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Materials Chemistry and Physics 118 (2009) 37–45

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

liding wear and electrochemical corrosion behavior of plasma sprayedanocomposite Al2O3–13%TiO2 coatings

ei Tian a, You Wang a,∗, Tao Zhang b, Yong Yang a

Nano surface engineering laboratory, Department of Materials Science, Harbin Institute of Technology, Harbin 150001, PR ChinaCorrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering University,inistry of Education, Nantong ST 145, Harbin 150001, PR China

r t i c l e i n f o

rticle history:eceived 13 February 2009eceived in revised form 21 June 2009ccepted 28 June 2009

eywords:

a b s t r a c t

The conventional Metco 130 coating, densified nanostructured coating and undensified nanostructuredcoating were deposited by plasma spray. There were typical lamellar splats composed of columnar grainsexisted in the conventional Metco 130 coating. Whereas, the lamellar structures and columnar grainswere not obvious in densified and undensified nanostructured coatings. The densified nanostructuredcoating had the highest sliding wear and corrosion resistance. Micro fracture, adhesive wear and tribo-

lasma sprayl2O3–13%TiO2 coatingliding wearlectrochemical corrosion

chemical reaction occurred in all three coatings during their sidling against Si3N4 balls. The dominantwear mechanism in Metco 130 coating was micro fracture along splat and columnar grain boundaries.While, in nanostructured coatings the damage was intergranular fracture and grains pull-out. Duringthe corrosion of densified nanostructured coating in 3.5%NaCl solution, the diffusion process of reactantthrough the coating was a determining factor. The NaCl solution could reach the steel substrate throughpermeable defects in coatings. The corrosion mainly occurred at the steel substrate near the NiCrAl bond

e.

coating/substrate interfac

. Introduction

It is well known that many metal facilities suffer from wear andorrosion in their service [1,2]. One of the most common routeso prevent metals from corrosion and wear is to deposit a pro-ective coating onto the surface of metals [3]. The plasma sprayedl2O3–TiO2 composite coatings have attracted much attention in

his field for their super resistance to heat, corrosion and wearompared to metals [4–6]. Usually, the plasma sprayed coatingsossess lamellar structures with a high density of defects, e.g. inclu-ions, pores and cracks. These defects could significantly affect theerformance of coatings [7–9].

Nanomaterials are experiencing a rapid development in recentears due to their existing and/or potential applications in a wideariety of technological areas such as electronics, catalysis, ceram-cs, magnetic data storage and structural components etc [10]. The

tudy of nanomaterials also has been extended to coatings processsing the thermal spray technique [11–14]. In particular, plasmaprayed nanocomposite Al2O3–13 wt%TiO2 coatings derived fromgglomerated feed stocks had much higher abrasive wear, thermal

∗ Corresponding author at: Nano surface engineering laboratory, Department ofaterials Science, Harbin Institute of Technology, No.92, West Da-Zhi Street, Harbin

50001, PR China. Tel.: +86 451 86402752; fax: +86 451 86413922.E-mail address: wangyou@hit.edu.cn (Y. Wang).

254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2009.06.035

© 2009 Elsevier B.V. All rights reserved.

shock and fretting resistance than the corresponding conventionalcoatings. The improved performances of the nanocomposite coat-ings were mainly attributed to their unique microstructures e.g.grain refinement, reduced splat boundaries and/or formation ofbimodal structure [15–19].

The aim of the present work was to study sliding wear andcorrosion behavior of nanostructured and conventional Al2O3–13 wt%TiO2 coatings prepared by plasma spray. Wear performanceof the composite coatings was tested using a ball-on-disk weartester. Furthermore, the corrosion behavior of coatings was exam-ined in 3.5% NaCl solution. Based on the analyses results, an attemptwas made to discuss the wear and corrosion mechanisms of com-posite Al2O3–13 wt%TiO2 coatings.

2. Experimental procedure

2.1. Preparation of plasma sprayed Al2O3–13%TiO2 coatings

Three kinds of composite Al2O3–13 wt%TiO2 coatings were fabricated by plasmaspray in the present study. These three coatings were conventional Metco 130coating, densified nanostructured coating and undensified nanostructured coating,which were deposited with commercial Metco 130 feed stock, plasma treated nanos-tructured feed stock and sintered nanostructured feed stock respectively. Two kinds

of nanostructured feed stocks were fabricated by a reconstitution process usingindividual nanoparticles as raw materials. Critical preparation steps for sinterednanostructured feed stock included: nanoparticles dispersion by wet ball milling,spray drying and sintering. In order to reduce porosity and improve flowability, thesintered nanostructured feed stock was treated by plasma spray system and then theplasma treated nanostructured feed stock was obtained. The Metco 130 feed stock

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as manufactured through a so-called cladding process, in which fused and crushedl2O3 powders (10–45 �m) were clad with submicronsized TiO2 particles.

The carbon steel (0.45 wt.% carbon) coupons were used as substrates. Beforepraying, the substrates were degreased and grit blasted with brown corundum. Aond coating of NiCrAl with thickness about 60 �m was deposited between ceramicoating and steel substrate. The plasma sprayed Al2O3–13 wt%TiO2 ceramic coatingsere about 300 �m in thickness [15].

.2. Microstructure characterization

The fracture surfaces of coatings were observed by the scanning electron micro-cope (SEM, S-4700, Hitachi, Japan). The microstructures of Metco 130 and densifiedanostructured coatings were characterized by transmission electron microscopeTEM, Philips Tecnai20, Netherlands). The TEM specimens were plan-view foils ofl2O3–13 wt%TiO2 coating.

.3. Sliding wear test

Sliding wear behavior of three coatings against commercial bearing grade Si3N4

alls (HV: 16 ± 0.6 GPa, �f: 775 ± 25 MPa, KIC: 6 ± 1 MPa m1/2, E: 310 ± 10 GPa, Diam-ter: 3.969 mm) was examined using a ball-on-disc tester (Zhong Ke Kai Hua Sciencend Technology Development Co., Ltd., Lanzhou, China). The coating specimens wereolished to 1 �m finish using routine metallographic procedures for wear testing.uring test, the Si3N4 balls bore normal load of 2N, 4N and 6N respectively and theoating samples rotated at the fixed speed of 955 rpm (0.3 m s−1). The sliding wearest was carried out in ambient condition of 20 ± 2 ◦C temperature and 50 ± 5% rel-tive humidity. No lubrication was used during wear test. The friction coefficientas continuously recorded in the test. After testing, the width and depth of wear

racks on each coating was measured in three different locations perpendicularly tohe sliding direction using a profile meter. The cross-sectional areas of wear tracksere determined from the data measured by profile meter. The wear rates were

alculated using cross-sectional areas of wear tracks and in terms of the volume ofaterial removed (in mm3) per unit load (N) and distance of sliding (mm), in unit

f mm3 N−1 mm−1.

.4. Electrochemical corrosion test

All the electrochemical corrosion tests were performed in a three-electrode cellsing a Pt plate as counter electrode, Ag/AgCl as reference electrode, and the coating

Fig. 1. SEM micrographs of fracture surfaces of (a) Metco 130 coating (b) densi

nd Physics 118 (2009) 37–45

sample as the working electrode. Prior to test, the coating samples were clad withteflon and paraffin only left an exposed area of coating about 0.6 cm2.

The electrochemical impedance spectroscopies (EIS) of coatings were measuredwith an electrochemical analyzer (IVIUM, Netherlands) in 3.5% NaCl solution. Thedisturbance signal was 20 mV and the measurement frequency ranged from 0.01 Hzto 100,000 Hz. The EIS of densified nanostructured coating was recorded after dif-ferent times (0.5 h, 24 h, 48h. . .192 h) of immersing the sample in NaCl solution.The measured EIS of coatings were fitted and interpreted by EIS fitting software ofZSimpWin.

The potentiodynamic polarization investigations were carried out by anelectrochemical analyzer (IM6ex, Zahner, Germany). The initial potential was−600 mV (Ag/AgCl), final potential was +1600 mV (Ag/AgCl) and the scan rate was0.333 mV s−1. The potentiodynamic polarization curves were recorded after 30 minof immersing the samples in 3.5% NaCl solution.

3. Results and discussion

3.1. Microstructures of coatings

The SEM micrographs of fracture surfaces of three coatings areshown in Fig. 1. It can be seen that, the plasma sprayed Metco130 coating had typical lamellar structures and straight colum-nar grains. In addition, there were many pre-existing cracks andvoids between splat boundaries in the Metco 130 coating. Thelamellar splats and columnar grains were not obvious in the twonanostructured coatings. Some micro spherical pores existed innanostructured coatings. Simply, the pores in undensified nanos-tructured coating were more and bigger than those in densifiednanostructured coating.

Fig. 2 illustrates the typical TEM micrographs of Metco 130 coat-ing and densified nanostructured coating. It can be seen that theMetco 130 coating consisted of different sized grains which wereon the order of 200 nm to several micrometers. Due to the resid-ual stresses, some intergranular cracks were formed in Metco 130

fied nanostructured coating and (c) undensified nanostructured coating.

W. Tian et al. / Materials Chemistry and Physics 118 (2009) 37–45 39

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Fig. 2. TEM micrographs of (a) Metco 130

oating. As shown in Fig. 2b, there were some amorphous phasesnd many nanosized grains distributed in the densified nanos-ructured coating. The selected area diffractions (SAD) revealedhat the observed grains in both coatings were �-Al2O3. As indi-

ated in our previous research, �-Al2O3 was the major phase inlasma sprayed Al2O3–13 wt%TiO2 coatings. They were formed inlasma spray process due to the high cooling rate of molten alumina20].

ig. 3. Measurement of wear scar profiles (I Metco 130 coating; III densified nanostructub) under normal load of 4N (c) under normal load of 6N and (d) showing the wear rate o

ng (b) densified nanostructured coating.

The different microstructures of the three coatings were mainlyrelated to their feed stocks. The different compositions in two kindsof nanostructured feed stocks were uniformly mixed during recon-stitution process. So, the compositional difference was minimized

in nanostructured coatings. The Metco 130 feed stock was manu-factured through fused-crushed-cladding process, submicronsizedTiO2 clad the micronsized Al2O3 powders. Accordingly, there werenon-homogeneity of chemical composition and typical laminar

red coating; II undensified nanostructured coating) of (a) under normal load of 2Nf three coatings under different normal load.

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plats in Metco 130 coating. Some pores were formed in reconsti-uted nanostructured feed stocks. During plasma spray process, their in pores could be retained in nanostructured coatings from feedtocks and then micro pores formed in coatings. The pores wereore and bigger in the sintered nanostructured feed stock than

hat in plasma treated nanostructured feed stock; accordingly, theorosity in undensified nanostructured coating was relative high.he Metco 130 feed stock was dense and angular. In spray process,he molten state, spread deformation and adhesion of Metco 130eed stock were not as good as nanostructured feed stocks. Many

icro cracks were formed in Metco 130 coating. The formation oficro cracks was related to the rapid cooling rate of the dropletshile spreading out over the substrate [8].

.2. Sliding wear behavior of coatings

.2.1. Sliding wear resultsThe friction coefficients of three coatings were similar. They all

anged from 0.55 to 0.65 under the normal loads of 2N, 4N andN.

ig. 4. SEM micrographs of worn surface of (a) Metco 130 coating (b) densified nanostrN; (d) Metco 130 coating (e) densified nanostructured coating and (f) undensified nananostructured coating and (i) undensified nanostructured coating under normal load of

nd Physics 118 (2009) 37–45

The wear scar profiles of three coatings after different normalload test measured by the profile meter are shown in Fig. 3. The mea-surement range of the surface profile meter used in this researchwas 25 �m. So when the wear scar was deeper than 25 �m, theprofile became straight (as Fig. 3c shown). Based on the wear scarprofile, the wear rate were calculated (as Fig. 3d shown) and used torepresent the wear resistance of three coatings at different normalload. It can be seen that the densified nanostructured coating pos-sessed much superior wear resistance to other two coatings. Thewear resistance of undensified nanostructured coating was higherthan that of Metco 130 coating with the exception under normalload of 4N.

3.2.2. Sliding wear mechanismThe sliding worn surfaces of three coatings under different nor-

mal load are given in Fig. 4. As shown in the SEM micrographs, allthe worn surfaces were composed of smooth regions and roughregions. With the increasing of normal load, the area of smoothregions reduced and rough regions increased. At the same normalload test, the areas of smooth region of Metco 130 coating were

uctured coating and (c) undensified nanostructured coating under normal load ofostructured coating under normal load of 4N; (g) Metco 130 coating (h) densified6N.

W. Tian et al. / Materials Chemistry and Physics 118 (2009) 37–45 41

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ig. 5. SEM micrographs of wear scars and EDS analysis result. (a) is the smooth regiohe morphology of cylindrical debris. (b) is the EDS analysis result of cylindrical deboating after sliding wear test under normal load of 6N.

igger than those of the other two nanostructured coatings. Thatay be attributed to the columnar grains and preferential growth

f �-Al2O3 in Metco 130 coating [17]. In addition, for Metco 130oating, a mass of wear debris accumulated on the brim of wearrack and adhered to the surface. The wear debris in wear tracks ofwo nanostructured coatings is much less than that of Metco 130oating. For densified nanostructured coating, there is only a littlef scattered wear debris present on the wear track.

The high resolution SEM micrographs reveled there was no obvi-us difference for smooth regions among three coatings. The typicalicrograph of smooth region of densified nanostructured coating

nder normal load of 6N is presented in Fig. 5a. The grooves with theidth and depth smaller than 1 �m were most probably formed by

lastic deformation. There were few fractures found on the smoothegions. In addition, much small cylindrical debris was observedn the surface. This cylindrical debris was only found on smoothegion, which ranged from 1 to 5 �m in length and less than 300 nmn diameter. As pointed in other researches [21,22], the cylindricalebris could be formed by tribochemical reactions during slidingf either silicon nitride or alumina. The EDS analysis result indi-ates that the cylindrical debris is mainly composed of Al and Olement (Fig. 5b). That means the cylindrical debris was derivedrom Al2O3–13 wt%TiO2 coatings. X. Dong et al. [23] proved that

ribochemical reactions between water vapor and the alumina sur-ace may produce a thin aluminum hydroxide film, which was thenolled to a cylindrical form by the sliding and reciprocating actiont the contact. The aluminum hydroxide films were much softerhan the alumina and they could distribute the stresses, accommo-

nsified nanostructured coating under normal load of 6N and the insert image shows) and (d) are the rough regions of Metco 130 coating and densified nanostructured

date the shear movement and, therefore reduce surface fracture andresult in low wear rate. As mentioned above, aluminum hydroxidefilm would be formed in smooth regions during sliding of compos-ite Al2O3–13 wt%TiO2 coatings. Materials in smooth regions weremainly removed by form of aluminum hydroxide cylindrical debris.

Fig. 5c and d show the damage characteristics of rough regionsof Metco 130 coating and densified nanostructured coating at nor-mal load of 6N. During sliding, the micro fracture occurred in roughregion of all three coatings. In Metco 130 coating, the materialsremoved with micro sized sheet debris. Some of this debris wereswept out and accumulated on the brim of wear track (Fig. 4a, dand g). Some other debris remained in the wear track was sub-jected to continued fracture, deformation or chemical reaction insubsequent sliding and constituted a very fine powder [22]. In thedensified nanostructured coating, the damage was mainly in theform of intergranular fracture and grains pull-out due to the con-tact fatigue. Obviously, the damage in rough regions of Metco 130coating was serious than that of densified nanostructured coat-ing.

It can be seen that in Metco 130 coating, the materials wereremoved with micro sized debris. While, intergranular fracturewas dominant in the densified nanostructured coating. The dam-age characteristics would be related to their microstructure. As

shown in Fig. 1a, lamellar splats and columnar grains existed inMetco 130 coating. So, fracture could take place along the colum-nar grains and splat boundaries since their strength usually werenot high enough [24–26]. There were no obvious columnar grainsand lamellar splats in densified nanostructured coatings. The frac-

4 istry and Physics 118 (2009) 37–45

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had the highest corrosion resistance among three coatings. Andthe Metco 130 coating showed lower corrosion current densitythan that of undensified nanostructured coating. In the potentio-dynamic polarization curves of three coatings, the anodic currentdensity of three coatings increased with the scanned potential.

2 W. Tian et al. / Materials Chem

ure mainly occurred along gain boundaries and without regularrientation.

Based on the above discussion, it could be concluded that theailures in the rough regions were much more serious than those inmooth regions during sliding wear. Increasing the normal load, therea of smooth region reduced and rough area increased. Accord-ngly, the wear rate of three coatings increased with normal load.espite the areas of smooth region of Metco 130 coating was larger

han that of densified nanostructured coating under the same nor-al load, the materials removal in rough regions of Metco 130

oating was very serious. That also could be proved by the fluc-uation of wear scar profile of Metco 130 coating. As a result, theensified nanostructured coatings exhibited superior wear resis-ance than Metco 130 coating. The smooth regions of Metco 130oating were much more than those of undensified nanostruc-ured coating under 4N normal load, accordingly Metco 130 coatingxhibited higher wear resistance.

The worn surface of Si3N4 ball wearing against densified nanos-ructured coating under normal load of 6N is shown in appendix.

ost areas of the worn surface were smooth. There were somerotrusions on the worn surface with about 50 �m in width. TheDS analysis of line scanning exhibited there were much Al ele-ent in the protrusions. The similar protrusions were also observed

n worn surface of Si3N4 balls wearing against other two coatingsnder different normal load. That means adhesion and materialsransfer occurred during the sliding of Al2O3–13 wt%TiO2 coatingsgainst Si3N4 balls.

.3. Electrochemical corrosion behavior of coatings

.3.1. Electrochemical impedance spectroscopy characteristicA number of works revealed that EIS could be used as a

on-destructive tool for the detection of structural defects and elec-rochemical corrosion behavior of ceramic coatings [27–29]. The

easured EIS for Metco 130 coating, densified nanostructured coat-ng and undensified nanostructured coating are shown as Bodepectra in Fig. 6. It can be seen that the impedance magnitude (|Z|) ofhree coatings were different. The densified nanostructured coatingossessed the highest impedance magnitude, which means the cor-osion resistance of densified nanostructured coating was higherhan that of other two coatings. The corrosion resistance of unden-ified nanostructured coating was the lowest among three coatings.t high frequency (HF) range, the impedance spectra reflected theielectric behavior of coatings, while the low frequency (LF) rangeharacterized the corrosion process of substrate [30]. The differ-nces of impedance magnitude of three coatings were observed inoth high frequency (HF) range and low frequency (LF). This indi-ated that the coating properties and corrosion process of threeoatings were different.

.3.2. Potentiodynamic polarizing analysisThe protectiveness of three composite coatings evaluated

hrough potentiodynamic polarization techniques in 3.5% NaCl

olution is shown in Fig. 7. The corrosion potential (Ecorr) and corro-ion current density (icorr) were derived from the data. The resultsf potentiodynamic polarization are summarized in Table 1.

It can be seen that the corrosion potentials of two nano-tructured coatings were more positive than that of Metco 130

able 1esults of potentiodynamic corrosion test of different coatings in 3.5% NaCl solution.

oatings Ecorr (mV) icorr (nA cm−2)

etco 130coating −263.7 522.4ensified nanostructured coating −209.2 126.8ndensified nanostructured coating −184.3 2647.9

Fig. 6. EIS (Bode spectra) of three coatings in 3.5%NaCl solution: (1) Metco 130coating (2) densified nanostructured coating and (3) undensified nanostructuredcoating. (a) Bode spectra of impedance magnitude vs. angular frequency and (b)Bode spectra of phase angle vs. angular frequency.

coating. That means the thermodynamic tendency of corrosion ofnanostructured coatings was low. The corrosion current density ofdensified nanostructured coating was the lowest, which means it

Fig. 7. Potentiodynamic polarization curves for three coatings in 3.5%NaCl solution.

W. Tian et al. / Materials Chemistry and Physics 118 (2009) 37–45 43

Fig. 8. (a) EIS (Nyquist spectra) of densified nanostructured coating after differentiQp

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mmersion times in NaCl solution and (b) equivalent circuit diagrams for the EIS.c, CPE of coating; Qdl CPE of substrate; Rpore resistance to current flow through theores; Rt charge transfer resistance; Rs solution resistance; W Warburg impedance;.

here was no obvious passivation observed for all three coat-ngs. The corrosion resistance of three coatings evaluated throughotentiodynamic polarization techniques was consistent with thatnalyzed by EIS.

The different corrosion resistance of three coatings was mainlyue to their different microstructures. The permeable defects inoatings were detrimental to the corrosion performance, as theyrovide direct paths to allow the corrosive electrolyte to reach theteel substrate. The Metco 130 coating was a built-up of lamel-ar splats composed of columnar grains. Some pores and cracksxisted between and through the splats. Thereby, the corrosive

ntermediate (Cl−) would be rapidly transferred through the cracksnd columnar grain boundaries then reach the bond coating andteel substrate. The densified nanostructured coating had muchess through-going channels compared to Metco 130 coating. So,

Fig. 10. (a) SEM micrograph and (b) EDS analysis result of the corrosion products of

Fig. 9. Rpore and Rt of densified nanostructured coating as a function of immersiontime in 3.5%NaCl solution.

the densified nanostructured coating had much higher corrosionresistance than Metco 130 coating. As to undensified nanostruc-tured coating, due to the high porosity and lose microstructure itexhibited lowest protectiveness.

3.3.3. Anti-corrosion behavior of densified nanostructuredcoating during immersion

In order to further understand the anti-corrosion mechanismof densified nanostructured coating, the corrosion process duringimmersion was studied by EIS measurements in 3.5% NaCl solution.

The measured EIS of densified nanostructured coating afterdifferent times of immersion in 3.5%NaCl solution are shown asNyquist spectra in Fig. 8a. The equivalent circuit showed in Fig. 8bwas used to fit and interpret the EIS. Among electrochemical ele-ments, Rs was the solution resistance, Rpore was the resistanceto current flow through the pores of coatings; Rt was the chargetransfer resistance of substrate, Qc and Qs were the constant phaseelement (CPE) to describe the dielectric behaviors of coating andsubstrate. Here, the constant phase element was used as a substitutefor the capacitor to fit more accurately the impedance data of theelectrochemical capacitance [31]. W was the Warburg impedancewhich was used to describe the semi-infinite length diffusion pro-cess during electrochemical corrosion of coatings [32].

The equivalent circuit of the EIS for the coatings showed thebase structure of the coatings, and the fitting values reflected thechanging regularity of the corrosion behavior as the immersiontime was increased. The solution resistance (Rs) decreased dramat-

densified nanostructured coating after 192 h immersion in 3.5%NaCl solution.

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cally in the first 48 h immersion and then it was almost stable.hat may be related to the penetration of NaCl solution throughhe coatings.

The element values of Rt and Rpore represented the corrosionesistance and protectiveness of coating, respectively [33,34]. Thehange of fitted values of Rt and Rpore is illustrated in Fig. 9. It cane seen that, the changing regularities of Rt and Rpore were similar.t the first 48 h immersion, the Rt and Rpore decreased dramati-ally. That means the corrosive intermediate (Cl−) could penetratehe coatings and reach the steel substrate quickly during that stage.ecause the corrosion potential of Ni is higher than that of Fe in NaClolution, galvanic corrosion would be formed on carbon steel sub-trate. At later stage (after 48 h immersion), Rt and Rpore increasedlowly and almost kept constant at last. The little increasing of Rt

nd Rpore related to the blocking effect of corrosion products. Duringhis stage, the diffusion of corrosive intermediate was decelerated.ccordingly, the values of Rt and Rpore had a little increasing. At

ast, all the pre-existing permeable channels in coatings were satu-ated by corrosive intermediate (Cl−). And no cracks or other defectsere formed during corrosion. So, the Rt and Rpore of coatings got

onstant gradually.As we known that, the corrosion reactions of ceramic-coated

etals were initiated at the coating/substrate interface [5]. In thisesearch, NiCrAl bond coating were relative passive compared toteel substrate. So the electrochemical corrosion mainly occurredn the steel substrate near the NiCrAl bond coating/substrate inter-ace. Fig. 10 illustrates the SEM micrographs of top surface and EDSnalysis result of densified nanostructured coating after immersionn 3.5%NaCl solution for 192 h. It can be seen, some of corrosionroducts were formed at the coating defects (i.e. pores and cracks).he EDS analysis (Fig. 10b) revealed that the corrosion productsere mainly composed of Fe and O. It proved that the corrosionroducts were mainly from steel substrate.

. Conclusion

The conventional and nanostructured plasma sprayed Al2O3–3 wt%TiO2 composite coatings showed different microstructuralharacteristics, wear and corrosion behavior. The following conclu-ions were drawn:

1) The as-sprayed conventional Metco 130 coating was a built-up of lamellar splats composed of columnar grains. Whereas,the lamellar structure and columnar grains was very few indensified and undensified nanostructured coatings.

2) The densified nanostructured coating exhibited highest slidingwear resistance. The wear resistance of undensified nanostruc-tured coating was higher than that of Metco 130 coating withthe exception under normal load of 4N.

3) There were both smooth regions and rough regions on the weartracks of three coatings under different normal load wear tests.In smooth regions, the tribochemical reaction, plowing andadhesive wear occurred during sidling wear. In rough regions,the damage of Metco 130 coating was micro fracture alongsplat and columnar grains boundaries; the damage of densifiednanostructured coatings was intergranular fracture and grainspull-out.

4) The densified nanostructured coating showed highest corro-sion resistance in 3.5%NaCl solution. The diffusion impedance

appeared in densified nanostructured coating during its immer-sion in 3.5%NaCl solution.

5) The 3.5%NaCl solution could penetrate the Al2O3–13 wt%TiO2and NiCrAl coatings during immersion. The electrochemicalcorrosion mainly occurred at the steel substrate near the NiCrAlcoating/substrate interface.

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nd Physics 118 (2009) 37–45

Appendix A.

SEM micrographs of worn surface of Si3N4 ball: (a) is the lowmagnified image showing contact area and accumulated weardebris, the insert image illustrating the protrusions. (b) is the linescanning results corresponding inserted image in panel (a).

References

[1] X.Y. Wang, D.Y. Li, Wear 259 (2005) 1490.[2] G. Ruhi, O.P. Modi, A.S.K. Sinha, I.B. Singh, Corros. Sci. 50 (2008) 639.[3] C.X. Shan, X.H. Hou, K.H. Choy, Surf. Coat. Technol. 202 (2008) 2399.[4] C.S. Zhai, J. Wang, F. Li, J.C. Tao, Y. Yang, B.D. Sun, Ceram. Int. 31 (2005) 817.[5] E. Celik, I. Ozdemir, E. Avci, Y. Tsunekawa, Surf. Coat. Technol. 193 (2005) 297.[6] E.P. Song, J. Ahn, S. Lee, N.J. Kim, Surf. Coat. Technol. 201 (2006) 1309.[7] S. Liscano, L. Gila, M.H. Staia, Surf. Coat. Technol. 188–189 (2004) 135.[8] R. Westergård, N. Axén, U. Wiklund, S. Hogmark, Wear 246 (2000) 12.[9] S. Lathabai, M. Ottmüller, I. Fernandez, Wear 221 (1998) 93.

[10] S.C. Tjong, H. Chen, Mater. Sci. Eng. R 45 (2004) 1.[11] R.S. Lima, B.R. Marple, J. Therm. Spray. Technol. 16 (2007) 40.12] X.H. Lin, Y. Zeng, X.M. Zhou, C.X. Ding, Mater. Sci. Eng. A 357 (2003) 228.

[13] H. Chen, Y. Zhang, C. Ding, Wear 253 (2002) 885.[14] B. Liang, C.X. Ding, Surf. Coat. Technol. 197 (2005) 185.[15] Y. Wang, W. Tian, Y. Yang, Surf. Coat. Technol. 201 (2007) 7746.[16] D. Goberman, Y.H. Sohn, L. Shaw, E. Jordan, M. Gell, Acta Mater. 50 (2002) 1141.[17] Y. Wang, S. Jiang, M. Wang, S. Wang, T.D. Xiao, P.R. Strutt, Wear 237 (2000) 176.[18] P. Bansal, N.P. Padture, A. Vasiliev, Acta Mater. 51 (2003) 2959.[19] W. Tian, Y. Wang, Y. Yang, Wear 265 (2008) 1700.20] X.H. Lin, Y. Zeng, S.H. Lee, C.X. Ding, J. Eur. Ceram. Soc. 24 (2004) 627.

21] T.E. Fischer, H. Tomizawa, Wear 105 (1985) 29–45.22] T.E. Fischer, Z. Zhu, H. Kim, D.S. Shin, Wear 245 (2000) 53.23] X. Dong, S. Jahanmir, S.M. Hsu, J. Am. Ceram. Soc. 74 (1991) 1036.24] G. Bolelli, V. Cannillo, L. Lusvarghi, T. Manfredi, Surf. Coat. Technol. 201 (2006)

458.25] G. Bolelli, et al., Wear 261 (2006) 1298–1315.

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W. Tian et al. / Materials Chem

26] Z. Chen, et al., Degradation of plasma-sprayed yttria-stabilized zirconia coat-ings via ingress of vanadium oxide, J. Eur. Ceram. Soc. (2008), doi:10.1016/j.jeurceramsoc.2008.10.003.

27] R. Heung, X. Wang, P. Xiao, Electrochim. Acta 51 (2006) 1789.28] R. Brown, M.N. Alias, R. Fontana, Surf. Coat. Technol. 62 (1993) 467.29] C. Liu, Q. Bi, A. Matthews, Corros. Sci. 43 (2001) 1953.

[[[

[[

nd Physics 118 (2009) 37–45 45

30] C. Liu, Q. Bi, A. Leyland, A. Matthews, Corros. Sci. 45 (2003) 1243.31] H.J. Zhao, L. Liu, Y.T. Wu, W.B. Hu, Com. Sci. Technol. 67 (2007) 1210.32] C.N. Cao, J.Q. Zhang, An Introduction to Electrochemical Impedance Spec-

troscopy, Science Press, Beijing, 2002, in Chinese.33] C. Liu, Q. Bi, A. Leyland, A. Matthews, Corros. Sci. 45 (2003) 1257.34] Z.P. Yao, Z.H. Jiang, S.G. Xin, X.T. Sun, X.H. Wu, Electrochim. Acta 50 (2005) 3273.