hotcorrosionof$ sicbased$ …12 joe hagan.pdfoctober 7-11, 2012 – pittsburgh, pennsylvania...
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October 7-11, 2012 – Pittsburgh, Pennsylvania
Hot Corrosion of SiC-‐Based Ceramic Matrix Composite
Materials
Joseph Hagan, Elizabeth Opila University of Virginia – Materials Science
Engineering
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
Hot Corrosion of SiC
•
• 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
October 7-11, 2012 – Pittsburgh, Pennsylvania
Objec?ves
Na2O·∙SiO2 Figure 12137, ACerS Phase Equilibria Diagrams Na2O·∙SiO2·∙B2O3 Figure 515, ACerS Phase Equilibria Diagrams
B2O3 SiO2 B2O3 SiO2
Na2O
October 7-11, 2012 – Pittsburgh, Pennsylvania
Materials • Substrate Materials
– Hexoloy -‐ sintered α-‐SiC (SASC) with C and B4C 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 0.125”
• SiC substrates evaluated uncoated and with
coaEngs to simulate
interphase materials – C coaEngs ~0.8 µm thick
– BN coaEngs ~1.0 µm thick
• fiber
• matrix
• interphase
Opila and Boyd, Unpublished
• 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
October 7-11, 2012 – Pittsburgh, Pennsylvania
Hot Corrosion Exposure
~ 1.25 cm
October 7-11, 2012 – Pittsburgh, Pennsylvania
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 – H2O to remove residual Na2SO4 and Na-‐(B)-‐Silicates
– HCl to remove soluble Na-‐(B)-‐Silicates
– HF to remove SiO2
• 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
October 7-11, 2012 – Pittsburgh, Pennsylvania
-‐10
-‐8
-‐6
-‐4
-‐2
0
2
1000°C 900°C
Mass Ch
ange (m
g)
Hexoloy Hexoloy CVD CVD
Mass loss aMer HF diges?on includes loss of residual
Na2SO4
• 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
October 7-11, 2012 – Pittsburgh, Pennsylvania
-‐14
-‐12
-‐10
-‐8
-‐6
-‐4
-‐2
0
2
4
Uncoated C-‐Coated BN-‐Coated
Mass Ch
ange (m
g)
• Greater weight loss with coated samples
• C-‐coated sample lost more weight than BN-‐coated
Mass loss aMer HF diges?on includes loss of residual Na2SO4
October 7-11, 2012 – Pittsburgh, Pennsylvania
SEM of Surface • Largely silica formaEon
• Pools of residual Na2SO4
remain
• Some larger silica
features present
• Typical of both CVD and Hexoloy samples
SiO2 Na2SO4
Hexoloy, uncoated, 1000°C for 24hr
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
CVD, uncoated, 1000°C for 24hr Hexoloy, uncoated, 900°C for 24hr
Silica Bubble
Na2SO4
Substrate Substrate
October 7-11, 2012 – Pittsburgh, Pennsylvania
PiRng Morphology • Pit depths on the order of 20-‐30 microns
– More than the surface recession of 8-‐10 microns
• Large distribuEon of pit sizes
CVD, uncoated, 1000°C for 24hr Hexoloy, uncoated, 1000°C for 24hr, aBer HF
Substrate
SiO2 in Pit
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
• Corrosion products largely water and HF soluble
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
ICP Results – Water Diges?on • Most of the products
being removed are Na and S
• Residual Na2SO4 from the
tests
• Some Si removed as well
in the 1000°C exposures
• Some B removed from the
BN-‐coated sample
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
ICP Results – HCl Diges?on • 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
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
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
October 7-11, 2012 – Pittsburgh, Pennsylvania
Acknowledgements • ONR Award No. – N000141110601 • Program Manager – Dave Shifler, Propulsion Materials Program
• Jayme Curet at Thermo Fisher ScienEfic
• Elise Poerschke