Volume 6 Preprint 78
The Cyclic Oxidation Behaviour of Several Aluminide and Platinum Aluminide Diffusion Coatings at 1150Ã?Â°C
A. Littner and M. SchÃƒÆ’Ã‚Â¼tze
Keywords: Cyclic oxidation, Aluminide coatings, Platinum aluminide coatings
Because you are not logged-in to the journal, it is now our policy to display a 'text-only' version of the preprint. This version is obtained by extracting the text from the PDF or HTML file, and it is not guaranteed that the text will be a true image of the text of the paper. The text-only version is intended to act as a reference for search engines when they index the site, and it is not designed to be read by humans!
If you wish to view the human-readable version of the preprint, then please Register (if you have not already done so) and Login. Registration is completely free.
Volume 6 Paper H023
The Cyclic Oxidation Behaviour of Several
Aluminide and Platinum Aluminide Diffusion
Coatings at 1150Â°C
A. Littner, M. SchÃ¼tze
Karl-Winnacker-Institut der DECHEMA e.V., Theodor-Heuss-Allee 25,
60486 Frankfurt am Main, Germany, email@example.com
Hot-section components of aero gas turbines are made of nickel
superalloys and are essentially protected by aluminide and platinum
aluminide coatings. In this paper, we report about the cyclic oxidation
behaviour in air at 1150Â°C of several aluminide and platinum aluminide
coatings. Gravimetric investigations of the specimens were used in
order to characterize the cyclic oxidation resistance of each specimen.
Aluminide coatings exhibited extensive spallation leading to early
mass losses and preventing the formation of a protective oxide scale.
X-ray diffraction of spalls as well as examination of specimens at
several stages of the tests showed formation of mixed oxide layers
such as Î±-Al2O3/NiAl2O4/TaO2 mixtures. SEM investigations revealed
the formation of extensive quantities of voids. Gravimetric
measurements on platinum aluminide coatings confirmed the
beneficial effect of platinum promoting selective oxidation of
aluminium resulting in the formation of a pure alumina scale with
better scale adherence. Spallation was delayed up to about 1250
cycles. Then, due to the wrinkling of the coating surface and to the
formation of voids, spallation led to the rapid degradation of the
Keywords: Cyclic oxidation, Aluminide coatings, Platinum aluminide
The use of nickel-base superalloys as turbine blade materials requires
their protection against environmental attack. It has been shown that
diffusion coatings can significantly improve the high-temperature hot
corrosion and oxidation resistance of gas turbine components. Of all
the diffusion coatings known from the literature, aluminide coatings
are the most widely used. During oxidation they may form a protective
Î±-Al2O3 scale acting as a barrier against oxidising species present in
the atmosphere. Unfortunately, under service conditions, the oxide
scale can rapidly spall as a consequence of stresses due to thermal
cycling and oxide growth. Then, the continuous re-oxidation of the
coating surface associated with interdiffusion between coating and
substrate progressively leads to aluminium depletion of the coating.
Finally, the Al-concentration near the oxide-metal interface is too low
to ensure the selective formation of alumina and non-protective mixed
Platinum additions are well known to improve the high temperature
cyclic oxidation behaviour of the coatings. However, mechanisms by
which platinum improves the oxidation resistance of the diffusion
coatings are still not well understood. Many studies attributed
beneficial effects to platinum promoting the selective oxidation of Al
, increasing the adherence of the Î±-Al2O3 scale , delaying the
diffusion of deleterious alloying elements towards the coating surface
 or reducing the amount of voids at the scale/metal interface .
The present work reports about the cyclic oxidation behaviour at
1150Â°C of aluminide and platinum aluminide coatings deposited on
two different substrates by several industrial partners under the frame
of the ORDICO European Growth contract . The purpose was to
investigate the specimen weight change as well as the nature and
morphology of the oxide layers formed on the surface of the coatings
as a function of time. Optical investigations, SEM, EPMA as well as XRD
were used to characterise the cyclic oxidation behaviour of the
2. Experimental Procedures
The chemical composition of the alloys used as substrate materials is
given in table1.
Tab. 1: Chemical composition of the alloys (wt.%) measured by EPMA (10 Âµm
Button samples (18 mm diameter and 5 mm thickness) were coated
with different qualities of aluminide and platinum aluminide coatings.
Table 2 shows the type of the coatings that have been tested. Two
different qualities of aluminide coatings and two platinum aluminide
coatings were tested. The production of the latter coatings involved a
standard process with platinum electrolytic deposition, aluminisation
and heat treatment at high temperature.
NiAl (PWA 73)
(Ni,Pt)Al + PtAl2
(Ni,Pt)Al + PtAl2
Tab. 2: Type and substrate of the coatings tested.
b. Cyclic Oxidation Test Procedure
The samples were placed on a motionless mullite sample-holder
whereas the movement of the tubular furnace occurred by horizontal
linear motion. The hot furnace was moved over the sample within 2
min. A thermocouple of type S was used to periodically calibrate the
working zone temperature of the furnace during the tests. Precision of
temperature was maintained within 1150 Â±8Â°C. In order to be sure to
measure the total mass change of the samples, including spalls, each
sample was introduced into individual Al2O3 crucibles. Mass change of
the crucible containing the specimen was recorded separately from
that of the specimen. Measurements were repeated three times in
order to allow accurate thermogravimetric curves.
Cyclic oxidation tests were performed at 1150Â°C in laboratory air. Each
cycle consisted of 1h at 1150Â°C, 44min at ambient temperature and
16min reheating (see fig. 1). These parameters were determined in
accordance with the COTEST programme .
The sample temperature reached after cooling was about 80Â°C. The
temperature profile was measured by a thermocouple in close contact
with the tested specimens.
Fig. 1: Temperature-time profile for the cyclic oxidation tests at 1150Â°C
c. Microscopy, Microanalysis and X-Ray Diffraction
After a certain number of cycles the tests were interrupted in order to
investigate the oxide scale morphologies and compositions of the
coatings. Optical, SEM and EPMA investigations have been carried out
for imaging of microstructural features including quantitative phase
Spalls were also periodically taken from the alumina crucibles and
investigated by X-ray diffraction. Diffractograms were taken with and
without silicon reference powder. The U-fit software  has been used
to fit the lattice parameters of the different structures. In order to
determine the volume fractions of the individual crystalline phases as a
function of cycle number, a method close to that proposed by
Chmielova has been used . If A is the oxidation product with the
highest X-ray peak, then the following equations allowed the
quantitative determination of the fraction volumes of the n different
= i , for i = 1 to n-1
x A + x1 + ... + x n âˆ’1 = 1 ,
where xA, x1, ..., xn-1 are the volume fractions of the n oxidation
products and IA, I2, ..., In-1 the highest peak intensities for each phase
determined from XRD patterns.
3. Results and Discussion
a. Microstructure in the As-Received Conditions
Cross sections of the as-coated specimens are shown in Fig. 2.
Backscattered electrons micrographs showed that Al 1 and Al 2
coatings had a Î²-(Ni,Al) outer zone (OZ) rich in substrate element
precipitates. Between the OZ and the substrate, an interdiffusion zone
(IZ), consisting of large alloying element precipitates was apparent.
Average thicknesses of the two coatings including the interdiffusion
zone were 72 and 83 Âµm, respectively. Because of the high density and
the small size of the precipitates, it was not possible to investigate
separately the Î²-(Ni,Al) matrix and the alloying element precipitates.
Then, compositions of the coatings were determined by EDS analyses
of 5x70 Âµm2 areas within 5 Âµm of the coating surface (see table 3). It
has been shown that the presence of 2 at.% Ti close to the surface of
the Al 2 coating coming from the alloy 2 clearly differentiated both
coatings (see Fig. 3). EDS investigations also revealed that the surface
of the Al 1 coating consisted of Al-rich Î²-(Ni,Co)Al particles (up to 55
at.% Al) with poor adherence to the coating whereas the Al 2 coating
presented a 2-3 Âµm Î²-(Ni,Co)(Al,Ti,Cr) layer poor in alloying element
Al 1 36.5
Al 2 36.1
Table 3: Chemical composition of the Al 1 and Al 2 coatings within 5Âµm of
the surface (EDS analyses).
Fig. 2: SEM micrographs of the as-coated a) Al 1 and b) Al 2 coatings.
Fig. 3: Ti-distribution in the Al 2 coating in the as-coated conditions.
Platinum aluminide coatings
Figure 4 shows the microstructures of the two platinum aluminide
coatings. From the SEM micrographs it was possible to distinguish for
each coating three different zones. The outermost layer consisted of a
Î²-(Ni,Pt)Al/PtAl2 fine-grained zone dissolving in both, Î²-(Ni,Pt)Al and
PtAl2 structures, Co, Cr and, in the case of PtAl 2, a low amount of Ti
(see tables 4 and 5). The intermediate layer was composed of a Î²-NiAl
matrix with Cr- and Ta-rich precipitates. The interdiffusion zone
consisted of Cr-, Ti-, Ta-, W- and Re-rich precipitates. X-ray
mappings showed that platinum was mainly localised in the outermost
layer of the coating.
The thicknesses of the coatings PtAl 1 and PtAl 2 including the
interdiffusion zone were 60 and 58, respectively. For each coating, a
row of pores evidencing the original substrate surface was apparent
above the interdiffusion zone.
Fig. 4: SEM micrographs of the as-coated a) PtAl 1 and b) PtAl 2 coatings.
Table 4: Chemical composition (at.%) of the Î²-(Ni,Pt)Al phases present in the
OZ of the platinum aluminide coatings (within 5 Âµm of the coating surface).
Table 5: Chemical composition (at.%) of the PtAl2 phases present in the OZ of
the PtAl 1 and PtAl 2 coatings (within 5 Âµm of the coating surface).
b. Cyclic Oxidation Tests at 1150Â°C
Results of the weight change measurements as a function of the
number of cycles are given in figure 5. Curves show that coatings Al 1
and Al 2 had a very similar behaviour up to the end of the tests (2269
Weight change [mg/cmÂ²]
Number of cycles
Fig. 5: Results of the weight-change measurements of aluminide coatings Al
1 and Al 2.
The optical inspection of the crucibles containing the specimens
showed that the spallation starting point of the oxide layers lies after
about 10 and 40 cycles for Al 1 and Al 2, respectively.
Coating Al 1 revealed the early formation of a blue oxide scale,
characteristic of the spinel NiAl2O4. From X-ray diffraction patterns of
the spalls collected in the crucibles, the presence of Al2O3, TaO2 and
NiO particles could also be evidenced. As shown in Fig. 7a, the spall
volume fraction of Al2O3 and TaO2 decreased progressively during the
first 1600 cycles whereas the formation of NiAl2O4 showed an
increase. NiO appeared only after longer exposure times and was
found to be predominant in the spalls collected after about 2200
cycles. It was also obvious that the appearance of NiO coincided with
the formation of a dark-grey oxide layer in the centre of the samples.
Coatings Al 2 revealed more complex oxidation mechanisms. The early
stages of oxidation (<40 cycles) led to the formation of a yellow oxide
layer (see Fig. 6). X-ray patterns evidenced in the spalls the presence
of the compounds Î±-Al2O3, NiAl2O4 and TaTiO4 (see Fig. 7b). Then,
TaTiO4 was assumed to be responsible for the yellow colour of the
oxide layer since NiAl2O4 and Al2O3 are known to be blue and grey,
respectively. The early extensive formation of TaTiO4 is also in
accordance with the Ti-distribution micrograph (see Fig. 3) exhibiting
a high Ti-concentration within 3 Âµm of the surface. After consumption
of the titanium close to the coating surface, the volume fraction of
TaTiO4 progressively decreased whereas NiAl2O4 predominantly
formed. In the case of coating Al 2, the oxide layer became
progressively green (due to a mixture of yellow and blue oxides) after
about 500 cycles and then completely blue after about 1500 cycles
(see Fig 6c and d).
A considerable amount of Al2O3 has also been evidenced as oxidation
product of both coatings. Nevertheless, the extensive spallation of the
oxide layers during cooling never allowed the formation of a protective
alumina scale. The continuous re-oxidation of the coating surface led
to progressive aluminium depletion of the coating. Finally no more Alrich oxide (Al2O3, NiAl2O4) was formed and oxidation was dominated
by fast growing NiO.
Comparing Al 1 and Al 2, it was obvious that the appearance of NiO
occurred later on coating Al 2 than on coating Al 1 (see Fig. 7).
Moreover, the Al2O3 volume fraction in the spalls of Al 2 decreased
slower than for coating Al 1. However, the weight change curves of
both coatings were very similar and did not seem to be affected by the
fact that Al2O3 formed more easily on coating Al 2. SEM inspections
also evidenced the detrimental effect of Ti-rich oxides (TaTiO4)
causing cracking of the upper oxide layer as shown in figure 8.
Fig. 6: Appearance of the surface on coating Al 2 a) as-received and after b) 22, c)
501 and d) 1515 cycles.
Number of cycles
Number of cycles
Fig. 7: Volume fraction of the oxidation products collected in the crucibles of a) Al 1
and b) Al 2 after different numbers of cycles.
Fig. 8: Formation of Ti-rich oxides on the surface of coating Al 2 causing
spallation of the Al-rich oxide layer.
In table 6, a comparison of the measured structural parameters of the
different oxidation products with those from the JCPDS-files in some
cases showed a distortion of the expected lattices. This effect is
explained by the substitution of Ni, Al or Ta by other coating/substrate
transition metal elements. Indeed, EDS-analysis of the spalls showed
the presence of Co and Cr in NiAl2O4 (up to 4 at.% for both elements),
TaO2 and TaTiO4 structures. TaO2 and TaTiO4 particles were too fine to
determine their exact compositions. Nevertheless, analyses confirmed
that TaTiO4 was the only structure where Ti was found. Analyses of
alumina particles evidenced the formation of pure Al2O3. No Cr, usually
in solid solution in this structure, could be detected.
a = 4.7589
a = 8.050
a = 13.32
a = 4.709
c = 6.120
c = 3.067
c = 12.9919
a = 4.76
a = 8.07
c = 12.99
a = 4.76
a = 13.20
c = 6.05
a = 8.08
a = 4.62
c = 2.99
Tab. 6: Comparison between parameters taken from JCPDS-data and those
measured from the spall X-ray diffraction patterns after 520 cycles (fitted
using the U-fit software).
SEM investigations showed in Fig. 9 on the surfaces of both coatings
the formation of three different zones that could be associated with
areas where the oxide scale is still adherent (zone 1), where reoxidation occurred after spallation (zone 2) and where spallation had
just taken place (zone 3) respectively. Investigations also revealed the
formation after about 1500 cycles of extensive quantities of voids with
sizes up to about 20 Âµm. Such voids are known from the literature
[9,10] and are assumed to be due to either a Kirkendall effect induced
by different diffusivities of Ni and Al in the Î²-NiAl structure or to
condensation of vacancies during cooling. They are detrimental for the
adherence of the oxide scale and promote its spallation during cooling
of the specimens.
Fig. 9: Surface of coating Al 1 after 1550 cycles.
Platinum aluminide coatings
The results of the weight change measurements are shown as a
function of the number of cycles in figure 10. Curves of each coating
exhibited two different regimes: an increase of the weight gain due to
the formation of an adherent and protective oxide scale followed by a
rapid weight loss due to extensive spallation of the oxide layer.
Weight change [mg/cmÂ²]
Number of cycles
Fig. 10: Results of the weight-change measurements of aluminide coatings PtAl 1 and
Coating morphology before the beginning of spallation
Optical inspections before the beginning of spallation revealed for
each coating the presence of a brown-grey oxide layer identified by
XRD as Î±-Al2O3. SEM investigations showed that this oxide scale
consisted of Al2O3 platelets (see Fig. 11). After 160 cycles, coating
surfaces of both coatings revealed the formation of wrinkles  (see
Fig. 12 a).
As shown in figure 12 b, the growth of the wrinkles progressively led
to the formation of cracks assumed to be responsible for the
beginning of oxide scale spallation. Magnification of the weight
change curves just before specimens started to loose weight showed
that spallation began for both coatings after approximately 1250
cycles (see Fig. 13). It was difficult to attribute any chemical effect to
the coating platinum concentrations since the coating performances
were dominated by the mechanical properties of the oxide layer
Fig. 11: SEM micrograph of the PtAl 2 coating after 36 cycles.
Fig. 12: Surface morphology of coating PtAl 2 after a) 160 and b) 500 cycles.
Weight change [mg/cmÂ²]
Number of cycles
Fig. 13: Results of the weight-change measurements for the platinum aluminide
coatings PtAl 1 and PtAl 2.
Coating morphology after the beginning of spallation
After the beginning of spallation, the oxidation resistance of the
coatings was exclusively dominated by the spallation kinetic of the
oxide layer. For each coating, blue zones, revealing the formation of
NiAl2O4, appeared preferentially in the centre of the specimens (see
Fig. 14 b). Then, progressively the blue oxide layer covered the entire
surface of the coatings. Finally, a dark-grey NiO-rich oxide layer
formed on the specimens (see Fig. 14 c).
Fig. 14: Appearance of coating PtAl 1 after a) 500, b) 1500 and c) 2145 cycles.
Weight change curves, that were very similar for both coatings up to
about 1300 cycles, revealed relevant differences in the mass loss rates
after the beginning of spallation.
The marked difference in the spallation rates of coating PtAl 1 and PtAl
2 was surprising. PtAl 2, deposited on alloy 2, clearly showed a slower
weight loss compared to PtAl 1 deposited on alloy 1. This indicated a
beneficial influence of alloy 1 as substrate material since PtAl 1 and 2
were deposited with the same coating parameters. After 2300 cycles,
X-ray diffraction patterns revealed the rapid formation of NiO in
accordance with observations of GÃ¶bel et al.  on CMSX-4
substrates coated with a Pt-Al diffusion coating. The volume fractions
of each oxide calculated from the XRD patterns confirmed the better
behaviour of PtAl 2 compared to PtAl 1. Indeed, the volume fraction of
NiO was much higher in the spalls coming from PtAl 1 (see Fig. 15),
which in turn indicates a more rapid aluminium depletion of the
coating preventing the formation of Al2O3 or NiAl2O4. SEM
investigations of PtAl 1 and PtAl 2 spalls also evidenced the presence
of a few Pt and (W, Ta) oxide particles that could not be detected by
Fig. 15: Volume fraction of the oxidation products collected in the crucibles of PtAl 1
and PtAl 2 after 2300 cycles.
Surface investigations of the specimens after 2300 cycles revealed a
similar surface morphology as that observed on the aluminide coatings
(see Fig. 16). Only small quantities of platinum could be detected by
SEM on the coating surfaces.
Fig. 16: SEM micrograph of the surface of coating PtAl 2 after 2300 cycles.
Investigations of the cyclic oxidation behaviour of aluminide coatings
evidenced that due to thermal cycling, the coatings were not able to
form a protective Î±-Al2O3 scale. It has been shown that as a function
of the coating composition, many different types of mixed oxides can
form during the oxidation tests. The aluminide coatings deposited on
alloy 1 and alloy 2 up to about 1600 cycles exhibited
NiAl2O4/Al2O3/TaO2 and NiAl2O4/Al2O3/TaTiO4 oxide layers,
respectively. Longer exposures led to the rapid growth of NiO. The
detrimental effect of Ti has also been evidenced by the growth of Tirich oxides leading to cracking of the upper Al-rich oxide scale.
Although the oxide layers formed on both coatings tested showed
many compositional differences, the weight change curves evidenced
that both coatings had up to about 2269 cycles a very similar
From the weight change measurements, it has been demonstrated that
the cyclic oxidation resistance of the platinum aluminide coatings was
much better than that of the aluminide coatings. Addition of platinum
could delay the beginning of spallation up to about 1250 cycles by
promoting the formation of an adherent and protective Î±-Al2O3 scale.
The start of spallation was due to the wrinkling of the coating surfaces
leading to rapid degradation of the platinum aluminide coatings. After
2000 cycles, the coatings showed a similar morphology as that
observed on aluminide coatings. Particularly, both types of coatings
exhibited extensive quantities of voids at their surfaces.
Finally, the investigations also showed that the combination of alloy 2
as substrate material and platinum aluminide coatings was the best
among the systems tested.
This work was supported by the ORDICO European-funded R&D
programme NÂ°G4RD-CT-2000-00319. The authors thank MTU Aero
Engines and Fiat Avio for providing specimens, the whole project
consortium (MTU Aero Engines/Germany, Fiat Avio/Italy, IAM-JRC
Petten/Netherlands, Archer Technicoat Ltd/UK, SIFCO Turbine
Components/Ireland, IOPW/Germany, Lufthansa Technik AG/Germany,
CRF/Italy, Techspace Aero/Belgium) for very useful discussions, M.
Schorr, P. Gawenda, M. Jusek for EPMA, SEM and X-ray diffraction
 E.J. Felten, Oxidation of Metals, 10, 1, 23-28, 1976
 Y. Zhang, W.Y. Lee, J.A. Haynes, I.G. Wright, B.A. Pint, K.M. Cooley,
P.K. Liaw, Metallurgical and Materials Transactions A, 30A, 2679-
 J. Schaeffer, G.M. Kim, G.H. Meier, F.S. Petit, in E. Lang (ed.) The
Role of Active Elements in the Oxidation Behaviour of High
Temperature Metals and Alloys, Elsevier, Amsterdam, 231-267, 1989
 J.A. Haynes, Y. Zhang, W.Y. Lee, in N.B. Dahotre, J.M. Hampikian
(eds.), Elevated Temperature Coatings: Science and Technology III,
TMS, Warrendale, PA, 186-196, 1999
 European Union-funded Research Programme NÂ° G4RD-CT-200000319, see www.ordico.eu.com for more details
 European Union-funded Research Programme NÂ° GRD1-200140037 M-T, see http://cotest.dechema.de for more details
 M. Evain, Institut des MatÃ©riaux de Nantes, France, Version 1.3,
 M. Chmielova, J. Seidlerova, Z. Weiss, Corrosion Science, 45, 883889, 2003
 C. Leyens, B.A. Pint, I.G. Wright, Surface and Coatings Technology,
133-134, 15-22, 2000
 D. Oquab, D. Monceau, Scripta materialia, 44, 2741-2746, 2001
 V.K. Tolpygo, D.R. Clarke, Acta mater., 46, 14, 5167-5174, 1998
 M. GÃ¶bel, A. Rahmel, M. SchÃ¼tze, Oxidation of Metals, 41, 3/4,