NBSIR 82-2569(R)

Div. 561

HIGH TEMPERATURE APPLICATIONS OF STRUCTURAL CERAMICS

QUARTERLY PROGRESS REPORT

April - June 1982

Samuel J. Schneider

Project Manager

Center for Materials Science

National Bureau of Standards

U. S. Department of Commerce

Washington, D. C. 20234

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

Under Contract No. DE-A105-800R20679

"This report was prepared as an account of work sponsored by the United

States Government. Neither the United States nor the United States

Department of Energy, nor any of their employees, nor any of their

contractors, subcontractors, or their employees, makes any warranty,

expressed or implied, or assumes any. legal liability or responsibility

for the accuracy, completeness, or usefulness of any information,

apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights."

TABLE OF CONTENTS

PAGE

INTRODUCTION 1

Subtask A. High Temperature Fracture of Structural Ceramics. . . 2

Subtask B. Crack Growth Mechanism Maps 3

Subtask C. Microstructure and Phase Alteration 6

Subtask D. Microstructure and Fracture in Reactive

Environments 12

*

i
1

Jl
1

NBS-5.12 - HIGH TEMPERATURE APPLICATIONS

OF STRUCTURAL CERAMICS

E. R. Fuller, R. J. Fields, T. J. Chuang,

R. Krause, A. Dragoo and S. J. Schneider

National Bureau of Standards

Center for Materials Science

Washington, D. C. 20234

INTRODUCTION

The achievement of higher efficiency thermochemical engines and

heat recovery systems requires the availability of high temperature,

high performance structural materials. Structural ceramics such as

SiC, Si^N^ and certain A^O^-Si^N^ combinations have received particular

attention for these applications due to their basic characteristics of

good strengths coupled with good corrosion and thermal shock resistances.

Even with these positive attributes, improved reliabilities and extended

lifetimes under service conditions are necessary for structural ceramics

to gain industrial acceptance and use. The problems are mechanical

and/or chemical in nature and are enhanced by the fact that these

materials 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 addresses

these problems through the determination of the critical factors which

influence mechanical and microstructural behavior. The activities of

the program are grouped under four major sub tasks with each designed to

develop key data, associated test methods and companion predictive

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

sections

.
NBS-5.12(A) - HIGH TEMPERATURE FRACTURE OF STRUCTURAL CERAMICS

DISCUSSION OF CURRENT ACTIVITIES

Chevron-notched beam specimens of NC-430 were fractured under fourpoint bending for a range of loading rates from 0.0002 in/min to 0.02 in/

min and temperatures from RT to 1350 °C. The majority of the specimens

were found to fail in transgranular mode.

Table I lists the raw data obtained for fracture toughness (K ) determination. Note that the K values in the last column of the

table are only preliminary results derived from the K expression

ILi using peak load and minimum Y (K^ = Pmax Y) . True values must

wait until SEM work is done so that the actual crack length at which

fast fracture occurs can be determined.

Two figures can be derived from Table I. Figure 1 shows K vs. T

J- L

plot for several loading rates. The data indicates increases when

T increases. This result is consistent with test data obtained from

other ceramic materials. It is thought that creep deformation near the

crack tip might be responsible for this effect.

• Figure 2 demonstrates K versus 6 at various temperatures. The

data at 1350 °C show a linear decrease of K as 6 increases on a semilog scale. It is believed that crack tip blunting may play a role in

the increase of K especially in the low 6 region.

J. L* The construction of a specially designed fixture for double-torsion

testing has been completed. This special fixture assures good alignment

of the loading system and soon will be used to test the double-torsion

specimens under constant load condition.
3

Table I. Rapid Screening Test Data of NC-430

Specimen

No.

Width

B

(mm)

Height

W

(mm)

Temperature

T

(°C)

Loading

Rate 6

(in/min)

Peak Load

Pmax

(kg)

Apparent

K

IC

(MPav'm)

24A 2.023 4.971 RT 0.02 4.1 4.66

24B 2.024 4.972 RT 0.02 3.60 4.76

23B 2.021 4.963 RT 0.02 3.85 4.51

22B 2.018 4.971 RT 0.02 3.95 4.39

19B 1.991 4.972 1050 0.0002 4.87 5.41

7B 2.009 4.974 1050 0.02 4.63 5.16

2 IB 2.019 4.972 1050 0.02 4.10 4.66

15B 1.989 4.974 1350 0.0002 5.68 6.13

16B 1.983 4.972 1350 0.0002 6.74 7.32

18B 1.995 4.974 1350 0.002 5.47 6.01

17B 1.990 4.970 1350 0.02 4.97 5.37

2 OB 2.006 4.970 1350 0.02 4.15 4.72

NBS-5. 12(B) - CRACK GROWTH MECHANISM HAPS

DISCUSSION OF CURRENT ACTIVITIES

Five notched specimens of KT-SiC were loaded under dead weight in

four-point bend fashion for a constant applied stress intensity of

3.2 MPai/m. These tests were interrupted before failure at different

durations. Table II lists the holding times and testing temperatures

for these specimens. The apparent fracture toughness (K.^) for this

material (KT-SiC) has also been determined to be approximately

4.0 MPa/m

.

These specimens are being polished and will be examined to see

if any subcritical crack growth or cavitation took place.
4

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2.

Plot

ofK

vs.

6

for

NC-430

at

various

temperatures.
6

Table II. Test Durations and Temperatures for Creep Bending Test of

KT-SiC (K = 3.2 MPav^m)

Specimen No. Temperature (°C) Duration (hr)

101 1300 95.5

102 1300 164.3

103 1300 65.0

104 1350 235.5

106 1350 66.0

NBS-5. 12(C) - MICROSTRUCTURE AND PHASE ALTERATION

DISCUSSION OF CURRENT ACTIVITIES

Phase Analysis of As-Received Materials

X-ray diffraction data for samples of Carborundum silicon car- bides —alpha-SiC and KT-SiC—and Norton silicon carbides—NC-203 and

NC-430—were analyzed for SiC polytypes and impurity phases. The

O observed diffraction results, expressed as d-values (A), were compared

with reference values catalogued in the JCPDS card file. The indexed

phases and polytypes of SiC contained in the file are listed in Table III.

Table III. JCPDS Indexed SiC Phases and Polytypes

Phase Polytype Card Number

a- SiC 2H 29-1126, 29-1130

a-SiC 4H 29-1127

a-SiC 6H 29-1128, 29-1131

a-SiC 15R 22-1301

a-SiC 21R 22-1319

a-SiC 33R 22-1316

a-SiC 51R 4-0756

3-SiC 29-1129
7

In addition, the cards for the indexed d-values for silicon (27-1402)

and for a number of tungsten and aluminum carbides, oxycarbides, and

oxides were used

.

Because of the great structural similarity of all of the SiC polytypes and phases, a delineation of all of the polytypes and phase components of commercial silicon carbide materials is not possible.

Moreover, analysis of a given material is complicated by compositional

variations throughout the material and by preferred orientation of

grains which can result, in the extremes, in abnormally large diffraction intensities or in absence of expected diffraction intensities.

Nevertheless, it has been possible to obtain a "best fit" to one polytype and to presume the absence of some polytypes based on the absence

of one or more characteristic diffraction peaks.

For all of the samples, the 6H polytype of a-SiC appears to be the

dominant structural type. In most cases, except as noted below, it was not possible to distinguish 3-SiC from the a-SiC s since possible

3-diffraction peaks were obscured by the a-SiC peaks. The presence of

> important a-SiC peaks which were not associated with 3-SiC indicated

that the a-SiC’ s were the principal components of these materials.

Minor amounts of a-SiC polytypes other than 6H will be discussed now

for the specific materials.

1. Alpha-SiC : Alpha-SiC is a reaction bonded silicon carbide which

contains no free silicon but presumably has A1 and B incorporated in

some form, since A^O^ and B reportedly are used as sintering aids. In

addition to the 6H polytype, a small amount of the 4H polytype appears

to be present. The presence of other polytypes is doubtful. This

material showed a number of weak diffraction peaks, particularly in the

O O range 1.73 A < d < 1.78 A, which suggest the presence of aluminum car- bide and aluminum oxycarbide.

2. KT-SiC ; This silicon carbide-based material is injection molded

SiC, impregnated with free Si. The sample examined here was cut with

the two examined surfaces transverse to the pressing direction. In

addition to the 6H poly types, small amounts of 2H, 4H, and 15R polytypes

appear to be present in this material. The general agreement of ob-
8

served SiC diffraction intensities with the published values suggests

that the SiC grains were not oriented with respect to the pressing

direction.

This material was notable for the large amount of free silicon

which appeared to be present. Moreover, the silicon distribution was highly variable, and the silicon grains appeared to be strongly oriented. The sample was examined at two positions on each of two opposite

sides. The ratio, I /I of the major silicon peak to the major

jl olL

silicon carbide peak varied between one and three. Furthermore, the

observed diffraction peaks for the free silicon showed enhanced diffraction by (lll)-planes, and to a lesser extent, by (311)- and (511)-

planes, indicating that these grains are oriented with their <111>-

axis approximately parallel to the pressing direction.

3. NC-203 : This is a hot-pressed silicon carbide which appears to

contain minor amounts of 2H, 4H, and 15R polytypes and of 8-SiC. Some

weak to moderate diffraction peaks were observed which were assigned to

WC. Evidence for other tungsten carbides was not found. A broad, weak

O diffraction peak at about d = 3.45 A suggested the presence of a small

amount of graphite, but since the position of the observed peak is

shifted slightly from the expected position for the graphite diffraction

O peak (3.35 A), this assignment is not certain.

4. NC-430 ; This siliconized silicon carbide material appears to be

composed of 6H with perhaps a minor amount of $-SiC. The diffraction

peaks for free silicon are not as strong as those for SiC (silicon

content was estimated to be about 20 percent by volume based on x-ray

data) . The silicon distribution does not appear to be as variable or the silicon grains as highly oriented as in the case of KT-SiC.

Reactions of SiC in Gaseous Environments

The results reported here were obtained in conjunction with the

work reported in Section (D) below. A sample of NC-203 and one of NC-430

were annealed at 1350 °C for four hours in the simulated coal-combustion
gas mixture (gas mixture A).* In addition, a fracture specimen of KT-SiC

was examined

.

As in previous investigations of the oxidation of silicon carbide

materials in air, each of the specimens exhibited a peak or a composite

O peak in the range of 4.04 to 4.07 A, which is evidence for an Si02

product phase—cristobalite or tridymite. The d-values for the product

diffraction peaks are given in Table IV. Other than the assignment of

the SK^-related peaks, the product phases have not been identified.

Previously obtained oxidation results for the air oxidation of

NC-430 at 1400 °C for seven hours were re-examined. It was found that

the major SiC diffraction peak, which is observed in the range of 2.515

O to 2.519 A for unoxidized samples, had split into two peaks, one at

O O 2.512 A and the other at 2.519 A. This is apparently associated with a transformation of the polytypes.

*The gas was introduced at a rate of 500 ml/min with a nominal composition

of 75% N

2

, 11% C0

2

, 7% 0

2

, 7% H

2

0 and an excess of + .1% S0

2

.
10

Table IV. Diffraction d-Values for Product Phases on SiC Reacted in

Combustion Gas Mixture A at 1350 °C for Four Hours: d-Values O

7 in A

NC-203 NC-430 KT-SiC Phase

6.32

4.10

4.15

4.07 4.06

4.04

Low cristobalite

or tridymite

4.01

3.74

2.973

2.596

2.238

2.110

1.998

1.608

1.581

1.443

1.357

1.245

1.108

0.960

*

High-Temperature X-Ray Diffraction

Machining of the bonded ceramic fiber insulation and assembly of

the x-ray diffraction furnace was completed. A second heating method

was tested. This method uses a heater-sample holder which is machined

from an SiC heating element. A test of a heating element in a test rig

is shown in Figure 3. Although the heater performed satisfactorily,

high contact resistances between the heater and the silicon carbide

electrical power leads presented difficulties. An attempt to eliminate

these resistances by coating the junctions with a mixture of Si and

MoSi^ powder, which then was arc-welded to form a bond, succeeded in

reducing the resistance so that sufficient current could be passed

through the heater to bring it to temperature. However, the welds

tended to crack on cooling, and the MoSi^ corroded the SiC. Subsequently, we have learned that it may be possible to form low resistance
11

Figure 3. Test of silicon carbide heating element.
12

junctions by coating the junctions with Si powder only and heating in excess of 2000 °C.

A talk entitled, "High-Temperature X-Ray Diffraction and the

Oxidation of Silicon Carbide", was given at the 85th Annual Meeting of

the American Ceramic Society.

Models for Induced Microstresses and Microstrains

A general bulk material contains cracks and inclusions of varying

sizes and placements. Towards the goal of describing mathematically

the stress/strain states of a given material, a very idealized problem

is under study as a first approximation. A thin slab of material

containing a periodic array of cracks and inclusions can be discussed

in terms of the solutions to the biharmonic equation if the regions of

plasticity are ignored. Thus, a solution is being sought for the

equation,

2 2 V VI = 0,

for an infinite slab containing periodically placed extended elastic

inclusions and nonplastic pores. While this problem is highly idealized, it has several useful and interesting features. First, the com- plex function formalism can be used along with the general solution

techniques that are well discussed in the literature. Second, the

interaction of the pores and inclusions can be studied. Third, the

change in overall bulk properties, such as the compressibility, can be

estimated. Fourth, the consequences of the differential thermal

characteristics of the constituents can be estimated. And, fifth, it

might be possible to consider plasticity in a subsequent perturbation

treatment

.

NBS-5. 12(D) - MICROSTRUCTURE AND FRACTURE IN REACTIVE ENVIRONMENTS

DISCUSSION OF CURRENT ACTIVITIES

During the past quarter, we completed the construction of an apparatus for fracture testing of ceramic materials in controlled

gaseous environments at elevated temperatures. Verification of the

capabilities and operation of this apparatus was demonstrated with a set of initial fracture experiments on alpha silicon carbide in an
13

atmosphere of simulated combustion product gases. More controlled

experiments, in terms of specimen orientation and machining, were then

conducted on a siliconized silicon carbide material. These studies

are described below.

Construction of an apparatus for fracture testing of selected

structural ceramics in controlled gaseous environments at elevated

temperatures was completed during the past quarter. The furnace for

this apparatus is a commercially manufactured box furnace which we had

specially designed. It is electrically heated with molybdenum disilicide

heating elements and is capable of temperatures up to 1600 °C. The

hot zone is approximately a 15 cm cube which is insulated with a refractory alumina. Load is transmitted into the hot-zone cavity by

silicon carbide rods which are appropriately secured to special bellows

assemblies attached to the top and bottom parts of the furnace. The

test chamber, hot zone region and insulation, are enclosed in a gas

tight steel jacket. Appropriate parts of this jacket and the furnace

shell are water cooled, but in all cases the cooling water .temperature

is above the required dew point for the steam component of the gaseous

atmosphere. The furnace is mounted in the frame of a gear driven

universal testing machine. Typical load ranges are 100 to 500 N full

scale with a 0.5 percent precision. Typical crosshead displacement

-4

rates range from 5 mm/min to 5 x 10 mm/min.

The test environment for this apparatus is a gaseous mixture of

nitrogen, oxygen, steam, carbon dioxide and sulfur dioxide. The total

pressure is nominally one atmosphere and the flow rate thorugh the

furnace cavity is nominally 500 mL/min (STP) . All the gaseous components

except steam are supplied from tank gases with their flow rate and

proportion established by mass flow controllers. A steam generator was constructed to generate the water vapor component of the gaseous

environment. A thermostat bath is used to control both the steam

partial pressure, via a condensor, and the temperature of the cooling

water in the furnace shell and bellows assemblies to prevent water

condensation in the furnace.
14

Having completed construction of the environmental testing apparatus,

initial fracture experiments were begun on a miscellaneous collection

of alpha silicon carbide specimens to verify operation of this system.

f* Chevron-notched, four-point flexural specimens (nominally, 2 x 5 x 50 mm)

were used to determine the fracture toughness (K ) at temperatures of

1 1200 °C and 1500 °C. The gas composition for these measurements was

75 percent nitrogen, 7 percent oxygen, 11 percent carbon dioxide,

7 percent water vapor and 0.1% sulfur dioxide, which simulates the

gaseous combustion products in an oxygen-rich fossil fuel system.

Before the fracture test was conducted, the specimen was equilibrated

at temperature and exposed to the flowing environment for four hours.

The fracture tests were conducted at a constant displacement rate to

failure, with both a high rate (0.5 mm/min) and a low rate (0.005 mm/min)

.

The lower rate indicates the presence of subcritical crack growth

and/or creep shielding at the crack tip. The fracture toughness was calculated using a variation of an analysis for chevron-notched

specimens by D. G. Munz, J. L. Shannon, Jr., and R. T. Bubsqy [Int.

Journ. of Fracture 16, R137-R141 (1980)]. Using this analysis, the

fracture toughness (K ) was determined from the maximum fracture load

1L* assuming that the crack went catastrophic at the minimum of the curve

in stress intensity factor versus crack length. This assumption is

only valid when subcritical crack growth is minimal. Accordingly, the

values of K at the rapid loading rate give a better measure of

it* toughness and the values at the slower rates should be viewed as giving an indication of the extent of subcritical crack growth and/or

creep blunting, giving lower values in the former case and higher

values in the latter case.

iMaterial for these and subsequent tests was supplied in the form of

square plate billets with nominal dimensions of 12 x 50 x 50 mm. (Most likely, these billets were cold pressed and sintered.) Two

test orientations were used for the flexural specimens : a TP orientation

and a TT orientation where T denotes a transverse direction and P

denotes the plate direction. In both cases the crack propagated

on a transverse (T) plane, but in the former case (TP orientation)

the propagation direction was perpendicular to the plate (P) , whereas

in the latter case it was parallel to the plate (T)

.
15

Results of these fracture experiments for an alpha silicon carbide

(HEXALLOY SA) are given in Table V. Comparison of these values with

those measured in air, both here and in Task A, indicate that this

environment has little synergetic influence with stress, as indicated

by the fracture toughness. It, however, does not address any long term

alterations in both microstructural and mechanical properties due to

prolonged exposure. Having verified operation of the system with the

miscellaneous alpha silicon carbide specimens, carefully machined and

ground specimens of a siliconized silicon carbide (HEXALLOY KT) were

tested using the same procedure. Results of these measurements are

given in Table VI. There appears to be a slight effect of the environment in this case.

Table V. Critical Stress Intensity Factor (K ) for alpha-SiC

(HEXALLOY SA)

L

Environment/

Temperature

Crack

Orientation

Loading

Rate/ (mm/min) K^/MPa'in 2

Air/25 °C TP 0.5 7.8

TT 0.5 6.1

TT 0.005 6.3

Combustion Gas/ TP 0.5 9.1

1200 °C 10.6

TP 0.005 9.3

8.7

TT 0.5 9.0

TT 0.005 7.2

Combustion Gas/ TT 0.5 6.1

1500 °C TT 0.005 5.7

TT 0.001 5.3

[Combustion Gas: 75% N

2

; 7% 0

2

; 11% C0

2

; 7% H

2

0; 0.1% so

2

]
Table VI. Critical Stress Intensity Factor (K ) for Siliconized

SiC (HEXALLOY KT)

10

Environment/

Temperature

Crack

Orientation

Loading

Rate (mm/min) Kic/MPa

Combustion Gas/ TT 0.5 6.1

1050 °C 0.005 15.7

Combustion Gas/ TT 0.5 12.7

1200 °C TT 0.005 22.8

Combustion Gas/ TT 0.5 12.7

1350 °C 0.005 18.5