AlSiCrFeCoNi alloys have also generated some traction primarily due to their high-temperature oxidation resis-tance. Bhattacharya et al. generated AlsoSi2Cr23.Co22Ni23 coatings using commercially produced GA powder. The coarser cut(-75+45 um) was used as feedstock for APS, and the finer(-45+20 um) cut for HVOF. BCC phase in the feedstock transformed to BCC+FCC+Al2O3in the APS coating, whereas a BCC/B2+FCC+A12O3constitution was achieved via HVOF. The APS coating was reportedly inhomogeneous, exhibited larger oxide and porosity content, and was rejected in favor of the denser HVOF coatings for subsequent oxidation studies. Interestingly, two ' layers' wereobserved in the HVOF coating, the lower layer appearing denser than the top one. This morphological effect may be due to a tempering and shot-peening-like treatment expe-rienced by the bottom layers during deposition of the toplayers. This study is novel because the authors used ther-modynamically calculated phase diagrams to choose the TS-HEA alloy composition. This method reduces the critical compositional variable of TS-HEA coatings property-parameter optimization.
Tian et al. studied equiatomic AlSiCrFeCoNi ball-milled for 30 h and coated via APS. Elemental Si was detected in the 30 h MA powder, which dissolved during the melting-resolidification routine during spraying, and B2+BCC+FCC phases were detected in the coating.Jin et al. explored the effect of varying Si content in AlosSi, CrFeCoNi(x=0.5,1,1.5,2) APS coatings.Phase separation into a Fe-Co-Cr-Al-rich BCC,a Fe-Co-rich FCC, and CraSi was observed in the coating, attributed to the use of blended elemental feedstock. The BCC phase fraction was observed to increase with Si content, and at x>1, no FCC or CrsSi was observed, due to the formation of a supersaturated solid solution during APS. Oxide splatswere also detected in the coating cross section along with porosity and interlamellar cracks. Thus, while some supersaturation may be achieved in-flight during plasma spray, it is not the recommended method for alloying using blended powders. It is noted, on the other hand, that supersaturation may be achieved in laser cladding since the time at temperature is significantly greater.
The effect of Al content on phase formation and wear behavior was examined by Xiao et al. in MA-APS Al, SiCrFeCoNi(x=0.5,1,1.5 molar ratios) coatings. BCC+FCC phases observed in the MA powder were translated into the coating with some IFO. An increase in the Al content did not affect the phases significantly. Heattreatment of the coatings led to evolution of the CraNisSi2 phase, which improved the wear resistance of the coatings.
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