Surface and Coatings Technology 155(2002) 146–151

0257-8972/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž02.00060-9

Mechanical characterization of DC magnetron sputtered amorphous Ti–Al–Cr coatings

Feng Huang , John A. Barnard , Mark L. Weaver *a b a,

Department of Metallurgical and Materials Engineering and Center for Materials for Information Technology, The University of Alabama,a

Box 870202, Tuscaloosa, AL 35487-0202, USADepartment of Materials Science and Engineering, School of Engineering, University of Pittsburgh, 848 Benedum Hall, Pittsburgh, PA 15261,b

USA

Received 28 June 2001; accepted in revised form 22 February 2002

Abstract

Recent research efforts have established that Laves phase reinforced gamma titanium aluminides(i.e.gqLaves) offer significantpotential as oxidation resistant coating in high-temperature structural applications. In this study TiAlCr coatings were d.c.magnetron sputtered from a Ti-51Al–12Cr alloy target onto silicon substrates. The microstructures, mechanical properties, andstress evolution have been investigated in as-deposited films and in films annealed in an argon atmosphere. The amorphousstructure found in the as-deposited coatings was retained after thermal cycling to 5008C. After annealing the coatings exhibitedimproved hardness, higher reduced modulus, and smaller CTE values, which likely result from densification of coatings inducedby thermal cycling and surface oxidation.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: TiAlCr; Coatings; Mechanical properties

1. Introduction

In recent years, various ternary Ti–Al–Cr alloysbased ong-TiAl and Laves-Ti(Al,Cr) phases, in which2

the Cr content was 8–28 at.%, have been developed foruse as oxidation resistant coatingsw1–9x. These protec-tive coatings offer the potential for extending the appli-cation temperatures ofg-TiAl alloys and conventionalTi-based alloys up to 10008C in air. For these coatingsto be effective, they will need to exhibit high oxidationresistance, good mechanical compatibility with theunderlying substrate and resistance to cracking underoften extreme loading conditionsw1,7x. As such, thecoefficient of thermal expansion(CTE), Young’s mod-ulus, as well as the residual stress, should also beamongst key considerations in evaluating the coatingspotential, especially for coatings that will undergo ther-mal cycling between room temperature and servicetemperatures. However, in sharp contrast to the intensiveattention paid to oxidation resistance, the mechanical

*Corresponding author. Tel.:q1-205-348-7073; fax:q1-205-348-2164.

E-mail address: mweaver@coe.eng.ua.edu(M.L. Weaver).

properties of such Ti–Al–Cr coatings have yet to beinvestigated. The objective of the present paper is tocharacterize mechanical response of magnetron sputteredTi–Al–Cr coatings in the as-deposited and the annealedstates, respectively.

2. Experimental

Ti–Al–Cr coatings were d.c. magnetron sputteredfrom a Ti–51Al–12Cr alloy target at ambient tempera-ture using argon plasma. Detailed deposition conditionsare listed in Table 1.As noted in our previous study, a strong correlation

between the coating thickness and mechanical propertieshas not been establishedw10x. Consequently, the thick-ness of Ti–Al–Cr coatings for mechanical characteri-zation was fixed in our current studies at 1.5mm.Thinner films were also prepared for transmission elec-tron microscopy(TEM) and X-ray reflectivity(XRR)studies. TEM studies were performed using a HitachiH-8000 transmission electron microscope operated at200 keV. Specular XRR measurements, which yield thethickness(for deposition rate calibration) and the refrac-

147F. Huang et al. / Surface and Coatings Technology 155 (2002) 146–151

Table 1Sputtering conditions for the Ti–Al–Cr coatings used in our present study

Target materials Ti–51Al–12Cr(nominal atomic ratio,101.6 mm in diameter, 99.9 at.%)

Base pressure -3.2=10 Pay5

Deposition power 200(W)Deposition pressure 0.40 PaDeposition rate 67 nmyminSubstrates Oxidized Si(100) wafers and Corning 1737 glassTarget–substrate distance 60 mmNominal film thickness 1500 and 90 nmSputtering temperature Room temperature, no external heatingDeposition gas Ar

tive index(for the density evaluation), were taken on aPhilips X’pert-MRD system using CuKa radiation inthe line focus modew11x. A graded parabolic focusingmirror was installed in the primary optics to obtain aquasi-parallel, yet intensive incident X-ray beam with-0.058 divergence.For hard X-ray radiation, the refractive index of

condensed matter,n, is slightly less than unity and canbe written as , whered is the dispersionns1ydyibterm, andb the absorption term, both being on the order10 –10 w12x. Sincen is less than unity, there existsy5 y7

a critical angle of incidence,a , below which totalc

reflection of X-rays occurs. For films of known chem-istry, density can be determined from the critical anglethrough the dispersion termd by w13,14x:

2a SZc j10 2ds s2.72=10 rl (1)2 SAj

whereZ is the atomic number,A the atomic mass, andj j

nm. In our current study, the critical anglels0.154 ac

is chosen as the incidence angle at which the reflectivityis 0.5 w15x. The uncertainty inherent in such a treatmentis w15x, which is approximately 1% by noting2Da yai c

a 0.0028 step width used in the measurement. Thethickness of thicker films was also confirmed using aDektak II-A profilometer.The thicker coatings were also studied by grazing

angle X-ray diffraction(GAXRD) at an incidence angleof 48. The GAXRD patterns were obtained with aRigaku DyMax-2BX X-ray diffractometer using CuKaradiation.Mechanical characterization was performed on sam-

ples in the as-deposited state and after a thermal cyclingin argon between room temperature and 5008C (inter-rupted at 4008C for 30 min during heating). Thoughthe intended service temperatures are expected toapproach 10008C, the maximum annealing temperatureused in the present study was limited to 5008C, whichrepresents the maximum temperature for in situ stressmeasurements. The hardness and the reduced modulusof the coatings were evaluated through the nanoinden-

tation technique, with the measurements performed ona Hysitron nanomechanical testing system(TriboScope ) equipped with a sharp Berkovich dia-�

mond tip. The load–displacement curves were analyzedvia the Oliver and Pharr methodw16x. The residualstress was determined using the substrate–curvaturemethodw17x using a Flexus F2320 stress measurementsystem. Details on stress determination have been report-ed previously w10x. Coatings for stress studies weredeposited onto 2-inch oxidized Si(100) wafers. Thestress–temperature relation( ) was studied in situsyTby the F2320 system when the sample was thermallycycled in argon between room temperature and 5008C.The thermal cycling was interrupted at 4008C for 30min during heating to study isothermal relaxation at thistemperature. The heating rate and most of the coolingrate ()150 8C) were 58C min . Lower cooling ratesy1

were observed when the temperatures were lower than150 8C.

3. Results and discussion

The as-deposited TiAlCr coatings were amorphous,as is evident from the GAXRD pattern presented in Fig.1. The broad peak near 2us408 cannot be attributed tothe equilibriumg-TiAl or Ti (Al,Cr) phases. This obser-2

vation is further supported by TEM studies performedon the as deposited coatings(Fig. 2a,b). The amorphousstructure exhibits a high thermal stability as it persistseven after annealing in vacuum at 6008C for 1800 s.Similar amorphous structures were also observed in as-sputtered Ti–51Al–12Crw7x and Ti–50Al–10Cr coat-ings w8x, and have been attributed to limited adatommobility on the film surface at low deposition tempera-turesw8x.An experimental XRR pattern for a thin as-deposited

Ti–Al–Cr film is presented in Fig. 3. The reflectivitypattern is characterized by oscillations resulting frommore than one modulation, which is indicative of theexistence of more than one constituent lamella. This isdue to the existence of a much thinner oxide layer on

148 F. Huang et al. / Surface and Coatings Technology 155 (2002) 146–151

Fig. 1. GAXRD patterns for Ti–Al–Cr coatings under various states:(a) as-deposited; and(b) after the thermal cycling. The grazing anglewas set at 48. The diffraction data of TiCr andg–TiAl phases were2

also presented for comparison.

Fig. 2. TEM micrographs of as-deposited and annealed Ti–Al–Cr films.(a) Bright-field TEM (BF-TEM) micrograph;(b) selected area diffraction(SAD) pattern for as-deposited films;(c) BFTEM micrograph; and(d) SAD pattern for annealed film(600 8Cy1800 s in vacuum).

top of the Ti–Al–Cr layer, as its modulation period ismuch larger than the one corresponding to the Ti–Al–Cr layer. The film thickness was 89 nm, as estimatedtfrom the fringe spacing of the dominant oscillationDu

periods at high angles by . The critical angletsly2Du

determined from the experimental pattern wasa scwhich translated through Eq.(1) into a mass(0.282

density of gycm .3rs4.00Hardness was evaluated from load–displacement

( ) curves taken at various loads. A plot of hardnessPyhvs. displacement( ) is as shown in Fig. 4. TheHyhstable values at larger were selected as representativehcoating properties to minimize surface effects(e.g.roughness, oxidation layer, etc.). The hardness of theas-deposited coating was 9.4"0.5 GPa, and remainedrelatively constant over the range of indentation testloads applied in this investigation. After thermal cyclingin argon (i.e. argon annealing), the hardness of thecoating increased above that of the as deposited coatings.It was additionally observed that the hardness of thecoating increased at lowest test loads employed in thisstudy reminiscent of the extensively reported indentationsize effect (ISE). At higher loads corresponding todisplacements of greater than 60 nm, the hardness wasobserved to stabilize at 11.2"0.4 GPa. The hardnessincrease in annealed coatings could be ascribed in part,to densification of the amorphous coating. Such struc-tural relaxation generally leads to an increased Young’smodulus w18x and accordingly to increased hardness.The formation of intermetallic bonds giving TiAl and

149F. Huang et al. / Surface and Coatings Technology 155 (2002) 146–151

Fig. 3. X-Ray reflectivity pattern for a 90 nm Ti–Al–Cr film. Thecritical angleu is used to calculate the film density.c Fig. 5. Reduce modulus vs. displacement plots for the as-deposited

and the annealed coatings.

Fig. 4. Hardness vs. displacement plots for the as-deposited and theannealed coatings.

Fig. 6. Stress vs. temperature curve for the first thermally cycling inargon.

Ti(Al,Cr) phases could be another cause. It is also2

speculated that some of the hardness increase could alsobe attributed to the formation of hard surface oxidesduring annealing. Such oxidation could similarly influ-ence hardness measurements at low indentation testloads leading to the hardness increase observed in thisstudy. Further experiments are necessary to validate thishypothesis and to determine whether or not the ISE isa factor.The reduced modulus , where is the2Ž .E y 1yn Ef f f

Young’s modulus andn the Poisson’s ratio of thef

coating, can also be evaluated from the unloading partof the curves. An vs. plot is presented2Ž .Pyh E y 1yn hf f

in Fig. 5. The values of decreased with2Ž .E y 1ynf f

increasing in the as-deposited coating, which at max-himum was taken as representative coating propertiesh

(145"4 GPa). became more stable in the2Ž .E y 1ynf f

argon annealed coating, except at low displacementwhere the influence from surface oxides was significant.The value of in the argon annealed coating,2Ž .E y 1ynf f

174"3 GPa, is comparable with that for theg-TiAl(Es174 GPa andns0.23 w19x). This proximity wouldbe very beneficial concerning the thermo-mechanicalcompatibility between the coating andg-TiAl basedmaterials.The residual stress in the as-deposited coatings was

compressive. Fig. 6 illustrates the relation of thesyTfirst thermal cycling of the as deposited coating in theargon. Upon heating, the stress initially became morecompressive with increasing temperature. The observedinitial linear development in the compressive stress

150 F. Huang et al. / Surface and Coatings Technology 155 (2002) 146–151

Fig. 7. Stress relaxation at 4008C in argon.

indicates elastic deformation in the films as a conse-quence of a thermal expansion mismatch between thesubstrate and the film. Deviation from linearsyTprofile at near 1508C denotes the onset of plastic flow.Stress relaxation occurred during the isothermal hold at400 8C, which caused the compressive stress to decreasein magnitude.Fig. 7 illustrates the isothermal stress relaxation at

400 8C. It is interesting to note that the stress relaxationprocess can be fitted very well with a function

Ž . Ž .s t ss q s ys expŽytyt (2).` 0 `

where is the initial stress level, the final stresss s0 `

level, and denotes a characteristic relaxation timew20x.t

Eq. (2) was generally used in approximating limitedstrain relaxation due to grain–boundary diffusionw20x.Since the Ti–Al–Cr coating used in our current studieswas amorphous, such a grain–boundary diffusion relax-ation mechanism should be excluded. The quality offitting, nevertheless, indicates that there is a singledominant, i.e. limiting, relaxation process, which is oftena diffusion process in the solid state because otherprocesses such as localized atomic rearrangements occurmore rapidly. This could be due to the microstructuralcharacteristics in our coatings. The poor crystallinityindicates the existence of considerable highly disorderedareas in the coatings, which can act as fast diffusionpaths(vacancies, constituent atoms, etc.). The charac-teristic relaxation time is comparable with that reportedt

in Bruckner and Weihnachtw20x, which provides addi-¨tional support for fitting with Eq.(2).Elastic deformation during thermal cycling resulted

from the CTE mismatch between the substrate and thefilm. The values of CTEs can be calculated from theslope of linear portions of the plots, which aresyTgiven by

ds E Ef fŽ . Ž .Ž .s a ya s 1qn a ya , (3)s f f s f2dT 1yn 1ynf f

where is the substrate CTE and the film CTE.a as f

Assuming is the same as that forg-TiAl ( s0.23n nf f

w19x), the room temperature CTEs for the as-depositedand annealed coatings were determined to be14.7=10 and 11.1=10 8C , respectively. Smallery6 y6 y1

CTE values in the annealed coatings are likely due tothe annealing-induced structural relaxation.

4. Conclusions

In summary, the mechanical characterization of Ti–Al–Cr coatings has been performed. The followingconclusions can be drawn from our observations:The as-deposited Ti–Al–Cr coatings were amorphous.

This amorphous structure was retained after a thermalcycle to 5008C.The hardness, reduced modulus, and CTE values were

experimentally obtained for as-deposited and annealedcoatings. After annealing the coatings exhibited higherhardness, larger reduced modulus, and smaller CTEvalues, which likely result from densification of coatingsinduced by thermal cycling and surface oxidation.

Acknowledgments

This work was supported by the US Army ResearchOffice under grant no. DAAD 19-99-1-0152. The useof the facilities supported by the MRSEC Program ofthe NSF under award no. DMR-9809423 is also grate-fully acknowledged.

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