Fabrication of micro-/nano-structured superhydrophobic ceramic coating with reversible wettability via a novel solution precursor vacuum plasma spray process

A novel plasma spray process, solution precursor vacuum plasma spray, was developed and used to fabricate superhydrophobic ceramic coatings. By injecting Yb(NO3)3 solution into a plasma jet operated at low pressure, a crystalline Yb2O3 coating was deposited that exhibited hierarchical structures with nano-scale splats/particles superimposed on micron-sized spherical clusters. The coating exhibited a water contact angle of ~160°, a roll-off angle of ~7°, and multiple complete droplet rebound behavior. Tests to explore the self-cleaning behavior, the resistance to contamination, and performance under water flushing and jetting conditions were conducted and further demonstrated excellent and durable hydrophobicity for the coatings. The superhydrophobic coatings became superhydrophilic after long-term (5 h) exposure to temperatures of 200 °C and above; the superhydrophobicity was recovered after vacuum treatment, showing a reversible wettability. The surface concentrations of Yb, O, and C for coatings with different wettability were investigated by X-ray photoelectron spectroscopy, which showed that the reversible wettability could be attributed to the adsorption and desorption of hydrocarbon species on the coating surface. Mathematic models were developed which correlated the water contact angles with coating structures for the superhydrophobic and superhydrophilic coatings.