Abstract
The present study investigates the effect of flame-spraying process parameters on the microstructure and mechanical properties of Ni–Cr–B–Si-based self-fluxing coatings. Corrosion-resistant AISI 321 austenitic chromium–nickel steel was used as the substrate material. The coatings were deposited by flame spraying using a Metal Coat 6PM-II system. To vary the thermal conditions of the spraying process, the oxygen-to-acetylene (O₂/C₂H₂) ratio was adjusted to 1.0, 1.1, and 1.2. The spraying parameters were monitored using an Accuraspray 4.0 diagnostic system. The resulting coatings were characterized in terms of microstructure, porosity, microhardness, surface roughness, and adhesion strength. Increasing the O₂/C₂H₂ ratio resulted in densification of the coating structure and a decrease in porosity from 1.97 ± 0.13% to 0.90 ± 0.09%. Concurrently, the surface roughness decreased from 4.70 ± 0.09 μm to 2.99 ± 0.11 μm, while the adhesion strength increased from 22.14 ± 4.3 MPa to 35.02 ± 3.9 MPa. The highest average microhardness of 1051.7 HV was obtained at an O₂/C₂H₂ ratio of 1.1. Among the investigated spraying conditions, the coating produced at O₂/C₂H₂ = 1.2 exhibited the lowest porosity and surface roughness together with the highest adhesion strength. The results demonstrate that controlling the O₂/C₂H₂ ratio provides an effective means of tailoring the microstructural and mechanical characteristics of flame-sprayed Ni–Cr–B–Si-based self-fluxing coatings.