Meeting Summary for the

Fourth Annual Review of Environmental Barrier Coatings for Engine
Applications

November 15-16^th^, 2005, Nashville, TN

Summary Prepared by Dave Carruthers & Associates

**Introduction**

Advanced structural ceramics are an enabling technology for
significantly improving performance and reducing emissions of gas
turbines for power and combined heat and power generation. However,
achievement of the benefits of ceramics is hindered by in-service
environmental degradation that results in recession of the ceramic
material. Each year from 2002 through 2005 the DOE Distributed Energy
Resources Office of Electric Transmission and Distribution and Oak Ridge
National Laboratory sponsored workshops addressing the need for improved
environmental resistance for advanced silicon based ceramics for gas
turbine applications. The workshops provided a forum for turbine and
material developers and manufacturers to access the most recent
innovations and test results on silicon nitride and environmental
barrier coatings (EBC) development, mechanical reliability, and
environmental stability. The nature of the problem, the requirements for
protection, and a variety of approaches to providing the needed
protection were presented. This report summarizes the presentations from
the 2005 review meeting. Summaries of the presentations of the previous
workshops are available at

<http://www.ms.ornl.gov/EBC05/pdf/Summary02.pdf>

<http://www.ms.ornl.gov/EBC05/pdf/Summary03.pdf>

<http://www.ms.ornl.gov/EBC05/pdf/Summary04.pdf>

**Nature of the Problem:**

Silicon nitride and silicon carbide ceramics rely on the formation of a
self-replenishing surface layer of SiO~2~ to resist oxidation at
elevated temperatures. However, under the severe gas turbine conditions
of pressure, temperature and velocity, water vapor reacts with the
SiO~2~ to form gaseous silicon hydroxide species that are swept away.
The resulting and continuous **loss of the SiO~2~ layer allows oxidation
of the substrate resulting in surface recession**. The associated
dimensional changes result in a loss of functional efficiency of the
components. Further, field experience with ceramic nozzles in an APU
application has shown the potential for corrosion induced from the
deposition of ingested dust. The results indicate that **hot corrosion**
from CMAS or other deposits must be addressed in addition to the water
vapor recession issue. EBCs will potentially be required for all static
structures and some turbine rotor components. Blade tip and shroud
regions present a unique requirement for EBCs due to the occurrence of
rotor blade tip rubs. Requirements for a protective EBC were defined in
detail in the 2002 summary and updated in the 2003 summary.

**EBC Composition Developments**

**Carlos Levi**, UCSB discussed the evolution of TBC's under aggressive
environments and the implications for potential EBC+(TBC) systems.
**CMAS** is already a major issue for TBCs that will become further
exacerbated as engine temperatures increase. CMAS is amorphous, and
dissolves then reprecipitates t'-YSZ as monoclinic and cubic. Alumina
substrates also dissolve in CMAS to form a new crystalline form. The
solution for TBC remains elusive, and it will be an issue of concern for
EBC and TBC/EBC systems.

**Dongming Zhu, NASA**, identified key durability issues for thermal and
environmental barrier coatings (TEBCs) as sintering and CTE mismatches
induce surface cracks, surface cracking accelerates delamination under
mixed mode thermal stress, and interfacial pore formation due to
chemical reactions accelerate coating spallation under thermal
gradients. Low thermal conductivity PS HfO~2~-(Y, GD, Yb)~2~O~3~ and
Gd~2~Zr~2~O~7~, Sm~2~Zr~2~O~7~ and La~2~Hf~2~O~7~ have high thermal
stability and retained strain tolerance at 2800F. Multicomponent HfO~2~
coating with ceramic based bond coats demonstrated excellent cyclic
durability at Tsurface\>1650°C (3000°F) and Tinterface up to 1500°C
(2732°F). HfO~2~ and Sm~2~Zr~2~O~7~, and La~2~Hf~2~O~7~ are considered
to be very stable (also Gd~2~Zr~2~O~7~ with minor initial weight loss)
in high water vapor environment at 1650C. High pressure burner rig
testing also demonstrated the HfO~2~ based coating stability

**Vimal Pujari, St. Gobain** reported the current approach to use
alumino or RE silicates applied by aqueous or non aqueous slurry, or by
polymer precursor. Coating candidate coupons of unspecified composition
have been sintered and evaluated the Kaiser rig at ORNL with encouraging
results. Acceptable EBC candidates have been applied to NT154 material
with good adherence. Exposure tests at St Gobain continue to show
promise.

**Tania Bhatia, UTRC**: SAS and YS based systems are being evaluated for
improved steam recession resistance over the BSAS/mullite/Si EBC system.
APS SAS coated CMC Solar combustor lines showed only minor recession
where the EBC survived, but also showed spallation and non-optimum
microstructures. No SiO~2~-SAS reaction was observed. The surface of the
SAS showed Sr enrichment. Improvements have been achieved in the
microstructure, and the use of a mullite interlayer improves that
temperature capability by suppressing the eutectic formation between SAS
and the SiO~2~ TGO. Analytical results highlighted the importance of CTE
matching for thermal gradient conditions

The EBC strength debit on Si3N4 substrates has been correlated to the
CTE mismatch. Cracks in the coating act as strength controlling flaws.
Multilayer EBCs expected to protect monolithic Si~3~N~4~ without
debiting its strength. NLOS coating application techniques are required
for complex Si~3~N~4~ parts. Electrophoretic/electrostatic deposition,
dip coating, and CVD methods are being evaluated. The EBC microstructure
is key to achieving projected life targets. The ability to predict life
of EBCs is a critical requirement. EBC candidates have erosion
resistance similar to 7 YSZ.

**Roger Wills, UDRI**, reported that Sc~2~O~3~ is the best sintered
additive for oxidation resistance for Si~3~N~4~, and that it may also
have improved warter vapor resistance. The Sc^3+^ ion size is smaller
than the lanthanides, thus shows stronger bonding and better oxidation
resistance. Sc~2~Si~2~O~7~ forms a tie-line to both Si~3~N~4~ and
Si~2~N~2~O, and does not form a metal oxynitride. There is a good CTE
match between Si~2~N~2~O and Sc~2~Si~2~O~7~. The weight loss of
Sc~2~Si~2~O~7~ is much less than Lu~2~Si~2~O~7~ in air/water vapor at
1500C. Oxidized samples of densified Si~2~N~2~O and Sc~2~Si~2~O~7~
formed a multilayer oxide structure. The presence of aluminum impurities
was shown to be deleterious to oxidation resistance, thus, O'SiAlON is
less stable than Si~2~N~2~O. The most promising compositions for further
study lie on the Si~2~N~2~O-Sc~2~Si~2~O~7~ tie line.

**Charles Lewinsohn, Ceramatec** reported continued development of
single and 2-layer, powder loaded polymer derived ceramic (PDC) bond
coating plus a dip coated, geomimetic, alumino-silicate top coat.
Residual stress analysis guided the bond coating thickness selection.
Experiments identified the controlling factors (solids loading) for
achieving the desired bond coat thickness and peal strength.
Hydrothermal exposures at 1200C/18 atm (Kaiser Rig) supported the
promising corrosion resistance of a geomimetic composition. Good bond
coat to substrate adherence has been achieved, but 2-layer bond coatings
delaminated at the inter-bond interface. The dense bond coatings
exhibited good thermal cycling resistance to 1300C in wet air, and no
degradation in mechanical properties. Bond coatings plus NASA applied
mullite or Yb~2~Si~2~O~5~ coatings were also cycled to 1300C. Some
spalling due to debonding between the bond coating layers occurred, but
the bond coat-to- substrate and bond coat-to-top coating held. Steam
injection exposures of EBC coated SN bars (ORNL) showed low topcoat
durability due to low density. Improvements are in progress to develop
single layer bond coatings, co-fired bond and top coatings, and denser
top coatings.

**HT Lin, ORNL** is evaluating candidate bulk silicates after steam jet
exposures. At 1300C, Lu~2~Si~2~O~7~ formed Lu~2~SiO~5~ which formed from
the volatilization of SiO~2~. At 1500C, Lu~2~O~3~ volatized from the
Lu~2~SiO~5~ and forming Lu~2~Si~2~O~7~ and Lu(OH)~3~. Exposed zircon
formed monoclinic ZrO~2~ and volatile Si(OH)~4~. Decomposition of double
Lu~2~Si~2~O~7~ EBC layers led to oxidation and recession of SN282
substrate at 1300°C. Exposed mullite/ Lu~2~Si~2~O~7~ eutectic bulk
showed localized mullite recession, but no phase change observed in
Lu~2~Si~2~O~7~ region. The stability of EBC systems in a "Na" containing
environment needs to be addressed and evaluated

**Beth Armstrong, ORNL**, highlighted how the use of colloidal theory
and the use of additives is a useful tool to designing and controlling
specific properties of **slurry-based coatings**. The understanding can
lead to coating thickness, structure (flatness, density, defects),
adhesion (wetting) and composition control. Suspensions of Mullite and
BSAS powders were characterized. Poly(ethylene imine) (PEI) was
identified as the appropriate dispersant for these and RE-silicates. The
influence of the concentration of PEI on the coating quality of mullite
on Si, Si~3~N~4~ and SASiC was illustrated. Improved wetting by the use
of latex was also demonstrated. Uniform 20-40 micron mullite coatings on
SiC and SN282 appeared stable after sintering at 1400C, although an
interlayer of Si-Al-O formed on the SN282. A uniform but somewhat porous
20 micron BSAS coating was also applied to SASiC. Strength limiting
flaws were identified in the substrate. Mullite coatings remained after
500 hr steam exposures at 1200C, 100 RH at 1 atmosphere. Some SiO~2~ and
Si-Al-O formation occurred. SASiC bars coated with BSAS/Mullite were
also exposed to humidity. Some debonding and reaction were observed at
the SASiC-mullite interface. Si coated CMC material was also coated with
BSAS/mullite and exposed to humidity at 1315C. Results were similar to
those of APS sprayed coatings.

**Ted Paquette, RCI:** The use of custom synthesized organometallic
reactants has the potential to dramatically improve **CVD** processes
through rapid evolution of OM synthesis and CVD processes. - RCI has
utilized these advanced for oxide and non-oxide coatings, composite
interfaces and composite matrices. RCI has now developed a pre-ceramic
polymer SiBN interface coating approach that produces uniform thickness
filament coatings. Si content can be controlled and content levels much
higher than prior CVI/CVD processes can be achieved. Additionally, RCI
has developed oxide interfaces for composites and coatings: a) Mullite
(3Al~2~O~3~·2SiO~2~) and alumina-rich aluminosilicate CVD coatings (6:2
Al:Si ratio), b) Hafnium Silicate (HfSiO~4~) composite fiber interface
coatings and EBC's on Ultra High Temperature CMC's. RCI has also
developed a functionally graded fiber interfaces: a) multi-layer fiber
interface structure with an outer HfSiO~4~ surface, b) a 2 part CVD
process-OM species stick to surface then mild oxidizing agent removes
organic moieties. The current process produces Hf rich HfSiO4

**Dave Carruthers, Dave Carruthers & Associates**, reviewed approaches
not otherwise reported at the review meeting. **Nakayama**, Japan, has
tested stand alone APS films in humid environments to 1100C as potential
topcoats for EBC for SiC/SiC. Sprayed combinations of Y~2~O~3~,
Yb~2~O~3~, or Lu~2~O~3~ and SiO~2~ were exposed. Amorphous Lu~2~SiO~5~
had superior stability, even when it crystallized as Lu2Si2O7 during
exposure.

**Kyocera** and KHI have achieved over 400 hours of engine testing on
thin and thick slurry applied Yb~2~Si~2~O~7~ and a Zr~2~O~3~ topcoat on
an intermediate layer on SN282 combustor liners. The thin coating was
dense but contained Si~3~N~4~ which recessed. The thick coating was
porous, but did not contain Si~3~N~4~, thus, did not recess The
Zr~2~O~3~ showed reduced recession in a 1300C burner rig compared to the
Yb disilicate coating, and also showed no degradation after engine
testing. **Vassen**, (ITSC, Germany) has reported the successful
application of APS LaHf~2~O~7~/YSZ/Mullite on a CFCC substrate. 1200C
humid exposures are planned for late 2005. **Harada**, AIST has exposed
Alumina and YAG, GAP, EAG or YAG/Zr~2~O~3~ melt grown eutectics to 1500C
humid exposures. The Al2O3/YAG/Zr~2~O~3~ material showed the lowest
weight loss. All of the exposed material showed a strength loss. **Rishi
Raj** and **Linan-An** have reported results of humid exposures of PDC
based EBCs. Raj's porous PDC derived intermediate layer with a HfO~2~
topcoat and a HfO~2~+SiCN+HfSiO~4~ bond coating had a reduced weight
loss of the bare Si~3~N~4~ substrate. An's coating was applied with very
small amounts of aluminum. Exposures up to 1400C showed reduced
oxidation of the SiAlCN compared to SiCN, and showed it to have an
oxidation rate constant lower than CVD SiC and Si~3~N~4~. It was
proposed that the aluminum atoms center in the 6-membered SiO~2~ rings
and reduces the diffusion of oxygen. Humid exposures at 1100C also
showed improved water vapor resistance.

**Rig, Engine, Mechanical Properties and NDE Evaluations**

**Pete Meschter, GE**, reviewed the testing protocol and sequencing to
confirm the reliability of Si/mullite+BSAS/BSAS coated CMCs prior to
engine testing. Testing has followed a progressive process from lab
isothermal air and steam exposures, to combustor rig tests to component
rig tests before engine testing. Nine EBC coated CMC inner shroud
segments have now been installed in a utility owned 7FA engine, and
testing has been in progress since 2002. Shrouds have been exposed for
5000+ hours and 10+ start/stop cycles confirming the EBC capabilities.
The maximum shroud surface temperature is 2200 to 2300F. Removed shroud
show no structural damage, however cracking/chipping of the EBC along
the edges was common. No mechanical property degradation occurred in the
prepreg shrouds after engine testing. Slurry cast EBC spalled over CMC
tooling bumps. Controlling the bondcoat oxide morphology is critical to
assuring EBC spalling resistance. The bondcoat oxide quality on engine
test shrouds is mixed. The BSAS smoothed and recessed uniformly. Celsian
is retained during the engine test.

**Mark van Roode, Solar:** SiC/SiC and Hybrid Oxide Ceramic Matrix
Composite (CMC) combustor liners have accumulated over 74,000 hours
since 1997 in Centaur 50S at two field-test sites. High time tests of
13,937 hrs and 15,144 hrs have been achieved for SiC/SiC CMCs with EBCs.
EBCs improved the life of the SiC/SiC CMCs by 2-3x. Hybrid Oxide CMC
outer liner: have accumulated over 18,288 hrs in field testing since
2003. FGI mitigates loss of fiber mechanical properties of the
oxide/oxide above \~ 1100°C. Testing was interrupted after 12,582 hrs/63
starts in 2004. Erosion of FGI was observed at liner hot zones Minor FGI
damage was repaired and test resumed thru Oct. 2005. Some new damage at
detected at July boroscope but liner still functional. Plans are to
continue testing to 20,000-hr mark followed by full NDE and destructive
evaluation to analyze performance of CMC and FGI. SiC/SiC MI liners will
be tested at second site 2006. Areas for further development include the
CMC-to-metallic structures interface and cost reduction of CMC parts.

**Ram Bhatt, NASA**, reported the effect of coating variables for
coatings of Si, mullite, and Sic the strength of AS-800, SN282, Ceralloy
147A silicon nitrides, Hexalloy AS, and Coors AD998 Alumina. Coatings
were applied by PS, CVD, or EB/PVD. SiC was also applied by a polymer
derived (PD) process. Ceramic substrates (nitrides, carbide and oxide)
coated with PS, CVD, or EB/PVD coatings show strength degradation, but
the extant of degradation depends on coating processing methods.
Strength degradation is independent of coating thickness or composition.
Preliminary results indicate that the PD-SiCcoated silicon nitride show
strength retention, but degrade after annealing at elevated
temperatures.

**Peter Tortorelli, ORNL** has previously reported that Kaiser rig
exposures 1-2 atm H2O characterize the thermal stability, interactions
between layers, and assess the effectiveness as an oxygen barrier. The
SiO~2~ thickness at the substrate interface correlates with oxygen
permeation. Eighteen atm exposures at 1250C of SiO~2~, CVD SiC, BAS,
SAS, and BSAS have also been conducted to qualitatively confirm the
effectiveness of the test to rank volatility. Bulk and Coating samples
of Lu2Si2O7, Yb~2~Si~2~O~7~, Y~2~Si~2~O~7~, SrAl~2~Si~2~O~7~, and BSAS
and other compositions are being exposed on Si based substrates. The
bulk silicates and some bulk BSAS showed good resistance to volatility.

**Bill Ellingson**, **ANL**, reported that Optical Computed Tomography
(OCT) and Elastic Optical Scattering (EOS) have both been shown to
detect thickness variations and delaminations in EBCs. EOS uses
polarized laser light to probe the subsurface characteristics of
optically translucent ceramics. The OCT uses a time --gated
reflectometer to obtain cross-sectional images of subsurface features of
optically translucent ceramic materials in axial and transverse planes.
The OCT evaluated Si/Mullite/BSAS on SiC/SiC and a green slurry-cast
oxide based EBC on a quartz plate.