber which used to cost $300 now costs less than

$20, and new processes based on synthesis from

petroleum pitch promise to reduce the cost even

further. 63 If high-strength carbon fibers in the $3

bgThe Wor]d’s foremost manufacturer of pitch-based carbon fiber, Kureha Chemical Industry Co. of Japan, has developed a new

high-strength fiber said to cost one-third as much as polyacrylonitrile (PAN) -based fibers. See IrorI Age, June 20, 1986, p. J6.
6 8

to $5 per pound range became available, major

new opportunities would open up for composites

in automotive, construction, and corrosionresistant applications.

Two other economic factors which can affect

the competitiveness of ceramics and composites

deserve mention: they are energy costs and labor

costs. The cost of the energy required to manufacture ceramic and composite components is a

negligible fraction of the overall production costs;

however, energy savings in service are a major

reason for the use of advanced materials in such

applications such as heat exchangers, engines, and

transportation structures. Persistent low energy

costs are likely to reduce the incentives to introduce advanced materials into these applications.

Throughout this report, the vision presented of

the materials factory of the future has been one

of extensive automation and computer-controlled

processing. The reliability and reproducibility requirements of ceramic and composite structures

demand a minimum of human intervention. These

considerations indicate that the ceramics or composites factory of the future will not be a labor

intensive operation. In fact, labor costs, which are

currently a large fraction of overall production

costs of advanced structures, are a key target for

future cost reduction. Thus, rising labor costs

could accelerate the utilization of ceramics and

composites.

Technical, economic, and institutional factors

will all influence the incorporation of ceramics and

composites technologies into commercial production. Many of these factors have been discussed

in this interim report. However, several key issues are mentioned without discussion. These include: the need for improved cooperation between

industry, universities, and government labs; the

dramatic changes which new materials will bring

to manufacturing; and government policy options

which could enhance the international competitiveness of United States advanced materials users

and suppliers. The final report will explore these

issues in detail.
GLOSSARY

adiabatic: Referring to any process in which there is

no gain or loss of heat.

aggregate: Inert filler material such as sand or gravel

used with a cementing medium to form concrete or

mortar.

alloy: A material having metallic properties and consisting of two or more elements.

anisotropic: Showing different physical or mechanical properties in different directions.

aramid: Lightweight polyaromatic amide fibers having excellent high temperature, flame, and electrical properties. These fibers are used as high-strength

reinforcement in composites.

brittle fracture: A break in a brittle material due to

the propagation of cracks originating at flaws.

carbon/graphite: These fibers, which are the dominant

reinforcement in “advanced” composites, are produced by pyrolysis of an organic precursor, e.g.

polyacrylonitrile (PAN), or petroleum pitch, in an

inert atmosphere. Depending on the process temperature, fibers having high strength or high elastic

modulus may be produced.

cement: A dry powder made from silica, alumina,

lime, iron oxide, and magnesia which forms a hardened paste when mixed with water; it may be used

in this form as a structural material, or used as a

binder with aggregate to form concrete.

ceramic: An inorganic, nonmetallic solid.

chemically bonded ceramics: Used here to distinguish

advanced cements and concretes, which are consolidated through chemical reactions at ambient temperatures (generally involving uptake of water) from

high-performance ceramics, such as silicon nitride

and silicon carbide, which are densified at high temperatures.

chromatographic adsorption: Preferential adsorption

of chemical compounds (gases or liquids) according to chemical affinity onto a solid adsorbent material.

composite: Any combination of particles, whiskers,

or fibers in a common matrix.

compressive stress: A stress that causes an elastic body

to shorten in the direction of the applied force.

concrete: A mixture of aggregate, water, and a binder

(usually portland cement) which hardens to stonelike condition when dry.

consolidation of parts: Integration of a number of formerly discrete parts into a single part which encompasses several functions; a key advantage of engineered materials such as ceramics and composites.

continuous fiber: A reinforcing fiber in a composite

which has a length comparable to the dimensions

of the structure.

creep: A time-dependent strain of a solid, caused by

stress.

cross-linking: The formation of chemical bonds between the formerly separate polymer chains.

crystal: A homogeneous solid in which the atoms or

molecules are arranged in a regularly repeating

pattern.

deflection: Deformation of a material produced without fracturing the material.

deformation, plastic deformation: Any alteration of

shape or dimensions of a body caused by stresses,

thermal expansion or contraction, chemical or metallurgical transformations, or shrinkage and expansions due to moisture change.

delamination: Separation of a layered structure into

its constituent layers.

dielectric: A material which is an electrical insulator

or in which an electric field can be sustained with

a minimum dissipation in power.

diffusion: The movement of mass, in the form of discrete atoms or molecules, through a medium.

dispersion: Finely divided particles of one material held

in suspension in another material.

ductility: The ability of a material to be plastically

deformed by elongation without fracture.

E-glass: A borosilicate glass most used for glass fibers

in reinforced plastics.

elasticity: The property whereby a solid material deforms under stress but recovers its original configuration when the stress is removed.

engineered materials: Materials whose physical and

mechanical properties and function are tailored for

a particular application.

extrusion: A process in which a hot or cold semisoft

solid material, such as metal or plastic, is forced

through the orifice of a die to produce a continuousl y formed piece in the shape of the desired

product.

failure: Collapse, breakage, or bending of a structure

or structural element such that it can no longer fulfill its purpose.

fatigue: Failure of a material by cracking resulting from

repeated or cyclic stress.

filtration: A process of separating particulate matter

from a fluid, by passing the fluid carrier through

a medium that will not pass the particulate.

finishing: Final processing operations on a part.

6 9
70

flexure: Any bending deformation of an elastic body

in which the points originally lying on any straight

line are displaced to form a plane curve,

fracture stress: The minimum stress that will cause

fracture, also known as fracture strength,

glass: A state of matter that is amorphous or disordered like a liquid in structure, hence capable of continuous composition variation and lacking a true

melting point, but softening gradually with increasing temperature.

glass-ceramic: Solid material, partly crystalline and

partly glassy, formed by the controlled crystallization of certain glasses.

grain: One of many crystallite comprising a poly -

crystalline material.

green state, greenware: A term for formed ceramic articles in the unfired condition.

hardness: Resistance of a material to indentation,

scratching, abrasion, or cutting.

heat treatment: Heating and cooling of a material to

obtain desired properties or conditions.

holography: A technique for recording and later reconstructing the amplitude and phase distributions of

a wave disturbance.

hot isostatic pressing: A form of ceramic or powder

metallurgical forming or compaction process in

which the mold is flexible and pressure is applied

hydrostatically or pneumatically from all sides,

hot pressing: Forming a metal powder compact or a

ceramic shape by applying pressure and heat simultaneously at temperatures high enough for sintering to occur.

impact strength: Ability of a material to resist shock

loading,

inclusion: A flaw in a material consisting of a trapped

impurity particle.

injection molding: Forming metal, plastic, or ceramic

shapes by injecting a measured quantity of the material into shaped molds.

internal stress, residual stress: A stress system within

a solid (e. g., thermal stresses resulting from rapid

cooling from a high temperature) that is not dependent on external forces.

interphase, interface: The boundary layer between the

matrix and a fiber, whisker, or particle in a composite.

lay-up: A process for fabricating composite structures

involving placement of sequential layers of matriximpregnated fibers on a mold surface.

load: The weight that is supported by a structure, or

mechanical force that is applied to a body.

matrix: The composite constituent that binds the reinforcement together and transmits loads between

reinforcing fibers.

metal: An opaque material with good electrical and

thermal conductivities, ductility, and reflectivity;

properties are related to the structure in which the

positively charged nuclei are bonded through a field

of mobile electrons which surrounds them, forming

a close-packed structure.

microstructure: The internal structure of a solid viewed

on a distance scale on the order of micrometers.

The microstructure is controlled by processing, and

determines the performance characteristics of the

structure.

modulus of elasticity: A parameter characterizing the

stiffness of a material, or its resistance to deformation under stress. For example, steel has a relatively

high modulus, while Jello has a low modulus.

monolithic: Constructed from a single type of material.

near-net-shape The original formation of a part to a

shape which is as close to the desired final shape as

possible, requiring as few finishing operations as

possible.

nondestructive testing, evaluation: Any testing method

which does not involve damaging or destroying the

test sample; includes use of X-rays, ultrasonics, magnetic flux, etc.

phase: A region of a material that is physically distinct and is homogeneous in chemical composition.

plasticity: The property of a solid body whereby it undergoes a permanent change in shape or size when

subjected to a stress exceeding a particular value,

called the yield value.

polymer: Substance made of giant molecules formed

by the union of simple molecules (monomers); for

example, polymerization of ethylene forms a polyethylene chain.

pore, porosity: Flaw involving unfilled space inside a

material which frequentl y limits the material

strength.

prepreg: Fiber reinforcement form (usually tape, fabric, or broadgoods) which has been preimpregnated

with a liquid thermosetting resin and cured to a viscous second stage. Thermoplastic prepregs are also

available.

proof test: A predetermined test load, greater than the

intended service load, to which a specimen is subjected before acceptance for use.

radiography: The technique of producing a photographic image of an opaque specimen by transmitting a beam of X-rays or gamma rays through it onto

an adjacent photographic film; the transmitted intensity reflects variations in thickness, density, and

chemical composition of the specimen.

radome: A strong, thin shell made from a dielectric

material, used to house a radar antenna,

reciprocating (engine or machinery): Having a motion

that repeats itself in a cyclic fashion.

refractory: Capable of enduring high-temperature conditions.

S-glass: A magnesia-alumina-silicate glass that pro-
7 1

vides very high tensile strength fiber reinforcement.

Often regarded as the reinforcement fiber dividing

“advanced” composites from reinforced plastics.

shearing stress: A stress in which the material on one

side of a surface pushes on the material on the other

side of the surface with a force which is parallel to

the surface.

sintering: Method for the consolidation and densification of metal or ceramic powders by heating without melting.

slip casting, slip, slurry: A forming process in the manufacture of shaped refractories, cermets, and other

materials in which slip is poured into porous plaster molds. Slip or slurry is a suspension of ceramic

particles in water with a creamy consistency.

strain: Change in length of an object in response to

an applied stress, divided by undistorted length.

stress: The force acting across a unit area in a solid

material in resisting the separation, compacting, or

sliding that tends to be induced by external forces.

structural materials or assembly: Those parts of a system that support most of the loading on the whole

system.

substrate: Base surface on which a material adheres,

for example a surface to be coated.

systems approach (to cost or to design): Consideration of product design, manufacture, testing, and

lifecycle as an indivisible whole; see comoliclation

of parts.

tensile strength, ultimate tensile strength: The maximum stress a material subjected to a stretching load

can withstand without breaking.

thermal conductivity: The rate of heat flow under

steady conditions through unit area per unit temperature in the direction perpendicular to the area—

the ability of a material to conduct heat.

thermoplastic resin: A material containing discrete

polymer molecules that will repeatedly soften when

heated and harden when cooled; for example, polyethylene, vinyls, nylons, and fluorocarbons.

thermosetting resin: A matrix material initially having low viscosity that hardens due to the formation

of chemical bonds between polymer chains. Once

cured, the material cannot be melted or remolded

without destroying its original characteristics; examples are epoxies, phenolics, and polyamides.

toughness: A parameter measuring the amount of

energy required to fracture a material in the presence of flaws.

tribology: The study of the phenomena and mechanisms of friction, lubrication, and wear of surfaces

in relative motion.

turbocharger: A centrifugal air compressor driven by

the flow of exhaust gases and used to increase induction system pressure in an internal combustion

reciprocating engine.

ultrasonic testing: A nondestructive test method that

employs high-frequency mechanical vibration energy to detect and locate structural discontinuities or

differences and to measure thickness of a variety of

materials.

unibody: Integrated structure containing the chassis

as well as elements of the body of an automobile.

viscoelasticity: Property of a material that is viscous

but which also exhibits certain elastic properties such

as the ability to store energy of deformation, and

in which the application of a stress gives rise to a

strain that approaches its equilibrium value slowly.

wear: Deterioration of a surface due to material removal caused by relative motion between it and

another material.

nettability: The ability of any solid surface to be wetted when in contact with a liquid.

whisker: A short, single crystal fiber with a lengthto-diameter ratio of 10 or more, often used to improve the fracture toughness of ceramics.

yield strength: The lowest stress at which a material

undergoes plastic deformation. Below this stress, the

material is elastic.