Volume 6 Preprint 63
Corrosion Resistant CVD Mullite Coatings
S.N. Basu and V.K. Sarin
Keywords: mullite, environmental barrier coatings, chemical vapor deposition, oxidation, hot-corrosion, coal-slag corrosion
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Volume 6 Paper H003
Corrosion Resistant CVD Mullite Coatings
S.N. Basu and V.K. Sarin
Department of Manufacturing Engineering, Boston University, Brookline,
MA 02446, USA, email@example.com
Dense, crystalline mullite (3Al2O3.2SiO2) coatings of uniform thickness
have been deposited by chemical vapor deposition (CVD) on SiC
substrates, using the AlCl3-SiCl4-CO2-H2 system. A graded coating
composition has been achieved in the coatings, with the Al/Si ratio being
close to stoichiometric (~ 3) at the coating/substrate interface for CTE
match, and increasing monotonically towards the outer coating surface
for superior hot-corrosion resistance. These mullite coatings have
exhibited excellent high temperature oxidation, hot-corrosion and coalslag corrosion resistance and have proven to be very effective in
protecting the SiC substrates from corrosive atmospheres.
Keywords: mullite, environmental barrier coatings, chemical vapor
deposition, oxidation, hot-corrosion, coal-slag corrosion.
Over the next decade, the increase in demand for electricity in the United
States is projected to be around 80 GW . A major portion of this
increased demand will be met by advanced gas turbine systems. This
next generation of advanced gas turbine systems will have to operate
with improved fuel efficiency and reduced emissions, mandating the use
of higher operating temperatures. Currently, superalloys with thermal
barrier coatings have been driven close to their limit in high temperature
applications. For a significant increase in operating temperatures, a new
class of materials will have to be introduced. Silicon-based ceramics such
as SiC and Si3N4 are leading candidate materials for the next generation
of high temperature materials.
Although these Si-based ceramics have excellent oxidation resistance
due to the formation of a protective SiO2 layer at the surface during high
temperature exposures, they have two major limitations. In complex
combustion environments, the presence of elements such as Na, Va, and
S, lead to the formation of corrosive oxides such as Na2O, V2O5, SO2 and
SO3. These oxides react with the protective silica scales formed on the Sibased ceramics forming non-protective low melting temperature
silicates, leading to severe pit formation, material loss and increased
porosity . Also, in the presence of high-pressure water vapor, the
protective silica scale volatilizes to gaseous Si-O-H species, exposing the
ceramic surface . This leads to an accelerated oxidation of the ceramic
surface to SiO2, which in turn volatilizes. This repeated cycling of
oxidation and volatilization leads to a rapid recession of the surface of
the Si-based ceramic. To avoid the problems of hot corrosion and
recession, refractory environmental barrier coatings (EBC) on these Sibased ceramics need to be developed.
Environmental barrier coatings have several requirements . The
surface of the coating needs to be environmentally durable to the
aggressive atmospheres it is exposed to. The coating has to act as an
effective diffusion barrier, and must be mechanically tough to be free of
cracks in order to prevent exposure of the substrate to the environment.
The coating must be stable to avoid deleterious phase changes during
long-term high temperature exposures. The coatings should have a close
coefficient of thermal expansion (CTE) match with the substrate to
minimize thermal stresses during temperature cycling. And finally, the
coating should have a good chemical compatibility with the substrate for
adherent bonding at the interface.
Several coating systems have been explored for protection of SiC against
hot-corrosion and recession. Although alumina and zirconia have
excellent hot-corrosion and recession resistance, they lack toughness
and have a substantial CTE mismatch with the Si-based ceramics (SiC:
4.3-5.6 X106/oC, Al2O3: 7.2-9.1X106/oC, ZrO2: 7.0-10.1X106/oC).
Consequently, yttria stabilized zirconia coatings deposited by electron
beam assisted physical vapor deposition (EB-PVD) and layered multiphase
AlN/Al2O3+ZrO2, grown by CVD on SiC substrates were found to fail at
1200°C by blistering, cracking and spalling under thermal cycling and
contact stresses .
Mullite (3Al2O3•2SiO2) is a promising coating material for silicon-based
ceramics due to its excellent corrosion resistance, creep resistance and
high temperature strength. Mullite has a much better CTE (3Al2O3.2SiO2:
5.7X106/oC) match with the SiC. Plasma sprayed mullite and mullite-
based EBCs have been deposited on SiC substrates for hot-corrosion
protection . In this study, mullite EBCs have been deposited by the CVD
technique, which lends itself readily for control of coating microstructure,
uniformity and thickness, even on complex parts with edges, corners and
curvatures. In addition, the CVD process leads to coatings that have
substantially lower residual stresses due to the lack of rapid quenching
inherent to the plasma spray process, thereby avoiding porosity and
Growth of Mullite Coatings
Mullite coatings were grown on SiC substrates using the AlCl3-SiCl4-CO2-
H2 system in a hot-wall CVD reactor , with the overall reaction:
6AlCl3 + 2SiCl4 + 13CO2+ 13H2 Æ 3Al2O3•2SiO2+ 13CO + 26HCl
Figure 1. SEM micrographs of CVD mullite coatings on a) planar SiC and
b) SiC fibers.
Detailed thermodynamic analysis of the AlCl3-SiCl4-CO2-H2 system was
carried out to identify the parameters to be used for CVD mullite growth
. These results were used to identify the range of process parameters
within which coating optimization was experimentally carried out. The
optimal growth temperature was found to be 950°C with a deposition rate
of ~ 5 microns/hr. Figure 1a shows a fracture cross section of a typical
adherent and uniform mullite coating on SiC. As mentioned, the CVD
process has the ability to deposit uniform coatings on parts with complex
shapes with large curvatures. Figure 1b shows a 20 micron diameter SiC
fiber uniformly coated with CVD mullite .
It has been found that CVD mullite coatings can be grown with a range of
input AlCl3/SiCl4 ratios. The change in the gas-phase composition in the
CVD reactor leads to a change in the surface composition of the growing
coating. With an appropriate choice of gas-phase AlCl3/SiCl4 input ratio,
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Distance from interface (µm)
the coating composition can be graded , as shown in Figure 2 (the
composition is expressed as the Al/Si ratio, with the stoichiometric Al/Si
ratio in mullite being 3).
Figure 2. a) Composition (expressed as the Al/Si ratio) variation in a
functionally graded CVD mullite coating. b) Cross-sectional TEM
micrograph of crystalline mullite grains.
There are several interesting features in Figure 2, which need to be
elaborated upon. Firstly, when the composition is graded, mullite grains
do not nucleate during deposition unless the surface composition of the
growing coating is within a narrow range of 3.2±0.3 , which is
around the value of stoichiometric mullite. Interestingly, the same holds
true if the composition is graded to be alumina-rich, as was the case for
growth on Al2O3 substrates, in which the Al/Si ratio close to the interface
was high (>5). In either case, when the Al/Si ratio was outside the critical
range (3.2±0.3), mullite did not nucleate directly. Instead, co-deposition
of nano-sized (~5 nm) γ-Al2O3 crystallites (SAD pattern shown in Figure 3)
in a vitreous silica matrix occurs . Figure 3a shows the TEM
micrograph of the region of transition from the ‘nanocrystalline’ to
crystallne layer on a SiC substrate. However, once nucleated, the mullite
grains can be made to grow over a wide range of non-stoichiometric
compositions. Figure 2b shows a TEM micrograph of mullite grains
whose surface composition is highly alumina-rich (Al/Si ratio of ~ 8).
Figure 3. a) transition region between the nanocrystalline to the
crystalline region in an as-deposited CVD mullite coating. The inset
diffraction pattern of the crystalline layer is consistent with  mullite,
while that from the nanocrystalline layer is consistent with
nanocrystalline γ-alumina. b) Complete transformation of the
nanocrystalline layer to equiaxed mullite grains after a 100 h anneal at
1300°C. The SAD pattern from one of the grains is consistent with 
Phase Transformations in Mullite Coatings
The nanocrystalline region gets converted to mullite on annealing at
temperatures of 1100oC and above. Figure 3b shows such a
transformation to equiaxed mullite grains after a 100 h anneal at 1300°C.
A accompanying SAD diffraction pattern from one of the equiaxed grains
is consistent with mullite. It is important to note that this transformation
occurs without any microcracking, or porosity formation .
The adhesion of CVD mullite coatings was evaluated by cycling them
between 1250°C and room temperature. The test consisted of rapid
insertion and removal of the samples from the hot zone of the furnace
after holding the sample for 1 hour at temperature. After 500 cycles, the
coating exhibited no signs of cracking and/or spallation. The excellent
adhesion of the coating can be partially attributed to two reasons. The
first is the formation of equiaxed mullite grains in the nanocrystalline
layer surface leading to a close CTE match at the coating/substrate
interface. The second is the gradation of the CTE across the thickness of
the mullite coating, which avoids any abrupt changes of CTE across the
coating thickness, while allowing the coating surface to be highly
alumina-rich. Furthermore, as can be seen in the cross-sectional TEM
micrograph in Figure 4a, the high-alumina coating surface (Al/Si ~ 8)
showed no signs of phase separation after a total of 500 h at 1250°C.
However, if the coating was further annealed for 100 h at 1400°C, phase
transformations in the high-Al regions did occur. Figure 4b shows the
formation of 100∼300 nm sized α- Al2O3 precipitates in the highly Al-rich
regions of the coating. It is significant to note that this precipitation
phenomenon was not accompanied by the formation of any microcracks.
The precipitation of the corundum phase in Al-rich mullite is consistent
with reports by other researchers , suggesting that annealing Al-rich
CVD mullite may be a viable method of producing uniform finely
dispersed Al2O3/mullite nano-composite coatings.
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Figure 4. TEM micrographs showing a) no phase separation in the Al-rich
portion of the CVD mullite coating after 500 at 1250°C, while b)
nanosized α-alumina precipitates form after 100 h at 1400°C.
Oxidation and Corrosion Protection
The mullite coatings were excellent oxidation barriers for SiC substrates.
Figure 5 shows a plot of weight gain for mullite-coated and uncoated
Nicalon SiC fibers oxidized in flowing oxygen at 1300°C . The uncoated
fibers were found to gain weight due to the formation of SiO2 at the
surface, part of which spalled when the sample was cooled. In contrast,
the mullite coated SiC fibers showed almost no weight gain, and
exhibited no signs of cracking or spallation after oxidation.
% Wt 8
Figure 5. Weight gain versus time plots for uncoated and CVD mullite
coated SiC fibers oxidized at 1300°C for 100 h.
Mullite coated and uncoated SiC substrates have also been subjected to
hot corrosion tests by loading the surface with about 5 mg/cm2 of
Na2SO4 and subjecting the samples to flowing oxygen (200 sccm) at
1100°C for 300 hours . The uncoated SiC substrate showed
substantial weight gain after an initial weight loss. Weight loss occurs due
to the formation of gaseous products while weight gain can be attributed
to the formation of silica due to oxidation of SiC. The silica scale formed
by oxidation of SiC reacts with Na2SO4 to form a liquid phase, through
which transport is rapid enough to expose the surface of the non-reacted
SiC to further oxidation . The depth of hot-corrosion attack was found
to be in excess of 20 µm for the uncoated SiC substrate as shown in
Figure 6a. In direct contrast, the mullite coated SiC sample exhibited no
weight gain. Examination of the sample after oxidation showed no
formation of silica. The Al rich surface of mullite allowed the coating to
remain unreacted in the presence of molten Na2SO4, as shown in Figure
6b, indicating that the CVD mullite coating acted as a very effective hot
Figure 6. Cross sections of a) uncoated and b) CVD mullite coated SiC
after 300 h exposure to molten Na2SO4 at 1100°C.
The effectiveness of CVD mullite coatings against corrosion attack by an
acidic Fe-based coal slag was investigated . After a 300h exposure at
1260°C, the uncoated SiC substrates suffered severe material loss and
pitting due to coal slag corrosion, as shown in Figure 7a. Uniform CVD
mullite coatings were found to be very effective in protection against the
coal slag. The coating did not degrade in the presence of the liquid slag
and did not allow liquid slag seepage to the SiC substrate. As shown in
Figure 7b, the CVD mullite coating protected the substrate from pitting.
Although some diffusion of Fe to the coating/SiC interface was seen, no
cracking or spallation was observed in the uniform mullite coatings.
Dense, adherent and uniform CVD mullite coatings were deposited on SiC
substrates. These coatings had a graded composition, being close to
stoichiometric mullite at the interface, and Al-rich at the surface. The
coatings exhibited excellent high temperature oxidation and corrosion
resistance and were highly successful in protecting the SiC substrates
from aggressive environments.
Figure 7. Coal slag corrosion of a) uncoated and b) CVD mullite coated
SiC after 100 h at 1260°C.
The authors would like to acknowledge Dr. Michael Auger, Dr. Ping Hou
and Dr. Arun Pattanaik for their contributions to this research. The
electron microscopy studies were carried out at the Center for Electron
Microscopy at MIT. This research has been partially supported by the
National Science Foundation under contract No. CMS 01122539.
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