Medium entropy alloy CoCrNi coatings: Enhancing hardness and damage-tolerance through a nanotwinned structuring

Medium entropy alloys (MEA) are defined as alloys consisting of three equiatomic elements, such as CoCrNi. MEAs are reported to have superior mechanical properties and high thermodynamic stability, as well as excellent fracture toughness at cryogenic temperatures. Here, we investigate a series of equiatomic medium entropy alloy coatings, containing three elements, Co, Cr, Ni. These coatings were deposited onto M2 steel substrates with a range of coating thicknesses using a DC magnetron sputtering system using a CoCrNi alloy target (1:1:1 at.%). The microstructure and mechanical properties were examined by a number of characterization techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. XRD analysis showed that the coatings were dominated by a fcc CoCrNi phase, with a smaller amount of hcp Co. TEM analysis demonstrated that the elongated grains contained a high density of {111} nanotwins. In addition, the residual stresses in the coatings were analysed using X-ray diffraction by adopting the conventional sin2ψ method. A high hardness value, ~ 10 GPa, was determined by nanoindentation of these coatings. Exceptional damage-tolerance was also found in these coatings under contact loading. It is believed that the nanotwinned structure is responsible for the high hardness and damage tolerance observed in the new coatings.