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Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.11, pp.1037-1057, 2010
jmmce.org Printed in the USA. All rights reserved
Oxidation Behavior of Nanostructured TiAlN and AlCrN Thin C o a ti n g s o n
ASTM-SA213-T-22 Boiler Steel
Vikas Chawlaa*, Amita Chawlab, Buta Singh Sidhuc , S. Prakashd and D. Purid
a Mechanical Engineering Department, F.C.E.T., Ferozepur-152004 , India
b Chemistry Department, Government Brijindra College, Faridkot -151203, India
c Punjab Technical University (P.T.U.), Jalandhar -144001, India
d Metallurgical & Materials Engineering Department, I.I.T. Roorkee, Roorkee-247667, India
*Corresponding Author: email@example.com
Metals and alloys gets oxidized when exposed to elevated temperatures in air or highly oxidizing
environments, such as combustion gas with excess of air or oxygen. They often rely on the
oxidation reaction to develop a protective oxide scale to resist corrosion attack. In the present
study, nanostructured TiAlN and AlCrN thin films were deposited by physical vapour deposition
process on T-22 boiler steel (ASTM-SA213-T-22). Cyclic oxidation studies in air were conducted
at 900°C temperature in the laboratory using silicon carbide furnace. The weight gain was
measured after each cycle and visually examined the surface morphology of the oxidized samples
was studied using FE-SEM with EDAX attachment and XRD analysis. The results obtained
showed the better performance of AlCrN coated T-22 boiler steels then the TiAlN coated and
uncoated T-22 boiler steel.
Keywords: Nanostructured coating, High temperature oxidation, Oxide Scale, Physical vapour
deposition, Scale morphology.
Coatings play crucial role to safeguard the materials of high temperature equipments, particularly
for boilers and gas turbine engines against the oxidation and corrosion attacks . Recent studies
show that 80% of the total cost for the protection of metals is related to coating application .
Coatings provide a way of extending the limits of use of the materials at the upper end of their
1038 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
performance capabilities, by alloying the mechanical properties of the substrate materials to be
maintained while protecting them against wear, oxidation and corrosion . Although protective
surface treatments are widely used at low temperature, the use of these at elevated temperature is
relatively more recent . In many tribological applications, hard coatings of metal nitrides are
now commonly used . The major properties required for such coatings are hardness and wear
resistance. However, because of severe operating conditions, there is a need to combine
mechanical features with corrosion resistance properties.
Physical vapor deposition technique (ion plating, sputtering, and arc evaporation) provides a
promising ground for the deposition of these hard coatings by the formation of dense adhesive
film at low deposition temperatures. Corrosion protection capability of physical vapor deposited
(PVD) coatings is widely reported in literature . Since the commercialization of physical
vapor deposited (PVD) TiN coatings in early 1980s, transition metal nitrides based hard coatings
have been successfully used for the materials protection particularly to improve cutting tools
lifetime . This type of coating, however, suffered severe oxidation at the temperatures 550 or
600°C . A possible solution was found in consideration to the fact that both TiN and TiAlN
have the same crystallographic structure (FCC), and therefore addition of Al atoms to the TiN
matrix in the 1990s. At high temperature exposures, a very dense and strongly adhesive Al2O3
film is observed because of diffusion of Al atoms to the surface, which stops further oxidation [8,
9]. Since the addition of aluminum increases the oxidation resistance of Ti-N coatings, a similar
effect on the oxidation resistance of Cr-N coatings should be expected . It has been reported
that the hardness, oxidation resistance and the tribological properties improve with increasing
Al-content up to 70-75% in the AlCrN coatings as long as fcc-structure is predominant. For
higher aluminum content, hcp-structure starts to form and thus the oxidation resistance
Nanostructured coatings are reported to provide surface characteristics (hardness, wear resistance
etc) superior to those of conventional coatings. Despite that several potential advantages have
been noted, the technology is yet to be established for use in industrial applications . Present
study investigates the effects of nanostructured TiAlN and AlCrN thin coatings on the oxidation
behavior of T-22 steel under the cyclic heating conditions. Some power plants in India are using
T-22 grade as boiler tubes material due to its performance in stringent service conditions of
pressure and temperature. A front-loading Balzer’s rapid coating system (RCS) machine (make
Oerlikon Balzers, Swiss) was used for the deposition of the coatings. The purpose of this study is
to develop high temperature oxidation, erosion and corrosion resistant materials by thin film
2. EXPERIMENTAL DETAILS
2.1 Selection of Substrate Material
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1039
The substrate material used is: 2.25Cr-1Mo steel ASTM-SA213-T-22 (T22). This material is
used as boiler tube materials in some of the power plants in northern India. T-22 boiler steel has
a wide range of applications in boilers, especially where the service conditions are more stringent
from the point view of temperature and pressure. The chemical composition of T-22 boiler steel
is as reported in Table 1.
Table 1: Chemical composition (wt %) of T-22 Boiler Steel (ASTM code SA213-T-22) :
Elements C Mn Si S P Cr Mo Fe
Nominal 0.15 0.3-0.6 0.5 0.03 0.03 1.9-2.6 0.87-1.13 Bal.
Actual 0.165 0.355 0.115 0.00153 0.02026 2.646 0.90275 Bal.
Specimens with dimensions of approximately 20mm x 15mm x 5mm were cut from the alloy
sheet. Polished using emery papers of 220, 400, 600 grit sizes and subsequently on 1/0, 2/0, 3/0,
and 4/0 grades, and then mirror polished using cloth polishing wheel machine with 1μm
lavigated alumina powder suspension.
2.2. Development of Coatings
In the present study, the two coatings selected were TiAlN and AlCrN. The RCS system used to
apply the coatings is shown schematically in Fig.1. The machine is equipped with 6 cathodic arc
sources. Two of the six sources were used to deposit a thin, 0.3 μm thick TiN sub-layer to
improve adhesion of coating. The remaining four sources were employed to deposit the main
layer of the coatings, which was obtained using customized sintered targets. The compositions of
the targets used, coating thickness and the summary of the process parameters are presented in
For all coatings argon (Ar) and pure nitrogen atmosphere was used during deposition. Prior to
deposition all the substrates were cleaned in two steps: firstly with Ultrasonic Pre-Cleaner
(Imeco, Pune, India) and secondly with Ultrasonic Cleaning Machine with 9 Tanks including hot
air dryer (Oerlikon Balzers (India) Ltd.) for 1.5 Hrs.
The characterization of as coated specimens was done and will be reported in another paper i.e.
XRD (Bruker AXS D-8 advance diffractometer (Germany) with Cu Kα radiation), SEM-EDAX
analysis of surface as well as cross-section (FEI, Quanta 200F), surface morphology (2D and 3D)
of the thin films by Atomic Force Microscope (AFM, make NT-MDT, Ntegra) and micro
1040 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
hardness. The particle size of the thin films was estimated from Scherrer formula as well as from
AFM analysis, which was found to be 18 nm & 22 nm respectively for TiAlN coating, whereas
for AlCrN coating was 25 nm & 27 nm respectively.
Fig.1: Schematic illustration of the coating device used for the film deposition 
Table 2: Summary of deposition parameters
Machine used Standard balzers rapid coating system (RCS) machine
Make Oerlikon Balzers, Swiss
Targets composition for TiAlN coating: Ti, Ti 50Al50
AlCrN coating: Al70Cr30
Number of targets Ti (02), Ti 50Al50 (04) and Al70Cr30 (06)
Targets power: 3.5 kW
Reactive gas Nitrogen
Nitrogen deposition pressure 3.5 Pa
Substrate bias voltage -40V to -170V
Substrate temperature 450°C ± 10°C
Coating Thickness 4 µm ± 1 µm
2.3 Oxidation Studies in Air
Cyclic oxidation studies in air were conducted at 900°C temperature in the laboratory silicon
carbide furnace, calibrated up to the variation of ± 5°C for 50 cycles. Each cycle consisted of 1
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1041
hour heating at 900°C followed by 20 min cooling at room temperature. The aim of cyclic
loading is to create severe conditions for testing. The studies were performed for uncoated as
well as coated specimens for the purpose of comparison. The uncoated specimens were subjected
to mirror polishing, whereas coated specimens were subjected to wheel cloth polishing for 5 min.
After washing with acetone, the specimens along with alumina boats were then subjected to tube
furnace for oxidation studies. During oxidation runs, the weight of boats along with the
specimens were measured together after visual observations at the end of each cycle with the
help of Electronic balance Model CB-120 (Contech, Mumbai, India) with a sensitivity of 1mg.
Spalled scale, if any was also included in the weight change determination. The kinetics of
corrosion was determined from the weight change measurement. After the oxidation studies, the
exposed specimens were analyzed by XRD and SEM-EDAX analysis using Bruker AXS D-8
advance diffractometer (Germany) with Cu Kα radiation at the scan rate of 2°/min for 20° to
120° and FE-SEM (FEI, Quanta 200F) respectively. The oxidized specimens were then cut using
Buehler’s Precision Diamond saw (Model ISOMET 1000, USA make) across the cross-section
and mounted for the cross-sectional analysis using SEM/EDAX and elemental X-ray mapping.
The kinetics of the cyclic oxidation of coated as well as uncoated specimens was determined
using the thermogravimetric analysis and by evaluating the parabolic rate constants.
3.1 Visual Examination
The macrographs for Uncoated, TiAlN coated and AlCrN coated T-22 boiler steel subjected to
cyclic oxidation in air at 900°C for 50 cycles are shown in Fig.2. For the uncoated T-22 boiler
steel, a grey colored scale appeared on the surface right from the 1st cycle. The surface
appearance of the scale turned to brownish grey tone which remained till the end of 50th cycle.
This bare steel showed spalling of scale just after the 5th cycle, which continued till the end of 50
cycles. At the end of cyclic study, irregular and fragile scale was observed with deep cracks and
blackish grey color (with brownish grey appearance at some places) surface appearance, which
can be seen in Fig.2. (a).
In case of TiAlN coated T-22 boiler steel, color of the oxide scale at the end of the study was
observed to be grey with some blackish grey areas on the surface, as shown in Fig.2 (b). The
color of the scale after 2nd cycle was observed as whitish brown which changes to grey with
golden and black spots at few areas after subsequent cycles. A fragile scale appeared on the
surface of the specimen during the initial cycles. Subsequently, cracks were developed in the
scale and spalling was observed during the subsequent cycles. After 20th cycle severe swelling
and peeling of the oxide scale was observed. Some of the scale was seen getting detached from
the surface of the TiAlN coated T-22 boiler steel.
V. Chawla, A. Chawla, B. S. Sidhu
, S. Prakash and D. Puri Vol.9, No.11
Fig.2: Surface macrographs for the T-22 boiler steel subjected to cyclic oxidation in air at
900°C for 50 cycles: a) uncoated, b) TiAlN coated, (c) AlCrN coated
The AlCrN coated T-22 boiler steel has shown the formation of smooth scale without the
presence of cracks, when subjected to cyclic oxidation in air at 900°C for 50 cycles. Color of the
oxide scale at the end of the study was observed to be dark grey, as shown in Fig.2 (c). The scale
was found to be lustrous, with no tendency to spall. Golden and ink blue reflections were
observed in the scale, after the completion of 2
cycle, which turned to dark grey subsequently.
3.2 Thermogravimetric Data
Thermogravimetric data for coated and bare T-22 boiler steel subjected to cyclic oxidation is
presented in Fig. 3 in the form of a graph between weight gain per unit area (mg/cm
time expressed in number of cycles. It can be inferred from the plots that the uncoated and TiAlN
coated T-22 boiler steels have shown initially high rate of oxidation as compared to AlCrN
coated T-22 steel, followed by a nearly constant rate. After 20
cycle, the oxidation rate in case
of TiAlN coated sample increased abruptly up to the 50
cycle. The cumulative weight gains
after completion of 50 cycles (shown in Fig.4) of oxidation are found to be 177.24, 203.39, 55.27
for the uncoated as well as TiAlN and AlCrN coated T-22 boiler steel, respectively.
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1043
Fig.3: Weight gain per unit area vs. number of cycles plot for coated and uncoated T-22
boiler steel subjected to cyclic oxidation in air at 900°C for 50 cycles
Fig.4: Bar chart showing cumulative weight gain per unit area for coated and uncoated
T-22 boiler steel subjected to cyclic oxidation in air at 900°C for 50 cycles
051015 20 25 30 3540 45 50
Number of cycles
Weight gain/Area (mg/cm
Al Cr N Coated
1044 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
Fig.5: (Weight gain per unit area)2 vs. number of cycles plot for coated and uncoated T-22 boiler
steel subjected to cyclic oxidation in air at 900°C for 50 cycles
Table 3: Values of parabolic rate constant for bare and coated T-22 boiler steels exposed to
cyclic oxidation in air at 900°C for 50 cycles
Type of steel Kp (x 10-7 gm2/cm4/s)
Uncoated T-22 1.88
TiAlN coated 2.25
AlCrN coated 0.15
051015 20 25 30 3540 45 50
Number o f c ycl es
(Weight g ai n / Area)
TiA l N Coat ed
l CrN Coat ed
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1045
30 35 40 45 50 55 60 65 70
α Fe2O3 β Cr2O3 γ Al2O3
ε TiO2 ν MnO2 κ Mo O2
Intensity (arbitra ry units)
D iffra c tio n a n g le (2-th e ta )
Fig.6: X-ray diffraction pattern for coated and uncoated T-22 boiler steel subjected to
cyclic oxidation in air at 900°C for 50 cycles
As evident from Fig. 4, the overall weight gain is highest in case of TiAlN coated steel and is
lowest in case of AlCrN coated steel. In Fig. 5, the (weight gain/area)2 versus number of cycles
plot are shown for all the cases to ascertain conformance with the parabolic rate law. Although
some scatter in the data can be observed in the plots, yet it is apparent that these data can be
approximated by a parabolic relationship. There is a visible deviation from the parabolic rate law
in case of TiAlN coated T-22 boiler steel, whereas the uncoated and AlCrN coated T-22 boiler
steel follow the parabolic behavior up to 50 cycles. Ignoring the scatter in the data, the parabolic
rate constant Kp was calculated by a linear least-square algorithm to a function in the form of
(W/A)2= Kp t, where W/A is the weight gain per unit surface area (mg/cm2) and ‘t” indicates the
1046 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
number of cycles representing the time of exposure. The parabolic rate constants for the bare and
coated T-22 boiler steel calculated on the basis of 50 cycle’s exposure data are shown in Table.3.
3.3 X-ray DiffractionAanalysis (XRD)
XRD diffractograms for coated and uncoated T-22 boiler steel subjected to cyclic oxidation in air
at 900°C for 50 cycles are depicted in Fig.6 on reduced scale. As indicated by the diffractograms
in Fig.6, Fe2O3 & Cr2O3 are the main phases present in the oxide scale of uncoated T-22 boiler
steel. In case of TiAlN coated T-22 boiler steel, the formation of Fe2O3, Cr2O3, Al2O3, MnO2 and
TiO2 has been indicated by XRD peaks. Further, in case of AlCrN coated T-22 boiler steel, the
prominent phases are Fe2O3, Cr2O3, Al2O3 and MoO2.
3.4 Surface Scale Morphology
SEM micrographs along with EDAX analysis for coated and uncoated T-22 boiler steel
subjected to cyclic oxidation in air at 900°C for 50 cycles are shown in Fig.7. Micrograph as
shown in Fig.7 (a) for uncoated T-22 boiler steel indicates mainly white and dark phases. As
indicated by the EDAX analysis the white phase shows more amount of Cr (03.78%), O
(16.79%) and Mo (13.4 %) then the dark phase i.e. Cr (00.55%), O (16.47%) and Mo (11.55%),
whereas the dark phase is rich in Fe (62.21%) as compared to the white phase i.e. Fe (58.82).
The oxide scale was fragile having some cracks in it.
The SEM micrograph and compositions of oxidized TiAlN coated T-22 boiler steel is shown in
Fig.7 (b). The oxide scale is mainly consisting of three types of regions i.e. one is like white
needles dispersed in matrix, second is light grey matrix and third is dark grey matrix. The top
scale is rich in Fe, Mo, O, and Mn as analyzed by EDAX analysis. The small amount of Al, Ti,
Cr, C, Si, and P are also present. The region with white needles like appearance shows maximum
amount of Fe (61.46%), moderate amount of Mo (22.21%) and minimum amount of O (10.66%)
& Mn (02.30%). The dark grey matrix is rich in Fe (57.40%) with Mo (11.71%), O (17.36%) and
Mn (05.44%). Further, the light grey matrix shows maximum amount of Mo (42.31 %), O
(20.29%) & Mn (12.44%) and minimum amount of Fe (19.90%).
In case of AlCrN coated T-22 boiler steel oxidized in air at 900 °C for 50 cycles, the SEM
micrograph indicates the dense dimple like surface appearance along with light grey porous
surface like matrix, as shown in Fig.7 (c). The EDAX point analysis shows, the top scale is rich
in Fe, Mo and O. The small amount of Mn, Al, N, Cr, C, Si, and P are also present. The dimple
like region is rich in Fe (52.78%) along with Mo (13.27%), O (26.63%). The porous surface like
grey matrix is rich in Fe (58.35%) along with Mo (10.32%), O (20.16%). The white region on
the grey matrix shows maximum oxygen content i.e. O (30.30%) along with Fe (53.39%) and
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1047
Fig.7 (a): SEM/EDAX analysis along with EDS spectrum for uncoated T-22 boiler steel
subjected to cyclic oxidation in air at 900°C for 50 cycles (X 200)
03.78 % Cr
58.82 % Fe
16.79 % O
13.45 % Mo
01.16 % Mn
00.61 % Si
04.95 % C
00.55 % Cr
62.21 % Fe
16.47 % O
11.55 % Mo
00.82 % Mn
00.32 % Si
07.63 % C
1048 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
Fig.7 (b): SEM/EDAX analysis along with EDS spectrum for TiAlN coated T-22 boiler steel
subjected to cyclic oxidation in air at 900°C for 50 cycles (X 200)
57.40 % Fe
11.71 % Mo
17.36 % O
05.44 % Mn
00.32 % Ti
00.74 % Al
04.91 % C
00.59 % P
19.90 % Fe
42.31 % Mo
20.29 % O
12.44 % Mn
00.36 % Cr
00.27 % Ti
00.25 % Al
02.42 % C
00.42 % Si
00.64 % P
61.46 % Fe
22.21 % Mo
10.66 % O
02.30 % Mn
00.63 % Ti
00.27 % Al
01.28 % C
00.36 % P
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1049
Fig.7(c): SEM/EDAX analysis along with EDS spectrum for AlCrN coated T-22 boiler steel
subjected to cyclic oxidation in air at 900°C for 50 cycles (X 200)
53.39 % Fe
07.65 % Mo
30.30 % O
00.84 % Mn
00.52 % P
00.38 % Al
03.60 % C
02.70 % N
58.35 % Fe
10.32 % Mo
20.16 % O
01.12 % Mn
00.46 % P
00.34 % Al
05.96 % C
02.38 % N
52.78 % Fe
13.27 % Mo
26.63 % O
00.95 % Mn
00.42 % P
00.45 % Al
02.39 % C
02.12 % N
1050 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
3.5 Cross-Sectional Analysis
3.5.1 Scale thickness
Scanning electron back scattered micrographs for coated and uncoated T-22 boiler steel
subjected to cyclic oxidation in air at 900°C for 50 cycles, are presented in Fig.8. Very thick
scale is observed in case of uncoated T-22 boiler steel, which is around 1.5 times & and 7.69
times thicker than the scale measured for TiAlN coated and AlCrN coated T-22 boiler steel
respectively. The measured oxide scale thickness for the oxidized specimens is shown in Table.4.
3.5.2 Cross-sectional scale morphology
Back Scattered Electron Image (BSEI) micrograph and elemental variation across the cross-
section for coated and uncoated T-22 boiler steel subjected to cyclic oxidation in air at 900°C for
50 cycles are shown in Fig.8. The SEM micrograph in case of uncoated T-22 boiler steel shows
thick scale as shown in Fig. 8 (a). EDAX analysis reveals the presence of Fe, Cr and oxygen
throughout the scale. The EDAX analysis shows the contrast grey phase (point 4 and 6) mainly
consist of Cr and Fe. The existence of significant amount of oxygen points out the possibility
that this grey phase may be rich in Cr2O3 and Fe2O3. The top portion of the scale (point 2) mainly
contains Fe and O and little amount of Mn, Cr, P, and Si. BSEI micrograph and elemental
variation depicted in Fig.8 (b), for the exposed cross-section of TiAlN coated T-22 boiler steel
shows the thick scale. The EDAX analysis reveals the presence of Fe, Mo and oxygen through
out the scale. So, iron oxide and molybdenum oxide are the main phases in the oxide scale. At
point 4 about 6 wt % of Cr is present along with of good percentage of oxygen (about 32 wt %),
which indicates the possible presence of Cr2O3. The outer portion of the scale is rich in Fe and
Mo oxides, whereas in the middle of the scale existence of Cr2O3 is indicated (point 4). BSEI
micrograph and elemental variation for AlCrN coated T-22 boiler steel is shown in Fig.8(c). A
continuous and adherent oxide scale can be seen. The presence of oxygen at the scale substrate
interface may be due to the in flight oxidation of coating or oxygen might have penetrated during
initial cycles of oxidation run along the intersplat boundaries.
Table 4: Average scale thickness (mm) for uncoated and coated T-22 boiler steels exposed to
cyclic oxidation in air at 900°C for 50 cycles.
Type of steel thickness (mm)
Uncoated T-22 1.00
TiAlN coated 0.64
AlCrN coated 0.13
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1051
Fig.8: Oxide scale morphology and variation of elemental composition across the cross-section
of coated and uncoated T-22 boiler steel subjected to cyclic oxidation in air at 900°C for
50 cycles, (a) Uncoated (X 80); (b) TiAlN coated (X 130) ; (c) AlCrN coated (X 726).
Point of analysis
W eig h t % of each el ement
C K O K SiK
P K S K CrK
MnK FeK MoK
Poin t of ana lysis
W e igh t % of e a c h e le m e nt
CO Al Ti Fe Mo
Si PMn Cr
Point of ana lysis
Weight % o f each elem en t
CFe O N Al
Cr Mo Si PMn
1 345672 8
1 2 3 4 6 5 7 8
12346 57 8
1052 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
The EDAX analysis indicates the presence of Fe, Mo and oxygen through out the scale. The
outer portion of the scale is rich in Fe, Mo and oxygen. Thin band of AlCrN coating (point 7) is
seen between substrate and scale. This thin band at scale substrate interface (dark grey line)
mainly contains Al, Cr, N and very less Fe, Mo and oxygen (point 7). At some points thin film
has gaps; Fe and Mo have moved through the openings and got oxidized to form the scale.
3.5.3 X-Ray mapping
BSEI and X-ray mapping for a part of oxide scale of uncoated T-22 boiler steel oxidized in air at
900 °C for 50 cycles are shown in Fig. 9. The micrograph indicates a dense scale, which mainly
contains iron and oxygen with some amount of chromium, as indicated by X-ray mapping.
Oxygen is present throughout the scale. The BSEI and X-ray mapping analysis of the scale
formed on TiAlN coated T-22 boiler steel is presented in Fig. 10. The BSE image indicates the
formation of a dense scale consisting mainly of iron, oxygen and chromium with small amounts
of titanium and manganese. The X-ray mapping indicates thin irregular bands of Cr parallel to
each other, where Cr is present & Fe is absent.
In case of AlCrN coated T-22 boiler steel subjected to cyclic oxidation in air, the BSEI and X-
ray mapping are shown in Fig. 11. The scale formed is dense and adherent to the substrate
without any crack in the scale or at substrate-scale interface. The X-ray mapping indicates the
presence of iron and oxygen throughout the scale. A thin band of Al, Cr and N is also indicated
by X-ray mapping, where iron and oxygen are completely absent.
Nearly parabolic behaviour followed by the coated and bare steels during cyclic oxidation study.
The parabolic kinetic behavior is due to the diffusion controlled mechanism operating at 900°C
under cyclic conditions . Small deviation from the parabolic rate law might be due to the
cyclic scale growth. The higher weight gain during the first few cycles might be attributed to the
rapid formation of oxides at the splat boundaries and within the open pores due to the penetration
of the oxidizing species, further the subsequent increase in weight is gradual . The parabolic
rate constant for the TiAlN coated T-22 boiler steel is found to be greater then the uncoated and
AlCrN coated T-22 boiler steel.
AlCrN coating has been found successful in reducing the overall weight gain of bare T-22 boiler
steel by 69%. The oxidation rate (total weight gain values after 50 cycles) of the coated and
uncoated T-22 boiler steel follows the sequence as given below:
TiAlN coating > Uncoated > AlCrN coating
During cyclic testing, cracks in the oxide scale (Fig.7.a) and spalling of the coatings might be
attributed to the different values of thermal coefficients for the coating and the substrate as
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1053
reported by Buta Singh Sidhu et al. , Harpreet Singh et al. , Evans & Taylor , Wang et
al.  and Niranatlumpong et al .
Fig. 9: Composition image (BSI) and X-ray mapping of the cross-section of the uncoated T-22
boiler steel subjected to cyclic oxidation in air at 900°C for 50 cycles (X 80).
Further Niranatlumpong et al.  opined that spallation could be initiated by the rapid growth
of void–like defects lying adjacent to coating protuberances, at which tensile radial stress
developed during cooling as a result of the thermal contraction mismatch between the oxide and
coating is maximum. The formation of cracks in the coating originates from stresses developed
in the deposit or at the coating-base metal interface . Though these cracks the corrosive
environment can quickly reach the base metal and cut its way under the coating to result in
adhesion loss and spalling, whereas some elements may diffuse outwards through these cracks to
form their oxides or spinels . The presence of Fe, Mo and oxygen (Fig.7.a,b and c) in the top
of the scale, is believed to be due to the diffusion of iron and molybdenum through the pores
and cracks that appeared in the coating during the course of oxidation studies .
Inferior oxidation resistance of TiAlN coating may be as opined by Xing-zhao Ding et al. .
According to them, in an oxidation or corrosive environment Ti element often forms a porous
non-protective oxide scale, which limits the oxidation and corrosion resistance of titanium-based
coatings. This has further been reported by Kazuhisa Fujita , where authors have reported
1054 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
that the oxide scale of binary TiAl alloy is composed of a porous oxide mixture of TiO2 and
Al2O3, which has dominated by TiO2. This might be the reason for rapid increase in oxidation
rate of TiAlN coatings after 20th cycle (Fig. 3).
Fig.10: Composition image (BSI) and X-ray mapping of the cross-section of the TiAlN coated
T-22 boiler steel subjected to cyclic oxidation in air at 900°C for 50 cycles (X 130).
W. Kalss et al.  have reported, at 900 °C an oxidized layer of thickness of about 350nm for
TiAlN coating was measured. Due to high temperature the segregation of titanium and aluminum
atoms is probable. Evidently, high temperature oxidation involved diffusion of aluminum atoms
to the surface to form a thin aluminum oxide top layer while the remaining titanium under
layered formed titanium dioxide. At 950 °C, the whole coating was decomposed and pure
titanium dioxide was formed , which is supported by B.Y. Man et al. as they have
Vol.9, No.11 Oxidation Behavior of Nanostructured TiAlN and AlCrN 1055
reported at 900°C a peak ascribed to Fe2O3 oxide , which corresponds to a failure of the TiAlN
Fig.11: Composition image (BSI) and X-ray mapping of the cross-section of the AlCrN coated
T-22 boiler steel subjected to cyclic oxidation in air at 900°C for 50 cycles (X 726).
The presence of Fe2O3 in the upper layer of the scale is followed by a subscale in which Fe2O3
and Cr2O3 are present, as indicated by the XRD (Fig.6), EDAX (Fig. 8.b) and X-ray mapping
(Fig. 10). Figure 10 indicated the presence of Cr in the subscale along with oxygen and iron. This
can be attributed to the depletion of iron due to oxidation to form the upper scale, thereby leaving
chromium-rich pockets those have further oxidized to form parallel and irregular chromium
oxide bands . These bands of chromium oxide may have prevented the deep penetration of
the environment, as the scale thickness is less in case of oxidized TiAlN coated T-22 boiler steel
then that of uncoated boiler steel.
The surface morphology, Fig. 7(c), of T-22 steel oxidized after coating with AlCrN indicates the
dense dimple like surface appearance along with light grey porous surface like matrix. The
EDAX point analysis shows, the top scale is rich in Fe, Mo and O. The scale formed is dense and
adherent to the substrate without any crack in the scale or at substrate-scale interface. A thin
band of Al, Cr and N is indicated by cross-sectional EDAX analysis (Fig.8.c) and X-ray mapping
(Fig.11), where iron and oxygen are completely absent. It clearly shows that coating remains
adherents to the substrate during the course of oxidation run. At some points thin film has gaps;
1056 V. Chawla, A. Chawla, B. S. Sidhu , S. Prakash and D. Puri Vol.9, No.11
Fe and Mo have moved through the openings and got oxidized to form the scale. Further good
oxidation resistance for this type of coating has also been observed by W. Kalss et al. .
The AlCrN coating has provided good resistance against oxidation in air at 900 °C for 50 cycles
and provided the necessary protection to the base metal. The oxidation rate (total weight gain
values after 50 cycles) of the coated and uncoated T-22 boiler steel followed the sequence:
TiAlN coating > Uncoated > AlCrN coating
The AlCrN coating has sustained during the course of oxidation study. At some points thin film
has gaps; Fe and Mo have moved through the openings and got oxidized to form the scale. The
TiAlN coating has failed to provide the protection to the base metal during the oxidation. This
might be due to the formation of oxide scale which is composed of a porous oxide mixture of
TiO2 and Al2O3, with the domination of TiO2. The internal oxidation has been observed in all the
cases. The difference in thermal expansion coefficients between oxides, coating and base steel
perhaps led to the cracking of the oxide scale and coatings.
The authors wish to thank All India Council for Technical Education (A.I.C.T.E.), New Delhi,
India for providing National Doctoral Fellowship (NDF) to Dr. Vikas Chawla (corresponding
author) and Nationally Coordinated Project (NCP).
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