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Splattaxonomyofpolymericthermalspraycoating

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Surface & Coatings Technology 205 (2011) 5028–5034

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Splat taxonomy of polymeric thermal spray coating

Kadhim Alamara ⁎, Saeed Saber-Samandari, Christopher C. BerndtIRIS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC 3122, Australia

⁎ Corresponding author. Tel.: +61 3 9214 4336; fax:E-mail address: kalamara@swin.edu.au (K. Alamara)

0257-8972/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.surfcoat.2011.05.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 January 2011Accepted in revised form 5 May 2011Available online 12 May 2011

Keywords:Thermal spraySplat metricsPolymerStand-off distance (aka ‘SOD”)Splash

The geometry of a powder particle and the splat that it gives rise to during the thermal spray process are keyparameters that determine the physical attributes of a coating. The splat shape is an important feature thatrequires accurate characterization, preferably by means of a quantified metric, so that the rapid solidificationprocess of thermal spray deposition can be understood.In this work we report on a polypropylene (PP) powder that was sprayed onto a glass substrate at roomtemperature using the flame spray process, at various stand-off distances (SODs). Several statistical conceptswere employed using image analysis techniques that were complemented by optical microscopy, scanningelectron microscopy, and 2D profilometry: methods that measured the splat metrics of formation, includingan estimate of splash area. Measurement of equivalent diameter, degree of splashing, spreading factor,deposition efficiency and circularity have quantified splats so that thermal spray parameterization can berelated to the process efficiency.The results of this study indicated that increasing the SOD from 10 cm to 25 cm reduced the splat thicknessand deposition efficiency; while other metrics (as listed earlier) were not routinely correlated, though anincrease in splash area was demonstrated. Splat circularity was steady around 0.9, indicating that thedeposited particles exhibited a circular shape at all SODs.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Deposited particles are the building blocks of coatings and the firstdeposited layer plays a significant role in the coating/substrateadhesion. A deposited particle is often referred to as a ‘splat’; asomewhat generic term that encompasses distinct morphologies. Thesplat formation and flattening that occurs when a sprayed dropletimpacts onto a substrate is a key process during the production ofthermal spray coatings. Splat formation depends strongly on thetemperature and velocity of the impacting particles [1]. Substrateparameters such as its temperature and roughness, among otherrelevant parameters, and their interfacial interactions at impact, willalso influence splat formation.

The splat morphology is a reflection of the degree of polymerparticle melting achieved during processing of the feedstock. Polymersplat morphologies may be classified as (i) disc-like (also described asa pancake), (ii) shaped in the form of a ‘fried egg’ that is raised in thecentre, (iii) exhibiting a reasonably coherent but highly deformedsplash pattern, (iv) a fragmented and quite discontinuous splat, or(v) a fingered morphology that has also been described as flower-shaped [2–4]. Coatings formed of disc or fried egg splats tend toexhibit high adhesion and cohesion and low coating porosity, while

coatings composed of splashed splats demonstrate poor adhesion andcohesion with high porosity [2]. The occurrence of splashing duringpolymeric droplet deposition is, therefore, an undesired outcome forthe coating. Splat solidification has a significant influence on splashbehaviour [5], with splashing observed during rapid formation of theinitial solidified layer just after impingement on the substrate [6].Thus, the liquid portion of the impacting polymer particle is notrestrained by surface tension forces. Inversely, at higher substratetemperatures the particle remains liquid at the bottom and this allowsformation of a pancake shape [7].

In a thermal spray process, splashing can occur (i) upon impact(so-called impact splashing) that takes place immediately at thebeginning of impact and (ii) during spreading (so-called spreadingsplashing) that occurs at the end of the flattening process and the startof solidification [8]. Both splashing modes are related to theSommerfeld parameter (K) of the particle at impact; which is definedas follows [9]:

K =ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiWe

ffiffiffiffiffiffiRe

pqð1Þ

where, We and Re are the dimensionless Weber and Reynoldsnumbers respectively. If Kb3 then particle rebound occurs. A value of3bKb58 results in deposition and KN58 induces splashing.

The splashing phenomena can be minimized if the droplet doesnot commence solidification until the completion of spreading [5]. The

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necessary condition is achieved by (i) raising the deposition surfacetemperature, (ii) increasing the thermal contact resistance at thedroplet-substrate interface, or (iii) using a substrate with lowerthermal diffusivity [10].

Particle and splat size measurements can be employed to optimizethe thermal spray process parameterization. Computer imagingtechniques present an accurate method for measuring the particlesize and provide an opportunity for gathering metrics on the particleshape. Measurements of shape variations are necessary whenparticles are small and densely packed. Image analysis generatesstatistically relevant data with no subjective bias: that is, data isgenerated from each individual particle, the individual microstruc-tural constituents of a coating, rather than the coating as a whole.

The behaviour of individual molten droplets during the depositionprocess determines the overall quality of thermal spray coatings [11].The controls of the numerous parameters that govern splat formationare typically related to the impact, spreading, and solidification ofdroplets directed at the substrate or previously deposited layers [11].The two extreme cases of thermal spray splat morphology are(i) extensively splashed and highly fragmented or (ii) disc-like splats;however, there are other splat shapes that lie between these extremes.

The majority of splats involve some degree of splashing; i.e. theyare not disc-like. The degree of splashing (DS) is a splat metric thatneeds to be quantified ideally as a volume fraction; however is notwell documented in the field of thermal spray technology andparticularly in experimental studies with polymers. Although themajority of particles splash during coating formation, the quantifica-tion of the DS has remained elusive despite recent advancedsimulation models of the coating formation process [12].

Spreading of a droplet commences when the droplet touches thesubstrate and continues until it is fully deformed into a staticmicrostructural feature. Solidification of the lower part of the splatduringflattening leads to a decrease in the splat thickness. Reducing thespraying angle and contact wetting angle also yields a decreased splatthickness [13]. As impact velocity increases, droplet spreading time andsplat thickness both decrease [14]. It has been observed that a highersolid volume fraction leads to an increase in the splat thickness [15].

Several hypotheses and models have been proposed concerningthe splash event [16–19]. For a substrate held below the transitiontemperature; solidification starts at the contact points of the interface,and for a preheated substrate, the solidification occurs once the whole

Fig. 1. Fish net showing the thermal spray p

flattening process is complete. It was suggested that rapid solidifica-tion on cold substrates and splashing produced by the Rayleigh–Taylor instability causes the splat periphery to become unstable andthe splash morphology forms [20,21]. The Rayleigh–Taylor instabilityis the theory developed to predict the number of fingers around theperiphery of a splash. It states that the more rapidly a droplet flattens,then the more complex its shape such that a splash occurs.

Simulation studies proposed that the splashing of molten dropletsoccurs due to local solidification [22,23]. Other authors [9,24]suggested that the liquid particle flattens to a maximum area dictatedby its initial kinetic energy until this is completely dissipated, at whichpoint the surface tension causes surface shrinkage and jetting out ofliquid from the upper part.

The quality and the behaviour of the splat is a function of theparticle impact pressure at the splat–substrate interface. Thesethermo-physical conditions vary non-uniformly along the contactsurface during impact. The contact quality also depends on the dropletwetting on the substrate and desorption of adsorbates and conden-sates at the surface of the underlying layer [25]. These mechanismshave been observed to occur on polished substrates where theaverage surface roughness is several orders of magnitude smaller thanthe splat height.

The present work consolidates information related to splat geometryfor a specific polymer feedstock. Several geometric quantities wereinvestigated with the aid of imaging software to determine the splashbehaviour of polymer particles sprayed at different stand-off distances(SODs). Splat profiles were evaluated to determine the area of splashingfraction for different SODs and establish a relationship between the SODand the splash occurrence. Statistical tools were implemented to studythe effect of the SOD on the geometric characteristics of polymer particlesafter their impingement on a glass substrate. Other parameters thatimpact splat geometry are shown in Fig. 1.

2. Experimental procedures

2.1. Sample preparation

The feedstock material used in this study was the commerciallyavailable Moplen EP203N, PP manufactured by Basell Australia Pty. Ltd.The as-received feedstock material was in the form of millimeter sizedgranules. Thismaterial was further ground into powder that exhibited a

arameters involved in splat formation.

Table 1Flame spraying parameters used in this study.

Torch Sulzer Metco (thermospray) torch 6P-IIStand-off distance, (cm) 10, 15, 20 and 25Oxygen pressure, (KPa) 200Acetylene pressure, (KPa) 103Spray angle, (deg.) 90

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particle diameter range of 70–120 μm. The spray distance wasoptimized by performing experiments at four stand-off distances of10, 15, 20 and 25 cm. Note that changing the spray distance can alsoalter the thermal flux transferred to the substrate. The PP splat sampleswere produced using a Sulzer Metco, 6P-II flame spray torch with anacetylene/oxygen gasmixture and air as the carrier gas under the sprayparameters shown in Table 1. An external powder feeder with aninternal diameter of 2 mm was used to avoid injecting feedstock intothe high core temperature of the torch and, thereby, to limit feedstockdegradation. PP powder was fed normal to the direction of the flame.Standard microscope glass slides were held at room temperature(~23 °C) and used as substrates. Substrates were cleaned with acetoneand dried before spraying. Samples were collected from the completeparticle flow profiles and are detailed elsewhere [26].

2.2. Image analysis

The morphology and size of PP powder particles were charac-terised using a ZEISS Supra 40 VP field emission Scanning ElectronMicroscope (SEM) and optical microscopy. Different shape factorswere assessed. A measure of the data variability is represented by thestandard deviation of the mean that is also included on the graphs.

A two-step technique was employed to evaluate the splat shape.Using imaging analysis software, the splat periphery was manuallytraced and an enclosed shape inside the splat, without the splashes,measured as shown in Fig. 2a. The second step was to measure theperipheral projection of all splat material around the core of theflattened particle and hence obtain the area of the splat that included

Fig. 2. SEM images of the same splat showing the two different measurementtechniques using image analysis: a, measuring splat particle size without splashes andb, measuring actual splat size with splashes.

splashed material, Fig. 2b. Equivalent diameter (ED) is used to defineparticle size and offers a practical technique for measuring the size ofirregular splat shapes. It is defined as the diameter of a circle with thesame area as the selected feature and can be calculated aftermeasuring the area of a 2D image of the particle shape.

Degree of splashing (DS) is defined as the peripheral projection ofmaterial at impact. Spread factor (SF), is defined as the ratio ofmaximum spreading diameter to the initial droplet diameter. Thesplat areas, circularity and the length of the periphery of all splatswere determined using the software imaging technique. The ED wasthen calculated using the circularity approximation relation of Eq. (2).

ED = 2

ffiffiffiAπ

rð2Þ

The splat DS was calculated according to the method defined byMontavon et al. [27] by applying Eq. (3), and the circularity wascalculated using Eq. (4).

DS =P2

4πAð3Þ

Circularrity =4πAp2

ð4Þ

where A is the area of the splat (μm2) and P is the splat perimeter (μm).One hundred splats were analysed in this study for each processingparameter. Fig. 3 is a schematic of these size and shape factors.

Fig. 3. Schematic of particle and splat shape factors.

Fig. 4. SEM image of polypropylene showing the particle size used in this study.

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3. Results and discussion

3.1. Particle analysis

The particle size distribution and particle morphology are relatedto the characteristics of the deposit. It is expected that a larger particlesize will give rise to larger splats and splashes. However, if the velocityand temperature are high then this effect may be disguised due tosevere fragmentation on impact. The particle morphology willinfluence its flow within the flame spray process envelope. Thus, aspherical particle is more likely to exhibit ideal behaviour where theflow path is reasonably linear. On the other hand granular and angularfeedstocks will tend to tumble and twist in-flight; thereby giving riseto non-linear flow and probably a slightly larger divergent sprayangle. The heat transfer to non-symmetrical particles will also be non-uniform compared to spherical feedstock and the outcome would berevealed as angular features within the coating.

The characteristics of the feedstock evolve from their manufactur-ing process. Grinding processes, as in the case of this current work,will lead to rough, angular morphologies that are likely to lead tomore variability in the microstructures of the deposit. The majorattribute of such feedstocks is their availability and more economicalcost. In many instances such feedstocks are quite acceptable forthermal spray processes. The PP feedstock after grinding is shown in

Fig. 5. Particle size distribution of polypropylene feedstock.

Fig. 6. SEM images of PP splats deposited onto a glass slide, with SOD

Fig. 4, which depicts an irregular and angular morphology with smallflakes adhering to the larger particles. The larger particles manifestedthemselves by impeding the flow characteristics into the flame. Theparticle size distribution of PP powder, Fig. 5, was determined bycounting and measuring particles under microscopy.

3.2. Splat morphology

Fig. 6 demonstrates the different levels of splashing seen in thesplat formations of PP sprayed on a glass substrate, collected fromvarious SODs. Generally, the polymer splashing degree on the flatsubstrate at room temperature is minimal, but much more significantat the larger SOD. SEM images of PP splats sprayed at the lowest spraydistance of 10 cm exhibit the smallest degree of splashing, with alarge, symmetrical and nearly-hemispherical unmelted core; and afully melted wide thin rim around the splat, forming a shape of “friedegg” splat. No voids or cracks were distinguishable, with good splatedge adhesion to the substrate. Increasing the SOD led to aproportional increase in the splat areas and splash size [26].

The splat areas were generally proportional to the preheating ofthe substrate, i.e. increasing splat areas were observed when thesubstrate temperature was increased since there was an increase inthe thermal flux generated by the torch. Disc-like splats, showing friedegg characteristics with little or no splashing, were found under SEMobservation. The “fried egg” phenomenon that primarily occurs forpolymer splats is believed to be caused by the large radial difference inthe flow properties of the molten PP droplet surface. There is a lowviscosity fully melted rim layer, with the unmelted or still highlyviscose core that does not spread upon impact; an observationdocumented by others for polymer splats [3,4,28].

3.3. Splat equivalent diameters

One-hundred splats were assessed with regard to their equivalentdiameters and ranked within size classes, without splashes, for eachSOD, Fig. 7. A wide spread of splat diameters is demonstrated for allSODs. The lower and upper limits of the diameter sizes wereestablished; with these ranges increasing with the SOD. A maximumsplat diameter of 220 μm was found for an SOD of 25 cm.

Fig. 8 illustrates the second step in the analysis, where the splashportions of the splat are included. A similar trend is observed, with amaximum splat diameter of 250 μmobtained for the 25 cm SOD. Fig. 9shows the average splat diameters for each SOD and each analysisstage. Table 2 provides details of lower and upper limits for all splatEDs and SODs.

3.4. Degree of splashing (DS)

DS could offer an alternate method for identifying and optimisingthe SOD to produce circular splats. As splashing decreases so does DS,with a DS of 1 indicating an ideal circular splat. The DS of flattened

s of 10 cm, 15 cm, 20 cm and 25 cm for a, b, c and d respectively.

Fig. 7. Comparison of particle and splat size distributions, analysedwithout splashes, fordifferent SODs.

Fig. 8. Comparison of particle and splat size distributions, analysed with splashesincluded, for different SODs.

Table 2Equivalent diameter of splats* with and without splashes for different stand-offdistances.

Equivalent dia. of splats withoutsplashes

Equivalent dia. of splats withsplashes

Stand-offdistance (cm)

Lowerdia. (μm)

Upperdia. (μm)

Average(μm)

Lowerdia. (μm)

Upperdia. (μm)

Average(μm)

10 68.9 158.6 116.3 73.9 179.6 122.815 75.0 181.7 120.3 81.6 204.1 127.520 85.9 197.6 133.6 90.3 197.2 153.825 90.9 216.4 142.4 117.7 264.3 180.7

*100 splats were considered for each measurement at each stand-off distance.Measurements were made twice, first without splashes and second with splashesincluded.

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particles was used to compare splat morphologies obtained underdifferent spray conditions. The DS presented in Fig. 10 shows that thePP splats are not circular, with values ranging from 1.6 for the smallestSOD of 10 cm to 2.5 for an SOD of 25 cm, which demonstrates thedependence of DS on the SOD. The values obtained here are lowerthan that achieved by other researchers when measuring DS as afunction of preheat temperature [29]. This behaviour arises becausethe polymer splat thickness is typically much greater than those ofmetal and ceramic splats. The splashes of polymeric material areproduced as the first layer of polymer contacts the substrate, while theremaining bulk of the splat will solidify before reaching the splat edge.The low thermal conductivity of polymers controls the interplaybetween spreading and freezing of splats.

The anticipated particle behaviour during impact on a lowtemperature substrate, which gives rise to rapid solidification rate, isthe formation of a solid ring that restricts materials from spreadingfreely.Due to thehigh inertial energyof thedroplet, the upper portion ofthe liquid droplet escapes beyond the solid ring, resulting in theformation of fingers around the splat periphery. Generally, splatmorphology is dictated by the heat transfer between the splat and thesubstrate. Droplets with a low melting point, such as polymers, areobserved to solidify quickly during impact and, therefore, triggerfreezing-induced splashing [10]. Thus, the faster the heat is conducted

Fig. 9. Comparison between the splats average particle size with and without splashesfor different SODs.

from the splat to the substrate, then the higher will be the solidificationrate in the droplet.

The rate of solidification depends on many factors, includingthermal contact resistance at the splat-substrate interface, substratetemperature, substrate material, and droplet melting temperature[30]. A Mehdizadeh et al. [31] simulation has shown that a molten tindroplet impacting on a substrate at low temperature begins to freezefirst around its periphery, thereby obstructing liquid flow and, whenthe solid rim becomes sufficiently thick, this flow instability triggerssplashing.

3.5. Circularity

Circularity is a measure of the compactness of a shape. It is a valuethat allows estimation of splat formation and how far the splat shapeis from the ideal disc shape, as well as examination of how the DSdevelops during impact at different SODs. A circularity of 1 equates toa perfect circle and the closer to zero then the more likely that splatsare splashed, fingered or fragmented. All results of the circularityanalysis indicate some DS for both calculations of splats; i.e. with andwithout splashes as depicted in Fig. 2. However, the lowest SOD of10 cm presented higher circularity values than the largest SOD of25 cm. It is proposed that there was insufficient heating so that thepartially melted splats did not contain adequate molten material tocause significant splashing. This postulate follows the trend that theoccurrence of splashing is more likely due to the jetting of liquid outfrom the upper part of the flattening particle [9,24].

Fig. 11 shows the relationship between the splat circularities andthe SODs, and demonstrates that the shape of splats with splashes isless circular than splats without splashes. No significant difference incircularity between SODs is evident for splats without splashes, withall values around 0.9, indicating that these splat shapes are regularand almost circular.

3.6. Spread factor

The rate of droplet spreading was quantified by measuring thesplat diameter at successive stages during droplet deformation; from

Fig. 10. Relationship between the degree of splashing and SOD.

Fig. 14. Relationship between the number of splats per unit area (5 mm2) and the SOD.Fig. 11. Relationship between splats' average circularity and the SOD, for analysis withand without splashes.

5033K. Alamara et al. / Surface & Coatings Technology 205 (2011) 5028–5034

the initial droplet diameter to the maximum splat diameter. A nearlylinear relationship exists between the spread factor and the SODacross the range in this study, Fig. 12, with the smallest SOD of 10 cmhaving the lowest spread factor value. The splat with splashes analysisof 100 particles does not show a significant increase in the dropletspreading with increasing SOD from 10 cm to 15 cm. However, a largeincrease in the splash area occurs when extending the SOD from20 cm to 25 cm. Calculation of the splash areas for 100 splats can beachieved by subtraction of the sum of the areas of 100 splats withoutsplashes from the sum of the area of 100 splats with splashes. Thesplash area of 100 splats reached 1 mm2 at the 25 cm SOD, while it isless than 0.1 mm2 for the SOD of 10 cm, as shown in Fig. 13.

3.7. Deposition efficiency (DE)

DE can be determined by calculating the number of splats withconsideration of the different splat size. The significant effect of theSOD on the number of splats deposited on the surface is evident fromanalysis of a 5 mm2 area of the sprayed glass slide, Fig. 14. The numberof splats decreases proportionally to the increase in SOD distancebetween 10 cm and 20 cm, while there is a larger drop in the numberof splats seen when the SOD reaches 25 cm. This decline in thenumber of drops impacted orthogonally could be explained bydeviations in the particle trajectory over longer distances, withpolymer particle decomposition due to the excessive heat that the in-flight particle experiences because of the increased dwell time.

Fig. 12. Relationship between the splats' spread factor and the SOD.

Fig. 13. Comparison of the spread factors for splats with and without splashes at allSODs, including calculation of splash area for 100 splats.

3.8. Splat thickness

Splat thickness can be calculated from the flattening ratio byapplying a geometric equation, defined by Madejski's relationships[32], which links the diameter of an impinging particle with thediameter and thickness of splats. The effect of surface tension isconsidered as negligible towards the end of the spreading process. It isassumed that the solidified splat is a thin cylindrical disc with avolume equal to that of the initially spherical droplet [12]. Splatthicknesses (h) were calculated according to the following equation;

h =23d3

D2 ð5Þ

where d andD are the diameters of the in-flight particle and diametersof splat respectively.

The variation of the splat thickness with the SODwas investigated.As Fig. 15 illustrates, thicker splats were observed at shorter SODs. Itwas noted that the average splat thickness ranged from 15 to 35 μm,equivalent to 1/5 to 1/8 of the droplet diameter. The results confirmthe visual observations and diagnostic data that show the highestspread factor occurs for the highest SOD, explained by particle dwelltime and a higher particle temperature and velocity. The splatthickness trend is opposite to that observed for the diameters becausethe volume of the particle and splat are identical except in instanceswhen the particle is overheated or evaporated at longer SODs. Thisrelationship, i.e. smaller diameters with increased thickness, isillustrated in Fig. 16.

The splat thickness measurement was performed using 2Dprofilometry. The results agreed with the outcomes of statisticalcalculations for PP splat profiles and thicknesses. Fig. 17a depicts the2D profilometry data, across the point on the splat indicated by theinset, as a trace line of the profilometer. An alternative technique wasalso used to measure the splat thickness, which involved cutting theglass slide using a dicing saw diamond close to the edge of thedeposited splat, and viewing under the SEM. This SEM side viewimage is shown in Fig. 17b, and reveals a splat thickness of around20 μm, which has been validated through calculations on splats withthe same topology.

Fig. 15. Relationship between the splat thickness and the SOD.

Fig. 17. 2D Profile of splat cross section (along X–Y shown in inset) using data obtainedfrom XP-2 surface profilometry; note that the X and Y axes are different scales, and(b) SEM image of the splat from side view.

Fig. 16. Relationship between splat diameter and splat thickness for different SODs.

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4. Conclusions

The shape of feedstock and impacted particles was measured usingcomputer software analysis. The complexities of the particle shapewereanalysed using splat metrics; e.g., particle area, splat area and diameter;degree of splashing; deposition efficiency; circularity; spread factor andsplat thickness. The results indicate that as the stand-off distance isincreased from 10 cm to 25 cm, the degree of splashing, flattening ratioand spread factor increase, while the deposition efficiency and splatthickness decrease. Splat circularity is steady around 0.9 indicating thatthe splats analysed are close to circular for all stand-off distances.

Acknowledgements

The authors would like to thank Dr. J. Wang, Swinburne Universityof Technology for his help in using the SEM. Thanks to Dr. Sriram,RMIT University for performing the 2D profilometry.

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