PEER REVIEWED
Microstructural, Mechanical and Wear Behavior of HVOF
and Cold-Sprayed High-Entropy Alloys (HEAs) Coatings
A. Silvello1 • P. Cavaliere2 • S. Yin3 • R. Lupoi3 • I. Garcia Cano1 •
S. Dosta4
Submitted: 22 June 2021 / in revised form: 22 October 2021 / Accepted: 8 November 2021 / Published online: 21 January 2022
 ASM International 2021
Abstract HEAs powders, unlike standard alloys which
contain one or two base elements, are new alloys that
contain multiple elements in the same quantity. These
materials have outstanding physical and mechanical prop-
erties and for this reason, are of great interest in the
material science community for their application in
advanced industrial sectors. In this investigation, cold
spray (CS) and high-velocity oxy fuel (HVOF) processes
were used to deposit Cantor alloy (FeCoCrNiMn) coatings.
Starting feedstock powders were thoroughly characterized
in terms of size, shape, phase and elastic modulus. For CS
process, the coating deposition efficiency and porosity
could be optimized by varying gas pressure, gas
temperature and stand-off distance. In the case of HVOF
process, stand-off distance influenced the thickness of the
coatings. Besides, on the optimized CS and HVOF coat-
ings, corrosion tests in 3.5% NaCl solution, as well as
rubber wheel, ball on disk and jet erosion tests were carried
out to evaluate their wear behavior. Also, to benchmark the
corrosion and wear behavior of optimized coatings, the
results were compared to 316L and C-Steel bulks. The
tribological study shows that Cantor alloy coatings
deposited via CS and HVOF are promising to protect parts
and components in a harsh environment.
Keywords cold spray  corrosion protection  erosion
resistant coatings  HVOF  HEAs  tribology  wear
Introduction
HEAs powders are a new class of materials that contain
multiple principal elements in the same quantity, unlike
traditional alloys, which contain one or two base elements.
Combining the effect of high entropy, lattice torsion and
cocktail effect, HEAs have been reported to have excellent
physical, chemical and mechanical properties such as high
hardness, thermal stability, ductility, high wear and cor-
rosion resistance (Ref 1, 2). Yeh et al. (Ref 3) and Cantor
et al. (Ref 4) have defined HEAs and established the con-
vention of at least five principal elements, thus the total
number of possible HEAs composition is significantly high
(Ref 5). It is undeniable that HEAs have received the
attention of scientific community thanks to their potential
industrial applications. As a consequence of their special
phase structure, they have excellent mechanical properties,
as well as remarkable corrosion resistance performances.
Compared to conventional alloys, HEAs have been shown
This article is part of a special topical focus in the Journal of Thermal
Spray Technology on High Entropy Alloy and Bulk Metallic Glass
Coatings. The issue was organized by Dr. Andrew S.M. Ang,
Swinburne University of Technology; Prof. B.S. Murty, Indian
Institute of Technology Hyderabad; Distinguished Prof. Jien-Wei
Yeh, National Tsing Hua University; Prof. Paul Munroe, University
of New South Wales; Distinguished Prof. Christopher C. Berndt,
Swinburne University of Technology. The issue organizers were
mentored by Emeritus Prof. S. Ranganathan, Indian Institute of
Sciences.
& A. Silvello
asilvello@cptub.eu
1 Thermal Spray Center (CPT), University of Barcelona, Martı´
i Franque´s 1, 08028 Barcelona, Spain
2 Department of Innovation Engineering, University of
Salento, via per Arnesano, 73100 Lecce, Italy
3 Department of Mechanical, Manufacturing & Biomedical
Engineering, Trinity College Dublin, The University of
Dublin, Parsons Building, Dublin, Ireland
4 Materials Science Department, University of Barcelona,
Martı´ i Franque´s 1, 08028 Barcelona, Spain
123
J Therm Spray Tech (2022) 31:1184–1206
https://doi.org/10.1007/s11666-021-01293-w
many advantages. Zhang et al. (Ref 6) have reviewed and
discussed HEAs properties in terms of physical, magnetic,
chemical and mechanical behavior, concluding that the
corrosion resistance of Cu0.5NiAlCoCrFeSi alloy is better
than that of the conventional 304-stainless steel. In their
corrosion analysis, Qiu et al (Ref 7) preferred to term these
new alloys as CCAs (compositionally complex alloys),
because some HEAs did not have high configurational
entropies and present the formation of secondary phases.
Even if these authors have chosen to use a different defi-
nition, the conclusion of their study is the same; the cor-
rosion resistance of HEAs is in the same range of
austenitic/ferritic stainless steel.
On these premises, it is no doubt true that investigating
on advanced industrial applications of HEAs, such as
boilers and tubes in power or waste plants, could open up
new scenarios. According to Kumar et al. (Ref 8), the
failure of a boiler is big hazard to the effectiveness of the
boiler in the power plants. In their work, it is shown how
overall economic loss due to all the types of corrosion
accounts to US$ 6500 million annually. This aspect limits
the availability of a plant and hence rises its maintenance
and investment costs.
Coatings deposited via traditional thermal spray process,
such as HVOF, have always been used in harsh environ-
ment to improve corrosion and wear resistance of a sub-
strate material (Ref 9-13). To protect metallic boiler parts,
usually produced from carbon steel, Ni-based superalloys,
nickel aluminide and NiCrAlY coatings have been suc-
cessfully deposited (Ref 14-16). In (Ref 17), Singh et al.
have reviewed corrosion performance of different coatings
produced via different techniques, while a complete anal-
ysis about corrosion resistance of thermal spray coatings
was carried out from Sadeghi et al. (Ref 18). In particular,
they have addressed the issue not only from a technical
point of view, but also studying challenges, opportunities
and future developments of coatings for corrosion and wear
resistance. In general terms, Katranidis et al. (Ref 19, 20)
have investigated the influence of HVOF process parame-
ters on porosity, residual stresses, thickness and so on.
They have concluded that using a new-generation HVOF
torch, the decarburization and oxidation effects on
mechanical properties of the coatings are minimal.
Thus, it is clear that surface coatings can improve
mechanical properties of the coated part. For this reason,
the next advance for scientific and industrial communities
is to investigate new promising starting feedstock powders
deposited via thermal spray processes, such as HEAs. Li
et al. (Ref 21) have studied coatings based on HEAs alloys,
discussing design principles, process fabrication, post heat
treatments and potential applications. Also, these authors
have divided HEAs coatings into three categories (metallic,
ceramic and composite coatings), and then they have listed
both the most common and emergent processes to fabricate
them.
Among these emergent processes, cold spray (CS) is
different from traditional thermal spray processes because,
to deposit coatings, it uses kinetics energy rather than
thermal energy. CS is used to fabricate coatings or to repair
damaged parts (Ref 22-24), and it is well-known as a thick
metal layer deposition technique. It works at relatively low
processing temperatures, maintaining the initial phase of
powders and thus, can be avoided the defects of conven-
tional thermal spray processes (Ref 25-28). Moreover, CS
has been used for advanced industrial purpose such as to
improve mechanical and microstructural properties of
coatings to rise up efficiency, reliability and productivity of
boilers and tubes of power/nuclear plants. In particular, the
chambers of Tokamak (Ref 29) are coated with dense
coatings of W or W-Cr-based alloys. The advantages of
using CS process for this task have been no oxidation, low
porosity and high adhesion strength.
Recently, CS process has been used to deposit HEA
Cantor (FeCrMnNiCo) alloy (Ref 30, 31). These two
studies have confirmed that CS can be used to produce a
thick HEA coating with low porosity that are harder than
the as-received starting powders, without any phase trans-
formation and finally, with a lower wear rate than laser
cladded one. Additionally, the investigation of the high-
temperature oxidation behavior of this HEA coating at
700 8C and 900 8C has shown that it presents more
favorable internal oxidation than the similar compositions
HEAs bulk. Lehtonen et al. (Ref 32) sprayed via CS a four
components alloy (FeCrMnNi, without Co) to study if it is
possible to replace stainless steels coating in nuclear
industries. They have deposited a HEAs alloys without Co
because of nuclear applications, due to the possible acti-
vation to 60Co in the presence of neutrons, it is better to
avoid it. However, even without Co, the FeCrMnNi cold-
sprayed coating has shown high hardness, low porosity and
in general terms, excellent mechanical properties.
In this fast-moving scenario, the aims of this work are to
study microstructural, and mechanical properties, as well
as corrosion and wear resistance, of different coatings
produced via HVOF and CS processes and to evaluate their
potential application for advanced industrial purposes. To
reach these objectives, Cantor starting powder was char-
acterized in terms of size, phases, shape and so on, while
each produced coating was characterized carrying out dif-
ferent wear tests. The results obtained need further inves-
tigation but are encouraging and confirm that the use of
thermally sprayed HEAs powders is viable for innovative
industrial applications.
The aim of the present paper is to understand how
processing parameters of two different deposition tech-
niques, HVOF and CS, can be optimized in order to obtain
J Therm Spray Tech (2022) 31:1184–1206 1185
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sound bulk HEA coatings. In addition, how to improve the
coatings’ mechanical properties by varying process
parameters for these two different deposition techniques is
a focus issue of the present study. After focusing on dif-
ferent porosity levels, the obtained results in terms of wear
and corrosion resistance are underlined as fundamental
properties for the deposition processes optimization.
Materials and Methods
HEAs powders characterization, as well as coatings depo-
sition, characterization and testing (adhesion strength,
hardness, corrosion resistance, rubber wheel, ball on disk,
jet erosion test), were carried out at the facilities of the
Thermal Spray Center (CPT) at the University of Barce-
lona (Barcelona, ES). All the tests were performed on the
cold-sprayed and HVOF samples. Also, the results were
compared to 316L and low C steel bulks (CM4 Enginyeria
S.L., Barcelona, ES, chemical composition in Table 1) in
order to obtain benchmark values for the coatings; 316L
and low C steel bulks have the same dimensions of the
coated substrate (50 9 20 9 5 mm).
Powders Characterization
HEAs powders (FeCrCoMnNi known as ‘‘Cantor alloy’’
were provided by Vilory Advanced Materials Technology
Ltd, CN). Powders were characterized by particle size
distribution, shape, chemical composition, XRD phase
analysis, apparent density and flow rate.
Laser scattering (LS) technique was used for particle
size distribution determination in a Beckman Coulter LS 13
320 (Brea, CA, USA) equipment, in accordance with the
ASTM B822-02 on dry via mode. Inductively couple
plasma (ICP) technique was used for chemical composition
in an equipment PerkinElmer Optima ICP-OES 3200 RL
(Waltham, MA, USA). The scanning electron microscopy
(SEM, Thermo Fisher Phenom Pro Desktop, Eindhoven,
NL) was used for image obtention. The technique x-ray
diffractometry (XRD) in an equipment Malvern PANalyt-
ical X’Pert PRO MPD h/h Bragg–Brentano with X’Pert
software (Malvern, United Kingdom) was used for phase
analysis, with font of Cu Ka (k=1.5418 A˚) and work power
45 kV–40 mA. The apparent density of the powders was
measured in accordance with the ASTM B-212-99. The
used powders sample had a volume of 40 cm3
approximately and has been followed the procedure listed
in ASTM standard. The flow rate was measured in accor-
dance with the ASTM B-213-03, applying the ‘‘Materials
and Methods’’ This method consists in to weight out a 50g
mass sample, pour the powders sample into the center of
the funnel and start a stopwatch when the powders exit the
orifice of the flowmeter funnel. When the last of the
powders exit the funnel discharge orifice, the stopwatch is
stopped. Both ASTM B-212-99 and ASTM B-213-03 tests
were repeated 5 times. The equipment Mettler AE100
(Columbus, OH, USA) was used to weight the powders to
calculate deposition efficiency. Also, nanohardness mea-
surements of the starting particles were performed at 2.5 gf
load on MTS Nano Indenter XP machine; the average
value was taken from 25 indentations per sample (Table 2).
Coatings Deposition
For cold spray depositions, PCS-100 equipment (Plasma
Giken Co., Ltd., Osato, Saitama, JP) was used. HVOF
coatings were produced using Oerlikon (Sulzer) Metco
(Pfa¨ffikon, CH) Diamond Jet Hybrid DJH 2600 equipment,
with H as fuel gas. Before the deposition process, sub-
strates were grit-blasted with alumina (F24), up to rough-
ness Ra&7 lm. The optimized process parameters used
for cold spray and the HVOF process parameters are pre-
sented in Tables 3 and 4. The strategy deposition (the robot
path on the substrate) is shown schematically in Fig. 1.
Tables 3 and 4 only list the optimized parameters used
to produce CS and HVOF coatings. The optimization
process of cold spray deposition parameters is shown in
Table 8, while in the case of HVOF process, the different
process parameters used are listed in Table 9.
In the case of cold spray deposition, the deposition
efficiency (DE%) was measured taking into account the
starting mass of powders into feeder, the remaining mass of
powders in the feeder after the deposition, the mass of
coated sample after the deposition, the starting substrate
Table 1 Chemical composition
(wt.%) of 316L and C low C
steel bulks
Bulk C Mn Si P S Cr Ni Mo Fe
316L \ 0.035 \ 2.0 \ 0.075 \ 0.040 \ 0.030 17.00 12.00 2.50 Bal.
Low C steel \ 0.17 \ 1.4 \ 0.40 \ 0.035 \ 0.035 … … … Bal.
Table 2 Chemical composition (weight %) of FeCrCoMnNi (Cantor)
alloy
Fe Cr Co Mn Ni
FeCrCoMnNi 20.09 18.86 20.96 19.27 21.01
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mass, the deposition time and the path and the time of robot
over the substrate.
Coatings Characterization
Metallographic preparation was carried out in accordance
with the ASTM E1920-03. The coatings microstructure
was observed by employing the optical microscopy Leica
DMI 5000M (OM). The Leica microscope image analyzer
was used to calculate the coatings thickness. The porosity
was calculated by grayscale threshold in OM images of the
microstructure, according to the test method B of ASTM
E2109-14, employing the ImageJ software. For each
coating, seven porosity measurements were performed. The
Shimadzu HMV (Tokyo, JP) was used for ten measures of
microhardness in Vickers scale for each coating, with load
0.1 kgf (HV0.1). Also, etching of the optimized coatings
was carried out adding some H2O droplets to NH3 ? HF ?
H2SO4 acid solution (1:1:1) and immersing the polished
cross section of the sample.
Corrosion Test
Potentiodynamic polarization measurements were carried
out, in accordance with the ASTM G59-97 and ASTM
G102-89. To determine corrosion current density, polar-
ization resistance, and corrosion rate of the coatings, a 3.5
wt.% NaCl water solution at room temperature was used.
Two different samples of each coating and reference bulk
were used for corrosion tests as working electrode, with
exposed area of 1.0 cm2. The exposed surfaces were
ground up to 1200 mesh and maximum roughness of Ra 0.3
lm. A saturated calomel (3.0 M KCl) was the reference
electrode and a platinum was the counter electrode in the
tests. A scan rate of 0.05 mV.s-1 and a potential range from
Ecorr ±25 mV were used to acquire the polarization
resistance (Rp), and Ecorr from - 250 to 1050 mV was
Table 3 Optimized parameters used for CGS process
Coating Gas Pressure Temperature Standoff distance Robot speed Powder feed Step Number of layers
CGS_N2 N2 7 MPa 1100 8C 15 mm 500 mm/s 2 rpm 1 mm 2
Table 4 Parameters used for HVOF process
Coating H2 Flow rate O2 Flow rate Air Flow rate Powder feed Standoff distance Robot speed Step Number of layers
HVOF_250 10.6 l/s 3.6 l/s 5.7 l/s 0.50 g/s 250 mm 500 mm/s 5 mm 10
Fig. 1 Deposition strategy, robot path Fig. 2 Real set-up for corrosion testing
J Therm Spray Tech (2022) 31:1184–1206 1187
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used to acquire the anodic (ba) and cathodic (bc) Tafel
slopes. Figure 2 shows the experiment set-up.
Adhesion Test
The adherence of the coatings was measured according the
ASTM C633-13. The adhesive agent used was the HTK
Ultra Bond 100 (Hamburg, D), with measured adherence
73.7±1.2 MPa (four measures were done). The Servosis
MCH-102 ME (Madrid, ES) equipment was used for the
tests, and the results of the adherence were classified
interpreting as adhesive, if the failure is completely
between the coating and the substrate, cohesive, if the
failure is intern of the coating and bonding failure, if the
coating is completely not detached from the substrate.
Sliding Wear Test (Ball on disk)
Ball on disk testing consists in the friction between a disk
and a ball, without abrasive condition. The test was carried
out in accordance with the ASTM G99-04, using a CM4
Enginierya S.L. (Barcelona, Es) equipment, as shown in
Fig. 3. The surface of samples tested was previously pre-
pared by grinding and polishing until the maximum
roughness Ra 0.8 lm. The tests were performed at room
temperature (27±2˚C) and maximum 20% moisture without
any lubricant. Process parameters are shown in Table 5.
The coefficient of friction (CoF) between the WC-Co ball
and the HEAs coating was measured, and this value was
plotted (Table 6).
Abrasive Wear Test (Rubber wheel)
In accordance with the ASTM G65-00, the low-stress
abrasion test (rubber wheel) consists in an erosion wear
with third body, where a rubber wheel rotates against the
surface of the sample. This contact is done with determined
load and a constant feed of abrasive material between the
wheel and the sample. Figure 4 shows how this abrasive
test works and the equipment (CM4 Enginierya S.L.,
Barcelona, ES). SiO2 abrasive particles produced by
Sibelco (Barcelona, ES) were fed in dry condition (less
than 0.5% moisture). The mass of the sample was mea-
sured in different elapsed times of testing, using an
equipment Mettler AE100 (Columbus, OH, USA).
The parameters of rubber wheel test are presented in
Table X. The SiO2 has a granulometry of 200 lm.
The results of the rubber wheel test are expressed using
Eq. 1,
V ¼ Vol= F:x:2:p:R:tð Þ ðEq 1Þ
where V is the ratio of volume loss in mm3N-1m-1, Vol is
the volume loss (mm3), F is the radial force of the rubber
wheel against the sample (N), x is the rotation (rpm), R is
the radius of the rubber wheel (mm), and t is the elapsed
testing time.
Jet Erosion Test
In jet erosion test (ASTM G73-10), a sample is abraded by
repeated impacts of water jets until the degradation/de-
struction of the coating. The jet erosion apparatus (Fig. 5a,
CM4 Enginierya S.L., Barcelona, ES) in CPT facilities
consists of two water jets and a central rotating arm
(Fig. 5b) that can reach high rotation speed. At the end of
the arm, a sample holder keeps the sample parallel to the
water jets. The water jets diameter is 4 mm and the process
parameters are water pressure (variable from 0.1 to 2.0
Bar), rotation speed (variable from 50 Km/h up to 350 Km/
h) and test time. The experiments were carried out at 190
Km/h, with the pressure of water set up at 1 Bar, control-
ling every 30/60 minutes the sample to measure the weight
loss and the damaged area. The test was repeated 3 times
for each sample.
Fig. 3 (a) Scheme and
(b) Equipment for ball on disk
testing, (1) Sample, (2) Ball, (3)
Load, (4) Wear path on sample
and (5) Rotation of sample
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Result and Discussion
Powders Properties
Figure 6 depicts the SEM microstructure of the starting
feedstock powders used in this investigation. Particles have
a quasi-spherical shape due to the employed gas atomiza-
tion production process (Ref 28); some particles show the
presence of satellites (Ref 31). The particles size distribu-
tion is shown in Fig. 7. A narrow size distribution can be
underlined. XRD confirmed that the crystallographic
structure of the starting powders is the FCC phase (Fig. 8)
(Ref 30) (Fig. 8).
As demonstrated in (Ref 33), powders flow rate is a
fundament characteristic for the cold spray process because
an increase in flowability of the powders leads to an
increase in deposition efficiency of the process. It is well-
known that powders with spherical shape have a higher
flowability than irregular ones and for this alloy. The
Table 5 Ball on disk parameters
Diameter of path Ball Rotation Sliding speed Sliding distance Number of cycles
14 mm WC-Co diameter 11 mm 124 rpm 0.13 m s-1 999.5 m 22737
Table 6 Rubber wheel testing parameters
Rubber wheel rotation Rubber wheel peripheral velocity Load Interval for mass measurement (elapsed time) Total time
139 rpm 1.66 m.s-1 125 N 15 s (0–60 s) 1800 s
30 s (60–300 s)
60 s (300–600 s)
300 s (600–1800 s)
Fig. 4 (a) Scheme and
(b) Equipment for the rubber
wheel testing, (1) Abrasive
feeder, (2) Rubber wheel
diameter 9 in (228.6 mm), (3)
Controller, (4) Used abrasive
container, (5) Load, (6) Sample
and (7) Rotation of the wheel
Fig. 5 Jet erosion equipment
(a) and central rotating arm (b)
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medium value of flowability for the mean particles size and
the measured density is shown in Table 7.
Besides, splat tests were carried out to investigate not
only the influence of substrate material on particles
deformability, but also the coating/substrate interaction. In
Fig. 9(a), a splatted cold-sprayed particle is shown; in
Fig. 9(b), the splatted HVOF one can be observed. If in
(Ref 30) the cold-sprayed particle is surrounded and locked
by the aluminum substrate, in this case, the particle cannot
penetrate entirely the C-Steel substrate, due to its hardness.
Despite this aspect and even if the deformation at the top of
the particle is limited, it can be confirmed the severe plastic
deformation of downside of the cold-sprayed particles. In
the case of the HVOF splat test, the particle is well-melted,
and it is well-known that thanks to the impinging of the
molten particles, the formed coatings will have a lamellar
microstructure. The growth of this lamellar microstructure
depends on particles size, process parameters as well as in-
flight particles velocity (Ref 34).
The nanohardness of the particles was measured on the
cross-section surfaces (Fig. 10a). The obtained values of
nanohardness and elastic modulus are 1.5±0.17 GPa and
Fig. 6 SEM free surface of FeCrCoMnNi powders (a) and high magnification of a particle (b)
Fig. 7 Particles distribution
size
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130±3.5 GPa, respectively. Using the equation to convert
the nanohardness measured with a Berkovich indenter into
a Vickers hardness (HV) as in (Ref 35), the obtained value
is 139 HV and the ultimate tensile strength is 460 MPa. To
explain the difference between these values and those in
(Ref 35), it should be taken into account the different
applied loads. The used Poisson ratio was 0.29, which is
the same as the stainless steel and as underlined in (Ref
Fig. 8 XRD pattern of the as
received HEAs particles
Table 7 Particles distribution,
apparent density and flow rate
of the FeCoCrMnNi powders
Particle size distribution (lm) Apparent density (g cm-3) Flow rate (g s-1)
Mean 34.94 4.44 ± 0.02 3.04 ± 0.02
Fig. 9 Splat tests of CGS_N2 (a), HVOF_250 (b)
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36), the most important aspect is that in the plastic region
Cantor alloy is stronger than a single Ni particle because in
HEAs alloys, each element is represented in the same
quantity and so that, each element plays the same role in
lattice distortion. Even if the obtained results in terms of
nanohardness and elastic modulus are lower than other
HEAs (Ref 37, 38), it was shown how the Cantor alloy
behavior could be compared to stainless steel.
Coatings Deposition
The optimization of cold spray and HVOF coatings was
carried out by varying not only carrier gas pressure and
temperature, but also the stand-off distance between the
gun and the substrate. The followed procedure was to
detect the coatings porosity and the deposition efficiency as
a function of the tuned processing parameters. They were
chosen because in our previous experiences the coatings
properties are very sensitive to the variation of the selected
parameters (Ref 39, 40). To obtain thick and dense cold-
sprayed coatings, the tested gas pressures were 5 MPa, 6
MPa, and 7 MPa, while the temperatures were 1000 8C and
1100 8C (Table 8).
In Fig. 11(a), we can see the coating deposited at 5 MPa-
1000 8C coating, while in Fig. 11(b), the one deposited at 6
MPa-1000 8C one, both were sprayed at 30 mm of SOD.
As demonstrated in (Ref 24, 41-45), the increase in
carrier gas temperature and pressure leads to an increase in
particles in-flight velocity and consequently, to an increase
in deposition efficiency as well as a decrease in porosity.
Based on our previous experience with Inconel 625 (Ref
44) and also with stainless steel (Ref 45), the higher
pressure and temperature, the better results on density and
DE. Also, previous papers on HEAs deposited using cold
spray process (Ref 30, 32) are used to set starting process
parameters and to carry on the optimization process. TheFig. 10 Load–displacement curve
Table 8 Cold-sprayed coatings
optimization
Pressure (MPa) Temperature (8C) SOD (mm) Thickness (lm) Porosity (%) DE (%)
50 1000 30 237 ?/-37 8 52
60 1000 30 382 ?/- 28 3 65
70 1100 30 … … …
70 1100 15 593 ?/- 31 \1 87
Fig. 11 5MPa-1000 8C coating (a) and 6MPa-1000 8C coating (b)
1192 J Therm Spray Tech (2022) 31:1184–1206
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coating deposited at 7 MPa-1100 8C and 30 mm of SOD
showed a de-cohesive behavior; this was attributed to the
higher temperature of in-flight particles.
The higher thermal input transferred to the particles
reduces their deformation strain at impact, what can cause
a loose of cohesive strength, resulting in higher strain
produced at lower temperature. In these spraying condi-
tions, the previous layer of deposited particles does not
deform enough during the impact, leading to a non-optimal
cohesion between the different layers. The microstructure
of the optimized coating sprayed at 7 MPa and 1100 8C
and 15 mm SOD is shown in Fig. 12. In general terms, it is
well-known that the bow shock effect depends on standoff
distance and, as a result of a reduction in particle velocity,
it has a negative influence on deposition efficiency. Near
the substrate, the bow shock effect causes a strong increase
in the turbulent kinetic energy and for this reason, the
particles are slowed down. Obviously, smaller particles are
more affected than bigger ones. For example, smaller
particles behavior is like a dust that does not impact on the
substrate at all or is too slow for bonding. Thus, the bow
shock effect on the particles velocity is related to the
particles size, density and shape (Ref 46-49). In our study,
even if we have sprayed at a short standoff distance, the
bow shock effect is mitigated from the particle density and
size because the specific weight of Cantor alloy is similar
to that of another material previously sprayed from the
authors (Ref 50) at SOD of 10 mm and median particles
size is close to 35 lm. It is well-known that the density of
the cold-sprayed coatings depends on process parameters,
as well as the shape and the size distribution of the starting
powders (Ref 46). Even if in (Ref 47) is reported that
porosity increases with increasing the powder d10 value, in
this case, a less than 1% porosity level is reached thanks to
the high flattening of sprayed particles. It is worth
remembering that this porosity level is evaluated as a mean
value of seven images.
The aspect of the coatings deposited through HVOF in
different conditions is shown in Fig. 13, while Table 9 shows
the process parameters used. They were chosen because of
our previous experience with other materials, such as Ni-
based superalloys and stainless steel (Ref 53). To improve
the quality of the coatings, the focus was on SOD distance
using already known process conditions (Ref 54).
Figure 14 shows the etched aspect of the cold-sprayed
and HVOF coating deposited using the optimized process
conditions. The etching reveals the particle boundaries, the
dendritic microstructure as well as the deformation and the
width and height of individual splats. In the case of cold-
sprayed coating, it is well-shown the deformation and theFig. 12 HEA coating cold sprayed at 7MPa-1100 8C and 15 SOD
Fig. 13 HVOF_225 (a) and optimized HVOF_250 coating (b)
J Therm Spray Tech (2022) 31:1184–1206 1193
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pancake-like shape of the particles, and the image can be
used to definite the flattening ratio (Ref 46, 49), consider-
ing the larger over the smaller particle dimension of the
splatted particle.
Mechanical and Corrosion Properties
The XRD pattern of the CS and HVOF optimized coatings
is shown in Fig. 15; for comparison, the results belonging
to the as-received particles are provided. As mentioned
above, the peaks of as-received particles are characteristic
of FCC structure. As expected, cold-sprayed coating shows
no phase change, while in the case of HVOF coating, there
are new peaks. In particular, in Fig. 16, the high magnifi-
cation resolution shows that those peaks belong to iron
oxide and manganese-chrome oxide formation.
Based on XRD analysis, the cooling rate in the case of
HVOF process does not affect the HEAs structure because
it retained the starting FCC structure. Even if Fig. 16 shows
a certain degree of oxidation, there is no shifting of main
peaks, and so the dynamic amorphization process is avoi-
ded (Ref 56).
The nanoindentations performed on the optimized
coatings are shown in Fig. 17. The hardness value of
Table 9 HVOF process parameters
Coating H2 Flow rate O2 Flow rate Air Flow rate Powder feed Standoff distance Robot speed Step Number of layers
HVOF 225 12 l/s 2.45 l/s 6.5 0.50 g/s 225 mm 500 mm/s 5 mm 10
HVOF_250 10.6 l/s 3.6 l/s 5.7 l/s 0.50 g/s 250 mm 500 mm/s 5 mm 10
Fig. 14 Etching of CGS_N2 (a) and HVOF_250 coatings (b)
Fig. 15 XRD of different coatings
1194 J Therm Spray Tech (2022) 31:1184–1206
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HVOF coating is a slightly greater (390±10 Hv0.1) than the
hardness value of cold-sprayed coating (382±6 Hv0.1).
These similar values are due not only to the very low
porosity level of both coatings, but also to the new phase
formation (Ref 57, 58) during HVOF deposition process
(Fig. 16). These oxides make comparable the hardness of
HVOF coatings to the hardness obtained thanks to the high
plastic deformation and the work-hardening structure of the
cold spray process (Ref 59, 60). According to (Ref 46), also
powders morphology affects hardness because irregular
shape leads to higher hardness values, thanks to a greater
defect density. For comparison, the hardness value of 316L
and C steel bulks are 300±18 Hv0.1 and 220±17 Hv0.1,
respectively.
All the obtained results are summarized in Table 10
along with the indication of the measured adhesion strength
of the optimized coatings. In (Ref 60), the authors clearly
defined the three different formation mechanisms of the
Fig. 16 High magnification of
XRD HVOF
Fig. 17 High magnification indents of CGS_N2 (a) and HVOF_250 (b)
Table 10 Coatings characterization
Adhesion strength (MPa) Hv0.1
CGS_N2 27±5 382 ± 6.0
HVOF_250 25±5 390 ± 10.0
316L Bulk … 300 ± 18.0
C_Steel_Bulk … 220 ±17.0
J Therm Spray Tech (2022) 31:1184–1206 1195
123
bonding at the coating/substrate interface. Even if in this
case the intermixing phenomenon is not plainly visible due
to the substrate hardness, the adhesion strength value is
reached thanks to the severe plastic deformation of parti-
cles that generate mechanical interlocking. In the case of
coatings sprayed at 1100 8C and a standoff distance of 30
mm, the in-flight thermal softening of the particles could be
the reason for low adhesion strength. To avoid this issue,
the standoff distance was reduced, increasing the particle
impact effect on the substrate and on the layer previously
deposited. For this reason, the results reached at 7MPa-
1100 8C and a stand-off distance of 15mm, due to the
greater plastic deformation of the particles, are better than
those reached applying the same condition but at a stand-
off distance of 30 mm.
Table 11 shows not only the corrosion potential and the
corrosion current values of the two optimized coatings, but
also those of the C-Steel and 316L bulks used to carry out
the benchmark. Both the cold-sprayed and HVOF coatings
protect the C-Steel substrate, improving significantly its
performance because they well impede the path of the
electrolyte to the substrate due to their very low porosity
levels. In general, the cold-sprayed coatings show higher
Ecorr and higher Icorr than HVOF coatings, due to the dif-
ferences of the microstructure. Probably, in the case of
cold-sprayed coatings, the Icorr founds the path through the
grain boundaries of the deformed particles. Taking into
account the results shown in Table 11, we can classify from
high-to-low Ecorr as follows (Fig. 18): CGS_N2,
316L_Bulk, HVOF_250 and C_Steel_Bulk. These results
are explained by the Cr and Ni content percentage (20% for
HEAs, 17 and 12%, respectively, for 316L_Bulk) and by
the oxidation of HVOF deposition process. As demon-
strated above by XRD analysis, the presence of cobalt
oxide and Fe oxide slightly worsen the Ecorr performance
of the HVOF coatings. Thanks to its intrinsic properties,
the 316 bulk sample exhibited better Icorr resistance, fol-
lowed by the cold-sprayed and HVOF coatings. Supposing
that these coatings would be work as a protective layer
between the environment and the substrate, the results
reached are encouraging.
The coefficient of friction between a WC-Co ball and
both the coatings and the bulks was measured through the
ball-on-disk test. Figure 19 presents the evolution of the
coefficient of friction that was calculated when the system
reached a stationary behavior, after 15.000 cycles. Table 12
shows that the CoF of both coatings is higher than that of
both bulks due to wear mechanism.
Figures 20 and 21 show the wear tracks of optimized
CGS_N2 and HVOF_250 coatings, respectively, while
Figs. 22 and 23 show the occurred oxidation of both
optimized coatings through the EDS mappings.
The wear mechanism is adhesive type and
Fig. 20(b) and (b) show a large amount of debris rolled,
adhered and oxidized on the wear track. This mechanism is
more visible in the cold-sprayed coatings, due to lower
cohesion strength between the plastic deformed particles in
comparison with melted particles of HVOF process. For
this reason, to improve the mechanical properties of cold-
sprayed coatings, some authors have studied the effects of
heat treatments on the coatings after the cold spray depo-
sition (Ref 61-63). Also, Fig. 24(a) and (b) show the width
of the wear track for both coatings, and these values are
been used to calculate the friction wear rate, as recom-
mended by ASTM G99-04.
After 30 minutes of rubber wheel tests, the wear rate
found for both cold-sprayed and HVOF coatings was lower
than that of both 316L and carbon steel bulks. The abrasive
wear rates obtained after the stabilization of the mass loss
were 1.6 9 10-4, 1.8 9 10-4, 2.0 9 10-4 and 2.2 9 10-4
mm3/Nm for samples HVOF_250, CGS_N2, 316L_Bulk
and C_Steel_Bulk, respectively (Table 12). The better
performance of HVOF_250 is due to the formation of hard
new phases, as shown by the XRD analysis. The presence
of oxides in the HVOF coatings ensures a greater wear
resistance than the cohesion of mechanically bounded
particles deposited by the cold-sprayed process. It is well-
known that the abrasion mechanism causes the removal of
the softest material, and the wear resistance of coating
depends on its porosity, hardness and composition (Ref
50). Besides, the thermal-sprayed coatings showed a better
wear rate compared to both bulks due to the different
values of surface roughness, because in the case of the
coatings, it was higher than that substrates. In addition, the
SiO2 sand abrasive particles have a larger size (& 200 lm)
compared to the size of HEAs particles in the coatings, as
was revealed by the chemical etching (Fig. 14). As
demonstrated in (Ref 51), this means that the pressure
generated between the sand particles and the coatings pulls
out the HEAs particles from the coating. This ‘‘pull out’’
mechanism (see the rounded black holes in Fig. 25(a) and
(b)), combined with the higher hardness of the coatings,
allows for less material loss than the plastic deformation
mechanism observed for a substrate without coating (Ref
Table 11 Corrosion potential and current
E (mV) I (lA)
316L BULK - 195 0.04
C STEEL BULK - 730 0.50
CGS_N2 - 65 1.20
HVOF_250 - 440 0.29
1196 J Therm Spray Tech (2022) 31:1184–1206
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Fig. 18 Polarization curves
Fig. 19 CoF—ball on disk
Table 12 Wear behavior
Abrasion rate (mm3 N-1 m-1) CoF Friction wear rate (mm3 N-1 m-1)
316L BULK 2.0 9 -4 ± 7.0 9 10-5 0.65 ± 0.08 1.69 9 10-4
C STEEL BULK 2.2 9 10-4 ± 6.5 9 10-5 0.53 ± 0.01 3.06 9 10-5
CGS_N2 1.8 9 10
-4 ± 5.3 9 10-5 0.73 ± 0.04 2.80 9 10-4
HVOF_250 1.6 9 10-4 ± 5.6 9 10-5 0.81 ± 0.11 6.70 9 10-5
J Therm Spray Tech (2022) 31:1184–1206 1197
123
51). Thereby, both coatings improved the abrasive wear
properties of the substrates (Fig. 26).
The erosion by water droplets impact is another crucial
wear mechanism for different industrial sectors such as
wind turbine blades, powerplant, and aeronautic applica-
tions (Ref 51-53). According to ASTM G73-10, the water
jet erosion mechanism is the same observed during cavi-
tation erosion tests, and the ideal case study shows that the
Fig. 20 (a) Wear track on CGS_N2 coating and (b) High magnification
Fig. 21 (a) Wear track on HVOF coating and (b) High magnification
1198 J Therm Spray Tech (2022) 31:1184–1206
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erosion slopes usually divide into three zones. The first
zone is the so-called incubation zone, in which there is no
material erosion; the second zone is characterized by the
acceleration of erosion rate up to a maximum value and
finally, the third zone shows a steady-state mass lost rate.
As reported in (Ref 53-55) for other thermally sprayed
coatings, also in our investigation in no case the incubation
stage was observed (see Fig. 27), due to the brittle of both
coatings and both bulks. Between the tested materials, the
316L bulk has better erosion resistance, while the CGS_N2
coating is the worse. The poor erosion resistance of cold-
sprayed coating could be explained looking at Fig. 28,
where the SEM image of droplets impact zone is shown.
The continuous and prolonged impact of the water rips
away the rounded particles that are not melted as in the
case of HVOF deposition process. Here too, the erosion
resistance of cold-sprayed coatings depends on the cohe-
sion strength between the particles, which is not enough to
resist the liquid impingement. In the case of C steel bulk, it
is shown how it well-resists during the initial stage of the
Fig. 22 EDS mapping of (a) CGS_N2 coating and (b) C, (c) W, (e) O
J Therm Spray Tech (2022) 31:1184–1206 1199
123
Fig. 23 EDS mapping of (a) HVOF_250 coating and (b) C, (c) W, (e) O
Fig. 24 Wear track width of (a) CGS_N2 coating and (b) HVOF_250 coating used for friction wear rate
1200 J Therm Spray Tech (2022) 31:1184–1206
123
test but after 350 minutes, and it is well-visible to the
change of the slope. This is due not only to the erosion
mechanism, but also to the corrosion of the hitting zone.
Conclusion
• A complete HEAs starting powders characterization
was carried out, in terms of size, shape, flow rate,
density, microstructure, and nanohardness. In particu-
lar, obtained nanohardness results showed a behavior
similar to stainless steel.
• Dense and thick Cantor alloy coatings were obtained
onto a C steel substrate by cold spray and HVOF
processes. The microstructural and the mechanical
characterization of coatings were carried out, and their
wear resistance was investigated. In the case of cold
spray process, coatings optimization has been
Fig. 25 SEM images of (a) CGS_N2 and (b) HVOF_250 rubber wheel wear track
Fig. 26 Wear rate and rubber
wheel
J Therm Spray Tech (2022) 31:1184–1206 1201
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performed starting from 5 MPa-1000 8C, 6MPa-1000
8C, and 7MPa-1100 8C, stand-off distance 30 mm. It
was clear that at 5 MPa pressure, the quality of the
coating is poorer, and the deposition efficiency is
smaller. There is a clear trend of decreasing porosity
and increasing deposition efficiency when raising up
the pressure and the temperature, being better at 7 MPa
and 1100 8C, but then there was a problem with
adhesion. The thermal softening of the particle when
increasing temperature up to 1100 8C at 30 mm could
be the reason for decreasing adhesion, and that is why
the distance was reduced in this case to increase the
particle impact effect on the substrate and solve that
problem. The best results have been obtained at 7MPa-
1100 8C and a stand-off distance of 15mm due to the
greater plastic deformation of the particles. Under the
same conditions but at a stand-off distance of 30 mm,
the particles partially melt in flight, so that when they
reach the substrate, the plastic deformation level is
lower. For the cold-sprayed optimum conditions, a
deposition efficiency of 87% is obtained. The thickness
measured for a two-layer cold spray coating was
593±31 lm, and for a ten-layer HVOF coating was
210±20 lm. Both obtained coatings showed very low
porosity\1%.
• As expected, no phase changes are observed using the
cold spray process, while the formation of Fe and MnCr
oxides is visible in HVOF coatings. The presence of
these oxides leads to a hardness value similar for both
process deposition. A hardness of (382±6) HV0,1 has
been measured for the CS coating and a value of
(390±10) HV0,1 for the coating deposited by HVOF.
These values are very similar taking into account that
using the CS technique no oxides are obtained. For the
HVOF coatings, oxides provide an increase in surface
hardness and better wear resistance. It could be useful
to underline that in the case of HVOF process, the
obtained results depend strictly on torch used. Other
HVOF equipment may produce coatings of different
quality.
• The dense cold-sprayed coating does not allow the path
of the electrolyte toward the substrate, and its Ecorr
value depends on the high Cr and Ni content. For the
Fig. 27 Jet erosion test results
1202 J Therm Spray Tech (2022) 31:1184–1206
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HVOF coating, the presence of the oxides slightly
worse the Icorr value.
• In the case of the rubber wheel, both tested coatings
showed very similar values, due to the similar surface
hardness. HVOF coating suffered less wear than cold-
sprayed one.
• Due to its molten particles, the ball on disk test for the
HVOF coating showed a lower wear rate than the cold-
Fig. 28 Jet erosion impact zone SEM image of (a) CGS_N2, (b) HVOF_250, (c) C_Steel_Bulk and (d) 316L_bulk
J Therm Spray Tech (2022) 31:1184–1206 1203
123
sprayed one. The HVOF particles are characterized by
higher cohesive strength than cold-sprayed particles,
and the WC-ball has more difficulty pulling them out.
• In the jet erosion test, the HVOF coating and the 316L
bulk showed the same trend, even if the starting
behavior is different due to the different surface
roughness. The C-steel bulk showed a good erosion
resistance in the initial stage of the test, but its wear rate
worsening due to the combined erosion and corrosion
effect. For the cold-sprayed coating, it has been
observed the same behavior of rubber wheel test, due
to poor cohesion strength between the particles.
• Both cold-sprayed and HVOF Cantor alloy coatings
protected the substrate, although not in the same way.
The cold-sprayed coating was effective in increasing
corrosion resistance, but it did not withstand erosion.
By contrast, HVOF coating performed well in all tests,
including erosion tests, due to its molten particles.
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