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Ceramics International 41 (2015) 15272–15277www.elsevier.com/locate/ceramint

Short communication

Microstructural, mechanical properties and corrosion behavior of plasmasprayed NiCrAlY/nano-YSZ duplex coating on Mg–1.2Ca–3Zn alloy

H.R. Bakhsheshi-Rada,n, E. Hamzaha, A.F. Ismailb, M. Daroonparvara, M. Kasiri-Asgaranic,S. Jabbarzarec, M. Medrajd,e

aDepartment of Materials, Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru,Johor, Malaysia

bAdvanced Membrane Technology Research Center (AMTEC), Universiti Teknologi Malaysia, Skudai, 81310 Johor Bahru, Johor, MalaysiacAdvanced Materials Research Center, Faculty of Materials Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Isfahan, Iran

dDepartment of Mechanical and Industrial Engineering, Concordia University, 1455 De Maisonneuve Blvd.West, Montreal, QC, Canada H3G 1M8eDepartment of Mechanical and Materials Engineering, Masdar Institute, Masdar City, Abu Dhabi, P.O. Box 54224, United Arab Emirates

Received 10 June 2015; received in revised form 2 August 2015; accepted 4 August 2015Available online 14 August 2015

Abstract

In this study, microstructural evolution, mechanical properties and corrosion behavior of plasma sprayed NiCrAlY/nano-yttria stabilizedzirconia (nano-YSZ) dual-layered coating on Mg–1.2Ca–3Zn alloy were investigated. NiCrAlY underlayer is composed of large amount ofporosities and micro-cracks with thickness around 80–90 μm. However, nano-YSZ overlayer shows bimodal microstructure consisting ofcolumnar grains and some partially molten parts of the nanostructured powders with thickness around 270–300 μm. The microhardness of dual-layered NiCrAlY/nano-YSZ coating is significantly higher than that of single-layered NiCrAlY. Despite that, the bonding strength of dual-layeredcoating is slightly higher than single-layered plasma sprayed coating. Results also showed that both single-layer NiCrAlY and dual-layerNiCrAlY/nano-YSZ coatings decreased the corrosion current density of Mg alloy from 217.1 μA/cm2 to 114.5 and 82.4 mA/cm2, respectively.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Magnesium alloy; Plasma spray; Yttria stabilized zirconia; Corrosion behavior

1. Introduction

Magnesium and its alloys are suitable replacement for ironand aluminum alloys for structural applications due to theirexcellent physical and mechanical properties such as lowdensity (65% of aluminum and 25% of iron) and high specificstrength [1–3]. Mg-alloys also received great attention fromaerospace and automotive industries due to high weightreduction which leads to noticeable reduction in fuel con-sumption and CO2 emission [3]. However, poor corrosionresistance and high reactivity of Mg alloy limits their wideapplications [4–6]. Apart from alloying, surface modificationssuch as plasma electrolytic oxidation (PEO), atmospheric

/10.1016/j.ceramint.2015.08.02515 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ60 147382258.sses: rezabakhsheshi@gmail.com, bhamidreza2@live.utm.my (H.R

plasma spraying (APS), ion implantation, Ca–P coatings andpolymer coatings have been widely applied in order to reducecorrosion rate of Mg alloys [7]. Among them, APS is aneffective and environment-friendly method that rapidly depos-its high-quality ceramic coating [8]. Fan et al. [9,10] reportedthat thermal barrier coating (TBC) on Mg alloy with 8YSZ andAl can decrease the corrosion rate of the alloy. However, thereare very few reports on the corrosion behavior of NiCrAlY andNiCrAlY/nano-YSZ coated Mg−Ca−Zn in NaCl solution.Thus, NiCrAlY and NiCrAlY/nano-YSZ coatings have beendeposited on Mg–1.2Ca–3Zn alloy by APS and tested in thispaper. In addition, the relationship between microstructure andcorrosion behavior of these coatings has been evaluated.

. Bakhsheshi-Rad).

Table 1Air plasma spraying parameters.

Parameters NiCrAlY Nano-YSZ

Current (A) 400 400Voltage (V) 50 50Primary gas, Ar (l/min) 60 60Secondary gas, H2 (l/min) 8 8Powder feed rate (g/min) 15 20Spray distance (cm) 12 10

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2. Experimental procedures

Mg–1.2Ca–3Zn alloy samples (15� 15� 6 mm) are used assubstrates in this work. It should be mentioned that singlenano-particles are not able to be sprayed due to their low massand inertia. To address this issue, reconstitution of thenanoparticles into micrometer sized granules is vital [9]. Thus,in this study, the agglomerated nanostructured YSZ (ZrO2–

8 wt%Y2O3; Nanox Powder S4007, Inframat, USA) powderand a commercial NiCrAlY (Ni22Cr10Al1Y; AMDRY 962)powder with size in range of 44–65 mm were used to depositthe overlayer and underlayer, respectively. An atmosphericplasma spray (METCO, type 3MB) was used for plasmaspraying of the NiCrAlY underlayer and nanostructured YSZoverlayer. The spraying parameters are summarized in Table 1.X-ray diffractometry (Siemens-D500) was used for identifyingthe phases present in the samples using Cu Kα radiationgenerated at 40 kV and 35 mA. Microstructural observationwas performed using a scanning electron microscope (SEM;JEOL JSM-6380LA) and transmission electron microscopy(TEM; Hitachi). A three-electrode cell was used for potentio-dynamic polarization tests (PARSTAT 2263) in 3.5 wt% NaClsolution according to [11]. Using a Vickers hardness tester(Shimadzu) with a load of 500 g and a holding time of 15 s, anaverage of five microhardness readings of the plasma spayedcoatings was obtained from the polished cross-section. Thetotal porosity of single-layer NiCrAlY and dual-layer YSZ/NiCrAlY coatings was evaluated by image analysis from fivebackscattered micrographs at magnifications of 1000� . Thebonding strength tests were carried out according to [11].

3. Results and discussion

Fig. 1a shows that the ternary Mg–1.2Ca–3Zn alloy consistsof α-Mg and Ca2Mg6Zn3 or MI (Ca3MgxZn15�x 4.6rxr12)according to Islam et al. [12] which is consistent with the Mg–Ca–Zn phase diagram [13]. The presence of secondary phasescan significantly affect the corrosion behavior of the Mg alloydue to the formation of micro-galvanic cells between thematrix and secondary phases [14]. The EDS analysis showedthat triple junction between the grains boundaries wereenriched with zinc and calcium thus, indicating the formationof Ca2Mg6Zn3 (Fig. 1d). The surface of the single-layeredNiCrAlY coating is uneven and contains a certain level ofroughness. It can also be observed that the underlayer contains

a large amount of pores, voids and micro cracks due to theinduced residual stresses from the deposition process (Fig. 1b)[15]. However, dual-layered NiCrAlY/nano-YSZ coatingshowed fewer amounts of pores and micro cracks. Thecorresponding EDS analysis result can be also observed(Fig. 1e). The semi-molten particles (SM) and molten particles(M) are also shown in Fig. 1c and e. It is reported that thesemi-molten particles contain porous structure, while moltenparts bonded with each other forms a dense structure [16].Nanostructured YSZ is also observed in Fig. 1c which iscomposed of nanosized particles that can be retained from thenon-molten part of the powder [8]. This structure containslarge amount of nano-pores with a wide range of sizes whichhas significant effect on the corrosion behavior of the plasmaspray coated alloy. Di Girolamo et al. [17] demonstrated thatpores with different sizes exist in nano-YSZ. Fine poresoriginated from gas entrapped in the molten droplets, whilethe coarse pores are produced by filling structural defects andpull-out effects during grinding and polishing. In this view, theflight path and the thermal history of the agglomeratedparticles in the plasma jet affect the coating microstructure.In this regard, the melting process is associated to bothtemperature distribution in the plasma jet and to the heattransfer to the porous agglomerates [18]. Thus, with aoptimization process the level of nanozones embedded in thecoating microstructure which has significant effect on thecorrosion and mechanical properties of the coated samples canbe monitored [19]. The XRD results of the ternary Mg–1.2Ca–3Zn alloy show that the uncoated alloy consists of α-Mg andCa2Mg6Zn3 which is consistent with the SEM/EDS results.The NiCrAlY coating contain γ phase (Ni, Cr-rich), γ0 phase(Ni3AL), AlCr3 and some traces of β phase (AlNi) peaks.However, nanostructured YSZ include the tetragonal zirconia(t) only which can be attributed to rapid solidification duringthe process of APS (Fig. 1f).Fig. 2a shows cross-sectional SEM image of the NiCrAlY

coated Mg alloy sample, indicating large amount of voids,globular porosities and micro-cracks with different sizes. How-ever, YSZ shows bimodal structure consisting of lamellarstructure with columnar grains, which are surrounded withpartially molten parts of the nanostructured powders and someequiaxed grains (micro- and nano-sized). In this regard, DiGirolamo et al. [20] showed the lamellar structure consisted oflamellae almost parallel to interface of the coating substrate,separated by splat boundaries and embedded in a network ofcracks and voids. Formation of columnar grains can be due tothe solidification of the melted part of the powder [21]. As canbe seen that the dual-layer coating includes less linked pores,voids and micro-cracks because of the compactness of thenanostructured YSZ compared to single-layer coating. Thisleads to less infiltration of corrosive media to the Mg substrate(Fig. 2b). Single-layer NiCrAlY coating exhibited higherporosity (16–18%) compared to the dual-layer YSZ/NiCrAlYcoatings (11–13%) hence, dual-layer coating is denser than thatof single-layer coating. The molten and semi-molten regions, asone of the source of porosity in APS nanocoating, could becontrolled by adjusting the feedstock characteristics [19,22]. In

Fig. 1. Scanning electron micrographs of the surface of (a) uncoated Mg–1.2Ca–3Zn alloy, (b) NiCrAlY coating, and (c) NiCrAlY/nano-YSZ coated Mg–Ca–Zn;and EDS analysis of (d) Areas A and B and (e) Areas C and D and (f) X-ray diffraction patterns of uncoated, NiCrAlY coated and NiCrAlY/nano-YSZ coated Mg–1.2Ca–3Zn alloy.

Fig. 2. Cross sectional SEM micrographs of (a) NiCrAlY, and (b) NiCrAlY/nano-YSZ coated Mg–1.2Ca–3Zn alloy. Elemental mapping of (c) NiCrAlY, and (d)NiCrAlY/nano-YSZ coated Mg–1.2Ca–3Zn alloy.

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addition, the porosity in the single-layer coated sample is muchlarger than that in the dual-layer coating. The presence of thiscoarse porosity in the single-layer coating caused more

penetration of corrosive species to the substrate. Fig. 2c and dshows EDS elemental maps of the cross-sectional NiCrAlY andNiCrAlY/nano-YSZ coating samples, indicating formation of

Fig. 3. Transmission electron micrograph of NiCrAlY/nano-YSZ coated Mg–1.2Ca–3Zn alloy (a) low magnification and (b) high magnification of equiaxed grains.

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thick layers that homogeneously cover the surface of the Mgalloy. The dual-layer coating indicated two layer structures, theYSZ layer with thickness of 270–300 μm as overlayer andNiCrAlY with a thickness of around 80–90 μm as an under-layer. It seems that there is strong adhesion between thedeposited coating and the underlying substrate. Fig. 3 showsTEM micrographs of NiCrAlY/nano-YSZ coating indicating theequiaxed grains with different sizes in the range of 30–80 nm.Similar result was also found by Wang et al. [23]. Theorientation and size of these crystals strongly depend on thethermal condition during spreading of droplet [24].

The electrochemical polarization curves of plasma spraycoated and uncoated showed that the corrosion potentials(Ecorr) of the uncoated Mg–1.2Ca–3Zn, NiCrAlY andNiCrAlY/nano-YSZ coated samples were �1638.7 mVSCE,–917.3 mVSCE and �829.6 mVSCE, respectively (Fig. 4a). Themore negative corrosion potential of the uncoated Mg–1.2Ca–3Zn compared with the coated alloy is due to the formation ofa microgalvanic cell between α-Mg and the ternary secondaryphase. However, single and dual-layered coating samplespresented more positive Ecorr than uncoated sample. Accordingto the mixed standard electrode potential theory [25,26], it wasfound that the air plasma spray coated samples were noblerthan the uncoated Mg alloy. The corrosion current densities(icorr) of the uncoated, single-layer and dual-layer coatedsamples were 217.1, 114.5 and 82.4 mA/cm2, respectively.Both coated samples exhibit a lower icorr than the uncoatedsample indicating lower corrosion rates of plasma sprayedcoating samples. This can be attributed to the formation of athick coating that leads to delaying the contact between thecorrosive species and Mg substrate. Fig. 4b shows the Nyquistplots of uncoated and coated Mg samples thus, indicating asingle capacitive loop at all high frequencies. The chargetransfer resistance (Rt) of the coated samples are almost thesame; however, the NiCrAlY/nano-YSZ coated sampleshowed higher polarization resistance (2502.2 Ω cm2) thanNiCrAlY coated sample (2199.2 Ω cm2). The uncoated sampleexhibited the lowest polarization resistance of 1437.1 Ω cm2.This can be due to the protective nature of chromium andnickel, as well as the homogeneous coating composition [25].

More so, the dual-layered coated sample demonstrated largerloop diameter compared to the single-layered coated sample.This can be attributed to the formation of more compactcoating and presence of lesser amount of pores, voids andmicro cracks [27,28]. Microhardness value of uncoated alloywas 65.2 Hv and this value significantly increased to 210.4 and760.8 Hv after NiCrAlY and NiCrAlY/nano-YSZ plasmaspray coating respectively (Fig. 4c). This indicates that thedual-layered coating is approximately 12 times harder than thatof the uncoated alloy. The higher hardness values of nano-YSZcoating compared to the NiCrAlY coating is due to thepresence of semi-molten nanostructured particles that areembedded in the YSZ structure, which act as crack arrestersthereby, increasing coating toughness [18,19]. In NiCrAlYcoating, a crack propagates via the coating's weakest link,which is the well-defined layered structure such as the splatboundaries. Fig. 4d shows that the bonding strength of thedual-layer NiCrAlY/nano-YSZ coating is about 14.9 MPawhich is slightly higher than that of single-layer NiCrAlYcoated Mg alloy (12.8 MPa). Higher bonding strength of dual-layer coating can be due to better splat-to-splat contactobtained when using a nanostructured powder, which wouldalso hinder crack propagation. This is also related to the crackarresting effect by the dense nanozones which enhances thebonding strength [18,19].

4. Conclusions

In this study, plasma sprayed NiCrAlY and NiCrAlY/nano-YSZ duplex coatings were deposited on Mg–1.2Ca–3Zn alloywith the aim of reducing corrosion rate. The nanostructured YSZoverlayer, compared to the NiCrAlY underlayer contains loweramounts of porosities and micro-cracks. The microhardness of theuncoated samples was found to significantly increase from 65.2 to760.8 Hv after NiCrAlY/nano-YSZ plasma spray coating. How-ever, NiCrAlY coating showed lower bonding strength comparedto the NiCrAlY/nano-YSZ coating. Single-layer NiCrAlY anddual-layer NiCrAlY/nano-YSZ plasma spray coatings decreasedthe corrosion current density of the Mg–1.2Ca–3Zn alloy.

Fig. 4. (a) Potentiodynamic polarization curves, (b) electrochemical impedance spectroscopy (c) hardness of uncoated, NiCrAlY coated and NiCrAlY/nano-YSZcoated Mg–1.2Ca–3Zn alloy and (d) bond strengths of the plasma spray coatings.

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Acknowledgments

The authors would like to acknowledge the UniversitiTeknologi Malaysia (UTM) and Nippon Sheet Glass Foundationfor providing research facilities and financial support under Grantno. R.J.130000.7324.4B136. The authors also thank Dr. Gujba forreading and improving the quality of language of the paper.

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