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Splat morphology of plasma sprayed aluminum oxide reinforced with carbonnanotubes: A comparison between experiments and simulation

Anup Kumar Keshri, Arvind Agarwal

PII: S0257-8972(11)00709-2DOI: doi: 10.1016/j.surfcoat.2011.07.025Reference: SCT 16889

To appear in: Surface & Coatings Technology

Received date: 21 December 2010Revised date: 30 April 2011Accepted date: 11 July 2011

Please cite this article as: Anup Kumar Keshri, Arvind Agarwal, Splat morphologyof plasma sprayed aluminum oxide reinforced with carbon nanotubes: A compari-son between experiments and simulation, Surface & Coatings Technology (2011), doi:10.1016/j.surfcoat.2011.07.025

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Splat Morphology of Plasma Sprayed Aluminum Oxide Reinforced with

Carbon Nanotubes: A Comparison between Experiments and

Simulation

Anup Kumar Keshri

1, 2 and Arvind Agarwal

1,*

1Plasma Forming Laboratory Mechanical and Materials Engineering,

Florida International University, Miami, FL 33174, USA

2Manufacturing Division, School of Mechanical and Building Sciences Vellore Institute of Technology University, Vellore 632 014, Tamil Nadu, India

Abstract

This study elucidates the effect of carbon nanotube (CNT) addition on the splat formation

in plasma sprayed aluminum oxide (Al2O3) composite coating using experimental and

computational methods. CNT content was varied as 0, 4 and 8 wt. % in Al2O3 matrix.

With an increasing CNT content, splat morphology became more circular and disk-

shaped. The average diameter of disk-shaped splats increased from 28.6±1.4 µm for

Al2O3 to 43.2±1.3 µm for Al2O3-8 wt. % CNT. The population density of splats with

fingers, fragments, and voids was the lowest for the highest (8 wt. %) CNT content. The

addition of CNTs resulted in two simultaneously competing phenomena viz. increased

thermal capacity and increased viscosity of the melt. Increased thermal capacity delayed

the localized solidification resulting in higher splat diameter while agglomeration of

CNTs at the periphery of the splat results higher viscosity of the melt which suppresses

the splat fragmentation that leads to increased population density of disc shaped splats.

Splat morphology of three compositions was also simulated using SIMDROP software,

which showed a good agreement with the experimentally collected splats.

Keywords: Aluminum oxide, Carbon nanotube, Plasma spraying, Splat

* Corresponding Author: Arvind Agarwal, Ph: +1-305-348-1701, Fax: +1-305-348-1932 Email: agarwala@fiu.edu

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1. Introduction

Splat is the smallest unit of the microstructure of plasma sprayed coatings. The

properties of the coatings are largely dependent on the splat morphology and their

stacking [1-4]. Incomplete melting and improper stacking of splats can result in globular

voids, poor adhesion at the inter-splat boundary which can have adverse effect on the

mechanical, thermal and electrical properties of the coatings [5-7]. Hence splat formation

and its morphology play a significant role in tailoring the coating properties. Several

studies have been reported on the morphological aspect of splats [1, 3, 4, 8-10] which

suggest that splat morphology largely dependent on (i) feedstock material properties [8],

(ii) thermal and kinetic state of the in-flight particle [8, 10] and (iii) substrate chemical

state, roughness and temperature [3, 4, 11].

Elsebaei et al. [8] performed a study on the morphology of individual splats for

different set of plasma operating parameters (arc current: 700, 870 A, stand-off distance:

80, 100 mm) for the regular yittria stabilized zirconia (YSZ) (particle size: 45-100 µm)

and the spherical agglomerate of YSZ (agglomerate size: 20-40 µm) synthesized from the

nano-YSZ powder particle. Spherical agglomerate of nano YSZ was used with the intent

to melt the periphery of the agglomerate and to retain the nano-features in the core. Such

coating resulted in bimodal microstructure. Lima et al. [12] studied thermal spray

coatings synthesized from the nanostructured ceramic agglomerated powder and

concluded that it was necessary to avoid the full melting of the agglomerates to preserve

nanostructure in the coating. Elsebaei et al [8] concluded that circularity (degree of

roundness) and flattening degree (ratio of the diameter of splat to starting droplet) of YSZ

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splats synthesized from the spherical agglomerate were larger than the regular-YSZ

splats. This was attributed to the smaller particle size, higher particle temperature and

velocity of agglomerated-YSZ particles [8]. Splats synthesized using spherical

agglomerates of YSZ were more disk shaped (i.e. better circularity) as compared to

regular-YSZ splats at a higher stand-off distance [8]. This was due to the higher particle

velocity of agglomerated-YSZ compared to regular-YSZ particle [8].

Bianchi et al. [13] deposited single splats by spraying micron-size zirconia on

“cold” 304L stainless steel substrate (~100°C) and observed a highly fragmented

morphology. Perfect disc shaped splats were obtained for the substrate heated to 300°C

[13]. Fukumoto et al. [14, 15] studied the relationship between the splat morphology

(from the micron-sized feedstock particles) and the substrate temperature and observed

distinct changes in the splat morphology. Threshold transition temperature for the

substrate was first identified by Fukumoto et al. [15], beyond which splat morphology

changes from fragmented to disk shaped. Sampath et al. [3, 4] carried out a study on the

effect of substrate temperature on the splat formation for partially stabilized micron-sized

zirconia particles. The threshold transition temperature was found in the range of 250-

300°C [3]. In the case of “cold” substrates, initiation or localized solidification is

responsible for spreading instability which leads to flattening splashing [3, 4]. Preheated

substrate above the transition temperature provides better contact and uniform heat

conduction which minimizes the localized solidification [3, 4]. The substrate heating also

allows condensates and adsorbates desorbtion. Li et al. [16] studied the effect of substrate

preheating temperature and surface organic covering on splat formation. Splats

(aluminum, nickel, copper, Al2O3 and molybdenum) were deposited on polished stainless

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steel substrate surface covered with different organic substances (xylene, glycol and

glycerol). It was found that when the preheating temperature exceeded 50°C over the

boiling point of organic substance, the regular disk type splats were formed [16]. An

optimum substrate preheating is required to strike a balance between better splat

formation and minimal residual stress. These studies indicate that powder feedstock,

processing conditions and substrate conditions have significant effect on the splat

formation.

Our research group has worked extensively on the synthesis of CNT reinforced

aluminum oxide coatings by plasma spray technique [17-27]. Comprehensive process

maps have been successfully developed to synthesize “lowest porosity” plasma sprayed

Al2O3 composite coatings with 0, 4 and 8 wt. % CNT reinforcement [26]. Balani et al.

obtained ~200% improvement in the elastic modulus [19], 57% improvement in the

fracture toughness [17] and 49 times enhancement in dry sliding wear resistance [21] by

adding 8 wt. % CNTs in Al2O3 coatings. Improved elastic modulus, fracture toughness

and wear resistance was mainly attributed to excellent dispersion of CNTs in the Al2O3

matrix that promotes toughening mechanisms such as CNT bridging, crack deflection at

CNT/Al2O3 interface [17-21]. A majority of our past work on Al2O3–CNT coatings was

focused on studying the materials property as function of process, microstructure and

CNT content. However, the role of CNT in the splat formation was never addressed.

Bakshi et al. [27] did a preliminary study on the role of CNT in metallic Al-Si splat

formation and concluded that splat shape is governed by the viscosity and thermal

conductivity of the droplet which are dependent on the CNT content. Higher CNT

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content (10 wt. %) leads to disc shaped Al-Si splat as compared to lower CNT content (5

wt. %), which was attributed to increase in the viscosity of the melt due to increased CNT

content [27].

Motivated by this scenario, the objective of this study is to understand the role of

CNT in the Al2O3 splat formation. The effect of varying CNT content on the splat

morphologies has been investigated. Splat formation can be optimized by

experimentation but it requires extensive and time consuming experiments due to large

number of processing variables involved in plasma spraying. Splat morphology

simulation for the given processing variables can save considerable amount of time. In

this study, splat morphology simulation has also been performed using SIMDROP

(Simulent Drop 3.0, Simulent Inc, Toronto, Canada) and comparison has been made with

experimentally deposited splats.

2. Experimental

2.1 Powder Feedstock

Sub-micron sized Al2O3 (~150 nm, average diameter) powder and multiwall

carbon nanotubes (95% +purity, 40–70 nm outer diameter, 0.5–2.0 μm in length) were

used as starting materials. Since sub-micron sized fine powder and CNTs cannot be fed in

the plasma flow using conventional carrier gas due to their high interparticle friction and

resulting inconsistent flow, spray drying was implemented to manufacture micron-sized

agglomerates. Spray drying also allows homogeneous dispersion of CNTs in Al2O3

matrix. Sub-micron Al2O3 powder was spray dried (referred as A-SD) to obtain spherical

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agglomerates of 30±10 μm in diameter. The diameter of the spherical agglomerates was

measured from the 5-6 different SEM images. From each image, 15-20 measurements of

agglomerate diameter were taken. Powder size distribution of A-SD, A4C-SD and A8C-

SD are shown in Figure 1a-c respectively. Spray drying of sub-micron Al2O3 with 4 wt.

% CNTs (referred as A4C-SD) and 8 wt. % CNTs (referred as A8C-SD) resulted in

spherical agglomerates of 26±7 μm and 24±5 μm respectively. A-SD powder served as

the control sample to investigate the effect of CNT addition.

2.2 Synthesis of Single Splat

A-SD, A4C-SD, A8C-SD powders were plasma sprayed using SG 100 gun

(Praxair Surface Technology, Danbury, CT, USA) on polished (Ra= 0.03 m, RZ =0.098

m) AISI 1020 steel substrate (22 mm ×19 mm ×3.2 mm) to collect splats. Diameters of

the splats were measured using Image J software (http://rsbweb.nih.gov/ij/index.html). A

total of ~100 splats were taken into consideration from 5-6 different SEM images. Splats

were deposited at optimized plasma process parameters which showed lowest porosity in

the coating [26]. Details of the optimization of plasma process parameters can be found

elsewhere [26]. Substrate preheat temperature was maintained at 453 K which was same

as in the optimization study for the lowest porosity coating [26]. Table I summarizes the

plasma spray operating parameters for splat experiments. Carrier gas flow rate was

adjusted for three different powder feedstock to maintain a constant powder feed rate of 3

g/min. Figure 2a shows the set up for plasma spraying of single splats. Temperature and

velocity of the in-flight powder particle were measured using AccuraSpray™ in-flight

diagnostic sensor (Tecnar Automation Ltée, QC, Canada). In-flight particle flow pattern

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was diagnosed at 75 mm of spray distance which was same at which splats of Al2O3 and

Al2O3-CNT were deposited. The sensor was located at the middle of the particle flow

pattern. Accuraspray diagnostic system provides ensemble average data which represents

the particle characteristics in a measurement volume of approximate 75 mm3. The

minimum temperature that can be measured with the sensor is 900°C with 0.5% precision

while the minimum velocity that can be measured is 5 m/s with 0.5% precision. Error in

the velocity and temperature measurement is <1.5 m/s, and <15°C respectively at cross-

correlation factor > 0.9. Experimentally obtained ensemble surface temperature and

velocity of in-flight particles were used for the splat simulation of A-SD, A4C-SD and

A8C-SD.

A shield plate with a series of 2 mm diameter holes was used to collect well

dispersed splats as shown in Figure 2b. The holes in the shield plate were aligned with

the particle flow pattern. The steel substrate was positioned behind the shield plate at a

total spray distance of 75 mm. The substrate was preheated to 453 K using a heating gun

(STEINEL, HG 2510 ESD, MN, USA) as shown in Figure 2c. The substrate temperature

was continuously measured using K-type thermocouple (KMQSS-020U, Omega

Engineering Inc., wire diameter: 1 mm) inserted through the substrate thickness. Figure

3a is the picture of the substrate showing hole (1 mm diameter) at the center of the

substrate and through thickness. One end of the thermocouple was positioned into this

hole while the other end was connected to the data logger for the substrate temperature

measurement. The reading rate of the data logger was 12 measurements/minute. Figure

3b illustrates the schematic of substrate showing dimension of all faces and hole. Plasma

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gun speed was 25 mm/sec, which was same as used for the optimization study for the

lowest porosity coating [26].

2.3 Microstructural Characterization

FEI PHENOM (in back scattered mode) and JEOL JSM-633OF field emission (in

secondary electron image mode) scanning electron microscopes were used at an

operating voltage of 15 kV to investigate the powder morphology and splat morphology

of A-SD, A4C-SD and A8C-SD.

2.4 Splat Simulation

Splat morphology simulation has been performed using SIMDROP (Simulent

Drop 3.0, Simulent Inc, Toronto, Canada) software. SIMDROP software, developed by

Pasandideh-Fard et al. employs three-dimensional finite-difference algorithm which

solves the Navier-Stokes equation including heat transfer and phase change [28-31].

Volume of fluid (VoF) tracking algorithm is used in this model to track the droplet-free

surface [28-31]. Also, thermal contact resistance (usually 10-7 m2K/W for thermal spray

process) at the droplet-substrate interface is included in this model [28-31]. Splat

simulation was performed for A-SD, A4C-SD, and A8C-SD powders. Experimental data

from the plasma spraying was used as the input for the simulation. One of the major

challenges encountered in the simulation was the non-availability of materials properties

(Kinematic viscosity, thermal conductivity, specific heat, surface tension of the liquid

phase, thermal conductivity and specific heat of solid phase, heat of fusion etc.)

especially for CNT reinforced Al2O3, because high temperature thermophysical

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properties of CNTs are non-existent in the literature. Thermophysical properties of CNT

were approximated as of graphite [32]. Thermophysical properties for Al2O3 were

obtained from the literature [33, 34]. However, most of the thermophysical properties in

the literature are listed at temperatures different than in-flight temperature of A-SD, A4C-

SD and A8C-SD particles exiting from the plasma. SIMDROP software is capable of

making the best fit of the materials property as a function of temperature from the

available data [35]. Thus, it can generate the property at the desired temperature by

interpolating or extrapolating the fitted curve [35]. For example, in case of one of the

materials property i.e. solid phase specific heat, format of the input file is as following:

N = n T = T1, T2, T3, T4 ……………………..Tn C = C1, C2, C3, C4……………………..Cn where, N is the total number of paired data sets, T is the array of temperatures (K), and C

is the array of solid phase specific heat at corresponding temperature (in W/m-K).

SIMDROP can generate best fitting curve as C=f(T). Similarly, other thermophysical

properties were inserted in the above format for the splat simulation. Rule of mixtures

(ROM) was used to estimate the apparent thermophysical properties of the complex

Al2O3-CNT composite materials from intrinsic properties of both Al2O3 and CNT and

inserted as an input parameter for the splat simulation. Hence, splat simulation was

treated as a single phase flow problem. Table II shows the calculated ROM values of the

thermophyical properties for A-SD, A4C-SD, and A8C-SD. The accuracy of the

computed ROM values has been addressed by comparing the simulated results with the

experimental results in section 3.3. Table III lists droplet size, droplet surface

temperature, droplet velocity, substrate temperature and substrate roughness, which were

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considered as input parameters for A-SD, A4C-SD and A8C-SD splat simulation. The

average powder size for each composition was assumed to be the droplet impinging

diameter. The powder size distribution for all compositions ranged between 24-30 μm in

diameter (A-SD: 30±10 μm, A4C-SD: 26±7 μm, A8C-SD: 24±5 μm). Surface

temperature and velocity of the particles were obtained experimentally using an in-flight

particle diagnostic sensor, as explained in previous section 2.2. As a consequence of the

splat simulation, two output files were obtained: (i) substrate temperature vs. time (i.e.

solidification curve) and (ii) the volume fraction of the solid vs. time.

3. Results and Discussion

3.1 Powder Morphology

Figures 4 (a), (b), and (c) show high magnification SEM micrographs of A-SD,

A4C-SD and A8C-SD agglomerates, respectively. The inset shows lower magnification

images of spherical agglomerates of each composition. The spray dried agglomerates

have improved flowability into the powder hoses and injector, due to their spherical

shape and reduced interparticle friction. From Figure 4(b) and (c), it can be seen that the

CNTs were dispersed uniformly on the surface of agglomerate. Uniform dispersion of

CNTs is critical in splats morphology which is discussed later.

3.2 Effect of CNT Content on Splat Formation

Figure 5a-c shows SEM images of A-SD, A4C-SD and A8C-SD splats. Splat

morphology changed from splashed and fragmented to almost disc-shaped with an

increase in CNT content. In addition, relatively higher splat diameter and lower finger

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length was observed with the increasing CNT content. Figure 6a-c shows the splat size

distribution for A-SD, A4C-SD and A8C-SD splats. The average splat diameters are

plotted in Figure 7a. Diameter of the fingered splat was calculated by measuring the

radius of the splat from the center to the end of the finger. Only those fingered splats

were considered for the measurement which showed lowest variation in the finger length.

Since the frequency of the fingered splats with lowest deviation in finger length was

much higher, it is safe to consider only these splats for the diameter measurement. Figure

7a shows the variation in the splat diameter as a function of CNT content. Splat diameter

increases with an increasing CNT content. Similarly, Figure 7b shows that with the

increasing CNT content, the average length of the fingers radiating from the periphery of

the splat reduced. Splats obtained from these experiments can be categorized into four

different types based on their geometry and are shown in Figure 8a. These splat

categories are shown schematically in Figure 8b: (1) disc shape, (2) splats with finger, (3)

fragmented splats, and (4) splats with voids. Perfect disk shape splat is formed due to

longer time of the melt. The fragmented or splashed splat might be the result of localized

solidification of melt. The solidified layer could obstruct the outward spreading liquid

which leads to the fingers radiating out from the periphery of the splat. Splat with voids

might be formed due to the entrapped gas.

Experimental population density of these splat geometries was measured using

image analysis from several images and has been plotted as a function of CNT content.

Figure 9 show the population density for four splat geometries in A-SD, A4C-SD and

A8C-SD samples. The percentage of disc shape splats increased with an increasing CNT

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content. Population density of splats with finger, fragmented splats and splats with voids

is the lowest for the highest (8 wt. %) CNT content.

To understand the underlying mechanism, splats were observed in SEM at a

higher magnification. Figures 10a-b show the single splat of A4C-SD and A8C-SD,

respectively with CNTs distributed in the splat matrix. In addition, agglomeration of

CNTs at the periphery of A4C-SD (Figure 11a) and A8C-SD (Figure 11b) splats is also

observed. Since, splat diameter of A8C-SD was relatively larger, complete splat is shown

as an inset in Figure 11b.

Such a varying degree of CNT dispersion in the splat is responsible for the

increased splat diameter and increased percentage of disc shaped splat. “Distributed

CNTs” in the melt (Figure 10) leads to enhanced thermal capacity resulting in the lower

viscosity. Specific heat capacity of Al2O3 is 1358 Jkg-1K-1 while the specific heat capacity

for CNT is 2145 Jkg-1K-1 at 2327 K [32]. Hence, specific heat capacity calculated using

ROM (as shown in Table-II) for A4C-SD is 1404 Jkg-1K-1 while its 1444 Jkg-1K-1 for

A8C-SD respectively at 2327 K. Specific heat capacity for A4C-SD and A8C-SD are

3.38 % and 6.33 % respectively higher than A-SD at 2327 K. The higher thermal capacity

of the melt would increase the time required for the heat loss to occur for identical

thermal resistance at the substrate/splat interface. Hence, localized solidification of the

melt will be delayed, which results in enhanced spreading and larger splat diameter in

CNT reinforced splats (figure 7). “Agglomeration of CNTs” (Figure 11) at the periphery

of the melt will produce a counter effect resulting in an increased viscosity. Increased

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viscosity of the melt will suppress fragmentation and hence, higher percentage of disc

shaped splat was observed in case of CNT reinforced splats. Both of the above

phenomena are schematically elucidated in Figure 12, which shows the droplet of Al2O3

(Figure 12a) and Al2O3-CNT (Figure 12b) at t=0 sec i.e. just before the impact on the

steel substrate. At t=t1, higher spreading of Al2O3-CNT will take place due to higher

thermal capacity of the melt as a result of CNT addition. Higher thermal capacity of the

melt leads to lower viscosity which contributes towards higher spreading of CNT

containing splats. At t > t1, localized solidification occurs in Al2O3 splats, that restricts

their size and causes finger formation. In case of Al2O3-CNT splats, CNTs flow easily

towards the periphery of the splat due to their lower specific mass (2.1 g/cc) as compared

to Al2O3 (3.9 g/cc) and start agglomerating at the edges (as seen in Figure 11).

Agglomeration of CNTs at the edges will contribute towards increased viscosity and

suppresses the splat fragmentation resulting in higher population of the disc shape splats.

Thus, it is clear that CNTs play an important role in modifying the splat shape and

size which ultimately affect the final properties of the coating. The experimental

optimization of the splat geometry by varying in-flight temperature and velocity, CNT

content and substrate conditions requires lots of intensive effort and time. Computer

simulation of the splat geometry can provide valuable information about the processing

conditions. Simulation of Al2O3-CNT single splat is discussed in the following section.

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3.3. Simulated Splat Morphology

Figures 13a, c, and e depict the simulated views of A-SD, A4C-SD and A8C-SD

splats. All distances in the simulated view are in millimeter. The simulation was

performed at cells per radius (CPR) value of 15. The CPR is an indicator of the resolution

and can be calculated for each calculation domain from the following equation [35].

A higher CPR number is an indicator of a higher resolution. It is recommended that CPR

to be at least 10 and the optimum number of CPR is 15 [35]. Increasing the CPR number

greater than 15 requires lots of extra computational time and resources [35]. Figures 13b,

d, f are SEM images of the experimentally obtained splats of A-SD, A4C-SD and A8C-

SD at the same parameters as simulation. Relatively higher splashing with broken fingers

was observed in the simulated view of A-SD (Figure 13a) which is also seen in the

experimental A-SD splat (Figure 13b). Lowest splashing of the droplet was observed in

both simulated and experimental results of A8C-SD splat (Figures 13c and f). Hence,

splats geometries obtained from the experiment and simulation showed good qualitative

agreement.

It was observed experimentally that splat diameter increases with an increasing

CNT content. In order to compare the experimental results with the simulated results,

spreading ratio (D/D0) for experimental and simulated splats were plotted. D is the final

diameter of splat and D0 is the initial diameter of the droplet considered here as the

average diameter of the size distribution. Figure 14 shows the spreading ratio as a

Calculation domain

Radius of the impinging droplet No of grids × CPR =

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function of CNT content for experimental and simulated splats. Both, experimental and

simulation results show the similar increasing trend in spreading ratio (D/D0) with an

increasing CNT content. However, experimentally obtained spreading ratio is lower than

computed which is attributed to the powder feedstock characteristics. Spray dried

powder, which contains ~30-40% of voids [20, 24, 26] was used for synthesizing the

splats. Due to the presence of those voids in powder, molten droplet size is expected to be

smaller resulting in smaller splat diameter and lower spreading ratio. During simulation,

molten droplet size was assumed to be of as initial powder size which leads to larger

diameter of splats as compared to experimentally obtained splats. An accurate method to

obtain droplet size could be to collect the plasma sprayed powder in a liquid and then

analyze the particle size distribution. Mean diameter obtained by this size distribution can

be put as a more accurate input parameter (initial droplet diameter) into the simulation.

The simulation also provides the volume percentage of solid in A-SD, A4C-SD,

and A8C-SD splats at different time intervals, as shown in Figure 15. 0.24 µs of the after

impact, the volume percentage of solid in A-SD splat is 26% while it is 14% and 8% in

A4C-SD and A8C-SD, respectively. This indicates a delayed solidification process in

CNT reinforced splat which is attributed to enhanced thermal capacity of the melt as a

result of higher specific heat of graphite [32]. 0.75 µs of the after impact, the volume

percentage of solid in A-SD splat is 66%, while it is 58% and 53% in A4C-SD and A8C-

SD splat, respectively which becomes constant. Between 1.5 and 5 µs after impact, small

increment in volume fraction of solid was observed for all splats (ΔV=+ 2% for A-SD

splat, ΔV=+ 1.2% for A4C-SD splat and ΔV=+ 0.91% for A8C-SD splat). These results

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indicate that complete solidification may be occurring over a greater time interval, which

could be order of magnitude greater. This is in accordance with the solidification of the

real molten droplets in plasma spraying. Typically, molten ceramic droplet solidify at a

cooling rate of ~106 K/s during plasma spraying [36-38], which indicates that droplet

solidification time is ~ 0.5 milliseconds for the in-flight particle temperature of 2750 K.

Since, cooling does not follow a linear relationship (according to Newton’s law of

cooling) in between 1-5 µs, the solidification time is much lower (in microseconds) in the

present study.

Solidification behavior of the splat can also be understood from the change in the

substrate temperature obtained from simulation. Figure 16 shows the computed substrate

temperature variation vs. time for A-SD, A4C-SD and A8C-SD splats. Three different

zones can be observed in Figure 16. Region I shows a significant increase in the substrate

temperature at 0.1 µs for A-SD, A4C-SD and A8C-SD splats. This is attributed to the

heat transfer from the molten droplet to the substrate immediately after its initial impact.

Higher increase (ΔT= + 650°C) in the substrate temperature was seen for CNT reinforced

splats which is attributed to higher thermal capacity of CNT containing molten Al2O3

melt resulting in higher heat transfer to the substrate.

In region II, substrate temperature increases for A-SD splats whereas substrate

temperature remains constant for A4C-SD and A8C-SD splats. Increasing substrate

temperature in case of A-SD splat is due to higher rate of solidification (as shown in

Figure 15) resulting in higher amount of heat energy release. Released latent heat of

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solidification will be absorbed by the substrate and substrate temperature will increase.

CNT reinforced melt has higher thermal capacity and remains molten for a long time.

The substrate temperature in CNT containing melts has reached maximum in Region II.

The further change in the temperature of CNT containing melt will occur only if it loses

heat due to solidification. Since solidification rate is lower in CNT containing melt

(Figure 15), hence no appreciable change in the substrate temperature was observed in

region II for A4C-SD and A8C-SD splats.

In region III, substrate temperature decreased for all splats (A-SD, A4C-SD and

A8C-SD). However, higher rate of change (dT/dt) of temperature was observed for A4C-

SD and A8C-SD splats compared to A-SD. This is attributed to lower solidification rate

in CNT reinforced melt (volume of solid: 58% and 53% in A4C-SD and A8C-SD splat

respectively at 0.75 µs) compared to A-SD (volume of solid: 66% in A-SD splat at 0.75

µs), as seen in Fig. 15. Lower solidification rate will lead to lower latent heat release and

hence less heat is available for the substrate to absorb. While, higher solidification rate in

A-SD splat leads to higher latent heat release, which will be absorbed by the substrate.

Due to the additional higher heat absorbed by the substrate (i.e. substrate will loose heat

slowly with time); lower rate of change of temperature (dT/dt) was observed in A-SD

splat. In the case of CNT reinforced melt, since, there is less heat available for the

substrate to absorb (i.e. substrate will loose heat slowly with time), higher rate of change

of temperature was observed in case of CNT reinforced melt.

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Though, computed substrate temperature decreases in the 0.75-1.5 µs range (in

region III), whereas, volume percentage of solid remains almost constant as shown in

Figure 15. This can be understood from Figure 17 which shows a liquid droplet

impacting on the substrate and spreading into a thin splat at two different interval of time.

Once the droplet impacts on the substrate, molten liquid which is in contact with the

substrate, starts solidifying. A solid layer of thickness “s” exists between the top and

bottom surface of the splat as shown in Figure 14. As the time progresses from t1 to t2, the

thickness of the solid layer, which has already solidified and in contact with the substrate,

increases to “s+s1”. With the growth of solidified layer, a temperature gradient exists

between the top and bottom surface of the splat. Temperature at the bottom surface of the

splat keeps on decreasing with the increase in the fraction of solid, and hence reduction in

the substrate temperature is observed in region III. However, the percentage volume of

solid is constant in region III in Figure 15 which is attributed to the lost material due to

flattening splashing. These lost solidified particles do not contribute in the computations

of volume percentage of solid. Salimijazi et al. [39] studied the solidification behavior

and splat morphology of vacuum plasma sprayed Ti alloy by computational modeling

using SIMDROP software and by the experimental results and mentioned that

approximately 30% of the droplet material is lost due to flattening splashing. The

splashing mechanism of the droplet at the time of impact and during its flattening has

been well explained by Cedelle et al. [40].

It is concluded that CNTs play an important role in tailoring the morphology of

splat, which ultimately affect the coating’s final properties. CNT reinforcement resulted

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in relatively lower splashing and increased population density of disc shape, which will

contribute towards obtaining the denser coating due to effective packing. Further, splat

simulation is an effective tool for simulating the plasma sprayed single splat morphology.

This can contribute significantly towards optimization of splat morphology and can

reduce the large number of experiments which involves lots of time and money.

Conclusions

Plasma sprayed CNT reinforced (0, 4 and 8 wt. %) Al2O3 single splats were

obtained on the polished steel substrate at the preheat temperature of 453 K. Results

strongly indicates that CNTs play a critical role in the splat morphology. Splat diameter

increased from 28.6±1.4 µm to 43.2±1.3 µm with the reinforcement of 8 wt. % of CNT.

Increased splat diameter was due to enhanced temperature of the melt as a result of

higher specific heat of Al2O3-8 wt. % CNT (1444 Jkg-1K-1) compared to Al2O3 (1358 Jkg-

1K-1). “Agglomeration of CNTs” at the periphery of the melt produced a counter effect

resulting in an increased viscosity. Increased viscosity of the melt suppresses

fragmentation and hence, higher percentage of disc shaped splat was observed in case of

CNT reinforced splats. Simulation of single splat showed good match in splat

morphology with the experimentally obtained splat. Volume percentage of the solid and

substrate temperature during splat formation elucidates the solidification behavior of

CNT reinforced splats.

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Acknowledgements

Authors acknowledge the financial support received from Office of Naval

Research (N00014-08-1-0494). AKK also acknowledge the support from the Dissertation

Year Fellowship awarded by University Graduate School at FIU. The support from

Advanced Materials Engineering Research Institute (AMERI), FIU for facilitating the

characterization facilities is greatly appreciated.

References

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[30] M. Pasandideh-Fard, V. Pershin, S. Chandra, J. Mostaghimi, J. Therm. Spray

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in technology and application: Proceedings of the international thermal spray conference (2004) 1008-1013

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List of Tables

Table I: Plasma spray operating parameters for splat formation

Plasma process parameters for synthesizing single splat

Current

(A) Voltage

(V)

Primary

Gas, Argon

(slm)

Secondary

Gas, Helium

(slm)

Stand-off

(mm) from the

substrate

Powder feed

rate

(g/min)

Substrate

preheat

temperature

(K)

850 40 56.6 59.5 75 3 453

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Table II: Thermo-physical properties of A-SD, A4C-SD and A8C-SD

S. No.

Property

Al2O3

(A-SD)

Al2O3-4 wt.

%CNT

(A4C-SD)

Al2O3-8 wt.

%CNT

(A8C-SD)

1.

Specific mass

(kg/m3) 3990 3850 3720

2.

Liquid phase kinematic

viscosity (m2/s)

1.026 x10-5

at 2473 K

1.098x10-5 at

2473 K

1.1704x10-5

at 2473 K

3.

Liquid phase thermal conductivity (W/m-K)

7.86

at 2327 K

204.7 at

2327 K

377 at

2327 K

4.

Liquid Phase Specific Heat (J/kg-K)

1358 at

2327 K

1404 at

2327 K

1444 at

2327 K

5.

Liquid Phase Surface

Tension (kg/s2)

0.69 at

2327 K

0.69 at

2327 K

0.69 at

2327 K

6.

Solid Phase Thermal Conductivity

(W/m-K)

36.16 at 298 K

5.90 at 1312 K

231.2 at 298 K

202.9 at 1312 K

401.9 at 298 K

375.4 at 1312 K

7. Solid Phase Specific Heat (J/kg-K)

772 at 298 K

1273 at 1312 K

1358 at 2327 K

767.5 at 298 K

1320 at 1312 K

1404 at 2327 K

763.6 at 298 K 1361.8

at 1312 K 1444

at 2327 K

8. Melting Point (K) 2327 2327 2327

9. Heat of Fusion (J/kg) 1.16x 106 1.16x 106 1.16x 106

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Table III: List of molten droplet and substrate parameters used for simulation of A-SD, A4C-SD and A8C-SD splats

A-SD A4C-SD A8C-SD

Initial particle diameter (µm) 30 26 24 Particle surface temperature (K) 2745 2423 2397 Particle velocity (m/s) 338 319 314 Initial substrate temperature (K) 453 453 453 Substrate surface roughness( µm) 0.03 0.03 0.03

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List of Figure Captions:

Figure 1: Particle size distribution of (a) Al2O3 (A-SD) spherical agglomerates (b) Al2O3-4 wt. % CNT (A4C-SD) spherical agglomerates (c) Al2O3-8 wt. % CNT (A8C-SD) spherical agglomerates.

Figure 2: (a) Plasma spray set-up for collecting single splats (b) Shield plate in front of the plasma gun having a series of holes of 2 mm diameter (c) Heat gun used to preheat the steel substrate (22 mm×19 mm×3.2 mm) which is positioned on the back of the shield plate.

Figure 3: (a) Digital picture of the steel substrate showing through thickness hole (Ø=1

mm) for inserting thermocouple (b) schematic of steel substrate showing dimensions of all faces and hole.

Figure 4: High magnification SEM images of spray dried (a) Al2O3 (A-SD) powder

particle (b) Al2O3-4 wt.% CNT (A4C-SD) particle showing homogeneous dispersion of CNTs on the surface (c) Al2O3-8 wt.% CNT (A8C-SD) particle showing homogeneous dispersion of CNTs on surface. Inset shows low magnification image of spray dried agglomerate of each composition.

Figure 5: SEM images of plasma sprayed single splat on polished steel substrate for (a) A-SD (b) A4C-SD (c) A8C-SD. Substrate was preheated to 453 K.

Figure 6: Splat size distribution of (a) A-SD (b) A4C-SD (c) A8C-SD. Total of ~100 splats were considered from 5-6 different SEM images to plot the splat size distribution.

Figure 7: (a) Variation in average splat diameter with function of CNT content. (b)

Variation in finger length with function of CNT content. Figure 8: (a) Experimentally collected splats showing different morphology (b)

Schematic showing different morphology of splats Figure 9: Population density of different type of splats as function of CNT content.

Figure 10: SEM image showing distributed CNTs in (a) A4C-SD (b) A8C-SD splats. Figure 11: SEM image showing distributed CNTs in the matrix and agglomerated

CNTsat the periphery of (a) A4C-SD (b) A8C-SD splats.

Figure 12: Schematic of splat formation in the case of Al2O2 and Al2O3-CNT splat.

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Figure 13: (a) Simulated top view of A-SD splat (b) SEM image of experimentally obtained A-SD splat (c) Simulated top view of A4C-SD splat (d) SEM image of experimentally obtained A4C-SD splat (e) Simulated top view of A8C-SD splat (f) SEM image of experimentally obtained A8C-SD splat.

Figure 14: Spreading ratio with varying CNT content for experimental and simulated splats.

Figure 15: Volume percentage of solid as a function of time for A-SD, A4C-SD and

A8C-SD splats.

Figure 16: Substrate temperature as a function of time for A-SD, A4C-SD and A8C-SD splat.

Figure 17: Schematic of the liquid droplet landing on the substrate and spreading into a thin splat at two different interval of time.

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Figures

Figure 1: Particle size distribution of (a) Al2O3 (A-SD) spherical agglomerates (b) Al2O3-

4 wt. % CNT (A4C-SD) spherical agglomerates (c) Al2O3-8 wt. % CNT (A8C-SD)

spherical agglomerates. A total of ~100 particles were considered from 5-6 different

SEMimages. .

(c)

10 15 20 25 30 35 40 4502468

101214 Al2O3- 8wt% CNT (A8C-SD)

Mean : 24.04 mSD : 7.3 m

No

. o

f P

art

icle

s

Particles Size ( m)

(a)

20 25 30 35 40 45 500

5

10

15

20 Al2O3 (A-SD)Mean : 30.18 mSD : 7.72 m

No

. o

f P

art

icle

s

Particles Size ( m)

(b)

20 25 30 35 40 45 500

5

10

15

20Al2O3- 4wt. % CNT (A4C-SD)Mean : 26.48 mSD : 6.92 m

No

. o

f P

art

icle

s

Particle Size ( m)

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Figure 2: (a) Plasma spray set-up for collecting single splats (b) Shield plate in front of

the plasma gun having a series of holes of 2 mm diameter (c) Heat gun used to preheat

the steel substrate (22 mm×19 mm×3.2 mm) which is positioned on the back of the shield

plate.

(a) (b)

(c)

Shield plate

Steel

substrate

Heat gun

Shield

plate

25 mm

25 mm

25 mm

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Figure 3: (a) Steel substrates showing through thickness hole (Ø=1 mm) for inserting

thermocouple and (b) schematic of the steel substrate showing dimensions of all faces

and hole.

22 mm

19 mm

3.2 mm

1.0 mm

(a) (b)

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Figure 4: High magnification SEM images of spray dried (a) Al2O3 (A-SD) powder

particle (b) Al2O3-4 wt.% CNT (A4C-SD) particle showing homogeneous dispersion of

CNTs on the surface (c) Al2O3-8 wt.% CNT (A8C-SD) particle showing homogeneous

dispersion of CNTs on surface. Inset shows low magnification image of spray dried

agglomerate of each composition.

(a)

(b) (c)

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Figure 5: SEM images of plasma sprayed single splat on polished steel substrate for (a)

A-SD (b) A4C-SD (c) A8C-SD. Substrate was preheated to 453 K.

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Figure 6: Splat size distribution of (a) A-SD (b) A4C-SD (c) A8C-SD. Total of ~100

splats were considered from 5-6 different SEM images to plot the splat size distribution.

26 27 28 29 30 31 3202468

101214

No

. o

f S

pla

ts

Splats Diameter ( m)

A-SD SplatsMean Diameter : 28.6 mSD : 1.43 m

(a)

32 33 34 35 36 37 380369

121518

A4C-SD SplatsMean Diameter : 34.7 mSD : 1.48 m

No

. o

f S

pla

ts

Splats Diameter ( m)

(b)

40 41 42 43 44 45 46 4702468

10121416

A8C-SD SplatsMean Diameter : 43.3 mSD : 1.31 m

No

. o

f S

pla

ts

Splats Diameter ( m)

(c)

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Figure 7: (a) Variation in average splat diameter (based upon ~100 splats) with function

of CNT content. (b) Variation in the finger length with function of CNT content. Error

bars corresponds to standard deviations associated to average value.

0 4 807

1421283542

Sp

lat

dia

mete

r (

m)

CNT content (wt.%)0 4 8

05

1015202530

Fin

ger

len

gth

(m

)

CNT content (wt%)

(a) (b)

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Figure 8: (a) Experimentally collected splats showing different morphology (b)

Schematic showing different morphology of splats

Disc splat Fingered splat Fragmented splat

Splats with

voids

Void

(1) (2) (3) (4)

Splat with

voids

(b)

(a)

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Figure 9: Population density (based upon total ~100 splats) of different type of splats as

function of CNT content

0 4 80

12243648607284

Po

pu

lati

on

(%

)

CNT content (wt.%)

Perfect disc splats with finger Fragmented splats having voids

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Figure 10: SEM image showing distributed CNTs in (a) A4C-SD (b) A8C-SD splats.

A4C-SD

CNT

(a) (b) A8C-SD

CNT

(a)

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Figure 11: SEM image showing distributed CNTs in the matrix and agglomerated CNTs

at the periphery of (a) A4C-SD (b) A8C-SD splats.

(a) (b)

Distributed

CNTs

Distributed

CNTs

Agglomeration

of CNTs

Agglomeration

of CNTs

A4C-SD A8C-SD

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Figure 12: Schematic of splat formation in the case of (a) Al2O2 and (b) Al2O3-CNT splat.

(b)

(a)

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Figure 13: (a) Simulated top view of A-SD splat (b) SEM image of experimentally

obtained A-SD splat (c) Simulated top view of A4C-SD splat (d) SEM image of

experimentally obtained A4C-SD splat (e) Simulated top view of A8C-SD splat (f) SEM

image of experimentally obtained A8C-SD splat.

(a) (b)

(c) (d)

(f) (e)

Splashing Broken finger

Finger

Finger Finger

Splashing

Splashing

Broken

Finger

Splashing

Splashing

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Figure 14: Spreading ratio (ratio of final splat diameter to initial droplet diameter) with

varying CNT content for experimental and simulated splats.

0 4 80.51.01.52.02.53.03.54.04.55.0

Experimental results

Simulation resultsS

pre

ad

ing

rati

o (

D/D

0)

CNT Content (wt. %)

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Figure 15: Volume percentage of solid as a function of time for A-SD, A4C-SD and

A8C-SD splats.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

10

20

30

40

50

60

70

Vo

lum

e o

f S

olid

(%

)

Time ( s)

HD-A-SD-180C HD-A4C-SD-180C HD-A8C-SD-180C

A-SDA4C-SDA8C-SD

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

10

20

30

40

50

60

70

Vo

lum

e o

f S

olid

(%

)

Time ( s)

HD-A-SD-180C HD-A4C-SD-180C HD-A8C-SD-180C

A-SDA4C-SDA8C-SD

T= 0.75 µs

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Figure 16: Computed substrate temperature as a function of time for A-SD, A4C-SD and

A8C-SD splat.

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Figure 17: Schematic of the liquid droplet landing on the substrate and spreading into a

thin splat at two different interval of time.

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Research Highlights

The effect of carbon nanotube (CNT) addition on the splat formation in plasma

sprayed aluminum oxide (Al2O3) composite coating using experimental and

computational methods is studied in this article.

The addition of CNTs makes splat morphology more disk-shaped with an increasing

diameter and lower splashing.

The addition of CNTs resulted in two simultaneously competing phenomena viz.

increased heat content and increased viscosity of the melt, which were responsible for

higher splat diameter and increased population density of disc shaped splats

respectively.

Splat morphology was also simulated and showed a good agreement with the

experimentally obtained splats.