Surface & Coatings Technology 204 (2010) 2709–2715

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Correlation between nanoindentation and nanoscratch properties of carbonnanotube reinforced aluminum composite coatings

Yao Chen a,b, Srinivasa R. Bakshi a, Arvind Agarwal a,⁎a School of Mechanical and Electronic Engineering, Soochow University, 178 Ganjiang East Road, Suzhou 215021, People's Republic of Chinab Nanomechanics and Nanotribology Laboratory, Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA

⁎ Corresponding author. Tel.: +1 305 348 1701; fax:E-mail address: agarwala@fiu.edu (A. Agarwal).

0257-8972/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2010.02.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 October 2009Accepted in revised form 7 February 2010Available online 13 February 2010

Keywords:NanoindentationNanoscratchWear modelingCarbon nanotubeCold sprayAluminum coatings

Nanoindentation and nanoscratch experiments are conducted on cold-sprayed aluminum composite coatingsreinforced with and without carbon nanotubes (CNTs). Analytical model is developed to correlate nanoscalemechanical and tribological properties of the solid material. The geometry factor of Berkovich tip and itsorientation during nanoscratch is taken into account to describe the deformation behavior of the solidmaterialunderneath indenter. Nanoindentation and nanoscratch experiments in conjunction with the computedresults illustrate that the addition of carbon nanotubes into aluminummatrix contributes to increase in elasticmodulus, hardness, yield strength, shear strength and nanoscale wear resistance. The coefficient of friction isunaffected by the addition of CNTs up to 1 wt.%.

+1 305 348 1932.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) are expected to be ideal reinforcementsfor polymers, metals and ceramics due to their excellent mechanicalproperties [1], chemical stability, and electrical and thermal conductiv-ities [2,3]. CNTs also exhibit relatively low density ranging from ∼1.2 g/cm3 for singlewalledCNT to∼1.8 g/cm3 formulti-walledCNTs [3,4], andsubsequently have high specific strength of ∼4.8×104 kNmkg−1 [4].These unique mechanical properties of CNTs make them morepromising reinforcements for synthesizing light weight, high strengthmetal matrix (e.g. aluminum, magnesium) structural composites [4–10]. Previous studies [8,10] onaluminum–CNTcompositeshave reportedthe reaction of carbon nanotubes with aluminum at high temperatureto form needle-like Al4C3 phase that degrades mechanical properties.Cold spray is a relatively new coating technique wherein powderparticles are accelerated to supersonic velocities (600–1500 m/s) by acarrier gas flowing under large pressure difference through a de-Lavaltype of nozzle andmade to impact onto a substrate. Upon impact againsta substrate, the solid particles plastically deform and bond together,rapidly building up a layer of deposited material [11]. Cold spray hasunique advantages like minimal effects on the material sprayed likeoxidation, grain coarsening or phase changes, andproduces highly densecoatings. Since cold spraydoes not involvehigh temperature required formetals processing, it is a promising technique to fabricate CNT reinforced

aluminum matrix composites without the detrimental reaction to formAl4C3 phase.

Nanoindentation and nanoscratch techniques can record load anddepth continuously throughout the indentation and/or scratch cycleto evaluate mechanical (elastic modulus, hardness) and tribological(coefficient of friction) properties of a solid material. Mechanicalproperties of a solid are well known to strongly affect its tribologicalperformance [12,13]. However, a conclusive understanding of thecorrelation between mechanical and tribological properties is lacking[14]. In our previous work on nanoscratch studies of plasma sprayedAl-12 wt.% Si coating containing 5 and 10 wt.% CNTs, we have shownthat addition of CNTs increases the elastic recovery during scratching.A newmethod for computation of wear volumes taking Berkovich tipsattack angle in account was presented. Based on the calculations ofthe scratch volume, an improvement in nanoscratch resistance up to 4times for the 10 wt.% CNT coating was observed [15]. In the currentwork nanoindentation and nanoscratch experiments were conductedon cold-sprayed aluminum–CNT composite coatings. Wear volumecalculations and the recovery following scratch studies have beenstudied. An analytic model was developed to correlate nanoscalemechanical properties with tribological properties of the cold-sprayedaluminum–CNT composite coatings.

2. Experimental details

2.1. Synthesis of Al–CNT coatings

Spray drying was used to uniformly disperse the 5 wt.% multi-walled carbon nanotubes (CNTs), with a purity of more than 95% and

2710 Y. Chen et al. / Surface & Coatings Technology 204 (2010) 2709–2715

diameter of 40–70 nm and length of 2.1±0.4 μm, within theagglomerates of fine sized Al–Si eutectic alloy powders having aparticle size of 57±21 μm. These spray dried powders were thenmixed with 99.7% pure Al powder (particle size of 26±13 μm Alpoco,Minworth, UK) in proportions of 10 wt.% and 20 wt.% to generatefeedstock for cold spray, which contained an overall CNT content of0.5 wt.% and 1 wt.%, respectively. Cold spray experiments werecarried out at the University of Nottingham, UK which has an in-house built system comprising a high pressure gas supply, highpressure powder feeder, a converging–diverging nozzle and a X–Ytraverse unit. The details of the microstructure and the processinghave been explained in our previous work [16]. A brief description ispresented here for the better understanding.

2.2. Nanoindentation and nanoscratch experiments

Nanoindentation and nanoscratch tests were carried out usingHysitron Triboindenter (Hysitron, Minneapolis, USA) with a diamondBerkovich indenter tip of 100 nm tip radius. The same tip and identicalloading conditionswereused for both, nanoindentation andnanoscratchexperiments. The cross-section of the cold-sprayed coatings wasground and polished metallographically to a 0.1 μm finish prior tonanoindentation and nanoscratch experiments. Nanoindentation wasperformed at the loading rate of 25 μN/s up to a peak load of 1000 μNwhere itwas held for 2 s and then unloaded completely at negative rateof 25 μN/s. The hardness and elastic modulus were calculated by theOliver and Pharr method [17]. Nanoscratch tests were conducted at aconstant force of 1000 μN with a scratching speed of 0.33 μm/s and atotal scratch length of 10 μm. After the scratching has been performed,the same tipwas used to image the surface by applying a contact load of2 μN. The resultant scanning probe microscopy (SPM) images wereanalyzed using the SPM image processing software SPIP™ (ImageMetrology A/S, Horsholm, Denmark). Depth profiles were taken alonglines drawnparallel aswell as perpendicular to the scratchusing SPIP™.Coefficient of friction was calculated as the ratio of the instantaneous

Fig. 1. SPM images of the scratches on cold-sprayed Al–CNT coatings made using a

lateral and normal forces. Three scratch tests were carried on eachsample to ensure the reproducibility.

3. Results and discussion

3.1. Microstructure of the coatings

Aluminum composite coatings up to 500 μm thickwere successfullysprayedontoAA6061aluminumsubstratewithoverall CNTcontentof 0,0.5 and 1.0 wt.%. CNTs were successfully retained and located bothbetween the splat interfaces and also embedded in the matrix [16].However, CNTs were shortened in length (0.8±0.4 µm) due to thefracture that occurs due to impact and shearing between Al–Si eutecticparticles and the aluminum matrix during the deposition process. Thetypical microstructural characteristics of the cold-sprayed CNT rein-forced Al coatings consists of deformed Al particles, Al–Si particles fromcollapse of the spray dried particles and porosity, in which Al particleshad undergone a large amount of plasticflow to formelongateddisc likeparticles which are referred to as splats. The significant differencebetween the microstructures of cold-sprayed Al coating with andwithout CNTs is the presence of CNTs embedded in Al matrix forming amechanical bond, which is expected to contribute to the improvementof themechanical properties of cold-sprayed Al coating reinforced withCNTs. A detailed discussion on the microstructure evolution has beenreported in our previous work [16].

3.2. Nanoscratch behavior of cold-sprayed coatings

3.2.1. Wear volume of the coatingsFig. 1 shows the 2D-SPM images of the scratches on the three

coatings. It is observed that scratches look more or less same for thethree coatings. It is noted that there are two depths associatedwith theindentation, the instantaneous depth during scratching, hinst, andthe true depth of the scratch,htrue, after elastic recovery processes havetaken place. Fig. 2 shows the variation of true depth and instantaneous

Berkovich tip at 1000 μN load. Scale bar in all the images corresponds to 2 µm.

Fig. 2. Variation of hinst and htrue along the scratch for the cold-sprayed Al–CNT coatings.

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depth along the scratch distance for the three coatings. The averagevalues of hinst and htrue values for three scratches of each coating aretabulated in Table 1. It is observed that the instantaneous depth (hinst)is not much different for all the coatings. However, the true depthdecreases as the CNT content increases. From the difference betweenthe hinst and htrue values in Table 1 and Fig. 2, it is concluded that theelastic recovery following scratching also increases with addition ofCNTs. It is seen that the recovery in Al–1CNT is slightly greater thanAl–0.5CNT coating. The scratch volumes were calculated according to themethodology outlined in previous work [15]. The contact volume (VC)is the volume of the material displaced during scratching and isdependent on the hinst, while the true volume (VT) is the final volumeof the scratch groove and is dependent on htrue. The volumeof a scratchgrove created by a Berkovich indenter is given by the formula:

V = ∫l=2

−l=2

12

tanϕ + tan θ−ϕð Þ½ �h2dx = ∫l=2

−l=2

C·h2dx: ð1Þ

Here h is the depth which is dependent on the scratch distance x, θ isthe angle between the faces of the scratch grove and ϕ is the angle

Table 1Calculated values of the contact and true wear volumes for cold-sprayed Al–CNTcoatings.

Coating Avg. hinst(nm)

Avg. htrue(nm)

Contactvolume(VC), μm3

Truevolume(VT), μm3

Reduction inVT compared toAl coating (%)

Al 115±18 103±13 0.49±0.08 0.46±0.02 –

Al–0.5CNT 127±14 96±15 0.58±0.04 0.40±0.07 15Al–1CNT 116±26 87±14 0.50±0.12 0.33±0.02 28

between one of the faces and the normal direction [15]. The values of θand ϕ were shown to be a periodic function of the tip orientationangle. The orientation angle of the tip was 18° which leads to a valueof C=3.57 for calculation of contact volumes [15]. The values of θ andϕ for calculation of true volume were obtained from the depth profileof the scratches normal to the scratch direction. Although the angles θand ϕ varied from 151 to 154° and 71 to 74° respectively, the averagevalues were found to be same for all the three coatings. The averagevalue of θ, ϕ and Cwere equal to 152°, 73° and 4.21° respectively for VT

calculation. The contact volume and true volume have been calculatedusing the information above and the average of three values has beentabulated in Table 1. The values of the wear volumes have also beenplotted in Fig. 3. It is observed that there is not much difference in thecontact wear volume due to addition of CNTs. The true volumehowever shows a reducing trendwith addition of CNTs. The reason forthis is increased elastic recovery of the coatings as shown in Fig. 2.Increased elastic recovery decreases the htrue values which in turnreduced the true volume of the scratches. Based on the true volume itcan be said that the nanoscale wear resistance of pure Al improves by40% by addition of 1 wt.% CNTs.

3.2.2. Correlation between nanoindentation and nanoscratch resultsTypical load–displacement curves obtained fromnanoindentation of

three cold-sprayed coatings are shown in Fig.4a. The sudden slopechanges during unloading curves for three cold-sprayed Al coatings isascribed to the porosity (∼2%) within the cold-sprayed coatings [16].The measured elastic modulus and hardness are listed in Table 2. It isclearly seen that introducing CNTs into aluminummatrix contributes toincrease in both elastic modulus and hardness of cold-sprayed coatings.The lateral force and coefficient of friction for three cold-sprayedcoatings duringnanoscratchdonot vary significantly as shown in Fig.4b.

Fig. 3. Variation of a) contact and b) true wear volumes of the cold-sprayed Al–CNTcoatings as a function of CNT content.

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The scratch depth along its width, as measured from the surfacetopography profiles of scratch marks on three different coatings isshown in Fig.4c. The average residual depth of the scratch (htrue) is∼120 nm for cold-sprayed Al, ∼100 nm for cold-sprayed Al-0.5 wt.%CNT and ∼95 nm for cold-sprayed Al-1.0 wt.% CNT. Scanning probemicroscopy (SPM) image of the nanoscratchmade on cold-sprayed Al isshown in Fig. 4d and e for reference.

It is well known that the yield strength (σy) of a rigid perfectlyplastic material is correlated to the hardness (H) as the following [18]:

H = Cσy ð2Þ

in which C is the constraint factor, and C≈3 for metals. It is worth tonote that C varies from 1.5 to 3 and is a material property-controlledfactor, i.e., C is dependent on the extent of plasticity measured by E /σy

(E is the elastic modulus of the material), strain hardening and otherstrengthening mechanisms, etc [19]. During nanoindentation, numer-ical simulation summarizes the relationship between Er /σy, the elasticrecoverable displacement (he) and maximum displacement (hm) [20]as shown in Eq. (3):

hehm

= 2:98Erσy

!−0:68

ð3Þ

where Er is the reduced modulus, he is the elastic recoverabledisplacement, and hm is the maximum displacement obtained duringnanoindentation. Substituting the nanoindentation results (Er, he, andhm) of different cold-sprayed coatings into Eq. (3), the computed yieldstrength are 0.36±0.10 GPa for Al, 0.47±0.13 GPa for Al-0.5 wt.%CNT, and 0.61±0.12 GPa for Al-1.0 wt.% CNT, respectively. The

computed H /σy i.e. C is less than 1.5, which suggests that computedyield strength lies in the upper bound. This behavior is attributed tothe pile-ups around the indentationwhich is not taken into account inthis numerical simulation [20]. Taking into account the pile up can bevery complicated and is beyond the scope of this study. Pile up inscratching also depends on the attack angle. In this model, pile-upphenomenon is ignored to make the model simple. Nonetheless, thecomputed yield strength reflects the trend of the addition of carbonnanotubes on the yield strength of as-sprayed samples.

As shown in Fig. 5, it is well known that during nanoindentation,the maximum displacement (hm) at the peak load equals the sum ofthe contact depth (hc) and the elastic surface displacement at theperimeter of the contact (hs) [21],

hm = hc + hs: ð4Þ

Bao et al. [22] defined recovery resistance (Rs) as a materialproperty to be an indicator of energy dissipation during an indentationcycle,

Rs =Fmax

h2sð5Þ

and

Rs = 2:263E2rH

ð6Þ

where Fmax is the peak load during nanoindentation, Er and H are thereduced elastic modulus and the hardness in the nanoindentation.

When the Berkovich tip penetrates the solid material duringnanoscratch, the displacement consists of contact displacement andthe elastic surface displacement at the perimeter of the contact. Weintroduce the concept of recovery resistance (Rs,s) during thenanoscratch which is defined as following:

Rs;s =Fmax;s

h2s;sð7Þ

and

Rs;s = 2:263E2rH

ð8Þ

in which hs,s is the elastic surface displacement at the perimeter of thecontact during the nanoscratch, Fmax,s is the applied normal loadduring nanoscratch.

Combining Eqs. (7) and (8), the elastic surface displacementduring nanoscratch can be obtained as following:

hs;s =Fmax;sH

2:263×E2r

� �1=2

: ð9Þ

Subsequently, the contact depth (hc,s) during nanoscratch isobtained as following:

hc;s = hm;s−Fmax;sH

2:263×E2r

� �1=2

ð10Þ

where hm,s is the maximum penetration depth during nanoscratch. Tosimplify our proposed model, the effect of surface roughness andmaterial sinking-in/pile up on the real contact area during loading is

Fig. 4. (a) Load–displacement curves obtained from nanoindentation of three coatings, (b) variations of lateral force and coefficient of friction as a function of scratch distance forthree coatings, (c) surface topography profiles of scratch marks for three cold-sprayed coatings, (d) 2-D and (e) 3-D SPM images of nanoscratch made on cold-sprayed Al coating.

2713Y. Chen et al. / Surface & Coatings Technology 204 (2010) 2709–2715

neglected. Therefore, the ideal contact area during nanoscratch can beestimated

A≈24:5h2c;s = 24:5 hm;s−Fmax;sH

2:263×E2r

� �1=2" #2

: ð11Þ

Nevertheless, it is important to note that the stress distribution andthe deformation behavior of an isotropic material underneath indenterduring nanoindentation is independent of the orientation of theBerkovich tip, because of the three-fold symmetry along its verticalaxis. On the contrary, the stress distribution and deformation duringnanoscratch is different due to the orientation of Berkovich tip,(e.g. Berkovich-face forward and Berkovich-edge forward scratchdirection). Hence, it is important to quantify the effect of the geometryfactor andorientation of Berkovich indenter on the contact depth and/orcontact area during nanoscratch.

Pelletier et al. [23] introduced the geometry factor, termed as X fora perfect conical indenter as shown in Eq. (12):

X =Eσ0

tanβ ð12Þ

where E and σ0 are the elastic modulus and yield stress of the testedmaterial, respectively, and β is the attack angle of the tip. Eq. (12) is also

Table 2Nanomechanical properties of cold-sprayed Al–CNT composites.

Coating E (GPa) H (GPa) σ (GPa) τ (MPa)

Cold-sprayed Al 45.0±13.5 0.47±0.11 0.36±0.10 320±18Cold-sprayed Al–0.5 wt.% CNT 62.1±20.4 0.51±0.10 0.47±0.13 623±31Cold-sprayed Al–1.0 wt.% CNT 94.8±19.6 0.64±0.12 0.61±0.12 664±35

available for perfect tetragonal (Berkovich, Cube corner) or pyramidal(Vickers) indenter geometries that can be assimilated by an equivalentconical indenter [23]. For a perfect Berkovich tip in the process ofnanoscratch, the equivalent conical angle should be 66.3° for faceforward orientation and 72.8° for edge forward orientation [24]. Therelationship between equivalent conical angle (θ) for a perfectBerkovich tip and attack angle (β) is given by Eq. (13) [23]

θ = π=2−β ð13Þ

Therefore, the equivalent attack angle should be 23.7° forBerkovich-face forward orientation, and 17.2° for Berkovich-edgeforward orientation.

Fig. 5. Schematic illustration of the indentation deformation at peak load and afterunloading during nanoindentation (from ref. [17]).

2714 Y. Chen et al. / Surface & Coatings Technology 204 (2010) 2709–2715

On the other hand, Raymon-Angélis et al. correlated the geometryfactor X for Berkovich tip with deformation behavior of a given mate-rial by the following relationship [23]

X =ðhm = FmaxÞS−2:0106

0:1812ð14Þ

in which hm is themaximumdisplacement at the peak load, Fmax is thepeak load applied upon the indenter tip, and S is the contact stiffness,which can be obtained from nanoindentation results.

According to above-mentioned descriptions, the contact depth ofthe tested material underneath the indenter during nanoscratch canbe estimated using Eqs. (12)–(14) for different Berkovich forwardorientations if the yield strength of the testedmaterial is known. Usingthe above computed yield strength for the three cold-sprayed coatings,the computedmaximumdepth during nanoscratch is 155±23 nm forcold-sprayed Al, 112±16 nm for cold-sprayed Al-0.5 wt.% CNT, and108±15 nm for cold-sprayed Al-1.0 wt.% CNT, respectively, in whichthemaximumandminimumvalues for each sample correspond to theBerkovich-edge forward and Berkovich-face forward orientations,respectively. The calculatedmaximumdepth of scratch for Al coating islarger than the average hinst as shown in Table 1. But for the CNTreinforced Al coatings, computed and experimental scratch depthvalues are goodwith the difference being less than 10%. The accuracy ofthe depth during nanoscratch at smaller scratch force is limited due to(i) presence of surface contaminants/lubricants, (ii) resolution of theattached transducers and (iii) hardness of the tested solid material.Hence it will not be prudent to quantify the regime of applicability forsmaller loads. The schematic illustration (Fig. 6) of the nanoscratchemployed in the experiments shows that orientation of the Berkovich tipis between the face forward and edge forward orientations. Therefore,the actual maximum penetration depth during nanoscratch can beestimated by the average of the maximum value for the Berkovich-edgeforward orientation and the minimum value for Berkovich-face forwardorientation. Subsequently, the computed contact area during nanoscratchcan be computed using Eqs. (9)–(11).

The lateral force, Ff, can be expressed as the following based on thetheory of Bowden and Tabor [25]:

Ff = τA ð15Þ

Fig. 6. Schematic of the orientation of the Berkovich tip during nanoscratch employed inthe experiments.

where Tτ is the shear strength, and A is the contact area. It is noted thatthis relationship can be used to investigate the sliding behavior of bulkmaterials [26] and coating/filmmaterials [14]. Therefore, the relation-ship betweenmechanical and tribological properties can be correlatedas:

Ff = 24:5 hm−FmaxH

2:263×E2r

� �1=2� �2τ: ð16Þ

Hence, for the given indenter tip and tested material, it is evidentfrom Eq. (16) that lateral force (tribological properties) during scratchtest can be correlatedwith hardness, elastic modulus yield strength andshear strength (mechanical properties). Based on the lateral force valuesfrom the nanoscratch experiments, and elastic modulus and hardnessfrom nanoindentation experiments, the shear strength (τ) of eachcoating was computed using Eq. (16) and is found to be 320±18MPafor cold-sprayed Al coating, 623±31MPa for cold-sprayed Al-0.5 wt.%CNT coating and664±35MPa for cold-sprayedAl-1.0 wt.%CNTcoating,as listed in Table 2. Slightly higher computed shear strength is attributedto the deviation between the computed ideal contact area using Eq. (11)and the real contact area. The strengthening is attributed to theimpediment of dislocationmotion by CNTswhichwill cause an increasein the shear strength [4]. Fig. 7a shows uniformly distributed CNTs in thecold-sprayed aluminum which serves as reinforcements. Fig. 7b showsthat CNT reinforcements retain their original cylindrical graphiticstructure, after cold spraying. Polycarpou et al. [27,28] have alsodeveloped a simulation model for the nanoscratch using conical tipwhich is independent of tip orientation due to its geometry. The notablefeature of the analytical simulation developed in our research is thatshear strength of a solid material can be correlated to the experimental

Fig. 7. (a) SEMmicrograph showing uniform distribution of CNTs in the matrix, and (b)TEM image showing CNT cylindrical graphitic structure in the matrix.

2715Y. Chen et al. / Surface & Coatings Technology 204 (2010) 2709–2715

results from nanoindentation and nanoscratch taking Berkovich tip'sorientation into account.

Although the lateral force and coefficient of friction duringnanoscratch for the three cold-sprayed samples do not changesignificantly (Fig. 4b), it is evidenced from Fig. 4c that the residualdepth of the scratch imprints for the cold-sprayed Al coating reinforcedwith CNTs decreases as comparedwith the cold-sprayed Al coating. Theaddition of CNTs imparts good micro-cutting resistance, which isascribed to the combination of improved hardness and strength (yieldstrength and shear strength). The average coefficient of friction wasfound to be 0.17±0.01, 0.19±0.01 and 0.18±0.02 for Al, Al–0.5CNTand Al–1CNT coatings respectively. The small effect of CNT content oncoefficient of friction is ascribed to the fact that the CNTs are intact andgraphitic lubrication mechanism are not operational unless there issignificant damage to CNTs to generate graphite like debris [15]. Thecoefficient of friction was found to bemore or less similar to the plasmasprayed Al–Si–CNT coatings [15].

4. Conclusion

Nanoscratch measurements showed an improvement in wearresistance of cold-sprayed Al–CNT coatings by 40% by addition of 1 wt.%CNT. Coefficient of friction is not affected by addition of CNTs. Ananalytical model to correlate between nanoscale mechanical (indenta-tion) and tribological (scratch) properties of a solid material that cantake account of not only elastic modulus, hardness, but also yield andshear strength. Themodel predicts introduction of CNTs into aluminummatrix contributes to significant increase in its shear strength andmarginal decrease in the adhesion force between Berkovich tip andcoating surface. Therefore, this model is expected to assist infundamental understanding of indentation and scratch properties of asolid surface at nanoscale.

Acknowledgments

A. Agarwal and S. R. Bakshi would like to acknowledge funding fromthe National Science Foundation CAREER Award (NSF DMI-0547178)

and International Research and Education in Engineering (NSF DMI-0634949). A.A. also acknowledge DURIP (N00014-06-0675) grant toestablish nanoindentation laboratory at FIU. S. R. Bakshi acknowledgesthe Presidential Enhanced Assistantship and Dissertation Year Fellow-ship from Florida International University. The authors would like tothank Prof. D. Graham McCartney for help with cold spraying at theUniversity of Nottingham.

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