y 201 (2006) 168–173www.elsevier.com/locate/surfcoat

Surface & Coatings Technolog

Nanocrystalline gamma alumina coatings by inverted cylindricalmagnetron sputtering

Atul Khanna 1, Deepak G. Bhat ⁎

Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR 72701, USA

Received 25 July 2005; accepted in revised form 7 November 2005Available online 27 December 2005

Abstract

Alumina coatings were deposited on glass, silicon and stainless steel substrates by magnetron sputtering of two cylindrical aluminum targets inargon–oxygen plasma using a mid frequency AC power supply operated at 41 kHz and 5 kW. No substrate heating was used during the depositionprocess. Reactive sputtering was carried out in the poisoned mode of the targets at low deposition rates of about 0.09 nm s−1. The coatings werecharacterized by X-ray diffraction studies and found to be nano-crystalline, γ-phase of aluminum oxide. The coatings thickness was measuredusing the optical interference effects in the UV–visible spectrum of thin films deposited on glass substrates. Thin film thickness after 4 h ofreactive sputtering was found to be 1.3 μm. We further studied the oxidation resistance of the alumina coatings produced on stainless steelsubstrates at a temperature of 400 °C. Coatings were found to be completely stable at this temperature and prevented the oxidation of underlyingstainless steel substrate.© 2005 Elsevier B.V. All rights reserved.

Keywords: Alumina coatings; Inverted cylindrical magnetrons; AC sputtering

1. Introduction

Aluminum oxide is a very important ceramic which isextensively used as a high temperature, corrosion resistantrefractory material. Due to its high hot hardness, chemicaldurability and oxidation resistant properties, there has been anenormous interest in the development of crystalline aluminafilms for their application as wear resistant protective coatingson high-speed machine tools for milling and cutting of cast ironand low carbon steel [1]. Alumina thin films are also of interestin microelectronics industry as an insulating material due totheir excellent electrical properties like large bandgap and highdielectric constant. Alumina is known to exist in severaltransient, metastable structures like γ-, η-, θ-, δ- and κ-phases.The most desirable form of crystalline alumina coating forcutting tool application is thermodynamically stable α-phase

⁎ Corresponding author. Tel.: +1 479 575 4946.E-mail addresses: akphysics@yahoo.com (A. Khanna),

dgbhat@engr.uark.edu (D.G. Bhat).1 On leave from Department of Applied Physics, Guru Nanak Dev University,

Amritsar-143005, India.

0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.11.109

which is commonly known as corundum and has the hexagonalrhombohedral crystal structure with lattice parametersao=0.476 nm and co=1.299 nm [2,3]. α-alumina can withstandhigh temperatures of more than 1200 °C and has high hardness(24 GPa) and excellent corrosion/oxidation resistant properties[4]. α-alumina thin films have conventionally been producedfor commercial applications by chemical vapour deposition(CVD) which requires a substrate temperature of about 1000 °C[5–15] and hence the number of substrate materials that can beused at such high temperatures are very limited. Due to highdeposition temperatures, α-alumina coatings prepared by CVDhave large residual stresses which in turn deteriorates filmadhesion to the substrates. CVD coatings are often not smoothand have a surface roughness of 1–2 μm[9,14,15]. CVDcoatings on cemented carbide tools are generally applied with aprecoating of TiC or TiCN, since it is not possible to apply themto the uncoated carbide substrate directly. This is because of thesevere chemical degradation of the carbide surface due to theoxidizing depositing environment. The intermediate hardcoating of TiC or TiCN provides a crystallographic orientation(epitaxy) between TiC (TiCN) and alumina. As result, the CVD-alumina coated cutting tools typically have a total thickness

Fig. 1. Schematic of dual cylindrical targets used in the ICM-10 depositionsystem.

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exceeding 6–8 μm. This often limits the effectiveness of thecoating during metal-cutting, due to rapid wear of the roughcutting edges.

Due to these difficulties, there is great interest in thedevelopment of α-alumina coatings by low temperaturephysical vapour deposition (PVD) techniques. The physicalvapor deposition (PVD) methods afford the possibility ofdepositing hard, wear-resistant coatings on sharp-edged tools atsignificantly lower temperatures in the range of 300–500 °C.

A variety of techniques like e-beam evaporation [16,17],plasma spraying[18–22], gas detonation [23], sol–gel process[24], laser ablation [25], RF sputtering [26], pulsed DCmagnetron sputtering [27–37] and mid-frequency AC magne-tron sputtering [38] have been reported for the deposition ofcrystalline alumina coatings. PVD-grown alumina coatings aresmoother than those obtained by CVD techniques and have asmaller grain size. Also due to much lower depositiontemperature, it is quite possible to deposit alumina directly onthe cemented carbide (or tool steel) substrates. The simulta-neous ion bombardment during deposition usually leads to avery fine grained coating in the submicron range. PVD aluminacoatings have smaller grain size and are smoother than thoseobtained by CVD. Nanocrystalline coatings have an advantageof having higher fracture toughness and strength.

Aluminum oxide coatings obtained by reactive sputtering ofAl in oxygen partial pressure generally grow in the amorphousphase below a substrate temperature of 350 °C and have lowmicrohardness values of 10 GPa or less. It is known that thesubstrate temperature of about 500 °C is required to deposit γ-alumina phase which is the first member of the so-called gammaseries (γ, η, δ and θ phases) while the two phases from the alphaseries (κ and α) require a substrate temperature of at least700 °C when grown by pulsed magnetron sputtering [1,31].One of the earliest studies of alumina deposition by sputtering isby Thornton and Chi in which they investigated the effect ofpost-deposition heat treatment of RF sputtered alumina, and theevolution of the various crystalline forms as the temperature wasraised to 1200 °C. They suggested that the formation of thecrystalline α-alumina by PVD cannot be achieved at lowtemperatures, due to kinetic energy barriers associated withcovalent bond angle requirements which can be overcome onlyat elevated temperatures of about 1000 °C [26]. Indeed manysubsequent studies have shown that ion bombardment duringdeposition can indeed produce crystalline alumina coatings.Once formed, α-alumina is stable at all temperatures and thus thestructural transformations that occur in alumina, as a function oftemperature, are strictly speaking not an example of polymor-phism or a structural phase transition [39–42]. It is well knownthat critical parameters that determine the crystallinity, phasesand properties of the oxide coatings grown by sputtering areplasma ion current densities, oxygen flow rate, substratetemperature, sputtering power and ion plating (positive ionbombardment of the negatively biased substrate that impartsadditional energy to the growing film). Conventional DC planarmagnetron sputtering systems generally operate at very lowcurrent densities in the range of 0.1–1.0 mA cm−2 whichexpectedly produce non-crystalline alumina coatings. Pulsed

DC planar magnetrons which overcome the difficulties ofcharging effects due to the formation of thin insulating layer atthe target surface, can form crystalline alumina phases attemperatures as low as 300 °C when operated at high sputteringpowers of 15 kW or above [31]. A novel method of enhancingthe plasma current densities without the use of high sputteringpowers of 15 kW or more is to use the dual inverted cylindricalmagnetron configuration. Mid-frequency AC reactive sputteringof dual cylindrical targets has been reported earlier by Glocker etal. for the deposition of alumina coatings [38]. In this paper wereport the growth and characterization of alumina coatingsprepared by inverted cylindrical magnetron sputtering at afrequency of 41 kHz and power of 5 kW.

2. Experimental

2.1. Inverted cylindrical magnetron sputtering

We have used a Isoflux model ICM-10, Dual Target InvertedCylindrical Magnetron Sputtering system to deposit aluminacoatings on glass, silicon wafer and stainless steel substrates.Reactive sputtering was carried out in an argon–oxygengaseous mixture, using an Advanced Energy PE II AC powersupply operated at a frequency of 41 kHz and constant power of5 kW. Fig. 1 shows the schematic of the dual hollow cathodes(targets). The two targets are made out of Al metal with a purityof 99.5% and have an outer diameter of 19 cm and height of9.8 cm. Each target is powered by a mid-frequency AC powersupply, such that the targets are alternately cathode and anodeduring each cycle. This permits the targets to operate effectivelywithout any arcing effects due to the deposition of thininsulating oxide layer on them.

All the substrates were ultrasonically cleaned in methanoland acetone for 30 min and kept on a substrate holder whichconsisted of a horizontal steel plate in the centre of two hollowAl targets. The substrate to target distance was about 3 cm. Nosubstrate rotation or heating was done during the deposition runwhich lasted for 4 h. The chamber was evacuated to a basevacuum of 4.3×10−4 Pa before the sputtering and reactivegases (argon and oxygen, respectively) were introduced andmagnetron power supply was switched on to create a discharge.Gases with purity levels of 99.99% were used in the system.

Table 1Conditions used for the deposition of alumina coatings

Base vacuum: 4.3×10−4 PaO2 flow rate: 48 sccmAr flow rate: 48 sccmPressure during deposition: 0.24 PaPower: 5 kWSubstrate heating: No

Fig. 3. X-ray diffraction patterns for alumina coatings on glass and siliconsubstrates. Peaks marked as “o” are attributed to the formation of Al2SiO5

phases for the coatings deposited on Si substrates.

170 A. Khanna, D.G. Bhat / Surface & Coatings Technology 201 (2006) 168–173

Argon and oxygen flow rates were both kept constant at 48sccm during the entire deposition process. This oxygen flowrate was sufficient to completely poison the targets. Thedeposition parameters are summarized in Table 1.

3. Results

3.1. Hysteresis effects in reactive sputtering

Fig. 2 shows the variation of the targets' voltage and currentwith increase in the oxygen flow rate into the deposition chamber.During these measurements the argon flow rate was kept constantat 75 sccm. We note an abrupt fall in the target voltage around theoxygen flow rate of 40 sccm. As the oxygen flow rate or partialpressure is increased the rate of alumina formation exceeds itsremoval rate from the target due to the lower sputtering yield ofalumina as compared to metallic Al and the fact that compoundshave higher secondary electron emission yield than pure metals,the deposition rate and the AC potential difference between thetwo targets decreases. However, since our power supply wasoperated in the constant power mode, the discharge currentincreases due to the drastic increase in the concentration ofelectrons in the plasma. Although a very thin insulating layerof alumina is formed on each target, the discharge sustains dueto the fact that polarity on each target is reversed at afrequency of 41 kHz. We did not observe any arcing due to thedeposition of insulating layer of aluminum oxide during theentire deposition run which lasted for 4 h.

Fig. 2. Poisoning of the Al targets with increase in oxygen flow rate in thechamber.

3.2. X-ray diffraction studies

X-ray diffraction studies were carried out on aluminacoatings deposited on glass, silicon and stainless steel substratesusing Cu Kα X-rays at the Philips make PW 1830 powderdiffractometer. Fig. 3 shows the XRD patterns as recorded forthe coatings on glass and silicon substrates at a glancingincidence angle of 2.5°. Divergence slit of half a degree wasused while recording the X-ray diffraction patterns. Fig. 4shows the similar XRD patterns for alumina coatings onstainless steel substrates both before and after their heattreatment. The XRD patterns for the thin film samples on allsubstrates are similar and we observe two prominent but ratherbroad diffraction peaks at 2θ angles of approximately 46° and66° due to the reflection of X-rays from the (400) and (440)crystal planes respectively of the cubic γ-alumina. The averagesize of crystallites, a, in the coatings was calculated from thewidth of above two XRD peaks using the Scherrer equation,

Fig. 4. X-ray diffraction patterns for alumina coatings on stainless steelsubstrates both before and after heat treatment at 400 °C and 600 °C.

Table 2Size of crystallites in alumina coatings as determined from the XRD studies

Substrate Average crystallite size (nm)

Glass 10Silicon 6Stainless steel 6Stainless steel (400 °C) 7Stainless steel (600 °C) 8

Fig. 6. Photograph of uncoated stainless steel (left) and alumina coated stainlesssteel substrate (right) heat treated to 400 °C for 8 h in air. The uncoated stainlesssteel piece acquired a brownish coloration due to its oxidation while the aluminacoated one retained its metallic luster.

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a=0.9λ/Bcosθ, where λ=1.54 Å is the wavelength of Cu Kα X-rays, B is the width of the diffraction peak at half maximum andθ is one-half of the angle at which the peak is centered. Table 2shows the average size of crystallites in all coatings obtainedafter making corrections for the instrumental broadening of theX-ray diffractometer.

3.3. Thin film thickness measurement

The thickness of alumina coatings was determined using theoptical interference effects observed in the UV–visiblespectrum of the coatings on a glass substrate. Fig. 5 showsthe transmitted light spectrum for the alumina coating on one ofthe glass substrates. The UV–visible spectrum was recorded onthe Shimadzu make double beam spectrophotometer (Model2010PC). By noting the wavelength difference (λ2–λ1) betweentwo successive interference minima and using the reportedvalues of refractive index, n, of 1.65 for gamma phase alumina[30] we calculated the film thicknesss, t, from the followingwell known relation [43,44]:

t ¼ k1k22nðk2−k1Þ ð1Þ

The film thickness was found to be 1.3 μm for one suchalumina coating on a glass substrate. Using a total depositiontime of 4 h we estimated the thin film deposition rate to be0.09 nm s−1.

Fig. 5. UV–visible spectrum of the alumina coating on a glass substratesshowing optical interference effects in the transmitted light.

3.4. Heat treatment of alumina coatings

We heated the stainless steel substrates coated with γ-alumina to a temperature of 400 °C and 600 °C in air for 8 h totest the oxidation resistant properties of these coatings.

Fig. 4 shows the XRD patterns of the alumina coatings onstainless steel both before and after heat treatment. The XRDpeak intensities increase with heat treatment of the coatings dueto growth in the size of crystallites; however, we did not observethe transformation of the gamma phase to other metastablestructures of alumina. This indicates that gamma phase is stableat least up to a temperature of 600 °C. Fig. 6 is the photograph ofboth the uncoated stainless steel piece (left) and a stainless steelpiece top coated with alumina (right) after their heat treatment at400 °C for 8 h in air. While the uncoated stainless steel sampleacquired a brownish coloration due to its oxidation at thesurface, the alumina coated steel piece retained its metallicluster indicating that the film was dense, adherent andprotective.

4. Discussion

Film thickness after 4 h of reactive sputtering wasdetermined to be 1.3 μm on a glass substrate. We did notmeasure coating thicknesses on silicon and stainless steelsubstrates but expect similar values on these substrates as theywere kept at identical positions on the substrate holders and allsubstrates were at equal distances from the two targets.Deposition rate is very small at 0.09 nm s−1 due to the factthat we carried out sputtering in the poisoned mode of thetargets. In this mode, the target surface is partly covered with avery thin dielectric layer of aluminum oxide. The sputteringyield of aluminum, oxygen and clusters of Al–O atoms from theoxidized surface is very small as compared to the yield from apure Al target because of the large surface binding energy peratoms in compounds as compared to metallic targets. Stirlingand Westwood had earlier shown by atomic absorption

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technique that there was no detectable Al flux sputtered from anoxidized Al target [45], these are mostly the secondary electronsand a small flux of Al–O clusters that are sputtered from anoxidized Al surface.

It may be mentioned here that the momentum of bombardingions is not conserved during the process of sputtering. It is theenergy of the incident ions that is conserved and transferred tothe electrons and atoms in the target material. When a thininsulating layer of Al2O3 is formed on the target surface theenergy of the incident Ar+ is mostly transferred to the electronsrather than to the atomic species in the target, hence there is adrastic increase in the emission of secondary electrons from thetargets at the expense of atomic species. The increase in theelectron concentration in the plasma simultaneously producesan abrupt fall in potential difference between the targets as seenin Fig. 2. The emitted secondary electrons also give a negativefloating potential to the substrate holder which in turn causes itsAr+ bombardment [46]. While this Ar+ bombardment heats thesubstrates and aids in the growth of dense, crystalline aluminacoatings, it has an undesirable effect that it produces aresputtering effect. The negative bias will further repel O2−

and O− species likely to be present in the plasma from thesubstrates, and thus may introduce some oxygen deficiency inthe growing film. All these factors lead to large reduction in thethin film deposition rate.

Alumina coatings obtained on glass substrates weretransparent and adhesion was very good on all three type ofsubstrates. It is known from the studies carried out on aluminacoatings prepared by plasma enhanced chemical vapordeposition (PECVD) technique that single phase γ-aluminacoatings are highly smooth and transparent while the presenceof two or more phases in the coatings reduces both the filmsmoothness and transparency to visible light [13]. It may bementioned here that XRD patterns of the metastable phasesnamely, γ, η, δ and θ alumina are very similar and it is difficultto identify a particular phase by XRD studies alone. However, itis known that γ- and η-alumina coatings are generally ultrafinegrained, and therefore, show rather broad peaks at angles ofabout 46° and 66° with a peak width of about 2°. On thecontrary, powder XRD patterns of δ-and θ-alumina consist offairly sharp peaks at these two angles [47]. In case of siliconsubstrates, we observe very sharp and strong diffraction peaks at2θ angles of 12.6°, 51.2° and 90.3°. These peaks are very strongin case of alumina coatings deposited on Si substrates and arevery weak and completely absent in case of the coatingsdeposited on glass and stainless steel substrates, respectively.We attribute these three peaks to the formation of Al2SiO5

phases due to the reaction of Al2O3 coating with the underlyingoxidized Si substrate during the deposition process [48,49].

All alumina coatings were electrically insulating and ameasurement of their surface resistance showed they had aresistance of more than 20 MΩ, which was the maximumresistance that could be measured with the digital multimeterused for this purpose. X-ray diffraction studies showed that thecoatings are entirely γ-phase alumina and we did not detect thepresence of metallic Al or any other transient metastable phasesof alumina. This is an important new result as earlier Zywitzki et

al., who have extensively used the pulsed magnetron sputtering(PMS) technique, for the deposition of crystalline aluminacoatings at Fraunhofer-Institut fur Elektronenstrahel-und Plas-mtechnik (FEG), Germany, reported that a substrate tempera-ture of 350–500 °C and sputtering powers of 11 kW wasrequired for obtaining γ-phase Al2O3 coatings [31]. Schützeand Quinto also reported the growth of γ-alumina coatings withhigh hardness values of 25 GPa using pulsed DC reactivesputtering of dual planar targets at substrate temperature of550 °C and ion current densities of nearly 57 mA/cm2 [50].

Åstrand et al. recently reported the deposition of γ-Al2O3

coatings on cemented carbide cutting tools by bipolar PMStechnique at a rather high substrate temperature of 700 °C [51].We have been successful in depositing nanocrystalline γ-alumina coatings with a grain size of about 7nm without anysubstrate heating and at relatively low sputtering power of 5 kW.Table 2 presents the size of crystallites as determined from theXRD patterns for the coatings on glass, Si and steel substrates.Interestingly grain size is maximum in case of glass substrateand least for the coatings on stainless steel. It may be mentionedhere that alumina coatings prepared by plasma-spray techniqueare also mostly γ-phase and post deposition heat treatment isrequired to transform them into the α-phase [18,19].

The plasma ion current densities used in our system wasabout 16 mA/cm2. It may be noted that we are able to achievethis high current density of 16 mA/cm2 at relatively lowsputtering powers of 5 kW, due to the novel geometry of dualhollow cylindrical magnetrons. As a comparison, similarcurrent density values are possible in planar magnetrons atsputtering powers of only 15 kW and above [28,30,31]. Earlier,Glocker had reported that AC plasmas offer an advantage thatthey produce higher ion current densities and more heating ofthe substrates [52].

Ion plating or the bombardment of the substrates by positiveions is known to impart extra energy to the growing film andfurther reduce the deposition temperature for the growth ofvarious metastable and thermodynamically stable phases ofalumina [34]. The novel geometry of dual unbalanced invertedcylindrical magnetrons combined with substrate biasing duringthe sputtering process can possibly reduce the temperatures fordepositing the thermodynamically stable α-phase aluminacoatings to below 500 °C from the current values of 750 °Cby PMS technique.

Our post-deposition heat treatment, XRD and visual studiesof alumina coatings deposited on stainless steel substrates showthat the coatings are stable up to a temperature of 600 °C and donot transform to high temperature metastable phases like δ- orθ-alumina (Figs. 4 and 6). This indicates that although γ-phasealumina coatings are metastable, they can withstand tempera-tures of at least 600 °C for several hours and can be used asprotective coatings on cutting tools in applications where theydo not reach temperatures of above 600 °C in various metalcutting operations. Åstrand et al. have recently reported that γ-Al2O3/TiAlN and γ-Al2O3/TiN coated inserts exhibit tool lifelonger than the single coated inserts especially at higher cuttingspeeds [51]. In fact, metal cutting applications of γ-phase Al2O3

are known to be comparable to PVD grown TiN and CVD κ and

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α phases of alumina [50,53,54]. Earlier, Larsson and Rupicarried out detailed microstructural and wear testing investiga-tions on CVD grown γ-alumina coatings on cemented carbidesubstrates and found that this metastable phase of aluminashowed better crater wear resistance properties than the CVD κ-alumina coatings mainly due to their ultra-fine grain size whichgreatly enhance their hardness [55].

5. Conclusions

We report very smooth, crystalline coatings of γ-phasealumina with excellent adhesion on glass, silicon and stainlesssteel can be grown using inverted dual cylindrical magnetronsputtering technique without any substrate heating at sputteringpowers of 5 kW or even less. This “inverted “ flow of sputteredion species from the two targets which completely enclose thesubstrates placed at their centre, provides a very uniformdistribution of the depositing species and also enhances the ioncurrent densities. This geometry of targets and the substrates isideal for the deposition of crystalline alumina coatings onvarious cutting tool materials and can possibly produce highlysmooth, uniform, nanocrystalline κ- and α-phase aluminacoatings at much lower substrate temperatures than reportedby pulsed magnetron DC sputtering of planar targets. The γ-phase alumina coatings reported in this research work arethermally stable up to at least 600 °C and have a potential forapplication as wear resistant coatings on cutting tools.

Acknowledgements

The authors would like to thank Dr. David Glocker and Mr.Mark Romach of Isoflux Inc., Rochester, NY, USA, for manyuseful suggestions while this work was being carried out.

The authors are indebted to Arkansas Analytical Laboratoryand Mr. David Ray, for the valuable technical assistance. Theresearch was funded by National Science Foundation under theNSF GOALI project grant #DMI-00400167.

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