NiCrBSi-based coatings with WC-Co reinforcement and h-BN solid lubrication: microstructure and wear behavior after flame and HVOF spraying with flame fusing

Thermally sprayed self-fluxing NiCrBSi-based coatings modified with WC-Co as a reinforcing phase and h-BN as a solid lubricant were produced using conventional flame spraying and high-velocity oxygen-fuel (HVOF) spraying, followed by flame fusing to improve densification and interfacial bonding. The influence of the spraying energy input, coating composition, and post-deposition flame fusing on the microstructural evolution and tribological performance at room temperature (RT) and 400 °C are investigated. Novel insights into the stabilization and distribution of carbides (Cr 7 C 3 ), borides (CrB) and Ni-based intermetallic phases (Ni 3 B, Ni 31 Si 12 ), as well as the persistence, partial degradation, and anchoring behavior of h-BN particles are provided. Flame spraying leads to thicker coatings with splat-related inhomogeneities, whereas HVOF spraying produces thinner, denser coatings with finer microstructures. Flame fusing substantially modifies both coating series by eliminating splat boundaries, inducing partial remelting, and promoting the formation of eutectic structures and a dendrite-containing fusion zone that includes Mo 6 Ni 6 C. Fretting wear tests under severe reciprocating conditions at RT and 400 °C reveal a strong dependence of wear behavior on coating composition, microstructure, and processing history. Flame fusing the HVOF coatings induces a partial decomposition of WC and the formation of brittle W 2 CoB 2 , resulting in increased microcracking and higher wear rates of ~0.7–1.5 × 10 −4 mm 3 /Nm at RT. However, the wear rates at RT and 400 °C remain comparable due to embrittlement and the development of oxide-based tribolayers. Analyses of the worn surfaces indicate a temperature-dependent shift in the dominant wear mechanisms—from adhesive wear and surface fatigue at RT to mixed abrasive–adhesive wear combined with tribochemical oxidation at 400 °C, where oxidized tribolayers and plasticized splats facilitate lubrication and coefficient of friction (COF) stabilization. These findings provide new insights into the role of microstructural heterogeneities, WC decomposition, dendrite-containing fusion zones, and h-BN particle behavior in governing the tribological response of thermally sprayed coatings, outlining a pathway toward advanced coating solutions for high-performance valve components and related tribological applications. • h-BN and WC-Co particles exhibit distinct stability and interfacial bonding. • Flame fusing enhances densification and promotes metallurgical inter-splat bonding. • Partially degraded h-BN causes debris and reduces lubrication efficiency. • Flame fusing can form dendritic fusion zones and induces WC decomposition. • Process–structure design can improve wear resistance for high-temperature service.