Glenn Research Center Materials Division
Protective SiO2 scales react in hot gas environment, form volatile Si(OH)4, give paralinear (recession) kinetics.
kl gas transport model, Opila, et al:
Self-Healing EBC Scales on SiCJames L. Smialek, QuynhGiao N. Nguyen, R. Craig Robinson
422
222
)(22
OHSiOHSiOCOSiOOSiC
)/()/108exp(0.2)( 22121
22 hrcmmgv
PPRTkJHPBRk
Total
OHl
Glenn Research Center Materials Division
Linear Weight Loss of SiC in HPBR6 atm, 20 m/s
Glenn Research Center Materials Division
10,000/T (K)5.8 6.0 6.2 6.4 6.6 6.8
k l/(P
1.50 )(v
0.50 )
10-4
10-3(6)
(15) (5)(10)
1400 C 1200 C1300 C
kl=2.06(e-108kJ /mol*RT)(P1.50)(v0.50)
Atm.=
Normalized Recession Rate5-15 atm, 10-30 m/s
Glenn Research Center Materials Division
Present Solution: Plasma sprayed EBC oxides 4. BSAS topcoat (barium strontium alumino-silicate)-----------------------------------------------------------------------------3. Mullite + BSAS intermediate layer (furnace sprayed)-----------------------------------------------------------------------------2. Si bondcoat-----------------------------------------------------------------------------1. Molten salt etched, washed, and dried CMC surface
Alternate approach: EBC scales formed on additives
TiC (TixSiy,TiAl, TiCrAl) TiO2, Al2O3
Hot pressed SiC-%TiX compositesFurnace (air, 50% H2O), HPBRCharacterization, optimization, kinetic models
Glenn Research Center Materials Division
SiO2
SiC
Ti-X precursors:TiC, TixSiy, TiAl
Idealized EBC Scale Schematic
critical kp 2m2/hr for healing (100hr @ 10v/o, 10 m radius)
Self-healing scalesTiO2 (Al2O3) kp= 105 (102) SiO2 kp
Glenn Research Center Materials Division
Pertinent Oxidation Background(non-oxide, multiphase ceramics)
• Si3N4–TiN, TiC,… Gogotsi, Lavrenko, Yaroshenko…
Bellosi, Vincenzini, Tampieri…Buljan, Zilberstein…
• ZrC, HfC (-TiC), Shimada,....
– At 25 wt.% TiN, continuous TiO2 surface scale forms.
– TiO2 nodules are only over individual particle at lower loadings.
– HfTiO4 inner scales develop under TiO2 surface scale.– Carbon found in scale at carbide interface (titanium oxycarbide?).
Glenn Research Center Materials Division
Short Subjects
• 50-50 SiC-TiC furnace data vs T• 50-50 SiC-TiC, 1330oC HPBR oxidation, scale microstructure• 50-50, 90-10, 1000oC HPBR weight change• 50-50 SiC-TiC, 1000oC HPBR microstructure• TiSi2 furnace oxidation
Glenn Research Center Materials Division
Self-Healed TiO2 Scale Formed on SiC-50TiC
As-polished SiC-TiC After 500 hr oxidation at 1000oC
25x
Glenn Research Center Materials Division
Furnace Oxidation of SiC-50TiC Composites at 1000-1300oC
oxidation time, hours0 20 40 60 80 100 120
spec
ific
wei
ght c
hang
e, m
g/cm
2
0
10
20
30
40
50
60
goal
1000o
1100o
1200o
1300o
Glenn Research Center Materials Division
Furnace Oxidation of SiC-50TiC Composites at 1000-1300oC
time1/2, hr1/2
0 2 4 6 8 10
spec
ific
wei
ght c
hang
e, m
g/cm
2
0
10
20
30
40
50
60
goal
1000o
1100o
1200o
1300o
Glenn Research Center Materials Division
Arrhenius Plot of SiC-50TiC Oxidation Rates
1/T, 104 K-16 7 8pa
rabo
lic o
xida
tion
rate
, kp,
mg2 /c
m4 hr
0.01
0.1
1
10
goal
1100o
1200o
1300o
1000o170 kJ/moleoK
200 kJ/moleoK oxidation of Ti195 kJ/moleoK oxidation of TiN220 kJ/moleoK oxygen diffusion in Ti189 kJ/moleoK oxygen diffusion in TiO2
455 kJ/moleoK oxidation of Si3N4-Al2O3-TiN
Glenn Research Center Materials Division
CSi
Ti
C
SiCDark phase
TiCLight phase
Unaffected coreSiC-50TiC, HPBR,1330oC, 7.5 hr
(dispersed porosity)
Glenn Research Center Materials Division
6kV SiC spectra
C
Si
C
Si
C
Si
CSi
Glenn Research Center Materials Division1
3
C
TiO
6kV TiC spectrum
Glenn Research Center Materials Division
C
Ti, O
100 m
TiO2 External Scale Formed in HPBR SiC-50TiC, 1330oC, 7.5 hr
Surface facets Cross section
20 m
Glenn Research Center Materials Division
C
TiO
C
Ti, O
Si
Ti, k
6kV TiO2 spectra
15kV
Glenn Research Center Materials Division
1
2 10 m
C
Ti, O O1) TiO2
Si2) SiO2
immediate subscale
Glenn Research Center Materials Division
HPBR Oxidation of SiC-TiC Composites at 1000oC
oxidation time, hours0 20 40 60 80 100 120
spec
ific
wei
ght c
hang
e, m
g/cm
2
0
10
20
30
40
goal
50-50furnace90-10
50-50
Glenn Research Center Materials Division
SiC-50TiC, HPBR, 1000oC, 60 hr
10 m
10 m
SiO2in TiO2
SiC in SiO2
10 m
10 m
100 m
Glenn Research Center Materials Division
Furnace Oxidation of TiSi2 Composites at 1000-1300oC
oxidation time, hours0 20 40 60 80 100 120
spec
ific
wei
ght c
hang
e, m
g/cm
2
0
1
2
3
1000oC1100oC1200oC1300oCgoal
Glenn Research Center Materials Division
Preliminary Status Report
• Hot pressed SiC-TiC; oxidized at 1000-1300oC.• At 25 and 50%, distinct, oversealing TiO2 scales formed.• Oxidation rates greatly exceeded target; porosity. • HPBR tests indicate even higher growth rates.• Faceted, continuous TiO2 surface, mixed TiO2+SiO2
subscale (not effective), then ‘intact’ SiC+TiO2
• (No guarantee of moisture resistance)
