Assessment of HVAF thermally sprayed coatings: Unraveling microstructural, electrochemical, and tribological performance using glass former Fe-Cr-Mo-Nb-B feedstock powder

Abstract

In this paper the microstructural features of the glass former Fe₆₈Cr₈Mo₄Nb₄B₁₆ coatings are unveiled and related to their electrochemical and tribological responses. The coating was mostly glassy with some embedded borides (M₃B₂, M₂B-tetragonal; M being the metallic elements of the alloy) and ferrite. The tribological behavior of the HVAF coated sample, characterized by a thickness of about 200 µm, ∼6% porosity and a Vickers hardness of 357 HV_0.5, was assessed in a sphere-on-plate configuration, revealing a specific wear rate of approximately 5 ×10⁻⁴ mm³∙N⁻¹m⁻¹. The wear mechanism was dominated by delamination caused by fragile intersplats. The corrosion resistance of HVAF coatings was evaluated in 0.6 M NaCl solution and compared with the results obtained for the crystalline Fe₆₈Cr₈Mo₄Nb₄B₁₆ ingot, produced by melting in an induction furnace, and for the AISI 1020 steel substrate. The HVAF coating showed satisfactory corrosion resistance compared to the carbon steel substrate and the crystalline ingot, with the highest corrosion potential, E_corr, values (−533 mV_SCE) and the lowest corrosion current density, i_corr, (10⁻⁶ A∙cm⁻²) followed by a clear passivation window upon anodic polarization in 0.6 M NaCl solution. Evaluations of HVAF coating showed a higher glassy content compared to the gas-atomized feedstock powders. This suggests that during spraying, certain particles were molten and experienced cooling rates adequate to inhibit crystallization, resulting in the freezing of the supercooled liquid. This phenomenon contributes to the good corrosion resistance observed in the present work and offers an opportunity to enhance the electrochemical behavior of HVAF coatings.