**Workshop Summary**

"New Developments in Silicon Nitride and Environmental Barrier Coatings
for Microturbine and Industrial Gas Turbine Hot-Section Components"

November 6-7^th^, 2002, 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. The ceramic
material and application technologies have progressed to the point where
ceramic components have been tested in laboratory and field engine tests
for tens of thousands of hours. However, widespread realization of the
benefits of ceramics is hindered by in-service environmental degradation
that results in recession of the ceramic material. The Department of
Energy\'s Office of Distributed Energy and Electrical Reliability (DEER)
and NASA have sponsored a variety of industrial, government, and
university studies of this degradation and potential approaches to its
mitigation. This workshop 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. This
paper provides a summary of the workshop presentations addressing these
topics. The paper is organized topically to provide summaries of the
Nature of the Degradation Problem, the Requirements for Protection
Approaches, Approaches and Results, and Conclusions and Recommendations.
Specific approaches discussed include in-situ or self generating
approaches, reaction and diffusion treatments, traditional ceramic
powder based coatings, and polymer precursor ceramic approaches.
Presenter's names and organizations are highlighted in the text to
simplify locating them.

**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, but no critical loss of
strength or creep properties. The associated dimensional changes result
in a loss of functional efficiency of the components. The environmental
degradation has been studied and modeled through laboratory and rig
tests, and has been collaborated in field engine evaluations.
Significant recession was measured on silicon nitride turbine nozzles
after only 1818 hours of engine testing. The predicted maximum recession
on the leading edge of a vane after 25,000 hours is 3mm (0.127").

**Requirements for Protection**

Approaches to the resolving the recession problem all rely on the
formation of a surface barrier to water vapor. Each presenter provided a
list of protection system requirements that are summarized below. The
system must provide protection at operating conditions up to 2400F, 10
or more atmospheres of pressure, gas velocity of Mach 1 or more, and up
to 10% water vapor for from 14,000 to 30,000 hours in cyclic duty.
Further, **Mark** **van Roode** from **Solar Turbines, Inc**,
highlighted the need for solutions that also meet the overall needs of
gas turbine engines including Reliability, Availability,
Maintainability, Durability (RAMD). It is unlikely that a single layer
approach will provide the best protection and also meet requirements
identified below:

-   Durability and environmentally stability including:

    -   Resistance to reaction with oxygen, water vapor or other gas
        stream constituents

    -   Low silica activity for systems containing silica or silicates

    -   Low oxygen and water vapor permeability

    -   Must Inhibit substrate oxidation

    -   Low volatility (low vapor pressure)

    -   Resistance to small particle erosion or impact

    -   Rub tolerance and abradability when applied to turbine shrouds
        or blade tips.

    -   Stability against microstructure and phases changes; no damaging
        phase changes, grain growth, pore formation or coalescence,
        sintering, or cracking

    -   Creep resistance

-   Strong and stable adherence, including:

    -   Chemical compatibility with SiO~2~ that will form at the
        substrate interface.

    -   Closely matching coefficient of thermal expansion (CTE) with the
        substrate

    -   A low Youngs modulus to minimize CTE mismatch induced stresses
        and cracks

    -   The ability to withstand cyclic shear loads of high gas
        velocities, tip rubs, and centrifugal loads when applied to
        turbine blades or tip shrouds

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-   Compatibility with the intended function of the components
    including:

    -   Must be thin and thickness must be controllable to retain high
        performance aerodynamics

    -   Must not degrade the substrate strength or creep properties

    -   When blade tip rubbing is anticipated, the blade tip and shroud
        EBCs must be considered as system

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-   Cost effective

    -   Low-cost, non line-of-sight application methods

    -   Inspectable

    -   Repairable or strippable / re-coatable

-   Reliability

    -   Based on extended evaluation exposures under actual and
        duplicated engine conditions

    -   Database of properties and experience

    -   Understanding of recession of EBCs and life modeling

It is also desirable that the system provide both impact and
environmental resistance

**Approaches and Results**

**Dave Carruthers** provided a historical perspective of EBC
development. Current experience has been built on previous studies
including oxidation protection for carbon fiber or carbon matrix and
SiC-SiC composites. Glass forming compounds including Si, B, Hf, Cr, and
Ti have been evaluated, as have silicates. Further, the need for
corrosion protection prompted studies of plasma sprayed mullite, graded
mullite, cordierite, zircon, alumina, yttria, chromia, hafnia, and YSZ.
Most coatings did not adhere after exposure. Mullite coatings on SiC,
with a good CTE match performed best, particularly when methods to apply
crystallized coatings were developed. A base coating of Si or MoSi~2~
improved the adherence of the mullite. However, cracking of the mullite
continued to occur. The use of a low modulus BSAS was identified as
having a low silica activity, and crack healing capability, but reacted
with silica. A Si/BSAS+mullite/BSAS combination was subsequently found
to provide a good bonding, chemical reaction barrier, and protection
from water vapor.

[In-situ or self generating approaches]{.underline}

A protective barrier may be formed by making bulk modifications that
result in an in-situ, self generating protective system. This could be
accomplished by modifying the surface or near surface chemistry/phases
to create the barrier; by applying a surface coating; or by using
combinations of these approaches. The in-situ approach would tailor the
surface layers that form during the oxidation of silicon nitride and the
additives it contains. This approach may not induce any discernable
dimensional change to the components. RE disilicates have been sited as
having high temperature stability, and thus, they have been studied the
most.

**Beth Opila** of **NASA** examined the nature of the oxide formation of
two state-of-the-art-silicon nitrides, SN282, (lutetia doped) and AS800
(lanthena as major dopant) for self protective capability. Oxidation in
dry oxygen resulted in the formation of a silica scale with high aspect
ratio RE silicates grains on the surface. The RE cation diffusion rate
controls the oxidation rate. During oxidation the silica volatilized
preferentially leading to an enrichment of the RESi~2~O~7.~ The
Lu~2~O~3~ additive material oxidized at a rate (6 x 10^-5^ mg^2^/cm^4^h)
below 50% that of pure Si~3~N~4~, whereas the La~2~O~3~ additive
oxidized at the same rate as pure Si~3~N~4~ (3 x 10^-4^ mg^2^/cm^4^h).
The reduced oxidation rate of the SN282 may be due to reduced silica
volatility by enrichment of the Lu~2~Si~2~O~7~ surface phase. With water
vapor present, in static, burner rig, and engine exposures, the
volatility increased for both materials, but the SN282 still oxidized at
a slower rate. Subsurface damage occurred in the AS800 during
intergranular phase depletion. Neither material formed a continuous
surface coating of disilicate in an oxidizing atmosphere. Factors
affecting in-situ protection formation include RE mobility, CTE match,
and silica activity. The near ideal silica activity of RE~2~Si~2~O~7~
limits its protective capability. A successful in-situ system would need
to balance the positive effect of the additive migration for the
formation of the surface coating versus the depletion of grain boundary
phase which leads to microstructural subsurface damage. **Bjoern
Schenk** of **Honeywell** commented that the disilicates would
volatilize above 1200C, thus, are not viable for high temperature
protection.

**Inna Talmy, NSWC**, proposed controlling the surface oxide by
incorporating transition metal compounds, diborides or boron. This is
because borate and silicate glasses containing Group IV-VI transition
metal oxides can provide high liquidus temperatures and viscosity.
Additions of Cr~2~O~3~ or CrB~2~ reduced the oxidation weight gain
during exposures from 1400 to 1550C. 5% CrB~2~ provided the best
oxidation resistance in Si~3~N~4~ containing Y~2~O~3~ and Al~2~O~3~
additives. The Cr~2~O~3~ in the surface melt induced immiscibility and
catalyzed crystallization of Y~2~O~3~**^.^**2SiO~2~, which has a high
melt temperature of 1775C. This is expected to enhance high temperature
oxidation resistance. No confirmatory exposure testing has been
performed.

[Reaction and Diffusion treatments]{.underline}

**Rick Lowden, ORNL** proposed a pack cementation/conversion approach.
The pack cementation trials may include metals, chlorides, or nitrates,
reacted individually or in combinations. Reactants, including Al~2~O~3~,
AlCl~3~, Cr, Mg, MgCl~2~, SrCO~3~, SrCl~2~, Sr(NO~3~)~2~, YCl~3~, and
ZrCl~4~ have been applied in a powder bed, sometimes also with a NH~4~Cl
activator. Reactants have been applied to AS800, NT154, and SN282.
Graded surface reactions have resulted in a large number of conversion
chemistries. Results vary significantly with pack composition, Si~3~N~4~
composition, furnace atmosphere, and temperature. Some loss of strength
has been measured.

The approach is in the initial stages of exploration and holds promise
of a wide variety of low cost tailored bond coatings or as stand-alone
protective layers. It is suitable for complex shapes. However control of
the variety of reaction products will be required. This research is
on-going.

[Traditional ceramic powder based surface coating
approaches]{.underline}

Surface coating approaches have been explored the most. The three layer
Si/mullite+BSAS/BSAS system, where BSAS is a
barium-strontium-aluminosilicate, developed by NASA, GE, and UTRC
continues to be improved by all three. It was developed for use on
SiC/SiC has been tested the most, but primarily on SiC or SiC/SiC
composite substrates. The system constituents form a bond coat,
intermediate layer for chemical isolation, and a moisture barrier. The
system is the current "standard" for SiC/SiC protection.

**David Mitchell** reported **GE**'s interest in the
Si/mullite+BSAS/BSAS EBC primarily for SiC/SiC applications for shrouds,
combustor liners, blades, and nozzles. The EBC system has been applied
to prepreg and slurry cast CMCs. No spalling occurred after 4500 hrs of
isothermal steam exposure. Insufficient protection on coupon and hole
edges as well as around surface tool bumps resulted in oxidation
accompanied by lifting and cracking of the EBC. The EBC protected the
substrate from recession, and environmental degradation of mechanical
properties after a 1000 hour rig exposure to 1205C, 8.8 bar pressure,
130 m/s velocity, and 0.9 bar partial pressure of H~2~O. When the EBC
was pre-cracked, environmental attack occurred, accompanied by
degradation of mechanical properties. The EBC coating was demonstrated
to resist FOD and rub events.

**Ellen Sun** summarized **UTRC**'s continued development of the 3 layer
Si/mullite+BSAS/BSAS EBC system on SiC/SiC and on Si~3~N~4~. EBC
protected SiC/SiC combustor liners have accumulated over 40,000 hours of
2250F exposure in field testing in Solar's Centaur 50S engine. A high
time exposure of \>15,000 hours has been achieved. Exposures show the
EBC to be susceptible to oxidation at uncoated edges, spallation at
edges due to mechanical interference and at surface asperities, and
recession of the topcoat. One liner set has been refurbished after 7238
hours, and is currently in on-going testing. Lab exposures at 1500C, 90%
steam of SAS showed a reduced weight loss compared to BSAS. Combustor
liners have been coated with a Si/SAS and Si/mullite+SAS/SAS, and are
also in on-going field testing.

Standard EBC application on Si~3~N~4~ results in cracking due to
differential CTE. The cracking could result in substrate oxidation
and/or spallation in thermal cycles. The system has been applied to SN88
and AS800 turbine nozzles by air plasma spraying, and rig tested.
Uniform coating deposition on airfoils, transition areas, and around
cooling holes was demonstrated. The EBC provided protection for FT8
vanes exposed for 30 hrs at 1230C plus 1 hr at 1260C. The as-applied
coating resulted in a strength loss of over 50% at room temperature and
\<15% at elevated temperature due to cracks in the coating which act as
flaws.

A compression test, capable of high temperature use, has been developed
to measure the shear strength of the EBC-substrate bond.

**Bjoern Schenk**, **Honeywell**, and **Kathy** **Faber**,
**Northwestern Univ.**, proposed the use of a fine powder Ta~2~O~5~
based coating system as a coating on AS800 due to its water vapor
stability, and close CTE match. Air plasma spraying and EBC application
approaches are being evaluated. Ta~2~O~5~ inhibits silica volatization
in water vapor, but transports oxygen rapidly so it may function well as
a top coat for a multilayer system. Ta~2~O~5~ undergoes a beta to alpha
phase transformation at 1360C with an approximate 8.6% volume increase.
Also, grain growth occurs during exposures to 1200C. Co-doping with
Al~2~O~3~ (2%) and La~2~O~3~ (1%) inhibits both grain growth and the
phase transformation. The tailored EBC provided cyclic stability to
1204C, and protection in Keiser rig exposures to 1315C with water vapor
for 500 hours. The growth of a cracked cristobalite scale at the
substrate surface on both the coated and uncoated materials indicates
that further development is needed. Substrate chemistry may be a
critical aspect of EBC life. Non line-of--sight application methods will
be focused in future activities.

[Polymer precursor ceramic approaches]{.underline}

**Rishi Raj, Univ of Colo** and R. **Bordia, Univ of WA** proposed the
use of polymer-derived ceramics, typically Si-C-N or SiC-N-B as top
coats, graded interlayers, and/or bond coatings for EBCs. These
approaches may offer application, chemistry, and microstructural options
not available with conventional powders, but are typically early in
their development stage. The benefits of the precursor route are that
they have a good CTE match, low elastic modulus (high thermal shock
figure of merit), can be tailored by controlling the processing
atmosphere and filler material, can be sintered without additives, can
produce a homogenous distribution of elements, and can be applied as
liquids. They may also offer the potential for repair. The main
drawbacks are low solids yields, shrinkage, cracking, and porosity.
Polymers are typically filled with ceramic nanoparticles to overcome
some of the drawbacks.

**Rishi Raj, Univ of Co.,** proposed the use an oxide topcoat with a
graded SiCN/SiCN+oxide nanocomposite interlayer. The oxide topcoat will
provide stability with water vapor and have impermeable grain
boundaries. Topcoat candidates include transition metal oxides (ZrO~2~,
HfO~2~, Ta~2~O~5~, TiO~2~), base metal oxides (Al~2~O~3~, MgO, etc),
complex oxides (YAG, Perovskites, etc) or silicates (mullite, etc). The
simple oxides are process friendly and are more likely to be used.

The graded SiCN-nanoscale oxide dispersion interlayer will provide a CTE
gradient for low energy interfaces, and a diffusion and silica formation
barrier. Polymer derived ceramics are proposed because they are
compatible with oxides, have low energy interfaces with oxides and
Si~3~N~4~, and have ultra-slow diffusion at ultrahigh temperatures. For
example the incorporation of SiCN in Al~2~O~3~ produced nanocomposites
with retarded creep and grain growth, and low energy interfaces. The
reduced creep is due to the extremely low diffusion of the large mobile
molecules (1-2 nm). Further, ZrO~2~ nanocomposites exhibit extremely
small grains, high strength (2-3 GPa), and are thermally stable.

The SiCN bond coat on Si~3~N~4~ would provide a low energy covalently
bonded interface, and may inhibit the nucleation of silica. A 0.1-0.2 μm
thick CVD SiCN film on Si~3~N~4~ demonstrated low oxygen diffusion and a
good bond, preventing the nucleation of SiO~2~ at the interface. Basic
research is needed to confirm that these aspects can be achieved
together in an effective EBC.

**Rajendra Bordia, Univ of Wa.,** proposed the use of active filler
preceramic polymers proposed joint materials and as coatings for metals
and ceramics. Polyureasiloxane (Ceraset) identify as the preferred
polymer based on yield, low viscosity and low oxygen content. This
polymer can produce Si~3~N~4~, SiC, or SiC~x~N~y~O~z~ by controlling the
processing atmosphere. The Active Filler Controlled Pyrolysis (AFCOP)
approach is proposed to counteract the normal polymer shrinkage. TiB~2~
and TiSi~2~ fillers resulted in lowest shrinkage and porosity due to the
volume increase during carburization or nitridation. At T\<1400C, the
composite remains amorphous or is nanocrystalline. Coatings remain crack
free if under \~10-20 microns.

[Approach to selecting protective compositions]{.underline}

**Charles Lewinsohn, Ceramatec** proposed using geomimetic data as a
screening tool for selecting materials. Low silica activity polymer
derived Si-B-C-N is proposed for use in conjunction with low silica
activity EBC materials. The B addition to Si-C-N stabilizes the
amorphous state at high temperature, increases the crystallization
temperature to 1700C, increases the oxidation resistance, reduces C
activity, and shifts Si~3~N~4~ and SiC degradation reactions to higher
temperatures. Si-B-C-N has high creep & oxidation resistance and can be
filled with active fillers such as MoSi~2~. The oxidation resistance
should be enhanced by incorporating elements such as Yb, Ba, Sr, etc,
that form oxidation resistant products.

An alternative is a layered approach, combining the Si-B-C-N with a
geomimetic candidate. Geomimetics is based on the hydrothermal phase
stability studies in the geology literature. The approach identified
mullite as a poor candidate due to its lack of stability under
hydrothermal conditions, and the potential stability of mono-celsian
(BAS) if it contains excess alumina and strontium. EBC are suggested
that use the low silica activity geomimetic fillers with SiBCN polymers
to form multilayer and/or graded coatings.

**Roger Wills, UDRI** discussed the potential for EBCs from the
RE-Si-Al-O-C-N family. Alpha-SiAlONs are not preferred because they have
a higher thermal expansion than silicon nitride and are susceptible to
phase transformations. Likewise, the most common RE oxynitrides have
poor oxidation resistance and high thermal expansion. However,
RE-SiAlONs show excellent oxidation resistance, and Al~4~SiC~4~ oxidized
to form a potentially protective Al~2~O~3~. The RE-Si-Al-O-C-N family
provides flexibility due to extensive solid solution formation based on
several crystal structures, the potential for non-glassy grain boundary
phases, and the potential to form a protective alumina upon oxidation.
Data are not available for many compositions, and data is frequently
difficult to compare, so additional studies are recommended.
Standardized oxidation test conditions, evaluations of high purity
samples, and compositions with high Al-O content and a good thermal
expansion match are recommended.

[Application methods]{.underline}

A variety of applications approach are being used or proposed. Air
Plasma Spraying (APS) has been used the most to coat engine or rig
tested components. APS coatings which use very fine powders tend to
create more uniform and dense coatings (**Kathy Faber**). However, lower
cost, non-line of sight approaches are desired for small and complex
parts such as turbine rotors, blades, and vanes. Deposition of complex
composition coatings via CVD proved to be difficult. (**Rick Lowden**).

The development of in-situ systems would be the lowest cost and
component independent approach. Pack cementation (**Rick Lowden**) and
slurry approaches (**Beth Armstrong**) are very flexible approaches with
the potential for low cost. The viability of both approaches has been
demonstrated. **Subu** **Shanmugham, MicroCoating Technologies** ,
proposed the use of a Combustion Chemical Vapor Deposition (CCVD)
Process for thin film deposition. The approach uses a flame sprayed
liquid precursor solution to form 10 - \>60nm thickness coatings, is not
line of sight limited, and can be applied on a low temperature
substrate. It is well suited for single or multilayer coatings of
oxides, noble metals or composites. **Vimal Pujari, Norton** proposed a
novel concept of incorporating application of the coating/near surface
modification during the glass encapsulation-HIP process.

**Karren More, ORNL**, showed that EB-PVD columnar coatings were not
protective due the formation of sintering gaps, and actually reduced the
strength.

[Evaluation methods]{.underline}

The primary challenge to conclusive EBC testing is the difficulty in
achieving the needed combination of all critical variables: temperature,
pressure, velocity, and water vapor content. **Roger Wills** summarized
the limitations of current testing capabilities at **UDRI**, including
flexure testing, a pressurized tensile testing system and pressurized
tube test method. A proposed ring-on-ring biaxial flexure test that can
provide control of all of the critical variables as well as fast
fracture, dynamic fatigue, stress rupture, and strain measurements was
reviewed.

**Karren More, ORNL**, reviewed the capabilities and results of Keiser
rig exposures of EBC coated materials. While the Keiser rig cannot
reproduce the microturbine operating conditions, in has reproduced the
microstructural damage at an accelerated rate relative to engine
exposures. It is ideally suited for first stage screening of EBCs and
evaluating damage below the EBC. With the advent of the High velocity
Rig at **Honeywell**, accurate simulations of engine conditions will be
possible. It is planned for use for only approximately 200 hour
exposures. **GE** currently uses a high pressure/high velocity
combustion rig for long term simulated engine conditions to 1205C, 8.8
bar pressure, 130 m/s velocity, and 0.9 bar partial pressure of H~2~O.

**Matt Ferber, ORNL**, presented a new capability for exposing tensile
or flexural specimens to temperature, velocity, and water vapor while
under stress. The approach has simulated the recession and the
microstructural damage observed after engine and rig tests. The effect
of water vapor on creep of NT164 appears to be negligible.

**Bill Ellingson, ANL,** reviewed NDE techniques under development at
the Argonne National Laboratory using elastic optical backscatter, tuned
flash infrared imaging, and air coupled ultrasonic. These approaches
have detected erosion of EBC on Si~3~N~4~ engine tested vanes and
predicted spall locations on CMCs. Further objectives include providing
methods to characterize EBCs for size and location of delaminations,
size and depth of foreign object damage, and estimating the growth of
delaminated regions.

**Conclusions & Recommendations**

The development of environmental protection systems for ceramics in gas
turbine applications is relatively new and exceedingly complex. No clear
solution is evident, and many questions remain regarding the entire
scope of material selection, application, evaluation, and performance
issues. The lack of available data on experience, physical properties,
and chemical thermodynamic data results in many trial and error
approaches to the challenge. Initial efforts to understand and mitigate
the degradation have significantly improve the understanding of the
problem, have identified many of the critical issues and options, but
have not provided the insight into a significant down selection of
materials and processes. Protection systems evaluated to-date are life
extending, rather than prime-reliant solutions. Thus, the broad approach
of the current effort should continue. Additional studies are needed to
accelerate the protection development. Specifically, emphases should
include:

-   Continuing the broad approach including further maturation of the
    current most promising approaches, as well as the exploration of new
    ideas.

-   Establishing a material property database resource of critical
    properties, identify data that is needed, summarize results of prior
    evaluations, and provide references.

-   Identifying low cost test methods for experimentally screening
    material candidates for water vapor resistance.

-   Continue studies to identify materials and application methods with
    chemical and phase stability, bond strength, and retain or improve
    the substrate properties, and possibly provide impact resistance.

-   Identify the bond strength requirements, and continue methods
    development for assessing the coating adherence.

-   Continue modeling and testing methodology development to bonding
    integrity.

-   Emphasize low cost application processes suitable for complex gas
    turbine components.

-   Continue conducting long term and cyclic exposure stability in
    combustion atmospheres.

-   Continue the development of NDE methods for EBC coatings