NISTIR 88-3817

Structural Reliability and

Damage Tolerance of

Ceramic Composites for

High-Temperature

Applications

Semi-Annual Progress Report for

the Period Ending March 31, 1988

E. R. Fuller, Jr., T.W. Coyle, T.R. Palamides and R.F. Krause, Jr.

U.S. DEPARTMENT OF COMMERCE

National Institute of Standards and Technology

(Formerly National Bureau of Standards

Ceramics Division

Gaithersburg, MD 20899

August 1988

Interagency Agreement DE-AI05-800R2061 79

Prepared for

U.S. DEPARTMENT OF ENERGY

Fossil Energy

Advanced Research & Technology Development

Fossil Energy Materials Program

Oak Ridge National Laboratory

Oak Ridge, TN

NISTIR 88-3817

Structural Reliability and

Damage Tolerance of

Ceramic Composites for

High-Temperature

Applications

Semi-Annual Progress Report for

the Period Ending March 31, 1988

E. R. Fuller, Jr., T.W. Coyle, T.R. Palamides and R.F. Krause, Jr.

U.S. DEPARTMENT OF COMMERCE

National Institute of Standards and Technology

(Formerly National Bureau of Standards

Ceramics Division

Gaithersburg, MD 20899

August 1988

Interagency Agreement DE-AI05-800R2061 79

National Bureau of Standards became the National Institute of Standards and Technology on August 23, 1988, when the

Omnibus Trade and Competitiveness Act was signed. NIST

retains all NBS functions. Its new programs will encourage

improved use of technology by U.S. industry.

Prepared for

U.S. DEPARTMENT OF ENERGY

Fossil Energy

Advanced Research & Technology Development

Fossil Energy Materials Program

Oak Ridge National Laboratory

Oak Ridge, TN

U.S. DEPARTMENT OF COMMERCE

C. William Verity, Secretary

NATIONAL INSTITUTE OF STANDARDS

AND TECHNOLOGY

Ernest Ambler, Director
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STRUCTURAL RELIABILITY AND DAMAGE TOLERANCE OF

CERAMIC COMPOSITES FOR HIGH -TEMPERATURE APPLICATIONS

Semi-Annual Progress Report for the Period Ending March 31, 1988 *

Prepared for US Department of Energy, Fossil Energy

Advanced Research & Technology Development

Fossil Energy Materials Program

Oak Ridge National Laboratory

Oak Ridge, TN

E. R. Fuller, Jr., T. W. Coyle,

T.R. Palamides and R. F. Krause, Jr.

National Bureau of Standards

Institute for Materials Science and Engineering

Ceramics Division

Gaithersburg, MD 20899

INTRODUCTION

The achievement of higher efficiency heat engines and heat recovery

systems requires the availability of high- temperature , high-performance

structural materials. Structural ceramics, and more recently, ceramic

matrix composites have received particular attention for these

applications due to their high strength, and resistance to corrosion and

thermal shock. Even with these positive attributes, improved reliability

and extended lifetime under service conditions are necessary for

structural ceramics to gain industrial acceptance. The problems with

these materials are mechanical and chemical in nature and are enhanced by

the fact that they are subjected to high temperatures, reactive

environments and extreme thermal gradients

.

With an objective of improved performance for heat engine/heat

recovery applications, the NBS program on structural ceramics and ceramic

composites addresses these problems through the determination of critical

factors that influence mechanical and microstructural behavior. The

activities of the program are grouped under two major subtasks, each

designed to develop key data, associated test methods and companion

predictive models. The status of the subtasks are detailed in the

following sections.

* Also in ORNL/FMP-88/1 , Fossil Energy Materials Program,

R.R. Judkins, Manager, Oak Ridge National Laboratory,

Oak Ridge, TN, July 1988, pp . 103-116.
DISCUSSION OF CURRENT ACTIVITIES

Subtask A: Interfacial and Micros true tural Composite Properties

Measurement of Fiber/Matrix Interfacial Forces

An indentation technique has been developed to characterize the

mechanical behavior of the fiber/matrix interface in ceramic-ceramic

composites under applied shear loads. A force is applied on the polished

cross section of a specimen to a fiber parallel to the fiber axis. This

results in a displacement of the fiber relative to the matrix surface.

To determine the resistance to shear displacement along the fiber/matrix

interface it is necessary to know the force applied to the fiber, the

relative displacement between fiber and matrix, and the area of interface

over which the displacement occurs. A Vickers hardness indenter instrumented to measure the load applied to the fiber and the displacement of

the tip of the diamond indenter has been developed over the past year and

is currently operational. The measured load-displacement curves can be

used in conjunction with an appropriate mechanics analyses to characterize the behavior of the interface.

Preliminary measurements have been made on Nicalon fiber, glass

ceramic (LAS III) composite to check out the system. These measurements

are still in progress. Future studies will examine specimens of a

Nicalon fiber-reinforced SiC composites provided by R.A. Lowden of Oak

Ridge National Laboratory. These materials are 2-D reinforced composites

produced at Oak Ridge National Laboratory by a chemical vapor infiltration (CVI) process. The goal of the testing is to relate the strength of

the interface to the various interfacial structures produced during the

CVI fabrication process.

Characterization of Matrix Crack - Fiber Interactions

Summary . The double cleavage drilled compression (DCDC) configuration has been used as a model composite system to study the influence of

multiple fiber arrays and fiber coating on the toughening increment from

fiber reinforcement. Heretofore, only cracks interacting with a single
fiber have been investigated. In this reporting period multiple linear

arrays of fibers, up to three (3), have been investigated. In addition,

studies have been initiated to examine the influence of fiber coating and

coating thickness on the toughening increment. Metallic coatings of

nickel from 1 ^tm to 22 /xm have been examined. Results of these studies

are reported below.

Multiple Fibers . The double cleavage drilled compression (DCDC)

configuration has been used to study the influence of multiple fiber

arrays on the toughening increment resulting from individual fibers. As

previously reported [1,2], the specimen configuration is a DCDC specimen

in the Janssen geometry [3]. Model composite systems have been simulated

by the addition of individual SiC fibers (AVCO's SCS-6 fibers) in the

borosilicate glass matrix.

The relationship between applied stress intensity factor and crack

length for the DCDC specimens has been experimentally calibrated.

Results agree reasonably well with previous analyses of this specimen [3-

6]. There is still, however, an unexplained shift in the calibration

curves, which result in an arbitrary shift in the resulting stress

intensity factor curves. Although we are still working to determine the

cause of this shift, its existence for the current experiments is

unimportant, since we can normalize the original toughness to that of the

glass matrix.

Specimens with one, two, and three fibers in a planar array along

the crack front have been fabricated. Typical toughening increments for

these cases are shown in Fig. 1. The one-fiber specimens had the fiber

situated in the center of the specimen, which was 6.35 mm wide. The twofiber array was arranged with the two fibers 3.18 mm apart and 1.59 mm

from the edges. The three-fiber array was arranged with the fibers 1.59

mm apart and from the edges. As can be seen from Fig. 1, the two- fiber

configuration resulted in a substantial toughening increment. However,

the three-fiber configuration was less than what might be expected (i.e.,

less than the two-fiber array). This observation most likely resulted

from the different processing conditions which were used for the two configurations. The 1.59 mm and the 3.18 mm glass slabs used to fabricate
these specimens, although nominally the same composition, appeared to

have different surface conditions, which required different hot pressing

conditions

.

Fiber Coatings . In addition to the studies with multiple fiber

arrays, studies have been initiated to examine the influence of fiber

coating and coating thickness on the toughening increment. Nickel has

been electrodeposited on the SiC fibers to a thickness of 1 /im to 22 jim.

Typical results of these studies are shown in Fig. 2. Preliminary

scanning electron microscopy has revealed microstructures which agree

qualitatively with these results; namely, the 1 /ira Ni coating appears to

have made a strong bond with the glass matrix, whereas the thicker

coating are not bonded as well.

Quantitative information regarding the closure forces exerted by

bridging fiber requires knowledge of the interfacial shear resistance of

the fiber-matrix interface. These interfacial properties can be

evaluated by measuring the force required to push the fibers through thin

sections of the glass matrix using the indentation technique discussed

above. Such measurements will be included in further efforts to quantify

the effects of the fibers on the toughening in these model composites.

Subtask B: High-Temperature Damage Modes and Failure Mechanisms

Creep and Creep Rupture Behavior

Summary . Creep and creep rupture behavior of a 25 wt percent

silicon carbide whisker-reinforced alumina ceramic with 4.9 percent

porosity was measured in four-point flexure at temperatures between 1100°

and 1300°C. The applied stresses for these studies varied between 55 and

306 MPa. However, the applied stresses at each temperature varied over a

much narrower range, typically by a factor of one and a half to two, with

the high applied stresses at the low temperature, and vice versa. The

composite materials were provided as billets through the courtesy of

Dr. J. F. Rhodes of the Advanced Composite Materials Corporation [7].

Results of these experiments are reported below.
High-Temperature Flexural Strength . The flexural strength of this

material was determined last reporting period as a function of temperature between 25 and 1300*C [2]. The strength remained approximately

invariant at approximately 600 MPa to 900 C; beyond this temperature, it

regressed to approximately half as strong at 1300 C.

Creep and Creep Rupture Behavior . The flexural creep specimens,

3 mm x 4 mm x 50 mm, were cut and ground the same as those used in the

fracture toughness study [1,2,8] except they did not receive a finishing

polish. They were mounted in place on silicon carbide platens with 6 mm

diam loading rollers using nitrocellulose cement. A constant load was

applied to the specimen through a pneumatic cylinder with a lubricated

rubber diaphragm. A controlled- leak , pressure regulator was capable of

maintaining the applied load within ± 2 N. The load was transmitted

through 19 mm diam rods of 99 percent recrystallized silicon carbide,

bearing on a hemi-spherical compression tool of the same material.

Three sets of loading rods protruded through the roof and floor of

an alumina- insulated furnace cavity, 150 mm x 200 mm x 150 mm high, to

accommodate two test specimens simultaneously. The third (central)

loading rod was used as a reference leg to correct for strains resulting

from expansion of the frame due to room temperature fluctuations. Four

such furnace assemblies were used in these tests. All tests at a given

temperature were conducted in the same furnace.

A pre-load of approximately 10 N maintained the specimen alignment,

while heating to the test temperature, during which time the nitrocellulose cement burned away. The furnace cavity was heated to the test

temperature in approximately one hour by molybdenum disilicide heating

elements, radiating from the side walls within the furnace cavity. The

temperature was controlled and measured by calibrated platinum- rhodium

thermocouples. The loading rods were water-cooled where they were

connected to air cylinders and load transducers, which were supported in

the same frame above and below the furnace, respectively. The load

transducers were calibrated with known weights. After the specimens

reached the test temperature, the load was increased to a value

corresponding to the pre-chosen applied stress, generally approximately

half the material strength, or less.
The creep strain, e, was characterized by measuring the loadingpoint displacement, y, as a function of time, t, and using the relation

[9]:

€ = 6 yw/(i 1 2 +i 1 i 2 -2i 2

2

) , (1)

where the specimen height is w = 3 mm, and the major and minor loading

spans are = 40 mm and i 2 = 10 mm, respectively. The neutral axis of

bending was assumed to be at the geometric center of the specimen and the

stress exponent was assumed as unity. These assumptions provide a

convenience by which the data can be analyzed.

The loading-point displacement during creep was monitored by a linear voltage displacement transducer that was attached to each watercooled holder of an upper ram of the test equipment. The transducers

were calibrated with known gauge blocks, and the displacement was measured within ± 5 fim. To enhance the long-term accuracy of the displacement

readings, a block of silicon carbide, the same as that used for the rams,

was used in the middle test station of a furnace. Any displacement measured on the block is believed to reflect changes in the test facility

due mainly to fluctuations in the ambient temperature. These displacement fluctuations were substracted from the displacements measured on the

specimens in the other two stations of the same furnace so that errors

due to differential thermal expansion of the test facility were minimized. The failure of one specimen in a furnace was observed to have negligible effect on the other specimen, mainly because the pneumatic pistons and rubber shock absorbers were used to restrain the propagation of

stress-waves

.

Creep experiments on 32 specimens have been run to date, with some

lasting up to sixteen hundred hours (1600 h) . Table 1 gives a summary of

the experiments conducted during this reporting period. Two regimes of

behavior have been observed. See Figs. 3 and 4.

In the first regime, typified by the creep strain versus time curve

in Fig. 3, only "primary" creep behavior is observed. Failure occurs in

this regime with little or no evidence of steady-state or tertiary creep;

the material "work-hardens" right up to the time of rupture. This apparent work-hardening characteristic of creep strain can be represented by
€ = Cta

, ( 2 )

where a and C are constants. Least-squares fits of the logarithm of Eq

.

(2) show that the time exponent, a, appears to remain fairly constant

over a range of applied stresses at a given temperature. See Figs. 5

through 7. This regime of response occurs for the larger applied stresses

at a given temperature and for the higher temperatures at a given applied

stress. In this failure regime the time-to-failure and the strain- tofailure increase with decreasing applied stress. This type of behavior

has also been observed by Porter et al. [10] in the much higher

temperature range around 1500° to 1600°C.

The second regime of behavior occurs for applied stresses below what

appears to be an apparent threshold stress at each temperature. The creep

behavior is typified by a large region of steady-state behavior; for

example, see Fig. 4. The failure behavior is such that the strain at

failure decreases with decreasing applied stress, although the time-tofailure is increasing; for example, see Fig. 7. The reasons for this

behavior are not yet understood, but may be shown by in-progress

microstructural examination of these specimens.

The threshold behavior between these two regimes becomes an

important design consideration for this material. This is particularly

exemplified at 1300°C, where a specimen at an applied stress of 72 MPa

failed in less than 30 hours, while a specimen at 62 MPa has survived

longer than 500 hours, thus far.

As a final comment, we noted a substantial degree of surface

reactions on these specimens tested in air. Previously polished surfaces

had their finishes dulled. Numerous pits were formed on the surface,

which were on the order of 20 to 40 ^m in diameter. The pits obscured

microindents which were previously placed on the side of the creep

specimen to monitor any shifts in the neutral strain axis during creep.

Silica and mullite phases were also observed to form on surfaces under an

applied stress, probably as a result of the oxidation of the SiC whiskers

and a reaction with the alumina matrix.
REFERENCES

1. E.R. Fuller, Jr., T.W. Coyle, R.F. Krause, Jr., and T.J. Chuang,

"Structural Reliability and Damage Tolerance of Ceramic Composites for

High-Temperature Applications," Semiannual Progress Report ORNL/FMP87/1, Fossil Energy Materials Program . R.R. Judkins, Manager, Oak

Ridge National Laboratory, Oak Ridge, TN, July 1987, pp . 115-140.

2. E.R. Fuller, Jr., T.W. Coyle, and R.F. Krause, Jr., "Structural

Reliability and Damage Tolerance of Ceramic Composites for HighTemperature Applications," Semiannual Progress Report ORNL/FMP-87/2

,

Fossil Energy Materials Program . R.R. Judkins, Manager, Oak Ridge

National Laboratory, Oak Ridge, TN, February 1988, pp . 145-167.

3. C. Janssen, "Specimen for Fracture Mechanics Studies on Glass",

Proceedings of the Xth International Congress on Glass . Kyoto, Japan,

The Ceramic Society of Japan, 1974, pp . 23-30.

4. J.H. Henry, "Energie d'Avancement de Fissure, G, pour une Plaque

Percee d'un Trou Soumise en Compression' , Mechanics Research

Communications 10 (5) 253-257 (1983).

5. C.G. Sammis and M.F. Ashby, "The Failure of Brittle Porous Solids

Under Compressive Stress States", Act Metall, 34 511-526 (1986).

6. W.E. Warren, "Theoretical Analysis of the Double Cleavage Drilled

Compression Specimen", Int. Journal of Fract. 33 223-325 (1987).

7. J.F. Rhodes, formerly ARCO Chemical Company, Greer, SC, May 14, 1986.

8. R.F. Krause, Jr. and E.R. Fuller, Jr., "Fracture Toughness Behavior of

a Silicon Carbide Whisker-Reinforced Alumina Ceramic at Selected

Porosities," in Fossil Energy Materials Program Conference

Proceedings . ORNL/FMP-87/4 , ed. by R.R. Judkins, Oak Ridge National

Laboratory, Oak Ridge, TN, Aug. 1987, pp . 38-55.

9. G.W. Hollenberg, G.R. Terwilliger, and R.S. Gordon, "Calculation of

Stresses and Strains in Four-Point Bending Creep Tests," J. Amer.

Ceram. Soc. 54 196-199 (1971).

10 J. R. Porter and B. N. Cox, Bull. Am. Ceram. Soc. 67, 568 (1988).
Table 1. Creep rupture behavior of a 25 wt percent silicon carbide

whisker-reinforced alumina ceramic having 4.9 wt percent porosity.

Failure times and strains with a greater- than- symbol refer to specimens

that have not yet ruptured up to the time and strain indicated.

Temperature

°C

Stress

a

(MPa)

Failure

Time

(hours)

Failure

Strain

(percent)

r 231 139 0.175

1100 4

L 306 25.1 0.167

r 175 262.4 0.621

200 64.1 0.388

1145 - 224 35.6 0.390

248 24.9 0.339

r 123 >504 >0.75

132 52 0.681

147 60.8 1.178

1200 4 174 19.9 0.825

174 27.2 0.872

198 12.3 0.703

82 >504 >0.35

98 161.7 1.135

1245

122 27.0 1.858

147 7.5 1.169

r 55 >504 >0.247

62 >504 >0.955

1300

72 27.6 1.846

98 5.1 1.956
i.2 n

E

<0 CLs

>.c

«£

W

01«k. 55T3

Jg

Q.a

<

-o- 1 Faer

2 Fibers

-o- 3 Fibers

Figure 1. The toughening increment for double-cleavage drilled-compression

(DCDC) specimens with one, two, and three planar arrays of fibers at the

indicated position. The reduced toughening increment of the three fiber

configuration, from what might be expected, most likely resulted from

different processing conditions.

E->

(0 Q.5

>*c

V£

ininv

55•o£

a

a. <

-o- 8B as received

1 |xm Ni coat o- 9|im Ni coat

22|j.m Ni coat

Figure 2. The toughening increment for double-cleavage drilled-compression

(DCDC) specimens with a single nickel-coated fiber with various coating

thicknesses

.
20 40 60 80

Time, h

Figure 3. Typical creep curve of creep strain versus time in the "primary"

creep regime. The data are for a flexure specimen of a 25 wt percent

silicon carbide whisker-reinforced alumina ceramic of 4.9 percent porosity

under the given applied stress of 147 MPa at 1200°C.

0 250 500 750 1000

Time, h

Figure 4. Typical creep curve of creep strain versus time in the "steadystate" creep regime. The data are for a flexure specimen of a 25 wt

percent silicon carbide whisker- reinforced alumina ceramic of 4.9 percent

porosity under the given applied stress of 123 MPa at 1200°C.
log(e)

Time, t/h

0 1 2 3

log(t)

Figure 5. Logarithmic creep data at 1100°C (logarithm of creep strain

versus logarithm of time) for flexural specimens of a 25 wt percent silicon

carbide whisker-reinforced alumina ceramic of 4.9 percent porosity. Note

the transition in behavior between applied stresses of 200 and 126 MPa.

Strain,

lOOe
log(e)

1 - 1.5

3 10

Time, t/h

30 100 300 1000

- 2.0

- 2.5

- 3.0

- 3.5

0 1 2 3

log(t)

0.03

Figure 6. Logarithmic creep data at 1200°C (logarithm of creep strain

versus logarithm of time) for flexural specimens of a 25 wt percent silicon

carbide whisker-reinforced alumina 'ceramic of 4.9 percent porosity. Note

the transition in behavior between applied stresses of 147 and 123 MPa.

Strain,

lOOe
log(e)

Time, t/h

1 3 10 30 100 300 1000

3.0

1.0

0.3

0.1

0.03

0 1 2 3

log(t)

Figure 7. Logarithmic creep data at 1245°C (logarithm of creep strain

versus logarithm of time) for flexural specimens of a 25 wt percent silicon

carbide whisker-reinforced alumina ceramic of 4.9 percent porosity. Note

the transition in behavior between applied stresses of 122 and 98 MPa.

Strain,

lOOe
<1 2 i- : ' - A TT

3IBLCGRAPHIC DATA

iHEET See nstructionsi

J BL. CATION OR

I REPORT NO.

NISTIR 88-3817 August 1988

4. t ! T!_= and subtitle

'Structural Reliability and Damage Tolerance of Ceramic Composites for High-Temperature

Applications Semi-Annual Progress Report for the Period Ending March 31, 1988.

5. AUTHOR(S)

E.R. Fuller, Jr., T.W. Coyle, T.R. Palamides and R.F. Krause, Jr.

5. PE R FC RMING ORGAN I ZATI ON (I f joint or other than N 8S. see instructions) 7. Contract/Grant No.

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

8 , Type of Report 4 Pe'\od Covered

1 9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City . State, ZIP)

U.S. Department of Energy

Fossil Energy, Advanced Research & Technology Development

ossil Energy Material Program, Oak Ridge National Laboratory, Oak Ridge, TN

10. supplementary notes

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant in formation . If document includes a si gn i fi cant

bi bl iography or literature survey, mention it here)

The achievement of higher efficiency heat engines and heat recovery

systems requires the availability of high- temperature , high-performance

structural materials. Structural ceramics, and more recently, ceramic

matrix composites have received particular attention for these

applications due to their high strength, and resistance to corrosion and

thermal shock. Even with these positive attributes, improved reliability

and extended lifetime under service conditions are necessary for

structural ceramics to gain industrial acceptance. The problems with

these materials are mechanical and chemical in nature and are enhanced by

the fact that they are subjected to high temperatures, reactive

environments and extreme thermal gradients.

With an objective of improved performance for heat engine/heat

recovery applications, the NBS program on structural ceramics and ceramic

composites addresses these problems through the determination of critical

factors that influence mechanical and micros true tural behavior. The

activities of the program are grouped under two major subtasks, each

designed to develop key data, associated test methods and companion

predictive models. The status of the subtasks are detailed in the

following sections.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words oy semicolon S) Composites; double-cleavage-drilled-compression ; indentation; R-curve; SiC fibers;

SiC Whiskers; shear resistance; strength

13. availability

'3T U n 1 1 m i ted For Official Distribution. Do Not Release to NTIS

14. NO. OF , PRINTED PAGES

1 7 O'der From. Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

204Q2. 15. Price

X Order From National Technical 'nformation Service (NTIS), Springfield. VA. 2216 $9.95

_ SCOmm-DC -5043-=}
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