Deposition Kinetics and Structural Transformations in WC–12Co and Cr3C2–NiCr HVOF Coatings Depending on Powder Dispersion

HVOF coatings based on tungsten and chromium carbides are widely used in aerospace, energy, and oil and gas industries. However, the selection between the wear resistance of WC–Co and the corrosion resistance of Cr3C2–NiCr is often hindered by a lack of systematic data on the effect of powder particle size. In this study, WC–12Co and Cr3C2–NiCr coatings were deposited onto 12Kh18N10T stainless steel substrates by HVOF spraying using three particle size fractions: <20, 20–32, and 32–40 μm. Individual spray parameters were selected for each material to ensure high-quality deposition (standoff distance 350 mm, one pass for WC–12Co; 250 mm, two passes for Cr3C2–NiCr). The influence of powder particle size distribution on coating properties was investigated using a comprehensive set of methods, including scanning electron microscopy, X-ray diffraction with Rietveld quantitative phase analysis, microhardness testing, tribological testing, electrochemical measurements, and numerical modeling of particle in-flight motion. It was found that the 20–32 μm fraction is optimal for both materials. Different degradation mechanisms were identified: for WC–Co, decarburization of WC with the formation of W2C and η-phases dominates (maximum for the <20 μm fraction); for Cr3C2–NiCr, oxidation and carbide dissociation prevail (in the <20 μm fraction—13.7% Cr2O3 and 38.6% Cr7C3), leading to low coating thickness (~16 μm) due to loss of ductility and wettability. Numerical modeling confirmed that the high velocity of fine Cr3C2–NiCr particles (~680–720 m/s) does not compensate for their overheating, whereas for WC–12Co all fractions have velocities above the critical threshold, but coarse particles (32–40 μm) give porosity up to 3.15% due to insufficient deformation. Corrosion tests showed that under spraying conditions optimized for each material, the Cr3C2–NiCr coatings (corrosion rate 0.066–0.102 mm/year) are an order of magnitude superior in corrosion resistance to WC–12Co coatings (corrosion rate 0.353–0.651 mm/year), owing to the passivation of the γ-Ni(Cr) matrix; for WC–Co, the main protective barrier is provided by the structural density.