Subsonic-Flame-Sprayed CoCrFeNi, AlCoCrFeNi and MnCoCrFeNi-Based High-Entropy Alloy Coatings and Their Tribological Behaviors

High-entropy alloy coatings (HECs) possess important prospects for tribological applications. In this research, CoCrFeNi, AlCoCrFeNi and MnCoCrFeNi-based HECs were prepared by subsonic flame spraying (SFS) process, which is an extremely convenient and very low-cost thermal spraying technology. The microstructure, phase composition, Vickers hardness, tribological behavior and wear mechanisms (as well as the effect of counterpart ball material, i.e., GCr15 bearing steel or Al 2 O 3 ) of the HECs were studied under paraffin lubrication. The results indicated that the MnCoCrFeNi-based HEC had excellent tribological behaviors; the lowest friction coefficient (COF) (0.14) and wear rate (3.14 × 10 –7 mm 3 N −1 m −1 ) of the MnCoCrFeNi-based HEC were obtained with the GCr15 counterpart, where the dominant wear mechanisms were mild abrasive wear and tribo-oxidation wear. The combination of such factors as the higher oxide content, Vickers hardness and fracture toughness as well as the tribo-oxidation layer formed at the sliding interface significantly improved the wear resistance of the MnCoCrFeNi-based HEC sliding against GCr15 counterpart under micro-vibration reciprocating sliding mode. In contrast, the AlCoCrFeNi and CoCrFeNi-based HECs under the same sliding condition revealed much higher COF value and wear rate as well as quite distinct primary mechanisms of wear, i.e., abrasive, adhesive and tribo-oxidation wear for the AlCoCrFeNi-based HEC, adhesive wear with delamination for the CoCrFeNi-based HEC. When Al 2 O 3 was utilized as counterpart, slight and typical abrasive wear occurred in the MnCoCrFeNi and AlCoCrFeNi-based HECs, respectively, while the primary wear mechanisms of the CoCrFeNi-based HEC changed to severe adhesive wear.