Other compositions based on CrFeCoNi have also been explored via the TS route. Addition of Mo to the quartet was studied by Li et al. as CrFeCoNiMoo2 and by Mu et al. as equiatomic CrCoFeNiMo for APS and HVOF coatings. The CrFeCoNiMoo2 HVOF coatings showed a markedly lower oxide content of 12 vol.%compared to 47 vol.% for their APS counterparts, in addition to having <1% porosity. The FCC phase of the GA feedstock transformed to FCC+ iron oxides and spi-nels,a result of IFO during both processes. There is also the possibility of losing molybdenum as volatile oxidesduring APS, which further substantiates the use of opti-mized HVOF parameters when spraying oxygen-sensitive materials. Mu et al. obtained similar results andidentified the multiple oxide phases using x-ray photo-electron spectroscopy (XPS) and transmission electron microscopy(TEM). Nano-oxide reinforcement of the APS coating due to IFO was also observed. It has been noted that for the same spray parameters, the depositionefficiency of CrFeCoNiMo was lower than that for AlCr-FeCoNi, due to the higher melting point of the former alloy, resulting in fewer instances of melting and splatting.
Wang et al. developed (CrFeCoNi) osNbs coatings via vacuum plasma spray(VPS), employing GA FCC HEA powders. VPS is currently the preferred alter-native to APS for spraying oxygen-sensitive materials but is also an expensive process where the object to be coated is constrained by the size of the vacuum chamber. No oxide peaks were detected in the VPS HEA coatings, and a dense, homogeneous microstructure was observed. In comparison with the as-cast form, where an FCC matrix +HCP-ordered Laves phases were observed, VPS resulted in a supersaturated FCC solid solution with a much lower degree of precipitation. Phase separation into CrFeCoNi-rich, Cr-Nb-rich, and CoFeNb-rich phases was observed in the coating cross section.
Cantor alloy, CrMnFeCoNi, has also been deposited by Yin et al. via cold spray using GA feedstock. The FCC particles of dendritic morphology did not undergo any phase transformation, nor was there oxidation of the coating. The coatings were dense, aided by the ductile nature of the alloy, which is advantageous since bonding in CS depends on plastic deformation of feedstock particles.The coating also exhibited significant grain refinement from strains generated from particle deformation on impacting the substrate. Wang et al. also coated GA CrMnFeCoNi particles via APS and reported FCCphase being conserved in the coating. However, APS coatings exhibited porosity and inter-lamellar cracking. To mitigate this, laser remelting was conducted, as elucidated in the following section. In contrast, Xiao et al. reported similar microstructures for GA-APS CrMnFe-CoNi, but with a fluffy, porous top surface. This is believedto arise from volatilization of manganese during spraying, which has a boiling point 2061 C. This manganese vapor reacts with oxygen, resulting in the porous manganese oxides detected on the coating surface.
Finally,a Co-free CrMnFeNi coating was developed by Lehtonen et al. for applications in the nuclearindustry.A GA-cold spray route using N2 as the process gas was used. The major FCC phase in the powder wasobserved in the coating, while a minor BCC phase is believed to have dissolved during processing. As in the work of Yin et al., grain refinement along the particle boundaries was observed and attributed to theintense plastic deformation typical of the CS process.
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