Tribological Aspects of Thermally Sprayed Red Mud-Fly Ash and Red Mud-Al Coatings on Mild Steel

Abstract

The present research work explains the dry sliding wear characteristics of thermally
sprayed red mud and its composite coatings. Composite of red mud, fly ash and
aluminium are plasma sprayed at 6, 9, 12 and 15 kW operating power levels. The
coatings were experimented to study the tribological behaviour like dry sliding wear
behaviour, XRD phase transformation, coating thickness, coating morphology, wear morphology, wear mechanism and co-efficient of friction. 10% Fly ash and 5% aluminium
powder were mixed separately by weight with pure red mud and sliding wear test was
conducted using pin on disc wear test machine. The test was performed with track
diameter of 100 mm and at sliding speed of 100 rpm (0.523 m/s) at a normal load of 20 N.
Coating property was discriminated by variation of sliding time length. Operating power
was found to be remarkable variable for wear rate, coating thickness, coating morphology
and friction coefficient. Significant increase in wear resistance was observed with fly ash
and aluminium addition, resulting in an increase in interfacial bond strength and dense film
formation.

Full text

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*Corresponding author: Email: h.k.sutar@gmail.com;
American Chemical Science Journal
4(6): 1014-1031, 2014
ISSN: 2249-0205
SCIENCEDOMAIN international
www.sciencedomain.org
Tribological Aspects of Thermally Sprayed Red
Mud-Fly Ash and Red Mud-Al Coatings on Mild
Steel
Harekrushna Sutar1*, Subash Chandra Mishra1, Santosh Kumar Sahoo1,
Himanshu Sekhar Maharana1and Ananta Prasad Chakraverty1
1Department of Metallurgical and Materials Engineering, National Institute of Technology,
Rourkela, 769008, Odisha, India.
Authors’ contributions
This work was carried out in collaboration between all authors. Author HS designed the
study, performed the experiments wrote the protocol and wrote the first draft of the
manuscript. Authors HSM and APC managed the analyses of the study. Authors SCM and
SKS supervised. All authors read and approved the final manuscript.
Article Information
DOI: 10.9734/ACSj/2014/12691
Editor(s):
(1) Zhi-Cheng TAN, Thermo chemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian, China.
(2) Sang Hak LEE, Professor, Department of Chemistry, Kyungpook National University Daegu, 702-701, KOREA.
Reviewers:
(1) Anonymous, University of Ibadan; Nigeria.
(2) Anonymous, RIEIT, India.
(3) Anonymous, East Point College of Engineering and Technology, India.
Peer review History: http://www.sciencedomain.org/review-history.php?iid=528&id=16&aid=5884
Received 13th July 2014
Accepted 14th August 2014
Published 25th August 2014
ABSTRACT
The present research work explains the dry sliding wear characteristics of thermally
sprayed red mud and its composite coatings. Composite of red mud, fly ash and
aluminium are plasma sprayed at 6, 9, 12 and 15 kW operating power levels. The
coatings were experimented to study the tribological behaviour like dry sliding wear
behaviour, XRD phase transformation, coating thickness, coating morphology, wear
Original Research Article

American Chemical Science Journal, 4(6): 1014-1031, 2014
1015
morphology, wear mechanism and co-efficient of friction. 10% Fly ash and 5% aluminium
powder were mixed separately by weight with pure red mud and sliding wear test was
conducted using pin on disc wear test machine. The test was performed with track
diameter of 100 mm and at sliding speed of 100 rpm (0.523 m/s) at a normal load of 20 N.
Coating property was discriminated by variation of sliding time length. Operating power
was found to be remarkable variable for wear rate, coating thickness, coating morphology
and friction coefficient. Significant increase in wear resistance was observed with fly ash
and aluminium addition, resulting in an increase in interfacial bond strength and dense film
formation.
Keywords: Red mud; fly ash; aluminium; plasma coating; sliding wear behaviour.
ABBREVIATIONS
SEM: Scanning Electron Microscope; FESEM: Filed Emission Scanning Electron
Microscope; EDS: Energy Dispersion Spectroscopy; XRD: X-Ray Diffraction; N: Newton;
F: Frictional Force in N; µ: Co- efficient of Friction; R: Applied normal load in N; l: Length in
mm; Ø: Diameter in mm; ∆m: Mass Loss in mg.
1. INTRODUCTION
In the present scenario coating technologies manifest a promising momentum for emerging
materials. Wear resistive coatings claim to be better tribological applications. Surface
modification by improving wear resistance is most widely adopted by plasma spra ying
technique, which could affirm a great versatility and its application to a wide spectrum of
materials. Wear resistive coatings can protect against different wear mediums like abrasive,
adhesive and corrosive. Some common wear resistive coating materials are nickel, iron,
cobalt and molybdenum based alloys, carbides of ceramic and tungsten [1 -2]. Investigations
pertaining to the erosion wear behaviour of plasma sprayed ceramic coatings by using
Taguchi Technique being reported [3]. Evaluation and characterization of plasma sprayed
Cu slag-Al composite coatings on metal substrates has been outlined [4].
In retrospection, literatures made available regarding the wear behaviour of WC with 12%
Co coatings produced by Air Plasma Spraying method at different standoff distances [5].
Examinations on the basis of the wear behaviour of Mo and Mo+NiCrBSi thermally sprayed
coatings being performed for the application as next generation ring face coatings [6].
Almost all plasma sprayed ceramic coatings portrayed favourable tribological performance in
linear contact at high temperatures: high anti-wear resistance and easy to be lubricated
owing to the oil storage of pores in coatings [7-9]. But needful to say, plasma sprayed
ceramic coatings exhibit some failure mechanisms during sliding such as plastic
deformation, brittle fracture and polish effects [10], which in turn demands a few additives,
which could reduce the friction and wear of plasma sprayed ceramic coatings [11].
Several factors may influence the tribological behaviour of a coated surface ramified as: the
geometry of the contact including macro geometry and topography of the surfaces; the
material characteristics; basic mechanical properties as well the microstructure and finally
the operating parameters controlling the coating deposition [12].

American Chemical Science Journal, 4(6): 1014-1031, 2014
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Red mud as an industrial waste material is considered to be the material of choice for
coating applications. It is behoving to mention here that, red mud in present decade should
be considered as an alternative wealth for replacing some conventional expensive coating
materials. Utilization of red mud and its implications made available in literature [13] in great
details. Few results on the basis of wear behaviour of red mud were being reported by some
researchers. In addition to above, morphology and solid particle erosion wear behaviour of
red mud and fly ash composite were being available in literature [14]. Characteristics of
plasma sprayed pure red mud coatings were being reported [15]. Red mud as filling material
is also found to be the wear enhancing agent for metals [16].
Data pertaining to dry sliding wear behaviour of fly ash and aluminium added red mud
composite coatings are not available in literature and needs to be addressed. The present
investigation is an attempt to report the wear behaviour of fly ash, aluminium and red mud
composite coatings at different operating power subjected to normal laboratory conditions.
2. MATERIALS AND METHODS
2.1 Formulation of Coating Precursor
The coating powder was formulated considering the raw materials as red mud (RM), fly ash
(FA) and aluminium (Al). The powder mixture made up of red mud and different percentage
of fly ash and aluminium was being prepared separately. In addition, pure red mud powder
was also considered as coating material for the sake of comparison analysis. Coating of the
various combinations of mixed powders was conducted on one side cross section of the mild
steel substrate. Table 1 shows the detail amalgam of the powder composite.
Table 1. Powders used for coating deposition
Sl.no.
Coating material
Mixture composition (by weight %)
1
Red Mud
100
2
Red Mud +Fly Ash
90 + 10
3
Red Mud +Aluminium
95 + 05
The primary raw material as red mud was collected in powder form from National Aluminium
Company (NALCO) located at Damonjodi in the state of odisha, India. The as-received
powder was sieved to obtain particles in the required size range of 80 -100 µm. Raw fly ash
was collected from the captive power plant of Rourkela steel plant, India and sieved to
maintain the same size range as that of red mud powder. Aluminium powder is procured
from Rourkela market and sized to the same size range. Coating powders were extensively
prepared by thorough mixing using V shaped blender.
2.2 Development of Substrate
The principal source for substrate preparation is mild steel rod available commercially. The
bar was chopped to l = 42 mm and Ø= 15 mm each. The test pieces were grit blasted from
one side cross section at a pressure of 4 kg/cm2using alumina grits of grit size 80. The dead
lock distance in the shot blasting was in-between 120-150 mm. The mean roughness of the
cross sections was found to be 5.8 µm. Hereafter the sample pieces are scrubbed in an
ultrasonic cleaning unit followed by immediate plasma spraying.

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2.3 Plasma Spraying Operation
Plasma spraying was headed up at the Laser and Plasma technology division of Bhab ha
Atomic Research Centre, Mumbai, India by arrogating conventional atmospheric plasma
spraying (APS) set up. The plasma input power was ranged from 6 to 15 kW by steering the
gas flow rate, voltage and arc current. The powder feed rate was maintained to be constant
at 15gm/min by using a turntable type volumetric powder feeder. Plasma generation
demanded the suitability by purging Argon as primary and Nitrogen as secondary gas agent.
Spraying was done at an angle of 90º by maintaining the powder feeding as external to the
gun. The detailed operating parameters are revealed in Table 2.
Table 2. Operating parameters during coating deposition
Operating parameters
Values
Plasma arc current (Ampere)
200,225,250,300
Arc voltage (Volt)
30,40,48,50
Torch input power (kW)
6,9,12,15
Plasma gas(Argon), (litre/min)
20
Secondary gas (Nitrogen), (litre/min)
2
Career gas(Argon) Flow rate (litre/min)
7
Powder feed rate (gm/min)
15
Torch to base distance (mm)
110
Arc length range (mm)
2,3,6,8,11
Fig. 1. Comprehensive outline of pin on disc machine

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2.4 Pin on Disc Wear Trial
The experiments were conducted using a pin on disc type friction and wear monitor allotted
by MAGNUM Engineers, Bangalore, India. The detailed experimental set up is materialized
in (Fig. 1a above). The tool possesses a data acquisition system (Fig. 1b above). The
concerned machine was used to evaluate the wear behavior of the coatings against
hardened ground steel disc (En-32) having hardness of 65 HRC and surface roughness (Ra)
0.5 µm. The gadget is designed so as to study the wear behavior both at lubricated and un-
lubricated sliding condition, which occurs between a stationary pin and a rotating disc.
A D.C motor is implicated for rotating the disc with range of speed from 0 -250 rpm with wear
track diameter 0-150 mm; yielding a sliding speed of 0-15 m/s. Dead weight was applied on
the pin by means of pulley and string arrangement. The system has a maximum loading
capacity of 400 N. The device is fabricated to keep the pin specimen stationary and
perpendicular to disc, while the circular disc spins anti cloc kwise as shown in
(Fig. 1c above).
3. RESULTS AND DISCUSSION
3.1 SEM and EDS Analysis of Precursor and Coatings
A SEM (JEOL; JSM-6480 LV) was epitomized to outline the red mud. The SEM image and
EDS analysis of pure red mud powder were manifested in (Fig. 2). The atomic and weight %
of elements existent in pure red mud powder is laid in (Table 3). The EDS analysis of red
mud revealed the existence of elements like Fe, Al, Si, O and some other minor constituents.
The key constituent of red mud was proved to be iron and with its oxides. The EDS analysis
of red mud with 10% fly ash coatings prepared at 6 kW of operating power was shown in (Fig.
3). The corresponding elemental analysis was reported in (Table 4), indicating the increase in
silica and iron components in the composite coating. The EDS analysis for red mud and 5 %
aluminium composite coating made at 6 kW operating power level is shown in (Fig. 4). The
elemental analysis analogous to (Fig. 4) was reported in (Table 5), indicating the increase in
aluminium content.
Fig. 2. (a) SEM and (b) EDS analysis of red mud

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Table 3. Elemental analysis of red mud
Element
Weight%
Atomic%
C K
24.59
33.29
O K
21.65
24.54
Al K
9.41
4.47
Si K
11.21
7.07
Fe K
33.14
30.62
Totals
100.00
Fig. 3. EDS analysis of RM+10 % FA Composite coating at 6kW
Fig. 4. EDS analysis of RM+ 5 % Al Composite coating at 6 kW
3.2 XRD Analysis of Red Mud and Coatings
To ascertain the phases present and phase changes taken place during plasma spraying,
the X-ray diffractograms were captured for red mud powder and its composite coatings using
a Philips X-ray diffractometer. The XRD result for red mud powder is shown in (Fig.5).The
major phases of red mud powder such as Silicon dioxide (SiO2), Calcium Alumino ferite
(Ca2Fe1.28Al0.75O5), Hematite (Fe2O3), Titanium dioxide (TiO7) and Aluminide (AlFe3) were
noticed.

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Table 4. Elemental analysis of RM+10 % FA Composite coated at 6 kW
Element
Weight %
Atomic %
Fe K
38.13
25.90
O K
21.74
42.61
Ti K
14.02
9.18
Si K
15.10
7.93
Al K
6.59
7.66
Cr K
2.14
1.29
C K
1.99
5.20
Ca K
0.29
0.22
Au K
0.00
0.00
Mg K
0.00
0.00
Totals
100 %
100
Table 5. Elemental analysis of RM+5 % Al Composite coated at 6kW
Element
Weight %
Atomic %
Fe K
39.13
26.89
O K
22.74
41.61
Ti K
9.00
9.18
Si K
14.10
7.93
Al K
11.61
5.46
Cr K
1.14
2.39
C K
1.99
5.20
Ca K
0.29
0.22
Au K
0.00
0.00
Mg K
0.00
0.00
Totals
100 %
100
Fig. 5. XRD of Red Mud Powder collected from NALCO

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1021
For red mud with 10 % fly ash coatings at 6 and 12 kW operating power level the XRD
patterns are shown in (Fig. 6). For 6 kW power level (Fig. 6a) the major phases are silicon
aluminium oxides (Si0.9016 Al0.0984O2), whereas for 12 kW power level (Fig. 6b) the major
phase is changed to pyrophosphate (NaFeP2O7).
Fig. 6. XRD of RM + 10 % FA Coatings; (a) 6 kW, (b) 12 kW
XRD patterns differentiated for red mud with 5 % aluminium coating is shown in (Fig. 7). At 6
kW operating power level (Fig. 7a) the major phases are silicon dioxide (SiO2), hematite and
aluminium are observed, whereas for 9 kW (Fig. 7b) the major phases changed to calcium
carbonate (CaCO3), magnesium oxides (MgO) and manganese oxides (MnO2). Again for 12
kW power level (Fig. 7c) the major phases found are Calcium Carbonate and Hematite.
Significant changes in phases are perceived for 15 kW operating power level. Most of the
elements have formed oxides like aluminium iron oxides (AlFeO 3) and other elemental
oxides as perceptible in (Fig. 7d).
3.3 Investigation of Coating Morphology
The coating surface morphology depends on inter particle bonding of the sprayed powders
and coating surface adherence [17]. In case of red mud and 10 % fly ash composite coating
the surface morphology deposited at different operating power level are presented in (Fig. 8)
implementing FESEM (Nova Nano SEM-450).For coating made at 6 kW, the morphology
(Fig. 8a) shows non uniform distribution of bigger globular particles. Cavitation is observed,
which may be originated at the time of solidification of particle s. As the power level increases
the structural pattern significantly affected. Smaller globular particles and more flattened
regions are observed indicating proper melting of the particles during spraying. Cavitation
decreases and uniformity is improved. This might be due to the proper particle to particle
bonding and improved stacking rate to the substrate, which have been resulted in increased
interfacial bond strength and more dense film at boundary.
The coating morphology for red mud with 5 % aluminium is visible in (Fig. 9). In this case the
operating power has no significant effect on structural properties. For all operating power
level small globular grains are seen and fused together to form a splat area. Very few
cavities are seen, might be originated at inter particle boundary regions. Addition of

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aluminium to red mud might have helped in joining of molten particles during in flight
traverse as aluminium remains in molten state for longer time after leaving plasma jet.
Fig. 7. XRD of Red Mud + 5 % Al; Coatings (a) 6 kW, (b) 9 kW, (c) 12 kW, (d) 15 kW
3.4 Investigation of Coating Thickness
Thickness of Red Mud + 5 % aluminium coatings on mild steel substrate are measured on
polished cross-sections of the samples using FESEM. Five readings were taken on each
sample and the values are averaged. The mean coating thickness is increased with
operating power. The minimum coating thickness is found to be 195 µm for 6 kW operating
power. The maximum coating thickness is reported to be 873 µm for 15 kW power. Results
are revealed in (Fig. 10). With increase in power level thermal flux increases, this increases
the enthalpy of the system. When feed material passes through plasma most of the
precursor powder particles get melted and also some disintegrated which flies off. The
molten particles which are at much higher temperature when strikes the substrate get
splatted and deposition volume increases thereby causing the rise in coating thickness with
power. The influence of operating power level on coating thickness is graphically
represented in (Fig. 11).

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Fig. 8. Coating Morphology for RM + 10 % FA; (a) 6 kW, (b) 9 kW, (c) 12 kW (d) 15 kW
3.5 Diagnosis of Sliding Wear Rate
At incipient, the pin and disc surfaces are polished duly with emery papers for ensuring a
smooth contact with coating samples. The wear tests were conducted as per ASTM G-99
standard for different time range under un-lubricated (dry) condition in a normal laboratory
condition at ambience temperature and relative humidity. The specimens were weighed
before and after each wear test by using electronic weighing machine with accuracy up to
second decimal limit (0.001 mg). Specimens were taken care of in particular by cleaning with
woolen cloth to avoid entrapment of wear debris and to maintain a distinct wear mechanism.
The specimens were cleaned with tetrachloroethylene solution before and after each test.
Wear rate was expressed as mass loss (∆m) of the coated specimen after each test. Co -
efficient of friction was assessed using the correlation F = µR [18];

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Fig. 9. Coating Morphology for RM + 5 % Al; (a) 6 kW, (b) 9 kW, (c) 12 kW (d) 15 kW
Where F is frictional force in N. R is applied normal load in N. µ is co-efficient of friction. The
frictional force (F) was quantified directly from the data acquisition tool in N at each time
intervals.
All wear experiments were carried out at a constant normal load of 20 N and a fixed speed of
100 rpm (0.523 m/s).The track diameter was fixed at 100 mm. The total duration of sliding
time was retained according to the extent of coating layer occurrence. The minimum and
maximum duration of sliding time was 54 and 90 minutes respectively.
The first step of the experiment includes the sliding wear of pure red mud coatings for
different operating power level. Mass loss against sliding time is pictured in (Fig. 12). The
total duration of sliding time prevails up to 54 minute. At beginning, up to 6 minute a
moderate mass loss was observed. Afterwards from 6 to 36 minute a steady mass loss was
detected. Then the situation of constant mass loss initiates and titled as break -in phase. The
break-in phase continues up to end of the experiment.

American Chemical Science Journal, 4(6): 1014-1031, 2014
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Fig. 10. Snapshot of Coating thickness for RM + 5 % Al (a) 6 kW, (b) 9 kW, (c) 12 kW,
(d) 15 kW
It is interesting to notice that the mass loss rate declines with increasing operating power
level up to 12 kW. This is the optimum power level in our experiment. For 15 kW power level
the mass loss rate lies in between 9 and 12 kW. This is surprising and not obvious, may be
due to improper particle to particle bonding and poor stacking to the substrate which in turn
lowered the hardness as well as density due to poor interfacial bond strength formation at
border line.
(Figs. 13 and 14) represents the mass loss against sliding time for Red Mud + 10 % FA and
Red Mud + 5 % Al respectively considering all coating power level. The optimum power level
for minimum mass loss lied as the same as earlier for all coating type. The results are
differentiated by sliding time length and reaching the break-in phase in behind time. For Red
Mud + 10 % FA coating the sliding time prevailed up to 75 minute with break -in phases
launched at 54 minute. Whereas for Red Mud + 5 % Al coating the sliding time continued up
to 90 minute and break-in phase originated at 66 minute.

American Chemical Science Journal, 4(6): 1014-1031, 2014
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Fig. 11. Graphical presentation of RM+5% Al composite coating thickness with
operating power
Fig. 12. Mass loss curves for red mud

American Chemical Science Journal, 4(6): 1014-1031, 2014
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Fig. 13. Wear plot for RM + 10 % FA
Fig. 14. Wear plot for RM + 5 % Al
Fig. 15. Wear rate comparison for all coating type at different operating power level
(a) 6 kW, (b) 9 kW , (c) 12 kW (d) 15 kW

American Chemical Science Journal, 4(6): 1014-1031, 2014
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The variation of wear rates (mass loss) for each coating type with sliding time were
compared for different operating power level in (Fig. 15 above). This prominently displays a
comprehensive recognition to the readers. From the results it is obvious that a significant
growth in wear resistance was contrived with fly ash and aluminium addition to red mud
resulting in stronger interfacial bond strength and dense film formation at boundary layer.
Fig. 16. Coefficient of friction for all Coating type at 6 kW
Fig. 17. Comparison of coefficient of friction for RM + 5% Al at all power level
A graphical representation concerning the variation of coefficient of friction with sliding time
is also focused. At 6 kW operating power level and for all coating type the coefficient of

American Chemical Science Journal, 4(6): 1014-1031, 2014
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friction plot is presented in (Fig.16 above). For all sliding time the coefficient of friction is
maximum for red mud and minimum for Red Mud + 5 % Al coating. At incipient, the frictional
values are extreme followed by a curly trend and finally static values. Frictional values are
compared for different operating power level for Red Mud + % 5 Al coating in (Fig. 17
above). As like mass loss an optimum operating power level also observed for friction
coefficient. The effectiveness of frictional force is maximum at 6 kW and minimum at 12 kW
was detected.
Fig. 18. Wear images for Red Mud + 5 % aluminium coating for 9 kW operating power
level; considering (a) 15, (b) 30, (c) 45, (d) 60, (e) 75 and (f) 90 minutes time interval

American Chemical Science Journal, 4(6): 1014-1031, 2014
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Wear morphology for designated coating samples were emphasized using FESEM. (Fig. 18
above) illustrates the wear images for Red Mud + 5 % aluminium coating (for 9 kW operating
power level) considering for time intervals of 15, 30, 45, 60, 75 and 90 minutes. Wear scars,
debris and cracked sections formed are being visible. The wear morphology pattern modifies
with time impacting the surface roughness leading to interruption of its contact mechanism.
The wear characteristics recasts due to variation of hardness of the inter layers with time.
After the break-in phase, increase in sliding distance cannot reform the contact area causing
a relatively steady wear rate. Hence it can be winded up that the wear takes place by
abrasion and adhesion mechanism due to development of shear stress in between hard
particles of the two surfaces in contact. The same mechanism is relevant for all coating type.
4. CONCLUSION
The present experimental study remarks some noteworthy conclusions. Red mud, the waste
generated from alumina plant is significantly coat able on mild steel using thermal plasma
spraying technique with acceptable wear resistance. Addition of fly ash and aluminium with
red mud diminishes the wear rate by amplifying the coating mechanism. It is observed that
initially the wear rate increases at a slow pace and then boosted up with sliding distanc e for
all coating type and finally becomes still. Operating power level authenticates as remarkable
variable for coating property; which enriches the coating resistance, but eventually with
reaching an optimum value indicating some other governing parameters. This work is a
portfolio for future investigators to discover many other aspects of red mud and its composite
coatings. Thermal security of these coatings may be evaluated for better claiming in high
temperature applications. Corrosive wear behavior under different operating conditions may
be explored to identify suitable application areas. Post heat treatment of these coatings may
also be fetched to study regarding the improvement in coating features.
COMPETING INTERESTS
Authors have declared that no competing interests exist.
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