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Oxidation behaviour of SiC fibre reinforced SiC
jem'nalof ELSEVIER
Journal of Nuclear Materials 227 (1995) 130-137
Hdear ada'ials
Oxidation behaviour of SiC fibre reinforced SiC H. Kleykamp *, V. Schauer, A. Skokan Forschungszentrum Karlsruhe, Institut fiir Materialforschung, Pos(fach 3640, 76021 Karlsruhe, Germany Received 31 October 1994; accepted 6 July 1995
Abstract
The oxidation behaviour of one-dimensional and two-dimensional SiC fibre reinforced SiC was investigated up to 1520°C in air, as well as in water vapour saturated air and argon by calorimetry, DTA and TGA. The oxidation process takes place in three steps: (1) oxidation of free carbon in the carbon coated composites between 530 and 690°C with mass losses up to 7.5%; (2) a fast exothermal process associated with mass gain starting at 800°C and terminated below 1500°C within 1 h; (3) the diffusion controlled oxidation of bulk SiC. The processes follow a logarithmic rate law between 875 and 985°C with an effective activation energy Q = 84 kJ/mol. The amorphous reaction product SiO2 transforms to cristobalite at (930 + 50)°C. The oxidation kinetics follows a quadratic rate law above 1000°C with rate constants k = 3.5 x 10 - 7 g2/cm 4 h for 1D SiC/SiC and k = 5 x 10 -8 g2/cm4 h for 2D SiC/SiC at 1520°C in air. The rate constants are up to three orders of magnitude higher than that for high density monolithic SiC which is explained by the high porosity of the SiC matrix.
1. Introduction
Silicon carbide, SiC, in its different variations is being discussed as structural material for plasma facing components in a fusion reactor due to the low atomic number, high thermal conductivity, reasonable strength properties and excellent corrosion resistance in aggressive media. SiC fibre reinforced SiC is believed to be even superior to monolithic SiC. However, severe corrosive interactions are expected to occur in case of air and water vapour breakthrough to the plasma at high temperatures above 800°C. Therefore, the passive oxidation behaviour of different SiC fibre reinforced materials was investigated between 875 and 1520°C in synthetic air and water vapour saturated air and argon at 1 bar total pressure. SiC is a line compound and decomposes peritectically into graphite and a Si-C melt at 2830°C [1]. Numerous papers have been published on the oxidation behaviour of monolithic Si and SiC in air or oxygen under different total pressures. Basic papers appeared on the insight of the logarithmic, linear,
* Corresponding author. Tel.: +49-07247-822888; fax: + 49-07247-82-4567.
quadratic and cubic rate laws up to 30 years ago [2]. A comprehensive review was published recently [3]. However, only a few results have been reported in the literature [4-15] on the oxidation kinetics of monolithic SiC near 1520°C (see Table 1). This temperature was selected for our own isothermal experiments. The mechanism of the passive SiC oxidation is correlated with the reaction SiC + 202 3 ~ SiO 2 + CO. (1) The rate determining step is based on the diffusion of oxygen through the formed SiO 2 layer followed by the reaction of SiC at the SiC-SiO 2 interface. The formed CO is transported by opposite diffusion through SiO 2 to the surface and may further react to CO2. Different activation energies were reported in the various temperature ranges which may depend on the type of the diffusing oxygen (O 2 or O) and on the structure of SiO 2 (amorphous or crystalline, porous or dense). Below 1200°C, molecular oxygen would diffuse through the SiO 2 layer that is amorphous below 1000°C and crystallizes to cristobalite above this temperature. The SiO 2 layer is dense above 800°C, the oxidation follows a quadratic rate law [4]. Proportionality is observed between the isothermal rate constant and the logarithm of the oxygen partial pressure [16]. The activa-
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H. Kleykamp et al. / Journal of Nuclear Material 227 (1995) 130-137
Table 1 Literature data of the oxidation kinetics of monolithic SiC at 1520°C with quadratic rate law and rate constant k in different atmospheres under 1 bar total pressure Material
Atmosphere
k (gZ/crn4 h)
Ref.
Reaction bonded SiC, porosity 1.7% CVD SiC layer SiC spheres, 40-63 i~m diameter SiC spheres, 40-63 i~m diameter CVD SiC layer CVD SiC layer Reaction bonded SiC, 10% Si, porosity 1% Reaction bonded SiC, 2.5% Si, 0.35% C Hot pressed SiC, 1-2% AI CVI SiC layer Reaction bonded SiC, 0.5% B, 0.5% C Reaction bonded SiC, 0.5% B, 0.5% C Hot pressed SiC, 4% A1203,4% WC Reaction bonded SiC, porosity 10% Reaction bonded SiC, porosity 30%
air oxygen air oxygen air oxygen air air air oxygen air/oxygen Ar + 0.03 bar H20 oxygen air air
2 x 10-10 5 × 10- 10 5 x 10- l0 1 X 10 -9
[9] [15] [7] [7] [10] [14] [4] [8] [11] [6] [5] [5] [13] [12] [12]
tion energy below 1400°C is about Q = ( 1 0 4 + 20) kJ/mol and is dependent on the respective reaction mechanism. An increase of the activation energy was stated above 1400°C that would be due to increased viscosity and plastic deformation of the formed SiO 2 layer [4]. The passive oxidation of SiC is further observed in water vapour or in water vapour containing air. The mechanism is based on the decomposition of water into its components on the SiC surface and successive oxidation according to SiC + 3 H 2 0 ~ SiO 2 + 3H 2 + CO.
(2)
The formation of aliphatic hydrocarbons was not stated. The oxidation rate increases proportionally to the logarithm of the water vapour pressure [4]. Recent experiments show that 10% water vapour in oxygen enhanced the oxidation rate of SiC only very slightly between 1200 and 1400°C compared to rates found in dry oxygen [14], see Table 1. Very low oxygen partial pressures suppress the SiO 2 layer formation and an active oxidation takes place, a volatilization according to the reaction SiC + 0 2 ~- (SiO)gas q- CO.
(3)
Available literature data on the oxidation kinetics of monolithic SiC in air, oxygen and water vapour under 1 bar total pressure and at 1520°C are compiled in Table 1. The results are based on a quadratic rate law of the SiO z layer growth, ( A m / F ) 2 = kt. The mass increase is Am and F is the geometric area of the oxidized sample. The results indicate for the rate constant k a figure in the order of 10 -9 g2/cm4 h for high density monolithic SiC. A reaction layer thickness 0.14 ixm follows for cristobalite of density p = 2.33 M g / m 3 after
10 -9
1 × 10 -9 2-10 -9 10-s 10-8 1 × 10-8 1 × 10-8 2× 10-8 4 × 10- 7 2× 10 - 6 2 x 10-4
1 hour reaction time. Silicon infiltrated SiC exhibits no worse oxidation behaviour than monolitic SiC at 1520°C. Other additives may enhance the oxidation rate. There is a strong dependence of the oxidation rate on the porosity of SiC up to five orders of magnitude above that of 100% dense SiC, see Table 1. Further, the oxidation behaviour of SiC is deteriorated by the presence of water vapour. However, only few quantitative experiments are available, which deal with the influence of the water vapour partial pressure or the porosity on the oxidation behaviour of monolithic SiC at 1520°C. As the influence of a fibre reinforcement on the chemical properties of SiC is less known, three SiC fibre reinforced SiC qualities were investigated in this work with respect to the oxidation behaviour in air and in water vapour atmospheres. Details on this topic and on selected mechanical properties were reported in Ref. [17].
2. Experimental 2.1. Materials
One-dimensional SiC fibre reinforced SiC (Dornier, Friedrichshafen, Germany) was supplied in form of fiat samples with the dimensions 1.4 x 9.5 × 60 mm 3. The untwined fibres with a diameter 12-20 i~m which have been coated with a carbon layer approximately 0.2 I~m in thickness were aligned parallel to the plate surfaces. The material between the fibres and in the surface regions consists of SiC that was formed by pyrolysis of methyltrichlorsilane. Measurements of the geometric
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H. Kleykamp et al. / Journal of Nuclear Materia1227 (1995) 130-137
Fig. 1. Light-optical microstructure of one-dimensional SiC fibre reinforced SiC (Dornier) perpendicular to the fibre direction. density yield a figure for the porosity of the composite of ~.bout 35 vol% related to the theoretical density p = 3.22 M g / m 3 of pure SiC. The light-optical microstructure perpendicular to the fibre direction is illustrated in Fig. 1. The SiC fibre fraction is about 40% of the total SiC inventory. The oxygen content in the SiC matrix is less than 1 mass%. The chemical analysis of the SiC fibres is given in Table 2. The two-dimensional SiC fibre reinforced SiC (General Atomics, GA, San Diego, USA) was delivered by one fiat sample with the dimensions 2.0 x 45 x 45 mm 3. Every 500 single fibres with a diameter 10-20 Ixm were integrated within filaments which were two-dimensionally (00/90 °) woven to a fabric and were coated with a 0.3 Ixm thick carbon layer. Four SiC layers form a composite which was soaked by an organic silicon compound and reacted to SiC by decomposition at ll00°C. Measurements of the geometric density yield a figure for the porosity of the composite of about 32 vol% related to the theoretical density. The light-optical microstructure perpendicular to one of fibre directions is illustrated in Fig. 2. The SiC fibre fraction is about 35% of the total SiC inventory. The oxygen content in the SiC matrix is less than 1 mass%. The chemical analysis of the SiC fibres is given in Table 2. Other two-dimensional SiC fibre reinforced SiC (Refractory Composites Inc., RECOMP, California USA) was delivered in form of 5 x 100 x 200 mm 3 sheets. Every 500 single fibres with a diameter of
Fig. 2. Light-optical microstructrue of two-dimensional SiC fibre reinforced SiC (GA) perpendicular and parallel to the filament directions. = 15 ~m were integrated within filaments which were two-dimensionally (00/90 °) woven to a fabric. The fabrics were coated with a 0.3 I~m thick C - S i O 2 - C layer [18]. Fifteen fabrics form a composite which was soaked by chemical vapour infiltration with methyltrichlorsilane and reacted to SiC by decomposition at 1200°C. The SiC overlayers on the composite are 200 p~m thick. The matrix consists of 65% cubic SiC and 35% hexagonal SiC. The porosity of the uncoated material is 30 vol%. The chemical analysis of the fibres is given in Table 2. The fibres contain about 11% free carbon [18,19]. The fibres (NicalonTM) were produced by Nippon Carbon Co., Japan in different compositions. Separate phases could not be identified by scanning electron microscopy and X-ray microanalysis. The oxygen concentration in the three investigated materials was found to be evenly distributed within the fibres. 2.2. Instruments
Anisothermal and isothermal oxidation tests were conducted on 1D SiC/SiC (Dornier) and on 2D SiC/SiC (GA) by calorimetry and differential thermal analysis (DTA). An isoperibol high-temperature calorimeter HTC 1800 (Setaram, Lyon, France) with heating rates 2 and 5 K / m i n was used for the anisothermal tests, a D T A 404/3 unit (Netzsch, Selb,
Table 2 Composition of the NicalonTM SiC fibres in the composites of different manufacturers, values in mass% Manufacturer
SiC fibre
Si
C
O
N
B
Ref.
Dornier GA RECOMP
one-dim. two-dim. two-dim.
55.5 53.8 56.9
28.0 29.5 30.6
14.4 15.0 11.3
0.1
3.4 0.01 0.008
this work this work [18,19]
H. Kleykamp et al. / Journal of NuclearMaterial227 (1995) 130-137 Germany) for isothermal tests, both instruments operable up to 1530°C. The samples in the 100 mg range have a defined geometric surface and were oxidized under 1 bar total pressure with 9 cm3/min flow rate in synthetic air (p(O 2) = 0.21 bar), in synthetic air water vapour saturated at 25°C ( p ( H 2 0 ) = 0.032 bar) and in high purity argon water vapour saturated at 25°C ( p ( O 2) = 3 × 10 -6 bar, p ( H 2 0 ) = 0.032 bar). The detection limit of the calorimeter is 0.5 mJ/s. Heat effects could be quantitatively measured in the S i C - C - O 2 system above 500°C by calorimetry. Semiquantitative heat effects could be detected by DTA above 1450°C. The isothermal oxidation of the samples took place at 1520°C up to 24 h after initial heating under argon with 10K/min heating rate. The weight gain of the samples was measured off-line by gravimetry. The isothermal oxidation of 2D SiC/SiC (RECOMP) was performed by thermogravimetry (TGA) using a symmetrical microthermobalance, type MTB10-8 (Setaram, Lyon, France). The sensitivity is 0.5 Izg, the temperature range of operation extends to 1000°C. The samples in the 100 mg range were heated from room temperature up to 850-1000°C under helium with a heating rate 10 K/min. Isothermal thermogravimetry was followed on-line in synthetic air with a flow rate 1 1/h under 1 bar total pressure.
I"~I"~ 1300
133
Small samples between 3 and 20 mm edge lengths were cut off from the 2D SiC/SiC (RECOMP) material. The surfaces of the sheets and those perpendicular to the fabrics of the annealed and the reference materials were analyzed by X-ray diffraction (Seifert, Ahrensburg, Germany) at room temperature, which indicated the crystallinity in the SiC matrix surface and in the SiC fibres, resp.
3. Results
3.1. Calorimetry and differential thermal analysis on 1D SiC~SiC (Dornier) and 2D SiC~SiC (GA) The oxidation experiments on 1D SiC/SiC (Dornier) by anisothermal calorimetry in air up to 1450°C with a heating rate 5 K / m i n revealed an exothermal peak between 530 and 690°C which is based on the oxidation of the unprotected carbon coating according to the reaction C + 0 2 ~ CO 2. The recorded peak area of 20 mV s corresponds to - 2 9 J, see Fig. 3. This value is equivalent to the 7.35 × 10 -5 fold of the molar enthalpy of combustion of graphite AfH ° = - 3 9 4 3 0 0 J / m o l at the mean temperature 850 K [20]. From these results follows that 0.9 mg C was burnt and the investi-
'(C)
FEAT FLOW
1200 iiO0 iO00
a00
t80.
700
:tOO _ 80
60_ ~
g K 40_
20_
-20 tl000
13000
I " ZM E
t6000
Ill _,^
-
Fig. 3. Anisothermal oxidation (heating rate 5 K/min) of one-dimensional SiC fibre reinforced SiC (Dornier) in air up to 1450°C. The peaks characterize the combustion of the free carbon (maximum at 620°C) and the first rapid SiC oxidation (maximum at 1200°C).
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H. Kleykamp et al. / Journal of Nuclear Material 227 (1995) 130-137
gated 263.9 mg SiC sample contained not under 0.33% free C. The finding is in agreement with the mass loss of 0.4% observed by gravimetry on a 1D SiC/SiC sample heated in air up to 800°C. The carbon oxidation process results in an annular pore circumferential to the SiC fibre which is surface sealed by sucessive SiO 2 formation [21]. Anisothermal calorimetry in air with 5 K / m i n heating rate indicated further the onset of the first rapid SiC oxidation step at about 800°C of which the oxidation rate passed a maximum at 1200°C, see Fig. 3. The isothermal gravimetric experiments on 1D SiC/SiC were performed at 1520°C in air, in air 0.032 bar H 2 0 and in Ar - 0.032 bar H 2 0 at 1 bar total pressure up to 24 hours reaction time. The results are presented in Fig. 4. An initial mass loss of 0.3 0.4% by free carbon combustion is followed by a distinct mass increase of about 2% within 1 h after reaction beginning. The following true SiC oxidation is based on a quadratic rate law, ( A m / F ) 2 = a 0 + k t , independent of the composition of the three selected gas mixtures (see Fig. 4 and Table 3), where Am is the mass gain, F is the geometric surface of the sample, k is the isothermal rate constant and a 0 is an induction period constant which characterizes the two prereactions. The progress of the reaction, which is the fraction of SiC in mol% oxidized to SLOE, was calculated from the mass gain and is added to Table 3. It is higher for the oxidation with water vapour saturated air than with pure air. The higher reaction rate is restricted to the beginning and is terminated after 1 h at 1520°C. The oxidation experiments on 2 D S i C / S i C (GA) by anisothermal calorimetry in air up to 1500°C do not indicate a peak that can be attributed to the combustion of free carbon at about 600°C. This finding is in aggreement with the observation of a very low mass loss < 0.1% of a sample heated in air up to 800°C. Fig.
(a)20
1dim.SiC-SIC T=1520"C,pt=lbar • 0,21bar02 ~ 15 o0,21barOz/0,03barHzO * Ar/ 0,03barH20 "~ " ,o 10 ~. _
0
o
•
~..~.
~
[
°o
'-
; ~-1'o
l's-'
2'0
' ~s
tin h
(b) 1,4
"~"E
1.2
2dimSLC-SiC . T=1520'CPt , =1bar _c .~,o~ 0,80,61'0 • 0,21barO ~ 2
•
O
~
0,4
0,2
O0
~
5
L
L
10
~
0 25 tinh Fig. 4. Oxidation behaviour of one-dimensional (a) and t~odimesional (b) SiC fibre reinforced SiC (Dornier and GA) in air, air'water vapour and argon-water vapour at 1520°C.The oxidation follows a parabolic rate law after combustion of the free carbon and fast initial oxidation. The solid lines represent least-squares fits of the SiC oxidation in air. 15
2 indicates that the carbon coatings of the SiC fibres have no direct contact with the open porosity of the composite. Therefore, the carbon burning is shifted to higher temperatures and is superimposed to the first step to the SiC oxidation. This observation is in agree-
Table 3 Oxidation kinetics of one-dimensional and two-dimensional SiC fibre reinforced SiC with quadratic rate law (Am I F ) t >_1 h, at 1520°C and 1 bar total pressure Material
Atmosphere
a o (10-6 g2/cm4)
k (10-6 gZ/cm4 h)
Reacted SiC in mol% after 10 h
1-dim., Dornier
air
4.3
0.351
6.5
1-dim., Dornier
air + 0.03 bar H20
6.5
0.35
7.8
1-dim., Dornier
argon + 0.03 bar H20
2
0.3
5.5
2-dim., GA
air
0.06
0.05
1.1
2-dim., GA
air + 0.03 bar H20
0.06
0.05
1.1
2=
a o + kt,
H. Kleykamp et al. / Journal of Nuclear Material 227 (1995) 130-137
ment with isothermal mass loss experiments on 1D S i C / C / S i C which started within 20 minutes reaction time not below 800°C [21]. The onset of the first step of the SiC oxidation was stated above this temperature, however, the extent of reaction is only a third of that of 1D SiC/SiC. The isothermal gravimetric experiments of 2D SiC/SiC were performed under the same conditions as described for 1D SiC/SiC. After a negligible mass decrease < 0.1% by free carbon combustion and a low mass increase by a fast SiC oxidation process within 1 h after reaction beginning, the true oxidation follows again a quadractic rate law, ( A m ~ F ) 2 = a o + kt independent of the selected gas environments, see Fig. 4 and Table 3. The extent of reaction was calculated from the mass increase of the samples and is added to Table 3. The three-stage character of the total oxidation process was observed also by Narushima et al. [15] Naslain [22] and Lamouroux et al. [23] and is attributed to the combustion of interlayer carbon and to a fast chemical reaction between oxygen and the three components of the metastable SiC(O) NicalonT M fibres [15,22] and gas phase diffusion through microcracks in the SiC coating [23], resp. These processes are followed by the diffusion controlled SiC oxidation. 3.2. Thermogravimetry on 2D S i C ~ S i C (RECOMP)
Isothermal thermogravimetric experiments were performed on 2D SiC/SiC (RECOMP) in air at 875 and 985°C up to 18 h. A fast relative mass decrease A m / m o of the samples down to - 7 . 5 % was observed at short reaction times up to 1.4 h that is based on the combustion of a high fraction of free carbon in this material. The oxidation of SiC to SiO 2 dominates at longer reaction times. The results are illustrated in Fig. 5. The presentation in a logarithmic time scale shows clearly the logarithmic rate law in this temperature range according to A m / m o = a o + k In t. The temperature dependent constant, a0, considers the fraction of the burnt carbon in the material, k is the isothermal logarithmic rate constant of the SiC oxidation. The rate law gives at 875°C with t > 1.4 h for a 122.6 mg sample: A m / m o = -0.0661 + 0.00227 In t, t in h, and at 985°C with t > 1.4 h for a 76.3 mg sample: A m / m o= -0.0736 + 0.00491 In t, t in h. The plot of the logarithm of the logarithmic rate constant versus the reciprocal temperature yields the activation energy Q = 84 k J / m o l for the rate determining step of oxygen diffusion through the SiO 2 layer formed on the surface of the porous SiC matrix in ~he temperature range 875-985°C. The mass decrease by combustion of free carbon dominates the SiC oxidation process at short times. The first reaction is terminated after 1.4 h. Assuming the reaction given by Eq. (1) for the latter
135
process, the progress of reaction was calculated as 1.0% and 2.5% of total SiC oxidized at 875°C and 985°C, resp., after 17 h. The logarithmic rate law after the uniform degradation of the carbon interlayer was also observed in other experiments, e.g. Refs. [22,24]. 3.3. X-ray diffraction on oxidized 2D S i C ~ S i C (RECOMP)
The as-received strongly textured 2D SiC/SiC (RECOMP) consists of the cubic modification and of the hexagonal 4 H and 6 H polytypes. Oxidation in air for 17 h up to 875°C exhibits no further X-ray diffraction lines. Oxidation at and above 985°C shows one additional reflex which is related to the tetragonal room temperature modification of cristobalite SiO 2 (JCPDS11-695). This phase could have transformed from the originally produced high temperature modification. The X-ray diffractogram is illustrated in Fig. 6. The amorphous SiO 2 formed by oxidation of SiC at lower temperatures transforms to the crystalline modification at (930 + 50)°C. Samples oxidized at higher temperatures exhibit no additional cristobalite lines though the intensity of the (101)-reflex increases with oxidation temperature. A cristobalite formation was reported at 1070°C [5], < 1093°C [13], < 1200°C [25] and > 1200°C [16] in earlier oxidation experiments of SiC.
2,8h
5,6h
8,3h
11,1h
13,9h
16,7h
19.4h
(a) o
~ I
-1
;
875"C
1148K
-2
c
-5
~-6 -7 -8
10 000
Ib)
20 000
28h
0-
30 000 40 000 tins
50 000
50h
83h
139h
r
!
~-2
11 h
70 000
16,h
19,h
5'C
|
~-5
__
~--
~-6
i
I
~
-7 ,
0 5.
~
10 000 Oxidation
,
I
,
20 000 behaviour
,
,
30 000 40 000 tins
_
I
~~
-8
Fig.
60 000
, ,
i
50 000
of two-dimensional
,
,
60 000 SiC
fibre
,
70 000 rein-
forced SiC (RECOMP) in air at (a) 875 and (b) 985°C. The oxidation follows a logarithmic rate law for t > 1.4 h after combustion of the free carbon.
136
H. Kleykamp et al. / Journal of Nuclear Material 227 (1995) 130-137 I
I
I
I
I
cristoba I i r e tetragona I (181)
1158°C / 188h / a i r
1858°C /
18h / a i r
585°C / ll~h / a i r
875°C / fl
18h / a i r
C
~
D reference
Z8
I
I
38
48
i
Z thata
58 (deJjree)
I
I
68
78
material
tiff
Fig. 6. X-ray diffraction pattern of a partially oxidized 2D SiC/SiC (RECOMP) sample perpendiucular to the plate surface indicating cristobalite formation; the letters A to D denote SiC reflexes.
4. D i s c u s s i o n
The qualitative valuation of the investigated chemical properties of the SiC fibre reinforced SiC composites with a pore volume between 32 and 35 vol% indicates already a strong influence of the porosity and a more unfavourable oxidation behaviour than that of high-dense monolithic SiC (p = 3.22 Mg/m3). The oxidation experiments on 1D and 2D SiC/SiC materials between 500 and 1520°C have shown that the reactions proceed in three steps. The first step is characterized by the combustion of free carbon which, in an anisothermal experiment, starts at 530°C and is terminated at 690°C within 30 minutes. This is correlated with a mass loss markedly observed on 2D SiC/SiC of RECOMP with 11 mass% free carbon in the composite. The mass increase in the second step implies a fast oxidation process which starts at about 800°C, is terminated below 1500°C within 1 hour and is based on an equilibration of the oxygen containing metastable Nicalon TM fibre or of the total composite. The third step is the true diffusion controlled SiC oxidation which follows a logarithmic rate law with an effective activation energy Q = 84 k J / m o l between 875 and 985°C. The amorphous SiO 2 formed transforms to tetragonal cristobalite in this temperature range. The oxidation kinetics above 1000°C follows a quadratic rate law. The rate constant of 1D SiC/SiC (Dornier) at 1520°C in air, k = ( A m / F ) 2 ( 1 / t ) = 3.5 × 10 -7 g2/(cm4 h) is one order of magnitude higher than that of 2D SiC/SiC (GA), k = 5 × 10 -8 gE/cm4 h. The reason for the stronger oxidation of the 1D material is due to the
higher porosity, the larger specific surface of the SiC matrix and a higher fraction of unprotected SiC fibres. The secondary electron image and the oxygen distribution image within the oxidized 1D material in Fig. 7 indicate the strong oxidation in the SiC matrix region. The mean oxygen concentration in the bulk of the NicalonT M fibres remained nearly unchanged ( < 15 mass% O) whereas the averaged oxygen content in the SiC matrix increased to about 25 mass% O under the conditions given in Fig. 7. The oxidation enhancement of SiC/SiC at 1520°C in air, water vapour saturated at 25°C, is up to 50% higher compared to dry air. This is quantified by a 50% increase of the parameter a 0 in the quadratic rate law ( A m ~ F ) 2= a o + kt for 1D SiC/SiC material. The enhancement is insignificant for the 2D SiC/SiC (GA) material, see lower a 0 values in Table 3. The oxidation of 1D SiC/SiC at 1520°C in argon, water vapour saturated at 25°C, yields only half of the oxidation in dry air. This is again quantified by a 50% decreased parameter a 0 in the quadratic rate law. In both cases, the specific mass change A m / F in the different environments occurs in the first fast oxidation process which is characterized by the parameter a 0 in the quadratic rate law. However, this oxidation process was not investigated further in the experiments. In comparison with these rate law constants, the quantity for high-dense monolithic SiC is in the region k = 1 × 10 -9 g2/cm4 h at 1520°C, see Table 1. The reasonable corrosion resistance of the NicalonT M fibre is destroyed by the high porosity of the SiC matrix and
H. Kleykarnp et al. / Journal of Nuclear Material 227 (1995) 130-137
137
Acknowledgement The technical assistance of calorimetric work by ' Mr. W. Laumer is gratefully acknowledged.
References
I
I
I
Fig. 7. Secondary electron image and oxygen distribution image of a 1D SiC/SiC (Dornier) sample after oxidation in air - 0.03 bar H20 at 1520°C/1 h; the oxidation takes place predominantly in the porous SiC matrix. the total composite, resp., and by the free carbon in the interlayers that is burnt in the lower temperature range.
5. Conclusions SiC fibre reinforced SiC materials in the present qualities do not offer any advantages of the corrosion resistance compared to high-density monolithic SiC for the use as plasma-facing components in the case of air and water vapour breakthrough. Advancements of the chemical properties would be promoted by (1) reduction of the porosity of the composite, (2) application of oxygen poorer NicalonT M fibres, (3) increase of the fibre-matrix bonding by improvement of the corrosion resistance of the interlayer materials.
[1] H. Kleykamp and G. Schumacher, Ber. Bunsenges. Physik. Chem. 97 (1993) 799. [2] B.E. Deal and A.S. Grove, J. Appl. Phys. 36 (1965) 3770. [3] N.S. Jacobson, J. Am. Ceram. Soc. 76 (1993) 3. [4] J. Schlichting, Ber. Dt. Keram. Ges. 56 (1979) 196; 256. [5] J. Li, P. Eveno and A.M. Huntz, Werkstoffe und Korrosion 41 (1990) 716. [6] L. Filipuzzi, R. Naslain and C. Jaussaud, J. Mater. Sci. 27 (1992) 3330. [7] E. Fitzer and R. Ebi, in Silicon Carbide 1973, eds. R.C. Marshall et al. (University of South Carolina Press, 1974) p. 320. [8] I.A. Yavorskii, V.I. Elchin, G.G. Gnesin and G.S. Oleinik, Sov. Powder Metall. Metal. Ceram. (1968) 59. [9] U. Ernstberger, H. Cohrt, F. Porz and F. Thiimmler, Ber. Dt. Keram. Ges. 60 (1983) 167. [10] R.A. Giddings, Report No. GE75CRD060 (1975). [11] J. Schlichting and J. Kriegesmann, Ber. Dt. Keram. Ges. 56 (1979) 72. [12] V.M. Gropyanov, V.L. Kuznetsova and V.I. Laptum, Ogneupory 6 (1970) 52. [13] S.C. Singhal, in Gas Turbine Materials in the Marine Environment, eds. J.W. Fairbanks and I. Machlin, Battelle Metals and Ceramics. Inf. Centre (1976) p. 311; J. Mater. Sci. 11 (1976) 1246. [14] E.J. Ophila, J. Am. C4ram. Soc. 77 (1994) 730. [15] T. Narushima, T. Goto and T. Hirai, J. Am. Ceram. Soc. 72 (1989) 1386. [16] P.J. Jorgensen, M.E. Wadsworth and I.B. Cutler, J. Am. Ceram. Soc. 42 (1959) 613; 43 (1960) 209. [17] J. Halbritter, H. Kleykamp, V. Schauer, A. Skokan and H. Zimmermann, K/K-5321 (1994). [18] G. Grathwohl, A. Hiihnel, B. Meier, E. Pippel, G. Richter and J. Woltersdorf, J. Europ. Ceram. Soc. 10 (1992) 1. [19] B. Meier, Dissertation, University of Karlsruhe, Bericht IKM-006 (1992). [20] I. Barin, Thermochemical Data of Inorganic Substances (Springer, Berlin, 1989). [21] L. Filipuzzi, G. Camus, R. Naslain and J. Thebault, J. Am. Ceram. Soc. 77 (1994) 459. [22] R. Naslain, in Advanced Structural and Functional Materials, ed. W. Bunk (Springer, Berlin, 1991) p. 51. [23] J. Lamouroux, G. Camus and J. Thebault, J. Am. Ceram. Soc. 77 (1994) 2049. [24] P.F. Tortoreili, S. Nijhawan, L. Riester and R.A. Lowden, Ceram. Eng. Sci. Proc. 14 (6-8) (1993) 358. [25] T.J. Clark, R.M. Arons and J.B. Stamatoff, Ceram. Eng. Sci. Proc. 6 (1985) 576.
Journal of Nuclear Materials 227 (1995) 130-137
Hdear ada'ials
Oxidation behaviour of SiC fibre reinforced SiC H. Kleykamp *, V. Schauer, A. Skokan Forschungszentrum Karlsruhe, Institut fiir Materialforschung, Pos(fach 3640, 76021 Karlsruhe, Germany Received 31 October 1994; accepted 6 July 1995
Abstract
The oxidation behaviour of one-dimensional and two-dimensional SiC fibre reinforced SiC was investigated up to 1520°C in air, as well as in water vapour saturated air and argon by calorimetry, DTA and TGA. The oxidation process takes place in three steps: (1) oxidation of free carbon in the carbon coated composites between 530 and 690°C with mass losses up to 7.5%; (2) a fast exothermal process associated with mass gain starting at 800°C and terminated below 1500°C within 1 h; (3) the diffusion controlled oxidation of bulk SiC. The processes follow a logarithmic rate law between 875 and 985°C with an effective activation energy Q = 84 kJ/mol. The amorphous reaction product SiO2 transforms to cristobalite at (930 + 50)°C. The oxidation kinetics follows a quadratic rate law above 1000°C with rate constants k = 3.5 x 10 - 7 g2/cm 4 h for 1D SiC/SiC and k = 5 x 10 -8 g2/cm4 h for 2D SiC/SiC at 1520°C in air. The rate constants are up to three orders of magnitude higher than that for high density monolithic SiC which is explained by the high porosity of the SiC matrix.
1. Introduction
Silicon carbide, SiC, in its different variations is being discussed as structural material for plasma facing components in a fusion reactor due to the low atomic number, high thermal conductivity, reasonable strength properties and excellent corrosion resistance in aggressive media. SiC fibre reinforced SiC is believed to be even superior to monolithic SiC. However, severe corrosive interactions are expected to occur in case of air and water vapour breakthrough to the plasma at high temperatures above 800°C. Therefore, the passive oxidation behaviour of different SiC fibre reinforced materials was investigated between 875 and 1520°C in synthetic air and water vapour saturated air and argon at 1 bar total pressure. SiC is a line compound and decomposes peritectically into graphite and a Si-C melt at 2830°C [1]. Numerous papers have been published on the oxidation behaviour of monolithic Si and SiC in air or oxygen under different total pressures. Basic papers appeared on the insight of the logarithmic, linear,
* Corresponding author. Tel.: +49-07247-822888; fax: + 49-07247-82-4567.
quadratic and cubic rate laws up to 30 years ago [2]. A comprehensive review was published recently [3]. However, only a few results have been reported in the literature [4-15] on the oxidation kinetics of monolithic SiC near 1520°C (see Table 1). This temperature was selected for our own isothermal experiments. The mechanism of the passive SiC oxidation is correlated with the reaction SiC + 202 3 ~ SiO 2 + CO. (1) The rate determining step is based on the diffusion of oxygen through the formed SiO 2 layer followed by the reaction of SiC at the SiC-SiO 2 interface. The formed CO is transported by opposite diffusion through SiO 2 to the surface and may further react to CO2. Different activation energies were reported in the various temperature ranges which may depend on the type of the diffusing oxygen (O 2 or O) and on the structure of SiO 2 (amorphous or crystalline, porous or dense). Below 1200°C, molecular oxygen would diffuse through the SiO 2 layer that is amorphous below 1000°C and crystallizes to cristobalite above this temperature. The SiO 2 layer is dense above 800°C, the oxidation follows a quadratic rate law [4]. Proportionality is observed between the isothermal rate constant and the logarithm of the oxygen partial pressure [16]. The activa-
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(95)00133-6
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H. Kleykamp et al. / Journal of Nuclear Material 227 (1995) 130-137
Table 1 Literature data of the oxidation kinetics of monolithic SiC at 1520°C with quadratic rate law and rate constant k in different atmospheres under 1 bar total pressure Material
Atmosphere
k (gZ/crn4 h)
Ref.
Reaction bonded SiC, porosity 1.7% CVD SiC layer SiC spheres, 40-63 i~m diameter SiC spheres, 40-63 i~m diameter CVD SiC layer CVD SiC layer Reaction bonded SiC, 10% Si, porosity 1% Reaction bonded SiC, 2.5% Si, 0.35% C Hot pressed SiC, 1-2% AI CVI SiC layer Reaction bonded SiC, 0.5% B, 0.5% C Reaction bonded SiC, 0.5% B, 0.5% C Hot pressed SiC, 4% A1203,4% WC Reaction bonded SiC, porosity 10% Reaction bonded SiC, porosity 30%
air oxygen air oxygen air oxygen air air air oxygen air/oxygen Ar + 0.03 bar H20 oxygen air air
2 x 10-10 5 × 10- 10 5 x 10- l0 1 X 10 -9
[9] [15] [7] [7] [10] [14] [4] [8] [11] [6] [5] [5] [13] [12] [12]
tion energy below 1400°C is about Q = ( 1 0 4 + 20) kJ/mol and is dependent on the respective reaction mechanism. An increase of the activation energy was stated above 1400°C that would be due to increased viscosity and plastic deformation of the formed SiO 2 layer [4]. The passive oxidation of SiC is further observed in water vapour or in water vapour containing air. The mechanism is based on the decomposition of water into its components on the SiC surface and successive oxidation according to SiC + 3 H 2 0 ~ SiO 2 + 3H 2 + CO.
(2)
The formation of aliphatic hydrocarbons was not stated. The oxidation rate increases proportionally to the logarithm of the water vapour pressure [4]. Recent experiments show that 10% water vapour in oxygen enhanced the oxidation rate of SiC only very slightly between 1200 and 1400°C compared to rates found in dry oxygen [14], see Table 1. Very low oxygen partial pressures suppress the SiO 2 layer formation and an active oxidation takes place, a volatilization according to the reaction SiC + 0 2 ~- (SiO)gas q- CO.
(3)
Available literature data on the oxidation kinetics of monolithic SiC in air, oxygen and water vapour under 1 bar total pressure and at 1520°C are compiled in Table 1. The results are based on a quadratic rate law of the SiO z layer growth, ( A m / F ) 2 = kt. The mass increase is Am and F is the geometric area of the oxidized sample. The results indicate for the rate constant k a figure in the order of 10 -9 g2/cm4 h for high density monolithic SiC. A reaction layer thickness 0.14 ixm follows for cristobalite of density p = 2.33 M g / m 3 after
10 -9
1 × 10 -9 2-10 -9 10-s 10-8 1 × 10-8 1 × 10-8 2× 10-8 4 × 10- 7 2× 10 - 6 2 x 10-4
1 hour reaction time. Silicon infiltrated SiC exhibits no worse oxidation behaviour than monolitic SiC at 1520°C. Other additives may enhance the oxidation rate. There is a strong dependence of the oxidation rate on the porosity of SiC up to five orders of magnitude above that of 100% dense SiC, see Table 1. Further, the oxidation behaviour of SiC is deteriorated by the presence of water vapour. However, only few quantitative experiments are available, which deal with the influence of the water vapour partial pressure or the porosity on the oxidation behaviour of monolithic SiC at 1520°C. As the influence of a fibre reinforcement on the chemical properties of SiC is less known, three SiC fibre reinforced SiC qualities were investigated in this work with respect to the oxidation behaviour in air and in water vapour atmospheres. Details on this topic and on selected mechanical properties were reported in Ref. [17].
2. Experimental 2.1. Materials
One-dimensional SiC fibre reinforced SiC (Dornier, Friedrichshafen, Germany) was supplied in form of fiat samples with the dimensions 1.4 x 9.5 × 60 mm 3. The untwined fibres with a diameter 12-20 i~m which have been coated with a carbon layer approximately 0.2 I~m in thickness were aligned parallel to the plate surfaces. The material between the fibres and in the surface regions consists of SiC that was formed by pyrolysis of methyltrichlorsilane. Measurements of the geometric
132
H. Kleykamp et al. / Journal of Nuclear Materia1227 (1995) 130-137
Fig. 1. Light-optical microstructure of one-dimensional SiC fibre reinforced SiC (Dornier) perpendicular to the fibre direction. density yield a figure for the porosity of the composite of ~.bout 35 vol% related to the theoretical density p = 3.22 M g / m 3 of pure SiC. The light-optical microstructure perpendicular to the fibre direction is illustrated in Fig. 1. The SiC fibre fraction is about 40% of the total SiC inventory. The oxygen content in the SiC matrix is less than 1 mass%. The chemical analysis of the SiC fibres is given in Table 2. The two-dimensional SiC fibre reinforced SiC (General Atomics, GA, San Diego, USA) was delivered by one fiat sample with the dimensions 2.0 x 45 x 45 mm 3. Every 500 single fibres with a diameter 10-20 Ixm were integrated within filaments which were two-dimensionally (00/90 °) woven to a fabric and were coated with a 0.3 Ixm thick carbon layer. Four SiC layers form a composite which was soaked by an organic silicon compound and reacted to SiC by decomposition at ll00°C. Measurements of the geometric density yield a figure for the porosity of the composite of about 32 vol% related to the theoretical density. The light-optical microstructure perpendicular to one of fibre directions is illustrated in Fig. 2. The SiC fibre fraction is about 35% of the total SiC inventory. The oxygen content in the SiC matrix is less than 1 mass%. The chemical analysis of the SiC fibres is given in Table 2. Other two-dimensional SiC fibre reinforced SiC (Refractory Composites Inc., RECOMP, California USA) was delivered in form of 5 x 100 x 200 mm 3 sheets. Every 500 single fibres with a diameter of
Fig. 2. Light-optical microstructrue of two-dimensional SiC fibre reinforced SiC (GA) perpendicular and parallel to the filament directions. = 15 ~m were integrated within filaments which were two-dimensionally (00/90 °) woven to a fabric. The fabrics were coated with a 0.3 I~m thick C - S i O 2 - C layer [18]. Fifteen fabrics form a composite which was soaked by chemical vapour infiltration with methyltrichlorsilane and reacted to SiC by decomposition at 1200°C. The SiC overlayers on the composite are 200 p~m thick. The matrix consists of 65% cubic SiC and 35% hexagonal SiC. The porosity of the uncoated material is 30 vol%. The chemical analysis of the fibres is given in Table 2. The fibres contain about 11% free carbon [18,19]. The fibres (NicalonTM) were produced by Nippon Carbon Co., Japan in different compositions. Separate phases could not be identified by scanning electron microscopy and X-ray microanalysis. The oxygen concentration in the three investigated materials was found to be evenly distributed within the fibres. 2.2. Instruments
Anisothermal and isothermal oxidation tests were conducted on 1D SiC/SiC (Dornier) and on 2D SiC/SiC (GA) by calorimetry and differential thermal analysis (DTA). An isoperibol high-temperature calorimeter HTC 1800 (Setaram, Lyon, France) with heating rates 2 and 5 K / m i n was used for the anisothermal tests, a D T A 404/3 unit (Netzsch, Selb,
Table 2 Composition of the NicalonTM SiC fibres in the composites of different manufacturers, values in mass% Manufacturer
SiC fibre
Si
C
O
N
B
Ref.
Dornier GA RECOMP
one-dim. two-dim. two-dim.
55.5 53.8 56.9
28.0 29.5 30.6
14.4 15.0 11.3
0.1
3.4 0.01 0.008
this work this work [18,19]
H. Kleykamp et al. / Journal of NuclearMaterial227 (1995) 130-137 Germany) for isothermal tests, both instruments operable up to 1530°C. The samples in the 100 mg range have a defined geometric surface and were oxidized under 1 bar total pressure with 9 cm3/min flow rate in synthetic air (p(O 2) = 0.21 bar), in synthetic air water vapour saturated at 25°C ( p ( H 2 0 ) = 0.032 bar) and in high purity argon water vapour saturated at 25°C ( p ( O 2) = 3 × 10 -6 bar, p ( H 2 0 ) = 0.032 bar). The detection limit of the calorimeter is 0.5 mJ/s. Heat effects could be quantitatively measured in the S i C - C - O 2 system above 500°C by calorimetry. Semiquantitative heat effects could be detected by DTA above 1450°C. The isothermal oxidation of the samples took place at 1520°C up to 24 h after initial heating under argon with 10K/min heating rate. The weight gain of the samples was measured off-line by gravimetry. The isothermal oxidation of 2D SiC/SiC (RECOMP) was performed by thermogravimetry (TGA) using a symmetrical microthermobalance, type MTB10-8 (Setaram, Lyon, France). The sensitivity is 0.5 Izg, the temperature range of operation extends to 1000°C. The samples in the 100 mg range were heated from room temperature up to 850-1000°C under helium with a heating rate 10 K/min. Isothermal thermogravimetry was followed on-line in synthetic air with a flow rate 1 1/h under 1 bar total pressure.
I"~I"~ 1300
133
Small samples between 3 and 20 mm edge lengths were cut off from the 2D SiC/SiC (RECOMP) material. The surfaces of the sheets and those perpendicular to the fabrics of the annealed and the reference materials were analyzed by X-ray diffraction (Seifert, Ahrensburg, Germany) at room temperature, which indicated the crystallinity in the SiC matrix surface and in the SiC fibres, resp.
3. Results
3.1. Calorimetry and differential thermal analysis on 1D SiC~SiC (Dornier) and 2D SiC~SiC (GA) The oxidation experiments on 1D SiC/SiC (Dornier) by anisothermal calorimetry in air up to 1450°C with a heating rate 5 K / m i n revealed an exothermal peak between 530 and 690°C which is based on the oxidation of the unprotected carbon coating according to the reaction C + 0 2 ~ CO 2. The recorded peak area of 20 mV s corresponds to - 2 9 J, see Fig. 3. This value is equivalent to the 7.35 × 10 -5 fold of the molar enthalpy of combustion of graphite AfH ° = - 3 9 4 3 0 0 J / m o l at the mean temperature 850 K [20]. From these results follows that 0.9 mg C was burnt and the investi-
'(C)
FEAT FLOW
1200 iiO0 iO00
a00
t80.
700
:tOO _ 80
60_ ~
g K 40_
20_
-20 tl000
13000
I " ZM E
t6000
Ill _,^
-
Fig. 3. Anisothermal oxidation (heating rate 5 K/min) of one-dimensional SiC fibre reinforced SiC (Dornier) in air up to 1450°C. The peaks characterize the combustion of the free carbon (maximum at 620°C) and the first rapid SiC oxidation (maximum at 1200°C).
134
H. Kleykamp et al. / Journal of Nuclear Material 227 (1995) 130-137
gated 263.9 mg SiC sample contained not under 0.33% free C. The finding is in agreement with the mass loss of 0.4% observed by gravimetry on a 1D SiC/SiC sample heated in air up to 800°C. The carbon oxidation process results in an annular pore circumferential to the SiC fibre which is surface sealed by sucessive SiO 2 formation [21]. Anisothermal calorimetry in air with 5 K / m i n heating rate indicated further the onset of the first rapid SiC oxidation step at about 800°C of which the oxidation rate passed a maximum at 1200°C, see Fig. 3. The isothermal gravimetric experiments on 1D SiC/SiC were performed at 1520°C in air, in air 0.032 bar H 2 0 and in Ar - 0.032 bar H 2 0 at 1 bar total pressure up to 24 hours reaction time. The results are presented in Fig. 4. An initial mass loss of 0.3 0.4% by free carbon combustion is followed by a distinct mass increase of about 2% within 1 h after reaction beginning. The following true SiC oxidation is based on a quadratic rate law, ( A m / F ) 2 = a 0 + k t , independent of the composition of the three selected gas mixtures (see Fig. 4 and Table 3), where Am is the mass gain, F is the geometric surface of the sample, k is the isothermal rate constant and a 0 is an induction period constant which characterizes the two prereactions. The progress of the reaction, which is the fraction of SiC in mol% oxidized to SLOE, was calculated from the mass gain and is added to Table 3. It is higher for the oxidation with water vapour saturated air than with pure air. The higher reaction rate is restricted to the beginning and is terminated after 1 h at 1520°C. The oxidation experiments on 2 D S i C / S i C (GA) by anisothermal calorimetry in air up to 1500°C do not indicate a peak that can be attributed to the combustion of free carbon at about 600°C. This finding is in aggreement with the observation of a very low mass loss < 0.1% of a sample heated in air up to 800°C. Fig.
(a)20
1dim.SiC-SIC T=1520"C,pt=lbar • 0,21bar02 ~ 15 o0,21barOz/0,03barHzO * Ar/ 0,03barH20 "~ " ,o 10 ~. _
0
o
•
~..~.
~
[
°o
'-
; ~-1'o
l's-'
2'0
' ~s
tin h
(b) 1,4
"~"E
1.2
2dimSLC-SiC . T=1520'CPt , =1bar _c .~,o~ 0,80,61'0 • 0,21barO ~ 2
•
O
~
0,4
0,2
O0
~
5
L
L
10
~
0 25 tinh Fig. 4. Oxidation behaviour of one-dimensional (a) and t~odimesional (b) SiC fibre reinforced SiC (Dornier and GA) in air, air'water vapour and argon-water vapour at 1520°C.The oxidation follows a parabolic rate law after combustion of the free carbon and fast initial oxidation. The solid lines represent least-squares fits of the SiC oxidation in air. 15
2 indicates that the carbon coatings of the SiC fibres have no direct contact with the open porosity of the composite. Therefore, the carbon burning is shifted to higher temperatures and is superimposed to the first step to the SiC oxidation. This observation is in agree-
Table 3 Oxidation kinetics of one-dimensional and two-dimensional SiC fibre reinforced SiC with quadratic rate law (Am I F ) t >_1 h, at 1520°C and 1 bar total pressure Material
Atmosphere
a o (10-6 g2/cm4)
k (10-6 gZ/cm4 h)
Reacted SiC in mol% after 10 h
1-dim., Dornier
air
4.3
0.351
6.5
1-dim., Dornier
air + 0.03 bar H20
6.5
0.35
7.8
1-dim., Dornier
argon + 0.03 bar H20
2
0.3
5.5
2-dim., GA
air
0.06
0.05
1.1
2-dim., GA
air + 0.03 bar H20
0.06
0.05
1.1
2=
a o + kt,
H. Kleykamp et al. / Journal of Nuclear Material 227 (1995) 130-137
ment with isothermal mass loss experiments on 1D S i C / C / S i C which started within 20 minutes reaction time not below 800°C [21]. The onset of the first step of the SiC oxidation was stated above this temperature, however, the extent of reaction is only a third of that of 1D SiC/SiC. The isothermal gravimetric experiments of 2D SiC/SiC were performed under the same conditions as described for 1D SiC/SiC. After a negligible mass decrease < 0.1% by free carbon combustion and a low mass increase by a fast SiC oxidation process within 1 h after reaction beginning, the true oxidation follows again a quadractic rate law, ( A m ~ F ) 2 = a o + kt independent of the selected gas environments, see Fig. 4 and Table 3. The extent of reaction was calculated from the mass increase of the samples and is added to Table 3. The three-stage character of the total oxidation process was observed also by Narushima et al. [15] Naslain [22] and Lamouroux et al. [23] and is attributed to the combustion of interlayer carbon and to a fast chemical reaction between oxygen and the three components of the metastable SiC(O) NicalonT M fibres [15,22] and gas phase diffusion through microcracks in the SiC coating [23], resp. These processes are followed by the diffusion controlled SiC oxidation. 3.2. Thermogravimetry on 2D S i C ~ S i C (RECOMP)
Isothermal thermogravimetric experiments were performed on 2D SiC/SiC (RECOMP) in air at 875 and 985°C up to 18 h. A fast relative mass decrease A m / m o of the samples down to - 7 . 5 % was observed at short reaction times up to 1.4 h that is based on the combustion of a high fraction of free carbon in this material. The oxidation of SiC to SiO 2 dominates at longer reaction times. The results are illustrated in Fig. 5. The presentation in a logarithmic time scale shows clearly the logarithmic rate law in this temperature range according to A m / m o = a o + k In t. The temperature dependent constant, a0, considers the fraction of the burnt carbon in the material, k is the isothermal logarithmic rate constant of the SiC oxidation. The rate law gives at 875°C with t > 1.4 h for a 122.6 mg sample: A m / m o = -0.0661 + 0.00227 In t, t in h, and at 985°C with t > 1.4 h for a 76.3 mg sample: A m / m o= -0.0736 + 0.00491 In t, t in h. The plot of the logarithm of the logarithmic rate constant versus the reciprocal temperature yields the activation energy Q = 84 k J / m o l for the rate determining step of oxygen diffusion through the SiO 2 layer formed on the surface of the porous SiC matrix in ~he temperature range 875-985°C. The mass decrease by combustion of free carbon dominates the SiC oxidation process at short times. The first reaction is terminated after 1.4 h. Assuming the reaction given by Eq. (1) for the latter
135
process, the progress of reaction was calculated as 1.0% and 2.5% of total SiC oxidized at 875°C and 985°C, resp., after 17 h. The logarithmic rate law after the uniform degradation of the carbon interlayer was also observed in other experiments, e.g. Refs. [22,24]. 3.3. X-ray diffraction on oxidized 2D S i C ~ S i C (RECOMP)
The as-received strongly textured 2D SiC/SiC (RECOMP) consists of the cubic modification and of the hexagonal 4 H and 6 H polytypes. Oxidation in air for 17 h up to 875°C exhibits no further X-ray diffraction lines. Oxidation at and above 985°C shows one additional reflex which is related to the tetragonal room temperature modification of cristobalite SiO 2 (JCPDS11-695). This phase could have transformed from the originally produced high temperature modification. The X-ray diffractogram is illustrated in Fig. 6. The amorphous SiO 2 formed by oxidation of SiC at lower temperatures transforms to the crystalline modification at (930 + 50)°C. Samples oxidized at higher temperatures exhibit no additional cristobalite lines though the intensity of the (101)-reflex increases with oxidation temperature. A cristobalite formation was reported at 1070°C [5], < 1093°C [13], < 1200°C [25] and > 1200°C [16] in earlier oxidation experiments of SiC.
2,8h
5,6h
8,3h
11,1h
13,9h
16,7h
19.4h
(a) o
~ I
-1
;
875"C
1148K
-2
c
-5
~-6 -7 -8
10 000
Ib)
20 000
28h
0-
30 000 40 000 tins
50 000
50h
83h
139h
r
!
~-2
11 h
70 000
16,h
19,h
5'C
|
~-5
__
~--
~-6
i
I
~
-7 ,
0 5.
~
10 000 Oxidation
,
I
,
20 000 behaviour
,
,
30 000 40 000 tins
_
I
~~
-8
Fig.
60 000
, ,
i
50 000
of two-dimensional
,
,
60 000 SiC
fibre
,
70 000 rein-
forced SiC (RECOMP) in air at (a) 875 and (b) 985°C. The oxidation follows a logarithmic rate law for t > 1.4 h after combustion of the free carbon.
136
H. Kleykamp et al. / Journal of Nuclear Material 227 (1995) 130-137 I
I
I
I
I
cristoba I i r e tetragona I (181)
1158°C / 188h / a i r
1858°C /
18h / a i r
585°C / ll~h / a i r
875°C / fl
18h / a i r
C
~
D reference
Z8
I
I
38
48
i
Z thata
58 (deJjree)
I
I
68
78
material
tiff
Fig. 6. X-ray diffraction pattern of a partially oxidized 2D SiC/SiC (RECOMP) sample perpendiucular to the plate surface indicating cristobalite formation; the letters A to D denote SiC reflexes.
4. D i s c u s s i o n
The qualitative valuation of the investigated chemical properties of the SiC fibre reinforced SiC composites with a pore volume between 32 and 35 vol% indicates already a strong influence of the porosity and a more unfavourable oxidation behaviour than that of high-dense monolithic SiC (p = 3.22 Mg/m3). The oxidation experiments on 1D and 2D SiC/SiC materials between 500 and 1520°C have shown that the reactions proceed in three steps. The first step is characterized by the combustion of free carbon which, in an anisothermal experiment, starts at 530°C and is terminated at 690°C within 30 minutes. This is correlated with a mass loss markedly observed on 2D SiC/SiC of RECOMP with 11 mass% free carbon in the composite. The mass increase in the second step implies a fast oxidation process which starts at about 800°C, is terminated below 1500°C within 1 hour and is based on an equilibration of the oxygen containing metastable Nicalon TM fibre or of the total composite. The third step is the true diffusion controlled SiC oxidation which follows a logarithmic rate law with an effective activation energy Q = 84 k J / m o l between 875 and 985°C. The amorphous SiO 2 formed transforms to tetragonal cristobalite in this temperature range. The oxidation kinetics above 1000°C follows a quadratic rate law. The rate constant of 1D SiC/SiC (Dornier) at 1520°C in air, k = ( A m / F ) 2 ( 1 / t ) = 3.5 × 10 -7 g2/(cm4 h) is one order of magnitude higher than that of 2D SiC/SiC (GA), k = 5 × 10 -8 gE/cm4 h. The reason for the stronger oxidation of the 1D material is due to the
higher porosity, the larger specific surface of the SiC matrix and a higher fraction of unprotected SiC fibres. The secondary electron image and the oxygen distribution image within the oxidized 1D material in Fig. 7 indicate the strong oxidation in the SiC matrix region. The mean oxygen concentration in the bulk of the NicalonT M fibres remained nearly unchanged ( < 15 mass% O) whereas the averaged oxygen content in the SiC matrix increased to about 25 mass% O under the conditions given in Fig. 7. The oxidation enhancement of SiC/SiC at 1520°C in air, water vapour saturated at 25°C, is up to 50% higher compared to dry air. This is quantified by a 50% increase of the parameter a 0 in the quadratic rate law ( A m ~ F ) 2= a o + kt for 1D SiC/SiC material. The enhancement is insignificant for the 2D SiC/SiC (GA) material, see lower a 0 values in Table 3. The oxidation of 1D SiC/SiC at 1520°C in argon, water vapour saturated at 25°C, yields only half of the oxidation in dry air. This is again quantified by a 50% decreased parameter a 0 in the quadratic rate law. In both cases, the specific mass change A m / F in the different environments occurs in the first fast oxidation process which is characterized by the parameter a 0 in the quadratic rate law. However, this oxidation process was not investigated further in the experiments. In comparison with these rate law constants, the quantity for high-dense monolithic SiC is in the region k = 1 × 10 -9 g2/cm4 h at 1520°C, see Table 1. The reasonable corrosion resistance of the NicalonT M fibre is destroyed by the high porosity of the SiC matrix and
H. Kleykarnp et al. / Journal of Nuclear Material 227 (1995) 130-137
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Acknowledgement The technical assistance of calorimetric work by ' Mr. W. Laumer is gratefully acknowledged.
References
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Fig. 7. Secondary electron image and oxygen distribution image of a 1D SiC/SiC (Dornier) sample after oxidation in air - 0.03 bar H20 at 1520°C/1 h; the oxidation takes place predominantly in the porous SiC matrix. the total composite, resp., and by the free carbon in the interlayers that is burnt in the lower temperature range.
5. Conclusions SiC fibre reinforced SiC materials in the present qualities do not offer any advantages of the corrosion resistance compared to high-density monolithic SiC for the use as plasma-facing components in the case of air and water vapour breakthrough. Advancements of the chemical properties would be promoted by (1) reduction of the porosity of the composite, (2) application of oxygen poorer NicalonT M fibres, (3) increase of the fibre-matrix bonding by improvement of the corrosion resistance of the interlayer materials.
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