Abstract
This article focuses on the microstructure and mechanism analysis of the novel coating MgO– B2O3–SiO2–Zn prepared by plasma spraying and flame spraying on the SUS304 alloy substrate. Firstly, the crystal compositions of the powders were analyzed by XRD (X-ray diffraction). Secondly, the surface and cross section microstructures of the coating were observed by SEM (Scanning Electron Microscope) and EPMA (electron probe microanalysis) in order to study the mechanism of the thermal insulation performance. It is concluded that, the microstructure, different thickness and compositions have various effect on heat insulation.
1. Introduction
In the 21st century, the awareness of reducing environmental burdens and preventing resource depletion has continued to increase on a global scale. Indeed, it can be said that mankind should regard the Earth as a single life form. This is already a consensus. In response to these trends in the new era, the material field urgently needs new materials that can eliminate harmful substances, control global warming, and achieve safety, recycling, and longevity [1]. Surface technology is playing an unprecedented role.
On the one hand, magnesium materials have the characteristics of good heat insulation [2, 3]. Light material is an important part of magnesium heat insulation material, and its structure and performance directly affect the performance of magnesium materials [4]. According to the heat transfer mechanism, the scale of pore size have significance effect on the heat insulation of the material. Microporosity of materials helps improve the heat insulation performance [5]. Especially when pores are fine, spherical, and closed, the convective heat transfer inside the material can be greatly reduced. On the other hand, Thermal plasma spraying enables preparation of material coatings via a single-step process in which the material is melted and solidifies under controllable atmospheric conditions [6]. Spraying has been used in the fabrication of various coating, including alloy [7] and ceramic [8].
Facing the demand for heat insulation and temperature control of equipment in the automotive industry in order to solve the problem of the efficiency of thermal insulation coatings on metal substrates, we proposed a new type of MgO–B2O3–SiO2–Zn-based ceramic powder thermal insulation materials to improve the performance of gas turbines [9]. In this research, SUS304 is utilized as the substrate, which is a typical austenitic stainless steel with excellent corrosion resistance, toughness, ductility, workability, weldability, and a wide range of uses [10]. It is a sort of chromium-nickel stainless steel with 18% Cr-8% Ni as the main component and austenite with excellent corrosion resistance as the metal structure. In order to effectively suppress the heat dissipation of high-temperature components to save power, a heat-insulating layer is covered on the surface of the base materials by spraying technology. This technology can alleviate the difference in thermal expansion, and the thermal insulation layer contains many spherical or layered pores, which reduces the thermal conductivity. The purpose of this research is to provide a thermal insulation material with its manufacturing method that can be applied to plasma spraying, flame spraying, form a coating with a thermal insulation effect on the surface of the substrate, and provide a thermal insulation coating composed of thermal insulation and the insulation coating’s forming method. This article discusses the structural properties of the materials.
2. Materials and Methods
The EPMA uses characteristic X-ray produced by the action of electron beam to analyze composition [11, 12]. Since the device contains a scanning electron microscope, the measurement part can be observed in real time and the sample can be analyzed nondestructively [13]. The EPMA can be widely used in metal, ceramics, semiconductors, rocks, minerals, polymer materials, medicine, dentistry, biology, and other fields [14–17].
2.1. Experimental Material
2.1.1. SUS304 Substrate
In this study, SUS304 is used as substrate with a size of 30 mm × 30 mm × 3 mm (obtained from Nippon Steel Corporation in Japan). Table 1 shows the chemical compositions of SUS304. In order to remove the impurities on the surface, samples were cleaned by the mixture of acetone and ethanol (1 : 1) with ultrasonic cleaning machine for 3 minutes, and then were washed and dried.
2.1.2. Thermal Spray Materials Design
In this study, the coating is prepared by thermal spray method. MgO powder is used as the main ingredient of coating materials. Besides that, B2O3, SiO2, and Zn are mixed with MgO in different proportions. The purity of all used powders are 99.99%, which are from Arai Co., Ltd, Gunma, Japan.
Different proportions are discussed and compared in this paper, and the compositions of spraying materials are shown in Table 2. In this paper, compositions are all expressed by percentage of weight (wt%). For each spray material, thermal spray was conducted at different thickness including 100, 200, 300, and 500 μm.
2.2. Thermal Spray Method and Process
In the previous section, the composition and proportion design of the spray powder in this paper are given. The powder can be coated on the substrate by plasma thermal spray (conducted by TAFA model by Praxair Surface Technologies, USA) and flame thermal spray(carried on Castolin Eutectic CastoDyn DS8000, Lausanne, Switzerland), forming a layer of heat insulation coating on the surface of the substrate, which can achieve excellent heat insulation effect. Figure 1 shows the process of spraying coating fabrication. The specific process is provided in reference [9].
Preheating enhances the bonding strength between the coating and substrate. Otherwise, the temperature of the substrate is low and the spraying material cools rapidly when reaching the substrate, which have a bad effect on the diffusion between substrate and spraying material. Besides that, if the substrate is not preheated, the cooling rate is high. It leads to a difference between the coefficient of thermal expansion of substrate and spraying material, damage.
2.3. XRD Composition Analysis
In order to confirm the final composition of the powders designed, the crystal was analyzed by XRD. The material A is taken as an example to demonstrate the microstructure by XRD (carried on Regaku Corporation ZSX Primus II, Tokyo, Japan). The analysis was conducted for 100, 200, 300, and 500 μm thickness, respectively. Figure 2 shows the intensity spectrum of each composition in A for different thickness. Compared with Table 2, the final composition of A was confirmed.
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Figures 3 and 4 show the XRD spectrum of coating B and C with different composition design. According to Figure 2, the analysis result is similar for different thickness, hence, only XRD spectrums of B and C at 500 μm are demonstrated in this section. Compared with Table 2, the main compositions are clarified by the peak intensity.
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3. Results of Microstructure Observation
In section 2, the design of MgO– B2O3– SiO2–Zn series powders are provided, and its preparation methods on the SUS304 substrate materials by plasma and flame thermal spray are given. In reference [9], the heat insulation performance of coatings with different compositions were discussed at different thickness. In this paper, microstructure observation methods are utilized to analysis the mechanism of heat insulation performance.
3.1. SEM Observation of Microstructures
In reference [9], we have discussed the thermal insulation performance of the coatings with different compositions. Obviously, the increase of thickness help improve the insulation performance effectively. As an example, the temperature analysis of material A is showed in Figure 5. Figure 5 shows the temperature insulation measurement results of material A at different thickness. Similar with A, the insulation performance of B and C increases gradually along with the coating thickness. Here the entire temperature data of B and C is not listed to avoid redundancy. We gives the best thermal insulation result at 500 μm for A, B, and C in Figures 6 and 7.
In conclusion, by comparing the thermal images of coatings with different compositions, on the one hand, coating MgO–B2O3–Zn has significant performance under high temperature, and its performance grows along with the increase of Zn. On the other hand, in MgO–B2O3–SiO2–Zn, SiO2 also helps insulate heat conduction significantly, and the best percentage is 30%. But the effect from SiO2 is limited, more and less SiO2 both have a negative effect on heat insulation.
Scanning electron microscope (SEM) is utilized to observe the surface and cross section morphology. The advantage of SEM was that it can evaluate the shape of the sample three-dimensionally at a magnification that exceeds the resolution of the optical microscope [18–20]. In this section, similarly, the material A was taken as an example to demonstrate the microstructure by SEM (carried on JEOL JSM-7500F, Tokyo, Japan). The analysis was conducted at 100, 200, 300, and 500 μm. Figures 8 and 9 show the surface and cross section microstructures of coating A at different thickness, respectively.
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The surface with a thickness of 100 μm mainly shows the morphology of bulk tissue. As the thickness increases, the elements diffuse into the matrix significantly. With a thickness of 200 μm on the surface, high-speed particles hit the substrate surface and form layered structure. The surface morphology with a thickness of 300 μm was distributed in the form of clusters, with many small holes and uniform particle distribution.
In Figure 9, cross section consists of the coating, contact area, and SUS304 substrate, which are combined as layered structure from top to bottom. With the 200 μm thickness, the contact area is directly in contact with the substrate with lower temperature. Due to good thermal conductivity, the contact area forms a denser coat than the coating. With 300 μm thickness, the cooling time is very short when the powder come into contact with the surface of substrate, so it is easy to maintain the original state and form different melting areas in the coating. When the thickness of the coating reach 500 μm, the particle cannot be completely flattened, and it is difficult to enter the pores of other particles, forming small holes. And the porosity of the coating increases obviously.
In reference [9], we have concluded that compared with thinner coatings, 500 μm coatings have better heat insulation performance and are thick enough to insulate. Hence, in this part, only microstructure of 500 μm coatings are given and compared Figures 10–13 show the surface and cross section microstructure of coatings B and C with different compositions.
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As shown in Figure 10 to 13, the morphology and particles of coatings with a thickness of 500 μm is uniform, and the porosity grows with the increase in surface heat capacity of the particles . It is found that the coating and the substrate are well combined with the subdivision phenomenon of crystal particles, and the structure is fine. Compared with Figure 8 of material A, Figure 10 of material B shows more pores, and the pores are more uniform. This help form a better insulation layer. It is because there is SiO2 in B. Besides that, in Figure 12, the pores in coating C with SiO2 and Zn are dense and fine. This does not appear in coating A. In conclusion, SiO2 and Zn powders help improve the insolation performance.
3.2. EPMA Observation of Coating Microstructure
In this part, we use EPMA (carried on JEOL JXA-8900R, Tokyo, Japan) to perform element analysis on the surface and measure the distribution of each element of Mg, B, O, Zn, and Si.
Figure 14 shows the elements in coating A at thicknesses 100, 200, 300, and 500 μm. EPMA is the surface image analysis. The coating and the substrate bond well. The substrate molecular composite powder diffuses slightly, and the diffusion depth is 1–5 μm. The bonding strength between the coating and the substrate is improved, and a more dense organization is promoted. Similarly, the analysis result of coating B and C with different compositions are demonstrated by Figures 15 and 16.
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4. Conclusion
In this study, ceramic particles with low thermal conductivity are utilized to fabricate insulation coating, which help suppress the heat dissipation of high-temperature components in auto. We proposed a novel ceramic coating, which is fabricated with MgO, B2O3, SiO2, and Zn particles by plasma and flame spraying. By microstructure analysis, on the one hand, increasing thickness improves the porosity of the spraying coating. The particle is difficult to enter the pores of other particles. It helps to form small holes and improve insulation. On the other hand, it is proven that the thermal insulation performance of materials with SiO2 and Zn are obviously better than coating only consisting of MgO and B2O3. MgO–B2O3–SiO2–Zn coating performs well at a thickness of 500 μm. It is concluded that SiO2 and Zn are recommended to be added in the insulation material mainly consisting of MgO.
Data Availability
The experimental data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research is supported by Shenzhen Polytechnic.