TECHNICAL PAPER TP 2814
High Temperature Erosion Performance of Nanostructuredand Conventional TiAlN Coatings on AISI-304 Boiler SteelSubstrate
Jasmaninder Singh Grewal • Buta Singh Sidhu •
Satya Prakash
Received: 23 August 2013 / Accepted: 8 April 2014
� The Indian Institute of Metals - IIM 2014
Abstract According to modern design philosophy better
overall performance can be obtained with the modification
of the surface structure and their properties without dam-
aging underlying bulk material or substrate. The surface
engineering can be classified in two broad classes: surface
modification and surface coating. In the present research
TiAlN coating was deposited on AISI-304 grade boiler
steel using three different techniques, out of which two
were thin nano coatings deposited at different temperatures
of 500 and 200 �C developed by Oerlikon Balzers rapid
coating system machine under a reactive nitrogen atmo-
sphere. One conventional coating of TiAlN was deposited
by plasma spraying method. The coated samples were
characterized relative to their coating thickness, microh-
ardness, porosity and micro structure. The optical micros-
copy, the X-ray diffraction analysis and field emission
scanning electron microscope (FESEM with EDAX
attachment) analysis have been used to identify various
phases formed after coating deposition on the surface of
AISI-304 grade boiler steel. The erosion studies were
conducted on uncoated as well as coated specimens in
simulated coal fired boiler environment using an air jet
erosion test rig at various impingement angles of 30�, 60�and 90�. The alumina particles of average size of 50 lm
were used as erodent at a velocity of 35 m/s. The eroded
samples were analysed with SEM/EDAX and optical pro-
filometer. The main objective of this research work was to
increase the life of boiler tubes by using nanostructured and
conventional TiAlN coatings and at the same time to
compare the performance of coatings with respect to bare
AISI-304 grade boiler steel. The nanostructured TiAlN
coatings has shown minimum erosion rate as compared to
conventional TiAlN coating and uncoated AISI-304 grade
boiler steel. Maximum erosion was observed at an angle of
30� as compared to 60� and 90� indicative ductile
behaviour.
Keywords Solid particle erosion �Nanostructured coatings � Physical vapour deposition
1 Introduction
Solid particle erosion is the progressive loss of original
material from a solid surface due to mechanical interaction
between that surface and solid particles. Erosion is a seri-
ous problem in many engineering systems, including steam
and jet turbines, pipelines and valves used in slurry trans-
portation of matter and fluidized bed combustion systems
[1–3]. Gas and steam turbines operate in environments
where the ingestion of solid particles is inevitable. In
industrial applications and power generation systems such
as coal burning boilers, fluidized beds and gas turbines,
solid particles are produced during the combustion of
heavy oils, synthetic fuels and pulverized coal. Moreover,
the shape of the particles and angle of impingement plays
an important role in the erosion of materials [4–7].
It is now a generally accepted practice to apply coatings
to the components in fossil fuel energy generation
J. S. Grewal (&)
Department of Production Engineering, Guru Nanak Dev
Engineering College, Ludhiana, India
e-mail: [email protected]
B. S. Sidhu
PTU, Kapurthala, India
S. Prakash
Department of Metallurgical and Materials Engineering,
IIT Roorkee, Roorkee, India
123
Trans Indian Inst Met
DOI 10.1007/s12666-014-0413-8
processes to provide thermal insulation, erosion and wear
resistance and in chemical process plants or boilers to
protect the surface of structural steels against surface
degradation processes such as wear, corrosion and erosion
[8, 9]. However, thermal spraying is an effective and low
cost method to apply thick coatings to change surface
properties of the component [10]. For more than four
decades plasma spraying has been used to deposit a wide
range of metals, ceramics and even composite materials for
many different applications [11]. Bulk nanostructured
materials (in general referring to a grain size smaller than
100 nm) have exhibited outstanding mechanical properties
such as exceptional hardness, yield strength and wear
resistance [12, 13].
Plasma assisted physical vapour deposition processes
(PAPVD) allow the deposition of metals, alloys, ceramic
and polymer thin films onto a wide range of substrate
materials. Nanostructured materials indeed behave differ-
ently than their microscopic counterparts because their
characteristic sizes are smaller than the characteristic
length scales of physical phenomenon occurring in bulk
materials [14–16].
In this work nanostructured and conventional titanium
aluminium nitride coatings were deposited on AISI-304
grade boiler steel substrate. The emphasis has been put on
the influence of nanostructured TiAlN coatings in com-
parison with conventional TiAlN coatings on the erosion
behaviour of Fe-based AISI-304 grade boiler steel. Nitride
based coatings deposited on AISI-304 stainless steels,
reduce the total wear rate by half with respect to (wrt) bare
stainless steel [17].
2 Experimental Details
2.1 Selection of Substrate Material
AISI-304 grade boiler steel has been selected as substrate
after consultation with Guru Nanak Dev Thermal Plant
Bathinda (India). The nominal composition of the material
is compared with the actual composition analysed by using
Brammer Standard 84-E stainless steel with Atomic
Emission Spectrometer (AES-DV4) at Research and
Development Centre for Bicycle and Sewing Machine at
Ludhiana, India as shown in Table 1.
Specimens of approximate dimensions of 20 mm 9
15 mm 9 5 mm were cut from the substrate sheet and then
polished with emery papers of 220, 400, 600 grit sizes,
subsequently on 1/0, 2/0, 3/0 and 4/0 grades and finally
mirror polished using cloth polishing wheel machine with
1 lm lavigated alumina powder suspension.
2.2 Development of Coatings
A front loading Balzers rapid coating system (RCS)
machine was used for the deposition of nanostructured thin
coatings by PAPVD process at Oerlikon Balzers Coating
India Ltd., Gurgaon, India. Two coatings namely Balinit
Futura Nano i.e. thin nano TiAlN coating at 500 �C tem-
perature with composition of Ti-50 % and Al-50 % in the
atmosphere of nitrogen and Balinit Futura Nano Arctic i.e.
thin nano TiAlN coating at 200 �C temperature with again
the same composition of Ti-50 % and Al-50 % in the
atmosphere of nitrogen were deposited on the substrate. A
conventional thick coating TiAl was deposited by plasma
spraying technique at Anod Plasma Spray Ltd., Kanpur,
India. The nitriding of which was done in the laboratory at
IIT Roorkee. The summary of the coating deposition
parameters of both nano and conventional is given below in
Tables 2 and 3. The cross sectional image of as coated
nanostructured TiAlN coating deposited at 500 and at
200 �C and conventional TiAlN coatings deposited on
AISI-304 grade boiler steel are shown in Fig. 1.
2.3 Erosion Studies in Simulated Coal Fired Boiler
Environment
The erosion studies were carried out using a high temper-
ature air-jet erosion test rig (Fig. 2). The erosion test
conditions utilized in the present study are listed in
Table 4. The impact velocity of 35 m/s has been selected
as per the ASTM G7607 standard under clause 9.1.4. A
standard test procedure was employed for each erosion test.
The uncoated as well as the coated specimens were pol-
ished down to 1 lm alumina wheel cloth polishing to
obtain similar condition on all the samples before being
subjected to erosion run. The samples were cleaned in
acetone, dried, weighed to an accuracy of 1 9 10-5 g
using an electronic balance, eroded in the test rig for 3 h
and then weighed again to determine weight loss. In the
present study irregular shaped alumina (Al2O3) of 50 lm
size was used as erodent. The scanning electron micro-
scope (SEM)/EDAX of alumina (Al2O3) is shown in Fig. 3.
Erosion rates in terms of volumetric loss (mm3/g) for
Table 1 Chemical composition (wt%) of AISI-304 grade boiler steel
Elements C Mn Si Cr Ni P S Other elements Fe
Nominal 0.08 2.00 1.00 18.0–20.0 8.0–10.5 0.045 0.03 – Balance
Actual 0.07 1.14 0.33 18.46 8.12 0.028 0.012 – Balance
Trans Indian Inst Met
123
different uncoated and coated alloys were compared. The
eroded samples were analyzed with SEM/EDAX and
optical profilometer. The erosion rate data for each coated
alloy has been plotted along with uncoated alloy in order to
assess the coating performance. Efforts have been made to
understand the mechanism of erosion.
3 Results and Discussion
3.1 Coating Microstructure and Properties
The typical microstructure of PAPVD coatings of nano-
structured TiAlN deposited at 500 and 200 �C has been
shown in Fig. 4a, b, has a dense and nano-layered structure,
identical to the results of Yang et al. [18, 19]. All the
TiAlN coatings with different Al concentrations showed
small finest grains as reported by Yang et al. [18] and a
dense structure with a strong (111) texture of CrTiAlN
coating is reported by Yang et al. [19].
The mechanical properties of plasma and D-gun sprayed
coatings are anisotropic because of splat structure and
directional solidification as reported by Hocking [20]
which corroborates the results of plasma sprayed, post
nitrided conventional TiAlN coating (Fig. 4c) of the pres-
ent investigation which indicate the coating is homoge-
nous, massive and free from cracks. D-gun spray process is
a thermal coating process which gives good adhesive
strength, low porosity and coating surface with compres-
sive residual stresses. Hearley et al. [21] also reported
Fig. 1 Cross sectional image of as coated a nano-structured TiAlN
coating deposited at 500 �C, b nano-structured TiAlN coating
deposited at 200 �C and c conventional TiAlN coating deposited on
AISI-304 grade boiler steel
Table 2 Summary of nano coating deposition parameters
Machine used Standard Balzers rapid coating system
(RCS) machine
Make Oerlikon Balzers, Swiss
Targets composition For TiAlN coating: Ti, Ti50Al50
Number of targets Ti(02), Ti50Al50 (04)
Targets power (kW) 3.5
Reactive gas Nitrogen
Nitrogen deposition
pressure (Pa)
3.5
Substrate bias voltage (V) -170 to -40
Substrate temperature
(�C)
450 ± 10
Coating thickness (lm) 4 ± 1
Table 3 Summary of conventional coating deposition parameters
Arc current (A) 750
Arc voltage (V) 45
Powder feed rate (rev/min) 5.2
Spraying distance (mm) 90–110
Plasma arc gas (argon) pressure (psi) 58
Powder gas pressure (psi) 60
AUX gas pressure (psi) 10
Trans Indian Inst Met
123
identical results of distinctive splats or lamellae associated
with thermal spray process, in case of HVOF sprayed NiAl
intermetallic coating with good homogeneity and
uniformity.
Hardness is the most frequently quoted mechanical
property of the coatings. The microhardness of the con-
ventional thick TiAlN coating was found to be in the range
of 900–950 Hv which is almost identical to the findings of
Adachi and Nakata [22], Chen and Hutchings [23], Vuor-
isto et al. [24] and Westergard et al. [25]. The hardening of
the conventional TiAlN coating observed in the current
study might have occurred due to high speed impact of the
coating particles during plasma deposition similar to the
findings of Sidhu et al. [26] and Hidalgo et al. [27].
The bond strength of the conventional TiAlN coating
was measured on three specimens as per ASTM standard
C633-01. This test method covers the determination of the
degree of adhesion (bonding strength) of a coating to a
substrate or the cohesion strength of the coating in a ten-
sion normal to the surface. The test consists of coating one
face of a substrate fixture, bonding this coating to the face
of a loading fixture, and subjecting this assembly of coating
and fixtures to a tensile load normal to the plane of the
coating. A data acquisition system has continuously
recorded the tensile load exerted by the machine. It is
adapted particularly for testing coatings applied by thermal
spray, which is defined to include the combustion flame,
plasma arc, two wire arc, high-velocity oxygen fuel and
detonation processes for spraying feedstock, which may be
in the form of, wire, rod or powder. Average bond strength
Fig. 2 Experimental set-up for erosion–corrosion in simulated coal-fired boiler environment a air jet erosion tester and b interior view of
specimen loading chamber
Table 4 Erosion test conditions
Erodent material Alumina (irregular shape)
Erodent specifications 50 lm Al2O3
Particle velocity (m/s) 35
Erodent feed rate (g/min) 2
Impact angle (�) 30, 90
Test temperature Sample temperature 400 �C and air
temperature 900 �C
Nozzle diameter (mm) 4
Nozzle to sample distance (mm) 10
Test time (h) 3
Trans Indian Inst Met
123
Fig. 3 Analysis of alumina (Al2O3), a FESEM morphology and b EDAX compositional analysis
Fig. 4 Optical micrograph (9200) of the surface of as coated AISI-304 grade boiler steel a nanostructured TiAlN coating at 500 �C,
b nanostructured TiAlN coating at 200 �C and c conventional TiAlN coating
Trans Indian Inst Met
123
of 68.77 MPa was observed which is almost in agreement
with the results reported by Adachi and Nakata [22].
The grain size of nanostructured thin coatings was
estimated from Scherrer’s formula i.e. D = 0.9k/B cosh,
where B is the corrected full width at half maximum of
Bragg’s peak, k is X-ray wavelength and h is the Bragg’s
angle. The calculated grain size for nanostructured TiAlN
coating deposited at 500 and 200 �C was found to be 15
and 14 nm, respectively, which are nearly equal to the
values of nanostructured TiAlN coatings reported by Yang
et al. [19], Pei et al. [28], Yoo et al. [29], Falub et al. [30]
and Man et al. [31].
The porosity analysis is having prime importance in
high temperature erosion studies. The dense coatings are
supposed to provide good resistance as compared to
porous coating. The porosity measurements were made
with PMP3 inverted metallurgical microscope with ste-
reographic imaging. The porosity of as sprayed nano-
structured TiAlN coatings deposited at 500 and 200 �C
is lower than or nearly equal to the porosity values
reported by Chawla et al. [14], Braic et al. [32], Grzesik
et al. [33] and Alberdi et al. [34] for TiAlN nanocom-
posite coatings. The porosity of as sprayed conventional
TiAlN coating after gas nitriding was less than 0.6 %
which is in accordance with the results reported by
Lackner et al. [35], Ohnuma et al. [36] and Leyendecker
et al. [37].
The microstructural and mechanical properties of
nanostructured thin TiAlN coatings deposited at 500 and
200 �C are given in Table 5 and of the conventional TiAlN
thick coating are given in Table 6.
3.2 Erosion Rate Wrt Impingement Angle
Impingement angle is one of the most important parameters
for the characterizing the erosion behaviour of materials. In
general different materials exhibit different response to the
impingement angle. The macrographs for uncoated and
coated AISI-304 grade boiler steel subjected to erosion studies
in simulated coal fired boiler environment are shown in Fig. 5.
The volume erosion rate of the samples is given in Table 7.
The shape of the scar (developed by constant strike of
erodent) is circular in case of normal impact at 90�, semi-
elliptical at 60� and elliptical in case of oblique impact of
30� of the erodent. The uncoated AISI-304 grade boiler steel
has shown the thin scale. The erosion seems to clean off the
scale of the surface in the eroded region. The impact of
erodent removes the scale down to the substrates–scale
interface. Away from this eroded region a thin layer of scale
was observed on the surface and the eroded region showed
rust coloured discolouration and the scar was surrounded by
dark rust coloured thin ring further surrounded by brown
ring. The nanostructured TiAlN coated AISI-304 grade
boiler steel has shown clear marks of erosion. The colour of
the coated specimen has been changed from violet grey to
whitish around the scar and further surrounded by blackish
blue ring. The colour of the scars was observed as dark grey
surrounded by whitish grey colour ring. In the case of
conventional thick TiAlN coating the formation of dark grey
coloured scar surrounded a white coloured thin ring, further
surrounded by light brownish black ring was observed.
For erosion damage, there are two dominant mecha-
nisms i.e. the ductile type damage, through cutting and
Table 5 Microstructural and mechanical properties of nanostructured thin TiAlN coatings at 500 and 200 �C on AISI-304 grade boiler steel
Coatings Surface
roughness
(nm)
Particle size (nm) Porosity
(% age)
Coating
thickness
(lm)
Micro hardness
(Hv 0.05)
Elastic modulus,
E (GPa)
Coating
colorsScherrer formula
Nanostructured TiAlN
coating at 500 �C
3.70 15 \0.5 5.9 3,300 343 Violet–grey
Nanostructured TiAlN
coating at 200 �C
3.65 14 \0.5 6.0 3,300 375 Light violet–grey
Table 6 Microstructural and mechanical properties of conventional thick TiAlN coating on AISI-304 grade boiler steel
Coating Surface roughness
(lm)
Coating thickness
(lm)
Porosity (% age) Bond strength
(MPa)
Micro hardness
(Hv)
Coating
colorsAs
sprayed
After gas
nitriding
Conventional TiAlN
coating
10.35–15.45 174 2.30–4.25 \0.6 68.77 900–950 Grey
Trans Indian Inst Met
123
brittle type damage by cracking. Impact angle is defined as
the angle between the target material and the trajectory of
the erodent. Dependence of erosion rate on the impact
Fig. 5 Surface macrograph of
uncoated and coated AISI-304
grade boiler steel exposed to
high temperature erosion studies
in stimulated coal fired boiler
environment
Table 7 Volume erosion rate (10-3 9 mm3/g) at different impact angles
Impact
angles (�)
Uncoated AISI-304 grade
boiler steel
Nanostructured TiAlN coating
deposited at 500 �C
Nanostructured TiAlN coating
deposited at 200 �C
Conventional TiAlN
coating
90 0.2783 0.03741 0.03659 1.8217
60 0.3428 0.04343 0.04132 1.9837
30 0.499 0.05608 0.05439 2.103
Fig. 6 Impact angle influence on erosion rate in case of ductile and
brittle materials (Sundararajan et al. [48])
Fig. 7 Influence of impingement angle on erosion rate of nanostruc-
tured TiAlN coatings deposited at 500 and 200 �C and conventional
TiAlN coating
Trans Indian Inst Met
123
angle is largely determined by nature of the target mate-
rials. There is a dramatic difference between ductile and
brittle materials when the weight loss in erosion is mea-
sured as a function of impact angle as reported by Bhushan
and Gupta [38]. Brittle erosion deals with the material
removal due to crack formation whereas ductile erosion
occurs due to exclusion of microplatelets of the base
material from craters which then flattened and fractured.
The erosion rate of ductile materials (like metals and
alloys) is maximum at intermediate impact angles (15�,
30�) as shown in Fig. 6 whereas the maximum erosion rate
of brittle materials (like glass) is usually obtained at normal
impact angle i.e. 90�. This occurs because ductile materials
are capable of absorbing the large amount of energy pro-
duced by the impacting particles without fracture and
brittle materials are capable of withstanding large amount
of shear stress. In general ductile material fail in shear
before tension and brittle materials fail in tension before
shear. For semibrittle materials such as transition metal
nitrides, cutting is the major erosion mechanism at a low
impingement angle, while cracking plays a more significant
role in erosion damage process at a high impingement
angle. The erosion rate wrt impact angle of uncoated AISI-
304 grade boiler steel, nanostructured TiAlN coating
deposited at 500 and 200 �C and conventional TiAlN
coating has been shown in Fig. 7 indicated that maximum
erosion took place at 30� which indicate ductile behaviour
as proposed by Murthy et al. [39]. The authors Finnie et al.
[40–42], Shimizu and Noguchi [43], Oka et al. [44],
Wellman and Allen [45] recognized the basis of erosion
mechanism that maximum erosion occurs at shallow angle
of 20–30� for ductile materials and brittle materials reaches
higher erosion rates at higher angles of 80–90� which
corroborates the results of present research. Most of the
metallic materials irrespective of temperature exhibit a
ductile behaviour i.e. a maximum erosion rate at oblique
impact angles reported by Tabakoff and Vittal [46].
However, Hutchings [47] and Sundararajan and Roy [48]
reported the angular dependence of erosion is not a char-
acteristic of material alone but also depends upon the
conditions of erosion and hence suggests that the terms
brittle and ductile in the context of erosion should therefore
be used with caution. This leads to the further detailed
microscopic analysis.
Basically the erosion rate of the coatings was obtained
by dividing the weight loss per gram of erodent with the
coating density [19]. The erosion rate of nanostructured
TiAlN coatings due to continuous impact of erodent par-
ticles i.e. aluminium oxide of 50 lm size at 30� impact
angle is minimum as compared to conventional TiAlN
coating and bare AISI-304 boiler steel. The results of dif-
ferent impact angles i.e. 30�, 60� and 90� considered in the
present study are almost identical to the observations made
by Yang et al. [18, 19], which reported ductile behavior at
30� impact angle and poor erosion resistance.
In erosion testing the material is eroded by continuous
impact of eroded particles, the erosion starts at the centre
first then proceeds towards the edges of the samples as
exactly happened in the present investigation, similar to the
Fig. 8 Column chart showing
the volume wear rate of
uncoated and coated AISI-304
grade boiler steel eroded at
impact angles of 30�, 60� and
90�
Fig. 9 Surface scale morphology and EDAX patterns on different
locations on eroded uncoated and coated AISI-304 grade boiler steel
exposed to high temperature erosion in simulated coal fired boiler
environment at impingement angle of 30�, a uncoated AISI-304 grade
boiler steel, b nanostructured TiAlN coating deposited at 500 �C,
c nanostructured TiAlN coating deposited at 200 �C and d Conven-
tional TiAlN coating
c
Trans Indian Inst Met
123
Trans Indian Inst Met
123
findings reported by Mann and Arya [49], Stack et al. [50,
51]. The steady state erosion rate column chart (Fig. 8) has
shown that nanostructured TiAlN coatings are weakly
dependent upon the angle of impingement and has a small
difference in erosion rate at 30�, 60� and 90� impact angles
whereas in case of conventional TiAlN coating erosion rate
is quiet substantial. This is in the complete agreement with
the study of Tabakoff and Vittal [46]. Hearley et al. [21]
have shown similar behavior for NiCr coatings.
The impact velocity is one of the key parameters and
can be regulated by varying the air pressure. The impact
velocity of 35 m/s selected for the present study in accor-
dance with ASTM G7607 standard under clause 9.1.4 is
almost similar to the impact velocities selected by Mishra
et al. [52] i.e. 40 ± 3 m/s for coated cobalt based alloys for
boiler steels.
3.3 Morphology and Erosion Mechanism
The mechanism by which material is removed from coating
under erosive conditions may be either ductile or brittle.
Microstructural characteristics enhancing erosion resis-
tance include high compactness, low porosity, fine grain
size, good adhesion and absence of cracks [53–55]. From
Fig. 4 it is clear that higher erosion resistance of PAPVD
sprayed nanostructured coatings are closely correlated with
their fine structure, high compactness and low porosity in
addition to their favourable compositions. In contrast
conventional TiAlN coating exhibited the highest erosion
wastage among the coatings tested which was attributed to
its large splat size, high porosity and fine cracks network.
Figure 9 has shown the typical morphologies of the
surfaces of the uncoated AISI-304 grade boiler steel,
nanostructured TiAlN coating deposited at 500 and 200 �C
and conventional coating of TiAlN eroded at high tem-
perature of 900 �C with erodent aluminium oxide at an
angle of impingement of 30�. The erodent impingements
produced rougher elliptical surface at the centre area of the
erosion scar with deeper erodent cutting marks which
corresponds to a higher erosion rate. At glancing
impingement, erosion is dominated by cutting with no
coating removal through cracking in case of nanostructured
coatings of TiAlN. Due to its high hardness, the TiAlN
coating exhibits high resistance to the coating removal
through cutting. The identical findings are given by Yang
et al. [18, 19] i.e. in case of CrTiAlN coatings at low
impingement angles of 15� and 30�. Sidhu et al. [56] and
Bellman and Levy [57] also reported identical results as
solid particle erosion rate of substrate steels is maximum at
30� impact angle. The material subjected to erosion ini-
tially undergoes plastic deformation and is later removed
by subsequent impacts of the erodent on the surface. Due to
cutting by erodent particle, lips and ridges are formed at the
bank of the grooves which are fractured or removed from
the grooves with further erosion.
It is well known that the mechanism of high temperature
erosion always involves first the oxidation of the surface
and then the removal of the oxide pieces and this process is
effected by the oxidation rate of the material and the
adherence between the oxide layer and the substrate [27].
Alloys that are developed for heat and oxidation resistance
typically form a protective layer of chromia or alumina.
The more rapidly this layer is established, the better pro-
tection is offered [26]. However in case of conventional
TiAlN coating the sample surfaces at high temperature i.e.
900 �C were more irregular and showed small pieces of the
coating were chipped off. At this temperature the oxidation
of the coating has affected its erosion behavior and the
erosion of the oxide layer seems to be the main weight loss
mechanism.
It is clear from Fig. 9d that there are some radial and
lateral cracks as well as loosened pieces of the eroded
specimen surfaces. Cracking is thought to form first at splat
boundary during impact of particles. Under continual
impacting of particles the radial and lateral cracks are
developed. Finally small voids and pits are formed. On the
other hand the uncoated AISI-304 grade boiler steel
showed finer and smoother morphology (Fig. 9a) which
corresponds to its lower erosion wastage as compared to
conventional TiAlN coating. However the eroded surface
having some evidence of deformation indicated by small
craters, striations and indentations. Similar findings are
given by Wang and Verstak [58].
Figures 10 and 11 has shown the morphology of the
surfaces at 60� and 90�, respectively. The erosion is again
ductile in nature at both the impact angles, the coating
being hard and with uniform homogeneity, the results of
the present investigation are identical to the findings of
Hearley et al. [21] for erosion rate as a function of impact
angle for NiAl intermetallic coatings produced by thermal
spraying.
At impingement of angles higher than 30� the eroded
nanostructured TiAlN coatings surface has also shown
cutting marks produced by eroded particles whereas in case
of conventional TiAlN coating it appears to have local
valleys where the coating has experienced more material
removal through cracking as compared to nanocoatings of
TiAlN. Even more valleys were observed at 90� than 60�.
Fig. 10 Surface scale morphology and EDAX patterns on different
locations on eroded uncoated and coated AISI-304 grade boiler steel
exposed to high temperature erosion in simulated coal fired boiler
environment at impingement angle of 60�, a uncoated AISI-304 grade
boiler steel, b nanostructured TiAlN coating deposited at 500 �C,
c nanostructured TiAlN coating deposited at 200 �C and d conven-
tional TiAlN coating
c
Trans Indian Inst Met
123
Trans Indian Inst Met
123
Trans Indian Inst Met
123
The SEM examination revealed that TiAlN coating
behaves like a semiductile material which experiences
solid particle erosion damage involving both cutting and
cracking.
The lower erosion rate of nanostructured TiAlN coatings
implies that these coatings have a higher resistance to
cracking at 90� or in other words a higher toughness and
also should be attributed to their favourable microstructure
with more uniform distribution of smaller TiAl particles,
lower porosity and oxidation rates. These findings are
identical to the observations made by Yang et al. [19] and
Wang and Shui [59].
3.4 Effect of Porosity
The porosity content of the nanostructured coatings of
TiAlN has been measured less than 0.5 %, due to which
dense structured coatings gave good erosion resistance,
whereas conventional coating after nitriding was having
porosity value less than 0.6 %, which is in agreement with
Sidhu et al. [26], as the nitriding of TiAl has increased the
density of the coatings and this made the structure dense.
Hearley et al. [21] reported that coating porosity is often
located along the lamella boundaries. Not only it will
influence the strength of the inter lamella bonding but may
also initiate micro cracking leading to loss of lamellae and
thus removing the coating. Similar results are reported in
case of conventional TiAlN coating of the present inves-
tigation, thus indicating that nanostructured TiAlN coatings
are having better porosity performance than conventional
TiAlN coating wrt erosion rate.
4 Conclusions
(1) The conventional thick TiAlN coating (by plasma
spraying followed by gas nitriding process) and the
nanostructured thin coatings of TiAlN at temperatures
of 500 and 200 �C were successfully deposited on
AISI-304 boiler steel.
(2) During solid particle erosion test both the nanostruc-
tured TiAlN coatings performed better than the
conventional TiAlN coating and uncoated AISI-304
boiler steel at all the three impingement angles of 30�,
60� and 90�.
(3) Nanostructured TiAlN coatings show fine, dense,
uniform and nanolayered microstructure, whereas
conventional TiAlN coating showed homogenous,
massive structure and free from cracks.
(4) No evidence of macro or micro cracking was
observed in the nanostructured TiAlN coatings,
however conventional TiAlN coating showed some
micro cracks after erosion.
(5) Ductile behaviour is indicated in case of uncoated
AISI-304 boiler steel and nanostructured TiAlN
coatings whereas conventional TiAlN coating showed
semiductile behavior.
References
1. Kosel T H, Lubrication and Wear Technology. ASM Handbook,
Vol. 18, ASM International, Materials Park (1992), p 199.
2. Zhang Y, Cheng Y B S, and Lathabai S, Wear 240 (2000) 40.
3. Wood R J K, Mater Des 20 (1999) 179.
4. Getu H, Spelt J K, and Papini M, Wear 292–293 (2012) 159.
5. Shimizu K, Xinba Y, and Araya S, Wear 271 (2011) 1357.
6. Wang X, Fang M, Zhag L, Ding H, Liu Y, and Yang J, Mater
Chem Phys 139 (2013) 765.
7. Laguna-Camacho J R, Marquina-Charvez A, Mandez-Mandez J
V, Vite-Torres M, and Gallardo-Hernandez E A, Wear 301(2013) 398.
8. Wang B Q, Geng G Q, and Levy A V, Surf Coat Technol 54–55(1992) 529.
9. Ramesh M R, Prakash S, Nath S K Sapra P K, and Vanktaraman
B, Wear 269 (2010) 197.
10. Turunen E, Varis T, Gustafsson T E, Keskinen J, Falt T, and
Hannula S-P, Surf Coat Technol 200 (2006), 4987.
11. Ding C, Chen H, Liu X, and Zeng Y, in Thermal Spray 2003.
Advancing the Science and Applying the Technology, (eds) Mo-
reau C, and Marple B, ASM International, Materials Park (2003),
p 455.
12. Leblanc L, in Thermal Spray 2003. Advancing the Science and
Applying the Technology, (eds) Moreau C, and Marple B, ASM
International, Materials Park (2003), p 291.
13. Luo H, Goberman D, Shaw L, and Gell M, Mater Sci Eng A 346(2003) 237.
14. Chawla V, Sidhu B S, Puri D, and Prakash S, J Aust Ceram Soc
44 (2008) 56.
15. Fedrizzi L, Rossi S, Cristel R, and Bonora P L, Electrochimica
Acta 49 (2004) 2803.
16. Chawla V, Puri D, Prakash S, Chawla A, and Sidhu B S, J Miner
Mater Charact Eng 8 (2009) 715.
17. Alegria-Ortega J A, Ocampo-Carmona L M, Suarez-Bustamante
F A, and Olaya-Florez J J, Wear 290–291 (2012) 149.
18. Yang Q, Seo D Y, Zhao L R, and Zeng X T, Surf Coat Technol
188–189 (2004) 168.
19. Yang Q, Zhao L R, Cai F, Yang S, and Teer D G, Surf Coat
Technol 202 (2008) 3886.
20. Hocking M G, Surf Coat Technol 62 (1993) 460.
21. Hearley J A, Little J A, and Sturgeon A J, Wear 233–235 (1999)
328.
22. Adachi S, and Nakata K, Surf Coat Technol 201 (2007) 5617.
23. Chen H, and Hutchings I M, Surf Coat Technol 107 (1998) 106.
24. Vuoristo P, Niemi K, Makela A, and Mantyla T, in Proc 7th
National Thermal Spray Conference, Boston, Massachusetts
(1994), p 121.
Fig. 11 Surface scale morphology and EDAX patterns on different
locations on eroded uncoated and coated AISI-304 grade boiler steel
exposed to high temperature erosion in simulated coal fired boiler
environment at impingement angle of 90�, a uncoated AISI-304 grade
boiler steel, b nanostructured TiAlN coating at 500 �C, c nanostruc-
tured TiAlN coating at 200 �C and d conventional TiAlN coating
b
Trans Indian Inst Met
123
25. Westergard R, Erickson L C, Axen N, Hawthorne H M, and
Hogmark S, Tribol Int 31 (1998) 271.
26. Sidhu B S, Puri D, and Prakash S, Mater Sci Eng A368 (2004)
149.
27. Hidalgo V H, Varela J B, Menendez A C, and Martinez S P, Wear
247 (2001) 214.
28. Pei Y T, Galvan D, De Hosson J Th M, and Cavaleiro A, Surf
Coat Technol 198 (2005) 44.
29. Yoo Y H, Le D P, Kim J G, Kim S K, and Vinh P V, Thin Solid
Films 516 (2008) 3544.
30. Falub C V, Karimi A, Ante M, and Kalss W, Surf Coat Technol
201 (2007) 5891.
31. Man B Y, Guzman L, Miotello A, and Adami M, Surf Coat
Technol 180–181 (2004) 9.
32. Braic M, Balaceanu M, Braic V, Vladescu A, Pavelescu G, and
Albulescu M, Surf Coat Technol 200 (2005)1014.
33. Grzesik W, Zalisz Z, Krol S, and Nielslony P, Wear 261 (2006)
1191.
34. Alberdi A, Marin M, Diaz B, Sanchez O, and Galindo R, Vacuum
81 (2007) 1453.
35. Lackner J M, Waldhauser W, Ebner R, Keckes J, and Schoberl T,
Surf Coat Technol 177–178 (2004) 447.
36. Ohnuma H, Nihira N, Mitsuo A, Toyoda K, Kubota K, and Ai-
ziwa T, Surf Coat Technol 177–178 (2004) 623.
37. Leyendecker T, Lemmer O, Esser S, and Ebberink J, Surf Coat
Technol 48 (1991) 175.
38. Bhushan, B, and Gupta B K, Handbook of Tribology: Material
Coatings and Surface Treatments, McGraw-Hill, New York
(1991).
39. Murthy J K N, Rao D S, and Venkataraman B, Wear 249 (2001)
592.
40. Finnie I, Wear 3 (1960) 87.
41. Finnie I, Wolak J, and Kabil Y, J Mater 2 (1967) 682.
42. Finnie I, Wear 19 (1972) 81.
43. Shimizu K, and Noguchi T, Wear 176 (1994) 255.
44. Oka Y I, Ohnogi H, Hosohawa T, and Matsumura M, Wear
203–204 (1997) 573.
45. Wellman R G, and Allen C, Wear 186–187 (1995) 117.
46. Tabakoff W, and Vittal B V R, Wear 86 (1983) 89.47. Hutchings I M, Tribology: Friction and Wear of Engineering
Materials, Metallurgy and Material Science Series, Edward
Arnold Publications, London (1992).
48. Sundararajan T, and Roy M, Tribol Int 30 (1997) 339.
49. Mann B S, and Arya V, Wear 249 (2001) 354.
50. Stack M M, Stott F H, and Wood G C, Wear 162–164 (1993) 706.
51. Stack M M, Purandare Y, and Hovsepian P, Surf Coat Technol
188–189 (2004) 556.
52. Mishra S B, Chandra K, Prakash S, and Venkataraman B, Surf
Coat Technol 201 (2006) 1477.
53. Raask E, Erosion Wear in Coal Utilization, Hemisphere, Wash-
ington, DC (1988).
54. Stridh B, Hedenqvist P, Olsson M, and Soderberg S, in Proc 7th
International Conference on Erosion by Liquid and Solid Impact,
Cavendish Laboratory, Cambridge (1987), paper 19.
55. Levy A V, and Wang B Q, Wear 121 (1988) 325.
56. Sidhu H S, Sidhu B S, and Prakash S, Surf Coat Technol 202(2007) 232.
57. Bellman R Jr, and Levy A, Wear 70 (1981) 1.
58. Wang B Q, and Verstak A, Wear 233–235 (1999) 342.
59. Wang B Q, and Shui Z R, Wear 253 (2002) 550.
Trans Indian Inst Met
123