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# High Temperature Materials and Processes

Editor-in-Chief: Fukuyama, Hiroyuki

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Volume 38, Issue 2019

# The Morphology Analysis of Plasma-Sprayed Cast Iron Splats at Different Substrate Temperatures via Fractal Dimension and Circularity Methods

Qiulan Wei
• Department of Automotive Engineering, Shaanxi College of Communication Technology, Xi’an, Shaanxi 710018, China
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• Other articles by this author:
/ Li He
• Engineering Research Center of the Ministry of Education for Pavement Materials, School of Materials Science and Engineering, Chang’an University, Xi’an, Shaanxi 710061, China
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• Other articles by this author:
/ Zhang Liu
• Engineering Research Center of the Ministry of Education for Pavement Materials, School of Materials Science and Engineering, Chang’an University, Xi’an, Shaanxi 710061, China
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/ Xiao Feng
• Engineering Research Center of the Ministry of Education for Pavement Materials, School of Materials Science and Engineering, Chang’an University, Xi’an, Shaanxi 710061, China
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/ Ya-Zhe Xing
• Corresponding author
• Engineering Research Center of the Ministry of Education for Pavement Materials, School of Materials Science and Engineering, Chang’an University, Xi’an, Shaanxi 710061, China
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Published Online: 2019-07-29 | DOI: https://doi.org/10.1515/htmp-2019-0021

## Abstract

Plasma-sprayed cast iron splats were deposited onto polished aluminum substrates preheated to different temperatures ranging from 25°C to 250°C. The morphology of single splat was observed by a field emission scanning electron microscope. Quantitative characterization methods, including fractal dimension (FD) and circularity analyses of the splat profile, were employed to identify the difference in morphology of the splats with the change of the substrate temperature. The results showed that the substrate temperature has a significant effect on the spreading of molten droplets and the morphology of resultant splats through changing the solidification rate of the droplets. With the increment of substrate temperature, the homogeneous and sufficient spreading of the droplets resulted from low solidification rate reduces the splashing of the droplets. In addition, the evaporation of adsorbed moisture on the substrate improves the wettability between the spreading droplet and the substrate, then benefits the homogeneous spreading of the molten droplet. As a result, a distinct decline in the FD value was observed. It was also suggested that the FD analysis could be used to characterize the morphology of the splat more effectively while the circularity method was heavily dependent on the area of the splat.

## Introduction

In the field of mechanical manufacturing, atmospheric plasma spray technique has been widely used to prepare metallic and ceramic coatings on mechanical parts to protect the parts surfaces from abrasion, corrosion and heat [1, 2, 3, 4]. It is well known that the plasma-sprayed coatings are built-up by a stream of molten particles which coalesce and solidify after impacting on rough substrate or solidified coatings. It was also confirmed that the formation of a splat is nearly independent [5]. Obviously, the formation of individual splat has great effect on the properties of the coatings, and then affects the lifetime of machinery parts.

In conclusion, the morphology of the splats affects the microstructure of the coating, and then affects the performance of the coating. Therefore, it is significant to employ some feature parameters to quantitatively characterize the morphology of the splat. These parameters can establish some quantitative relationships, such as the relationship between the spray parameters and the splat morphology and the relationship between the splat morphology and the properties of the coating. That is to say, a quantitative description of the splat morphology can contribute to establishing a quantitative relationship between the spray parameters and the coating properties. Chen et al. [22] employed the image analysis method to identify the splat in the field emission scanning electron microscope (FESEM) image. The fractal dimension (FD) and the solidity were used to quantitatively characterize the morphology of the splat. Kang et al. [23] defined the aspect ratio and the fractional factor to investigate the influence of the impacting angle on the spreading and the solidification behaviors of a molten droplet. The results showed that both the aspect ratio and the fractional factor increase with the increase in the inclination angle of the substrate. Mulero et al. [24] developed an image inspection system to characterize the splat in terms of its area, perimeter, circularity, eccentricity and solidity, which can quantify the morphology of a thermally sprayed splat.

In the present work, polished aluminum substrates were preheated to different temperatures, then the cast iron splats were plasma-sprayed onto the preheated substrate surface. The morphologies of the splats were quantitatively characterized by the circularity and the FD of the splat periphery. The effect of the substrate temperature on the geometric parameters (circularity and FD) of plasma-sprayed splats was investigated.

## Materials and preparation of individual splats

Commercially available Fe-4.1C-1.5Si-1.5B-36.6Cr (in weight percentage) powder (DG.Fe-05, Chengdu Daguang Thermal Spraying Materials Co. Ltd., China) was used as the starting powder The powder consists of spherical particles with a range in size from 20 to 75 μm (as shown in Figure 1). A commercial plasma spray system (GP-80, Aerospace Research Institute of Materials & Processing Technology, China) was employed to deposit the cast iron splats. Pure aluminum disks (99.7%, 20 mm in diameter, 4 mm in thickness) were used as the substrate. Before spraying, all aluminum disks were mirror-polished until their surface roughness (Ra) reached about 0.03 μm, which was measured by a surface roughmeter (TR-240, Beijing Time Group, China). The traverse speed of the plasma torch was fixed at 1,200 mm/s. The flow rate of the powder was fixed at 4 L/min. The substrate was preheated by the plasma jet without carrying powders, and the preheating temperature was monitored using a pyrometer (RAYRPM30L3U, Raytek, USA). When the temperature of the substrate surface reached a set value shown in Table 1, the powders were immediately carried into plasma jet. Table 1 lists all the spray parameters for the deposition of the cast iron splats on the aluminum surface. Obviously, in addition to the difference of the substrate temperature, other preparation conditions for all samples were identical.

Figure 1:

The morphology of the feedstock powders.

Table 1:

Spray parameters for the deposition of cast iron splats on the aluminum surface.

## Morphology and geometric parameters of individual splats

After spraying, the morphology of the splats deposited under different conditions was observed by a commercial FESEM (S-4800, Hitachi, Japan). For each sample, ten images of individual splats were obtained with a resolution of 1,024 × 768 pixels and 24-bits per pixel. The magnification of all selected images is 500×. The calculation process of the geometric parameters of single splat contains the following steps: (1) image denoising, (2) binarization processing, (3) Canny algorithm edge detection, (4) FD calculation of the splat periphery using the box-counting method, (5) circularity calculation of the same splat after the FD calculation. All calculations were conducted with a commercially available software, MATLAB R2014. As reported by Ding and Goshtasby [25], the object boundaries in the image can be obtained by a Canny edge detector. The Canny edge detector classifies a pixel as an edge if the gradient magnitude of the pixel is larger than those of the pixels at both its sides in the direction of the maximum intensity change. Therefore, we used the Canny edge detector to obtain the boundaries of the splat. In the process of edge detection, the splashed parts produced during the spreading of the molten droplet were removed because they were not connected to the main part of the splat.

Circularity specifies the relationship between the area and the perimeter of a splat (see eq. (1) [24]) and signifies the degree of approximation of a splat to a circle. Furthermore, area is calculated by adding the actual number of pixels in the region. Perimeter is defined as the distance around the periphery of the region, and it is measured by calculating the distance between each adjoining pair of the pixels around the border of the region.

(1)

The FD was utilized to quantitatively describe the complexity of the periphery of a splat. A box-counting method was employed to evaluate the FD. In the process of the box-counting method, a series of squares were used to cover the profile of a splat. The relationship between the square size ε and the number of the squares N(ε) can be expressed as [26]:

$N\left(\epsilon \right)\propto {\epsilon }^{-D}$(2)

where D is the FD of the periphery of a splat, which can be calculated by the linear regression given as [26]:

$logN\left(\epsilon \right)=-Dlog\epsilon +C$(3)

## Effect of substrate temperature on geometric parameters of the splats

Figure 2:

The changes in the FD (a) and the circularity (b) of the splats with the substrate temperature.

Figure 3:

Typical splat deposited on the substrate preheated to (a) 25±2°C, (b) 150±8°C and (c) 250±10°C. The left micrographs are FESEM images and the right micrographs are the corresponding images after Canny algorithm edge detection.

As the substrate temperature further increased to 250 ± 10°C, the FD decreased continuously and the circularity increased continuously. With preheating the substrate to a higher temperature, the cooling rate of the droplets decreased due to the reduced temperature difference between the droplet and the substrate and the oxidation of the substrate surface [14], delaying the solidification of the spreading droplets and then causing the homogeneous and sufficient spreading of the droplets. As a consequence, a regular disk splat was expected (see Figure 3(c)) and both the minimum FD value and the maximum circularity value were obtained in the present temperature range.

## Relationship between the FD and the circularity

As mentioned in the “Effect of substrate temperature on geometric parameters of the splats” section, both a higher FD and a lower circularity correspond to an irregular splash splat. This indicates a potential relationship between the FD and the circularity. Therefore, we chose 30 splats (ten splats for each sample) to plot the relationship between the FD and the circularity of them, as shown in Figure 4. Meanwhile, the linear fitting of the data was also conducted. Apparently, there was a negative linear correlation between the FD and the circularity due to the high linear fitting correlation coefficient (R = 0.9772).

Figure 4:

Relationship between the FD and the circularity of the splats.

As mentioned in the “Morphology and geometric parameters of individual splats” section, the circularity signifies the degree of approximation of a splat to a circle, and the FD describes the complexity of the periphery of a splat. Despite the difference in the concepts of two geometric parameters, a strong correlation exists between them (see Figure 4). This is related to the formation mechanism of the splats. Figure 5 shows the schematic diagrams of forming processes of two typical splats with different morphologies. In Figure 5, the arrows signify the spreading direction of the melts, and the length of the arrows represents the spreading speed. When the spreading speeds of the droplet in different directions are identical, a disk splat (see Figure 4(a)), which is close to a circle without complex periphery, is prone to form. Inversely, when the spreading speeds of the droplet in all directions are different, a splash-shaped splat (see Figure 4(b)), which possesses an irregular shape with complex periphery, is easy to form. That is to say, the FD and the circularity are consistent in characterizing the morphology of a plasma-sprayed splat. High circularity of a splat signifies a uniform spreading of the molten droplet, which leads to the reductions in the complexity and the FD value of the resultant splat.

Figure 5:

Schematic diagrams of the forming processes of typical morphologies of two splats. The left images present the morphologies of the splats at an early stage of spreading, while the right images present the morphologies of final splats.

## Applicability of the circularity

According to the previous analyses, there was a negative correlation between the FD and the circularity (see the fitted line in Figure 4). However, some outliers in the data of individual splats were far away from the straight line. Moreover, these outliers were concentrated in a circularity range of 0.2586–0.5976 and a FD range of 1.2412–1.2918, which were marked by the dotted square box in Figure 4. This indicates that the values in this small range do not fit the straight line. Therefore, in this range, two typical splats (see Figure 3(a) and 3(b), marked by SP1 and SP2 respectively) were selected from sample A and sample B to reveal the changes in the FD and the circularity of the splats. The corresponding values were listed in Table 2. It was found that the FD values of two splats were approximately identical, while the circularity of SP2 was about twice that of SP1. Furthermore, the area of SP2 was 3.7 times that of SP1, and the perimeter of SP2 was 1.4 times that of SP1. These indicate that the area of the splat seriously affects the circularity value. Even if the complexities of the peripheries of two splats were approximately similar, the circularity value increased significantly with the increase in the area of the splat. Generally, the area of the splat is determined by both the particle size of the starting powder and the spreading of the molten droplet. With a certain flattening ratio, thereby, the particle size of the starting powder will be the main factor affecting the area of the splat. As for the present work, the size of the powder used ranges from 20 to 75 μm. Such a wide range of particle size makes the final area of the splat different, leading to a wide range of the circularity value. Therefore, in order to use the circularity to characterize the morphology of the splats, a small difference in the size of the starting powders is required. The comparability in the circularity is only valid for the powders with comparable sizes. In contrast with the circularity, the FD is more suitable for characterizing the morphology of the splats with different sizes.

Table 2:

The main parameters of two splats.

## Conclusions

Individual cast iron splats were prepared on the mirror-polished aluminum substrates preheated to different temperatures. The morphologies of the resultant splats were quantitatively characterized by both the FD and the circularity. With increasing the preheating temperature, the FD value of the splats decreased and the circularity value increased. For the splats deposited at room temperature, a maximum FD value, which corresponded to a complex periphery of the splats, was acquired due to the strong splashing of the droplet induced by the evaporation of the adsorbed moisture on the substrate surface. With preheating the substrate to 150 ± 8°C, most of the water on the substrate surface was removed at a high temperature and the wettability between the droplet and the substrate surface was improved. This caused the homogeneous spreading of the droplet and consequently led to a reduced FD value. When the substrate was further preheated to 250 ± 10°C, the homogeneous and sufficient spreading of the droplet caused by lower solidification rate significantly suppressed the splashing of the droplet. As a result, the FD value decreased while the circularity increased accordingly.

Compared with the circularity, the FD was more excellent in characterizing the morphology of splat. The FD and the circularity were intrinsically consistent when using them to characterize the morphology of the splat. However, the circularity is heavily dependent on the area of the splat. Therefore, reducing the size difference of the original powders can enhance the applicability of the circularity in characterizing the morphology of the splats.

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Accepted: 2018-12-05

Published Online: 2019-07-29

Published in Print: 2019-02-25

This work was supported by the Special Scientific Research Project of Shaanxi Provincial Department of Education (18JK0081).

Citation Information: High Temperature Materials and Processes, Volume 38, Issue 2019, Pages 692–698, ISSN (Online) 2191-0324, ISSN (Print) 0334-6455,

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