Oxidation resistance, and examples of oxidation of SiC, Si3N4 and sialon are reviewed. A description is given of the oxidation mechanism, including the oxidation product, oxidation reaction and the bubble size. The oxidation reactions are represented graphically. An assessment is made of the oxidation process, and an oxidation example of silicon ceramics is given

Hirota, K.

Yasutoshi, H.

English

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NASA TM-77429NASA TECHNICAL MEMORANDUM
S	 ,
	
1
OXIDATION RESISTANCE OF SILICON CERAMICS
Hasegawa Yasutoshi and Kazushi Hirota
(NASA-TM-77429) OXIDATION RESISTANCE OF
	 N85-15858SILICON CERAMICS (National Aeronautics and
Space Administration) 14 p RC A02/ME A01
CSCL 11C
	 Uncias
G3/27 13474
Translation of "Silicon Ceramics no tai Santasei";
Seramikkusu, Vol. 18, No. 7, 1983, pp. 580-585
FU 1985
RX41Y1b
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGT0N, D. C. ` 0546	 APRIL 1984
RAWn
1., ANNA ere.
	
1. DeNrMM I ANN.IM ere. RNIpIMI's CNdef me.
HASA TM-77429
d. Tolle .nd ldRIM !. Rger1 Date
APRIL 398 4
OXIDATION RESISTANCE OP SILICON CERAMICS' r. /.d.rnln^OgM1u11M C.I.
1. M •berl.l f. L/romint ofs"11e11ee R..NI N..
Hasegawa Yasutoshi and xazushi Hirota
If. Welk Unit Me.
I I. C.nnN1 N DIM I N6
NASM	 3542:f. ►.rl.r.nme ory priI.eUM Na.e w1 ASMeee
SCLTRAN
Doi 5456
If. Type of Rge.1 sell ►erred Covered,
Translation
u. 
lWitionatrn^ arocautice alW Space Administration
Washington, D'.C.
	 20546 14. Spono leg Afeee/ code
If. 1eglNnelN/NNN
	 •
Translation of "Silicon ceramics no tai
Seramikkusu, Vol. 	 18, No.	 7,	 1983, pp.
Santasei",
580-585.
la.	 Abe1r	 *
The article provides a review with eight
oxidation resistance, and examples of oxidation
references on the
of SiC, Si3N4
and sialon.	 A description is given of the
including the oxidation product, oxidation
oxidation mechanism,
reaction and the
bubble size.	 The oxidation reactions are represented
graphically.	 An assessment is made of the oxidation process,'
and an oxidation example of silicon ceramics is given.
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fTANeARD TITLE rASI
ii
OXIDATION RESISTANCE OF SILICON CERAMICS
,t
Hasegawa Yasutoshi and Kazushi Hirota
1. PREFACE
The development of scientific technology demands increas-
ingly high standards for basic materials. Fire-resistant
materials composed mainly of metallic oxides have been used
widely in various high-temperature industries. Recent
research has focussed on nitride, carbide, silicide and boride
as a fire-resistant material. Much research has been done
especially on nitride, carbide and acid nitride as high-
temperature-resistant structural materials. From the energy
conservation perspective, these materials are expected to be
improved. These materials possess superior characteristics that
other oxidized substances do not have. However, they tend to
oxidize easily in a high-temperature environment. Unless a
solution to this problem characteristic of the materials can
be found, the economical use of these materials as a pyrogenic
heat-resistant structural material in an oxidizing environment
cannot be realized. Among the substances that are being dis-
cussed, Si 3N4 , SiC and sialon are the first to be used as a
superior fire-resistant material or partial heat-resistant
structural material because of their relatively cheap prices and
comparatively high purity. Their defects caused by oxidation
during their usage, however, are the major obstacles hindering
their further improvement, in this article, Si 3N4 , SiC and
sialon are all considered as silicon ceramics and their oxida-
tion process at a high temperature is discussed.
Numbers in margin indicate foreign pagination.
(National Institute for Research in Inorganic Materials);
On the Oxidation Properties of Silicon Ceramics
/580
f,
s
I
I
l	 '
1
2. OXIDATION MECHANISM
(a) The oxidation of Si 3N4
 proceeds in a reaction as
indicated below..
S4N,+30, --P 3SiO, 1\;;r;'	
/581Si,N, +O, 
--P Si3ON.+SiOj-i.Nti:"
(b) Similarly, oxidation of Sic takes place as follows:
sic+3/2O,
	
S:O,+COi
(c) Sialon's oxidation goes through the following reaction.
A(Si4_.A1.O.N,_.) +BO;
—+ C(3A1,O,•2SiO,)+DSiO, ' EV,1'
As shown in the above formulas, all of the materials
increase in weight as the reaction proceeds. Generally, weight
increase is the basis for an oxidation resistance assessment
i
which will be discussed in detail in the following chapter.
4	 To give an example, sintered Si 3N4 's (Y2 0 3 and Al2 0 are added
as sintering auxiliary agents) oxidation process is shown in
Figure 1. An analysis on the surface oxidation product by X-ray
diffraction indicates that the strength of crystalline barite
increases drastically at an early stage of oxidation, but as
oxidation progresses, the strength decreases. After many hours
^n
of oxidation, the strength of crystalline barite drops to a
point when it is no longer measurable. in inverse proportion
to decreasing strength of crystalline barite, the glassy phase
increases its strength. Y2 0 3 that was added as sintering agent,
{	 together with Al2 0 3
 reacts to SiO 2
 which is produced by oxida-
tion and forms Y2 0 3 . 2Si0 2 with gradually increasing strength.
Similarly, after a long period of oxidation, Al 2 0 3
 can be found
in the ,form 2Y 2 0 3 -Al2 0 3 . These oxidation product actions are
easy to understand when you look at them as surface accumula-
tions of impurities in a sinter by pyrogenic oxidation.
2
ion film'
body
ORIGINAL PAGE
'BLACK AND WHITS PHOTOGRAPH
r.
'oxidation time
1) Oxidation product (comparison of intensity of diffracted
X-ray) crystalline barite < crystalline barite >
crystalline barite > N.D.
Y2S < Y2S < Y2S < Y25 '	 ..
'.MD.	 N. D.	 N.U.	 (2YA).
sinter < sinter < sinter < sinter
2) Oxidation reaction
Sf,N.+3 0;--»3 SIO:}^?7:. 3120;+2 N''—+3 0'-{ N,
	
2S10:r2Y"-6301	YA•2 Sio,
4Y o'+2 A:" +D 08n-2 Y,0,-A1,0,
SIO^•^\u'+K'tCu"+al"—¢laeny Phase
s) Bubble size (um) (AME sample, 1300°C oxidation):
— 10-30	 200-500
Figure 1. Oxidation reaction and oxidation chart of 	 f
pressurized sinter S1 3N4
 (sintering agent, Y203);
5, Al 2 0 3 : 3 wt %)
In a similar manner, Figure 2 shows the oxidation process
	
':	 1
of pressurized Si 3N4
 sinter. (Sinter agent, MgO'- 5 wt % added). 	 i'	 P
A scanning electron microscope shows an aggregate of crystals
I
in the post oxidation surface, which looks like a porous oxida-
tion film. However, further analysis of the section by a
scanning electron microscope and an optical microscope reveal
that the closer the oxidation layer gets to the surface, the 	 h`r
more porous it gets. The part of the layer close to the basis	 ^^(
is very fine. This is because impurities in the base accumulate
on the surface during the oxidation process, forming a layer
of fine glassy phase near the base and a porous layer as it
goes to the tap due to the on-going crystalization process [3].
3
A
N. L,
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
.1:7 1 .'top layer (porous)
middle'layer
",.L
(relatively'fine)l
bottom *layer (fine) G'•
^\\^	 base Si N
\^\\ -0•	 _	 1	 3
fl^nr N•!
SI,N,+30,-3S10.+2N.
	 .^
S102
--crystalline barite
N; orb • 1120,+N'—;01-+1/2N,
—+MaSIO,+413N''
2/3 SI,N,+Mje-+2Ce-+70-
" /„	 y —M¢O.MO-2510,+813N`'
313 SI,N4+1%I9"+Ca"+80"
---caQMaO .2SI0,+8/3 N'-	 -
0	 0, or 01'. •1/3SI,N,+2M¢"+401'
"Ma,Si0,+413 N'-
i
C
Figure Z. Oxidation reaction and chart of Si3N4 pressurized
sinter (sinter agent Mq0): 5 wt _%. added, 1300"C,_,30d),
0.
C
1	 f"Y )^i	 :._^ .'.oxidation film',
/ 7et''CK ;rn •2:a : ir N	 ;	 , .^a 
	 8ialon (z=J-4^) base
1) Oxidation weight increase
2) Oxidation film thickness	 9, < P, < A. < A,
3) Oxidation products (comparison	 c.b. > c.b. > c.b. > N.D.
of intensity of diffracted X))
	 ..
4) Oxidation reaction	 Si,\, 30n—»3 S,Or}2Ni• 3I'Or+3 N' —•30''+I^,
2 S!o,+G AP'+30''—•3 A1,0,•2 sI0,
.5) Bubble size (}lm) 	 2-15 10-15 porous more porous
Figure 3. (3-sialon pressurized sinter oxidation reaction
(z=1-4) and chart (1300 °C, 30d)	 (2]
4
Figure 3 shows oxidation of sialon which sinters relatively
easily without an agent (can be considered as a single phase).
An oxidation experiment with sialon sinter offered an interest-
ing result: from Z=1 to Z=4, the oxidation weight increase
becomes smaller as the number goes up, whereas the thickness of
the film increases as the number goes up [2]. The one with the
smallest oxidation weight increase, in this case sialon Z=4,
is believed to possess the best oxidation resistance. But in
the case of sialon, Z-4 type composition does not necessarily
have better oxidation resistance. From the point of oxidation	 1
film thickness, the Z=1 type composition is considered to have
better oxidation resistance. This experimental finding demands /583
a basic re-analysis of conventional oxidation resistance, namely
the assessment based on oxidation weight increase. Based upon
a study of the oxidation mechanism of SiC, SiC sinter forms
crystalline barite on the surface while a part of sinter agents
(normally B, C and Al) accumulate on the surface as well, in the
same way as Si 3N sinter which is indicated in Figure 1. B
oxidizes easily and evaporates as B 2 0 3 at or above 1300°C. As
an example, the oxidation weight increase process with B: 0.5 	 -a
wt % and Al: 1 wt % added is shown in Figure 4. Considering
the above discussion, silicon
u y a,l	 uoo'c	 ceramic oxidation may seem
vi	 simple, but in actuality, it is
N 01 o	 quite a complicated process.
i . ^ I	 laooc
.y ,d
	 -0.1
	
1 ,4. 6 8 10. 12 ,IA 14
oxidation time (h)
Figure 4. Pressurized sinter
SiC (B oxidation mechanism [5].
(B: 0.5 wt %, AL: 1.0 wt %)
3. ASSESSMENT STANDARD OF OXIDATION PROCESS
In the previous chapter, it was pointed out that the con-
ventional oxidation resistance assessment by means of the
5
oxidation weight increase entails problems. As assessment
criteria of oxidation resistance, aside from weight increase,
one may consider the thicknesses of the oxidation film, oxida-
tion product and the nature of oxidation film surface and bubbles.
When the primary component of oxidation products is crystalline
barite alone, oxidation film exfoliates when it cools down to
room temperature, because the crystalline barite a^b transition
takes place around 27 °C. When the coefficient of thermal expan-
sion of an oxidation product differs suh:.trntially from that of
the base, shear strain occurs on the surface of contact between
the oxidation film and the base, which causes exfoliation. The
nature of the oxidation film surface is closely related to the
oxidation products. When the impurities of the base accumulate
"f
IIf
	
on the surface, glass formation with a low melting point among
the impurities, causes very uneven oxidation of the film surface
due to N2 , CO gas generated by the oxidation reaction. We have
experienced a case where Rmax reached 100 um. As discussed, it
involves great risk to assess the overall oxidation characteristic
from one particular characteristic and it is inevitable to use a
comprehensive assessment method. To do so, we made Figure 5 by
applying the characteristic factor often used for quality control
for oxidation assessment. The factors in this figure are
insufficient; therefore, this serves as a reference of the basic
assessment method. Generally, the same oxidation experiments do
not result in accurately overlapping findings, which makes it
extremely difficult to obtain data usable as a data bank. This
is because too many factors are involved. For the assessment of
oxidation characteristics, we have to specify the characteriza-
i
tion of sinter, and to do so we need to characterize the original
material.
4. OXIDATION EXAMPLE OF SIL ICON 'CERAMICS
As an example, I would like to discuss Davidge and other
experiments on reactive sinter Si 3N 4 I s oxidation. Organic
6
I`roay,
4J
C.'
F+
N
N
fn
N
N
ro
0
.ri
+J
ro
O
w
O
N
H
O
41
U
rd
w
U
4J
N
N
4J
U
ro
H
ro
x
U
)n
N
H
O
W
f
I
i	 :^7
w
H
O
N
ppU
CWA
M
w
•rl N • i O
41 u
A M0 1-pi ° ?(9
w w o 4
M H
O O
4j Jj LJ 0)	 N
8107 ttl OJ H	 H
k k
-14
1^0
	 N
f	 ^
HuCK AND ORIGINAL
 
TE PNOTpGRAPH
	
• ..j	 binder was added to silicon powder
	
" 46	 which is available commercially.
°o ,^ ! a ,	 iooaa	  
Then it was pressurized at 500
	
^;	 12C%. -o MN 2 . Then nitriding was done at
o
	
J400C	 1350°C for 20 hours first, followed
z	 a	 s	 e	 i	 i,	 by
	
)oxidntion time ii,
	
20 more hours of nitriding at
Figure 6. Reactive sinter 1450°C, which created a test mater-
	
S19N4 oxidation weight 	 ial with a density of 2.53 g/cm 3 .
increases in relation to	 The material was then cut into spe-
oxidation time (h) [6]
cific sizes, which was oxidized in
an open furnace at three different temperatures (1000, 1200 and
1400°C) for 150 hours. Figure 6 shows the oxidation weight
increase observed in the experiment.
Providing the oxidation mechanism proceeds by a simple
diffusion mechanism, two clear steps of oxidation can be observed.
At 1000°C, the early stage of oxidation (stage I) takes 3 to 4
days. At 1200°C or 1400°C, however, stage I is completed in
about 15 minutes. Moreover, at higher temperatures, the oxida-
tion weight increase M during stage I is greater, but the
oxidation rate is lower. In this experiment by Davidge and others,
low stages (early stage and mid-stage) of the oxidation mechanism
were observed. In a series of our experiments on silicon cera-
mics oxidation, several stages were found as shown in Figure 7.
These stages change depending on oxidation conditions
(especially oxidation atmosphere). Note that unit of oxidation
time in Figure 7 is days. It has been reported that oxidation
is completed primarily in stages I and II and the mechanisms
can be explained as simple diffusion mechanism. However, in
oxidation over a long period of time (unit = month), stages that
cannot be explained by simple diffusion start to appear (stages
III-IV in the figure). When there is partial steam pressure,
the material reaches stages III-IV within relatively a few days.
8
o	
If
Sea
QOPQP^4o^GVQ
S,P Qy;^^
!O
^P
O^G
i r 'IStage UAL Stage (a) Stage (y)
Sa
w"
Ia
u	 L
(oxidation time (day)) 7/z
Figure 7. Thickness (um) of oxidation film when oxidized at
a set temperature (oxidation time (day)) 1/2
x---x repetitive: cooling and heating while maintaining
the specific temperature
o---o keeping a general temperature steady
(dry Od
u(
20
d0 u
^ N 1U CI Jr . L 	.,	 IS
,oxidation time
Figure 8. S-sialon (x=2) sinter
60
IJ00•C
so	
I+
	
--J	 (dry Cr)
}' 60
^°	
1gg^	 . j.
u
y^.t.N.10
.;	 IJ	 ]	 ,.	 is	 ,70 .
oxidation time
Figure 9. S-sialon (z=2) sinter
9
SAC41i::'C ;M 04
01
r,o
L1 6
I
7.0
^tCd	
coarseness
81'
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
re now discuss how strength changes
by oxidation. As an example,
Figures 8 and 9 show data on 0-sialon
(z=2) pressurized sinter.
d 1 y^
U d
w
+	 i
Figure 10. 0-sialon (x=2) 	 As shown in the figures, strength
pressurized sinter surface decreases as the oxidation time
coarseness - oxidation
time	 innreases. At higher, temperatures,
the data becomes more inconsistent.
It is nearly impossible to explain these phenomena in the limited
number of words here, but for reference purposes, we touched upon
the relation .between strength and coarseness of the surface in
Figure 1).
5. POSTSCRIPT
Oxidation resistance of anti-oxidizing materials depends
mainly on the basic body crystals phase type, grain shape, grain
distribution, grainy phase type and its distribution, porosity
chars,;-teristics and distribution. Moreover, the resistance is
determined mostly by physical and chemical properties of the
oxidation film on the surface. Thus, accurate research on oxida-
tion should be initiated with a study on the basic sintering pro-
cess and the compound phase distribution and properties. Based
upon the oxidation characteristics of the oxidation mechanism,
better resistance is observed in the following conditions.
1) Oxidation weight increase: Small weight increases.
Weight increases in the parabolic manner in relation to oxidation
time witho:,t drastic change of pace.
2) Oxidation film thickness: Like the weight increase,
the oxidation film is thin, while the thickness increases in a
parabolic manner in relation to oxidation time without drastic
change of pace.
10
i
3) Oxidation product: They are non-transferable oxides.
They possess a coefficient of thermal expansion O.MAlar to that
of the basic body. They are not complex oxides v nsisting of
many different components. They form fine layers.
A) Surface of oxidation film: even and smooth surface with
no complicated protrusions or deflections.
A sinter with oxidation film which follows the above condi-
tions most clearly is desirable from the viewpoint of oxidation
resistance. However, an oxidation film which meets all the above
conditions is not available yet. in the end, we would like to
express our appreciation to Dr. Hiroshige Suzuki, honorary pro-
fessor at Tokyo Kogyo University, ^ for providing us supervision.
11
e(1] M. E. Washburn, Bull. Am. Ceram. Soc., Vol. 46, p. 667,
(1967) .
[2] Y. T. Hasegawa, H. Tanaka, M. Tsutsumi and H. Suzuki,
Yogyo-Kyokai-Shi, Vol. 88, p. 292,, (1980).
[31 Y. T. Hasegawa, T. Yamane, K. Hirota, M. Tsutsumi and H.
Suzuki, Yogyo-Kyokai-Shi, Vol. 88, p. 46 (1981).
[4] Yasutoshi Hasegawa, Kazushi Hirota, Nbriko Yamane, G.
Mitomo, Hiroshige Suzuki, Yogyo-Kyokai-Shi, Vol. 89,
p. 148 (1981).
[5] J. Schlichting and J. Kriegesmann. Ber. Dt. Keram. Ges.,
Vol. 56, pp. 72-75 (1979).
[6] R. W. Davidge, A. G. Evans, D. Gilling and P. R. Wilyman.
Proceedings of the Fifth Symposium on Special Ceramics,
p. 329 (1970).
[7] Yasutoshi Hasegawa, and others, Yogyo-Kyokai-Shi, ready to
be published.
[8] Yasutoshi Hasegawa, G. Mitomo, Kazushi Hirota, Hidehiko
Tanaka, Yoji t'ujii, Hiroshige Suzuki, Yogyo-Kyokai-Shi,
Vol. 89, p. 533, (1981).
AUTHORS' BACKGROUND
Yasutoshi Hasegawa
Graduated from Tokyo Kogyo University, Chemical Engineering
Department, in 1955 and joined Onoda Cement Co. Ltd in the same
year. Since 1966, he has been working at the National Institute
of Research ..n Inorganic Materials. He is a chief research
officer and engineer of departments of chemistry, ceramics .and
production control, quality control. He is also a professor of
engineering at Tokyo Kogyo Un-,rersity.
Kazushi Hirota
He graduated from Tokyo Kogyo University, Science Department,
Chemistry Div. in 1957. Completed MA courses at the same univer-
sity in 1959. In 1960, he became an assistant instructor at the
university. In 1971, he joined the National Institute for
Research in Inorganic Materials of Science and Technology Agency.
Currently; he is a chief research officer of a research group,
no. 15.
12


Oxidation resistance of silicon ceramics

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