Plasma sprayed equi-atomic AlCoCrFeNi HEA coatings are composed of a lamellar microstructure constituted by a mixture of three types of phases exhibiting distinct zcontrast; (i) a black (B) phase that is an Al-rich oxide phase, (ii) a grey (G) phase that is an AleCreFe rich oxide, and (iii) a white (W) phase that is an Al-depleted multicomponent HEA phase.
The ultra-high speed nanoindentation XPM technique was used for evaluating the localized hardness and reduced elastic modulus, in combination with in-situ scanning probe microscopy and scanning electron microscopy.
Statistical analysis of observed nanoindentation datasets revealed that the black phase exhibited the highest hardness and the white phase lowest. Also, pile-up around the white phase indicated that the alloy phase endured extensive localized plastic deformation. Moreover, Weibull regression analysis showed that hardness within the black phase is least homogenous compared to the other two phases. However, the reduced elastic modulus displayed only a minor difference among the phases.
Superimposing the concepts of statistical analysis and nanoindentation XPM mapping has been shown to be a powerful tool to establish relationships between local mechanical properties and multi-phase microstructures. This technique can also be exploited to evaluate the influence of interphase, neighbouring cracks and inter/intra splat behaviour, to understand the overall performance of coatings.
Residual stress in a plasma sprayed AlCoCrFeNi HEA coating is tensile since the quenching stress mechanism dominates over that of thermally generated stress from thermal expansion mismatches between the coating and substrate materials.
In nano-scale scanning wear test, the coating cross-sectional area with a higher fraction of the softer white phase exhibited a high wear volume loss and vice versa. The wear resistance of grey and black oxide phases generated by inflight oxidation (IFO) was superior to that of the multicomponent HEA phase (white), as evident by their substantial differences in wear depth. Concurrently, white phase with the lowest nanohardness suffered significant plastic deformation, as evident from pile-up around edges of the worn area; implying that cutting wear was the dominant wear mechanism operating at nanoscale.
Plasma sprayed AlCoCrFeNi HEA coatings exhibited superior wear resistance at high temperature (500 C) than at room temperature. A two-fold decrease in volume wear was observed. The wear mechanism changes from a combination of delamination wear and adhesive wear at room temperature, to oxidative and abrasive wear at elevated temperature.
Potentiodynamic polarization tests show that plasma sprayed AlCoCrFeNi HEA coatings displayed slightly lower corrosion resistance than SS316L substrate, in both general and localized corrosion. However, identical polarization curve characteristics indicated that AlCoCrFeNi-based HEAs are promising corrosion resistant materials.
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