microstructure, hardness, resistivity and thermal stability of sputtered oxide films of...

Materials Science and Engineering A 457 (2007) 7783

Microstructure, hardness, resistivity and the h

Yu i c, MiangmTechnsity, Ng Hua

embe

Abstract

The sputt ted oand subsequ filmworking gas e oxysputtered ox 0 nmoxide film d s maxduring anne owesize among grains increases with the annealing temperature. The resistivity of the oxide film steps up with annealing temperature, whereas thehardness value decreases. The oxide-film thickness changes very little during annealing. 2006 Elsevier B.V. All rights reserved.

Keywords: High-entropy alloy; Oxide film; Microstructure; Resistivity; Hardness

1. Introdu

In the pon one psubstantialproperty/prthemselveswas limitedare made wintermetallthe databasincreases sken with thmixed in a[5,6], multand nano-sal. [7] annocompositio

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ast the practical alloy systems were based mainlyrincipal element as the matrix even though aamount of other elements was incorporated for

ocessing enhancement [1,2]. Metallurgists confinedto systems where the number of alloying elementsin any one phase. This was partly because as alloys

ith an increasing number of component metals, manyic compounds and complex microstructures form ande necessary in terms of phase diagram informationharply [3,4]. However, this impasse has been bro-ree startling discoveries of recent vintage. Metals aremulti-metallic cocktail to make bulk metallic glassesi-functional, superelastic and superplastic alloys [7]tructured high-entropy alloys [3]. In 2003, Saito etunced the discovery of a multi-functional alloy withns such as Ti23Nb0.7Ta2Zr1.2O alloy.

ding author. Tel.: +86 750 3021406; fax: +86 750 3501211.dress: [email protected] (Y.-S. Huang).

High-entropy alloys were developed in recent years by Yehet al. [3,8]. Nano-structured high-entropy alloys with multipleprincipal elements are of equimolar or near-equimolar ratiosand concentrations between 35 and 5 at.%. High-entropy alloysmight possess simple crystal structures, ease of nanoprecipita-tion, and promising properties in high hardness and superiorresistance to temper softening, wear, oxidation and corro-sion [920]. Following Boltsmanns hypothesis the relationshipbetween the entropy and system complexity, the change in con-figurational entropy during the formation of a solid solution fromthree elements with an equimolar ratio is already larger thanthe entropy changes for fusion of most metals. Consequently,alloys containing a higher number of principal elements willmore easily yield the formation of random solid solutions dur-ing solidification, rather than intermetallic compounds or othercomplicated phases [1012]. Mackay [21] had reported that thenumber of intermetallics as a function of components increasesat first but drops off after four components. Solid solutionswith multi-principal elements will tend to be more stable at ele-vated temperatures because of their large mixing entropies [20].AlCoCrCu0.5Ni, AlxCoCrCuFeNi, AlCoCrCuFeMoNiTiVZr,

see front matter 2006 Elsevier B.V. All rights reserved..msea.2006.12.001oxide films of AlCoCrCu0.5NiFan-Sheng Huang a,, Ling Chen b, Hong-Wei Lu

a Department of Materials Technology, Jiangmen Polytechnic, Jb College of Mechanical Engineering, South China University of

c College of Mechanical Engineering, Guangxi Univerd Department of Materials Science and Engineering, National Tsin

Received 14 September 2006; received in revised form 30 Nov

ered oxide films of AlCoCrCu0.5NiFe high-entropy alloy were deposiently were annealed at 500, 700 or 900 C. Surprisingly, the sputtered, the AlCoCrCu0.5NiFe high-entropy alloy film is amorphous. When thide films are HCP with lattice constants of a = 0.3583 nm and c = 0.495ecrease with increasing oxygen content and the hardness value reachealing, indicating the oxide films are very stable at high temperature. Hermal stability of sputteredigh-entropy alloying-Hong Cai d, Jien-Wei Yeh d

en 529000, Guangdong, PR Chinaology, Guangzhou 510640, PR Chinaanning 530004, PR ChinaUniversity, Hsinchu 300, Taiwan, ROC

r 2006; accepted 1 December 2006

n the silicon wafer using radio frequency sputter system,s are of simple structure. With no oxygen addition to thegen content in the working gas is between 10 and 50%, the. Before annealing, both the resistivity and thickness of theimum at 30% O2. No new phases in the oxide films form

ver, the crystal grains tend to grow up and the micro-hole

78 Y.-S. Huang et al. / Materials Science and Engineering A 457 (2007) 7783

Fig. 1. XRD patterns of: (a) target, (b) AlCoCrCu0.5FeNi alloy film, (c) oxidefilm deposited at CO2 = 10%, (d) oxide film at CO2 = 30%, and (e) oxide filmat CO2 = 50%.

AlCoCrFeMo NiSiTi and AlCrFeMo NiSiTi high-entropyalloys are eture and pralloys investructure. Msynthesizedcipal matriwith the Alfrequencyties were mAlCoCrCuhave rather

2. Experim

The AlCthe arc metarget. RefeThe sputtealloy were(microscopsilicon waf

Fig. 2. XRDannealing, (b)900 C.

Fig. 3. XRD patterns of the oxide film deposited at CO2 = 30%: (a) beforeannealing, (b) annealed at 500 C, (c) annealed at 700 C, (d) annealed at 900 C.

XRD patterns of the oxide film deposited at CO2 = 50%: (a) beforeg, (b) annealed at 500 C, (c) annealed at 700 C, (d) annealed at 900 C.

o deposition. The power on the AlCoCrCu0.5FeNi target0 W. The depositions were performed at a total pressure3 Torr in a mixed Ar and O2 atmosphere. The ration the oxygen partial pressure and the total working

essure (CO2 = pO2/(pO2 + pAr)) was varied from 0 ton order to obtain different oxygen concentrations in theThe substrate temperature was kept constant at 573 K.l was carried out on a vacuum furnace. Both the surface0.5 0.5xamples exhibiting a quite simple as-cast microstruc-omising properties [10,1316]. Among the previousstigated, the AlCoCrCu0.5NiFe alloy is of BCCost present and former oxide-film systems werewith targets that are based mainly on one prin-

x element. In this paper, oxide film was depositedCoCrCu0.5FeNi high-entropy alloy target using radiosputter system, and its structure and some proper-easured. Surprisingly, the sputtered oxide films of

0.5FeNi high-entropy alloy are of simple structure andfine properties and a good thermal stability.

ental details

oCrCu0.5FeNi high-entropy alloy was prepared bylting and casting method, and then was made intor to [9] for further details of this preparation method.red oxide films of AlCoCrCu0.5FeNi high-entropydeposited on Si (1 0 0) single crystal wafers and glassic slides) using radio frequency sputter system. Theers and glass were cleaned with water and acetone

Fig. 4.annealin

prior twas 15of 10betweegas pr50% ifilms.Anneapatterns of the oxide film deposited at CO2 = 10%: (a) beforeannealed at 500 C, (c) annealed at 700 C, and (d) annealed at Fig. 5. TEM m

30%.

orphology and EDP of the typical oxide film deposited at CO2 =

Y.-S. Huang et al. / Materials Science and Engineering A 457 (2007) 7783 79

morphology and film thickness of the sputtered films wereobserved with scanning electron microscope (SEM, JEOLJSM-5410). The chemical compositions were analyzed by SEMenergy dispersive spectrometry (EDS). An X-ray diffractometer(XRD, Rigaku ME510-FM2, Tokyo, Japan) was used for theidentification of structure, with the 2 scan ranging from 20 to90 at a rate of 0.5 min1. The radiation was from a 30 kV,20 mA copper target. The resistivity of the oxide films wasmeasured using four-point probe (Keithley Model 236). Thefilm hardness was analyzed using micro/nanoindention tester(CSM, Swiss) and represented with Vickers hardness value.Thin-foil specimens were prepared by mechanical thinningfollowed by ion milling at room temperature, and subsequentlywere observed under a transmission electron microscope (TEM,JEOL JEM-100CXII, Tokyo, Japan).

3. Results and discussion

3.1. XRD and TEM analysis

Fig. 1 shows the XRD patterns of the sputtered alloy film oroxide films of AlCoCrCu0.5NiFe high-entropy alloy. The XRDpattern of AlCoCrCu0.5FeNi alloy target is also provided in

Fig. 1. It is clear that the target is of BCC structure. When nooxygen is added to the working gas, the AlCoCrCu0.5FeNi alloyfilm is amorphous. When the oxygen content in the working gasis between CO2 = 10 and 50%, the sputtered oxide film is HCP.The lattice constants of the HCP phase are a = 0.3583 nm andc = 0.4950 nm. When the oxygen content increases from CO2 =30 to 50%, XRD peaks become lower and broader, indicatinggrains tend to have a smaller size (about 8.12 nm determined byScherrer expressions).

The sputtered oxide films of AlCoCrCu0.5NiFe high-entropyalloy were annealed at different temperature, and subsequentlywere analyzed by XRD. The XRD results are shown inFigs. 24. It is seen that after annealing, the oxide films are ofHCP structure. No obvious phase transformation in the oxidefilms occurs during annealing. It is obvious that the oxide filmhas a good thermal stability. This is similar to high-entropyalloys, that is, following Boltsmanns hypothesis the relation-ship between the entropy and system complexity, the changein configurational entropy during the formation of a solidsolution from three elements with an equimolar ratio is alreadylarger than the entropy changes for fusion of most metals.Consequently, alloys containing a higher number of principalelements will more easily yield the formation of random solid

Fig. 6. SEM m ) annorphology of the oxide film deposited at CO2 = 10%: (a) before annealing, (b ealed at 500 C, (c) annealed at 700 C, (d) annealed at 900 C.


solutions during solidification, rather than intermetallic com-pounds or other complicated phases [1012]. Solid solutionswith multi-principal elements will tend to be more stable atelevated temperatures because of their large mixing entropies[20]. Therefore, it is believed that the oxide film having a simplestructure and a good thermal stability is relevant to high-entropyeffect. In order to further confirm the phase of the oxide films,TEM was employed to analyze the films. The results of typicaloxide film deposited under CO2 = 30% are shown in detail. TheTEM morphology of the oxide film is shown in Fig. 5. Inset isthe corresponding electron diffraction patterns (EDP). Similar toXRD result, TEM analysis also indicates the oxide film is HCP.

3.2. SEM analysis

Figs. 68 show the SEM morphology of the sputtered oxidefilms of AlCoCrCu0.5NiFe high-entropy alloy. It is observedthat the oxide film is composed of nanograin. Before annealing,the grain size is about 30 nm at CO2 = 10 and 50% as shown inFigs. 6(a) and 8(a). When the oxygen content is CO2 = 30%,there are two kinds of particles with different sizes as shown inFig. 7(a). The smaller is 40 nm in size and the bigger between70 and 150 nm. From Figs. 68, the grain size increases withthe anneal temperature and the micro-holes among gains are

apparently enlarged during annealing. This is due to the factthat grains grow up during annealing and the number of grainsdecreases. The piling of bigger grains results in bigger micro-hole formation. It is also seen that the surface roughness of filmtends to increase with the anneal temperature. This is also relatedwith the piling of bigger grains.

The chemical compositions of the sputtered alloy film oroxide films of AlCoCrCu0.5NiFe high-entropy alloy were ana-lyzed using EDS. The results are shown in Table 1. It is seenthat the oxygen concentration of the oxide film increases withthe oxygen content (CO2 ) in the working gas. After annealing,the films deposited under CO2 = 10 and 30% tend to absorb oxy-gen and cause the oxygen concentration of film to increase. Thisis reasonable since, although anneal was carried out on a vac-uum furnace, there still existed oxygen in the anneal furnace.However, the oxygen concentration of the oxide film depositedunder CO2 = 50% changes very little during annealing, indicat-ing oxygen is saturated.

3.3. Hardness analysis

Fig. 9 shows the hardness value of the sputtered oxide filmsof AlCoCrCu0.5NiFe high-entropy alloy. Before annealing, thefilm deposited at CO2 = 30% has the highest hardness value

Fig. 7. SEM ) annmorphology of the oxide film deposited at CO2 = 30%: (a) before annealing, (b ealed at 500 C, (c) annealed at 700 C, (d) annealed at 900 C.


Fig. 8. SEM m

while that dthere existsresults in ttion in thisvalue of thature increCO2 = 30%

Table 1Chemical com

Oxygen conte

0101010103030303050505050orphology of the oxide film deposited at CO2 = 50%: (a) before annealing, (b) ann

eposited at CO2 = 10% has the smallest, indicatingan optimal oxygen concentration of oxide film that

he highest hardness value. The optimal concentra-paper is 38.5% as shown in Table 1. The hardness

e oxide film steps down when the annealing temper-ases. However, the hardness of film synthesized at

is reduced most quickly with increasing the anneal-

ing temperdeposited atends to deing annealthe changeof oxide filtemperatur

position of the sputtered alloy film or oxide film of AlCoCrCu0.5NiFe high-entropy

nt in the sputtering gas Anneal temperature (C) Al Co 20.9 17.9 14.5 12.4

500 14.8 11.8700 14.0 12.1900 14.2 11.6

12.9 10.5500 13.5 10.2700 12.7 10.4900 12.5 10.7

11.8 10.2500 11.9 10.4700 11.5 10.3900 11.8 10.6ealed at 500 C, (c) annealed at 700 C, (d) annealed at 900 C.

ature. After the 900 C annealing of the oxide filmst CO2 = 10, 30 and 50%, their hardness differencecrease. Since there is no phase transformation dur-ing as discussed in Section 3.1, it is believed that

of both the oxygen concentration and grain sizem determines the film-hardness change. After high-e annealing, the grain size of the oxide films becomes

alloy in atomic percentage

Cr Cu Ni Fe O

16.4 8.3 18.0 18.5 11.0 5.9 12.6 13.0 30.611.4 5.3 12.5 12.9 31.310.8 5.5 12.3 12.3 33.011.2 5.4 12.4 12.7 32.510.0 5.1 11.2 11.8 38.510.3 4.9 10.9 11.4 38.8

9.8 4.9 10.8 11.2 40.29.8 5.0 10.9 11.4 39.79.2 4.6 10.0 10.7 43.59.5 4.5 9.8 11.2 42.78.8 4.3 9.7 10.9 44.59.2 4.6 9.8 10.8 43.2


Fig. 9. Hardness vs. substrate temperature or annealing temperature of the sput-tered oxide films of AlCoCrCu0.5NiFe high-entropy alloy.

similar as shown in Figs. 68 and the oxygen concentration dif-ference decreases as given in Table 1. Thus, after annealing, thehardness d

3.4. Resist

Fig. 10of AlCoCrthe AlCoC320 cm0 to 10%, tAfter that(CO2 ). Thethe AlCoCreasonablephous as shtype semicin the oxidconductivitFig. 10 alsoat different

Fig. 10. Resisputtered oxid

Fig. 11. Thicsputtered oxid

increase wes ndurof o

re steong

er de

hickn

thicigh-ownses with

de isurve

ter anhat tin Flittlreasifference of the oxide films decreases.

ivity analysis

shows the resistivity of the sputtered oxide filmsCu0.5NiFe high-entropy alloy. The resistivity ofrCu0.5NiFe-alloy film is also measured, about. As the oxygen content increases from CO2 =he resistivity has a large enhancement by 20 times.the resistivity decreases with the oxygen contentresistivity of the oxide film is larger than that of

rCu0.5NiFe-alloy film deposited at CO2 = 0. This issince the alloy film is metallic although it is amor-own in Fig. 1. The oxide film is similar to oxidation

onductor. With the oxygen content increase, oxygene is surplus and metal vacancies increase. Thus, they of the oxide film is dependent on metal vacancies.

provides the resistivity of the oxide films annealedtemperature. The resistivity of the oxide film tends to

ture dochangedensityperatusize amin low

3.5. T

TheNiFe hare shdecreasince wlic oxithree clar. AfAfter tshownis veryThis isstivity vs. substrate temperature or annealing temperature of thee films of AlCoCrCu0.5NiFe high-entropy alloy.

grains resuwhereas aduring ann

4. Conclu

The spualloy are oworking gaamorphousbetween 10lattice conannealing,decrease wreaches maform durinkness vs. substrate temperature or annealing temperature of thee films of AlCoCrCu0.5NiFe high-entropy alloy.

ith the annealing temperature. Since the phase struc-ot change with the annealing temperature, resistivitying annealing is mainly relevant to the grain size andxide films. From Figs. 68, when the annealing tem-ps up, grain size becomes similar but the micro-holegrains is enlarged. The larger micro-hole size resultsnsity and larger resistivity.

ess analysis

kness of the sputtered oxide films of AlCoCrCu0.5-entropy alloy was measured using SEM. The resultsin Fig. 11. Before annealing, the film thicknessith the oxygen content (CO2 ). This is reasonable

increasing CO2 , along with exhaust gas more metal-pumped out of deposition chamber. The tendency ofs of thickness versus annealing temperature is simi-nealing at 500 C, the oxide-film thickness increases.

he thickness decreases with the anneal temperature asig. 11. However, the variation of the thickness valuee. In other words, the thickness is at the same level.onable since the bigger micro-hole formation among

lts in the thickness increase as shown in Figs. 68,little part of oxide was sublimed and pumped outealing.

sions

ttered oxide films of AlCoCrCu0.5NiFe high-entropyf simple structure. With no oxygen addition to thes, the AlCoCrCu0.5NiFe high-entropy alloy film is. When the oxygen content in the working gas is

and 50%, the sputtered oxide films are HCP withstants of a = 0.3583 nm and c = 0.4950 nm. Beforeboth the resistivity and thickness of the oxide filmith increasing oxygen content and the hardness valueximum at 30% O2. No new phases in the oxide filmsg annealing, indicating the oxide films are very stable


at high temperature. However, the crystal grains tend to growup and the micro-hole size among grains increases with theannealing temperature. The resistivity of the oxide film stepsup with annealing temperature, whereas the hardness valuedecreases. The oxide-film thickness changes very little duringannealing.

Acknowledgements

The authors gratefully acknowledge the financial supports forthis research by the National Science Council of Taiwan undergrant no. NSC-91-2120-E-007-007, the Ministry of EconomicAffairs of Taiwan under grant no. 92-CE-17-A-08-S1-0003 andthe Guangdong Provincial Natural Science Foundation undergrant no. 04300026. We also thank Doctor Davison for criticallyreading and editing the manuscript.

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Microstructure, hardness, resistivity and thermal stability of sputtered oxide films of AlCoCrCu0.5NiFe high-entropy alloyIntroductionExperimental detailsResults and discussionXRD and TEM analysisSEM analysisHardness analysisResistivity analysisThickness analysis

ConclusionsAcknowledgementsReferences

microstructure, hardness, resistivity and thermal stability of sputtered oxide films of...

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