High temperature electrical property a nd oxidation resistance of [622436]

46th International Conference on
Metallurgical Coatings and Thin Films
Manuscript Draft

Manuscript Number: ICMCTF_2019 -D-19-00054

Title: High temperature electrical property a nd oxidation resistance of
V-Nb-Mo-Ta-W High entropy alloy thin films

Article Type: Full Length Article

Section/Category: Symposium TS1 – High entropy and Other Multi -principal –
element Materials (SCT)

Keywords: high entropy alloy thin film, V -Nb-Mo-Ta-W, electrical
resistivity, electrochemical impedance spectroscopy, oxidation

Abstract: High entropy alloys (HEA) show outstanding thermodynamic,
mechanical or thermal properties as compared with pure metals or binary
alloys. Among several kinds of HEAs, the refractory element containing
HEAs exhibit relatively high thermal stability and better mechanical
properties at elevated temperature. In this study, the
V19.2Nb19.4Mo20.3Ta19.5W21.6 high entropy alloy thin films were deposited
on the AISI 304 s tainless steel substrates by a magnetron sputtering
process. The oxidation behavior and electrical conductivities of HEA thin
films at different temperatures were evaluated. The body -centered cubic
(BCC) structure of HEA thin film can be kept up to 500oC o xidation, and
it transferred to V -Nb-Mo-Ta-W based oxides and then turned to iron
contained oxide phases when the oxidation temperature increased to 700oC.
The electrical resistivity of thin film increased with oxidation
temperature. The electrochemical im pedance spectroscope (EIS) analysis
showed the total apparent resistivity of 304SS substrate coated with
V19.2Nb19.4Mo20.3Ta19.5W21.6 high entropy alloy thin film at 600, 700 and
-cm. The electrical resistance of the
HEA coated sample was contributed by the oxide resistance and interface
polarization. Through the electrical property analysis at high
temperatures, the HEA thin film can provide a protection layer to the
304SS substrate and kept the sample's high temperature ap parent
resistivity to a relatively low value implying its possible application
as a protective coating for electrical conduction components at high
temperature.

 A refractory V -Nb-Mo-Ta-W high entropy alloy thin film is grown by sputtering
method
 V19.2Nb19.4Mo 20.3Ta19.5W21.6 thin film has BCC structure below 500oC
 V19.2Nb19.4Mo 20.3Ta19.5W21.6 film has low resistivity below 800oC oxidation
 V19.2Nb19.4Mo 20.3Ta19.5W21.6 film has good high temperature electrical property *Research Highlights

High temperature electrical properties and oxidation resistance of V-Nb-Mo-Ta-W high
entropy alloy thin films
Yen-Yu Chen1, Sheng -Bo Hung2, Chaur -Jeng Wang2, Wen-Chung Wei3, Jyh-Wei Lee1,4,5,6*
1 Department of Materials Engineering, Ming Chi University of Technology
2 Department of Mechanical Engineering, National Taiwan University of Science and Technology
3 Department of Materials Science and Engineering, National Taiwan University
4 Cent er for Plasma and Thin Film Technology, Ming Chi University of Technology
5 Department of Mechanical Engineering, Chang Gung University
6 Plastic and Reconstructive Surgery, and Craniofacial Research Center, Chang Gung Memorial
Hospital, Taoyuan, Taiwan
*Corresponding author’s e -mail: [anonimizat]

Abstract
High entropy alloys (HEA) show outstanding thermodynamic, mechanical or thermal
properties as compared with pure metals or binary alloys . Among several kinds of HEA s, the
refractory element containing HEAs exhibit relatively high thermal stability and better mechanical
properties at elevated temperature. In this study, the V19.2Nb19.4Mo 20.3Ta19.5W21.6 high entropy alloy
thin film s were deposited on the A ISI 304 stainless steel substrates by a magnetron sputtering
process. The oxidation behavior and electric al conductivities of HEA thin films at different
temperature s were evaluated. The body -centered cubic ( BCC ) structure of HEA thin film can be
kept up to 500oC oxidation , and it transfer red to V-Nb-Mo-Ta-W based oxide s and then turned to
iron contained oxide phase s when the oxidation temperature increased to 700oC. The electrical
resistivity of thin film increased with oxidation temperature. The electrochemical impedance
spectroscop y (EIS) analysis showed the total apparent resistivity of 304SS substrate coated with
V19.2Nb19.4Mo 20.3Ta19.5W21.6 high entropy alloy thin film at 600, 700 and 800oC were 5.80, 6.91, and
7.74 -cm. The electrical resistance of the HEA coated sample was contributed by the oxide *Manuscript

resistance and interface polarization. Through the electrical property analysis at high temperatures,
the HEA thin film can provide a protection layer to the 304SS substrate and kept the sample’s high
temperature apparent resistivi ty to a relatively lo w value implying its possible application as a
protective coating for electrical conduction component s at high temperature.
Keywords: high entropy alloy thin film , V-Nb-Mo-Ta-W, electrical resistivity, electrochemical
impedance spectroscop y, oxidation ,

1. Introduction
Multi -principal element alloys , especially the high entropy alloys (HEA), show outstanding
thermodynamic, mechanical or thermal properties as compared with pure metals or binary alloys
[1-3]. The concept of HEA is basically considered as an alloy containing at least five principal
elements with the configurational entropy of mixing per mole higher than 1.5R (R is the gas
constant) [4 -6]. The core effects of HEA are summarized including high-entropy effect, sluggish
diffusion effect, severe lattice distortion effect, and cocktail effect [1, 2] . Based on these core effects,
the HEA materials showed excellent mechanical properties as compared with traditional stainless
steels and other sup eralloys [ 7]. Among several kinds of HEA s, the refractory high entropy alloys is
mainly composed of refractory elements such as Cr, Hf, Mo, Ta, V , W, and Zr . The refractory HEAs
show relatively good thermal stab ility and adequate high temperature mechanica l properties at
elevated temperature. For example, the V-Nb-Mo-Ta-W refractory HEA can keep a body -centered
cubic (BCC) phase structure and maintain ed a yield strength as high as 421 MPa after exposed to
1400oC in vacuum [8] .
The HEA thin films were fabricated to improve the corrosion resistance, oxidation resistance ,
and wear resistanc e of substrate materials successfully [2,3,9] . Among several kinds of thin film
deposition methods , the magnetron sputtering technique was frequently reported to grow the
HEA -based thin films, such as FeCoNiCuVZrAl [10], AlCrSiTi Zr [11], CoCrCuFiNi [12] thin films,
as well as their ni tride or carbide coatings [13,14]. These HEA thin films typically exhibit higher
mechanical properties as compared with these of the traditional stainless steel s or some super alloys
at high temperature environments [13]. Meanwhile , the thermal stability and oxidation resistance of
these HEA thin films are also better. One of the possible application s of these protecting HEA thin
films is the high temperature electrical conduction, such as coatings for thermoelectric elements [15]
or interconnectors of solid oxide fuel cell [16]. The electrical conduction component requires a
material has a high electrical conductivity to transfer electr ic currents under both oxidation and
reduction environments. For high temperature solid oxide fuel cells or thermoelectric devices,

several coatings , such as rare earth pervoskite oxides, spinel oxides, MAlCrYO (M = Co, Mn or Ti),
etc. [16], were deposited on stainless steels to form the protective oxides offer ing the corrosion
resistance of thermal oxidation and providing the high electrical conduction at the same time [16,
17].
Based on our previous experiences on the fabrication of multi -components thin films , such as
ZrNiAlSi [18], TiCrSiN [19], and TiWNbTa [20] coatings, we attempt ed to grow the
V-Nb-Mo-Ta-W refractory HEA thin films for the application on high temperature and oxidation
environment in this work . The high temperature electrical properties and the oxidation resistance of
the V -Nb-Mo-Ta-W refractory HEA thin films were evaluated . Crystal phase e volutions and
microstructure developments of the HEA thin films after high temperature oxidation were also
investigated.
2. Experimental procedure
The V-Nb-Mo-Ta-W refractory HEA thin films were fabricated on p -type (100) Si wafer,
polished sapphire, and AISI 304 stainless steel (304SS) substrate using a direct current ( DC)
sputtering system. The power of a V-Nb-Mo-Ta-W equimolar target (3 inch in diameter) was 500W,
and the deposition time was 90 min. The detail deposition parameters are listed in Table 1 . The
HEA thin films were oxidized in a tubular furnace from 300oC to 800oC for 1h with an air flow rate
of 1 standard liter per minute ( SLPM ).
The glancing angle X -ray diffractometer (PANalytical, X'pert, Holland) with an incidence
angle of 1° was adopted to explore the phase of thin films. Cu Kα radiation generated at 30 kV and
40 mA from a Cu target was used. A n optical microscope (BX53M, Olympus, Japan) and a field
emission scanning electron microscope (JSM -6701, JEOL, Japan) were employ ed to examine the
surface and cross -sectional morphologies of thin films . The chemical composition s of thin films
were analyzed by a field -emission electron probe microanalyzer (FE -EPMA, JXA -8500F, JEOL,
Japan). The electrical resistance and electrochemical impedance spectroscopy (EIS) of thin films
coated 304SS or sapphire samples were measured by using a potentiostat (SP -200, Bio -Logic,

France) under room temperature. A four point probe was used to measure the room temperature
electrical conductivity of as-deposited films and thin films after oxidation at different temperatures
ranging from 300oC to 700oC. Meanwhile, a two point probe was adopted to in -situ measure the
electrical conductivity of bare 304SS and the 304SS coated with HEA thin films at different high
temperature s. In this high temperature electrical conductivity measurement, the thin film was
deposited on the top surface of 304SS substrate. The back surface of 304SS was deposited with Pt
film for high temperature oxidation protection. Two silver electrode wires were connected to the Pt
coated 304SS surface and the HEA thin film coated surface , respectively . The silver paste was used
to glue the Ag wires on two surfaces for rigid contact. The same sample setup method was also
adopted for the EIS in-situ measurements at different high temperatures.
3. Result and discussion
3.1 Chemical composition and c rystal phase of V -Nb-Mo-Ta-W Thin Film
The composition of the as -deposited HEA thin films measured by FE -EPMA is 19.17% V ,
19.38% Nb, 20.36% Mo, 19.47% Ta, and 21.61% W (all in at.%). Fig. 1 shows the XRD pattern s of
V-Nb-Mo-Ta-W HEA thin films deposited on sapphire before and after oxidation test at 300oC and
500oC in air for 10 min . It appears that all thin films show a BCC crystal structures with diffraction
peaks of (110) and (211) planes . When the oxidation temperature raised to 500oC, a broaden
diffraction peak ranging from 20o to 30o was found , which was possibly due to the partially
oxidation of thin film and form ed the nucl ei of oxide species or amorphous phases . Based on the
XRD results, t he average grain size of as -deposited fil m is around 14.3 nm and it slightly reduced to
13.7 nm after oxidation at 500oC. The lattice constants of BCC structure for the as -deposited thin
film is 0.3199 nm and it slightly increased to 0.3211 nm after oxidation at 500oC.
The XRD results of V-Nb-Mo-Ta-W HEA thin films deposited on 304SS substrates before and
after oxidation at 300o to 800oC in air for 1h are depicted in Fig. 2. The crystal phases of the HEA
thin films before and after oxidation at 300oC and 400oC in air are mainly BCC structures. The
average grain size is around 11.8 nm for the as-deposited one and it slightly reduced to 10.1 nm

after oxidation at 400oC for 1 h. The lattice constant of BCC structure increased from 0.3203 nm to
0.3223 nm after the HEA thin film oxidize d at 400oC. As compared with that deposited on sapphire
substrates, the average grain size on 304SS substrate is slightly smaller, but the lattice constant is
slight ly larger , which is possibly caused by the coefficient of thermal expansion (CTE) mismatch
induced residu al stress between HEA thin films and two different substrates .
When the oxidation temperature is higher than 500oC, the thin film crystal phase s were
changed and became nanocrystalline phases . These crystal phase s were further identified as oxides
of V-Nb-Mo-Ta-W element s, such as the binary oxides of V and Nb ( Nb18V4O55, JCPD S 46-87,) Nb
and W ( Nb12W11O63, JCPD S 20-813; Nb2W3O14, JCPDS 25-1357 ; or Nb22W20O102, JCPDS 75-560),
V and Ta ( Ta18V4O55, JCPD S 42-432), Ta and W ( Ta16W18O94, JCPD S 29-1323). We propose that
during the oxidation temperature between 500oC to 600oC, the crystalline structure of the
V-Nb-Mo-Ta-W thin films were mainly mixed oxides containing V , Nb, Mo, Ta, and W element s.
When the oxidation temperature was as high as 700~800oC. The thin film crystalline phases were
changed to the Fe contained refractory metal (V , Nb, Mo, Ta, W) oxides, such as FeVO 4 (JCPD S
29-736), FeNbO 4 (JCPD S 71-1849), FeMoO 4 (JCPD S 22-6229), FeTaO 4 (JCPDS 25-1401), and
FeWO 4 (JCPDS 46-1446). The formation of Fe contained oxide maybe caused by the outward
diffusion of Fe species from 304SS substrate into V-Nb-Mo-Ta-W thin film and further reacted with
oxygen and these refractory elements . The lattice constants , a, of V, Nb, Mo, Ta and W are 0.3039,
0.3301, 0.3147, 0.3303, 0.3158 nm [22], respectively. Therefore, the lattice constant of these Fe
contained ternary oxide will shift a little bit as compared with that of these pure refractory binary
oxides of these species.
As compare d with the lattic e constant of the HEA bulk (a =0.3183 nm) reported by Senkov et
al. [22], the lattic e constant of the as -deposited V-Nb-Mo-Ta-W HEA thin films (a =0.3199 nm on
the sapphire substrate, and a =0.3223 nm on the 304SS substrate) are slightly larger in this work ,
which may be due to the substrate effect during sputtering processes to constraint the growth of the
thin films. The chemical composition of bulk HEA reported by Senkov et al. [22] were 21.0 % V,

20.6% Nb , 21.7 % Mo , 15.6% Ta , and 21.1% W (all in at.%) . The h igher Ta content (a =0.3303 nm
for pure Ta metal) , ~3.87 at.% higher, of the V19.2Nb19.4Mo 20.3Ta19.5W21.6 HEA thin film can be the
reason responsible for its higher lattice constant in this study.
3.2 Microstructures of V-Nb-Mo-Ta-W Thin Films
Fig. 3 illustrate s the SEM micrographs of top -view of HEA thin films deposited on 304SS
substrates before and after oxidation at 300 to 800oC for 1 h . The top-view morpholog y of the HEA
thin films did not change when the oxidation temperature increased to 500oC. However, the surface
morphology of thin film show ed granular microstructure after oxidation at 800oC in air. The lower
magnification SEM micrographs of thin film s after oxidation at 800oC for 1 h are shown in Fig. 4.
In Fig. 4(a), the oxidized thin film surface shows lots of pores with the dimension of several tens
micrometers, and the oxidized 304SS substrate is exposed from the se pores. The grain size s of the
oxidized thin film are around several micrometers, which is larger than the grain size, around 1 m
or less, of 304SS substrate in side the pores .
In order to further confirm the composition difference between the grains of the thin film
surface and th ese in the pores , the X-ray mapping technique by EDS analysis were conducted and
illustrated in Fig. 5. From the X-ray mapping s of V, Nb, Mo, Ta, W, Cr, Fe and O elements on the
oxidized thin film at 800oC in air for 1 h , the Fe , Cr and O elements are rich inside the pores,
whereas the V , Nb, Ta, W, and Mo elements are depleted . This result indicates that the HEA thin
film was partially spalled during oxidation and the exposed 304SS subst rate was further oxidized .
The SEM cross -section al micrographs of V19.2Nb19.4Mo 20.3Ta19.5W21.6 HEA thin films before
and after the oxidation at 300oC and 500oC are illustrated in Fig. 6. The Si substrate is also shown .
The average thickness of each thin film is around 1.64 m and shows no significant thickness
change after oxidation at 500oC in air for 1h . The columnar microstructures can be seen for the
as-deposited thin film and these films oxidized at 300 and 500oC. Fig. 7 depict s the optical top-view
micrographs of HEA thin film after oxidation at 500oC in air . The remained HEA thin films (darker
region), exposed 304SS substrates (lighter area) and the oxidized 304SS (spherical gray area) in Fig.

7(a) can be clearly seen . We suggest that some small spherical portions of HEA thin film
delaminated during oxidation at 500oC in air, which made the exposed 304SS substrate further
oxidized and became the spherical gray area. On the other hand, the HEA thin film spalled away
during the cooling stage and the exposed 304SS substrate did not oxidized due to its lower
temperature environment . In Fig. 7(b), the partially delaminated HEA thin film can be observed ,
which might be due to the thermal stress induced by the CTE mismatch between HEA thin films
and 304SS substrate during cooling .
3.2 Electrical Properties of V -Nb-Mo-Ta-W Thin Films
Fig. 8 depict s the e lectrical resistivity of V19.2Nb19.4Mo 20.3Ta19.5W21.6 HEA thin films deposited
on either sapphire or 304SS substrates before and after oxidation from 300 to 700oC in air for 1 h.
The resistivity data of thin films on the sapphire substrate before and after oxidation at 300oC in air
for 1h are 3.34 x 10-5 and 3.87 x 10-5 -cm, respectively . However, the resistivity raises to 1.29 x
10-3 -cm when the oxidation temperature increases to 500oC. Due to the extremely high resistivity
of sapphire, the contribution of electron transport is mainly from the HEA thin film. Therefore, the
resistivity of V-Nb-Mo-Ta-W H EA thin film can be measured directly . As c ompared with th ese
deposited on sapphire substrate s, the resistivi ty data of thin films deposited on 304SS su bstrate s
show lower values , i.e., 8.04 x 10-6 -cm for as-deposited thin film, 9.64 x 10-6, 1.76 x 10-5, and
3.63 x 10-5 -cm for these oxidize d at 300oC, 500, and 700oC, respectively .
It is suggested that the electrical resistivity of thin films deposited on 304SS substrates should
be dominant by both the HEA thin films and 304SS substrate. Therefore, the resistivity values are
lower than these deposited on sapphire substrates. As c ompared with the resistivity values of thin
films after oxidation at 500oC, the resistivity of thin film deposited on 304SS substrate is much
lower than that grown on sapphire substrate. Th is result indicates that although the resist ivity of
HEA thin film increased due to the oxidatio n at 500oC in air, the oxidized HEA thin film still can
act as a p rotection layer to prevent the further oxidation of 304SS substrate . Meanwhile, the
electrical resistivity increases a little bit to 3.63 x 10-5 -cm when the oxidation temperature

increases to 700oC.
The in -situ high temperature electrical resistivity plots for the bare 304SS substra tes and the
304SS substrate s with V19.2Nb19.4Mo 20.3Ta19.5W21.6 coatings oxidized from 300oC to 800oC are
depicted in Fig. 9 . It should be noticed that t he apparent resistivity in -situ measurement in Fig. 9
was made under differ ent high temperature condition s, which is different from that measured after
the sample was oxidized and cooled down to room temperature , as shown in Fig. 8 . Besides, the
data as shown in Fig. 8 is mainly the resistivity of HEA thin films, whereas the data as depicted in
Fig. 9 is the apparent resistivity of the bulk bare 304SS substrate or the bulk sample containing
304SS substrate and HEA thin film on the sur face. Fig. 9 illustrate s that the apparent resistivity
increased with increasing temperature . The apparent resistivity values of the samples consisting of
304SS substrate and HEA thin film tested at 600oC, 700oC and 800oC in air are 7.33, 7.85 and 13.7
-cm, respectively . For comparison, the apparent resistivity of bare 304SS substrate raise s to 59
-cm when the tempera ture is up to 7 00oC. From the above observation s, it appears that the
V19.2Nb19.4Mo 20.3Ta19.5W21.6 HEA coating can provide an effectively protection to the 304SS
substrate and keep s the sample’s apparent resist ivity t o a relatively lower value until 800oC.
Fig. 10 shows the Nyquist plot of samples consisting of 304SS substrates and HEA thin film s
coated on the top, which were in-situ measured at 600, 700 and 800oC in air , respectively . It is
obvious that the radius of semi -circle is gradually larger when the testing temperature increase s
from 600oC to 800oC. According to the XRD in Fig. 2, the oxide of HEA thin film started to form at
600oC and further transformed into the Fe contained refractory metal oxides at 800oC. Therefore,
the simulated equivalent circuit model is plotted in Fig. 10, which can be simplified as R 1(R2/CPE),
where R 1 represents the ohmic resistance , R2 is the charge transfer resistance , and CPE is the
constant phase e lement contributed to the capacity of double electric layer [23,24]. In this study, R 1
should be dominant by t he bulk resistance from the oxide film , and R 2 might be from the interface
polarization caused by charge transfer . The fitting results are also inserted in Fig. 10, which show
that the R 1 of HEA thin films tested under 600, 700 and 800oC are 0.91, 0.90, and 0.89

-cm2respectively, whereas the R2 are 0.57, 0.86, and 1.08 -cm2, respectively. The R 1 values
show a decreasing tendency with increasing test temperature, which can be related to the charge
transfer by ion species. In contrast, the R2 value decrease s with increasing test temperature, which
may be dominated by the electron transfer mechanism on the interface [23,24]. According to the
electrical analysis by the Nyquist plot in Fig. 10 , the total apparent resist ivity values , the resistance
value divided by the thickness of 304SS (0.2 cm), of the 304SS substrates with HEA coatings at 600,
700 and 800oC are 5.80, 6.91, and 7.71 -cm, respectively . Comparing with the results in Figs. 9
and 10, the in -situ measured apparent resistivity of the sample at high temperature by 2-point
method is slightly higher than that measured by EIS method. We suggest that the difference might
be due to the high contact resistance or interface polarization between HEA thin film and Ag
electrode in the 2-point method . Finally, we can conclude that the high temperature a pparent
resistivity of the 304SS substrate can be kept at low values by the coating of
V19.2Nb19.4Mo 20.3Ta19.5W21.6 refractory high entropy alloy thin film , which demonstrat es the possible
application of such HEA film as a protective coating for electrical conduction component s at high
temperature.

4. Conclusion
In this study, V19.2Nb19.4Mo 20.3Ta19.5W21.6 refractory high entropy alloy (HEA) thin films were
deposited on 304SS substrate by a direct current sputtering process to evaluate the ir crystal phases,
microstructure, and electrical properties after oxidation at different high temperature s in air
environment. The HEA thin films show ed BCC structures and partial ly chang ed to nanocrystalline
or amorphous phases after oxidation at 500oC in air for 1h. The crystal phases of thin films
transfe rred to refractory metal oxide s and then further reacted with iron species from the 304SS
substrate to form the Fe -contained refractory oxides when the temperature was higher than 700oC.
The high tempe rature in -situ electrical properties analysis, including two probe method and
electrochemical impedance spectroscopy method, reveal ed that the apparent resistivity of 304SS

substrate can be maintained at lower values when it was coated with HEA thin films even at 800oC.
The total apparent resist ivity of 304SS substrate s coated with V19.2Nb19.4Mo 20.3Ta19.5W21.6 thin films
at 600, 700 and 800oC are 5.80, 6.91, and 7.74 -cm. The possible application of such HEA film as
a protective coating for electrical conduction component s at high temperature was proposed in this
work.
5. Acknowledgement
The authors would like to thank the funding given by Ministry of Science and Technology
(MOST) in Taiwan under the Project numbers of MOST 106 -2218 -E-131-003, MOST
107-2218 -E-131-001, and MOST 107 -2221 -E-131-002-MY3 . This work was also financially
supported by the “High Entropy Materials Center” from The Featured Areas Research Center
Program within the framework of the Higher Education Sprout Project by the Min istry of Education
(MOE) and from the Project MOST 107 -3017 -F-007-003 by Ministry of Science and Technology
(MOST) in Taiwan.
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List of Table
Table 1 . Deposition parameters of HEA thin film sputtering process
Figure Caption
Figure 1. XRD patterns of V-Nb-Mo-Ta-W HEA thin films deposited on sapphire substrates before
and after oxidation at 300oC and 500oC in air.
Figure 2. XRD patterns of V-Nb-Mo-Ta-W HEA thin films deposited on 304SS substrates before
and after oxidation from 300oC to 800oC in air.
Figure 3. SEM top -view micrographs of V-Nb-Mo-Ta-W HEA thin films coated on 304SS
substrates , (a) as -deposited and af ter oxidation at (b) 300oC, (c) 500oC, (d) 800oC.
Figure 4. SEM top -view micrographs of V -Nb-Mo-Ta-W HEA thin film coated on 304SS substrates
after oxidation at 800oC, (a) lower magnification (300X) image and (b) enlarged image of
the white rectangle in (a).
Figure 5. Surface morphology of V-Nb-Mo-Ta-W HEA thin film oxidized at 800oC in air for 1h and
elemental X-ray mapping s.
Figure 6. SEM cross -sectional micrographs of V-Nb-Mo-Ta-W HEA thin film s, (a) as -deposited,
and oxidized at (b) 300oC, and (c) 500oC in air for 1h.
Figure 7. Optical top-view micrographs of spalled V-Nb-Mo-Ta-W HEA thin film after oxidation at
500oC, (a) low magnification image and (b) high magnification image reveal ing the
exposed 304SS substrates .
Figure 8. Electrical resistivity d ata of V-Nb-Mo-Ta-W HEA thin film as a function of oxidation
temperature after thin films were oxidized at different temperatures in air .
Figure 9. In-situ high temperature apparent resistivity data of bare 304SS substrate and the 304SS
substrates with V-Nb-Mo-Ta-W HEA coatings as a function of oxidation temperature.
Figure 10. Nyquist plots and equivalent circuit analysis data of 304SS substrates with
V-Nb-Mo-Ta-W HEA coatings in-situ measured at 600, 700 and 800oC in air.

Table 1
Target power (W) 300 W
Input Ar gas (sccm) 30
Background pressure (Torr) 6.7×10-4
Working pressure (Torr) 6.7×10-1
Substrate DC bias ( -V) -100
Working temperature (oC) 100
Deposition time (min) 90
Rotational speed (rpm) 30

Table 1

Fig. 1

20 30 40 50 60 70 80as-deposited
2θ (o)Intensity (arb.unit)
300oC
500oC

(110)
(211)
BCCFigure 1

Fig. 2

20 30 40 50 60 70 80
2 (o)HEA TF 400oC
HEA TF as-depositedHEA TF 300oCIntensity (arb. unit)HEA TF 700oC
HEA TF 600oC
HEA TF 500oCHEA TF 800oC
(110)
(211)BCC(V,Nb,Mo,Ta,W ) FeO4
(V,Nb,Mo,Ta,W ) OX
304SSFigure 2

(a) (b)

(c) (d)
Fig. 3

Figure 3

(a)

(b)

Fig. 4

Figure 4

Fig. 5
Figure 5

(a)

(b)

(c)

Fig. 6
Figure 6

(a)

(b)

Fig. 7

Figure 7

Fig. 8

Figure 8

Fig. 9

0 100 200 300 400 500 600 700 800 9000102030405060708090100Apparent resistivity (-cm)
Temperature (oC) VNbMoTaW Thin Film
304SS substrate
VNbMoTaW Thin Film by EISFigure 9

Fig. 10
Figure 10