Hot Corrosion of SiC- Based Ceramic Matrix Composite Materials

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1 Hot Corrosion of SiC- Based Ceramic Matrix Composite Materials Joseph Hagan, Elizabeth Opila University of Virginia Materials Science Engineering

2 Mo?va?on SiC/SiC ceramic matrix composites (CMCs) are now being introduced into aircrab turbine engines NaCl from marine environment ingested into engine combine with sulfur impuriees in jet fuel resuleng in hot corrosion condieons Hot corrosion of monolithic SiC is well studied and rapid ahack rates are known to occur under some condieons Hot corrosion of composites is not well characterized SiC- based composites will likely all be coated with Environmental Barrier CoaEngs (EBCs) Understanding hot corrosion of SiC- based composites is needed in case of coaeng imperfeceons or spallaeon

3 Hot Corrosion of SiC

4 Objec?ves Determine the effects of chemistry on the corrosion of CMCs as compared to monolithic SiC C and BN interphases, excess Si from melt infiltraeon process Determine the effect of CMC architecture on corrosion Na 2 O B 2 O 3 SiO 2 B 2 O 3 SiO 2 Na2O SiO2 Figure 12137, ACerS Phase Equilibria Diagrams Na2O SiO2 B2O3 Figure 515, ACerS Phase Equilibria Diagrams

5 Materials Substrate Materials Hexoloy - sintered α- SiC (SASC) with C and B 4 C sintering aids High purity CVD SiC Silicon Real composites (MI and CVI with both HNS and Sylramic fibers) Coupons are ~ 0.5 x 0.5 x SiC substrates evaluated uncoated and with coaengs to simulate interphase materials C coaengs ~0.8 µm thick BN coaengs ~1.0 µm thick matrix interphase Opila and Boyd, Unpublished fiber

6 Hot Corrosion Exposure Samples loaded with Na2SO4 on top surface Salt loading of ~2-3 mg/cm2 Samples exposed in a tube furnace in pairs One sample for chemical and one for microstructural characterizaeon 24 hour exposures, controlled dry 0.1% SO2/O2 atmosphere, 100 sccm flow rate through a 46 mm tube Mass change measured aber every step As- received, aber salt- loading, aber exposure Recession measured with a micrometer ~ 1.25 cm

7 Sample Characteriza?on Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) used to determine morphology of corrosion products in plan view and cross- seceon Corrosion products removed via step- wise digeseon procedure H 2 O to remove residual Na 2 SO 4 and Na- (B)- Silicates HCl to remove soluble Na- (B)- Silicates HF to remove SiO 2 InducEvely Coupled Plasma - OpEcal Emission Spectroscopy (ICP- OES) used to analyze composieon of corrosion products removed in each of these steps Atomic concentraeons (ppm levels) and raeos of elements Analyzed for Na, Si, S, and B

8 2 Hexoloy CVD Hexoloy CVD 0 Mass Change (mg) Mass loss amer HF diges?on includes loss of residual Na 2 SO C 900 C Dark bars: change vs. mass aber salt deposieon Light bars: change vs. mass aber salt deposieon aber HF digeseon More mass loss at 1000 C than at 900 C More mass loss in Hexoloy than in CVD

9 4 2 0 Mass Change (mg) Mass loss amer HF diges?on includes loss of residual Na 2 SO 4 Uncoated C- Coated BN- Coated Greater weight loss with coated samples C- coated sample lost more weight than BN- coated

10 SEM of Surface Largely silica formaeon Pools of residual Na2SO4 remain SiO2 Na2SO4 Some larger silica features present Typical of both CVD and Hexoloy samples Hexoloy, uncoated, 1000 C for 24hr

11 SEM of Cross Sec?on Bubble formaeon on leb typical at 1000 C Thick layer of residual sodium sulfate on right with lihle surface ahack at 900 C Silica Bubble Na 2 SO 4 Substrate Substrate CVD, uncoated, 1000 C for 24hr Hexoloy, uncoated, 900 C for 24hr

12 PiRng Morphology Pit depths on the order of microns More than the surface recession of 8-10 microns Large distribueon of pit sizes SiO 2 in Pit Substrate CVD, uncoated, 1000 C for 24hr Hexoloy, uncoated, 1000 C for 24hr, aber HF

13 ICP- OES Samples digested (dissolved) in a known volume of liquid Liquid pumped into a spray- atomizer Atomized sample injected into an Ar plasma Atomic emissions are analyzed using a spectrometer Trace element analysis possible DetecEon limits of between 0.5 and 5 ppb ConcentraEons in normalized by volume 1 ppm = 1 μg/ml = 1 mg/l

14 ICP Challenges QuanEtaEve results rely on very careful procedures Since trace concentraeons are measured, small errors or impuriees can affect results significantly Na is one of the most prevalent contaminants found Very difficult to ensure complete accuracy with Na Si and B are secky elements They adhere to the sample introduceon system An acidic rinse must be used to remove remnants Incomplete rinse can result in inaccurate calibraeons and measurements Inaccurate concentraeons May result in negaeve values in some cases

15 Corrosion products largely water and HF soluble

16 ICP Results By Diges?on Step ID- 3 and Hex are baselines ID- 6: Hexoloy at 1000 C ID- 8: CVD at 1000 C ID- 9: Hexoloy with BN at 1000 C ID- 21: Hexoloy at 900 C ID- 22: CVD at 900 C ID- 25: Hexoloy with C at 1000 C

17 ICP Results Water Diges?on Most of the products being removed are Na and S Residual Na 2 SO 4 from the tests Some Si removed as well in the 1000 C exposures Some B removed from the BN- coated sample

18 ICP Results By Diges?on Step ID- 3 and Hex are baselines ID- 6: Hexoloy at 1000 C ID- 8: CVD at 1000 C ID- 9: Hexoloy with BN at 1000 C ID- 21: Hexoloy at 900 C ID- 22: CVD at 900 C ID- 25: Hexoloy with C at 1000 C

19 Most of the products being removed are siliactes Rich in Na and S Total mass removed in HCl is four orders of magnitude less than in water ICP Results HCl Diges?on

20 ICP Results By Diges?on Step ID- 3 and Hex are baselines ID- 6: Hexoloy at 1000 C ID- 8: CVD at 1000 C ID- 9: Hexoloy with BN at 1000 C ID- 21: Hexoloy at 900 C ID- 22: CVD at 900 C ID- 25: Hexoloy with C at 1000 C

21 ICP Results HF Diges?on Most of the products being removed are Si- based Hexoloy had over three Emes as much silica as CVD C and BN coated samples had worse oxidaeon than uncoated Much less silica at 900 C than 1000 C

22 Summary and Conclusions CVD SiC is more resistant to hot corrosion than Hexoloy Hot corrosion is much greater at 1000 C than at 900 C Current results indicate BN and C degrade SiC corrosion resistance Hot corrosion presents a non- uniform ahack on the sample surface Bubbles and pits ICP appears more robust and versaele than measuring mass change and morphology characterizaeon Allows for analysis of the amount of corrosion products formed InformaEon about the chemistry of corrosion products as well ICP in conjunceon with tradieonal characterizaeon will allow for a greater understanding of the hot corrosion of SiC- SiC CMCs

23 Future Work Further explore the dependence of corrosion rate on temperature Explore higher temperatures, up to 1200 C InvesEgate the kineecs of the corrosion and how they develop at both shorter and longer Emes Gain a greater understanding of how B and C affect the hot corrosion behaviour of SiC Determine phase equilibria present in the system Characterize the pits and measure a staesecal number of pits to help inform life- prediceon models Move into the more complicated architecture of composite materials

24 Acknowledgements ONR Award No. N Program Manager Dave Shifler, Propulsion Materials Program Jayme Curet at Thermo Fisher ScienEfic Elise Poerschke