Review: chemical compatibility of SiC/SiC composites with the GFR environment C. Cabet Laboratoire of Non Aqueous Corrosion, CEA Saclay, FRANCE

1
Review chemical compatibility of SiC/SiC
composites with the GFR environmentC.
CabetLaboratoire of Non Aqueous Corrosion, CEA
Saclay, FRANCE
2
GFR and SiC/SiC composites
Fuel assembly
850C
HeateXchanger
Helium
Introduction
3
Concepts of fuel assembly
Needle concept
Plate concept
Introduction
4
Requirements on material for fuel assembly

• Containment of fuel and FP
• Refractory behaviour
• Resistance to normal operating temperatures
  (about 900-1200C) on extended lifetimes
• Confining of FP during a transient incident up to
  1600C
• Mechanical integrity after a major accident up to
  2000C
• High thermal conductivity (gt10 W/m.K)
• Transparency to fast neutrons
• Mechanical strength and creep resistance
• Ability to dissolve in nitric acid
• Workability and assemblage
• Resistance to corrosion/ oxidation

Introduction
5
GFR environment

• High temperature 900-1200C
• short term transitory up to 1600C (confining)
  and accident up to 2000C (integrity)
• Long in-core times
• No inspection, no repair
• Cooling gas

impure helium
Introduction
6
SiCf/SiCm usual applications
Turbines
Rocket engines

• High temperature
• Oxidative atmospheres
• Inspection and repair
• Short term

Aircraft engines
Introduction
7

• SiCf/SiCm compatibility with GFR physico-chemical
  conditions over long term ?

• Thermal stability
• Oxidation resistance
• Consequences of thermal aging and oxidationon
  the mechanical (and confining) properties
• Improvement strategies

Lifetime prediction
Introduction
8
Content

• Introduction on the GFR application
• SiCf/SiCm structure and fabrication
• Thermal stability
• Oxidation propertis
• Composite resistance
• RD needs to qualify SiC/SiC for GFR applications

9
SiCf/SiCm structure
SiC-basedfibre 10µm
SiC-based matrix
crack
interphase (C) 0.1µm
Fibres in bundle UD or 1D 2D 3D
SiCf/SiCm structure and fabrication
10
SiC based matrix (SiC Si)

• CVI Chemical Vapor Impregnation
• PIP Polymer Impregnation and Porolysis
• RMI Reative Melt Infiltration
• SI-HPS Slurry Infiltration and
  High Pressure Sintering

porosity
additives
11
SiC-based fibres fabrication

• 1st generation
• cure in oxygen at T1200C
• Si-C-O 2nm SiC C SiCxOy

• 2nd generation
• cure by electron beam in inert atm at T1400C
• Si-C C ( 0.5 O)
• 3rd generation or nearly stoichoimetric
• cure at 1800-2000C optimization
• thin C layer on the surface

SiCf/SiCm structure and fabrication
12
SiC-based fibres 3 generations

• Exemple of the development of the Nicalon fibres
  by Nipon Carbon

13
Interphase

• Compliant material
• Thin layer 100nm
•  leaf  structure
• pyrocarbon
• hex-BN
• Multilayer

14
Content

• Introduction on the GFR application
• SiCf/SiCm structure and fabrication
• Thermal stability
• Monolithic SiC
• Matrix
• Fibres
• Oxidation properties
• Composite resistance
• RD needs to qualify SiC/SiC for GFR applications

15
SiC phase diagram

• Stoichoimetric
• no other intermediate compound
• SiC (SiC)(l) C

2540C
Thermal stability
16
Thermal stablity of SiC
in vacuum or inert atmopshere

• Thermodynamic calculation SiC C Si(g)
  recrystalisation
• Kinetic factors SiC stable up to 1600C

SiC Si
104/T (K)
Thermal stability
17
Thermal Stability of the matrix
in vacuum or inert atmopsheres

• SiC and SiC/C matrixes are stable up to about
  1600C

Thermal stability
18
Thermal Stability of fibres
Fibres of the 1st generation Si-C-O

• Basically instable à Tgt1200C
• (SiC, C, SiC2xO1-x) ? w SiC x C y CO(g) z
  SiO(g)

Porous C/SiC (large grains)
Mass loss
Decrease the creep strength
1300C
1200C
Creep curves for Nicalon fibres tested in pure Ar
under 0.7 GPa
Mass loss for Nicalon fibres tested in pure Ar
Bodet et al. J Amer Ceram Soc 79 (1996) 2673
Thermal stability
19
Thermal Stability of fibres
Fibres of the 2nd generation Si-C(0.5 O)

• Stable up to 1350C
• (SiC, C) Otrace(g,s) ? SiC CO(g) C

Large grains
Mass loss
??r
Si-C-O Nicalon NL202 and Si-C Hi-Nicalon
(as-received and heat treated) fibres under
100kPa Ar (heating rate 10C/min) Chollon et
al., J Mater Sci 32 (1997) 333
Tensile strength and Youngs modulus at RT of
Si-C Hi-Nicalon after annealing under 100kPa Ar
for tp1hrs exept tp10hrs) Chollon et al., J
Mater Sci 32 (1997) 333
Thermal stability
20
Thermal Stability of fibres
Nearly stoichiometric fibres

• Stable up to very high temperatures 1800-2000C
• Some SiC grain growth
• Good mechanical properties up to 1400-1500C

Strengh as a function of temperature for 3rd gen
fibres with a 250mm gauge length Bunsell and
Piant, J Mater Sci 41 (2006) 835
Thermal stability
21
Content

• Introduction on the GFR application
• SiCf/SiCm structure and fabrication
• Thermal stability
• Oxidation properties
• Monolithic SiC
• passive oxidation
• active oxidation
• Matrix
• Fibres
• Interphase
• Composite resistance
• RD needs to qualify SiC/SiC for GFR applications

22
Oxidation of SiC at high Po2 passive oxidation

• Same mechanism that the oxidation of Si and other
  ceramics
• SiC(s) 3/2 O2(g) SiO2(s) CO(g)
• SiC(s) 2 O2(g) SiO2(s) CO2(g)
• Linear-parabolic kinetics

Very protective
Tgt800C Monolithic SiC
Parabolic rate constant
linear rate constant
Scale thickness
a-SiC in 1 atm air Costello Tressler, J Am
Ceram Soc 64 (1981) 327
Oxidation - SiC
23
Oxidation of SiC at high Po2 mechanism
Tgt800C Monolithic SiC
Growth rate oxygen transport through the SiO2
scale
Tgt1400C Ea 150-300 kJ/mole atomic
diffusion cristobalite
Tlt1400C Ea 300 kJ/mole molecular
diffusion amorphous SiO2
90µm
MEB image of sintered a-SiC 6hrs at 1400C in 1
atm air Costello Tressler, J Am Ceram Soc 64
(1981) 327
KP for the oxidation of single-crystal SiC under
0.001 atm O2 Zheng, J Electrochem Soc 137
(1990) 854
Oxidation - SiC
24
Oxidation at high Po2 polycrystalline SiC

• Determining factors for Kp
• Polytype
• Porosity (fabrication process)
• Additives and impurities
• Formation of a silicate with a lower viscosity(?
  transport of O )
• Modify the crystallisation

HP SiC with different Al2O3 at 1370C in 1 atm
O2 Opila Jacobson, in Materials science and
technology Vol. 19, RW. Cahn et al. Ed. (2000)
Kp from the literature for different type of
SiC Narushima et al., J Am Ceram Soc 72 (1989)
1386
Oxidation - SiC
25
Oxidation at high Po2 effect of water vapour

• Passive oxidation by water vapour SiC 2
  H2O(g) SiO2 CH4(g) SiC 3
  H2O(g) SiO2 CO2(g) 3 H2(g)

Tlt1400C
Tgt1400C

• Some water vapour increases the oxidation rate

• Higher oxidation rate in pure water vapour
• SiO2(s) H2O(g) SiO(OH)2(g) SiO2(s) 2
  H2O(g) Si(OH)4(g)

CVD-SiC at 1200C in pure CO2, pure O2 and
50H2O/50O2 Opila Nguyen., J Am Ceram Soc 81
(1998) 1949
Oxidation - SiC
26
Oxidation of SiC at low Po2 active oxidation

• Same mechanism that the oxidation of Si and other
  ceramics
• SiC O2(g) SiO (g) CO(g)

Volatilization
Mass Change
ka
CVD-SiC in 0.1 MPa at 1600C Po2 in Ar from 0
to 160Pa
Corresponding rate constant for active oxidation
at two gas flow rates
Goto et al., Corrosion in advanced ceramics, KG
Nickel Ed. (1993) 165
Oxidation - SiC
27
Oxidation of SiC at low Po2 active oxidation

• Transition point between active and passive
  oxidation

• Determining factors for transition
• Temperature
• Po2
• SiC purity
• Vgas
• Total pressure

Theory (Wagner)
Active to passive transitions from the literature
for different types of SiC Opila Jacobson, in
Materials science and technology Vol. 19, RW.
Cahn et al. Ed. (2000)
Theory (Volatility diag.)
Oxidation - SiC
28
Oxidation at low Po2 effect of water vapour

• Active oxidation by water vapour SiC 2
  H2O(g) SiO(g) CO(g) 2 H2(g)

Active to passive transition
Corrosion rate
Flexural strength at RT
active
passive
1400C
PLS a-SiC at 1300 and 1400C 10min in H2 with
different P(H2O) Opila Nguyen., J Am Ceram Soc
81 (1998) 1949
Oxidation - SiC
29
Oxidation of SiC-based matrixes at high Po2

• Under oxidizing atmosphere CVD-SiC
  (representative of CVI-SiC Passive
  oxidation

Thickness of the SiO2 scale
Crystallisation
Amorphous SiO2
CVD-SiC representative of CVI-SiC at 1000C and
100 kPa Naslain et al. J Mater Sci 39 (2004)
7303
Oxidation - matrix
30
Oxidation of fibres passive mode at high Po2

• Growth of silica around the fibre surface
  (2nd and 3rd generation fibres)

SiO2
Flexural strength
Mass change in Ar-25O2
Mass change at 1300C
Hi-Nicalon S
Hi-Nicalon
Oxidation in Ar-O2 at 1500C Shimoo et al., J
Ceram Soc Japan 108 (2000) 1096)
Nicalon
Hi-Nicalon fibres (SiC-C) in Ar-25O2
Hi-Nicalon fibres (SiC-C) in Ar-O2 at
1300C Shimoo et al. J Mater Sci 35 (2000)
3301)
Oxidation - fibres
31
Oxidation of fibres active mode at low Po2

• Volatilization of SiO(g)
• SiC(s) O2(g) SiO(g) CO(g)
• recrystallisation of SiC

Mass change at 1500C
Passive oxidation
Active oxidation
Lox M fibres in Ar-O2 at 1500C Shimoo et al. J
Mater Sci 37 (2002) 4361)
Oxidation - fibres
32
Oxidation of fibres case of 1st generation
Mass change

• Passive oxidation with SiO2 growth
• SiC 3/2O2(g) SiO2 CO(g)
• No thermal decomposition of Si-C-O

• Thermal decomposition of Si-C-O
• SiCO SiO(g) CO(g) SiC C
• recrystallisation of SiC

• Active oxidation
• SiC O2(g) SiO(g) CO(g)
• recrystallisation of SiC

Nicalon CG fibres in Ar-O2 at 1500C Shimoo et
al. J Amer Ceram Soc 83 (2000) 3049
Oxidation - fibres
33
Oxidation of fibres active to passive transition

• As for pure SiC, there is an active to passive
  transition

Mass change
Active to passive transition
Fibres heated 72 ks in Ar-O2 at 1500C Shimoo et
al. J Mater Sci 37 (2002) 1793
Po2 for active to passive transition
Oxidation - fibres
34
Oxidation of fibres effect of water vapor at
high Po2

• As for pure SiC, H2O increases the oxidation rate

Ln (Kp) (h-1)
Tensile strength of SiC fibres after 10h at
1400C in dry or wet (2H2O) air Takeda et al. J
Nucl Mater 258-263 (1998)1594
Kp for Hi-Nicalon fibres tested in N2/O2/ H2O
under 100 kPa and Po220 kPa Naslain et al. J
Mater Sci 39 (2004) 7303
Oxidation - fibres
35
Oxidation of the interphase at any Po2

• Carbon is highly oxidizable at Tgt600C
• C O2(g) CO2(g)
• C ½ O2(g) CO(g)
• C 2 H2O(g) CO2(g) 2 H2(g)
• C H2O(g) CO(g) H2(g)
• Oxidation rate is dertermined by
• Temperature
• Po2
• Total pressure
• Gas flow rate

Oxidation - interphase
36
Content

• Introduction on the GFR application
• SiCf/SiCm structure and fabrication
• Thermal stability
• Oxidation properties
• Composite resistance
• Thermal aging
• Oxidation
• Improvement of the HT oxidation resistance
• RD needs to qualify SiC/SiC for GFR applications

37
Thermal aging of UD SiCf/SiC (inert gas)

• UD-SiCf/C/PIP-SiCm
• Nicalon CG - 1st generation SiCO thermal
  decomposition
• Hi-Nicalon - 2nd generation SiC-C (0.5 O)
  stable up to 1350C
• Hi-Nicalon S - 3rd generation nearly
  stoichiometric

Mass change
Residual oxygen
Fracture strength
UD SiCf/C/PIP-SiCm 3.6ks in vacuum Araki et al.
J Nucl mater 258-263 (1998) 1540
composite thermal aging
38
Thermal aging of 2D SiCf/SiC (inert gas)

• 2D Nicalon CG/C/CVI-SiC
• 1st generation SiCO thermal decomposition
• SiCO SiO(g) CO(g) SiC C
• Interaction with the interphase
• SiO(g) 2 C SiC CO(g)

Tensile strength
coarse SiC
Interfacial decohesion (weakening of the
fibre-matrix bounding)
Partial consumption of the interphase with
formation of coarse surface SiC-grains (weakening
of the fibres)
Total consumption of the interphase with
decomposition/crystallisation (fully brittle)
Stress-strain curves of 2D Nicalon/C/SiC
composite at RT after thermal aging in vacuum
under various conditions Labrugère et al. J Eur
Ceram Soc 17 (1997) 623
composite thermal aging
39
Passive oxidation of model SiCf/SiCm (high Po2)

• Passive oxidation of fibres and matrix
• SiC 3/2 O2(g) SiO2 CO(g)
• SiC 2 O2(g) SiO2 CO2(g)
• Oxidation of the interphase
• C O2(g) CO2(g)
• C ½ O2(g) CO(g)
• Model UD Nicalon/C/CVI-SiC no coating on the back
  and front surfaces
• ? gas phase diffusion of O2 and CO in the pore
• ? reaction of O2 with the C interphase
• ? diffusion of O2 in SiO2 and reaction with SiCf
• ?diffusion of O2 in SiO2 and reaction with Sim

Filipuzzi et al. J Amer Ceram Soc 77 (1994) 459
composite oxidation
40
Passive oxidation of 2D SiCf/C/SiCm (high Po2)

• Oxidation of the interphase
• C O2(g) CO2(g)
• C ½ O2(g) CO(g)
• Passive oxidation of fibres and matrix
• SiC 3/2 O2(g) SiO2 CO(g)
• SiC 2 O2(g) SiO2 CO2(g)

• Sealing of the pore
• Passive oxidation of the matrix
• SiC 3/2 O2(g) SiO2 CO(g)
• SiC 2 O2(g) SiO2 CO2(g)

Residual Youngs modulus
Mass change
2D Nicalon / C (d0.1 µm)/ CVI-SiC without an
anti-oxidation coating heated for 35hrs in air at
different temparatures Huger et al. J Amer Ceram
Soc 77 (1994) 2554
composite oxidation
41
Active oxidation of 2D SiCf/C/SiCm (low Po2)

• SiC-based fibers are basically instable
• SiC O2(g) SiO(g) CO(g) recrystallisation
  of SiC
• Strong impact on the fibre strength that provides
  the mechanical properties of the composite
• Surface flaws ? cracks

Fully brittleno test
RT tensile strength of fibres heated for 3.6ks in
Ar-O2 at 1500C Shimoo et al. J Mater Sci 37
(2002) 4361)
composite oxidation
42
Oxidation of composites under load

• Even for coated specimens
• At ?gt0-100MPa ? matrix cracking
• At 500-1000C
• Jones et al. proposed a Po2/T map

SiO2 on the fibres
Interphase removal
Fibre creep only
Crack velocity for model composite with Nicalon
fibres at 1100C Jones et al. Mater Sci Eng A198
(1995) 103
Jones et al. J Amer Ceram Soc 83 (2000) 1999
composite oxidation
43
Improvement of the oxidation resistance EBC

• Environmental Barrier Coating

• Boron forms an oxide with a low melting point
  Tf(B2O3)450C
• 2B O2 ? B2O3
• 2BN O2 ? B2O3 N2(g)
• B4C 4 O2 ? 2 B2O3 CO2(g)
• SiB6 11/2 O2 ? 3 B2O3 - SiO2
• Fusible boron oxide or boron silicate seal the
  porosity and the crack tips

?r (MPa)
RT flexural strength of a 2D-Nicalon/C/CVI-SiC
with and without a CVD-SiC seal coat after
oxidation in air at 1000C Lowden, in Designing
Ceramic Interfaces II, Peteves Ed. (1993) 157
Time at 1000C in air (h)
composite oxidation
44
Improvement of the oxidation resistance
self-healing matrixes

• Matrix with dispersed particles
• Boron-based particles B4C, BN, SiB6
• Forms a healing oxide
• Matrix fabricated by PIP

• Multilayer matrix
• Low melting phase X B, B4C, Si-B-C
• Compliant material Y PyC, C(B), hex-BN
• Matrix fabricated by P-CVI (X-Y-X-Y)n

Lamouroux et al., Composites Sci Technol 59(199)
1073
composite oxidation
45
Improvement of the oxidation resistance
alternative interphases

• B-based interphases hex-BN or C(B)
• 2BN O2 ? B2O3 N2(g)
• 2B O2 ? B2O3
• Forms a healing oxide
• Multilayer interphase
• Oxidation resistant material SiC, TiC
• Compliant material Y PyC, hex-BN
• Deposition by P-CVI (X-Y-X-Y)n

Fatigue life (4-point bending) of 2D-Nicalon/PyC
or BN/CVI-SiC in air at 600 and 950C Lin et
al., Mater Sci Eng A321 (1997) 143)
composite oxidation
46
Content

• Introduction on the GFR application
• SiCf/SiCm structure and fabrication
• Thermal stability
• Oxidation
• Composite resistance
• RD needs to qualify SiC/SiC for GFR applications

47
RD needs for qualifing SiC/SiC composite for GFR

• Corpus of data on the thermal aging and oxidation
  behaviour of composites
• All studies are on a very short term!
• For monolithic SiC wide ranges of temperature
  and P(O2) were covered
• Widespread results (strong dependence to SiC
  purity and nature)
• Few data on the effect of water in relevant
  ranges
• For components some domains of temperature and
  P(O2) were investigated
• Strong influence of chemistry, structure and
  fabrication processes
• Pre-selection of candidate technologies and
  systematic study
• For whole composites some particular studies
  at high P(O2)

Helium O2, H2O
900-1200C Very long times Short time at
1600C (even 2000C)
conclusion
48
RD needs for qualifing SiC/SiC composite for GFR
Helium O2, H2O

• Choice of best state of the art materials
• Stoichiometric fibres
• Low-porosity matrix (dispersed particles) or
  multilayer matrix
• Environmental Barrier Coating
• Multilayer interphase
• Acceptability ofadditives and B ?

• Control of the environment
• Control of the Po2 (lower and upper limit)
• Control of the PH2O (upper limit)
• Limit on the temperature
• Design

900-1200C Very long times
conclusion