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Surface & Coatings Technology

Characterization of mechanical properties of suspension plasma sprayed TiO2

coatings using scratch test

Roman Jaworski a, Lech Pawlowski a,⁎, Francine Roudet b, Stefan Kozerski c, Fabrice Petit d

a Service of Thermal Spraying at Ecole Nationale Supérieure de Chimie de Lille, B.P. 90108, 8, av. Mendeleïev, 59650 Villeneuve d'Ascq, Franceb Laboratory of Mechanics Lille, URM CNRS 8107, IUT A, BP 179, rue de la Recherche, 59653 Villeneuve d'Ascq, France

c Faculty of Mechanics, Wroclaw University of Technology, ul. Lukasiewicza 5, 50-371 Wroclaw, Polandd Belgian Ceramic Research Center, 4, av. Gouverneur Cornez, 7000 Mons, Belgium

Received 6 July 2007; accepted in revised form 24 September 2007Available online 6 October 2007

Abstract

The study aimed at characterizing mechanical properties of TiO2 coatings obtained by the use of aqueous suspensions of fine rutile and anataseparticles onto metal substrates. Thickness of the coatings was found with the use of optical microscope observations of metallographical cross-sections. The coatings morphology was found with scanning electron microscope (SEM) and their phase composition was determined by X-raydiffraction method. The mechanical properties were characterized using scratch test. The test enabled to characterize the adhesion of coatings bydetermination of critical force necessary to peel off the coatings from the substrate and, on the other hand, to estimate their cohesion by themeasurement of the scratch hardness. The hardness was calculated using the width of the scratch and the value of force applied. The design ofexperiments (DOE) of spraying with the use of a 23 full factorial plan was applied to rutile powder. The coatings were sprayed onto aluminiumsubstrate. The DOE enabled to find the effects of three principal parameters, i.e. electric power input to plasma, spray distance and suspension feedrate onto coating adhesion expressed by critical force. A preliminary data for anatase powder sprayed with one set of operational parameters ontostainless steel substrate are also shown.© 2007 Elsevier B.V. All rights reserved.

Keywords: Suspension plasma spraying; Titania coatings; Scratch test

1. Introduction

Titanium oxide films and coatings have been studied recentlybecause of their interesting physical, chemical and electricalproperties.Application of coatings as hydrogen and oxygen sensors[1,2], self-cleaning and photocatalytic surfaces, useful in degradingorganic pollutants [3] but also electron emitters for light emittingdevices [4,5] have been considered. The TiO2 coatings have beensynthesized by many methods including physical and chemicalvapour deposition, sol–gel and more recently by electrophoreticdeposition [6]. Thermal spraying techniques have been frequentlyused to obtain thick titania coatings sprayed using coarse powderparticles reviewed recently by Berger [7]. Thin layers (10–30 μm)have been synthesized by injection water suspension of nanometricand submicrometer titania powder particles into high velocity

⁎ Corresponding author. Tel./fax: +33 320 33 61 65.E-mail address: lech.pawlowski@ensc-lille.fr (L. Pawlowski).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.09.044

combustion flame [8] or plasma jet [9–11]. The obtained depositshad mainly been characterized for their microstructure, namely:

• morphology using scanning electron and optical microscopes;• chemical composition using X-ray photoelectron spectroscopy;• phase composition using X-ray diffraction method andRaman microscopy.

Mechanical properties and especially, adhesion of suspen-sion thermally sprayed layers, have not yet been characterized.The main difficulty results from the thickness of deposits that isintermediate between thin films and thick coatings. The tensiletest, frequently used in thermal spraying and normalized by thenorm ASTM 633-01, needs 380 μm thick coatings. Otherpossibilities offer, reviewed recently [12], tests based onfracture mechanics, such as three and/or four points bendingtest, indentation test and, on the other hand, scratch test. Thelatter has been used to assess the adhesion of thin hard coatings

Table 1Boundaries of the experimental space including low and high level values ofvariables

Variable Spray process parameter Low level Central point High level

Xi=−1 Xi=0 Xi=+1

X1 Suspension feed rate, ml/min 20 30 40X2 Spray distance, cm 8 10 12X3 Power input to plasma, kW 38 39 40

Table 3Principal parameters of scratch test

Parameter Value

Initial load, N 0.05Final load, N 30Rate of load increase, N/min 15Speed of indenter, mm/min 5.22Scratch length, mm 10

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for some time. In the test a diamond stylus is drawn across thecoated surface under an increasing load until a defined failureoccurs under a critical load, Lc. The failures may include [13]:

• coating delamination;• through-thickness cracking and plastic deformation;• cracking in the coating or substrate.

The mechanism of scratch is suggested to correlate well withdamage mechanism occurring at abrasive wear [14]. Anappropriate interpretation of load vs. acoustic activity at thetest can be also useful to estimate the thin coating toughness[15]. In the present paper the scratch test is used to estimate theadhesion of the coatings sprayed using suspension withdifferent spray parameters using a 23 full factorial plan enablingstatistical treatment of results. Moreover, the scratch hardness iscalculated and related to the coating cohesion. Scratch hardnessnumbers are also considered as providing an appropriatemeasure of the damage resistance of a material to surfacedamage processes like two-body abrasion [16].

2. Experimental

2.1. Suspensions preparation

In the experiments, suspensions were formulated with theuse of rutile TiO2 (Tioxide R-TC90 of Huntsman Tioxide, USA)and of 99.8% anatase (Aldrich) in the following proportions:360 g of distilled water +40 g of rutile or anatase +0.12 g ofdispersant. The dispersant was an aqueous solution of sodiumpolyacrylate (Hydropalat N, Congis). The size distribution ofrutile powder used in the suspension formulation is shownelsewhere [10]. The average diameter (volume–surface mean

Table 2Matrix 23 of suspension plasma spraying experiments (samples T1-T8) with oneexperiment in the centre (sample T9)

Experiment no. X1 X2 X3

T1 −1 −1 −1T2 1 −1 −1T3 −1 1 −1T4 1 1 −1T5 −1 −1 1T6 1 −1 1T7 −1 1 1T8 1 1 1T9 0 0 0

defined elsewhere [12]) equals to dVS=0.33 μm. Thecharacteristics of anatase powder are shown in the Resultssection. The suspension was atomized and injected into plasmajet from the distance of 15 mm of jet axis. The injector was inthe distance of 13 mm from the torch. The atomizing gas was airunder pressure of 0.6 bar. More details about suspensioninjection are shown elsewhere [17].

2.2. Plasma spraying

Aluminum plates of size 15×15×3 mm were used assubstrates to spray rutile and stainless steel substrates of size20×50×0.55 mm to spray anatase. They were plasma sprayedusing Praxair SG-100 torch with anode 03083-175 and cathode03083-129 mounted on 5-axis ABB IRB-6 industrial robot.Throughout all the experiments the following parameters werekept constant:

• composition of plasma working gas: Ar+H2;• flow rates of plasma working gas: qAr=45 slpm (for sprayrutile); qAr=49 slpm (for spray anatase) and qH2=5 slpm;

• linear torch velocity: v=500 mm/s;• trajectory of torch passes over substrates is shown elsewhere[17];

• total number of passes was 50 (5 cycles of 10 passesincluding 1 min of cooling down).

The anatase was sprayed using electric power of P=40 kWand spray distance of D=10 cm. The variable spray parametersused to spray rutile, called experimental domain, are collected inTable 1. The full matrix of 23 experiments was realized forsamples sprayed using water suspension and two experimentswere made in the center. All experiments done are summarizedin Table 2. The experiment T1 was repeated two times.

2.3. Powders and coatings characterization

The distribution of powder particles sizes were analyzed withMastersizer laser setup of Malvern. Morphology of powders,splats and coatings surfaces and cross-sections was investigatedusing JEOL-JSM 6480L microscope, equipped with a detectorof secondary electrons and an energy dispersive spectrometer(EDS). XRD analysis of powders and coatings was carried outusing Bruker diffractometer of type D8 using CuKα radiation.The phases were identified with use of Diffracplus EVA softwarewhich uses database of International Centre of Diffraction DataJCPDS-ICDD. Raman spectroscopic investigations were

Fig. 1. Typical scratch curve obtained for the sample T1 with the critical load of Lc=11.2 N.

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carried out using Dilor-Jobin Yvon-Spex spectrometer of typeLabRam, which uses HeNe laser (632.8 nm, 8 mW) asexcitation source. The scratch test was realized with a Micro-

Fig. 2. X-ray diffraction diagrams of fine powders used to formulate suspensionwith: rutile (a) and anatase (b).

CombiTester of CSM Instruments equipped with a Rockwelldiamond indenter having a tip radius of 0.2 mm. The scratcheswere linear with progressively increasing load. The experimen-tal conditions are collected in Table 3. As the coatings are veryporous, the acoustic signal could not be used to the estimation ofcritical load, Lc. Instead, the following indications enabled theload to be found:

• the curve of penetration was showing a change of slope at thecontact of the indenter with the coating/substrate interface(see Fig. 1);

• the optical microscope observations of the interface.

In present work, the critical load was defined at the onset ofthe coating loss associated with a beginning of visibility ofmetallic substrate inside the scratch channel. This measurementwas made with a help of optical microscope. The tester enabledalso to find the friction coefficient at the critical load. Thescratch hardness HSL (Pa) was estimated following thespecification of ASTM G171-03 norm:

HSL ¼ 8d Lpd d2

ð1Þ

where L (N) is the applied normal force (the load of 30 N wasselected) and d (m) is the corresponding scratch width. Thescratch hardness is an indicator of coating cohesion.

2.4. Crystallographic analyses

The content of rutile and anatase in the sprayed coating aswell as the size the anatase crystals in the coatings wereestimated by the method described elsewhere [17].

Fig. 3. Size distribution of fine powders: rutile (a) and anatase (b).

Fig. 4. SEM (secondary electrons) of fine powders: rutile (a) and anatase (b).

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2.5. Statistical treatment of data

The effect of operational spray parameters on the followingmechanical properties of coatings sprayed using rutile finepowder was estimated:

• critical load, Y1;• scratch microhardness, Y2;

The effects were calculated following to the model valid inthe experimental space1:

Ym ¼ b0 þX

biXi þX

bijXiXj þ b123X1X2X3 ð2Þwhere i, j are equal to 1, 2 and m can be equal to 1 or 2. Thecoefficient b0 is the average of responses of all experiments; bicoefficient represents the effect of the variable Xi, and bij andb123 are the coefficients which represent the effects ofinteractions of variables XiXj and X1X2X3 respectively. Experi-ments were carried out by considering a 23 full factorial design

1 Defined as −1≤Xk≤+1, k=1 to 3, variables are normalized with regard tospray parameters in such a way that Xi=−1 corresponds to the lower limit ofvariable, Xi=0 corresponds to the centre of experimental domain and Xi=1corresponds to higher limit of the variable.

with two levels and three factors, i.e. a total of 8 experiments(Table 2). The advantage of this type of experimental design isthat the structure is perfect for estimating the model (Eq. (2)).The analyzes were made using Nemrod software [18]. Theyaimed at determining significant (active) effects influencingquantitative properties of sprayed deposits. The method ofstatistical analyzes was a creation of the regression equation

Fig. 5. Raman spectrum of anatase powder.

Fig. 6. X-ray diffraction diagram of coatings suspension plasma sprayed usinganatase fine powder.

Fig. 8. SEM (secondary electrons) of the cross-section of the coating suspensionplasma sprayed using rutile.

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describing the property and an analysis of the coefficients of thisequation. Initially, all effects and their first and second orderinteraction (as shown in Eq. (2)) were considered. Then themost active effects and their interactions were selected byapplication of the Student criterion. The selected coefficientsshould be greater than the incertitude calculated usingreproducibility variance and Student criterion, taken at 95%confidence level and for a degree of freedom that depends, inturn, on number of experiments, number of their repetition, and

Fig. 7. SEM (secondary electrons) of coatings surface suspension plasmasprayed using: rutile with parameters T8 (a) and anatase (b).

on the number of regression equation coefficients as shown byLazić [19]. Later on, the analysis was repeated by taking onlythese effects in the regression equation into account (andnarrowing the incertitude in coefficients determination). Finally,the probability that the effect is active was calculated with theuse of Bayes approach and the final statistical model of theanalyzed property was established.

3. Results

3.1. Microstructural characterization of powders

The X-ray diffraction diagrams of initial powders used toformulate suspension are shown in Fig. 2. The present phases arerutile in powder of Huntsman and anatase in Aldrich powder.The sizes distributions of the powders are shown in Fig. 3. Therutile powder distribution has a maximum at about 0.4 μm andthe one of anatase has two maxima: at 0.2 and 0.9 μm. Thepowder morphology is shown in Fig. 4. The rutile particles are

Table 4Results of mechanical properties of coatings plasma sprayed using suspension ofrutile (experiments T1-T9) and of anatase

Experiment no Lc, N Frictioncoefficient a

HSLc

Meanvalue

Standarddeviation

Meanvalue

Standarddeviation

Meanvalue

Standarddeviation

T1 10.68 0.47 0.29 0.04 0.64 0.10First repetition ofT1

12.99 0.66 0.27 0.00 0.73 0.06

Second repetitionof T1

13.93 1.31 0.29 0.06 0.76 0.05

T2 13.10 1.23 0.37 0.05 0.59 0.02T3 10.15 1.96 0.27 0.04 0.60 0.07T4 12.64 0.34 0.31 0.02 0.58 0.03T5 14.49 1.35 0.34 0.06 0.66 0.06T6 16.56 2.10 0.38 0.04 0.65 0.04T7 10.55 0.85 0.27 0.05 0.69 0.04T8 11.08 0.09 0.27 0.01 0.68 0.05T9 11.54 1.01 0.27 0.04 0.71 0.04Anatase N30 – 0.11 0.01 3.6 b 0.03

a At critical load.b At 30 N.

Fig. 9. Optical micrograph of the scratch in the coating suspension plasma sprayed using anatase at the load of 30 N.

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rounded and homogenous and the anatase one are irregular. TheRaman spectrum of rutile powder described elsewhere [10]shows only peaks corresponding to this phase. The spectrum ofanatase powder, shown in Fig. 5 shows the anatase peaks at 397,516 and 640 cm−1. A small intensity peak at 830 cm−1 indicatestraces of rutile.

3.2. Microstructural characterization of coatings

The X-ray diffraction diagram of coating sprayed usingsuspension formulated with anatase fine powder is shown inFig. 6. The content of anatase in the coatings was estimated on44 wt. % and the size of anatase crystals is about 90 nm. Themorphologies of coatings’ surfaces are different. The coatingssprayed using rutile have some agglomerates having typical sizeof a few tenths of micrometers (circle in Fig. 7a) and smallspherical particles which solidified before impact with previouslydeposited coating (arrow in Fig. 7a). The surface of anatasesprayed coating indicates that the particles are better connectedtogether and the coating seems to be slightly denser. The cross-section of rutile sprayed coating confirms important porosity of

Fig. 10. Optical micrograph of the scratch in the coating suspension plasma

the rutile sprayed deposits (Fig. 8). The thicknesses of coatingssprayed using rutile formulated suspension were estimated, usingobservations of metallographic cross-sections with the use ofoptical microscope and from the analysis of the scratch test data,to be in the range 8–33 μm. The thickness of anatase coatings,estimated from the scratch test data, is about 5–7 μm.

3.3. Analysis of scratch test results

The results of all mechanical parameters are collected inTable 4. A typical scratch curve is shown in Fig. 1. For the anataseformulated suspension sprayed coating, the critical load wasgreater thanLcN30N (30N is the greatest load that can be appliedby used scratch tester). Cracks running perpendicular to thescratch edges were observed (Fig. 9) and the scratch remainedinside the coating. By their aspect, those cracks are similar totensile cracks which form in the wake of the moving indenter induplex coatings [20]. Obviously, the adhesion between the steelsubstrate and the anatase formulated suspension sprayed coatingis good. On the contrary, the critical loadwas lower than Lcb30Nin rutile formulated suspension coatings sprayed onto aluminium

sprayed using rutile and processing parameters T1 at the load of 30 N.

Fig. 11. Optical microscope views of the scratches in coatings suspensionplasma sprayed using: rutile with parameters T6 (a) and anatase (b).

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substrates. The typical aspect of the inside of a scratch is shown onFig. 10 (at the load of 30 N). Ductile deformation of the metallicsubstrate is revealed through the existence of many longitudinalmicro-scratcheswhichwere absent in the anatase sprayed coating.Despite scattering, it reveals that the critical loads are alwayscomprised between Lc=10 N and 15 N only. These low valuesdemonstrate relatively poor adhesion of these coatings to thealuminium substrate. The results of critical load and scratchhardness for coatings sprayed using rutile were submitted to astatistical analysis. The scratch hardness of anatase suspensionsprayed coating is significantly greater than that of rutiles ones. Infact, the width of scratch in anatase coating is much narrower thanthat in rutile ones (Fig. 11). Finally, the friction coefficientdependence on processing spray parameters is hard to interpretthat is why the statistical treatment was not carried out. Again, forthis property the coefficient for sprayed anatase was clearly lowerthan that for rutile.

3.4. Statistical analysis of critical load

The initial analysis shows that the coefficients b2, b3 and b12are greater than 95% significance level (Fig. 12a). The refinedanalysis with the calculation of the effects of only three

principal variables and to one interaction, i.e. X1X2 confirms theinitial analysis (Fig. 12b) but the probabilities that b3 and b12interaction are smaller than 50% (Fig. 12c). Consequently, thefinal regression equation has the following form:

Y1 ¼ 12:5� 1:5X2: ð3Þ

3.5. Statistical analysis of scratch hardness

The initial analysis reveals that the effect of electric power b1is greater that 95% significance level and the effect ofsuspension feed rate b3 is close to the level (Fig. 13a). Therefined analysis with the calculation of three principal effectsshowed that only the effect b3 is greater than significance level(Fig. 13b). The probability that this effect is active is lower than50% (Fig. 13c). Consequently, it is probable that no significanteffects of selected variables influence scratch hardness.

4. Discussion

4.1. Injection of suspension into plasma jet

The droplets of atomized liquid are injected into plasma jet.Knowing, that the size of suspension delivery outlet is about0.7 mm and that the liquid is subsequently atomized, the sizes ofdroplets must have diameter ranging from a few tens to a fewhundreds of micrometers. The injection of suspension intoplasma is a crucial problem. In fact, smaller droplets maypossibly remain on the periphery of the plasma jet. The solidparticles contained in such droplets remain most probably solidin-flight. On the impact with substrate or previously depositedcoating, they rebound without building up the coating or getincluded weakly bound to it. Consequently, such small dropletswould weaken cohesion of coatings and their hardness. Anumber of solid particles inside a droplet can be estimated on105 to 106 [17]. The size distribution of the particles may play arole. In fact, a particle size distribution with a greater maximumacts in favour of obtaining of denser coatings [21]. As anatasepowder has the distribution shifted in the direction of greatersizes (Fig. 3b), the mentioned effect may be correlated with itshigh scratch hardness comparing to that of rutile (Table 4 andFig. 11b). A part of droplets is injected in the plasma jest. Waterstarts to boil and gets evaporated. The solid particles inside anydroplet get closer and may start to agglomerate by sintering in-flight. The process can be compared to that of, well known inchemical technology, the spray-drying one. The plasma getscolder because of evaporation of water. As the atmospherearound solid precursors contains a lot of vapours of water thesolidification of precursors in-flight is probable (see Fig. 7a —arrow). The fine particles may get agglomerated by sinteringwith other particles and/or evaporated. The agglomerationprogresses until an impact with the substrate. This can beconfirmed by aspect of surface of the coatings sprayed usinggreater spray distance, which has slightly more agglomeratesvisible (Fig. 7a — circle). The phase transformations of TiO2

sprayed as rutile are discussed extensively elsewhere [17].

Fig. 12. Statistical analysis of active effects influencing critical force at scratch test of coatings suspension plasma sprayed using rutile: regression equation coefficientsfor a model including all effects and their interaction of second and third order (a); refined regression equation coefficients for most active effects (b); determination ofthe probability that the effect in the refined regression equation is active (c).

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Fig. 13. Statistical analysis of active effects influencing scratch hardness of coatings suspension plasma sprayed using rutile: regression equation coefficients for amodel including all effects and their interaction of second and third order (a); refined regression equation coefficients for most active effects (b); determination of theprobability that the effect in the refined regression equation is active (c).

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4.2. Effect of spray distance mechanical properties

Adhesion of rutile formulated suspension plasma sprayedcoatings depends clearly on processing distance. Shorter distancepromotes adhesion. This can be explained by a greater number ofmolten particles impacting the substrate at shorter distances. Thisfinding coincides well with the recommendation of Fauchais et al.[21]. The molten particles can adapt themselves to the substrateirregularities promoting better contact, which is a prerequisite forhigh adhesion. Also, the scratch hardness of anatase coating issignificantly greater than that of rutile ones what results from anarrower scratch track width (Fig. 11b). The greater hardness ofsprayed anatase can be associated with stronger cohesion strengthof the coating comparedwith the rutile ones. Low scratch hardnessof rutile formulated suspension sprayed coatings is very smallwhat must result from their very porous microstructure. Theanatase sprayed coating is considerably harder what results fromtheir denser microstructure (Fig. 7b). The results of adhesion foranatase and rutile sprayed coating are difficult to compare becauseof different substrate these coatings were deposited on (aluminiumfor rutile coatings and stainless steel for anatase one). Moreover,the deposition temperature was not controlled and it can beexpected that the temperature vary with operational sprayparameters. Since the friction coefficient dependence on proces-sing spray parameters reveals hard to interpret, the statisticaltreatment of this quantity was not carried out. Nevertheless, it isnoteworthy that the average friction coefficient for anatase is lowerthan that of rutile. Although surprising, this result can be related tothe lower penetration depth of the indenter in the anatase coating(5–7 μm against 30 μm to 70 μm in rutile coatings at 30 N).

5. Conclusions

A statistical study was carried out to find out the influence ofoperational spray parameters of TiO2 on critical load at scratchtest and scratch hardness. The spray distance has the strongestinfluence on the load. Suspension feed rate does not seem to playany significant role in influencing these properties of TiO2

coatings. Scratch hardness of rutile sprayed coatings was verysmall what should have resulted from very porous microstructureof coatings. The important porosity can be an advantage for suchapplication of the coatings as photo-catalyse. In order to decreaseporosity, the key problem is an injection of suspension to plasmajet. The atomization of suspension could have resulted in manysmall droplets remaining on the periphery of the plasma jetwithout being molten and contributing in the coatings build up.The future experiments will be carried out without atomizer by

injecting suspension directly into plasma jet. On the other hand,the control of the deposition temperature will be added to theprocess.

Acknowledgments

Prof. Elisabeth Bordes-Richard contributed in the paper withvery useful discussions. Ms. Hakima Zarriou helped in studieson anatase powder and coatings. Ms Ophélie Meunier andMarie Aguila helped in X-ray investigations of coatings.

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