Effects of Nb Content and Heat Treatment on the Microstructure and Properties of Plasma-Sprayed CoCrFeNiNbx High-Entropy Alloy Coatings on Ductile Iron Substrates

Ductile iron suffers from insufficient wear resistance under heavy-load service conditions. Surface engineering technologies offer effective solutions to this problem. However, current research on the application of atmospheric plasma-sprayed (APS) CoCrFeNiNbx high-entropy alloy (HEA) coatings on ductile iron and the systematic study of compatible heat treatment processes with the substrate are still insufficient. In this study, CoCrFeNiNbx HEA coatings (x = 0.25, 0.50, 0.75, 1.00) were deposited on QT800-5 ductile iron by APS, and the effects of Nb content and low-temperature annealing (400–600 °C) on coating microstructure and properties were investigated. The x = 0.25 coating exhibited a single face-centered cubic (FCC) solid solution structure, while coatings with x ≥ 0.50 comprised an FCC solid solution and Cr2Nb-type Laves phase; hardness increased with Nb content, and as-sprayed wear resistance peaked at x = 0.75. Post-deposition annealing at 500 °C yielded a peak hardness of 477.45 HV and reduced the wear rate by 45% relative to the as-sprayed condition, with no measurable degradation of the substrate. These findings offer a practical reference for developing wear-resistant coatings on ductile iron components.