Surface & Coatings Technology 248 (2014) 74–80

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Oxidation behaviour of new electrolytically synthesized ceria modifiedplatinum γ/γ′ coatings

Baptiste Bouchaud ⁎, Fernando PedrazaLaboratoire des Sciences de l'Ingénieur pour l'Environnement LaSIE UMR CNRS 7356, Université de La Rochelle, Pôle Sciences et Technologie, Avenue Michel Crépeau,17042 La Rochelle cedex 01, France

⁎ Corresponding author. Tel.: +33 5 46 45 86 00; fax: +E-mail addresses: baptiste.bouchaud01@univ-lr.fr (B. B

(F. Pedraza).

http://dx.doi.org/10.1016/j.surfcoat.2014.03.0340257-8972/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 February 2013Accepted in revised form 18 March 2014Available online 27 March 2014

Keywords:Aluminide coatingsCeria coatingsPlatinumNi-based superalloyHigh temperature oxidationRaman spectroscopy

This study investigates the influence of an electrodeposited ceria coating on the high temperature performance ofa raw and platinum-diffused Ni-based superalloy. Both the CeO2 and the Pt/CeO2 coatings drastically improvedthe oxidation resistance of the substrate under isothermal and cyclic conditions at 1100 °C. In particular, whereasPt-free CeO2-coated samples grew a duplex oxide scale composed of a top NiAl2O4 layer and an inner Al2O3,Pt/CeO2 coatings exhibited an exclusive α-Al2O3 scale that tended to rumple. None of these systems developeda secondary reaction zone (SRZ) because of the absence of any chemical gradient of aluminum between thesubstrate and the coating/oxide scale system.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Nickel aluminide coatings are widely used as oxidation resistantcoatings to protect hot section parts from turbine engines made ofNi-based superalloys [1–6]. Many studies have been dedicated to theeffect of electrodeposited Pt, either to improve the high temperatureoxidation resistance of such coatings or to be used as bondcoats forthermal barrier coatings [7–12]. Other alternatives focused on the addi-tion of reactive elements [13–18], whose precise content and uniformdistribution are difficult to control in spite these features are criticalto improve the adherence of the protective oxide scales [19,20]. Yet,upon exposure at high temperatures, the great chemical potentialgradient between the nickel aluminide coating and the substrate entailsinterdiffusion of the elements that results in the β to γ′ phase transfor-mation [6,21–23] and in the appearance of a secondary reaction zone(SRZ), the latter being detrimental from a mechanical point of view[24–26]. More thermodynamically stable platinum modified γ/γ′[27–29] or EQ [30,31] coatings offering compositions close to those ofthe underlying substrates have been therefore studied, based on amuch lower chemical gradient between the substrate and the coating.However, despite these coatings provide adequate oxidation resistanceunder isothermal and cyclic exposure, high Pt contents and additionsof Hf (difficult to incorporate in precise amounts) during a post

33 5 46 45 82 41.ouchaud), fpedraza@univ-lr.fr

Al-enrichment are required to ensure the formation of an exclusiveα-Al2O3 scale [27,32–35].

A new concept of electrodeposited CeO2 overlay coatings onto a Ni-based superalloy was therefore studied [36]. It appeared that the lowoxygen partial pressure at the coating/substrate interface allowed theexclusive growth of theα-Al2O3 scale upon annealing the electrodepos-ited coating. In addition, such annealed coatings provided oxidationprotection to the substrate well over 1000 h at 1100 °C in air with noSRZ developing within the substrate [37,38]. However, the potentialbeneficial effect of Pt was not investigated. Therefore, the presentwork focuses on the effect of the addition of a Pt layer between theNi-based superalloy substrate and the electrodeposited ceria overlaycoating. The subsequent isothermal and cyclic oxidation behaviour at1100 °C is compared to Pt-free ceria-coated samples.

2. Experimental

A René N5 Ni-based superalloy (Ni–7Cr–8Co–2Mo–5W–7Ta–8Al–2Re wt.%) was used in the present study. Coupon samples of 12 mmdiameter and of 2 mm thick were grit blasted with 220 mesh aluminaparticles and cleaned with ethanol under ultrasonic agitation beforeapplying the three different coatings to be investigated.

The aluminide coating was produced by a low-activity out-of-packaluminizing at 1080 °C for 6 h from a 70Cr–30Al donor/NH4F activatorunder a flowing argon atmosphere. The resulting coating was 50 μmthick on average and exhibited a typical morphology of an outwardlygrown aluminide with an additive layer (content of 50 at.% Al at the

75B. Bouchaud, F. Pedraza / Surface & Coatings Technology 248 (2014) 74–80

outermost surface) on top of an interdiffusion zone [23,37,39] and isconsidered as a baseline coating for comparative purposes.

The platinum-diffused coating was processed from a 4 μm thickelectroplated Pt layer (Q salt) [8] followed by a diffusion treatment at1080 °C for 1 h under vacuum to enrich the superalloy surface with Pt[40].

The ceria coating was generated by cathodic electrodeposition in athree-electrode set-up for 20 min from 0.1 M concentrated cerium-nitrate fresh solution [41] and subsequent thermal treatment (TT)under vacuum at 1050 °C for 1 h to enhance adherence and providepre-oxidation of the substrate [36,42–44] onto both raw and platinizedRené N5.

The isothermal tests were performed in a tubular PYROX furnace at1100 °C up to 1000 h. The samples were heated up to the oxidizingtemperature at 10 °C/min under flowing argon gas before introducingthe synthetic air (Messer). Cooling to room temperature was then car-ried out at 50 °C/min to keep the microstructure from the oxidizingtemperature. Cyclic oxidation tests were accomplished in a horizontalautomatic rig equippedwith a CARBOLITE tubular furnace for 1 h cyclesconsisting of 45min at the oxidizing temperature (1100 °C) followed bycooling at room temperature for 15 min (T b 80 °C). The samples weresystematically weighed to assess the mass variation associated withthe growth and degradation mechanisms of the coating systems usinga 10−5 g accurate balance.

The coated samples were characterized in the as-coated + heattreated and oxidized conditions by X-ray diffraction in the Bragg–Brentano mode using a Bruker AXS D8 Advance (λCu). Top view andcross-section analyses were carried out by scanning electron microscopycoupled with EDS analyses (SEM/EDS, FEI Quanta 200F/EDAX and JEOL5410LV/Rontec). Complementary micro-Raman spectrometry analyseswere also performed at room temperature with a Jobin Yvon HoribatLabRam HR Raman spectrometer (He–Ne laser source— λ =632.8 nm) to substantiate oxide phases at local scale.

Fig. 1. (a) Cross-section morphology and (b) EDS profile of Pt-electroplated René N5 afterdiffusion treatment at 1080 °C for 1 h.

3. Results and discussion

3.1. Platinum-diffused coatings

Fig. 1(a) shows the cross-section morphology of the Pt-diffusedcoating after diffusion annealing at 1080 °C for 1 h. The overall thicknesswas about 25 μm and can be divided into two major zones: (i) an outerlayer, which developed above the original surface of the substrate(marked by black arrows) by outward diffusion, and (ii) an inner layerresulting from interdiffusion with the substrate. The original interfacewas evidenced by the presence of pores (Kirkendall) and contrastedparticles, onto which EDS analyses revealed Al enrichment due to theincorporation of alumina coming from the grit particles as typically ob-served in other studies [8,9,45].

The concentration profile (Fig. 1b) indicates that the Pt contentof about 20 at.% near the surface progressively slowed down whilediffusing towards the internal interface. This concentration was lowerthan those observed by Pedraza et al. [9,40] and Mollard et al. [46]onto CMSX-4 and PWA1483 or CM247 Ni-based superalloys, respec-tively. These differences were ascribed to both the different structureand composition of the underlying superalloy and the initial thinnerPt layer in the present case. According to XRD and SEM/EDS, thePt-diffused coatings displayed similar microstructures and phases tothose described previously [9,40,47]. Indeed, higher Pt contentsappeared in the brightest areas related to the Pt3Al phase. In agreementwith Benoist et al. [8] and Pedraza et al. [9,40], Cr and Co were mostlyassociated with Ni in the darkest areas and diffused to the top surfaceto stabilize the γ-Ni phase. Ta and Re enrichment in the diffused layerwas related to their greater diffusion coefficients in Ni compared tothose of Pt and W observed by Karunaratne and Reed [48]. Therefore,the diffused layer/substrate interface appeared richer in W.

3.2. Morphology of the ceria-coated systems after thermal treatment

Both ceria coatings on the raw and Pt-diffused René N5 exhibited a“dry-mud” morphology after annealing at 1050 °C for 1 h in flowingargon. Such morphology has been reported to result from the internalstresses generated upon the fast electrodeposition [41] and the subse-quent shrinkage of the wet electrodeposited coatings upon annealing[36,41–44]. The XRDpatterns and the Raman spectra confirmed the for-mation of CeO2 (with fluorite structure) as the only cerium-containingspecies.

In the cross sections, the CeO2 overlay was composed of tiny grains(Fig. 2a) underneath which mostly α-Al2O3 with traces of θ-Al2O3

(Fig. 3a) grew by a mixed diffusion mechanism onto the substrate[42], the latter leading to the loss of the γ′ precipitates of the substratedue to the growth of the oxide scale from this phase (the black linein Fig. 2a underlines the frontier between the top γ-Ni matrix and theγ/γ′ matrix beneath).

In contrast, the Pt-diffused substrate displayed a CeO2 coatingwith aneedle-like morphology (Fig. 2b). These needles were strongly orientedand seemed to grow perpendicularly to the electrical field. Previousstudies by XRD on aluminized N5 samples also evidenced the develop-ment of oriented deposits in the as-deposited conditions and wereascribed to the growth of more crystalline layers of CeO2, H2O [41].Compared to the Pt-free ceria coatings, the micro-Raman analysesrevealed a greater contribution of θ-Al2O3 at the coating/Pt-layer inter-face (Fig. 3b), thus suggesting that an outward Al diffusion mechanismpredominated. This has been ascribed to the presence of the Pt3Alphase in the matrix of the Pt-diffused coatings providing a greater Al

Fig. 2. Cross-section morphology of the ceria-coating after thermal treatment at 1050 °Cfor 1 h under flowing argon grown onto (a) raw René N5 and (b) Pt-diffused coating.

Fig. 3. Raman spot analyses performed at different locations (1 to 9) along the interfacialoxide grown in the (a) CeO2-coated and (b) Pt/CeO2-coated René N5 substrates after 1 hthermal treatment at 1050 °C.

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reservoir [7,49]. Jiang et al. [50] also showed that Pt has a strong prefer-ence for occupying Ni sites in γ′ (L12), thereby promoting an effectiveAl enrichment and promoting the formation of Al2O3. Therefore, thegreater chemical potential of Al in the Pt-diffused coating matrix mayaccount for the quicker growth of a more continuous and thickeroxide scale after 1 h treatment under inert atmosphere at 1050 °C,and is in agreement with the hypothesis of Krishna et al. [51] in whichPt increases the Al activity.

3.3. Oxidation mechanisms and morphology of the oxide scales

After 1000 h of isothermal exposure at 1100 °C, the aluminidecoating showed the development of an even, adherent and protectivealumina scale. However, a few cracks and spalled off areas wereobserved due to the accumulation of growth stresses in the α-Al2O3

[52,53] and where a critical thickness was reached [54]. At the metal/oxide interface, pores resulting from the coalescence of cation vacanciesonto the grains [55] were observed and are indicative of prior Aloutward diffusion [56–58]. Underneath, the β-NiAl columnar grainstructure of the additive layer was underlined by brighter grain bound-aries of the γ′-Ni3Al phase (noted “GB” on Fig. 4a) driven by enhancedoutward diffusion of Ni during the first oxidation stages. In addition,the β to γ′ phase transformation occurred especially at the interdiffu-sion zone by major diffusion of Al into the substrate [6,23]. This micro-structural change is also accompanied by volume contractions [21,59]often put forward to explain surface rumpling. However, because ofthe limited extent of γ′ phase formation within the additive layer, onecan presume that the wavy oxide scale results from creep relaxationupon cooling [23]. At the coating/substrate interface, the formation

and extension of the secondary reaction zone (100 μm within thesubstrate) beyond the pre-existing precipitate zone (interdiffusionzone) occur due to interaction between the inwardly diffusing Aland the refractory elements from the substrate (e.g. W, Ta and Re)[24,25,60,61].

In the case of CeO2 and Pt/CeO2 coatings, both systems developedprotective and adherent oxide scales. For comparison purposes, theevolution of the nature and of the averaged oxide scales thicknesseswere gathered for both coating systems after oxidation in Fig. 5.Where-as very seldom spallation occurred after 500 h of isothermal oxidation at1100 °C on ceria coatings, no detachment was observed onto the Pt/CeO2 coating even after 1000 h of oxidation at 1100 °C. In the former,spallation mostly initiated at the ceria coating/oxide interface by adecohesion mechanism related to the extensive growth of NiAl2O4 spi-nel nodules (1.8±0.8 μmafter 500h) as shown in cyclic conditions [38].Underneath, at highermagnification, a columnar scale that grewprimar-ily by inward oxygen diffusion was observed (Fig. 4b), similar to thosereported for adherent alumina scales observed on RE-doped substratematerials [62–65].

By extending the oxidation time until 1000 h (Fig. 4b), a duplex scalewith a top NiAl2O4 spinel (1.2 ± 0.5 μm) and a bottom dense α-Al2O3

subscale (3.5 ± 0.5 μm) similar to those grown from Pt-poor γ/γ′ coat-ings [33] was revealed. In contrast, the growth of an exclusive aluminascale was observed onto Pt/CeO2 substrates (Fig. 4c) where an inwarddiffusion mechanism of oxygen seemed to predominate. Its thickness(4.5 ± 0.5 μm) was rather similar to that on the Pt-free ceria coatingand for aluminide coatings. Both CeO2 and Pt/CeO2 systems also showedthe formation of nano-grains between the dark alumina and the bright

Fig. 4. Cross-section morphology of the isothermally oxidized coatings at 1100 °C for1000 h: (a) aluminide coating, (b) Pt-free CeO2 coating and (c) Pt diffused CeO2 coating.A detail of the CeAlO3 nano-grains [36,37] developed between the top ceria layer andthe alumina scale is displayed in the inset.

Fig. 5. Evolution of the nature and thickness of the oxide scales grown with time on CeO2

and Pt/CeO2 coatings under (a) isothermal and (b) cyclic regimes at 1100 °C in air.

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grains at the ceria-containing layers (inset in Fig. 4c) as recently reported[36]. Such nano-grains have been ascribed to CeAlO3 oxides and accountfor the enhanced adherence [37,38] compared to the spallation of thescale observed in conventional aluminide coatings.

In addition, none of these systems developed a SRZ (Figs. 4b and c)because of the absence of any chemical gradient of aluminum betweenthe substrate and the coating/oxide scale system (Fig. 6a). Nevertheless,the corresponding composition profiles across the Pt/CeO2 coatingsshowed the flattening of the Pt concentration (Fig. 6b) as a result ofinward diffusion with increasing oxidation exposure time (about

120 μm after 1000 h). However, the presence of the Pt3Al phase(Fig. 4c) underneath the near surface provides suitable conditions forselective oxidation of aluminium and the growth of a single aluminascale, as often proposed for NiPtAl coatings [12,66–68], without signifi-cantmicrostructural changes observed in the subscale of the underlyingCeO2-modified Pt γ/γ′ coating.

The cyclic oxidation resistance at 1100 °C in air of both CeO2 andPt/CeO2 coated samples (Fig. 7) was drastically improved with respectto the uncoated superalloy and was rather similar to that provided byconventional vapour phase aluminides on the same substrate (bluecurve) [36]. However, the ceria-coated curve (red curve) [38] appearedmore irregular compared to the very even curves of both the Pt/CeO2

(green curve) and aluminide coatings. This was mostly ascribed to thespallation of the spinel nodules grown in the Pt-free ceria coatings,whose duplex scale structure was similar to the one grown under iso-thermal conditions (Fig. 8a). Similarly, an exclusive α-Al2O3 grew inthe Pt/CeO2 coatings (Fig. 8b). However the alumina subscale appearedto be thicker in the Pt-diffused than in the Pt-free systems [37,38],which may explain the overall greater mass gain of the former.

As with γ/γ′ (or EQ) coatings [27–31], no SRZ developed in eitherthe CeO2 or Pt/CeO2-coated superalloys, whether under isothermal orcyclic conditions, and this is believed to be a direct consequence of theabsence of an aluminium concentration gradient in these systems. Incontrast, such SRZ formation and growth of nodules are suggested asthemain factors of degradation of conventional aluminides under cyclicconditions [6,23]. In the coating systems studied here, a relatively wavyscale developed (Figs. 8a and b), although this phenomenon was morepronounced in the presence of Pt. Indeed, although Pt is well known

Fig. 6. Evolution of the concentration profile (at. %) of (a) Al and (b) Pt with oxidation timeof Pt/CeO2 coatings at 1100 °C.

Fig. 8. SEM cross-sections of the (a) CeO2 and (b) Pt/CeO2 coatings after cyclic oxidation at1100 °C up to 1000 × 1 h cycles.

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to promote the formation of adherent α-Al2O3, oxidation studies ofPt-modified coatings also revealed that scales are more prone torumpling than Pt-free systems [22,59,69-72], especially under cyclicconditions [21,22,73,74].

Based on the Pt–Al content map proposed by Littner et al. [7]describing the cyclic oxidation performance of different NiPtAl coatings,the oxidation resistance of the CeO2 and Pt/CeO2 coatings of this studywas added to the map (Fig. 9) by just considering the N1 parameter

Fig. 7. Evolution of the mass variation per surface unit during cyclic oxidation of thedifferent coating systems at 1100 °C for 1 h-cycles.

(defined as the number of cycles after which the mass variation slopebecomes negative). Additional data from the development studies ofGleeson et al. [75], Mu et al. [76], Izumi et al. [27,77] and Haynes et al.[29,34] for β, β/γ′ and γ/γ′ alloys and coatings literature have beenincluded to also consider the thickness and the nature of the oxide scale.

Littner et al. [7] demonstrated that a critical Pt content of 4 at.% wasrequired to obtain extended coating lifetimes for 1-h cycled samples at

Fig. 9. Composition map summarizing the cyclic oxidation behaviour of Pt-free and newPt/CeO2 coatings and interrelationships between the average Pt and Al composition andthe coating lifetime. (Note that stars represent material growing an exclusive α-Al2O3 scale,whereas other symbols represent duplex or multi-layered oxide scales).

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1150 °C. This has been since confirmed by several authors when inves-tigating the oxidation resistance of various compositions of NiPtxAly(Hf)alloys and coatings [27,76,77]. Indeed, low performance of the coatingswas observed when the Pt content was under this 4 at.% Pt threshold(represented by a grey line in Fig. 9). Above this critical Pt content, themap clearly highlights two trends regarding the nature of the oxidescale (thick thermally grown oxide “TGO” and thin oxides), hencedemonstrating that the Al and Pt contents must be balanced in orderto reduce oxidation of the γ/γ′ and NiPtAl systems. In essence, Pt signif-icantly decreases the critical Al content required for exclusive Al2O3

formation, thereby improving the oxidation resistance and decreasingthe growth kinetics at high temperature.

By considering this, Fig. 10 summarizes the thicknesses and compo-sition of the scales of the coatings studied here with the mass gainsafter a given number of cycles of oxidation at 1100 °C. The single CeO2

coatings led to the growth of a duplex oxide scale composed of anouter NiAl2O4 and an inner Al2O3 layer comparable to those grownontoNi–15Al–2Pt, Ni–12Al–10Pt andNi–10Al–25Pt systems investigatedby Gleeson et al. [75]. However, for the Pt/CeO2 coatings an exclusivealumina scale grew like in Ni–14Al–10Pt and Ni–12Al–18Pt [75] inspite of a lower Al content (8 at.%). Therefore, it appears that theelectrosynthesized coatings behaved rather well when considering themass gain variation as they gave rather similar values than protectivesystems growing exclusive alumina scales [29,34,38,43,72]. No furtherstudies have yet been carried out at this stage on the optimal lower Ptcontent to reach a beneficial effect of Pt/CeO2 combinationwith thinneroxide scales on the oxidation resistance of a Ni-based superalloy.

4. Conclusions

The potential beneficial effect of Pt in the presence of an electro-synthesized ceria coating onto a René N5 Ni-based superalloy wasdemonstrated. During exposure at high temperature, the oxidationbehaviour of the substrate upon both cyclic and isothermal conditionswas remarkably improved. In particular, the presence of Pt promotedthe formation of an exclusive α-Al2O3 scale and of nano-grainedCeAlO3 layer at the ceria/alumina interface. In contrast, Pt-free coatingsdeveloped a duplex oxide scale composed of a top NiAl2O4 layer and aninner Al2O3, the latter undergoing very seldom spallation and limited tothe spinel interlayer breaking apart. However, the alumina scale grownfrom the substrate was prone to rumpling, in particular in the presenceof Pt. The oxide scales remained adherent in both cases under theconditions investigated. After 1000 h of oxidation at 1100 °C, none ofthese Pt-free or Pt-containing CeO2 coating systems developed anySRZ, because of the absence of an Al gradient with the underlying sub-strate and offer a protection similar to conventional aluminide coatings

Fig. 10. Literature comparison of the mass gain after oxidation at 1100 °C for 1000 ×1 h cycles of different alumina-forming coating systems [29,34,38,43,72].

and new developmental γ/γ′ coatings, and so are expected to providegood performance with limited spallation phenomena.

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