Cold spray HEA coating surface microstructural characterization and mechanical testing

Novel High Entropy Alloy (HEA) coatings in the Al0.1-0.5CoCrCuFeNi and MnCoCrCuFeNi multi-materials systems on Mg substrate were prepared from mechanical alloyed HEA powder feedstocks and by three different Cold Spray (CS) process gas (N2) temperatures (650, 750 and 850 °C). Macro and microstructural characterization of mechanically alloyed and cold sprayed HEA coatings were carried out by macro photography, OM, SEM + EDS study, micro-hardness testing, roughness, and porosity measurements.

Mechanical alloying (MA) caused plastic deformation and fracture in harder particles. Relatively soft and ductile A1 phase and Cu-rich region particles were coarser and globular in shape. Some separate Cu-rich regions were also observed apart from A1 particles. Mn-HEA powder showed a different trend with finer particle sizes due to the more brittle nature of the powder and acicular shape. During MA, a loose structure with lots of gaps, cracks, plastic deformation signs, and small particles adhering to the particle surface is generated.

Based on the experimental data obtained, it cannot be concluded that the chemical composition of the high entropy alloy influences the roughness of the coating. The deposited volume increases with temperature only for Al_0.1 and Mg-based HEA, while for the other Al-based HEA no noticeable influences can be observed. The micro-hardness of a coating depends significantly on its chemical composition: as the percentage of aluminum increases in the samples micro-hardness increases. The hardness of the coating is significantly higher than that of the substrate, and the hardness measured at the interface is intermediate between the two values.