Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 853-857
Published Online August 2012 (
Kinetics Growth and Oxidation Resistance of Aluminide
Coatings Deposited by the CVD Method on
Re 80 Superalloy
Marek Goral, Maciej Pytel, Kamil Dychton, Andrzej Nowotnik
Research and Development Laboratory for Aerospace Materials, Rzeszow University of Technology, Rzeszow, Poland
Received July 2, 2012; revised August 4, 2012; accepted August 19, 2012
The preliminary results of research on forming the aluminide coatings using CVD method were presented in the article.
The coatings were obtained in low activity process on the surface of Rene 80 superalloy. The microstructure analysis
and chemical composition analysis were performed applying different values of aluminizing process parameters. The
authors present in the article the results of oxidation resistance analysis of aluminide coatings which were obtained on
the surface of Rene 80 superalloy using various techniques. It was shown that the coating created during the CVD
process was characterized by a good oxidation resistance at the temperature of 1100˚C.
Keywords: CVD Aluminizing; Aluminide Coatings; Nickel Superalloy; Re 80
1. Introduction
Diffusion aluminizing is one of the basic techniques of
protecting the turbine blades surfaces against the oxidiz-
ing influence of fumes environment. The pack cementa-
tion-, out-of-pack-, slurry- and CVD methods of alumin-
izing is one of most commonly used [1,2]. Regardless of
the aluminizing technique the mechanism of the coating
formation consist in the chemical reaction between the
halide containing aluminium and the base material. The
β-NiAl phase is created as a result of aluminium diffu-
sion. Goward and Boone [3] used pack cementation me-
thod and showed two possible courses of growth mecha-
nism for aluminide coatings. Using the pack containing
15% of Al, 3% of amonium chloride and 82% of granular
Al2O3 it has been observed that the Ni2Al3 phase is cre-
ated in the first instance on the surface on the nickel su-
peralloy. During this high activity process, the inward
aluminium diffusion is the dominant growth mechanism.
By using the powder with 15% of Ni2Al3, 3% of NH4Cl
and 82% of Al2O3 it has been shown, that β-NiAl is the
basic phase constituent. During this low activity process,
the outward aluminium diffusion is the dominant growth
mechanism. During the high activity process it is neces-
sary to perform the additional thermal treatment in order
to obtain the transformation of the Ni2Al3 phase into the
β-NiAl phase. The out of pack and vapour phase alumin-
izing techniques are considered as the extension of pack
cementation method. The Al powder and Cr-Al granules
are the source of aluminium placed at at least 100 mm
away from the coated surface. The process is conducted
in the retort furnaces in the atmosphere of argon or hy-
drogen. The aluminium fluoride or ammonium fluoride is
used as the activating agent [3,4]. It has been proven that
there is a possibility of conducting the aluminizing proc-
ess of blades cooling channels with a use of out-of-pack
There is a limitation of parameter control during the
aluminizing process using out-of-pack method. It is a
result of application of the activating agent introduced in
the solid state before the process. The chemical vapour
deposition method provides the largest ability of control-
ling the parameters values. Sun et al. [5] conducted the
aluminizing process with a CVD method applying the
flow of AlCl3 and hydrogen which are created in the ex-
ternal generator over the powder made of aluminium and
aluminum oxide. Howmet company develops the alu-
minizing process by chemical vapour deposition [6]. The
technological process is conducted as a high or low ac-
tivity process, like in the case of pack cementation. Dur-
ing the low activity process, the aluminium chloride is
created in the external generator by transferring hydrogen
chloride through pure aluminium granules. The obtained
aluminium chloride was introduced to the retort inside
the pit furnace. The Cr-Al were inside the baskets placed
around the central pipe which distributed the reactive gas
from the external aluminium generator. They were the
Copyright © 2012 SciRes. JMMCE
additional source of aluminium during the high activity
process [7]. It resulted from the influence of hydrogen
which caused the sulfur removal from the base material.
Kohlscheen and Storck [8] proposed different idea of
the aluminizing process using CVD method. The authors
used the internal generator, in which the Al-Cl granules
were the source of aluminium. AlCl3 was obtained by
transferring HCl through those granules. The model-
based calculations proved, that the increase of hydrogen
chloride amount didn’t ensure the thickness growth of
the aluminium coating. It was explained by the fact that
surface of the granules which reacted with hydrogen
chloride was to small to create AlCl3. The experimental
verification confirmed that the increase of hydrogen par-
ticipation ensured the thickness of the aluminide coating.
The literature analysis showed, that during the low ac-
tivity aluminizing process, it is necessary to decrease the
amount of used hydrogen chloride. The recent work fo-
cuses on the assessment of influence of the basic alu-
minizing parameters using CVD method on the micro-
structure of aluminide coating deposited on the high
temperature creep resisting nickel alloys. Application of
new BPXPro 325S device, which is available in the Re-
search and Development Laboratory for Aerospace Ma-
terials at the Rzeszów University of Technology required
the correlation of available literature data with the device
2. Experimental
The authors analysed the high temperature creep resisting
Rene 80 nickel superalloy. It’s chemical composition
was presented in the Table 1. Samples were made form
bar; they had thickness of 4 mm and diameter of 14 mm.
The samples were polished with water-resistant abrasive
paper with gradation of 500, degreased in isopropyl al-
cohol with a use of ultrasonic cleaner and dried before
the process.
The aluminizing process using CVD method was
conducted in the Research and Development Laboratory
for Aerospace Materials at Rzeszow University of Tech-
nology. The Bernex BPX Pro 325S device was used. It is
a conventional hot-wall CVD device. The aluminizing
process was performed during the HTLA process (high
temperature low activity). The aluminizing processes were
conducted for different parameters values to analyse the
growth kinetics of aluminide coating (Table 2). Parame-
ters used till now were used as a base values: HCl flow
of 1.4 NLPM, hydrogen flow of 10.5 NLPM, tempera-
ture of 1000˚C and pressure of 150 mbar. For the pur-
poses of further tests the pressure value from 100 mbar to
359 mbar and the time from 2 h to 8 h were chosen. The
HCl flow of 1.2 - 2.0 NLPM and the hydrogen flow of
10.5 NLPM was applied. The parameters concerning
particular processes are presented in Table 2. After fin-
ishing the analysis of the sample, the microstructure was
investigated with a use of S-3400 Scanning Electron Mi-
croscope (Hitachi) equipped with electron probe micro-
analysis (Thermo).
The oxidation test was conducted in static laboratory
air at the temperature of 1100˚C. 23 hours cycles of ex-
posure to high temperature were conducted. The mass
measurements of samples were taken after each cycle.
Table 1. The nominal chemical composition of Rene 80 alloy (wt%).
Material Ni Co Cr W Mo Al Ti Zr C
Rene 80 Bal.9.5 14 4 4 3 5 0.06 0.17
Table 2. The list of parameters used for aluminizing process implemented by CVD method.
Run No. Temp (˚C) P (mbar) HCl flow (l/min) H2 flow Time (h)
1 1000 150 1.2 10.5 4
2 1000 150 1.4 10.5 4
3 1000 150 2.0 10.5 4
4 1000 100 1.4 10.5 4
5 1000 350 1.4 10.5 4
6 1000 150 1.4 10.5 2
7 1000 150 1.4 10.5 8
Copyright © 2012 SciRes. JMMCE
3. Results
3.1. Microstructure of Coating
The conducted aluminizing processes implemented with
CVD method resulted in total coverage of the surface of
all samples made of Re 80 alloy (Figure 1 by the diffu-
sion coating. For the base parameters of aluminizing
process (1.4 NLPM HCl, 10.5 NLPM H2, 150 mbar, 4 h,
1000˚C) the aluminium content on the alloy surface was
of 44.5 at %.
Two characteristic zones could be observed in the
coating microstructure for the base parameters. Their
structure was typical for coatings obtained in low activity
process. The average aluminium content in the outer
zone (area 1 on Figure 2, Table 3) was approximation
39 at %. In the area near surface, the aluminium content
was higher and was at the level of 45 at % (point 3 on
Figure 2, Table 3). In the lower part of the outer zone
(point 4 on Figure 2, Table 3) the aluminium content
was of 35 at %. Chromium and cobalt were present, ex-
cept the aluminium and nickel, in the outer area of the
coating. In the outer diffusion zone the average alumin-
ium content was of 23 at %. The separations in compo-
nents of Re 80 alloy—Cr, Co, W, Ti were present (area 2
on Figure 2, Table 3). The white-coloured separations
contained 10 - 11 at % of Cr, Co and W for aluminium
content of 23 at %. The aluminium content was higher in
the base of diffusion zone and was at the level of 27 at %
(point 6 on Figure 2, Table 3).
3.2. Kinetic Growth of Coating
The performed microstructure analysis of aluminide
coatings created during CVD process conducted with
different parameters values didn’t prove that there is a
significant difference in their chemical composition. It
proved however that there is a difference as far as coating
thickness is concerned. The influence of pressure value
Figure 1. The surface morphology of aluminide coatings
deposited on Rene 80 superalloy by the CVD method.
Figure 2. The microstruture of aluminide coatings deposited
on Rene 80 superalloy by low-activity aluminizing. Process
parameters: 1.4 NLPM HCl, 10.5 NLPM H2, 150 mbar, 4 h,
Table 3. The results of EDS microanalysis of areas pre-
sented on Figure 2.
Chemical composition (at %)
point Al Ti Cr Fe Co Ni W
1 39.290.63 2.84 1.09 6.67 49.48-
2 23.091.8410.64 - 10.18 43.4210.83
3 45.14- 0.81 1.15 4.21 48.510.18
4 35.181.42 4.16 0.87 7.02 50.590.76
5 23.101.4511.76 - 11.93 40.9510.82
6 27.711.92 8.51 0.86 9.52 46.275.21
in the retort, HCl flow and time on the thickness of the
coatings obtained during different processes is presented
on Figures 3-5. The thickness of coatings was in the range
of 16 - 18 μm. It has been showed, that decrease of pres-
sure in the retort during aluminizing process causes a small
increase of thickness of the aluminide coating by ap-
proximation 1 μm (Figure 3). It applied to the whole
coating as well as to particular zones (external and the
diffusion one). The authors noticed also a slight influence
of hydrogen chloride flow on the coating thickness. A
small decrease of the coating thickness (by approximation
1 μm) was observed in case of HCl flow of 1.4 NLPM
(Figure 4). The duration of aluminizing process had the
largest influence on the thickness of obtained coatings
(Figure 5). During the two-hour process, the thickness of
obtained coating was of 9 μm. Increase of aluminizing
time to 8 hours caused growth of thickness to 19 μm.
Copyright © 2012 SciRes. JMMCE
3.3. Oxidation Resistance
The cyclic oxidation test was conducted at the tempera-
ture of 1100˚C. The test results are presented on Figure 6.
Resistance to oxidation of the alloy without protective
coating and the previously investigated aluminide coat-
ing obtained using slurry method were investigated dur-
ing the research. The decrease of sample mass below the
initial value was a criterion of a process termination. The
obtained results showed a high oxidation resistance of
obtained coating during the CVD method with standard
parameters values. The trial was terminated after 49 cy-
cles (1127 hours) of exposure at high temperature. The
coatings fromed using slurry method, in spite of high
aluminium content, exceed the value of initial mass after
230 hours of oxidation and for Rene 80 alloy—after 4
cycles (92 hours).
Figure 3. The influence of pressure in retort on thickness of
aluminide coating obtained on Re 80 superalloy by CVD
Figure 4. The influence of HCl flow on thickness of alu-
minide coating obtained on Re 80 superalloy by CVD
Figure 5. The influence of aluminizing time on thickness of
aluminide coating obtained on Re 80 superalloy by CVD
0510 15 20 25 30 35 40 45 50
Figure 6. The results of cyclic oxidation test at 1100˚C of aluminide coatings deposited on Rene 80 nickel superalloy.
Copyright © 2012 SciRes. JMMCE
4. Summary
The CVD method is one of most modern techniques of
aluminizing of turbine blades in aircraft engines. The
conducted technological trials made with BPX Pro 325S
device showed a possibility of creating the aluminide
coatings with a structure typical for low activity process.
The coating consisted of the outer layer (β-NiAl phase)
containing approximation 45 at % of aluminium and the
transition diffusion zone. The coating structure indicates
the growth resulting from outward nickel diffusion. The
analysis of coating growth kinetics didn’t show the sig-
nificant influence of pressure in the retort and the HCl
flow on the thickness of formed coating. It is necessary
to change a value of hydrogen chloride flow in order to
increase the coating thickness.
The conducted research proved that the aluminizing
using CVD method is an efficient way of surface protec-
tion against oxidation for Rene 80 alloy. It was con-
firmed by the result of cyclic oxidation test. High heat
resistance of the coating obtained with CVD method can
be a result of application of the hydrogen atmosphere. The
method provides high quality of obtained coating [7].
5. Acknowledgements
The research was supported by National Centre for Re-
search and Development under grant No. PR NR15-
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