**1. Introduction**

High-entropy alloys (HEAs), which are usually composed of at least five principal metallic elements in equimolar or near-equimolar ratios, were defined by Yeh et al. in 2004 [1]. Due to their outstanding properties (e.g., high strength, sufficient hardness, and excellent wear resistance) [2–6], HEAs are a kind of promising coating materials used in harsh environments, especially at high temperature. So far, most of the HEA coatings have been prepared by electrochemical deposition [7], magnetron sputtering [8,9], and laser cladding [10,11]. However, the preparation and industrial application of HEA coatings are limited owing to the low deposition efficiency, high residual stress, high dilution, and high cost of these techniques.

Compared with the above coating techniques, plasma spraying, which has been widely used to prepare coatings because of its low production cost and high efficiency, is an appropriate method to deposit HEA coatings. However, few investigations on plasma-sprayed HEA coatings have been done until now [12,13]. An AlCoCrFeNi coating with a Vickers hardness of about 421 HV and a MnCoCrFeNi coating with that of about 451 HV were deposited by plasma spraying [12]. Plasma-sprayed Ni*x*Co0.6Fe0.2Cr*y*Si*z*AlTi0.2 HEA coatings were investigated and a microhardness of 450 ± 30 HV was obtained for the NiCo0.6Fe0.2Cr1.5SiAlTi0.2 coating [13].

In addition, in the authors' previous study [14], an AlCoCrFeNiTi HEA coating (hereinafter referred to as HEA coating) was deposited by plasma spraying using ball-milled powder as a feedstock. Investigation results showed that a higher bonding strength of 50.3 MPa and an average microhardness of 642 HV were obtained for the coating at 25 ◦C. Meanwhile, the coating also exhibited better wear resistance at higher temperatures of 700 ◦C and 900 ◦C. However, at test temperatures of 25 ◦C and 500 ◦C, the volume wear rates were 0.77 ± 0.01 × 10−<sup>4</sup> mm3·N−1·m<sup>−</sup><sup>1</sup> and 0.93 ± 0.02 × 10−<sup>4</sup> mm3·N−1· m<sup>−</sup>1, respectively. These results indicate that the wear resistance of the coating at lower temperatures was not satisfactory.

For the plasma-sprayed AlCoCrFeNiTi protective coating on components of industrial machineries, improvement of its performances, such as hardness and wear resistance is quite necessary to meet the strict requirements of much severer conditions. Generally, to design and prepare a composite, i.e., adding a secondary material into the matrix is an effective approach to further enhance the hardness and wear resistance of the matrix materials. Until now, investigations on high-entropy alloy matrix composite materials and coatings have not been reported.

Usually, oxides, nitrides, borides, and carbides are chosen as reinforcements of composites [15,16]. However, owing to their brittleness, the toughness of the composites can be deteriorated significantly. As a conventional coating material with high hardness, sufficient strength, excellent wear, and heat resistances, Ni-based self-fluxing alloy has been widely used in fields of petroleum and chemical industries, power, and national defense [17,18]. By now, a large number of coating technologies, such as overlay welding, laser cladding, flame spraying, high velocity oxy-fuel spraying, and plasma spraying have been employed to deposit Ni-based self-fluxing alloy coatings [17–19]. In addition to the high hardness and excellent wear resistance at both room temperature and high temperature, good toughness makes Ni-based self-fluxing alloy a more appropriate reinforcement than the brittle traditional ones. However, no studies on composite materials and coatings reinforced by Ni-based self-fluxing alloy have ye<sup>t</sup> been reported.

Therefore, in this comparative study, to enhance the hardness and wear resistance of the AlCoCrFeNiTi coating, Ni60 was used as a reinforcement and added into the matrix. An AlCoCrFeNiTi/Ni60 coating (hereinafter referred to as HEA/Ni60 coating) was prepared by plasma spraying. Mechanically-blended AlCoCrFeNiTi/Ni60 powder (hereinafter referred to as HEA/Ni60 powder) was used as a feedstock. The microstructure of the plasma-sprayed coating was observed and analyzed. Bonding strength, microhardness, and wear resistance at 25 ◦C and 500 ◦C of the coating were comparatively investigated.

#### **2. Experimental Materials and Procedures**
