Exploring the impact of Mo addition in high-velocity oxygen-fuel (HVOF) sprayed CoCrFeNiMo0.5 high-entropy alloy coatings

In this research, the microstructural and mechanical aspects of a CoCrFeNiMo 0.5 high-entropy alloy coating (HEAC) deposited via high-velocity oxygen-fuel (HVOF) spraying are analysed. The investigation explores the cross-sectional and plan-view properties of the coating, which consists of unmelted splats (USs), semi-molten splats (SMSs), fully-molten splats (FMSs), oxides, and pores. Within the USs, Ni-Co-Fe-rich face-centred cubic (FCC) dendrites containing deformation twins are identified, accompanied by Mo-rich σ interdendrites. In SMSs, heavily deformed impact regions lead to ultra-fine elongated grains, while dynamic recrystallization (DRX) promotes (ultra-)fine equiaxed grains in their interior sections, both adopting a FCC structure. FMSs, on the other hand, exhibit fine columnar FCC grains with a completely uniform elemental distribution. Beyond the extremely hard σ phase, strengthening mechanisms, such as lattice distortion, grain refinement, and twinning-related strengthening, contribute to the mechanical strength of the coating. Notably, oxide dispersion strengthening occurs predominantly through spinels formed during the HVOF spraying process, playing a crucial role in improving the coating’s strength further. In contrast, the presence of small amounts of pores has a detrimental effect on mechanical properties, though this effect remains localized. • CoCrFeNiMo 0.5 HVOF-HEAC exhibits remarkable mechanical properties. • Spinel-type in-flight oxides yield oxide dispersion strengthening. • Deformation twins in FCC dendrites and σ interdendrites contribute to hardness.