Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 719-723
Published Online July 2012 (
The Microstructure and Oxidation Resistance of
Aluminide MeCrAlY-Modified Coatings Obtained by
Slurry Method on Rene 80 Superalloy
Marek Goral
Research and Development Laboratory for Aerospace Materials, Rzeszow University of Technology,
Rzeszow, Poland
Received April 16, 2012; revised May 31, 2012; accepted June 20, 2012
The slurry method is one of the oldest techniques of deposition of aluminide coating on the nickel superalloy, titanium
alloys and steel. It is characterized by relatively low costs of its realisation and necessary equipment. This method en-
ables a simple modification of chemical composition of the coating through addition of different powders. The author
showed study on the possibility of modification of the Al-Si slurry chemical composition used for aluminide coating
deposition by addition of MeCrAlY powder. The slurry was deposited by immersion than the diffusion treatment at
950˚C for two hours was applied. The thickness of obtained coatings was in the range of 30 - 65 μm.
Keywords: Slurry Method; Aluminide Coatings; Nickel Superalloy
1. Introduction
The diffusion aluminide coatings are used for protection
of the turbine blades made from nickel superalloys and
used as elements of aircraft and industrial gas turbines,
against oxidizing influence of exhaust gases [1]. The
most common technique used for aluminizing is the pack
cementation method. It involves putting the elements in
metal container and covering them with a powder which
contains an active powder (source of aluminium), neutral
filler material as well as halide activator. The process is
performed in protective atmosphere of argon, in the
chamber furnace [2]. The gas methods (out of pack, gas
phase aluminizing) are currently used in the aerospace
industry. The blades are located in those methods over
the powder or granules containing aluminum. The alu-
minum fluoride is most commonly used as an activating
agent. The aluminizing is conducted in retort furnace in
the temperature of 900˚C - 1050˚C and the argon and
hydrogen protective atmosphere for 2 - 6 hours [3,4]. The
most modern method of obtaining aluminide coatings is
the chemical vapour deposition (CVD method). The
halogenide Al activator (aluminium chloride) is created
in CVD process—as opposed to other methods—during
chemical reaction between HCl and aluminum granules,
in the external heated generator. The generated gas is
introduced to the retort, where the coated elements are
placed. The further development of diffusion aluminizing
methods is focused on the modification of aluminide
coatings by other elements—zirconium, hafnium, palla-
dium, platinum and silicon [5,6]. The slurry method has
been used, among other described methods, for a couple
of decades. It involves the deposition of organic or non-
organic slurry, which contains the aluminum powder, on
the blades surface, followed by the drying process. The
aluminide coatings are formed as a result of a diffusion
heat treatment process, in the atmosphere of argon, hy-
drogen or in vacuum. The slurry method is used for
preparation of coatings on the turbine blades. It enables
to create the aluminide coating on the surface of regener-
ated blade. It is possible to modify the chemical compo-
sition of the slurry by adding the powders with different
chemical composition mainly by silicon. Praxair com-
pany is one of the companies which manufactures this
kind of slurry (J. Sermaloy) [7]. The requirements con-
cerning the usage of chromium compounds, due to their
carcinogenic properties, led to development of new kinds
of slurries, based on organic binders. Product manufac-
tured by Ceral company—LSR SIALOY is an example
of this kind of slurry [8]. The slurry method enabled the
production of high temperature resistant coatings on dif-
ferent materials. This technique is used to obtain, except
the nickel superalloys, the diffusion coatings on the cast
steel or intermetallic γ-TiAl alloys [9,10]. The conducted
till now research on modifying the diffusion aluminide
coating obtained with slurry method, by addition of Me-
Copyright © 2012 SciRes. JMMCE
CrAlY to commercial slurry containing aluminum and
silicon powders [11]. The authors present the techniques
of obtaining the annealed coatings in a reductive hydro-
gen atmosphere (instead of argon used so far) involving
the modification of components ratio.
2. Experiments
The process of aluminide coating deposition was per-
formed on the Rene 80 nickel superalloy. Its chemical
composition is presented in Table 1. The cylinder-shaped
samples, with a diameter of 14 mm, were sandblasted
and washed. The MeCrAlY powder was obtained from
the typical AMDRY 995C powder used for plasma spray-
ing during the milling process. The chemical composi-
tion of powder is presented in Table 1. After the milling
process, the final grain size in the MeCrAlY powder was
approx. 20 μm. The samples were coated by immersion
in the slurry. Addition of the MeCrAlY powder into a
typical commercial slurry Ceral 10 were done in 1:2 ratio.
The drying process was conducted for 24 h in the tem-
perature of 80˚C. The diffusion heat treatment process
was conducted in the retort furnace for 2 hours into hy-
drogen atmosphere condition (flow 2 l/min) and in the
temperature of 950˚C. The last stage of coating deposi-
tion process was sandblasting and washing to remove the
slurry residue.
The microstructure analysis was conducted with a use
of the Hitachi S-3400 scanning electron microscope con-
nected with chemical composition microprobe analysis
(EDS) manufactured by Thermo company.
The cyclic oxidation test was performed in chamber
furnace in static laboratory air in the temperature 1100˚C.
The 23-hour heating cycles were used. The mass meas-
urements were conducted on the analytical balance, with
accuracy of the order 104 g. The moment of mass de-
crease below the initial mass is considered to be a resis-
tance criterion.
3. Results
The microstructure analysis of obtained coatings con-
firmed a presence of many cracks. The thickness of coat-
ing was influenced by addition of MeCrAlY powder. In
case of slurry obtained from Al-Si, the thickness was
approx. 65 μm. With an increase of MeCrAlY powder
content, there was a decrease of coating thickness. In the
case of adding the MeCrAlY powder into Al-Si slurry
with the 1:2 ratio, the coating thickness was approx. 34
3.1. The Microstructural Analysis
Three zones could be observed (Figure 1) in a structure
of the coating obtained from the commercial Al-Si slurry.
The thickness of the outer zone presented on Figure 1(a)
(Zone 1) was of approx. 5 μm. The chemical composition
analysis, confirmed presence of 44 at% of silicon and 34
at% of chromium in this point. Moreover, the titanium,
molybdenum and nickel in the amount of 4 - 5 at% were
also present in this area. In the lower Zone 2 with a
thickenss of approx. 55 μm many precipitates with the
high silicon content (Figure 1(a), Table 2 pt 2) were
observed. Those precipitates were located uniformly in
the whole area of Zone 2 (Figure 1(b), Table 2 pt 4). The
precipitate matrix (Figure 1(a), Table 2 pt 5) contained
Table 1. The nominal chemical composition of Rene 80 alloy [wt%].
Material Ni Co Cr W Mo Al Ti Zr C Y
Rene 80 Bal. 9.5 14 4 4 3 5 0.06 0.17 -
MeCrAlY AMDRY 995C powder Bal. 38.5 21 - - 8 - - - 0.5
Table 2. The results of chemical composition analysis in areas presented on Figure 1.
Element at%
Point Al Si Ti Cr Co Ni Mo W
1 6.04 44.13 4.79 34.13 - 1.43 5.32 4.16
2 40.89 13.47 1.26 3.86 5.08 34.60 0.50 0.33
3 42.30 10.32 1.10 3.29 5.39 36.33 0.55 0.73
4 28.96 11.13 1.17 28.53 3.06 23.24 1.89 2.01
5 47.22 0.25 0.72 2.76 4.84 43.48 0.43 0.30
6 20.82 - 11.10 27.76 6.64 26.72 3.27 3.70
7 11.31 - 10.83 6.69 6.27 62.30 1.24 1.36
Copyright © 2012 SciRes. JMMCE
M. GORAL 721
Zone 1
Zone 2
Zone 2
Zone 3
Figure 1. Microstructure of the coating obtained on the
Rene 80 superalloy from a slurry contained 100 wt% of
Al-Si powder. The coating was obtained during diffusion
heat treatment at 950˚C/2 h under hydrogen atmosphere
condition with marked points of chemical composition mi-
approx. 47 at% of aluminum, 43 at% of nickel, approx. 5
at% Co as well as 3 at% of Cr. Presence of Ti, Si, Mo
(with a concentration <1 at%) was also confirmed. In the
inner zone (marked as Zone 3 on Figure 1(b) pt 6, 7) the
existence of silicon was not observed. In the 6th point
(Figure 1(b), Table 2) the aluminum concentration was
approx. 20 at%. For chromium and nickel it was approx.
27 at%.
In the interface zone, between inner Zone 2 and base
material (Figure 1(b), Table 2 pt 7), the concentration of
aluminum was approx. 11 at%, and for nickel –62 at%.
The titanium content in this area was approx. 10 at%. For
chromium and cobalt it was approx. 6 at%.
The coating obtained form the slurry which contains
the addition of MeCrAlY powder were characterized by
a lack of outer zone rich in silicon and chromium. Its
smaller thickness was also observed. Two basic zones
were observed in theirs structure (Figure 2). The layers
with 33% (mass) content of MeCrAlY powder had pre-
cipitates in its outer zone (marked as “1” on Figures 2
and 3) and in the inner zone with the thickness of approx.
5 μm (marked as “2” on Figure 2). The chemical com-
position analysis conducted for precipitates in the outer
zone (marked as “1”) of a coating formed form the slurry
with an addition of 33% (mass) of NiCrAlY powder re-
vealed a presence of the silicon (approx. 19 at%) and
chromium (approx. 45 at%, Figure 2(a) pt 1, Table 3).
In the analyzed area, the aluminum content was approx.
19 at% and approx. 12 at% for nickel. The analysis of the
basis in the outer zone “1” of the coating (Figure 2(a) pt
2, Table 3) revealed a high aluminum content—approx.
52 at% and 37 at% for nickel. The chemical composition
Zone 1
Zone 2
Figure 2. Microstructure of the coating obtained on Rene 80
superalloy from slurry containe d 66 wt% Al-Si and 33 wt%
of NiCrAlY powder. The coating was created during diffu-
sion heat treatment at 950˚C/2 h under hydrogen atmos-
phere condition with marked points of chemical composi-
tion microanalysis.
Copyright © 2012 SciRes. JMMCE
analysis conducted in the area of the inner Zone 2 (Fig-
ure 2(b) pt 3, 4, Table 3) showed a aluminum concentra-
tion of 44 - 51 at%. The precipitate matrix in this area
was characterized by nickel content of approx. 40 at% (pt
4 on Figure 2(b), Table 3) as well as by the existence of
chromium and cobalt. In the inner zone (Figure 2(b) pt 5
and 6) the aluminum content was approx. 13.6 - 14.88
at%. Moreover, the chemical composition analysis by
EDS method, revealed a high chromium amount (34 - 45
at%) and a presence of titanium, molybdenum and tung-
3.2. Results of Oxidation Test
The results of mass change measurement during cyclic
oxidation is presented on Figure 3. Tests performed in
the temperature of 1100˚C revealed the increase of cor-
rosion resistance of the Rene 80 alloy with deposited
aluminide layers. The mass of the sample without the
protective coating dropped below the initial mass after
the 4th cycle. The samples with the protecting coating
increased above the initial mass after the 4th cycle. There
were no mass change observed for the samples covered
with commercial Ceral slurry as well as the for those
modified with the multicomponent MeCrAlY powder.
4. Discussion
In the article authors presented new method of chemical
composition modification of the aluminide coatings ob-
tained by slurry method. The conducted research showed
a possibility of introduction of the conventional Al-Si
Table 3. The results of chemical composition analysis in areas presented on Figure 2.
Element at%
Point Al Si Ti Cr Co Ni Nb Mo W
1 52.08 1.60 0.83 3.09 5.28 37.12 - - -
2 19.42 19.34 1.85 44.55 2.06 11.54 0.04 1.20 -
3 44.41 - 6.49 19.11 3.78 19.06 - 3.07 4.08
4 51.46 - 0.87 2.52 5.52 39.64 - - -
5 14.88 - 7.67 45.18 4.92 17.67 - 5.22 4.46
6 13.60 - 8.85 33.85 7.69 26.88 - 4.87 4.26
Figure 3. The results of mass change measurement during the cyclic oxidation process in the temperature of 1100˚C.
Copyright © 2012 SciRes. JMMCE
M. GORAL 723
slurry consisting of multicomponent MeCrAlY alloy.
However, one has met some difficulties resulting from a
high density of introduced MeCrAlY powder. One has
observed the sedimentation of heavier powder compo-
nents on the bottom of the vessel during the application
process. All obtained coatings were characterized by a
structure, which is typical for highly active process and
was created during the inward diffusion of aluminium.
After the diffusion heat treatment process one has also
observed a presence of many fractures in the coating re-
sulting from internal stresses. It could be caused by a
short time of diffusion annealing process (2 h), fast cool-
ing and using of hydrogen atmosphere, instead of argon
atmosphere used till now. It caused in the coatings with
the addition of NiCoCrAlY powder fractures and exfo-
liation of the outer layer with high content of silicon and
chromium. In the structure of all obtained aluminide
coatings, a few characteristic zones can be observed. In
the layer obtained form a commercial slurry containing
only aluminum and silicon powders, the outer zone was
rich in silicon and chromium, which could be a proof of
silicide formation. In all layers, one observed the zone
with fine porous precipitates with the diameter less than
1 μm. They contained silicon on the basis of the β-NiAl
intermetallic phase, with 50 at% of aluminum. In all lay-
ers, the inner zone with a thickness of approx. 5 μm was
observed. The result of cyclic oxidation tests showed an
increase of oxidation resistance of samples with alu-
minide layers. The research did not proved a significant
difference in oxidation resistance between layer made
from commercial Al-Si slurry and layer modified with
MeCrAlY powder. It was caused by the observed sedi-
mentation process of the powder. One observed no in-
fluence of the additional outer layer with high silicon and
chromium content, on the increase of oxidation resis-
tance of the layer created from the commercial Al-Si
slurry. The presented results indicate the significant dif-
ficulties in obtaining the aluminide coatings modified by
powders, which are made of metals being components of
MeCrAlY alloy and have higher density. It is necessary
to prepare a new binder, which ensures the possibility of
homogenization of the slurry and prevents the sedimen-
tation process. Searching for new methods of obtaining
thermal barrier coatings is a significant reason for con-
tinuing the research. The European Project “Particoat” is
a good example of it.
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