Y. JUAN et al.: WEAR-RESISTANT SINGLE-PHASE HIGH-ENTROPY ALLOY COATINGS ...
751–758
WEAR-RESISTANT SINGLE-PHASE HIGH-ENTROPY ALLOY
COATINGS PREPARED ON TI6AL4V WITH LASER CLADDING
OBRABNO OBSTOJNA ENOFAZNA VISOKO-ENERGIJSKA
PREVLEKA, NANE[ENA Z LASERSKIM NAVARJANJEM NA
OSNOVO IZ ZLITINE Ti6Al4V
Yongfei Juan, Jun Li
*
, Yunqiang Jiang, Wanli Jia, Zhongjie Lu
Shanghai University of Engineering Science, School of Materials Engineering, No. 333 Longteng Road, Songjiang District,
201620 Shanghai, China
Prejem rokopisa – received: 2019-01-22; sprejem za objavo – accepted for publication: 2019-05-17
doi:10.17222/mit.2019.018
Excellent wear-resistant high-entropy alloy (HEA) coatings were successfully synthesized on Ti6Al4V with laser cladding using
FeCrCoNiAlMo x (x = 0.5, 1.0, 1.5) as the cladding materials. Investigations into the effect of Mo on the microstructures and me-
chanical properties (microhardness and wear resistance) of the coatings were carried out. The results showed that all the coat-
ings exhibited good metallurgical bonding with the substrate where no clear defects were found. The dilution rate of the coat-
ings increased with an increase in the content of Mo (60.39 % for x = 0.5, 64.09 % for x = 1.0, 72.68 % for x = 1.5). The coating
for x = 1.0 was mainly composed of a single solid solution (BCC). Besides the solid solution, traces of TiC dendrites were also
synthesized in the other two coatings (x = 0.5 and x = 1.5). The solid solution exhibited two different morphologies, correspond-
ing to fine rod-like particles and coarse equiaxial grains. Due to the solution- and dispersion-strengthening effects, the hardness
of the coatings (759.4 HV0.2 for x = 0.5, 857.2 HV0.2 for x = 1.0, 844.3 HV0.2 for x = 1.5) was significantly superior to that of the
substrate (330 HV0.2). The friction coefficient of the coatings (0.80 for x = 0.5, 0.61 for x = 1.0 and 0.78 for x = 1.5) was obvi-
ously reduced when compared with that of the substrate (1.04). The wear volumes of the coatings were decreased by 51 %,
65 % and 48 % (1.145 ± 0.021 mm
3
for x = 0.5, 0.823 ± 0.012 mm
3
for x = 1.0, 1.219 ± 0.018 mm
3
for x = 1.5) when compared
with that of the substrate (2.329 ± 0.025 mm
3
). It can be confirmed that the coating with x = 1.0 exhibited the optimum wear re-
sistance.
Keywords: laser cladding, high-entropy alloy, Ti6Al4V, wear resistance
Avtorji so uspe{no sintetizirali obrabno obstojno prevleko na osnovi visoko-entropijske zlitine (HEA; angl.: High Entropy Al-
loy) na podlago iz Ti6Al4V, z laserskim navarjanjem. Za navarjanje so uporabili zlitino na osnovi FeCrCoNiAlMo x (x = 0,5, 1,0,
1,5). Izvedli so tudi raziskave vpliva Mo na mikrostrukture in na mehanske lastnosti (mikrotrdoto in odpornost proti obrabi)
prevleke. Rezultati so pokazali, da imajo prevleke dobro metalur{ko povezavo s podlago, na kateri sicer niso na{li jasnih napak.
Hitrost raztapljanja prevleke s podlago je nara{~ala s pove~evanjem vsebnosti Mo (60,39 % za x = 0,5, 64,09 % za x=1,0 in
72,68 % za x = 1,5). Prevleka z x = 1,0 je bila v glavnem enofazna homogena trdna raztopina s telesno centrirano kubi~no
zgradbo (BCC; angl.: Body Cubic Centred). Poleg homogene trdne raztopine so se v drugih dveh sintetiziranih prevlekah,
x = 0,5 in x = 1,5, nahajali {e sledovi dendritov TiC. Trdna raztopina je imela dve razli~ni morfologiji s finimi pali~astimi delci
in grobimi enakoosnimi kristalnimi zrni. Zaradi u~inkov raztapljanja in disperzije so imele prevleke v primerjavi s podlago (330
HV0.2) precej vi{je trdote: 759,4 HV0,2 za x = 0,5, 857,2 HV0,2 za x = 1,0, 844,3 HV0,2 za x = 1,5). Koeficient trenja prevlek: 0,80
za x = 0,5, 0,61 za x = 1,0 in 0,78 za x = 1,5, se je o~itno zmanj{al v primerjavi s podlago (1,04). Volumska obraba je bila manj{a
za 51%, 65 % in 48 % (1,145±0,021 mm
3
za x = 0,5, 0,823±0,012 mm
3
za x = 1,0 in 1,219±0,018 mm
3
za x = 1,5) v primerjavi s
podlago (2,329±0,025 mm
3
). Avtorji zaklju~ujejo, da je imela prevleka s sestavo x = 1,0 optimalno odpornost proti obrabi.
Klju~ne besede: lasersko navarjanje, visoko-energijska zlitina, Ti6Al4V, odpornost proti obrabi
1 INTRODUCTION
In modern industry, benefited from the excellent
properties such as low density, high strength, excellent
corrosion resistance and biocompatibility,
1
titanium al-
loys have been applied to various fields, such as aero-
space, marine industries, petrochemical and medical ap-
plication. However, poor wear resistance and low
hardness seriously restrict their wider application.
2
At
present, nitriding is the main method for improving the
wear resistance of titanium alloys. Yang et al. applied in-
termittent vacuum gas nitriding (IVGN) to improve the
wear resistance of the TB8 titanium alloy.
3
A dense
nitride layer and a nitrogen diffusion zone formed on the
alloy surface after heat treatment at 780 °C for 4 h. The
hardness of the sample reached 850–900 HV. The fric-
tion coefficient of the nitrided sample (0.064–0.082
within 12 min) was clearly lower than that of the un-
treated sample (0.483), indicating that the wear resis-
tance was greatly improved due to the formation of a
gradient hardened layer.
S. Takesuea et al. developed a rapid nitriding process
for titanium alloys at a low temperature, described as gas
blow induction heating (GBIH) nitriding accompanied
with fine-particle peening (FPP).
4
Results showed that
the nitrogen diffusion into titanium was accelerated by
the pre-treatment with FPP during the GBIH nitridig pro-
cess. Due to the formation of a hard nitrogen compound
Materiali in tehnologije / Materials and technology 53 (2019) 5, 751–758 751
UDK 621.791.725:621.793.8:669.295:669.71 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(5)751(2019)
*Corresponding author's e-mail:
jacob_lijun@sina.com

layer on the alloy surface, its wear resistance increased
by about three times compared with that of the substrate,
and its hardness (5.8±1.3 GPa) was higher than that of
the substrate (3.4±0.3 GPa). In addition, other sur-
face-engineering techniques such as physical vapor de-
position (PVD) and micro-arc oxidation (MAO) are also
adopted to enhance the wear resistance of titanium al-
loys. Unfortunately, the coatings synthesized with the
above processes are usually thin (0.1–5 μm for PVD,
10–20 μm for MAO, 20–30 μm for nitriding),
5–7
which
greatly shortens their service life when a large impact
load is applied to them in critical working conditions.
Comparatively speaking, laser cladding has attracted
more attention in recent year. A good metallurgical
bonding can be formed between the coatings and the
substrates, the thickness of the coatings can reach up to
the millimeter level, while their mechanical properties
such as hardness and tensile strength can be flexibly ad-
justed by selecting different cladding materials.
8–11
Many
metal matrices reinforced with ceramic particles have
been fabricated to improve the wear resistance of tita-
nium alloys.
12
However, the presence of a large number
of intermetallic compounds in laser-clad coatings can
cause a decrease in plasticity, which reduces their
machinability and increases their cracking susceptibility.
In recent twenty years, a new alloy-design concept of
a high-entropy alloy (HEA) has attracted considerable at-
tention. The main phase of such an alloy is a solid solu-
tion without or with traces of hard and brittle inter-
metallic compounds.
13
The two shortcomings mentioned
above can be overcome without significantly weakening
the wear resistance. N. Tüten et al.
14
prepared a
TiTaHfNbZr HEA coating on Ti6Al4V using laser clad-
ding. The average values of the Vickers hardness and
elastic modulus of the HEA coating (12.51±0.34 GPa
and 181.3±2.4 GPa) were higher than those of the sub-
strate (3.46±0.17 GPa and 115±1.4 GPa). The friction
coefficient of the coating (about 0.2) was about one third
of that of the substrate (about 0.6), which indicated that
the wear resistance of the coating was better than that of
the substrate. H. Li et al.
15
synthesized AlB
x
CoCrNiTi
HEA coatings on Ti6Al4V using laser cladding. The
coatings were mainly composed of the BCC phase. And
due to the addition of B, the grain size of the coatings
was refined. When x was 1.0, the hardness of the coating
reached the highest value (814 HV) and the wear rate
(mm
3
·h
–1
) was one seventh of that of the coating without
the addition of B. Obviously, deposition of protective
HEA coatings on their surfaces with laser cladding is a
very practical way to improve the wear resistance of tita-
nium alloys.
Present investigations into a laser-clad HEA coating
mainly focus on the selection and optimization of con-
stituent elements for the cladding material so that the
coating can be endowed with an excellent wear resis-
tance on the basis of the solid solution as the main syn-
thesized phase.
16
Many studies have confirmed that simi-
lar atomic radii of different elements are beneficial to the
formation of the solid solution for HEAs. Cr, Co, Fe and
Ni are often selected as the main elements of HEAs due
to their similar atomic radii (0.1248 nm for Co, 0.1241
nm for Fe, 0.1246 nm for Ni).
17–20
However, the solu-
tion-strengthening effect is weakened to a certain extent
for a solid solution composed of elements with similar
atomic radii. Therefore, some non-metallic elements
(such as B) and metallic elements with a great difference
in the radius are often added to improve the solu-
tion-strengthening effect.
21
The non-metallic elements
easily react with the metallic elements, resulting in the
formation of intermetallic compounds.
22
When compared
with the non-metallic elements, the metallic elements not
only enhance the solution-strengthening effect, but also
inhibit the formation of compounds.
23
Mo with excellent
high-temperature strength and high hardness can be cho-
sen as the other main strengthening element owing to its
significant difference from Cr, Co, Fe, Ni in the atomic
radius (0.1362 nm). However, excessive Mo in alloys
may lead to the formation of unfavorable intermetallic
compounds; therefore, it is extremely important to deter-
mine the appropriate Mo content in HEA coatings. Un-
fortunately, the effect of Mo on the microstructure and
wear resistance of the laser-clad HEA coatings prepared
on titanium alloys is hardly noted in present investiga-
tions.
Based on the above analyses, Cr, Co, Fe, Ni and Mo
were chosen as the main constituent elements for the
HEA coatings on Ti6Al4V applied with laser cladding in
this work. Al was also added due to its positive effect in
refining the grain size and improving the strength. The
work comprehensively investigated the effect of Mo in
FeCrCoNiAlMox (x = 0.5, 1.0, 1.5) on the microstruc-
ture, phase constituents, microhardness and wear resis-
tance of the coatings. After that, the optimum amount of
Mo was confirmed.
2 EXPERIMENTAL PART
The Ti6Al4V titanium alloy (annealed at 800 °C for
one hour) was selected as the substrate and processed
into a cylinder with a diameter of 50 mm and a height of
10 mm, then polished with 150-grit SiC abrasive papers
and cleaned ultrasonically in acetone for 15 min before
laser cladding. Pure Fe, Cr, Co, Ni, Al and Mo commer-
cial powders were weighed in the three molar ratios
(FeCrCoNiAlMox, x = 0.5, 1.0, 1.5) and mixed for 8 h in
a ball grinding mill. A modified pre-placed method was
applied to prepare the layer (Figure 1).
24
The substrate
was placed into a hollow cylinder with a height of
11.0 mm and an inner diameter of 50.2 mm to allow the
empty space with a height of 1.0 mm above the substrate.
The organic binder (4 % polyvinyl alcohol) was pasted
onto the substrate surface and then the empty space was
filled with the mixed powder. Finally, the mixed powder
was pressured by a tablet machine at 30 MPa with a
Y. JUAN et al.: WEAR-RESISTANT SINGLE-PHASE HIGH-ENTROPY ALLOY COATINGS ...
752 Materiali in tehnologije / Materials and technology 53 (2019) 5, 751–758

holding time of 3 min to form a pre-placed layer with a
thickness of approximately 1.0 mm. An YLS-5000 fiber
laser system was applied to obtain single-track coatings.
The cladding parameters were as follows: a laser power
of 3.0 kW, a spot diameter of 6 mm and a scanning speed
of 5 mm/s.
The macromorphology and microstructure of the
coatings were observed with a VHK-600 optical digital
microscope and an S-3400 scanning electron micro-
scope. A PANalytical X’Pert X-ray diffraction diffracto-
meter and an EDAX energy dispersive spectrometer
were used to analyze the phase constituents and chemical
compositions of the coatings. The microhardness of the
coatings was measured with an HXD-1000 TMSC/LCD
Vicker’s microhardness tester with a load of 1.96 N ap-
plied for 15 s. A CFT-1 ultra-functional wear-test ma-
chine was used to investigate the wear resistance of the
coatings in dry-sliding conditions. Dry-sliding-wear test
parameters were as follows: an applied load of 30 N, a
reciprocating distance of 5 mm, a reciprocating sliding
speed of 0.1 m·s
–1
, a wear test time of 180 min and the
total distance of 1080 m. A 4-mm-diameter Si
3
N
4
ball
was chosen as the counterpart due to its excellent
high-temperature stability (up to 1000 °C) and high hard-
ness (up to 35 GPa). The former makes Si
3
N
4
retain its
structural stability when exposed to a high temperature
resulting from a lot of friction heat during dry-sliding.
The latter can accelerate the wear of the test coatings in a
short period. The contact stress between the friction pairs
is closely related to the wear rate of the coatings. The
initial contact stress can be calculated since the initial
contact area is approximately constant. Based on the es-
tablished contact model between the rigid sphere and the
plane,
25
the initial Hertzian contact stress was calculated
as 765 MPa.
3 RESULTS AND DISCUSSION
Figure 2 shows the cross-sectional macromorphol-
ogies of the coatings. The coatings are relatively smooth
and no clear defects such as holes and cracks can be
found. It can be observed that the thickness of the coat-
ings increased with the increase in the content of Mo
(2.64 mm for x = 0.5, 2.73 mm for x = 1.0 and 2.82 mm
for x = 1.5). Further calculations show that the dilution
rate also presents the same change trend (60.39 % for
x = 0.5, 64.09 % for x = 1.0, 72.68 % for x = 1.5), illus-
trating that an addition of the Mo element can signifi-
cantly improve the bonding strength between the sub-
strate and the coatings. This phenomenon is closely
related to the high laser absorptivity of Mo. The laser ab-
sorptivity of Mo (0.43) is higher than those of the other
elements (0.09 for Al, 0.26 for Ni, 0.33 for Co, 0.36 for
Fe and 0.42 for Cr), meaning that more laser energy can
be absorbed by the cladding material with a higher Mo
content, causing a more serious melting of the substrate.
Figure 3 shows high-resolution micrographs of the
interfaces in the coatings with different contents of Mo.
Y. JUAN et al.: WEAR-RESISTANT SINGLE-PHASE HIGH-ENTROPY ALLOY COATINGS ...
Materiali in tehnologije / Materials and technology 53 (2019) 5, 751–758 753
Figure 2: Macromorphology of the coatings with different contents of
Mo: a) FeCrCoNiAlMo
0.5
, b) FeCrCoNiAlMo
1.0
, c) FeCrCoNi-
AlMo
1.5
Figure 1: Schematic drawing of the preparation of the pre-placed
layer

All the coatings exhibit typical melting-solidifying char-
acteristic, being free of cracks and porosities at the inter-
faces. A fusion line can be clearly observed at the inter-
face between the substrate and the coatings, which
further indicates that a good metallurgical bonding was
formed.
Figure 4 shows X-ray diffraction patterns of the coat-
ings. A comparison between the experimental data and
the standard values from JCPDS cards (Joint Committee
on Powder Diffraction Standards) show that the coatings
are mainly composed of the BCC solid solution (no.
03-065-9394) accompanied with the traces of TiC (no.
00-031-1400). When x is increased from 0.5 to 1.0, the
diffraction peaks related to TiC are significantly de-
creased in intensity (2 mm
3
= 35.9° and 41.7°) and they
even disappear (2 mm
3
= 72.2°). However, with x in-
creased to 1.5, the diffraction peaks associated with TiC
appear again and they even increase in intensity. It can be
concluded that a laser-clad HEA coating composed of a
single solid solution is successfully synthesized when x
is 1.0.
Figure 5 shows the cross-sectional microstructures of
the coatings with different contents of Mo (x = 0.5, 1.0,
1.5). It is obvious that the coatings are mainly composed
of three phases with different morphologies, correspond-
ing to fine, gray, rod-like particles (marked as 1), coarse,
gray, equiaxial grains (marked as 2) and fine black den-
drites (marked as 3). The gray equiaxial phase with the
highest fraction volumes can be regarded as the matrix of
the coatings, and the other two phases can be seen as the
reinforcements. The dendrites are uniformly distributed
throughout the whole matrix; however, the rod-like parti-
cles are mainly located along the grain boundaries of the
matrix. EDS was applied to analyze the chemical compo-
sitions of the phases with different morphologies in the
coatings. As shown in Table 1, the dendritic phase is rich
in Ti and C (53.11 at/% Ti and 43.84 at/% C). Combined
with the XRD results, the phase can be confirmed as
TiC. The chemical compositions of the two gray phases
are rich in Ti, accompanied by Al, Mo, Cr, Co, Fe and Ni
with approximately the same atomic content of about
7 at/%. It is clear that their chemical compositions are
nearly identical, indicating that they belong to the same
phase (the BCC solid solution).
The SEM results agree well with the XRD results.
For the same phase, the difference in the morphology is
closely related to the distribution of metal elements with
high molten points during solidification. According to
the basic theory of material science, the elements with
high molten points concentrate in the pre-crystallized re-
gion and are scarce in the post-crystallized region.
Y. JUAN et al.: WEAR-RESISTANT SINGLE-PHASE HIGH-ENTROPY ALLOY COATINGS ...
754 Materiali in tehnologije / Materials and technology 53 (2019) 5, 751–758
Figure 4: X-ray diffraction patterns of the coatings with different con-
tents of Mo
Figure 3: Micrographs of the interfaces in the coatings with different
contents of Mo: a) FeCrCoNiAlMo
0.5
, b) FeCrCoNiAlMo
1.0
,c )
FeCrCoNiAlMo
1.5

Among the elements used in this study, the molten point
of Mo (Mo at 2617 °C) is much higher than those of the
other elements (Cr at 1857 °C, Fe at 1535 °C, Co at
1495 °C, Ni at 1453 °C, Al at 660 °C), its content in the
pre-crystallized equiaxial phase (8.84 at/%) is higher
than that in the post-crystallized rod-like phase (7.05
at/%). It is clear that the content of black dendrites is de-
creased when x increases from 0.5 to 1.0, showing an in-
creasing tendency with x being further increased to 1.5.
The microstructural-observation results coincide well
with the XRD results.
Figure 6 shows the microhardness profiles through-
out the cross-sections of the coatings. According to the
change in the microhardness, the sample can be divided
into three zones, namely, the coating, the transition area
and the substrate. The average hardness of the three coat-
ings is 759.4 HV
0.2
(x = 0.5), 857.2 HV
0.2
(x = 1.0) and
844.3 HV
0.2
(x = 1.5). The hardness of the coatings is su-
perior to that of the substrate (330 HV
0.2
), which should
be attributed to the solution- and dispersion-strengthen-
ing effects. The hardness of coating (x = 1.5) is consider-
ably close to that of coating (x = 1.0), which is higher
than that of coating (x = 0.5). This phenomenon is
mainly associated with the difference in the content of
alloying elements in the coatings with the change in the
content of Mo. The dilution rate is similar (about 62.24
%) for two coatings (x = 0.5 and 1.0). Therefore, a larger
addition of Mo (x = 1.0) means that the higher content of
Mo in the coating can improve the solution-strengthen-
ing effect. As a result, a higher hardness can be obtained
in coating (x = 1.0) when compared with that of coating
(x = 0.5). When x is further increased to 1.5 in the clad-
ding material, the coating should contain a higher con-
tent of Mo. However, as it is accompanied with a signifi-
cant increase in the dilution rate from about 62.24 % to
72.68 %, the content of Mo in coating (x = 1.5) is even
slightly lower than that in coating (x = 1.0). Conse-
quently, the hardness of coating (x = 1.5) is similar to
that of coating (x = 1.0).
Besides, it should be noted that the hardness of the
coatings prepared in this study is not inferior to the
products synthesized with other surface-engineering
techniques such as the process of nitriding. W. H. Wang
et al.
26
synthesized a porous titanium nitrided coating
Y. JUAN et al.: WEAR-RESISTANT SINGLE-PHASE HIGH-ENTROPY ALLOY COATINGS ...
Materiali in tehnologije / Materials and technology 53 (2019) 5, 751–758 755
Figure 5: Microstructures of the coatings with different contents of
Mo: a) x = 0.5, b) x = 1.0, c) x = 1.5
Table 1: Chemical compositions of the marked zones from the coating
shown in Figure 5
Areas
Compositions (at/%)
Al Mo Ti C Cr Fe Co Ni
1 9.53 7.05 51.57 0.32 7.27 7.79 8.07 8.40
2 6.83 8.84 51.69 0.29 8.52 8.08 7.94 7.81
3 0.80 0.42 53.11 43.84 0.26 0.35 0.46 0.77
Figure 6: Microhardness profiles along the cross-sections of the coat-
ings

with a thickness of 20 μm on a Ti surface, and the
hardness of the coating was 618.68 HV. In the research
of C. Yang et al.
3
, the hardness of the nitrided coating
prepared on a new type of metastable  titanium alloy
(TB8) was about 850-900 HV. However, the hardness of
the coating was extremely unstable and rapidly fell to
700 HV when the distance from the surface was 70 μm.
For a nitrided coating, a high hardness can be maintained
throughout the whole coating (about 2.7 mm in
thickness).
Figure 7 shows the changes in the friction
coefficients of the substrate and coatings with different
contents of Mo. The friction coefficient of the substrate
fluctuates more violently, which indicates that the
friction environment between the friction pairs is
extremely unstable. Its average value (1.04) is also
significantly higher than those of the three coatings (0.80
for x = 0.5, 0.61 for x = 1.0 and 0.78 for x = 1.5). The
optimal antifriction effect can be obtained for coating (x
= 1.0) where the friction coefficient is reduced by 41 %
compared with that of the substrate. D. S. She et al.
27
applied a series of plasma-nitriding processes to improve
the tribological properties of titanium. At the optimized
nitriding conditions (850 °C for 8 h), a hard nitrided
coating with a thickness of about 80 μm exhibited the
lowest friction coefficient of about 0.54, which was
reduced by 35 % compared with that of the untreated
samples (about 0.73). M. Lepicka et al.
28
deposited TiN
coatings on the Ti6Al4V alloy using the cathodic arc
evaporation/physical vapor deposition (CAE–PVD)
method. The friction coefficient of the coatings (0.46)
was reduced by 8 % compared with that of the substrate
(0.50). It can be seen that the friction coefficient of the
laser-clad coating (0.61 for x = 1.0) is slightly lower than
those of the above coatings prepared with the other
methods.
Figure 8 shows the wear profiles of the coatings and
the substrate. The wear volumes of the coatings with dif-
ferent Mo contents (x = 0.5, 1.0, 1.5) are 1.145±0.021
(a deviation of 0.014), 0.823±0.012 (a deviation of
0.011) and 1.219±0.018 mm
3
(a deviation of 0.015),
which are decreased by 51, 65 and 48 %, respectively,
when compared with that of the substrate (2.329±0.025
mm
3
with a deviation of 0.014). It is also confirmed that
the coating with x = 1.0 exhibits the optimum wear resis-
tance among the three coatings. Moreover, the wear rates
for the substrate and coatings (x = 0.5, 1.0, 1.5) can be
calculated as follows: 7.19×10
–5
, 3.53×10
–5
, 2.54×10
–5
,
3.76×10
–5
mm
3
N
–1
m
–1
, respectively. The change in the
wear rate of the samples mainly depends on their friction
coefficients. When the same load is applied to the sam-
ples, a high friction coefficient indicates that a high fric-
tion force is generated between the friction pairs, pro-
ducing a serious micro-cutting effect on the samples’
surfaces. More debris is peeled off from the samples,
causing an increase in the wear rate. Among all the sam-
ples, the wear rate of coating (x = 1.0) is the smallest due
to its lowest friction coefficient. When compared with
coating (x = 1.0), the friction coefficients of coatings (x =
0.5 and 1.5) are approximately same and higher, result-
ing in higher wear rates. For the substrate with the high-
est friction coefficient, the highest wear rate is obtained.
Figure 9 shows the wear morphology of the coatings
and the substrate. The wear surface of the substrate is
very rough and a large amount of debris can be clearly
observed. This indicates that the substrate suffers from
very serious micro-cutting during sliding. Comparatively
speaking, the wear surfaces of the three coatings are
smooth and some fine debris is sporadically distributed,
indicating that the wear resistance of the coatings is sig-
nificantly improved when compared with that of the sub-
strate. Our observation reveals that the wear morphol-
ogies of the coatings present a regular change with the
increasing Mo content in the cladding materials. On the
coating with x = 0.5, some fine grooves can be observed,
accompanied with some spalling pits and debris. With
the increase in the Mo content (x = 1.0), only slight
scratch traces can be observed and the amount of debris
is also significantly reduced. When the content of Mo is
Y. JUAN et al.: WEAR-RESISTANT SINGLE-PHASE HIGH-ENTROPY ALLOY COATINGS ...
756 Materiali in tehnologije / Materials and technology 53 (2019) 5, 751–758
Figure 8: Wear profiles of the substrate and coatings
Figure 7: Changes in the friction coefficients of the substrate and
coatings

further increased to x = 1.5, the wear morphology of the
coating is very similar to that of coating (x =0 . 5 )a s
again grooves appear on it. The SEM observation clearly
indicates that the optimum wear resistance of the coating
is obtained when x = 1.0, which is completely in accor-
dance with the results of the friction coefficient and wear
volume.
4 CONCLUSIONS
In this study, high-entropy-alloy protective coatings
of FeCrCoNiAlMox (x = 0.5, 1.0, 1.5) were successfully
prepared on Ti6Al4V using the laser-cladding process.
The coatings were mainly composed of the BCC-solid-
solution phase accompanied with traces of TiC. A single
solid solution (BCC) was successfully synthesized when
x = 1.0. Benefited from the effects of solution strength-
ening and dispersion strengthening, the coatings exhib-
ited excellent mechanical properties. The hardness val-
ues for the coatings were 759.4 HV
0.2
, 857.2 HV
0.2
and
844.3 HV
0.2
, respectively, with the increase in x (0.5, 1.0,
1.5). The wear volumes were 1.145 mm
3
, 0.823 mm
3
and
1.219 mm
3
and the friction coefficients were 0.80, 0.61
and 0.78. The properties of the coatings were superior to
those of the substrate (330 HV
0.2
, 2.329 mm
3
, 1.04). The
optimum wear resistance of the coating was obtained
when x = 1.0.
Acknowledgment
This work was financially supported by the National
Natural Science Foundation of China (51471105), Shu
Guang project of the Shanghai Municipal Education
Commission and the Shanghai Education Development
Foundation (12SG44).
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