Fracture toughness enhancement mechanism of multimodal WC-10Co-4Cr coating sprayed by HVOF

WC-10Co-4Cr coatings are widely used for surface protection of aircraft landing gear due to their high hardness and wear resistance. Generally, reducing the WC grain size in the feedstock powder enhances the hardness of coatings prepared by high-velocity oxy-fuel (HVOF) spraying. However, this grain size refinement also increases the surface area and promotes severe decarburization, leading to the formation of brittle phases, which in turn limits the fracture toughness of WC-10Co-4Cr coatings. In this study, three conventional WC-10Co-4Cr coatings (with WC grain sizes of 1.5 μm, 0.8 μm, and 0.3 μm) and one multimodal WC-10Co-4Cr coating were prepared via HVOF spraying. The results show that with decreasing WC grain size, the hardness of the conventional coatings increased from 1059.09 HV to 1182.07 HV, while the fracture toughness decreased by approximately 36 %. In contrast, the multimodal coating exhibited an 18 % improvement in fracture toughness compared to the conventional coatings, while maintaining a high hardness of 1178.18 HV. The enhanced toughness of the multimodal WC-10Co-4Cr coating is mainly attributed to its multimodal microstructure, which facilitates the formation of Cr-rich transition layers and Cr-rich precipitates at the WC interface. These Cr-enriched features significantly inhibit the decarburization process, thereby improving the coating's structural stability and fracture resistance.