*3.1. TiB<sup>2</sup> Coating Microstructure*

Figure 1a,b are the SEM images of the TiB<sup>2</sup> coating surface prepared via APS at two different magnifications. Figure 1a shows that the coating internal structure was relatively dense and uniformly distributed. The coating mainly showed two colors of dark gray and white. Moreover, small cracks occurred in the coating. The formation of such microcracks depends on the melting of TiB2, the volume shrinkage of the particles, and the TiB<sup>2</sup> coating cooling process. Figure 1b shows that after the TiB<sup>2</sup> powder was remelted and recrystallized, the particle size of the coating was not uniform. The distribution of such particles led to the generation of pores in the coating, and the increase in porosity will reduce the coating hardness and the bonding strength between the coating and the substrate; moreover, it will destroy the conductive network of the TiB<sup>2</sup> coating and increase the coating resistivity.

Figure 2a,b are the cross-sectional topography images of the TiB<sup>2</sup> coating under two different magnifications. The figure shows a clear boundary between the substrate and the coating. The gray area is the TiB<sup>2</sup> coating, and the black area is the carbon block substrate. The coating was formed by injecting TiB<sup>2</sup> powder into a high-temperature plasma jet and then transporting it to the substrate. The temperature at the center of the plasma flame was as high as 32,000 K, and the temperature at the nozzle outlet was still up to 20,000 K. The TiB<sup>2</sup> powder particles were instantly heated to a molten or semi-melted state in the high-temperature plasma jet. When the particles hit the carbon block substrate surface, they spread into a flat liquid covering and clung to the concave and convex points on the surface of the carbon block substrate. It shrank and bit the anchor point during condensation. Therefore, the combination of the TiB<sup>2</sup> coating and the carbon block substrate was mainly mechanical embedded-type. Because the TiB<sup>2</sup> coating was formed by solidifying a single

particle as a unit to the substrate surface in a layered accumulation, the difference in particle size between the droplets resulted in some voids in the coating. Moreover, it can be seen from the cross section of the coating that the substrate and the coating. The bonding interface of the layer was clear, smooth, and tortuous, which shows that the coating was tightly bonded. *Minerals* **2022**, *12*, x FOR PEER REVIEW 4 of 11 **Figure 1.** Scanning electron microscopy (SEM) image of TiB2 coating surface morphology. (**a**) SEM of coating under 600 times, (**b**) SEM of coating under 1000 times. Figure 2a,b are the cross‐sectional topography images of the TiB2 coating under two

different magnifications. The figure shows a clear boundary between the substrate and

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**Figure 1.** Scanning electron microscopy (SEM) image of TiB2 coating surface morphology. (**a**) SEM of coating under 600 times, (**b**) SEM of coating under 1000 times. **Figure 1.** Scanning electron microscopy (SEM) image of TiB<sup>2</sup> coating surface morphology. (**a**) SEM of coating under 600 times, (**b**) SEM of coating under 1000 times. coating. The bonding interface of the layer was clear, smooth, and tortuous, which shows that the coating was tightly bonded.

the difference in particle size between the droplets resulted in some voids in the coating. Moreover, it can be seen from the cross section of the coating that the substrate and the **Figure 2.** TiB2 coatings sectional SEM micrographs. (**a**) SEM of coating under <sup>600</sup> times, (**b**) SEM ofcoating under <sup>1000</sup> times. **Figure 2.** TiB<sup>2</sup> coatings sectional SEM micrographs. (**a**) SEM of coating under 600 times, (**b**) SEM of coating under 1000 times.

#### coating. The bonding interface of the layer was clear, smooth, and tortuous, which shows *3.2. X‐Ray Diffraction Pattern of TiB2 Coating 3.2. X-ray Diffraction Pattern of TiB<sup>2</sup> Coating*

that the coating was tightly bonded. Figure 3 is the X‐ray diffraction (XRD) analysis result of the TiB2 coating prepared via APS. The figure shows that the coating was mainly composed of TiB2, TiO2, and a small amount of B2O3, indicating that TiB2 was oxidized during the spraying process. This was mainly due to the air involved in the plasma jet and the molten TiB2 particles during the Figure 3 is the X-ray diffraction (XRD) analysis result of the TiB<sup>2</sup> coating prepared via APS. The figure shows that the coating was mainly composed of TiB2, TiO2, and a small amount of B2O3, indicating that TiB<sup>2</sup> was oxidized during the spraying process. This was mainly due to the air involved in the plasma jet and the molten TiB<sup>2</sup> particles during the plasma-spraying process. Contact causes TiB<sup>2</sup> particles to be oxidized to form oxidation products such as TiO<sup>2</sup> and B2O3.

**Figure 2.** TiB2 coatings sectional SEM micrographs. (**a**) SEM of coating under 600 times, (**b**) SEM of

Figure 3 is the X‐ray diffraction (XRD) analysis result of the TiB2 coating prepared via APS. The figure shows that the coating was mainly composed of TiB2, TiO2, and a small amount of B2O3, indicating that TiB2 was oxidized during the spraying process. This was mainly due to the air involved in the plasma jet and the molten TiB2 particles during the

coating under 1000 times.

*3.2. X‐Ray Diffraction Pattern of TiB2 Coating*

plasma‐spraying process. Contact causes TiB2 particles to be oxidized to form oxidation

plasma‐spraying process. Contact causes TiB2 particles to be oxidized to form oxidation

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**Figure 3.** X-ray diffraction (XRD) patterns of plasma-sprayed TiB<sup>2</sup> coating. **Figure 3.** X‐ray diffraction (XRD) patterns of plasma‐sprayed TiB2 coating.

#### **Figure 3.** X‐ray diffraction (XRD) patterns of plasma‐sprayed TiB2 coating. *3.3. Wettability of TiB<sup>2</sup> Coating*

inert cathode in this study has good wettability.

products such as TiO2 and B2O3.

products such as TiO2 and B2O3.

*3.3. Wettability of TiB2 Coating* Figure 4 compares the peeling of molten aluminum from two samples. Figure 4c shows that the liquid aluminum spreading on the surface of the carbon block cathode was easily peeled off from the substrate and will not peel off with the carbon block material. Figure 4d shows that it is desired to spread the It is very difficult for the liquid aluminum on the surface of the layer to peel off from the TiB2 substrate. The adhesion of the liquid aluminum to the coating is strong, and during the peeling of the coating, part of the coat‐ ing materials peel off with the substrate; this also shows that the coating prepared by TiB2 Figure 4 compares the peeling of molten aluminum from two samples. Figure 4c shows that the liquid aluminum spreading on the surface of the carbon block cathode was easily peeled off from the substrate and will not peel off with the carbon block material. Figure 4d shows that it is desired to spread the It is very difficult for the liquid aluminum on the surface of the layer to peel off from the TiB<sup>2</sup> substrate. The adhesion of the liquid aluminum to the coating is strong, and during the peeling of the coating, part of the coating materials peel off with the substrate; this also shows that the coating prepared by TiB<sup>2</sup> inert cathode in this study has good wettability. *3.3. Wettability of TiB2 Coating* Figure 4 compares the peeling of molten aluminum from two samples. Figure 4c shows that the liquid aluminum spreading on the surface of the carbon block cathode was easily peeled off from the substrate and will not peel off with the carbon block material. Figure 4d shows that it is desired to spread the It is very difficult for the liquid aluminum on the surface of the layer to peel off from the TiB2 substrate. The adhesion of the liquid aluminum to the coating is strong, and during the peeling of the coating, part of the coat‐ ing materials peel off with the substrate; this also shows that the coating prepared by TiB2 inert cathode in this study has good wettability.

**Figure 4.** Characterization of wettability of TiB2 coating prepared using cathode carbon block and plasma spraying to liquid aluminum. (**a**) SEM image wetting effect diagram of cathode carbon block **Figure 4.** Characterization of wettability of TiB2 coating prepared using cathode carbon block and plasma spraying to liquid aluminum. (**a**) SEM image wetting effect diagram of cathode carbon block **Figure 4.** Characterization of wettability of TiB<sup>2</sup> coating prepared using cathode carbon block and plasma spraying to liquid aluminum. (**a**) SEM image wetting effect diagram of cathode carbon block on molten aluminum, (**b**) SEM image wetting effect diagram of TiB2 coating on molten aluminum, and comparative characterization of wettability of liquid aluminum after peeling, (**c**) SEM image of the peeling of the molten aluminum from the cathode carbon block, (**d**) liquid aluminum peeling from TiB<sup>2</sup> coating.

#### *3.4. Corrosion Resistance of TiB<sup>2</sup> Coating Analysis of Solubility Loss of TiB<sup>2</sup> Coating in Molten Aluminum 3.4. Corrosion Resistance of TiB2 Coating Analysis of Solubility Loss of TiB2Coating in Molten Aluminum*

on molten aluminum, (**b**) SEM image wetting effect diagram of TiB2 coating on molten aluminum, and comparative characterization of wettability of liquid aluminum after peeling, (**c**) SEM image of the peeling of the molten aluminum from the cathode carbon block, (**d**) liquid aluminum peeling

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from TiB2 coating.

Figure 5 depicts the change curve of the TiB<sup>2</sup> coating prepared via plasma spraying in molten aluminum. The figure shows that the quality of the TiB<sup>2</sup> coating remained unchanged and basically stable within 48 h of corrosion in high-temperature molten aluminum, demonstrating a good resistance to corrosion by molten aluminum. This is mainly because the TiB<sup>2</sup> material had good wettability to liquid aluminum and low solubility in liquid aluminum. Moreover, the TiB<sup>2</sup> coating prepared via plasma spraying was uniform and dense, demonstrating the good resistance of the TiB<sup>2</sup> material to liquid aluminum erosion. Figure 5 depicts the change curve of the TiB2 coating prepared via plasma spraying in molten aluminum. The figure shows that the quality of the TiB2 coating remained un‐ changed and basically stable within 48 h of corrosion in high‐temperature molten alumi‐ num, demonstrating a good resistance to corrosion by molten aluminum. This is mainly because the TiB2 material had good wettability to liquid aluminum and low solubility in liquid aluminum. Moreover, the TiB2 coating prepared via plasma spraying was uniform and dense, demonstrating the good resistance of the TiB2 material to liquid aluminum erosion.

**Figure 5.** Variation curve of TiB2 coating quality with erosion time after aluminum alloy erosion. **Figure 5.** Variation curve of TiB<sup>2</sup> coating quality with erosion time after aluminum alloy erosion.

According to the XRD analysis, after the TiB2 coating was corroded in high‐tempera‐ ture liquid aluminum (960 °C) for 120 min, no Ti component was detected, and the main component in the liquid aluminum was Al (Figure 6). The TiB2 coating prepared via plasma spraying could be well wetted by molten aluminum and had excellent corrosion resistance to molten aluminum. Moreover, the elemental analysis method was used to detect the Ti content in the molten aluminum. The Ti content of the molten aluminum was 0.0042%, which was only 0.0016% higher than that of the original aluminum (0.0026%). It was found that the titanium content in the original aluminum was 0.0026% (mass percent), and after the TiB2 coating was immersed in the aluminum solution for 48 h, the titanium content in the analyzed aluminum was 0.0042% (mass percent), which was only 0.0016% higher than the original aluminum content of 0.0026%. According to this calculation, the industrial tank was coated with 1 mm. The service life of thick pure TiB2 coating should be over four years. According to the XRD analysis, after the TiB<sup>2</sup> coating was corroded in high-temperature liquid aluminum (960 ◦C) for 120 min, no Ti component was detected, and the main component in the liquid aluminum was Al (Figure 6). The TiB<sup>2</sup> coating prepared via plasma spraying could be well wetted by molten aluminum and had excellent corrosion resistance to molten aluminum. Moreover, the elemental analysis method was used to detect the Ti content in the molten aluminum. The Ti content of the molten aluminum was 0.0042%, which was only 0.0016% higher than that of the original aluminum (0.0026%). It was found that the titanium content in the original aluminum was 0.0026% (mass percent), and after the TiB<sup>2</sup> coating was immersed in the aluminum solution for 48 h, the titanium content in the analyzed aluminum was 0.0042% (mass percent), which was only 0.0016% higher than the original aluminum content of 0.0026%. According to this calculation, the industrial tank was coated with 1 mm. The service life of thick pure TiB<sup>2</sup> coating should be over four years.
