indentation properties of plasma sprayed al2o3–13% tio2 nanocoatings

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Acta Materialia 57 (2009) 3148–3156

Indentation properties of plasma sprayedAl2O3–13% TiO2 nanocoatings

J. Rodriguez a,*, A. Rico a, E. Otero a, W.M. Rainforth b

a Departamento de Ciencia e Ingenierıa de Materiales, Universidad Rey Juan Carlos, C/Tulipan s/n 28933 Mostoles, Madrid, Spainb Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK

Received 29 July 2008; received in revised form 19 February 2009; accepted 15 March 2009Available online 9 April 2009

Abstract

Young’s modulus and hardness were determined by depth sensing indentation in plasma sprayed Al2O3–13% TiO2 nanocoatings.Results were compared to conventional coatings and the relevance of the nanostructure was analyzed. An indentation size effect wasobserved. Data provided by indentation tests at different maximum loads were used to estimate size-independent hardness and elasticmodulus. Enhanced properties were observed in the nanostructured coating compared to the conventional one. Partially melted zonesin the nanocoating, which act as reinforcements in the ceramic matrix composite, are likely responsible for the enhancement.� 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Ceramic coatings; Plasma spraying; Nanoindentation; Nanostructure

1. Introduction

Ceramic coatings are used in applications where goodtribological properties, elevated hardness and high thermalresistance are required. Coatings based on alumina are agood choice when wear resistance is the controlling factor.Alumina is hard, but its main drawback is brittleness [1–3].The addition of titanium oxide leads to balanced proper-ties, maintaining sufficient hardness but substantiallyincreasing the coating toughness. Titanium oxide has alower melting point than alumina and plays a role in pro-moting coatings with a higher density [4–6].

The coatings are usually applied by a thermal spray pro-cess because a very high temperature is required to melt theceramic powder. Atmospheric plasma spray (APS) is oneof the most commonly used techniques because, in compar-ison to other thermal methods such as flame spraying, itprovides coatings with better quality, good adherence tothe substrate and a lower level of porosity. During the

1359-6454/$36.00 � 2009 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2009.03.020

* Corresponding author.E-mail address: [email protected] (J. Rodriguez).

spraying process, the ceramic particles are injected intothe plasma jet, where they should be completely meltedand accelerated against the substrate to form a compactcoating. Unfortunately, it is very difficult to achieve thisideal situation and many particles remain only partiallymelted, leading to poor coating properties, even thoughgreat efforts have been made to evaluate the dependencyof the behavior of the coating on the atmospheric plasmaprojection parameters [1,6,7].

During recent years, enhanced mechanical properties inbulk materials and coatings with nanoscale microstructureshave been reported, leading to a growing interest in thestudy and analysis of this type of material [7–14].Plasma-sprayed alumina–titania coatings have also beenprepared from nanocrystalline powders. Two main difficul-ties should be addressed during the manufacture of thecoating: the small size of the nanoparticles impedes thedirect projection in conventional plasma equipment andthe high temperatures reached in the plasma jet maydestroy the original nanostructure during thermal sprayingif the process is not carefully controlled [7,9]. To circum-vent the first of these obstacles, a new technological processwas developed in which weak agglomerates of nanoparti-

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J. Rodriguez et al. / Acta Materialia 57 (2009) 3148–3156 3149

cles are prepared by spray drying, with agglomerate aver-age sizes similar to those of the conventional powders(approximately 30–50 lm). These agglomerates can be pro-jected using standard plasma equipment [15]. The nano-structure is maintained if the temperature during plasmaspraying is set up at intermediate values between the melt-ing points of alumina and titania. Absolute control of theatmospheric plasma projection parameters is needed and,by modifying the so-called critical plasma sprayed param-eter (current � voltage/gas flow), a large variety of micro-and nanostructures are attainable [7–14].

Mechanical characterization of a coating is a rather prob-lematic task and, unfortunately, a systematic approach islacking in many of the results reported showing improve-ments in the mechanical performance of ceramic coatings.Depth sensing indentation is one of the few methods fromwhich reliable properties can be determined if the appropri-ate methodology is followed. A remarkable size effect is usu-ally observed and a sound procedure is needed to providerepresentative values of the mechanical properties of thematerial.

In this work, a microstructural analysis of conventionaland nanostructured Al2O3–13% TiO2 coatings has beencarried out. The similarity and disparity between bothtypes of coatings are highlighted to explain the differentmechanical properties observed. Size-independent Young’smodulus, hardness and fracture toughness were determinedafter indentation tests. Toughness results are explained interms of the physical mechanisms controlling the fractureprocess.

2. Materials

Al2O3–13% TiO2 coatings deposited on SAE-42 steel byatmospheric plasma spraying were studied. Conventionalcoatings were fabricated from Metco 130 commercial pow-der, provided by Sultzer MetcoTM. The average size of theparticles is approximately 50 lm in diameter.

Nanostructured Al2O3–13% TiO2 coatings were pre-pared from agglomerates, supplied by Inframat AdvancedMaterials. The agglomerates, comprising nanometric parti-cles (NanoxTM S2613S) with an average size of 200 nm, wereprepared by spray drying. The difference between this pow-der and the conventional powder was in the particle size(average diameter of 30 lm) and the presence of small addi-tions of ZrO2 and CeO2. With this size, the agglomerates areable to be projected with standard APS equipments.

In both the standard and nanostructured coatings, aNi–Al–Mo 90/5/5 (wt.%) bond coat was deposited withthe same equipment between the substrate and the ceramic

Table 1Projection parameters used in the deposition of the coatings.

Material Current (A) Voltage (V) Ar ga

Ceramic coatings 150 100 40Bond coat 500 80 60

coating to enhance the adherence. The final coating thick-ness was approximately 500 lm, including the bond coat.The parameters controlling the APS process were the samefor both types of coatings and are indicated in Table 1,together with those corresponding to the bond coat.

3. Experimental techniques

3.1. Microstructural analysis

To perform the microstructural analysis, several sampleswere cross-sectioned using a diamond disc and then metal-lographically prepared. These samples were analyzed byscanning electron microscopy (SEM), using a PhilipsXL30 and a JEOL 6400 microscope. Both were equippedfor energy-dispersive X-ray microanalysis. To resolve themorphology and composition of the material, samples wereprepared for observations by transmission electron micros-copy (TEM), in a FEI Tecnai 20 microscope operating at200 kV. Finally, X-ray diffraction measurements were alsocarried out to determine the dominant phases present in thecoating. To this end, a Philips PW3040/00 X’Pert diffrac-tometer was used.

3.2. Depth sensing indentation tests

Depth sensing indentation tests were carried out using adiamond Berkovich indenter with a nominal edge radius of100 nm. The experimental device used was a NanoindenterXP (MTS System Co.). The instrument applies load via acalibrated electromagnetic coil with a resolution of 50 nN.The displacement of the indenter was measured using acapacitive transducer with a resolution of 0.01 nm. A seriesof 20 indentation tests were performed at different maxi-mum loads, from 1 mN to 500 mN, for both standard andnanostructured coatings. Each series consisted of 15 repeti-tions under the same conditions.

The indentation samples were prepared in plan view fromtheas-sprayedcoatings.Thesampleswerecutwithadiamondsawandpolishedusing1200 gritSiCpaper.Thiswasfollowedby subsequent polishing in diamond slurry up to a 0.25 lmfinish. Times and pressures were carefully controlled to min-imize any artificial increase in surface porosity or excessiveremoval of those poorly bonded regions. Finally, polishedsurfaces were cleaned first in deionized water, then byultrasound in acetone and propanol.

The Oliver and Pharr method was used to analyze theload–depth of penetration curves and to obtain theYoung’s modulus and hardness of the sample [16]. Theslope of the unloading branch of the load–penetration

s flow (SCFH) Feed rate (g/min) Spray distance (mm)

80 15080 150

Fig. 1. Size distribution of the feeding powders and the partially meltedparticles.

ry U

nits

Al2O

3

TiO2

CeO2

ZrO2

3150 J. Rodriguez et al. / Acta Materialia 57 (2009) 3148–3156

depth curves, S, is calculated at maximum load, providinga reduced modulus, E*, through the projected contactarea, A:

S ¼ 2ffiffiffipp � 1

b� E� �

ffiffiffiAp

ð1Þ

where b is a correction factor, estimated by finite elementsimulations to be 1.034 for a Berkovich indenter. The the-oretical contact area for a Berkovich tip can be calculatedfrom the following expression:

A ¼ 24:5 � h2p ¼ 24:5 � ht �

2 � ðp� 2Þp

P max

S

� �2

ð2Þ

where hp and ht are the contact depth and total depth,respectively. The reduced modulus, E*, can be correlatedwith the Young’s modulus of the sample using the follow-ing equation:

1

E�¼ 1� m2

Eþ 1� m2

i

Eið3Þ

where E, Ei, m and mi are the Young’s modulus and the Pois-son’s ratios of the sample and the indenter, respectively.Hardness can be calculated as:

H ¼ P max

Að4Þ

An accurate knowledge of the indenter geometry is neces-sary in order to determine the contact area, A, betweenthe indenter and sample surface at maximum load.Changes in the actual tip geometry with respect to the con-sidered one in the data analysis could produce errors in H

and E values obtained from indentation tests. Normally acalibration procedure using a reference material is followedto estimate the actual geometry of the tip. Before the tests,the contact area was calibrated using fused silica as the ref-erence material.

3.3. Vickers indentation

Vickers indentations at different loads were performedon the plan view of the nanostructured coating to createand propagate cracks. Selected samples were imagedby SEM to observe the different interactions betweencracks and microstructural features of the nanostructuredcoating.

20 30 40 50 60 70 80

Arb

itra

2θ

Fig. 2. X-ray diffraction of the nanoparticles.

4. Results

4.1. Microstructural analysis

4.1.1. Nanoparticle characterization

The agglomerates obtained after spray drying exhibiteda spherical shape, with the size distribution shown in Fig. 1and an average diameter of about 30 lm. Agglomeratesconsisted of irregular-shaped and polygonal nanoparticles.TEM study performed on these particles revealed that theyconsisted of c-Al2O3 and anatase-TiO2. The nanopowders

are composed of particles with a bimodal distribution ofsizes. Usually, anatase particles are the smallest ones, rang-ing from 50 to 70 nm, while c-Al2O3 particles present sizesof approximately 200 nm.

These results were supported by X-ray diffraction exper-iments carried out on the nanostructured powders. Peaksdue to the presence of c-Al2O3 and anatase are identifiedin Fig. 2. Additional peaks corresponding to traces ofCeO2 and ZrO2 can also be observed.

4.1.2. Nanocoating characterization

Fig. 3a shows the cross-section of the nanostructuredcoating. Three layers can be recognized. The upper layeris the ceramic coating and the intermediate layer corre-

Fig. 3. Cross-section of the nanostructured coating. (a) General view showing the multilayer structure of the coating system. (b) Detail showing the fullyand the partially melted zones.


sponds to an Ni/Al/Mo bond coat used to minimize theresidual stresses due to the mismatch between the thermalexpansion coefficients of the metallic substrate and theceramic material. The lower layer is made of SAE-42 steelused as substrate. In Fig. 4b a detail of the ceramic coatingis shown. The microstructure of the coating fabricatedfrom nanopowders consists of two main zones. Splatsformed by deposition of individual molten droplets gener-ate a lamellar fully melted structure (FM) which is typicallyobtained when thermal projection techniques are used.Nevertheless, partially melted (PM) zones correspondingto the deposition of semi-molten droplets can also beobserved in Fig. 3b. The size distribution of these PMzones is compared to the size distribution of feeding parti-cles in Fig. 1, showing that the average size is also 30 lm,thus maintaining the initial size of the agglomerates. Toretain the original nanostructure of the starting particlesin the coating, the projection parameters should be selectedand controlled with the aim of fixing the temperature of theplasma jet between the melting temperatures of both com-ponents (c-Al2O3 and TiO2). The projection parametersused to deposit the ceramic coatings lead to a volume frac-

Fig. 4. Partially melted microstructure. (a) General view. (b) Electrondiffraction pattern from an Al2O3 particle with an orientation B <111>. (c)Electron diffraction pattern showing the nanostructure of the matrixsurrounding the particles.

tion of partially melted zones close to 30%, which is inagreement with previous works [7–11].

TEM studies developed in the literature [7,9–11,13,14] insimilar nanostructured coatings indicated that the FMphases are composed by c-Al2O3. On the other hand, PMzones exhibit a composite microstructure (Fig. 4). The par-ticles observed, with an average size of 200 nm, correspondto the alumina particles previously identified in the nano-powders. However, the electron diffraction pattern shownin Fig. 4b indicates that a phase transition occurred duringthe plasma process. PM zones are essentially composed ofa-Al2O3, while the primary phase in the powder was c-Al2O3. Although the particles maintain their initial size,they lose the polygonal contour and a smooth, round finalshape is observed. The high temperature reached duringthe projection promotes the phase transition but is not highenough to completely melt the particles. Fig. 4c shows thata-Al2O3 particles are embedded in a matrix composed of c-Al2O3 nanocrystallites similar to those presented in the FMzones. The only difference is that this matrix is supersatu-rated in Ti4+ because of the formation of a solid solutionbetween the c-Al2O3 and the TiO2. These results are inagreement with those obtained in previous works by Shawet al. [7,9] and Bansal et al. [13].

4.1.3. Standard coating characterization

Conventional coatings were also studied following thesame methodology. Fig. 5 shows an SEM micrograph ofthe overall aspect of the microstructure. The morphologyand composition are similar to the FM zones shown inthe nanostructured coating. In this case, PM zones arenot identified even though projection parameters employedto deposit these standard coatings were identical to thoseused in the nanostructured ones. A possible explanationto this phenomenon is given in Ref. [11] regarding the mor-phology of the conventional and nanostructured startingpowders. Unlike Metco 130 powders, comprising denseparticles, the nanopowders are in fact agglomerated parti-cles with a high degree of porosity. Therefore, it is reason-able to expect that the conventional powders exhibitedgreater heat conduction, leading to a better temperaturedistribution in the plasma jet, and subsequently to com-pletely melted particles.

Fig. 6. Young’s modulus and hardness vs. total penetration depth forboth coatings.

Fig. 7. (a) SEM micrograph showing a crack generated during a Vickersindentation being deflected by a partially melted particle. (b) A newdeflection mechanism is observed inside the partially melted zones. Cracksare deflected by the alumina nanoparticles placed inside the partiallymelted regions.

Fig. 5. Cross-section of the conventional coating. (a) General view presenting the multilayer structure of the coating. (b) Detail showing the lamellarstructure of fully melted slats.


4.2. Mechanical properties

Experimental data of the coating hardness and Young’smodulus are included in Fig. 6 for both the conventionaland nanostructured materials. The first important resultis the significant size effect exhibited by these two proper-ties, increasing as the size of the indentation falls.

SEM images around a Vickers indentation performedat 3 N are shown in Fig. 7a. Cracks emerging from theedges of the residual imprint were arrested and deflectedby the PM particles (these features are marked withwhite arrows). The PM particles constitute obstacles tothe cracks propagation, which are forced to deflectaround them, thus improving the indentation fracturetoughness of the nanocoating compared to the conven-tional one.

When the load is high, deflection is also detected, butmost cracks pass through the PM particle, avoiding thecrack plane tilting. Nevertheless, a new deflection mech-anism may be active on a different scale when cracksencounter a-alumina nanoparticles inside the PM zones.The existence of this crack deflection on a smaller scale

is shown in Fig. 7b, although its significance is uncer-tain, since most of the crack path is located in theFM region.

5. Discussion

There are several aspects arising from the experimentalresults that require additional discussion: the actual signif-icance of the small differences observed in Young’s modu-lus and hardness between the nanostructured and theconventional coatings; and the significant size effect exhib-

Fig. 8. (a) Representative area of the nanostructured coating. A partially melted particle is shown in the middle of the micrograph. (b) Young’s modulusmapping of the area detailed in (a). The color scale is set in GPa. (c) Hardness mapping of the area detailed in (a). The color scale is set in GPa. Mechanicalproperties were extracted by superposing a matrix of indentations on (a) and by interpolating the values of the indented positions.



ited by the hardness and Young’s modulus measured bydepth sensing indentation.

5.1. Effect of nanostructure on the Young’s modulus and

hardness

Given the overlap of the error bars shown in Fig. 6, theexperimental differences observed are not conclusive. Amore detailed analysis is needed to discriminate the effectof the nanostructure, i.e. of the PM particles on theYoung’s modulus and hardness of the coatings. With thisaim, a matrix of 204 (17 � 12) indentations was performedinside a representative area (Fig. 8a) at 200 mN. Contourmaps of mechanical properties were done after interpolat-ing the results obtained in the indented locations (Fig. 8band c). Several important conclusions can now beestablished:

Fig. 9. Histograms showing the statistical distribution of the mechanicalproperties in the area presented in Fig. 10. (a) Young’s modulus. (b)Hardness.

� nanoindentation tests are able to discriminate betweenthe different microstructural regions;� the PM particles are clearly stiffer and harder than the

FM regions; and� the experiments are sensitive to material defects such as

pores and cracks.

To assess and quantify the last conclusions, a statis-tical analysis was performed. The resultant histogram ispresented in Fig. 9, where two peaks are easily identi-fied, the first one corresponding to the FM regionsand the second one associated to the PM particles.The same methodology can be applied to the originaldata included in Fig. 6. The conclusions mentioned

Fig. 10. (a) Slope of the unload branch of the load–displacement curve, S

vs. the contact depth, hp, for conventional coating, and for nanostructuredcoating constituents. (b) Maximum load, Pmax vs. contact area, A, forconventional coating, and for nanostructured coating constituents.

Table 2Mechanical properties derived from Fig. 10b.

Material Elastic modulus (GPa) Hardness (GPa)

Partially melted 300 ± 16 14.0 ± 0.9Fully melted 269 ± 18 9.7 ± 1.1Conventional 265 ± 15 9.6 ± 0.7


above are confirmed under all experimental conditions.The mechanical properties related to the FM regionsare indistinguishable from those corresponding to theconventional coating. The role played by the PM parti-cles is now clarified. Interestingly, the results included inFig. 6 can be derived from these new data by assuminga simple rule of mixtures.

5.2. Size independent hardness and Young’s modulus

The indentation size effect (ISE) is a phenomenon thathas been widely studied in the literature [17–19]. Most ofthe work has been dedicated to metals. Nix and Gao [17]developed a model based on the strain gradient theoryand the generation of geometrically necessary dislocationsaround the indentation to justify the tendencies observedin hardness when a self similar tip is used. Nevertheless,Young’s modulus should not depend on dislocation fields.This is why the size effect in Young’s modulus is consideredin the scientific community of depth-sensing indentationusers as a warning signal of artifacts in the method itself.Several factors are candidates for causing an undesirableISE: surface roughness, tip roundness and indenter defor-mation are probably the most commonly recognized [20–26]. Surface and tip roughness are preventable throughproper sample preparation and the accurate calibrationof the indenter in an adequate reference material. Whenrelatively stiff materials are to be characterized, the thirdfactor is inevitable. The estimation of the contact area isinaccurate and the use of Eq. (1) leads to inexact results.This is, unfortunately, the case of the ceramic coatingsstudied in this work.

As a consequence of this undesirable ISE, a methodol-ogy should be followed to obtain size-independent mechan-ical properties. In this respect, it would be useful to employdata provided by indentation tests at different maximumloads. This approach has previously been applied in ceram-ics and led to much more consistent results [27], and a sim-ilar line of attack was successfully used by the authors totreat the phenomenon of pile-up in depth-sensing indenta-tion of metals [28]. Thus, a size-independent value of themechanical properties can be extracted from the indenta-tion tests if data provided by tests performed at differentpenetration loads are used. In Fig. 10a the slope of theunloading branch of the load–displacement curve calcu-lated at maximum load, S, is plotted vs. the contact depth,hp, for conventional and nanostructured (FM and PMregions) coatings. The data seem to fit linear behavior,although very different mechanical properties can beobtained if the Oliver and Pharr method is directly appliedto the individual tests. The slope of these lines can be cor-related to the reduced elastic modulus through Eqs. (1) and(3), providing a unique size-independent value representa-tive of each material.

A similar approach could be used to estimate size inde-pendent hardness. Eq. (4) relates the maximum load of

indentation, Pmax, to the contact area, A. The slope ofthe Pmax vs. A plot provides a size-independent hardness.As can be seen in Fig. 10b, the first region of the plot doesnot obey a linear fit. This is a consequence of the inaccura-cies made in the estimation of the contact area, by not tak-ing into account the geometrical effects mentioned before.When high penetration depths are reached, these effectsare dissipated and the experimental data seems to fit muchbetter to a linear function.

The results obtained (Table 2) indicate that the nano-structured material has better mechanical properties thanthe conventional one. The PM zones may work as rein-forcement particles in a ceramic matrix composite, ham-pering the deformation in the surrounding regions andthereby increasing the global stiffness and hardness ofthe coating.

6. Conclusions

Microstructure and indentation properties of Al2O3–13% TiO2 plasma-sprayed nanocoatings were analyzed,leading to the following conclusions:

– By controlling the composition and morphology of thefeeding particles and the plasma projection parameters,it is possible to obtain a hierarchical structure in a cera-mic coating. When nanoparticles agglomerated by spraydrying are used, a composite microstructure is reached.Fully melted lamellar zones composed of c-Al2O3 nano-crystallites and partially melted regions formed by a-Al2O3 particles embedded in a c-Al2O3 matrix areobtained. If conventional micrometer-sized particlesare employed, only fully melted splats can be observedin the coating.

– Both nanostructured and conventional coatings showan indentation size effect on the mechanical proper-ties evaluated from depth-sensing indentation. Geo-metrical deviations from the ideal tip and indenterdeformation seem to be the most relevant factorsexplaining this behavior. A methodology based ondata obtained from tests performed at different inden-tation loads has been applied to get size-independentYoung’s modulus and hardness values.

– Nanoindentation tests are able to discriminate betweenthe different microstructural regions. The partiallymelted particles (those maintaining the original nano-structure after thermal projection) are stiffer and harderthan the fully melted regions. They act as reinforcementsin a ceramic matrix composite.


Acknowledgements

The authors are indebted to Comunidad de Madrid forthe financial support through program ESTRUMAT andGrant 5963.

References

[1] Ananthapadmanabhan PV, Thiyagarajan TK, Satpute RU, Venkatr-amani N, Ramachandran K. Surf Coat Technol 2003;168:231–40.

[2] Ramachandran K, Selvarajan V, Ananthapadmanabhan PV, Sreeku-mar KP. Thin Solid Films 1998;315:144–52.

[3] Erickson LC, Hawthorne HM, Troczynski T. Wear 2001;250:569–75.[4] Pantelis DI, Psyllaki P, Alexopoulos N. Wear 2000;237:197–204.[5] Gessasma S, Bounazef M, Nardin P, Sahraoui T. Ceram Int

2006;32:13–9.[6] Normand B, Fervel V, Coddet C, Nikitine V. Surf Coat Technol

2000;123:278–87.[7] Shaw LL, Goberman D, Ren R, Gell M, Jiang S, Wang Y, et al. Surf

Coat Technol 2000;130:1–8.[8] Wang Y, Jiang S, Wang M, Wang S, Xiao TD, Strutt PR. Wear

2000;237:176–85.[9] Gell M, Jordan EH, Sohn YH, Shaw L, Xiao TD. Surf Coat Technol

2001;146–147:48–54.[10] Jordan EH, Gell M, Sohn YH, Goberman D, Shaw L, Jiang S, et al.

Mater Sci Eng 2001;A301:80–9.

[11] Goberman D, Sohn YH, Shaw L, Jordan E, Gell M. Acta Mater2002;50:1141–52.

[12] Liu Y, Fischer ET, Dent A. Surf Coat Technol 2003;167:68–76.[13] Bansal P, Padture NP, Vasiliev A. Acta Mater 2003;51:2959–70.[14] Lin X, Zeng Y, Lee SW, Ding C. J Eur Ceram Soc 2004;24:627–34.[15] Cao XQ, Vassen R, Schwartz S, Jungen W, Tietz F, Stoever D. J Eur

Ceram Soc 2000;20:2433–9.[16] Oliver WC, Pharr GM. J Mater Res 1992;7:1564–83.[17] Nix WD, Gao H. J Mech Phys Solids 1998;46:411–25.[18] Huang Y, Zhang F, Hwang KC, Nix WD, Pharr GM, Feng G. J

Mech Phys Solids 2006;54:1668–86.[19] Feng G, Nix WD. Scripta Mater 2004;51:599–603.[20] Swadener JG, George EP, Pharr GM. J Mech Phys Solids

2002;50:681–94.[21] Peng Z, Gong J, Miao H. J Eur Ceram Soc 2004;24:2193–201.[22] Yu N, Polycarpou AA, Conry TF. Thin Solid Films

2004;450:295–303.[23] Alkorta J, Martınez-Esnaola JM, Gil-Sevillano J. Acta Mater

2006;54:3445–52.[24] Jeong SM, Lee HL. Thin Solid Films 2005;492:173–9.[25] Knapp JA, Follstaedt DM, Myers SM, Barbour JC, Friedmann TA. J

Appl Phys 1999;85:1460.[26] Rico A, Garrido MA, Otero E, Rodriguez J. Key Eng Mater

2007;333:247–50.[27] Gong J, Miao H, Peng Z. Acta Mater 2004;52:785–93.[28] Rodrıguez J, Garrido-Maneiro MA. Mech Mater 2008;62:69–72.

indentation properties of plasma sprayed al2o3–13% tio2 nanocoatings

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