Ž .Surface and Coatings Technology 135 2001 166]172

Evaluation of microhardness and elastic modulus ofthermally sprayed nanostructured zirconia coatings

R.S. Lima, A. Kucuk, C.C. BerndtU

Department of Materials Science and Engineering, State Uni ersity of New York at Stony Brook, 306 Old Engineering Building,Stony Brook, NY 11794-2275, USA

Received 8 February 2000; accepted in revised form 2 August 2000

Abstract

Ž .Results concerning microhardness and roughness R of plasma sprayed coatings fabricated from nanostructured partiallyaŽ . Ž .stabilized zirconia PSZ feedstock are presented. Nanostructured zirconia particles were plasma sprayed ArrH at three power2

levels, with two argon flow rates at two spray distances. The results indicate that the microhardness, elastic modulus androughness of the nanostructured zirconia coatings exhibit the following trends: the smoother the roughness, the higher themicrohardness and elastic modulus. It was found that roughness is an indicator of the coating state that reflects the intrinsicmicrostructure of the coatings. It was ascertained that a surface profilometer could be used to determine the level ofmicrohardness and elastic modulus as a non-destructive and in situ test by simple comparison with standard samples. Q 2001Elsevier Science B.V. All rights reserved.

Keywords: Thermal spray; Microhardness; Elastic modulus; Roughness; Nanostructured zirconia-yttria; Thermal barrier coatings

1. Introduction

ŽNanocrystalline materials also referred to asnanostructures, nanophase materials, or nanometer-

.sized crystalline solids are single-phase or multi-phasepolycrystals. The crystal size is typically approximately

w x1]100 nm in at least one dimension 1,2 . Nanostruc-Ž .tured materials come in two general morphologies: i

nanolayered materials deposited by physical vapourŽ .deposition or electrodeposition processes and; ii

nanograined materials, which are usually consolidatedw xfrom nanostructured powders 1,2 . As the grain size

U Corresponding author. Tel.: q1-631-632-8507; fax: q1-631-632-8525.

Ž .E-mail address: cberndt@notes.cc.sunysb.edu C.C. Berndt .

becomes smaller, there are an increasing number ofatoms associated with grain boundary sites comparedto crystal lattice sites. The unique properties ofnanograined materials are associated with the finenessof structure as well as the enhanced solubility andincreasing atomic mobility associated with grain boun-

w xdaries 1,2 . It has also been demonstrated thatnanostructured ceramics can be sintered at relativelylow temperatures, exhibit improved ductility, and evenpotential superplasticity, in the nanocrystalline statew x1 .

Ž .Thermal barrier coatings TBCs consist of a bondcoat and a zirconia coating overlay. They are primarilyused for aerospace applications. TBCs, due to their lowthermal conductivity and thermal diffusivity combinedwith proper chemical stability at high temperatures,

Ž .provide a means for i raising the operating tempera-

0257-8972r01r$ - see front matter Q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 2 5 7 - 8 9 7 2 0 0 0 0 9 9 7 - X

( )R.S. Lima et al. r Surface and Coatings Technology 135 2001 166]172 167

Žture of hot part sections e.g. turbine blades, nozzles. Ž .and combustion chambers or ii they enable the un-

derlying material to operate at lower temperatures dueto a temperature drop through the ceramic coating.TBCs can also increase engine efficiency either byincreasing the working temperature whilst maintaininga constant component temperature, or by decreasing

w xthe use of cooling systems in components 3,4 .Individual nanoparticles cannot be successfully ther-

mal sprayed because of their low mass. They do nothave enough inertia required to cross the streamlinesin the spray jet, being projected to its periphery withoutdepositing on the substrate. To overcome this problem,the feedstock was developed by creating a dispersion ofnanosized particles into a colloidal suspension, fol-lowed by the addition of a binder and subsequent spraydrying into granules, followed by sintering.

The final powder structured consists of agglomeratedŽ . w xmicronsized particles spherical shape 5 formed from

the coalescence of many nanosized particles. Thispowder can be fed by conventional mechanical powder

w xfeeders 5 . Since the particles are porous, the plasmagas could penetrate into the inner part of the particles;thereby melting its surface. The air which is trappedinside the particle heats up; thereby exploding thepreviously agglomerated particles into several tiny par-

w xticles during their residence time in the plasma 5 . Theunmelted cores of these particles will keep the powder

w xnanostructure intact in the coating 5 . As a conse-quence, the presence of non-molten particles in thecoatings is very important.

In the current work, the influence of different sprayparameters on the microhardness, elastic modulus androughness of nanostructured coatings is investigated asa continuing study to better understand the microstruc-

w xtural characteristics 6]9 of this new class of sprayedmaterial.

It is important to comment that the thermal sprayindustry does not have simple effective methods toevaluate the coating quality because the traditionalmethods are destructive. In this work, a surface pro-filometer was used to determine the quality of a coat-

ing in situ. These roughness measurements were corre-lated to microhardness and elastic modulus.

2. Experimental procedure

Ž .The nanostructured PSZ ZrO ]7 wt.% Y O ex-2 2 3Žperimental powder Nanox 4007 Inframat Corp., North

.Haven, CT, USA was plasma sprayed under differentparameters in air on mild steel substrates with a 40-kW,

Žstandard SG100 plasma torch Praxair, Appleton, WI,.USA . The substrates were grit-blasted with alumina

Ž .just before thermal spraying R s4.1"0.3 mm . Theasubstrates were not pre-heated. Typical coating thick-ness was 0.9]1.1 mm. The spray parameters applied arelisted in Table 1.

The Vickers microhardness measurements were per-Žformed at 500 and 1000 g loads for 15 s Buehler

.Micromet II, Buehler Ltd., Lake Bluff, IL, USA onthe top-surface and the cross-section of coatings. TheKnoop microhardness measurements were performed

Žat 1000 g load for 15 s Tukon, Instron, Canton, MA,.USA on the cross-section of coatings. The cross-sec-

tions of the samples were polished before the indenta-tions. For the cross-sections, the indentations wereapplied near the center line of the coating thickness.The distance between the indentations was at leastthree times the diagonal to prevent stress-field effectsfrom nearby indentations for both the top-surface and

w xcross-section studies 10 .Ž .The arithmetic mean roughness value R of thea

coatings was determined by a mechanical profilometerŽ .T1000 Hommel America Inc, New Britain, CT, USA

performed along two orthogonal directions on the as-sprayed coating surfaces, with the following specifica-

Ž .tions; type of roughness filter: M1 DIN 4777 , tracinglength: 4.8 mm, cut-off length: 0.8 mm and tracingspeed: 0.5 mmrs. Roughness measurements near thecoating edges were avoided. A total of 10 measure-ments were taken for each test condition of hardness,elastic modulus and roughness.

Table 1The spraying parameters used for the PSZ coatings

Parameters Set 1 Set 2 Set 3 Set 4 Set 5 Set 6

Ž .Power kW 40 40 40 40 32 24Ž .Current A 800 800 800 800 800 600Ž .Voltage V 50 50 50 50 40 40

Ž .Ar flow-plasma slpm 48 38 48 38 48 48Ž .H flow-plasma slpm f5 f5 f5 f5 f4 f42

Ž .Ar flow-carrier gas slpm 5 5 5 5 5 5Ž .RPM-Hopper % 30 30 30 30 30 30Ž .Spray distance cm 6 6 8 8 8 8

( )R.S. Lima et al. r Surface and Coatings Technology 135 2001 166]172168

Ž .Fig. 1. Vickers microhardness]roughness relationship 500 g loadŽ .for different sets of spray parameters Table 1 .

3. Results and discussion

3.1. Roughness

Ž .The arithmetic mean roughness value R is theaaverage deviation of a surface profile from the center-

Ž .line over the measuring length, defined by Eq. 1 ;

xsL1< Ž . < Ž .R s y x d x 1Ha L xs0

where y is the deviation of the surface profile from thecenterline.

The average roughness varied significantly withŽ .change in spray conditions Figs. 1]4 . The R in-a

Žcreased with increasing spray distance set 1 vs. set 3.and set 2 vs. set 4 . Similarly, increase in torch power

Žresulted in decrease in roughness of the coatings set 6,.set 5 and set 3 . However, change in H rAr ratio made2

a slight difference in the R of the coatings.a

Ž .Fig. 2. Vickers microhardness]roughness relationship 1000 g loadŽ .for different sets of spray parameters Table 1 .

Ž .Fig. 3. Knoop microhardness]roughness relationship 1000 g loadŽ .for different sets of spray parameters Table 1 .

3.2. Microhardness]roughness

Figs. 1 and 2 show a trend whereby the roughnessdecreases as the hardness increases. A dashed line wasplaced in Figs. 1 and 3 as a ‘guideline for eye’ to showthis trend. The data points on these figures indicateplasma spray parameter sets where the plasma powerdecreases and the spray distance increases from set 1through set 6. It is speculated that these responses arealso a manifestation of the physical principles that are

w ximplicit in the Madejski equation 11 . Madejski formu-lated a theoretical model on the impact of a moltendroplet with a solid substrate by making a relationship

Ž .between the splat diameter D and the diameter ofŽ .the initial droplet d ;

0.2r¨ dD 0.2d Ž . Ž .s1.2941 s1.2941 Re 2ž /d m

Fig. 4. Elastic modulus]roughness relationship for different sets ofŽ .spray parameters Table 1 .

( )R.S. Lima et al. r Surface and Coatings Technology 135 2001 166]172 169

where r, m and ¨ are the density, viscosity and thedimpact velocity, respectively, of the particle. TheReynolds number is represented as Re. According to

Ž .Eq. 2 , when the velocity of the particles is increasedandror the viscosity is decreased then particle spread-ing tends to increase. It becomes apparent that highlyflattened particles will form a coating with low rough-ness, while low flattened particles will form a coating

w xwith high roughness. According to Vardelle et al. 12 ,when particle velocity and temperature increase, theflattening degree increases in a linear trend. The parti-cles sprayed with parameter set 1 should exhibit thehighest velocity and the lowest viscosity, since they areprocessed under conditions of the highest power andshortest spray distance. The opposite effect is demon-strated by particles sprayed with parameter set 6; i.e.these particles will have the lowest velocity and highestviscosity since they are sprayed at the lowest power andwith the shortest spray distance. Experimental observa-tions of splat formation and solidification of zirconiaAPS particles show that the Reynolds number alsoinfluences the flattening degree in an almost linear

w x w xtrend 12 . But according to theoretical works 13,14the flattening degree increases non-uniformly with theincrease of Reynolds number, in the regions of smallvalues of Reynolds number.

Other physical relationships can be ascertained fromthe observations the spray parameters. Thus, increasingargon flow will increase the particle velocity becausethe velocity of the plasma gas flame is proportional to

w xthe working gas mass flow rate 15 . Also, increasingthe plasma gas flow will increase the length of the

w xplasma flame 16,17 , extending the high temperatureŽzone and, thereby, preventing resolidification decreas-

.ing of viscosity of the sprayed particles. At the sametime, increasing plasma power increases the tempera-ture and the extent of temperature and velocity isocon-

w xtours 17 . Clearly, increasing the plasma gas tempera-ture causes enhanced conditions for particle meltingand, thereby, lowering particle viscosity.

w xVardelle et al. 18 deals with the plasma phenomenaexhibited when the spray distance is decreased. For anArrH plasma, alumina particle temperatures and ve-2locities were measured as a function of spray distancefor the same plasma gas flow. A change in spraydistance from 8 to 6 cm, increases average particlevelocity by ;60 mrs, while the gain in temperature

w xwas ;1000 K 18 . This difference of 60 mrs in 2 cmmay be reflected in the particle flattening behavior,because the maximum plasma gas velocity for regularair plasma spray systems is approximately 300]400 mrsw x15 . The difference of 1000 K in 2 cm is sufficient tocause a drop in the viscosity, thereby preventing partialor total resolidification of the sprayed particles priorimpact against the substrate. This effect can be morepronounced with the nanostructured powder after it

defragments within the plasma stream. Vardelle et al.w x w x12 and Pawloski 15 detail results of particle flatten-ing calculations as a function of the impact tempera-ture at the impact. It was found that the flatteningdegree is enhanced for higher impact temperatures.The dual effects of particle velocity and temperaturecan be linked and also show that the particle tempera-ture increases at the moment of impact due to thetransformation of the kinetic energy into heat andthereby contributing to a higher flattening degree.

It has been suggested that the microstructure ofplasma-sprayed ceramic coatings consists of regions ofperfect contact separated by thin regions of no contact.The regions of imperfect contact arise from gas en-trapped beneath spreading liquid droplets during coat-ing formation and is aided by factors such as lowviscosity and low velocity. These thin regions of poor

Žcontact are, physically, very narrow pores approx.. w x0.01]0.1 mm 9,19]21 with a real area of contact

w xsplat]splat is in fact approximately 20% 9 . The influ-ence of microstructural factors on the mechanicalproperties of coatings has led to the suggestion that themechanical behavior of a coating is limited by thedegree of contact between splats within the coating, orbetween the splat and substrate, rather than the nature

w xof the bond in regions of good contact 9,19]21 .The elastic modulus of coatings is much lower than

that of the bulk material. For example, the elasticmodulus of plasma sprayed alumina is approximately

w x20% that of a sintered ceramic 9,21 . This is a muchlarger reduction than can be explained on the basis ofrandomly distributed spherical pores but is consistentwith the concept of narrow planar pores between splats

w xwith a low ‘true contact’ area 9,19]21 .Large changes in the elastic modulus of plasma

sprayed ceramic coatings have also been observed afterw xheating 22 . This effect can be explained by changes in

the shape of the intersplat pores from an essentiallycontinuous thin gap, containing small regions of truecontact, to more rounded pores and a considerableincrease in the area of true contact. This could occurwith negligible change in the total porosity of the

w xcoating 9,21 . The interfaces between splats or betweensplats and the substrate must be regarded as the ‘weaklinks’ with respect to mechanical properties and, there-fore, improvement of the mechanical behavior of coat-ings must be aimed at enhancing interfacial bondingw x9 .

The mechanical properties of the coatings, will beruled by how effectively the sprayed particles can becompacted during spraying, among other influences.On the basis of this discussion and the experimentalresults with respect to roughness, it is apparent thatlow roughness coatings will exhibit splats that are well-packed compared to coatings with a high R . Hardnessais usually defined as resistance to penetration, defor-

( )R.S. Lima et al. r Surface and Coatings Technology 135 2001 166]172170

w xmation, scratching and erosion 23 , and can be con-sidered to reflect the splat-to-splat cohesion of thermalspray coatings; i.e. a higher hardness coating implies agreater degree of splat-to-splat cohesion.

Vickers microhardness measured on cross-sectionand top surface for 500 and 1000 g are shown in Figs. 1and 2, respectively. The values measured at top surfacehave slightly lower values than those measured atcross-section. Microhardness measurements in thermalspray coatings with respect to the planar or cross-sec-tional aspects of individual splats will correspond area

w xmeasurements on circles or ellipses 10 . These twotesting orientations would be reflective of anisotropicmodes of deformation, which do not produce identicalhardness values. The planar hardness values were nor-malized with respect to the cross-section hardness val-ues for indenting loads of 500 and 1000 g. These ratioswere ; 0.78 " 0.13 and demonstrate the relativeanisotropic degrees of splat packing with respect to thetwo testing directions.

Knoop microhardness values measured using 1000 gon the cross-sectional area are given in Fig. 3. TheKnoop values are lower than the Vickers values at thesame load. In addition, the variation in Knoop values ishigher than that in Vickers. Both phenomena areprobably related to the difference in geometry of thetwo indenters, which provide different force fields inthe coatings.

Observing Figs. 1]3, the following trend is noticed.Following the R axis from left to right, starting ataspray parameter 1 throughout 6, the torch power isdecreasing and the spray distance is increasing. Whenthe plasma torch power is decreased, particle tempera-ture and velocity will tend to drop, which will impede alarger spreading of the sprayed droplet, as discussedabove. Also, when the spray distance is increased, theparticle temperature and velocity will drop due to theirinteraction with the air. This also will impede a largerparticle spreading and its effects on mechanical proper-ties as already discussed.

When the plasma power decreases, the number ofnon-molten particles should increase. The presence ofnon-molten particles will also increase the roughness ofthe coating, and it will lower the values of hardness dueto low particle cohesion. This also should increase theporosity of the coatings. An increase in porosity willlower the coating stiffness, producing a decrease in thevalues of hardness.

3.3. Elastic modulus-roughness

References in literature propose and use indentationtechniques for measurement of elastic modulusw x10,24]26 . The elastic modulus of the coatings was

w xdetermined via Knoop microhardness tests 10,24 . The

elastic modulus is determined by measuring the majorŽand minor diagonals of a Knoop indenter 2a and 2b,

.respectively .The orientation of the indenter main diagonal was

parallel to deposition surface, i.e. the main diagonalwas at 908 with respect to the splashing direction of thedroplets. As the microstructure of thermal spray coat-ings is anisotropic, the mechanical properties will havedifferent values for different orientations, as experi-mentally observed in the preceding section for hard-ness. The elastic modulus determination technique de-

w xveloped by 24 is based on the measurement of elasticrecovery of the in-surface dimensions of Knoop inden-tations, i.e. the length of the major and minor diagonalsof the indentation after the unloading is measured. Theelastic recovery reduces the length of the minor diago-nal as well as the residual indentation depth, whereasthe length change in major diagonal is negligible. Asthe measurement of the elastic modulus is based on theminor diagonal, the elastic modulus values obtained inthis work represent the in-plane orientation of thecoating.

The advantage of this type of test is that the valuesof elastic modulus and Knoop microhardness can beobtained simultaneously and a small specimen can be

w xused for a large number of tests 10 . The Knoopindentation test enables elastic modulus values of ther-mal spray coatings to be obtained in a simple fashionand has a potential to be used as a quality-control tool

w xin industry and laboratories 10 . The values of elasticmodulus and their relationship with roughness areshown in Fig. 4.

Again a same trend is observed: the smoother thecoating, the higher the Knoop microhardness and elas-tic modulus. A dashed line was placed in Figs. 3 and 4as a ‘guideline for eye’ to show this trend. The elasticmodulus slightly changes from spray parameters 1 to 4,where the same plasma power was applied for differentH rAr ratios and a difference in spray distance of 22

cm. This experimental observation agrees with the elas-tic modulus measurements of PSZ coatings via four-

w xpoint bending method 27 , which the PSZ coatings didnot present a significant change in elastic modulusvalues varying the spray distance in 2 cm keeping thesame plasma power. But when the plasma power was

Ž .decreased spray parameters 5 and 6 , the drop in theelastic modulus values is clearly noticed.

Specifically for the elastic modulus, when the splatsŽ .are highly flattened low roughness they have a large

area of contact and as a consequence they have highcohesion. High splat cohesion will imply high stiffnesswhich is measured as elastic modulus. The oppositebehavior takes place for the splats which are not highflattened.

( )R.S. Lima et al. r Surface and Coatings Technology 135 2001 166]172 171

3.4. Other influence factors

This method used to evaluate the microhardness andelastic modulus by relating them with roughness for thenanostructured coatings may be also applied to othercoatings. Each group feedstock and thermal sprayprocess combination will demonstrate their own char-acteristics. Also factors such as substrate roughness,

w xpreheating and substrate temperature 28,29 , sprayw xangle 30 , bond coat and coating thickness may have

an influence on the final roughness. Therefore, therewould be the necessity to calibrate each system prior toemploying this simple method for evaluating mechani-cal properties. Modern surface profilomers are able todistinguish between roughness and waviness. Thus, evennon-flat surfaces can be tested and analyzed.

w xBianchi et al. 29 corroborates with way of thinkingdiscussed in this work. Preheating the substrate prior tospraying always induces higher values of coating adhe-sion regardless of particle size and plasma parametersw x29 . When the substrate is preheated to temperaturesgreater than 3008C before spraying, contact of splats tothe substrate or previously deposited layers is im-proved, probably due to increased wettability of the

w xceramic droplets 29 , which lead to better spreading.w xThe same phenomenon was observed for hardness 29 .

Also the influence of particle velocity on splat contactwith hot substrates was noted. A corresponding im-provement of coating adhesion on preheated substratesalso occurs when sprayed with a mean particle velocityof approximately 250 mrs compared to a velocity of

w x130 mrs 29 . Again, the same phenomenon wasw xobserved for hardness 29 .

Beyond splat]splat cohesion, microcracking can alsoplay a role in the elastic modulus of zirconia APScoatings. Microcracking changes the load vs. displace-ment curves during four-point bend tests of TBCsw x27,31]33 . These results were also studied with acous-

w xtic emission 31]33 . Extensive microcracking along thecoating changes the behavior of stress vs. displacementcurves, lowering the stiffness of the coating, decreasingthe value of the yielding load of the system substrate qTBC.

w xA new reference 34 characterized the microstruc-ture of APS PSZ coatings by the measurement ofsurface roughness and Vickers microhardness. The au-thors observed that the Vickers hardness decreases

w xwith an increase in roughness. Yasuda et al. 34 statethat the surface roughness of the APS PSZ coatingsreflected the state of piling up of the melted powder

w xparticles. Yasuda et al. 34 also discuss the possibilitythat the greater the surface roughness of the zirconiacoatings, the lower the elastic modulus; a factor whichwas proven during this work.

4. Conclusions

The experimental observations of this work concludethe following:

v The microhardness]roughness relationship has thefollowing trend; smoother the coating, higher themicrohardness; in all cases.

v The elastic modulus]roughness relationship has thefollowing trend; smoother the coating, higher theelastic modulus; in all cases.

v When the roughness is smooth, the splats present ahigh degree of flattening, creating more points ofcontact or anchors between splats at a microscopiclevel, increasing the cohesion of the coating. Theenhancement of cohesion will increase the mi-crohardness and the elastic modulus of the coating.

v A surface profilometer can be used to evaluate themicrohardness and the elastic modulus of a coatingin situ without destroying it by simple comparisonbased on a standard sample.

v The ratio between top-surface and cross-section ofŽthe Vickers microhardness at both loads 500 or

.1000 g is 0.78"0.13.

Acknowledgements

This work was supported by the US Navy-Office ofNaval Research under grant number N00014-97-0843and the Center of Thermal Spray Research underNSF-MRSEC DMR grant number 9632570.

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