Compressive mechanical properties of thermal sprayed AlCoCrFeNi high entropy alloy coating

Atmospheric plasma spraying (APS) was used to deposit an AlCoCrFeNi high entropy alloy (HEA) coating on stainless steel substrate. The as-deposited coating was about 300 mu m thick and the microstructure consisted of a nickel solid-solution matrix, together with a number of secondary phases/intermetallics. In addition, phase and splat boundaries were also prevailing. However, the extent of intermetallics and secondary phases were supressed, compared to other processing techniques (e.g., melting) of HEA, due to fast solidification in the APS process. In-situ micro-pillar compression was employed to evaluate the mechanical properties of the coating. The experimental results show that, mechanical properties of the coating in the cross-sectional direction (963.14 f 28.58 MPa of yield strength and 1005.58 f 22.08 MPa of compressive strength) is marginally higher than that of planar direction (802.33 f 43.76 MPa of yield strength and 817.73 f 43.84 MPa of compressive strength). The presence of secondary phases and/or intermetallics in the coating microstructure acts as a reinforcement medium to allow effective load bearing capacities of the coating. On the hindsight, phase and splat boundary areas are the 'weakest link', that acts as a slip/shear plan initiation site. The orientation of these phase/splat boundaries to that of direction of loading plays a significant role on the coating failure mode.