Thermodynamic analysis of the high-temperature stability of silicon nitride and silicon carbide

123

CERAMURGIA INTERNATIONAL, Vol. 2, n. 3, 1976

THERMODYNAMIC ANALYSIS OF THE HIGH-TEMPERATURE STABILITY OF SILICON NITRIDE AND SILICON CARBIDE S. C. SINGHAL Metallurgy and Metals Processing, Westinghouse Research Laboratories, Pittsburgh, Pennsylvania 15235

A thermodynamic analys'is of the stability of S i3N4 and SiC is presented which can be employed to assess their suitability for use at high temperatures in various environments. Specifically, the decomposition and the volatilization of S'i3N4 and SiC, and of SiO2, which is the major constituent in the oxide scales formed on their surfaces in oxidizing atmospheres, are discussed in terms of ambient environment and temperature. The calculated values of the lowest oxygen partial pressure up to which a protective S,iO2 layer should be maintained on the surface of Si3N4 and SiC are also presented.

1 - INTRODUCTION

Silicon nitride and silicon carbide have generated considerable interest in recent years as potential materials for many high temperature engineering applications 1~. In their dense high-strength forms, these materials are being proposed for use as structural materials in power generating gas turbines to increase their efficiency by allowing gas inlet temperatures to about 1400°C ~,'. These materials are also being considered for automotive gas turbines 6.7, small automotive thermal reactorsS, roller bearings', 1o, advanced combustion chambers", and radome applications 1,. Their potential usefulness and the maximum use temperature depend upon their chemical stability at high temperatures in gas turbine or various other environments. The attractiveness of SigN, and SiC for use as high temperature structural materials lies in the fact that a protective layer, containing predominantly silica (SiO=), is formed on their surfaces in highly oxidizing atmospheres '3-". Thus, the usefulness of these materials at high temperatures depends not only upon their intrinsic stability, but also upon the stability of this protective oxide layer. This SiO2 layer could be consumed from the surface of SigN, and SiC either by straight volatilization or by reaction with Si,N, or SiC at the Si3N~-SiO2 or the SiC-SiO2 interface. This paper presents a thermodynamic analysis of the decomposition and volatilization of Si,N,, SiC and SiO,. and of the interfacial reactions of Si,N, and SiC with surface SiO=. The Si3N, and SiC being developed for high-temperature

structural applications are prepared by hot-pressing or sintering respective powders with a densification aid. Dense Si3N, is prepared by hot-pressing with MgO '~ or Y203 '8, while SiC is prepared by hot-pressing with AhOy" or sintering with boron 20. These additives cause the formation of small amounts of extraneous phases, e.g., oxynitride and mixed silicates, in hot-pressed Si,N, and SiC materials, as well as in the surface oxide during oxidation '~. ". The existence of the phases complicates the situation requiring detailed phase equilibria and thermodynamic properties information on these phases for a complete analysis of the stability of commercial SigN, and SiC materials. In the absence of such information, the analysis presented here is performed for pure SigN,, SiC and SiO2. Any departure from this analysis is expected to be minor if the concentration of the extraneous phases is small. The present analysis, thus, can be used to assess the potential performance of commercial Si,N, and SiC materials in various environments. 2 - GENERAL PRINCIPLES OF T H E R M O D Y N A M I C CULATIONS

CAL-

The Gibbs free energy, AG, of a chemical reaction is related to the mass action constant, K, by the equation: AG = AG° -t- RT In K I'i] where AG° is the standard Gibbs free energy change of the reaction at temperature T (in OK), and R is the

124

s.c. $1NGHAL

gas constant. The mass action constant, K, is the ratio of the activities of the reaction products and the reactants in the chemical reaction. For example, for the reaction ~A + 13B--> 1"C + 6D, K--

a~c + a~D a= , + a p B

~G ° = --RT In K,

0

I

1600

1800

'

-1

rii]

For gaseous species in a constant pressure system, the activities may be taken as equal to respective partial pressures. When ~G < O for a reaction, that reaction occurs spontaneously; when Z~G > 0, the reverse reaction is possible; and when ~G = 0, the chemical reaction is at equilibrium. Thus, at equilibrium, AG = 0 and K = K,, the equilibrium constant at constant pressure, and the equation (i) can now be written as:

1-e~nperature, °C 1200 1400 I

1000

-2 -3 E

~-4 -5

-6

[iii]

The values of AG° for a reaction at different temperatures are obtained from the corresponding values for the different reactants and products through the relation: Z~Go(.=.o.) = ~ t~Go<,,od.o,,~__ T. ~Go(. . . . . ) [iv]" The thermodynamic analysis presented in this paper was carried out using equations [ i ] to [iv] for various reactions. The values of AG° for the reaction under consideration were calculated using the literature values of t~G° for various reactants and products in the reaction. All free energy data used in this paper, unless mentioned otherwise, have been taken from the JANAF Tables z,

-7

-8

8.0

]

I

7.5

7.0

t

6.5 104 / T, K-1

I

1

b.O

5.5

4.5

5.0

FIGURE 1 - Dissociation pressure o f Nz over Si;N 4 (s}. Temperature, °C 1000

1200

1400

1600

I

I

I

I

0

t /

-5

3 - DECOMPOSITION IONS

3.1

- SILICON

AND

VOLATILIZATION

REACT-

NITRIDE

Silicon nitride exists in two forms, = and 13, which for many years were thought to be the low temperature and the high temperature modifications, respectively, of the same compound =.~. However, Wild, et al. u, suggested that only ~-Si,N, is pure SisN,, while =-SigN, is an oxynitride of silicon containing ~ 1 at % oxygen. Based on this thesis, Wild, et al. ".~, determined separate thermodynamic data for c-and ~-Si,N,. Recently though, Priest, et al. =, and Kohatsu and McCauley = showed that ~-phase is not an oxynitride of silicon but also a true Si3N,. Their conclusion has also been substantiated by Blegen2' who found that c-phase may form even in the practically complete absence of oxygen and that it contains only minute amounts of oxygen, if any at all. The differences in the results of various investigators are difficult to reconcile, although not completely unexpected in view of the great experimental difficulties in working with very low oxygen partial pressures encountered in studying silicon nitride phases. Even though further experimentation is required to conclusively prove its nature, bulk of the evidence suggests that c-phase is not an oxynitride of silicon, but a true Si,N~ which usually only forms via a vapor phase reaction between a silicon-bearing gaseous species and nitrogen. Accepting that ~-phase represents the pure stoichiometric nitride at high temperatures, only ~-Si,N, need to be considered in the present thermodynamic analysis. Pehlke and Elliot ~ determined the Gibbs free energy of Si,N4 (of unspecified ~/~ ratio) by direct measurement of its dissociation pressures. Their data have been adopted by and are summarized in the JANAF Tables'L Blegen =' determined the Gibbs free energy of pure ~-SisN, by equilibrating Fe-Si alloys with nitrogen, and her data are in close agreement with the earlier data summarized in the JANAF Tables. The

g

-25 ~.o

I / 7.5

I 7.0

I 6.p z04/T, K-z

I 6.0

I ~.p

5.0

FIGURE 2 • Partial pressures o f various volatile specie= over SIsN4 {s) in 1 atm pressure Nz environment.

Gibbs free energy data of Wild et al." and Colquhoun et al. = are in poor agreement with the data of other investigators. However, several experimental inadequacies have recently been pointed o u t a,3t in their investigations. For this reason, their data are ignored, and the Gibbs free energy values for Si,N, summarized in the JANAF Tables are used in the present analysis. Silicon nitride dissociates into Si (solid or liquid depending on the temperature) and Nz gas according to the reaction =:" Si3N4(s) --> 3 Si(s, I) Jr 2 N=(g)

[1]

The pressures of Nz(g) formed by the dissociation of Si3N, are shown in Fig. 1 as a function of temperature. The dissociation N, pressure at 1600 K, a temperature where Si,N, is being proposed to be used, is only 2.8x 10-4 atm *. At 2151 K, Si3N, dissociates completely and the N2 pressure reaches 1 atm. This dissociation will be suppressed if Si,N4 is used in a high ambient N= pressure environment, raising in effect the dissociation temperatUre.

125

THERMODYNAMIC ANALYSIS OF THE HIGH-TEMPERATURE STABILITY OF SILICON NITRIDE AND SILICON CARBIDE

Si/SiC

Temperature, °C ]200 l

1000

0

T

1400

IO00

I

I

6

1

A

-5

SiC/C I

E -10

-8 ~

.~ ~ -- 10 --

aS

E

"

--

o_

_9 -12 SiC2(g) -25 ,K" 8.0

I 7.5

,..,r 7.0

i 0.5 104 / I, K-1

I 6.0

I 5.5

SiC (g)

l 5.0

-16 I I I I I/ -17.0 -16.6 -16.2 -15.8 -15.4 log PC' atm

FIGURE 3 - Partial pressures of various volatile species over Si;N4 (s) in 7.75 etm pressure N2 environment. The vaporization behavior of Si3N, is complex and remains only partly characterized as regards the nature of the volatile species 33. The various possible reactions in the volatilization of Si3N, include: 2 Si,N,(s) --->6 SiN(g) + N2(g)

[2]

4 Si3N,(s) ..-, 6 SGN(g) + 5 N2(g)

[3]

Si,N,(s) --* 3 Si(g) + 2 Ns(g)

[4]

2 Si3N,[s) --> 3 Sh(g) + 4 Ns(g)

[5]

Si3N,(s] --* Sis(g) + 2 Ns(g)

[6]

The partial pressures of the volatile species, SiN(g), SisN(g), Si(g), Sis(g) and Sis(g), calculated according to reactions [2] to [6] in a N, environment of 1 atm pressure are shown in Fig. 2 as a function of temperature. The predominant volatile species over Si3N, is SiN(g) up to about 1600 K, above which Si(g) becomes the predominant species. The partial pressure of both SiN(g) and Si(g) at 1600 K is 4 x 1 0 -'° atm, which is extremely small. The partial pressures of SisN(g), Sis(g) and Si,(g) are even smaller. High ambient N, pressures will further reduce the volatilization of Si,N,. The partial pressures of the volatile species in a N= atmosphere of 7.75 atm pressure, which is the case in a gas turbine operating at 10 atm total pressure, are shown in Fig. 3. The partial pressures of SiN(g) and Si(g) at 1600 K are now reduced to 3 x 1 0 -'° and 1 x l0 -'° atm, respectively. The partial pressures of other volatile species are similarly reduced. Thus the extent of volatilization of Si3N, at temperatures where Si,N, is being proposed to be used is extremely small. Furthermore, the formation of volatile species by the diss.ociation and volatilization of Si,N, in environments like that of gas turbines will, at best, be a localized condition, since a protective oxide layer should form very quickly over SigN, in oxidizing environments. 3.2 - SILICON CARBIDE Various investigators have reported conflicting decomposition temperatures and decomposition species for SiC. According to Baumann" and Baird anc~ Taylor 3s, SiC dissociates into Si and graphite at 2325-2475 K. Novikov" maintains that SiC rapidly dissociates only at 2575-2975 K. Lely 3, states that at temperatures above " 1 atm =

101325 Pa.

t:IGURE 4 - Partial pressures of various SiC(s) at 1600 K.

volatile

species

Si/SiC

SiC/C I

I

i

:

I

I

Si(g)

-5-

E

over

-7

-10-11

SiC(g) I -11.6

L -11.4

I -11.2

I -11.0

I -10.8

I -10.6

volatile

species

log Pc arm FIGURE 5 - Partial SiC(s) at 2000 K.

pressures of various

over

3023 K, SiC passes almost completely into the gaseous phase, and the solid residue (5-10% of the original quantity) consists only of graphite. However, careful mass spectrometric studies". 3' have shown that the main components produced by the dissociation of SiC are Si(g), Si2C(g) and SiC,(g), and the amount of other species (SiC(g), Si,(g), Si3Cg) in the vapors is negligibly small. Stull et al.2' have reviewed the data of various investigators, and according to their selected values, 13-SIC decomposes at 3259 K into a vapor mixture consisting of Si (32.1 mol %), SisC (20.1%), SiCs

126

s.c. SINGHAL

[45.4%), Si= (1.8%), SiC (0.28%), and Si, [0.23%). The Gibbs free energy data in the JANAF Tables" have been used in the present analysis, and this data predict decomposition of SiC into graphite and pure liquid Si at ~ 3337 K. The various possible reactions in the volatilization of SiC can be written as: SiC(s) --> SiC(g}

[7]

2 SiC(s) --> Si,C(g) + C(g)

[8]

SiC(s) --> Si(g) + C(g)

[9]

2 SiC(s) --> Sis(g) + 2 C(g)

[10]

3 SiC(s} --* Si,(g} + 3 C(g}

[11]

2 SiC(s) --* SiC=(g) 4- Si(s, I} [12] The partial pressures of various volatile species over SiC at 1600 and 2000 K, calculated based on reactions [7] to [12], are shown in Figs. 4 and 5, respectively, as a function of carbon potential. The carbon potential ranges chosen for Figs. 4 and 5 represent the regions over which SiC(s) is stable at each temperature. The carbon potential at the extreme left in both figures represents the Si/SiC phase boundary, while the extreme right carbon potential represents the SiC/C phase boundary. It is seen from these figures that the predominant volatile species over SiC(s) is Si[g), both at 1600 and 2000 K. At 1600 K, the maximum partial pressure of Si(g) over SiC[s) is 1 x 10--' atm, which is insignificant for most applications. The partial pressures of other volatile species are even smaller. However, at 2000 K, the partial pressure of Si(g} could be as high as 4.2x 10-~ atm. This might limit the use of SiC at such high temperatures in certain environments, e.g., under dynamic vacuum conditions where the volatile species will be continuously removed from the system. It is also seen from Figs. 4 and 5 that the partial pressures of Si[g), Si,(g), Sis[g) and Si,C(g} over SiC(s) decrease as the carbon potential increases. For example, at 1600 K, the partial pressure of Si(g) at the SiC/C interface is only 1 x 1 0 ' atm compared to 1 x 10-7 atm at the el/SiC interface. Therefore, any carbon deposits on bare SiC from the ambient environment will tend to lower its volatilization. 4. - STABILITY OF OXIDE FORMED OVER SLN, AND SiC

4.1 - NATURE OF OXIDE FORMED OVER SisN, AND SiC The attractiveness of Si,N, for use as a high temperature structural material is mainly due to a protective oxide layer which forms on its surface at high temperatures in oxidizing environments. Two oxides are known to exist in the Si-N-O system: silicon oxynitride [Si=ON,) and silica (SiO,}. The thermodynamic data used in the present analysis for these two oxides are reviewed below. Widely differing values have been reported by different investigators for the Gibbs free energy of silicon oxynitride. Ryall and Muan ~ determined the Gibbs free energy of Si,ON, by measuring the equilibrium pressure ratios of oxygen to nitrogen required for the existence of Si,ON,. These investigators observed the formation of Si=ON= from SiO,-saturated CaO-MgO-SiO2 and CaOSiO2 melts in N,/H,/H,O gas mixtures at 1400 and 1500°C, and from Si-Ni (alloy}-SiO= mixtures in nitrogen. However, in their study, they failed to take into account the formation of SiO[g} at the surface of the CaO-SiO, or CaO-MgO-SiO, melt which could have significantly altered the oxygen pressure in the N=/H=/H=O gas mixture. The SiO(g) pressures in their experiments would have roughly equaled the H,O[g) pressures in the bulk gas mixture causing great errors in their

interpretation of the data. Ryall and Muan themselves described their results as • approximate ,. Wild, Grieveson and Jack"." obtained the thermodynamic data on Si=ON= by studying the onset of its formation on powdered Fe-Si alloy in N~/H,/H=O gas mixtures. This gas mixture was generated by equilibrating N~/H2 mixture with Cr-Cr,O3 at temperatures between 700 and 900°C, and the resulting oxygen partial pressure was calculated from the known nitrogen content of the gas and the H,/H,O ratio characteristic for the Cr-Cr,O3 equilibrium. However, the nitrogen pressure in their experiments was several orders of magnitude higher than where chromium nitride can begin to form. Thus, one would expect that a CrN/Cr,O, equilibrium controlled the oxygen potential in their gas mixture rather than the Cr-Cr,O3 equilibrium as assumed by the investigators. In addition, the evolution of water vapor from the refractory tubes in their experiments could also have significantly altered their calculated oxygen pressure. It also seems doubtful that these investigators could have really detected the first onset of the formation of the equilibrium phases by x-ray diffraction technique when the amount formed is extremely minute. Recently, Blegen 2' determined Gibbs free energy of Si~ON, by equilibrating Fe-Si alloys with SiO2 and N,, and also by studying its thermal decomposition according to the reaction. Si,ON,(s)--> Si(s) + SiO(g) + N,(g) [13] in a vacuum by effusion measurements. The values for free energy of Si,ON2 obtained from these two sets of experiments different by at least 16 kcal mol -x, the values obtained from Fe-Si equilibration experiments being lower. However, the agreement between these two sets of values and the data of other investigators mentioned above =. ~,'° was poor. In view of the experimental shortcomings in the earlier investigations =. 26.4o and the fact that the values obtained by Blegen 2' from the equilibration experiments with Fe-Si alloys are likely to be low because of slow approach to equilibrium, it is felt that the free energy data from Blegen's effusion experiments represent the best available data for Si,ON,. These data are, therefore, used in the present analysis. The Gibbs free energy data for SiO2 in its various forms (amorphous as well as crystalline) have been reviewed by Stull et al. 2'. The exact nature of SiO2 formed by oxidation of Si,N, depends upon various factors such as impurities and additives in the material, its porosity, the composition of ambient environment, and temperature. Both amorphous and crystalline silica scales have been reported"'l~' for different silicon nitrJde materials. Fortunately, the Gibbs free energy data for different forms of SiO, are almost identical [within -+ 1 kcal moP') 5,. For this reason, all free energy data used for equilibrium calculations in this analysis are those for high-cristobalite [crystalline SiO~)". The error introduced by this assumption, even if a form of SiO2 other than cristobalite forms, should be negligibly small. Using the selected Gibbs free energy data for ~-Si3N,, Si,ON,, and SiO~, a thermochemical phase diagram has been constructed for the Si-N-O system at 1600 K, and this is shown in Fig. 6. This diagram, which is substantially identical to one presented by Blegen", represents the regions of thermodynamic stability for SisN4, SimON2 and SiO2 as a function of oxygen and nitrogen partial pressures. It follows from this diagram that on heating Si,N, in an oxidizing environment, a duplex oxide layer consisting of el,ON2 and Si,N4 should form on its surface. Depending upon kinetics of the reaction and the duration of oxidation, the Si,ON2 in the oxide layer may almost completely oxidize to SiO,. In various experimental investigations on oxidation of different SigN, materials, such duplex layers of Si,ON= and SiO,, however, have never been identified, and,

127

THERMODYNAMIC ANALYSIS OF THE HIGH-TEMPERATURE STABILITY OF SILICON NITRIDE AND SILICON CARBIDE

0 /

-1

/

T

I

I

I

I

I

0

Si3N4ls)

Siofgj

N

-3

8"~" -4

~

~

-5 -10

Si 021s1

Si02(g)

E

-5 .m Q_

-15

8~

-N -23 -22 -21 -20 -19 -18 -17 -16 log , atm Po2

-20

-

-25

FIGURE 6 - Thermo©homical diagram for the SI-O-N system at 1600 K.

furthermore, only in a few investigations Si=ON= has been detected in the oxide scales. In the oxidation of pure Si3N, powder ~1,'2 although both amorphous and crystalline SiO= have been reported to form, no Si,ON, was detected in the oxide scales. In the oxide scales over Si,N, hot pressed with MgO, enstatite [MgSiO,) has been found to be the major constituent with minor amounts of cristobalite and amorphous silica and only traces of SimON2'3,. Evans and Davidge'3 reported the formation of only cristobalite when reaction-sintered Sj3N4 was oxidized in air up to 1400°C. However, Themmler et al. '5 found that during oxidation of reaction-sintered Si3N, in air, large concentrations of Si2ON2 formed in the interior of the material when oxidized at 1400°C, though no increase in Si2ON= concentration, over that in the starting material, was observed for oxidation at 1200 and 1300°C. It appears that, due to about 30% porosity in their reaction-sintered material, the interior of the material oxidized rapidly to SimON,, closing the pores and limiting the subsequent supply of oxygen to the interior. Thus, Si,ON2 in the interior of the material could not oxidize further to SiO,. However, bulk of the evidence suggests that the oxide layer formed on Si3N4 consists predominantly of SiO=, which provides protection against further oxidation in oxidizing environments. For this reason, the stability of SiO2 layer is discussed in Sections 4.2 and 4.3. In the Si-C-O system, the only known condensed phases are SiC and SiO,, and, hence, on oxidizing SiC, a surface layer of SiO, is formed on it. Both crystalline and amorphous SiO, scales have been reported to form on oxidation of different SiC materials", % However, in the analysis on the stability of this surface SIC)= layer in subsequent sections, the Gibbs free energy data for high cristobalite has been used as mentioned previously. 4.2 -

-20

i -16

As mentioned in previous sections, a protective film consisting predominantly of SiO2{s) forms on the surfaces of Si,N, and SiC in oxidizing atmospheres according to the reactions: Si3N,[s) + 30,[g) --~ 3 SiO~(s) + 2 N,[g) [14] 2 SiC(s) + 30,(g) --* 2 SiO,(s) + 2 CO(g) [15] The volatilization of this surface SiO=(s) can result in the formation of various gaseous species according to the reactions: SiO,(s) --> SiO,[g) [16] [17] [18]

-12

-8

-4

0

4

IogP02, atm FIGURE 7 - Partial SiOz(s) at 1600 K.

pressures

I

of

I

0-

various

volatile

I

species over

I

~

-5

Si02(g )

~ -]0

Z

-20 r

-25

-12

FIGURE 8 - Partial SiOz(I) at 2000 K.

VOLATILIZATION OF SiO=

2 SiO,(s) --->2 SiO[g) + O,(g) SiO,(s) --> Si(g) + O,(g)

-30

-8 -4 0 logP02 ' atm

pressures

of

various

volatile

2 SiO,(s) --, Si,(g) + 2 0 , ( g ) 3 SiO,(s) - , Si,(g) + 30,(g)

4

species over

[19]

[20]

The partial pressures of the gaseous species formed according to reactions [16] to [20] have been calculated, and are shown in Figs. 7 and 8 at 1600 and 2000 K, respectively, as a function of ambient oxygen partial pressure. The extreme left oxygen partial pressure in these figures (3.2x 104' and 4.0x 10-1' atm at 1600 and 2000 K, respectively) represents the oxygen partial pressure below which SiO, is not thermodynamically stable. It is seen from these curves that SiO,(g) is the predominant gaseous species over SiO,(s) in high ambient oxygen pressures, while SiO(g) becomes

128

s.c. SINGHAL

the predominant species in low oxygen partial pressure environments. Also, the partial pressure of SIO2(g) formed over S|Oz(s) at 1600 K is only 3.8x10-" atm, and remains constant irrespective of the ambient oxygen pressure. The partial pressure of SiC(g] is lower than that of the SiO2(g) down to about 1 x 10~ arm oxygen pressure, but increases rapidly at lower oxygen pressures. Similar behavior is indicated at 2000 K. Thus the volatilization of SiO2(s] i s insignificant in highly oxidizing environments, e.g., in air, in gas turbines, etc., but becomes very critical in reducing environments, e.g., under vacuum, due to the formation of SiC(g). Hence the usefulness of Si,N, and SiC in reducing environments will be limited to rather low temperatures. Temperature, °C 1200 1400

lO00

o

The drastic difference in the volatilization behavior of SiO2(s] in oxidizing and in reducing atmospheres is also illustrated in Figs. 9 and 10, where the partial pressures of gaseous species formed in 1 arm O2 pressure and in 1 x 10-' arm 02 pressure environments, according to reactions [16] to [20], are shown as a function of temperature. Again, it is clear that in 1 arm 02 pressure, SiO2(g) is the predominant vapor species with rather low partial pressures. However, in 1 x l0-' arm O, pressure (reducing conditions), SiC(g) becomes the predominant species at temperatures above - 1250 K. Under such reducing conditions, the SiC(g] partial pressure could be as high as 2 x 10-4 arm at 2000 K. 4.3 - REACTIONS BETWEEN SURFACE SiO= AND ShN, OR SiC In addition to its volatilization, the protective surface layer of SiO2(s) could also be consumed by reaction with substrate Si3N, or SiC according to reactions:

1600

Si3N,(s) + 3 SiO2(s) --, 6 SiC(g) + 2 N , ( g ) SiC(s) + 2 SiO2(s) -+ 3 SiC(g) + CO(g)

£~-15

--

m Q_

-20 -25

-3O 8.0

I

I

7.5

7.0

~IT 6.5 104/f, K-I

I 6.0.

I 5,5

5.0

FIGURE 9 - Partial pressures of various volatile species over SiO2(s, I) in I arm pressure 02 environment. Temperature, °C

0

1000

1200

1400

1600

1

I

I

I

~

-5

.,0

y

-25

/ -30

,/ 8.0

I 7.5

J I 7.0

/

,<

-

/

I 6.5

[22]

These reactions result in the formation of SiC(g) and other gaseous species at the Si,N,[s)-SiO2(s) and SiC[s]-SiO2(s) interfaces. Assuming stoichiometric formation of gaseous products, the equilibrium partial pressures of SiC(g) formed according to reactions [21] and [22] have been calculated. These equilibrium SiC(g) pressures for Si3N4 and for SiC substrates are shown in Figs. 11 and 12, respectively, as a function of temperature. High SiC(g) pressures at high temperatures could cause rupture of the protective SiO=(s) layer with subsequent accelerated oxidation. More importantly, if all the surface SiO,(s) reacts according to reactions [21] and [22], then a bare SigN4 or SiC surface will be exposed causing = active, oxidation of Si3N, or SiC by the formation of SiC(g) according to reactions: 2 Si,N,(s) + 3 O2(g) --> 6 SiC(g] + 4 N2(g) [23] SiC(s) + O,(g)--* SiC(g) + CO(g] [24] This could only happen in environments with very low oxygen partial pressures. Wagner" has developed a theory to interpret the transition between • active • oxidation (by formation of gaseous SiC) of Si found at low oxygen partial pressures, and • passive, oxidation (by formation of a protective layer of SIC2] which occurs at high oxygen partial pressures. He derived the following equation for calculating the value of the 02 partial pressure, Pc,*, below which no oxide is maintained on the surface of silicon: Pc=* ~ 0.4 Psych.q=)

S\Ok°)

-I0

[21]

It" 6.0

I 5.5

/ 5.0

104/f, K-1 FIGURE 10 - Partial pressures of various volatile species ever SIO2(I, I) in 10-s arm pressure 02 environment.

where Ps,o,,~=) is the equilibrium partial pressure of SiC(g) at the Si-SiO2 interface. Extending this theory to the oxidation of Si,N, and SiC, the values of the O= partial pressure, Pc=*, below which all the surface SiO~ reacts to form SiC(g} according to reactions [21] and [22], and above which a protective SiC2 layer is maintained, have been calculated. These values of Pc=° are also shown in Figs. 11 and 12 as a function of temperature. According to these figures, at 1600 K, a protective layer of SiO,(s] should be maintained on Si,N, down to ambient oxygen pressure of 8 x 10~ arm, and on SiC down to 3.6x 10~ arm. However, at 2000 K, this protective layer of SIC, is expected to be maintained only down to O2 pressure of 3.5x 10-' arm on Si,N,, and 1.8x10-' arm on SIC. Below these oxygen pressures, • active • oxidation of ShN4 and SiC c a n occur limiting their usefulness as structural materials. Thermodynamically, the above analysis for Si,N, is n o t completely correct, since as shown in Fig. 6, the oxide in equilibrium with Si3N, should be ShON, end not SIC2. However, as already discussed in Section 4.1,

129

THERMODYNAMIC ANALYSIS OF THE HIGH-TEMPERATURESTABILITY OF SILICON NITRIDE AND SILICON CARBIDE

Temperature, °C 1200 1400

0 1000 I

~

I

Si=ON2(s) + S i O , ( s ) - * 1600

i

3 S i O ( g ) + N~(g}

[25]

t o f o r m SiO(g) at the Si2ON2-SiO, interface. The equi. l i b r i u m SiO(g) p r e s s u r e at this i n t e r f a c e at 1600 K w o u l d be 8 . 0 4 x 10-~ a t m ; t h i s is c o m p a r a b l e t o t h e e q u i l i b r i u m SiO(g) p r e s s u r e at t h e = a s s u m e d . Si,N,-SiO2 i n t e r f a c e s h o w n in Fig. 11.

i

-2 5 - CONCLUSIONS

A t h e r m o d y n a m i c analysis of t h e s t a b i l i t y of Si3N+ and SiC has been p r e s e n t e d w h i c h can be used to assess t h e i r b e h a v i o r at d i f f e r e n t t e m p e r a t u r e s in v a r i o u s envir o n m e n t s . The v o l a t i l i z a t i o n of both Si+N, and SiC results in gaseous s p e c i e s w i t h r a t h e r l o w partial p r e s s u r e s at 1600 K. F u r t h e r m o r e , a p r o t e c t i v e o x i d e f i l m , c o n s i s t i n g p r e d o m i n a n t l y of SiO2, is f o r m e d in o x i d i z i n g a t m o s p h e r e s on both SigN, and SiC, w h i c h should be m a i n t a i n e d d o w n t o a m b i e n t o x y g e n partial p r e s s u r e s of 8 x 10-' and 3.6 x 10-' arm, r e s p e c t i v e l y , at 1600 K. B e l o w t h e s e pressures, w h i c h are d e p e n d e n t on t e m p e r a t u r e , no p r o t e c t i v e SiO~ layer is e x p e c t e d to be f o r m e d , and both Si,N, and SiC can d e t e r i o r a t e due to the f o r m a t i o n of gaseous silicon m o n o x i d e .

(A) E o_'-4

BI

-6

-8.0

7.5

7.0

6.5

6.0

5.5

5.0

ACKNOWLEDGMENTS

104/T, K - ] FIGURE 11 - Equilibrium pressures of SiO(g) at the Si3N4-SI02 inter. face (A); and the O5 partial pressures, Po2*, above which a protective SiOz layer should be maintained on Si~N4(B).

Temperature, °C 1200 1400 I I

1000 0 I

REFERENCES 1000 i

-2

-4

-6

-8 8.0

I 7.5

I 7.0

L 6.5 104/T, K - I

L 6.0

This work was supported by the Advanced Research Projects Agency, Department of Defense, under Contract DAAG 46-71-C-0162.

L 5.5

5.

FIGURE 12 - Equilibrium pressures of SiO(g) at the SiC-SiOz Inter. face (A); and the 02 partial pressures, Po2*, above which a protective SiOz layer should be maintained on SiC (B}.

e x p e r i m e n t a l i n v e s t i g a t i o n s have s h o w n t h a t SiO, is the p r e d o m i n a n t o x i d e f o r m e d on Si,N,. For t h i s reason, t h e above analysis can be used t o at least approxim a t e l y assess t h e b e h a v i o r of Si3N, in e n v i r o n m e n t s w i t h d i f f e r e n t o x y g e n partial p r e s s u r e s . From t h e purely t h e r m o d y n a m i c e q u i l i b r i u m p o i n t of v i e w , though, Si2ON2 and SiO2 c o - e x i s t e n c e should be analyzed r a t h e r than t h e Si,N+-SiO2 interface. Silicon o x y n i t r i d e and SiO2 could react according to reaction:

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130

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s.c. SINGHAL

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