Process, Microstructure, and Properties of Continuous Thermal Jet-PVD Zinc Coatings on Steel Strips
Traditional continuous galvanizing of steel strips faces escalating energy consumption, wettability limitations on advanced high-strength steels, and hydrogen embrittlement risks. This study establishes and experimentally validates a coupled Hertz-Knudsen evaporation and choked-flow jet deposition model for a continuous thermal jet physical vapor deposition (PVD) process. Using SPCC cold-rolled steel strips at a substrate temperature of 160 °C, zinc vapor generated in a crucible (913–943 K) is transported through a vapor distribution box and accelerated to supersonic velocity (Mach > 1) via a Laval nozzle. Theoretical coating thicknesses (9.49–15.42 µm) agree with measured values (9.22 ± 0.2 to 13.2 ± 0.6 µm), confirming model validity. Compared with non-continuous vacuum evaporation, the continuous thermal jet-PVD process increases deposition rate from 5 nm/s to 1.32 × 10^4 nm/s (2,600-fold), coating density from 86.3% to 97.4%, gloss from 3.7 GU to 30 GU (8-fold), and adhesion critical load from 24,316 mN to 34,618 mN (42% increase). Corrosion current density decreases from 66.4 µA/cm² to 27.8 µA/cm² (58% reduction), and neutral salt spray red-rust time extends from 48 h to 72 h. Microstructural analysis reveals a transition from hexagonal crystallites with random orientation to a microcrystalline structure with a dominant (102) texture. This dense (102)-oriented coating acts as a physical barrier against chloride ion penetration and promotes a protective corrosion product layer, offering a high-speed, clean alternative to conventional hot-dip and electro-galvanizing.