AlxCoCrFeNiSi high entropy alloy coatings with high microhardness and improved wear resistance

AlxCoCrFeNiSi (x = 0.5, 1.0, 1.5, and 2.0 in molar ratio) high-entropy alloy (HEA) coatings are prepared by atmospheric plasma spraying and laser re-melting. The effects of Al addition on the formation of different crystal structures and the mechanical properties of this series of HEA coatings are evaluated in terms of microhardness and wear resistance. The phase formation of the AlxCoCrFeNiSi coatings is analysed based on the rules of HEAs. With an increase in Al content, the crystal structures transform from body-centred cubic (BCC) and face-centred cubic (FCC) to B2/BCC and Cr3Si phases. The formation of a simple solid solution is governed by the combined effects of mixing entropy (∆S), mixing enthalpy (∆H), atom-size difference (δ) and valence electron concentration (VEC). For high Al contents (x = 2.0), the maximum microhardness reaches 1255 HV. Simultaneously, the coefficient of friction reaches its lowest value of 0.25 when the worn surface is smooth with minimal debris. Moreover, the main wear mechanism is abrasive wear, and the wear resistance of the HEA coatings is enhanced.