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Rare Metals2018年第8期

Amorphous phase stability of NbTiAlSiN_x high-entropy films

Wen-Jie Sheng Xiao Yang Jie Zhu Cong Wang Yong Zhang

State Key Laboratory for Advanced Metals and Materials,University of Science and Technology Beijing

Center for Condensed Matter and Materials Physics,Department of Physics,Beihang University

作者简介：*Yong Zhang e-mail:drzhangy@ustb.edu.cn;

收稿日期：23 November 2015

基金：financially supported by the National Natural Science Foundation of China (No.51471025);

Amorphous phase stability of NbTiAlSiN_x high-entropy films

Wen-Jie Sheng Xiao Yang Jie Zhu Cong Wang Yong Zhang

State Key Laboratory for Advanced Metals and Materials,University of Science and Technology Beijing

Center for Condensed Matter and Materials Physics,Department of Physics,Beihang University

Abstract：

In this study,high-entropy films with the composition of NbTiAlSiNx were prepared by a reactive direct current(DC) magnetron sputtering technique,with different nitrogen flow rates(0,4 and 8 ml·min^-1).The microstructures and properties were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),high-resolution transmission electron microscopy(HRTEM),nano-indenter and spectrophotometer.All of the as-deposited NbTiAlSiN_X films are shown to have an amorphous structure,and the films exhibit high thermal stability up to 700 ℃.The maximum hardness and modulus values of the films reach 20.5 GPa(4 ml·min^-1) and 206.8 GPa(0 ml·min^-1),respectively.The films exhibit high absorption of the solar energy in the wavelength of 0.3-2.5 μm,which indicates that NbTiAlSiN_X nitride film is a potential candidate solar selective absorbing coating for high-temperature photo-thermal conversion in the concentrated solar power project.

Keyword：

High-entropy film; Sputtering; Amorphous; Photo-thermal conversion; Phase stability;

Received： 23 November 2015

1 Introduction

High-entropy alloys (HEAs) [ 1, 2, 3] have four main core effects,i.e.,(1) thermodynamically,high-entropy effects;(2) kinetically,sluggish diffusion;(3) structurally,severe lattice distortion and (4) properties,cocktail effects,which were first introduced by Yeh [ 2] ,and are also known as equi-atomic ratio alloys,multi-component alloys,compositional complex alloys,multi-principal-elements alloys,etc.Increasing attention has been focused on HEAs due to their unique compositions,microstructures and properties,compared to conventional alloys [ 1, 4, 5] .HEAs tend to form disordered solid solutions and amorphous structures,even with nano-precipitates.In recent years,HEA nitride,oxide and carbide thin films have been studied,such as(AlCrTaTiZr)N_X,(AlCrTaTiZr)O_X and (TiZrNbHfTa)C [ 6, 7, 8] .The HEA nitrides,carbides and carbonitrides [ 9] are still at the preliminary stage of study and have been widely used as protective applications due to their unique properties,e.g.,their good biomedical properties [ 8] ,good oxidation [ 10, 11] ,high hardness [ 9, 12] ,excellent wear resistance [ 13, 14] ,outstanding thermal stability [ 15, 16, 17, 18, 19, 20] .

Expected concentrated solar power (CSP) solar-thermal collector must have high solar energy absorption in the wavelength range of 0.3-2.5μm,and low thermal emissivity in the infrared range (λ≧2.5μm) at high temperatures [ 21, 22] .From the substrate to the top surface,a typical double interference absorption cermet layer structure consists of an infrared-reflective metallic layer,biabsorption layer (including a high metal volume fraction(HMVF),a low metal volume fraction (LMVF) cermet layer) and an anti-reflective layer [ 23] .According to the Carnot cycle thermal efficiency,the greater the temperature difference is between the system and the ambient,the higher the efficiency of the thermal conversion obtained will be.The temperature of the films for the current parabolic trough CSP is usually below 600℃ [ 24] .It is vitally important that the coatings can be phase stable above600℃.At present,the double ceramic absorption layer structure has been widely studied,and transition metal nitride cermet is usually used in solar selective absorbing coatings for middle-to high-temperature applications,such as NiAl-Al₂O₃,Mo-Al₂O₃,TiAlON,TiZrON [ 25, 26] .However,the optical properties degrade significantly at high temperatures,especially when the temperature is higher than 400℃for prolonged durations.The thermal stability of the coating has become a critical problem,which directly determines the working lifetime of the solar absorber.To our knowledge,HE As and HEA nitride films exhibit excellent thermal stability [ 15, 16, 17, 18, 19, 20] ,which would be potential candidates as solar selective absorbing coatings for concentrated solar power applications.

In the present paper,NbTiAlSiN_X HEA films were prepared using a magnetron sputtering technique in a mixture gas of argon and nitrogen,and the microstructures and properties of these films were studied.

2 Experimental

2.1 Target preparation

Two methods were used for the preparation of sputtering targets,arc-melting and powder metallurgy.(1) Arc-melting:the constituent equi-molar elements of Nb,Ti,Al and Si (with purities of>99.99%) quaternary were melted at least five times to ensure homogeneity,followed by cutting and polishing of the solidified bulk into a disk of 60 mm in diameter and 4 mm in thickness.(2) Powder metallurgy:equi-molar Nb,Ti,Al and Si powders were mixed intensively before being placed in a sheath serving as a mold,then undergoing sintering hot-extrusion,and finally the target is cut and polished into a disk with the predetermined dimensions.When the melting point of constituent elements varies widely,powder metallurgy method is preferred and considered to be more practical than arcmelting.So the sputtering targets were prepared by powder metallurgy method in this paper.

2.2 Films preparation

Films were deposited on different substrates (dimensions26 mm×26 mm×1 mm) using a reactive high-vacuum direct current (DC) sputtering technique.Before placing all of the substrates into the vacuum chamber,the substrates were ultrasonically cleaned and rinsed with acetone,ethanol and deionized water for 15 min.High purity argon was placed into the vacuum chamber when the base pressure was higher than 2.0×10^-3 Pa.The target was cleaned by argon ion bombardment for 15 min to remove the oxide or contaminant of the surface,and then the gas flow rate and the paddle valve of the molecular pump were adjusted to the predetermined working pressure.All of the films were deposited in a mixture atmosphere of Ar and N₂ at room temperature with a working distance of 60 mm,and the total flow of the argon and nitrogen flow rates was fixed at24 ml·min^-1.The relative experimental parameters are listed in Table 1.

2.3 Film characterization

The structure of the film was identified using X-ray diffractometer (XRD,Dmax) with Cu Kα(40 kV,20 mA)radiation and 2θrange of 20°-80°at a scanning speed of 3(°)·min^-1 and an incidence angle of 1°.The microstructures of the top view and cross section of the as-deposited films were characterized by scanning electron microscope(SEM,Auriga,Carl Zeiss,Jena,Germany) equipped with energy dispersive X-ray spectrometer (EDS) operated at10 kV.The surface imaging of the as-deposited and heattreated samples was characterized by atomic force microscope (AFM,multimode,VEECO,America).Transmission electron microscope (TEM) observations were carried out with a 300 kV Tecnai F30 at a resolution of 0.205 nm.The films (50-100 nm in thickness) deposited on the singlecrystal NaCl (100) were used for TEM observation.The films fell off as the substrates were dissolved in the deionized water.Then they were ultrasonic ally vibrated into fragments,and the preparation of the specimen for the TEM observations is shown in Fig.1.This is one of the methods for preparing samples for TEM observations,which is easily realized,and could thus be used to reduce the damage of the films to a large degree.The hardness andmodulus of the as-deposited films were tested by a nanoindenter with a Berkovich triangular pyramid indenter.In order to test the thermal stability of the films,the samples were placed in a quartz tube with a vacuum of5×10^-3 Pa·heated up to the predetermined temperatures(500,700 and 1000℃) with a heating rate of 15℃·min^-1from room temperature,then held there for 24 h,and finally cooled in air.The reflective spectra were measured using a UV3100 spectrophotometer equipped with an integrating sphere coated with barium sulfate in the wavelength range of 0.3-2.5μm and a Fourier transform infrared reflectance (FTIR,Bruker Tensor 27) spectrometer equipped with an integrating sphere coated with gold in the wavelength range of 2.5-25.0μm.The solar absorption (α)and thermal emissivity (ε) were calculated as follows,respectively.

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Table 1 Experimental parameters

Fig.1 Schematic image of films for TEM observations

where R(λ) is the measured spectral reflectance at a specific wavelength (λ),I_s(λ) is the solar spectra radiation power at AM 1.5,and I_b(λ) is the spectral black body emissive power at room temperature.

3 Results and discussion

3.1 Microstructures

The chemical compositions of the NbTiAlSi metallic film are listed in Table 2.The content of Al in the NbTiAlSi alloy film is much higher,and those of Nb and Ti are lower than nominal content,as the melting point of Al is lower than those of the other three elements.In addition,Al is more reactive than the other elements.The compositions of each element in the film have certain deviation from those of the nominal ones;this is difficult to control,as well as unavoidable,and thus poses a great challenge in the present work.

No evident peak is observed from XRD patterns of asdeposited NbTiAlSiN_X film,thus indicating that all of the as-deposited films are amorphous structures,and afterward the structure is confirmed by TEM observations.There are four reasons that could be proposed to account for the formation of an amorphous structure.(1) The sluggish diffusion effect plays a significant role in the formation of the amorphous structure.(2) The mixing enthalpy of the constituent is equal to-41.1 kJ·mol^-1,especially the mixing enthalpy between Al and Si,and the other two elements are very negative.(3) Severe lattice distortion,i.e.,the atomic radii of Nb,Ti,Al and Si are 1.429,1.462,1.423 and 1.153 nm,respectively [ 27] .According to the prediction of HE As phase formation proposed by Zhang et al. [ 5, 28] ,the calculating parametersΩandδ(Ωis also known as Yang-Zhang parameter,indicating the entropy effect balances the enthalpy effect at melting temperature,whileδis the atomic-size difference) for the film are shown in Fig.2,indicating that the calculated results are in agreement with the experimental results.(4) There is no sufficient time for crystallization due to the rapid cooling rate of the sputtering deposition process,and crystallization requires sufficient time and energy,but deposition is an atom-by-atom process [ 29, 30] .

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Table 2 Compositions of NbTiAlSi metallic thin film (at%)

XRD patterns of as-deposited films are shown in Fig.3a.No significant changes are found from XRD patterns of the films before or after heat treatment at 500℃(data are not shown here).Figure 3b shows XRD patterns of NbTiAlSiN_X films annealed at 700℃for 24 h in vacuum,but the diffraction peak appears after heat treatment at 700℃and becomes more obvious especially in the metallic film.Moreover,the diffraction peak shifts to a lower angle,and the intensity decreases with the increase in nitrogen flow rate.Both the concentration of nitrogen and the degree of disorder increase significantly in the amorphous structure with the increase in nitrogen,leading to the expansion of the structure and causing the film to be less dense.It can be demonstrated that all of the films still retain an amorphous structure and the amorphous structure is very stable even at a high temperature of 700℃.

Fig.2 Relationship between parametersδandΩ,for high-entropy alloys (parametersδandΩfor NbTiAlSiN_X films denoted by a five-pointed star)

SEM cross-sectional images of as-deposited NbTiAlSiN_X thin films are shown in Fig.4,and the deposition time is 30 min.The morphology appears very smooth and dense,typical of a dense amorphous film.With the increase in nitrogen flow rate,the deposition rates at different nitrogen flow rates can be obtained according to the thickness and deposition time (nitrogen flow rates of0,4 and 8 ml·min^-1,deposition rates of 27.5,17.5 and11.6 nm·min^-1,respectively).The deposition rate decreases with the increase in the nitrogen flow rate,which results from the fact that the ionization of Ar reduces,eventually leading to the reduction of the sputtering target atom.The surface topographies of as-deposited NbTiAlSiN_X films were characterized by threedimensional AFM,indicating that the surfaces are quite smooth and dense,similar to tiny domed columnar grains(Fig.5).The as-deposited metallic film has a surface roughness value of 11.9 nm,but as nitrogen flow rate increases from 4 to 8 ml·min^-1,the surface roughness of the nitride films increases from 9.11 to 20.50 nm.This trend indicates that more nitrogen can induce less dense and smooth amorphous films.

Fig.3 XRD patterns of a as-deposited amorphous and b NbTiAlSiN_X thin films after annealing at 700℃for 24 h with different nitrogen flow rates

Fig.4 SEM cross-sectional images of as-deposited NbTiAlSiN_X thin films with different nitrogen flow rates:a 0 m·min^-1,b 4 ml min^-1 and c 8 ml·min^-1

Fig.5 Three-dimensional AFM images of as-deposited NbTiAlSiN_X thin films with different nitrogen flow rates:a 0 m·min^-1,b 4 ml·min^-1and c 8 ml·min^-1

Figure 6 shows high-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) patterns of as-deposited films.It is clear that all of the as-deposited NbTiAlSiN_X films are amorphous,which is in agreement with the results from XRD patterns.HRTEM images show a typical amorphous structure with a highly disordered yet homogeneous morphology.

3.2 Mechanical properties

The hardness and modulus of as-deposited thin films were tested using a nano-indenter.Each sample was tested with five points,and the maximum displacement into the sample surface is 200 nm.The load-displacement curve,average hardness and modulus are shown in Fig.7.The hardness of the films increases from 18.1 GPa (0 ml·min^-1) to a maximum value of 20.5 GPa (4 ml·min^-1) and then decreases to 17.4 GPa (8 ml·min^-1).For the metallic film,the maximum modulus is 206.8 GPa,but the modulus decreases with the increase in N₂ flow rate.It is proposed that the content of nitrogen in the nitride thin film exceeds the corresponding saturation at nitrogen flow rate of8 ml·min^-1;thus,the softening process takes place [ 31] .With the increase in nitrogen flow rate,the films become looser and rougher than the metallic film (Fig.5).Overall,the addition of nitrogen into the films does not show significant strengthening effects.Conversely,a greater amount of nitrogen added into the films would not offer a strengthening behavior,but rather would have a negative effect on the enhancement of the hardness and modulus.

3.3 Optical properties

NbTiAlSiN_X HE A nitride thin films deposited on stainlesssteel substrates exhibit different colors with varied thicknesses (Fig.8).Some people prefer to choose colored solar collectors for placement in buildings,despite the fact that the black ones have higher absorption,and this is compatible with the architectural design from the esthetic perspective [ 31] .The absorption increases to a maximum value and then decreases sharply with the increase in nitride film thickness,as shown in Fig.9.With the increase in thickness,the color becomes darker that is sensitive to wavelength [ 32] ,but when the films are too thick,the adhesion strength of the films and substrates becomes worse,which leads to the film being detached from the substrate,as can be seen in Fig.8f.

Fig.6 HRTEM images and corresponding SAED patterns of as-deposited NbTiAlSiN_X films with different nitrogen flow rates:a 0 ml·min^-1,b 4 ml·min^-1 and c 8 ml·min^-1

Fig.7 Load-displacement curve a and hardness and modulus b of as-deposited NbTiAlSiN_X films

The bi-absorbing layer of HMVF/LMVF was deposited on stainless-steel substrates,in order to characterize the optical properties (nitrogen flow rates of 4 and 8 ml·min^-1,denoted by HMVF and LMVF,respectively).The absorptions of the absorbing layers under the corresponding thicknesses are listed in Table 3.The optimized thicknesses of HMVF and LMVF layers are 260 and 175 nm,respectively.It is clear that the absorption has no change compared to that of the as-deposited samples (Table 3),mainly because the films of HMVF and LMVF have no change in the phase structure and still remain in amorphous conditions,as can be seen in Fig.3.Interestingly,the absorption of the samples increases after annealing at1000℃for 1 h.Figure 10 shows XRD patterns of NbTiAlSiN_X films after heat treatment at 1000℃.There is a change from amorphous to crystal structure.The metallic film shows an intermetallic structure with Nb₅Si₃,TiSi and Al₃Ti,which are due to the extremely negative mixing enthalpy between Al and Si,Ti and Si,and Al and Ti,respectively.Moreover,the nitride films form fcc solid solution structure,but when nitrogen flow rate increases to8 ml·min^-1,the film has simple fcc solid solution structure and a small amount of an unknown phase with nano-sized particles.Particles differing in size have formed during the course of crystallization and have produced an appropriate surface roughness that can increase the light absorption [ 24] .Yeh et al. [ 33] found that an anomalous decrease exists in XRD intensities of CuNiAlCoCrFeSi multi-principal alloy systems.Severe lattice distortion effects with different atomic sizes account for the anomalous phenomenon.Similarly,the reflectance of the appropriate surface with multi-scaled particles decreases accordingly and there is an increase in the light absorption.

Fig.8 Variation of absorption of thin film with deposition time.Thickness increasing gradually from a to f and each thickness presenting different colors

Fig.9 Absorption of mono-layer with various thicknesses

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Table 3 Absorption of films before and after heat treatment.HMVF,LMVF and HMVF/LMVF deposited on stainless-steel (SS) substrates

Fig.10 XRD patterns of NbTiAlSiN_X thin films after annealing at1000℃for 1 h

4 Conclusion

In this study,a series of NbTiAlSiN_X high-entropy films were prepared using magnetron sputtering method in a mixture atmosphere of nitrogen and argon.All of the asdeposited films exhibit an amorphous structure which remains stable even at the high temperature of 700℃for24 h.The addition of nitrogen does not provide a significant enhancement on the hardness and modulus of the films.On the contrary,a greater amount of nitrogen would have a negative effect on the enhancement of the hardness and modulus.Moreover,the film exhibits a potential candidate as solar selective absorbing coatings for the photothermal conversion in the concentrated solar power.The colored films may cater to our pursuit of esthetics,especially in the building integration with solar collectors.The optimization of the multi-layer films,including the infrared-reflective layer,bi-absorbing layer and anti-reflective layer,still requires extensive experiments,and the thermal emissivity of the films deposited on stainless-steel substrates shall be further investigated.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No.51471025).