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Self-Healing EBC Scales on SiC James L. Smialek, QuynhGiao N. Nguyen, R. Craig Robinson

Self-Healing EBC Scales on SiC James L. Smialek, QuynhGiao N. Nguyen, R. Craig Robinson. Protective SiO 2 scales react in hot gas environment, form volatile Si(OH) 4 , give paralinear (recession) kinetics. k l  gas transport model, Opila, et al:. Linear Weight Loss of SiC in HPBR

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Self-Healing EBC Scales on SiC James L. Smialek, QuynhGiao N. Nguyen, R. Craig Robinson

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  1. Self-Healing EBC Scales on SiC James L. Smialek, QuynhGiao N. Nguyen, R. Craig Robinson Protective SiO2 scales react in hot gas environment, form volatile Si(OH)4, give paralinear (recession) kinetics. kl gas transport model, Opila, et al:

  2. Linear Weight Loss of SiC in HPBR 6 atm, 20 m/s

  3. Normalized Recession Rate 5-15 atm, 10-30 m/s

  4. 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 composites Furnace (air, 50% H2O), HPBR Characterization, optimization, kinetic models

  5. Idealized EBC Scale Schematic Self-healing scales TiO2 (Al2O3) kp= 105 (102) SiO2 kp SiO2 Ti-X precursors: TiC, TixSiy, TiAl SiC critical kp 2m2/hr for healing (100hr @ 10v/o, 10 m radius)

  6. 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?).

  7. 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

  8. As-polished SiC-TiC After 500 hr oxidation at 1000oC 25x Self-Healed TiO2 Scale Formed on SiC-50TiC

  9. 170 kJ/moleoK 200 kJ/moleoK oxidation of Ti 195 kJ/moleoK oxidation of TiN 220 kJ/moleoK oxygen diffusion in Ti 189 kJ/moleoK oxygen diffusion in TiO2 455 kJ/moleoK oxidation of Si3N4-Al2O3-TiN

  10. Unaffected core SiC-50TiC, HPBR,1330oC, 7.5 hr (dispersed porosity) C Si C SiC Dark phase TiC Light phase Ti

  11. 6kV SiC spectra Si Si Si Si C C C C

  12. 1 6kV TiC spectrum C Ti O 3

  13. Ti, O C 100 mm Surface facets Cross section TiO2 External Scale Formed in HPBRSiC-50TiC, 1330oC, 7.5 hr 20 mm

  14. Ti, ka O Ti Ti, O C Si C 15kV 6kV TiO2 spectra

  15. immediate subscale 1 2 10 mm Ti, O O 1) TiO2 2) SiO2 Si C

  16. 10 mm 10 mm 100 mm 10 mm 10 mm SiC-50TiC, HPBR, 1000oC, 60 hr SiO2in TiO2 SiC in SiO2

  17. 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)

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