hotcorrosionof$ sicbased$ …12 joe hagan.pdfoctober 7-11, 2012 – pittsburgh, pennsylvania...

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


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


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


Objec?ves

Na2O·∙SiO2 Figure 12137, ACerS Phase Equilibria Diagrams Na2O·∙SiO2·∙B2O3 Figure 515, ACerS Phase Equilibria Diagrams

B2O3 SiO2 B2O3 SiO2

Na2O


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Opila and Boyd, Unpublished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


Hot Corrosion Exposure

~ 1.25 cm


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


-‐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


-‐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


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


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


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


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


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


•  Corrosion products largely water and HF soluble


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


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




baselines


•  ID-‐8: CVD at 1000°C



900°C

•  ID-‐22: CVD at 900°C



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




baselines


•  ID-‐8: CVD at 1000°C



900°C

•  ID-‐22: CVD at 900°C



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


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


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


Acknowledgements •  ONR Award No. – N000141110601 •  Program Manager – Dave Shifler, Propulsion Materials Program

•  Jayme Curet at Thermo Fisher ScienEfic

•  Elise Poerschke

hotcorrosionof$ sicbased$ …12 joe hagan.pdfoctober 7-11, 2012 – pittsburgh, pennsylvania...

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