201 (2007) 7425–7431www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

The effect of yttrium addition on the isothermal oxidation behavior ofsputtered K38 nanocrystalline coating at 1273 K in air

Wen Wang a,⁎, Ping Yu a,b, Fuhui Wang a, Shenglong Zhu a

a State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui Road 62, 110016 Shenyang Chinab Shenyang Institute of Chemical Technology, 110142 Shenyang China

Received 14 December 2006; accepted in revised form 5 February 2007Available online 20 February 2007

Abstract

The influence of various amounts of Y addition on the isothermal oxidation behavior of sputtered Ni-based superalloy K38 nanocrystallinecoating at 1273 K was investigated. The result indicated that the addition of 0.05 and 0.1 wt.% Y in nanocrystalline coating enhanced the selectiveoxidation of Al, reduced scale growth rate and retarded the phase transformation from metastable to stable alumina. In contrast, 0.5 wt.% Yaddition significantly increased the oxidation rate of the coating, the incorporation of Ti in the external alumina scale accelerated the phasetransformation to stable alumina.© 2007 Elsevier B.V. All rights reserved.

Keywords: High temperature oxidation; Nanocrystalline coating; Reactive-element; Superalloy

1. Introduction

A dense, stable, slow-growing, and adherent oxide scale,such as Al2O3 and Cr2O3, is essential for alloys in an applicationat high temperatures. Numerous studies have revealed that asmall amount addition of reactive elements (RE), such as Y, Hf,Zr, significantly improves the high temperature oxidationresistance of Al2O3-, Cr2O3-formers. Based on a large numberof experimental results, the mechanisms of reactive-elementeffects have been reviewed by several authors [1–5].

For Al2O3-forming alloys, the formation of metastablealumina in a transient oxidation stage has been identified. Themetastable alumina scale is of importance for high temperatureoxidation behavior of an alloy since it has a higher oxidationrate and the subsequent phase transformation to α-aluminaresults in a 13% volume reduction. Several studies [6–8] haveindicated that the RE retards the phase transformation. A modelproposed by Burtin and co-workers [6] suggested that largerions, such as Y, Hf, Zr, La, inhibit the transformation from cubic

⁎ Corresponding author. Tel.: +86 24 23904856; fax: +86 24 23893624.E-mail address: wen@imr.ac.cn (W. Wang).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.02.015

to hexagonal structure when they enter the cubic lattice, whilesmaller ions, such as Ti, have the inverse effect.

Once a protective α-alumina scale is formed, the segregationof RE ions at the scale grain boundaries and metal-scaleinterface affects the scale. The first is the large RE ions retardthe outward diffusion of cations along scale grain boundaries.Thus, the growth of scale is controlled by inward diffusion ofoxygen ion along scale grain boundaries, which results in thereduction of oxidation rate. The second is RE additionsimproves scale adhesion by modifying the scale microstructureand preventing the harmful elements, especially sulfur, fromsegregating to the scale-alloy interface [9]. Also, the incorpo-ration of yttrium in the external scale improves the sinteringcharacteristics and plasticity of the oxides, which reduces thestress in the scale, and therefore promotes its adhesion [10].

To date, most studies regarding the effect of RE were carriedout on cast alloys. Wang [11–14] reported the effect ofnanocrystallization on the oxidation behavior of sputteredcoatings. Similar to the effects of RE, nanocrystallizationremarkably enhances selective oxidation and improves the scaleadhesion. Subsequently, several publications have reported theinfluence of RE addition on the oxidation behavior ofnanocrystalline coatings. Peng et al. [15] suggested that Y

Table 1Nominal chemical composition of K38 superalloy (wt.%)

C Cr Co W Mo Al Ti Fe Nb Ta Zr Ni

0.1–0.2 15.7–16.3 8–9 2.4–2.8 1.5–2 3.2–3.7 3.0–3.5 =b0.5 0.6–1.1 1.5–2.0 0.05–0.15 Bal

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addition retards the phase transformation from θ to α-aluminaformed on nanocrystalline CoCrAl coating. Liu et al. [16]reported 0.3 wt.% Y promotes the scale adhesion onnanocrystalline Ni3Al coating. However, more investigationsare needed to further assess the influence of RE addition on theoxidation behavior of sputtered nanocrystalline coatings.

In the present work, nanocrystalline coatings of K38 withvarious amounts of Y addition were deposited with themagnetron sputtering technique. The isothermal oxidationbehavior of the coatings was investigated at 1273 K to studythe effect of Y contents on the development and growth ofalumina scale on sputtered nanocrystalline coating.

2. Experimental procedures

The cast K38 superalloy was chosen as the substrate forthe coating deposition; the nominal chemical composition islisted in Table 1. The alloy was melted in a vacuum-induc-tion furnace and then cut into 15×10×3 mm pieces by spark-

Fig. 1. Cross-section views of (a) cast k38 alloy with 0.5 wt.% Y and (b) itssputtered coating.

erosion machining. The sputtering targets with dimension of380×126×10 mm were fabricated in the same manner withyttrium additions. Thirty-micron-thick coatings were depositedby magnetron sputtering and the detailed sputtering parameterswere the same as used in a previous study [17]. The coatedsamples are denoted as K38-0%Y, K38-0.05%Y, K38-0.1%Y,and K38-0.5%Y corresponding to the yttrium additions of 0,0.05, 0.1 and 0.5 wt.% (nominal content) respectively. The as-deposited coatings were nanocrystalline with grain size lessthan 100 nm [18].

After ultrasonically cleaning in acetone, the isothermaloxidation test was performed on the coated samples in athermo–gravimetric analysis (TGA) system at 1273 K instatic air for 50 h. The oxidized samples were characterizedusing X-ray diffraction analysis (XRD), scanning-electronmicroscopy (SEM) with energy-dispersive X-ray analysis(EDS), and electron-probe microanalysis (EPMA).

3. Results

3.1. Microstructure

The cast K38 superalloy consists mainly of a γ-Ni solidsolution, γ′-Ni3Al, and refractory elements enriched precip-itate (white phase in Fig. 1a). With the addition of 0.5 wt.%yttrium, an Y-rich phase, which chemical composition was Ni-5.9Al-4.3Y-3.9Cr-5.1Co in atomic percent (EDS), wasobviously precipitated in cast alloy (grey phase in Fig. 1a).Whereas, the sputtered coatings consisted of γ phase, no suchY-rich precipitates were observed even though the Y contentin the coating was high as 0.5 wt.% (Fig. 1b).

Fig. 2. Isothermal oxidation kinetic curves of sputtered coatings at 1237 K in air.

Table 2The fitting results of oxidation kinetics

Coating Transient stage Static stage

n Range of oxidation time (h) n Range of oxidation time (h)

K38-0%Y 5 0.7–11.8 2 11.9–50.0K38-0.05%Y 3 0.8–16.3 5 16.4–50.0K38-0.1%Y 4 0.7–14.7 7 14.8–50.0K38-0.5%Y 4 0.6–7.8 2 7.9–50.0

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3.2. Oxidation kinetics

Fig. 2 shows the oxidation kinetics curves of nanocrystal-line coatings with and without yttrium additions in static airat 1273 K for 50 h. It clearly shows that the weight gains ofK38-0.05%Y and K38-0.1%Y were the smallest after 50 hoxidation, while the weight gain of K38-0.5%Y was thegreatest among the coatings. The weight gain of K38-0%Ywas less than those of K38-0.05%Y and K38-0.1%Y duringthe early oxidation stage and then became greater after 36 hexposure.

The oxidation kinetics were analyzed by assuming thegrowth rate of oxide scales followed power law in the formof (ΔW/A)n =Kt where ΔW is the change in weight, A thesurface area, K the oxidation rate constant, and t the exposuretime. The exponent numbers were calculated and summarizedin Table 2. The results show that the oxidation kinetics of all thecoatings consisted of two stages: a transient stage and a staticstage. It can be seen that K38-0%Y and K38-0.5%Yapproximately followed the parabolic rate law (n=2) in thestatic stage, while the exponent n were 5 and 7 for K38-0.05%Yand K38-0.1%Y which values indicated a subparabolic timedependence of the scale growth rate.

3.3. Oxide scales

Surface morphologies of the oxide scales formed on thesputtered nanocrystalline coatings after 50 h exposure at 1273 Kare shown in Fig. 3. The XRD analysis indicated that thescales formed on K38-0%Y, K38-0.05%Y and K38-0.1%Yconsisted mainly of alumina, while TiO2 peaks were detected onthe coating K38-0.5%Y. It can be seen that a smooth andcontinuous scale formed on coating K38-0%Y (Fig. 3a and e).However, the Y additions significantly modified the scalesurface morphology. A bladelike alumina network formed onK38-0.05%Y (indicated by arrow in Fig. 3b and f ) and K38-0.1%Y (indicated by arrow in Fig. 3c and g); nodular Ti-richoxide formed on coating K38-0.5%Y (Fig. 3d) and no bladelikealumina was observed (Fig. 3h). The bladelike morphologygenerally implies the formation of the transient alumina.

Cross-sectional morphologies of the oxide scales are shownin Fig. 4. It can be seen that a thin external alumina scaleformed on K38-0%Y, K38-0.05%Y, and K38-0.1%Y after 50 hexposure at 1273K. Internal oxide, enriched in Al and Cr, wasfound beneath the external scale on K38-0%Y (Fig. 4a), whileno internal oxidation occurred on K38-0.05%Y (Fig. 4b) andK38-0.1%Y (Fig. 4c). In contrast, the external scale formed on

K38-0.5%Y (Fig. 3d) was significantly thicker; in which Ticontent was around 5 at.% (EDS). Internal attack was observed,particularly along the coating defects, and external Ti-rich oxideformed on the top of those defects, which implies that the scaleformed on K38-0.5%Y was less protective. The substrate-coating interface was penetrated by a needle-like precipitate(indicated by arrow in Fig. 4d) which has not been previouslyreported. The EDS and EPMA analysis indicated that theprecipitate was enriched in Ti. No Y-rich particles wereobserved in the scale and coating after 50 h exposure byelemental mapping analysis (Fig. 5).

4. Discussion

Previous works [12–14] studied the oxidation behavior ofsputtered K38G coating, which indicated that the externalprotective scale is α-alumina and its growth rate approximatelyfollows a cubic rate law. In the present work, the nanocrystallineK38-0%Y coating, with lower Al content comparing withK38G coating (4 wt.% Al), approximately followed a parabolicrate law in the static stage. The observation of internal oxideindicated that the scale formed on K38-0%Y coating was lessprotective compared to K38G although the phase structureanalysis identified the external scale consisted mainly ofalumina. This result implies that the Al content of K38 alloymight be close to the critical Al concentration for nanocrystal-line coating to form a protective alumina scale.

In general, addition of reactive elements, such as Y, Hf, Zr,can enhance the selective oxidation and reduce the protectivescale growth rate for Cr2O3-forming alloys [19,20]. In thepresent work, a protective alumina scale with low growth rateformed on K38-0.05%Yand K38-0.1%Y, and no internal attackoccurred. Based on this observation, it is suggested that theaddition of an appropriate amount of yttrium can enhance theselective oxidation of Al and decrease the growth rate ofprotective scale formed on the nanocrystalline coatings.However, a question remains regarding the influence of Y onthe oxide scale growth mechanism in the static stage. Thekinetics results indicated that the oxidation rates of K38-0.05%Y and K38-0.1%Y did not follow parabolic or cubic rate lawwhich is followed by coarse-grained Al2O3-forming alloys andcoatings. The reason is not known.

The increase in the Y contents to 0.5 wt.% in nanocrystallinecoating had a detrimental effect. In previous work [21], the Y-rich phase precipitated in cast K38 alloy acted as a second phasewith a fast oxidation rate, resulting in oxide penetration into thealloy along this phase. However, no Y-rich phase was observedin the as-deposited nanocrystalline coatings, and, within thedetection limits, EPMA analysis showed the distribution of Ywas quite uniform in the coating and the scale during the wholeexposure time. Experimental results related to diffusion in bulkalumina have shown an increase in cation transport with REdoping [22,23], rather than a decrease. By comparing theseresults with those from oxidation tests, Pint [5] proposed thedynamic-segregation theory and suggested RE-rich oxideparticles could nucleate on the grain boundaries in the static,bulk oxides, which could enhance cation grain-boundary

Fig. 3. Surface morphologies of sputtered coatings (a) K38-0%Y, (b) K38-0.05%Y, (c) K38-0.1%Y, and (d) K38-0.5%Yafter 50 h isothermal exposure at 1237K; (e),(f ), (g), and (h) are their close-up views to show the alumina scale morphologies.

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Fig. 4. Cross-sectional images of sputtered coatings after 50 h isothermal exposure at 1237K, (a) K38-0%Y, (b) K38-0.05%Y, (c) K38-0.1%Y, (d) K38-0.5%Y.

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diffusion. He suggested that RE ion segregation in RE-dopedscales decreased cation diffusion. Therefore, there are twopossibilities concerning the high oxidation rate of K38-0.5%Y.The first is Y-rich oxide might form in the scale and contributeto the high scale growth rate as a result of enhancing grain–boundary diffusion in the scale. The second is ultra-fine Y-richparticles would be precipitated in the coating during the hightemperature oxidation and this second phase led to a highoxidation rate, similar to the situation which occurred in the castalloy. In this case, it is suggested, the finely-dispersed particleswould each give rise to oxygen penetration to the metal, but theoverall effect would look as if the scale itself had grown fasterby solid state transport. The Ti-rich needle-like precipitates havenot previously been reported. The phase structure and formationmechanisms of that precipitate need further investigation.

A significant proportion of the literature concerning the effectof reactive elements on the oxidation behavior of alumina-forming alloys is related to the transition from transient alumina tostable alumina. Burtin et al.[6] suggested that larger ions, such asCa, La, and Th, enter the open, cubic lattice and there inhibit thetransformation to the α structure, while smaller ions, such as Mg,Ga, and Ti, accelerate the transformation. The model proposed byBurtin et al. appears to account for the present observation. Asmooth alumina scale formed on the coating without Y addition,while a blade-like alumina, the typical morphology of transientalumina, obviously formed on the coatings with 0.05 wt.% and0.1 wt.% Y. This observation suggests that the addition of Y

retarded the θ→α transformation, although no obvious differ-ence was obtained from XRD patterns after 50 h exposure.Generally, the phase transformation from θ to α results in 13%volume reduction, and leads to scale cracking. Based on thealumina scale morphology observation on Zr-doped NiAl,Doychak and co-workers [24–26] suggested the formation ofsurface ridges in the alumina scale is a result of rapid oxidation inthe transverse cracks through the scale. In the present work, theformation of a bladelike alumina network on K38-0.05%Y andK38-0.1%Yappeared to be in agreement with themodel proposedby Doychak et al. In contrast, no bladelike alumina was observedon the coating K38-0.5%Y, with the highest Y content in thiswork, which implies that the θ→α transformation was notinhibited. The EDS analysis revealed that the Ti content in theexternal scale was around 5 at.%. According to Burtin model, theincorporation of the smaller Ti ion in the alumina lattice couldaccelerate the phase transformation to α-alumina, thus nometastable alumina morphology was observed on K38-0.5%Y.

5. Conclusions

K38 nanocrystalline coatings with various amount yttriumadditions were deposited by magnetron sputtering. Isothermaloxidation behavior of the coatings were investigated in air at1273 K, the following conclusions can be drawn:

The addition of 0.05 and 0.1 wt.% yttrium had beneficialeffects on the oxidation resistance of the sputtered coating,

Fig. 5. X-ray maps of a cross section of K38-0.5%Y after 50 h isothermal exposure at 1273 K.

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such as enhancing the selective oxidation of Al, reducingalumina scale growth rate. Meanwhile, the addition of yttriumretarded the phase transformation from meta-stable to stablealumina.

The addition of 0.5 wt.% yttrium significantly increased theoxidation rate of the sputtered coating, Ti-oxide formed on thesurface. The incorporation of Ti in the alumina latticeaccelerated the phase transformation to α-alumina.

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Acknowledgment

Financial support by the NSFC (Project No. 50671115) isgratefully acknowledged.

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