Synergistic Enhancement of Magnetorheological Fluid Sedimentation Stability via Plasma Surface Engineering and Micro–Nano Dual-Dispersed Particles

Poor sedimentation stability remains a critical bottleneck hindering the widespread application of magnetorheological fluids (MRFs). To address this challenge, we propose a synergistic strategy that combines plasma surface engineering with micro-nano dual-dispersed particles to simultaneously tailor interfacial properties and suspension microstructure. Graphite-coated Fe/FeN core-shell magnetic nanoparticles (Fe/FeN@G) were synthesized through the pyrolysis of ferrocene in high-temperature plasma and subsequently fluorinated in a low-temperature CF₄ plasma to obtain Fe/FeN@PFG. Unlike previous studies that individually employed plasma-treated carbonyl iron particles or bidisperse micro-nano systems, the present work integrates fluorinated Fe/FeN@PFG nanoparticles as a nanosized secondary phase with micron-sized carbonyl iron (CI) at a fixed solid loading of 50 wt % without additional stabilizing additives, thereby constructing an additive-free, plasma-engineered bidisperse MRF. Plasma fluorination introduces a fluorine-containing organic layer (C-F/C-F₂ bonds), reduces the contact angle with silicone oil from 65.7° to 40.3°, and promotes the formation of a shear-sensitive three-dimensional thixotropic network. The resulting Fe/FeN@PFG-CI MRF exhibits a high zero-field viscosity of 73.2 Pa·s at 0.01 s^-1, providing structural antisedimentation support, while maintaining a low viscosity of 0.111 Pa·s at 1000 s^-1 to ensure good flowability. Despite the enhanced off-state viscosity and elasticity, the system retains reversible magnetorheological responses and fast thixotropic recovery. Sedimentation stability is markedly improved, as reflected by a turbiscan stability index of only 0.51 after 5.3 h and a sedimentation ratio of 85% after 72 h. These findings demonstrate that coupling core-shell nanoparticle design with CF₄ plasma fluorination offers a cost-effective and scalable route to high-stability, high-performance MRFs that complement and extend existing stabilization strategies.