A novel super-fine nanograin Al2CoCrFeNi coating deposited via the suspension high velocity air fuel (HVAF) spraying method

Al₂CoCrFeNi is a candidate material for thermal barrier coatings due to its high-temperature oxidation resistance and high hardness. Suspension high velocity air fuel (SHVAF) is a novel thermal spray technique that can fabricate coatings employing a finer powder, but with reduced risk of thermal degradation. In this study, Al₂CoCrFeNi coatings were deposited on to Hastelloy®X using SHVAF using a 1-4 µm mechanical alloyed feedstock. XRD, TEM and SEM analyses of the powder revealed the presence of B₂ and BCC phases, promoted by the presence of Al. Grain sizes of both phases were ~3-20 nm. In contrast, B₂, BCC, Al₂O₃ and FCC phases were detected in the coatings. The coatings contained two types of splat morphologies. Firstly, partially melted splats, exposed to higher temperatures, exhibited grain sizes of up to ~200 nm, with local partitioning of elements between the B₂ and BCC phases, as well as Al₂O₃ derived from in-flight oxidation. Secondly, unmelted splats, which comprised finer grains (~5-40 nm) that exhibited B₂ and BCC phases, as well as a low volume fraction of a FCC phase, but fewer oxides due to reduced IFO. The coating exhibited 9.4 ± 0.6 GPa hardness, attributed to the fine grain sizes and B₂/BCC phases.