NBSIR 76-1153

Microstructural Aspects of

Deformation and Oxidation of

Magnesia-Doped Silicon

Nitride

N. J. Tighe

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

September 1976

Interim Report for Period 1 July, 1976 to September 30, 1976

Prepared for

United States Air Force

Air Force Systems Command

Aeronautical Systems Division

Wright Patterson Air Force Base, Ohio 45433

NBSIR 76-1153

MICROSTRUCTURAL ASPECTS OF

DEFORMATION AND OXIDATION OF

MAGNESIA-DOPED SILICON

NITRIDE

N. J. Tighe

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

September 1976

Interim Report for Period 1 July, 1976 to September 30, 1976

Prepared for

United States Air Force

Air Force Systems Command

Aeronautical Systems Division

Wright Patterson Air Force Base, Ohio 45433

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

MICROSTRUCTURAL ASPECTS OF DEFORMATION AND OXIDATION OF

MAGNESIA DOPED SILICON NITRIDE*

N. J. Tighe

Institute for Materials Research, National Bureau of

Standards, Washington, D. C. 20234

ABSTRACT. The microstructural changes that occurred in magnesiadoped silicon nitride as a result of slow crack growth, plastic

deformation and oxidation were studied by transmission electron

microscopy. Specimens which exhibited slow crack growth showed

extensive crack branching along the fracture path and ahead of the

primary crack tip. These primary and secondary cracks followed

intergranular paths. In samples which were deformed by bending

at 1400°C, dislocation arrays were found as well as intergranular

cracks and voids. Silicon nitride oxidized during heating in air

at 1400°C and enstatite and cristobalite were present in the oxide

layer. At lower oxidation temperatures, crystalline and amorphous

silica formed a semiprotective layer on the silicon nitride surfaces.

1. Introduction

Silicon nitride compacts have heterogeneous microstructures

because the grains vary in morphology and in phase. Grain sizes

within some compacts range from a few tenths to several micrometers

in diameter. The sintered grain phases are a-silicon nitride,

8-silicon nitride and silicon carbide, and there are large inclusions of silicon oxy-nitride and tungsten silicide. Tne tungsten

particles are picked up during the grinding of the powder prior to

compaction

.

It is necessary to use electron microscopy to study the

morphology and structure of individual grains in fine-grained

silicon nitride compacts. This method requires special techniques

that are not available in all laboratories, and microstructural

* Presented at NATO-ASI Institute Canterbury, England

August 15-27, 1976
Fig. 1 Grinding damage (a) scanning electron micrograph of a thin foil specimen which curled-up as a result of strain on one

ground side. (b) TEM showing sub-surface damage.

analyses of as-received and of tested specimens have previously

been limited in scope [1-3]. Without such analyses it will

become increasingly difficult to tailor silicon nitride materials to the desired applications in turbine engines. In the

present paper we identify some microstructural features characteristic of deformation, of fracture and of oxidation in

specimens made from magnesia-doped compositions.

2. Materials and Procedures

Samples were made from hot-pressed silicon nitride (HPSN)

billets which had a hot pressing additive of MgO or of an MgO

precursor. Spectrographic analysis by the manufacturer showed

typical impurities as: A1 0.2 w/o, Ca 0.05 w/o, Fe 0.40 - 0.64 w/o

Mg 0.70 - 0.74 w/o, W 2.6 - 2.7 w/o.

Microstructural examination was carried out on specimens

from: (a) 4-pt. bend bars tested at 1400°C; (b) double torsion

specimens [4] tested at 1400°C; (c) bulk and thin foil samples

oxidized by heating in air at 1000°C, 1200°C and 1400°C; and

(d) samples ground with 400 mesh SiC abrasives. Thin foils for

transmission electron microscopy (TEM) were prepared by the ion

sputtering technique [5]. In making foils to show the grinding

damage it was necessary to sputter away ^1/2 - 1 pm from the

ground surface to keep the foil from curling-up as shown in Fig. lb.

Double torsion specimens were sectioned parallel to the tension

surfaces and 3 mm disks for thin foils were cut out along the

crack length.

2
Fig. 2 Light micrographs of cracks in specimens deformed at

1400°C (a) tension surface of double-torsion specimen, (b) tension

side of 4-pt. bend bar, (c) area between edge cracks in (b)

.

3. Results

3.1 Surface Damage

Test specimens usually are ground to produce uniformly flat

surfaces. Cracks and chips which occur during the grinding process

can penetrate several tens of micrometers. Such surface cracks

become the strength limiting flaws of fracture mechanics theories.

The damage found near a ground surface is shown in Fig. lb.

The dislocation density is high and the strain contrast in the

image reduced the resolution. The micrograph shows, however, the

dislocations and cracks that constitute some of the near surface

grinding damage.

3.2 Slow Crack Growth

At temperatures above 1200°C and at low strain rates magnesiadoped silicon nitride exhibits slow crack growth [4, 6] and samples

deform plastically at relatively low stress.

Specimens which exhibited slow crack growth during crack • propagation experiments of 1400°C were examined by light and electron

microscopy. Cracks which had formed in the tension side of specimens

were filled partially with oxide; and, therefore were easily visible.

The segmented primary crack shown in Fig. 2a demonstrates the characteristic fracture pattern of slow crack growth in HPSN.

Cracks were produced along the tension side of a 4-pt. bend bar

which was tested at 1400°C. When the surface oxide was polished

away the deep cracks shown in Fig. 2b were seen along the specimen

edge; however, near the center of the bar cracks penetrated only a

few tenths of a mm. Short crack segments which occurred between the

deep edge cracks are shown in Fig. 2c. These observations suggest

that the crack growth was influenced by edge effects and by

surface oxidation.

3
Fig. 3 Electron micrographs of cracks produced in double-torsion

specimens tested at 1400°C, (a) primary (P) and secondary cracks

(S) , (b) dislocation arrays at the tip of a secondary crack.

In double-torsion specimens, electron microscopy showed

that cracks occurred along grain boundaries. The primary crack

and the secondary cracks were oriented approximately parallel

to each other and were separated by one or more grains. An

example of this configuration is shown in Fig. 3a. Figure 3b

shows a crack tip which is several grain diameters away from

the primary crack. The crack is several grains long and dislocations are seen emanating from the crack tip into the two

adjacent grains.

Because of severe cracking along the tension surface of

bend specimens thin foil specimens were made from regions

several micrometers beneath the oxide layer. These samples

showed primarily the microstructure similar to that seen in

double-torsion specimens, namely intergranular fracture, and

dislocation arrays near grain boundaries.

Grains on the compression side of the bent bars had extensive

arrays of bubbles, (B) , dislocation arrays, (P) associated with

grain boundaries and in sub-grain boundaries. The pictures in

Fig. 4a, b show typical areas in these samples.

3.3 Oxidation

Silicon nitride oxidizes readily in air above 1000°C, however

the effects of the oxide on mechanical properties has not been

established. The thickness and the structure of the oxide layer

are related to the temperature and time of heating. The x-ray

diffraction patterns obtained from oxidized specimen surfaces showed

that S-cristobalite was present in samples oxidized at 1000°C,

1200°C and 1400°C that enstatite (MgSi 03 ) and possibly pigeonite

(Mg 2 Fe 2 Ca) [S^Og] were present in samples heated at 1400°C. When

the oxides were examined by electron microscopy, two amorphous

phases were found in addition to these crystalline phases.

4
Fig. 4 Compression side of a bent sample showing (a) Bubbles,

and dislocation arrays within a grain, (b) Extensive array of

voids

.

A cross section of the oxide formed on a specimen heated 96 hrs

at 1400°C is shown in Fig. 5a. The oxide is polycrystalline and

polyphase and has many pores and cracks. This specimen was heated

cyclically and the oxide appeared to have melted during this heat

treatment

.

Electron microscopy on the sample in Fig. 5a showed that the

oxide nearest the matrix was S^^O. This phase was distinguished also by light microscopy, and, from the light microscopy

examination it appeared that the oxide formed nodules which grew

to cover the surface. Large grains of silicon oxy-nitride were

found in the thin sections, and these grains were heavily faulted

as shown in Fig. 5b.

In the layer adjacent to the silicon oxynitride, enstatite and

cristobalite were found. The electron micrograph in Fig. 6a shows

an enstatite grain with its characteristic faulting and a partially

vitrified cristobalite grain.

Cristobalite was found in all the oxidized specimens. In thin

foils which were oxidized at 1000°C and at 1200°C, 8-cristobalite

occurred as grains within a glassy matrix. However, under the influence of the electron beam, the 6-cristobalite decomposed first to

a-cristobalite an amorphous phase. The grain shape did not

change during this vitrification. The vitrification occurred in

less than two minutes and Fig. 6b shows both transformed and nontransformed grains. The diffraction pattern taken from the vitrified

6-cristobalite was used to differentiate the amorphous silica phase

from the other amorphous phase. It is possible that the second

amorphous phase is a magnesium silicate glass. Cristobalite and

various pyroxene phases can occur by devitrification of a magnesium

silicate glass at 1400°C as well as during cooling to room temperature.

5
Fig. 5 Oxide produced at 1400°C, (a) light micrograph showing

oxide (0) on both surfaces of Si 3 N4 specimen (M) , (b)

grain

.

Enstatite is one phase in the pyroxene group of calcium, magnesium

iron silicates which form an impressive array of mineral structures.

These structures are difficult to identify from electron diffraction

patterns alone. The Ca,Mg and Fe are the major impurities in the

magnesia-doped silicon nitride and their presence in an oxide layer

indicates that considerable diffusion occurred during oxidation.

4. Discussion and Conclusions

4.1 Microstructural Factors Affecting Strength

Other investigators attributed deformation of magnesia-doped

silicon nitride to grain boundary sliding from a glass binder

between grains [1, 6]. However this glass phase was not evident

in the samples discussed in this paper. Cracks did propagate

primarily along grain boundaries indicating that grain boundaries

presented easy fracture paths. It is possible that impurities

can segregate near grain boundaries and can weaken their bonding

without forming a distinct glass phase.

The electron microscopy results show that slow crack growth,

which occurs at low strain rates, is characterized by branching

of grain boundary cracks and by localized plastic deformation at

crack tips and along grain boundaries. Oxide formation during the

deformation may influence the crack growth.

The surface oxide can have an initial strengthening effect by

filling up and healing flaws produced during machining. However,

the oxide forms preferentially at fresh fracture interfaces and

penetrates deeply into the matrix during continued deformation.

6
Fig. 6 Phases found in oxide produced by heating at 1400°C

(a) enstatite, MgSi03 (E) and B-cristobalite grains in the

mid-oxide layer. (b) B-cristobalite formed on a thin foil

oxidized 1 hr at 1200°C; area shows faulted grains and some

vitrified grains.

This sub-surface oxide penetration can weaken the structure during

prolonged stressing at elevated temperatures. Changes in oxide

composition from silica to magnesium silicate decrease the viscosity

and the melting point and thus modify the oxide behavior at temperature.

Consideration of the microstructure suggests that the effects

of the oxide growth and of the structural inhomogenities which

contribute to uneven thermal stress distribution should be accounted

for in the development of deformation models for HPSN. It will

be useful to compare the microstructures of silicon nitride compacts

having different sintering additives and to examine specimens

after deformation in inert as well as in oxidizing atmospheres.

Acknowledgments

This work was supported by the USAF under grant no.

F33615-76-F-6752 . We thank Mr. L. Russell for performing mechanical

tests, and Mr. G. Garrett for printing the photographs.

References

1. R. Kossowsky, "The Microstructure of Hot-Pressed Silicon Nitride"

J. Matl. Sci. 8_ 1603-1615 (1973).

2. P. Drew and M. H. Lewis, "The Microstructure of Silicon Nitride

Ceramics During Hot-Pressing Transformation" J. Matl. Sci. 9_ 261-269

(1974).

3.

N. J. Tighe, "Microstructure of Oxidized Silicon Nitride" Proc.

EMSA 420-471 (1974).

7
4. A. G. Evans and S. M. Wiederhorn, "Crack Propagation and Failure

Prediction in Silicon Nitride at Elevated Temperatures" J. Matl.

Sci. 9_, 270-278 (1974).

5. N. J. Tighe, Experimental Techniques in Electron Microscopy in

Mineralogy, (ed.) H. R.-Wenk, Springer Verlag, Berlin 1976.

6. F. F. Lange, "High-Temperature Strength Behavior of Hot-Pressed

S^N^: Evidence for Sub-Critical Crack Growth” J. Am. Ceram. Soc.

57, 84-89 (1974).

8
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR RK PORT NO.

NBSIR 76-1153

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Microstructural Aspects of Deformation and Oxidation of

Magnesia Doped Silicon Nitride

5. Publication Date

September 1976

6. Performing Organization Code

7. AUTHOR(S)

N. J. Tighe

8. Performing Organ. Report No.

NBSIR 76-1153

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Pro ject/Task/Work Unit No.

3130459

11. Contract/Grant No.

F33615-76-F-6752

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

United States Air Force

Air Force Systems Command

Aeronautical Systems Division

Wright Patterson AFB, Ohio 45433

13. Type of Report & Period

Covered

Interim Jul/Sept 1976

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The microstructural changes that occurred in magnesia-doped silicon nitride as a result of slow crack growth, plastic deformation and oxidation were studied by transmission electron microscopy. Specimens which exhibited slow crack growth showed

extensive crack branching along the fracture path and ahead of the primary crack tip.

These primary and secondary cracks followed intergranular paths. In samples which were

deformed by bending at 1400 C, dislocation arrays were found as well as intergranular

cracks and voids. Silicon nitride oxidized during heating in air at 1400°C and

enstatite and cristobalite were present in the oxide layer. At lower oxidation tempera

tures, crystalline and amorphous silica formed a semiprotective layer on the silicon

nitride surfaces.

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, deformation, oxidation, microstructure, slow

crack growth enstatite, cristobalite

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