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1. A brief review of high-entropy films
Xue Hui Yan a
, Jin Shan Li b
, Wei Ran Zhang a
, Yong Zhang a, *
a
State Key Laboratory of Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 10083, China
b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
h i g h l i g h t s g r a p h i c a l a b s t r a c t
Composition design and processing
of high-entropy films are discussed.
Various properties of high-entropy
films are summarized.
High-entropy films for high-
throughput experiments are put
forward.
a r t i c l e i n f o
Article history:
Received 2 April 2017
Received in revised form
18 July 2017
Accepted 23 July 2017
Available online 24 July 2017
Keywords:
High-entropy films
Magnetron sputtering
High-throughput experiments
Wide-temperature-range applications
Irradiation resistance
a b s t r a c t
High-entropy films are a new type of films which have been developed based on high-entropy alloys. It
features many excellent properties, such as high strength and hardness with remarkable strain hardening
rate, excellent wear and corrosion resistance, thermal stability, irradiation resistance, high toughness for
wide-temperature range applications, etc., which traditional films are incomparable with. In recent years,
high-entropy films have shown a fast development potential in various fields. This paper reviewed the
recent development about high-entropy films. Preparation methods, composition design, phase struc-
tures, and various properties are mainly discussed in the paper, and the prospect of high-entropy films
for high-throughput experiments is put forward.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
Materials are the foundation of social civilization, and alloy
materials occupy an indispensable position. In recent years, high-
entropy alloys (HEAs) have attracted a great deal of attention
from researchers due to their unique multi-element solid-solution
structures, and excellent properties. The high mixing-entropy al-
lows them to have a lower free-energy and higher phase-stability.
Therefore, HEAs tend to form a single phase solid-solution struc-
ture. This type of multi-component solid-solution has a large solid
solubility, and its existence allows HEAs features high strength,
excellent corrosion resistance, oxidation resistance, irradiation
resistance, and wide-temperature range applications. HEAs have
become another new research hotspot, following bulk amorphous
alloys and intermetallic compounds.
In recent years, binary or ternary film has been widely explored.
* Corresponding author.
E-mail address: drzhangy@ustb.edu.cn (Y. Zhang).
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
http://dx.doi.org/10.1016/j.matchemphys.2017.07.078
0254-0584/© 2017 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 210 (2018) 12e19

2. However, due to the limitations of the alloy entropy, the traditional
metal nitride, carbide, and oxide film which are low-entropy films,
could not meet the increasing requirements criteria. With the
development of HEAs, researchers have focused on study about
high-entropy films (HEFs). HEFs not only possess the excellent
performance as HEAs, even some of the performances have been
enhanced. These alloy films have shown major application poten-
tials in the fields of solar thermal conversion systems [1], high
hardness coating [2], corrosion resistant coating [3], diffusion bar-
riers in the integrated circuit (IC) systems [4], etc.
2. Emergence and development of HEFs
Since the concept of HEAs was published in 2004, the study of
HEFs has been subsequently developed. HEFs was reported in 2005,
when Chen et al. [5] first prepared FeCoNiCrCuAlMn and FeCo-
NiCrCuAl0.5 nitride film through magnetron sputtering, and
explored the change of phase structure with the nitrogen flow rate.
Then, Huang et al. [6] prepared AlCoCrCu0.5NiFe oxide film, and
analyzed the phase structure, hardness, and phase stability. With
the continuing development of HEFs, Tsai et al. [7] applied
AlMoNbSiTaTiVZr film to the diffusion barrier between Cu and Si
wafer. The experimental results showed that the films have major
application potential. In recent years, high-entropy coatings pre-
pared by laser cladding have gradually developed. Through laser
cladding, Zhang et al. [8,9] prepared NiCoFeCrAl3 and FeCoNiCrAl2Si
high entropy coatings.
At present time, the HEFs are mainly including nitride-films and
oxide-films. The principle of the HEFs composition design is similar
to that of HEAs. Generally speaking, the elements of HEAs can be
divided into “basic element” and “functional element” as shown in
Fig. 1. For example, Cr, Fe, Co, Ni, and Cu, along with other elements,
can be referred to as the base elements, since they do not have large
differences in atomic size, and tend to form simple face-centered
cubic (FCC) or body-centered cubic (BCC) solid-solution struc-
tures. Meanwhile, Ti, V, and W, which have excellent thermal sta-
bility and corrosion resistance properties, can be referred to as the
functional elements. According to the performance requirements,
the function elements can be added into the base elements. In
addition, a number of non-metallic elements, such as C, N, and B,
can also be added, which can fill interstitial positions of films, in
order to improve the hardness characteristics.
3. Preparation of HEFs
The preparation methods of films can mainly be divided into
two categories: physical methods, and chemical methods, which
are shown in Fig. 2. Physical preparation mainly refers to physical
vapor deposition (PVD), including vacuum sputtering, vacuum
evaporation, and ion plating. Meanwhile, chemical preparation
mainly includes chemical vapor deposition (CVD), and the liquid
phase deposition (LPD).
At the present time, the following methods of preparing HEFs
have been reported: magnetron sputtering method [10e13], laser
cladding [8,9,14,15], electrochemical deposition [16], arc thermal
spraying [17], cold spraying [18], electron beam evaporation
deposition [19], and plasma cladding [7]. Among them, magnetron
sputtering and laser cladding are the most mature techniques for
the preparation of HEFs. Table 1 summarizes the characteristics of
the two technologies.
The principle of magnetron sputtering deposition is a sputtering
effect. The high energy particles bombard the surface of the target
(Fig. 3), making the target atoms escape and move along a certain
direction, and eventually thin film forms on the substrate [20]. The
dual function of magnetic and electric fields increases the collision
probabilities of the electron, charged particles, and gas molecules.
In the usual cases, the magnetron sputtering target material is
prepared by the arc melting and powder-metallurgy methods. If
the melting point of each principal component is relatively
different, a powder metallurgy method is usually the priority
choice. Also, it is difficult to obtain an equiatomic ratio thin film,
since the different elements have different sputtering output ca-
pacities. Therefore, the “multi-target sputtering” has been pro-
posed [21]. According the composition design of HEFs, many single
element targets or alloy targets can be prepared, and the atomic
ratio of elements can be controlled by adjusting the target sput-
tering power.
Laser cladding technology is pointed out that melting metal
powder which features certain physical, chemical, or mechanical
properties by high-power and high-speed laser. A layer combining
with the matrix in the way of metallurgy bonding can improve the
mechanical properties between the layer and matrix. Laser clad-
ding technology is divided into two types of methods, referred to as
pre-powder and synchronous feeding, as shown in the schematic
diagram of Fig. 4.
4. Phase structure
The composition design of the HEFs is consistent with the HEAs.
Therefore, the HEFs also has a “high entropy effect”. In addition, the
film cooling rate is relatively fast, and it tends to form an amor-
phous phase or simple FCC, BCC solid solution phase. In this section,
Fig. 1. Composition design of HEFs.
Fig. 2. Classification of thin film processing (Electrochemical (EC) deposition, Liquid
phase epitaxy (LPE), Arc ion plating (AIP), Molecular beam epitaxy (MBE), Direct cur-
rent sputtering (DCS), Radio frequency sputtering (RFS)).
X.H. Yan et al. / Materials Chemistry and Physics 210 (2018) 12e19 13

3. the current study on the phase structure of HEFs prepared by
magnetron sputtering and laser cladding are mainly analyzed. In
particular, during the process of magnetron sputtering, different
process parameters such as nitrogen flow rates, substrate bias, and
substrate temperatures can play an important role on the phase
structure of the HEFs.
4.1. Phase structure of the HEFs prepared by magnetron sputtering
4.1.1. N2 flow rate
The N2 flow rate has a significant effect on the phase structure of
the HEFs. Under the conditions of low nitrogen gas flow rate, it has
been found that the HEFs tends to form an amorphous structure.
Chang et al. [13] reported that the TiVCrAlZr films exhibited
amorphous structures at low nitrogen gas flow rates (Fig. 5).
However, with increases the N2 flow rate, the HEFs phase structure
transforms from an amorphous to a simple FCC solid solution
structure. Liang et al. [22] found a similar phenomenon in the
preparation of TiVCrZrHf nitride films. When the N2 flow rate was
between 0 and 2 standard cubic cm per minute (SCCM), the XRD
pattern of the film showed a broad diffraction peak. This phe-
nomenon indicated that the phase structure of the TiVCrZrHf film
was amorphous as shown in Fig. 6. With increases in the nitrogen
flow rate, the phase structure also changed from an amorphous
phase to a FCC solid solution phase. In addition, many HEFs have
been reported to have similar results, such as AlCrTaTiZr [23],
AlCrMoSiTi [24], and TiAlCrSiV [25] films.
It could be seen that the phase structures of the HEFs were
transiting from an amorphous to simple solid solution with the
increases of N2 flow rates. The main reason for this phenomenon is
that, as the nitrogen flow rate increased, more and more binary
nitrides were formed. For example, the (TiVCrZrHf)N thin films
contain many binary nitrides, such as TiN, VN, CrN, ZrN and HfN,
which were formed with the increase of the nitrogen flow rate.
These nitrides are FCC structures, and therefore the structure of the
FCC is obvious in the XRD pattern. Another reason is that the larger
nitrogen flow rate tended to increase the mobility of the atoms, and
ensured the growth and diffusion of the grains. In addition, with
the increase of the N2 flow rates, the thicknesses of the film became
thinner, which was mainly due to the increased nitrogen content.
Meanwhile, the argon content decreased, and the number of argon
ions reduced. Therefore, the sputtering frequency was reduced, and
the deposition rate was slow.
Table 1
Features of the magnetron sputtering and laser cladding.
Methods of preparation Advantages Disadvantages
Magnetron sputtering 1) The substrate temperature rises slowly, and the
deposition rate of the film is high;
2) The film has good consistency and a compact structure;
3) Good compatibility with the substrate;
4) The performance and thickness of the film can be
flexibly controlled by the parameters.
1) Target utilization is low;
2) Film thickness is limited
Laser cladding 1) High heating and cooling rates;
2) The thermal effect on the substrate is low;
3) The grains are small and uniform;
4) The coating is combined with the matrix for metallurgy,
and a high bonding strength is achieved;
5) The thickness is up to several millimeters.
1) Easily produces various defects;
2) The surface is not flat;
3) A cracking sensitivity of the cladding layer is evident.
Fig. 3. Schematic of the magnetron sputtering process.
(a) (b)
Fig. 4. Schematic of laser cladding: (a) Synchronous powder; (b) Pre-powder layer.
X.H. Yan et al. / Materials Chemistry and Physics 210 (2018) 12e19
14

4. 4.1.2. Substrate bias
The substrate bias is an effective method to improve the struc-
ture and properties of films. Shen et al. prepared
(Al1.5CrNb0.5Si0.5Ti) Nx thin films by magnetron sputtering tech-
niques [11] and investigated the effects of substrate bias (Vs) on the
phase structures of films. It was found that the phase structures of
the films tended to form simple FCC structures. With the change of
Vs, the crystal diffraction peak became gradually weakened, and
the structure slowly changed from simple columnar crystal to a
dense phase structure. At the same time, the grain was refined, and
the grain size decreased from 70 nm to 5 nm. Huang et al. [12] also
studied the effects of substrate bias on the phase structure of HEFs.
The results show that when the Vs were weakened, the phase
structure tended to be amorphous.
With the increase of the Vs, the energy of ion bombard the
target was increased, therefore both the ability of target atoms
diffusion and participation in the chemical reactions can be facili-
tated. The film's density and film-forming ability were also
improved. With the improvements in the film's density and film-
forming ability, the high energy bombardment induced various
defects in the film, and also inhibited the growth of columnar
crystals and refinement of the film grains.
4.1.3. Substrate temperature
Along with the substrate bias and the N2 flow rates, the tem-
perature of the substrate also was found to have a significant effect
on the phase structure of the HEFs. Huang et al. [26] prepared
AlCrNbSiTiV nitride film with a substrate temperature between
100 C and 500 C. The results indicated that the phase structure
was a single-phase FCC structure (Fig. 7a). In addition, as the sub-
strate temperature increased, the size of grain became larger
(Fig. 7b), especially at between 100 C and 300 C. Liang et al. also
got the same conclusion, with the increase temperature of the
substrate, the high entropy nitride film still presented a simple FCC
solid solution phase structure. Overall, the grain size shows an
increasing trend.
It was concluded that the main reason for the phenomenon was
that the increasing substrate temperatures enhanced the adsorp-
tion capacity and surface mobility of the atoms. Therefore, the
grains grew easily. The lattice constants show a tendency to
decrease, which may have been due to multiple factors, such as
phase separations and residual stresses.
4.2. Phase structure of the HEFs prepared by laser cladding
Currently, laser cladding technology is one of the most common
ways to prepare the surface coating of a work piece. Similarly, due
2 (degree)
Intensity
(Arb
Units)
Fig. 5. XRD patterns of TiVCrAlZr films with different N2 flow rates [13].
2 (degree)
Intensity
(Arb
Units)
Fig. 6. XRD patterns of the TiVCrZrHf films with different N2 flow rates [22].
(a) (b)
2 (degree) Substrate temperature
Intensity
(Arb
Units)
Grian
size
(nm)
Lattice
constant
(A)
Fig. 7. (a) XRD patterns of the AlCrNbSiTiV nitride films at different substrate temperatures; (b) Variations of the grain sizes and lattice constant of the AlCrNbSiTiV nitride film at
different substrate temperatures [26].
X.H. Yan et al. / Materials Chemistry and Physics 210 (2018) 12e19 15

5. to the “high entropy effect”, HEFs which are prepared by laser
cladding technology tend to form simple FCC and BCC solid solution
structures. Yeh et al. [27] prepared a CrMnFeCoNi coating by laser
cladding, and analyzed its microstructure and corrosion resistance.
The XRD results indicated that the phase structure of the HEAs
coating was mainly a simple FCC solid solution phase.
Boron is a common element which is used to improve the
hardness of a coating. After adding the B element, the phase
structure of the HEFs will not only have the simple solid solution
phase of an FCC and BCC, but also an alloy boride phase will exist.
Lin et al. [28] reported that, with increases in boron content, the
phase structure of FeCoCrNiAlBx HEAs coating had both FCC and
BCC phases, along with a large amount of boride (Fig. 8). It has
shown the FeCoCrNiB HEFs with high wear resistance, Huang et al.
[29] also reported the presence of alloy boride phases.
Titanium is a common element which is used to improve the
corrosion resistance and high temperature performance of
coatings. Following the addition of Ti, a titanium alloy was pre-
sented in the HEFs phase structure. Zhang et al. [30] studied
high-temperature TiZrNbWMo HEAs coatings, and the XRD pat-
terns showed that the phase structure of the coating included a
mainly simple BCC solid solution phase, and a TixW1-x precipi-
tated phase.
5. Mechanical properties
Comparing with traditional alloy films, the HEFs have excellent
mechanical properties. Table 2 summarizes the hardness and
modulus of HEFs which have been reported. As mentioned in Part 2,
by adding N and other elements, the hardness of the HEFs can be
further enhanced. It can also be seen in Fig. 9 the hardness and
modulus of the nitride films showed a rising trend.
In addition to the effects of the elemental content, the
hardness of the HEFs is affected by the sputtering power and
substrate temperatures. Ren et al. [42] explored the effects of
the magnetron sputtering parameters on the hardness and
modulus of HEFs. It was found that, as the sputtering power
increasing, the hardness and modulus increased from 13.1 and
200 GPa to 15.2 and 221 GPa, respectively (Fig. 10a). Subse-
quently, the hardness and modulus display slightly downward
trends. However, with increases in the substrate temperature,
the hardness and modulus of the film display a single rising
trend (Fig. 10b).
6. Thermal stability
The HEFs has displayed excellent thermal stability. Even at high
temperatures, it has been found to still maintain high strength and
phase structure stability. Zhang et al. [43] reported that the
hardness of a CoCrCuFeNi coating displayed almost no changes
after annealing at 500 C for five hours (Fig. 11a). That which
annealed at 750 C for five hours decrease in hardness by 5.5%,
and it was maintained at 1000 C for five hours, the hardness was
higher than the alloy prepared by vacuum melting. In addition,
their study also explored phase stability at high temperatures
(Fig. 11b). The phase structure of the CoCrCuFeNi HEAs coating
was a simple FCC solid solution phase, after annealing at 500 C,
and at 750 C for five hours, the phase structure did not change
significantly. Table 3 summarizes the high temperature perfor-
mance of the HEFs.
7. Corrosion resistance
The HEFs are usually composed of elements such as Ni, Cr, Co, Ti,
and Cu, therefore HEFs not only feature excellent mechanical
properties, but also can show a good corrosion resistance to high
concentrations of acid. Ren et al. [50] studied the corrosion
behavior of a CuCrFeNiMn coating by immersion test and potenti-
ometric polarization measurement method. The results showed
Intensity
(Arb
Units)
2 (°)
2 (degree)
Fig. 8. XRD pattern of FeCoCrNiAlBx coatings [28].
Table 2
Hardness and modulus of the HEFs.
Composition Hardness (GPa) Modulus (GPa) Reference
FeCoNiCuVZrAl 12.0 166 [31]
Al0CoCrCuFeNi 15.4 203.8 [32]
Al2.5CoCrCuFeNi 16.5 218.3 [32]
AlCrNiSiTi 12.9 141.2 [33]
AlCrSiTiZr 11.5 ~203 [34]
(AlCrSiTiZr)N 17 ~231 [34]
(AlCrTaTiZr)N 23.9 234.77 [35]
(AlCrMoTaTiZr)N 40.2 ~385.0 [36]
(AlCrTaTiZr)N 30.0 293.1 [37]
(TiVCrZrHf)N 23.8 267.3 [22]
(AlCrNbSiTiV)N 42 350 [12]
(TiHfZrVNb)N 44.3 384 [38]
(AlCrTaTiZr)N 20 242 [39]
(NbTiAlSi)N 20.5 206.8 [40]
(AlCrTaTiZr)N 30 350 [41]
Yong’s modulus (GPa)
Hardness
(GPa)
Fig. 9. The hardness and Yong's modulus of HEFs with and without N.
X.H. Yan et al. / Materials Chemistry and Physics 210 (2018) 12e19
16

6. that when the coating immersed in 1 mol/L of sulfuric acid solution
for 100 h at the 25 C, the corrosion rate of the CuCrFeNiMn coating
was only 0.074 mm/y, which was much lower than that of 304
stainless steel (1.710 mm/y). Yeh et al. also did a similar work [51].
Both CrMnFeCoNi coating and 304 stainless steel was immersed in
a 3.5% NaCl solution, and 0.5 mol/L of H2SO4. The experimental
results showed that the corrosion resistance of the HEAs coating
was better than that of A316 steel, and the corrosion current was
even lower than that of the 304-stainless steel. Qiu et al. [52]
compared an AlCrFeNiCoCu coating with 304 stainless steel, and
also obtained similar conclusions.
8. HEFs for high-throughput experiments
As we all know, material science is developed on the basis of
experiments. In the process of scientific exploration, the method
which relies on scientific intuition and “trial error” method
has been formed. The kind research method can inevitably lead
to the shortcoming of time-consuming, long cycle of research
and low-efficiency, etc. In the 1970s, Hanak proposed an
experimental scheme with a “high-throughput” feature firstly
[53]. So far, it has been developed 40 years for high-throughput
preparation, various forms of materials such as film, bulk, and
(a) (b)
Hardness
(GPa)
Yong
’
s
Modulus
(GPa)
Hardness
(GPa)
Yong
’
s
Modulus
(GPa)
Sputtering power (W) Substrate temperature (K)
Fig. 10. Variations hardness and modulus of the (AlCrMnMoNiZr)N films with: (a) Sputtering power; (b) Substrate temperature [42].
(a) (b)
fcc
2 (degree)
Annealing temperature ( )
Intensity
(Arb
Units)
Hardness
(Hv)
Electrical
resistivity
cm)
cm)
Fig. 11. (a) Electrical resistivity and hardness changes with annealing temperature of CoCrCuFeNi films; (b) XRD patterns of CoCrCuFeNi films [43].
Table 3
Phase structures of the HEFs after annealing.
Composition Temperature (
C) Time (hours) Phase structure Ref.
Before annealing/after annealing
(TiVCrZrHf)N 300 2 FCC/FCC [44]
(AlBCrSiTi)N 700 2 Amorphous/Amorphous [45]
(NbTiAlSiW)N 700 24 Amorphous/Amorphous [1]
TaNbTiW 700 1.5 BCC/BCC [46]
6FeNiCoCrAlTiSi 750 5 BCC/BCC [47]
FeCrNiCoMn 900 2 FCC þ BCC/FCC þ BCC [48]
FeCoCrNiB 900 5 FCC þ M3B/FCC þ M3B [49]
X.H. Yan et al. / Materials Chemistry and Physics 210 (2018) 12e19 17

7. powder, have achieved the high-throughput preparation.
The film deposition is the most usual method for high-
throughput preparation. It mainly includes co-deposition method
[53], Separate template coating method [54], continuous phase
diagram template coating method [55]. Among them, the co-
deposition is the most suitable high-throughput preparation
method for HEAs with multi-elements. Target atoms have a “dis-
tance advantage” in the process of deposition, taking advantage of
the different distance between targets and substrate, the proba-
bility of target atoms depositing nearby is larger, therefore a film
with a continuous composition gradient is formed on the substrate.
Co-deposition technology is divided into single target co-
deposition (Fig. 12) and multi-target co-deposition (Fig. 13). Both
methods can achieve co-deposition of multiple elements. In com-
parison, multi-target co-deposition is more controllable for each
component content than single target co-deposition, and a larger
gradient of composition can be obtained easier. According the
composition design of HEAs, many single element targets or alloy
targets can be prepared, and the atomic ratio of elements can be
controlled by adjusting target sputtering power. And finally HEFs
with a continuous composition gradient is obtained. Combined
with high-throughput characterization techniques, we can achieve
rapid screening of HEAs and HEFs.
9. Prospects
The HEFs has been proven to have excellent mechanical prop-
erties, and high temperature performance, as well as wear and
corrosion resistance. The HEFs has shown great development po-
tential in the fields of solar thermal conversion, surfaces of work
piece engineering, and integrated circuit diffusion barriers. How-
ever, there are still some problems involved in the research of the
HEFs, as follows: (1) The composition design of the alloy is still in
the form of a “cocktail”; (2) The studies about HEFs are mainly
focused on the microstructure. The properties about corrosion
resistance, wear resistance, and high temperatures stability have
been less studied; (3) To date, there has been no complete phase
formation rule to support the study of HEFs. Finally, this study
believed that during the processes of HEFs film research, the
method of “trial and error” should be abandoned. The composition
selection and design of HEFs should be combined with simulated
calculations. In addition, high throughput experiments should be
taken into account. Developing purposeful and efficient research is
the most effective way to promote the development of the HEFs.
Acknowledgment
The authors would like to thank the financial supports by the
National Science Foundation of China (NSFC, Granted Nos.
51471025 and 51671020).
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