Accepted Manuscript

Dense and smooth amorphous films of multicomponent FeCoNiCuVZrAl high-

entropy alloy deposited by direct current magnetron sputtering

L. Liu, J.B. Zhu, C. Hou, J.C. Li, Q. Jiang

PII: S0261-3069(12)00758-3

DOI: http://dx.doi.org/10.1016/j.matdes.2012.11.001

Reference: JMAD 4915

To appear in: Materials and Design

Received Date: 15 September 2012

Accepted Date: 1 November 2012

Please cite this article as: Liu, L., Zhu, J.B., Hou, C., Li, J.C., Jiang, Q., Dense and smooth amorphous films of

multicomponent FeCoNiCuVZrAl high-entropy alloy deposited by direct current magnetron sputtering, Materials

and Design (2012), doi: http://dx.doi.org/10.1016/j.matdes.2012.11.001

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Dense and smooth amorphous films of multicomponent FeCoNiCuVZrAl high-entropy

alloy deposited by direct current magnetron sputtering

L. Liu, J. B. Zhu, C. Hou, J. C. Li*, Q. Jiang

Key Laboratory of Automobile Materials of Ministry of Education, and School of Materials

Science and Engineering, Jilin University, Changchun 130025, China

Abstract: The multicomponent amorphous nitride films of FeCoNiCuVZrAl high-entropy

alloy were deposited by direct current magnetron sputtering in the mixture atmosphere of Ar

and N2. The systematical investigations demonstrate that the chemical composition,

microstructure, and mechanical properties of the amorphous films intimately rely on the

concentration of N2 in the atmosphere mixture. When N2 flow ratio increases from 0 to 50%,

the thickness of the films decreases, whereas the roughness firstly decreases and then

increases. At the N2 flow ratio of 30%, a perfect dense and smooth amorphous nitride film

could be achieved. While the hardness and Young’s modulus of the film reach the maximum

values of 12 and 166 GPa, respectively.

Keywords: High entropy; Amorphous; Film; Properties; Magnetron sputtering

* Corresponding author. Tel: +86-431-85095371; Fax: +86-431-85095876; E-mail: ljc@jlu.edu.cn

http://ees.elsevier.com/jmad/viewRCResults.aspx?pdf=1&docID=17139&rev=3&fileID=648166&msid={1E4C2444-F2C4-4FFF-AFA1-4E67C7779DDF}

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1. Introduction

The conventional alloys generally consist of one principal element associated with a

substantial amount of other elements to enhance the properties and processing [1], which

differ from the high-entropy alloy (HEA) recently proposed by Yeh et al. [2, 3]. HEA is a

novel concept for the alloy system that has multiple principal elements with equimolar or

near-equimolar ratios in the rang of 5~35 at.%. In recent years, HEA films have been widely

studied, such as TiVCrZrHf film [4], AlCrMoTaTiZr film [5, 6], AlCrMoSiTi film [7] and

FeCoNiCrCu film [8]. Moreover, HEA films have been proposed for the potential applications

as protective films [9-11], wear-resistant materials [12], corrosion-resistant materials [13], and

coatings in communication devices [14]. That is due to their interesting properties, such as

high hardness [15], strength [16, 17], wear resistance [18, 19], and microstructure stability

against heat treatment [20-22].

It is known that the transition metal nitride coatings are usually much harder, more

chemically inert, but brittler than the original metals or alloys. This has stimulated recent

explorations for improving the mechanical properties of alloys. In previous work [23, 24], we

have developed the novel HEA alloys (i.e. FeCoNiCu system) with a single FCC crystalline

structure, which exhibits good plastic properties with the tensile strain up to 18%. Musil et al.

reported the hard and super-hard Zr-Ni-N nanocomposite films with the hardness of 40 GPa

[25]. The similar strengthening effect was also revealed in VN [26]. Moreover, the addition of

other element, such as Al, was found to further improve the thermal stability of the film [27,

28]. Based on the above observations, the multicomponent nitride FeCoNiCuVZrAl film is

expected to possess excellent mechanical properties.

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In this contribution, we report the fabrication of amorphous nitride film of

FeCoNiCuVZrAl HEA by using direct current (DC) magnetron sputtering system at a low

depositing temperature. The amorphization occurs probably due to the enhanced

glass-forming ability for the alloys containing more than three elements [29], as well as the

limited diffusion of the elements at the low depositing temperature [30]. In addition, the

composition, microstructure, hardness and Young’ modulus of the amorphous film were

systematically investigated and discussed.

2. Experimental methods

The FeCoNiCuVZrAl HEA was melted for at least 5 times by the arc melting-method

under a purified argon gas atmosphere. Then, it was shaped into a disc of 60 mm in diameter

and 5 mm in thickness as a target. Table 1 lists the composition of the target and the atomic

ratios of each element measured by energy dispersive spectrometry (EDS). Quartz glass

wafers were cleaned sequentially in de-ionized (DI) water, acetone and DI water, for the

following deposition of the nitride films of FeCoNiCuVZrAl by DC magnetron sputtering. In

a mixture atmosphere of Ar and N2, the nitride films with the thickness of about 12 μm were

deposited under a plasma power of 30 W, a constant working pressure of 0.9 Pa, a substrate

bias of 98 V, a deposited time of 2 hours at room temperature. The distance between the

substrate and the target was set as 75 mm. The nitrogen flow ratio RN = N2/(Ar+N2) was

precisely adjusted by varying the N2 flow rate from 0 to 30 sccm with a constant Ar flow rate

of 30 sccm.

The phase structure analysis of the target and nitride films was performed on Rigaku

D/max 2500 X-ray diffractometer at 50 KV and 250 mA (XRD, D/Max 2500pc) with the

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scanning angles ranging from 20 to 90 degree at a scanning rate of 2 degree/min. Both the

surface morphology and thickness of the deposited films were observed by field-emission

scanning electron microscopy (FESEM, JEOL JSM 6700F). The chemical compositions of

the films were analyzed by EDS. The hardness and Young’s modulus of the films were

measured by a nanoindenter (XP nanomechanical testing system, MTS Corporation)

according to ISO standard 14577-4: 2007 [31], during which the penetration depth of the

indenter was controlled at about 1/10 of the film thickness to avoid substrate effect.

3. Results and discussion

Fig. 1 shows the XRD pattern of the target of FeCoNiCuVZrAl HEA. As indicated, the

crystalline structures are composed of α (cubic), β (cubic) and γ (monoclinic) phases with the

space group of Fm3m (225), Im3m (229) and C2/m(12), respectively. Fig. 2 illustrates the

XRD patterns of the nitride films of FeCoNiCuVZrAl alloy deposited under different N2 flow

ratios (RN). It can be clearly seen that all the films exhibit amorphous structure, dramatically

different from that of the as-melted HEA target shown in Fig. 1. This phenomenon was also

observed in the preparation of the nitride film of the AlBCrSiTi HEA [32].

The formation of the amorphous films during the fabrication is reasonable according to

the rules proposed by Inoue [29] that the glass-forming ability can be strengthened for

multicomponent systems. From a thermodynamical point of view, the large glass-forming

ability is obtained under the condition of low Gibbs free energy ΔG (T) for the transformation

from liquid to crystalline phase, where ΔG = ΔHf - TΔSf with ΔHf and ΔSf being the enthalpy

and entropy of fusion, respectively. According to the above equation, the low ΔG value is

obtained in the case of low ΔHf and large ΔSf. The large ΔSf is expected to be obtained in

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multicomponent alloy systems, because ΔSf is proportional to the number of microscopic

states. The ΔG at a constant temperature also decreases in the cases of low chemical potential

caused by the low enthalpy, high reduced glass transition temperature, and large solid/liquid

interface energy. Based on these thermodynamical considerations, it can be concluded that the

multicomponent increases the dense random packing, which is favor of the decrease of ΔHf

and the increase of solid/liquid interface energy. This is consistent with the result that a larger

glass-forming ability has been obtained for the above multicomponent systems. In conclusion,

the low enthalpy, the great difference between the depositing temperature of films and the

glass transition temperature, and the large solid/liquid interface energy benefit the formation

of the amorphous films.

From Fig. 2, we can find that the film without nitrogen is also amorphous. Thus, the RN

has almost no effect on the amorphous formation. However, the RN has great effects on the

chemical compositions, thickness, morphology, hardness and Young's Modulus of the films,

which will be discussed later.

Fig. 3 shows the chemical compositions of the multicomponent FeCoNiCuVZrAl films

deposited under different RN. When RN = 0, the concentration (atomic ratio) of each metallic

element in the film is rather close to that in the FeCoNiCuVZrAl target, whose composition is

listed in Table 1. Since the sputtering yield data of Cu is higher than other elements in the

films [33], the content of Cu atoms is relatively lower than other elements at RN = 0. In

addition, the concentrations of all elements are not constant in the range of 0-10%, because

the elements, such as, Fe, Co, Ni, Cu, V, Zr and Al, are strong nitride formers and prone to

form nitrides with the addition of N2 [33], which gives rise to the dramatical changes of

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concentrations of all elements in comparison with the specimen without any N2 addition (RN =

N2/(Ar+N2) = 0), as shown in Fig. 3. While the further increases of N2 concentration does not

result in evident variation of concentrations of all elements [28, 32]. The concentrations of Al

and Ni atoms little changed with further increasing RN, because the bonding strength between

Al and N atoms is higher than that between other elements and N atoms [34], Al atoms can

combine with N atoms more easily than other elements at higher RN. While Ni atoms have the

smallest mixing enthalpy with N atoms [35], they combine with less N atoms than other

elements. Thus, as RN increases, the relative concentrations of Al atom increase and Ni atom

decreases in substrate.

Fig. 4 shows the cross-section microstructures of the FeCoNiCuVZrAl nitride films

deposited under different RN. A typical columnar microstructure could be observed for RN

lower than 30% [see Figs. 4(a) and 4(b)], while a glass-like feature microstructure can be

clearly characterized in the films deposited with RN larger than 30% [see Figs. 4(c) and 4(d)].

Table 2 gives the thickness of the FeCoNiCuVZrAl nitride films deposited under different RN.

The largest and smallest thicknesses are 2.100 µm (RN = 0) and 0.875 µm (RN = 50%),

respectively. It is also found that the thicknesses of the films decrease with the increasing RN.

This originates from the formation of the nitride film at the target surface due to the good

affinity of all the target elements with nitrogen [33], which hinders the atoms from sputtering.

It is a typical result of target poisoning [36]. Additionally, nitrogen ions are known as less

effective as argon ions for sputtering [36]. Therefore, as the two gas species have comparable

collision cross-sections for ionization [37], a greater proportion of the target current will be

carried by nitrogen ions at higher nitrogen partial pressures, inducing the decrease in the

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sputtering rate. Moreover, when more N2 are added, more metallic nitride are formed on the

substrate, which enhances the internal stress of the film and inhibits the growth. Another

aspect is probably due to the fact that more metallic nitrides are pumped out of deposition

chamber along with exhaust gas [38].

Fig. 5 shows the top-view FESEM images of the FeCoNiCuVZrAl films deposited under

different RN, illustrating the evolution of morphology. It can be see that the films are

composed of nanoparticles with different geometric morphology as RN changes. When RN = 0,

an amount of nanoparticles with the size of about 10 to 25 nm gather together, as shown in

Fig. 5(a). With increasing RN, the surface of the film becomes smoother and forms a smooth

coating [see Fig. 5(b)]. Fig. 5(c) shows a morphology for a perfect dense amorphous

FeCoNiCuVZrAl nitride film obtained at RN = 30%, where the surface is much smoother and

cleaner than other films. This decrease in the roughness at the higher nitrogen flow rate could

be attributed to the reduction in the arrival rate of the sputtered species, which can be reflected

by the decrease of the film thickness and corresponds to the structure zone models [39, 40].

There is more energy of per depositing atom that leads to the formation of denser coatings

with reduced surface roughness. However, when the RN is increased to 50%, the surface

become rougher [Fig. 5(d)], and the dense amorphous coating seems to be destructed. That is

because with further increasing RN, the target poisoning is enhanced and the arrival rate of the

sputtered species becomes so low that there are not enough sputtered species arriving at the

substrate. Therefore, the film surfaces become looser and have more micro-holes. Therefore,

RN of 30% is the just right ratio to form the smooth and dense amorphous FeCoNiCuVZrAl

nitride film.

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Fig. 6 plots the hardness and Young’s modulus of the FeCoNiCuVZrAl nitride films as a

function of RN. For the FeCoNiCuVZrAl film without the addition of N2, the hardness and

Young’s modulus are 8.6 and 153 GPa, respectively. They are relatively superior to typical

films of pure metals and alloys, which is mainly caused by the great solid-solution

strengthening effect from the addition of a large amount of different-size atoms. With addition

of N2, the hardness and modulus increase and reach maximum values of 12 and 166 GPa for

the film deposited at RN = 30%. That might be due to the perfect dense and smooth

amorphous structure. With further increasing RN, the surface becomes rougher and looser, as

shown in Fig. 4(d). Meanwhile the micro-holes among particles increase in size. Therefore,

the hardness and modules decrease.

4. Conclusions

The nitride films of multicomponent FeCoNiCuVZrAl high-entropy alloy have been

deposited successfully using direct current magnetron sputtering. All the films exhibit

amorphous structure, dramatically different from the as-melted HEA target. The morphology

and mechanical properties of these amorphous films depend on the nitrogen flow ratio. The

thickness of films decreases with the increasing N2 flow ratio. The biggest and smallest

thicknesses are 2.100 µm (RN = 0) and 0.875 µm (RN = 50%), respectively. At RN = 30%, a

perfect dense and smooth amorphous film is obtained with the hardness and Young’s modulus

up to the maximum values of 12 and 166 GPa, respectively.

Acknowledgements

The authors gratefully acknowledge the financial supports from NNSFC (Grant No.

50571040) and National Foundation of Doctoral Station (Grant No. 20100061110019).

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Table and Figure Captions

Table 1 Composition of the FeCoNiCuVZrAl high-entropy alloy target (at.%)

Table 2 Thickness of the FeCoNiCuVZrAl nitride films deposited under different N2 flow

ratios.

Fig. 1 X-Ray diffraction curve of the FeCoNiCuVZrAl target.

Fig. 2 X-Ray diffraction curves of the FeCoNiCuVZrAl nitride films deposited under

different N2 flow ratios (RN).

Fig. 3 Chemical compositions of the FeCoNiCuVZrAl nitride films deposited under different

N2 flow ratios (RN).

Fig. 4 FESEM cross-section microstructures of the FeCoNiCuVZrAl films deposited under

different N2 flow ratios (RN): (a) RN = 0; (b) RN = 10%; (c) RN = 30%; (d) RN = 50%.

Fig. 5 FESEM morphology of the FeCoNiCuVZrAl nitride films deposited under different N2

flow ratios (RN): (a) RN = 0; (b) RN = 10%; (c) RN = 30%; (d) RN = 50%.

Fig. 6 Hardness and Young's modulus of the FeCoNiCuVZrAl films deposited under different

N2 flow ratios (RN).

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Table 1 Composition of the FeCoNiCuVZrAl high-entropy alloy target (at.%)

Element Fe Co Ni Cu V Zr Al

Nominal

composition

14.28 14.28 14.28 14.28 14.28 14.28 14.28

Composition

By EDS

13.96 13.95 13.20 13.83 14.57 15.19 15.31

Table 2 Thickness of the FeCoNiCuVZrAl nitride films deposited

under different N2 flow ratios (RN).

RN 0 10% 30% 50%

Thickness (µm) 2.100 1.738 1.200 0.875

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Fig. 1 X-Ray diffraction curve of the FeCoNiCuVZrAl target.

Fig. 2 X-Ray diffraction curves of the FeCoNiCuVZrAl nitride films deposited under

different N2 flow ratios (RN).

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Fig. 3 Chemical compositions of the FeCoNiCuVZrAl films deposited under different N2

flow ratios (RN).

(a) (b)

(c) (d)

Fig. 4 FESEM cross-section microstructures of the FeCoNiCuVZrAl nitride films deposited

18

under different N2 flow ratios (RN): (a) RN = 0; (b) RN = 10%; (c) RN = 30%; (d) RN = 50%.

(a) (b)

(c) (d)

Fig. 5 FESEM morphology of the FeCoNiCuVZrAl nitride films deposited under different N2

flow ratios (RN): (a) RN = 0; (b) RN = 10%; (c) RN = 30%; (d) RN = 50%.

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Fig. 6 Hardness and Young's modulus of the FeCoNiCuVZrAl nitride films deposited under

different N2 flow ratios (RN).

We prepare a perfect dense and smooth amorphous nitride high entropy film.

The formation mechanism has been discussed based on thermodynamic theory.

The hardness and Young’s modulus of the film can reach to 12 and 166 GPa.

We discuss the effects of N2 flow ratios.