B. G. NAGULAN et al. INFLUENCE OF THE PROCESS PARAMETERS ON HIGHVELOCITY OXYGEN-FUEL-SPRAYED ...
653–659
INFLUENCE OF THE PROCESS PARAMETERS ON
HIGH-VELOCITY OXYGEN-FUEL-SPRAYED 75Cr
3
C
2
-25NiCr
COATINGS
VPLIV PROCESNIH PARAMETROV NA IZDELAVO
75Cr
3
C
2
-25NiCr PREVLEK S POSTOPKOM NAPR[EVANJA PRI
VISOKIH HITROSTIH
BalaGanesh Nagulan, SenthilKumar Thamilkolundu, Chandrasekar Murugesan
Anna University, University College of Engineering, Department of Mechanical Engineering, Bit-Campus, Tiruchirappalli 620024,
Tamil Nadu, India
balaganeshnagulan@gmail.com
Prejem rokopisa – received: 2017-12-28; sprejem za objavo – accepted for publication: 2018-05-25
doi:10.17222/mit.2017.224
In this work 75Cr3C2-25NiCr powder was sprayed on stainless steel with various pressures and powder-feed rates utilizing the
HVOF method. The microstructure of the coating and the micro-hardness and porosity, micro-hardness, indentation fracture
toughness, adhesion strength and wear resistance at 490 °C of the surface coatings were examined. The outcomes demonstrated
that HVOF sprayed 75Cr3C2-25NiCr coatings had low porosity, high micro-hardness and enough adhesion strength. The powder
feed rate had an evident impact on the porosity, micro-hardness and indention fracture toughness of the coatings, and the coating
sprayed under the powder feed rate of 35 g/min had the ideal execution. The wear-resistance test showed that the HVOF
indicated that the 75Cr3C2-25NiCr surface coatings had great influence on the wear resistance at 490 °C, in which the coating
sprayed at the powder flow rate of 35 gram/min had the best wear resistance because of its thick and dense structure and having
enough fracture toughness.
Keywords: 75Cr
3
C
2
-25NiCr, coating, adhesion strength, powder feed rate, wear resistance
Avtorji v ~lanku opisujejo, kako so prah zlitine 75Cr3C2-25NiCr napr{evali na nerjavno jeklo s postopkom plamenskega
napr{evanja pri visokih hitrostih (HVOF; angl.: High-Velocity Oxy-Fuel Method). Pri tem so uporabili razli~ne tlake oz. hitrosti
nana{anja kovinskega prahu. Nato so preu~evali mikrostrukture nastalih prevlek, njihovo mikrotrdoto in poroznost, lomno
`ilavost, dolo~eno z metodo vtiskovanja indenterja (piramide), adhezijsko trdnost ter odpornost proti obrabi pri 490 °C.
Rezultati so pokazali, da imajo s HVOF postopkom izdelane 75Cr3C2-25NiCr prevleke majhno poroznost, visoko mikrotrdoto in
zadovoljivo adhezijsko trdnost. Hitrost nana{anja (napr{evanja) kovinskega prahu je imela opazen vpliv na poroznost,
mikrotrdoto in lomno `ilavost prevlek. Idealna hitrost napr{evanja je bila 35 g/min. Pri teh pogojih je imela 75Cr3C2-25NiCr
prevleka tudi najve~jo odpornost proti obrabi pri 490 °C, ker je bila tako izdelana prevleka primerno debela, z ustrezno gosto
strukturo in zadovoljivo lomno `ilavostjo.
Klju~ne besede: 75Cr3C2-25NiCr, prevleka, adhezijska trdnost, hitrost napr{evanja, odpornost proti obrabi.
1 INTRODUCTION
Cermet coatings are generally utilized as a part of
modern applications against tribological degradation,
even in the eroding condition, because of their high
hardness and the great resistance of the utilized metal
matrix. Chromium-carbide-based materials are, by and
large, utilized to create hard coatings for high-tempe-
rature wear applications, including sliding, fusing,
scraped spot, and disintegration in the powder generation
industry, avionic business, oil refining industry, heat-
treatment rolls, and coal-consuming heater tubes.
1,2
The
chromium-carbide nickel chromium (75Cr
3
C
2
-25NiCr)
cermet coatings have been broadly practiced in industry,
due to the profitablemix of the 75Cr
3
C
2
phase of ceramic
with great wear resistance and the 75NiCr metal stage,
which gives high oxidation resistance at high tem-
perature.
3,4
The procedure of high-speed oxygen fuel
(HVOF) splash ends up being the most encouraging
procedure, which implies it is desirable over store cermet
coatings, for example, Cr
3
C
2
-NiCr and WC–Co. This
method joins high-speed powder particles with low
temperature to develop a thick and firmly adhesive
coating with residual stress and low oxidation.
5
Coatings
kept by the HVOF procedure show high density, low po-
rosity, and brilliant adhesion strength, with substantially
more carbide particles held in the matrix in contrast to
those saved by the plasma splashing process. It has been
accounted for that HVOF-sprayed Cr
3
C
2
-NiCr coatings
display great wear resistance and erosion resistance in
different high-temperature conditions.
6–9
Numerous
specialists revealed that the execution and microstructure
of cermet coatings was overwhelmingly impacted by the
spray conditions.
10
The research of Qun Wang et al.
11
outlined that a difference in the spraying parameters
demonstrated little impact on the phase structure of the
WC–12Co coatings, however, it affected different perfor-
mances, for example, hardness, porosity, fracture
toughness, and the per-pass thickness of the coatings. It
Materiali in tehnologije / Materials and technology 52 (2018) 5, 653–659 653
UDK 669.018:621.793.7:669.14.018.8:544.3.032 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(5)653(2018)

additionally reasoned that the arrangements of signifi-
cance of coating parameters on the performances of
coatings are as per the following: fuel flux > spray
distance > feed rate > oxygen flow for hardness; spray
distance > fuel flow > feed rate > oxygen flow for
porosity; oxygen flux > kerosene flux > feed rate > spray
distance for fracture toughness; and feed rate > spray
distance > oxygen flow > kerosene flow for per-pass
thickness. H. Fukutome et el.
12
examined the impact of a
low proportion of oxygen to propene on the phase and
abrasive wear of a HVOF Cr
3
C
2
-20%NiCr coating and
presumed that the best wear performance could be
acquired when the flow proportion moved towards the
chemical ratio, above which the Cr
2
O
3
would exist. It has
been accounted for that the hardness, fracture toughness
of the matrix, fortifications, and the interfacial strength
of the support/matrix are the most vital mechanical
variables that decide the wear resistance and the dis-
appointing properties of cermet-based coatings.
13,14
The
impact of fracture toughness on coating wear perfor-
mance was explored generally. High fracture toughness
was useful for the wear resistance of the coatings.
14
Gang
Chang Ji announced the evacuation of carbide particles
in the coating was chiefly responsible for the abrasive
wear of the coating. The substance and molecule size of
the Cr
3
C
2
carbides were the two key variables controlling
the abrasive wear of the HVOF-coated Cr
3
C
2
-NiCr
coatings.
15,16
In the present work, three levels of
75Cr
3
C
2
-25NiCr coatings produced by HVOF were
coated under various powder feed rates utilizing Hipojet
2700 system, Jodhpur, India, Aum Surface Technologies,
Bangalore. All the coatings were analysed by X-ray
diffraction (XRD), optical microscope, image analyser,
scanning electron microscope (SEM) including energy-
dispersive spectroscopy (EDS), micro-hardness analyser,
and nano-indenter. The wear resistance and wear me-
chanisms of the 75Cr
3
C
2
-25NiCr coatings at 490 °C were
researched and investigated.
2 EXPERIMENTAL PART
The 75Cr
3
C
2
-25NiCr feedstock powders of circular
shape and a grain size of approx. 45 μm to 10 μm were
coated on SS304 stainless-steel substrates. Three sorts of
coatings were acquired by HVOF and the spraying
parameters were recorded as Table 1. The Hipojet 2700
was utilized to obtain the coating. The powder was
axially fed via the air. Prior to spraying, the stainless-
steel plates of 10 mm × 10 mm × 5 mm were grit-blasted
with alumina to accomplish a surface roughness (R
a
)of
around 7 μm to 8 μm and after that cleaned in a ultra-
sonic bath of acetone in order to improve the adhesion
strength between the coatings and substrate. The sub-
strates are shown in Figure 7. The thickness of the
as-sprayed coatings was around 200 μm.
The wear performance of the coated specimens
measurements, i.e., 10 mm × 10 mm × 5 mm, was tested
using a ball-on-disk (BOD) high-temperature wear
analyser. Before the test, the coated examples were
cleaned and afterward cleaned ultrasonically with ace-
tone and alcohol. The coated specimens were mounted
immovably in the example holder and were permitted to
be pressed against the rim of the rubber wheel with a
normal pressure by applying a known dead weight
(21 N). The test was performed at 490 °C for 3550 s to
acquire the friction coefficient of the coatings. The wear
counterpart was the WC ball, whose width was 6 mm.
The wear rate of the coatings was measured by volume
wear loss. After the HVOP spraying, the examples were
cut into pieces of 10 mm × 10 mm×5mmandag ain
cleaned ultrasonically with acetone and alcohol. The
cross-sectional micrograph of the coatings was obtained
by optical microscopy. The well-worn surface of the
coatings was likewise analysed by SEM/EDS and a sur-
face profiler. The porosity was measured with an
imaging apparatus. For each specimen, the mean estima-
tion of the eight readings was ascertained for measuring
the porosity. The X-ray diffraction (XRD) investigation
of the coating was made at first glance with a Vega3
Tescan, Czech Republic. It is operated at 40 kV and 40
mA, utilizing Cu-K
 as radiation. The composition of the
coating powder were examined utilizing Energy-Dis-
persive X-beam Spectroscopy (EDS) and an Oxford
Instruments framework, crest potentially excluded 3.698
KeV, handling alternative: all components broke down,
number of cycle = 3 The bond strength of the coatings
was tested with the ASTM adhesion test standard.
17
The
microhardness of the as-sprayed coatings was measured
by a Wilson-402 MVD microhardness tester at 1 kg and
for a stacking span of 15 s. The normal estimation of the
five readings for every example was figured. The
indentation fracture of the coating toughness was tried
by indentation mechanical assembly of Vickers indenter
Model: VH 1150, Wilson. The indentation was made on
the cross-segment of the coatings in the mid-plane locale
to limit the edge and interface impact. The indenter was
stacked so one of the even diagonals was parallel to the
interface of the coating and substrates. A weight of 2 kg
was connected for a dwell time of 20 s at the settled rate.
A nano-indention test was used on the surface of coating
with a G200 Nano-indenter For each example, 15 read-
ings were obtained on the cleaned surface using a similar
parameter, and the indention was acquired by a micro-
scope. Meanwhile, the empty to stack curve was
acquired.
B. G. NAGULAN et al. INFLUENCE OF THE PROCESS PARAMETERS ON HIGHVELOCITY OXYGEN-FUEL-SPRAYED ...
654 Materiali in tehnologije / Materials and technology 52 (2018) 5, 653–659
Table 1: Spray parameters of the three coatings.
Coating
Powder
Feed Rate
(g/min)
Fuel
(LPM)
Oxygen
(LPM)
Spray
Distance
(mm)
A 30 30 150 200
B 35 33 160 225
C 40 36 170 250

3 RESULTS
The XRD examples of the 75Cr
3
C
2
-25NiCr powder
and three coatings are shown in Figure 1.P e a k so f
carbides (Cr
3
C
2
) and binder phase (NiCr) were observed
in both the powder and coatings. The carbides phase of
Cr
7
C
3
was found in the coating, which was framed by the
decarburization of Cr
3
C
2
. In the examination of the
powder and coatings, there was an exceptionally wide
peak focused cycle 43° in the example of the coatings.
All the peaks of nickel had a strong arrangement and the
Cr
3
C
2
was noticeably expansive, and an extremely wide
peak revolved at around 43° showed up in the as-sprayed
state of three coatings. The extremely wide peak demon-
strated the arrangement of an undefined phase because of
the high cooling rate of HVOF process. In this way, it
was demonstrated that the nickel–chromium alloy and
some Cr
3
C
2
carbide were dissolve and some amorphous
phase framed. The comparable outcomes were likewise
detailed in those past investigations observed the change
of the Cr
3
C
2
phase to the Cr
23
C
6
phase while spraying
75Cr
3
C
2
-25NiCr powder by the jet procedure. Nonethe-
less, this change was not found in this examination. One
conceivable reason was that the living arrangement time
of the particles in the flame was significantly less for the
spraying system with fluid fuel.
18,19,20
Three XRD exam-
ples of coatings were about the same and demonstrated
little decarburization of the Cr
3
C
2
, which showed little
impact of spraying parameters on the coating phase
structure. It outlined the coating-B had more Cr
3
C
2
stage,
which implied moderately little decarburization of the
Cr
3
C
2
into Cr
7
C
3
of the coating B. It has been accounted
for that Cr
3
C
2
and binder NiCr were the real phases of
HVOF sprayed chromium carbide-based coating, and the
decarburization of Cr
3
C
2
into Cr
7
C
3
or Cr
23
C
6
did not
have an inconvenient impact on the wear resistance of
the coatings.
21
The cross-sectional micrographs of three coatings are
shown in Figure 2. The micrographs demonstrated that
the three coatings had comparative permeable micro-
structures and the chromium carbide particles were
encompassed by the matrix phase in the coatings. The
thickness of the coatings is around 200 μm. No splits can
be seen on the cross-sections of the coatings. It is
accounted for that there are two sorts of pore in the
HVOF sprayed coating. One is striated and the other is
dispersed consistently all through the covering.
22
Com-
parable pores could likewise be found in the present
research. The coatings had a thick structure and the
carbides conveyed consistently in the coating. It could be
seen that the covering B was more dense and uniform
than others, which was the least permeable of the three
coatings.
The coating porosity is an essential issue in the ther-
mal spraying strategy as it identifies with various coating
mechanical properties.
23
The metallographic examination
of the coatings demonstrates a low porosity. As shown in
Table 2, HVOF-sprayed 75Cr
3
C
2
-25NiCr coatings have
B. G. NAGULAN et al. INFLUENCE OF THE PROCESS PARAMETERS ON HIGHVELOCITY OXYGEN-FUEL-SPRAYED ...
Materiali in tehnologije / Materials and technology 52 (2018) 5, 653–659 655
Figure 2: Micrographs of the coatings: a) coating A, b) coating B, c) coating C
Figure 1: a) SEM Image of Cr
3
C
2
-NiCr powder, b) XRD pattern of powder

low porosity. It reflects that coating B has the least poro-
sity among the three coatings. It is accounted for that the
HVOF procedure shows the most reduced porosity
caused by its high impact velocity, contrasting with the
other spraying systems, for example, electric arc, flame,
plasma spraying, and detonation gun.
Table 2: Characteristics of the three coatings.
Coating
Porosity
(%)
Adhesion
strength
(mpa)
Indention frac-
ture toughness
(mPa m
1/2
)
Hardness
(hv)
A 0.9 62 4.14 875.4
B 0.60 73.7 5.59 884.2
C 1.14 62.2 4.89 862.3
The microhardness estimations of the coatings are
given in Table 2. The outcomes demonstrate that the
HVOF sprayed 75Cr
3
C
2
-25NiCr coating displays a high
microhardness (around 800 HV). In like manner, high
hardness is of advantage to wear protection in light of
the fact that the hard carbide could balance the external
stress successfully. The adhesive strength is a standout
amongst the most vital factors in thermal spray coating
since it directly identifies with the coating.
24
The adhe-
sion of coating to the substrate was explored by the
tensile test. As Table 2 appears, the adhesion strength of
HVOF sprayed Cr3C2-25NiCr coating could accomplish
more than 60 MPa. The coatings indicate better adhesive
performance, perhaps in light of the fact that the particle
was immovably inserted into the substrate at high speed
by utilizing HVOF spraying and the residual compres-
sion could likewise be a reason. It has been accounted
for that the higher adhesion strength (>80 MPa) of
coatings could be achieved in both the HVOF and the
HVAF frameworks.
25
As Table 1 appears, the fuel flow
and powder feed are close and the spraying parameters
of the three coatings are for the most part extraordinary
in the powder feed rate. The proportion of the com-
bustion weight to the stream of fuel is 0.417 for every
one of the coatings. Along these lines, it is important to
examine the impact of the powder feed rate on the
coating’s properties. Figure 3 demonstrates the impact
of powder feed rate on the coating’s properties. As
Figure 3 shows, it could be reasoned that the powder
feed rate greatly affects the porosity, adhesion strength,
and fracture toughness of the coating. With the feed rate
diminishing, the molten level of the spray particles
expanded, which brought about the expansion of the
hardness and the abatement of the porosity. In the
interim, the aggregate particles kept on the substrate
diminished and it brought about a reduction of the per-
pass thickness.
11
In this way, in the present examination,
the coating kept at a powder feed rate of 35 g/min had
the optimal basic properties.
Figure 4 demonstrates the regular indentations on the
transverse area with in-plane cracks for the three coat-
ings, separately. Because of the qualities of thermally
sprayed coatings, the cracks parallel to the coating sub-
strate interface are less likely to be formed in comparison
with those in the perpendicular direction.
26
It comes from
the elongated nature of the splats and the residual stress
fields introduced.
20
The length of crack c, from the
middle to the indent, was utilized for deciding the frac-
ture toughness of the coatings. The fracture toughness,
KIC, was computed through the accompanying condi-
tions.
13
kc = 0.193 (HVd)(E/HV)2/5(a) – 1/2, c/d  2.5 (1)
kc = 0.711 (HVd1/2)(E/HV)( a) – 3/2, c/d  2.5 (2)
where E is the Young’s modulus and HV is the Vickers
hardness, d is the half diagonal of the Vickers inden-
tation. The radial crack length c, is equal to a, the
B. G. NAGULAN et al. INFLUENCE OF THE PROCESS PARAMETERS ON HIGHVELOCITY OXYGEN-FUEL-SPRAYED ...
656 Materiali in tehnologije / Materials and technology 52 (2018) 5, 653–659
Figure 4: Micrographs of Vickers indentations of coatings A, B, C
Figure 3: Effect of powder feed rate on: a) porosity, b) microhardness,
c) adhesion strength and d) indention fracture toughness of
75Cr
3
C
2
-25NiCr coating

indentation crack length d, the half diagonal of the
Vickers indentation. Both the Vickers diagonal and
crack length are analysed and measured from an optical
image. The Young’s modulus of the coating was cal-
culated by a nano-indenter. Five readings were tested for
each sample and the typical fracture toughness values of
the coatings are shown in Table 3. The fracture tough-
ness is of the three 75Cr
3
C
2
-25NiCr coatings. It has
been indicated that the scatter of fracture toughness was
also observed in the HVOF thermal sprayed coatings,
which indicated the microstructural properties of non
homogeneous coatings.
26
The scatter was also brought
into being in this work and the result indicates, com-
pared with the other coatings, that coating B had
superior fracture toughness, which may be endorsed by
its lowest porosity.
Table 3: The wear rates of the coatings.
Coating
type
Friction
coefficient
Wear rate
(10–15 m
3
/N m)
A 0.34 7.06
B 0.31 6.48
C 0.37 7.32
There are three possibilities of the nano-indenter
pressed into the surface, namely, hard phase, middle
zone between hard phase and binder phase, binder phase
or defect. The micrographs of the nano-indentation
showed these three possibilities, which showed a larger
indentation successively. There are hard phase of higher
hardness and smaller indentation on the binder phase and
fewer defects in coating B. As Figure 5 shows, there are
hard phases of higher hardness and smaller indentation
on binder phase and fewer defects in coating B. which
demonstrates a dense structure with few defects and
more hard carbide content.
In addition, special interest is given to the unload-to-
load-ratio of the nano-indentation, which represents the
ratio between the total work (Wt) done and the recovered
(elastic) work during indentation.
27
It can separate the in-
dentation work into elastic (We) and plastic components
(Wp) since Wt = Wp+We.
28
The determination of the
We/Wt ratio is straightforward from the load–unload
curves, no need for any model application. The ratio
cannot be affected by the shape of the stylus, the state of
material (bulk or thin film), or the applied load, which
are generally critical for the estimation of hardness and
elastic modulus.
29
In the present, the average value of the
We/Wt of the three coatings was obtained by calculating
15 curves. The result showed the We/Wt of the three
coatings is 0.28, 0.43, 0.32, respectively. It illustrates
that the coating B possesses a higher elasticity, which
could be an advantage to improve the resistance against
the deformation and the mechanical properties. Figure 5
provided the morphology of the cross-section of the wear
scar tested with the step profile The wear rate was
calculated according to Equation (3):
k
S
Ln
=
× ()
(3)
B. G. NAGULAN et al. INFLUENCE OF THE PROCESS PARAMETERS ON HIGHVELOCITY OXYGEN-FUEL-SPRAYED ...
Materiali in tehnologije / Materials and technology 52 (2018) 5, 653–659 657
Figure 6: Coated specimens before and after adhesion strength
Figure 5: Micrographs of the worn surfaces of coatings A, B, C

where S is that the area of the cross-section of the wear
scar, L is that the load (N), n is the revolutions. The
wear rate of the three coatings is shown in Table 3. The
coating B has a higher wear resistance than the others,
drawing this conclusion from its lower volume loss
when tested. Coating B possesses the most effective
wear resistance as a result of the dense structure, addi-
tional hard carbide, high fracture toughness, and sen-
sible elasticity.
Those factors are all helpful to the coating resistance
against a recurrent external force. The coating B was
chosen to research the wear mechanism of the HVOF-
sprayed 75Cr
3
C
2
-25NiCr coating seen in Figure 7. There
is very little peeling, crack, and deformation on the worn
surface. The scratches ensuing from the action of the
abrasive particle and also the particle will be clearly
recognized through an in-depth examination. For the
Cr
3
C
2
-NiCr coating, an individual splat within the
coating will be thought of as composites during which
NiCr alloy could be a continuous matrix part with
chromium carbides as the reinforcement hard phases.
Because of the microhardness of metal carbides being
abundant above that of NiCr matrix, the carbides are
additional proof against cutting or gouging than matrix
alloy phase. Consequently, the inorganic compound part
with higher wear resistance would be removed at a lower
rate, and also the carrying off of NiCr alloy binder also
happens preferentially. Throughout the method of
carrying, the high hardness of surface protuberance of
the WC ball may insert into the as-tested coating surface,
resulting in the removal of the metal binder-NiCr alloy
and also the exposed Cr
3
C
2
ceramic particles. Then, the
exposed Cr
3
C
2
particles suffered from shear removal as a
result of the high-contact stress and cyclic stress, ensuing
from the reciprocatory vibration and sliding wear
method. As a result, the detached wear debris containing
exhaust particles from the coatings and WC balls acted
as abrasive particles, leading to severe, three-body
abrasive wear at high temperature. The examination of
the worn surface morphology recommended that the
abrasive wear behavior of the HVOF sprayed Cr
3
C
2
-NiCr
coating was dominated by the selective gouging or
cutting of the NiCr binder matrix phase, followed by the
removal of the carbide particles.
4 CONCLUSIONS
HVOF-coated chromium-carbide-related coatings
possessed a high microhardness, a low level of porosity,
and a high level of adhesion strength. The powder feed
had an evident effect on the coating substrate surface
area and a mechanical property like the porosity, inden-
tion fracture toughness and microhardness. The coating
sprayed at 35 g/min possessed a relatively lower poro-
sity, a higher fracture toughness, and better elasticity.
Thus, the HVOF-sprayed Cr
3
C
2
-NiCr coating using
Hipojet 2700 system (Mec Jodhpur, India) had a suitable
powder feed rate of 35 g/min. The wear test method
showed that the HVOF sprayed 75Cr
3
C
2
-25NiCr carbide
coatings possessed a good level of wear resistance at
elevated temperature, which resulted from the steady
friction coefficient and small crack and peeling and
deformation on the coated worn surface area. The
coating at the powder feed of 35 g/min had the best level
of wear resistance due to the dense structure and
sufficient fracture toughness.
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