In Situ Oxidation of Y2 03-doped Si3 N

4

N. J. Tig he

Fracture and Deformation Division

National Measurement Laboratory

Center for Materials Science

i r National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

I

K. Kuroda, T. E. Mitchell and A. H. Heuer

Case Western Reserve University

Cleveland, Ohio

Interim Report

April 1980

Issued August 1 980

Prepared for

[*-QG —«Jepartment of Energy

| 2qq Iffice of Coal Utilization

; nr . r Washington, D.C.

. Ubb

I 80-2075

\ 1980

-

NBSIR 80-2075

IN SITU OXIDATION OF Y 2 0 3 -DOPED Si

3

N

4

N. J. Tighe

Fracture and Deformation Division

National Measurement Laboratory

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

KfttonxJ Bureau of Standartt

Ubrtry. E-01 Admin. BWi

OCT 2 9 1980

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K. Kuroda, T. E. Mitchell and A. H. Heuer

Case Western Reserve University

Cleveland, Ohio

Interim Report

April 1980

Issued August 1 980

Prepared for Department of Energy

Office of Coal Utilization

Washington, D.C.

r< . ,<*• ^EAU Of

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
. ?

.
1 .

In Situ Oxidation of Y 2 03 -doped Si 3 N 4

N. J. Tighe, National Bureau of Standards

Washington, D. C.

K. Kuroda, T. E. Mitchell and A. H. Heuer

Case Western Reserve University

Cleveland, Ohio

Hot-pressed SijN^+8-13%Y

2 02 compacts, which exhibited catastrophic

oxidation in air at 740°C but only passive oxidation at 1380°C, were

studied by HVEM using an environmental cell. The in situ experiments

were started as part of a larger program to establish a model for this

catastrophic oxidation process. The low temperatures required for

initiation of oxidation are within the capability of the hot stage and

the reaction products are sufficiently complex to require analysis by

electron diffraction. This yttria-doped silicon nitride ceramic is of

interest for use in ceramic turbine engine components because of its

good mechanical properties at temperatures above 1200°C [1] . However,

until the cause of the low temperature catastrohpic oxidation is identified

and corrected the material cannot be manufactured successfully for these

components. The low temperature oxidation is similar in appearance to

the so-called "pest condition" and occurs in some billets and not in

others which were manufactured under the same apparent commercial,

powder processing and hot pressing conditions. Lange et al. [2] found

that billets they prepared in the Si

;,N^-Si2N ::) 0 -Y2Si

2 0

7 compatibility

triangle of the phase equilibrium diagram, shown in Fig. 1, were oxidation resistant; and, that billets with compositions outside this triangle

were unstable in oxidizing conditions.

In this paper, the material designated -A- exhibited only passive

oxidation during heating in air from 600°C to 1400°C; while the material
designated -B- exhibited catastrophic oxidation during heating in air at

* 740°C. Both billets were from the same manufacturer [3] but were purchased

at different times. A bar of material -B- when heated in a gradient

furnace from 735°C to 1380°C for 20 hours oxidized catastrophically at

the low temperature end, and passively at the high temperature end. The

major crystalline phases in both materials were identified by powder

x-ray diffraction as SSi^N^, (H phase) and WSi^ (there were

some unidentified lines) . The tungsten phase (3-4%) is a result of

contamination from WC balls used in ball-milling of the silicon nitride

powder, and thus is a common impurity phase in hot-pressed Si^N^. The

WSi^ phase was not present in the castrophically oxidizad portion of the

material B, but was present in the portion oxidized at 1380°C. Although

the compositions of A and B were similar, there was considerable phase segregat

ion and inhomogeneity in material B as seen in the light microscope.

Both materials appear to have compositions outside the compatibility

triangle discussed by Lange et al. [2].

The Swann enviromental cell in the CWRU 650kV electron microscope

was used with a platinum strip heater, in flowing oxygen. Specimens

of materials -A- and -B- were prepared as ion-thinned discs with one

flat side and one dished side. This configuration gave good contact

with the grid heater; and, resulted in better heat conduction to the

specimen.

The microstructure of the yttria -A- is shown in Fig. 2, and is

similar to that found in other billets [4,5]. The large tabular grains

are Si^N^, the triangular shaped phases surrounding the grain corners

are the 4- Si^N^ phases and the dark particles along the grain

boundaries are the WS^ and/or WC phases. During hot-pressing at 1750-1800 C,

* The specific materials identified are not the only available material.
3.

the yttrium oxynitride phases can melt and form a series of solid solutions

with the impurity phases and with the silica in the starting powder [5]

.

Both crystalline and non-crystalline phases of variable composition can

fill the interstices around SiJ^ grains.

Specimens were heated gradually to 750°C in 40 torr of 0^. Under

these conditions oxide platelets were seen to nucleate on the surfaces

of the gSi^N^ grains, around holes and at the yttria phases as in Fig.

3. The platelets, which were randomly oriented and incoherent with the

matrix, gave spotty ring patterns both at temperature and after cooling

to room temperature. The platelets in Fig. 4 were identified from these

ring patterns as a mixture of SiO^ and Si^N^O. During the oxidation

considerable drift and flexure of the specimens occurred, as is evident

in the figures

.

After a few minutes of heating, cracks appeared along some grain

boundaries; and, as oxidation proceeded a film was observed to grow by

the merging of nucleating clusters. These clusters changed diffraction

contrast as they merged. It is assumed at this time, that this process

represented the gradual reaction and volatilization of the tungsten rich

phase. The film thickened and the crack widened to form a sizeable hole

as seen in Fig. S. Iron rich and tungsten depleted phases around reacted

thin foil edges were detected with an EDX system on a 200 KV instrument [7]

.

The yttria-silicon oxynitride phases became porous during the in

situ oxidation. This type of reaction can be seen at Y in Fig. 5 and in

Fig. 6. The morphology of the phases suggests that the initial, yttriarich regions were multiphase or possibly an amorphous -crystal line mixture.

The phase in Fig. 6 was identified from its electron diffraction pattern

as YSiO^N or the K-phase of Fig. 1.
4.

The in situ specimens were bent and slightly pink when removed

from the strip heates. Later TEM examination showed that the oxidation

had not occured uniformly over the entire thin area of the specimens.

Some of the differences reflect temperature gradient and oxygen partial

pressure differences between parts of the specimen that, are exposed through

holes in the grid heater and other parts of the specimen that were under

the grid bars. In the region shown in Fig. 7 few oxide platelets

are seen, and the boundaries between Si^N^ grains have opened. The

yttria phase visible on one edge of the grain was identified as Y^S^O.^

or the J phase. The oxidation rate differences could reflect also the

inhomogenity of the phase distribution mentioned previously.

Additional in situ experiments are needed to verify the mechanism

of the loss of W and the observed reaction differences. Video or cine -"

recording will be used to make a more complete record of the

observations.

The results of the in situ experiments demonstrate that: castrophic

low temperature oxidation in yttria-silicon material -B- started at

surfaces exposed to oxygen; volatilization of a W containing phase created

voids; continuous films were produced over some grains; yttria rich

phases became porous; and considerable strain was associated with the

oxidation.

References

1. G. E. Gazza, J. Amer. Ceram. Soc. 56, 662 (1973).

2. F. F. Lange, S. C. Singhal and R.C. Kuznicki, Westinghouse Report

76-904 (1976).

3. The Norton Co. Material Designation NCX-34.

4. D. R. Clarke and G. Thomas, J. Amer. Ceram. Soc. J50 491-5 (1978).
5.

5. 0. L. Krivanck, T. M. Shaw and G. Thomas, J. Amer. Ceram. Soc., 62_,

585-89 (1979).

6. K. H. Jack in Nitrogen Ceramics, F. L. Riley ed. Noordhoff, 1977,

p. 109-125.

7. G. Nord, USGS, Reston, Va.

This work is supported partially by DOE Office of Coal Utilization under

Task Order No. A0861PS and by AFOSR grant 49620- 78C-0053. The material -Bwas supplied by J. Wimmer of Garrett AiResearch, Phoenix, Az.
y

.
Fig. 1. Phase diagram for the yttria sialons [1J.

Fig. 2. Microstructure of hot-pressed

Si 3 N 4 + 8%Y 2 0 3 showing 3Si 3 N 4 (8), grains

Y Si o N phase (Y) WSi particles (W).

1 © 9 2 4

Fig. 3. Material -B- at 750°C in 5200 Pa after

heating for ^5 min., showing oxide platelets

(arrowed) and reacting yttria phase (Y).
Fig. 4. pSisNit grain with Si

O

2 and Si 2 N 2 platelets produced during in situ oxidation. Yttria phase at Y. (Material-B-)

Fig. 6 . Material-B-. After oxidation

the yttria phase (Y) is porous and layered,

BSi 3 N 4 gains (B) have oxide platelets.

Fig. 5. Specimen at 750°C in 5300 Pa O 2 . The hole (H) and the film around it (Ox) were produced in <30 min (Material-B-).

Fig. 7. Portion of in situ specimen which

was under the grid, yttria phase (Y) is

Y 4 S

2 O 7 N 2 .
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-2075

2.Getft Accession No. mm WM:—

3. Recipients Accession No.

4. TITLE AND SUBTITLE

In Situ Oxidation of Y^-doped SigN^

5. Publication Date August 1980

__ | I $

WT'ES -

7. AUTHOR(S)

N.J. Tighe, K. Kurodo, T.E. Mitchell, A.H. Heuer

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

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

Department of Energy

Office of Coal Utilization

Washington, D.C.

13. Type of Report & Period Covered Interim Report

April 1980

15. SUPPLEMENTARY NOTES

|I I

I Document WVVUIIIWIIV describes UVVVI I UVJ aU VVIlipWkWI computer program; pi VglUUI t SF-185, wi * vv, FIPSi W Software WVIW.WIV* Summary, WUIIIIMW. J I 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.) Specimens of -doped Si^N^ were oxidized in an environmental cell in a 600 kV

electron microscope. This ceramic oxidizes passively at temperatures greater than

1000°C but oxidizes catastrophically at a/750°C. The in situ experiments showed that

oxidation occurs by nucleation and growth of SiCL and Si N? 0 on the 6Si

qK surfaces and

by volatilization of W inclusions. The paper discusses the HVEM methodology and the

oxidation results

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)

Ceramic; high voltage electron microscopy; in situ ; oxidation; silicon nitride;

yttria-doped silicon nitride

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