J . Am. Cerum. Soc.

, 71 [lo] 837-40 (1988)

Stability of Phases in the Si-C-N-0System

HARUE WADA,* MING-JONG WANG,* and TSENG-YING TIEN"
Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109

The stability of the phases in equilibrium is calculated for the Two carbon activity levels, ac = 1 and which are referred
Si-C-N-0 system in order to analyze and predict the reactions to solid graphite as a standard state, are used for the calculation.
in ceramic whisker formation and sintering of silicon nitride The temperature is from 1573 to 2073 K. This is a fairly close
composites. Equilibria among Sic, SiJN4,Si2N20, SO2, Si, approximation of the actual experimental conditions; for
and the gas phase are evaluated at different carbon activities, example, when a graphite crucible or furnace is used for Si3N4
nitrogen pressures, and temperatures. Phase stability dia- sintering, the carbon activity would be unity or very close to
grams are constructed as a function of nitrogen and oxygen unity, whereas it would be very low in an elevated pressure,
pressures for two levels of carbon activity. Silicon nitride be- nongraphite furnace.
comes a stable phase with increasing nitrogen pressure or de-
creasing carbon activity and temperature, whereas silicon (2) Three-Phase Equilibria
carbide becomes a dominant phase at lower nitrogen pres- Equilibrium between two solid phases and a gas phase is calcu-
sures or at higher temperatures when carbon activity is unity. lated first. For instance, equilibrium between Si3N4and S i c is
The maximum sintering temperature of the SiC/Si3N4 com- expressed as follows for the present temperature range:
posite is higher with an elevated nitrogen pressure or a re- P-Si3N4 + 3C(s) = 3P-Sic + 2N2(g) (1)
duced carbon activity.
The ratio of the activities of Si3N4and Sic, which are referred to
1. Introduction solid Si3N4and Sic, respectively, is written as
log = log + 3 log U C - log PNI (1)
B
(a&/aS13N4) K1
OTH silicon nitride and silicon carbide are advanced engi-
neering materials that are expected to be used in a wide where K , is the equilibrium constant of Eq. (1). Since Si3N4and
range of applications. The stability of the phases in the Si-C-N-0 S i c form almost no solid solution, their activities are either unity
system, including the gas phase, is calculated to predict and ana- or less than unity-the activity of unity showing that the species
lyze the reactions in the formation and sintering of silicon nitride exists as a solid phase, whereas an activity of less than unity
and silicon carbide. Since the sintering or the formation of pow- shows nonexistence of the solid phase of that species. There
ders and whiskers of silicon nitride is usually performed with ei- are three possibilities for Eq. (I): If log ( a ~ , c / u s 1 3 N=4 0, )
ther a graphite resistance furnace or a nongraphite furnace under then uS,C = a S , 3 N 4 = 1; if log ( C I : , C / U S , ~ ~ J > 0, then aS,c = 1
a nitrogedargon atmosphere, carbon activity and nitrogen pres- and < 1 ; and if log ( ~ ~ ~ ~ /<a 0,~ then , , ~ , )< 1
sure are the main variables in this calculation. Other important and = 1. In the first case, the S13N4 and S i c phases co-
factors are oxygen partial pressure and temperature. Temperature exist, whereas S i c is the stable phase in the second case and
is limited to a practical range which is commonly applied for sin- Si,N4 is the stable phase in the third case.
tering. The oxygen partial pressure in the gas phase controls the Equation (I) is calculated as a function of temperature for
formation of certain phases such as silicon oxynitride and silica. ac = 1 and pNZ= 1 and 10 atm (0.10 and 1.01 MPa) as shown
The equilibrium oxygen partial pressure and the partial pressure in Fig. 1 . At pN2= 1 atm (0.10 MPa), Si3N4and S i c are in equi-
of silicon monoxide (SiO) -which controls the high-temperature librium at 1647 K. S i c is stable at higher temperatures, and
decomposition of silicon nitride -can be determined by thermo- Si3N4is stable at lower temperatures, whereas the temperature for
dynamic calculations. equilibrium between the two phases moves upward to 1809 K
Phase equilibria in the system based on silicon nitride have when the nitrogen pressure is increased to 10 atm (1.01 MPa).
been calculated by Weiss et ~ l . ' The
* ~ Si-N-0, Si-C-0, Si-C-N, This information can be applied as a guideline to the sintering of
and Si-C-N-0 systems have been calculated by the Lukas pro- Si3N4ceramic or SiC/Si3N4composite.
gram3 and the Eriksson p r ~ g r a m .These
~ calculations cover a
high-temperature range and a wide range of compositions. The
gas-phase compositions and the phases in equilibrium with the
gases are summarized in Table 3 of Ref. 1. Weiss et al. did not Temperature, O c
include carbon activity as a variable in their calculations. There- 1700 1600 1500 MOO 1300
I
fore, the equilibrium gas compositions in the systems without
5-
I I
-
graphite should be considered separately from those calculated
for a carbon activity of unity.
To clearly demonstrate a relationship between the solid and gas
phases, the solid phase stability is calculated as a function of
gas composition and pressure at known temperatures and car-
bon activities.

11. Stability of Phases J

(1) Phases in Equilibrium

The equilibrium phases in the system are P-Si3N4, Si2Nz0,
-5 -
P-Sic, SiOz (crystobalite), Si, C , and a gas phase. The main I I I L

components of the gas phase are N2, 0 2 ,SiO, and CO/COz. 5 .O 5.5 6.0 6.5

i/T lo4,K-'
Manuscript No. 199373. Received December 31, 1987; approved March 28,
1988.
Supported by the National Science Foundation under Grant No. MSM-8700568. Fig. 1. Equilibrium between p-Sic and P-Si3N4at a , = 1 with
Member, the American Ceramic Society. pN2= 1 and 10 atm (0.10 and 1.01 MPa).
837
838 Journal of the American Ceramic Society-Wada et al. Vol. 71, No. 10

Table I. Equilibrium Reactions in Si-C-N-0 System Table 11. Standard Free Energies of Formation of
Reaction Condensed Phases
(I) P-Si3N4 + 3C(s) = 3P-Sic + 2N2(g) Species A,Go (kJ/mol)* Reference
(2) 4P-Si3N4 + 302(g) = 6Si2N20(s) + 2N2(g) P-Si3N4 -925.2 + 0.450T 6
(3) P-Si3N4 + 302(g) = 3Si02(c) + 2N2(g) Si2N20 -658.3 + 0131T 6
(4) 2Si2N20(s)+ 4C(s) = 4 P - K + 02(g) + 2N2(g) P-Sic -72.832 + 0.007T 5
(5) 2Si2NzO(s) + 302(g) = 4Si02(c) + 2N&) SiO,(c) -900.384 + 0.171T 5
(6) +
Si02(c) C(s) = P-Sic + 0 2 ( g ) Si(Z) -42.825 + 0.06477 - 0.24 X 10-4T2 5
(7) 3Si(Z) or (s) + 2N2(g) = P-Si3N4 *Standard states are Si(s), C(s), pN, = 1 atm (0.10 MPa), and po, 1 atm
4Si(Z) or (s) + 2N2(g) + O&) = 2Si2N20(s)
=
(8) (0.10 MPa).
(9) Si(Z) or (s) + 02(,q)
= Si02(c)

Table 111. Invariant Points at Given ac and Temperature

Other equilibria, such as S i 3 N 4 / S i 2 N 2 0 ,S i 3 N 4 / S i 0 2 , librium is calculated as follows from reactions (1) and (2) in
Si2N20/Si02,Si2N20/SiC, SiC/Si02, Si/Si3N4, Si/Si2N20, and Table I:
Si/Si02 are calculated by the same principle. The equilibrium log K1 + 3 log U C - 2 log p N Z = -36909.385/T
reactions in the Si-C-N-0 system in the temperature range con-
cerned in this study are summarized in Table I. The standard free + 22.406 + 3 log a, - 2 log p N Z = 0 (11)
energies of formation are obtained mainly from JANAF Tables.s
The free energies of formation of P-Si3N4 and SizN20 are not log K2 + 3 log po2 = 13004.648/T
- 2 log p N Z
listed in JANAF. These two phases are important phases in the + 52.959 - 2 log p N Z + 3 log po2 = 0 (111)
Si-C-N-0 system and their free energies should be consistent
with each other. Two values are accepted from Hendry.6 The At known ac and temperature, the oxygen and nitrogen pressures
standard free energies of formation of the solid phases are sum- are calculated for this invariant point. Other invariant points are
marized in Table 11. also calculated as summarized in Table 111.
Examples of calculated stability diagrams are shown in Figs. 2
(3) Four-Phase Equilibria and 3. Figure 2 shows the stability diagram for a, = 1 and
When three solid phases and a gas phase are in equilibrium, T = 1673 and 1973 K, which are shown with solid and dashed
the number of degrees of freedom determined from the phase rule lines, respectively. The most significant feature of the diagram is
is two. Combining the equations shown in Table 11, phase stabil- the effect of nitrogen pressure and temperature on the phase sta-
ity is calculated at uc = 1 and and at temperatures 1673 and
1973 K. For instance, the Si3N4/Si2N20/SiC/gas-phase equi-

- 26 -21 -16 = 10-4

I -1673K
I4 ---1973K

-2.0
0
a
H
- -3.0 2
m
1

--4.0

- - 5.0

-2.01
-25
' I
- 20
I I
-15
! ,b.o I I
-25
I
-20
I I
-15
i
-10
Log ~ , ~ , o t m Log po2.0tm

Fig. 2. Phase stability diagram as a function of partial Fig. 3. Phase stability diagram as a function of partial pres-
pressures of nitrogen and oxygen at a, = 1, T = 1673 and sures of nitrogen and oxygen at a , = T = 1673 and
1973 K. 1973 K.
October 1988 Stability of Phases in the Si-C-N-0 System 839

Temperature, OC
1800 1600 I400

I \ mi
--- oC=

I
I I I I
5.0 6.0 7.0
1 / T ~ ~ 0 K4- 1,
I I I I
Fig. 4. Changes in equilibrium nitrogen partial pressure at in- 5.0 6.0 2.0
variant points as a function of temperature at a , = 1 and
yTX404,K-'
Fig. 5. Changes in equilibrium oxygen partial pressure at invari-
ant points as a function of temperature at a , = 1 and
bility, especially at the two invariant points where the effects can
be seen simultaneously. A large increase in the nitrogen pressure
is required to make SilN4 the stable phase at 1973 K, and avoid-
ing Si2N20phase formation is still difficult with increasing tem- relatively low -approximately lo-' atm ( MPa) at a , =
perature. This effect of temperature is shown more dramatically 1 -and increases 1 order of magnitude with the reduction of car-
in Fig. 3 when the carbon activity is reduced to ac = bon activity to This increase in pslo is less with increased
These stability diagrams can be a valuable guide in various nitrogen pressure. This implies that the decomposition of Si3N4
fields. One such field is ceramics sintering, which we will ad- by a graphite crucible can be reduced by increasing nitrogen pres-
dress in a later section. sure and keeping the SiO gas in the crucible by packing it with
(4) Gas Phase auxiliary Si3N4powders.
The composition and pressure in the gas phase affects the sta- (5) Application as a Guideline
bility of solid phases, as expected from Table I. Changes in the Densification of ceramic powders or composites by solid-phase
nitrogen and oxygen pressures at both invariant points for the sintering is a complicated process which involves powder prepa-
Si3N4/Si2N20/SiCand Si02/Si2N20/SiC or Si3N4/Si2N20/Si ration, green compact preparation, and sintering. There has been
and Si02/Si2N20/Siequilibria are shown as a function of tem- considerable active research on the parameters in these processes,
perature in Figs. 4 and 5, respectively. Both Si3N,/Si2N20/SiC such as, sintering temperatures, powder sizes, and compositions.
and SiO2/Si2NzO/SiC or Si3N4/Si2N20/Siand Si02/Si2N20/Si Since the gas phase of the sintering process is usually controlled
equilibria reach equilibrium with the same gas composition at by the flowing gas, the stability diagram is constructed as a func-
1417.8 K, as shown in Figs. 4 and 5. tion of temperature for known nitrogen pressures and carbon ac-
The stability diagram at 1417.8 K is shown in Fig. 6. The tivities. Flowing nitrogen gas is commonly used for silicon
phase stability is governed by the following reaction:
Si3N4 + Si02 = 2Si2N20 ( 10)
Hence, the silicon oxynitride (Si,N20) phase is present on the
same line at both carbon activities regardless of nitrogen and oxy-
gen pressures until the S i c or Si(s) phase appears at lower nitro-
gen pressures for ac = 1 and respectively.
All Sic, Si3N4,and Si2N20formations from SiOz occur via in-
termediate SiO(g) formation, and a high-temperature decomposi-
tion of Si3N4is also related to SiO(g) as
Si02(s) + C(s) --j SiO(g) + CO(g) (11)

SiO(g) + 3CO(g) + SiC(s) f 2C02(g) (12)

3SiO(g) + 3CO(g) + 2N2(g) + Si3N4(s) + 3c02(g)
(13)
2SiO(g) + CO(g) + N2(g) 4 Si2N20(s)+ C02(g) (14)
Therefore, the partial pressure of SiO in the gas phase is closely
related to phase stability and materials processing. Log pOz1atm
The Si3N4/Si2N20/SiCand SiO2/Si2N20/SiCo r Si3N4/
Si2N20/Siand Si02/Si2N20/Siequilibria are shown in Fig. 7 as Fig. 6. Phase stability diagram as a function of partial
a function of SiO partial pressure and temperature. The SiO pres- pressures of nitrogen and oxygen at T = 1417.8 K and
sure in the gas phase in equilibrium with Si3N4/Si2N20/SiCis ac = 1 and
 840 Journal of the American Ceramic Society-Wada et al. Vol. 71, No. 10

Temperature, OC Temperature, O C
I800 1600 1400 I200

\ \.

-pN2=I a t m (0.10MPa)

E
0

ON
a
cp
-20-
I:

P-Si3N4
-25 - 26
5.0 6.0 7.0
’/T x lo4, K-‘
Fig. 8. Phase relationships in the Si-C-N-0 system as a function of
oxygen partial pressure and temperature at ac = 1 , p N I = 1 atm
(0.10 MPa), andpN2= 10 atm (1.01 MPa).
-6 ) I
5.0
I
6.0 7.O
‘/T lo4,K - f
Temperature,“C
Fig. 7. Changes in equilibrium SiO partial pressure at invariant
points as a function of temperature at ac = 1 and

-15 -16

nitride sintering. Typical results are shown in Figs. 8 and 9. These

results, combined with gas compositions, can be applied as a
guideline to sintering both Si3N4and SiC/Si3N4 composite.
Figure 8 shows that for a nitrogen pressure of 1 atm
(0.10 MPa), Si3N4cannot be sintered without forming p-Sic at
temperatures higher than 1374”C, even if the oxygen pressure is
kept below atm MPa). However, if the nitrogen
pressure is increased to 10 atm (1.01 MPa), the sintering tem-
perature of Si3N4increases to 1536°C without S i c formation.
Furthermore, using a BN crucible instead of a graphite crucible 5.O 6.0 7.0
for sintering would increase the possible sintering temperature to l/T x 104, K-1
more than 1800°C (Fig. 9). For the sintering of Si3N4, at each
level of carbon activity, the nitrogen pressure must be always Fig. 9. Phase relationships in the Si-C-N-0 system as a
higher than the Si3N4/Si2N20/SiCline in Fig. 4, whereas the function of oxygen partial pressure and temperature at
oxygen pressure must be lower than the Si3N4/Si2N20/SiCline ac = and pN2= 10 atm (1.01 MPa).
in Fig. 5. If a carbon container is used for sintering Si3N4at
1700”C, nitrogen pressure as high as 70 atm (7 MPa) is required.
Appropriate nitrogen pressures are much higher at higher tem-
peratures. If the activity of carbon is reduced to ac = Reducing carbon activity would allow an increase in the sinter-
however, Si3N4can be sintered with atmospheric nitrogen pres- ing temperature. However, reducing the carbon activity too much
sure at 1700°C. would introduce metallic Si formation and increase the possibility
On the other hand, the SiC/Si3N4 matrix composite should of Si02 phase formation. If the carbon activity is reduced as low
be sintered at conditions for which the two phases coexist. as a, = the S i c phase will deteriorate and the metallic Si
Therefore, the carbon activity, the nitrogen pressure, and the gas phase will be formed in a composite sintered at 1870”C, even
phase should be well controlled. For instance, Si3N4 and S i c when the nitrogen pressure is 10 atm (1.01 MPa).
a r e in e q u i l i b r i u m at p N 2= 1 . 4 8 a t m ( 0 . 1 5 M P a ) a n d
poz < 7.41 X atm (7.59 X MPa) at 1673 K when a
graphite crucible is used, whereas the equilibrium condi- References
tions change to p N z = 7 0 . 7 9 atm (7.24 MPa) and ‘J. Weiss, H. L. Lukas, J. Lorenz, G. Petzow, and H. Krieg, “Calculation of
poz < 2.40 X atm (2.45 X lo-*’ MPa) at 1973 K (Fig. 2). Heterogeneous Phase Equilibria in Oxide-Nitride Systems,” CALPHAD: Comput.
Nitrogen pressure must be increased almost 50 times with an in- Coupling Phase DlQgrQms Thermochem., 5 [2] 125-40 (1981).
’J. Weiss, H. L. Lukas, and G. Petzow, “Calculation of Phase Equilibria in Sys-
crease of 300°C in the sintering temperatures. If the nitrogen tems Based on Si,N,”; pp. 77-87 in Progress in Nitrogen Ceramics. Edited by F. L.
pressure is lower than the values at SiC/Si3N4 equilibrium, the Riley. Martinus Nijhoff, The Hague, Netherlands, 1983.
’E.-Th. Henig, H. L. Lukas, and G. Petzow, CALPHAD VII Meeting, Stuttgart,
S i c phase becomes more stable and the Si3N4matrix would be FRG, April 1978, extended abstract pp. 235-44, 1978.
deteriorated. When the composite is sintered in a graphite %. Eriksson, “Thermodynamic Studies of High-Temperature Equilibria,” Chem.
Scr., 8, 100-103 (1975).
crucible under 1 atm (0.10 MPa) of nitrogen gas flow, the sinter- 5JANAF Thermochemical Tables, 3d e d . , American Chemical Society and
ing temperature should be close to 1374”C, whereas it can be in- American Institute of Physics for National Bureau of Standards, 1986.
creased to 1536°C when sintering is conducted at a high nitrogen 6A. Hendry, ‘Thermodynamics of Silicon Nitride and Oxyninide”; pp. 183-84 in
Nitrogen Ceramics. Edited by F. L. Riley. Noordhoff International, Leyden, Nether-
pressure of 10 atm (1.01 MPa). lands, 1977. 0