Analysis of Oxide and Oxide/ Matrix

Interfaces in Silicon Nitride

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Materials Science

Fracture and Deformation Division

Washington, DC 20234

July 1982

Interim Report

NBS-DOE Interagency Agreement EA-77-A-01 -601

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Prepared for 'department of Energy

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NBSIR 82-2574

ANALYSIS OF OXIDE AND OXIDE/MATRIX

INTERFACES IN SILICON NITRIDE

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Materials Science

Fracture and Deformation Division

Washington, DC 20234

July 1982

Interim Report

NBS-DOE Interagency Agreement EA-77-A-01 -601

Prepared for Department of Energy

Fossil Fuel Utilization Division

Washington, DC

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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ANALYSIS OF OXIDE AND OXIDE + MATRIX INTERFACES IN SILICON NITRIDE

Nancy J. Tighe

National Bureau of Standards

Washington, DC 20234

ABSTRACT

In order to understand the strength and microstructural changes that are

produced during oxidation, it is necessary to examine the oxide scale, the

oxidersil icon nitride interface and the silicon nitride below the oxide: matrix

interface. In the present study, these three interfacial layers were removed

and analyzed using transmission electron microscopy, light microscopy, x-ray

energy analysis and x-ray diffraction. Oxide scales were produced on hotpressed silicon nitride samples by heating in air at 1000 °C, 1200 °C, and

1400 °C for 1/2 to 1000 hr. The phases in the oxide scale were found to occur

in layers that were ordered according to the phase diagrams for the oxide

mixtures. Crystalline and amorphous phases were present in all specimens

examined. The oxynitride and amorphous phases are present in as-pressed

billets at triple junctions and along grain boundaries. The elements in the

amorphous phases were identified using energy dispersive x-ray analysis In

this paper, the phases found in the oxide scales are characterized and the

relationships between the oxide scale, the oxide:matrix interface and the

mechanical properties are discussed.

1
I. INTRODUCTION

Oxide phases occur in hot-pressed silicon nitride billets as a result of

processing and in silicon nitride components as a result of exposure to air at

elevated temperatures. During processing, the oxides act as sintering aids

and form as thin (approx. Inm) films around grains [1,2]; as massive second

phases at triple junctions; and, as fillers of void space [3]. The internal

oxides and the oxide scale formed during heating in air at elevated

temperatures consist of several discrete phases which can be either amorphous

or crystalline [3,4]. The oxide phases modify the silicon nitride

microstructure and can have the effect of strengthening or of weakening a

particular component depending on the formation parameters.

Silicon nitride is a brittle ceramic which fails under stress by crack

propagation from a flaw which has reached a critical size as defined by the

stress intensity factor [5]. At sufficiently high temperature the material

can creep under an applied stress and can fail by crack propagation and

plastic deformation. Cavitation and separation of grains along their

boundaries eventually leads to creep rupture [6,7]. The flaw population in

machined silicon nitride specimens includes cracks produced during surface

finishing; and, internal inclusions and pores which were produced during

processing of the original billet. Extensive measurements of strength and

analyses of the durability and reliability of hot-pressed silicon nitride

billets have demonstrated conclusively that the flaw population changes

continuously during heating in air [6,8,9]. The changes in flaw population

are divided into two categories (a) flaw healing, whereby the size and

effectiveness of initial flaws are reduced; and, (b) flaw generation, whereby

2
new flaws such as surface pits, grain boundary cracks and internal oxide

inclusions are produced. The effects of the new flaw population on strength

distribution are less predictible than effects of initial machining flaws

because they are produced in a time and stress dependent manner and are

changing continuously. New techniques have been developed to predict the

reliability of the materials [10] but will not be discussed in this paper.

The techniques used to identify flaw populations and their influence on the

material strength will be discussed.

In order to identify the microstructural changes that occurred during

heating in air we examined the oxide scale, the oxide : silicon nitride

interface and the silicon nitride beneath the oxide : matrix interface.

Samples were oxidized by heating in air at 1000°C, 1200°C and 1400°C for 1/2

to 1000 hours. The interfacial layers were removed and examined by

transmission electron microscopy, light microscopy, energy dispersive x-ray

spectroscopy, and in x-ray diffraction procedures. The strength changes that

were measured for silicon nitride after heating in air to cause oxidation

resulted from changes in microstructure near the oxide : matrix interface.

Because of the overwhelming evidence for diffusion from the matrix to the

outer oxide interfaces in this paper and in earlier papers [3] we have termed

this interface area the diffusion reaction zone.

II. EXPERIMENTAL PROCEDURE

Specimens examined during this study were made from billets of

hot-pressed magnesia-doped Si^N^ designated HS130 and NC132. Specimens thin

3
enough for examination in a transmission electron microscope were made from

oxidized specimens by sectioning them parallel to the oxide/sil icon nitride

interface; grinding the section to include the desired portion of the oxide

layer, polishing the thin section to show the oxide phases, and then

ion-thinning until the desired area was electron transparent.

The oxidation treatments were carried out on thin foil specimens as well

as on bulk samples. Small specimens were held vertically in a silicon nitride

boat and larger bars or plates were supported at the ends on silicon nitride

or silicon carbide plattens. Thus, there was no direct contact with an oxide

ceramic or a metal foil which would cause an undesirable chemical reaction.

Specimens were weighed and measured before and after oxidation in order to

record the oxide scale growth. The oxide scale on some specimens was

dissolved in HF+H

2 S 0^

in order to establish the depth of oxide penetration

into the matrix. Specimens from mechanical tests had been heated in air at

1000°C, 1200°C, and 1400°C for times of 1/2 to 1000 hr.

III. EXPERIMENTAL RESULTS

(1) OXIDE PHASES

When the surface and cross-sections of heavily oxidized specimens were

examined it was found that the oxide scale had formed in layers that differred

in com,position (Si2N20+Si02 and Si02+MgSi02) and in crystallinity. As shown

in Fig. 1, certain oxide phases could be distinguished easily by light

microscopy because of large differences in the index of refraction and the

reflectivity. In samples that were machined to have sharp edges^ corner or

4
edge cracks were extended during bending. The crack interfaces oxidized

preferential ly and were seen easily in polished sections. A plate 1 mm thick

which was oxidized at 1400 ‘‘•'C for 112 hours sagged as a result of the exposure

indicating that creep and plastic deformation had taken place. Thickness

measurements of the oxide scale, which were made from cross-sections of

hot-pressed (NO 130) samples heated at 1400 °C, are shown in Fig. 1(c). It is

seen that the scale thickness did not change appreciably from 4-24 hours

exposure; but, did increase for the 48 and 112 hour exposures. This is

consistent with the appearance of magnesium silicate phases in these specimens

as determined from X-ray powder diffraction patterns. The oxide scale on a

plate that was heated cyclically at 1400 °C for 96 hours showed globules of

silicate which were considered evidence for melting (Fig. lb). Such melting

can be attributed to the gradual change in composition of the oxide surface

which enhanced the oxidation rate compared with continuously heated samples.

Silicon Oxynitride : Si2N20[ll ,12] Silicon oxynitride, was found in the

hot-pressed billets in sufficient quantity to be identified in x-ray powder

diffraction patterns. When polished sections were examined in the light

microscope, the' oxynitride could be distinguished from the silicon nitride

matrix by its darker grey color which results from the difference in

reflectivity (r), and index of refraction (n): Si^N^ r=12.2, n=2.1; Si

2

N 20

r=8.5, n=1.82 [13]. In the light micrograph of Fig. Id, spherical oxide

nodules are visible below the matrix/oxide interface. The smallest nodules

are oxynitride. In the center of the larger nodules the silicate phase is

visible; thus, indicating a m.ore complete oxidation reaction. The position of

the nodules below the reaction interface is strong evidence for significant

diffusion of oxygen into the matrix at 1400 °C.

5
The electron micrographs in Fig. 2a, b show the structure of the

oxynitride phase in a thin foil specimen prepared from a section similar to

m-m in Fig. Ic. The lower magnification picture of Fig. 2a shows two

oxynitride grains with a region of silicon nitride grains between them. In

Fig. 2b the faults, pores and inclusions within one of the oxynitride grains

are imaged. It is apparent that the initial silicon nitride grains were

consumed by the growing oxynitride.

Cristobal ite-BSi02- Beta-cristobal i te was found along the silicon nitride

interfaces in specimens oxidized at 1000 ®C and throughout the oxide scale in

specimens oxidized at 1400 °C. The cristobalite was found in quenched samples

as well as in heated samples that were oxidized at 1000 °C and 1200 ®C in a

special high temperature x-ray furnace. The phase coexisted with an amorphous

phase which appeared to be silica from its diffraction pattern.

Figure 3 shows 6-cristobal i te which was produced in a thin foil by

heating for one hour at 1000 °C. The oxide grains, which were contained

within the amorphous layer, were identified by their diffraction patterns.

This figure demonstrates the major difficulty associated with examining Si02

in an electron microscope, namely the vitrification that occurs rather

quickly[14]. As shown in Fig. 3, the B-cri stobal i te transformed to a

disordered a-cristobal i te which then transformed to amorphous silica without

changing shape. A similar decomposition occurred in all of the samples

regardless of the oxidation conditions. This diffraction pattern of the

transformed amorphous silica was used as a standard pattern for silica glass.

6
Magnesium Si 1 icate-MgSiO^. The magnesium silicate phase, enstatite, was found

in sections of specimens that were oxidized at 1200 °C and 1400 °C. The

enstatite was found to coexist with 3-cristobal ite forming a wel 1 -sintered

layer. Figure 5 shows an example of the layer morphology. In this figure the

B-cristobal ite has vitrified as expected, while the enstatite remained

unchanged during long time electron irradiation. The enstatite grains were

usually faulted and had a microstructure similar to the natural mineral [15].

Both Cristobal ite and enstatite can be produced by devitrification of

magnesium silicate glass at 1400 °C [16]. At 1400 °C the stable crystalline

phase is protoenstatite while cl inoenstatite forms on cooling below 1000 °C.

Enstatite belongs to the mineral class of pyroxenes which vary in structure

according to the impurities in them. The element analysis shows that the

magnesium silicate layer contains Fe, Ca, A1 and other impurities commonly

found in hot-pressed silicon nitride. The presence of these impurities in the

oxide scale in quantities sufficient to form varients of the pyroxenes is

evidence for diffusion from the matrix to the oxide scale. In sampling

specimens from the mechanical test specimens, it was noted that the occurrence

of enstatite coincided with the rapid degradation of strength and the rapid

dissolution of the ground surface [8,9].

(2) DIFFUSION REACTION ZONE

The microstructure of the diffusion reaction zone (DRZ) was examined using

thin sections of specimens that were oxidized by heating in air 50 hours at

1100®C, 200 hours at 1200°C and 1 and 14 hours at 1400°C. Oxidation in the

diffusion reaction zone appeared to initiate at grain boundaries as evidenced

7
either by porosity or by thickening of a grain boundary phase. Figure 5 shows

an elongated pore between two grains in a sample oxidized at 1400 ®C. An

example of thickening of the oxide phase between grain boundaries is shown in

Fig. 6 where the breadth of the phase is indicated by the moire fringe

pattern. At this temperature, the evidence for creep and plastic deformation

in the bulk specimens was shown by extensive dislocation interactions in the

diffusion reaction zone. There is a significant increase in the occurrence of

grain boundary dislocations, sub-boundaries, and dislocation interactions with

inclusions as in Figs. 5 and 6b. Rapid growth of oxide at the grain

boundaries can produce considerable stress; and, this extended phase can

fracture on cooling to produce extensive grain boundary cracking and a new

flaw population.

Evidence for nucleation and growth of Si2N20 within the silicon nitride

grains is shown in Fig. 7. This foil was made from a specimen that was

oxidized for 24 hr. at 1000 ®C and had an oxide scale composed of amorphous

silica and 6-cristobal ite. The presence of such plates in the matrix grains

could reduce fracture toughness by providing easy intragranular fracture paths

along the fault planes.

The early stages of void or particle nucleation within silicon nitride

grains is seen in Fig. 8a. The specimen was oxidized for 50 hours at 1100°C.

The area shows dislocation loops and stacking faults lying in rhombohedral

planes. The small bubbles or precipitates are intersected by dislocations

passing through the bulk of the grain. Similar defects are seen in Fig. 8b

which shows a specimen that was oxidized during a static load test that lasted

8
1000 hours at 1200°C. These grains are approximately 20 ym from the oxide :

matrix interface.

(3) INCLUSIONS

The grinding particle inclusions seen in foils made from as-received

specimens, are usually polyhedral and range in size from a few nanometers to a

few micrometers. The W-based inclusions can be seen as dark masses in thin

sections ('v30ym) examined with a light microscope as in Fig. 9a. The spacial

distribution of the individual particles can be seen using the back-scattered

electron mode and the transmission mode of imaging in the scanning

transmission electron microscope. In the scanning transmission mode as in the

normal transmission mode the particles are imaged with the microstructural

background of grain boundaries as in Fig. 9b. By using the back-scattered

mode the heavy element particles are imaged in Fig. 9c, as dark against the

light background and the stereo pictures show three-dimensional distribution

of particles. Small particles are found along grain boundaries and within

grains. The larger particles are close to or larger than matrix grains.

Energy dispersive analysis shows Fe, Cr, Co, A1 , and W from the WC and WSi

2

particles and the metal alloy binder.

Oxidation at the inclusions broken off from the WC grinding media can

lead to the formation of oxide filled pits in the matrix and mounds or

depressions in the oxide scale. Such pits were shown to be active fracture

sites in specimens broken at 1200 °C; and the oxide scale at the pit was found

to have a high Fe content [8]. In oxidized specimens these inclusions may be

polyhedral or spherical and examples are shown in Fig. 10. The inclusion in

9
Fig. 10a is surrounded by an amorphous film and this early oxidation reaction

is a precursor to an oxide-filled pit. The large spherical particles in Fig.

10b had a high Fe and Cr content but low W content; and, these differences in

morphology and composition indicate possible melting or rapid volatization

during the exposure at 1400 ®C. The oxidation pits are new flaws which affect

the strength in a time dependant manner because they require an incubation

time to be produced. Since the distribution of the grinding particle

inclusions varies somewhat from billet to billet, the nucleation and growth of

the oxidation pits will vary also to change the flaw population.

IV. DISCUSSION

The results presented here represent the starting point for further

analyses of the relationships between microstructure and high temperature

mechanical properties of silicon nitride. Because this ceramic oxidizes, its

high temperature behavior as a structual material must be catagorized with its

oxidation behavior. By the time cavitation creep and finally creep rupture

occur the material has exceeded the limits of usefulness in any structural

application. The defects identified in the diffusion reaction zone appear to

be related to impurity content and therefore can give some clues to the

differences in behavior between billets of the same nominal composition and

manufacturing conditions. For example, the defects shown in Fig. 8 were not

found in a companion billet which exhibited slightly different strength

behavior. Cavities that nucleate and grow near and along grain boundaries

.create a new flaw population that can cause weakening and failure when the

cavities link up to produce grain boundary cracks.

10
The effect of flaws, such as machining cracks which intersect the

surface, on the strength can be changed by oxidation which blunts them or

shortens them. When the matrix is consumed by the growing oxide as shown in

Fig. 2 then the crack size is reduced and the strength is increased. The time

frame for such crack healing is related to the transformation of oxide scale

from oxynitride and silica to the silica and silicate. A sharp crack can be

blunted when the interfacial matrix grains oxidize. A blunt crack requires a

higher stress intensity factor for propogation which leads to a higher

strength. Additionally the strain relief resulting from annealing at 1200°C

can change the effective stress in ground surfaces and at crack tip. Thus any

of these three effects can cause a strengthening. We have shown here that, in

the billets studied oxidation can change the strength by reducing the

effectiveness of old flaws and by generating new flaws. The specific causes

of billet to billet variation in strength and oxidation behavior have not been

determined but are believed to be related to impurity content. The oxide

scales are similar in composition but have grown at considerably different

rates apparently as a result of minor impurity differences. The real effect

of annealing in the absence of oxygen will have to be examined in neutral

atmosphere tests.

The interaction of the different oxide phases with each other and with

the silicon nitride matrix is time and temperature dependent. As long as only

silicon oxynitride and crystalline and amorphous silica cover the exposed

surfaces, the oxidation rate is slow and the oxide scale is protective.

Diffusion of oxygen through the oxide scale is then rate controlling. When

sufficient diffusion of Mg and Ca impurities from the matrix has occurred to

permit growth of the pyroxene phase then the oxidation reaction can change

11
rapidly. The diffusion reaction zone illustrated in this paper is more

complex than the original matrix structure; and, more examples taken from

oxidized and deformed samples must be examined to fully characterize this

zone. Recent experiments with sialons (silicon-aluminum-oxynitride) have

shown that oxidation at 1400 °C for 1000 hr. can be beneficial to the strength

as a result of diffusional changes in the microstructure [16]. Specimens of

magnesia-doped hot-pressed silicon nitride which have undergone such long

annealing treatments during static fatigue tests show some strengthening but

these specimens have not been characterized completely to identify the sources

of the strengthening.

V. CONCLUSIONS

The oxide scale on magnesia-doped hot-pressed Si^N^ forms in layers

composed of amorphous and crystalline SiO^, Si2N20, and MgSiO^ (Mg0*Si02). A

diffusion reaction zone in the Si^N^ matrix is produced during oxidation; and,

structural changes such as cavitation in this zone affect strength. Flaw

healing and flaw generation occur as a consequence of the growth of oxide

phases at and below the Si^N^ ground surfaces during heating at temperatures

> 1000 °C.

12
REFERENCES

1. D. R. Clarke and G. Thomas, Grain Boundary Phases in flot-Pressed

MgO Fluxed Silicon Nitride, J. Amer. Ceram. Soc. 491-95 (1977).

2. A. H. Heuer, V. Lou, L. Ogbugji, and T. E. Mi.tchell, Lattice

Resolution Studies of Engineering Ceramics: SiC and Si^N^, J.

Micros. Spec. Electron 475 (1977).

3. N. J. Tighe, Microstructure of Oxidized Silicon Nitride, Proc. EMSA p.

470-71 (1974) Claitor Publ. Baton Rouge.

4. S. C. Singhal, Thermodynamics and Kinetics of Oxidation of Hot-Pressed

Silicon Nitride, J. Mat. Sci. U^, 500 (1976).

5. A. G. Evans and S. M. Wiederhorn, Proof Testing of Ceramic Materials, An

Analytical Basis for Failure Prediction, Int. J. Fracture, 379-92

(1974).

6. S. M. Widerhorn and N. J. Tighe, Effects of Oxidation on the Reliability

of Si^N^, in "Fracture Mechanics of Ceramics", Vol . 5 & 6, R. C. Brandt,

A. G. Evans, D. P. H. Hasselman and F. F. Lange, eds. Plenum Publ. 1982.

7. F. F. Lange, "Evidence for Cavitational Crack Growth in Si^N^^, J. Am.

Ceram. Soc. 62 [3-4], 222-23 (1979).

8. S. M. Wiederhorn and N. J. Tighe, Proof-Testing of Hot-Pressed

Silicon Nitride, J. Mat. Sci. j^, 1781 (1978).

9. S. M. Wiederhorn and N. J. Tighe, Effect of Flaw Generation on

Proof Testing, MCIC Report, March 1978 p. 689.

10. S. M. Wiederhorn, A Probabilistic Framework for Structural Design in

"Fracture Mecahnics of Ceramics, Vol. 5 & 6, R. C. Bradt, A. G. Evans,

D. P. H. Hasselman and F. F. Lange, eds. Plenum Publishing Corp., 1982.

13
11. I. Idrestedt and C. Brosset, Structure of Si2N20, Acta Chem. Scand.

1879 (1964).

12. C. A. Anderson, K. Keil, and B. Mason, Silicon Oxynitride: A Meteoritic

Mineral , Sci. 256 (1964).

13. D. Taylor, Reflected Light Microscopy of Silicon Nitride and Oxynitride,

T. Brit. Ceram. Soc. 7^, 319-21 (1973).

14. N. J. Tighe and J. M. Christie, Deformation Structures in Quartz Rocks,

Proc. EMSA p. 60-61 (1969) Claitor Publ . Baton Rouge.

15. P. E. Champness and G. W. Lorimer, Precipitation in an Orthopyroxene, J.

Mat. Sci. 8, 467 (1973).

16. L. Atlas, The Polymorphism of MgSiO^ and Solid-State Equilibria in the

System MgSi02"CaMgSi20g , J. Geol. 125 (1952).

17. M. H. Lewis and P. Barnard, Oxidation Mechanisms in Si-Al-O-N Ceramics,

J. Mat. Sci. 15, 443 (1980).

ACKNOWLEDGEMENTS

This work was supported in part by the Office of Coal Utilization, Div.

of Heat Engines and Heat Recovery, Department of Energy Contract EA77-A01-6010

Task A086. We thank C. M. McDaniel for the careful X-ray diffraction

analyses.

14
Fig. 1. Hot-pressed silicon nitride samples which were oxidized at 1400°C (A)

specimen heated continuously for 100 hr., (B) specimen heated and

cooled to room temperature several times during 96 hr., (C) polished

section of cyclically heated specimen note thickness variations in

oxide scale -0- and in Si^N. -S-. (D) nodules of Si2N20 below the

primary oxide-matrix interface spec. (E) plot of scafe'^thickness vs time for NO 130 specimens.

15
Fig. 2. Electron micrographs of sections from specimen heated 264 hr. at

1000°C showing (A) 2 Si^N^O grains within the Si^N^, (B) enlargement

of oxynitride grain showing faults, pores and precipitates.

16
Fig. 3. Thinned Si.N, foil after ^"9 (B)

:ii?]ffed'st!lc':D!E)'5]ffraction patterns of the transformation

eSiO<, aSiOp ^ amorphous Si 02 .

17
I

}

i

Fig. 4. Oxide scale from bulk specimen heated at 1400°C showing grains of

Si02 and MgSiO^, note the Si02 grain has vitrified.

18
Fig. 5. Silicon nitride grains in specimen heated 24 hr. at 1400°C, large

pore is seen along grain boundary (arrowed).

19
Fig. 6. Section from bar heated cyclically for 96 hr. at 1400°C showing;

(A) Silicon nitride grains in the diffusion reaction zone, moire

fringes along the boundary show position of the growing oxioe film.

(B) Dislocation interactions with inclusions near area in (A).

20
Fig. 7. Grain of Si^N, containing plates of Si2N20 in the diffusion reaction

zone of a specimen oxidized 24 hr. at io6o°C.

21
Fig. 8. Oxidation induced cavity and particle nucleation in the diffusion

reaction zone of NC132. (A) Oxidized 50 hours at 1100°C dislocation

loops lying in rhombohedral plans and are pinned at cavities. (B)

Oxidized 1000 hours at 1200°C during static load test. The cavities

are distributed through the grain thickness.

22
Fig. 9. Spacial distribution of inclusions in silicon nitride (a)

transmitted light showing dark inclusion masses, (b) scanning

transmission images showing inclusions and silicon nitride grains,

(c) stereo-pair of back-scattered electron images showing 3-D view

of inclusions.

23
Fig. 10. Inclusions in Si^N* specimen heated at 1400°C (A) in 1 hr. inclusion

is surrounded by porous and amorphous phase, indicating that

oxidation has started. (B) After 96 hr. of cyclic heating,

inclusions are spherical and show no W.

24
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4. TITLE AND SUBTITLE

Analysis of Oxide and Oxide + Matrix Interfaces in Silicon

Nitride

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Nancy J. Tighe

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

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i Document describes a computer program; SF-135, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual summary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here.) In order to understand the strength and microstructural changes that are produced

during oxidation, it is necessary to examine the oxide scale, the oxide; silicon

nitride interface and the silicon nitride below the oxide: matrix interface. In the

present study, these three interfacial layers were removed and analyzed using transmission electron microscopy, light microscopy, x-ray energy analysis and x-ray

diffraction. Oxide scales were produced on hot-pressed silicon nitride samples by

heating in air at 1000 ®C, 1200 °C, and 1400 °C for 1/2 to 1000 hr. The phases in

the oxide scale were found to occur in layers that were ordered according to the phase

diagrams for the oxide mixtures. Crystalline and amorphous phases were present in all

specimens examined. The oxynitride and amorphous phases are present in as-pressed

billets at triple junctions and along grain boundaries. The elements in the amorphous

phases were identified using energy dispersive x-ray analysis. In this paper, the

phases found in the oxide scales are characterized and the relationships between the

oxide scale, the oxide:matrix interface and the mechanical properties are discussed.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Silicon nitride; electron microscopy; oxidation; microstructure; analysis; TEM; STEM;

enstalite; 3 crystabal ite; silica; silicates.

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