Engineering, 2010, 2, 602-607
doi:10.4236/eng.2010.28077 Published Online August 2010 (http://www.SciRP.org/journal/eng).
The Effect of Initial Oxidation on Long-Term Oxidation of
NiCoCrAlY Alloy
Chao Zhu, Xiaoyu Wu, Yuan Wu, Gongying Liang
MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter,
School of Science, Xi’an Jiaotong University, Xi’an, China
Received March 8, 2010; revised June 3, 2010; accepted June 5, 2010
Abstract
The initial oxidation behavior of Ni-6.5Co-17.8Cr-3.7Al-0.5Y alloy is investigated at 800°C-1000°C. X-ray
diffraction results show that the dominant Cr2O3 phase and secondary α-Al2O3 and NiO phases are observed
on the surface of samples at all initial stages (oxidized for 16 hours). YAlO3 and θ-Al2O3 can only be de-
tected at low temperature (800°C) while the spinel NiCr2O4 is only observed at 900°C and 1000°C. Though
the growth rates of α-Al2O3 and Cr2O3 are comparable at 900°C, the former becomes much lower than the
latter when the temperature changes to 1000°C. Scanning electron microscopy (SEM) images show that the
α-Al2O3 grows from some irregular ditches in the chromia scale at 900°C. However, cracking and spalling
are more serious at 1000°C without α-Al2O3-grown-ditches, which is in accordance with the growth rates of
these oxides at different temperatures. The cracking can be explained by the results of Raman determination
which indicate that the stress on the surface of specimen oxidized at 1000°C is higher than that at 900°C.
Owing to this condition, a preoxidation treatment on the NiCoCrAlY alloy for 16 hours is prepared at 900°C,
and then thermal cycling oxidation test is conducted at 1000°C for 200 hours. The result indicates that the
initial preoxidation treatment at 900°C improves the oxidation resistance of alloy at 1000°C.
Keywords: NiCoCrAlY, Oxidation Kinetics, Initial Oxidation, Al2O3, Cr2O3
1. Introduction
NiCrCoAlY alloys are often used as bond coatings of
thermal barrier coatings (TBCs) to protect the substrate
from oxidation at high temperature and to provide the
necessary adhesion of the ceramic to the substrate [1-3].
Some authors [4-7] indicated that the protection offered
by MCrAlY (M=Ni, Co or a combination) alloys against
high temperature oxidation relies on the ability of the
alloy to develop and maintain a continuous, dense and
slow growing α-Al2O3 scale. The formation of a con-
tinuous of alumina (Al2O3) layer during the oxidation of
the substrate at high temperatures could result in a dra-
matic slowing down of the oxidation process, because
Al2O3 formation has a slower rate of oxidation compared
to other oxidations. Generally, the protectiveness of the
alloy surface at long-term stage is frequently determined
by the initial stage of oxidation [8-10].
Besides alumina, chromia (Cr2O3) also plays an im-
portant role during the high temperature oxidation
[11-13]. However, the effect of the interaction among the
oxides on the oxidation resistance of alloy has not been
discussed in detail. In particular, there has been a lack of
attention to the initial stages of oxidation on NiCoCrAlY
alloy to date.
Raman spectroscopy has been used as a non-destru-
ctive technique for determining the stresses in oxide
scales for decades due to the bands in the Raman spectra
of specimens shift with pressure [11-13]. The salient
features of the Raman technique are that it does not re-
quire a special environment, and it provides a high reso-
lution.
The aim of this study is to improve the service life of
the alloys. The evolution of the oxide scale on the sur-
face of NiCoCrAlY alloy at initial stage oxidized at
800°C-1000°C were investigated. After reporting the
experimental results, a method to improve the oxidation
resistance was proposed by utilizing the interaction cha-
The project was supported by the State Key Development Program fo
r
Basic Research of China (Grant No. 2007CB707700).

C. ZHU ET AL. 603
racteristics of Al2O3 and Cr2O3 growth.
2. Experimental
The original powder was commercially available, and its
component was 6.5%Co, 17.8%Cr, 3.7%Al, 0.5%Y,
balance Ni (wt.%) with an average particle size of 16.34
μm.
The powder was heated and compacted into the form
of a cylindrical rod of green density equal to 89 ± 5% of
its theoretical density at inner temperature of 800°C for 2
hours with a pressure of 300 MPa. Subsequently, the rod
was annealed in vacuum at 1000°C for 1 hour in order to
homogenize and recrystallize the alloy.
Disc shaped specimens (diameter 15 mm and thick-
ness 1 mm) were cut from the rod using spark-machining.
The specimen surface was ground and polished. After
each preparation step the specimens were thoroughly
cleaned ultrasonically with alcohol.
Isothermal oxidation was performed in static air at 1
atm pressure in a resistance furnace which has a maxi-
mum operating temperature of 1300°C. All the speci-
mens were put into the furnace at the same time after the
test temperature was reached. Then, oxidized specimens
were removed out from the furnace after a chosen time
and air-cooled to room temperature. The initial oxidation
tests were performed at 800°C, 900°C and 1000°C for 2,
4, 8 and 16 hours.
A thermal cycling oxidation test was conducted at
1000°C for 200 hours. In order to keep consistent of
oxidation condition in the test, the specimens were pre-
oxidized at 900°C and 1000°C for 16 hours. Then the
oxidation behavior of the specimens was evaluated by
measuring the weight gains of the samples for 184 hours.
The 12-hour cycle consisted of 11 hours holding at
1000°C, followed by cooling in air for 1 hour. The preci-
sion of the balance was 0.1 mg.
Raman spectroscopy was used here in order to deter-
mine the stresses in chromia scales formed at 900°C and
1000°C on NiCoCrAlY alloy. Chromia has the same
structure as corundum, and therefore it should have
seven Raman active bands (A1g + 5Eg) [11-13]. The most
intense mode is the 549 cm-1 A1g vibration [11] and this
one was used for monitoring the stress.
The spectroscopy was measured at room temperature
using the Renishaw Ramanscope 1000 (Renishaw™,
Gloucestershire, UK) in conjunction with an Olympus
BH-2 microscope. During the measurements, the laser
(He–Ne, 632.8 nm) was focused at a position on the sur-
face of the sample and the laser spot size was set about
3–5 μm. The Raman spectroscopy acquired was analyzed
by the commercial Renishaw WiRe software to obtain
the peak shift fitted by Gaussian–Lorentzian function.
The surface morphologies and polished cross sections
of the specimens were observed using a scanning elec-
tron microscopy (SEM) (JSM-7000F). The chemical
composition of the oxides was determined qualitatively
by energy-dispersive X-ray analysis (EDX). The phases
in the oxide scales were analyzed using an X-ray diffrac-
tion (XRD) (Rigaku D, CuKα radiation).
3. Results and Discussion
3.1. The Oxides on the Surface after Initial
Oxidation
X-ray diffraction patterns of NiCoCrAlY alloy after oxi-
dation for 16 hours at 800°C, 900°C and 1000°C are
shown in Figure 1. Results from the study show that the
oxides on the surface of alloy which was heated to 800°C
for 16 hours are composed of Cr2O3, a few θ-Al2O3 and
YAlO3, trace α-Al2O3 and NiO. It was found that how-
ever, oxides θ-Al2O3 and YAlO3 did not form at 900°C
and 1000°C. The spinel oxide, NiCr2O4, began to exist
after 16 hours of oxidation at 900°C.
According to the intensity of diffraction peaks, the
relative oxidation rates of Cr2O3 and α-Al2O3 phases on
the surface of alloy oxidized from 2 hours to 16 hours at
800°C, 900°C and 1000°C are shown in Figure 2.
From Figure 2(a), Figure 2(b), it can be seen that the
relative quantities of Al2O3 and Cr2O3 increased quickly
in the first two hours. After that, the oxidation rate of
Al2O3 rises slowly while the relative quantities of Cr2O3
at 900°C and 1000°C keep fluctuating. The increase of
quantities of α-Al2O3 resulted from dense α-Al2O3 oxide
forming and θ-Al2O3 transforming. The fluctuation of
quantities of Cr2O3 at 900°C and 1000°C may have been
caused by spinel oxides NiCr2O4 formation which con-
sumed Cr2O3. With the increased temperature or pro-
longed exposure time, NiO eventually became destabi-
lized and reacted with Cr2O3 to form a thin spinel layer
of NiCr2O4, which was thermodynamically more stable
[9,14-16]. The competition between the consumption of
Figure 1. X-ray diffraction patterns of Ni6.5Co17.8Cr3.7
Al0.5Y alloy (oxidized for 16 h) at (a) 800°C; (b) 900°C and
(c) 1000°C.

C. ZHU ET AL. 604
(a)
(b)
(c)
Figure 2. Relative quantities of Cr2O3 and α-Al2O3 phases
on the surface of alloy vs. time: (a) α-Al2O3; (b) Cr2O3; (c)
the relative oxidation rates of Cr2O3 and α-Al2O3 at the
second hour at 800°C, 900°C and 1000°C.
Cr2O3 and the formation of Cr2O3 determined the outline
of curve.
Figure 2(c) shows that the relative oxidation rates of
α-Al2O3 and Cr2O3 increased with temperature in the
second hour from 800°C to 1000°C. Also, it can be ob-
served that the growth rate of α-Al2O3 is similar to that of
Cr2O3 at 900°C. However, at 1000°C, the increase of
growth rate of α-Al2O3 is not as much as the growth rate
of Cr2O3.
3.2. The Cracking and Closure of the Oxide
Scale in Initial Oxidation
A SEM image of the specimen surface oxidized in air for
up to 16 hours at 800°C is shown in Figure 3(a). Figure
3(b) is at a higher magnification. These images show that
some protrudes, pores and pits are presented on the sur-
face, but few cracks appeared.
The whisker or needlelike oxide phase is observed.
The EDX analysis (Figure 3(c), Figure 3(d)) of the ox-
ide whiskers produced Al, Cr and O peaks, which is
qualitatively identified as Al2O3 and Cr2O3 phases. As
the θ-Al2O3 phase usually grows in a needlelike, whis-
kerlike or bladelike morphology and α-Al2O3 grows in a
weblike or dense equiaxed structure [14,17], these blade-
like oxides should be θ-Al2O3.
(c)
(d)
Figure 3. SEM images of the specimen surface (oxidized for
16h at 800°C) (a) surface image; (b) higher magnification;
(c) and (d) EDX analysis in selected region.

C. ZHU ET AL. 605
The surface images of specimens exposed at 900°C
and 1000°C in a static atmosphere for 16 hours are
shown in Figure 4(a), Figure 4(b). It was found that
specimens covered with fine oxide particles. It was also
observed irregular ditches on the surface at both tem-
peratures and cracks in the oxide scale. The cracks be-
came more serious as the temperature increased. The
spallation on the surface of the oxidized specimen was
unavoidable.
It may have resulted from the stress during the cooling
and heating process and the mismatch between the ex-
pansion coefficients of oxides and alloys.
(e)
(f) (g)
Figure 4. The surface images of specimens (oxidized for 16h
at 900°C and 1000°C) (a) specimens oxidized at 900 °C; (b)
specimens oxidized at 1000°C; (c) higher magnification of
Figure. 4(a); (d) higher magnification of Figure. 4(b); (e), (f)
and (g) EDX analysis in selected regions.
EDX analysis (Figure 4(f), 4(g)) shows that the oxides
around the cracks are Cr2O3 (Figure 4(d)). This indicates
that these cracks were caused by the Cr2O3 oxide scale
broke during the oxidation. At the same time, it was ob-
served that some oxides were growing from the cracks in
Figure 4(c). By EDX analysis, it is confirmed that these
oxides are α-Al2O3 and Cr2O3. It can be speculated that,
at 900°C, α-Al2O3 grew form the bottom of the Cr2O3
oxide scale cracks, which filled in the cracks and made
the oxide scale dense. Contrasting Figure 4(a) with Fig-
ure 4(b), it is can be seen that the cracks in the oxide
scale at 1000°C are more than those at 900°C. There are
more irregular ditches observed among the Cr2O3 oxide
scale but not so much α-Al2O3 fill in the cracks.
Form Figure 2, we observe that the growth rate of
α-Al2O3 is similar to that of Cr2O3 at 900°C. Though the
spallation on the Cr2O3 scale was unavoidable, α-Al2O3
could preferably nucleate during oxidation on the surface
in some cracks of the oxide scale at 900°C where they
could grow and fill in those ditches. With the density of
the oxide scale increased, both the oxygen and cation
diffusion rate decreased. Thus the ability of oxidation
resistance would be improved. However, the growth rate
of Cr2O3 is much larger than that of α-Al2O3 at 1000°C.
When the oxides on the surface of the alloy grew at a
larger rate, the oxide scale cracked and spalled easily.
Because of the cracks, oxygen diffused through the oxide
scale easily to contact the oxide–alloy interface, which
speeded up the oxidation of alloy greatly.
The cross-sectional microstructure and the elemental
maps of the NiCoCrAlY specimens obtained by SEM
and EDX after oxidation at three different temperatures
for 16 hours are shown in Figure 5. The elemental con-
centration regions of O, Cr and Al are presented in the
Figures 5(a2)-5(c4).
From Figure 5(a2), Figure 5(b2) and Figure 5(c2), it
was found that the thickness of the oxide layer increases
with the temperature rising. A continual oxide layer
formed at 900°C, and it became thicker at 1000°C. The
Cr2O3 phase dominated in the oxide layers (Figure 5(a3),
Figure 5(b3) and Figure 5(c3)) while the Al2O3 phase
was not abundant in these environments (Figure 5(a4),
Figure 5. The cross-sectional microstructure and the ele-
mental maps of NiCoCrAlY alloy (oxidized for 16 h) at (a)
800°C; (b) 900°C and (c) 1000°C.

C. ZHU ET AL. 606
Figure 5(b4) and Figure 5(c4)). This indicated that Cr2O3
formed at the initial oxidation stage. Some brightness
Al-containing regions denote the Al2O3 which filled in
the Cr2O3 ditches (Figure 5(b4)). This is in good agree-
ment with the XRD and surface microstructure results.
Though a continuous layer of Al2O3 was not appeared
at the onset of the oxidation, an inner zone of isolated
Al-containing phase could be observed at all three tem-
peratures in Figure 5(a4), Figure 5(b4) and Figure 5(c4).
This was in good agreement with the literature [10] and
the Al-containing phase should be α-Al2O3. The lateral
growth of α-Al2O3 precipitates occurred until they coa-
lesce into a continuous α-Al2O3 layer.
3.3. Stress Determination
Figure 6 shows the Raman spectrum obtained from the
oxide scale formed on the surface of NiCoCrAlY alloys
oxidized at 900°C and 1000°C for 16h. The only band
which is well-defined in both spectra is the A1g mode at
549 cm-1 in Mougin et al.’s work [11]. In the scales, this
mode shifts to 549.53 cm-1 for 900°C specimen and
554.34 cm-1 for 1000°C specimen respectively, resulting
in the observed shifts are equal to 0.53 cm-1 and 5.34
cm-1 respectively. Using the law given by Mougin et al.
[11] for the frequency dependence with pressure, i.e.,
0.307 ± 0.005 GPa/cm-1, it gives the stress values of
0.163 ± 0.005 GPa for 900°C specimen and 1.639 ±
0.005 GPa for 1000°C specimen respectively. The shift
direction corresponds to compressive stress.
The results exhibited here agree with the previous dis-
cussion. The higher growth rate of Cr2O3 at 1000°C re-
sulted in a higher stress than the stress generated at a lower
temperature. The structure under high stress condition was
easier to crack, spall and fracture and was more difficult
to self-healing by the Al2O3 growth simultaneously.
Figure 6. Raman spectrum for the chromia formed on Ni-
CoCrAlY alloys oxidized for 16 h at 900°C and 1000°C.
3.4. The Effect of the Preoxidation Treatment on
the Oxidation Resistance
Due to the different growth characteristics of α-Al2O3
and Cr2O3 at different temperatures at initial stage of
oxidation, two groups of specimen were conducted. One
was directly oxidized at 1000°C for 200 hours. The other
one was subjected to a preoxidation treatment at 900°C
for 16 hours first, aiming to repair micro-cracks in the
Cr2O3 scale by subsequent growth of α-Al2O3, then oxi-
dized at 1000°C for 184 hours. The oxidation behavior of
the specimens was evaluated by a cyclic oxidation test.
Figure 7 represents the weight gain as a function of
time for the cycle oxidation at 1000°C. In the figure,
curve (a) indicates the specimen directly oxidized at
1000°C for 200 hours and curve (b) indicates the speci-
men which preoxidixed at 900°C. At the onset of the
oxidation, the rate of weight gain of the specimen pre-
oxidized at 900°C was slower than that of 1000°C. After
the sharp increase of weight gain at the initial oxidation
stage, both of the kinetic curves showed an extensive
period of very slow weight gain. Obviously, the alloy
which preoxidised at 900°C showed lower weight gains
than that of 1000°C.
With oxidation depth increased, oxygen activity re-
duced unceasingly, Al2O3 precipitates would nuclear in
subsurface of the alloy but no longer for Cr2O3. This
could explain that why the relative quantity of α-Al2O3 at
900°C was larger than that at 1000°C. At 1000°C, a great
amount of Cr2O3 formed by the contact between Cr irons
and the oxygen through the severe cracks.
4. Conclusions
Initial oxidation tests of Ni-6.5Co-17.8Cr-3.7Al-0.5Y
alloy specimens was performed at 800°C, 900°C and
1000°C for 16 hours. Cr2O3 was the predominant phase
at all three temperatures and the dense Cr2O3 scale play-
Figure 7. Weight gain of specimens vs. time in the cyclic
oxidation at 1000°C for 200 h (a) directly oxidized at
1000°C; (b) preoxidation treatment at 900°C for 16 h.

C. ZHU ET AL. 607
ed an important role in protecting against cracking and
oxidation in the first 16 hours of the isothermal oxidation
at 800°C. YAlO3 phase was only observed at 800°C.
That the growth rate of α-Al2O3 was similar to that of
Cr2O3 at 900°C lead to the α-Al2O3 could grow and fill in
the ditches on the Cr2O3 scale. However, the growth rate
of Cr2O3 at 1000°C was much larger and produced higher
stress than that at 900°C, so that the α-Al2O3 grown from
the Cr2O3 oxide ditches was not enough to fill in these
cracks. Though the spallation on the surface of oxidized
specimen was unavoidable at the higher temperature
(900°C and 1000°C), a preoxidation treatment at 900°C
for 16 hours can cause α-Al2O3-dispersions-in-Cr2O3
scale formed on the surface of the specimen. This could
improve the oxidation resistance of NiCoCrAlY alloy in
the thermal cycling oxidation.
5. Acknowledgment
The project was supported by the State Key Develop-
ment Program for Basic Research of China (Grant No.
2007CB707700).
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