Sliding Wear of Spark Plasma Sintered CrFeCoNiCu High-Entropy Alloy Coatings: Effect of Aluminum Addition

High-entropy alloys (HEAs) are considered as attractive coating materials due to their high hardness, good wear and corrosion resistance. Al_xCrFeCoNiCu (x = 0, 1, 2, 3) high-entropy alloy coatings were fabricated on carbon steel substrate using spark plasma sintering. The microstructure evolved from FCC to FCC–BCC mixed structure. Al₃CrFeCoNiCu coating possesses an average hardness of approximately 682 HV_0.2, which is the highest hardness in all of the HEA coatings. The dry sliding wear behavior of the HEA coatings was investigated using a ball-on-disk sliding tribo-meter with a silicon nitride ball. In all of the HEA coatings, Al₃CrFeCoNiCu presents the lowest wear rate 8.81 × 10⁻⁵ mm³ m⁻¹ and Al₂CrFeCoNiCu has the lowest coefficient of friction 0.195 during 1000 m sliding distance at 0.1 m s⁻¹ sliding velocity under 20 N normal load. Comparing with AISI 52100 steel, spark plasma sintered Al₂CrFeCoNiCu and Al₃CrFeCoNiCu HEA coatings exhibit outstanding sliding wear resistance and extremely low friction coefficient. The wear resistance of Al₃CrFeCoNiCu HEA coating is approximately four times better than the bearing steel, showing a promising application as a wear-resistant material.