Synthesis of nanostructured Cr3C2-25(Ni20Cr) coatings

On the basis of the nanocrystalline Cr₃C₂-25 (Ni20Cr) feedstock powders produced by mechanical milling, a nanostructured coating has been synthesized using high velocity oxygen fuel (HVOF) thermal spraying. The properties of the nanostructured coating were compared to those of the conventional coating of the same composition using scanning electron microscope (SEM), transmission electron microscope (TEM), and microhardness tests. The nanostructured Cr₃C₂-25 (Ni20Cr) coating synthesized in this study had an average carbide particle size of 24 nm. Discontinuous elongated amorphous phases were observed in the nanostructured coating. The conventional Cr₃C₂-25 (Ni20Cr) coatings produced using blended elemental powders exhibited an inhomogeneous microstructure. The observed homogeneity of the nanostructured coating is attributed, in part, to the microstructural improvement of the starting powder. The nanostructured Cr₃C₂-25 (Ni20Cr) coating yielded an average microhardness value of 1020 DPH₃₀₀, which corresponds to a 20 pct increase in microhardness over that of the conventional coating. The nanostructured Cr₃C₂-25 (Ni20Cr) coating also exhibited a higher apparent fracture toughness relative to that of the conventional coating. The apparent mechanical property improvements in the nanostructured coating were thought to result from the uniformity of the microstructure and the high performance associated with a nanostructured structure. In addition, the mechanism that is present during the milling of a system containing nondeformable particles is discussed in light of the TEM observations.