HIGH TEMPERATURE APPLICATIONS OF STRUCTURAL CERAMICS

QUARTERLY PROGRESS REPORT

July - September 1980

Samuel J. Schneider

Project Manager

Center for Materials Science

National Bureau of Standards

U. S. Department of Commerce

Under Contract No. DE-AI01 5-800R20679

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

Washington, D. C. 20234

TABLE OF CONTENTS

PAGE

I. SUMMARY OF PROGRESS TO DATE 1

II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS 2

Task 1. High Temperature Fracture of Structural

Ceramics 2

Task 2. Crack Growth Mechanism Maps 5

Task 3. Thermochemical Data 40

Task 4. Kinetics of Oxidation 43

iii

I. SUMMARY OF PROGRESS TO DATE

1. High Temperature Fracture of Structural Ceramics

Further studies of Tien's Si 3 N 4 materials were conducted to obtain

critical stress intensity factors and to clarify some inconsistencies

in earlier results. Sialon specimens received from Kobayashi were annealed at 1430 °C for 60 hours with little degradation of the specimen.

Billets of Si C materials were ordered from Norton and Carborundum.

A literature survey of available data on fracture toughness, crack

growth behavior, and creep of SiC, Si 3 N 4 , and sialon was completed.

2. Crack Growth Mechanism Maps

A preliminary crack growth mechanism map for commercial, hot pressed

SiC materials was constructed from published information. The literature survey of fracture and crack growth mechanisms was completed for

silicon carbide, silicon nitride, and sialons. A critical review of

these mechanisms is now underway.

3. Thermochemical Data

The experimental results obtained this quarter clearly indicate that

it is very necessary to first understand the more simple two, three,

and four component systems involved before expecting to obtain useful

thermochemical data from specimens in complex five and six component

systems. Therefore it will be necessary to first examine such systems

as Si-Mo-N 2 , Ce-Ta-N 2 , Si-Ta-N 2 , Si-Ce-N2 , Si-Ce-Ta-N2 as well as the

corresponding metal plus 02 systems before studying the metal N 2 -02 systems.

4. Kinetics of Oxidation

X-ray analysis has been completed on 17 e-Si 3 N 4 plus garnet samples

before and after high temperature fracture testing. The surface film

thickness after the fracture testing did not show any obvious correlation

with the past history of the samples. New phases observed after the high

temperature fracture testing include 3-cristobalite, e-Y 2 Si 2 0 7 , mullite,

and a well developed but unidentified phase. There was no correlation

between the amounts of the different phases and the processing schedule.

1
II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS 1.

High Temperature Fracture of Structural Ceramics (E. R. Fuller, Jr.,

R. J. Fields, L. Chuck, and D. Harne)

Progress : The first major milestone for this task (literature survey

of available data) has been completed. This literature survey of available data on fracture toughness, crack growth, and creep behavior of

SiC, Si 3 N 4 , and sialon materials was carried out using a computerized

literature search facility available at NBS. The results of this

search are described in an attached draft of this report. In this

report, the lists of papers and reports found by a computerized search

were winnowed to extract only that literature having immediate bearing on high-temperature applications of structural ceramics. A critical discussion of these papers is included. This report together with the

computer output lists of titles is being prepared for publication as an NBS internal report, and will be submitted to DOE following internal

review.

Eighty-eight papers having information relevant to this survey were partially or completely copied. The Fracture and Deformation Division

at NBS has this collection available for the use of all interested

parties. The major conclusions of the survey were:

1. There is a considerably larger body of literature on the

relevant mechanical properties of Si 3 N 4 than on SiC.

2. Theoretical studies on fracture mechanisms may be the most

active area of current research.

3. The areas where research would be particularly fruitful are:

a. Effect of atmosphere on creep rate.

b. Creep mechanisms in SiC and Reaction bonded Si 3 N 4

.

c. High temperature v-K plots: the effect of temperature,

atmosphere, and structure on all three materials.

d. Model experiment on cavitation crack growth in grain

boundaries.

e. Direct measurement of the physical and chemical properties

of grain boundary phases.

Crack lengths at the onset of rapid fracture were determined by

microscopic examination of the fracture surfaces of the material

obtained from Professor Tien of the University of Michigan. This

information together with the load-displacement curves obtained in

the last quarter allows the critical stress intensity factor, Kjto be calculated. The results are listed in Table I. The toughness

so obtained does not appear to correlate with trends in either the

fabrication procedures employed at the University of Michigan or the

the x-ray results obtained at NBS. The unreasonably high Kj

C

values

2
recorded for some of these silicon nitrides indicate that rapid creep

deformation is occuring and obscuring the true toughness in some samples.

To avoid significant creep deformation, the fracture test must be

carried out at a more rapid cross-head speed. However, it is necessary

to pre-crack the specimen prior to these rapid tests. A series of tests on Tien's materials are now being performed on pre-cracked specimens to

eliminate the effect of creep on toughness in order to check the corre- lation between a material's composition, fabrication history, and its

mechanical properties.

There is some evidence that long time anneals in air greatly improve

the mechanical properties of sialon material. To investigate this

possiblity, one of Kobayashi's sialon materials (Z=l) was aged at 1430 °C

for 60 hours to see if it could withstand 1000 hours at this temperature.

The oxidation that occurred was not significantly greater than that

which occurred in 2 hours at 1500 °C, so it appears that this sialon

will remain relatively stable during long time anneals at 1430 °C.

Billets of Crystar, NC 203, and NC 435 were ordered from Norton and

billets of Alpha Si C and reaction sintered SiC were ordered from

Carborundum.

Plans : Having considered the results of the literature survey, attention

will be centered on testing silicon carbide as this material shows

excellent combination of creep resistance, crack growth resistance,

and toughness. The work on Tien's silicon nitrides will be finished

and the results will be communicated to him. Kobayashi's sialons will

be tested with and without a 100 hour anneal to see if the effect of

heat treatment is as beneficial as preliminary work by other investigators indicate it to be.

3
Crit.

Stress

ec

No.

Thickness

Width

Crack

Length

Fracture

Load

Intensity*

Fabrication

Notes

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4
2 . Crack Growth Mechanism Maps (R. J. Fields and E. R. Fuller, Jr.)

Progress : The first major milestone of this task (compilation of data

and construction of crack growth mechanism maps) has been completed.

The available crack growth data and fracture toughness data for all

types of silicon carbide, silicon nitride, and sialon materials have

been collected and are discussed in the attached draft of this literature

survey. The greatest amount of data is for hot-pressed silicon nitride,

followed by hot-pressed silicon carbide. Crack growth mechanism maps

for these two materials have been constructed using the data in Table

II and are presented in Figs. 1 and 2.

The purpose of these preliminary maps was to determine areas of fracture

behavior in which research was needed and for which research would be

most fruitful. The construction of these maps for NC 203 silicon

carbide and HS 130 silicon nitride was beneficial in that regard.

First, it is clear for both silicon carbide and silicon nitride that more crack growth behavior needs to be examined at higher temperatures

(>1400 °C) and at lower crack velocities (<10 -8 m/s). In the case of

silicon carbide, this work should also be carried out at lower temperatures

(<1000 °C). Secondly, Kj« needs to be measured at higher temperatures

as it is still changing rapidly for silicon nitride even at the highest

temperatures for which it has been measured. There is some evidence

in silicon carbide that there is a minimum Kj below which there is no crack growth. This possibility should beinvestigated further in

both materials. The existence of an intergranular crack growth mechanism change (brittle to viscous) has largely been inferred from gross

mechanical behavior and a few isolated microstructural studies. The

boundary between mechanisms should be investigated in much more detail

to prove the existence of the purported mechanisms. From the map for

NC 203, the mechanism change occurs between 1200 and 1300 °C at lower

crack velocities.

The literature survey for the second major milestone of this task

(i.e., literature survey and critical review of crack growth mechanisms

in structural ceramics) has been completed and a draft is attached.

The formal report is being prepared.

Plans : Complete the critical review of fracture and crack growth

mechanisms in structural ceramics. This review will focus on three

• main mechanisms in crack growth: 1) diffusional crack growth; 2) crack

growth by viscous flow; and 3) lattice trapping and stress corrosion

cracking. Data from Task 1 will be analyzed to generate v-K diagrams

and crack growth mechanism maps. In light of the preliminary map for

SiC, crack growth in NC 203 will be investigated below 1000 and above

1400 °C. Also, microstructural analyses will be made on specimens broken

at 1200 and at 1300 °C to verify the existence of the possible mechanism change.

5
Silicon

Nitride

CHS

130),

tested

In

air

Silicon

Carbide

(NB203),

tested

In

sir

<3 "3

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sf cm o cm o«nm PM m

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6

Henshall,

J.L.

,

Rowcllffe,

D.J.,

and

Edlngton,

J

.

W

Cer.

Soc.,

62,

p.

36,

(1979).
CRACK

GROWTH

MECHANISM

MAP

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500

1000

1500

TEMPERATURE

(°C)
DRAFT

LITERATURE SURVEY OF THE HIGH-TEMPERATURE CREEP

AND FRACTURE PROPERTIES OF SILICON NITRIDE,

SILICON CARBIDE AND SIALONS

John S. Nadeau
1, Objectives of Survey;

1.1 To obtain all available literature on subcritical

crack-growth and creep at high temperatures for the compounds

SiC, Si 3 N^ and Sialons.

/

1.2 To extract from the literature all information on

V-K plots (crack velocity versus stress intensity) for SiC,

Si 3N4 and sialon, for the purpose of constructing fracture

mechanism maps.

1.3 To obtain all relevant literature on fracture mechanisms in ceramics at high temperatures

.

1.4 To provide a critical evaluation of the literature

on high temperature creep and fracture of SiC, Si 3 N4 and sialons,

in order to identify areas where additional experiments would

be fruitful or where the understanding of mechanisms is lacking.

10
2. Scope of Survey:

The following abstract sources were used:

2.1 Compendex (Engineering Index) - 1970 to present.

2.2 Chemical Abstracts - 1970 to present.

2.3 National Technical Information Service (NTIS) - 1964

to present.

2.4 Ceramic Abstracts 1970 to present.

2.5 An Annotated Bibliography on Silicon Nitride for Structural

Applications: D. Messier and M.M. Murphy, MCIC Battelle,

Columbus, 1979, 1961 to 1978.

The first three sources were surveyed by computer, while

the remaining two were surveyed directly. Reference lists in

individual articles and reports were also examined to see if

they contained references not otherwise identified.

*

3 . Survey Method

:

In the computer searches, the computer was asked to identify all references to SiC, Si 3 N4 , sialons, creep, fracture or

crack growth. A typical result, that from Chemical Abstracts,

was as follows

:

11
Table 1. Results of Computer Search in Chemical Abstracts.

Description

1. Silicon Carbide

2. Silicon Nitride

3. Sialon

/ * 4. Crac1

'. growth or fracture or creep

5. ( 1+1 )

6. ( 2+4 )

7. ( 3+4 )

Number of Items

2148

1161

6

7254

41

38

0

Thus the last three items, constituting 79 papers, contained information on both the materials and the properties of

interest. The computer was then directed to print the titles of

the 79 articles so identified.

The list of identified items from the three computer sources

contained a total of 375 references as shown below:

Table 2. Iteris identified by Computer Search.

NTIS 135

Compendex 161

Chemical Abstracts 79

Total 375

Unfortunately, there was no way of rejecting from one source

items that had already been identified in another source. Thus,

there were many cases where a single article was identified by

12
all three sources. This was also true of the two non-computer

sources and it became incumbent upon the searcher to recognize

articles that had been previously examined.

The raw materials for this literature survey thus consisted

of five lists of titles, the three computer produced lists, the

annotated bibliography of Messier and Murphy (1,386 entries) and

the list obtained by searching the index of Ceramic Abstracts

(44 items). These lists were then examined and winnowed to

yield the titles of articles fitting the criteria of this search.

The resulting winnowed lists were then examined directly by going

to the library shelves. Those items which upon direct examination were found to have information that satisfied the criteria

of this survey were partially or completely copied.

I

4. Results of che Survey;

*

The main product of the survey is a collection of copies of

titles, abstracts, data and entire papers and reports, fitting

the criteria of the search. A list of the titles of these items

is appended to r.his report. They are comprised as follows:

13
Table 3. Number of published articles fully or partly copied

or on hand.*

Articles and Reports

Si 3 N4 51

SiC 18

Sialons 3

Fracture Mechanisms 16

Total 88

* Items containing information on both Sic and Si^N^ are counted

twice.

According to Table 3, silicon nitride has received much

more attention than silicon carbide (within the framework of this

study). This attention has been fairly recent. Table 4 shows

that the output of papers on the high temperature creep and fracture of silicon carbide has been more or less constant for the past

decade, averaging about one to two papers per year. Similar papers

on silicon nitride began to appear in 1972, and the output quickly

increased to ten per year in 1975. It appears that the production

of papers on silicon nitride is now falling, but the data for 1980

are incomplete.

The publications in which papers relevant to this study have

appeared are shown in Table 5. The major journals are Journal of

the American Ceramic Society and Journal of Materials Science.

Papers on fracture mechanisms tend to be in the metallurgical

literature rather than in the ceramics literature. All of the

14
articles are in the English language except for four in German.

Two articles in Japanese were rejected from the search because

of the searcher's inability to deal with that language. The

penalty for confining the search essentially to English appears

to be negligible, since most non-English speaking workers seem to

publish some of their results in English.

Table 4. Dates of Copied Articles

:

Date Sir*N4* SiC Sialon Mechanisms

Before 1970 - 2 - -

1970 - - - -

1971 - 2 - -

1972

si 1 - - -

1973 6 1 - -

1974 4 - - . -

1975 10 4 1 3

1976 5 2 - 1

1977 5 3 - -

1978 9 1 - 4

1979 8 1 - 2

1980

+

3 2 2 6

Total

"^Incomplete

51 18 3 16

Articles having information on both SiC and are counted twice

15
Table 5. Sources of Published Articles Having Data Relevant

to this Survey:

Publication

Materials

• Si ?NA SiC Sialon

J. Amer. Ceram Soc. 13 6 -

Bull. Amer. Ceram. Soc. 6 1 -

J. Mater. Sci. 14 3 1

Powder Met. Int. 3 3 1

Ber. Deut. Keram. Ges. 4 1 -

Met. Trans. A.I.M.E. 1 - -

Special Ceramics 1 - -

Matls. Sci. and Eng. - 1 -

Symposia 6 3 -

Acta Met. - - -

Phil. Mag. - - -

Reports and unpubl. man. 3 1 1

Scripta Met. - — —

Mechanisms

2

5

1

6

2

16
5 . Critical Evaluation of the Available Literature:

The purpose of this evaluation is to summarize the amount

and type of information that is available, to identify areas in

which more complete knowledge would aid the understanding of

creep and fracture mechanisms, and to examine critically the prevailing views on high temperature failure mechanisms.

It must be pointed out, however, that the materials of

interest in this survey are still under development. There are

several methods of producing them and a host of possible compositions. Thus, in making generalizations about them, it is

usually necessary to treat broad classes, and only occasionally

is it possible to refer to a particular material with a wellcharacterized structure and set of properties. A simple nomenclature that seems to pervade the literature defines the following

materials:

Silicon Nitride

A. Reaction-bonded (RBSN) - parts are made by cold-pressing

or injection-molding silicon powder and reacting it with

nitrogen at approximately 1200°C.

B. Hot-pressed (HPSN) - billets are made by hot-pressing

Si3N4 powder with a few percent of MgO, AI2O3 or other

oxides as sintering aids.

C. Sintered - this is a fairly recent development of G.T.E.

laboratories. The material contains 6% Y2O3 as a sintering aid.

17
Silicon Carbide

A. Hot-pressed (HPSC) - sintering aids, such as boron or

oxides, are required.

B. Sintered (SSC) - boron and carbon additions permit

pressureless sintering.

| C. Direct-bonded (DBSC) ("KT" or "Refel" are trade names) -

a porous, partially sintered body of Sic is infiltrated

with molten Si and reacted with carbon.

Sialons

These materials can be made by hot-pressing or sintering,

and a wide range of compositions is possible. The starting

materials are AI 2 O3 , AlN and sintering aids such as MgO

5.1 Creep

The creep test is a simple and direct measure of the rate of

deformation of a specimen under stress. The "creep strength" is

usually the controlling engineering property of materials used at

high temperatures. The two objectives of creep testing are to

answer the questions:

a) What is the life expectancy of a structure under

load? and

b) What is the deformation mechanism?

Both questions can usually be answered by a combination of the

simple creep test, a well established analytical method and an

18
elaborate theory rooted in absolute reaction-rate theory. The

analytical method, in its simplest form, employs two ways of

plotting creep data. The first plot (on logarithmic axes), of

creep-rate versus stress with temperature as a parameter, usually

gives straight lines, the slopes of which are related to the

deformation mechanism. These plots also clearly display the

relative creep resistance of different materials and are therefore an indispensable part of engineering design.

The second plot, of creep rate versus reciprocal temperature with stress as the parameter, reveals the activation energy

of the rate-controlling process and thereby helps to identify

the deformation mechanism.

The analysis of creep data is a time-honoured activity and

has resulted in good empirical methods for characterizing the

-effects of temperature, stress and some of the microstructural

variables, such as grain-size and porosity, on creep-rate. The

theory of creep has evolved steadily over the past thirty years

and has become more complex and detailed as it tried to embrace

the growing wealth of experimental data. It has provided mechanistic models for the processes of creep and analytical methods

for testing a set of creep-data to identify the mechanisms. It

is when the theories are validated by comparison with experiments,

that certain misgivings arise. The validation process involves

comparison of measured thermodynamic quantities, such as activation

19
enthalpies and activation-volumes, and of course creep-rates,

with those predicted by calculation from a model. The basic

physical mechanism is seldom observed directly, it is only perceived through its thermodynamic properties. The model may be

too simple or even wrong and still have approximately the correct

. thermodynamic behaviour. For the materials of this report, the

analysis is even more awkward, because the basic thermodynamic

properties (activation-enthalpies and volumes for self-diffusion)

are not known. Furthermore, while the models assume essentially

homogeneous microscopic processes, direct observations seem to

reveal heterogeneous, macroscopic phenomena. Nevertheless, careful metallographic studies in conjunction with creep-tests have

developed a consensus that creep in silicon nitride (particularly

HPSN) is controlled by a glassy phase in the grain boundaries.

This has led to elaborate efforts to remove or to crystallize the

gi assy phase and to devise theoretical models for creep in materials

that deform in this manner. There has been some success in both

areas, although it would appear that success in the first might

. render the second unnecessary.

The actual mechanism of creep in HPSN (and to varying degrees

I in the other materials covered by this report) is rather complex.

The creep deformation may be due largely to extensive cracking

and cavity formation, processes that are difficult to model. The

creep rate is almost ten times faster in tension than in compression

I 20
(in HPSN) , suggesting the need for tensile stress to open cavities. The creep rate of HPSN is about five times faster in

air than in argon or vacuum, which is very difficult to reconcile with a bulk process. (Sialons also creep faster in air than

in argon.) No tests of the effects of environment on the creep

of SiC were discovered in this survey. Since HPSC has good creepresistance, this might be a fruitful area for research.

The following generalizations can be made about creep in

the materials considered in this study.

A. The order of creep resistance is roughly as follows

1. HPSC

2. Sintered SiC

3 . RBSN

4. Sialons

5. HPSN - Y203

6. HPSN - MgO

However, there is considerable variation among materials

of a particular class.

B. The deformation mechanism involves boundary sliding,

cavitation and possibly a small amount of plastic

. deformation at triple points.

C. The rate of creep in some of these materials is sensitive to environment.

21
5.2 Subcritical Crack-Growth.

One of the early disappointments with the silicon compounds

was that they were not as refractory as expected. The strength

fell precipitously in the temperature range 1200° to 1400°C.

Even then it was recognized that the weakening was probably due

to an intergranular phase. Now it is widely accepted that the

drop in strength as well as the low creep resistance of some

grades of these materials is due to subcritical crack-growth (SCG)

in the boundary phase. As a result, the relatively new techniques

of fracture mechanics have been mobilized to study this phenomenon. The objectives of such studies are to develop an understanding of this type of subcritical crack-growth and to use the analytical failure-prediction methods that fracture mechanics provides.

Fracture mechanics tests yield two types of data, V-K plots

and Klc values. The first is just a plot of crack velocity

versus stress intensity, usually on logarithmic co-ordinates,

while the second is the stress intensity at which fast fracture

occurs. Table 6 shows the number of papers having data of this

kind

:

22
Table 6. Number of Papers in which High Temperature Fracture

Mechanics Measurements have been Reported:

Material

HPSN

V-K K

1SL

13

RBSN

HPSC

SSC

Sialons

3

*

Three attempts, no SCG observed.

The preponderance of data has been obtained from HS-130,

which is a hot-pressed silicon nitride containing about 5% MgO.

No V-K plots for RBSN were found. Since the strength of this

material is insensitive to temperature, it may be relatively immune to subcritical crack-growth. The name appears to be true

of sintered silicon carbide, in which several attempts were made

but no SCG was found.

The effects of environment on SCG were reported only in one

study on two grades of silicon carbide, HPSC and DBSC. Subcritical crack-growth was observed on the hot-pressed material at

temperatures below 1000°C. It was not observed in the directbonded materia] . The effects of environment on SCG appear to

have been neglected and may provide useful information about the

fracture mechanisms. The fact that the creep-rate in silicon

23
nitride is faster in air than in inert environments, and that

creep may be no more than generalized cracking, suggests a process of environment-assisted crack-growth.

One of the objectives of this survey was to identify the

data suitable for the construction of fracture mechanism maps.

Although Table 6 suggests that a substantial amount of data is

available, it i:? obtained from a large number of different materials, including numerous examples of HPSN. The data are generally

adequate only for very rudimentary maps. The real test of what

is available and what is needed will have to await the mapping

process itself.

5.3. Mechanisms of Subcritical Crack-Growth.

The phenomenon of grain boundary separation by cavity formation was recognized about twenty years ago. It occurs in many

materials, metals and ceramics. Several contributing phenomena

are believed to be involved. One is simply the ability of grain

boundaries to attract second phases, and another is the related

ability of boundaries to act as paths for rapid diffusion.

The presence of second phases on grain boundaries, when the

second 'phase is weak or undeformable, gives rise to two phenomena,

cracking and void formation. If the second phase is weak or has

a weak interface with the matrix, cracks may form even though the

matrix is quite ductile. If the second phase is simply undeformable.

24
the plastic deformation in the surrounding grains may result in

microvoid formation around the second phase particles.

The most persistent and invidious effect of grain boundary

phases in ceramics is that they tend to soften at a lower temperature than the grains. This permits shear stresses between grains

to cause sliding (relative displacement of neighboring grains),

and tensile stresses to cause cavitation in the boundary phase.

Grain boundary sliding can only be accomodated if the grains

themselves deform, and in the silicon-based ceramics this is

very difficult. In fact, the grains remain essentially rigid

and wedge each other apart at the corners, producing cavities.

Even when grain boundary phases are absent, when the grains

are directly bonded to each other, a sort of sliding is possible.

The boundary material itself, constituting only of only a few unit cells

on each side of the interface, is less resistant to shear stresses

at high temperatures than are the grain interiors. Thus, even

the elimination of second phases on grain boundaries does not

remove the possibility of sliding and cavitation. This has led

to heroic measures to control microstructure in refractory metals.

These include directional solidification, to avoid having any

grain boundaries normal to the maximum tensile stress, and single

. crystal structures, to avoid having grain boundaries at all. The

silicon ceramics do not appear to be amenable to these measures

as yet, although the possibility of fabricating oriented components

25
by vapour deposition should probably receive more attention.

The objective of theoretical treatments of the deformation

of ceramics at high temperatures is to develop constitutive

equations, i.e. equations that describe internal phenomena, such

as strain and strain rate, in terms of external conditions such

as stress and temperature. Success in this endeavour has two

benefits. The first is that it gives the theoretician satisfaction in knowing that he has probably modelled the system correctly.

The second and more practical benefit is that it gives working

relationships that can be used to predict behaviour under varied

conditions

.

Two types of models are used to account for the growth of

cavities on grain boundaries in materials. One relies on the

flow of .matter away from the cavity by diffusion, and the other

relies on the viscous flow of a semi-liquid grain boundary phase.

The diffusive growth of cavities and crocks has been studied for

the longest time. Early attempts to understand this phenomenon

were based on simple one-cavity models, and these have been

steadily elaborated through the years. They now incorporate the

concepts of fracture mechanics to model the stress field and the

effects of interactions between cavities. In a recent statistical

model (Evans and Rana) , an attempt has been made to include the

probability of occurrence of boundary cavities and a mechanism

for linking them to form cracks.

26
These approaches to the problem of cavity growth by diffusive flow appear to be based on sound principles and to be evolving toward a widely acceptable theory. They do, however, appear

to suffer from a scarcity of model experiments against which to

test their constitutive relations. There appears to be ground

here for fruitful interaction between experimenters and theoreticians.

The growth of cavities and cracks by viscous flow appears

to be even more difficult to model than the dif fusive-flow case.

However, attempts have been made to calculate the constitutive

relations (Evans and Rana, Evans), and to do model experiments

(Raj and Dang) . The calculations are difficult to verify,

because much of the needed experimental information, such as

viscosity and its temperature dependence., and the distribution

of void ^nuclei, are simply lacking. Even the distribution of

the viscous phase is known only in a few cases, and these are

the poorest materials, in which the viscous phase is relatively

thick. Thus, a great need in this area is a more detailed knowledge of the physical and chemical properties of grain boundary

phases.* A further need is for model experiments in which fracture mechanisms are carefully examined.

27
6. Summary of the Survey;

6.1 Eighty-eight papers having information relevant to this

survey were partially or completely copied. There were three

times as many papers on as on SiC and only three papers

on Sialons. Theoretical studies on fracture mechanisms may be

the most active area of current research.

6.2 The following appear to be areas where research would be

particularly fruitful:

A. Effect of atmosphere on creep rate.

B. Creep mechanisms in silicon carbide and RBSN.

C. High temperature V-K plots: the effects of

temperature, atmosphere and structure in all

three materials.

D. Model experiments on cavitation crack growth

in grain boundaries.

E. Direct measurement of the physical and chemical

properties of grain boundary phases.

28
Appendix : A list of titles of items relevant to this report. A set of

partial or complete copies is supplied with the report.

Silicon Nitride

1. R.M. Arons, J.K. Tien and E.M. Lence, "Observation of

Viscoelastic Strain Recovery in Hot-Pressed Silicon

Nitride", J. Am. Ceram. Soc., Vol. 62, No. 3-4,

216-217 (1979).

2. J.M. Birch and B. Wil shire, "The Compression Creep Behaviour of Silicon Nitride Ceramics", J. Mater. Sci.,

Vol. 13, 2627-2636 (1978).

3. J.M. Birch, B. Wilshire, D.J.R. Owen and D. Shantaram,

"The Influence of Stress Distribution on the Deformation and Fracture Behaviour of Ceramic Materials Under

Compression Creep Conditions", J. Mater. Sci., Vol. 11,

1817-1825 (1976).

4. W.C. Bourne and R.E. Tressler, "Molten Salt Degradation of

Si 3 N4 Ceramics", Bull. Amer. Ceram. Soc., Vol. 59, No. 4,

443-452 (1980).

5. D.R. Clarke and G. Thomas, "Grain Boundary Phases in a HotPressed MgO Fluxed Silicon Nitride", J. Am. Ceram, SOc.,

Vol. 60, No. 11-12, 491-495 (1977).

29
6 . J. A . Coppola, R.C. Bradt, D.W. Richerson and R.A. Alliegro,

"Fracture Energy of Silicon Nitrides", Bull. Amer. Ceram.

Soc., Vol.51, No. 11, 847-851 (1972).

7. W.J. Croft and I.B. Cutler, "Review of Silicon Nitride",

ONR Report, No.R-16-73, July 1973.

8 . P.J. Dixor.-Stubbs and B. Wil shire, "Creep of Hot-Pressed

Silicon Nitride Ceramics", J. Mater. Sci., Vol.14,

2773-2774 (1979).

9. W. Engel and F. Thtlmmler, "Kriechverhalten von reaktionsgesintertem Si-^^ bei 1200° bis 1400°C" , (Creep Behaviour of Reaction-Sintered Si^N^ Between 1200° and

1400°C), Ber. Dt. Keram. Ges., Vol.50, No. 6 , 204 (1973).

10. A.G. Evans, "High-Temperature Slow Crack Growth in Ceramic

Materials", 373ff., in Ceramics for High-Performance

Applications , edited by J.J. Burke, A.E. Gorum and

R.N. Katz, Proc. Conf., Hyannis, Mass., 1973.

11. A.G. Evans, L.R. Russell and D.W. Richerson, "Slow Crack

Growth in Ceramic Materials at Elevated Temperatures",

Met. Trans., V0I. 6A, 707ff. (Apr. 1975).

12. A.G. Evans and S.M. Wiederhorn, "Crack Propagation and Failure

Prediction in Silicon Nitride at Elevated Temperatures",

J. Mater. Sci., Vol.9, 270-278 (1974).

30
13. S.T. Gonczy and D.L. Johnson, "Impact Fracture of Ceramics

at High Temperature", 495ff., in Fracture Mechanics

of Ceramics , Vol.3, edited by R.C. Bradt, D.P.H. Hasselman

and F.F. Lange, Plenum Press, N.Y., 1978.

14. R.K. Govila, K.R. Kinsman and P. Beardmore, "Phenomenology

of Fracture in Hot-Pressed Silicon Nitride", J. Mater.

Sci., Vol.14, 1095-1102 (1979).

15. G. Grathwohl, F. Porz and F. Thtlmmler, "Beitrag zur Entwicklung von kriechfestem reaktionsgesintertem Siliciumnitr id"

(Development of creep-resistant reaction-sintered silicon

nitride), Ber. Dt. Keram. Ges., Vol.53, No. 10, 346 (1976).

16. G. Grathwohl and F. Thtlmmler, "Kriechen von unter

oxidierenden und nichtoxidierenden Bedingungen" (the creep

of Si^N^ under oxidizing and non-oxidizing conditions),

Ber. Dt. Keram. Ges., Vol.52, No. 8, 268 )1975).

17. G. Grathwohl and F. Thtlmmler, "Creep of Reaction-Bonded

Silicon Nitride", J. Mater. Sci., Vol.13, 1177-1186 (1978).

18. J.L. Henshall, D.J. Rowcliffe and J.W. Edington, "The Fracture

Toughness and Delayed Fracture of Hot-Pressed Silicon

Nitride", in Special Ceramics 6, edited by P. Popper,

Brit. Ceram. Res. Assoc., Stoke-on-Trent, 1974.

19. G. Himsolt, H. Knoch, H. Huebner and F.W. Kleinlein,

"Mechanical Properties of Hot-Pressed Silicon Nitride

with Different Grain Structures", J. Am. Ceram. Soc.,

Vol.62, No. 1-2, 29-32 (1979).

31
20. H.M. Jennings, B.J. Dalgleish and P.L. Pratt, "Relationship of Microstructure and Temperature to Fracture

Mechanics Parameters in Reaction Bonded Silidon Nitride",

Ber. Dt. Keram. Ges., Vol.55, No. 8, 394-397 (1978).

21. R. Kossowsky, "The Microstructure of Hot-Pressed SiliconNitride", J. Mater. Sci., Vol.8, 1603-1615 (1973).

22. R. Kossowsky, "Creep and Fatigue of S^N^ as Related to

Microstructures", 347ff., in Ceramics for High-Performance Applications , edited by J.J. Burke, A.E. Gorum

and R.N. Katz, Proc. Conf. Hyannis, Mass., 1973.

23. R. Kossowsky, D.G. Miller and E.S. Diaz, "Tensile and Creep

strengths of Hot-Pressed SijN^', J. Mater. Sci., Vol.10,

983-997 (1975).

24. O.L. Krivanek, T.M. Shaw and G. Thomas, "The Microstructure

and Distribution of Impurities in Hot-Pressed and

Sintered Silicon Nitrides", J. Am. Ceram. Soc., Vol.62,

No. 11-12, 585-590 (1979).

25. F.F. Lange, "Evidence for Cavitation Crack Growth in. S^N^",

J. Am. Ceram. Soc., Vol.62, No. 3-4, 222-223 (1979).

26. F.F. Lange, "High Temperature Strength Behaviour of HotPressed Si^N^: Evidence for Subcritical Crack Growth",

J. Am. Ceram. Soc., Vol.57, No. 2, 84-87 (1974).

27. F.F. Lange, E.S. Diaz and C.A. Anderson, "Tensile Creep

Testing of Improved Si-jN4

", Bull. 'Amer. Ceram. Soc.,

Vol.58, No. 9, 845-846 (1979).

32
28. L.K.V. Lou, T.E. Mitchell and A.H. Heuer, "Discussion of

'Grain Boundary Phases in a Hot-Pressed MgO Fluxed

Silicon Nitride'", J. Am. Ceram. Soc., Vol.61, No. 9-10,

462-464 (1978).

29. J. A. Mangels, "High Temperature, Time Dependent Physical

Property Characterization of Reaction Sintered Si-^N^"

,

in Nitrogen Creamics , edited by F.L. Riley, NoordhoffLeyden, 1977.

30 . M.G. Mendiratta and J.J. Petrovic, "Slow Crack Growth from

Controlled Surface Flaws in Hot-Pressed Si^N^",

J. Am. Ceram. Soc., Vol.61, No. 5-6, 226-230 (1978).

31. M.G. Mendiratta, J. Wimmer and I. Bransky, "Dynamic Kjc

and

Dynamic Flexual Strength in HS-130 S^N^', J. Mater.

Sci., Vol.12, 212-214 (1977).

32 . D.R. Mosher, R. Raj and R. Kossowsky, "Measurement of Vis-

' cosity of the Grain-Boundary Phase in Hot-Pressed

Silicon Nitride", J. Mater. Sci., Vol.ll, 49-53 (1976)

33. J.A. Palm and C.D. Greskovich, "Thermomechanical Properties

of Hot-Pressed Si 2 ^ gBe0 # #8 0

q 2 Ceramic", Bull. Amer

Ceram. Soc., Vol.59, No. 4, 447-452 (1980).

34 . J.J. Petrovic, L. A. Jacobson, P.K. Talty and A.K. Vasudevan

"Controlled Surface Flaws in Hot-Pressed Si^N^",

J. Am. Ceram. Soc., Vol.58, No. 3-4, 113-116 (1975).

33
35. B.D. Powell and P. Drew, "The Identification of a GrainBoundary Phase in Hot-Pressed Silicon Nitride by Auger

Electron Spectroscopy", J. Mater. Sci., Vol.9, 1867-1870,

(1974).

36. W.A. Sanders, "High Temperature Compressive Cracking in

Hot-Pressed Silicon Nitride", J. Am. Ceram. Soc.,

Vol.61, No. 5-6, 278-279 (1978).

37. M.S. Seltzer, "High Temperature Creep of Silicon-Base

Compounds", Bull. Amer. Ceram. Soc., Vol.56, No. 4,

418-423 (1977).

38. H. Suzuki, T. Iseki and M. Mori, "Fracture of Hot-Pressed

Silicon Nitride in Bending", unpublished manuscript, 1980.

39. N.J. Tighe, "Examination of Fracture Interfaces in Silicon

Nitride", from 33rd Ann. Proc. Electron Microscopy Soc.

Amer., Las Vegas, Nevada, 1975, editor G.W. Bailey.

40. N.J. Tigho, "The Structure of Slow Crack Interfaces in

Silicon Nitride", J. Mater Sci., Vol.13, 1455-1463 (1978).

41. G.G. Trantina, "Strength and Life Prediction for Hot-Pressed

Silicon Nitride", J. Am. Ceram. Soc., Vol.62, No. 7-8,

377-330 (1979).

42. A. Tsuge and K. Nishida, "High Strength Hot-Pressed S^^

with Concurrent Y2O3 and AI 2 O3 Additions", Bull. Amer.

Ceram. Soc., Vol.57, No. 4, 424 (1978).

43. S. Ud Din and P. Nicholson, "Creep of Hot-Pressed Silicon

Nitride", J. Mater. Sci., Vol.10, 1375-1380 (1975).

34
44. S. Ud Din and P. Nicholson, "Creep Deformation of ReactionSintered Silicon Nitrides", J. Am. Ceram. Soc., Vol.58,

No. 11-12, 500-502 (1975).

45. J.E. Weston and P.L. Pratt, "Crystallization of Grain Boundary

Phases in Hot-Pressed Silicon Nitride Materials",

J. Mater. Sci., Vol.13, 2147-2156 (1978).

Silicon Carbide

1. J.A. Coppola and R.C. Bradt, "The Fracture Energy of Silicon

Carbide from -195. 8°C to 1500°C" , in Mechanical Behavior of • Materials , Vol.IV, Proc. Int. Conf. on Mech.

Behavior of Mat., Kyoto, Japan, 1971.

2. A.G. Evans and F.F. Lange, "Crack Propagation and Fracture

in Silicon Carbide", J. Mater. Sci., Vol.10, 1659-1664,

(1975).

3. T.D. Gulden, "Mechanical Properties of Polycrystalline /9 -SiC"

,

J. Am. Ceram. Soc., Vol.52, No. 11, 585-590 (1969).

4. J.L. Henshall, D.J. Rowcliffe and J.W. Edington, "Fracture

Toughness of Single-Crystal Silicon Carbide", J. Am.

Ceram. Soc., Vol.60, No. 7-8, 373-375 (1977).

5. J.L. Henshall, D.J. Rowcliffe and J.W. Edington, "Kxc and

Delayed Fracture Measurements on Hot-Pressed SiC"

,

J. Am. Ceram. Soc., Vol.62, No. 1-2, 36-41 (1979).

35
6. V. Krishnamachari and M.R. Notis, "Interpretation of HighTemperature Creep of SiC by Deformation Mapping Techniques", Mater. Sci. and Eng., Vol.27, 83-88 (1977).

7. J. Kriegesmann, "Biegefestigkeitsuntersuchungen an heissgepresstem Siliciumcarbid" (Bending strength of hotpressed Sic), Ber. Dt. Keram. Ges., Vol.55, No. 8,

391-430 (1978).

8. P. Marshall and R.B. Jones, "Creep of Silicon Carbide",

presented Powder Met. Int., Vol.12, No. 23, 193ff. (1969).

9. K.D. McHenry and R.E. Tressler, "High Temperature Dynamic

Fatigue of Hot-Pressed SiC and Sintered ct-Sic",

Bull. Amer. Ceram. Soc., Vol.59, No. 4, 459-461 (1980).

10.

K.D. McHenry and R.E. Tressler, "Subcritical Crack Growth in

Silicon Carbide", J. Mater Sci., Vol.12, 1272-1278 (1977).

11. K.D. McHenry and R.E. Tressler, "Fracture Toughness and

High- Temperature Slow Crack Growth in SiC" , J. Am.

Ceram. Soc., Vol.63, No. 3-4, 152-156 (1980).

12. R. Stevens, "Temperature Dependence of Fracture Effects in

Self-bonded SiC", J. Mater. Sci., Vol.6, 324-331 (1971).

13. G.G. Trantina and C.A. Johnson, "subcritical Crack Growth in

Boron-Doped SiC", J. Am. Ceram. Soc., Vol.58, No. 7-8,

344-345 (1975).

36
Sialon

1, B.S.B. Karunaratne and M.H. Lewis, "High-Temperature

Fracture and Diffusional Deformation Mechanisms in

Si-Al-O-N Ceramics", J. Mater, Sci., Vol.15, 449-462,

(1980)

.

2 , M.H. Lewis and B.S.B. Karunaratne, "Determination of HighTemperature Kj-V Data for Si- M-O-N Ceramics", unpublished manuscript, 1980.

Combined Silicon Nitride and Silicon Carbide or Sialons

1. N. Claussen and C.P. Lahmann, "Fracture-Behavior of Some

Hot-Pressed Si 3 N4 Ceramics at High Temperatures",

Powder Met. Int., Vol.7, No. 3, 133-135 (1975).

2. J.W. Edington, D.J. Rowcliffe and J.L. Henshall, "The

> Mechanical Properties of Silicon Nitride and Silicon

Carbide. Part Is Materials and Strength.", Powder

Met. Int., Vol.7, No. 2, 82-96 • (1975)

.

3. J.W. Edington, D.J. Rowcliffe and J.L. Henshall, "The

Mechanical Properties of Silicon Nitride and Silicon

Carbide. Part II: Engineering Properties.", Powder

Met. Int., Vol.7, No. 3, 136-146 (1975).

4. F. F. Lange and J.L. Iskoe, "High-Temperature Strength Behavior

of Hot-Pressed Si 3 N4 and SiC: Effect of Impurities",

223-237, in Ceramics for High-Performance Applications .

edited by J.J. Burke, A.E. Gorum and R.N. Katz, Proc. Conf

Ityannis, Mass., 1973.

37
5. K.D. McHenry, T.M. Yonushonis and R.E. Tressler, "LowTemperature Subcritical Crack Growth in Sic and SijN^"

,

J. Am. Ceram. Soc. , Vol.59, No. 5-6, 262-263 (1976).

6. K.D. McHenry, T.M. Yonushonis and R.E. Tressler, "Subcritical Crack Growth in SiC and S^N^ Ceramics",

1503-1507, in Proc. of Second Int. Conf. on Mech.

Behavior of Mat., Boston, Mass., 1976.

Mechanisms

1 .

2 .

3 .

4 .

5 .

6 .

7 .

T-j. Chuang, K. Kawaga, J. Rice and L.B. Sills, "Non-Equilibrium Models for Diffusive Cavitation of Grain Interfaces", Acta Met., Vol.27, 265-284 (1979).

F.W. Crossman and M. F. Ashby, "The Non-Uniform Flow of

Polycrystals by Grain-Boundary Sliding Accomodated

by Power-Law Creep", Acta Met., Vol.23, 425-440 (1975).

A.G. Evans, "High Temperature Cavity Growth in Ceramics",

unpublished manuscript, 1980.

A.G. Evans and A. Rana, "High Temperature Failure Mechanisms

in Ceramics", Acta Met., Vol.28, 129-141 (1980).

R.J. Fields and E.R. Fuller, Jr., "Crack Growth Mechanism

% Maps", to appear in Int. Conf. Frac. #5, Cannes, 1981.

R. Raj and M.F. Ashby, "Intergranular Fracture at Elevated

Temperature", Acta Metallurgica, Vol.23, 653ff. (1975).

R. Raj and S. Baik, "Creep Crack Propagation by Cavitation

Near Crack Tips", to appear in Metal. Sci. Jour.

38
8. R. Raj and C.H. Dang, "De-adhesion by the Growth of Pennyshaped Bubbles in an Adhesive Layer", Phil. Mag.,

Vol.32, No. 5, 909-922 (1975).

9. J.R . Rice and T-j. Chuang, "Energy Variations in Diffusive

Cavity Growth", U.S. Dept, of Energy: E(ll-l) 3084-70,

June 1979.

10. K. Sadananda, "A Theoretical Model for Creep Crack Growth",

Mfit. Trans, a, Vol.9A, 635 (1978).

11. T. Takasugi and V. Vitek, "Effect of Surface Diffusion on

Creep Fracture", unpublished manuscript, 1980.

12. V. Vitek, "A Theory of Diffusion Controlled Intergranular

Creep Crack Growth", Acta Met., Vol.26, 1345-1356 (1978).

13. V. Vitek, "Diffusional Growth of Intergranular Cavities in

a Uniform Stress Field and Ahead of a Crack-Like Stress

Concentrator", unpublished manuscript, 1980.

Charles vs. Dutton:

14. R.J. Charles, "Diffusion Controlled Stress Rupture of Polycrystalline Metals", Met. Trans A, Vol.7A, 1081-1089 (1976).

15. R. Dutton and M.P. Puls, "Comments on 'Diffusion Controlled

Stress Rupture of Polycrystalline Materials'", Scripta

•• Met., Vol.12, 847-852 (1978).

16. R.J. Charles, "Reply to 'Comments on Diffusion Controlled

Stress Rupture of Polycrystalline Materials'", Scripta

Met., Vol.12, 853-856 (1978).

39
3. Thermochemical Data (R. S. Roth, J. L. Waring, H. S. Parker,

and T. Negas)

Progress : Work was continued with Si 3 N 4 related compositions in

accordance with subtasks 3A, 3B. As mentioned previously, to minimize

volatility and to maintain chemical composition, a radio frequency

heated apparatus was constructed to heat specimens at pressures up to

300 psi. A Mo crucible and cover with an appropriate size hole for

pyrometer measurements served as the susceptor. This apparatus was calibrated against the melting point of platinum (1773 °C). A small

piece of high purity platinum was placed on single crystal sapphire

and the fusion characteristics were observed. An appropriate temperature

correction was applied for the system. Since Si 3 N 4 is known to be

volatile at high temperatures, a series of experiments were to be con- ducted to determine the weight loss at various pressures of gas

( 95%N 2

: 5%H 2 ) . A commercially prepared crucible of Si 3 N 4 was placed

in the Mo susceptor. The susceptor was covered with a Mo lid. The

test sample pellet of Si 3 N 4 was placed on another pellet of the same composition in the Si 3 N 4 crucible. The apparatus was heated to 1784 °C

for 1 hour at 15 psig of 95%N 2

:5%II 2. In this experiment the bottom

surface of the Si 3 N 4 crucible was in contact with the Mo susceptor and

apparently reacted catastrophically.

.It is possible that the 15 psig pressure was not sufficient to inhibit

the Si 3 N 4 decomposition to Si + N 2 and the Si reacted with the Mo susceptor. It is more likely, however, that Si 3 N 4 and Mo do not form an equilibrium join in the ternary system Si-Mo-N2

. Instead, MoSi 2 , Mo 3 Si 5 , etc. are in equilibrium with N 2 and modest over-pressure of gas would

merely enhance the equilibrium reaction between Si 3 N 4 and Mo. It is

desirable, therefore, to reduce the amount of contact between the

Si 3 N 4 specimens and the Mo crucible.

From the above, it was decided to forego the lower pressure experiment

and redo the experiment at a higher pressure of 200 psi. For this

experiment a Si 3 N 4 test pellet was weighed and placed on a similar Si 3 N 4 pellet was served as a setter. The setter in turn was placed on a Mo

ring in the crucible. The material was heated to about 1800 °C/200 psi

of gas ( 95N 2: 5%H 2 ) for one hour and the weight loss was found to be about

1.92%. On the basis of these results it was concluded that this is a feasible testing technique, since volatility and container reaction

problems were minimized. Si 3 N 4 mixed with other materials can now be

heated with some hope of successfully understanding the thermochemical

reactions occurring in the specimens.

Most sintering aids and inherent impurities lead to poor mechanical /thermal

properties of finished Si 3 N 4 as secondary phases are formed during processing.

Although contributing to densification these cause mechanical problems

when the ceramic undergoes in-service oxidation as such reactions produce

phase changes and large positive molar volume changes. Cerium oxide is an excellent densification aid. However, complex Ce 3+ silicates develop

during fabrication via the reduction process:

12Ce0 2 + ySi02 + zSi 3 N 4

-> 6Ce 2 0 3 + (3+y) Si02 + (z-1) Si 3 N 4 + 2N 2

40
During service, reduced cerium silicates oxidize to Ce02 + Si

2

- Associated

volume changes contribute to loss of mechanical integrity, therefore, at

least for this example, it would be desirable to maintain the Ce 3+ state

within an oxidizing environment. This can be accomplished by the addition

of Ta 2 0 5 which fixes or locks in Ce 3+. Additionally, pure Ta 2 0 5 does

not reduce under fabrication conditions and, therefore, might be expected

to interact only with cerium oxide and not the Si 3 N 4

.

Two compositions, 6Si 3 N 4 :Ce0 2 :Ta 20 5 and 3Si 3 N 4 :CeTa 3 0 9 , were weighed, mixed,

and pellets were calcined at 1350 °C for 20 hours in 95%N 2

: 5%H 2

. These

pellets were buried in powder of the same composition. The composition

6Si 3 N 4 :Ce0 2 :Ta 2 0 5 previously calcined at 1350 °C (see Table 1 for x-ray

diffraction data) was pressed into pellets for high temperature heat

treatments. The experimental procedure used was the same as outlined

previously. In this experiment both the test pellet and the setter melted

and reacted with the Mo setter when heated to 1800 °C in 200 psi of

95%N2 :5%H 2 for one hour. The process was repeated at about 1725 °C for

one hour at 200 psi 95%N 2 :5%H 2 . The resulting test pellet probably had

undergone a small amount of melting as it was distorted and adhered

to the setter. Although the distorted pellet was only 1 mm thick it was extremely hard and resistant to fracture.

Another pellet of the 1350 °C calcine mixture was heated similarly to about

1650 °C for one hour in 95%N 2 :5%H 2

. The reaction product was a rea- sonably dense, well formed pellet which did not appear to be affected

by the Mo environment. The x-ray pattern of this material (Table 1)

showed a new phase + $-Si 3 N 4

.

A test pellet and setter of 3Si 3 N 4 :CeTa 2 0 9 previously calcined at 1350 °C

were heated similarly to about 1700 °C for one hour at 200 psi 95%N 2: 5%H 2. The material when examined was found to be melted. A small portion

of this 1350 °C calcine material was reheated in a small Mo tube to • about 1550 °C for an hour in 200 psi 95%N 2

: 5%H 2. The x-ray diffraction

powder pattern appeared to be essentially single phase. This unknown

phase appears to be the same as that found to occur in the 6Si 3 N 4 :Ce02 :Ta 2 0 5 specimen heated to 1650 °C. The major d-spacings of this phase apparently

can be indexed on a hexagonal unit cell related to TaN. From these

results it would appear that the Si 3 N 4 reacted with the Mo tube and

that the remaining CeTa 3 0 9 perovskite was chemically reacted with N 2 to

form a (Ce,Ta)N solid solution.

Two compositions, 75Si 3 N 4:1 2. 5Y 2 0 3

: 12. 5Cr 2 0 3 and 75Si 3 N 4 :12.25La 2 0 3 :12.5Cr 2 0 3 were weighed, mixed, and calcined at 1350 °C - 16 hours, in 1 atm of

95%N2 : 5%H 2

. The x-ray diffraction data for these compositions is given

in Table 1

41
Table 1. Experimental Data for the System Si-N.-CeOo-Ta.jO,-.

Starting Material Temp. Time

°C hr

3Si

3

N

4

:CeTa

30g 1350 20

1350 20

1550 1

6Si

3

N

4

:Ce0

2

:Ta

2

0

5

1350 20

1350 20

1650 1

1350 20

1725 1

75Si'N

4

:12.5Y

2

0

3

: 12.5Cr

2

0

3

1350 20

75Si*N

4

:12.5La

2

0

3

: 12.5Cr

2

0

3

1350 20

Environment X-ray Diffraction Results

95%N2 :5%H2

(1 atm)

perovskite + (Ce,Ta)N

solid solution + a-Si

3

N4

95%N2 : 5%H 2

(13.6 atm]

(Ce,Ta)N solid solution

95%N2

:5m>

(1 atm)

a-Si

- 3 N

4 + (Ce,Ta)N solid

solution

95%N2 :5%H2

(13.6 atm)

(Ce,Ta)N solid solution +

6-Si

3

N

4

95%N2 :5%H2

(13.6 atm)

B-Si 3N4 + (Ce,Ta)N solid

solution

95%N2 :5%H«

(1 atm)

a + perovskite

95%N2 :5%Ho

(1 atm)

a + perovskite

42
From the results of the current quarter it is apparent that it is

very necessary to first understand the more simple two, three, and four

component systems involved before expecting to obtain useful thermochemical

data from specimens in the complex five and six-component systems. Therefore

it will be necessary to first examine such systems as Si-Mo-N 2 , Ce-Ta-N 2 , Si-Ta-N2 , Si-Ce-N 2 , Si-Ce-Ta-N 2 as well as the corresponding metal plus

02 systems before studying the metal -N 2 -02 systems.

Two commercially available forms of Si 3 N 4 were obtained for these studies.

The first was found to be a mixture of a-Si 3 N 4 and 3-Si 3 N 4

. The second was amorphous by x-ray diffraction. In an attempt to determine the

polytype(s) which form when this material crystallizes, amorphous Si 3 N 4 samples were heated to various temperatures. Some of the samples con- tained impurities to serve as "mineralizers" to enhance crystallization.

The results of these studies are given in Table 2. From these results

it can be seen that additional work will be necessary to properly

characterize this amorphous material.

Plans : (1) Study thermochemical reactions in the systems Si-Mo-N2 and

Ce-Ta-N2 to obtain knowledge of themochemical relations between test

specimens, container, and atmosphere. (2) Study high temperature

thermochemical reactions in the calcined Si 3 N 4 -La 2 0 3 -Cr 2 0 3 and

Si 3 N 4 -Y 2 0 3 -Cr 2 0 3 specimens. (3) Continue study of formation of

crystalline Si 3 N 4 in molten Si and silicon alloys to determine the

thermochemistry of the a and 3 phases.

43
Table 2. Experimental data for amorphous Si^

Starting Material

Amorphous Si ?NA +

b wt % Li0H7H

2

0

Amorphous ^i 3N4

Temp.

°C

Time

hr

Envi ronment X-ray Diffraction Results

700 16 N

2 amorphous

700 143 air Si0« + Li* Si* 0c + Li* SiO + unknown

1375 1 sealed- Mo tube a-SigN^ + Mo (silicides)

500 20 95%N

2

:5%H

2 amorphous

600 19 11 n 11

700 20 11 n n

800 20 11 n n

900 69 11 11 11

1000 21 ti 11 n

1375 64 n 11 cristobal ite

44
Task 4. Kinetics of Oxidation (T. Hahn)

Progress :

X-ray analysis has been completed on 17 3-Si 3 N4 plus garnet samples

prepared by R. Muller of Professor Tien's group at the University of

Michigan. The analyses of the "as-received" samples were made on a portion of the sample that was cut off the original billet obtained in

preparing the high temperature test specimens. After the samples were fracture tested at 1300 °C in air (total exposure at temperature approximately one hour), they were re-analyzed to determine what new phases formed due to the extra heat treatment. This analysis was limited to the surface of the samples in order to preserve the samples

for future SEM analysis. Thus the results reported are for the surface

film which may not be characteristic of the bulk sample. The change of

the 3-Si 3 N 4 peak intensities as compared to the as-received samples is a rough indication of the film thickness. Grading of the film thickness is as follows:

Thickest V13

' V2 , V 5, Vll, V 22, V 23

' V6, VI 4, -VI 7, V 20

' V3, V8, V9

' VI

6

Thinest VI, V4, V10

From Table 4.1, which list the composition, presintering heat treatment, hot pressing temperature, and post sintering heat treatment for

each of the samples, there was no obvious correlation between the past

history of the samples and the film thickness.

The various phases observed in each sampleare also listed in

Table 4.1. The major phase observed in the "as-received" samples was

3-Si 3 N 4 , as expected, while a distinct garnet phase was only observed in a few of the samples. A glassy phase was also optically observed on a few of the samples. A number of the samples contained some other

phases; however, their x-ray pattern was not intense enough for positive

identification.

After the 1300 °C fracture test in air a number of new phases were positively identified. These phases along with their relative amounts

are listed in Table 4.1. These amounts were determined by the intensity

of the x-ray pattern relative to the same phase in the other samples of

this set. The phases that were positively identified include 3-Si 3 N 4 3-cristobal ite, 3-Y 2 Si 20 7 (garnet), mullite, and a well developed but as yet unidentified phase. In general the samples with 20 wt. °l garnet,,

as opposed to the samples with 10 wt. % garnet, contained more of the

3-Y 2 Si 20 7 garnet phase, mullite, and the unknown phase. There was,

however, no obvious correlation between the amounts of the different

phases and the reported processing schedule supplied with the samples by

R. Muller of the University of Michigan. After further analysis of these

samples, the surface layer will be removed by polishing to better determine

the bulk composition of these samples.

45
Plans:

Work will continue on the high temperature furnace for exposing

the samples at elevated temperatures and the high temperature x-ray

furnace.

46
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