J. LAUZUARDY et al.: MICROSTRUCTURE CHARACTERISTICS OF Cr3C2-NiCr COATINGS DEPOSITED ...
137–145
MICROSTRUCTURE CHARACTERISTICS OF Cr
3
C
2
-NiCr
COATINGS DEPOSITED WITH THE HIGH-VELOCITY OXY-FUEL
THERMAL-SPRAY TECHNIQUE
KARAKTERISTIKE MIKROSTRUKTUR Cr
3
C
2
-NiCr PREVLEK
IZDELANIH S TEHNIKO NAPR[EVANJA V TOKU PLINSKE
ME[ANICE KISIKA IN PROPANA Z VELIKO HITROSTJO
Jason Lauzuardy
1,2
, Eddy Agus Basuki
1
, Erie Martides
2
, Selly Septianissa
3
,
Budi Prawara
2
, Dedi
4
, Endro Junianto
5
, Edy Riyanto
2,
*
1
Department of Metallurgical Engineering, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung,
Bandung 40132, Indonesia
2
Research Center for Advanced Materials, National Research and Innovation Agency, Serpong 15314, Indonesia
3
Department of Mechanical Engineering, Faculty of Engineering, Widyatama University, Bandung 40125, Indonesia
4
Research Center for Electronics, National Research and Innovation Agency, Bandung 40135, Indonesia
5
Research Center for Smart Mechatronics, National Research and Innovation Agency, Bandung 40135, Indonesia
Prejem rokopisa – received: 2023-05-03; sprejem za objavo – accepted for publication: 2024-01-22
doi:10.17222/mit.2023.869
With the goals of protecting boiler tubes from hostile surroundings, increasing thermal efficiency, and minimizing time losses
from damage, thermal-spray coating methods for high-temperature operations were created. Ceramic-metal composite materials
(e.g., Cr3C2-NiCr) are well known for protecting components from erosion decay in a high-temperature environment. In this in-
vestigation, the high-velocity oxy-fuel (HVOF) thermal-spray technique was employed to successfully deposit several variations
of feedstocks containing Cr3C2-NiCr and NiCr powders onto a medium-carbon steel substrate, with and without filtering
through a 400-mesh screen. Utilizing X-ray fluorescence (XRF), scanning electron microscopy (SEM), energy dispersive spec-
troscopy (EDS) and X-ray diffraction (XRD), the microstructure features of the deposited coatings were assessed. The experi-
ment results demonstrate that the crystallite and grain sizes of the deposited coatings can be increased by reducing the powder
size through a sifting process using a 400-mesh sieve. This procedure also resulted in a coating with a higher density and lower
porosity. Furthermore, new compounds including Cr2O3 and MnCr2O4 were formed in the coating layers as indicated by the
XRD spectra. These phenomena are in good agreement with the EDS mapping of Cr and O, which reveals highly similar distri-
butions. Manganese was originally a part of the substrate composition. Manganese could diffuse rapidly across the Cr2O3 layer
and form the MnCr2O4 compound, indicating the manganese diffusion from the substrate into the Cr3C2-NiCr coating. The for-
mation of MnCr2O4 can be attributed to the prior emergence of the Cr2O3 compound.
Keywords: termal-spray coating, high-velocity oxy-fuel coating, ceramic-metal material, Cr3C2–NiCr coating
Z namenom, da bi se cevi bojlerjev za{~itile pred {kodljivimi vplivi oklice, pove~ala njihova toplotna u~inkovitost in zmanj{ale
~asovne izgube zaradi po{kodb oziroma vzdr`evanja, se v novej{em ~asu za nana{anje tankih za{~itnih slojev uporabjajo metode
termi~nega napr{evanja v toku plinske me{anice kisika in propana z veliko hitrostjo (HVOF; angl.: high velocity oxy-fuel ther-
mal spray coating). Za ta namen so danes poznani kot naju~inkovitej{i kompozitni materiali keramika-kovina (to je:
Cr3C2-NiCr), ki uspe{no prepre~ujejo propadanje komponent (cevi) zaradi visoko temperaturne korozije. V tem ~lanku avtorji
opisujejo uporabo termi~nega napr{evanja za{~itnih plasti s HVOF postopkom. Uspe{no so izdelali ve~ vrst za{~itnih plasti
(slojev), ki so jih izdelali z omenjenim postopkom. Na podlago iz srednje oglji~nega jekla so nana{ali Cr3C2-NiCr in NiCr
nefiltrirane prahove ter predhodno presejane prahove na filtru z velikostjo odprtin cca 37 mikrometrov (400 mesh). Za
karakterizacijo mikrostrukture izdelanih za{~itnih slojev so uporabili rentgensko fluorescenco (XRF), vrsti~no elektronsko
mikroskopijo (SEM), spektroskopijo na osnovi energije sipanja elektronov (EDS), in rentgensko difrakcijo (XRD).
Eksperimentalni rezultati so pokazali, da nara{~a velikost dendritov in kristalnih zrn z zmanj{anjem koli~ine doziranja pra{ne
me{anice skozi 37 mikrometersko sito. Ta postopek je zmanj{al poroznost in povi{al gostoto izdelanih za{~itnih slojev. V XRD
spektru so avtorji zaznali tudi nastanek novih spojin, kot sta Cr2O3 in MnCr2O4. To se je dobro ujemalo tudi z izdelano vzdol`no
in pre~no EDS porazdelitvijo vsebnosti kroma (Cr) in kisika (O). Mangan (Mn) originalno izvira oziroma je difundiral v
napr{ene plasti iz jeklene podlage preko Cr2O3 in tvoril MnCr2O4. To nakazuje, da se je med napr{evanjem Cr3C2-NiCr prahu pri
visoki temperatur najprej tvoril Cr2O3, nato pa je zaradi difuzije Mn iz jeklene podlage v plasteh za~ela nastajati {e faza
MnCr2O4.
Klju~ne besede: prevleke izdelane s postopkom termi~nega napr{evanja z veliko hitrostjo, plinska me{anica kisik (zrak)-propan,
keramika-kovina, prevleka na osnovi me{anice prahov Cr3C2 in NiCr
1 INTRODUCTION
Surface erosion is a common problem for compo-
nents that are exposed to environments with sand parti-
cles, such as those used in slurry pumps, boiler tubes,
hydro turbines, and pulp handling components.
1,2
WC-Co, NiCr, WC-CoCr and other cermet-based materi-
als with excellent corrosion resistance and high wear re-
sistance are used for high-velocity oxygen-fuel (HVOF)
spraying in a variety of technical applications.
3,4
To
strengthen the resistance to wear caused by abrasion and
Materiali in tehnologije / Materials and technology 58 (2024) 2, 137–145 137
UDK 669.058.67:621.793.7 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(2)137(2024)
*Corresponding author's e-mail:
edy.riyanto@brin.go.id (Edy Riyanto)

lengthen the product life and efficiency of steels, particu-
larly in hostile situations with high temperatures, ther-
mal-spray coating has become a vital technique.
5,6
In
boiler applications, for example, wear and corrosion oc-
cur at high temperatures, one of the most difficult condi-
tions.
7
Because there are so many different coating appli-
cations, it is necessary to adapt coating qualities to
ensure that the substrate and coating work at their best in
each particular environment.
7
Thermal spraying, which creates and projects drop-
lets of molten or semi-molten material onto a substrate to
form a coating, is widely used in industries to alleviate
the effects of erosion oxidation. Hard-metal coatings
(e.g., Cr
3
C
2
-25NiCr, Cr
3
C
2
-37WC-18NiCoCr, WC-
10Co4Cr) are often applied using high-velocity flame
spray methods such as high-velocity oxygen-fuel or
high-velocity air-fuel (HVAF) spray procedures.
7,8
To
produce combustion with a fuel, the HVOF spray method
uses pure oxygen as the oxidizer, whereas the HVAF
spray process uses compressed air.
7
In general, the
HVAF spray process achieves faster particle velocities
than the HVOF spray process; however, particle tempera-
tures are lower due to a decreased oxygen content, re-
sulting in a lower combustion temperature.
1,7
For this
reason, the HVAF spray process is a viable alternative to
HVOF, while the application equipment and component
type define the best spray technique.
For the creation of carbide-based cermet coatings,
high-velocity oxygen-fuel technology has gained wide-
spread use.
9,10
Due to its ability to generate coatings with
strong adhesion (bond strength > 70 MPa), low porosity
(< 1 %), and low oxide content (< 1 %), the HVOF ther-
mal spraying is one of the best substitutes for chro-
mium.
11,12
High kinetic energy powder particles acquired
during an HVOF process ensure strong coating cohesive-
ness and enable the production of carbide-based coatings
with little decarburization and porosity.
11
The HVOF
thermal-spray coating method facilitates the production
of a very dense layer with low porosity and low residual
stress.
13,14
The process can be used to achieve a
high-hardness coating that is very dense and has a high
cohesive strength.
10,15,16
Furthermore, it was found that
the coating obtained with 60CrC-40NiCr powder is
denser and more homogeneous than that obtained with
75CrC-25NiCr powder as the microhardness of
75CrC-25NiCr is greater than that of 60CrC-40NiCr.
17
On the other hand, new phases can emerge in the coat-
ings obtained with HVOF thermal spraying due to the
chemical reaction among the constituents.
18
It was shown
that the manganese element that originates from the sub-
strate can diffuse across the HVOF Cr
3
C
2
-NiCr layer.
Furthermore, this diffusion occurs due to the prior for-
mation of a chromium oxide compound in a Cr
3
C
2
-NiCr
HVOF coating as manganese is able to diffuse rapidly
across Cr
2
O
3
and develops an MnCr
2
O
4
compound over
the surface.
2 EXPERIMENTAL PART
Medium-carbon Fe-based boiler steel (ASTM SA210
grade C), commonly used for boiler tubes, was employed
as the substrate material. The composition of the steel
used as the substrate, which was observed using X-ray
fluorescence (XRF), is shown in Table 1. The chemical
configuration is generally matching the ASME
SA210/SA210m standard for medium-carbon steel tubes
for boilers and superheaters.
19
A boiler tube was cut in
the form of a coupon with dimensions of  4 0×3 0×
8mm( Figure 1d), which was grit blasted using alumi-
num oxide (Al
2
O
3
) with a particle size of 24 mesh before
the HVOF spraying process.
Figure 1 shows the substrate preparation from the
boiler-tube raw material (ASTM SA210 grade C). The
initial raw material is represented in Figure 1a as a tube,
which was subsequently cut into three portions (Figure
1b) to create the substrate (Figures 1c and 1d). To en-
J. LAUZUARDY et al.: MICROSTRUCTURE CHARACTERISTICS OF Cr3C2-NiCr COATINGS DEPOSITED ...
138 Materiali in tehnologije / Materials and technology 58 (2024) 2, 137–145
Table 1: Chemical composition of the substrate material
Chemical composition
Content, w/%
Fe Mn Cr S Si
ASTM SA210 Grade C 98.90 0.74 0.034 0.057 0.27
Standard
19
Allowance 0.29–1.06  0.35  0.035  0.10
Figure 1: Raw materials and substrate: (a) boiler-tube raw substrate
material with ASTM SA210 grade C specifications; (b) a diagram of
the boiler tube being sliced to produce the substrate; (c) the substrate’s
inner side; (d) partially flattened surface of the outer pipe, which
serves as the coated substrate

able the coating performance measurement, the curved
surface of the substrate was flattened prior to the deposi-
tion process as illustrated in Figure 1d.
The powders used in this study were Cr
3
C
2
-NiCr
(PAC 131) and NiCr (43C-NS). The deposited powders
included 5 powder variations as shown in Table 2.A
powder was sifted using a 400-mesh sieve agitated by a
sieve shaker (450 watt, 50 Hz). The amount of each pow-
der variation was 500 grams. For the mixing process,
blending was carried out using a V-type blending ma-
chine for 8 hours with a rotational speed of 64 rpm and
an ambient temperature of ~28 °C.
Table 2: Variations of the coating powder
Coating powder Type
Weight
(grams)
Cr3
C
2
-NiCr Original 500
60%Cr 3
C
2
-NiCr + 40 % NiCr Original 300 + 200
80%Cr 3C2-NiCr + 20 % NiCr Original 400 + 100
60%Cr 3C2-NiCr + 40 % NiCr 400 mesh 300 + 200
80%Cr 3
C
2
-NiCr + 20 % NiCr 400 mesh 400 + 100
The TECKNOTHERM Hipojet-2700 HVOF was
used to perform HVOF spraying at the Coating Labora-
tory of the National Research and Innovation Agency of
the Republic of Indonesia. Table 3 provides an illustra-
tion of the process parameters.
Table 3: HVOF parameters
Parameter Unit Value
Air pressure bar 6.2
Pressure
O
2
bar 8
N2
bar 5
Propane bar 5.5
Flow rate
O
2
L/min 271
N2
L/min 8
Propane L/min 62.4
Powder feeder rotation min
–1
5
Stand-off distance mm 200
Spray angle ° 90
Substrate preheating °C  150
Images of the powder, cross-section of the coating
and surface view of the coated Cr
3
C
2
-NiCr were taken
with a high-resolution scanning electron microscope
(JSM-IT300, JEOL-Japan). This instrument was also
employed to detect the chemical composition of the pow-
ders and HVOF coatings through energy dispersive spec-
troscopy (EDS). The formed compounds of the coating
were detected using X-ray diffraction (D8 Advance Eco,
Bruker, Bragg-Bentano Difraction). Furthermore, XRD
data were also used to measure the crystallite size of the
deposited coatings using the Debye-Scherer method. In
addition, the pore formation levels were evaluated from
SEM images using ImageJ software.
3 RESULTS AND DISCUSSION
Figure 2 shows SEM-EDS micrographs of coat-
ing-powder variations. Figure 2a shows Cr
3
C
2
-NiCr, 2c
shows 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr, 2e shows
80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr. In addition, EDS
mapping of these powders can be seen in Figures 2b, 2d
and 2f, respectively. Characterisation of the coating pow-
ders is one of important tasks before they can be applied
onto the substrate with the thermal spray method.
20
It
clearly reveals that the shape of nickel is spherical (Fig-
ures 2b, 2d and 2f). Chromium and carbide have an ir-
regular shape (Figures 2b, 2d and 2f). The Cr
3
C
2
-NiCr
powder is the only one that originates from the powder
maker (Figure 2a). It is shown that there is no visible
oxidation element (oxygen) in the EDS mapping of the
powder (Figure 2b). Meanwhile, the powders that under-
went the mixing process have identical chemical con-
tents of Cr, C, Ni and O (Figures 2d and 2f). The pres-
ence of O indicates the existence of oxidation that
occurred during the mixing process.
Figure 3 shows the surfaces of HVOF coatings (Fig-
ures 3a, 3c, 3e, 3g and 3i) and the surface elemental
contents measured using EDS (Figures 3b, 3d, 3f, 3h
and 3j). It is shown that the surfaces are rough. The
microstructures consist of oxide particles, pores, splats,
partially melted particles, and unmelted particles (Fig-
ures 3a, 3c, 3e and 3g). There are also splats with cracks
as shown in Figures 3a and 3g. The cracks of the splats
are attributed to the residual stress formation during the
J. LAUZUARDY et al.: MICROSTRUCTURE CHARACTERISTICS OF Cr3C2-NiCr COATINGS DEPOSITED ...
Materiali in tehnologije / Materials and technology 58 (2024) 2, 137–145 139
Figure 2: SEM and EDS images of powders: (a–b) Cr
3
C
2
-NiCr, (c–d)
60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr, (e–f) 80 w/% Cr
3
C
2
-NiCr +
20 w/% NiCr

cooling process after the melted particles hit the sub-
strate. The effect of the melted particles that spread to
the surrounding of the hitting point causes the pore for-
mation (Figure 3i).
Based on the EDS measurement, it was found that Cr
increased in the coatings consisting of 60 w/%
Cr
3
C
2
-NiCr + 40 w/% NiCr and 80 w/% Cr
3
C
2
-NiCr +
20 w/% NiCr without sieving (Figures 3d and 3f), con-
trary to the HVOF coating formed with the original pow-
der (Figure 3b). On the other hand, the Cr element con-
tent was reduced in the HVOF coating formed with the
powder that was sifted with a 400-mesh sieve (Fig-
J. LAUZUARDY et al.: MICROSTRUCTURE CHARACTERISTICS OF Cr3C2-NiCr COATINGS DEPOSITED ...
140 Materiali in tehnologije / Materials and technology 58 (2024) 2, 137–145
Figure 4: Cross-sectional images of the coatings made by HVOF thermal spray: (a) Cr
3
C
2
-NiCr, (b) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr,
(c) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr, and the HVOF coatings composed of the powder that was sifted with a 400-mesh sieve including
(d) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr and (e) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr
Figure 3: Top-view SEM images and elemental contents of HVOF coatings: (a–b) Cr
3
C
2
-NiCr, (c–d) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr, (e–f)
80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr, and the HVOF coatings composed of the powder that was sifted with a 400-mesh sieve including (g–h) 60
w/% Cr
3
C
2
-NiCr + 40 w/% NiCr and (i–j) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr

ures 3h and 3j). In general, the nickel element is re-
duced, due to the mixing of Cr
3
C
2
-NiCr with NiCr, in
both the coatings with and without sieving with a
400-mesh sieve (Figures 3d, 3f, 3h and 3j). On the other
hand, the carbide element was increased due to the mix-
i n go fC r
3
C
2
-NiCr with NiCr in comparison with the
original Cr
3
C
2
-NiCr powder (Figures 3d, 3f, 3h and 3j).
Cross-sectional images of the following ther-
mal-spray coatings are shown in Figure 4:C r
3
C
2
-NiCr
(Figure 4a), 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr (Figure
4b), 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr (Figure 4c), the
coating obtained with the powder that was sifted with a
400-mesh sieve, i.e., 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr
(Figure 4d) and 80 w/% Cr
3
C
2
- N i C r+2 0w/% NiCr
(Figure 4e). Through SEM observations, it was shown
that the Cr
3
C
2
-NiCr coating layers can be successfully
deposited onto the substrate (Figures 4a to 4e). Gen-
erally, it can be seen that the coating materials can ad-
here well to the substrate surfaces. The morphology of
the layer shows the gradation of dark and light colours,
indicating the splats and the unmelted particles, respec-
tively.
21–23
The light colour of the coating represents the
NiCr alloy and the dark colour represents the Cr and C
elements. The cross-sectional images show the coating
structures, including splats, unmelted particles, partially
melted particles, pores, overblasted areas, embedded
grit-blasting media and intra-lamellar pores. A splat is a
fully melted particle that hits the substrate. To get a good
HVOF layer, a splat formation is required. The prerequi-
sites for a splat formation are a flow of powder particles
that are small enough and the heat resulting from the
HVOF gun flame, able to melt these particles in a very
short time. However, when the particle size is high
enough but without enough HVOF heat flame energy,
particles will remain unmelted or be partially melted.
Porosity, one of the primary structural flaws in coat-
ings created during thermal spraying, is primarily caused
by two factors: shrinkage and gas dynamics in a coat-
ing.
24–26
Due to the substrate (coating) surface roughness,
pores are created as a result of gas capture (i.e., at an
overblasted area) (Figure 4d) and during the impact,
molten droplets deform to create intra-lamellar holes
(Figure 4a).
26,27
The embedded grid-blasted Al
2
O
3
in a
coating structure indicates the presence of residual parti-
cles that were previously used as an abrasive material
during the roughing of the substrate surface.
Figure 5 depicts the thickness of the HVOF thermal
spray coatings as estimated from the cross-sectional pho-
tographs of the deposited coatings. It is demonstrated
that the typical coating thickness values of (a)
Cr
3
C
2
-NiCr, (b) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr, (c)
80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr and the HVOF coat-
ings achieved with the powder that was sifted with a
400-mesh sieve including (d) 60 w/% Cr
3
C
2
-NiCr +
40 w/% NiCr and (e) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr
are ~93.35, 166.9, 220.8, 151.67, 83.3 μm, respectively.
Figure 6 shows the cross-sectional EDS mapping of
the HVOF coatings. The elements found in the layer are
chromium (Cr), nickel (Ni), carbide (C), oxygen (O),
aluminum (Al) and silicon (Si). It is shown that the main
coating elements including Cr, Ni, C and O can be seen
clearly in the mapping, indicated by different colours in-
cluding red, pink, green and turquoise, respectively. The
mapping shows the general structures of the coatings.
The inserted figure at the bottom right corner of each
EDS image relates to the Fe element indicated by green
(Figures 6a to 6e). The main element of splats is Cr,
shown in red (Figures 6a to 6d). Pink indicates the Ni
element, whose shape is mainly spherical and irregular
(Figures 6a to 6e). Furthermore, these shapes are likely
to indicate the unmelted and partially melted particles.
The mapping of the O element is very similar to the
mapping distribution of the Cr element (Figures 6a to
6e). This indicates that the Cr element forms a com-
pound with the O element to form MnCr
2
O
4
as indicated
by the XRD spectra at 2 of around 33.72, 36.29, 42.37°
(Figures 7a to 7e) and Cr
2
O
3
at 2 of around 24.56,
33.72, 36.33, 44.19, 97.49° (Figures 7a, 7b, 7d and 7e).
These EDS mapping images also reveal that a splat for-
mation is mainly composed of a compound with Cr (Fig-
ures 6a to 6e). The presence of Al and Si in the coatings
is due to the residual blasting particles (i.e., Al
2
O
3
, SiO
2
)
on the surfaces that still stick during the substrate rough-
ing. This is consistent with the cross-sectional SEM im-
ages which show that the embedded grit-blasted media,
indicated with white, are found in the coatings to have
sporadic distributions (Figures 4a to 4e).
Figure 7 shows the XRD spectra of the Cr
3
C
2
-NiCr
coating with a variety of powders including Cr
3
C
2
-NiCr
(Figure 7a), 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr (Figure
7b), 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr (Figure 7c),
and the coatings achieved with the powder that was
J. LAUZUARDY et al.: MICROSTRUCTURE CHARACTERISTICS OF Cr3C2-NiCr COATINGS DEPOSITED ...
Materiali in tehnologije / Materials and technology 58 (2024) 2, 137–145 141
Figure 5: Thickness of the coatings obtained with HVOF thermal
spraying: (a) Cr
3
C
2
-NiCr, (b) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr,
(c) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr and the HVOF coatings
achieved with the powder that was sifted with a 400-mesh sieve in-
cluding (d) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr and (e) 80 w/%
Cr
3
C
2
-NiCr + 20 w/% NiCr

sifted with a 400-mesh sieve including 60 w/%
Cr
3
C
2
-NiCr + 40 wt.% NiCr (Figure 7d) and 80 w/%
Cr
3
C
2
-NiCr + 20 w/% NiCr (Figure 7e), deposited with
HVOF thermal spraying. The major compounds of the
coatings are mainly Fe
0.27
Ni
0.73
,C r
0.22
Ni
0.78
,C r
7
C
3
and
Cr
2
O
3
at the 2 of around 44.17 (Figures 7a, 7b, 7d and
7e). It is shown that in comparison to the Cr
3
C
2
-NiCr
coating, the intensity of these phases are generally re-
duced for 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr and 80
w/% Cr
3
C
2
-NiCr + 20 w/% NiCr coatings (Figures 7b
and 7c). Conversely, the phases are increased for the
coatings obtained with the powder that was sifted with a
400-mesh sieve including 60 w/% Cr
3
C
2
-NiCr + 40 w/%
NiCr and 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr (Figures
7d and 7e). As the main phases are Cr compounds (i.e.,
Cr
0.22
Ni
0.78
,C r
7
C
3
,C r
2
O
3
,C r
0.1
Fe
0.9
), the particle size of
the powder with Cr
3
C
2
is smaller than that of the powder
with Ni. This is consistent with the EDS mapping, which
J. LAUZUARDY et al.: MICROSTRUCTURE CHARACTERISTICS OF Cr3C2-NiCr COATINGS DEPOSITED ...
142 Materiali in tehnologije / Materials and technology 58 (2024) 2, 137–145
Figure 6: Cross-sectional EDS mapping of the coatings obtained with HVOF thermal spraying: (a) Cr
3
C
2
-NiCr, (b) 60 w/% Cr
3
C
2
-NiCr +
40 w/% NiCr, (c) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr and the HVOF coatings achieved with the powder that was sifted with a 400-mesh sieve in-
cluding (d) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr and (e) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr

shows that the Ni particles are bigger than the Cr parti-
cles (Figures 2b, 2d and 2f). This condition makes the
Cr
3
C
2
powder able to pass through the 400-mesh sieve
and eventually we can obtain the powder with a Cr
3
C
2
content that is larger than that of the powder obtained
without sieving. The powder with a larger content of
Cr
3
C
2
can be used for a HVOF coating with a higher in-
tensity of the phases with a Cr compound (Figures 7d to
7e).
We can be see the emergence of Mn compounds
MnO
2
and MnCr
2
O
4
in the HVOF coatings (Figures 7a
to 7e). As originally manganese existed in the substrate
(Table 1), this phenomenon indicates that Mn diffuses
into the coatings layer. It is similar to some other ele-
ments (i.e., Si, C, Fe, Mn) that can move from the sub-
strate to the coating. In the previous research carried out
by Premkumar and Balasubramanian, it was found that
Si, C and Fe emerge in the scale, which spreads from the
substrate to the coating subjected to cyclic oxidation.
28,29
Furthermore, the study carried out by Sundararajan et al.
shows that Mn and Si can also spread from the substrate
into the coating due to a high-temperature steam oxida-
tion (i.e., 600–750 °C).
30,31
XRD spectra revealed the for-
mation of Cr
2
O
3
as shown in Figures 7a, 7b, 7d and 7e.
The following can be used to explain how Cr
2
O
3
forms in
NiCr-based coatings: despite the fact that NiO and Cr
2
O
3
are stable oxides at a 1 atm oxygen pressure, other fac-
tors, particularly kinetics and thermodynamics, may have
an impact on the overall scale growth.
30,32
Compared to
nickel, chromium has a stronger affinity for oxygen and
produces a more stable oxide.
30,33
A high temperature
during thermal spraying may lead to a reaction of the ox-
ygen in the surroundings with dissociated chromium to
form stable Cr
2
O
3
. According to a report, manganese
generally has a negative impact on the oxidation resis-
tance of alloys that create Cr
2
O
3
.
30,31
While it does not
help the protective film formation, it can diffuse across
the Cr
2
O
3
film fairly quickly and form a MnCr
2
O
4
layer
on the surface.
30,31
It is worth noting that the diffusion of
manganese from the substrate into the HVOF
Cr
3
C
2
-NiCr coating and MnCr
2
O
4
formation are attrib-
uted to the prior emergence of the Cr
2
O
3
compound in
the HVOF coating.
Figure 8 shows the crystallite and grain sizes as well
as the porosity level of the coatings obtained with HVOF
thermal spraying. It shows that the crystallite and grain
sizes of 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr are bigger
J. LAUZUARDY et al.: MICROSTRUCTURE CHARACTERISTICS OF Cr3C2-NiCr COATINGS DEPOSITED ...
Materiali in tehnologije / Materials and technology 58 (2024) 2, 137–145 143
Figure 8: Average crystallite size of the coatings achieved with HVOF
thermal spraying: (a) Cr
3
C
2
-NiCr, (b) 60 w/% Cr
3
C
2
-NiCr + 40 w/%
NiCr, (c) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr, and the HVOF coating
obtained with the powder that was sifted with a 400-mesh sieve in-
cluding (d) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr, (e) 80 w/%
Cr
3
C
2
-NiCr + 20 w/% NiCr; and the grain size in relation to the level
of porosity of (f) Cr
3
C
2
-NiCr, (g) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr,
(h) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr, and the HVOF coatings ob-
tained with the powder that was sifted with a 400-mesh sieve includ-
ing (i) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr, (j) 80 w/% Cr
3
C
2
-NiCr +
20 w/% NiCr
Figure 7: XRD diffraction patterns of the coatings obtained with
HVOF thermal spraying: (a) Cr
3
C
2
-NiCr, (b) 60 w/% Cr
3
C
2
-NiCr +
40 w/% NiCr, (c) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr and the HVOF
coatings achieved with the powder that was sifted with a 400-mesh
sieve including (d) 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr and
(e) 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr

than those of 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr, for
both the powders with and without sifting with a
400-mesh sieve (Figures 8b to 8e and 8g to 8j). Gen-
erally, the crystallite and grain sizes increase due to the
sifting with a 400-mesh sieve as shown for the crystallite
sizes of 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr, which are
31.56 nm and 27.62 nm when the coating is obtained
from the powders with and without a sifting process, re-
spectively (Figures 8b and 8d). This trend is consistent
with the crystallite sizes of the 80 w/% Cr
3
C
2
-NiCr +
20 w/% NiCr coatings, which are 28.41 nm and
26.83 nm for the coatings obtained with the powder with
and without sifting with a 400-mesh sieve, respectively
(Figures 8c and 8e).
These phenomena show that the grain sizes are in-
creased due to the sifting with a 400-mesh sieve of both
60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr and 80 w/%
Cr
3
C
2
-NiCr + 20 w/% NiCr coatings (Figures 8g to 8j).
The sifting with a 400-mesh sieve enables us to obtain a
powder with a size of 400 mesh and lower. During the
deposition with HVOF thermal spraying, a powder with
a low size can absorb the heat of the HVOF gun flame
better than a powder with a bigger size. A powder with a
small size has a larger surface area than a powder with a
bigger size. As a result, a larger number of particles from
the sifted powder can be melted enough to form a fully
melted droplet, which eventually leads to crystallites and
grains with bigger dimensions. The grain sizes of all
coatings are bigger than crystallite sizes as a grain can be
either a single crystalline or polycrystalline structure as
shown in Figure 8.
One of the important characteristics of a coating, ob-
tained with thermal spraying is porosity. Although pores
are unavoidable in the coatings, obtained with thermal
spraying, they can be minimized through a proper depo-
sition. Normally the porosity level of an HVOF coating
is around 0.1 % to 2 %.
34
It was shown that the porosity
of the Cr
3
C
2
-NiCr, 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr,
80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr coatings and the
HVOF coatings achieved with the powder that was sifted
with a 400-mesh sieve including 60 w/% Cr
3
C
2
-NiCr +
40 w/% NiCr and 80 w/% Cr
3
C
2
-NiCr + 20 w/% NiCr are
(0.76, 0.63, 0.77, 0.54, 0.58) %, respectively (Figures 8f
to 8j). These values indicate a successful deposition as
the pore formations in all deposited coatings are rela-
tively low, in a range of 0.54–0.77 %. This clearly shows
that the powder size obtained with the 400-mesh sieve
can reduce the porosity level of the coating (Figures 8i
to 8j). A powder with a small grain size leads to the pro-
duction a large quantity of fully melted droplets, which
eventually results in an HVOF-deposited layer with a
higher density of formed splats. The intensified contact
among the splats can reduce the pores and oxide forma-
tions.
35,36
As a result, the pore formation can be mini-
mized. It can be deduced that decreasing the powder size
with a sifting process improves the quality of HVOF
coatings. In relation to the grain size, it can be seen that
the porosity level decreased by increasing the grain size.
4 CONCLUSIONS
A successful deposition of Cr
3
C
2
-NiCr layers onto a
medium-carbon steel substrate was achieved using
HVOF thermal spraying, as demonstrated by substrate
surfaces that can hold coating elements firmly, exhibiting
a porosity of up to 0.77 %. When compared to 80 w/%
Cr
3
C
2
-NiCr + 20 w/% NiCr, it was found that the crystal-
lite and grain sizes of 60 w/% Cr
3
C
2
-NiCr are larger. Fur-
thermore, the crystallite and grain sizes can be increased
through a sifting process using a 400-mesh sieve. As
smaller-sized powders have more surface area, more par-
ticles from a sifted powder can combine to create totally
melted droplets, ultimately leading to crystallites and
grains with larger dimensions. In terms of pore develop-
ment, better melted droplets allow for the production of
an HVOF layer with a higher density. Voids and oxide
deposits can be lessened by an enhanced splat contact. It
is obvious that the powder sifted with a 400-mesh filter,
i.e., 60 w/% Cr
3
C
2
-NiCr + 40 w/% NiCr, has the best
powder composition as it achieves the lowest porosity of
0.54 %. The oxidation resistance of the alloys that form
Cr
2
O
3
is typically adversely affected by manganese. This
element does not promote the formation of a preservative
coating, but it has a tendency to disperse over the Cr
2
O
3
film fairly quickly to form a MnCr
2
O
4
layer on the sur-
face. It should be noted that the emergence of the Cr
2
O
3
compound in the HVOF layer causes the diffusion of
manganese from the substrate into the Cr
3
C
2
-NiCr layer
and the establishment of MnCr
2
O
4
.
Acknowledgment
The authors thank Advanced Characterization Labo-
ratories Bandung, National Research and Innovation
Agency E-Layanan Sains for providing the facilities and
scientific and technical support. This work was sup-
ported by Rumah Program Nanoteknologi dan Material
Maju Tahun 2023 - ORNM BRIN, National Research
and Innovation Agency of the Republic of Indonesia
(Grant No. 3/III.10/HK/2023).
Declarations
Jason Lauzuardy and Edy Riyanto are the main con-
tributors. The authors declare that they have no compet-
ing interests.
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