*2.2. Preparation of Plasma-Sprayed TiB<sup>2</sup> Coating*

During the TiB<sup>2</sup> coating preparation, first, the carbon block and TiB<sup>2</sup> powder were placed into the drying box and heated up to 100 ◦C for 4 h to remove the moisture, improve the TiB<sup>2</sup> powder fluidity, and improve the bonding strength between the carbon block and TiB<sup>2</sup> coating. Furthermore, TiB<sup>2</sup> powder was deposited on the graphite cathode carbon block using the DH-2080 plasma spray gun(Shanghai Ruifa Spraying Machinery Co., Ltd., Shanghai, China), with argon as the main gas, hydrogen as the secondary gas, and argon and hydrogen as the powder carrier gases. Before the preparation of the TiB<sup>2</sup> wettable cathode coating via spraying, the DH-2080 plasma spray gun was used to preheat the surface of the roughened and clean carbon block. The preheating temperature was maintained between 100 ◦C and 200 ◦C to shorten the temperature difference between the substrate and the TiB<sup>2</sup> powder and reduce the residual stress between the coating and the carbon block substrate. After the preheating, the plasma-spraying process parameters (Table 1) were adjusted, and the TiB<sup>2</sup> coating was sprayed on the carbon block substrate to obtain a TiB<sup>2</sup> coating with ~1000 µm thickness.

**Table 1.** Parameters of TiB<sup>2</sup> coating prepared via atmospheric plasma spraying (APS).

