## 2 Materials and Methods  
Q245R steel plates (American grade: SA515 Gr60), which were provided by Baowu Steel Company (Wuhan, China), were used as the substrate material. Q245R steel plates were cut into rectangular specimens of dimensions 120 mm \(\times\) 60 mm \(\times\) 10 mm by a wire cutting machine. The sizes of the substrate plates are shown in Figure 1. The chemical composition was measured using the PDA-5500S Shimadzu optical emissions spectrometers (Shimadzu Corporation, Kyoto, Japan). The Q245R substrate plate surface was treated by abrasive papers and then dried after using alcohol to remove the dirt and oil. A comparison of measured chemical composition of the Q245R substrate plate and the standard of GB713-2014 (national standard of the people's republic of China for alloy structural steel) for Q245R steel is presented in Table 1. It can be seen in Table 1 that the chemical composition of the specimen used in this study meets the requirement of the national standard. Alloy power mixtures with a nominal composition of FeCoCrNiB\({}_{x}\) (\(x\): molar ratio, \(x\) = 0.5, 1, and 1.5, denoted by B\({}_{0.5}\), B\({}_{1.0}\), and B\({}_{1.5}\), respectively) were prepared by mechanically mixing commercially pure elemental powders (99.99 wt.%) with particle sizes below 400 mesh. The mixed powders were put into a vacuum oven at 80 \({}^{\circ}\)C to dry for 1 h after vacuum ball milling in order to remove moisture and enhance the liquidity of the powder. The plasma spraying technology [27] was adopted in the preparation of the FeCoCrNiB\({}_{x}\) HEA coatings. An appropriate amount of alcohol was taken as the adhesive and mixed with the aforementioned ball milling powder to obtain the viscous powder, which was coated on the Q245R steel matrix with a coating size of 32 mm \(\times\) 10 mm \(\times\) 1.6 mm. The coating was then put into the vacuum oven at 120 \({}^{\circ}\)C to dry for 2 h. Subsequently, Laser cladding was carried out using the YLS-4000-S2T-CL fiber laser system (IPG Photonics Corporation, Oxford, USA). After a series of optimization runs, the processing parameters were as follows: The laser radiation was at 980 nm wavelength, 4000 W output power, 1.5 mm \(\times\) 12 mm rectangular beam, and 180 mm/min scanning speed velocity. High-purity argon gas was used as shielding gas through the coaxial nozzle to prevent oxidation. Figure 2 presents the macroscopic morphology of FeCoCrNiB\({}_{x}\) HEA coatings, and the three different B contents were 0.5, 1.0, and 1.5, respectively. The crystal structures of the three different thickness HEA coatings were identified on the cross-section surfaces, using Bruker D8 Advance X-ray diffraction equipment AXS (Bruker AXS GMBH, Karlsruhe, Germany). The working voltage and current were 30 kV and 15 mA, respectively. The scan range was from 20\({}^{\circ}\) to 100\({}^{\circ}\), and the scanning rate was 4\({}^{\circ}\)/min. For microstructural observation, the microstructures and chemical compositions of the HEA coatings were analyzed with the use of an S-4800 scanning electron microscope (Hitachi, Tokyo, Japan) and energy dispersive spectroscopy (EDS). The Vickers micro-hardness of the polished longitudinal-section surfaces of the HEA coatings was measured using an HMV-G21 ST Vickers hardness tester (Shimadzu Corporation, Kyoto, Japan), and the test parameter was applied for 5 s under a load of 100 g. Each longitudinal-section surface was tested for multi points from the surface to the substrate with an equal interval. The wear tests were carried out using the M-2000A roller friction wear tester (YNS) Test Instrument co., LTD, Jinan China) \begin{table}
\begin{tabular}{c c c c c c c} \hline
**wt.\%** & **C** & **S** & **P** & **Mn** & **Si** & **Alt** \\ \hline Q245R & 0.18 & 0.012 & 0.022 & 0.65 & 0.022 & 0.031 \\ GB/T3077-2015 & \(\leq\)0.20 & \(\leq\)0.015 & \(\leq\)0.025 & 0.50–1.0 & \(\leq\)0.35 & \(\geq\)0.020 \\ \hline \end{tabular}  
S: sulfur, P: phosphorus, Alt: aluminum total.  
\end{table}